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2131   Sun Jul 2 21:21:39 2017 KojiUpdateWOPOMode Matching Woes - Fibre issues

Jenne's laser at the 40m PD testing table is a fiber coupled 1064nm DL.

But you just can couple 5~10% of the beam from the other side of the fiber to know the mode at the input side.
It does not require too much effort if you have the fiber testing illuminator to align the beam.

2132   Wed Jul 5 16:44:34 2017 KojiUpdateWOPOPDs for homodyne detector

Correction: If the diode is 3mm x 3mm, it is Excelitas C30665.

2133   Thu Jul 6 15:03:07 2017 DhruvaUpdateWOPOHomodyne Design

I am attaching a schematic of the proposed design for the Homodyne detector. The damping capacitance in the two photodetectors has to be decided based on what we estimate the capacitance of the photodiode to be.

Please let me know if there's anything I can do to make this better suited for homodyne detection.

Attachment 1: Homodyne.pdf
2135   Mon Jul 10 12:12:26 2017 DhruvaUpdateWOPOEfficient Fibre Coupling and Phase Matching Plot

Using the 632nm laser provided by Gautam to back couple the 1064nm light into the fibre, I managed to finally get efficient mode matching. For 25.4mW of light going into the collimator, the fibre outputs 19.3mW (~76%).  Now that efficient mode matching has been achieved, I finally managed to plot the SHG Phase matching curve for the PPKTP Waveguide (a plot of output power (532nm) against the temperature). I fit the data obtained to the function

$P_0sinc((T-T_0)/a)^2$

Here, the normalised sinc(x) fiunction is given by

$\frac{sin(\pi x)}{\pi x}$

The optimised values are

$P_0 = 65.3 \mu W$ Maximum SHG Power

$T_0 = 60.99$  Optimum Phase Matching Temperature.

$a = 2.609$

That gives the FWHM as 2.32 C.

The maximum output is much lower than expected as that is because of loss at the fibre coupling at the waveguide. This loss is visible and I am attaching a couple of pictures.

Attachment 1: Phase_Matching.pdf
Attachment 2: Green_Loss_1.jpg
Attachment 3: Green_Loss_2.jpg
2140   Mon Jul 17 22:19:36 2017 DhruvaSummaryWOPOLast Week in WOPO

1. I have achieved 50% fibre coupling efficiency for the 532nm beam. I have attached the simulation of the estimated solution in ABCD and the profiles after the first two lenses.

Solution - 50cm lens at 73.4cm, 35cm lens at 105.4cm 17.5cm at 150.2 cm.

The beam profiles approximately agree wth the given solution.

2. I measured the attenuation of 1064nm light in the 532 nm single mode patch cable. These are the reaadings I took of input 1064nm power vs output power.

5mW   -0.07uW

10mW - 0.10uW

20mW- 0.12uW

30mW - 0.14uW

40mW - 0.15uW

50mW - 0.26uW

I will later calculate how this would lead to a loss in measured squeezing.

3. While trying to characterise the fibre beam splitter, I found that for the for an input power of 50mW in the 1064nm fibre, there was a total of 22mW coming out of the output ports. On checking the 1064m fibre, we found that the mode shape coming out of the fibre wasn't clean. Inspection through a fibre microscope showed no damage to the fibre ends. We have changed the fibre, but lost the 75% mode-matching in the process. I'm getting a maximum of 50% with the new fibre.

TODO -

Noise analysis of photodetectors in the ATF Lab (I have collected data from the FFT network analyser. I will plot it and put it up on the elog)

Make the Homodyne Setup.

See Squeezing.

Measure Squeezing.

Calculate Squeezing Losses.

Convince people that these losses can be reduced.

Successfully finish SURF project

Attachment 1: 532_MM.pdf
Attachment 2: 532_f1.pdf
Attachment 3: 532_f2.pdf
Attachment 4: sm1064att.pdf
Attachment 5: sm1064out.pdf
2141   Tue Jul 18 22:09:14 2017 DhruvaUpdateWOPOPhotodetector Dark Noise

These are the plots of the dark noise of the Thorlabs photodetectors lying in the ATF lab using the FFT network analyser. For higher frequency ranges, I have to configure the other network analyser.

These are the settings that the analyser was running on -

Measure Group: FFT
Measurement: FFT 1
Num of extracted Points: 401
FFT Lines:  400
Window: Hanning
Averaging Mode: RMS
Averaging Type: Exp. / Cont.

Edit : I've added a stiched plot of all the collected data. The noise from 10-100KHz is around the order of 40nV. We're hoping to see if we can do better by designing our own photodetectors. We also see a lot of peaks that correspond to the harmonics of the 60Hz mains.

Edit 2: The Photodetector is the Thorlabs PDA55 detector.  We CANNOT use this detector as it is silicon and has a terrible quantum efficiency at 1064nm.

Attachment 1: 20170714-pda55noise.pdf
2142   Thu Jul 20 18:16:33 2017 ranaUpdateWOPOPhotodetector Dark Noise

Ugh - I deleted those 1000 bad plots. Just give us 1 trace per PD, all on one plot. Each trace should also include the model # of the PD. Just 'stuff we have laying around' is not useful.

Also, what are the requirements on the PD? Describe how these are computed.

2144   Sat Jul 22 14:25:55 2017 DhruvaUpdateWOPOTable Noise Issues

Yesterday, with a lot of help from Koji, we built the transimpedance circuit (Gain = 10k) for the photodetectors of the homodyne circuit.

While doing so, we encountered a most bizzare issue. The circuit shows a significantly larger amount of noise (especially in the 10-100kHz band) when it is on the table as opposed to when it is suspended in air. I'm attaching pictures if the setup as well as a comparative plot. We still cannot ascertain the reason behind this extra noise.

Attachment 1: 20170721-_table_noise.pdf
Attachment 2: 20170721_Circuit_in_Air.jpg
Attachment 3: 20170721_Circuit_on_Table.jpg
2145   Sat Jul 22 14:41:07 2017 DhruvaUpdateWOPOPhotodetector Dark Noise

 Quote: Yesterday, with a lot of help from Koji, we built the transimpedance circuit (Gain = 10k) for the photodetectors of the homodyne circuit.a

I connected the Excelitas C30665 photodiode to the above transimpedance circuit and measured the dark noise while suspended in air (please refer previous elog on table noise).

For a mW of light and a quantum efficiency of about 87%, we expect to see about 0.68mA of current. This gives the shot noise to be 14.7pA/sqrt(Hz) which corresponfs to about 147nV/sqrt(Hz) for a 10k gain which is significantly higher than the noise floor of the circuit between 10 and 100KHz.

Attachment 1: 20170721-dark_noise.pdf
2146   Sat Jul 22 14:43:16 2017 DhruvaUpdateWOPOSubtractor Circuit Noise

I also made a op-amp subtractor circuit for the homodyne detector and measured the noise. It is significantly lower than the shot noise.

Attachment 1: 20170721-subnoise.pdf
2187   Wed Jan 3 20:20:07 2018 awadeDailyProgressWOPOReboot WOPO

## Laser restarted + 1064 for Shotnoise + SQZ measurment

I've restarted the Diabolo and am checking the alignment into fibers. The current configuration coming out of the WOPO breadboard is a fiber 50:50 beam splitter followed by two matched F240APC-1064 nm fiber collimators.  There is a HT532HR1064 dichroic mirrors in each of the split arms remove any remaining residual green.  The plan is to use a single NF1811 in one arm to see if we can see SQZ out at RF.  It will be lossy and susceptible to RIN, but we will be measuring at very high frequency.

Power of 1064 nm after the power-control PBS is 3.12 mW, at the other end of the fiber I am seeing 300 uW.  At the output of the HD fiber colimators there is an even split of about 148 uW: about what we should expect. I will try to check the alignment tomorrow and see if I can identify shot noise on the NF1811 above the dark noise. I haven't don the calculations, will check these number tonight.

---

## Temperature control WOPO

I also tracked down the Newport 3040 temperature controller (found in the PSL lab).  I've reattached this to the WOPO butterfly mount and am able to get a temperature readout from the 10kΩ thermistor with a 10 µA test current (this should deliver 0.1V to the NP3040 ADC). There is an option for 100 µA excitation of the sensor (have used this in the past), but I figured less current means less self heating. Not sure what the situation is with S/N inside the box, its an expensive mystery.

Settings on the Newport 3040 are basically the same as before, see ATF:2124,  for good measure here are the full settings list:

Newport 3040 settings WOPO
Setting Value [units]
Sense type 10 [µA]
Mode Const T
Gain 2 Slow
Limit Ite 0.65 [A]
Tol time 1.0 [s]
Tol Temp 0.1 [C]
Limit Tl 18.00 [C]
Limit Th 70 [C]
C1 1.0445e-3
C2 2.5075e-4
C3 0
Ts (set point) 61.93 [C]

The NP3040 does give you explicit gain levels for P and I terms in the feedback loop real values. It just has mystery numbers 0.2, 0.6, 1, 2, 3, 5, 6, 10...300 with either "fast" or "slow". I used 2 Fast, and then gain 10 Fast.  Integration doesn't seem to be aggressive enough as its not reaching the set point.  Any more proportional gain and it overshoots and hits the shutdown rail on loop startup. A current of 0.4 A is needed to reach a set point of 61.93 C, so there is plenty of actuation headroom.  Its not an ideal PID loop but I'll leave this for now, it is enough to just move the set point a little higher.

---

## LO phase scanning

There is a 1064 nm mirror mounted on a PZT just before coupling into the fiber. Wires have been soldered to a BNC and solidly mounted to a L-bracket on the table. I have obtained a thorlabs HV driver that can do upto 150 V from 10 V .  There is an adjustable range with a switch on the back (75V, 100V or 150V), I need to check the voltage range allowable for this PZT before powering up.  The plan is to scan 1064 nm phase over a few wavelengths to scan the detected SQZ phase. About 100V will do it.

Something to check is the impact of banana shapped motion of the mirror+PZT, in the past this changed power through modecleaners by misaligning with the voltage scan. However, that was on very long PZT stacks. Might expect a similar effect coupling into fiber, its just something to calibrate out in the baseline shotnoise curve as a function of scan voltage.

---

I haven't checked the 532 nm coupling efficiency or made a shot noise measurement.  I have a  NF1811 + power supply and will try to look at this tomorrow with a spectrum analyzer.

2188   Wed Jan 10 17:03:55 2018 awadeDailyProgressWOPOWOPO some shot noise numbers

Summary, here are some numbers relating to requirements for for the WOPO squeezing detection:

In order to obtain some coherent light inducing some measurable shot noise, 1064 nm light is coupled in to fiber and injected into one of the legs of the fiber 50:50 beam splitter; the other leg of the 50:50 splitter is to be connected to the WOPO directly. If we can see shot noise then injecting a 1 dB squeezed state with 50% loss should give use a roughly 0.47 dB of squeezing. The variance of the prepared state will go as

$V^\textrm{(out)} = \eta_\textrm{total} V^\textrm{(in)}+(1-\eta_\textrm{total})$

where $\inline \dpi{100} \eta_\textrm{total} = \prod_i \eta_i$ is the total loss from the point of amplification. Computing dB of squeezing is then found by normalizing the squeezing the the LO shot noise level, taking 10*log10(V_in/V_shot).

Initially I will try to see squeezing with a single detector out of one of the legs of the fiber beam splitter.  This means a 50% hit in terms of loss and also that the detector is susceptible to intensity noise of the 1064 nm local oscillator (although we might expect this to be much low in the >1MHz range).   With about 1 mW of light on an ideal (eta=1, lossless) photo diode we should we photocurrent of order

$i_\textrm{DC} = \frac{Pe^\text{-}\lambda}{hc} = 0.856 P \approx 0.856\textrm{ mA}$

The shot noise current  at this power is given by:

$i_\textrm{sn} = \sqrt{2e^\text{-} i_\textrm{DC}} = e^\text{-} \sqrt{\frac{2\lambda}{hc}P} = 0.52 \sqrt{P} \textrm{ nA/}\sqrt{\textrm{Hz}} \approx 16.6 \textrm{ pA/}\sqrt{\textrm{Hz}}$

We want to look at the quantum noise at around 1 MHz and for it to be above all of the typical noise sources with reasonable margin (i.e. 6 to 10 dB clearance).  A New Focus 1811 detector is an OK choice as its quoted dark noise, or NEP, is 2.5 pW/sqrtHz. This is given for peak responsivity 1.05 A/W @ 1550 nm.  Scaling to equivalent NEP at 1064 nm amounts to rescaling NEP by the relative responsivity ratio

$NEP_\textrm{@1064 nm} = \textrm{NEP}_\textrm{1550nm}\frac{\mathcal{R}_\textrm{1550nm}}{\mathcal R_\textrm{1064nm}} = 2.5 \textrm{ pW/}\sqrt{\textrm{Hz}} \frac{1.05 \textrm{A/W}}{0.80\textrm{A/W}} \approx 3.3 \textrm{ pW/}\sqrt{\textrm{Hz}}$

The equivalent current is 2.6 pA/sqrtHz. Which gives a shot noise clearance above PD dark noise of 8.0 dB.  For order of 10 dB shot noise clearance we would need 2.5 mW: this is still within a factor of two of the maximum power of the detector.

At the moment I can't find at spare working NF1811. Here are some options that don't work (for the record):

A New Focus 1611 is not such a good choice. Peak responsivity NEP is 20 pW/sqrtHz; gain is also lower 700V/A. Here the current dark noise for the given responsivity of 1.05 A/W is is 21 pA/sqrtHz.  To get shot noise equivalent to detector dark noise we would need 1.48 mW. To get to 10 dB clearance I would need 148 mW. So no good here.

The Thorlabs PDA10CF has a NEP of 1.2e-11 W/sqrtHz @ 1.04 A/W.  At this current noise of 12.5 pA/sqrtHz we would need 58 mW for 10 dB clearance.

Maybe...

Looking at Zach's M2 ISS board (ATF:1888) it looks like it will clear the dark noise with 10 dB clearance, but nobody can tell me where that already built unit is or the spare PCB.  From what I know, the main issue with ordering more is sorting out whether using a MAX333A in place of the MAX333 will have sufficiently low through resistance (the MAX333A is about 20 Ω, compaired to the 100Ω MAX333 that Zach used in the initial test model.  There was also an issue with the size of some of the inductors compared to the design footprint, not sure if that is resolved.  Its also not RF.

---

### Coupling into fibers

At the moment with the best alignment (30 minutes effort) the efficiency of coupling into the fibers is 75% for 1064 nm and 50 % for 532 nm.  The 50:50 fiber beam splitter appears to have close to 1-2% loss.  Its rated up to 1W so we can easily get 1-10 mW out the other end.

Some more effort needs to be put into the 532 nm in coupling.  We will need order 10s mW.  I just don't want to burn the ends by just jamming a bunch of power in.  Need to check the cleanness of fiber ends with a microscope before upping the power: it seems this is how the last fibers were damaged.

Draft   Wed Jan 17 10:41:45 2018 awadeSummaryWOPOMaking a transformer coupled photodiodes

[DRAFT]

These are some some summary notes on bandwidth and noise budget of transformer coupled photodiode detectors and and example made for the WOPO experiment.

## PD transformer coupling: what and why?

A lesser known strategy in photodetector design is to transformer couple the photocurrent generated by a photodiode before applying amplification.  It offers a potential benefit in terms of improved input current signal-to-noise at the amplifier input. Monolithic style RF amplifiers only come in a couple of fixed input impedances, determined mostly by mass market commercial imperatives. Transformer coupling their inputs allow us to adapt common 50 Ω, low-noise figure, wide bandwidth RF amplifiers for use in TransImpedance Amplifiers (TIAs), boosting their current signal to noise at a tradeoff of bandwidth.  For the addition of this simple passive element we get an AC coupled output with added overall gain that adds (almost) no additional noise.

We don't, however, get a free lunch. There are tradeoffs in bandwidth and dirt effects of transformers that need to be carefully considered against the requirements. We also miss out on a DC coupled output that is useful for diagnostics and initial alignment.

A schematic of the basic circuit is shown below:

The photodiode is reverse biased and connected to the primary coil of a low resistance RF transformer of coil ratio* N:1, where N is the integer ratio of extra turns on the primary coil.  With less turns on the output secondary coil the voltage is stepped down but the current is scaled by a factor of N.  This means that for a fixed current noise at the input of a given amplifier we are able to increase the relative size of the input current: this means better signal to noise.

### Shot noise

Shot nosie is given by

$i_\textrm{sn} = \sqrt{2eI_p}$

where e is the charge per electron (1.602•10-19 C) and Ip is the photocurrent. Ip is given by

$\inline I_p = \eta\frac{eP}{h\nu} = \eta\frac{eP\lambda}{hc}$

where eta is the quantum efficiency of the diode (from 0 to 1.0), P is power incident, h is plank constant (6.626•10-34 J.s) and c is the speed of light. At some choice of incident laser power shot noise will become the dominate noise source.

Vendors tend to quote responsivity of diodes which is current per unit laser power

$\inline \mathcal{R} = \eta\frac{e\lambda}{hc}$

for 1064 nm light the maximum is 0.858 A/W and responsivity is scaled by the quantum efficiency of the diode.

### Input noise of the amplifier

Op amp first stage amplifiers have quoted voltage and current noise. Typically we choose FET input op amps for their lower current noise: these can be order 0.8 fA/rtHz and 5 nV/rtHz. For monolythic RF amplifiers there is a terminating impedance at the input. Thermal noise of this termination sets the limiting signal to noise.

Thermal noise (Johnson–Nyquist) associated with a 50 Ω termination resistance at the input of the RF amplifier given by

$i_\textrm{th} = \sqrt{\frac{4k_B T_n}{R}}$

where kB is Boltzmann's constant (1.38•10-23 J/K), Tn is the amplifier noise temperature (Ideal ~ 293K) and R (=50Ω) is the input resistance of the amplifier. For 50 Ω termination at room temperature, current noise is 18 pA/rtHz. Referenced to the input at the photodiode, the transformer reduces this apparent noise by a factor of N (the number of coils on the primary side).

$i_\textrm{th, in ref.} = \frac{1}{N}\sqrt{\frac{4k_B T_n}{R}}$

Also, there is a voltage induced by thermal noise at the input termination.  This goes as $\inline \sqrt{4k_B T_n R}$ and these fluctations in voltage accross the diode capactitace induce a $\inline \dpi{100} C\mathrm dV/\mathrm dt$  convertion to current noise, where C is the inherent capacitance of the PD.  At the diode the voltage is stepped up going from the amplifier to the diode. In the frequency domain this means the input refered current noise induced from amplifier voltage noise is given by

$\inline \dpi{100} i_\textrm{cv} = 2\pi fCN\sqrt{4k_B T_n R}$.

Thus, there is some tradeoff between current and voltage noise. Ignoring dark current of the PD, the total noise is

$\inline i_n = \sqrt{i^2_\textrm{sn}+(i_\textrm{th}/N)^2+ (i_\textrm{cv}N)^2}$.

There is another bonus here: reduction in the equivalent capacitance of the PD as seen by the TIA. Voltage noise at the input pins of the first stage op amp can be converted to equivalent current noise by the intrinsic capacitance of the photodiode.  Applying a voltage across a capacitive junction induces a current

$\inline \dpi{100} i_\textrm{in} = C_\textrm{in} \frac{\textrm d V_\textrm{in}}{\textrm d t}$

that, in the Fourier domain, is

$\dpi{100} \tilde{i}_\textrm{input}(f) = 2\pi e_n C_\textrm{in} f$

where $\inline \dpi{100} e_n$ is the amplifier voltage noise density [V/sqrtHz]. The effective step up of the amplifier voltage noise going back to the PD input g is N for a N:1 transformer the source impedance apprent at the amplifier input stage for a N:1 is N

$\tilde{i}_\textrm{input}(f) = 2\pi e_n NC_\textrm{in} f$

The migration of charge carriers to the two sides of a photodiode N-P region means that natural l capacitor is formed across the active region of the photodiode

## Noise Budget

• Effective input referred noise from apparent transformed impedance
• Insertion loss of typical and best in class transformers
• Hysteresis of ferromagnetic core devices
• Inherent thermal noise of transformer coils
• Bandwidth limitations of transformers themselves
• Bandwidth relating to impedance ratio of transformer configuation (N:1 turns)
• Amplifier stage noise

Optimization given desired noise profile and bandwidth

Dynamic range estimate

## WOPO Requirements

[xxx] Chaoyong Chen, Shaoping Shi, and Yaohui Zheng, "Low-noise, transformer-coupled resonant photodetector for squeezed state generation", Review of Scientific Instruments,  Volume 88, Issue 10 (2017). https://doi.org/10.1063/1.5004418

[xxx] Shreyas Potnis and Amar C. Vutha, "Broadband low-noise photodetector for Pound-Drever-Hall laser stabilization", Review of Scientific Instruments 87, 076104 (2016) https://doi.org/10.1063/1.4960088 https://arxiv.org/pdf/1607.01816.pdf

[xxx] Malcolm B. Gray, Daniel A. Shaddock, Charles C. Harb and Hans-A. Bachor, "Photodetector designs for low-noise, broadband, and high-power applications", Review of Scientific Instruments 69, 3755 (1998); https://doi.org/10.1063/1.1149175

Some typical 'chip' sized RF transformers: https://www.coilcraft.com/pdfs/pwb.pdf Range is limited, you might want to wind your own but this is challenging

* Note: in this post I refer to the primary coil as the one with more turns connected to the input side.  Some of the literature makes a habit of defining the secondary coil as the larger number of terms (the assumption is that you always want to step up voltage) and refer to hooking up the transformer with secondary on the input.

Attachment 1: BasicSchematic_TranCoupledPD.pdf
2199   Wed May 23 20:38:10 2018 ranaSummaryWOPOTransformer coupled photodiodes

What is going on with this fossilized squeezing experiment? I reckon that there is a loss diagram out there somewhere. Le'ts see some ellipses and arches:

https://youtu.be/WyF8RHM1OCg

2225   Thu Aug 2 15:01:21 2018 awadeDailyProgressWOPOBuilding a SN limited detector for WOPO experiment

For WOPO squeezing detection I've ditched the idea of using a transformer coupled PD: its a nice idea but more complicated that we need.  As we only need a BW of < 2 MHz I've opted to build a simple trans-impedance amplifier (TIA) that will have 10 dB clearance (in voltage noise) for 1 mW of incident LO power.

## Optical setup at WOPO output

Basic optical setup at the output of the WOPO is as below:

I have a strong feeling that a single ported homodyne detection will be dominated by laser intensity noise from the LO. I'll go back and make a second TIA if that is the case configure it properly with some subtraction.

## Photodetector design

The PD design is very basic.  The schematic is illustrated below.

Here I'm using a 1 mm^2 Excelitas InGaAs detector C30641GH photodiode.  I chose this one because we had it in stock at the 40m, it has a reasonably high Quantum Efficiency (QE) at 90%,  and, the smaller area makes the capacitance relatively low. The schematic is accurate as of now.  It is missing some series impedance on the output and and could really do with a buffer to provide a little more output.  Maybe a MAX4106.  We don't have any of these chips in the WB EE shop, I'll check at the 40m. Although, from 40m:13960 it sounds like having a couple in close proximity causes some bad crosstalk.

The goal is to use 1 mW of power on the detector and have a clearance of about 10 dB electronic noise below the shot noise level.  For 1 mW of power the shot noise photocurrent will be

$i_\textrm{SN} = \sqrt{2e^- i_\textrm{DC}} = \sqrt{2e^- \mathcal{R} P_\textrm{DC}} = \sqrt{2\times1.602\times10^{-19}C * 0.772 \textrm{A/W} * 1 \textrm{mW}} = 15.7 \textrm{ pA}/\sqrt{\textrm{Hz}}$

For this case an OP27 will have more then enough clearance between its current noise floor (0.4 pA/rtHz) and the expected shot noise level.  Assuming the detector has a junction capacitance below 50 pF then the voltage noise should meet our factor of ten clearance as well.

Current thermal noise induced across the feedback resistor will also be a concern.  The Johnson current noise across the feedback resistor is

$\delta n_\textrm{thR} = \sqrt{\frac{4 k_B T}{R_\textrm{fb}}} \qquad \textrm{[A/rtHz]}$.

Where k_B is  Boltzmann's constant (1.38e-23 J/K), T is temperature (300 K at room temp) and R_fb is the feedback resistor value. If the goal is to have input referred noise current a factor of ten below 16 pA/rtHz then the ideal resistor value is about 6.1 kΩ.  I chose 6.8 kΩ because that is what I had available in SMT 0805 thin film (better than thick metal film for excess noise) and it is still small enough not to saturate the DC level at the output.

To make sure I didn't have any weird stability issues I also put a phase compensation cap across the op-amp feedback path.  This is to ballance out the input side capacitance that forms a virtual zero with the feedback path and ensure the OLG has an acceptable stable slop at the UGF.  The choice of cap is often given as

$C_\textrm{FB} = \frac{1}{4\pi R_\textrm{FB}f_\textrm{GBWP}} (1 + \sqrt{1 + 8 \pi R_\textrm{FB}C _\textrm{PD}f_\textrm{GBWP}})$

For a 1 mm^2 Excelitas InGaAs detector C30641GH photodiode the capacitance is 40 pF (100 pF) for the biased (unbiased) case.  This would suggest a good value should be 12 pF (19 pF) for biased (unbiased) case.  After entering the circuit into LISO and playing around a bit I found that 24 pF was a good safe value for this specific case.  In the end I used 22 pF, the closest value available.  I've attached my interactive python notebook with slider widgets below if you want to explore the parameter space.

The detector is biased directly from the positive supply rail.  The plan is just to battery power the circuit for now with ±12V from a SR560, so there should not be too much noise coupling from there. To be safe I put a 1 kΩ + 1 µF passive LP filter to the cathode. This will give a corner of 159.0 Hz. For about 1 mA (equivalent to about 1 mW of power) this will drop the bias voltage by about 1 V.  This is fine as we still have more than enough before the diode capacitance keeps going up: see datasheet for C30641GH

Bandwidth of the circuit is a function of the op amp GBWP, photodiode capacitance, and the choice of feedback gain. Its given by

$f_\textrm{pf} = \sqrt{\frac{\textrm{GBWP}}{2\pi R_\textrm{FB} C_\textrm{PD}}}$

For OP27 the GBWP is 8 MHz and the R_FB = 6.8 kΩ and C_PD = 22 pF in our case.  This gives a PD BW of 2.2 MHz, sufficient for my needs.  A set of plots generated from LISO output is given below: inset in the figure shows the chossen TIA component values.

## Construction

Nothing fancy.  Its made onto a standard layout proto-board.  What I did do to minimize parasitic capacitance was to solder the 6.8kΩ feedback resistor to the back of the SOIC8 to DIP8 converter board (pictured below).  The old fashioned op amp circular footprint in the center gave pads just the right spacing for a 0805 resistor.  I then soldered the compensation cap directly on top of the resistor​.  The whole thing makes a nice all in one TIA that I then soldered to a larger proto-board.

The OP27 on the DIP adaptor was then a complete TIA that I soldered to the board and added all the usual bypass caps etc.  Its not pretty but it will do the job.

The noise as measured on SR785 checks out but I need to connect it to the network to get data out.  More to come.

Attachment 1: OutputDetectorOpticalConfig.pdf
Attachment 3: 20180802_SimpleTIA_C30641GH_Detector_full.pdf
2229   Mon Aug 6 17:30:53 2018 awadeDailyProgressWOPOToo much RIN, making a balanced detector

I tweaked up the alignment into the 1064 nm fiber and directed 1 mW onto my new detector.  The noise I see is well in excess of the expected shot noise.  I'm guessing this is just purely RIN as there is no stabilization of the Diabolo (a Mephisto on the inside).

## Noise higher than expected shot noise level

Below is a plot of the input referred current noise as see by a SR785.  Although the BW of the photo detector is 2 MHz it is easier to see on the network analyzer (with its lower input noise) without further pre-amplification. All of the audio and other LF noise pickup is below 1 kHz leaving plenty of the band that is relativity low noise.

Here the detector is operating on the ±12 V lines provided out of the back of a SR560.

There are some discontinuities at the points where the plots were stitched.  Not really sure what happened there but the SR785 was re-ranging a bit over the measurement period.  I haven't overlayed the LISO model, I'll wait till I get some better data.  For now it looks about the right level for the dark noise of the detector.

## Making a balanced detector

I built an identical copy of the first detector over the weekend.  However, for the second detector I flipped the bias to the negative rail to invert the output. The two PDs now put out approximately equal DC voltage of 5.2V. I then built a simple inverting summing amplifier using an OP27.  The schematic is illustrated below.  It makes use of a potentiometer to trim the summed balanced signal.  We need some fine tuning here as we are trying to place the amplitude noise into common mode.

A quick measurement of the summing circuit put the output noise at about 12 nV/rtHz: this makes sense for the 1 kΩ resistors and OP27 voltage/current noise all together.  This would add an additional 1.8 pA/rtHz of additional noise (input referred) on the homodyne readout signal. This still places me well clear of the 16 pA/rtHz level of shot noise.

The variable resistor element is not ideal.  It is probably introducing excess noise and thermal drift over 5-10 minute time scales. Its also not linear gain as a function of resistance (except over very short ranges). Not sure if there are any good ways to fine trim with an analog circuits without a pot. I can rebuild if something is forthcoming. For now I just used the best quality pot I could find.

That said, I was able to quickly get a Common Mode Rejection (CMR) of 60 dB almost strait away. The method for checking common mode rejection involves injecting an amplitude modulation, in this case modulating laser diode current at 1.234 kHz, and trimming ballance until the peak in the output spectrum is nulled​.  This is most easily seen in swept sine mode, which gives a nice time domain plot of the CMR as the trim is adjusted.  The traces are uninteresting so I haven't included them here.

The noise level at the output was then on the order of 160 nV/rtHz. This is pretty much exactly what we would expect for shot noise for 2 mW of incident power.

I'll post a plot when I have some good data.

## Scatter

There is no need to do much about this at this stage.  I can pick a measurement span well above the scatter opto-mechanical pickup region which seems to be around 1 kHz and below.  Somewhere in the 10-100 kHz region should be fine.

## Setting up the 532 nm pumping

I've realigned the 532 nm into the end of the waveguide again.  For about 6 mW of input green I get about 4-5 mW at the output of the fiber.  I made an initial attempt to see if I could see anything but what really need is a zero span scan with a spectrum analyzer.  I've recovered the Agilent from the 40m but haven't quite got to this measurement yet.  The LO 1064 nm light used for the readout is bounced off a PZT mounted mirror and we are able to modulate the phase with a function generator and HV amp.

I'll check up on the power handling thresholds of the WOPO and then see what I can see.

---

Data and notebook used for plotting are attached below. Iris was used to stitch data.

2231   Thu Aug 9 16:51:43 2018 awadeDailyProgressWOPOTesting balanced detector

I did a number of initial tests and then attempted to inject 30 mW of light into the WOPO to see if I could at least some amplification (anti-squeezing). I saw nothing but relize now that I have not done enough to control the polarization either going into the WOPO or for the LO beam in the homodyne.

Combination is being done in a TN1064R5A2A 50:50 narrowband fiber beam splitter. Wagga Wagga.

2235   Mon Aug 13 01:48:48 2018 ranaDailyProgressWOPOSR560 battery as a voltage supply

When powering from the SR560 bananananas, you should disconnect AC power, else it may produce extra lines due to the charger.

 Quote: Here the detector is operating on the ±12 V lines provided out of the back of a SR560.

2236   Mon Aug 13 01:53:21 2018 ranaDailyProgressWOPOzero-span spec OR RF bandpass

I know the squeezing people often use the zero-span feature of a spectrum analyzer to produce their McDonalds plots, but why not just use a LC bandpass (e.g. a 1 MHz wide mini-circuit filter)?

The transimpedance amp could drive a mini-circuits amp which then drives the bandpass into a RMS->DC circuit (some diodes). Then you can plot it on a scope easily. Maybe not worth it for this first measurement.

2237   Mon Aug 13 15:43:55 2018 ChrisDailyProgressWOPOzero-span spec OR RF bandpass

 Quote: I know the squeezing people often use the zero-span feature of a spectrum analyzer to produce their McDonalds plots, but why not just use a LC bandpass (e.g. a 1 MHz wide mini-circuit filter)? The transimpedance amp could drive a mini-circuits amp which then drives the bandpass into a RMS->DC circuit (some diodes). Then you can plot it on a scope easily. Maybe not worth it for this first measurement.

2241   Wed Aug 15 09:57:56 2018 awadeDailyProgressWOPOSR560 battery as a voltage supply

I think this was unplugged from the wall at the time. I guess I should be careful about the grounding in connections to the laser too.  If we are looking out above the 100s of kHz then maybe a lot of these lines don't matter as much.

Quote:

When powering from the SR560 bananananas, you should disconnect AC power, else it may produce extra lines due to the charger.

 Quote: Here the detector is operating on the ±12 V lines provided out of the back of a SR560.

2242   Mon Aug 20 12:08:17 2018 awadeDailyProgressWOPOzero-span spec OR RF bandpass

zero-span is just for a quick and dirty measurement.  As long as the there is ok noise clearance its usually enough to see what is going on.  Chris's AD8361 seems good as well, will just require some extra building.

Not sure what the best BW is to work with 300 kHz - 1 MHz is ok I guess.

 Quote: I know the squeezing people often use the zero-span feature of a spectrum analyzer to produce their McDonalds plots, but why not just use a LC bandpass (e.g. a 1 MHz wide mini-circuit filter)? The transimpedance amp could drive a mini-circuits amp which then drives the bandpass into a RMS->DC circuit (some diodes). Then you can plot it on a scope easily. Maybe not worth it for this first measurement.

2243   Mon Aug 20 12:11:45 2018 awadeDailyProgressWOPOTesting (freespace) balanced detector

Small polarization drift seems to have a significant impact on splitting ratio of the TN1064R5A2A.  over the course of 5 - 20 min the CMMR of the detector is appreciably degraded to about 30 dB. I've swapped out the fiber beam splitter for a good quality 50/50 free space beam splitter (pictured below).

I also installed waveplates at the WOPO launch (532 nm) and exit (1064 nm) to align polarization to the axis of the PPKTP.  I was hoping to see at least a little antisqueezing by adjusting these blind but in hindsight this will be much easier if I have some seed light (1064 nm injected) to work with.  The WOPO temperature must also be tuned at the same time, which makes this a rather large parameter space to search. The plan today is to install a 1064 nm coaligned at the WOPO input and use this to measure the classical NL gain using the HD.  This will need to be tapped off before the PZT at the 1064 nm fiber input.  This stratergy will also help confirm if the PZT is actually working.

2244   Mon Aug 20 22:09:27 2018 awadeDailyProgressWOPOPZT broken no slow phase modulation on LO

This afternoon I constructed a Mach–Zehnder around the PZT to confirm if it was actually providing modulation.  It isn't.

This was a very old unit with multi element actuation that Steve gave us. It had been wired up to push all in the same direction along the normal axis of the mirror. I thought we did test it, but can't find a record in the elog.

I switched it out the non-working PZT mounted mirror and installed another 1064 nm mirror mounted on a PZT stack that I found in one of the ATF lab cupboards. For the ~ 0-60 V voltage ramp signal applied I only saw about 0.75 of a fringe from this second unit.  This seems like much less than I would expect for a multistack element with ~8 PZT elements stacked together.  I took a while to make sure I was really getting the fringe I though I was getting but both these PZTs seem dead or underperforming.

The HV amp being used is a standard Thorlabs HV amp (MDT694B), I switched it out for another unit borrowed from the PSL lab and I got the same result.  So it doesn't appear to be that either.

The conclusion is that both of these PZTs are likely broken. There are no more units in the ATF or PSL labs.  I'll check with the 40m tomorrow.

2245   Wed Aug 22 00:42:42 2018 awadeDailyProgressWOPONew PZT mounted mirror

Koji showed me the collection of PZTs at the 40m.  I've epoxied a 9 mm mirror to the end of a P-810.10 peizo element.

Tomorrow I'll epoxy the other end to a heavier reaction mass.

I used EPO-TEK 353ND epoxy which I though Aidan was curing at room temperature.  12 hours later it was still pretty liquid.  Apparently it takes a very long time to work at at 20 C. Aidan et al. have reported apparent slips/hysteresis after epoxying their mirrors to heating elements, it would seem that even very long room temperature cures may not be doing the job.

## Baking epoxy element

I built an oven out of a clean aluminum box and Kapton heaters, pictured below. I lined the inside of the box and the table underneath with clean aluminum foil. Total resistance of the heaters is about 47 Ω, so 30V (.64 A) can deliver ~19W of heat.  With some foil packed around the outside it should easily get to the minimum recommended cure temperature of 80 C.

As a bonus I used the remaining not-quite-cured epoxy to stick a AD592 sensor to the box.  This is the cheaper cousin of AD590, basically the same specs but in a TO-92 package.  I modified Kira's AD590 temperature preamp circuit (see PSL:1883) to use a 5 V regulator to offset the the output signal to 0V at 0C. Also I flipped the bias accross the Temperature-to-Current element so the TIA produced positive voltage of 10 mV/K.

## Mount backing for PZT element

There weren't any blocks of the right hight/shape an any of the labs. I took an old 1" post that someone had hacked the end off (for some reason) and turned it down to a reaction mass that will fit into a standard 1" optic mount. I'll glue the PZT to this today.

Attachment 4: 18-08-23_12-02-38_3684.jpg
2247   Thu Aug 23 12:49:17 2018 awadeDailyProgressWOPONew PZT mounted mirror mounted

The bake overnight settled at 64 C.  This wasn't quiet the minimum recomended for EPO-TEK 353ND but was enought with the 12 hours of bake to bond the mirror well.

Bad news is that I stupidly clamped the mirror/PZT combo in such a way that I can't get it out of the mount without cutting it out.  There is not much I can do about this now. However, the mount has a tapped 8-32 hole that can be used to clamp the PZT element in and mount it on top of a post.  It actually works ok as a mounting (for slow actuation) and reminds me of the The Priest and the Beast episode of Mighty Boosh

I count 9 fringes with 0-50 V ramp drive.  So I now have a working phase actuation.

 Quote: Koji showed me the collection of PZTs at the 40m.  I've epoxied a 9 mm mirror to the end of a P-810.10 peizo element.  Mirror being epoxied to PZT element.

2255   Fri Oct 12 19:05:51 2018 awadeDailyProgressWOPOGluing 532 nm PZT mirror

I'm making a PZT mounted 532 nm mirror, so I have some independant control of the pumping light phase for optimizing the classical gain of the WOPO.

Previously I machined a simple reaction mass out of an old 1 " post (see ATF:2245).  This fits neatly into a standard 1" adjustable mount. To this I have glued a PA44LEW PZT chip and am curing it overnight at 80 C. To this I will stick a broadband visable mirror (Thorlabs BB03-E02) in the next few days and will replace it into the path that I am currently re-modematching into fiber.

Epoxy being used is EPO-TEK 353ND.

2257   Mon Oct 29 19:05:58 2018 awadeDailyProgressWOPOFixing MM into 532nm collimator

After a number of changes to the path for the pump light (532 nm) it has be very difficult to get any decent amount of pumping power coupled into the waveguide.  I'm re-mode matching light back into the fiber.

The main motivation for the changes was to get a co-propagating 1064 nm beam going into the WOPO so that the non-linear gain could be directly optimized with some seed light.  I installed a dichroic mirror through which the 532 nm is coupled into the fiber and from which 1064 nm is reflected into the coupler from a second path (see picture below).

The fiber is P3-460B-FC-2, but it was just possible to get a couple of µW of power out at 1064 nm out the other end (we probably don't need much).  But after the dichroic was installed and pump path optics were moved around a little to accommodate the new configuration, I found with best pointing the pump light was struggling to get much more than 10% coupling efficiency.

Re mode matching:

I double checked 532 nm beam coming out of the laser with the beam profiler. I confirmed that there was an x-axis waist of 89.0+/-1.2 um at z=-320+/-8 mm and a y-axis fitted waist of 82.7+/-1.0 um at z=-328+/-8 mm (z = 0 is marked on the table after the faraday isolator, the front of the laser head is z = -0.381 m).

Best MM solution was one lens f=350mm@0.37m and a second f=175mm@1.225m.  This was for the F240APC-532 (532 nm) collimator (assuming a 1.48 mm beam diameter).  This is assuming a 460HP fiber which has a slightly different mode size in the fiber; this changes the MM solution slightly but should be close enough.  After a bit of walking of the lenses I found I could bring the 532 nm coupling back up to about 50%.  This can be improved by more walking.

2262   Wed Nov 28 12:14:35 2018 awadeDailyProgressWOPOMode field diameter choosing best patch for WOPO input

There is some loss in fiber-to-fiber connection when there is a different mode field diameter between the two.  Ideally one would use the same type of fiber when patching but close enough ussually has lowish losses.  Just to check the numbers, to make sure I'm not getting larger than expected loss going into the WOPO device I looked at the missmatch loss between the single mode patch fibers have have (both single mode polarization maintaining and non-polarization maintaining) and the WOPO's Coastalcon 480PM (1628-10-19) input fiber on the 532 nm side.

## For reference

Mismatch in Mode Field Diameter (MFD) between single mode fibers leads to losses at their interface according to

$Loss[dB] = 10 log_{10}\left[\frac{4}{\left(\frac{MFD_1}{MFD_2}+\frac{MFD_2}{MFD_1}\right)^2}\right]$

## Some patch fiber parameters

Summary: 488PM-FC-2 (Blue Panda 2 m patch fiber)
Parameter Value
Fiber used PM460-HP
Mode Field Diameter (MFD) 3.3±0.5 @ 515 nm
Propagation loses ≤100 dB/km@ 488 nm
Cutoff wavelength 410±40 nm
Numerical aperture (NA) 0.12
Core (cladding) diameter  3.0 (125 ± 2) µm
Beatlength (of polarization) 1.3 mm @ 460 nm

Summary: P3-460B-FC-2  (Yellow SM 2 m patch fiber)
Parameter Value
Fiber used SM450
Mode Field Diameter (MFD) 3.45±0.65 @ 515 nm
Propagation loses ≤50 dB/km@ 488 nm
Cutoff wavelength 410±60 nm
Numerical aperture (NA) 0.12±0.02
Core (cladding) diameter  Not listed (125 ± 2) µm
Beatlength (of polarization) N/A

And the fiber input to the AdvR waveguided device

Summary: Coastalcon 480PM (1628-10-19)
Parameter Value
Fiber used Coastalcon 480PM (1628-10-19)
Mode Field Diameter (MFD) 4.0±? @ 515 nm
Propagation loses ≤30 dB/km@ 488 nm
Cutoff wavelength 435±35 nm
Numerical aperture (NA) 0.075±?
Core (cladding) diameter  Not listed (125 ± 2) µm
Beatlength (of polarization) ? Not listed
Extinction Ratio (pol) (dB) ≥25 dB
Insertion loss ≤1.15 dB

## Mode Field Diameter Mismatch Loss (Between Fibers)

Based on the above specs of mode field diameter the loss due to mismatch is order 0.15 dB for the blue PM460-HP polarization maintaining fiber and -0.1 dB for the singla mode yellow SM450 patch cable.  We can conclude that the mode field dameter mismatch is probably not going to be the dominant loss at the fiber interface with insertion loss likely to be larger at about ~ 1.0dB loss at the plugged interface of the cables.

2268   Tue Dec 4 19:34:22 2018 awadeDailyProgressWOPODiabolo 532 nm power drop and retuning

I restarted the Diabolo laser after it had been shut down for a lab tour.  The using the throlabs S130C head the 1064 nm was 355 mW which is about what we expect (previously it was measured to be 302.3 mW @ 1064 nm. see ATF:2103).  However I was unable to recover the 532 nm full power from the SHG; power from the SHG was down to 5.9 mW (previously ~300 mW).

It seems like mode of the 532 nm is not that clean.  Its likely that the PDH servo is not locking to the optimal mode any more. Because the locked cavity self heats with the high circulating power, often the steady state operation the SHG has a different lock point from the cold start.  Some days I kind of scan it for a bit, lock on the best mode and then come back after a few minutes and relock on the brightest mode.

The last record I made of the Diabolo and its SHG settings was in ATF:2103.  It seems like the doubling crystal temperature has been changed at some stage from 102.87 C to 108.84 C.  We could have done this with Dhruva but it might not have been noted down.  In this original post I noted down values and the SHG was set to its lower bound, this didn't match up with what was in the manual that I had at the time.  Later Koji found the correct manual for this unit (see ATF:2104).  It seems that the manufactures value for peak phase matching was 109.20 C.

## Snapshot of settings

Before I mess up the settings by tuning some nobs, here is the present settings for future reference:

Laser diode A temp = 19.69 C
Laser diode B temp = 20.32 C
Injection current  = 2.1 A
Laser crystal temp  = 23.42 C

SHG unit settings were
Double crystal temp = 108.83 C
Offset = 5.09
Gain  = 0.4
Scan amplitude  = 0 (you turn this up to scan SHG cavity)

## Fixing alignment of 1064 nm into SHG

I then scanned the SHG cavity to see the transmission peaks and PDH error signal (BNC outputs are on the back of the control unit).  It looked about the same as that reported in ATF:2103.  I then popped the lid on the laser head and tuned the alignment of the two steering mirrors into the SHG to maximize the dominant mode while minimizing the secondary peaks.

Trace is displayed below (second attachment).  The lopsidedness of the main peaks are heating effects in the non-linear crystal.  Compare to ATF:2103, ratio of secondary peaks to primary has gone from ~0.25 to <0.05. This should give me a much better power output when relocked.

## Still no full power on lock

After replacing lid on laser and attempting to relock the SHG unit I still get the weak mode.  It doesn't look like a HOM but its hard to tell.

Possible issues are: that we might be getting bad phase matching temperature after locking; self heating is pulling pulling SHG cavity mode to saturate the PZT range; and maybe during the power cycle of the SHG unit, knobs were changed that have messed up the stability of the PDH loop.

I checked the error signal.  Its lopsided, like the transmission peaks, because of the self heating of the Lithium Niobate crystal as we cross resonance. However, zooming in on the oscilloscope seems to show the peak corresponding to the zero crossing point of the error signal. I also played around with the loop gain to find that it is nicely below the point at which it starts ringing.  Its a bit hard to get get an openloop transfer function without getting into the guts of the control box.

Bad locking of the SHG is still unresolved.  I will do more troubleshooting today as I can't proceed without 532 nm light.

Attachment 2: plot20181204_DiaboloSHGRaligned1064nm.pdf
Attachment 3: plot20181204_DiaboloSHGRErrSig.pdf
Attachment 4: 20181204_RetuneDiaboloSHGAlignment.zip
2273   Thu Dec 6 20:04:50 2018 awadeDailyProgressWOPODiabolo 532 nm power drop and retuning

I've continued to have problems getting the Diabolo laser SHG to lock.

I get nice clean fundamental resonance peaks when fast scanning the SHG cavity length and optimizing the 1064 nm alignment into the SHG . However, when I switch to engage I see an initial power jump before the power dips and then settles to what looks like a higher order mode.  It seems like the heating of the NL crystal when locked is misaligning the eigen-mode of the cavity so that the cold scan of the cavity is optimal until locking and the higher circulating power heats things up.

I tried a different approach, slow scanning close about the fundamental (to keep things warm) and attempted to maximize the fundamental peak.   This was done by narrowing the scan range and then tuning the laser frequency (using NPRO crystal temp) to be close to resonance.  This improved the power output by a few tens of percent to ~27 mW, but is still well short of the previous performance.  This current power output is unworkable because we need at least 60 mW to overcome losses coupling into the WOPO and also because it is not stable in power and drifts around a bit and sometimes unlocks (probably because it is on the edge of some thermal state of the system).

I also tried tuning 1064 nm alignment into the cavity with the SHG locked.  This is tricky because that for each adjustment it takes a little time for the thermal state of the crystal to stabilize and settle. It improved power output maybe 10% but wasn't clearly leading to good operating point quickly.

## Fixing this

So it seems the unit was previously in a combination of states that worked once the SHG was locked.  Its been hard to reproduce that operating point. Its possible that since 2004, when the unit was manufactured, that there has been some degradation to the crystal.  Also, maybe the reason it has failed to perform to spec might be that the SHG cavity itself is slightly mialigned.  I'm reluctant to open up the SHG unit as it is hermetically sealed.  I assume they flood it with N2 to keep everything in a dry no oxygen controled environment but at this stage maybe it doesn't matter.  Tweaking the input coupler on the SHG will be super sensitive so would be a delecate task, but I'm at a loss as to how to find an optimal operating point to return to normal operation.

2277   Fri Dec 7 20:31:06 2018 awadeDailyProgressWOPODiabolo: cleaning dust out of SHG unit, self heating issues

I've managed to vastly improve the power output of the SHG by doing a little cleaning inside the box.

After some consideration I decided that the only remaining possibility was something wrong inside the SHG. Its a hermetically sealed box so I cleaned around the area (to keep dust down) and then removed the lid.  Some of the screws were loose, so its likely that somebody has opened this before.  Below are a few pictures of the inside of the Diabolo SHG.

Looking through an IR viewer I could see that there was a large absorber on the front face of the crystal.  I didn't get a picture but it was dead center and scattering a lot of light given the tightness of the beam. There was also other smaller scatters around the edge of the front surface. An clean N2 ionizing air gun was used to blow in and around the cavity and its electronics. There was a lot of dust blown out from inside the unit; the SHG has evidently been opened at some point in a dirty lab.  The spot in the middle of the front face of the SHG came loose with very gentle bushing with a corner of a lens cleaning tissue (MC-5).  I thoroughly flushed the inside of the unit again with 40 psi clean N2 and then replaced the lid.  Just before sealing the box I cracked the lid and flushed again with the N2 gun to fill the box with clean dry air.  The screws were sealed down with 2.25 Nm of torque.

I ramped the laser back on and after some small small alignment tweaks I was able to get an initial large boost in 532 nm laser output but it quickly dropped to the lower ~12 mW output again. It looks like, after lock, the output light goes through a few fringes and then either settles on a HOM or a weaker fundamental mode.  My guess is that in the locked state the crystal/mirror is heating up and mudslinging the cavity.  I don't really want to pull the unit apart into is constituent pieces to inspect.  I might need to have a closer look at the crystal front face absorber and the state of the input coupler mirror.  Over time some non-linear crystals do develop either grey tracking or permanent damage along the tighly focused points of 532 nm light.  It could be that the unit has gone past a tipping point but this is hard to see from just peaking in from the top of the unit.

Not sure what to do next.

---

Side note: Its seems very hard to make any alignment changes from the inside: my guess is that alignment is done from the three outside exposed hex screws on the front coupler. Its unlikely with such a short cavity much alignment changes would be needed.  For now its best to leave it as it is.

Attachment 5: IMG_4766.MOV
2280   Thu Dec 13 14:01:13 2018 awadeDailyProgressWOPODiabolo: contacted coherent

I've contacted the people at Coherent about getting instructions for repair or whether they can still service this unit as I don't want to waste a lot of futile time on trying to optimize a unit that might be broken/degraded.

The technicians I spoke to didn't seem to know a lot about the Diabolo unit but will forward on to their manufacture for further information/assistance.

 Quote: I've managed to vastly improve the power output of the SHG by doing a little cleaning inside the box. After some consideration I decided that the only remaining possibility was something wrong inside the SHG. Its a hermetically sealed box so I cleaned around the area (to keep dust down) and then removed the lid.  Some of the screws were loose, so its likely that somebody has opened this before.  Below are a few pictures of the inside of the Diabolo SHG.         Looking through an IR viewer I could see that there was a large absorber on the front face of the crystal.  I didn't get a picture but it was dead center and scattering a lot of light given the tightness of the beam. There was also other smaller scatters around the edge of the front surface. An clean N2 ionizing air gun was used to blow in and around the cavity and its electronics. There was a lot of dust blown out from inside the unit; the SHG has evidently been opened at some point in a dirty lab.  The spot in the middle of the front face of the SHG came loose with very gentle bushing with a corner of a lens cleaning tissue (MC-5).  I thoroughly flushed the inside of the unit again with 40 psi clean N2 and then replaced the lid.  Just before sealing the box I cracked the lid and flushed again with the N2 gun to fill the box with clean dry air.  The screws were sealed down with 2.25 Nm of torque. I ramped the laser back on and after some small small alignment tweaks I was able to get an initial large boost in 532 nm laser output but it quickly dropped to the lower ~12 mW output again. It looks like, after lock, the output light goes through a few fringes and then either settles on a HOM or a weaker fundamental mode.  My guess is that in the locked state the crystal/mirror is heating up and mudslinging the cavity.  I don't really want to pull the unit apart into is constituent pieces to inspect.  I might need to have a closer look at the crystal front face absorber and the state of the input coupler mirror.  Over time some non-linear crystals do develop either grey tracking or permanent damage along the tighly focused points of 532 nm light.  It could be that the unit has gone past a tipping point but this is hard to see from just peaking in from the top of the unit. Not sure what to do next. --- Side note: Its seems very hard to make any alignment changes from the inside: my guess is that alignment is done from the three outside exposed hex screws on the front coupler. Its unlikely with such a short cavity much alignment changes would be needed.  For now its best to leave it as it is.

2281   Thu Dec 13 16:24:06 2018 awadeDailyProgressWOPODiabolo: baseline numbers for coherent

Some numbers for the Diabolo laser for reference (to quote to Coherent, if they get back to us with help).

Diabolo numbers (Dec 13, 2018)
Parameter Value
Fundamental (1064 nm) laser power exiting laser from tap off BS 372 ± 2 mW
Fundamental (1064 nm) incident on SHG (measured after steering mirror 1) 1.32 ± 0.02 W
Total fundamental (summed from above two mesurements) 1.692 ± 0.02W
SHG cavity scan TEM00 peak (voltage trans diode) 1.21 V (1 MΩ imp oscilloscope)
SHG cavity scan second HOM peak 38 mV (1 MΩ imp)
SHG cavity scan third HOM peak 17 mV (1 MΩ imp)

To characterize the phase matching temperature I fast scanned the SHG and measured the peak 532 nm output for a range of temperatures in 0.05 C steps.  Acoss a single sweep of the cavity resonances there is some change in the peak 532 nm output.  This change accross the sweep range is likely an alignment thing with the shape of the PZT sweep path.  Plot below shows the peak 532 nm output power measured on a Thorlabs PD (PDA100A, 0 dB gain with 1.3 ND filter in front). Vertical axis units are less relivant than the shape and peak possition of the curve.  Error bars were estimated based on spread of peak amplitudes (the largest error in the mearument).

We see a peak conversion at 109.8 C with a FWHM of about 0.2 C

I've attached data and python notebook for plotting in a zip below.

 Quote: I've contacted the people at Coherent about getting instructions for repair or whether they can still service this unit as I don't want to waste a lot of futile time on trying to optimize a unit that might be broken/degraded. The technicians I spoke to didn't seem to know a lot about the Diabolo unit but will forward on to their manufacture for further information/assistance.

Attachment 1: plot20181218_PhaseMatchingCurveFastScanDiaboloSHG.pdf
Attachment 2: 20181213_SHGScanWithLNTempTuning.zip
2283   Wed Dec 19 16:48:18 2018 awadeDailyProgressWOPODiabolo: baseline numbers for coherent

### Contact with Coherent re Diabolo

I got in contact with Coherent they have provided some instructions on aligning the laser cavity, something I've already attempted and knew how to do.  For future reference I've attached the instructions on the wiki (they are confidential): they can be found attached HERE.

More details on Diabolo manual and documentation can be found on the Manuals-And-Datasheets ATF wiki page.

### Temperature tuning

I've been attempting to find sweet spot for phase matching temperature between oven and self heating.  The FWHM of the phase matching temperature of the SHG lithium niobate crystal appears to be on the order of about 0.2 C which makes it quite narrow.  Given there is quite a bit of self heating in the cavity when it is locked up, it would be very easy to miss the correct temperature when setting the crystal's oven set point.

Koji/rana has suggested turning down the laser power, locking and then gradually tuning the oven temperature to get to ideal set point as we increase power.  I've attempted this and found that the autolock requires some minimum amount of laser power to engage.  I opened up the controller box and traced a few things through the PCBs, but it is not immediately apparent which pot adjusts the autotune threshold value for power.  I've emailed the contact at Coherent and they are making inquires to see if they can tell me what to adjust to lower this threshold manually.  For now I will attempt a few more full power locks at carefully spaced values of crystal temperature to see if I am able to happen upon a sweet spot.

2284   Thu Dec 20 19:20:50 2018 awadeDailyProgressWOPODiabolo: trialing SHG oven set temperature points to see if there is a maxima when locked

The strategy locking at lower laser power and optimizing phase matching temperature by slowly walking the temperature and laser power up won't work: the autolocker mode of the Diabolo SHG won't engage at lower laser power. Instead I tried a brute force approach of stepping temperature and re-locking the cavity in 0.05 C increments of set point temperature.  I've plotted the result below.  Error bars are based on standard deviation on 32 averaged measurements over 10 seconds.

I stopped at 106.00 C as  power output was not improving beyond that point. Maximum temperature is clipped at the controller at 110.26 C. Peak output of 66 mW is actually  at 110.20 C. This is counterintuitively higher than the previous set point but might be good enough to work with, as long as its stable over longer periods.

Still not clear what has changed to shift the ideal SHG over set point to 1.4 C higher than it was before.  Maybe the SHG oven needs to go even a little hotter, but we can only access tempertures up to 110.26 C. I'll take what I can get.

Some other laser settings:

• Injection current: 2.077 A
• Laser crystal temperature: 26.32 C
• SHG PDH gain: 0.5
• PDH offset: 4.95
• Noise eater: off

Data and plotting notebook are attached in a zip below: 20181220_SHGLockedPowerVsSetPointTemp.zip

Attachment 1: plot20181220_SHGLockedOutputVsOvenSetpointTemperature.pdf
Attachment 2: 20181220_SHGLockedPowerVsSetPointTemp.zip
2286   Fri Dec 21 15:33:26 2018 KojiDailyProgressWOPODiabolo: trialing SHG oven set temperature points to see if there is a maxima when locked

This means that you want to make the SHG crystal longer. Is that true? If so, can you change the temperature for the optimal phase matching by tuning the 1064 crystal temperrature? I suspect you need to cool the YAG crystal, but I am not sure what is the thero-optic constant of the SHG crystal, and how much you can gain from this.

2287   Fri Dec 21 15:51:27 2018 awadeDailyProgressWOPODiabolo: trialing SHG oven set temperature points to see if there is a maxima when locked

No I don't think so.  The temperature of the oven is important for the phase matching condition.  Its the nob you can turn so that the refractive index for both 532 nm and 1064 nm is just right so that both traveling waves stay in phase as they propagate in the non-linear crystal.  Otherwise the phase precesses in one wavelength relative to the other causing repeated amplification and de-amplification as the light propagates (rather than just amplification when they are in phase).

Its true that temperature change induces expansion​ and dn/dt changes that will change the round trip effective length/phase. However, the PZT in the SHG should pick up the slack of from changes in the crystal round trip phase due to temperature.  We can tune the laser temperature to bring the PZT to the center of its range but at this stage it doesn't look like its railing the range of this actuator.

My guess is that self heating affects the relative alignment of the crystal HR surface relative to the PZT mounted (curved) mirror, this small change in alignment will shift the eigen axis of the cavity and mean that the locked hot cavity will have a slightly different optimal alignment to the unlocked case.  Its much harder to walk alignment of 1064 nm while keeping cavity locked.

 Quote: This means that you want to make the SHG crystal longer. Is that true? If so, can you change the temperature for the optimal phase matching by tuning the 1064 crystal temperrature? I suspect you need to cool the YAG crystal, but I am not sure what is the thero-optic constant of the SHG crystal, and how much you can gain from this.

2289   Mon Dec 24 10:08:49 2018 awadeDailyProgressWOPODiabolo: back to power with deliberate pre-misalignment

I think I've managed to bring the power of the Diabolo back to a usable level for WOPO.

## More power with misalignment

I had another look at the alignment into the SHG.  This time I systematically trialed a range of cavity misalignments to deliberately set the pointing into the cavity to be wrong for the fast scanning mode of the control unit (cold) but to allow for the locked (hot) cavity to expand into correct alignment.  This video shows the SHG output as the cavity is locked IMG_4766.MOV,  this seems to show the expansion affecting mostly the horizontal axis. However, eventually I found that the locked cavity had a preferred misalignment on the vertical axis (in terms of the nobs that I needed to turn).  In reality its a mixture of horizontal and vertical once all the DOF are traced through the lenses and mirrors.

I was able to get a steady 160 mW of power with this new misalignment/alignment strategy.  The photos below shows the 1064 nm transmission peaks and error signal for a combination of (mostly) vertical and (a little) horizontal misalignment​ that gave greater power.

The oven operating temperature was at 110.14 C with a laser diode current of 2.102 A.  PDH gain was set to 0.56 and offset was set to 5.1.

## Upping the oven temperature limit

The Diabolo SHG oven set point temperature range doesn't put the ideal phase matching temperature at the center of the available values.  In fact, the optimal power for the above-mentioned power boost was achieved pretty much of the edge of the available range (top of range is 110.26).  I popped the lid on the controller box and traced through all the PCBs to find out how the temperature range was trimmed. The front panel potentiometer (1k) is trimmed with a small set screw (blue) potentiometer located directly to the low left of the temperature knob (when viewed from the component mounting side).  I trimmed the set point temperature value so that the upper value was 111.00 C, giving an extra 0.75 C of headroom above the ideal phase matching set point.

I found that at the very upper edge of the new range the thermal control loop started to loose some stability.  I limited the increase of temperature to the 111.00 C point and will use some caution when adjusting temperatures at these upper ranges to make sure I don't get oscillations in power.

 Quote: The strategy locking at lower laser power and optimizing phase matching temperature by slowly walking the temperature and laser power up won't work: the autolocker mode of the Diabolo SHG won't engage at lower laser power. Instead I tried a brute force approach of stepping temperature and re-locking the cavity in 0.05 C increments of set point temperature.  I've plotted the result below.  Error bars are based on standard deviation on 32 averaged measurements over 10 seconds. I stopped at 106.00 C as  power output was not improving beyond that point. Maximum temperature is clipped at the controller at 110.26 C. Peak output of 66 mW is actually  at 110.20 C. This is counterintuitively higher than the previous set point but might be good enough to work with, as long as its stable over longer periods. Still not clear what has changed to shift the ideal SHG over set point to 1.4 C higher than it was before.  Maybe the SHG oven needs to go even a little hotter, but we can only access tempertures up to 110.26 C. I'll take what I can get. Some other laser settings:  Injection current: 2.077 A Laser crystal temperature: 26.32 C SHG PDH gain: 0.5 PDH offset: 4.95 Noise eater: off Data and plotting notebook are attached in a zip below: 20181220_SHGLockedPowerVsSetPointTemp.zip

2290   Mon Dec 24 12:07:18 2018 KojiDailyProgressWOPODiabolo: back to power with deliberate pre-misalignment

Great recovery job!

2291   Mon Dec 24 16:46:41 2018 awadeDailyProgressWOPOMM into 532 nm patch fiber (to be hooked up to WOPO)

I had another look at MM the 532 nm light into the P3-460B-FC-2 patch cable.  After walking some lens positions and mirror pointing based on ATF:2257 I found that I could get 1.07 mW output from launch of 2.3 mW (46.5 %) through the fiber.  I expect the most I'll need is about 30 mW at the output which will put the amount of required power at about 64 mW.  There is now enough 532 nm laser light to do this and I think its within the tolerances of the fiber to have that much launched in.

One thing I did notice, on closer inspection of the fiber ends, is that there is a little bit of damage on one end (labeled A) of the P3-460B-FC-2 patch.  This is pictured below.

I tried a bit of gentle cleaning with some fiber cleaning cloth (Thorlabs FCC-7020)  but this appears to be a burn mark when zoomed in. Sorry doesn't capture very clear on my phone through the fiber microscope. I swapped the end that was in the fiber collimator (F240APC-532) but didn't increase the fiber launch efficiency​.  Not sure when this got damaged, but I might have exposed it to excessive power at some stage.

I still think that the fiber through put should be sufficient, if it turns out to be a problem then re-ording shouldn't be an issue.  Thorlabs usually seems to make these fiber items next day.

## Next steps

Next step is to tune up the 1064 nm alignment into the LO launch fiber and see if I can get the new PZT mounted mirror to scan as expected.  Need to figure out if there is a way to calibrate the phase on this so that I can check it is actually scanning phase (the last PZT broke and I had no idea).

Need to figure out how much 1064 nm light can be co-propagated in the patch waveguide.  The previous dismal power values might be because I made very little effort to mode match,  I'll look at what lenses I have available and see if I can boost the power through put to something respectable.

2296   Wed Feb 20 10:38:46 2019 awadeDailyProgressWOPOFixing 532 nm polarization linearity issues

Alignment of the pumping 532 nm polarization into the WOPO is important to getting the correct phase matching condition.  For the periodically polled Lithium Niobate (LN) waveguide the phase matching is type-0: and pumping and fundamental wavelengths are in the same polarization.  The AdvR non-linear device is coupled with polarization maintaining fibers (Panda style), which are keyed at their FC/APC ends.  This means that with the correct launch polarization we should be correctly aligned with the proper crystal axis for degenerate down conversion (at the right chip temperature).

Till now I was using non-pol maintaining patches to coupling into the WOPO fiber ends.  This should have been ok, but it is hard to figure out exactly which polarization is optimal so I switched to a pol-maintaining patch because it can be aligned separately and then the keyed connectors give you automatic alignment.  I had some issues trying to find the optimal polarization going into the fiber and I've now traced this back to the polarizing beam cubes.  I've been using Thorlabs PBS101 which is a 10x10x10 mm^3 beam cube that is supposed to be broad-band (420-680 nm).  When I checked the extinction ratio I saw Pmax=150 mW, Pmin=0.413 mW on transmission between extremes.  This is an extinction ratio of Tp:Ts = 393:1 which is much less than the spec of >1000:1.  Not sure what's going on here, the light going into the BS is coming directly from a Faraday isolator and a half-wave plate.  With some adjustment to the angle of the wave plate I can do a little better but it should be nicely linearly polarized to start with.

I've switched out the PSB101 for the laser line PBS12-1064 I remeasured extinction ratio (Pmax=150 mW, Pmin=27.6 µW) Tp:Ts = 5471:1 (better than the quoted 3000:1 spec).  This is good, at least now I know what is going on. I am also putting in an order for a 532 nm zero order quarter-wave plate, so that we can be absolutely sure we are launching in linear light always.

## Aligning light into pol-maintaining fiber

I previously thought I might be able to use the frequency modulation technique to align the light through the polarization maintaining fiber.  There is a birefringence in PM460-HP fiber of  3.5 x 10-4.  The phase between ordinary and extraordinary axes over the whole fiber length is

$\Delta\phi = \frac{2 \pi \Delta n L}{c}f$

Where L is fiber length, $\Delta n$ is the birefringence and f is the laser frequency.  The idea is to launch linearly polarized light into the fiber and then at the readout place a polarizer rotated to be 90°: ramping frequency will produce an amplitude modulation on the dark fringe.  However, even with 1 GHz of frequency ramp this is only a 15 mrad effect for a 2 m fiber, its likely to be too small to see over other effects.  This is not enough to be able to fine align polarization.

Instead I'll use the heat gun method.  I'll fire linearly polarized light into the fiber and measure the output with a crossed polarizer.  If the input polarization is correct there should be no power changes on the output as the fiber is thermally cycled. Its only two meters long so hopefully this effect is easy to see.

2297   Thu Feb 21 17:02:24 2019 awadeDailyProgressWOPOMixing down homodyne output from 1 MHz

I want to get signal from about 1 MHz down to around DC from my subtracted  homodyne photodetectors. I'm planning to do something like this:

This mixer has a 4.78 dB conversion loss and should do the job.  Only issue is that to operate the mixing down the low pass filter needs to be lowered from 1.9 MHz down to something pretty low to ensure the harmonic (2 MHz term is removed).  These are 4th order filters, we'd probably want the cut off to be an order of ten below the mixing frequency... 100 kHz.  I don't see this in the minicircuits catalog don't know how doable that is to make one.  I'll have a look at what the 40 m has.

Attachment 1: IMG_5106.JPG
2298   Fri Feb 22 14:05:09 2019 awadeDailyProgressWOPOMixing down homodyne: 5th order elliptic filter (100 kHz)

## Making 5th order Elliptical Filter

I couldn't find any filters that would cut off above 100 kHz so I made my own using a Thorlabs EEAPCB1 generic filter PCB in a Thorlabs EEA14 enclosure. I used a 5th order elliptic design with a pass band up to 100 kHz and a stop band of 40 dB from 150 kHz.  To speed things up I used the Coilcraft Filter Designer software  (4.0.1, Windows) and chose closest standard values from parts we had in EE workshop kits. The Coilcraft designer is nice because it has the full physical model of the inductors built in.

The schematic is illustrated below:

Actual values selected were closest available and I didn't try to do any mixing or matching to get fine tuned correct values. Values are as follows:

• L2 and L4 27 µH => 1812CS-273XJL (27µH @ 2.5 MHz, tol 5%, Q_min 15 @ 2.5 MHz, SRF_min 11 MHz, DCR 11.04 Ω)
• C1 20.47 nF => 12065C223KAT2A (0.022 µF, 50 V, 10%)
• C2 14.38 nF => 12065153KAT2A (0.015 µF, 50 V, 10%)
• C3 47.42 nF => 12061C473KAT2A (0.047 µF, 50 V, 10%)
• C4 4.792 nF => 12065C472KAT2A (4700 pF, 50 V, 10%)
• C5 27.3 nF => 12065C273KAT2A (0.027µF, 50 V, 10%)

The built filter is shown below.

The transfer function was taken from 10 Hz up to 5 MHz (IF BW of 10 Hz).  The Coilcraft Filter Designer software seems to export all the filter scattering parameters except S12 and S21, not the best. Instead I used LISO to model the filter's predicted response and this is plotted alongside measured TF.  Here I have scaled the filter model's response by 2 to match the impedance condition of the TF measurement.  The coilcraft 0805LS-273X_E 27 µH inductors series resistance was modeled as 11 Ω (as per spec sheet) and values of capacitors were those used in the actual circuit.  In the original ideal 5th order elliptical circuit there is a double dip above the corner frequency.  The series resistance of the non-ideal inductors dampens these.  I don't really want to spend much more time on mixing and matching capacitor values.  It looks like for now that the pass band ripple is acceptable and the attenuation is >40 dB at 1 MHz and 2 MHz where we are trying to block harmonic signals. I'll leave optimization of this part for now and write this off as done.

Edit Mon Feb 25 19:22:14 2019 (awade): Fixed the phase pane of the bode plot, had accidentally used magnitude and wrong units. Also fixed some spelling.

Attachment 3: Elliptical5thOrder_MeasuredVsModel.pdf
Attachment 4: 20190222_5thOrderEllipticFilter100kHz.zip
2300   Thu Feb 28 16:00:59 2019 awadeDailyProgressWOPOPol launch into PM fiber

I've set up a rotating PBS and half-wave plate to provide polarization adjustment into the 532 nm fiber without misalignment the spatial alignment.  Here I've used a PRM1 rotation mount with a SM1PM10 lens tube mount for beam cube prisms.  The lens tube mount is supposed to be for pre-mounted cubes but I've inserted some shims to hold it in place and it seems to work well like that.  It means I can get a nice clean linear polarization at all rotations.

After spatially aligning the input beam I stepped the rotation of the PBS (and accordingly the L/2 wave plate) and pulsed the temperature of the fiber using a heat gun.  After some walking I found that for the current fiber rotation (0 deg) the linear polarization was aligned with the fiber axis at 88 deg PBS rotation (here 0 deg PBS rotation is aligned for p-pol transmission, well almost). I made some adjustments to the alignment of the fiber collimator in the fiber launch, I aligned the slow axis key with the vertical so that the fast axis of the fiber is p-pol.

## Keying on PM fibers

As a side note the keying of PM fiber patches is typically with the slow axis aligned with the key notch. The WOPO's PM fibers are keyed so that the alignment key is along the slow axis of the fiber (i.e. aligned with the stress rods). Figure below illustrates the configuration.

## Replacing the 532 nm patch with fresh PM fiber

I was getting a large jitter in the power levels as measured at the output of the old SM and PM fibers (on the order of 10%).  These power fluctuations were not present on the input side.  I thought this was an alignment jitter or a polarization effect.  However, I was unable to minimize it by improving the input polarization at the launch.  When I tapped various mounts there didn't seem to be a corresponding correlation with output power jitter of the fiber.  When I checked the end of the PM fiber (P3-1064PM-FC-2), I saw that there was damage about the core (see pictured below).  It seems like maybe I had some kind of etalon effect from this burn mark and the launch.  After replacing the 532 nm PM fiber with a fresh one that arrived last week the power is much more stable and I was able to easily​ find the pol alignment going in.

Next job is to replace SM fiber for the 1064 nm delivery with PM fiber so there is a well defined polarization for launching into the homodyne detector.

Quote:

Alignment of the pumping 532 nm polarization into the WOPO is important to getting the correct phase matching condition.  For the periodically polled Lithium Niobate (LN) waveguide the phase matching is type-0: and pumping and fundamental wavelengths are in the same polarization.  The AdvR non-linear device is coupled with polarization maintaining fibers (Panda style), which are keyed at their FC/APC ends.  This means that with the correct launch polarization we should be correctly aligned with the proper crystal axis for degenerate down conversion (at the right chip temperature).

Till now I was using non-pol maintaining patches to coupling into the WOPO fiber ends.  This should have been ok, but it is hard to figure out exactly which polarization is optimal so I switched to a pol-maintaining patch because it can be aligned separately and then the keyed connectors give you automatic alignment.  I had some issues trying to find the optimal polarization going into the fiber and I've now traced this back to the polarizing beam cubes.  I've been using Thorlabs PBS101 which is a 10x10x10 mm^3 beam cube that is supposed to be broad-band (420-680 nm).  When I checked the extinction ratio I saw Pmax=150 mW, Pmin=0.413 mW on transmission between extremes.  This is an extinction ratio of Tp:Ts = 393:1 which is much less than the spec of >1000:1.  Not sure what's going on here, the light going into the BS is coming directly from a Faraday isolator and a half-wave plate.  With some adjustment to the angle of the wave plate I can do a little better but it should be nicely linearly polarized to start with.

I've switched out the PSB101 for the laser line PBS12-1064 I remeasured extinction ratio (Pmax=150 mW, Pmin=27.6 µW) Tp:Ts = 5471:1 (better than the quoted 3000:1 spec).  This is good, at least now I know what is going on. I am also putting in an order for a 532 nm zero order quarter-wave plate, so that we can be absolutely sure we are launching in linear light always.

## Aligning light into pol-maintaining fiber

I previously thought I might be able to use the frequency modulation technique to align the light through the polarization maintaining fiber.  There is a birefringence in PM460-HP fiber of  3.5 x 10-4.  The phase between ordinary and extraordinary axes over the whole fiber length is

$\Delta\phi = \frac{2 \pi \Delta n L}{c}f$

Where L is fiber length, $\Delta n$ is the birefringence and f is the laser frequency.  The idea is to launch linearly polarized light into the fiber and then at the readout place a polarizer rotated to be 90°: ramping frequency will produce an amplitude modulation on the dark fringe.  However, even with 1 GHz of frequency ramp this is only a 15 mrad effect for a 2 m fiber, its likely to be too small to see over other effects.  This is not enough to be able to fine align polarization.

Instead I'll use the heat gun method.  I'll fire linearly polarized light into the fiber and measure the output with a crossed polarizer.  If the input polarization is correct there should be no power changes on the output as the fiber is thermally cycled. Its only two meters long so hopefully this effect is easy to see.

Attachment 2: PMFiber.pdf
2303   Tue Mar 12 16:35:43 2019 awadeDailyProgressWOPOPol launch into PM fiber 1064 nm

I've replaced the SM fiber in the 1064 nm launch with a PM fiber (P3-1064PM-FC-5). I also moved the fiber collimator (F240APC-1064) back 2.54 cm back to give more space for a PBS cube (to check linearly of the light).

For the 1064 nm launch it seemed to be a lot harder to find the initial alignment of the collimator using the alignment of the back propagated 650 nm fiber laser source. Here I aligned a pair of irises in the forward propagating direction and then back propagated through the PM fiber using 650 nm to get the initial​ pointing of collimator. I don't know why this is so much harder than the 532 nm case.  I suspect one of the steering mirrors is not really reflecting off the front dielectric surface.  In the end I did a bunch of systematic walking of the fiber launch mount and eventually fount the alignment.

From 4.44 mW of input light I get 2.74 mW of light out the other end of the fiber.  This is an efficiency of 62 % which is more than enough for my needs.  I expect the HD will only need 1 mW (2 mW max), so this is fine. Getting this in coupling higher will require a bit of lens walking, not really worth it at this stage.

I had already carefully aligned the collimator orientation to put the fast axis on aligned to p-pol (wrt the table), by eye.  It seems like the launch pretty much hit the correct launch polarization on the first go.  I see little variation in the polarization when I pulse the heat on the fiber.  This is now good to go for optimizing the homodyne visible and polarization overlap output from the SQZ.

2304   Tue Mar 12 16:58:50 2019 awadeMiscWOPOTodo list W11

These are things to get do this week on WOPO experiment:

☑️ Reinstall 50:50 fiber splitter into homodyne setup and go back to fiber launch of both ends of the HD directly onto photodetectors (rather than free space)

☐ Check visibility of HD by launching 1064 nm into both arms of HD using splitter and extra length on one arm, ramp laser frequency to get fringes and look at Vpp on each diode seperatly

☑️ Optimize subtraction of HD for max Common Mode Rejection (CMR) of LO amplitude noise.  Inject 3.21 kHz line into laser BNC port and minimize this peak on the subtracted output of the HD.

☐ Check 1064 nm -> 532 nm conversion in WOPO device to establish polarization basis for correct pol alignment into fiber (change 532 nm launch polarization if necessary), should be along the fast axis but for some reason this isn't in the datasheet explictly

☐ Double check how much power change there is from 2 pi modulation depth from PZT mounted 1064 nm mirror.  We don't want to over actuate on this element as it slightly misaignes into the fiber launch and causes some change in power, this is ok over a small range as the CMR of the homodyne will reject this.  We just want to be sure that this isn't the dominant effect that we are seeing at the output once we thing we should be seeing SQZ.

☑️Tick mark when done.

2305   Wed Mar 13 12:44:41 2019 awade, anchalDailyProgressWOPOPol launch into PM fiber 1064 nm

After a bit of reading I've realized that the standard use of these PM fibers is to launch along the slow axis (see for example Thorlabs and OzOptics info on fiber beam splitters).  It should be much of the sameness for patch cables, but polarization sensitive elements like beam splitters are mostly tested and specified for slow axis launch unless they are custom made to order.

We are switching the polarization alignment to slow axis in the 1064 nm and 532 nm fiber coupling.  Anchal is re-optimizing​ the 1064 nm launch to get the PM fiber extinction ratio back to a good place.  We've also changed input launch to use a laser line PBS mounted in a rotation mount for clean linear polarization.  With the optimized setup the for the 1064 nm fiber path the output polarization signal goes from 3700 mV to 39.3 mV which is an extinction​ ratio of -19.7 dB.

Here the max theoretical extinction​ ratio is

$ER = -10 \log_{10}[\tan^2(\theta)]$

which would place our goodness of alignment to with 0.61 deg.

 Quote: I've replaced the SM fiber in the 1064 nm launch with a PM fiber (P3-1064PM-FC-5). I also moved the fiber collimator (F240APC-1064) back 2.54 cm back to give more space for a PBS cube (to check linearly of the light).   For the 1064 nm launch it seemed to be a lot harder to find the initial alignment of the collimator using the alignment of the back propagated 650 nm fiber laser source. Here I aligned a pair of irises in the forward propagating direction and then back propagated through the PM fiber using 650 nm to get the initialâ€‹ pointing of collimator. I don't know why this is so much harder than the 532 nm case.  I suspect one of the steering mirrors is not really reflecting off the front dielectric surface.  In the end I did a bunch of systematic walking of the fiber launch mount and eventually fount the alignment.   From 4.44 mW of input light I get 2.74 mW of light out the other end of the fiber.  This is an efficiency of 62 % which is more than enough for my needs.  I expect the HD will only need 1 mW (2 mW max), so this is fine. Getting this in coupling higher will require a bit of lens walking, not really worth it at this stage. I had already carefully aligned the collimator orientation to put the fast axis on aligned to p-pol (wrt the table), by eye.  It seems like the launch pretty much hit the correct launch polarization on the first go.  I see little variation in the polarization when I pulse the heat on the fiber.  This is now good to go for optimizing the homodyne visible and polarization overlap output from the SQZ.

2306   Wed Mar 13 17:49:14 2019 awadeSummaryWOPODamage threshold estimates for optical fibers

This is just a note about damage tolerances for fibers so we have a reference of the amount of power that can be used.

Thorlabs gives the maximum theoretical CW power as 1 MW/cm² and a 'practical' safe power level of 250 kW/cm² (or a quarter of the max).  They don't seem to provide information about wavelength dependence and assume its the same for all wavelengths.  We don't expect to be affected by any of the exotic ultra high power effects like bend loss induced damage and photodarkening.  The air/glass interface is where the damage will occur.  This can either be because of heating of the ferrule/connector (causing epoxies etc to break down and damage the interface by depositing on the optical surface) or regular mechanism that are the same as bulk optics (dielectric break down and thermal effects).

The intensity profile of light confined in the fiber is defined by the Mode Field Diameter (MFD) -- the cross-sectional diameter of the light that includes the core of the fiber and a region just beyond the cladding the mode occupies. MFD of 1064 PM fiber (PM980-XP) is 6.6 ± 0.5 μm @ 980 nm and for the 532 nm PM fiber (PM460-HP) is 3.3 ± 0.5 µm @ 515 nm.

Fiber effective area is

$\dpi{100} A = \pi (\textrm{MDF}/2)^2$

which is 8.5x10^-8 cm^2 for PM460-HP and 3.4x10^-7 cm^2 for PM980-XP.  Taking the conservative 'practical' damage threshold this indicates a maximum power of 21.4 mW into the 532 nm fiber and 85.5 mW into the 1064 nm PM fiber. The absolute maximum is just a factor of four more than this: 85.6 mW into 532 nm PM fiber and 342 mW into 1064 nm fiber. If the fiber ends are kept clean then we should be fine if the power level is kept below 85 mW.

## Fiber beam splitter

Of particular concern is the power handling capability of the fiber beam splitter (PN1064R5A2).  There will be some waste 532 nm light coming out of the WOPO and I don't want these potentially multi-mode components to exit the cladding at the point of the coupler and damage the surrounding material. The 1064 nm maximum power rating of this 50:50 PM beam splitter is listed as 1 W (for the connectorized fiber), so we should be well clear of that threshold for the LO light.  For 532 nm its less clear. The equivalent 532 nm PM 50:50 beam splitter (PN530R5A2) has a rated power of 100 mW @ 530 nm for bare or connectorized fibers. As the MFD of the 1064 nm version of this PM beam splitter has a much larger MFD and the exiting 532 nm light will already be expanded in the cable patching the WOPO to the BS, we should be well clear of this damage threshold point.

So bottom line is that we need to keep power below 85 mW going into the WOPO device and keep all the end connectors super clean and it will be fine.

2316   Wed Mar 27 20:37:38 2019 awadeDailyProgressWOPOMeasuring TF Homodyne Photo Detector for WOPO experiment

## Initial measurment of PD TF

I realize I never really measured the signal transfer function for each of these Photo Detectors (PD).  This post summarizes measurements of the optical to electrical signal output signal transfer function. Here I used the Jenne rig at the 40m and took a transfer function using an Aglent 4395A from 30 kHz to 5 MHz.

I've labeled the two PD in the homodyne as unit A and unit B.  Below (first attached)  is the raw TF as measured by the Aglient taking the ratio of the detector to the reference NF1611 detector:

## Fixing differences between output impedance unit A and B

After looking a bit at the circuit I realized that I had added 100 Ω in series in detector B (to limit current draw when driving 50 Ω loads).  This had not been added to the detector A, which means that with more power than 300 µW on this detector the op amp would be drawing more than 30 mA when driving a 50 Ω.  This shouldn't have been an issue when driving the summing circuit, but is good practice.

I added 100 Ω in series with op27 output (thin film 1206 size) in detector A to match the output to that of detector B.  I remeasured the TF and get a much better match between the paths.

## Calibrating TIA Measurment

The following is used to calibrate the  transfer function (See PSL:2247):

$\dpi{100} Z_\textrm{AC,PD} = Z_\textrm{AC,Ref} R_\textrm{PC}e^{i\delta\phi} T_\textrm{meas}$

where ZAC,PD is Calculated RF Transimpedance, Z AC,Ref is the known RF transimpedance of the reference photodiode, 𝛿ϕ is arbitrary phase delay due to light and cable length and RPC  is the photocurrent ratio at DC of reference PD to RFPD under test given by

$\dpi{100} R_\textrm{PC} = \frac{V_\textrm{DC,Ref}/Z_\textrm{DC,Ref}}{V_\textrm{DC,PD}/Z_\textrm{DC,PD}}$

For these measurements the calibration factors were as follows:

Unit A TF (after adding 100 Ω series​ output): measured NF1611 DC voltage of 780 mV (@ 1 MΩ impedance), PD DC output voltage was -1.20 V (@ 1 MΩ impedance).  Here the photodetector has transresistance is 6.8 kΩ. The NF1611 detectors have a DC path gain of 10 kΩ and an AC path gain of 700 Ω. From this the R_PC is 0.4496.  Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).

Unit B TF: measured NF1611 DC voltage of 791 mV (@ 1 MΩ impedance), PD DC output voltage was -1.18 V (@ 1 MΩ impedance).  Here the photodetector has transresistance is 6.8 kΩ. The NF1611 detectors have a DC path gain of 10 kΩ and an AC path gain of 700 Ω. From this the R_PC is 0.4558.  Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).

Results of calibrated transimpedance measurements are plotted below*. I also added the LISO estimated noise curve.  The 3 dB of unit A and B were 1.93 MHz and 1.72 MHz respectively.  There is some discrepancy with the LISO model here, some of this might be artifacts in the reference detector and some is likely dirt effects in the proto board circuit that I'm not going to debug for now.  The biggest discrepancy between the two detectors starts around 900 kHz.  When I looked again at the detectors I realized I had used a Wima foil cap in detector B for the bias LPF. It should be fine for these frequencies, but I switched the foil cap out and replaced it with a 1 µF + 100 nF ceramic cap to match the unit A.  I haven't gotten a chance to remeasure this since as Gautam is still using the Jenne rig to do some stuff. I'll remeasure later, we should then find the two responses match.

*I didn't do the many IRIS measurement​ uncertainty analysis, its not necessary at this stage.

Attachment 1: TFUnitAandB_BeforeMatchingOutImp.pdf
Attachment 2: TFUnitAandB_FixingDetectorAImp.pdf
Attachment 3: WOPO_HD_TranImpGain_UnitAandBandLISO.pdf
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