40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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  2556   Fri Apr 16 11:27:01 2021 AidanUpdate2um Photodiodes2um DC photodiode voltage weird drift with no incident power

[Aidan, Radhika, Nina]

We noticed that the DC channel readout (FM30) of the JPL A1 photodiode is drifting around. What we observe with no light on the photodiode, is the DC output drifiting around. It gets particularly bad when we apply voltage to other DAC channels.

For example, the attached plot shows the DC voltage from the photodiode as I change the set voltage to the laser diode driver. To be absolutely clear, the laser driver itself was completely powered off. I'm just varying the voltage going into the set point BNC connector on the back of it.

For reference, the set up is:

DAC (300mV bias) > relay > PD > relay  > FEMTO preamp (1000x gain) > ADC channel FM30

 

Attachment 1: DC_2um_voltage_weirdness.png
DC_2um_voltage_weirdness.png
  2557   Fri Apr 16 13:05:05 2021 AidanUpdate2um Photodiodes2um DC photodiode voltage weird drift with no incident power

We put the preamp output directly into a multimeter and observed the same fluctuating behavior as the DAC channel was changed.

We're bypassing the relay to see if that makes any difference. The old relay wiring (to be bypassed) is shown in the attached diagram. That didn't do anything.

We're looking at filtering the DC output by 5kHz to see if there are any resonances at higher frequencies that might go away. Changing SR560 output for AC path to DC and setting gain to 1 on that unit. Also changing gain in FM31 filter bank from 1E-3 to 1. The results are shown in the attached time series. The channels FM30 and FM31 see the same thing. The only difference is that FM31 goes through an SR560 with a 0.03Hz pole (6dB).

Success by bypassing the DAC bias voltage. We switched to a 300mV bias voltage from a function generator. Doing that removed the causal PD voltage drift induced by changing the laser diode current set voltage (see the last time series). So the issue is some weird coupling into the DAC bias voltage.

Quote:

[Aidan, Radhika, Nina]

We noticed that the DC channel readout (FM30) of the JPL A1 photodiode is drifting around. What we observe with no light on the photodiode, is the DC output drifiting around. It gets particularly bad when we apply voltage to other DAC channels.

For example, the attached plot shows the DC voltage from the photodiode as I change the set voltage to the laser diode driver. To be absolutely clear, the laser driver itself was completely powered off. I'm just varying the voltage going into the set point BNC connector on the back of it.

For reference, the set up is:

DAC (300mV bias) > relay > PD > relay  > FEMTO preamp (1000x gain) > ADC channel FM30

 

 

Attachment 1: IMG_2132.jpg
IMG_2132.jpg
Attachment 2: FM30_31_w_latter_filtered.png
FM30_31_w_latter_filtered.png
Attachment 3: IMG_2134.jpg
IMG_2134.jpg
Attachment 4: bias_bypassed.png
bias_bypassed.png
  2589   Wed Jun 16 17:17:12 2021 KojiUpdateGeneralI2 cell

I was searching an I2 (Iodine) cells back to the days of the laser gyro.

I found a likely box at a very tricky location. Took the photos and returned to this tricky place.
 

2021/Jul The box was moved to the OMC lab (KA)

Attachment 1: P_20210616_170104.jpeg
P_20210616_170104.jpeg
Attachment 2: P_20210616_170038.jpeg
P_20210616_170038.jpeg
Attachment 3: P_20210616_170021.jpeg
P_20210616_170021.jpeg
  2590   Fri Jun 18 10:15:14 2021 StephenUpdateCryo vacuum chamberTemperature sensor considerations

RTD thoughts - we have just been using the sensors that were provided, without noticing their constraints or deficiencies.

  • our RTD p/n is 615-1123-ND (Digikey), which is a Littlefuse PPG102A6 platinum RTD and has an apparent temperature range of -200 °C (73 K) to + 600 °C. The primary data sheet does not have a Resistance vs Temperature curve, simply presenting the slope of the temperature dependence in a parameter "Temperature Coefficient of Resistance" (TCR), but a table is available in an auxiliary document called "RT Chart". An image of this chart is attached below.
  • we have not violated that 73 K lower limit yet, but we are about to as we mount an RTD directly to the coldhead. Let's see how that RTD on the coldhead will withstand the lower temperatures.
  • the CTC100 manual indicates that any arbitrary calibration curve may be input, but there are a number of sensors' calibration curves built in - it might be a good idea to make our next sensor decision based on that list.
  • there is a Lakeshore guide and category page for temperature sensor selection - reconfiguring our 4 RTDs would cost over a kilobuck through Lakeshore, but perhaps we can learn general ideas from the guide as well.
    • it seems that switching to their Cernox line would be helpful in terms of packaging options, and would be the most accurate.
    • their line of silicon diodes would be suitable and has flexible packaging as well.
    • their platinum RTDs also have low temperature range and would be suitable. Packaging is cylindrical, so might be best to pursue the aluminum housing with a bolt hole.

Planning for next steps:

  • for now, it seems that we could get by with our generic and 73 K limited RTDs, and this option is tempting as it requires no additional effort.
  • if we decide we really want to have reliable sensing down to sub-50 K temperatures, we should move to one of the Lakeshore product lines (hopefully one which the CTC100 is configured with a calibration curve for) for about a kilobuck.
  • we should engage in more serious sensor design before Mariner, regardless of whether we take any action now.
    • as a starting point, the Lakeshore catalog and appendices (ref. product info page) and other resources should be absorbed, for considerations like thermal anchoring, lead length, benefits of 4-lead wiring, polyimide leads causing less conduction to the sensor than teflon, phosphor bronze having lower thermal conduction than copper, etc. Most of these topics are gathered from Appendix C.
Attachment 1: RTD_Resistance_vs_Temperature_for_PPG102A6.png
RTD_Resistance_vs_Temperature_for_PPG102A6.png
  2668   Wed Sep 15 08:33:39 2021 AidanUpdate2um PhotodiodesVideo review of 2um testing setup post A1 testing

https://dcc.ligo.org/LIGO-G2102040

 

  2672   Tue Sep 28 08:23:11 2021 AidanUpdate2um PhotodiodesDistances between optics, collimating lens, focussing lens and photodiode

Precise distances required between:

  • fiber launcher and collimating lens
  • focussing lens and beam waist 

accounting for thickness of optic mounts, sunken fiber launcher plane, back focal length of lenses, dispersive variation in focal lengths of lenses  from nominal and distance between PD surface and base of PD mount. Also shown are the distances between the steering mirrors (PZT steering mirror, lower periscope mirror and upper periscope mirror).

Beam propagation through this system is shown in the attached PDF. The upper plot shows a paraxial beam propagation as the collimating lens is displaced from the nominal position. The purpose is to indicate the beam size (radius) all the way through the system. We would like this to be less than about 6mm radius (12mm diameter) on all of our 1 diameter optics. The second plot shows the waist size at the PD as the collimating lens is moved by +/- 2mm. The purpose is to allow us to tune the beam size on the PD without clipping the beam on intervening optics. 

Keeping the collimating lens Delta Z to a range of +/- 2mm is safe for beam propagation in terms of clipping on apertures or on the 1.5mm diameter PD.

Attachment 1: JPL_PD_collimating_lens_Optical_layout.pdf
JPL_PD_collimating_lens_Optical_layout.pdf
Attachment 2: JPL_PD_optical_propagation_and_beam_size.pdf
JPL_PD_optical_propagation_and_beam_size.pdf
  2686   Mon Oct 25 16:10:51 2021 AidanUpdateCDSRestarted computers and front-end model following campus power outage

Rebooted the workstations and FB4.

 Restarted the model on the FB4:

  • sudo /opt/rtcds/caltech/c4/target/c4iop/scripts/startupC4rt
  • sudo /opt/rtcds/caltech/c4/target/c4tst/scripts/startupC4rt
  2687   Mon Oct 25 16:13:47 2021 AidanUpdateCryo vacuum chamberHeater left on - chamber got warm

The cryocooler was switched off last Thursday to do testing on the JPL_PD. I turned the heater back on during this testing and neglected to turn it off when I finished at the end of the day. As a result, the workpiece reached ~400K over the weekend.

We are now allowing it to slowly cool down.

The CTC100 has a feature to specify an upper limit on temperature and then shut off the heater if that temperature is exceeded. We should engage this going forward.

Attachment 1: Screenshot_from_2021-10-25_16-19-39.png
Screenshot_from_2021-10-25_16-19-39.png
  2689   Tue Oct 26 07:32:52 2021 AidanUpdateCryo vacuum chamberHeater left on - chamber got warm

We're at 300K as of 7AM this morning.

Quote:

The cryocooler was switched off last Thursday to do testing on the JPL_PD. I turned the heater back on during this testing and neglected to turn it off when I finished at the end of the day. As a result, the workpiece reached ~400K over the weekend.

We are now allowing it to slowly cool down.

The CTC100 has a feature to specify an upper limit on temperature and then shut off the heater if that temperature is exceeded. We should engage this going forward.

 

  2690   Tue Oct 26 08:43:38 2021 AidanUpdateCryo vacuum chamberCTC100 temperature alarm and heater shutoff

Instructions on how to enable the alarm and heater shut off for the CTC100.

Status: This reports the status of the alarm. If LATCH is enabled, this must be manually set to OFF once it has been enabled.

Mode: Set to "Level"

Latch: Optional to set to "YES" if desired.

Output: Set to "Heater"

Max: Set to desired maximum temperature.

The attached photos show:

  • the menu where the settings are ALARM entered
  • the main display just before the alarm is enabled (at 300.350K with a 1s delay)
  • the main display just after the alarm is enabled - note that the Heater Output has been set to 0W.
Attachment 1: Screenshot_from_2021-10-26_08-42-57.png
Screenshot_from_2021-10-26_08-42-57.png
Attachment 2: IMG_5371.jpg
IMG_5371.jpg
Attachment 3: IMG_5396.jpg
IMG_5396.jpg
Attachment 4: IMG_5401.jpg
IMG_5401.jpg
  2723   Tue Feb 22 07:53:06 2022 YehonathanUpdateWOPOWaking up WOPO

{Shruti, Yehonathan}

On Friday, we came down to QIL to poke around the WOPO setup. The first thing we noticed is that the setup on the wiki page is obsolete and in reality, the 532nm light is coming directly from the Diablo module.

There were no laser goggles for 532nm so we turned on the 1064nm (Mephisto) only. The pump diode current was ramped to 1A. We put a power meter in front of Mephisto and opened the shutter. Rotating the HWP we got 39mW. We dialed it back so that 5mW is coming out of the polarizer.

The beam block was removed. We disconnected the LO fiber end from the fiber BS - there is light coming out! we connected a power meter to the fiber end using an FC/PC Fiber Adapter Plate. The power read 0.7mW. By aligning the beam into the LO fiber we got up to 3.3mW.

We connected the BHD PDs to the scope on the table to observe the subtraction signal. Channel 2 was negative so we looked at the sum channel.

Time ran out. We ramped down the diode current and turned off Mephisto.

Next time we should figure out the dark current of the BHD and work toward observing the shot noise of the LO.

  2725   Fri Feb 25 17:09:53 2022 shrutiUpdateWOPOWaking up WOPO - green beam

[shruti, yehonathan]

SHG and 532 nm beam alignment

Yehonathan brought over 532nm/1064nm laser goggles from the 40m.

  • We turned on the 1064 nm Mephisto, and set the pump current initially to 1.5 A (which gave us ~30 mW of output)
  • We then turned on the doubling crystal. It took a while to reach its setpoint temperature of 110 C. Initially (without adjusting the SHG cavity parameters) we saw less than a microW of power after removing the beam block and opening the shutter.
  • Playing around with the SHG cavity settings, we realized that
    • Setting the switch to "Auto" is how to nominally operate the 532 nm beam
    • "Scan" is useful for scanning the cavity, the amplitude of which can be controlled by the "Scan amplitude" knob. "Standby" seems to effectively turn off second harmonic generation
    • "Offset" knob tends to change the amount of green power generated and adjusting the "Gain" simultaneously helps stabilize this power (with some difficulty)
  • On increasing the laser driver current to 2 A and adjusting the temperature setpoint to 109.7 C, we were able to see 100 mW of green power!
  • Next, we played with the alignment into the fiber, seeing that initially there was barely any power at the other end of that patch cable
    • We adjusted the waveplate near the laser head to give us 5.3 mW of power right before coupling into the fiber
    • Adjusting a single mirror (the nearer one) did not result in much gain, so we simultaneously adjusted the two steering mirrors and achieved about a 50% coupling. (Our readings suggested it could be the max)
    • At  the end of the alignment: 2.74 mW at the output and 5.46 at the fiber input
  • Reconnecting the fiber to the waveguide, without adjusting the temperature of the advr waveguide, we saw that the fiber at the output of this crystal seemed to glow.
    • The power measured at the end of the output fiber (fiber after the waveguide) was 0.6 mW. Not entirely sure what the contribution of loss was in the decrease from 2.74 mW through the waveguide.
  • The laser is still ON although the shutters to the green and IR paths are closed. Safety glasses required before opening shutters.

 

Questions about the setup

  1. The spec sheet on the wiki mentioned PM980 and PM480 input and output fibers, respectively for the waveguide operating as an SHG, what were being used instead were P3-1064PM-FC2 and P3-488PM-FC2. Is this a significant source of loss that can be easily remedied?
  2. Yehonathan mentioned that stimulated Brilluoin scattering occurs in all fibers above a threshold. What is the threshold for the the ones used in the setup? We probably want to operate below this threshold.

 

Our next step would be to measure the LO shot noise.

 

 

 

  2727   Wed Mar 2 14:38:55 2022 YehonathanUpdateWOPOWaking up WOPO - some more fiddling and a plan

{Shruti, Yehonathan}

We made some a list of some random questions and plans for the future. We then went down and found answers to some of those:

1. Why is there no Faraday isolator in the 1064nm beam path? (edit: turns out there is, but inside the laser, see pictures in this elog).

2. Do the fibers joined by butt-coupling have similar mode field diameter? If not it can explain many loss issues.

a. In the green path we find that according to the SPDC datasheet the 532nm fiber (coastalcon PM480) is 4um while the input thorlabs fiber (P3-488PM-FC2) coupled to it has an MFD of 3.3um. This mismatch gives maximum coupling efficiency of 96%. Ok not a big issue.

b. At the 1064nm output the SPDC fiber is PM980 with MFD of 6.6um while the BS fiber is 6.2um which is good.

3. What is the green fiber laser damage threshold? According to Thorlabs it is theoretically 1MW/cm^2 practically 250kW/cm^2 for glass air interface. With 3.3um MFD the theoretical damage threshold is ~ 80mW and practically  ~ 20mW. It doesn't sounds like a lot. More so given that we could only get 50% coupling efficiency. How much is needed for observable squeezing? There is the possibility to splice the fiber to an end cap to increase power handling capabilities if needed.

4. Is stimulated Brillouin back scattering relevant in our experiment? According to this rp photonics article not really.

5. How much green light is left after the dichroic mirrors? Is it below the shot noise level? Should check later.

In addition, we found that the green fiber input and the 1064nm fiber output from the SPDC were very dirty! We cleaned them with a Thorlabs universal fiber connector cleaner.

 

 

 

 

  2728   Fri Mar 4 11:49:45 2022 shrutiUpdateWOPOWaking up WOPO - attempts at readout

[Yehonathan, Shruti]

1. Doubling cavity and green beam

Since we had left the lasers ON with the shutters closed we wanted to see if the powers measured after opening the shutter would be similar to what it was when we left. We realized that opening and closing the green shutter destabilizes the doubling cavity (the FI is after the shutter and the shutter does not seem to be a good dump), which in turn changes the SHG crystal temperature (possibly because of the power fluctuation within the crystal). Re-opening the shutter requires some tuning of the temperature and offset to recover similar output power. Finally, after some tuning, we were able to see 156 mW of green light.

 

2. Attempt at measuring LO shot noise

  • We want to measure the BHD output A-B channel, which we expect to be dominated by shot noise since all the classical noise would be canceled when optimally balanced, but found that one of the PDs was inverted so that the sum of the two channels would be what we needed to measure. Since the SR 560 has only an A-B option, we used a second SR 560 to invert the B channel before subtracting
  • Operating at an LO power of ~4 mW did not give us sufficient clearance from the dark noise in each channel with the SR 560s, which was around the max power I believe we're supposed to use for the BHD LO
  • Somehow measuring the output directly, without the SR 560, gave us some clearance over the dark noise at ~1 MHz and higher (possibly because 1 MHz is the SR560's BW) so we decided to measure the time series of both channels together and do the optimized subtraction and FFT offline

3. Subtracted noise signal spectra [Attachment 1]

  • The plots show the noise spectra of the channels measured individually. The 'gain adjusted' means that A was multiplied by 1.05 and B by 0.95 in order to get the two plots to more or less line up
  • We used the Moku to measure the timeseries at a sampling rate of 10.2 MS/s for a period of 1.2 ms with AC coupling and 50 Ohm impedance. Elog 2324 suggests the designed measurement was for a 50 Ohm load so we should be impedance matched but I'm yet to convince myself
  • Our estimate for the shot noise was 77 nV/rtHz for 4 mW of power and 87% QE, using a TI gain of 2kOhm (the black dashed line in the plot). If we were impedance matched the yellow trace must be higher than this estimate
  • In our next measurements, we will also record the dark noise, carefully measure the power. There is obviously sonething wrong with the plot

 

 

30 Mar 22 edit: script here, data here

Attachment 1: LO_shot_noise.pdf
LO_shot_noise.pdf
  2731   Thu Mar 10 17:12:01 2022 awadeUpdateWOPOWaking up WOPO - attempts at readout

[awade]

Good to see this experiment being revived.

1. The design of this laser had a number of flaws and one of them is this sensitivity to backreflections at 532 nm. I mostly just disabled the doubler's lock and closed the shutter for good measure, but probably best not to leave flickering around in an unstable state when you're away.

2. I built in the inversion in the second channel to give myself the option to electronically subtract: something that didn't end up being very practical compared to just digitally recording channels and subtracting in post.

  • I'm surprized the SR560 don't given you clearance there.  Nominally these units are 4 nV/rtHz if you operate in low noise mode AND gain=100 (see p21 of the manual), below this gain gives you 10-60 nV/rtHz noise. When I built the PD circuits I did verify that I was getting the clearanceI expected.  At 1 mW on each photodiode one would expect order =sqrt(2*h*c/lambda0*Responisvity*Power)*Gain ~ sqrt(2*6.626e-34*3e8/1064e-9*1e-3*0.7)*2e3 = 32 nV/rtHz.  
  • A few things to verify:
    • check the DC voltage (just with a multimeter) to see true power picked up by diodes, this should be 1.4 V for about 1 mW of 1064 nm;
    • Make sure you're AC coupled into SR560, there is no way you operate at gain 100 or above and also not saturate for 1 mW (~1.4 V amp output DC) of light
    • at 2kΩ gain you should expect the noise floor to be of order =sqrt(4*kB*T*G)=sqrt(4*1.38e-23*300*2e3)~5.8 nV.  Only just clear of the SR560 spec, and about equal to typical actual performance.  To see this level you might want to pre-amplify with a Femto amplifier, the 40m Busby box or the ganged amplifier box I made for the CTN lab (its black with gold writing, Anchal knows the one). A dark measrument like this may have a little offset that you can either null or just AC couple with a minicircuits DC block;
    • Take a terminated (50 Ω) measurment of your ADCs when you collect your PD 'dark' data. Even better also collect terminated SR560 data. And put these on the plot.  Moku:Labs have ~30 nV/rtHz @100 kHz and above.  Just be sure you're measure photodetectors and not Pre-amp or ADC noise. Moku nominal input refered noise is 13.9 nV/rtHz * sqrt(1+220kHz/f).
    • If you can't get any quick progress, try all the above with minicircuits lower noise amplifiers.  They have plenty of bandwidth and go to higher frequencies.
  • Just measureing the output of the PD directly, with no subtraction or amplification, I'd say you are looking at laser technical noise at about 1 MHz: this is what the subtraction is for, to null the LO noise effects to only listen to signal port.  Maybe somethings burried in the subtracted signal offline, but you need some simultaneous termianated measurements back long the signal chain to put some bonds on what is the limiting noise here.

3. Subtracted noise spectra

  • These gain numbers sound right to me.
  • The AD829 is designed to drive this combined 150 Ω load, I would stick to 50 Ω terminations for now. On the topic of mokus: again verify its input referred noise and pre-amplify accordingly.  Also there is a choice of "Normal" or "Precision" acquisition mode, I think the right choice is precision (this should have the right filters to kill aliasing from the downsampling)
  • ~70 nV/rtHz shot noise sound about right to me.  Not clear why a subtracted signal doesn't seem to reach this.  Once again, measure the actual DC voltage output from the TIA to get the true absorbed photons and use that to calibrate your estimate.  

We should chat some time on zoom about more details (rana can forward my details).  Hope this enought to go on for at least the homodyne part of the experiment.  

  2733   Wed Mar 16 12:22:44 2022 YehonathanUpdateWOPOWaking up WOPO - attempts at readout

{Shruti, Yehonathan}

Yesterday, we measured a bunch of noises.

We wanted to have as reference the Moku noise, the PDs noise, and measure the shot noise of the LO again.

Attachment 1 shows the Moku noise measured by just taking data with no signal coming in. We tried both the spectrum analyzer (SA) and the oscilloscope tools, with and without averaging, and the difference between the channels.

For some reason, the SA has a worse noise figure than the oscilloscope and the difference channel doesn't give us any special common-mode rejection. Also more averaging doesn't help much because we are already taking 1.2ms of data which is way longer than 1/RBW=0.2ms we are taking here.

From now on we use the oscilloscope as the spectrum analyzer and to its noise we refer as the Moku noise floor.

Moving on, we try to measure the PD dark noise. Given that the PD dark noise floor is ~ 6nV we don't expect to see it with the Moku without amplification. Attachment 2 shows that indeed we couldn't resolve the PD dark noise.

We then opened the LO shutter. We measured with a power meter 1mW and 1.15mW coming impinging on the PDs. The voltage readings after the preamp were 1.66V for the white fiber, and 1.93 V for the red fiber. These values suggest responsivities of 0.830 and 0.834 respectively.

The PDs were measured using the Moku scope and subtracted digitally with some small gain adjustment (0.93*ch1-1.07*ch2) between the channels. The result is shown in attachment 3 together with the expected shot noise level.

1. There is not enough clearance for detecting squeezing.

2. Expected shot noise level is still too high. Does the 2kohm preamp gain go all the way above 1MHz??

Attachment 1: Moku_Noise.pdf
Moku_Noise.pdf
Attachment 2: PD_Connected_no_light.pdf
PD_Connected_no_light.pdf
Attachment 3: Diff_Channel.pdf
Diff_Channel.pdf
  2735   Tue Mar 22 09:19:38 2022 shrutiUpdateWOPOWaking up WOPO - back to green path

[Yehonathan, Shruti]

Yesterday we went back to fiddling with the green path. Soon after opening the green shutter and then switching the doubling cavity to 'AUTO' we were able to see 150 mW of green light. We were able to replicate this a couple of times yesterday.

Since we had earlier removed the green fiber from the fiber launch to clean its tip, the coupling into the fiber turned out to be quite poor. As can be seen in Attachment 1, Yehonathan pointed out that a lot of green light was being lost to the cladding due to poor coupling. He then played around with the alignment and finally was able to see 65% coupling efficiency. This process seemed to involve a great amount of trial and error through several local power minima.

Attachment 2 shows that the coupling between the two fibers at the 532 nm input of the waveguide is quite poor (there is visible light being lost in the cladding). Furthermore, this light intensity decreases as we get closer to the waveguide meaning this light is being dissipated in the fiber. Even at the 1064 nm output where we expect to see squeezing there is some remnant green light.

We wanted to test if the green leakage reaching the PDs were causing additional noise. For this we just looked at the spectrum analyzer on the Moku (after amplifying 100x with the SR 560) and saw no difference in the noise spectrum with and without the green shutter being open. Although, we're not convinced with this measurement since we were not able to find good quality SMA cables for the entire path. Moving around the BNCs seemed to change the noise. Also, near the end, we noticed some coupling between the two channels on the Moku while measuring the noise that seemed to cause additional noise in one of the channels. We did not have sufficient time yesterday to probe this further.

Attachment 1: before_opt.jpg
before_opt.jpg
Attachment 2: around_waveguide.png
around_waveguide.png
  2759   Wed Apr 20 00:12:12 2022 YehonathanUpdateWOPOStill figuring out the readout electronics and fixing of some stuff

{Yehonathan, Shruti}

1. Grabbed 30Hz-3GHz HP spectrum analyzer from the Cryolab. Installed it in the WOPO lab under the optical table. We figured out how to do a zero-span measurement around 10MHz. The SA has only one input so we try to combine the signals with an RF splitter. We test this capability by sourcing the RF splitter with 10MHz 4Vpp sine waves from a function generator and measuring the output with a scope. We measure with the scope 1.44Vpp for each channel. The combined channel was 2.73Vpp. We then realized that we still don't have a way to adjust the gains electronically, so we moved on to trying the RF amplifiers (ZFL500 LN).

We assemble two amps on the two sides of a metal heatsink. We solder their DC inputs such that they are powered with the same wire (Attachment 1). We attach the heatsink to the optical table with an L bracket (Attachment 2).

We powered the amps using a 15V DC power supply and tested them by feeding them with 10MHz 10mVpp sine waves from a function generator. We observe on a scope an amplification by a factor of ~ 22. Which makes a power amplification of ~ 26db consistent with the amplifiers' datasheet.

We couldn't find highpass filters with a cutoff around 1MHz, so we resumed using the DC blocks, we test them by feeding white noise into them with a function generator and observing the resulting spectrum. First, we try the DC blocks with a 50 Ohm resistor in parallel. That happened to just cut the power by half. We ditch the resistor and get almost unity transmission above 20kHz.

Moving on to observing LO shot noise, we open the laser shutter. We find there is only 0.7mW coming out of each port of the fiber BHD BS. We measure the power going into the BS to be 4mW. This means the coupling between the LO fiber and the BS fiber is bad. We inspect the fibers and find a big piece of junk on the BS fiber core. We also find a small particle on the LO fiber side. We cleaned both fibers and after butt coupling them we measure 1.6mW at each port. We raise this power to 2mW per port.

We connect the outputs of the PDs to the amps through the DC blocks. The outputs of the amps were connected to the Moku's inputs. The PDs were responding very badly and their noise was also bad. We bypass the amps to debug what is going on. We connect the PDs to a scope. We see they have 300mV (attachment 3) dark noise which is super bad and that they hardly respond to the light impinging on them (attachment 4). We shall investigate tomorrow.

Attachment 1: 20220419_153109.jpg
20220419_153109.jpg
Attachment 2: 20220419_164600.jpg
20220419_164600.jpg
Attachment 3: 20220419_184248.jpg
20220419_184248.jpg
Attachment 4: 20220419_184233.jpg
20220419_184233.jpg
  2760   Thu Apr 21 10:33:33 2022 shrutiUpdateWOPOStill figuring out the readout electronics and fixing of some stuff

[Yehonathan, Shruti]

 

First we turned on the relevant instruments for this experiment after the power shutdown:

- Main laser drivers and doubling cavity controller. We set the current to 2 A as we had it before.

- The waveguide TEC. We tried setting it to 60.99 C (for maximum efficiency) but the temperature ramps up much faster and over shoots the setpoint. So we had to do what we did earlier which was to adiabatically change the setpoint from room temperature and finally set it to something like 63 C so the actual measured temperature stabilizes at ~60.9 C. How do we change the PID parameters on this controller? The settings don't seem to allow for it.

- PD power supply, oscilloscopes, function generator, SR 560s lying nearby

 

Then we tried to probe further what was going on with the PDs (TL;DR not much made sense or was reproducible)

  • Initially we were sending in 12 V from Ch1 of the Tenma power supply and \pm15 V to the mini-circuits RF amplifiers from Ch2 of the same Tenma. When we tried to observe the DC voltage levels (before amplification) and the noise (after amplification) but they did not make sense to us as described in the previous elog.
  • Then we disconnected the RF amplifiers and switched the power supply for the PD transimpedance amps to Ch2 of the Tenma. Briefly we were able to see 2 V DC at each output (as expected for the ~1.3 mW of light incident  on each PD) when measured with the multimeter. On the scope we were seeing that one of the PDs was dominated by 60 Hz fluctuations with a much lower
  • We tried to switch it back to Ch1 but even the DC levels seen by the multimeter were bogus (~1 V for one of the channels and 2 V for the other)
  • When we switched the power supply to a HP one taken from CTN, the DC levels on both channels seemed ~1V without any light and the noise somehow seemed lower with some incident light

 

Possible next steps

  • Even though the AD 829 data sheet says it can be operated at up to 15 V, it is possible that we damaged both of them somehow when cycling the power. Elog 2324 also says that it was designed to be powered at \pm 5 V. We could replace both op-amps and measure teh transimpedance TFs before and after the change
  • Use different PDs. Maybe temporarily try with just the ThorLabs PDA20CS or similar with ~20 mW of LO power and measure the shot noise (possibly also squeezing).
  2762   Mon Apr 25 11:08:35 2022 YehonathanUpdateWOPOStill figuring out the readout electronics and fixing of some stuff

{Shruti, Yehonathan}

We realized that the PD amp circuit only requires a 5V DC supply so we try that. One of the PD had the right response, although only after cycling the input impedance from 50ohm to 1Mohm which is weird. The other one (which produces the negative signal) was complete bonkers.

We remove the home-built PDs and put 2 Thorlabs PDs (forgot the model) with a bad dark current but a decent response and high saturation current. With these PDs we are limited by the PD noise to about 1.25db od squeezing when 30mW LO is detected on each PD without using electronic amplifiers. Attachment 1 shows the different noise spectra we measured.

We maximize the coupling efficiency before boosting the LO power. For some reason, the coupling between the LO fiber and fiber BS deteriorated but there was no apparent dirt on them upon inspection. We crank up the power and measure PD outputs using the Moku oscilloscope. The PD signals were subtracted digitally, but now we were not able to get the shot noise even after fine-tuning the gains. What went wrong? maybe it's because the PDs have separate power supplies?

Details later...

 

Some analysis in this notebook

  2764   Thu Apr 28 14:12:20 2022 YehonathanUpdateWOPOStill figuring out the readout electronics and fixing of some stuff

{Shruti, Yehonathan}

We went to the e-shop to investigate the PD circuits. Completely confused about the behavior of the PDs we decide to gain some sanity by testing a sample AD829 on a breadboard with resistors and capacitors similar to those in the design of the PD circuits shown here. The PD is replaced by a voltage source and a 2kOhm resistor such that 0db gain is expected. We first measure the TF of the opamp with the Moku just with the resistors (attachment 1) then with the compensation capacitors.

We tried powering the opamp with shorting V- to ground like we did in the WOPO lab (for some reason this was how it was connected) and got garbage results (attachment 3).

We then turned to retesting the PD circuits with a proper powering scheme. However, connecting +/-5V and ground from a power supply resulted in the output of the PD circuit being ~ -2V even when the PD is taken out which might suggest that the opamps have really gone bad.

Attachment 1: JustTIR.pdf
JustTIR.pdf
Attachment 2: TIR_WithCapacitors.pdf
TIR_WithCapacitors.pdf
Attachment 3: OnlyV_plus.pdf
OnlyV_plus.pdf
  2777   Thu Jun 2 10:28:26 2022 YehonathanUpdateWOPOInstalling 1811 PDs

[Shruti, Yehonathan]

We took newport 1811 PDs, one from CTN lab (suspicious) and one from (I forgot) for their high gain and low dark noise.

The detector diameter is small 0.3mm, but our focusing is sufficient:

The mode field diameter of the PM980 fiber is ~ 6.6um. The beam is collimated by a Thorlabs F240APC-1064 with f = 8.07mm and focused with an f = 30mm lens. It means that the diameter at the focus should be roughly 6.6um*30/8.07 = 0.024mm which is well within the PD active area.

 

We place the PDs at the focal point of the lens at the BHD readout. The impinging optical power was set to be ~ 0.6mW at each port. In one of the PDs, we measure the DC response with a scope to be ~ 5.5 V/0.6 mW ~ 9e3 V/W. According to the specs, the DC monitor as a response of 1e4 V/A while the responsivity of the PD itself is ~ 0.8 A/W at 1064nm so the overall responsivity is ~ 8e3 V/W.

However, the second PD's DC response was bonkers: we measured it to be ten times less. The AC response might still be OK since it is a different port but we haven't measured it yet.

  2796   Wed Jul 20 15:55:41 2022 YehonathanUpdateWOPOInstalling 1811 PDs

{Yehonathan, Paco}

We comfirmed that the DC ouput of one of the 1811s is bad. We set out to measure the AC response of the PDs.

For this, we decided to use the current modulation on the Diabolo laser which is rated to have 0.1 A/V and a bandwidth of 5kHz. We calibrated the current to optical power by swiping the current and measuring the power at the homodyne PDs using a power meter. The laser power before the 1064nm PZT mirror was measured to be 5mW.

Attachment shows the measurement and a linear fit with slope=0.97 mW/A.

We drove the current modulator using a sine wave from a function generator with 1kHz 0.5Vpp. When we looked on the PD AC signal port in the Oscilloscope we saw 2 Vpp 12Mhz signal. We passed the signal with a low pass filter but again we saw mostly noise.

We took the PDs to the 40m PD test stand but we accidently fried Jenne's laser.

Next, we should just use the Moku network analyzer instead of the scope to measure the response in the QIL using the Diabolo current modulator.

Attachment 1: Diabolo_Current_Power_at_PDs.png
Diabolo_Current_Power_at_PDs.png
  2800   Tue Aug 23 22:23:06 2022 awadeUpdateWOPOInstalling 1811 PDs

I recall at one point we had one of these NF1811 with a broken power suply pin.  It was from a limited production run with the smaller micro 3-pin power connectors. Maybe check yours is not that one.  

Long story short it still responded with only the positive rail but DC will gave a bad photovoltaic mode response and the AC had a large unstable oscillation that was only viewable on a high speed scope (if I recall right higher than the 125 MHz nominal bandwidth).  I would check the power-in pins aren't bent/broken and also check the AC out on a higher speed scope (i.e. >=500 MHz).

ELOG V3.1.3-