After reviewing the Brimrose AOM driver mnual, yesterday, it turns out I was previously using it incorrectly. It is a VCO with the frequency port accepting a DC voltage, between 0 and 10 V to control the frequency of the AOM - note that the mapping is not intuitive so one should refer to the manual. The modulation port is used for amplitude modulation, not frequency modulation. This port, modulation, has 50 ohm input impedance and accepts signals between DC and 10 MHz - modulating the power output. Table 1 below shows the operating parameters we should use:
Following this I characterised the phase noise of this VCO. Results are shown in attachment 1, for various powers. The setup is shown in figure 2, with the data for M = +0.85 V provided in attachment 3. These results show that this VCO has poor phase performance. A value of M = +0.02 V gives the same RF power as when a 0.5 Vpp signal @ 80 MHz was input into the modulation port - as I was previously doing.
This has a few implications for previous measurements:
The script used to create the smoothed ASD can be found here.
Following up from my previous post I measured the phase noise of the Mach Zehnder setup with the AOM driven properly with its VCO. The setup is shown in attachment 1, and compressed data in attachment 2 (goto here to get the python library to decompress this data). The value of M for this data shown is +0.85 V, though I have data for other voltages - however it should affect the performance. Using the low frequency preview I was able to see that I would need to coherently subtract the phase measurements of both measurements, which restricted me to a maximum sampling frequency of 15 kHz.
The measured data is shown in attachment 3. Already you can see that the phase noise of the VCO limits the measured phase from the IFO. This also tells me that previously, when I was modulating the VCO, the AM was affecting the measured phase. When I convert this difference into frequency noise the result can be seen in attachment 4. One upside of this setup is that the signal from the IFO is much more robust, so the PLL can stay locked for longer. This is a consquency of increased drive, 24 dB more, on the AOM.
To me this demonstrates that the way forward is to replace the VCO driving the AOM with a RF amplifier, driven by a low noise (or lower noise) signal generator. The Moku is able to drive at 80 MHz. We have a Mini Circuits ZHL-5W-1 (ZHL--5W) amplifier in the laboratory. This has 46.4 dB of gain, and a maximum power output of 37 dBm. The maximum power that can be input into the AOM is 27.8 dBm (0.6 W). Thus with an appropriate setup this miniciruits amplifier should be a viable repalcement for the current VCO.
Attached are the results from measuring the phase noise of the Moku with a SRS DS345, which was the available signal generator in the QIL lab. Attachment 1 is the estimate of the Moku phase noise, with attachement 2 containing the data. Attachment 3 shows the data this was obtained from, and attachment 4 the experimental setup used to measure this. 15.625 kHz is the maximum frequency that the Moku will let you acquire two channels of data, in the phasemeter setup. In this setup I generated a 20 MHz signal on the DS345. This was then split, via a T piece, and sent through cables of length 0.6414 m to inputs 1 and 2 of the Moku. Concurrently the 10 MHz synchronisation signal was sent, also through a 0.6414 m cable, with an additional elbow piece, from the DS345 output to the Moku 10 MHz input. The phase difference between input 1 and input 2, shown in yellow orange in attachment 3, should be sqrt(2) times the phase noise of the Moku.
Attachment 5 shows the same setup, but with the spectrum straight from the Moku, which can be acquired at a maximum speed of 488 Hz. They are relatively consistent in the overlapping frequencies. Attachment 6 shows what happens when the 10 MHz synchronisation is removed. The low frequency performance of individual channels suffers but there is not a substantial change in the difference, yellow orange.
Attachment 8 shows the phase difference performance at 10 MHz, with attachment 7 showing the setup for this measurment. The cable length for this measurment was 0.9993 m for all paths. Again the difference, yellow orange, is comparable to other measurements.
We adjusted the cable lengths going into IN-LOOP mixer in the fiber experiment so that the phase difference between the 80MHz signal of both cables was minimized (probably now < 5 degrees). This was done by looking at a long term average of these signals on the network analyzer. I suppose we could also look at the DC value coming from IF port of the mixer as well when the loop is closed.
Anyway, this allowed us to increase the gain by 500x on the error signal and we saw a huge stabilization in the noise on the out-of-loop PD.
Isn't it nice when things just work ...
(No spectra right now ... no DTT )
Johannes and I took the phospor coated CCD camera (from the cryo lab) down to the ATF to see the response to 2004nm. We tested it was working by shining an incandescent light onto it and confirming that we could see a signal on the monitor. Then we took the output of the 2004nm laser diode and put about 600 microWatts directly onto the CCD camera and ...
We saw no response at all. Nothing - on any of the gain settings.
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
Averaging Mode: RMS
Averaging Type: Exp. / Cont.
Overload reject: On
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.
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.
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.
The PDA10CS from Thorlabs appears to have a peculiarity, or I am just confused. The Spec Sheet says that it should have a 10V max output for a Hi Z load. This appears to not be true for its 0 dB gain setting. It rails at 5.44V with zero dB of gain, and 10.something V at higher gain settings. This caused some modicum of confusion.
It was plugged into a Tektronix 1001B which is high Z (I measured to double check - 1MOhm).
Is it fine to use ND filters with my photodiodes? It seems fine, but I don't know if there is somet hing I am missing there.
I just ordered a 1811 photodiode from Newport for temporary transmission readout duty. I've ordered the one with the AC coupled output. It's in stock so should be with us soon.
Quick update, more detailed update to follow.
Still to do:
Embellished Chris's PD MEDM screen a bit to illustrate controls in a diagram. The representation of the RELAY SWITCH between the Keithley and the SR560 is a bit off - I think the transimpedance amplifier is switched out as well.
Also - Keithley bright PD sweep output is attached.
I am measuring the power of my PMC transmission with a Thorlabs PDA10CS. I have one after a 50:50 Beamsplitter on the output.
I measured 603 mV through a 2.0 ND filter on a 0dB gain setting with an oscilloscope (1MOhm load). This should be half my transmitted power.
I calculate: 0.603 V *(1A/1.5*10^3V)*(1W/.7A)*100 (for ND filter) = 58 mW...
I measure again with a 10 dB setting for sanity:
1.94 V * (1/4.75*10^3)/\*(1/.7)*100 = 58 mW....
BUT when I put a power meter in the same part of the beampath, I get a reading of 30 mW. My input power is 90 mW...I think I am getting at least 2/3 transmission, but am confused by my photodiode reading. It seems to imply I am getting more power out than I put in. (hmm.)
I've taken some pictures and tried to label the beam paths. They're not great, but hopefully they're understandable.
Green is the unsplit laser beam, Red is the CCW beam, Blue is CW (on the AOM side) beam, and Purple is both beams together.
The first three show the incoming and reflected beams. Arrows indicate a reflected beam. The last picture shows the beams in transmission onto the PD.
*Edited to correct first picture
Nice shots. I noticed that on one picture you indicate a "curved input mirror". Was the input mirror really changed to a curved one? If so, why? My (perhaps-flawed) intuition tells me that it will be more difficult to get an exact modematch to the weird phasefront on that mirror.
Sorry, the input/output mirrors are flat, and it's the other two mirrors that are curved with a ROC of 9m. I'll try to fix it on the picture tomorrow.
It's fixed now, in the original entry.
Pictures galore of my Mach Zehnder doubling noise setup, with and without the box.
Images, in order:
Here is the first version of a design for a mounting block for the pin vise. The concept is that the pin vise will be screwed into this aluminum block and the block will be screwed into the optical breadboard (i.e. inertial mass). We want to have the flexure point of the wire be located (ideally) at the center of mass of the system, so I computed where the center of mass would be using the estimation that both the breadboard and mounting block are solid Aluminum (density = 2.7 g/cm^3).
For the dimensions as follows:
optical breadboard = 12" x 10" x 0.5" (2655 grams)
mounting block = 2" x 2" x 1.75" (310 grams)
the center of mass would be:
below the center of mass (also geometric center) of the breadboard. I haven't yet calculated the flexure length for the wire, but the consensus between Alastair and Koji is that it won't be more than a couple mm. This means the tip of the pin vise which grasps the wire should be yet another few mm lower, or approximately co-located with the bottom surface of the breadboard. However, since the flexure point will need to be slightly above the center of mass for stability and because we will load the breadboard with some weights (i.e. mirrors for optical readout of the mass's motion and balancing weights) which will raise the center of mass, I decided to make the tip of the pin vise somewhere in the middle of the lower half of the breadboard.
I brought this design to the machine shop in the sub-basement of Lauritsen, but the guy I talked with there said the 1 mm thick wall of the pin vise handle is too thin to die (add threads to) with any of the machines they have. We discussed an alternate solution of tapping two holes into the side of the block and just grasping the pin vise with the force of screws. He also suggested filling the pin vise handle with aluminum to make it stronger.
Working with Kevin on LabJack U3-HV to get statistics about ADC and DAC noise/inaccuracies for Rana. For preliminary testing, created an analog input into the ADC that broadcasts a sine wave (generated by a computer or anything else that can generate a sine wave). The U3 can receive commands and send responses at around 150 Hz. However, there is a separate module in Python that allows you to open a stream from the device with will stream packages from the ADC anywhere up to 20KHz. Above 20KHz packets begin to drop. These packages are then interpreted and used to calculate an Amplitude Spectrum using Welch’s method. This information can be used to calculate peak-to-peak noise and RMS noise. The following information was taken with an audio output from a laptop. It serves as preliminary data as a proof of concept that the scripts are working the way they should. Attachments 1 and 2 correspond to the Amplitude Spectrum and Time Series of sine waves of 440 Hz and 20 Hz. Of course, we see a peak in Attachment 1 at 440 Hz and at 20 Hz and subsequent harmonics in Attachment 2.
With this method written, I will now use a Phase-locked loop (PLL) to measure the Rayleigh statistic over a long time period (days to weeks) of two DAC conversions to observe any variation in two different phases. This will test the accuracy of the LabJack over long time spans and ensure that there is no variation in long-term usage between different DACs on the device. This begins with digitally inputting two sine waves of the same frequency through the devices digital inputs. The method for this is described in the next paragraph. They are then converted using the DAC into an analog signal, which we can analyze with some of the previously built methods. I will use a phase detector, which generates a voltage signal which represents the difference between our two analog (or two digital) signals in a PLL application. Using this data, we can then use the Rayleigh statistic, which measures the continuous probability distribution for random variables. In the application of our data, it can be used as a test for non-uniformities in a set of periodic points. This method is available through the stats module of SciPy. Using Rayleigh statistic data, I will make plots of a spectrogram and a whitened spectrogram, which normalizes with the median in each frequency bin. I can use methods available on GWpy (from LIGO algorithm library) with some minimal changes to build the spectrograms.
I will use two Raspberry Pi’s - one to generate the signal and another to run scripts for the PLL, Rayleigh statistic, and spectrograms. The signal generator will have a USB-output to a DB15 cable, which will connect to the bottom of the LabJack. From there, I can generate signals on pins 1 and 2 of the DB15 connector which will connect to the dedicated Digital I/O ports CIO1 and CIO2 on the LabJack, respectively. Next, I will use the onboard DAC to convert the signals to analog and feed them through I/O ports FIO4 and FIO5 which will then be connected back into AIO0 and AIO1, respectively, using wires. This means that we can receive the analog feed from AIO0 and AIO1 and pass them through the same scripts that were previously used to measure analog signals onto the interpreter Raspberry Pi to run the methods specified above. Attachment 3 is a diagram of these connections.
[Stephen, Clare, Hiya, Radhika]
We started a cooldown on Thursday with an undoped Si wafer in Megastat . The chamber does not have the Maglite flashlight installed, due to struggles with soldering the ground wire to the aluminum surface of the flashlight. We decided to pivot to drilling a hole in the aluminum, wrapping the ground wire through and around, and then applying solder to the joint. This will be completed by next week.
These tasks will ideally lead up to a Si wafer cooldown next week with optimal heat excitation and with an RTD re-installed on the cold head, and potentially with the finalized baseplate and aquadag-coated Al sheet on the cold plate. It is with this cooldown that we can start to estimate the sample’s emissivity, first with least-squares fitting and eventually with MCMC analysis.
Below are the outlined steps towards building an MCMC model for estimating the emissivity of a surface inside Megastat, and determining achievable uncertainty bounds:
I've used the following model of heat transfer between a suspended Si sample (1) and the inner shield (2) in Megastat:
, where I have not assumed that A1/A2 << 1.
For this analysis, I simulated temperature data of a sample using models of e1 and e2. I simulated e1 and e2 as linear in T:
, and generated test mass temperature data using these emissivity models and inner shield temperature data from a previous cooldown. My goal was to determine how uncertainty in the emissivity of the inner shield and heat capacity of silicon would propagate to the calculated emissivity of the sample.
I back-calculated the emissivity of the sample using a procedure similar to this paper: https://www.sciencedirect.com/science/article/pii/S0017931019361289?via=ihub. To summarize, I used a Savitzky-Golay (SG) filter in scipy to calculate dTdt from the temperature of the sample, and rearranged the model above to solve for e1.
The uncertainty of e1 can be found by:
I considered Cp_Si and e2 as uncertain parameters of interest, and assumed we do not have significant uncertainty on the geometric parameters such as the areas of the inner shield or the sample, or mass of sample.
The results from this analysis are:
Plots of these uncertainties can be found in Attachments 1 and 2.
Next steps are to add an additional radiative heating term to the model (more realistic given what we see in MS) and repeat this analysis, adding uncertain parameters such as size of heat leak. Transfering this analysis to MCMC is also in progress.
I've now used the following model of heat transfer between a suspended Si sample (1), inner shield (2), and aperture heat leak (3) in Megastat:
where the first term is the heat transfer from the inner shield to the sample, and the second term is the heat transfer from an aperture opening to the sample. Note that the second term contains the geometric view factor Fleak, which is dependent on the radii of and separation between the 2 surfaces. (This parameter is also implicit in the first term, but because we approximate the inner shield as nearly completely surrounding the sample, we set this to 1.)
Once again I simulated typical/expected values for the emissivities of silicon and rough aluminum:
, for the sample,
, for the inner shield and outer shield (source of heat leaks).
I further parametrized both Aleak and Fleak in terms of rleak, in the assumption of circular aperture openings. This reduced the number of parameters to consider to 4 (e1, e2, e3, rleak).
As before, I simulated T1 using these emissivities, previous inner shield T data, and the above model. I numerically evaluated dT1/dt from the simulated T1 data.
The resulting evaluations for e1 using the dimensions of a large, cylindrical test mass (like the one we've been cooling in Megastat) can be found in Attachment 1. The uncertainty bounds from e2 and rleak were found by evaluating partial derivatives of e1 with respect to those parameters. (The uncertainty bound from e3 was not even resolvable on the plot, so I excluded it as it is a negligible source of e1 error.) This process is mathematically equivalent to evaluating the 2x2 Fisher matrices formed by e1 and e2, e1 and e3, and e1 and rleak.
The resulting uncertainties can be summarized by:
where I have used reasonable guesses for our uncertainties in these parameters. This tells us that in the high mass/area sample case, the uncertainty in the emissivity of the inner shield is our largest source of potential error in e1 determination, even with heat leaks into the system. This makes sense qualitatively, since the first term in the model dominates and view factor from heat leak apertures is small.
Attachment 2 shows the same analysis but using dimensions of a thin, 2" wafer. The resulting uncertainties can be summarized by:
This tells us that in the low mass/area case, the uncertainty of the size of the heat leak is the largest contributor to error in e1 evaluation. Now, the first term in the model no longer dominates, and heat leaks play a non-negligible roll in the cooldown of a wafer.
I next verified these results by simulating my own form of MC, aka defining a prior distribution on e2 and rleak and observing the posterior on e1. I specified the prior on e2 as a 2-dimensional gaussian (m vs. b) with standard deviations of 1e-4 and 1e-2, respectively. I specified the prior on rleak also as a gaussian with standard deviation 1e-2. The results for both the high mass/area case and low mass/area case can be found in Attachments 3 and 4. These serve to verify the previous error analysis and create groundwork for MCMC analysis.
To design our experimental setup with this analysis in mind, I propose contacting the company/manufacturer of the shields and requesting an inner shield with only 1 or 2 apertures. (1 for the copper linkage to pass through, and maybe 1 for the RTD feed through if they can't be passed in through the first aperture). This can be the inner shield used when we have no use for the optical viewports, AKA for emissivity testing. Since minimizing uncertainty in the emissivity of the inner shield is in our best interest especially in the high mass case, this might also be a good opportunity to specify the roughness/polishing level of the inner shield that we have the best model for. (We could still paint the inner surface of the inner shield with Aquadag, but my only concern with this is not fully being able to quantify Aquadag emissivity vs. temperature.)
I took my earlier measurements of the phase noise of the marconi and computed what it would produce as the phase noise of the plant output of the fiber stabilization setup at a site. I used a transfer function of G = 3kHz/f to simulate the gain of the loop. I also integrated to find the total phase noise from DC to 3kHz to be 280.9 mHz. As in my earlier plots, the red line is the measurement noise.
EDIT (11/5/09): I was off by a factor of 1/2pi in my conversion from phase noise to frequency noise; I originally used freq = 1/2pi* phase/f. The frequency noise plot is updated, and the total noise is now 280.9 mHz over the 0-3kHz range
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.
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.
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.
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.
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
Where L is fiber length, 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.
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.
[awade, anchal ]
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
which would place our goodness of alignment to with 0.61 deg.
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.
For measuring the polarization, the setup as shown in Fig. 1 was prepared.
The angle of the HWP #2 was 30 degree.
Rotating the angle of the HWP #3, I measured the laser power with a power meter and a PD.
And I fitted the measured data to the function, .
Here \theta is the angle of the HWP #3.
The result was shown in Fig. 2 and the paremeters were determined as
(with the power meter) a = 7.965 +/- 0.0005 mW, b = -0.002 +/- 0.003 mW, phi = -40.65 +/- 0.01,
(with the PD) a = 1373 +/- 3, b = -1 +/ 2, phi = 40.52 +/- 0.03.
Accoding to this result, the S-pol. and the P-pol are obtained at 40.6 degree and 85.6 degree of the angle of the HWP #2, respectively.
And the calbration constant of the PD from voltage to power is determined roughly as 5.8*10^(-3) W/V. (Systematic errors have not yet been concerned.)
% Polarizer calibration / Rana's lab
% Tobin Fricke 2007-10-26
% Experimental setup:
% +-------+ |
% | Laser |-------|lambda/2|----|PBS|----[Power Meter]
One possible vacuum chamber solution for the Gyro is to use long tubes for the Gyro arms and then to have a small chamber with ports at each corner.
I looked a little at using stock MDC vacuum parts for this; its not out of the question.
For the tubes, we could use something like their NW50 Kwik-Flange nipple. It has an OD of 2" and a length of 6.5". Its $63.
For the corners, we can use one of their '5-way crosses' like the 406002. Its basically 5 flanges welded onto a shell. Depending on the size its ~$250 ea.
I would prefer to get one long tube for each arm, rather than stick a bunch of short ones together, so I'll get a quote from MDC on a custom job.
Uncoated quartz viewports are ~$250 ea. I expect that we will want AR coated and angled viewports. Maybe $400 ea then.
So the total cost, without pumps would be ~5k$.
We're not looking for super high-vacuum though are we? Maybe we can get away with borrowing the small turbo pumping station from the suspensions lab to pump it down and then we can just valve it off.
Also, we have the cylinder head for a tank of helium now so we should order in a tank to try that (the tanks get delivered really fast so that shouldn't be a problem). Of course we'll at least need windows before doing that.
I've asked Gina to check on the CVI W2 window order. The order went in on 22nd July and CVI said that they had them in stock.
I think that's right - we won't need any better than 1 mTorr. As long as there is no huge leak, we should be fine. It would be handy to have a (EPICS trendable) gauge on the system so that we can know if its leaking.
The system's total volume will be ~20 liters. So we need the leak rate to be below ~1e-6 Torr-liters / hour. A suggestion from Mike Z is to use the usual flexi-hosing from Norcal instead of the rigid nipple type of tube
I was talking about before. flexi hose link ..... I think we can use the 2" ID, 24" long tubes and make up for the difference in the length in the corner chambers. The length of each side of the gyro should be 29.5" with a 100 MHz FSR.
Power budget of the PMC has been considerd.
AR Reflectivity (rAR2): 7.50% (<- HUGE)
Round trip mirror (3 loss) : 0.499% (<- HUGE)
Transmission of the flat mirrors (t12): 1.65% (<-OK)
Transmission of the curved mirrors (t22): 302ppm (<-OK)
Modematching (1-Rjunk):: 86.5% (<-hmm, OK for now)
Raw cavity transmission (t1 gcav): 86.2%
I wonder how the loss of 5000ppm comes from.
Frank suggested that there may be the spot not at the center of the curved mirror.
I checked the spot position at the end and it is actually very close to the edge of the hole. (See photo)
Of course the beam has the angle because of the short triangular cavity. So it is difficult to say this is still ok or not.
The AR seems to have huge reflection, but this is real.
- Two flat mirrors are identical
- Losses are distributed on the three mirrors equally.
- AR surface has no loss.
- AR of the end transmission is ignored as the beam will not be separated on the PD
Those conditions give us the five undetermined variables: rAR, t1, t2, loss, Rjunk
where they are amplitude reflectivity of AR, that of the flat mirrors, that of the curved mirrors, loss in power, power ratio of the unmatched light in the incident beam, respectively.
We have 5 independent measurement after the normalization of the power measurements by the incident power. So there is a unique solution.
After a bit painful solutions serach, the numbers below have been obtained.
rAR -> 0.2739, t1 -> 0.1283, t2 -> 0.01737, Rjunk -> 0.1347, loss -> 0.00166271
Converting this result into useful numbers, we obtain the following quantities:
AR Reflectivity (rAR2): 7.50%
Round trip mirror (3 loss) : 0.499%
Transmission of the flat mirrors (t12): 1.65%
Transmission of the curved mirrors (t22): 302ppm
Modematching (1-Rjunk): 86.5%
Raw cavity transmission (t1 gcav): 86.2%
==> Finesse: 163 +/- 6
Cavity incident: 168 +/- 1mW
Forward Transmission: 92.5 +/- 0.2 mW
End Transmission: 1.80 +/- 0.02 mW
Reflection: 21.7 +/- 0.2 mW
AR reflection: 12.6 +/- 0.1 mW
==> Transmission: 60%, Loss: 25%
One of the factors we're taking into account when figuring out the optimal fiber cable length to use in the 2um laser characterization project is the power loss present as a function of such length. Andrew and I worked through some figures and came up with the following plots, sampling a few values of the attenuation coefficient alpha. The process was relatively straightfoward, we introduced some loss, , into a signal. Thus, at one of the outputs of the MZ, the signal we receive would be:
Next, since we ideally want our signal to be locked at mid-fringe, we take the derivative of the function with respect to frequency and observe the maxima.
In order to best visualize the points at which the slope is of highest sensitivity, we take the derivative once more and observe the zero points.
Through ThorLabs data on the SM2000 fiber optic cable ( https://www.thorlabs.com/drawings/d7a7404567d69154-FBD8C6B1-0D0E-7F1A-14D2F3A96ED2FF2E/SM2000-SpecSheet.pdf ), we came to a good approximation that our attenuation coefficient is approximately 8.63*10^-3 dB/m. The orange line in the above graph is a close approximation to this value, but the sensitivity slope for the approximation we obtained is shown in the following graph:
When considering power loss in the fiber optic cable, the optimal fiber cable length is roughly 116.8 meters. If we are willing to sacrifice roughly 10% of the calculated sensitivity*, then we can drop the cable length to approximately 72 meters.
*This was done by subtracting 10% of the maximum value of the derivative of the output power w.r.t frequency (using the actual attenuation coefficient from ThorLabs). Maximum was 8.776*10^-8 W/Hz , 90% of max = 7.893*10^-8, which falls around 72 meters.
**First elog, critiques are very much welcome!
In order to pick a length, we'll have to go beyond this optimization and consider cost and acoustic sensitivity.
Also, we have to start by making a guess at the frequency noise PSD of this laser, and also what sensitivity we want the MZ to have, as well as the PD electronics noise.
Please upload a noise budget plot in units of Hz/rHz showing a bunch of these noises as well as the frequency noise sensing requirement. Every 2 meters of fibers means 1 less pizza, so we'd like to take the length down from 100 meters to roughly 10 meters.
In our mission to characterize our 2micron laser, I calculated the changes of power at different points within the experiment- the points are shown in the schematic below. I kept the input current constant at 50.02 mA, and the temperature of the laser diode at 8.657k.
The excess power loss, , at either beam splitter can be expressed in dB as:
Running this through gives us an excess loss of 0.351dB at Beam Splitter #1 and 0.327dB at Beam Splitter #2.
We're finishing up the thermal sensor to be placed in the ATF! Schematics and pictures will be provided later on today.
We characterized the power output vs. drive current of the 495 mW NPRO laser for the gyro experiment. The current supply goes up to 1.00 A. Here is our data:
C = [0.46 0.52 0.60 0.66 0.74 0.80 0.88 0.94 1.00];
Pd = [2 18 38 55 71 93 110 125 146];
Pu = [2 20 39 56 72 94 110 126 147];
C is drive current, Pd is lower limit on power output in mW, and Pu is upper limit.
We graphed the results and fit a line to it. The slope is 261.5 mW/A, and the intercept is -118.2 mW. The graph is attached.
directory is: \users\cmooney\Power495mW.m
Rana suggested that the DC power steps were due to a bistability of the NPRO inside the MOPA.
To try and diagnose this, I turned off the diodes...I could do nothing with the mode coming out of the laser, the thermal lensing of the MOPA is needed. There is a diagnostic PD sitting inside the PSL, undpowered. It has a 3 pin power connector on the outside of the box saying "+/- 24V DC", but I am not plugging anything into it until I know what to put where...
For now, there are still glitches coming out of the PMC, presumably from the PSL. I will toss a diode upstream of the PMC to confirm this. There are DC steps, and 2-5% spikes in the data, which make it unusable. Running the MZ overnight, I was able to use a stretch of data with 1 arm blocked, and the HEPA on.
Below is what the current AC coupling looks like, SUM is the recombined signals. AC and DC are self explanatory. alpha and beta are 532, gamma and delta are 1064.
I have added the following two channels:
C2:ATF-PSL_OUT Pickoff of the PSL output power (upstream of the PMC)
C2:ATF-PMC_TRANS PMC Transmission - this is my in loop ISS PD
State as of p[ost - HEPA off lights off one arm blocked
The DC power step was seen upstream of the PMC in the new PSL output monitor
Used DMass's bidirectional coupler setup and measured the reflected power from the lab AOMs for a sweep between 30 and 110MHz. Also tested a 50Ohm terminator and an unterminated setup to calibrate the equipment.
Results shown in the attached plot.
We have managed to pre stabilise the laser using a few SR560's. It is not as stable as the one that would be implemented once the digital system is in place. But it should be good enough for some preliminary data. As described in a previous log we had locked the cavity to track the laser. The PZT actuator in the cavity is driven by a SR560 which has a limited output voltage range from (-4V to +4V) and if due to slow frequency drifts the frequency of the laser drifts beyond the limit to which the PZT can compensate the cavity would unlock itself. Also, currently the intensity noise of laser has not been stabilised, this passes directly through the cavity and will appear at the transmission if its not accounted for. This feedback has been achieved using a AOM. The AOM is driven by a RF function generator and the output power of thr RF function generator can be modulated by this in turn changes the power in the carrier frequency by pushing some power into the first order diffracted beam, thereby stabilising the laser intensity fluctuation by almost 2 orders of magnitude. A brief description of the feed-back loops is given below.
The laser was frequency stabilised for its slow drifts, this could lead to SR650 controlling the cavity not being able to compesnsate for it due to its limited output range. The output of cavity stabilisation was low passed and then fed to the frequency control of the laser. But the gain had to be adjusted appropriately, as otherwise the loop could become unstable. This was done by using a simple resistive divider circuit (potentiometer) to first attenuate the control signal for the cavity stabilisation and then low passed and fed back to counter for the slow frequency drifts of the laser.The cutoff freqeucncy of the secondary feedback loop is also important, so as to ensure that at cross-over frequency nyquist stability condition is satisfied. The cutoff for this was kept at 30mHz.
Intensity fluctuations from the laser will appear at the output port of the PMC because of the fact that the cavity is locking itself to laser's frequency. These have to be corrected for in the final setup and this has been achieved using an AOM. A AOM splits power in the beam into a diffracted beam and this splitting of power depends on the power that is injected into the AOM and also the direction in whihc the AOM is oriented. When no power is injected in the AOM the carrier beam passes through unaffected and when some non-zero power is injected a part of power goes into another diffracted beam. The AOM we use has a maximum power input of 2W at 80MHz. For achieving the required functionality, a RF signal from a RF signal generator is amplified using a high power RF amplifier and this drives the AOM. Now to stabilise the intensity fluctuations going into the PMC we can setup a positive feedback by sensing the power at thePMC's output and using that to modulate the signal generated by RF signal generator thereby modulating the power with which the AOM is driven and finally controlling the way power is split between the two output rays. Hence this way the power entering the PMC has been stabilised. In the actual setup this feedback will be provided by the common mode output of the two photodiodes being tested.The entire pre-stabilisation of laser implemented is shown in the schematic below.
The total stabilisation schematic is shown below-this includes the locking of the cavity, feedback to supress the laser frequency drift and supression of intensity fluctuations.
We were also able to get some more optics fixed, since the table will also be used for another experiment, we divided the beam using a halfwave plate and a polarising beam splitter to control the power going into each of the experiments. A image of the setup assembled is attached below, the read trace is the path of the laser.
I have begun designing a precision temperature controller for use in (at least) the following three applications:
The idea is to have one controller suitably adaptable to each of the three situations. In effect, the only major difference will be the type or specifications of the resistive heaters used in each case, so it is logical to make one design. Here is a basic functional diagram (the actual schematic can be found at the bottom of the post):
A quick walkthrough:
You can find some more details in the schematic below.
Please reply with your questions and/or comments on the design. I am happy to take them (for now), because this is something we may all need at one point or another. I'm talking to you, 40m'ers(!).
What is the precision of the temperature controller? May be it is possible to create a feed-forward correction in temperature caused devices, such as knife-edge tiltmeter.
Here are the Arbcav-predicted cavity transmission and HOM spectra for the new cavity and current modulation frequency.
In case you haven't used this tool yet, note that this was all produced with one line:
[finesse, coefs, df] = arbcav([200e-6 20e-6 200e-6 20e-6],[0.75 0.75 0.75 0.75],[1e10,9,1e10,9],[45 45 45 45],29.189e6,20e-6,1064e-9,1000);