This set of measurments turned out to be bad. The OP27 (±15V supplies) with 50 Ω series at the output + 50 Ω load impedance seemed to be saturated.
The rest of this post is a stream of conciseness.
As noted in previous post (ATF:2316) the PD unit B in the homodyne had a slightly reduced HF response, possibly due to the use of a foil cap in the PD bias supply low pass. I removed this and resmeasured.
Unit B TF: measured NF1611 DC voltage of 312 mV (@ 50Ω impedance), PD DC output voltage was - 566 mV (@ 50 Ω 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 xxx. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible). This was a bad measurment, something was up with the circuit saturating way too low.
Changed 100 Ω series on output to 50 Ω to make it more strait forward to understand how it works with 50 Ω load.
Unit A TF: measured NF1611 DC voltage of 658 mV (@ 50Ω impedance), PD DC output voltage was 1.42 mV (@ 50 Ω impedance). Here the photodetector has transresistance of 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 0.3151. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible). This measurment could have be saturating the current output of the OP27, likely that numbers here are not right.
After examining the output of detector B it became apparent that the output was saturated for powers of more than ~20 mA laser diode drive.
Unit B TF: measured NF1611 DC voltage of 312 mV (@ 50Ω impedance), PD DC output voltage was -566 mV (@ 50 Ω 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 -0.3748. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible). 100 Ω (50Ω + 50 Ω) loading is too low, OP27 open loop gain and voltage swing range slew starts to sag below 150Ω loading. These results are bogus.
This time I measured with 50 Ω on the oscilloscope for apples-to-apples comparison with what the Aglient 4395A was seeing. Turns out I think I was saturating the photodetector current wise in the first round of measurments. All of the above is probably not right and should be disregarded. Also, later I switched to a AD829 as the op amp of choice so the above is not much use for characterizing the detector performance anymore. The OP27 isn't really spec'ed to operate with loads this low and only has its full band with for output loads of >2 kΩ.
This post details the rebuild I made of the transimpedance amplifiers (TIA) for the homodyne detectors. My previous iteration of this design failed to take into account the output swing limitations and current draw limitations at the output of the op amp. It turns out that the OP27 was not the best choice here. On the face of it its GBP and input referred noise were fine for a ~ 1 MHz detection. However, operating the chip with 5V output voltage (to get 10 dB shot noise clearance from thermal noise) and with output impedance on order of 150 Ω meant that the frequency response was not as expected (see ATF:2316).
A better choice is something like AD829 which is designed for driving signals directly into 50Ω terminated outputs. The updated design is illustrated below.
Here the transimpedance gain has been lowered to 2 kΩ to lower the relative input referred noise contributed by the AD829's slightly higher current noise. Here the optical power can be increased to give the same effective output signal. In this case increase optical power and lowering gain keeps the thermal noise clearance below shot noise constant but improves the clearance of the AD829 input current noise.
For reference the thermal noise of the TIA is given by the Johnson noise
where k_B is the Boltzmann's constant, T is the temperature (300K) and Rfb is the feedback resistor value. Also here the shot noise is
Where e is the charge on an electron (1.602e-19 C), R is the photodiode responsivity (~0.8 A/W), and P_PD is the power on the photodetectors (0.5-5 mW). Keeping in mind that there is a maximum voltage swing (V_max) on the output of the op amp, this comes from its current driving capability into a given load, then the feedback resistance (TIA gain) is limited to
The noise clearance, given a maximum output voltage, is then given by the ratio
Power on the photodiode and responsivity cancel out for the noise clearance. Assuming we are not optical power limited then the thermal noise ratio (clearance below shot noise) is wholly determined by V_Max and the physical constants e, k_B and T. For room temperature (300 K) this reduces to the rule of thumb
Keeping the V_max to within 3 V means that we will get a thermal noise clearance factor of 7.6 below shot noise (8.8 dB). V_max of 5.2 volts will give 10 dB clearance. Ok I guess.
Koji suggested the AD829 as a replacement to OP27. AD829 has applications in driving 50 Ω/75 Ω loads in video applications and has some nice noise characteristics. The bottom line is that it has 1.7 nV/√Hz input voltage noise, 1.5 pA/√Hz input current noise, can do a ±3 V voltage swing into 150 Ω load (DC coupled), and is fast (600 MHz uncompensated, with 230 V/µs slew). There are some quirks. AD829 seems have have a weird 80 MHz feature that causes oscillations if not compensated properly. I did a bit of modeling in LISO and then just decided to build it once I found that ~2 kΩ gain was about right for ensuring that the dark noise wasn't dominated by the op amp current noise.
Because I was building on proto-board I wanted to ensure that there was as little parasitic capacitance as possible. I built the whole amplifier + PD onto a single SOIC-14 to 0.1 pitch adaptor (Adafruit SMT Breakout PCB for SOIC-14, Part No. 1210), pictured below. The small footprint and short trace lengths between critical pins means that there is a lower chance of parasitic impedance causing instability or impact on bandwidth.
The feedback resistor (2 kΩ) and feedback compensating capacitor (15 pF ceramic + 3 pF nominal pin parasitic) were soldered directly to the reverse side where the TSSOP-14 pins were connected through vias to the top side. This minimized the path length of this electrical signal. I scratched off pads that were unused in case they shorted or were a source of crosstalk. The photodiode mount pin was soldered directly to the edge of the board to minimize distance. The power bypassing capacitors were SMT 100 nF ceramics (5%) that were soldered directly to the SMT breakout with the shortest path to ground on that board (see pictured). The other photodiode pin was soldered to +5 V (-5V) supply pin in unit A (unit B). The opposite biasing makes subtracting the signals using a summing circuit easier but may affect the overall TF response. There is no power regulation, I'm planning to use batteries to power these circuits.
In my initial tests I had no compensating capacitor attached to pin 5. I inserted a 40 pF ceramic capacitor in place of the photodiode and looked at the AD829 output pin with a high impedance probe on an oscilloscope. I immediately saw 80 MHz oscillations. I don't really need a super high bandwidth so I went strait to the maximum recommended choice for pin5 compensating capacitor of 68 pF. This makes AD829 unity gain stable with 66 MHz bandwidth (slew 16V/µs) but is more than enough for my needs. This killed the high frequency RF ringing junk. Maybe less capacitance here would have worked, but this performance is enough for my needs and gives some certainty about the op amps stability (ignoring input capacitance compensation).
At the output I added 100 Ω of series resistance to limit the loading on the op amp and current draw when 50 Ω terminated. With 50 Ω terminating impedance this makes a 150 Ω to ground. Providing the output DC swing is kept within 3V the op amp should behave as expected.
The whole thing fits together very nicely on a single SMT breakout board. Making it compact will hopefully avoid any issues with stray capacitance messing up the performance. I have then mounted this on a larger proto board for ease of installing in the experiment. Paths to the output SMA output are thin wire and not routed through the underlying board. Only power and ground is routed through the board, all other pins to the op amp disconnected.
I measured the signal transfer function of the above TIA units using the Jenne rig at the 40 m. It took me a number of tries: I made the mistake of loading the DC port of the NF 1611 detector with 50 ohms (which resulted in too much current draw and affected the LF response of the witness detector); there were also some issues with the polarization/positioning onto the beam splitter there that made it not 50:50 (it was 69:33 ), I moved the fiber launch a bit to get the splitting right and retook measurements after that; also I initially had the current set too high on the laser current driver which I think was saturating its response. I'll skip over the details and just present the final measurement. I've included a zip in the attachments with the data that logs some more of the details of my failed attempts.
I used the same calibration as used in PSL:2247. Here the laser diode current was set to 25.0 mA, which is 1.51 mW of 1064 nm, and the AC excitation into the laser was set to -21 dBm (19.9 mV rms). Power on the witness detector was measured to be 0.79 mW and that on the PD under test was 0.75 mW (as measured with a power meter). Units of measurement were linear magnitude and phase in degrees. DC voltages were measured with 1 MΩ impedance.
Unit A: using 25 mA current into laser and -21 dBm sweep I get DC on PD of 1.20 V (into 1 MΩ) and a DC voltage of NF1611 detector of -2.13 V (into 1 MΩ).
Unit B: using 25 mA current into laser and -21 dBm sweep I get DC on PD of 1.07 V (into 1 MΩ) and a DC voltage of NF1611 detector of -2.12 V (into 1 MΩ).
The calibrated TIA current to voltage gain is plotted below:
A notebook detailing all the attempts is attached below.
Andrew measured the output voltage noise of photodiodes through an AC coupled pre-amplifier (G=200, 0.7 nV/rtHz) while varying the intensity of white light falling on it. The setup uses a regular 2xAA flashlight (torch) with an incandescent bulb to illuminate the photodiode. The flashlight focus is adjusted to give a non-expanding beam and a 25.4 mm (1") lens is then used to focus the collimated beam down. The total power of light was controlled by trimming it with an iris.
Below is the fitting of data to estimate the shotnoise intercept current. Here the fitted function is the quadrature sum of input referred shot noise and dark noise modeled as an equivalent shot noise quanitiy i_snint (called intercept current):
I have included zip file with data and notebook in it as well. With measured shotnoise intercept current of 48 ± 7 uA and 31 ± 7 uA, we are good to go to use these for measuring shot noise with light power in the order of 5.0 mW falling on the photodiodes. With 5.0 mW (~3.9 mA) on each PD, this gives us a clearance of 10dB from the dark noise floor on average for each detector. Plots with fits are attached below.
As I couldn't find this on the 40m elog/wiki I'm putting notes here on how to configure a fresh webserver + python server for the Zurich box (model HF2LI) that runs an autobooting service job.
The Zurich box (model HF2LI) has no front panel interface. All controls are done from a USB interface to a host computer (running windows or Ubuntu/Debian). This computer then hosts a web-server that runs an interface in a browser (through http) or responds to requests from python/matlab/labview/C API over the network.
Set up on a Linux box is fast and easy from a fresh install. A thin low powered computer is ideal. Start with a fresh install of Debian, use the latest stable release. Then download the latest stable release for the HF2LI, HF2IS, HF2PLL instruments from the Zurich downloads page. You’ll probably want the 64bit linux. You can untar this file with
> tar xzvf LabOneLinux<arch>-<release>-<revision>.tar.gz
tar xzvf LabOneLinux<arch>-<release>-<revision>.tar.gz
(Insert your filename details above)
Then cd into the directory it created and run
> sudo bash install.sh
It will ask a bunch of questions, just hit enter and y all the way through. The install should immediately boot the HF2 data server at the end. You can check the data server is running with
> ziService status
Now plug in your Zurich box using the USB cable. Note: there are ethernet ports on the box, but these are NOT for network ethernet. If the box is powered up the data server should find the box.
In another terminal you’ll want to to start a Webserver so you can view the box status, settings and live data. Go to another terminal and run
Your Zurich device should now be exposed on port 8006 of that machine. You are read to go.
Now on that computer you can got to http://localhost:8006, alternatively on any other computer on the network you can go to the host machine’s IP address and see your Zurich interface. I.e. if the computer IP is 192.168.1.121 then go to http://192.168.1.128:8006.
If you wanted to access the Zurich interface remotely (outside network) you can do some SSH forwarding if you have a gateway machine to outside the lab. The standard command for this is
> ssh -nNT -L <clientport>:localhost:<hostport> <controls>@<Host-IP-Address>
where controls is the username, Host-IP-Address is the IP address of the machine to be forwarded from, clientport is the port on your machine (the one you are ssh'ing from) and hostport is the port on the remote machine (for Zurich this is 8006). Then when you go to http://localhost:<clientport> and you'll see Zurich interface.
Since the release 2.18 (Nov 2018) the python access tools are now just in Pypi. You can install them with pip:
> pip install --upgrade zhinst
This should now allow you to use python API to access your Zurich box.
You’ll want to set up a service on your Debian box with systemd. Make a unit
> cd /etc/systemd/system
> sudo vim ziServer.service
You must use sudo to be able to save the file here.
Paste the following in, this assumes that your username and its group is called controls on your machine:
Description=Zurich Instruments HF2LI data server
ExecStart=/bin/bash -c "exec ziServer"
Tip: in vim you can type “:set paste” to get exact paste without auto indent etc.
Save the file and exit. You'll also need to create the working director, or change the above to your desired directory
> mkdir /home/controls/services/
Finally, tell systemd to look for new things:
> systemctl daemon-reload
and enable the service,
> systemctl enable ziServer.service
and manually start the service,
> systemctl start myservice.service
It should be running, try to see if this is so:
> ps | grep ziServer
It should run on reboot of the machine and restart every 30 seconds if it crashes for whatever reason. Try rebooting the host machine to check it is still running
Follow the above steps to also auto load the webserver with a unit file named /etc/systemd/system/ziWebServer.service:
Description=Zurich Instruments HF2LI web server
ExecStart=/bin/bash -c "exec ziWebServer"
Now you have a local network ready server for accessing the Zurich box through a web browser.
The detectors have been verified to have a low intercept current, that will give us good clearance of our shot noise to dark noise (at least as measured in the <100 kHz band). The question is if we can make a ~ 1 Mhz measurement of squeezing without extra noise in that readout chain dominating over our expected 1 dB squeezing.
The plan is to use the Zurich box (model HF2LI) to mix down quantum enhancement from 1 MHz in our homodyne detector. This will be well above most of the classical noise of the laser and other acoustic pickup from the environment.
The Zurich box has a input referred noise of 5 nV/rtHz above 10 kHz and operating at at the higher end of its sampling rate. It has a couple of nice features that would make it well suited to the task. We can use the lock in amplifier instrument to digitally mix down signal from 1 MHz and output one or both of the i/q quadratures to its digital output ports. From this instrument we can also set the bandwidth of the
Output noise of Zurich is 25 nV/rtHz, so I'll need some gain factor to boost signal if readout is done from the external ports. Relative gain of the two paths can be changed manually by typing values into boxes, but might be tought to do a direct ballenced subtraction within the box live if the real time drift is large.
Laser diode modulation port on Diabolo has a signal bandwidth of 5 kHz, so can't really be used to generate a reference signal.
The Zurich box can also be accessed via a python API. Its a little bit opaque in some points on how this works as their documentation is a little thin on actual examples. I've included a jupyter notebook and equivalent .py file for future reference. This code is tested and has worked with the Zurich box data server at IP address 10.0.1.23. It shows exactly how settings can be modified and how data can be read back using their poll command.
Notebook includes all instructions for setting the Zurich HF2LI box up from scratch.
For the detection of squeezed light the homodyne detector is now configured for digital subtraction in post processing. The measurement scheme is now to take the output of the two TIA amplifiers (see QIL:2324 and QIL:2327), digitize them directly at 210 MSa/s (5 nV/rtHz input ref noise) with the Zurich box and demodulate both detector streams using its internal FPGA at about 1 MHz. The Zurich box allows for direct sampling of the signal out of the FPGA which can then be downloaded either through the web interface or the python API. The signal chain is illustrated below:
The basic approach is to mix down the shot noise limited light from some higher frequency (somewhere in the range 100kHz - 8 MHz) and then compute the ideal subtraction factor from a short (10 sec) segment of data. The method for optimization is to compute the time domain subtracted signal for a given balancing factor (SF) and compute the goodness of subtraction from the RMS of the signal. An fmin search is done to find the ideal subtraction factor (SF) for shot noise limited light.
It wasn't immediately clear to me what demodulation frequency to choose, the low pass filter and the digitization sampling rate. So I set up a python notebook and did a few sweeps of these parameters to find the best combination of parameters. This book is attached below as 20190514notebook_ZurichDemodeCompairTests.ipynb in a zip. I've pickled data for replotting if needed later.
At present the detectors are illuminated with 4.5 mA worth of light (as measured from DC voltages / TI gain). This should give about 25 nV/rtHz at the output of the photodetector, as measured by the Zurich box. Fixing the demodulation frequency at 500 kHz and the sampling rate at 30 kHz I found empirically that the when the LP filter (4th order) was kept at 10 kHz and below the minimum subtracted signal converged to about 30 nV/rtHz (about what I expected). With the demodulation frequency set much higher than this, the minimum subtracted signal no longer converged to the predicted SN level. Maybe there is some aliasing thing going on here. The outcome of this experiment is that clearly the demodulation LP filter should be set to at least 1/3 of the sampling rate for the time domain rms minimization to work.
I did a similar sweep with the sampling rate while keeping the demodulation frequency fixed at 500 kHz and the demodulation low pass fixed at 10 kHz. The conclusion was basically the same: as long as the demodulator low pass was kept within 1/3 of the sampling frequency then the method of computing the ideal subtraction factor using the total time domain rms as a cost function returned the minimum possible differential noise.
The demodulation frequency choice is a tricker one. Initially I selected 1 MHz but was getting excess noise above what I expected for shot noise. I then looked at a single channel on the Zurich's scope in FFT mode. Data is plotted below
Basically the plot shows that there is a strong peak at about 900 kHz. This is due to the relaxation oscillation of the laser. When the laser's noise eater is turned on this peak is damped with some tradeoff of slightly higher noise at lower frequencies. So when choosing a demodulation frequency it is best to select something well below 1 MHz. I'm not sure if it is best to turn the noise eater off if the demodulation frequency is set well clear of the 900 kHz peak. It seems like there is a noise penalty for the noise eater loop at all other frequencies.
With the noise eater on I then did a sweep of the demodulation frequency while keeping the low pass filter at 10 kHz and the sampling rate at 30 kSa/s. I've plotted the median ASD value in the band between 100 Hz and 1 kHz as an indication of the minimum SN level after digital subtraction verses the demodulation frequency. Plot below. There is a hump at about 1 MHz that should be avoided, and spikes at 87 kHz and just below 300 kHz but anything else in the range of 100 kHz to 700 kHz should be fine.
So for now, based on the above sweeps of parameters, I have selected demodulation frequency of 600 kHz that is low passed at 10 kHz with a 4th order filter and sampled at 30 kSa/s. As an example of optimized subtraction I have plotted the two homodyne channels along with the subtracted differential signal (optimized by minimizing subtracted time domain rms) and the dark noise (using the same subtraction value).
Here the clearance from the dark noise floor doesn't seem so great. The input referred noise of the Zuirch box is 5 nV/rtHz per channel. Also the equivalent output noise from the PDs at this point is 2.5 nV/rtHz. As these are uncorrelated noise processes this would place the noise floor at about sqrt(5^2*2+2.5^2*2) = 7.9 nv/rtHz, pretty close to what we are actually seeing.
One option here is to pre-amplify the PDs before going into the zurich box. A standard mini-circuits amplifier like ZFL-500LN+ has an input referred noise of 1.6 nV/rtHz which would bring the dark noise floor down to something closer to 4.2 nV/rtHz. I'll keep going without any pre-pre-amplification for now and see what I can do with 532 nm pumping. There should be enough clearance to see at least something if it is there.
In order to see squeezing I want to scan the phase of the homodyne relative to the squeezed light and see the variations in noise power as a function of time.
From my initial scans it seems like there is excess noise of a factor of 1.5 above shot noise (see below). This is only present with 532 nm pump injected. It could be anit-squeezing washing out with some phase noise bluring across the sample time of 0.1 seconds or maybe RIN from residual 532 nm present at one of the photo detectors.
After injecting light into the WOPO for some initial tests it was apparent that there was about 1.4 mW of waste 532 nm light exiting the fiber launch on the detector A path. About 16 µW gets through to the detector from the dichroic mirror reflection. A quick measurement of 532 nm power on detector B show that there was about 49 µW coming out of that fiber end with 0.2 µW making to the photodiode. The 50:50 splitting doesn't apply for non-design spec wavelengths of fiber splitter. The InGaAs detectors have a pretty poor responsivity at this wavelength but the pumping light on detector A was enough to create a DC voltage of 1.36 mV. After dividing through by detector gain of 2kΩ this is equivalent to about 0.68 µA of DC power on the detector A. This suggests a responsivity of order 0.014 A/W. There isn't enough light on detector B to create any DC voltage.
The imbalance here, with the 532 nm light, means that there is a mechanism for coupling in 532 nm RIN into the measured output signal. Not sure if the RIN would be all that high at the 600 kHz (that I'm planning to mix down from) but it would be a good idea to remove it anyway. I'll look for a another HR1064/HT532 dicroic mirror to attenuate this 532 nm component a little more.
For the detection of squeezed light the homodyne detector is now configured for digital subtraction in post processing. The measurement scheme is now to take the output of the two TIA amplifiers (see QIL:2324 and QIL:2327), digitize them directly at 210 MSa/s (5 nV/rtHz input ref noise) with the Zurich box and demodulate both detector streams using its internal FPGA at about 1 MHz. The Zurich box allows for direct sampling of the signal out of the FPGA which can then be downloaded either through the web interface or the python API. T
There are a few other possibilities for the excess noise when injecting pump, this is a checklist for me to run through tomorrow:
I had a look at a few of these things. I've found that it doesn't seem to be caused by heating of the beam splitter, while turning 532 nm on an off I see no change in the balancing of the beam splitter. It appears that on remeasuring the data on Tuesday I found that there had been some glitching of the digitized data readout.
Suspicions for now is that there is not as much pumping light making it to the chip as a thought and that bandwidth resolution of the time scan scan was a little too wide. I have shortened the scan, increased the noise bandwidth of the demodulation and will widen the width of the low pass filter on the noise-time scan to increase the time resolution.
I'm also implementing proper subtraction of the signal that uses both the I and Q quadratures so that all the information about the relative phase and amplitude of the digitized signals is properly mixed down by the FPGA inside the Zurich.
Notebook with demo of the subtraction is attached in a zip below.
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.
Yehonathan brought over 532nm/1064nm laser goggles from the 40m.
Our next step would be to measure the LO shot noise.
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.
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.
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.
3. Subtracted noise spectra
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.
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??
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.
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.
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):
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?
Some analysis in this notebook
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.
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.
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.
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).
This happened mysteriously and had absolutely nothing to with me. The fact that I was the last person to open the filing cabinet before this happened is circumstantial and beside the point.
Will get the lockshop onto this in the next couple of days. In the meantime, just try and exercise your clairvoyance.
Fixed! The key is now hanging in the second bay from the right above the main bench.
I tried to make a change to the front-end in Simulink and compile it on fb0 - which is supposedly now our front-end machine. For some reason it won't build the .rtl file that is the front-end itself. When I tried to revert to the backed-up model I had the same problem. It doesn't look like anyone has tried to rebuild the front-end since late September and there have been some changes to the network since then.
I'm going to track down Alex and sort this out.
Aidan and I began circling the holes in the ventilation system with Sharpies this afternoon, only to find that the situation was even worse than we thought before (which was already bad). Below is a picture of one particularly bad section (compare with Frank's previous post). We can continue to highlight each one, but by the time we are done the entire lab will look like this--maybe I should have chosen 'fugly'. The worst leaks are not the small ones that can be seen on the sides and bottom of the ducts, but rather the HUGE gaping holes on the top sections of nearly every joint in the system. I am fairly sure that even if we were to highlight each and every hole, we will not get PMA to fix each one. Rana suggests that we at least get them in to fix the largest ones for the time being.
I was attempting to replicate the pwelch command using the numerical recipes formula for the Welch's Periodogram PSD estimate. Koji helped me figure out that the calculation was missing a factor of bandwidth, and even though it said explicitly it was a PSD, it was a power spectrum. He also helped me figure out some stupid errors I was making in indexing.
I have recreated the algorithm now, and am going to use this, along with TFestimate to do frequency domain subtraction. I think this could still be a good thing even with the MZWino code, but it will be good to at least compare.
I restarted the ELOG on NODUS just now. Our attempt to set up error logging worked - it turns out ELOG was choking on the .ps file attachment.
So for the near future: NO MORE .PS files! Use PDF - move into the 20th century at least.
matlab can directly make either PNG or PDF files for you, you can also use various other conversion tools on the web.
Of course, it would be nice if nodus could handle .ps, but its a Solaris machine and I don't feel like debugging this. Eventually, we'll give him away and make the new nodus a Linux box, but that day is not today.
To restart the elog: http://lhocds.ligo-wa.caltech.edu:8000/40m/How_To
- apparently PDFs work okay. Frank is reporting continual crashes from last night when uploading a graph of the particle counter in different formats from Firefox in Windows.
I added some instruction to the ATF internals/scripting bit of the wiki for installing liso. Since it needs gnuplot and this requires all kinds of other stuff it seemed like a good plan to document it for future generations.
In short the message is simple. If you're using a mac then you should install X-code from your OSX dvd and then install MacPorts. Your life will be greatly enriched and future dealings with all things linux will be nicer.
RIP Old Whiteboard
I found the old white board and got a photo of the info before the goons who removed it can throw it away.
As a tribute to the old whiteboard, I think that Aidan in particular will appreciate this
Why did we switch whiteboards?
Why would we let them throw this one away?
Don't ask me George, I have no idea. We now have a huge door sized white board though, and a sort of fake wall covering up the door. I'm just assuming that at some point the old one will be gone since it no longer has a wall to live on. Any more questions?
This is what one of the stainless steel dogclamps looked like when I removed it from the vacuum chamber today. This is most likely from the improper configuration we have to use them in due to space constraints, but could also be as a result of improperly pitching them (by using the right counter-height). This is one reason I have opted to use the longer screws from now on.
I ordered some lenses for the lens kit.
What focal lengths/coatings
I chased this up this morning. Even though I made the order directly through techmart (not as a spot buy) Newport have not received it. I'll put it in again. The lenses will have the AR18 anti reflection coating for 1000-1550nm.
I'm ordering two each of the following:
KPX103 - f=175
KPX106 - f=200
KPX109 - f=250
KPX112 - f=300
KPX115 - f=400
KPX118 - f=500
KPX121 - f=700
KPX124 - f=1000
I'm ordering one each of the following:
KPX100 - f=150
KPX097 - f=125
KPX094 - f=100
KPX091 - f=88.3
KPX088 - f=75.6
KPX085 - f=62.9
KPX082 - f=50.2
KPC037 - f=-75
KPC034 - f=-100
KPC028 - f=-200
Gina has checked through the PO and Newport should definitely have received it, but they claim they did not. This is the third time that I know of recently that we've had trouble with ordering this way from Newport.
--- The new order has now been put through and has arrived at Newport ---
I thought we wanted the straight up 1064 only coating on these (AR.33)?
According to Newport:
That would probably be better, but I'm just replacing like with like in the lens kit just now.
the laser data is not available since i left last week. Can some plz check the IBM laptop computer sitting in the rack on top of the HV power supply. plz reboot/turn it on and boot windows (not linux). if you don't choose it from the boot menu while booting it will be running linux by default.... thanks