Yuri Levin emailed us pointing out that we didn't use an updated version (2009 vs 2004) of substrate thermorefractive noise that includes contributions from the standing wave in the beamsplitter. Unfortunately, this has pretty major implications for our noise budget. Attached is the updated noise budget and a 2 page summary of my calculations today and some future outlook.

My attempt at measuring the FSR of the cavity is off by ~20% of the expected value.

I scan the frequency of the laser and record the time between two 0,0 modes. The voltage between the two modes can be calculated from the scan amplitude and frequency. Then, the FSR of the cavity from experimental data should be:

t_start = -48.12e-3 #sec

t_end = 13.8383232e-3 #sec

scan_amp = 2 #volts

scan_freq = 5 #Hz

period = 1/scan_freq #sec

miliamp_per_V = 2e-3

meters_per_miliamp =  2.625e-10 #5e-12/20e-3

cav_len = 2.4

lambda_1 = 1550.08e-9

c = 3e8


delta_t = t_end - t_start

delta_V = delta_t/(period/2) * 2 * scan_amp


FSR = delta_V*(miliamp_per_V)*(meters_per_miliamp)*(c/lambda_1**2)

This gives an FSR of 162 MHz which is clearly way off. I think the discrepancy comes from the uncertainty in the number used for the wave wavelength change of the laser as a function of the pump current. I was eyeballing this before, but here i used the number from the data sheet orginally emailed along with our purchase of the laser. See Attached. He did not provide the raw data. I used a web plot digitizer to get the raw data. The increasing/decreasing.csv are the results of this. The slope of these give the number used in the above calculation.

TLDR: I think the frequency of the laser is a poor way to calibrate the FSR.

See attached photos for latest version of the custom end mirror mount.

Since the update to firmware 591, the python API for the laser lock box seems to have all the functionality required to lock the laser via the API. I have successfully initialized the laser lock box and read off the PDH error signal and the transmission peak. Not sure why the error signal is crap.

I have continued my testing of the voltage sources we have for the SNSPD's, including the new DAC VME. I have below some data that was taken using the SRS voltmeter as a dedicated measuring device. I then stepped through various voltages within the range we will be using the source for the SNSPD's to read out the measured output vs the set voltage. To better see the minute changes of each source, I then subtracted the "set voltage" from the reading and plotted the difference vs the set voltage shown below. The variations are small, but it is clear that the filtered channel has some extra amount of variation between the set voltage and what is being output. 

In my previous post I showed a plot that has PCR tests for each of the voltage sources. The filtered channels (ch 2) seemed to have a shift to the right indicating that the voltage source may be outputting less voltage respective to what is being set to. To test if this may be the case, I replotted this PCR chart using the measured voltages for each step of data that was taken. The results still show that the filtered DAC channel continues to have a shift, which we believe may be due to some type of voltage division happening with the lowpass filter on its output. For the time being, we will be utilizing the new DAC VME but on channels without these filters as they seem align well with the SRS source, while we are aware of the obvious aliasing occuring on its ramped output due to its digital nature. (see plots below)

Attached are some high quality images of the 3D MOT I have been designing.

Wrote up derivation of more accurate Readout Diffusion Noise calculation

Here are svg renderings of the lab space, with GQuEST renderings superimposed. Note that inkscape is functional, but struggles with these exports from the design PDFs.

I took the wiki down to see if other parts of the site would then perform better. Will put it back up at some point.

[Alex, Boris, Andrew] 

I have been working with Boris and Andrew Muller to do testing on the GQuEST SNSPD using 3 different options we have for voltage sources in the lab. The following plots will further detail the results of using each of these sources which include an old battery powered SRS voltage supply, and the new DAC VME that was developed by Lautaro. We also compare two different channels on the DAC VME where one of the two includes a low pass filter on the output voltage. tests of biasing the SNSPD with positive and negative voltages were also done to find any likely voltage offsets between the two. 

In our testing we have found that the filtered output on the DAC VME has a voltage shift, such that the output voltage is slightly less than what is being set with our code. Thus the PCR and DCR curves are shifted to the right on the plot bellow.

The channel without the LP filter seems to have a very similar PCR response to that of the SRS which is what we hoped to see for both. 

This issue will be addressed with Luataro and we will also run callibrations next week to determine the exact voltage outputs of each system relative to what is being set via SCPI commands. 

As mentioned all tests have been done using the GQuEST SNSPD, and thus we are seeing dark count rates that apear to be in the range where the SNSPD rails due to overstimulation of current. Thus, it is expected that in our lower bias ranges (where efficiency should be highest) we will see dark counts well within our 10^-4 regieme (outlined in pink shaded region). This will require the dark box being reinstalled and tests ranging from 1-3+ hours per data point to achieve this resolution. 

In the meantime, we will be diagnosing the voltage supply variations and attempting to get each of the PCR curves to overlap. 

[Ian, Daniel]

We compared our analytical calculations to the calculations made by the coatings optimization package. We plotted an exponential decay and the product of an exponential decay and a sinusoid to the output of the controls package. The analytical calculations that partly motivate some of the results in the GQuEST paper so we wanted to double check them. We get almost a perfect fit. The plot of the three and the code that generated them is attached.

I noticed some drift in the frequency of the laser that may prove to be harmful for the experiment down the road. Untitled.png shows the voltage offset over time of the slow controller output while the cavity is locked. As you can see, it is roughly centered around the middle of its range and drifts out of range eventually losing lock once it goes about its maximum offset of 1V. The change in optical path length the piezo is compensating for is approximately (1.1V - .6 V) * 15  * 3.3e-6 m/150V = 165 nm. This seems like a large amount of drift, but the main problem I think is the output for the controllers in the laser lock box of the moku is limited to 1 V. So we are limited to roughly 10% of the entire 150 V range allowed by the piezo. Although it would work for the demonstrator, I think a reference cavity is out of the question for end design since the two lasers will eventually need to be phase locked to each other.

As followup to log post 11511, a transfer function of the band pass filter was taken and the delay calculated from the derivative of the phase. This gave a delay of 3.1 microseconds, in agreement with the pulse timing measurement. 

The two attatched pictures are of the band pass filter described in log post 11511.

The amplitude of the signal going into the filter must be kept below 70mVpp to avoid strange effects. Sin waves in the passband with amplitudes of 50mVpp were observed to pass through the RFSoC without noticeable distortion.

Conclusion: Performing a freqency sweep response measurement of the digital filter box and taking the derivative of the phase with respect to frequency gives a value of 1.2 microseconds, in agreement with the measurement of logpost 11512.

The digital filter box of one moku pro was set to an all pass filter using the API. The sampling frequency is 39.063 MHz. A second Moku measured the frequency response. The phase data of this frequency response was numerically differentiated using next neighbors, and the delay of 1.2 microseconds is the median time over all pairs.

When the same measurement is performed using a single moku in multi instument mode and the signal is routed through the internal bus, a delay of 950 nanoseconds is measured. 

An independent measurement of the DAC + ADC delay gives 250 nanoseconds. A pulse from the oscilloscope is split, half is routed outside the moku and the other half along the signal bus, the arrival times of the two pulses is then compared.

This leads to the conclusion that the 1.2 microseconds of delay is approximately 950 nanoseconds of filtering time plus 250 nanoseconds of ADC/DAC time.

See attached photo

Update to 11481. 

Improved how the cavity bandwidth is calculated and cleaned everything up. Attached is the python notebook used to calculate this. Data can be found in \Nextcloud\GQuEST\B102\Output Filter Cavity\Cavity Piezo Calculations\cav_piezo_data\ . The associated spectra is also in this directory. These plots now show the piezo TF for the PA44M3KW glued to a mirror and in daniels compressed design, PA44LEW (small thorlabs piezo), and cylindrical Noliac piezo.