I have calculated the predicted angular and linear deflection from changing the AOM drive frequency. 

The setup:

Assume the beam is aligned for some nominal offset frequency such that the angle of incidence on the (thin) crystal is equal to \theta_{B, i} (B for Bragg, i for initial setup). This angle. is linear with frequency. Assume the unshifted beam goes through and is blocked and the frequency shifted beam, which has an angle of 2*\theta_{B, i} with the unshifted beam, is reflected back to the same spot on the crystal. Then assume this beam is either unshifted again, or shifted again (so twice the frequency shift). See the images from Steck's Optics Notes section 13.2.

Now assume the rf frequency is changed. Now the shifted beam makes an angle of  2*\theta_{B, f} with the unshifted beam in both directions. Assume nothing else, such as the alignment of the beam incident on the crystal or the alignment of the mirror, has changed. I have calculated the  angle of transmission of the once and twice shifted beams. See my sketch attached.

Twice shifted:

\[\theta_2 = 4\theta_{B, f} -   3\theta_{B, i}\]

Once shifted:

\[\theta_{1, \text{out}} = 3\theta_{B, i} -   2\theta_{B, f}\].

So the twice shifted beam is twice as sensitive to the frequency shift. However, the frequency shift required is half, so it does not matter which beam is used.

The linear deflection, for a AOM to mirror distance l = 5 in, a wavelength \lambda = 775 nm, a crystal speed of sound v_{rf} = 4200 m/s, and a frequency shift of 40 MHz.

\[\Delta h = \frac{l \lambda}{v_{rf}}\Delta f_{rf} \approx 1.4 mm\].

There are lots of triangles and some AOM physics that is new to me, so there is a decent change I have messed up somewhere.

We are imaging the beams after they go through a lens, so this might distort the picture presented.

-Total power input on AOM - 1.3mW

-Use iris to block unshifted light - 250 uW

-Current AOM settings: 2W amplifier, 350 mVpp, 80 MHz - 20% efficient on first pass.

-74uW total power after PBS

-17uW blocking main beam with iris - ~23% efficient on second pass.

-optimized the quarter wave plate to maximize reflection back through the PBS. total power after PBS is 190 uW

-39uW blocking main beam with iris.

-Attempted to improve efficiency by increasing RF drive power. 600 mVpp RF drive now. This should be providing approximately a watt to the AOM now.

-1.5 mW total light

-600 uW blocking with iris to give 40% efficient first pass.

-585uW total on PBS reflection, 260 uW blocking with iris to get ~44% efficient with second pass.

-A tube based BS holder is probably the best bet to see the shape of the first pass and second pass beam simultaneously.

[Lee, Torrey]

-Low voltage high amp draw from RF power source

-Checking the power of the RF drive on the aom. Within limits with some loss through BNC and factor of two from termination.

-Tested everything. The homeade power source was shorted.

-Re-soldered power source, plugged everything in, and we have frequency shifted light. More to follow after lunch.

Because the location of the curved super optic has changed inside the cavity from its initial position, we must change the mode matching for the 1550nm light in the filter cavity. Previously, the waist was calculated to be at the third mirror the 1550nm light encounters, whereas now the waist is on the input coupler. The new MM solution is approximately f1 = .250 @ .902m and f2 = 1.5 @ 1.546m, where the distances are counting from the 1550nm fiber collimator. I placed new lenses according to the solution and measured the beam to be slightly too big at the measure point. This means L1 was too far back, it was moved forward <1 inch to match the predicted beam size. We can see the first pass beam on the SWIR camera but alignment will need to be improved before we see any coresonance.

[Torrey, Ian]

-Noticeable bump on SHG sled, things were realigned.

-Decent initial alignment into AOM. Going for power on now. 24 V, 0.9 A DC supply per the requirements of the 2 W amplifier we have. https://www.minicircuits.com/pdfs/ZHL-1-2W+.pdf 

-According to RF amplifier the absolute input Vpp is 2 V (10 dBm). We are going to start with a 20th of this, 0.1Vpp. This should correspond -16 dbm to start with a ~35 dbm boost giving 19 dbm. This corrersponds to 0.080W.  No observed diffraction.

-Bump it up to 0.317 Vpp. This is -6 dbm. +33 dbm boost min gives 27 dbm, which is .5 W. No observed diffraction. Going to change alignment now.

-No visible difference when changing alignment on PY005. Need a different strategy for now. Not sure if we're under driving the AOM either. RF input signal is powered off and disconnected for now.

First, I downloaded the image found here (DSP sandbox PYNQ-Red Pitaya) and wrote it to a micro SD card. After that I connected the Red Pitaya to:

1. Power (micro USB)

2. Serial connection to computer (micro USB)

3. Ethernet connection to router

Then, I went to "http://pynq:9090" I was met with a jupyter notebook password screen. The password is "xilinx".

In the jupyter menu you can open a terminal to access the red pitayas Linux system. I followed the instructions here (PYNQ getting started) to change the host name to "pynqpitaya"

I ran the command "ifconfig". There are multiple IP addresses. Why?

The address that allows one to ssh or scp onto this red pitaya is 192.168.50.14, which corresponds to "eth0" rather than "eth0:1"

After using scp to get the analog echo bitfile and notebook from Chris' logpost onto the board, I ran the notebook. The first cell runs successfully, and a blue light comes on the red pitaya. However the cell with the write statement never executes. When I measure the DAC outputs on an oscilloscope, they measure about +1V and -1V. No response is seen to an input pulse on ADC 1.

[Torrey, Daniel, Sander]

Continuing AOM progress. A f= 100mm lens is in place, with the AOM, and power supply ready. We ran into the problem of the input for the AOM being SMB, of which we do not have an adapter for. This will be on hold until we can procure one from another lab. Digikey sells these but have a long lead time. Amazon also has these.

[Torrey, Sander, Daniel]

Third basler camera is now in use and is on the SHG sled (SN 24839320). Tested as is working.

Current list of RF power amplifiers we have in B102:

-Mini circuit 5 W amp x2

-Mini Circuits 2 W amp x2 (one in use on SHG sled)

-ENI 3W amplifier x1 

These are all on the top shelf of the beige cabinet to the left of the toolbox.

Attached is the plot for a number of controllers calculated by buzz for the ASC DHARD Yaw DOF. The plot's axis represent the total RMS on the Y-axis in Radians and a weighted RMS on the X-axis that is proportional to the BNS detection range. This is just an estimate using the square of the BNS FOM that I was using previously. The number by each point represents the phase margin of each controller. The outline color is also supposed to represent this but there is some bug that gets the colors slightly wrong from the scale. I will get to fixing this. A recent hand tuned controller is also present for comparison. The plot was calibrated using the 5.2e-11 rad/ct from Elenna's Alog post on ASC Calibration. The hand tuned controller shows an RMS of approximately 1.6 nrad RMS. given that our model represents is modeling a day with higher than normal low frequency noise this compares favorably with the 0.36 nrad RMS measured recorded in the same post by Elenna. This indicates that our model is a good representation of the system, at least in this limited way. Given the calibration it is possible to get a controller that with similar phase margin with an order of magnitude less overall RMS.

The next step is to add the updated BNS FOM so that the BNS RMS value has a physical representation. Currently it seems that the updated BNS FOM causes problems with the current solver.

To one of the AOM mounts we ordered from 3D Hubs, I added two tapped #6 holes to the following two locations, assuming the center of the mount is the origin: (+/- 0.875, 0.25). The y coordinate is not zero because there are existing holes at (+/- 0.75, 0) which would interfere.

The Gooch & Housego 3080-125 AOM fits very well onto these holes and it mounted in B102.

[Torrey, Ian]

We have an AOM to use while we obtain our own. We have decided on using this to power it. Elie Bataille quotes it at 5W max so we will under drive this amplifier. We took an intial profile of what the beam looks like near where the AOM will be. Space is very limited to build a proper double lens solution (i.e. sum of the focal lengths = the distance between them) as seen in this. Because of this we are proposing a single lens solution where the mirror is placed approxiamtely at the focal length, seen here.

I borrowed a Gooch & Housego 3080-125 AOM from the Endres Lab. My point of contact is Elie Bataille. It is currently in East Bridge B102.

Behavior:

Attached is a block diagram describing the behavior of a DSP system permforming feedback control for the Pound-Drever-Hall (PDH) frequency stabilization technique. This block diagram is based on Liquid Instrument's laser lock box, which we have been using to lock our current cavity.

There are two modes of operation: 'scanning mode' and 'locking mode.' When the toggle (on the right side of the diagram) is down, we are in 'locking mode' and the control loop is closed.

In locking mode there are two main tasks. First, we must obtain the error signal from the incoming photodiode signal by demodulation. We mix the incoming signal with the modulation tone, (with an adjustable phase offset) then we low-pass filter to remove unwanted high-frequency signal coming from the mixing. Then we subtract the desired setpoint from this signal to obtain the error signal. At this location, the error point, we want to be able to take time series of the values. These timeseries will be used to evaluate the characteristics of the loop and to make the transition from scanning mode to locking mode. Next, the error signal is passed through a series of biquad filters (the controller) to create the signal to be sent to the actuators. A constant offset is added to this signal and it is sent out to the actuator.

In scanning mode there is no feedback. Instead, the actuator voltage is swept by the sawtooth generator, and the error signal is monitored at the error point. Using the timeseries of the error signal during this sweep we can adjust the value of the constant offset and toggle the switch, exiting sweep mode and closing the loop.

Specifications:

To meet the requirements of the GQuEST experiement, we want to lock the cavities with a 10kHz unity gain frequency, and a 'low enough' root-mean-squared displacement. We will continue to work to understand what is 'low enough' and how this translates to FPGA specs, but until then here is a list of our best guesses at the approximate desired stats. These are based on the stats of the Moku and a few calculations, some of which are found in this log post and its comments. In particular, as the filter sample rate is increased, the number of bits required for the filter coefficients will increase as well. 

I 3D printed a Holder for the Basler ace GigE C camera, which we are using to image the 775 nm light. The holder interfaces with 3 M3 screws on the bottom of the camera and allows for a #8 socket head cap head screw to be added. The bottom of the holder is 1 inch from the center of the camera. The holder has some slots to allow for air to flow below the camera for ventilation and hopefully keep it cooler. I'm not sure of the efficacy of this since the other 5 faces were open on the other camera and it still got quite hot (although still within spec).

Attached are SolidWorks and STL files for the part I made and the camera, plus a PDF design of the camera.

[Torrey, Daniel]

Torrey thinks that the central axes of the piezo and the mirror are misaligned. I designed a part that aligns the small piezo and a 1/4 in thick spacer with a #8 through hole with the "piezo top". This piezo top should be well aligned with the piezo bottom that holds the mirror. I believe this should give alignment of the axes to within ~5 thou rms (3 thou from the piezo top to the base, 4 thou from the mirror in the SM1 threads, and ~1 thou from this tool).

Attached is the part file with CAM as well. I decided to make this part in a CNC Lathe for its precision compared to a 3D print. An important consideration is the radius of curvature of the cutting tool. This is why there is a notch toward the thickest part of the tool and why the levels of the tool don't match the levels of each part. If one were to 3D print this part, they should remove the notch so that there are no overhung sections.

The blue colored photo is the CAM simulation.