[Daniel, Mai ,Torrey]

The leading theory on the ~520 Hz resonance seen in the spectrum of the cavity error signal is some part of the flexture mount mirrors resonating. To test this theory we pushed on the mirror with viton to see if the resonance is damped/moved at all. Our initial test has a viton O ring pushing on M2 of the cavity only. We did this with and without pushing on the mirror, results seen here image.png. We are skeptical of these results and tomorrow will:

a) repeat this test several times. It is time consuming as the force on the cavity mirrors misaligns the cavity.

b) instead of relying on the ambient noise of the room to ring up, use phones to drive at say 520 +/- 20 Hz at constant locations between tests.

[Torrey, Lee, Daniel]

After some investigation we have found a mystery resonance around ~520 Hz. IMG_0015.png is the set up used to take a spectrum of the error signal and the control signal. As seen from mysteryresonance.png (blue is error, orange control) you can see a resonance peak at ~520. This exibits resonance type behavior, i.e. drive a sound at 520 Hz and measure the level of excitation and compare it to frequencies around this, say +/- 20 hz it is much reduced. 

Additionally we found some source of phase delay from our last post by taking a transfer function accros the piezo controller, as seen in IMG_0020.png. Lee mentioned it would be useful to create a filter that inverts this amplitude loss.

The same sd card in a different RedPitaya and power supply works.  

Yellow is the input signal to the RedPitays.  Blue is the output, with offset register set to 100 mV.  The delay seems to be 100 ns.  The gain seems to be a bit more than 1.

So, we can read and write analog signals and control registers with pynq.

[Torrey, Daniel]

We aligned light from the misc category into a fiber on the power distribution sled. Instead of the 5 axis mount, we used a simple 2 axis mirror mount. We also tuned up the alignment into the other 3 fibers to maximize the power. We measured the power going into the fiber and out of the fiber to see if and how it drifts over time. Potentially, the 5 axis mounts will drift more than the 2 axis mount.

I built the example analog feedback from https://github.com/dspsandbox/Pynq-Redpitaya-125

On one RedPitaya board the firmare loads, but setting the offset value via pynq causes the board to freeze up.  Identical firmaware and test on a second board succeeds.  Now will diagnose the "bad" board.  First suspect is the power supply.

[Lee, Torrey, Daniel]

On Friday, we adjusted the loop shaping on the filter cavity to try and make the lock better. With a 1/f loop, it will lock, but goes between flickering out of lock, to oscillating as the gain is increased.

The new shaping uses the "slow" output of the "laser lock box." It shapes the fast output to have a 10Hz - 200Hz boost (pole - zero), giving 20x gain at low frequencies. The overall 1/f is then put at 1Hz in the slow path. We are able to get a UGF of 1kHz, with 30-40deg remaining. We lose 90deg by 2kHz from either the actuator or the Moku, so 1kHz is around the max UGF we can currently get.

Even with that loop, we get dropouts while talking loudly or while the fans are on. Problematic. That mean the loop can supress the peak length noise below 1.5um / 2  / (2 * finesse). Our current input coupler is 1%, so our finesse is around 2pi / 0.01 = 600 (it might be as low as 300, depending on the losses in the cavity). So our peaks/RMS are around 0.5nm - 1nm of noise.

+1 to the piezo transfer teams. both 11380 and 11373 have the transfer function data along with the plots. It looks like all of the phase we are losing in the cavity lock is from the piezo (though could still be from Moku). We'll need to fit and at least slightly invert this actuator transfer function to make a loop above 1kHz UGF. The fit can help tell us how much of the phase is from the piezo roll-off.

We are going to need it - the current loop & cavity  has marginally too much noise, with an RMS around the linewidth. That is going to be a problem once we increase the finesse of the cavity. So we'll need to reduce the noise and/or improve the loop a respectable amount.

our voices are driving 100-300Hz, so we need gain there. If we can get the UGF up as high as 3kHz, then we should also be able to move the boosts up to 600Hz or so. That should give 9x more loop gain and cover much of the need.

options:

frequency lock - good for acquiring and transitioning to aggressive lock, but not an option for operating the experiment

optimal control - we can determine how good we can do with what we have

fitting/inverting actuator - probably needed to get UGF higher. May vary with time in bad ways

sound attenuation or active noise cancellation - may be possible and should look into engineering this.

We should determine if the sound driving is from pushing the mirrors or from the index fluctuation as the pressure modulates. 11290 has calculations about the pressure pushing on a 1kg mass. My worry is that the small alignment adjust set-screws in the MKS flexture mounts are fairly compliant. Could sound drive them 1nm?
My guess is that the light going through 2m of air is the culprit.

The sound pressure of 40dB should be around 2000uP. Air pressure is 101kPa. The index of air is 1.000293

2m / (1.000293**2) * 2000e-6Pa / 101kPa * (1.000293 - 1) = 1.2e-11m

So that is a little small, but if our sound is at 80db, which would be quite loud, then it would be enough to cause this.

For the mirror motion, the mirror mounts are about 1.5in square. On the pressure level that is

(.0254m * 1.5)**2 * 2000e-6Pa = 3e-6N

of the mirror+panel is about 35g (very rough estimate). For a free mass motion that is

1 / ((2 * np.pi * 100Hz)**2 * 0.03534291735288516kg) * 3e-6N = 2e-10m which is getting in the ballpark with 60db of noise rather than 40db. The mirrors aren't free though.

and now I'm curious how that is against the spring constant of the tiny screw.

[Torrey, Daniel]

We measured the reflectivity of 775 nm light on the Newport 10D20DM.8 (1550 nm HR mirror). At a 45° AOI, there is roughly a 40-50% reflection. +/- around 5° away from 45°, the reflection drops off sharply. Thus, these mirrors cannot be used to make a bowtie cavity for 775 nm light.

Low AOI: 3% reflection, 70% "transmission" but definitely a lot of this is scattered light

45°: 40-50% reflection depending on polarization, ~40% "transmission" but definitely a lot of this is scattered light

The frame is assembled.  Custom-machined parts for the walls and inside shields are on order.

I 3D printed two aligment tools that slide into the corners of the bowtie cavity. The beam should pass through all 4 holes. The diameters are tight, so probably not the best to use for lining up the reflected beam and instead just use for the incident beam.

Following instructions here:  GitHub - dspsandbox/Pynq-Redpitaya-125 leads to better results.  

Built a project from scratch in Vivado 2023.2 to control the LEDs.  Include the QICK procedures extracted to ip.py to instantiate drivers. 

[Daniel, Lee, Torrey]

First filter cavity lock!

No REFL dips were found after alignment from the new mode matching solution. Lee suggested instead use the very sensitive SWIR cameras to check in transmission of M2. We first replaced M2 with a 50/50 beam splitter to roughly align the cavity and see the beam making a round trip. Scanning the piezo mirror (M3) allows us to see higher order modes flashing. We use the mirrors before the cavity to align and maximize the 0,0 mode. We then put the HR mirror back in the M2 position instead of the BS. The reason this wasn't done before is we thought the frosted backs of the mirrors in use might obscure the beam, but this was not the case. You can still see a beam through the frosted back. We used the laser lock box to control the cavity with the piezo mirror and achieve a fairly robust lock. 

First steps to customize pyrpl on the RedPitaya. Does it compile?

Install Vivado 2015.4 on Windows laptop. Clone https://github.com/lneuhaus/pyrpl.git In the directory pyrpl/pyrpl/fpga run this command in windows power shell:

C:\Xilinx\Vivado\2015.4\bin\vivado.bat -nolog -nojournal -mode batch -source red_pitaya_vivado.tcl

This created the files:

/fpga/red_pitaya.bin

pyrpl/fpga/project/pyrpl.srcs/sources_1/bd/system/hw_handoff/system.hwh

Installed pynq on a RedPitaya following https://github.com/dspsandbox/Pynq-Redpitaya-125 Trying to load the bitfile gives this error:

File /usr/local/share/pynq-venv/lib/python3.10/site-packages/pynqmetadata/frontends/hwh_frontend.py:452, in HwhFrontend._resolve_subordinate_addressing(self) 450 for i in self._root.iter("MEMRANGE"): 451 if i.get("MEMTYPE") == "REGISTER" or i.get("MEMTYPE") == "MEMORY": --> 452 core = self.blocks[i.get("INSTANCE")] 453 port = core.ports[i.get("SLAVEBUSINTERFACE")] 454 if isinstance(port, SubordinatePort): KeyError: 'M_AXI_GP0'

I added a lens between the two EOMs to ensure no clipping on the second EOM. I then reprofiled and found a MM solution to put the waist of 600 um at M3 (3.003 m from colimator output) in the cavity. This has been implemented and measured to have an average beam waist and location of waist (average of x and y, beam is slightly astigmatic) of 599.5 um @ 2.97 m. This should more than suffice to see some amount of resonance in the cavity. The nextcloud server is currently down but "Profiling after l1 12072023.ipynb" has this data on the lab computer. Will also update the layout mockup when the server is back up.