I reran the latest simulation with the same parameters as (https://mccullerlab.com/logs/lab/index.php?callRep=11430) but with a fused silica coating material instead of silicon. The Q of the coating remained 1e4. The noise floor from the coating goes up, but the qualitative behavior is very similar as silicon coatings with the same loss.

Potential reason for the loss in power over time for the fibers on the power distribution center: This says:

Wide-Key-Slot Mating Sleeves - While this configuration is acceptable for patch cables with FC/PC connectors, for FC/APC applications, we recommend using narrow-key-slot mating sleeves to ensure optimum alignment. Our patch fibers are FC/APC. Additionally the adjustable fiber collimators we purchased (pdf attached) are wide key. They don't sell the collimator as narrow key, but we could potentially request it.

[Torrey, Daniel, Sander]

While trying to lock a readout filter cavity using a setup that enabled reliable and stable locking before, we noticed mode hops (i.e. we saw different modes flashing in the cavity in random succession) during scanning and locking. There was also a much larger drift of the error signal than previously observed. I suspected this could be due to mode hops in the seed laser, caused by back-reflection into the seed laser. A seed laser output beam pick-off after the fibre-PBS is currently used as input for a Michelson setup, and this output has no Faraday isolator to prevent back reflections. Blocking this output with a beam dump resolved all problems and enabled locking of the filter cavity as before. 

Going forward we should install a Faraday isolator at the fibre output or only pick off beams after the amplifier's Faraday isolator.

I set up a Michelson Interferometer with a 17 inch Schnupp Asymmetry to test the noie from the flexure mount. A 750 mm focal length lens is added 6 inches away from the long arm end mirror so that the beams are approximately the same size at the beam splitter. The visability is approximately 50% without alignment to maximize this.

Before the input port, I added a 10:90 (R:T) BS and photodiode to act as a power reference. When the laser's frequency is modulated, the power is also modulated. By dividing the Michelson output power by this reference power, the direct frequency dependence can be seen. This will allow us to put the Michelson on a midfringe and to test the noise from the flexure mount.

[Torrey, Daniel]

We set up a Michelson interferometer with a 14 inch Schnupp Asymmetry to lock the output to midfringe. We added a lens so that the beams are the same size at the beamsplitter and roughly the same Guoy phase. However, modulating the laser's frequency also modulates its power. Next week, we will set up a photodiode with a reference power before the beamsplitter and divide the output with the input to see if the fringe changes.

Locked the cavity with the DC modulation port of the ULN15TK laser. Couple take aways:

-The UGF is pushed to ~2kHz (was ~1kHz).

-Some very strong peaks introduced via this port at ~830 Hz, 1.6 kHz, etc...

-the lock is more resilient against sound, but not immune.

I set up a piezo assembly with a 10D20DM.8 1550 nm HR mirror, a viton o-ring, and a Noliac NAC2125-H08 piezo. This assembly is mounted on a 5-axis stage so that it can be aligned . I moved the flexure mount assembly to a new, fixed mount since the flexure mount itself can be moved. Once the mirrors are aligned, the michelson should be able to be locked. This should hopefully provide more sensitivity for our noise test.

I am setting up a second Noliac NAC2125-H08 for the Michelson and also eventually for the 2nd output filter cavity.

Capacitance: 2.3684 uF

Resistance: 4.132 kOhm

When I press on the piezo, there is a positive voltage when I assign the lead with the black dots to be the positive lead.

I have flagged this lead with kapton tape.

[Daniel, Sander, Torrey]

We wanted to see if we could lock our michaelson in the lab using the frequency of the laser. However, the external modulation ports on the turnkey laser we have has an input voltage requirement of +/- 5V. The manual says this modulates the current at 2 mA/V. The graphs on thorlabs say changing the current by ~20mA should correspond to changing the wavelength of the laser light by order 10 pm. If our interferometer has interference fringes lambda/2 = 775 nm apart and we can only modulate by about 10 pm maximum, we will never be able to lock a michaelson using the laser frequency alone.

We were able to inject a 150 kHz signal into the AC modulation port of the laser and can directly read that out on a PD, but only by blocking one of the arms of the michaelson, suggesting this is purely modulating the amplitude and not frequency. We also tried using the temperature unsuccessfully, which does not have specs online of its mA/V response.

I using Levin's method (Internal thermal noise in the LIGO test masses: A direct approach) of using the suceptability of a mirror to a force resembling the laser beam profile to model the thermal noise, I modeled the GQuEST end mirrors as 2 mm thick, 1 in side length squares made of silicon with a Q of 10^6. I added a coating 20 um thick made of silicon as well but with a Q of 10^3. I constrained the 4 side of the mirror to only move in the z-axis. The spot radius is 2 mm. The face of the mirror had 52 x 52 mesh points, the substrate had 10 layers of mesh, and the coating had 3 layers of mesh. I simulated from 500 kHz to 16.5 MHz with 100 kHz frequency spacing. It look 5 hours and 17 minutes. Attached is the data converted into an ASD. The coating thermal noise, due to its low Q and that the force is applied on it, dominates the "bottom of the bucket".

[Torrey, Daniel]

We set up a Michelson Interferometer using a Flexure Mirror Mount and a standard tip-tilt mirror mount as end mirrors to test if there was a resonance that produced the noise source around 520 Hz that is observed in the output filter cavity. The beamsplitter was a 50:50 non-polarizing plate, and the readout was done by a Thorlabs PDA50B2 amplified photodetector. The alignment was such that the contrast from hitting the table was above 80%. On the frequency trace, there was no discernable increase in noise around 520 Hz (see photos). I also played sinusoids at 520 Hz and did not observe any increase in the noise. This indicates this noise is not inherent to all flexure mounts. Perhaps some flexure mounts are manufactuered or mounted sufficiently differently as to produce a 520 Hz noise source. The large peaks appear to be 60 Hz noise and its overtones.

[Jeff, Torrey]

This is a continuation from 11423. We set to improve upon our characterization of how well pushing a viton ring against the mirror (M2) of the cavity damped out the mystery 520 Hz resonance. 

We first drove sound from a phone at 490 Hz, 520 Hz, 550 Hz with and without pushing on the mirror, seen in driving_3_lines.png. The amplitude of these oscilations seem to be damped ~15 dB. There seems to be an additional peak around 450 Hz that is more broadband that should be investigated.

Jeff had an idea to play some white noise and try the same thing. Result can be found in white_noise_injection.png. Here we can see the resonance is completely damped out. It should be noted that the amplitude of the white noise the phone app I was using was quite small, so this is probably just damping out the oscillations due to the ambient noise of the room. Daniel found a louder app, will repeat this with bigger white noise. We should buy a speaker and microphone for future tests. Data for these can be found in the zip.

I find this result quite strange as we are only damping one mirror. I am going to repeat this process doing the same thing to M4. 

[Ian, Daniel]

We went to the Caltech VWR Stockroom to get solvents. We got

• 1101 Acetone 99.5% ACS 4L
• 1133 Isopropyl alcohol 99% ACS: 4L
• Water: 4L

To get them we went to the stockroom linked above, filled out the PTA number, and loaded them onto the cart.

We still need to buy funnels to fill our smaller bottles. we need to buy appropriate funnels such that they are safe for these chemicals. We have a bottle for Methanol but we decided to hold off on purchasing that until later as we don't really need it yet. 

We need to move them to a chemical cabinet. Right now they are stored in a fume hood that is not turned on I want to temporarily store them in one of the chemical cabinets (see image) in one of the Adhikari labs. I want this to happen today so I will ask for space ASAP.

Another minor step towards pynq on the RedPitaya control system.  Here is the result of uploading an arbitrary wave form, playing it, and reading it back.  This is the demo described here:  https://github.com/dspsandbox/FPGA-Notes-for-Scientists/wiki/DMA-transfer

One vhdl bug had to be fixed, putting a 0 value in quotes to help the compiler.  Vivado compilers are more careful now about types.