EDIT (ZK): Koji points out that (1 - Ti) should really be the non-resonant reflectivity of the aligned cavity, which is much closer to 1. However, it should *actually* be the non-resonant reflectivity of the entire OMC assembly, including the steering mirror (see bottom of post). The steering mirror has T ~ 0.3%, so the true results are somewhere between my numbers and those with (1 - Ti) -> 1. In practice, though, these effects are swamped by the other errors.

More information about the power-dependent visibility measurement:

As a blanket statement, this measurement was done by exact analogy to those made by Sam and Sheon during S6 (c.f. LHO iLog 11/7/2011 and technical note T1100562), since it was supposed to be a verification that this effect still remains. There are absolutely better ways to do (i.e., ways that should give lower measurement error), and these should be investigated for our characterization. Obviously, I volunteer.

All measurements were made by reading the output voltages produced by photodetectors at the REFL and TRANS ports. The REFL PD is a BBPD (DC output), and the TRANS is a PDA255. Both these PDs were calibrated using a Thorlabs power meter (Controller: PM100D; Head: S12XC series photodiode-based---not sure if X = 0,2... Si or Ge) at the lowest and highest power settings, and these results agreed to the few-percent level. This can be a major source of error.

The power was adjusted using the HWP/PBS combination towards the beginning of the experiment. For reference, an early layout of the test setup can be seen in LLO:5978 (though, as mentioned above, the REFL and TRANS PDs have been replaced since then---see LLO:5994). This may or may not be a "clean" way to change the power, but the analysis should take the effect of junk light into account.

Below is an explanation of the three traces in the plot. First:

• TRANS: TRANS signal calibrated to W
• REFL_UL: REFL signal while cavity is unlocked, calibrated to W
• REFL_L: REFL signal while cavity is locked, calibrated to W
• Psb: Sideband power (relative to carrier)
• Ti: Input mirror transmission (in power)

Now, the traces

1. Raw transmission: This measurement is simple. It is just the raw throughput of the cavity, corrected for the power in the sidebands which should not get through. I had the "AM_REF" PD, which could serve as an input power monitor, but I thought it was better to just use REFL_UL as the input power monitor and not introduce the error of another PD. This means I must also correct for the reduction in the apparent input power as measured at the REFL PD due to the finite transmission of the input coupler. This was not reported by Sam and Sheon, but can be directly inferred from their data.
   • trans_raw = TRANS ./ ( REFL_UL * (1 - Psb) * (1 - Ti) )
   • Equivalently, trans_raw = (transmitted power) ./ (input power in carrier mode)
2. Coupling: This is how much of the power incident on the cavity gets coupled into the cavity (whether it ends up in transmission or at a loss port). Sheon plots something like (1 - coupling) in his reply to the above-linked iLog post on 11/8/2011.
   • coupling = ( REFL_UL * (1 - Ti) - REFL_L ) ./ ( REFL_UL * (1 - Psb) * (1 - Ti) )
   • Equivalently, coupling = [ (total input power) - (total reflected power on resonance) ] ./ (input power in carrier mode)
3. Visibility: How much of the light that is coupled into the cavity is emerging from the transmitted port? This is what Sam and Sheon call "throughput" or "transmission" and is what is reported in the majority of their plots.
   • visibility = TRANS ./ ( REFL_UL * (1 - Ti) - REFL_L )
   • Equivalently, visibility = (transmitted power) ./ [ (total input power) - (total reflected power on resonance) ]
   • Also equivalently, visibility = trans_raw ./ coupling

The error bars in the measurement were dominated, roughly equally, by 1) systematic error from calibration of the PDs with the power meter, and 2) error from noise in the REFL_L measurement (since the absolute AC noise level in TRANS and REFL_L is the same, and TRANS >> REFL_L, the SNR of the latter is worse).

(1) can be helped by making ALL measurements with a single device. I recommend using something precise and portable like the power meter to make measurements at all the necessary ports. For REFL_L/UL, we can place a beam splitter before the REFL PD, and---after calibrating for the T of this splitter very well using the same power meter---both states can be measured at this port.

(2) can probably be helped by taking longer averaging, though at some point we run into the stability of the power setting itself. Something like 30-60s should be enough to remove the effects of the REFL_L noise, which is concentrated in the few-Hz region in the LLO setup.

One more thing I forgot was the finite transmission of the steering mirror at the OMC input (the transmission of this mirror goes to the QPDs). This will add a fixed error of 0.3%, and I will take it into account in the future.

I found that, in fact, I had lowered the modulation depth since when I measured it to be 0.45 rads --> Psb = 0.1.

Here is the sweep measurement:

This is Psb = 0.06 --> gamma = 0.35 rads.

This changes the "raw transmission" and "coupling", but not the inferred visibility:

I also measured the cavity AMTF at three powers today: 0.5 mW, 10 mW, and 45 mW input.

They look about the same. If anything, the cavity pole seems slightly lower with the higher power, which is counterintuitive. The expected shift is very small (~10%), since the decay rate is still totally dominated by the mirror transmissions even for the supposed high-loss state (Sam and Sheon estimated the roundtrip loss at high power to be ~1400 ppm, while the combined coupling mirrors' T is 1.6%). I have not been able to fit the cavity poles consistently to within this kind of error.

[Koji, Zach]

Tonight, we locked the "fauxMC". We obtained a visibility of >99%.

Koji had aligned it roughly last night, but we wanted to have a couple steering mirrors in the path for this practice cavity (the periscope mirrors will serve this function in the real setup), so we marked the alignment with irises and installed two extra mirrors.

After obtaining flashes with the WinCam placed at the output coupler, we removed the WinCam and put a CCD camera at one of the curved mirror transmissions and used this to get a strong TEM₀₀ flash. Then, we installed the REFL PD/CCD, swept the laser PZT and optimized the alignment by minimizing the REFL dips. Finally, we connected the RF electronics and locked the cavity with the LB box. We used whatever cables we had around to trim the RF phase, and then Koji made some nice SMA cables at the 40m.

One thing we noticed was that we don't have enough actuation range to keep the cavity locked for very long---even with the HV amp (100V). We are going to offload to the NPRO temperature using an SR560 or pomona box circuit. We may also make an enclosure for the cavity to protect it from the HEPA blasting.

Tomorrow, after we do the above things, we will practice measuring the transmission, length (FSR) and mode spectrum of the cavity before moving on to the real McCoy.

That is an expanding fire pillow, also known as firebrick.  It is used to create a fire block where holes in fire-rated walls are made and prevents lab fires from spreading rapidly to adjacent labs.  I had to pull cable from B254 to our labs on either side during a rather narrow window of time.  Some of the cable holes are partially blocked, making it difficult to reach the cable to them. The cable is then just guided to the hole from a distance.  With no help, it's not possible to see this material getting shoved out of the hole.  I can assure you that I took great pains not to allow the CYMAC coax to fall into any equipment, or drag against any other cables.   

[Camille, Thejas, Koji]
16 August 2023

We met in the lab to try to understand the mode-match discrepancy we see in our measurments. Adjusted the fiber coupler and the periscope to minimize the REFL PD signal. (REFL PD signal was 0.116 when locked.) The shape of the beam on the REFL CCD looked the same as in https://nodus.ligo.caltech.edu:8081/OMC_Lab/607.
We observed the transmission spectrum on the scope to identify higher order modes and side bands (need to attach plot). We closely examined the signal intensity of the weaker peaks in addition to the stong TEM00 peaks and exported the data from the scope. We also locked the cavity on the other modes to observe the shaped of these other modes (we see some pitch and yaw misalignment in the other modes).
The intensity signals of the other modes estimates ~1.8% mode-mismatch. (Still does not explain the 2% discrepancy between we mode-mismatch we calculate in our power budget analysis.)

We also varied amplitude of the phase modulation (from ~8-17dB) but this showed no improvement to the REFL PD signal.

Our plans moving forward:
-Center the beam path through the lenses to try to improve the mode-matching
-Further reduce REFL PD signal (~70mv?)
-Quick check: Attenuate the TRANS PD signal and compare ration between TEM00 signal and other modes.

For Set 1 of the data in https://nodus.ligo.caltech.edu:8081/OMC_Lab/604,

V_REFL(unlocked) = 3.226V
V_REFL(locked) = 0.13V
P_in = 20.56mW

Fraction of light that is reflected (mode-mismatched) = 0.135/3.226 = 4.03%
P_junk = 0.0403*20.56mW = 0.83mW

From T1500060 Section 3.3, "The incident beam power to the cavity (P_in) can be split into the mode-matched (coupled) and mode-mismatched (junk) light power (P_coupled and P_junk, respectively)."
P_in­ = P_coupled + P_junk
20.56mW = P_coupled + 0.83mW
P_coupled = 19.73mW

This suggests that our cavity has nearly no loss, and the mode-matching efficiency is ~96%

However, this mode-matching efficiency is very different from the mode-matching efficiency determined from our transmitted PD signal on https://nodus.ligo.caltech.edu:8081/OMC_Lab/603.

From the PD_trans signal, the TEM00 signal is ~7.0V
There was only one higher-order mode observed with a signal of 0.070V.

0.070/7.0 = 1% mode-mismatch

[Camille, Thejas, Masayuki]

This afternoon we finished the realignment that we started after the FC cleaning in https://nodus.ligo.caltech.edu:8081/OMC_Lab/605.
We wanted to try to improve mode-matching before taking new power measurements. We used the signal from the transmission PD to characterize the mode-matching. We observed the TEM00 peak and one additional HOM peak:
TEM00 signal: 6.9V
HOM signal: 0.095V
--> mode-matching efficiency is ~1.4%

We observed the REFL CCD and include an attached picture. We recorded pictures of the beam spots using the CCD video camera (pictures attached).

We took one set of power budget measurements (measured values and outputs are shown in the attached screenshot).
The fraction of light that is reflected is
0.012V/3.35V = 3.6%
This is very similar to our previous data.
Similarly, our reflected power, incident power, and transmittd power are very similar to our previous values (P_refl=0.79mW, P_in=21.82mW, and P_OMCT=20.73mW)
This would seemingly indicate that we have very little loss in the cavity, however we still plan to further investigate the 3.5% loss observed by the REFL signal.

[Camille, Thejas] 4 August 2023

-REFL PD signal (unlocked): 3.5V
We optimized the alignment of the steering mirror on the REFL PD.
-REFL PD signal (locked): 1.5V (initially) --> 0.15V after we optimized alignment into the cavity.

-We finished setting up the table optics so we monitor the transmitted beam with a photodiode. (Previously, we were only monitoring the transmitted beam with a CCD camera.) A mirror is used to steer the beam to the PD.

-We checked the beam spot positions on the mode-matching lenses and adjusted the fiber mount. (It is centered on the first lens. It is somewhat off-center on the second lens [attach picture].)

-We optimized the alignment to reduce the REFL PD signal when locked. (We slightly pitched the periscope mirror, adjusted the fiber mount, and slightly walked the beam with the periscope. Lowest achievable REFL PD signal was 0.15V
Our mode-matching ratio is ~95.8%

-We unlocked the laser and used the frequency offset to sweep and observe shape of the transmitted modes on the CCD camera. TEM00 modes are the most prominent. The other observed modes appear to be 1st/2nd/3rd order rectangular modes. [attach pictures of modes]

Plans for this week:
-If mode-matching ratio is acceptable, we will proceed to move the cavity axis between CM1 and CM2. (The axis needs to move towards FM1 and FM2.)
-If mode-matching ratio needs to be improved, we will re-characterize the beam waist position and re-verify the location of our beam waist. (We will temporarily place a mirror or beamsplitter after the mode-matching breadboard, so we can steer the beam away from the OMC optics and use the beam profiler without disrupting the OMC optics.)

With the current level of the mode matching, the spot positions can be optimized. I'd work on it first as the mirror RoC and loss depends on the spot positions on the curved mirrors.

----

I'd also look at the reflection CCD image:

With the previous OMC, we saw the reflection to be 60~70mV. We saw ~85mV with the previous alignment when I went down to the lab.
Today it is ~150mV.

- If this increase is coming from the worse reflection of the cavity (lossy cavity somehow), the reflection CCD image should definitely show TEM00 mode.
  -> The cavity needs to be cleaned
- If this is coming from the mode matching, the image seen with the room lights off should show (somewhat symmetric) higher-order modes.
  -> I'd try moving the lenses.

We will plan to look at the reflection CCD image this afternoon.

[Camille, Thejas]

This morning, we continued the efforts from http://nodus.ligo.caltech.edu:8080/OMC_Lab/602.
While monitoring the REFL PD, we walked/optimized the alignment after cleaning the optics with FC. After optimization, our PD signals were:
REFL PD: 0.11V (locked)
REFL PD: 3.3V (unlocked)
We also have a picture of the REFL CCD (attached). The REFL CCD does not appear to show so much TEM00 as it did previously.

We also checked the mode scan on the transmission PD and recorded the signals of the TEM00 mode and another higher-order mode that was still observed.
TEM00: 7.0V
HOM: 0.070V
--> ~1% mode mismatch.
From this we estimate that we should be able to reduce the REFL PD signal to ~30mV.
 

We used the CCD video cameras to record pictures of the beam spots on the cavity optics (attached). The scatter looks much better post-cleaning and the beam spots are closer to center. However, we can still steer the beam closer to center (need to move away from FM1 and FM2).

[Camille, Thejas]

We lowered the setting to MEDIUM on the HEPA furthest from the entrance. (The two HEPA settings are now LOW and MEDIUM.) We measured the particle count for all particle sizes at different locations in the enclosure (next to the OMC fixture and at the edge of the table closest to the entrance). All counts read zero.

Yesterday, we measured the particle count in the enclosure to ensure that the lower setting on one of the HEPAs is still acceptable. The particle count is still zero for all measured particle sizes (0.3um, 0.5um, 1.0um, 2.0um, and 5.0um).

We also relocked the cavity and repeated the optimization efforts in https://nodus.ligo.caltech.edu:8081/OMC_Lab/584. The reflected signal was around 110mV (compared to 80mV on Tuesday).

We used a CCD camera to view the beam spots on the curved mirrors (pictures attached). We will compare the spot positions to the scatter plots for these mirrors and try to steer the beam spots accordingly.
(*Note: We did see some stray light scattering on the edge of FM2. We will further examine this and see where the stray light is coming from.)

 

Plans for this week:
1) Verify electronics setup for cavity locking.
     Current state: Output from function generator is going directly to laser driver (not currently using the Newport Servo Module). We will need to set up the servo module.

2) Finish setting up the table optics so we can monitor the reflected beam (need to set up photodiode and CCD camera to monitor this beam).

3) Once the cavity is locked, we can use the steering mirrors to optimize the cavity while monitoring the CCD cameras for the transmitted and reflected beams. We need to walk the beam so that more of the TEM00 mode is transmitted. (Currently still seeing a lot of higher order modes transmitted, as shown in attachment).

[Camille, Thejas]
10 August 2023

We want to evaluate the loss in the cavity, so we recorded the power measurements needed to enter into the power budget analysis (see attached picture with recorded values). We collected three sets of measurements to be averaged.

The screenshot below shows the output from the python analysis. (OMC throughput values are surprisingly high: 100.0%, 103.3%, and 99.8%) We plan to try to improve the mode-matching and re-evaluate the power throughput.

[Camille, Thejas]
16 August 2023

We reconvened in the afternoon to begin realignment of the beam path through the mode-matching lenses. Before doing so, we placed two iris to mark our current beam path to the OMC. (one iris after the steering mirror, one iris right in front of the OMC (picture attached))
We made a few slights adjustments to the fiber coupler and the lenses: We used a level to adjust the height of the second lens so that it is at the same height as the first lens. We slightly adjusted the height of the fiber coupler mount so that the fiber height matches the height of the center of the lenses. We translated the fiber coupler slighly to adjust the centering while maintaining the same distance from the first lens.
After centering the path through the lenses, we repositioned the periscope mount and the steering mirror accordingly so that the beam path hits the centers of these mirrors.

Tomorrow, we will lock the cavity and repeat power measurements to determine if there is any improvement to the mode-matching.

[Camille, Thejas]
We rechecked the alignment this morning and re-optimized the resonance a bit. There was some horizontal drift in the alignment from yesterday.
 

Plan for this afternoon:
Camille, Thejas, and Masayuki will meet with Koji in the OMC lab to go over electronics setup. (Need to set up EOM driver? Laser is not sweeping.)

[Camille, Thejas]
7 August 2023

After optimizing the alignment, we used the CCD video cameras to observe the beam spots on CM1 and CM2 (pictures attached). For both mirrors the beam spot is off center in the direction away from FM1 and FM2.
In order to steer the beam axis towards FM1 and FM2, we rotated CM1 clockwise (as viewed from above) and we rotated CM2 counter-clockwise (as viewed from above).
We aimed to steer the beam axis by 1mm, so we displaced the micrometers on both curved mirrors by 10um.
After displacing the micrometers, we attempted to recover TEM00 using only the fiber coupler and the steering mirrors, but we were unable to achieve this. We made an additional adjustment to the CM2 micrometer so that the beam reflected from CM2 would better overlap the incoming beam on FM1. After doing this, we were able to observe TEM00 and proceeded to lock and optimize using the fiber coupler and steering mirrors. (REFL PD was 0.12V, REFL CCD looked similar to the one in https://nodus.ligo.caltech.edu:8081/OMC_Lab/597.
We observed the beam spots on CM1 and CM2 again and didn't observe any noticeable change (pictures attached).

We repeated this process. (We rotated CM1 clockwise and CM2 counter-clockwise each by the same amount.) We tried to recover TEM00 using only the steering mirrors and fiber coupler, but were unable to find TEM00. We made an additional small adjustment to the CM2 micrometer and found TEM00. After locking and optimizing, the beam spots on the curved mirrors still appear to be in the same locations.

The spots look quite bright on the CMs, we need to move the spots towards the center of the mirorrs to avoid scatter points.

[Camille, Thejas, Masayuki]
8 August 2023

We continued our efforts to steer the beam spots on CM1 and CM2 towards FM1 and FM2. We rotated CM1 clockwise by displacing the micrometer 20um (2 small divisions). We rotated CM2 counter-clockwise by the same amount by displacing the micrometer 20um.
We then walked the beam with the periscope until we recovered the TEM00 mode. Once recovered, we locked the cavity and continued to optimize using the periscope and the fiber coupler output. We checked the beam spot positions with an IR card and viewer and recorded images with the CCD video camera (images attached).

We also monitored the transmission PD on the scope (green trace in attached picture, pink trace is the laser freq sweep) to compare the voltage signal from TEM00 (4.3V) and the voltage signal from the higher-order mode (76mV). (1.7% from mode-mismatch = 76 mV *100 / 4.3 V + 0.76 mV) This means we should be able to resonate the cavity at 0.017*3.4 V ~ 70 mV of refl power with the current level of mode-mathcing

Our refl power (refl.jpeg) shows significant relfection of about 400 mV, suggests the cavity is lossy. (See scatter on CM1). A separate elog will look at the scatter plot.

For this afternoon, we will plan to clean the optic surfaces with First Contact and see if this reduces the cavity loss.

Summary of Zygo setups

Initial Zygo Setup:
Our initial Zygo setup consisted of a flat transmission sphere with the 0.5" curved mirror mounted against a 1" flat mirror.
Mounting procedure:
The bottom part of the gluing fixture was attached to a mounting plate using two screws. The 1" reference flat was placed on the gluing fixture. The reference flat was inspected with a green flashlight to ensure that there was no dust on the mirror surface. Any dust was removed using top gun. If any dust remained after using top gun, it was removed with a swab.
The back surface of the curved mirror was inspected and cleaned using the same method (flashlight inspection, followed by top gun if necessary, followed by swab if necessary).

After ensuring that both surfaces are clean, the back surface of the curved mirror was placed on the front surface of the reference flat. The fiducial of the curved mirror was positioned at 12:00. (12:00 is defined as the top of assembly.) The two mirrors were held in place using a mounting plate with a 0.4" aperture. The mounting plate was fixed to the bottom part of the gluing fixture using two screws and a spring for each screw (see attached picture).

The mounting plate holding this assembly was then attached to a optical mount with tip/tilt adjustments (see attached picture).
This assembly was placed facing the Zygo transmission flat (see attached picture) and the mount was pitched/yawed until the fringes on the 1" reference flat were nulled. After nulling the fringes, the data was then recorded.

The mounting plate was then removed from the tip/tilt mount and dissassembled so that the curved mirror could be rotated so that the fiducial is in the 3:00 position. The procedure is then repeated and the data recorded.
This was repeated again with the fiducial in the 6:00, 9:00 and 12:00 (again) positions.

Review of this data shows that the positions of the curvature minimums was not reproducible with sufficient precision. A teflon mounting plate was added to clamp the 1" reference flat more securely to the gluing fixture (See attached pictures). Data was collected in the same manner (twice with the fiducial at 12:00 and once with fiducial at each of the positions 3:00, 6:00, and 9:00).
Additional data collected still failed to produce reproducible results and the removing/remounting process of the curved mirror was time-consuming, so we attempted a new setup for the Zygo measurments.

Final Zygo Setup:
The new setup used a fold mirror mounted at 45degrees to direct the Zygo beam downwards into the plane of the table. A 3" flat was used as our reference flat. The reference flat was placed on some lens tissue parallel to the plane of the table. The same inspection and cleaning method was used to ensure that there was no dust on the reference flat (flashlight inspection, followed by top gun if necessary, followed by swabbing if necessary).
The back of the curved optic was inspected and cleaned using the same method. The curved mirror was placed on the 3" reference flat with the fiducial at the 12:00 position. (12:00 here is defined as the direction ponting towards the Zygo instrument.) (See attached picture of this setup.)
The fold mirror was pitched/yawed so that the fringes on the 3" reference flat were nulled. (An additional advantage of this setup is that more surface of the reference flat was viewable.) After nulling the fringes, the curved mirror was picked up and replaced a few times to verify that the fringe pattern on the curved mirror appeared reproducible. The data was collected with the fiducial at the 12:00 position. This process was repeated with the fiducial at 3:00, 6:00, 9:00, and again at 12:00.
Results from this setup were reproducible and this setup was used to measure the surface profile of all the curved mirrors.

[Camille, Koji]

Friday, Nov 11th, 2022

Setting up OMC #1 for transmission measurements:

The laser beam was aligned to the OMC cavity. The OMC cavity was locked and the transmission measurements were recorded.

The measured total optical loss of the OMC was

1st:   0.015 +/- 0.003
2nd: 0.085  +/- 0.005
3rd:  0.0585+/- 0.0008
4th:  0.047  +/- 0.002

In avegrage the estimated loss is
Loss = 0.055 +/- 0.014

This is unchanged from the measurement at LLO after the FC cleaning
Loss = 0.053 +/- 0.010