[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]
11 August 2023

We cleaned the cavity optics again using the top gun and First Contact. (FC was used on the S1 and S2 sides of the FMs and on the S1 side of the CMs.) The bottom surfaces of the optics were swabbed with IPA. The breadboard was also swabbed with IPA where the optics are positioned.

The optics were returned to their same positions on the breadboard.
On Monday, we will finish slight realignments to recover TEM00 lock. We will take pictures of the beam spots and see if we can improve mode-matching before taking more power efficiency measurements.

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, 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.

[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]
18 August 2023

We continued the work started on https://nodus.ligo.caltech.edu:8081/OMC_Lab/609. The beam is well-centered through the mode-matching lenses. We used the periscope to optimize cavity alignment while locked on the TEM00 mode.

We checked the steering on the REFL PD and the TRANS PD to make sure both are aligned.
The REFL PD signal was 3.0V (unlocked) and 46mV (locked). (This is the lowest REFL power we have had with this cavity.) A picture of the REFL CCD is attached.

We also checked the intensity of the HOMs on the mode spectrum (pictures attached). The TEM00 signal was ~7.2V while the observed HOMs had a signal of 23mV and 2 mV.

We proceeded to take 2 sets of power budget measurements (measurements and screenshot attached). After running the measurements in the power analysis script, we have and OMC throughput of ~99% and mode-matching efficiency of 98%. This seems to agree better with our mode-spectrum. (The excel spreadsheet with the analysis is attached as Attachment 5.)

This is WOW!

Excellent mode matching work.

The measurement is still consistent with the low loss even with the different mode-matching level.

99.5%? The IFO commissioners will cry.

Edit: Wait a sec. The incident * mode matching = 20.14mW. This is the cavity-coupled power.
And you have the transmission of 9.78*2=19.56mW.
The ratio of these is ~97% and not 99.5%. Did I miss something?

=> Ah, understood. You have the incident power measurement with a significantly different reference voltage from the one at the transmission measurement. (4.21V vs 4.11V)
This is because the laser output power depends on the laser PZT feedback.
The quick hack to reduce this is that check the laser PZT feedback voltage (on the Thorlabs driver) right before the unlock, and bring the "output offset" close to that value after unlocking.
This brings the laser frequency back to the one during lock. At the same time, the laser freq is now close to the cavity resonance. So reading the unlocked REFL voltage, you need a bit of care.

Thanks for clarifying that. We will repeat some power measurements and check the output offset voltage to the laser.

You should proceed to the next steps. You'll need to repeat the power measurements before the bonding. Next time, the measurement procedure will be improved.

Thanks Koji, we did observe this discrepency and thought that the code normalizes the power/voltage readings to the reference power value. We will then proceed towards FSR and TMS measurements today.

During the OMC(004) assembly, the stock situation for Excelitas C30655 PDs was checked.

PD Cage H
Slot 1: Laser Components 3mm ->
Slot 2: Laser Components 3mm -> one of them is broken
Slot 3: C30655
Slot 4: Empty

PD Cage I
Slot 1: C30655 PD window unopened - Sourced from 40m
Slot 2: C30655 PD window unopened - Sourced from 40m
Slot 3: C30655 PD window unopened - Sourced from 40m
Slot 4: C30655 PD window opened - collected from photon-recycling experiment / state unknown

OMC DCPD bag
C30665 #4/#5/#6/#8/#13

- Some of the PDs were still in OMC (004) -> will be used for 40m BHR
- More PDs will be used for 40m BHR

[Camille, Thejas]
22 August 2023

We used the network analyzer to measure the FSR of the cavity using the method described in section 3.2.1 of T1500060. We locked the OMC cavity and maximized transmission the TEM00 mode. (REFL PD signal was ~45-50mV and REFL CCD looked the same as in 610). We adjusted to input offset on the servo module (REFL PD signal ~95mV) and recorded the transfer function between the modulation signal (channel R) and the transmission PD signal (channel A). (See attached picture of transfer function and phase.) We fit the FSR data to the code to get a value of 264.658982 MHz.

We also recorded the TMS of the cavity (with 0V to the PZTs). We measured the horizontal and vertical mode spacing separately. After maximizing transmission of TEM00, we then used the fiber coupler to misalign in the vertical direction first (REFL PD signal ~100). Using the network analyzer, we observed the peak at ~58 MHz. We then misaligned the mirror that steers the transmited beam to the PD. We clipped the transmitted beam so as to maximize the peak at ~58 MHz. (See attached spectrum.)
We recoved vertical alignment and then repeated this process for the horizontal direction. (See attached spectrum.)

Analysis in the next elog entry. 

File names:
test_22-08-2023_160812 --> FSR
test_22-08-2022_165728 --> TMS vertical
test_22-08-2022_170543 --> TMS horizontal
https://gla-my.sharepoint.com/personal/t_seetharamu_1_research_gla_ac_uk/_layouts/15/onedrive.aspx?id=%2Fpersonal%2Ft%5Fseetharamu%5F1%5Fresearch%5Fgla%5Fac%5Fuk%2FDocuments%2FA%2B%20OMC%20%231%2F22%5F08%5F2023&view=0

Attached are the analysis results from the measurements in 616.
FSR: 264.657354 +/- 0.003444 MHz

Pitch TMS: 58.45691858660249 MHz

Yaw TMS: 58.55821902092523 MHz

The attached plot shows the HOM spectrum with their sidebands. We see that there is overlap between TEM00 and one of the 9th-order modes which means this higher order mode will resonate with the TEM00 carrier.
We estimate that by increasing the FSR to ~266.7 MHz, we will avoid this as shown in the next attached plot (HOM_scan.pdf).
This will require us to decrease the cavity length by 16mm (4 mm each mirror). We plan begin adjusting the micrometers this afternoon.

[Camille, Thejas]

We checked the length-to-angle coupling of each PZT by monitoring the position of the transmitted beam on the CCD camera. The CCD camera was placed behind the steering mirror that guides the transmitted beam to the PD. We used a ThorLabs piezo controller to actuate the PZT.

We first tested PZT2. We increased the voltage to PZT2 in 50V increments from 0V to 150V. We did not observe any change in the position of the transmitted beam. We monitored the signal of the TRANS PD on the scope and did not see any change. (The signal was between 191-195V.) We monitored the REFL CCD and did see changes in the beamshape, which was expected (see pictures). The REFL PD signal also increased slightly with PZT actuation (see attachment).

We repeated this process for PZT1, which showed similar results (see attachment). We did not observe any movement in the position of the transmitted beam. Increasing PZT voltage shows increasing pitch misalignment in the REFL CCD and increasing REFL PD signal.

Here are the dimensions of the LED strips and their gaps.

[JC, Koji]

We've installed the LED strips on the HEPA frame. We tried not to touch the OMC there. But please check if everything is still ok.

Attachment 1: Installed LED light. Notice the room light is off. At the max brightness, it's still sufficient to work with the room light off.

Attachment 2: The strips are connected at the south side of the HEPA booth. LEDs are attached to the frame with the default double-sided tape. We can improve how the wire is fixed on the frame by more tapes.

Attachment 3: The switch is close to the TOPGUN unit. The single click does turn on/off, and the long touch makes the brightness go up and down. At the max and min brightness, it blinks.

Examination into bonding template confimred the limitation of space available to change cavity length by at least 10 mm to improve the cavity HOM spectrum. Here's an anlysis of HOM spectrum for various possible ROCs and corresponding required cavity length change for optimum HOM spectrum. 

Assume: Astigmatism: Rx-Ry = 8mm

Current cavity length: 1.132 m

Ry = measured, Desired Cavity length: <1.12476 m

Ry = 2.5 m, Desired cav length: >1.13876 m

Ry = 2.55 m, “: No change 

Ry = 2.6 m, “:<1.12

So travel range of ~ (1.138 - 1.12) / 4 = 5 mm on each CM is required. WIth a safety factor for alignemnt say we require 30 mm/4 ~ 7 mm on each curved mirror.

PZTs 31, 32, 35, 40 and 45 have had their DC responses measured (pre-reliability test) and have been through the reliability test (see 564). We still need to measure their DC responses post-reliability test.

Camille and Thejas will plan to do these measurements tomorrow afternoon (15 Sept. 2023).
Notes for setup:
-The setup will be the same as 542 and 551.
-0.5 Hz triangle wave 0-150V
-8s aquisition time (128Hz sample rate) on SRS785 spectrum analyzer
-62g washer on top of the PZT
-Same laser, collimation lenses (beam size 0.34mm at edge of washer), and photodiode
-Record photodiode response and voltage to the PZT

I went in lab today and turned the HEPA filter (clsoe to the entrance) to high since we are not doing any measurements at the moment. 

New piezo actuated micrometers from thorlabs arrived last week. 

4 x MPIA10

1 x KPS201

1 x KIM101 

One of the micrometers' fucntioning was checked (SN..229). The micrometer was checked in jogging mode and velocity control mode using the software. 

Aidan and Hanna wanted to use the Agilent 4395A Network Analyzer at the WB251.
All the connections were removed. Unrelated units including an HP FFT analyzer and the Linux PC were also removed.

The Cage H&I, as well as OMC DCPD bag, were brought to the 40m for the 40m BHD.