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. 

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. 

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

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.

[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.

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

[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.

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]
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

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

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.

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

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.

[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.)

[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, 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, 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.

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]
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.

[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]

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 inspected surfaces of the cavity optics with a halogen lamp and noticed a particulates on optic HR surfaces. We cleaned all 4 optics with top gun, this did not seem to reduce the cavity loss upon locking. The cavity loss seemed to be higher with refl power now at 600 mV.

So we proceeded to apply First Contact on all the four optics. 

The bottom surfaces of the prisms and tombstones were swabbed with IPA. We also cleaned the breadboard positions for these four optics with an IPA swab. (The CM1 mirror looked better after cleaning (no scatter observed).)
After cleaning the optics were returned to their positions on the breadboard. The spot positions on the curved mirrors still look roughly centered, but the steering mirrors need to be optimized.
When viewed through the IR viewer, the spots on the curved optics are not as bright as they were before FC. (The brightness is more or less equal to the brightness on the FMs.)
We observed the transmission PD which was mainly dominated with TEM00 modes, and also some higher-order modes:
TEM00: 6.85V
HOMs: 0.174V and 0.054V
--> 3.2% mode-mismatch

REFL PD: 3.6V (unlocked)
REFL PD: 0.340V (locked) (We estimate we should be able to reach 0.115V for the same level of mode-matching.) (The refl mode shape looks similar to last time on the CCD (some TEM00 mode).)

This implies there is  still ~200 mV of loss from the cavity.

[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.

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

It has a higher-order mode, but you can still extract TEM00 from this beam.
e.g., See a past reflection image in Attachment 4 of this entry. In this case, it's apparently difficult to extract more TEM00 mode from this spot.

The meaning of this can be
- You need to adjust the PDH locking offset (error offset) on the servo box to tune the locking point.
- The cavity loss is so high (transmission is low), and this causes a significant amount of reflection.

[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.

[Camille, Thejas]
7 August 2023

REFL PD signal (unlocked): 3.42V
REFL PD signal (locked): 0.16V
While monitoring the REFL PD and the REFL CCD, we adjusted the the fiber coupler and the steering mirrors to optimize alignment into the cavity. Attached is the image we observed on the REFL CCD camera. This image shows some higher-order modes in the reflected beam. To further improve this, we might consider to move the lenses.

 

OMC (004) Production completed. The OMC together with the transport fixture is still placed at the 40m clean room.

The writing of this elog is still on going

[Dean, Stephen, Koji]

4th OMC production was completed.
 

Today's Menu

• PZT connector assembly
• DCPD/QPD cable installation to the DCPD/QPD housings
• Cable installation on the connector harness
• Routing of the cables / Installation of the cable ties
• Storage

== PZT connector assembly ==

Dean and Stephen already crimped the pins to the PZT wires in the previous session.
We wanted to complete this cable by inserting the pins into the mighty mouse connector
(803-003-07M6-4PN-598A). 

This MM connector has the pin1 at the top left and numbered clock wise when it is seen from the mating side with the center notch up. 

We looked at D1300589. This drawing has some inconsistency in the description (LV Piezo at the left side turns to HV Piezo at the right side), but it is not an issue as we are supposed to have two identical (or similar) PZTs.

Here the arrangement of the MM connectors:
- Pin 1:  PZT1 Pos
- Pin 2:  PZT1 Neg
- Pin 3:  PZT2 Pos
- Pin 4:  PZT2 Neg

== DCPD/QPD cable installation to the DCPD/QPD housings ==

We opened the cable bags and found that they were not completed. The cables didn't have face parts. We found the face parts in the cable parts plastic box, but the screws were missing. We had to send one to pick up screws from the OMC lab while the others were working on inserting helicoils into the cable ASSYs.

We also found the ancient elog about the cables. This told us that the cable set we had was not the proper one, but rather spares with all the long variants of the cables. This meant that the routing needed to be different from the past 3 OMCs.

The flex PCB boards have solder bumps sticking out. This prevented the front pieces from being flush with the back piece. It was slightly improved by clipping a few solder marks (Attachments 1/2). The cables were inserted into the DCPD/QPD housings. To align the connector heads properly, we installed dummy (i.e., non-high-QE PDs with caps) PDs to the housing. The QPDs were necessary to be removed from the housing as it was known that the insertion was very difficult.

DCPD cable for DCPD1 (Trans side): D1300372 S1301809
DCPD cable for DCPD2 (Refl side): D1300372 S1301807

QPD cable for QPD1 (Refl side): D1300375 S1301816
QPD cable for QPD2 (Trans side): D1300375 S1301814

Cable installation on the connector harness

We flipped the OMC (manually) so that we could work on the suspension side. The connectors were attached to the cable harness. The connector nuts were fastened by a nut spanner and/or a long-nose plier. (Attachment 3)

Routing of the cables / Installation of the cable ties

As mentioned above, some cables are longer than they used to be. So the routing required some creativity. The cables were attached to the cable pegs. We used Kapton sheets to distribute the cable tie's pressure to the wires and also protect the wires from the ties.

Storage

The OMC (004) and the transport fixture were so far placed in the 40m clean room.

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

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.

[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.)

[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.

The amount of the reflected beam is a function of the mode matching and the cavity loss.

- The noise could show up larger when the PDH lock has the error offset. Try to tune the input offset knob (which has no knob dial ;) ) to minimize the reflection DC.

- The mode matching tends to get worse if the beam is not going through the center of the two mode-matching lenses after the fiber coupler.
  If you find the beams are too far off from the centers, bring them back towards the center and use the steering mirrors after the lenses to align the beam to the cavity.

- After this, you can try to move the position of the mode matching lenses using the micrometers on the base stages. This again misaligns the beam a bit, this can be compensated by the fiber mount.

- If the large mode mismatch seems removed by the input optics, you don't need to pursue it too much. Just continue to get the satisfactory beam axis, beam spot positions, camera alignment, and reflection PD alignment. We can still make the power budget measurement with this level of mode matching.

The cavity falls out of lock very often when subjected to slight movements of objects on the optic table. On Wednesday, we tried to bring the cavity back to resonance when one of the curved mirrors' position was slightly disturbed. And now the reflected cavity power is higher than that achieved on Tuesday (see below). This means mode-matching efficiency is about 96.5%. The laser control signal and reflected PD signal also seem noiser than Tuesday (refer to attachment). We want to improve this before continuing to monitoring the beam spot positions on the mirrors. Is that a good idea or is it okay to continue with this level of mode-matching? Also the noise level is concerning, this wasn't present earlier, we want to investigate this.

[Dean, Stephen]

Unit #4 aLIGO OMC unit was transported to the 40m Bake Lab.

We unwrapped the Transport Fixture and identified the orientation which would place the Light Side up and accessible. We removed the 4 long SHCS (Dark Side up), then rolled the Transport Fixture to the Light Side Up orientation. The PEEK pads were kept in the locked state, with set screws pushed forward. We loosened the set screws, retracted the PEEK pads, lifted the transport fixture lid, and saw no issues with the breadboard. While rotating the Transport Fixture, Stephen had allowed the Transport Fixture to tilt down a little bit harder than intended.

We noticed the oxidation of the PZT lead which had happened during acetone debonding (OMC_Lab/378).

We opened and inspected the Class A bulkhead 4-pin male Mighty Mouse connectors (Glenair p/n 803-003-07M6-4SN-598A) from Bake-7594. There are quantity 13 in the bag, with approximately the right amount of pins. We also opened and inspected the PEEK cable ties, aluminum Cable Pegs D1300057-v2, and additional button head screws for cable pegs (Koji had considerately installed some, while waiting for the Cable Pegs).

Dean added, to the PZT leads, the pins for Mighty Mouse connectors using a wiped-down crimper. He then bundled the PZT leads and routed the PZT wire bundles to Dark Side of the breadboard.

Koji supplied us with the on-board DCPD and QPD cables, and we anticipate that he will bring the DCPDs when we install the on-board cables. We are ready to install the on-board cables and complete Dark Side cable routing, and we will coordinate a work session with Koji.

Photos are posted to this Google Photos album: aLIGO Unit 4 Build, Cable Routing - https://photos.app.goo.gl/LAJ1Q6Fw9X8TrUBu6

Koji mentioned that previously sarisfactory particle count was not acheived with the Med/Med HEPA filter setting once the enclosure was expanded, so a higher setting was used. This imparted noise to the cavity from the turbulent air flo. We noticed that the noise level in the laser control signal reduced with turning the HEPA filter next to the entrance to low and the current setting is Low/High. The particle count is zero (as below).

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.)

 

[Camille, Thejas, Masayuki, Koji]

Previously, we were using the function generator to drive the laser without using the servo module. On Tuesday, we incorporated the servo module (output from function generator to sweep input). Slow laser freq scan: 8Hz, 3 Vpp ramp.  We were able to see the TEM00 mode after increasing the span or by adjusting the offset from the servo module (usually between 50-90 V on PZT driver). If the span knob is set to zero, drive from the signal generator to the laser PZT driver is suppressed (This was the reason why we couldn't scan the laser frequency through the servo module last week). the EOM drive freq is ~ 31.23 MHz, 13 V 

Once the cavity was locked, we set up a photodiode to monitor the reflected beam. (Reflected beam signal was ~3.4V).

It was observed that the control signal from the servo module to the laser was noisy. HEPA air filter was the source and we reduced the speed of on of the HEPAs (close to the entrance).

Process for optimizing alignment once cavity was locked:

-First maximise the power on the reflection PD using the steering  mirror infront of the PD. 
-Use cavity steering mirrors to minimise reflected PD signal.
-When PD signal is low like ~0.3V or less, switch to fiber output alignment.
-Continue to optimize using fiber adjustment. (Best was ~80mV).
-Make sure that the reflected light is still coupling 100% into the PD using the steering miiror. 
-Check reflected beamshape. Since the OMC cavity is a critically coupled cavity the Transmitted light = Incident light, & Reflected lgiht = 0 at resonance and when the mode-matching is perfect. Since we have a some amount of light reflecting, current mode-matching efficiency = 3.4-0.080/3.4 i.e. 97.6 %. May be we can translate the mode-matching optics bench to improve this at some point. 

We then set up two more cameras so we can monitor the beam spot positions on the curved mirrors.
We also moved the CCD w/ ND filter that was monitoring the output at FM2. We moved it so that it is monitoring the leaked transmission through CM2 instead (no need to use ND filter).

Next steps for this week:
-Save data from camera showing the beam spot positions on the curved mirrors.

- Look at the scatter plots, and steer the cavity beam spots on the mirrors as needed (refer to the cavity matrix)
- Beam transmitted through the cavity should availble for incidence on the PDs at the tranmission side. If needs be steer the cavity axis to attain this.
-Measure power transmission through the cavity. The target efficiency is ...

- If qualified measure TMS, FSR else swap subassemblies. 

[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.)

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]

Steps followed to acheive this resonance:

1. Make sure the input beam is roughly aligned to the center of the flat mirrors (FMs). The height of the beam should be roughly the height of the curved mirrors

2. When you see incident spot on the CM1, use one of the beam steering mirrors to yaw the input beam such that the position the beam spot on the CM1 such that it is centered. (different from the assembly procedure)

3. Now use the micrometer on CM1 to adjust the spot on CM2 such that it's centered there. (different from the assembly procedure)

4. Now use micrometers on CM2 to merge the spot created by the beam reflected from CM2 and incident beam. 

5. Now use one of the steering mirrors and yaw the mirrors in a particular direction. Now repeat processes 3-5 unitl you see an improvement on the beam spots and they start merging. If this is not helping, change the direction in which you yawed the steerting mirror in the starting of this step.

6. At this point you should have the laser frequency scanning, say about 8 Hz, this will make the transmitted beam appear flashing (https://photos.google.com/share/AF1QipPWevx1_4bMFYei6ZQybmhTETlgGgJVTV6RMMv6gh_IxFMd0PUnmdXBu674QlhhNg/photo/AF1QipPalJGieVv80msC9x7jxCvDslgpPUaDY8JGbr5d?key=UlNGdUpQMUpiT1JMQ2ZxTXFYc2FVdTZaaTlSMm9R). Also maybe use a CCD camera at the transmission to optimize the alignment unitl you see TEM00 (there will be other modes, https://photos.google.com/share/AF1QipPWevx1_4bMFYei6ZQybmhTETlgGgJVTV6RMMv6gh_IxFMd0PUnmdXBu674QlhhNg/photo/AF1QipM1NCCZfC_N0wdzl8QnPgJPccTibM2q5LfvoUfK?key=UlNGdUpQMUpiT1JMQ2ZxTXFYc2FVdTZaaTlSMm9R)

Next week, we will

1. Lock the cavity and improve the power in the TEM00 transmission. 

2. Assess beam spot positiosn on the mirrors, take photographs. 

3. If not happy, swap the sub-assemblies. Earlier last week, we took put the subassembly with CM #30, maybe bring this back adn align the cavity. 

The bonding efforts were conducted on Thursday, July 13, 2023 (https://nodus.ligo.caltech.edu:8081/OMC_Lab/561)

The duration of the air bake in ABO A was recorded and results can be found in the graphs below. The raw data can be found in the .xlsx file.

Post-bake log (https://nodus.ligo.caltech.edu:8081/OMC_Lab/563) for reference.

While using mirror SN14 and mirror SN30 for CM1 and CM2 respectively, we monitored the two beam spots on FM2 and continued to see ~5mm of vertical displacement between the two spots. We swapped the subassembly containing SN30 for the one containing SN02 and we see that the pitch misalignment is resolved. We will proceed to lock the cavity using the following mirrors:
FM1 --> SN11
FM2 --> SN17
CM1 --> SN14
CM2 --> SN02

With CM #30 at CM1 position and CM #14 at CM2 postion, when the beam spot on the CM1 is a little lower than the central line the spot position on the CM2 is higher. This not expected given that the ROC of these mirrors is large. We swapped the position of these mirrors and the spot positons on these CMs is not consistent i.e. whne the spot on CM1 is lower the same is true for CM2. But we see that the first reflection spot from FM1 is higher than the incident beam spot on FM2. We were unable to merge these spots by walking the beam. 

The reason for this could be:

1 Positive pitch of CM #30 subassembly

2 Prism at FM1 has a positive pitch

This readjustment to the input beam involved adjusting the persicope mirorrs and sterring mirror. We estimate that this will have changed the waist position by approx. half an inch, a compensating adjustment to the position of the transport fixture will be made eventually. 

Thanks Koji

7/24/23

The cavity input was re-adjusted to make sure the beam was parallel to the breadbaard and close to the cavity axis. Beam spot on the CM1 is about a mm off from the centre, clsoer to the FM1. The opposite was seen for CM2. This is desirable. But we also observed that the spot was lower that expected (centre line) on CM1 and higher than the centre line on CM2. Tomorrow we will walk the beam optic axis a bit higher so that this is rectified and we get resonance.