This measurement has been done on Dec 1st, 2015.

The phase noise added by the EOM driver was tested.

The test setup is depicted in the attached PDF. The phase of the RF detector was set so that the output is close to zero crossing as much as possible with the precision of 0.5ns using a switchable delay line box. The phase to voltage conversion was checked by changing the delayline by 1ns. This gave me somewhat larger conversion factor compared to the sine wave test using an independent signal generator. This was due to the saturation of the phase detector as the LO and RF both have similar high RF level for the frequency mixer used.

The measurement has done with 1) no EOM driver involved, 2) one EOM driver inserted in the RF path, and 3) EOM drivers inserted in both the LO and RF paths.

I could not understand why the measurement limit is so high. Also the case 2 seems too low comsidering the noise level for 1) and 3).

At least we could see clear increase of the noise between the case 1) and 3). Therefore, we can infer the phase noise added by the EOM driver from the measurements.

Note: The additional phase noise could be associated with the original amplitude noise of the oscillator and the amplitude-to-phase conversion by the variable attenuator. This means that the noise could be corellated between two EOM drivers. The true test could be done using a PLL with a quiet VCO. Unfortuantely I don't have a good oscillator sufficient for this measurement.

The dimensions of a high QE PDs was measured as well as the ones for C30665. (Attachment 4, Unit in mm)
They seemed to be very much compatible.

The PDs came with the designated case (Attachment 1). The bottom of the case has a spongy (presumably conductive) material.

Diodes have no window. Each came with an adhesive seal on it. (Attachment 2)
There is a marking of a serial at the side.

I opened one (Attachment 3). The sensitive area looks just beautiful. The seal was reapplied to avoid possible contamination.

Dark current of the HQE PD and other PDs were measured.

- The HQE PDs were loaded on the new PD transportation cages (Attachment 1)
The PDs are always shorted by a clean PD plugs. The PD element is still capped with Kapton seals.

- The assignment of the container/slot and the PDs are as follows

- The measurement has been done with KEITHLEY sourcemeter SMU2450.

- The result is shown in Attachment 2. Most of the PDs show the dark current of ~3nA at 15V bias. C1-05 and C1-07 showed higher dark current at high V region. We should avoid using them for the aLIGO purpose. I hope they are still OK at low bias V if there is no noise issue (TBC). You can not read the PD names on the plot for the nominal ones, but that's OK as they are almost equivalent.

- As a comparison, the dark current of a C30655 (serial #10) was measured. Considering a DC current due to an anbient light (although the PD was covered), the dark current of the HQE PD seems double of C30655.

- Taking an advantage of having the setup, I took the same measurement for the Laser Comp. PDs in ATF. I gave the identification as #1 and #2. #1 has full-length legs while #2 has trancated legs. As Zach reported before, they showed significantly high dark current. (Attachment 3)

On friday, I removed a glass cover of a G30655 with a PD can cutter.

When a beam shoots a Perkin Elmer/Excelitas PD, we usually observe three reflections.
We always wonder what these are.

When the glass window is illuminated by a beam, I could see two reflections. So they are the front and rear reflection from the glass windows.

Transfered for RGA scan

B4 (C1-05) -> F1
C1 (C1-07) -> F2

Direct comparison of the PD responsibities

We can expect 5% increase of the QE with the new PD.

P-pol 10deg incident

Power meter Ophir RM9C (Systematic error +/-5%)
Vbias = 6V

C30665GH (#07)
Incident: 7.12mW
Reflection: 0.413mW (=> R=5.8%)
PD output: 5.690+/-0.006V
=> Responsibity 0.799+/-0.001 A/W
=> QE = 0.931+/-0.001

HQE PD (A1-23)
Incident: 7.15mW
Reflection: 0.020+/-0.1mW (=> R=0.28%)
PD output: 6.017+/-0.007V
=> Responsibity 0.842+/-0.001 A/W
=> QE = 0.981+/-0.001

Note that there is a 5% systematic error with the power meter.

Calibration of the transimpedance

Use KEITHLEY 2450 as a calibrated current source. Model 2450 has the current source accuracy of 0.020%+1.5uA at 10mA range. For 6mA current output, the error is 3uA (0.05%).

The output of the current amp at 10³ Ohm setting was 6.0023V when -6.000mA current was applied. i.e R_trans = 1000.4 +/- 0.5 Ohm. This is a negligible level.

QE of the diodes (As of 07/30/2016)

Refer E1800372

Dark noise measurement for 6 HQEPDs and 1 C30665. All of these showed sufficiently low dark current noise levels compared with the noise level of the DCPD preamp. The measurement was limited by the input noise (ADC) noise of the FFT analyzer as the line noises were too big.

The measurement has been done with the transimpedance of 1e7. The bandwidth of the measurement was 50kHz.

For the production of the aLIGO PDs, the following transfer of the PDs were carried out
A1-23 Cage A1 -> G1
A1-25 Cage A2 -> G2

The cage A will be baked at 75degC to see if this improves AMU=64 emission.

At the same time, we will put C1-05 (F1) and C1-07 (F2) into the contamination test cavity.

Possible reduction of the QE was observed after air-bake at 75degC.

Yesterday I received Cage G from Bob for intermediate test of the PD performance after air bake but before vacuum bake.
This cage was prepared to be the production pair.

According to the ICS, https://ics-redux.ligo-la.caltech.edu/JIRA/browse/Bake-8047
the PDs were air baked at 75degC for 48 hours.

I took the PDs to my lab to check if there is any issue in terms of the performance.
- Dark current: No change observed
- Dark noise: No noise increase observed
- QE: Probably reduced by ~0.5%.

Here I attached the result of the QE measurement. I have measured the QEs of the baked ones (A1-23 and A1-25) and the reference. Since the reference PD has not been baked, this gives us the measure of the systematic effect. The reference showed the reduction of ~0.1%. Assuming this reduction came from the systematic effect of the measurement system, I observed at least 0.5% QE reduction (A1-23). Note that the previous measurement of 99.8% for A1-25 was too high and dubious. But both A1-23 and A1-25 showed ~0.4% lower QEs.

So I believe the air-baking process reduced the QE.

Another evidence was that now I could clearly see the beam spots on these air-baked-PDs with an IR viewer when the PDs were illuminated with a 1064nm beam. Usually it is difficult to see the spot on the PD. The spot on the reference PD was still dark. So this difference was very obvious. I was afraid that something has been deposited on the surface of the photosensitive element. The surface of the diodes looked still very clean when they were checked with a green LED flash light.

All survived PDs have been measured.

1. Calum and GariLynn checking the CM1 defect from the front side.
2. Same as above
3. Close up of the defect
4. Using dino-lite microscope to get a close up view of the defect from the front surface.
5. Same as 4
6. Finished for the day and setting up a safefy clamp
7. Finally a tefron cover was attached.

Initial inspection results by Calum, et al.
https://dcc.ligo.org/LIGO-E1600268

Attachment 1: PDH amp RF part (before the preamp was installed)

Attachment 2: RF-AF transmission

Attachment 3: Attachment 3: LO dependence

Attachment 4: RF amp gain (saturation)

Attachment 5: Input/output noise level

Attachment 6: Attachment 6: Preamp/DCPD out buffer AF circuit

attachment 6: DCPD preamp looks like the opamp is wired for positive feedback?

The polarities indicated in the right circuits were opposite, obviously.

POSTED to 40m ELOG

2nd optical test http://nodus.ligo.caltech.edu:8080/40m/13725

https://awiki.ligo-wa.caltech.edu/wiki/EOM_Mount_Stability

3IFO EOM dark microscope images courtesy by GariLynn and Rich

Attachment1/2: Hole #1
Attachment3/4: Hole #2
Attachment5: Hole #2

[Rich Koji]

3IFO EOM (before any modification) was tested to measure the impedance of each port.

The impedance plot and the impedance data (triplets of freq, reZ, imZ) were attached to this entry.

[Rich Koji]

The impedances of the individual components from the 3IFO EOM (before modification) were tested.
Each component was modeled by LISO. The LISO model (in PDF and txt) are attached at the end of the entry.

Coils
There are three inductors taken from the EOM unit. They showed the Q ranging from 150~300.
Their impedances are compared with the coil taken from the 9MHz port of the spare EOM (=current LHO EOM).
The inductance of the 8.7MHz inductor indicated higher L but still higher Q.

Todd made a replica of the 45.3MHz coil. He used a silver plated wire and it actually showed highest Q of ~400.

Crystal capacitance
The crystal capacitances were measured by attaching a test rig on the DB15 connector of the crystal housing. The rig was calibrated such that the impedances of the attched components on the rig were measured. They showed somewhat similar feature with parasitic resonances at ~50MHz. Above this frequnecy the capacitance went down (i.e. Abs(Z) went up). This indicates there are stray series LCR in pararrel to the crystal. Not sure what is the cause of this.

The central (24.1MHz) port showed smaller capacitance. This probably means the plates for the central port is smaller. Not sure the actual dimensions of the plates for this unit.
 

Attached please see my notes summarizing the models for the electrodes and inductors within the 3rd IFO EOM

The circuit models for the 3IFO EOM (before mods) were made using LISO.
Then the modification plan was made to make it a new LLO EOM.

Impedance data, LISO model, Mathematica files are zipped and attached at the end.

[Rich Koji]

The 9MHz port was tuned and the impedance was measured.

[Rich, Dean, Koji]

Stuffed all inductors for the new LLO EOM. As the impedances were sensitive to the positions of the inductors in the housing, they were glued with a glue gun.
Also the lid of the housing significantly change the stray capacitance and lowers the resonant frequency (meaning lowers the Q too), we decided to tune the matching circuit without the lid.

The attached plots show the measured impedances. They all look well tuned and matched. We will prepare and perform the optical measurement at the 40m.

Attached is a block diagram of the test setup used in the 40m lab to measure the modulation index of the IO modulator

[Rich Koji]

The impedances of the new LLO EOM were measured with the beat note setup at the 40m PSL (as described in the previous ELOG entry.

At the target frequencies (9.1MHz, 24.1MHz, 45.5MHz, 118.3MHz), the modulation responses were (0.09, 2.9e-3, 0.053, 0.021) rad/V.

This corresponds to the requirement for the driving power as follows.

OMC SN002 = Former LHO OMC which CM1 was destroyed by the lock loss pulse in 2016. This OMC needs to be optically tested before storage.

The test items:

• [done] FSR measurement with offset PDH locking (FM->AM conversion)
• [done] FSR/finesse measurement with the EOM RFAM injection
• [done] TMS measurement with input miaslignment and the trans RFPD misalignment: with no PZT offset
• [done] TMS measurement with input miaslignment and the trans RFPD misalignment: with PZT offsets
   
• PZT response
• Mirror cleaning
• Power budget
• Diode alignment: shim height
• PD/QPD alignment

As OMC SN002 already has the PZTs connected to the Mighty-Mouse connector, a test cable with a female mighty-mouse connector was made.

A small imperfection: When the cable was inserted to the connector shell, I forgot to mirror the pin out. Therefore the color and pin number do not match.

Mode cleaning capability of an optical fiber was measured. The conclusion is that the leakage of the non-fiber mode to the fiber output is insignificant and also practically negligible.

The tested fiber was Thorlabs 5-m Polarization Maintaining Single-Mode fiber (P3-1064PM-FC-5, PM Patch Cable, PANDA, 1064 nm, FC/APC, 5m).

The output mode cleaner was used as a mode analyzer. The fiber input was aligned and the misaligned so that the amount of higher order mode for the fiber is changed. The fiber output has been mode matched to an output mode cleaner. Therefore excess mode mismatch when the fiber input was misaligned, was accounted as the leakage higher order mode.

For each alignment state, the OMC transmission (in V), the OMC reflection (in V), and the OMC reflection with the OMC unlocked were measured. The voltages were measured with a digital multimeter (non-portable unit). With the fiber input beam aligned to the fiber, the fiber input and output powers were measured with a power meter.

With the input beam aligned
- Fiber input: 52.5 +/- 0.2 [mW]
- Fiber output: 35.5 +/- 0.2 [mW] (~68% coupling)
- Reflection PD offset: -0.00677 +/- 0.00001 [V]

- Refl PD reading with the OMC unlocked: 6.32 +/- 0.01 [V]
- Refl PD reading with the OMC locked: 0.133 +/- 0.002 [V]
- OMC Trans PD with the OMC locked: -1.72 +/- 0.01 [V] 

With the input beam misaligned
- Refl PD reading with the OMC unlocked: 3.63 +/- 0.01 [V]
- Refl PD reading with the OMC locked: 0.0752 +/- 0.001 [V]
- OMC Trans PD with the OMC locked: -1.00 +/- 0.01 [V] 

The naive mode matching was 0.9779 +/- 0.0003 and 0.9775 +/- 0.0003 without and with misalignment. We initially had roughly 17mW of non-fiber mode incident. And it was increased by roughly 15mW. For the misaligned case, the amount of the OMC-matched carrier was also reduced due to the misalignment. So the actual fiber mode cleaning effect needs more careful quantitative analysis.

The power budget at each part of the setup was modeled as shown in Attachment 1. The blue numbers are the measured values.
The factor a is the ratio of the leakage non-fiber mode into the fiber transmission.
The factor (1-b) is the mode matching of the fiber mode into the OMC mode.

and

With the calibration between the refl PD and the power meter measurement,
 

The solution of the equations is

So, the leakage of the non-fiber mode to the fiber output is insignificant. Moreover, the number is practically negligible because the mismatching between the fiber and OMC modes is of the order of percent and dominated by the aberration of the collimator (i.e. the OMC reflection looks like concentric higher-order LG modes) with the order of 1~2%.
 

Each peak of the transfer function measurement was fitted again with a complex function:

OMC (001)
History: Measurement date 2013/5/31, Installed to L1 2013/6/10~

OMC (002)
History: Measurement date 2013/10/11, Installed to L1 2013/XX

OMC (003)
History: Measurement date 2014/7/5, Stored for I1, Installed to H1 2016/8 upon damage on 002

OMC (002) after repair
History:Mirror replacement after the damage at H1. Measurement date 2019/1/10

OMC #002 Optical tests

• FSR measurement (done, 2019/1/8-9, 2019/4/1)
• TMS measurement (done, 2019/1/9)
• TMS measurement (with DC voltage on PZTs) (done, 2019/1/10)
• Cleaning (done, 2019/3/19)
• Power Budget (done, 2019/3/19, 2019/4/1)
• PZT DC response (done, 2019/3/27)
• PZT AC response (done, 2019/3/27)
• QPD alignment (done, 2019/4/5)
• DCPD alignment (done, 2019/4/4)
• Beam quality check (done, 2019/4/4)

(Backscattering test)

(Cabling / Wiring)

• (Attaching cable/mass platforms)
• (PZT cabling)
• (DCPD cabling)
• (QPD cabling)

(Baking)
(First Contact)
(Packing / Shipping)

As performed in the ELOG 202, the PZTs of the OMC 002 were tested.

DC response was measured by sweeping each PZT with 0-150V triangular voltage at 11Hz. Acquire 0.2sec of the tie series using an oscilloscope to get the PDH error, cavity transmission, and the sweep signal.

The voltage where the tranmission peaks were observed were fitted were recorded. One fringe corresponds to the displacement of 532nm. So the displacement and the applied volatagewere fitted witha linear function.

This gave the PZT response for PZT1 and PZT2 to be 14.9nm/V and 14.4nm/V.

AC response was measured with SR785. The PZT was shaken with 1~50mVpp signal with the DC offset of 5V while the OMC was locked with the feedback to the laser fast PZT. The transfer function from the applied PZT voltage to the servo output were measured. The closed loop TF was also measured to remove the effect of the servo control.  The DC levels of the responses were calibrated using the values above.

There was a concern that the transmission from CM1 has additional fringes. The shape of the transmitted beams from CM1, CM2, and FM2 (main) werecaptured with WinCamD.
Indeed CM1 and CM2 have the fringes, but it does not exist in the main transmission. So it seems that the fringes are associated with the curved mirrors. But how???

The beam height in the cavity became totally different from the previous one and the shims needed to be much thicker than before. This is probably because of the alignment of the newly-glue curved mirror.

As the beam height is 2~2.5mm higher, two shims need to be stacked. The preliminary check of the heights using the alignment disks (dummy PDs) suggested the following combinations.

QPD1(SHORT)  D1201467-03 (SN 007) + D1201467-03 (SN 008) (2.0 mm + 2.0 mm = 4 mm)
QPD2(LONG)   D1201467-01 (SN 001) + D1201467-01 (SN 002) (1.5 mm + 1.5 mm = 3 mm)
DCPD1(TRANS) D1201467-02 (SN 006) + D1201467-03 (SN 005) (1.75mm + 2.0 mm = 3.75 mm)
DCPD2(REFL)  D1201467-02 (SN 002) + D1201467-03 (SN 006) (1.75mm + 2.0 mm = 3.75 mm)

This resulted that the fixing button head socket screws for the PD housings to be replaced from 5/16" to 7/16". Stephen kept CLASS A spare screws from Jeff's time.

For the DCPD alignment, a cap-removed Excelitas 3mm InGaAs PD is used. -> This needs to be returned to the PD stock next time.

- DCPD1 was aligned using the zoomed CCD image (Attachment 1). Once the beam is aligned, the angle was tweaked to have the reflection nicely dumped by the glass beam dump (Attachment 2).

- DCPD2 was aligned too. (Attachment 2/3)

- The two housings were fastened by a torque wrench at 2 inch lb.

Next step:

Continue with the QPDs. The QPD amp was already set.

Notes:
The cable of the CCD monitor has a problem -> need to check what's wrong
The servo box probably have large offset at the output stage or somewhere (but not input stages).

QPD#              QPD1       QPD2
Housing#          #004       #008
Diode#            #44        #46
Shim              (see OMC ELOG 323)
-------------------------------------
Power Incident    252.3 uW  266.0 uW
Sum Out           174.2 mV  176.0 mV   +0.3
Vertical Out      + 4.7 mV  +19.0 mV   +0.2
Horizontal Out    -16.1 mV  - 8.0 mV   +0.0
SEG1              -52.4 mV  -53.  mV   -0.1
SEG2              -37.6 mV  -47.  mV   -0.1
SEG3              -41.8 mV  -34.  mV   -0.1
SEG4              -43.7 mV  -36.  mV   -0.1
-------------------------------------
Spot position X   +39   um  +15. um  (positive = more power on SEG1 and SEG4)
Spot position Y   - 8.1 um  -56. um  (positive = more power on SEG3 and SEG4)
-------------------------------------
Responsivity[A/W] 0.69      0.66
Q.E.              0.80      0.77
-------------------------------------

Arrangement of the segments
View from the beam
/ 2 | 1 X
|---+---|
\ 3 | 4 /

---------------

I(w,x,y) = Exp[-2 (x^2 + y^2)/w^2]/(Pi w^2/2)

(SEG_A+SEG_B-SEG_C-SEG_D)/(SEG_A+SEG_B+SEG_C+SEG_D) = Erf[sqrt(2) d/w]

d: distance of the spot from the center
w: beam width

[Koji, Philip]

Today we tested the functionality of the four remaining PZTs (11,12,13 and 22) .  Each PZT was placed within a collimated 500um beam.
Roughly half of the beam was blocked by the PZT. The PZT and a PD then acted as shadow sensor. Each PZT was tested with 0 and
150 V. The resulting power change then could be converted into a displacement of the PZT using the beam diameter.

The open light value for each of these tests was 3.25 V.

PZT 11:
0 V supply voltage     --> 1.717 V on PD
150 V supply voltage --> 1.709 V on PD
delta = 0.008 V

PZT 12:
0 V supply voltage     --> 1.716 V on PD
150 V supply voltage --> 1.709 V on PD
delta = 0.007 V

PZT 13:
0 V supply voltage     --> 1.702 V on PD
150 V supply voltage --> 1.694 V on PD
delta = 0.008 V

PZT 22:
0 V supply voltage     --> 1.770 V on PD
150 V supply voltage --> 1.762 V on PD
delta = 0.008 V

0.008 V --> 0.24% change in power on PD --> about  3.8 um displacement assuming no light which is blocked
by the PZT is hitting the PD.

We further started to drive all four PZTs over night with 100 V (half of their range) at 100 Hz.
We additionally display the impedance to ensure none of them degrades.

All four PZTs seem to be connected to Teflon coated wires. It needs to be checked if these
fulfill the vacuum compatibility requirements.

Attachment 1: Shadow sensor setup for the PZT displacement test

Attachment 2: PZT endurance test. 4 PZTs were shaken at once.

Attachment 3~5: Function generator setup 100Hz, 3.5Vpp 1.75Voffset (meant be displayed for 50Ohm load)

Attachment 6: The above setting yields 7Vpp unipolar signal @Hi-Z load

Attachment 7: The output was monitored with a 1/10 probe with the PZTs connected. This shows 10Vmax 0Vin -> Good. This photo was taken at 17:35.

Attachment 8: The test is going well @9:15 next day. (t=15.7hours = 5.6Mcycles)

Attachment 9: The test went well. The modulation was stopped @15:35. (t=21hours = 7.6Mcycles)

After having dug into the past email, it turned out that these wires were the ones already replaced from the original teflonwires. The length of them were confirmed to be ~19" (480mm). 

Similarly to OMC ELOG 349 the spot positions after the replacement of CM2 were measured (Attachment 1)
Also, the spot positions after the realignment were measured. (Attachment 2)

[koji,philip,joe,liyuan,stephen]

need to add spot positions.

Mirrors: PZT11,PZT22, A14, A5

Mode matching = 97.72%

15.66-> 15.30mW coupled.

~100uW for QPD

->15.2mW in cavity

Trans = 14.55mW -> 95.7% transmission

The flat mirrors were the ones with the most scattering, so we thought about how to improve it. We tried to move the first flat mirror by pushing it with our finger so that he beam would move along the optic. We tried this a couple of times, however the second time we moved it we lost our alignment and could not retrieve it. We looked at the mirror and we could see quite a lot of newtonian rings. We could see a small fibre on the glass bread board. We cleaned the optics base and the gbb, and we could get the alignment back. The beam was aligned to the cavity, the spots no longer hit the centre of the CM2. 

We measured the power budget again.

mode matching = 1-47/2680 = 0.9824, 98.2% mode matching

same p_normalise so

15.66-> 15.34mW coupled.

~15.24mW in cavity

transmission = 14.45, so 94.8% transmission.

Koji noticed that FM1 wasn't touching the template correctly, so he re-aligned the cavity.

Afternoon session - UV Bonding (E1300201-v1 procedure 6.4.4 "Gluing" using procedure in section 7.2 "UV Gluing")

Wiped down UV PPE, UV Illuminator, and UV Power Meter

Applied Optocast 3553-LV Epoxy to sample fused silica optics, to test quantity of glue needed and to become familiar with the process and tools. Philip and Joe each created a successful bond. Joe's had 3 visible spots in the bulk of the bond. Acetone was used to scrub some residue of epoxy from the surface near the OD, which was likely cured. Short duration exposure (seconds) to acetone at the perimeter of the bond did not yield any weakening of bond.

While test pieces were bonded, Koji was making some adjustments to the cavity alignment in preparation for gluing of the steering mirror BS1.

Koji noticed that the spring clamp was causing pitch in the BS1 mirror, so he recommended that we utilize the "restrain by allen key" technique to load the mirror during curing.

Once aligned, we tried taking the BS1 mirror out of the template and then putting it back. We did this twice and both times the cavity needed realigning (with the curved mirrors as well as the input steering periscope). Why is this? Since the mirror was touching the template it should not have become misaligned right? Maybe the template moves slightly? I think before glueing in the cavity mirrors we should find out why probably? Koji took a look and claimed that a few optics may have been unconstrained. 

Planning between Koji and Joe led to placement of 5 drops of epoxy on the BS1 surface, to match the bonding area. At this point we noticed that the template was not secured very well, by poking down on it we could see it move. This might explain why we are becoming misaligned very easily. Once the prism was back on the board, Koji used allen keys to move around the prism. This was done until we could align it again (i.t looked too pitched). The beam was aligned back into the cavity, and the UV light was used to cure the bond. The reflected DC when locked was

• pre-cured = 47mV
• cured = 55 mV

so it looks ok still.

[koji, joe]

The template or glass breadboard was wobbling, and we noticed that the caivty alignment became worse/better when it was pressed down. We saw that it was the glass breadboard, so it was fixed into the transport fixture more securely. Now its alignement didn't change when it was pressed down. We took a pzt mirror out and replaced it, the alignment din't change much so that was good. We set up posts to hold the pzt wires.

We noticed that the bottom of the mirrors were dirty, so we cleaned them, and once we were happy with the newton rings, we aligned the cavity

Took a photo of CM2, the spot is maybe 1 beam diameter vertically and horizontally from the centre, and quite a bright spot could be seen. The same problem with CM1. We thought it would be good to see a measurement of higher order mode spacing dependence on PZT DC voltage rather than doing the full characterisation since the alignment seems to change quite a lot when ever we do anything, and this cavity arrangement probably isn't very good anyway (can see scattering on both curved mirrors with the IR camera). 

did measurements of FSR, = 2.64835MHz

did HOM spacing for 0,75,150V on CM1 in pitch and yaw.

we want to come up with a work flow for how to do these measurements, and make automate parts of the analysis?

Tried a few things.

1. Replaced CM1 (PZT ASSY #10=M21+PZT#22+C12) with PZT ASSY #7 (=M1+PZT#13+C13)

We tried PZT ASSY #7 at the beginning and had the spots at almost at the top edge of the curved mirrors. As we found a particle on the bottom of the M1 prism (and removed it), I gave it a try again. Resulting spots are again very high. This results in rejecting PZT ASSY #7 and we set the combination of the PZT ASSYs as #8 (M7+P11+C11) and #10 (M21+P22+C12). This combination nominally gives the spot ~1mm above the center of the curved mirrors.

2. Swapped FM1 and FM2. Now FM1=A5 and FM2=A14.

No significant change of the scattering features on the FMs. The transmitted power was 14.85mW (Ref PD Vin = 3.42V), Reflection PD Vrefl,lock = 54.3mV and Vrefl,unlock = 2.89V (Vin=3.45V), Vrefl,offset = -6.39mV. The incident power was 17.43mW (Vin 3.69V).

==> Coupling 0.979 , OMC transmission 0.939 (This includes 0.6% loss to the QPD path) ...Not so great number

3. Built better camera setups to check the spot position and the scattering from the cavity mirrors.

Now the spot heights are fixed and safe to move the camera up for inches to obtain better views of the mirror faces. The camera was set 15" away from the mirrors with 1.5" height from the beam elevation. This is 0.1rad (~ 5 deg) and Cos(0.1)~0.995 so the distortion (compression) of the view is negligible. (Attachment) The spot photo were taken with the fixed CCD gain, the focus on the glass, and  lens aperture F=8.0. Later the focus and aperture were adjusted to have clear view of the scattring points.

The intensity of each scattering was constant at different views. I suppose this is because the scattering is coming from a spot smaller than the wavelength. The bright spots does not show any visible feature on the mirror surfaces when they were inspected with a green flash light.

CM2 has the excellent darkness and we want to keep this spot position. FM1, FM2, and CM1 showed bright scattering.

The spot at CM1 is not well centered on the mirror. And this is the way to avoid this scattering point. So let's think about to move the spot on CM1 by 1.3mm towards the center while the spot on the CM2 is fixed. Note that this is going to be done by the micrometers for CM1 and CM2.

By turning right micrometer of CM1 forward (50um = 5div = 1/10 turn) and the left micrometer of CM2 backward (60um = 6div) moves the spots on FM1, FM2, CM1, and CM2 by (0.43, 0.87, 1.3, 0)mm. This basically moves the spots toward the center of each mirror. Let's give it a try.

Experiment on 5/1
- CM1 right knob was moved 1div (10um) backward such that the spots were better centered on the mirrors 
FM1 (A5): h=-0.2mm -> 0.4mm made the spot much darker but still it has a few scattering spots.
FM2 (A14): h=-0.8mm -> 0.2mm reduced the number of spots from 2 to 1. And it is darker. The remaining spot at the center.
CM1 (C11): h=-1.3mm -> +1.0mm made the spot much darker.
CM2 (C12): h=-0.7mm -> +0.2mm remains dark.

Note: CM1 h=1mm and CM2 h~0mm are good locations. h+ is the good direction to move. Avoid h-.
FM1 and FM2 has the scat spots at the center. Want to go h+ more.

Uniformly go h+ is the good move. => This can be done by rotate CM1 positive => CM1 right knob CCW.

Improvement of the transmission from 93.9%->95.3%

- Further moved CM1 right knob 0.5div (0.5um) backward such that the spots were moved to h+ directions.
FM1 (A5): h=0.4mm -> 1.1mm (there is only one spot rather than multiple spots)
FM2 (A14): h=0.2mm -> 1.1mm (darker but multiple spots)
CM1 (C11): h=1.0mm -> 1.8mm (brighter but single spot)
CM2 (C12): h=0.2mm -> 1.5mm (dark multiple spots)

Not much improvement of the transmission but kept 95% level.

- Replaced FM1 (A5) with A1 mirror (No photo)

Good news: This did not change the cavity alignment at all.

Transmission 95.4%

- Tweaked the CM1 angle

Transmission 95.3%

=> A1 mirror does not improve the transmission much.

Next Plan: Use A5 (or something else) as FM2 and see if A14 caused the dominant loss.

(Now the CCD image is captured as a movie and the screen capture is easier!)

Various spot positions on CM1 and CM2 were tried to test how the transmission is dependent on the spot positions. CM1 has a few bright spots while CM2 shows very dark scattering most of the case. Attachment 1 is the example images of one of the best alignment that realized the transmission of ~96%. FM1 and FM2 also showed bright spots. The replacement of the FM mirrors does not improve nor degrade the transmission significantly. The transmission is still sensitive to the spot positions on the alignment. This indicates that the loss is likely to be limited by CM1.

Attachment 2 shows the distribution of the (known) scattering spots on CM1. The bright spots are distributed every ~1mm on the spot height and the beam (with beam radius of .5mmm) can't find a place where there is no prominent spots.

We will be able to examine if the transmission can be improved or not by replacing this CM1 mirror.

An Excelitas C30665 PD with the cap removed (SN07 in Case H slot #2) was exposed to the beam with the optical power of 1.4mW to 334mW.
After each illumination, the dark current and the dark noise level were tested. Also the photo image of the PD surface was taken each time.

- No significant change of the dark current after each illumination.

- No significant change of the dark noise after each illumination.

- No visible change of the surface observed.