We want to make sure the responses of the PZT actuator does not change after the EP30-2 gluing.

A shadow sensor set up was quickly set-up at the fiber output. It turned out the ring PZTs are something really not-so-straightforward.
If the PZT was free or just was loosely attached on a plane by double-sided tape, the actuation response was quite low (30% of the spec).
After some struggle, I reached the conclusion that the PZT deformation is not pure longitudinal but some 3-dimensional, and you need to
use a "sandwitch" with two flat surfaces with some pressue.

I turned the setup for horizontal scans to the vertical one, and put the PZT between quarter-inch spacers.
Then two more spacers are placed on the stack so that the weight applies the vertical pressure on the PZT.
This is also use ful to adjust the height of the shadow.

The calibration plot is attached. It gives us ~21k V/m.
Voltage swing of 150V results the output voltage change of ~50mV.  This is pretty close to what is expected from the spec (16nm/V).
The PZT#3 (which had the mirror glued on) showed significantly large response.

Test PZT #1: 17.4nm/V
Test PZT #2: 17.2nm/V
Test PZT #3: 30.6nm/V
UHV PZT #24: 17.6nm/V

These numbers will be checked after the heat cure of EP30-2

The fiber output was matched with the lenses on a small bread board.
The detailed configuration is found in the following elog link.

http://nodus.ligo.caltech.edu:8080/OMC_Lab/105

[Zach, Koji]

The measurement setup for the transmission measurement has been made at the output of the fiber.

- First, we looked at the fiber output with a PBS. It wasn't P-pol so we rotated the ourput coupler.
  What we found was that it wasn't actually linearly polarized.
  So the input coupler was rotated to correct it. This terribly misaligned the input coupling.
  After some iteration of rotating and aligning the input/output couplers, we obtained reasonable
  extiction ratio like 10mW vs 100uW (100:1) with 11mW incidence. (Where is the rest 0.9mW!?)

- The P-pol (transmission) out of PBS goes into the mirror. Here we tested mirror A1.
  The mirror is mounted on the prism mount supported by a rotational stage for precise angle adjustment
  We limited the input power down to 5mW so that we can remove the attenuator on the power meter.
  The reading of the power meter was fluctuating, indeed depending on MY position.
  So we decided to turn off the lighting of the room. This made the reading very stable.

  The offset of the power meter was -0.58uW

  The transmitted power for the normal incidence was 39.7uW with the incident 4.84mW.
  [39.7-(-0.58)] / [4.84*1000-(-0.58)] *10^6 = 8320 ppm

  The transmitted power for the 4deg incidence was 38.0uW with the incident 4.87mW.
  [38.0-(-0.58)] / [4.87*1000-(-0.58)] *10^6 = 7980 ppm

 cf. The specification is 7931ppm

We attached fused silica windows on the test PZTs. http://nodus.ligo.caltech.edu:8080/OMC_Lab/93

The glued assemblies were brought to Bob's bake lab for the heat cure. There they are exposed to 94degC heat for two hours (excluding ramp up/down time).

After the heat cure, we made the visual inspection.
The photos are available here.

Pre-bake
Test PZT #1: 17.4nm/V
Test PZT #2: 17.2nm/V
Test PZT #3: 30.6nm/V

Post-bake
Test PZT #1: 27.2 nm/V
Test PZT #2: 26.9 nm/V
Test PZT #3: 21.4 nm/V

Measurement precision is ~+/-20%
Spec is 14nm/V

More Ts of the mirrors were measured.

A mirror specification:
Request: 8300+/-800 ppm
Data sheet: 7931ppm

C mirror specification:
Request: 50+/-10 ppm
Data sheet: 51.48ppm or 46.40ppm

Mirror | P_Incident P_Trans  P_Offset | T_trans
       | [mW]       [uW]     [uW]     | [ppm]
-------+------------------------------+---------
A1     | 10.28    82.9       -0.205   | 8.08e3
A2     | -----     -----     ------   | ------
A3     | 10.00    83.2       -0.205   | 8.34e3
A4     | 10.05    80.7       -0.205   | 8.05e3
A5     |  9.94    81.3       -0.205   | 8.20e3
A6     | 10.35    78.1       -0.205   | 7.57e3
A7     | 10.35    77.8       -0.205   | 7.54e3
A8     | 10.30    78.0       -0.205   | 7.60e3
A9     | 10.41    84.1       -0.205   | 8.10e3
A10    | 10.35    77.3       -0.205   | 7.49e3
A11    | 10.33    77.9       -0.205   | 7.56e3
A12    | 10.34    78.7       -0.205   | 7.63e3
A13    | 10.41    85.4       -0.205   | 8.22e3
A14    | 10.34    84.4       -0.205   | 8.18e3
-------+------------------------------+---------
C1     | 10.30     0.279     -0.225   | 48.9
C2     | -----     -----     ------   | ------
C3     | 10.37     0.240     -0.191   | 41.6
C4     | 10.35     0.278     -0.235   | 49.6
C5     | 10.40     0.138     -0.235   | 35.9 => PZT assembly #2
C6     | 10.34     0.137     -0.235   | 36.0 => PZT assembly #1
C7     | 10.37     0.143     -0.229   | 35.9
C8     | 10.41     0.224     -0.237   | 44.3
C9     | 10.36     0.338     -0.230   | 54.8
C10    | 10.39     0.368     -0.228   | 57.4
C11    | 10.38     0.379     -0.209   | 56.6
C12    | 10.28     0.228     -0.238   | 45.3
C13    | 10.36     0.178     -0.234   | 39.8
-------+------------------------------+---------

• Mirror List
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/152
   
• Mirror T measurement:
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/100
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/112
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/114
   
• Curved mirror curvature center test:
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/91
   
• Curved mirror thickness:
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/50
   
• Curved mirror curvature measurement:
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/49
   
• Mirror A/B/E and mounting prism perpendicularity measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/65
   
• Mirror A/B wedge measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/60
   
• Mirror E wedge measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/66
   
• PZT wedge measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/53
   
• PZT actuator response & L2A coupling
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/102
   
• Diode test
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/73
   
• Diode QE measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/78
   
• Breadboard dimension measurement
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/27
   
• Mirror list for L1OMC
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/113
   
• Mirror list for H1OMC
  http://nodus.ligo.caltech.edu:8080/OMC_Lab/151
   

Test result of the PZTs by Valera and Ryan

PZT  Length Angle
 #   [nm/V] [urad/um]
 11  14.5   17.6
 12  13.8   17.8
 13  11.2   25.0
 14  14.5    6.6
 15  12.5   10.6

 21  14.5    9.7
 22  13.8   28.8
 23  14.5    6.8  ==> Assembly #2
 24  18.5   51.7  ==> Used for prototyping
 25  17.1   13.8
 26  14.5    6.6  ==> Assembly #1

[Zach Koji]

The first attempt not to touch the curved mirrors did not work. (Not surprising)
The eigenmode is not found on the mirror surface.

We decided to touch the micrometers and immediately found the resonance.
Then the cavity alignment was optimized by the input steering mirrors.

We got the cavity length L and f_TMS/f_FSR (say gamma, = gouy phase / (2 pi) ) as
    L=1.1347 m        (1.132m nominal)
    gamma_V = 0.219176    (0.21879 nominal)
    gamma_H = 0.219418    (0.21939 nominal)

This was already sufficiently good:
- the 9th modes of the carrier is away from the resonance 10-11 times
  of the line width (LW)
- the 13th modes of the lower f2 sideband are 9-10 LW away
But
- the 19th modes of the upper f2 sideband are 1-3 LW away
  This seems to be the most dangerous ones.
and
- The beam spots on the curved mirrors are too marginal

So we decided to shorten the cavity round-trip 2.7mm (= 0.675mm for each micrometer)
and also use the curved mirrors to move the eigenmode toward the center of the curved mirrors.

After the movement the new cavity length was 1.13209 m.
The spot positions on the curved mirrors are ~1mm too close to the outside of the cavity.
So we shortened the outer micrometers by 8um (0.8 div).
This made the spot positions perfect. We took the photos of the spots with a IR sensor card.

The measured cavity geometry is (no data electrically recorded)
    L=1.13207 m        (1.132m nominal, FSR 264.8175MHz)
    gamma_V = 0.218547    (0.21879 nominal, 57.8750MHz)
    gamma_H = 0.219066    (0.21939 nominal, 58.0125MHz)
- the 9th modes of the carrier is 11-13 LW away
- the 13th modes of the lower f2 sideband are 5-8 LW away
- the 19th modes of the upper f2 sideband are 4-8 LW away

The raw transmission is 94.4%. If we subtract the sidebands and
the junk light contribution, the estimated transmission is 97.6%.

Note:
Even if a mirror is touched (i.e. misaligned), we can recover the good alignment by pushing the mirror
onto the fixture. The fixture works pretty well!
 

T&Rs of the B mirrors and some of the E mirrors are measured.

I found that these BSs have high loss (1%~3%) . As this loss will impact the performance of the squeezer
we should pick the best ones for the DCPD path. B5, B6, and B12 seems the best ones.

Mirror | P_Incident   P_Trans     P_Refl      | T             R             loss          |
       | [mW]         [mW]        [mW]        |                                           |
-------+--------------------------------------+-------------------------------------------+
B1     | 13.80+/-0.05 7.10+/-0.05 6.30+/-0.05 | 0.514+/-0.004 0.457+/-0.004 0.029+/-0.005 |
B2     | 14.10+/-0.05 6.50+/-0.05 7.15+/-0.05 | 0.461+/-0.004 0.507+/-0.004 0.032+/-0.005 |
B3     | 13.87+/-0.05 7.05+/-0.05 6.55+/-0.05 | 0.508+/-0.004 0.472+/-0.004 0.019+/-0.005 |
B4     | 13.85+/-0.05 6.78+/-0.05 6.70+/-0.05 | 0.490+/-0.004 0.484+/-0.004 0.027+/-0.005 |
B5     | 13.65+/-0.05 6.93+/-0.05 6.67+/-0.05 | 0.508+/-0.004 0.489+/-0.004 0.004+/-0.005 |
B6     | 13.75+/-0.05 6.70+/-0.05 6.92+/-0.05 | 0.487+/-0.004 0.503+/-0.004 0.009+/-0.005 |
B7     | 13.83+/-0.05 7.00+/-0.05 6.60+/-0.05 | 0.506+/-0.004 0.477+/-0.004 0.017+/-0.005 |
B8     | 13.90+/-0.05 6.95+/-0.05 6.68+/-0.05 | 0.500+/-0.004 0.481+/-0.004 0.019+/-0.005 |
B9     | 13.84+/-0.05 6.95+/-0.05 6.70+/-0.05 | 0.502+/-0.004 0.484+/-0.004 0.014+/-0.005 |
B10    | 13.97+/-0.05 6.98+/-0.05 6.72+/-0.05 | 0.500+/-0.004 0.481+/-0.004 0.019+/-0.005 |
B11    | 13.90+/-0.05 7.05+/-0.05 6.70+/-0.05 | 0.507+/-0.004 0.482+/-0.004 0.011+/-0.005 |
B12    | 13.90+/-0.05 6.98+/-0.05 6.78+/-0.05 | 0.502+/-0.004 0.488+/-0.004 0.010+/-0.005 |
-------+--------------------------------------+-------------------------------------------+

Mirror | P_Incident   P_Trans         P_Refl       | T            R             loss          |
       | [mW]         [uW]            [mW]         | [ppm]                                    |
-------+-------------------------------------------+------------------------------------------+
E4     | 13.65+/-0.05 0.0915+/-0.0005 13.50+/-0.05 | 6703+/-44ppm 0.989+/-0.005 0.004+/-0.005 |
E12    | 13.75+/-0.05 0.0978+/-0.0005 13.65+/-0.05 | 7113+/-45    0.993+/-0.005 0.000+/-0.005 |
E16    | 13.90+/-0.05 0.0975+/-0.0005 13.30+/-0.05 | 7014+/-44    0.957+/-0.005 0.036+/-0.005 |
-------+-------------------------------------------+------------------------------------------+

Since the previous measurement showed too high loss, the optical setup was checked.
It seemed that a PBS right before the T&R measurement setup was creating a lot of scattering (halo) visible with a sensor card.

This PBS was placed to confirm the output polarization from the fiber, so it was ok to remove it.

After the removal, the R&T measurement was redone.
This time the loss distributed from 0.2% to 0.8% except for the one with 1.3%. Basically 0.25% is the quantization unit due to the lack of resolution.

At least B7, B10, B12 seems the good candidate for the DCPD BS.

The AR reflection was also measured. There was a strong halo from the main reflection with an iris and sense the power at ~.5mm distance to separate the AR reflection from anything else. Now they are all somewhat realistic. I'll elog the measurement tonight.

33.6 +/- 0.2 uW out of 39.10+/-0.05 mW was observed. The offset was -0.236uW.
This gives us the AR reflectivity of 865+/-5ppm . This meets the spec R<0.1%

Mirror | P_Incident   P_Trans      P_Refl       | T             R             loss          |
       | [mW]         [mW]         [mW]         |                                           |
---------------------------------------------------------------------------------------------
B1     | 39.10+/-0.05 19.65+/-0.05 19.25+/-0.05 | 0.503+/-0.001 0.492+/-0.001 0.005+/-0.002 |
B2     | 39.80+/-0.05 19.90+/-0.05 19.70+/-0.05 | 0.500+/-0.001 0.495+/-0.001 0.005+/-0.002 |
B4     | 39.50+/-0.05 19.70+/-0.05 19.30+/-0.05 | 0.499+/-0.001 0.489+/-0.001 0.013+/-0.002 |
B5     | 39.50+/-0.05 19.70+/-0.05 19.50+/-0.05 | 0.499+/-0.001 0.494+/-0.001 0.008+/-0.002 |
B6     | 39.55+/-0.05 19.50+/-0.05 19.95+/-0.05 | 0.493+/-0.001 0.504+/-0.001 0.003+/-0.002 |
B7     | 40.10+/-0.05 19.80+/-0.05 20.20+/-0.05 | 0.494+/-0.001 0.504+/-0.001 0.002+/-0.002 |
B8     | 40.15+/-0.05 19.80+/-0.05 20.20+/-0.05 | 0.493+/-0.001 0.503+/-0.001 0.004+/-0.002 |
B9     | 40.10+/-0.05 19.90+/-0.05 19.90+/-0.05 | 0.496+/-0.001 0.496+/-0.001 0.008+/-0.002 |
B10    | 40.10+/-0.05 19.70+/-0.05 20.30+/-0.05 | 0.491+/-0.001 0.506+/-0.001 0.002+/-0.002 |
B11    | 40.20+/-0.05 19.80+/-0.05 20.20+/-0.05 | 0.493+/-0.001 0.502+/-0.001 0.005+/-0.002 |
B12    | 40.20+/-0.05 19.90+/-0.05 20.20+/-0.05 | 0.495+/-0.001 0.502+/-0.001 0.002+/-0.002 |
---------------------------------------------------------------------------------------------

Measurement Order: DCPD2->DCPD1->QPD1->QPD2

DCPD1: 1.50mm+0.085mm => Beam 0.027mm too low

DCPD2: 1.75mm+0.085mm => Beam 0.051mm too high (...less confident)

QPD1:   1.25mm+0.085mm => Beam 0.077mm too low

QPD2:   1.25mm+0.085mm => Beam 0.134mm too low
          or 1.00mm+0.085mm => Beam 0.116mm too high

Remeasured the spot positions:

DCPD1: 1.50mm+0.085mm => Beam 0.084mm too high

DCPD2: 1.50mm+0.085mm => Beam 0.023mm too high

QPD1:   1.25mm+0.085mm => Beam 0.001mm too low

QPD2:   1.25mm+0.085mm => Beam 0.155mm too low
 

[Koji / Jeff]

This is the elog about the work on May 9th.

We made two glass brackets glue on the junk 2" mirrors with the UV glue a while ago when we used the UV bonding last time.

On May 7th:

We applied EP30-2 to the glass brackets and glued invar shims on them. These test pieces were left untouched for the night
and brought to Bob for heat curing at 94degC for two hours.

On May 9th:

We received the test pieces from Bob.

First, a DCPD mount was attached on one of the test pieces. The fasteners were screwed at the torque of 4 inch lb.
It looked very sturdy and Jeff applied lateral force to break it. It got broken at once side of the bracket.

We also attached the DCPD mount to the other piece. This time we heard cracking sound at 2 inch lb.
We found that the bracket got cracked at around the holes. As the glass is not directly stressed by the screws
we don't understand the mechanism of the failure.

After talking to PeterF and Dennis, we decided to continue to follow the original plan: glue the invar shims to the brackets.

We need to limit the fastening torque to 2 inch lb.

Measurement for pitch

The followings are the previous values before the bake
[from this entry]

- After everything was finished, more detailed measurement has been done.

- FSR&TMS (final)
  FSR: 264.963MHz => 1.13145m
  TMS(V): 58.0177MHz => gamma_V = 0.218966
  TMS(H): 58.0857MHz => gamma_H = 0.219221
  the 9th modes of the carrier is 10.8~11.7 LW away
  the 13th modes of the lower f2 sideband are 7.3~8.6 LW away
  the 19th modes of the upper f2 sideband are 2.6~4.5 LW away

 D1300507

This is the analysis of the cavity finesse data taken on  Apr/13/2013 (before baking), May/30/2013 (after baking), and Jun/02/2013 (after cleaning).
If we believe this result, baking contaminated the cavity, and the first contact removed it. That agrees with the power measurement of the transmitted light.

Analysis of the PZT scan / TF data taken on May 31st and Jun 1st.

[DC scan]

Each PZT was shaken with 10Vpp 1Hz triangular voltage to the thorlabs amp.
The amp gain was x15. Abut 4 TEM00 peaks were seen on a sweep between 0 and 10V.

The input voltage where the peaks were seen was marked. Each peak was mapped on the
corresponding fringe among four. Then the each slope (up and down) was fitted by a iiner slope.
Of course, the PZTs show hystersis. Therefore the result is only an approximation.

PZT1: PZT #26, Mirror C6 (CM1)
PZT2: PZT #23, Mirror C5 (CM2)

PZT arrangement [ELOG Entry]

PZT1:
Ramp Up        13.21nm/V
Ramp Down   13.25nm/V
Ramp Up        13.23nm/V
Ramp Down   13.29nm/V

=> 13.24+/-0.02 nm/V

PZT2:
Ramp Up        13.27nm/V
Ramp Down   12.94nm/V
Ramp Up        12.67nm/V
Ramp Down   12.82nm/V

=> 12.9+/-0.1 nm/V

[AC scan]

The OMC cavity was locked with the fast laser actuation. Each PZT was shaken with a FFT analyzer for transfer function measurments.
(No bias voltage was given)

The displacement data was readout from the laser fast feedback. Since the UGF of the control was above 30kHz, the data was
valid at least up to 30kHz. The over all calibration of the each curve was adjusted so that it agrees with the DC response of the PZTs (as shown above).

The response is pretty similar for these two PZTs. The first series resonance is seen at 10kHz. It is fairly high Q (~30).

Alignment of the H1 OMC cavity mirrors

- The cavity mirrors as well as the first steering mirror were aligned on the cavity side template.

- The locking of the cavity was not so stable as before. Some high freq (several hundreds Hz) disturbance makes the cavity
  deviate from the linear range. This can be mitigated by turning off the HEPA units but this is not an ideal condition.

- FSR and TMS were measured.

FSR: 264.305MHz
TMS(V): 58.057MHz
TMS(H): 58.275MHz

These suggest the cavity length L and f_TMS/f_FSR (say gamma, = gouy phase / (2 pi) ) as
L=1.1343 m        (1.132m nominal)
gamma_V = 0.219659    (0.21879 nominal)
gamma_H = 0.220484    (0.21939 nominal)

- the 9th modes of the carrier is away from the resonance 6-9 times of the line width (LW)
- the 13th modes of the lower f2 sideband are 11-15 LW away
- the 19th modes of the upper f2 sideband are 0.6-7 LW away

We still need precise adjustment of the gouy phase / cavity length, this was enough for the gluing of the flat mirrors

After the PZT test, the curved mirrors were aligned to the cavity again.

In order to check the height of the cavity beam, the test DCPD mount was assembled with 2mm shim (D1201467-3)
The spot position was checked with a CCD camera.

According to the analysis of the picture, the spot height is about 0.71mm lower than the center of the mount.

Cavity parameter was measured with 50V bias on PZT1 (CM1)

- PZT combination was changed: PZT1 #21 (PZT ASSY#6) / PZT2 #25 (PZT ASSY #4)

- 19th HOMs of the USB makes accidental resonance with the nominal cavity length.
  Because of the mirror astigmatism, HOMs spreads more than the design.
  In order to avoid these modes, the cavity length had to be moved from the nominal value (1.134m).

- The clearance between the fixture and the prism was limited. This prevents to shorten the cav length.
  The cavity length was made longer about 10mm.

-----

Cavity parameter obtained from the pitch misalignment

Free Spectral Range (FSR): 261.777947 +/− 0.000299 MHz
Cavity roundtrip length: 1.145217 +/− 0.000001 m
Lock offset: 1.636183 +/− 0.238442 kHz
Transverse mode spacing (TMS): 57.581950 +/− 0.000163 MHz
TMS/FSR: 0.219965 +/− 0.000001
Cavity pole (1st order modes, avg and stddev): 353.465396 +/− 0.657630 kHz
Finesse (1st order modes, avg and stddev): 370.302940 +/− 0.688585

Carrier 9th-order HOM: -8.1 line width away
Upper Sideband 13th-order HOM: 13.3 LW away
Lower Sideband 19th-order HOM: 2.2 LW away

-----

Cavity parameter obtained from the pitch misalignment

Free Spectral Range (FSR): 261.777106 +/− 0.000226 MHz
Cavity roundtrip length: 1.145220 +/− 0.000001 m
Lock offset: 0.215937 +/− 0.183434 kHz
Transverse mode spacing (TMS): 57.875622 +/− 0.000116 MHz
TMS/FSR: 0.221087 +/− 0.000000
Cavity pole (1st order modes, avg and stddev): 356.862001 +/− 0.448102 kHz
Finesse (1st order modes, avg and stddev): 366.776766 +/− 0.460598

Carrier 9th-order HOM: -4.1 line width away
Upper Sideband 13th-order HOM: 19.1 LW away
Lower Sideband 19th-order HOM: 10.8 LW away

-----

We could avoid hitting the 19th modes of the 45MHz sidebands.

First accidental hit is the 28th order modes of the lower sideband.

Red: Carrier
Blue: Upper sideband (45MHz)
Green: Lower sideband (45MHz)

Beam heights on the diodes

DCPD1: 14.459mm -> With 1.5mm shim, the beam will be 0.038mm too low.

DCPD2: 14.221mm -> With 1.25mm shim, the beam will be 0.026mm too low.

QPD1: 14.691mm -> With 1.75mm shim, the beam will be 0.056mm too low.

QPD2: 14.379mm -> With 1.5mm shim, the beam will be 0.118mm too low.

Since the middle of September, the optical tests of H1 OMC were took place.
Here is summary of the progress.

TEST1: FSR/FINESSE measurement before applying First Contact
TEST2: Power budget

MIrror cleaning with First Contact

TEST3: FSR/FINESSE measurement after First Contact application
TEST4: Power budget

TEST5: N/A

TEST6: HOM measurement @PZT V=0
TEST7: HOM measurement @PZT V=0-200

TEST8: DC response of the PZT
TEST9: AC response of the PZT

TEST10: PD/QPD alignment / output check

LHO OMC power budget

1) Deburr the bottom surfaces of the QPD housings

2) Aligned the QPDs

QPD#              QPD1       QPD2
Housing#          #004       #008
Diode#            #44        #46
Shim              1.75mm 001 1.25mm 001
-------------------------------------
Power Incident    125.7 uW  126.4 uW
Sum Out            80.1 mV   78.9 mV
Vertical Out      + 3.4 mV    0   mV
Horizontal Out    -23.7 mV  -26   mV
SEG1              -15.6 mV  -13.2 mV
SEG2              -13.1 mV  -13.3 mV
SEG3              -29.0 mV  -26.4 mV
SEG4              -23.2 mV  -26.3 mV
-------------------------------------
Spot position X   -13   um  - 0.8 um  (positive = more power on SEG1 and SEG4)
Spot position Y   +93   um +107   um  (positive = more power on SEG3 and SEG4)
-------------------------------------
Responsivity[A/W] 0.64      0.62
Q.E.              0.74      0.73
-------------------------------------

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

 shim 1.5mm 001/002

Spot positions on CM1 and CM2 scanned according to the recipes provided by the previous entry.

The best result obtained was:

Transmission from FM2: 32.7mW
Incident on BS1: 34.4mW

Reflection (Unlocked): 5.99V
Reflection (Locked): 104mV
Reflection (Dark): -7.5mV

to accomodate the spot on BS1 it had to be about a mm moved from the template.

This gives us:
- Portion of the TEM00 carrier: R = 1-(104+7.5)/(5990+7.5) = 0.981
- Raw transmission: 32.7/34.4 = 0.950
- TEM00 transmission 0.950/R = 0.969
- Excluding the transmission of BS1: 0.969/0.9926 = 0.976
  => loss per mirror ~40ppm

Spot positions were inferred from the photos

Data analysis of the FSR/TSM measruement last week.

1. FSR was measured with "the golden arches" technique.

FSR = 263.0686 MHz +/- 900Hz

Lcav = 1.1396 m --> 7.6 mm too long! (nominal 1.132m)

2. Transverse mode spacings for the vertical and horizontal modes were measured.

TMS/FSR = 0.219366 (V) / 0.220230 (H) (Predicted value with the current cavity length 0.2196/0.2202 very close!)

We want to make this to be ~0.219 (~3% less)

With the current parameters, the 19th-order lower sideband make the coincident resonance.

1. FSR was adjusted and measured with "the golden arches" technique again.

FSR = 264.8412 MHz +/- 1400Hz => Lcav = 1.13197 m. (nominal 1.132m)

2. Transverse mode spacings for the vertical and horizontal modes were measured.

TMS/FSR = 0.218144 (V) / 0.219748 (H)

This is almost perfect!

The 19th-order lower sideband hit the resonance. Next step is to glue some of the flat mirrors.

BS1/FM1/FM2 for I1OMC were glued.

FM1 had to be intentionally rotated.
FM1 had to be intentionally shifted to avoid scattering spot.

Pin: 36.3 / Ptrans: 33.7 = Raw transmission 92.8%
Vunlock = 6.30 / Vlock = 0.120

Mode matching (estim) 0.98
Loss per mirror 84ppm
Cavity transmission 0.947

ummm

Tomorrow:
- Transmission needs to be optimized
- Apply 50V to a PZT
- Cavity FSR/HOM should be optimized
- gluing

Put a cover
Return power meter / DC supply

I1OMC cavity mirrors were glued.

FSR = 264.82MHz => Lcav = 1.132m (nominal 1.132m)

TMS/FSR for Vpzt1=Vpzt2=0: 0.2185 (V) and 0.2196 (H) (nominal 0.219)

aLIGO OMC: Power Budget 2014/5/16

<<<Measured Values>>>
Input Power: 35.7 [mW]
Transmitted Power through FM2: 33.5 [mW]
Transmitted Power through CM1: 0.188 [mW]
Transmitted Power through CM2: 0.192 [mW]
Reflection PD DC output (Unlocked): 6.2 [V]
Reflection PD DC output (Locked): 0.096 [V]
Reflection PD DC output (Dark Offset): -0.00745 [V]
Assumed cavity finesse : 400.

<<<Results>>>
Input Power: 35.7 [mW]
Uncoupled light Power (Junk light + sidebands): 0.575698 [mW]
Input TEM00 Carrier Power: 35.1243 [mW]  (Ratio: 0.983874)
Cavity reflectivity (in power): 548.319 ppm
Cavity transmission (in power): 0.953756
Loss per mirror: 70.1183 ppm
FM1 power transmission: 7640.17 ppm
FM2 power transmission: 7640.17 ppm
CM1 power transmission: 43.2093 ppm
CM2 power transmission: 44.1337 ppm

3rd OMC power budget (2014/7/2)

Input power: 34.8mW

REFLPD dark offset:  -7.57mV
REFLPD unlocked: 6.22 V
REFLPD locked: 110mV

Transmitted Power: 16.8mW (T) and 15.9mW (R)
CM1 transmission: 0.176mW
CM2 transmission: 0.181mW

Cavity Finesse: 399.73

Junk light: 0.64mW (out of 34.8mW)
Coupled beam: 34.16 mW (out of 34.8mW)
Mode Matching: 0.982
Cavity reflectivity: 467ppm
Loss per mirror in ppm: 63.8ppm
Cavity transmission (for TEM00 carrier): 0.957

FM1: R = 0.992277, T = 7659.46
FM2: R = 0.992277, T = 7659.46
CM1: R = 0.999895, T = 41.5461
CM2: R = 0.999893, T = 42.7309

Compare the above number with the best result obtained during the alignment trials

Input power: 34.4mW

REFLPD dark offset:  -7.5mV
REFLPD unlocked: 5.99 V
REFLPD locked: 104mV

Transmitted Power: Total 32.7mW (T+R)
CM1 transmission: 0.194mW
CM2 transmission: 0.194mW

Cavity Finesse: 400

Junk light: 0.631mW (out of 34.4mW)
Coupled beam: 33.77 mW (out of 34.4mW)
Mode Matching: 0.982
Cavity reflectivity: 255ppm
Loss per mirror in ppm: 39.7ppm
Cavity transmission (for TEM00 carrier): 0.968

Cavity FSR/TMS measurement (2014/7/5) with PZT voltages swept from 0V to 200V (50V step)

3rd OMC, HOM diagram at PZT1=0V and PZT2=50V.

First coincidence with the carrier is the 32nd-order carrier mode. Very good.

Each PZT was swept with 0-150V 11Hz triangular wave.
Time series data for 0.2sec was recorded for each PZT.

The swept voltage at the resonances were extracted and the fringe number was counted.
Some hysteresis is seen as usual.

The upward/downward slopes are fitted by a linear line.

The average displacement is 11.3nm/V for PZT1 and 12.7nm/V.

The PZT response was measured with a FFT analyzer. The DC calibration was adjusted by the above numbers.

QPD#              QPD1       QPD2
Housing#          #006       #007
Diode#            #50        #51
Shim              1.25mm 03  1.25mm 02   (1.25mm = D1201467-10)
-------------------------------------
Power Incident    123.1-13.0 uW  124.5-8.0 uW
Sum Out            77.0 mV   82.5 mV
Vertical Out      -24.0 mV  - 8.8 mV
Horizontal Out      4.2 mV    9.0 mV
SEG1              -11.6 mV  -16.0 mV
SEG2              -12.6 mV  -18.0 mV
SEG3              -25.2 mV  -24.4 mV
SEG4              -21.4 mV  -21.4 mV
-------------------------------------
Spot position X   -21   um  -19   um  (positive = more power on SEG1 and SEG4)
Spot position Y   +102  um  +47   um  (positive = more power on SEG3 and SEG4)
-------------------------------------
Responsivity[A/W] 0.70      0.71
Q.E.              0.82      0.83
-------------------------------------

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

DCPD#             DCPD1      DCPD2
Housing#          #009       #010
Diode#            #07        #10
Shim              1.00mm 01  1.00mm 02   (1.00mm = D1201467-09)
-------------------------------------
Power Incident     11.1 mW   10.6 mW
Vout                7.65 V    7.33 V
Responsivity[A/W]   0.69      0.69
Q.E.                0.80      0.81
-------------------------------------
photo              2nd        1st

PD alignment confirmation

Backscattering reflectivity of the 3rdOMC was measured.

Attached: Measurement setup

1) A CVI 45P 50:50 BS was inserted in the input beam path. This BS was tilted from the nominal 45 deg so that the reflection of the input beam is properly dumped.
This yielded the reflectivity of the BS deviated from 45deg. The measured BS reflectivity is 55%+/-1%.

2) The backward propagating beam was reflected by this BS. The reflected beam power was measured with a powermeter.

3) The powermeter was aligned with the beam retroreflected from the REFL PDH and the iris in the input path. The iris was removed during the measurement
as it causes a significant scatter during the measurement.

4) While the cavity was either locked or unlocked, no visible spot was found at the powermeter side.

The input power to the OMC was 14.6mW. The detected power on the powermeter was 66.0+/-0.2nW and 73.4+/-0.3nW with the cavity locked and unlocked, respectively.
This number is obtained after subtraction of the dark offset of 5.4nW.

Considering the reflectivity of the BS (55+/-1%) , the upper limit of the OMC reflectivity (in power) is 8.18+/-0.08ppm and 9.09+/-0.09ppm for the OMC locked and unlocked respectively. Note that this suggests that the REFL path has worse scattering than the OMC cavity but it is not a enough information to separate each contribution to the total amount.

Impact on the OMC transmission RIN in aLIGO:

- The obtained reflectivity (in power) was 8ppm.
- For now, let's suppose all of this detected beam power has the correct mode for the IFO.
- If the isolation of the output faraday as 30dB is considered, R=8e-9 in power reaches the IFO.
- The IFO is rather low loss when it is seen as a high reflector from the AS port.
- Thus this is the amount of the light power which couples to the main carrier beam.

When the phase of the backscattered electric field varies, PM and AM are produced. Here the AM cause
the noise in DC readout. Particularly, this recombination phase is changing more than 2 pi, the fringing
between the main carrier and the backscattered field causes the AM with RIN of 2 Sqrt(R).

Therefore, RIN ~ 2e-4 is expected from the above of backscattering.

Now I'm looking for some measurement to be compared to with this number.

First, I'm looking at the alog by Zach: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=8674

I'm not sure how this measurement can be converted into RIN. Well, let's try. Zach told me that the measured value is already normalized to RIN.
He told me that the modulation was applied at around 0.1Hz. The maximum fringe velocity was 150Hz from the plot.
At 100Hz, let's say, the RIN is 2e-6 /rtHz. The fringe speed at 100Hz is ~70Hz/sec. Therefore the measurement stays in the 100Hz freq bin
only for delta_f/70 = 0.375/70 = 5.3e-3 second. This reduces the power in the bin by sqrt(5.3e-3) = 0.073.

2e-6 = 2 sqrt(R) *0.73 ==> R = 2e-10

This number is for the combined reflectivity of the OMC and the OMC path. Assuming 30dB isolation of the output Faraday
and 20% transmission of SRM, the OMC reflectivity was 5e-6. This is in fact similar number to the measured value.

If I look at the OMC design document (T1000276, P.4), it mentions the calculated OMC reflection by Peter and the eLIGO measurement by Valera.
They suggests the power reflectivity of the order of 1e-8 or 1e-7 in the worst case. This should be compared to 8ppm.
So it seems that my measurement is way too high to say anything useful. Or in the worst case it creates a disastrous backscattering noise.

So, how can I make the measurement improved by factor of 100 (in power)

- Confirm if the scattering is coming from the OMC or something else. Place a good beam dump right before the OMC?

- Should I put an aperture right before the power meter to lmit the diffused (ambient) scatter coming into the detector?
  For the same purpose, should I cover the input optics with an Al foil?

- Is the powermeter not suitable for this purpose? Should I use a PD and a chopper in front of the OMC?
  It is quite tight in terms of the space though.

- Any other possibility?

Presence of the misaligned SRM (T=20%) was forgotten in the previous entry.
This effectively reduces the OMC reflectivity by factor of 25.

This is now reflected in the original entry. Also the argument about the power spectram density was modified.

Backscatter measurement ~ 2nd round

Summary

- The backscatter reflectivity of the 3rd OMC is 0.71 ppm

- From the spacial power distribution, it is likely that this is not the upper limit but the actual specular spot from the OMC,
propagating back through the input path.

Improvement

- The power meter was heavily baffled with anodized Al plates and Al foils. This reduced many spourious contributions from the REFL path and the input beam path.
  Basically, the power meter should not see any high power path.

- The beam dump for the forward going beam, the beamsplitter, and the mirrors on the periscope were cleaned.

- The power meter is now farther back from the BS to reduce the exposed solid angle to the diffused light

- The REFL path was rebuilt so that the solid angle of the PD was reduced.

Backscattering measurement

- P_in = 12.3 +/- 0.001 [mW]

- R_BS = 0.549 +/- 0.005

- P_back = 4.8 +/- 0.05 [nW] (OMC locked)       ==> R_OMC(LOCKED) = 0.71 +/- 0.01 [ppm]

- P_back = 3.9 +/- 0.05 [nW] (OMC unlocked)   ==> R_{OMC(UNLOCKED)} = 0.57 +/- 0.01 [ppm]

Note that the aperture size of Iris(B) was ~5.5mm in diameter. 

V-dump test

- Additional beam dump (CLASS A) was brought from the 40m. This allowed us to use the beam dump before and after the periscope.

- When the beam dump was placed after the periscope: P = 0.9+/-0.05nW

- When the beam dump was placed before the periscope: P=1.0+/-0.1nW

===> This basically suggests that the periscope mirrors have no contribution to the reflected power.

- When the beam dump was placed in the REFL path: P=2.1+/-0.1nW

Trial to find backward circulating beam at the output coupler

The same amount of backreflection beam can be found not only at the input side of the OMC but also transmission side.
However, this beam is expected to be blocked by the beamsplitter. It was tried to insert a sensor card between the output coupler
and the transmission BS, but nothing was found.

In order to see if the detected power is diffused light or not, the dependence of the detected light power on the aperture size was measured.
Note that the dark offset was nulled during the measurement.

IRIS B
aperture   detected
diameter   power
[mm]       [nW]
 1.0        1.1
 2.5        2.6
 4.25       4.0
 5.5        4.6
 8.0        5.3
 9.0        6.1
11.0        6.3
15.0        7.0

We can convert these numbers to calculate the power density in the each ring. 
(Differentiate the detected power and aperture area. Calculate the power density in each ring section, and plot them as a function of the aperture radius)

This means that the detected power is concentrated at the central area of the aperture.
(Note that the vertical axis is logarithmic)

If the detected power is coming from a diffused beam, the power density should be uniform.
Therefore this result strongly suggests that the detected power is not a diffused beam but
a reflected beam from the OMC.

According to this result, the aperture size of 2.6mm in raduis (5.5mm in diameter) was determined for the final reflected power measurement.

Summary

- The breadboard has a resonance at 1.2kHz. The resonant freq may be chagned depending on the additional mass and the boundary condition.

- There is no forest of resonances at around 1kHz. A couple of resonances It mainly starts at 5kHz.

- The PZT mirrors (CM1/CM2) have the resonance at 10kHz as I saw in the past PZT test.

Motivation

- Zach's LLO OMC characterization revealed that the OMC length signals have forest of spikes at 400-500Hz and 1kHz regions.

- He tried to excite these peaks assuming they were coming from mechanical systems. It was hard to excite with the OMC PZT,
but actuating the OMCS slightly excited them. (This entry)

Because the OMC length control loop can't suppress these peaks due to their high frequency and high amplitude, they limit
the OMC residual RMS motion. This may cause the coupling of the OMC length noise into the intensity of the transmitted light.
We want to eventually suppress or eliminate these peaks.

By this vibration test we want to:

- confirm whether the peaks are coming from the OMC or not.
- identify what is causing the peaks if they are originated from the OMC
- correct experimental data for comparison with FEA

Method

- Place a NOLIAC PZT on the object to be excited.
- Look at the actuation signal for the OMC locking to find the excited peaks.

Results

Breadboard

- This configuration excited the modes between 800-1.2kHz most (red curve). As well as the others, the structures above 5kHz are also excited.

- The mode at 1.2kHz was suspected to be the bending mode of the breadboard. To confirm it, metal blocks (QPD housing and a 4" pedestal rod)
  were added on the breadboard to change the load. This actually moved (or damped) the mode (red curve).

- Note that the four corners of the breadboard were held with a PEEK pieces on the transport fixture.
  In addition, the installed OMC has additional counter balance mass on it.
  This means that the actual resonant frequency can be different from the one seen in this experiment. This should be confirmed with an FEA model.
  The breadboard should also exhibit higher Q on the OMCS due to its cleaner boundary condition. 

DCPD / QPD

- Vibration on the DCPDs and QPDs mainly excited the modes above 3kHz. The resonances between 3 to 5kHz are observed in addition to the ubiquitous peaks above 5kHz.
  So are these coming from the housing? This also can be confirmed with an FEA model.

- Some excitation of the breadboard mode at 1.2kHz is also seen.

CM1/CM2 (PZT mirrors)

- It is very obvious that there is a resonance at 10kHz. This was also seen in the past PZT test. This can be concluded that the serial resonance of the PZT and the curved mirror.
- There is another unknown mode at around 5~6kHz.

- Some excitation of the breadboard mode at 1.2kHz is also seen.

FM1/FM2 and Peripheral prism mirrors (BSs and SMs)

- They are all prism mirrors with the same bonding method.

- The excitation is concentrated above 5kHz. Small excitation of the breadboard mode at 1.2kHz is also seen. Some bump ~1.4kHz is also seen in some cases.

Beam dumps

- The excitation is quite similar to the case of the peripheral mirrors. Some bump at 1.3kHz.

Other tapping test of the non-OMC object on the table

- Transport fixture: long side 700Hz, short side 3k. This 3K is often seen in the above PZT excitation

- Fiber coupler: 200Hz and 350Hz.

- The beam splitter for the back scattering test: 900Hz

Improved vibration measurement of the OMC

Improvement

- Added some vibration isolation. Four 1/2" rubber legs were added between the OMC bread board and the transport fixture (via Al foils).
  In order to keep the beam height same, 1/2" pedestal legs were removed.

- The HEPA filter at the OMC side was stopped to reduce the excitation of the breadboard. It was confirmed that the particle level for 0.3um
  was still zero only with the other HEPA filter.

Method

- Same measurement method as the previous entry was used.

Results

Breadboard

- In this new setup, we could expect that the resonant frequency of the body modes were close to the free resonances, and thus the Q is higher.
  Noise is much more reduced and it is clear that the resonance seen 1.1kHz is definitely associated with the body mode of the breadboard (red curve).

  As a confirmation, some metal objects were placed on the breadboard as tried before. This indeed reduced the resonant frequency (blue curve).

DCPD / QPD

- Vibration on the DCPDs and QPDs mainly excited the modes above 2~3kHz.
  In order to check if they are coming from the housing, we should run FEA models.

- Some excitation of the breadboard mode at 1.1kHz was also seen.

CM1/CM2 (PZT mirrors)

- Baseically excitation was dominated by the PZT mode at 10kHz. Some spourious resonances are seen at 4~5kHz but I believe this is associated with the weight placed on the excitation PZT.

FM1/FM2 and peripheral prism mirrors (BSs and SMs)

- The modes of the FMs are seen ~8k or 12kHz. I believe they are lowered by the weight for the measurement. In any case, the mode frequency is quite high compared to our frequency region of interest.

- As the prism resonance is quite high, the excitation is directly transmitted to the breadboard. Therefore the excitation of the non-cavity caused similar effect to the excitation on the breadboard.
  In fact what we can see from the plot is excitation of the 1.1kHz body mode and many high frequency resonances.

Beam dumps

- This is also similar to the case of the peripheral mirrors.

Prisms

Fundamental: 12.3kHz Secondary: 16.9kHz

DCPDs

Fundamental: 2.9kHz Secondary: 4.1kHz

QPDs

Fundamental: 5.6kHz Secondary: 8.2kHz

L1 OMC Cavity power budget

H1 OMC Cavity power budget

3IFO OMC Cavity power budget

Structural analysis of the PZT mirror with COMSOL.

Inline figures: Eigenmodes which involves large motion of the tombstone. In deed 10kHz mode is not the resonance of the PZT-mirror joint, but the resonance of the tombstone.

Attached PDF: Simulated transfer function of the PZT actuation. In order to simulate the PZT motion, boundary loads on the two sides of the PZT were applied with opposite signs.
10kHz peak appears as the resonance of the tombstone dominates the mirror motion. At 12kHz, the PZT extension and the backaction of the tombstone cancells each other and
the net displacement of the mirror becomes zero.

Motivation: Characterize the loss of the Calcite Brewster PBS.

Setup: (Attachment 1)

- The beam polarization is rotated by an HWP
- The first PBS filters out most of the S pol
- The second PBS further filters the S and also confirms how good the polarization is.

- The resulting beam is modulated by a chopper disk. The chopping freq can be 20~1kHz.

- The 50:50 BS splits the P-pol beam into two. One beam goes to the reference PD. The other beam goes to the measurement PD.

- Compare the transfer functions between RefPD and MeasPD at the chopping frequency with and without the DUT inserted to the measurement pass.

- The PBS shift the beam significantly. The beam can't keep the alignment on the Meas PD when the crystal is removed.
  Therefore the "On" and "Off" states are swicthed by moving the PBS and the steering mirror at the same time.
  The positions and angles of the mounts are defined by the bases on the table. The bases are adjusted to have the same spot position for these states as much as possible.

Device Under Test:

Brewster polarizer https://dcc.ligo.org/LIGO-T1300346

The prisms are aligned as shown in Attachment 2

Between the prisms, a kapton sheet (2MIL thickness) is inserted to keep the thin air gap between them.

Result:

Set1: (~max power without hard saturation)
PD1(REF) 10dB Gain (4.75kV/A) 6.39V
PD2(PBS) 10dB Gain (4.75kV/A) Thru 4.77V, PBS 4.75
Chopping frequency 234Hz, FFT 1.6kHz span AVG 20 (1s*20 = 20s)

Thru 0.748307, PBS 0.745476 => 3783 +/- 5 ppm loss
Thru 0.748227, PBS 0.745552 => 3575 +/- 5 ppm
Thru 0.748461, PBS 0.745557 => 3879 +/- 5 ppm
Thru 0.748401, PBS 0.745552 => 3806 +/- 5 ppm
Thru 0.748671, PBS 0.745557 => 4159 +/- 5 ppm
=> Loss 3841 +/- 2 ppm

Set2: (half power)
PD1(REF) 10dB Gain (4.75kV/A) 3.20V
PD2(PBS) 10dB Gain (4.75kV/A) Thru 2.38V, PBS 2.37
Chopping frequency 234Hz, FFT 1.6kHz span AVG 20 (1s*20 = 20s)

Thru 0.747618, PBS 0.744704 => 3898 +/- 5 ppm loss
Thru 0.747591, PBS 0.744690 => 3880 +/- 5 ppm
Thru 0.747875, PBS 0.744685 => 4265 +/- 5 ppm
Thru 0.747524, PBS 0.744655 => 3838 +/- 5 ppm
Thru 0.747745, PBS 0.744591 => 4218 +/- 5 ppm
=> Loss 4020 +/- 2 ppm

Set3: (1/4 power)
PD1(REF) 10dB Gain (4.75kV/A) 1.34V
PD2(PBS) 10dB Gain (4.75kV/A) Thru 1.00V, PBS 0.999
Chopping frequency 234Hz, FFT 1.6kHz span AVG 20 (1s*20 = 20s)

Thru 0.745140, PBS 0.741949 => 4282 +/- 5ppm loss
Thru 0.745227, PBS 0.741938 => 4413 +/- 5ppm
Thru 0.745584, PBS 0.741983 => 4830 +/- 5ppm
Thru 0.745504, PBS 0.741933 => 4790 +/- 5ppm
Thru 0.745497, PBS 0.741920 => 4798 +/- 5ppm
Thru 0.745405, PBS 0.741895 => 4709 +/- 5ppm
=> Loss 4637 +/- 2ppm

Possible improvement:

- Further smaller power
- Use the smaller gain as much as possible
- Compare the number for the same measurmeent with the gain changed

- Use a ND Filter instead of HWP/PBS power adjustment to reduce incident S pol
- Use a double pass configuration to correct the beam shift by the PBS

To be measured

- Angular dependence
- aLIGO Thin Film Polarizer
- HWP
- Glasgow PBS

Calcite Brewster PBS Continued

The transmission loss of the Calcite brewster PBS (eLIGO squeezer OFI) was measured with different conditions.
The measured loss was 3600+/-200ppm. (i.e. 900+/-50 ppm per surface)
The measurement error was limited by the systematic error, probably due to the dependence of the PD response on the spot position.

I wonder if it is better to attenuate the beam by a ND filter instead of HWP+PBS.

o First PBS power adjustment -> full power transmission, OD1.0 ATTN Full Power
   PDA20CS Gain 10dB
   Thru 0.746711, PBS 0.744155 => Loss L = 3423 +/- 5ppm

o Same as above, PDA20CS Gain 0dB (smaller amplitude = slew rate less effective?)
   Thru 0.748721, PBS 0.746220 => L = 3340 +/- 5ppm

o Same as above but OD1.4 ATTN
   Thru 0.744853, PBS 0.742111 => L = 3681 +/- 5ppm

o More alignment, more statistics
(PDA20CS 0dB gain =  0.6A/W, 1.51kV/A)
PD(REF, 0dB) 0.426V = 0.47W
PD(MEAS, 0dB) Thru 0.320V, PBS 0.318V = 0.35W, L = 6000+/-3000ppm

Chopping 234Hz, TF 1.6kHz AVG10
Thru 0.745152, PBS 0.742474 => 3594 +/- 5 ppm
Thru 0.745141, PBS 0.742467 => 3589 +/- 5ppm
Thru 0.745150, PBS 0.742459 => 3611 +/- 5ppm
Thru 0.745120, PBS 0.742452 => 3581 +/- 5ppm
Thru 0.745153, PBS 0.742438 => 3644 +/- 5ppm
=> 3604ppm +/-25ppm

o More power

Attenuation OD 1.0
PD(REF, 0dB) 0.875V = 0.97W
PD(MEAS, 0dB) Thru 0.651V, PBS 0.649V = 0.72W, L = 3100+/-1600ppm

Chopping 234Hz, TF 1.6kHz AVG10
Thru 0.746689, PBS 0.743789 => 3884 +/- 5ppm
Thru 0.746660, PBS 0.743724 => 3932 +/- 5ppm
Thru 0.746689, PBS 0.743786 => 3888 +/- 5ppm
Thru 0.746663, PBS 0.743780 => 3861 +/- 5ppm
Thru 0.746684, PBS 0.743783 => 3885 +/- 5ppm
=> 3890ppm +/- 26ppm

o Much less power

Attenuation OD 2.4
PD(REF, 0dB) 67.1mV = 74.0mW
PD(MEAS, 0dB) Thru 53.7V, PBS 53.5V = 59mW, L = 3700+/-1900ppm

Thru 0.745142, PBS 0.742430 => 3640 +/- 5ppm
Thru 0.745011, PBS 0.742557 => 3294 +/- 5ppm
Thru 0.744992, PBS 0.742537 => 3295 +/- 5ppm
Thru 0.745052, PBS 0.742602 => 3288 +/- 5ppm
Thru 0.745089, PBS 0.742602 => 3338 +/- 5ppm
=> 3371ppm +/- 151ppm

o Much less power, but different gain

Attenuation OD 2.4
PD(REF, 20dB) 662mV = 73.1mW
PD(MEAS, 20dB) Thru 501V, PBS 500V = 55.3mW, L = 2000+/-2000ppm

Thru 0.744343, PBS 0.741753 => 3480 +/- 5ppm
Thru 0.744304, PBS 0.741739 => 3446 +/- 5ppm
Thru 0.744358, PBS 0.741713 => 3553 +/- 5ppm
Thru 0.744341, PBS 0.741719 => 3523 +/- 5ppm
Thru 0.744339, PBS 0.741666 => 3591 +/- 5ppm
=> 3519ppm +/- 58ppm

Using the last 4 measurements, mean loss is 3596, and the std is 218. => Loss = 3600+/-200ppm

Brewster calcite PBS (eLIGO Squeezer OFI)

Loss L = 3600 +/- 200ppm
Angular dependence: Attachment 1

In the first run, a sudden rise of the loss by 1% was observed for certain angles. This is a repeatable real loss.
Then the spot position was moved for the second run. This rise seemed disappeared. Is there a defect or a stria in the crystal?

Wave plate (eLIGO Squeezer OFI?)

Loss L = 820 +/- 160ppm
Angular dependence: Attachment 2

Initially I had the similar issue to the one for the brewster calcite PBS. At the 0 angle, the loss was higher than the final number
and high asymmetric loss (~2%) was observed in the negative angle side. I checked the wave plate and found there is some stain
on the coating. By shifting the spot, the loss numbers were significantly improved. I did not try cleaning of the optics.

The number is significantly larger than the one described in T1400274 (100ppm).

Thin Film Polarlizer (aLIGO TFP)

Loss L = 3680 +/- 140ppm @59.75 deg
Angular dependence: Attachment 3

0deg was adjusted by looking at the reflection from the TFP. The optics has marking saying the nominal incident angle is 56deg.
The measurement says the best performance is at 59.75deg, but it has similar loss level between 56~61deg.

Glasgow PBS

It is said by Kate that this PBS was sent from Glasgow.

Loss L = 2500 +/- 600ppm
Angular dependence: Attachment 4

For the Glasgow PBS, the measurement has been repeated with different size of beams.
In each case, the PBS crystal was located at around the waist of the beam.
Otherwise, the measurement has been done with the same way as the previous entries.

Beam radius [um]  Loss [ppm]
 160              5000 +/-  500
 390              2700 +/-  240
1100              5300 +/-  700
1400              2500 +/-  600 (from the previous entry)
2000              4000 +/-  350

Linearity of the EOM driver was tested. This test has been done on Nov 10, 2015.

- Attachment 1: Output power vs requested power. The output start to deviate from the request above 22dBm request.

- Attachment 2: Ctrl and Bias voltages vs requested power. This bias was measured with the out-of-loop channel.
The variable attenuator has the voltage range of 0~15V for 50dB~2dB attenuation.

Therefore this means that:

- The power setting gives a voltage logarithmically increased as the requested power increases. And the two power detectors are watching similar voltages.

- And the servo is properly working. The control is with in the range.

- Even when the given RF power is low, the power detectors are reporting high value. Is there any mechanism to realize such a condition???