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 were inferred from the photos

STORY:

- The cavity mirrors have scattering spots. The cavity alignment should have been scanned to find a cavity mode to have lowest loss possible.
  BTW, We only have horizontal dof for the alignment scan.

- After some struggle nice cavity mode was found. The cavity transmission was 96% for the ideally matched TEM00 carrier.

- It turned out that this imposed too much beam shift in the input beam (~2mm).

- This big shift induces a lot of trouble for the peripheral optics (PDs, QPDs, sterring mirrors).

- What should we do???

Analysis:

- The beam needed to go up between CM1 and CM2 to have the right spots on them. ("UP" is the input side of the OMC).

- This imposed the beam between FM1 and FM2 moved up. In other word, for the given alignment of the FMs by the template,
  We needed to hit the upper part of the FMs to have the spots on the CMs up.

Solution:

- The above argument suggets that the nominal beam will give us the right spots on the CMs if we rotate the FMs.
  Of course this induces the spot move on the FMs. But this should not be the issue as the most of the loss seems to come from the CMs.

- How much misalignment show we give to the FMs? We want to shift the beam by 2mm on the CMs.
  The length of the optical lever is ~0.25m. Therefore the mialignment angle should be

  theta = 2e-3/2/0.25 = 4e-3 rad = 4mrad.

  The template pad has ~20mm separation. The thickness of the shim should be 20mm*4mrad = 80um

- Our aluminum foil seems to have the thickness of 30-40um. We can't have this minimum thickness on the template pad as there is not enough compression pressure
  => Just use a single layer of Al piece to shim the FMs.

Attempt:

- The shims were inserted at the upper pads of the FMs.

- Aligned the input beam and the CMs so that the spots on the CMs are approximately recovered.

- Measure the cavity power budget

Pin: 34.7mW
Refl PD: offset = -7.5mV, unlock = 6.07V, inlock = 89.7mV

Ptrans = 32.5mW

Ptrans(CM2) = 0.181mW
Ptrans(CM2) = 0.184mW

Assume finesse of 400

==>

Pin: 34.7mW
Pjunk: 0.534mW
Pcoupled: 34.1mW

Mode matching: 98.5%

Cavity reflectivity in power: 0.00061
Cavity transmission in power: 0.951 (This is not a best number but acceptable.)
Loss per mirror: 75.4ppm

FM power refl/trans: 0.9923 / 7630ppm
CM1 power refl/trans: 0.999882 / 42.8ppm
CM2 power refl/trans: 0.999881 / 43.5ppm
Total roundtrip loss of the cavity (Loss + CM leakage): 388ppm

Result:

How much the input beam is away from the left wall of the OMC breadboard?

40.88mm from the template edge
  8.36mm between the template edge and the bread board
=> 32.52mm

How much should this number be? 32.94mm from the solidworks model => With in 0.5mm! Nice!

Next:

- Just in case plce all of the optics and check if the beam is delivered within the alignment range of the optics

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

- All of the PRISM mirrors have been glued

- 4 out of 5 beam dumps have been glued

TODO

EP30-2 gluing of the INVAR blocks for the PDs

PDs/QPDs need to be slightly lower -> order more shims

Remove the templates

Glue the last beam dump

Vibration test?

Bring the OMC to the bake lab

Vacuum baking

Bring it back to the OMC lab

Cabling / Wiring

Optical tests

Backscattering test

Packing / Shipping

All of the INVAR blocks have been glued.

I found thinner shims in the stock.

On Monday, the template will be removed.

EP30-2 7g mixed with 0.35g of 75-90um sphere

TODO

EP30-2 gluing of the INVAR blocks for the PDs

PDs/QPDs need to be slightly lower -> order more shims

Remove the templates

Glue the last beam dump

Vibration test?

Bring the OMC to the bake lab

Vacuum baking

Bring it back to the OMC lab

Cabling / Wiring

Optical tests

Backscattering test

Packing / Shipping

The bottom-side templates were removed.

The last beam dump was removed

TODO

ICS entry

Bring the OMC to the bake lab

Vacuum baking

Bring it back to the OMC lab

Cabling / Wiring

VIbratin test

Optical tests

Backscattering test

Packing / Shipping

3rd OMC FSR / Finesse measurement

RF AM was injected by detuning a HWP.

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.

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.

L1 OMC Cavity power budget

H1 OMC Cavity power budget

3IFO OMC Cavity power budget

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

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.

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.

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

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

D1102211 OMC Diode Mount Glass Block (11pcs) have been given to Calum@Downs

- About 1.5 month ago, an 700mW LWE NPRO has been brought to OMC Lab.

- The SOP can be found here.

- The base was made for the beam elevation of 3" height. Four 1" pedestals were attached to rise the beam elevation to 4".

- The output from the laser is ~740mW

- After the faraday and the BB EOM, the output is ~660mW

- After the usual struggle, the beam was coupled to the SM fiber. The output is 540mW. The coupling efficiency is >80%.

- Will proceed to the OMC cavity alignment.

FIber Input Mount 132deg
Fiber output mount 275deg
-> 525mW P: 517mW S: 8mW extinction ratio: 0.016

Working on the SN002 OMC fix. Checked the inventory. I think I am using C8 mirror as the new temporary CM1 and PZT24 as the new temporary CM2.

[Alena, Koji]

Jun 21: Alena and Koji worked on gluing of the CM1 mirror on the OMC breadboard #002. This is an irregular procedure. Usually, the PZT mirror subassembly is prepared before the mounting prism is glued on the breadboard. In this occasion, however, a PZT and a mirror are bonded on an existing prism because only the damaged mirror and still functional PZT were debonded from the mouting prism.

For this purpose, the mirror and the PZT were fixed on the mounting prism with the modified fixture set (D1600338). The original PZT was reused, and the new mirror #8 was used. The alignment of the mirror was checked OK using the cavity beam before any glue was applied. The arrow of the CM mirror is facing up.

We mixed 8g EP30-2 (it was almost like 3~4 pushes) and 0.4g glass sphere bond lining. Along with EP30-2 procedure, the bond was mixed in an Al pot and tested with 200degF (~93degC) preheated the oven for 15min. The cured bond showed perfect dryness and crispness. The bond was painted on the PZT and the PZT was placed on the fixture. Then more bond was painted on the other side of the PZT. The mirror was placed in the fixture. The spring-loaded front plate was fixed, and the breadboard was left for a day. (Attachment 1~3)

Jun 22: The fixture was removed without causing any visible delamination or void. The attachment 4~6 show how wet the joint is (before baking). There were some excess of EP30-2, which bonded the fixture and the mounting prism as usual. The fixture was detached by prying the front piece against the rear piece with a thin allen key. Some of the excess bond on the mounting prism was removed by scratching.

The alignment of the cavity was checked with the cavity beam and it is still fine.

More photos can be found here: Link to Google Photos Album "OMC #002 Repair - CM1 gluing"

I wondered why the black glass beam dumps looked not as shiny as before. It was in fact a layer of dusts (or contaminants) accumulated on the surface.
The top part of the internal surface of the black glass was touched by a piece of lens tissue with IPA. The outer surface was already cleaned. IPA did not work well i.e. Required multiple times of wiping. I tried FirstContact on one of the outer surface and it efficiently worked. So I think the internal surfaces need to be cleaned with FC.

[Alena, Koji]

Report of the work on June 30.

1. Cleaning of the black glass beam dumps

As reported in the previous entry, the beam dumps on the OMC breadboard exhibited accumulation of dusts or contaminants on the black glass surfaces. We worried about transfer of the dusts over a period or of the contaminant during baking. It was already known that the contaminants are persistent and not easy to remove only by drag wiping with IPA. So Alena brought a set fo tools to try. Here is the procedure described.

- Inventory (Attachment 1): A small glass beaker, TX715 Alpha® Sampling Swab, plastic brushes, syringes with pure IPA, inspection flash light, Vectra IPA soaked wipes

- Apply clean IPA on a brush. Some IPA should be removed by the IPA soaked wipe so as not to splash IPA everywhere. Rub a glass surface with the brush while the surface is inspected by the flash light. The strokes migrate the contaminants to the direction of wiping. So the brush should be moved outward. This does some cleaning, but it is not enough to remove smudges on the surface. Occasionally clean the brush with IPA poured in the small beaker.

- Apply clean IPA on a swab. Rub the surface with the swab outward. This removes most of the visible smudges.

We decided not to apply FirstContact on the beam dumps at this occasion. In any case, we need to apply FC on all the optical surfaces after the baking. We judged that the current cleanliness level of the beam dump does not affect the over all contamination of the OMC considering the FC application after the baking.

2. Gluing of the reinforcement Al bars on the delaminated Invar mounting brackets
One of the mounting bracket (=invar shim) on the top side (= suspension I/F side) showed the sign of delamination (Attachment 3). This invar is the one at the beam entrance side (Attachment 2).

EP30-2 was mixed as usual: 6g of EP30-2 was mixed with 0.3g glass sphere. The glue was tested with a cooking oven and the result was perfect. The glue was applied to two Al bars and the bars were attached on the long sides of the invar shim with the beveled corner down (to avoid stepping on the existing original epoxy) (Attachments 4, 5). The photo quality by my phone was not great. I will take better photos with a better camera next week.

Glue condition was checked on Monday Jul 3rd. It was all good. New photos were taken. OMC #002 Repair - Gluing of reinforcement AL bars

POSTED to 40m ELOG

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

The 3IFO EOM test performed at the 40m. Result: 40m ELOG 13819

Qs' were estimated with a lorentzian function (eye fit)

Current LHO EOM (final version, modulation depth measurement 2018/4/5)
f0=9.1MHz, Q=18
f0=45.38MHz, Q=46
f0=118.05MHz, Q=30

Prev LHO EOM (RF transmission measurement 2018/4/13)
f0=9.14MHz, Q=53
f0=24.25MHz, Q=55
f0=45.565MHz, Q=62;

3IFO EOM (RF transmission measurement 2018/4/23)
f0=8.627MHz, Q=53
f0=24.075MHz, Q=60
f0=43.5MHz, Q=65

3IFO EOM dark microscope images courtesy by GariLynn and Rich

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

[Koji Gautam]


With Gautam's help, I ran a coating design code for an HR mirror with the standard quarter-wave design. The design used here has 17 pairs of lambda/4 layers of SiO₂ and Ta₂O₅ (=34 layers) with the fused silica as the substrate to realize the transmission of tens of ppm. At the AOI (angle of incidence) of 4 deg (=nominal angle for the aLIGO OMC), there is no significant change in the reflectivity (transmissivity). With 95% of the case, the phase difference at the AOI of 4 deg is smaller than 0.02 deg for given 1% fluctuation (normal distribution) of the layer design and the refractive indeces of the materials. Considering the number of the OMC mirrors (i.e. 4), the total phase shift between P and S pols is less than 0.08 deg. This makes P and S resonances matched well within 1/10 of the cavity resonant width (360/F=0.9deg, F: Finesse=400).

Of course, we don't know how much layer-thickness fluctuation we actually have. Therefore, we should check the actual cavity resonance center of the OMC cavity for the polarizations.

Attachment 1 shows the complex reflectivity of the mirror for P and S pols between AOIs of 0 deg and 45 deg. Below 30 deg there is no significant difference. (We need to look at the transmission and the phase difference)

Attachment 2 shows the power transmissivity of the mirror for P and S pols between AOIs of 0 deg and 45 deg. For the purpose to check the robustness of the reflectivity, random fluctuations (normal distribution, sigma = 1%) were applied to the thicknesses of each layer, and the refractive indices of Silica and Tantala. The blue and red bands show the regions that the 90% of the samples fell in for P and S pols, respectively. There are median curves on the plot, but they are not well visible as they match with the ideal case. This figure indicates that the model coating well represents the mirror with the transmissivity better than 70ppm.

Attachment 3 shows the phase difference of the mirror complex reflectivity for P and S pols between AOIs of 0deg and 45deg. In the ideal case, the phase difference at the AOI of 4deg is 1x10^-5 deg. The Monte-Carlo test shows that the range of the phase for 90% of the case fell into the range between 5x10^-4 deg and 0.02 deg. The median was turned to be 5x10^-3 deg.

Attachment 4 shows the histogram of the phase difference at the AOI of 4deg. The phase difference tends to concentrate at the side of the smaller angle.

I started some analytic calculations of how OMC mirror motion would add to the noise in the BHD. I want to make some prettier plots, and am adding the interferometer so I can also compute the noise due to backscatter into the IFO. However, since I've pushed the notebook I wanted to post an update. Here's the location in the repo.

I used Koji's soft limit of 0.02 degrees additional phase accumulation per reflection for p polarization.

I'm still not satisfied/done with the solution to this, but this has gone too long without an update and anyway probably someone else will have a direction to take it that prevents me spinning my wheels on solved or basic questions.

The story will have to wait to be on the elog, but I've put it in the jupyter notebook. Basically:

• I considered the polarization-separated OMC in several configurations. I have plots of DARM referred noise (measured free-running and controlled noise for the current OMC, thermal theoretical noise curve, scattered light) for the case of such an OMC with one lambda/2 waveplate oriented at 45 degrees. This is the base case.
• I also considered such an OMC with a lambda/2 both before and after the OMC, where their respective polarization axes can be arbitrary (I look at parameter space near the previous case's values).
  • I optimize the BHD angle to balance the homodyne (minimize the E_LO^2 term in the homodyne readout).
  • I then optimize the rotations of the lambda/2 polarization axes to minimize the noise
  • For the optimum that is closest to the base case, I also plotted DARM referred length noise.

It's clear to me that there is a way to optimize the OMC, but the normalization of my DARM referred noise is clearly wrong, because I'm finding that the input-referred noise is at least 4e-11 m/rt(Hz). This seems too large to believe. 

Indeed, I was finding the noise in the wrong way, in a pretty basic mistake. I’m glad I found it I guess. I’ll post some plots and update the git tomorrow. 

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%.