[Evan, Alexa, Koji]
We worked on the ISCT6 yesterday after noon before the vent recovery.
- AS_AIR and OMCR paths were realigned
- AS_AIR path attenuations (ND filter, BS) were removed.
- Power at various points on the table were measured.
- OMC PZT shutter and HAM6 fast shutter were tested. Their shutter speeds are 4usec and 2msec, and fast enough for higher power operation.
Optical path alignment
- The alignment of the beams in the AS_AIR and OMCR paths were revisited.
- The OMCR path had the SRC length setup which had undamped reflections.
A beam dump formed by two ND filters (OD1.0) and knife-edge dump were inserted in the path.
- Two power-attenuation elements in the AS_AIR path were removed.
A 50:50 BS common for AS_AIR_A (1f) and AS_AIR_B (2f) were replaced with an HR mirror.
The other was OD1.0 ND filter fro AS_AIR_A.
Power measurement
The naming of the optics is based on D1201210-v7.
AS-BS2 = 50:50 BS common for the PDs
AS-BS3 = 50:50 BS for the PDs
Before AS-BS2 replacement (but no ND filter for AS_AIR_A)
After the periscope/before the 1st steering mirror: 8.05 uW (@Apr 10 4:52PM local)
After the periscope/after the 1st steering mirror: 8.04 uW
AS-BS2 incident: 4.20uW
AS-BS2 refl: 2.53uW (R(AS_BS2)=60.2%)
AS-BS3 trans: 1.51uW (T(AS_BS3)=59.7%)
AS_AIR_B incident: 0.79uW (R(AS_BS3)=31.2%)
AS_AIR_A incident: 1.31uW (No ND filter)
ND filter (tested at OMCR)
ND incident 1.90 mW
ND trans 0.180mW (T(ND) = 9.5%)
After AS-BS2 replacement
AS-BS2(HR) incident: 4.15uW
AS-BS2(HR) refl: 4.08uW (R(AS-BS2) = 98.3%)
ASAIR_B incident 1.45uW (R(AS-BS3) = 35.5%)
ASAIR_A incident 2.44uW (T(AS-BS3) = 59.8%)
Previous
AS_AIR Periscope -> AIR_B thruput: 9.8%
AS_AIR Periscope -> AIR_A thruput: 1.6%
New
AS_AIR Periscope -> AIR_B thruput: 18.0%
AS_AIR Periscope -> AIR_A thruput: 30.3%
=> We decreased the OM1 transmission from 5% to 800ppm = 1/63
This was compensated by the inclease of the PD incident by
x1.8 for AS_AIR_B => x35 gain missing
x19 for AS_AIR_A => x3.3 gain missing
OMCR after the periscope (OMC unlocked): 1.95mW
Shutter test
We performed an end-to-end test of the fast shutter and the OMC PZT shutter.
- Procedure (See also LHO ALOG 16355)
Fast Shutter test:
1) Confirm the shutter threshold level. It was 2V.
2) Thorlabs PDA255 (50MHz) was set on the ICST6 table. The OMCR beam was aligned. Confirmed the OMC was unlocked.
3) Disconnect the analog signal to the trigger logic module, and plug it on a function generator.
QPD Transimpedance Amp for AS_C (Rack ISC-R5 U18) has QPD SUM1 output at the back side.
This cable is connected to the trigger logic module on the top of the ISCT6 enclosure.
4) Set the function generator to produce the triangular signal with an offset of 0.7V and the amplitude of 2V. The frequency was 100Hz.
=> The signal ramps from -1.3V to 2.7V.
5) First, the amplitude of the FG was set to be 0. Open the shutter if it is closed.
6) Set an oscilloscope to watch AS_C Sum, Thorlabs PD, and trigger signals. Set the scope to "Single Sequence" mode with "Normal" trigger.
7) Set the amplitude is to be 2V. This already triggers the shutter. Return the amplitude to 0V.
OMC PZT Shutter test:
1) Lock the OMC. Place the PD at the OMC trans (leak) beam.
2) Otherwise the same setup as above.
- Result
- The trigger occured at 2V and almost immediate.
- The fast shutter was closed with the shutter time of 2.3ms between the trigger and the complete close.
- The OMC shutter was closed with the shutter time of 3.9us between the trigger and the complete close.
- Analysis: Are the diodes safe now?
This threshold level shuts off the shutters when the incident power to HAM6 is 1.2W (cf. Dan's LHO ALOG 17787)
According to the lock loss observation at LLO, the AS port power during lock loss is not impulsive but rather slow.
1.2W x 4us = 4.8 uJ
We deliver half of this power on the OMC DCPD. => 4.8/2 = 2.4uJ
This is x400 smaller than the minimum pulse energy level (1mJ) tested in Frank's diode blasting experiment,
which did not show any (or significant) damage of the diode.
Therefore we can conclude that the OMC DCPDs are safe.
OMCR QPDs and OMC QPDs receive 1/20 and 1/250 attenuated pulse with 600 times longer time.
So the below calculation is for the OMCR QPDs.
1.2W x 2.3ms / 20 = 0.14 mJ
This is still x7 smaller than the above 1mJ.
Therefore the OMCR/OMC QPDs are also safe even the beam is on a single element by chance.
