Sudden move of coherent control laser's steering mirror

Participant: Marco and Yuhang

We found a sudden movement of the mirror while we were doing the experiment. We think two mirrors are suspicious, which are used for coherent control laser alignment. Because the alignment of p-pol didn't change, we think the guess above is reasonable.

The suspicious mirrors are marked with black circles in the attachment. Next time, we should try to recover alignment by moving only one mirror. So that we can know which one is causing the problem and replace it.

The fork is properly fixed.

accident change of OPO temperature

We tried to make s PLL working for 7MHz. At the same time, make p and s co-resonant. Then shift 7MHz of p. This is the procedure to lock PLL.

However, we found a good setting for that procedure brought us mode-hooping of p.

In the end, we found the temperature of OPO was changed accidentally from 7.05 to 7.9. After we brought back the temperature, we can lock PLL for coherent control without mode-hooping.

Next step is to check coherent control error signal. However, the reflection power drop for resonance is still not visible up to now.

Optimal laser temperature region

[Aritomi, Yuhang, Marco]

We found optimal temperature region which doesn't have mode hopping for three lasers and has beat note with 7 MHz for ML-CC PLL and below 400 MHz for ML-p pol PLL and overlapping of CC and p pol inside OPO.

	Current (A)	Temperature (deg)
Main Laser	1.834	23.12
CC	1.183	38.18
P pol	1.338	32.43

P pol beat note when CC and p pol is overlapping inside OPO is 69.8 MHz without green.

With green power of 68 mW, p pol overlapping beat note is around 7 MHz. The laser setting is as follows.

	Current (A)	Temperature (deg)
Main Laser	1.834	23.12
CC	1.183	38.1
P pol	1.338	32.41

We have to lock p pol at p pol beat frequency +- 7 MHz, so p pol beat note around 7 MHz is not good. To change the p pol beat frequency, we changed OPO temperature from 7.05 kOhm to 7.02 kOhm. The p pol beat frequency became 56 MHz.

Mode hopping region

Locking of ML-CC PLL with both fast and slow loop at 7 MHz

sideband amplification	20.8 dB	8.8 dB	0 dB
beat note	-38 dBm	-38 dBm	-38 dBm
lower sideband	-45 dBm	-52 dBm	-66 dBm
upper sideband	-55 dBm	-60 dBm	-

We can lock SHG with both 8.8 dB and 0 dB amplification, but SHG locking is not good for 0 dB amplification. So we decided to use 8.8 dB amplification.

Fiber BS for each PLL

p pol and main laser: PN1064R5F2

s pol and main laser: TN1064R5F2A

Setting for locking ML-CC PLL at 7 MHz

s-pol mismatching solved

Participant: Eleonora and Yuhang

As we said during the last filter cavity meeting, the matching of s-pol inside OPO is becoming worse. Today we checked again and found something different. Especially we checked the shape of the first higher order mode. And another important effect is the higher order mode becomes higher after moving screws for yaw. All of these prove that the higher order mode is because of yaw misalignment.

Then we aligned s-pol and also p-pol. The alignment condition is taken as a photo and attached. The first one is for s-pol and the second one for p-pol.

Locking both p pol PLL and CC PLL

Cables we need

0.5 m LEMO-SMA x 2

2 m SMA-SMA x 2

5 m LEMO-SMA x 2

Please check the cable before you use it !

Test of PLL servo box

Since we found that we cannot lock PLL, I did the test of PLL by sending signal inside and checking output signal.

The PLL servo box contains

Input	local oscillator, beat note
output	fast control, slow control, mux
function	filter, integrator(switchable)

(Before doing test, we succeed in connecting computer and servo. We also succeeded in writing a desirable command to servo.)

 

Input signal:

1.Beat note: (1) 20MHz Sine wave with 100Hz frequency modulation, deviation of 1MHz. (2)20MHz Sine wave with 100Hz frequency modulation, deviation of 1kHz. (see attached figure 1 and 2)

2. Local oscillator: 20MHz from DDS board.

 

The purpose of using these two signals is to check how PLL acts when we have a "beat note" signal deviating from local oscillator. The result is as following:

1. Close the fast control loop. Sending beat note (1) and local oscillator. We check on oscilloscope and found it almost give just an offset of 10V. If we look at the AC of this signal, there is something(20mV) and the frequency is 100Hz. So it is sensing the difference between LO and BEAT.(see attached figure 3 and 4)

2. Open the fast control loop. Others are the same with rsult 1. We found almost nothing. So this means the small AC signal we get is because of the comparison of LO and BEAT.

3. We also tried to reduce the deviation. Close the loop and send beat note (2) and local oscillator. Then we got an AC signal without a clear frequency.

 

Conclusion: The PLL board has a problem. Actually we did the same test when Chienming was here. At that time, the signal we get from output channel is quite large.

Floor repaired in TAMA circuit prefab

Last Monday, the damaged part of the floor of TAMA circuit prefab (a.k.a elecshop) has been repaired.

In order to allow for the floor replacement we moved away everything in the interested area and we took the chance to do some cleaning.

We will put everything back in the next days and possibly tidy up a bit.

PLL lock input signal magnitude measurement

local oscillator amplitude 16dBm(DDS3 CH0)

beat note amplitude (p pol-main laser before amplification) is -7dBm

beat note amplitude (s pol-main laser before amplification) is -10dBm

Sapphire Namiki 1inch x 20mm

Members: Marco Bazzan, Manuel

P=32mW
We aligned the red probe with the surface reference sample.
Then we calibrated the bulk absorption.
Ac= 0.1V; DC=4.75V. T_ref=55%
R=0.1/4.75/0.032/sqrt(0.55)/1.04=0.85 cm/W

at 17:45 - Mounted the Namiki sapphire sample
Imaging unit position corrected for sample thickness by Delta_z = 8.6 mm
Rised the power to 1 W by rotating the IPC HWP
T_sapp=86%

Although noisy, we can observe an absorption profile by scanning the sample along z (see first screenshot). The profile is confirmed by flipping the sample (see second screenshot).

AC max = 250uV
DC=5V
Max absorption level estimate:

250/5/1/sqrt(0.86)/R*3.34 = 212 ppm/cm

PLL fiber coupling

Re-alignment of fiber for PLL and fiber fluctuation

Replace of fiber splitter

We found the fiber for ppol-mainlaser PLL (FC1064-50B-FC) was broken. We replaced it by a new one (PN1064R5F2).

Locking procedure of coherent control

Original position of flipping mirror for Bright Alignment Beam

Attached picture shows micrometer of  flipping mirror for BAB when BAB is aligned to OPO.

Fiber coupling for coherent control PLL

[Aritomi, Yuhang, Matteo]