I injected noise on lenght d.o.f of the end mirror when FC is locked and measured its TF to legth oplev and correction signal to the laser piezo.

The TFs look similar (pic 1) even if they are not very clear and the coherence is quite low. (pic 2).

The noise amplitude was 10000 count (~3V) and the driving matrix was such that only vertical coils where used. (It should couple only with pitch)

Pic 3 shows the spectra of the other d.o.fs. The damping loops were closed at that time, but pitch shows a motion larger that usual.

1. For main laser power line. The board should have a hole 11.1cm away from border and 4cm high.

2. For green injection, the beam is at the edge of the board. Height of 7.55cm

3. For IR injection, the beam is 37.2 holes(each hole 2.5cm)  away from main laser side.

4. length of homodyne power cable should be 5m

(Please look at attached figure to make it clear about where should be the holes)

[Aritomi, Yuhang]

First we measured dark noise of homodyne since we had large 50Hz noise in shot noise spectrum. The dark noise already had large 50Hz noise and we found that when we removed ground floating of spectrum analyzer we are using, these peaks disappeared (Pic 1).

For CC, we checked noise in SHG, GRMC (entry 1514). Then we tried to lock CC1 and 2. For CC1, at the beginning of today we could lock CC1 in stable region between 500Hz and 7kHz resonance, but at the end of today, we couldn't find stable region somehow. For CC2, we couldn't lock it stably so we put rubber to CC2 mirror holder and 4kHz peak is damped (entry 1508), but we still cannot lock it because of 7kHz resonance. 4kHz resonance should be resonance of mirror holder, but we don't know where 7kHz resonance comes from.

Then we measured squeezing with 50mW green (Pic 2) when both CC loops are stable. Squeezing is ~5dB and anti-squeezing is ~20dB. Squeezing spectrum was not very good due to phase noise. We also measured it with 40mW green (OPO:7.169kOhm, PLL:150MHz). Squeezing spectrum is more clear while squeezing level is almost same.

Aritomi and Yuhang

We measured the open-loop transfer functions of CC1 and CC2 yesterday.

We could see from these two figures that 0dB line cross 500Hz and 7kHz for CC1. While 0dB line cross 600Hz, 4kHz and 7kHz peaks at the same time for CC2.

These peaks make locking not easy to achieve. We are thinking about the following things to try:

• to dump resonance by putting rubber
• to use a notch filter to not excite the resonance
• to glue PZT and mirror on the phase shifter

I tried to improve the lock stability by adjusting the gain of SR560.
However, I could not improve so much.
In the end, the lock last only 0.5sec.

So we need circuits for feedback control to achieve stable lock.
Anyway, we can lock the laser to temporal silicon cavity.

To confirm if we have more green phase noise(free-running), I performed the measurement of noise spectrum of SHG and GRMC transmission again.

The comparison is with entry 1276.

We could see we have much more noise now!

Add some points,

• We observed the unstable lock of green phase. And I think it is related to the unstable of squeezing measurement.
• From the phase control behavior, the phase noise(free-running) seems to be higher than before. Need to be confirmed.
• The green phase lock has two main resonance 500Hz and 7kHz. In the beginning, we could find a gain region between these two oscillation and lock green phase. But at the end of yesterday, we couldn't find this region when we change the gain of control servo.
• The error signal for the green phase lock usually change. As pointed out by Aritomi-san, once it is due to the misalignment of green(green power was changed). Actually, we observed a second change of error signal of green phase lock. We check PLL lock, green power(also GRMC alignment), OPO temperature. But they were fine. We should use more time to investigate what makes it change and try to avoid it.

[Aritomi, Yuhang]

First we made LO DC balanced and aligned LO and BAB into AMC. DC balance of BAB is 10mV. We measured shot noise of LO (Pic 1). Although there are large 50Hz peak and its harmonics , homodyne is shot noise limited above 20Hz. To remove the 50Hz peak, we floated ground of homodyne, but the shot noise was same. We also measured shot noise spectrum when LO and CC are injected into homodyne. Shot noise with LO and CC was same as shot noise with LO only.

Then we locked green and IR phase with SR560 (bandwidth  ~ 100Hz) and found squeezing spectrum is very bad. So we locked green and IR phase with Pierre's servo (bandwidth ~ 1kHz) and finally could recover the 5dB squeezing (Pic 2). 

We thought green power is 50mW, but actually green power was smaller than 50mW due to misalignment of GRMC at that time.

I reduced the gain to 2*10^2, and tried to lock.
The oscilation was improved than higher gain one, but the lock last only 0.1sec and DC REFL dip was ~0.1mW.

Anyway, it seems that it is possible to lock TEM00 mode.
Actually, feeding back to laser PZT induces laser power fluctuation.
Therfore, ISS is needed in this experiment.

Matteo, Simon

On Friday, we started to upgrade the absorption map to be capable of taking maps on the homogeneity of mirror-substrates in terms of their polarization.
The basic idea is to use a PBS nad two photodiodes (PSDs in our case) to distinguish between S and P polarization. Actually, we recognized that the incoming beam is 90% P and 10% S polarized so that we decided to put another PBS also in the incoming beam in addition to a half-wave plate which we use to check the differences in S and P polarization and the calibration of the system.

After some first checks, we found out that the PSD designated for sensing the S-polarization is very sensible to any scattered light which is automatically an issue due to the many ND filters we have to use for not saturating the AC channel (which is fed by the S-pol PSD). Therefore, we took long time to cover the path from the PBS to the PSD. We finally took a beam-pipe which does the job quite well (see attached pictures).

Over the weekend, we took two first maps. One with incoming-beam fully P-polarized and the other one having the half-wave plate rotated by 30 degrees. Qualitative pictures can already be calculated but we still need to calibrate the DC signal properly to calculate also a quantitative number.

This morning, I again tried to lock TEM00 mode.
The attached picture shows when the laser seemed locked (the label is same as before).
It seemes something is fed back to the laser, and it last several tens of seconds.
Actually, it is oscillating, and the dip is so small, so it needs to improve the alignment, gain, and some other things for more stable lock.

[Aritomi, Yuhang]

This is work on July 20th. First we made LO and BAB overlap and more or less DC balanced at homodyne. CMRR at 1 kHz is 82 dB which is similar to before. Then we measured shot noise and squeezing spectrum (data is not saved). Shot noise is worse below 100Hz and squeezing spectrum was very bad. 

After that we found large 7 MHz peak at homodyne DC signal (attached pictures). LO and CC before homodyne BS is 1.2 mW and 8 uW respectively. This 7 MHz seems beat between LO and CC.

[Aritomi, Yuhang]

This is work on July 19th. In current setup, we only have one steering mirror and one lens for alignment of BAB going to homodyne and it's difficult to align BAB into AMC. So we changed optical layout so that we can have three steering mirrors for alignment of BAB. I updated optical layout of our bench. After this modification, we could align BAB to AMC easily. Re-alignment of IR to FC after this modification is ongoing.

Today I tried to lock TEM00 mode with 10^3 gain.
The attached pictures show DC REFL (blue), error signal (green), and feedback signal (yellow).

Actually, the lock was not stable; it last only ~10msec.
This seemes due to the fluctuation of feedback siganl which oscillated about 20Hz.
Also the error signal oscillates about this frequency, and this hinders me to lock.
I will investigate from where this oscillation comes.

The excess of high frequency noise measured last summer when the cavity is locked (see entry #903) seems disapeared. This is good but the reason is not clear to me.

Note that in order to sum the line into the PZT  for the calobration we have reconnected the ramp potentiometer which was disconnected last summer,  following Pierre's advice,  in order to reduce the rampeato noise (see entry #875).

We have left it connected for now. It would be interesting to compare the difference in the error signal spectrum when it is connect and disconnected as done last summer (entry #883).

Can you do the same measurement with gain other than 10 (when error signal noise is larger)?

Today I increased the peal-to-peak voltage from 0.4Vpp to 0.8Vpp which is applied to the EOM for modulation.
This voltage corresponds to ~0.2[rad] modulation.
In addition, I adjusted the phase shift for demodulation.
The first picture shows before changing, and the second one shows after.

• Blue line : DC REFL
• Green line : error signal
• Red line : control signal (but not yet optimized)

On the other hand, the dip got worse than before.
In order to lock the laser, I have to improve the alignment.

Next step is to improve the alignment with two STMs, and increase the dip of reflected beam.
Then I will try to lock by feedbacking to PZT of the laser.

Eleonora and Yuhang

While I was doing this measurement, I checked the error signal on an oscilloscope. I found the error signal noise is smaller if I use gain of 10.

So I changed the gain of filter cavity locking to 10.

Following the method Matteo B and Eleonora, I characterized the filter cavity locking noise again. (Measuring sine-wave single frequency noise at a frequency higher than unity gain frequency (channel 'ramp mon' and channel 'eps1'). Calibration by considering ramp monitor factor of 100, PZT gain of 2e6 and filter cavity pole of 1+(f/f0)^2 with f0=1.45kHz )

• Take ramp mon signal, calibrate it to frequency. S_Hz = V_RMS (V) * 100 * sqrt(2) * 2e6 Hz/V = 6.477 Hz
• Take eps1 signal, compare it with ramp signal(already calibrate to frequency), to have the conversion factor from Hz to V. K(V/Hz) = Err_V/S_Hz*(1+ (f/f_0)^2) =   4.568e-3 V/Hz

Then we use this K to calibrate the measurement of error signal noise spectrum to Hz. The result is shown in the attached figure.

However, compared with the measurement of more than one year ago. This level is much lower(almost 50 times lower). We checked the calculation and measurement procedure. We couldn't find out what is the problem.

I installed 3 lenses in order to avoid clipping the beam at AOM.

• f=-50.0mm and f=150.0mm to reduce beam diameter at AOM
• f=100.0mm in one of double-pass AOM part

I am planning to re-install the RFPD and try to lock the laser tomorrow.