[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.

Matteo, Satoshi

We measured the transfer function of RFPD used for PDH locking.
I have thought it be a resonant RFPD, but not.
Attached picture shows the TF of the RFPD.
The gain is 33.5db at 29.1MHz which is the modulation frequency of EOM.

Here I attach the measurement of PLL phase noise. The measurement has some disconnect points although I used V/sqrt(Hz) as a unit. The result is summarized as following.

The lock of filter cavity make RMS phase noise increase by a factor of 1.5/3.5

Aritomi and Yuhang

Today, we checked together about the polarization again with PBS plate.

• The mirror was mounted as flipped(the uncoated side was facing laser). We flipped it back to the correct side.
• The angle of the PBS plate was quite far from 45 deg. We checked today this angle. When we have all the light reflected, the angle was quite far from 45 deg(at that time, around 35 deg). When we turn the mirror to the direction of 45 deg, this reflection disappeared.

So we concluded that the beam we were using is p polarization. And we also need to pay attention next time about the surface of mirror and the angle of PBS plate.

Matteo, Simon

Since the DC value seems to be depending on the longitudinal position of the mirror-substrate (Z), we repeated taking a XY-map at Z=112 with re-calibrated DC-value. It turned out that also the absorption-values increased. We now think that we would have to readjust the DC value every time the Z-stage moves, which would make it basically impossible to receive any quantitative results on XZ and YZ maps (however, qualitative results are anyway achievable).

We run another map at the center position with adjusted DC value to have the three maps (Z=46, 79, 112) all adjusted.

At the same time we started to reconfigure the absorption bench for measuring polarization maps.
Therefore, we aquired some PBS and a 2" lens (f=100) which we could put in the outgoing beam path. In a first test, we already recognized that the main part of the pump beam is p-polarized with ~10% s-polarization, although we thought it were merely s-polarized!
So, we put another PBS in the incoming beam-path to make it almost completely p-polarized and checked the effect on the photometer. When moving the mirror into the beam-path, there seems to be some changes happening but without more precise analysis with photo-diodes, we cannot be sure.
Anyway, we let it like this and will continue tomorrow when the other absorption measurement is finished.

At last, we removed also the first-contact from the ETMY substrate which we put there to clean its surface after the laser-burning accident last week. It seemed to be successful and we could not see any dirt anymore. In this state, we put the container over the substrate to protect it.

[Aritomi, Yuhang]

First we tried DC balance of homodyne with s pol, but power unbalance of BS was large for s pol. Reflection of BS is 630 uW and transmission is 600 uW for s pol while it's almost balanced for p pol. So we decided to use p pol for the moment. We made LO p pol with HWP and confirmed its polarization with cubic PBS (Newport,10BC16PC.9). However, when we put plate PBS (thorlabs, PBSW-1064) instead of cubic PBS, most of the LO was reflected by plate PBS. Actually I didn't check the direction of plate PBS and maybe it's wrong, but even in that case we cannot explain this behavior. We'll check the direction of plate PBS and check the polarization with AMC tomorrow.

Anyway, today we managed to make LO DC balanced with "p pol".

I checked the current assumption as suggested by Matteo. It is fine. The situation now is the same with entry 1426. At the same time, for -19V channel, the current assumption is 0.063A. Although we don't have reference for this negative channel, I think it is also fine.

After the optimization of locking servo parameter, we locked PLL_CC. We are interested in this PLL because this phase noise is coupled into squeezing measurement directly. However, the other PLL phase noise changes the parametric amplification and de-amplification factor. How much can they affect squeezing needs to be considered further.

Anyway, the measurement result is attached. There is two main difference:

• The high-frequency PLL noise when the filter cavity is locked is lower
• The low-frequency PLL noise when the filter cavity is locked is higher

I think this is because we are making the main laser to follow filter cavity. Filter cavity suspension makes low frequency noisier while high frequency quieter.