Yuhang, Michael

From Thursday last week. We made many fixes to just about all parts of the frequency independent squeezer.

SHG

The RFPD that detects the SHG error signal was moved to the reflection of the first dichroic in the green path, aided by a lens and a green block filter. The error signal becomes like the reference on the wiki. We took the open loop transfer function eps1/eps2 on the spectrum analyzer but it was strange compared to the reference version (Aritomi thesis). We tried on the old spectrum analyzer and it seemed fine, with the correct low frequency response and first mechanical mode at 21 kHz. So maybe we just missed some settings. We changed the servo gain to give 2.2 kHz unity gain frequency and 45 degree phase margin. Coherence was close to one apart from a spike at 500 Hz.

OPO optimisation

The OPO was optimized using a fast method: scan green phase (ramp signal sent to green phase shifter high voltage driver) without locking OPO and measure amplification/deamplification of BAB. We saw that for some reason the green beam was greatly misaligned in pitch on both the filter cavity and OPO paths, so we adjusted with the first steering mirror after the green FI. Then green to OPO was maximized using the second 45 degree incidence mirror after GRMC transmission to maximize nonlinear gain. We saw:

74 mV without green
268 mV with 25 mW green, before optimization
496 mV after optimization

giving a nonlinear gain of 6.7. This is really quite high compared with what we saw before. We did not see the need to adjust temperature (though maybe we should).

IRMC

We had issues with mutual lock of SHG and IRMC ever since switching to the one EOM scheme. Currently SHG and IRMC are both modulated with one DDS channel and demodulated with one DDS channel. We took some cables to add phase delay to the IRMC and got the good PDH signal. So now it locks properly, although the threshold needs to be optimized a bit.

Homodyne alignment

The homodyne was realigned and balanced. The coarse alignment steps are as usual:

1) Block all OPO side infrared signals from reaching the homodyne
2) Lock IRMC to send local oscillator to homodyne
3) Adjust pitch and yaw of lens closer to IRMC to bring the homodyne photocurrent close to zero (will go far down if misaligned)
4) Adjust pitch and yaw of lens closer to edge of table to bring homodyne photocurrent close to zero (will go far upif misaligned)
5) Adjust balancing beam splitter to finish bringing the HD signal to zero

Then local oscillator is aligned to alignment mode cleaner for fine alignment.

The spectrum of the homodyne output was seen to be flat to about 40 Hz, with 100 Hz peak from ceiling light. We turned off the ceiling light but then it somehow turned back on. We decided to investigate using the security camera system but could not login.

After LO is aligned, the squeezed path is aligned by sending BAB to the alignment mode cleaner, but first we have to lock OPO with ppol.

GRMC

For whatever reason GRMC misaligned a lot as well. Maybe had something to do with the green misalignment in pitch above. During this process we could see the filter cavity reflection moving a lot on the green injection mirror though (there was an earthquake), so the green beam shouldn't be too far misaligned to FC.

PPol

The ppol frequency was changed to optimize BAB transmission (without green). It goes to 220 MHz, down from 260 MHz some time ago.

CC

On this day we saw that the CC PLL had erratic locking. The sidebands would come close to lock and then eventually quickly be ejected away from the resonance point.

Nishino,

By scaaning frequency, I measured the mode-mtching ratio of the current setup.There were 4 HOMs (both HG and LG). The mode-matching ratio is:

4.90 / (4.90 + 0.337 + 0.097 + 0.097 + 0.097)*100 = 88.7 %

The injection current was 1.1 A.

Also I locked the cavity by Moku:lab. Environmental sound seems to be too big and the locking was not stable only with a feedback onto PZT. It would need something like a shield for a stable locking.

Yuhang, Marc, Michael

We once again tried to find transmission sidebands at 88 MHz for the SHG locking but could not see them, so we had to rethink the lock scheme.

We had some discussion of how to resolve the BAB direct reflection issue (in fact, the beam is forming an interferometer with OPO HR and SHG input as the two arms, and SHG refl as the output port).

Our first thought was to put a Faraday isolator to decouple the two beams.  For the SHG input path, there is not very much space for an FI, while on the BAB path the beam is quite large and would require a redesign of the mode matching telescope for BAB to OPO.

We had some thought about trying to decouple them using polarization. But given that SHG must be p-pol and BAB must be s-pol, we cannot decouple them using just HWPs and one PBS, and besides, we need those polarization optics for the Taiwan and Korea activities in ATC.

We then decided to try a pick-off from the SHG reflection before it reaches the SHG/BAB separation beam splitter and recombines with BAB. Since the current SHG refl PD has an ND2 attached and has about ~ 100 mW incident, we only really need about 1 mW of power to get a good signal. At this point we remembered that there are two dichroic mirrors on the green output of the SHG that reflect away residual IR into beam dumps. Actually, they reflect quite a lot, we measured 3.6 mW infrared reflected from the first dichroic after green generation.

And then we found that our attempts to see the transmission sidebands on the transmission PD were separately mistaken. On the first attempt, a low bandwidth PD was used in reflection, which is not fast enough to see RF sidebands. On the second attempt, an ND2 filter was left screwed on to the SHG refl RFPD when it was moved to the transmission side. So in both cases the sidebands were suppressed. Correctly implementing an RFPD in transmission of the SHG with sufficient incident power resulted in recovery of the error signal. So we didn't really have to change the locking scheme very much if at all.

We decided to put both SHG lock PDs in transmission. We selected a BSF10C beam sampler with an output wedge - this has ~ 1% reflection for p-pol at 45 degrees AOI, and maybe 5% at imperfect angle. To improve the visibility of the small transmitted power, the SHG correction signal SERVO OUT was disconnected, and an offset applied to the SHG PZT to see strong green generation. Then the IR transmission also becomes stronger, to about the level we would expect during SHG lock - about 3 mW transmission. In 99% transmission of the beam sampler I placed the RFPD Thorlabs PDA05CF2 which goes to the SHG error signal. In 1% reflection I placed the DCPD Thorlabs PDA36A-EC switchable gain, which goes to the servo threshold check and TRANSMIS OUT. I looked at TRANSMIS OUT on the oscilloscope -it was quite small so I increased the DCPD gain to 30 dB and it became about 550 mV. Then I checked the RFPD signal going to the mixer, which has 4.5 V max signal. I could see 95.7 % mode matching (4480 TEM00 + (96+56+48) HOMs). Then I placed a large beam dump at the former SHG refl PD point, which has quite large infrared power.

I set the threshold to slightly less than half of the DCPD signal (about 250 mV) and tried to lock, but the SHG would still only lock to a bad point (i.e. not to the level of the TEM00 peaks on TRANSMIS OUT), I looked at the error signal and tried to optimize the SHG demodulation phase but could not. From the reference signal on the wiki, it should have jump from a large negative point to a high positive point with a strong positive slope at the lock point (once again the NAOJ elog image upload issue is not very nice). However, instead it goes from large negative peak -> small positive peak -> small negative peak -> large positive peak, essentially making a small bump in the middle of the error signal with the opposite sign compared to what the polarity of the large peaks should give. I cannot get rid of this by changing the demodulation phase. Tomorrow we will try using the dichroic reflection for RFPD to see if that fixes the error signal.

Yuhang, Michael

We continued having SHG lock issues when investigating the stability of the green generation/stabilization path. We checked LO, it is at the correct frequency and level (5 dBm). The signal from the photodetector at the SHG mixer also looks fine on the spectrum analyzer (88 MHz peak going up and down with automated cavity scan). However, the output of the mixer PD x LO is quite bad.  It was eventually found that direct reflection from BAB is coming back the the SHG reflection PD that we use to lock.

Originally, locking of the SHG was done via transmission through the SHG modulated at 15.2 MHz. This is within the linewidth of the SHG, so the PDH sidebands have strong transmission. However, it also was causing phase noise in squeezing due to being inside the linewidth of other green optics, so we decided to replace the 15.2 MHz modulation with 88.3 MHz. Since we expected the PDH sideband transmission through the SHG to be much weaker (and it is), we placed a PD in reflection of the SHG to lock. However, the reflection signal is transmitted through the same BS that splits the main laser to BAB.

Since I can't upload images right now I will just draw text art:

                               ^

                               I
                               I
                               V
                              To and from BAB

                              \
<------- To SHG----- \ ---------> To SHG refl PD
                                \
                               ^
                               I
                               I
                             From main laser

In fact, the BAB is injected at the OPO HR surface, so it is also reflected quite strongly, and the BS is 80:20 R:T, so there would be quite a lot of BAB reaching the SHG refl PD.

I tried to switch the error signal acquisition PD to the SHG transmission one - I took the cable connected to SHG refl PD (the one with the DC block) and connected it directly to SHG tra. I could see a nice cavity mode sweep on the oscilloscope, about 1.4 V or so, however, I could not see any mixer signal, nor could I see any 88 MHz sideband on the spectrum analyzer. There was a 0.2 ND filter going to the PD which I removed, but no difference. I tried to lock with very low positive, or negative, threshold on the servo but still nothing. So we have to reconsider how we lock SHG, otherwise we cannot have BAB and green at the same time, which is necessary to optimize the OPO for green and infrared.

I returned the ND filter put the DC block cable back to SHG refl.

Measurement finished and could find voltages range where both azimuth and ellipticity are crossing 0.

Then, with help of Yiru Zi-Hao and Byeong-Jun we moved the laser source to ATC clean room.

We did not brought back any replacement source yet.

Yuhang, Chang-Hee Kim, Michael

BAB to OPO - the BAB alignment to OPO was optimized using the hom sub dc signal with one homodyne input blocked. Mode matching was increased to 1170 mV + (46, 34, 14, 18 mV) -> 91.8%. Then ppol was also aligned from a mode mismatch of about 50% and now OPO locks with ppol. However, since the OPO temperature hasn't been optimized for a long time we should re-optimize parametric gain.

PLL - there is still quite a lot of signal in the ppol PLL so probably it was not misaligned too much to the fiber (I accidentally misaligned it in pitch last time when I thought I was adjusting BAB to OPO).

Lasers - We adjusted the temperature and current for each laser to be consistent with the values in the wiki +/- 0.001

GRMC MZ - we want to measure OPO nonlinear gain by injecting green and measuring BAB transmission, but GRMC/MZ kept unlocking on short timescales for some reason. The offset for the MZ HVD was unintentionally left on by me but it should be off. But the system kept unlocking anyway. So we decided in the end to take transfer function measurement, but it was getting late, so we will do tomorrow

Measurement was finished at 23deg. I rotated the mirror back to 0deg. There was some discrepancy in measurement at this position. The measurement is ongoing in between 1.34-1.36 and 16.42-24.62 with 1mV resolution for first and second LC respectively. 

foldername=r'C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Coating measurement\BB1-E03\0 deg'

filename='Mon, Dec 18, 2023 9-16-57 AM.txt'

I designed 3 parts in SolidWorks for testing the hydrogel birefringence change vs applied force.

1 holder for the DM713 use to apply force, 1 cube to apply uniform force and 1 holder for the cell container.

Files are saved in my folder of the workstation in room 216/217.

Hirata-san is helping for the 3d printing that will be done at ATC.

I took a Thorlabs 1064 VLP Faraday isolator (4.7mm aperture, max power 1.7 W) from the speed meter experiment to the KOACH clean room. We still need an appropriate power meter - we would like to inject up to 1 W into the SHG.

The voltage scan was performed for the measurement. The scans started from 0-25V for each LC with a resolution of 0.1V and were finished with 1-1.3V and 22-23V with 0.001V resolution for the 1st and 2nd LC respectively. 

I have rotated the mirror by 23 deg. The voltage scans were started for 0-25V range with  0.01V 0.1V (edited) resolution.

measurement file is here:

C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Coating measurement\BB1-E03\23 deg\Wed, Dec 13, 2023 1-20-43 PM.txt

MZ and GRMC

Lock was recovered with the nominal level of stabilized output power (power meter in transmission of GRMC says 26.5 mW green to OPO at 4.2 on the MZ offset dial, which is our usual setting).

I tried mode matching on each arm - 
PZT arm: 384 mV TEM00, 8 mV HOM, 96.9%
Non PZT arm: 364 mV TEMO00, 33.2 mV total HOMs, 91.6 %
 

After adjusting them together, then maximising the mode cleaner transmission using the MZ HVD offset (different control knob to the above), I get:
GRMC: 1.5 V TEM00, 68 mV HOMs, 95.76%
but one other time I got 720 mV, 72.4 HOMs, 90.9%
 

The power meter autoranging should be off. Maybe I am just a bit confused about the MZ scan. A bit of explanation for people not familiar, the MZ is not locked using PDH since it is not a cavity, rather it is just locked to some reference voltage controlled by MZ offset dial. When the LOCK switches are turned on, the GRMC scans across the free spectral range quickly and acquires lock first using the PDH sideband signal from GRMC reflection PD, then the MZ scans slowly and locks second using the signal from GRMC transmission PD to keep the transmitted green power at a stable level. Also, the interference condition of MZ using the HVD offset can be changed to reflect or transmit TEM00/HOM - we want it to transmit mostly TEM00 and reflect HOM into a rejected port. Anyway, I don't remember how slow is "slow" scan, but I don't think it should be causing the slow change of MZ mode matching on the timescale I saw. Either way, that is only an issue for measuring mode matching when not locked. The system locks and sends stabilized power to OPO so we can fix it later.

BAB to OPO

I recovered bright alignment beam mode matching to about the level it was during Yuhang's last visit, which is to say, ok but not great (~ 60%). I measured using power meter in transmission of OPO - it is quite hard to see the BAB transmission with OPO unlocked since it is injected from the HR side of the OPO, but there was a conveniently placed holder at just the right spot for the power meter. I tried to measure with homodyne but the IRMC started having some issue where it wouldn't lock to the correct point. We have been having issue with SHG and IRMC mutual lock ever since we switched to the single 88 MHz EOM to do all of the squeezer table modulations. We should decouple these two signals some time.

FC green lock

I turned on the FC length control servo inside the clean room and then tried to lock FC on green but was not successful.

I could fix 2 out 3 metric screws in the KAGRA mir box (spareETMY). the last screw was harder to fix and I didn't had the proper key so I did not fix it yet..

The spare screws are in the PCI pre-clean room

The IRMC was not too badly misaligned, just tweaking a bit the lens in front of it I could recover about 94% mode-matching.

Further improvement would require to tune the IRPS but it's not high priority for now so I left it like that.

Then, could smoothly lock it.

[ByeongJun, Marc, Michael]

The GR beam was realigned on BS chamber using PR picomotors, on 1st and 2nd target using BS picomotors.

We tuned END coils actuators to get the beam on the back of 2nd target.

Then, using IN picomotors, we recovered fundamental mode in transmission of FC.

To do :

offload END pitch (now 280 is sent to coil)

replace the quad box for the camera display close to DGS

recover remote control of 2nd target

I replaced the newfocus 5104 with Thorlabs BB1-E03 mirror.

Actually, when I first checked the beam centering, I noticed that the beam was not centered on the 5104 mirror. The beam at this instance was at the center of the camera.

So, even though I changed the mirror, it was made sure that the beam was incident at the center of mirror, and the reflection was at the center of the camera. In, this process only the mirror was moved. All other components were left undisturbed.

The LC voltage scan was done from 0-25V for both with 0.02 resolution.

measuerment data location
C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Coating measurement\BB1-E03-10\Fri, Dec 8, 2023 5-48-31 PM.txt

Edited:

measurement file is here:

C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Coating measurement\BB1-E03\0 deg

I confirmed the alignment of the green AOM.

The (lower left on electronics rack) function generator outputs from channel 1: 109.036 035 615 MHz, 5 dBm voltage, to + 18 dB RF amp -> 23 dBm to AOM.

I reduced the main laser current from 1.833 to 1.750 to make the green power consistent with previous values of AOM green power. I confirmed the alignment of the first order diffracted beam from the AOM to the subsequent iris and the green is now hitting the PR tank targets.

SHG green out: 284 mW
AOM in: 48.8 mW
AOM out (RF on): 46.0 mW
AOM out (RF off): 45.2 mW
Iris out (RF on): 22.1 mW
Iris out (RF off): < 10 uW

[Shalika, Marc]

In continuation of elog 3361, the coating was characterized and compensated as mentioned in the table.

First the beam was positioned to be incident at the center and the measurement was done with the rotator mount at 0º. For further investigation, we rotated the mirror by 20º. Several voltage scans were done, starting from 0-25V of both LCs with the resolution of 0.02V. The final scans were performed with the resolution of 1mV around points which provided us with the minimum values in azimuth and ellipticity of polarization. 

The combination of voltages at which the minimum for both azimuth and ellipticity were obtained is mentioned in the table below. 

The power meter was installed in GRMC transmission in order to align the MZ. First I blocked the PZT arm of the MZ and aligned using the MZ mirror and subsequent steering mirror to optimise mode matching from GRMC transmission. Then I blocked the non-PZT arm and aligned the MZ to GRMC again. Afterwards I adjusted MZ offset on the high voltage driver to get the intereference condition that outputs TEM00 from the main MZ path and TEM10 from the rejected path. Currently we have:

TEM00 1540 mV
HOMs 56, 32, 12 mV
Mode matching = 1540/1640 = 94%

One time I noticed the level of TEM00 slowly going down over a timescale of minutes. Adjusting the MZ offset brings it back to where it was. Maybe something is drifting slowly?

power before shg a bit higher than usual ~840mW

installed power meter in transmission of SHG

tuned to recover fundamental mode

actually the lens installed on rail just before SHG was not fixed. I tried to tune it a bit but it seemed that the initial position was quite good (48mm).

it was a bit hard to tune as when it is unscrewed it is also tilted. If needed, could spend time to have a finer tuning.

installed power meter in reflection of SHG (in reflection of 1st dichroic before beam dump)

further tuned alignment

tuned SHG modulation phase to 70deg (from 0deg) to recover the usual shape.

can see weak green in reflection and error signal seems half of usual ~1VpK

found out that temperature control of SHG and OPO was not turned on (actual temp was around 11kOhm)

after thermalisation recovered error signal ~2.4Vpk and could lock SHG.

Green power before EOM is ~247mW (also a bit larger than usual due to higher input laser power)

A bit strange thing is the GRMC refl pd has a quite strong noise at 100 Hz. It disappears when ceiling light is off though.

I tried, mostly just for my own amusement, to again align the SHG. The reflected beam overlaps the incident and I brought the power to a reasonable level by beam walking the two steering mirrors just before the cavity, but still no clear mode structure. I guess due to the temperature the z-direction mode matching is not good enough - the beam waist inside the nonlinear crystal cavities is very small and extremely sensitive to tuning. So I guess we just wait for the air conditioning to be fixed.

I replaced a faulty 3x2 power plug board that was making an uncomfortable crackling noise with a 6x1 board. The connected items are the homodyne power supply, power meter power supply and one other item.