new measurement with 30 deg polarization finished.

The results (especially of the theta distribution) seems to agree with our hypothesis that the previous measurement with input polarization 30 deg was something like 30+90 deg.

I attached to this entry several results extracted from measurements with incident polarizations 90 deg (ie s-pol), 60 deg, 45 deg and 30 deg. They are made based on a python code kindly shared by Simon.

It seems this sample is more uniform than the SHINKOSHA #7.

The s0 component is far smaller while the s1 component is larger. A possible explanation could be (from MIR meeting discussion) that the birefringence appearing here is not due to stress but to the crystal axis orientation with respect to the pump beam optical axis.

The delta n distribution seems to be quite identical for all incident polarizations and the offset might be due to the cutting angle of this sample.

A possible explanation for the offset on the theta distribution for incident polarization of 30 deg could be explain by having rotating the incident polarization by more than 90 deg.

Indeed, the HWP software now is stuck at 15 deg whatever the polarization...

I started again this 30 deg incident polarization while only decreasing the value of the HWP angle.

Yuhang and Michael

We had some trouble aligning the OPO inside the holder (initially without the input coupler). A photo is shown in figure 1. The OPO assembly is contained on the white plastic mount and has a small ~ 5mm hole at both the entrance and exit. The mount past the OPO assembly is a beam splitter that goes to a camera and photodetector. The two mirrors on the bottoms left and right are the steering mirrors. However, these are used to control the horizontal and vertical alignment of the beam before placing the OPO. Our approach was to constrain the cavity axis prior to placing the OPO - we make sure the beam stays at constant height, send it to a camera and then mark the position of the beam on the TV (fig 2). The OPO cavity is placed so that the entry hole has the input beam centered and the exit hole aligns with the previous mark on the TV screen. Then, in theory, we only need one degree each of pitch and yaw (i.e. a rotation stage) to align the OPO to the cavity axis and match the incident and reflected beams. An illustration is shown in figure 3. As seen with the arrangement of the periscope, a yaw misalignment of the reflected beam when inspected at the prompt reflection translated to a pitch misalignment of the reflected beam when inspected past the periscope (fig 4).

Using the alignment setup described, we couldn't meet the alignment conditions. We tried again by moving the OPO assembly (and the OPO  curved HR surface) closer to the centre of rotation of the rotation stage. Actually, after all of this we found that the camera wasn't very stable on the mount as well, so we fixed it properly.

Initially we put the modulators on the edge of the table so that their connecting cables wouldn't get in the way during the experiment. However, in retrospect, it would have been better to put the OPO assembly on the edge of the table since the alignment of the OPO is the most delicate task.

This morning I put back the correct lockin parameters.

I started a measurement so that the polarization angle is close to 45 degrees.

This time, a new trouble with the HWP appeared : I can change the polarization angle (quite visible from the relative change of the p pol and s pol photodiodes) but the value on the angle value on the Kinesis software does not change.

I'll investigate this issue after all required measurements for this sample are finished.

I did a series of measurements while changing the polarization angle by 90 degrees.

I did not recorded the required max/min of s and p polarizations to have accurate calibrations of the measurements.

So today I restarted measurements with incident p pol, s pol and started a 30 degrees polarization angle measurement.

However, there was another WIndows update... As it is not possible (yet?) to fully control the 2 lockin amplifiers from remote, I can not restart the measurement for today.

Furthermore, the HWP2 controller is not recognized by the software after acting strangely in the past days (only jog by 1 degree otherwise it resets the current angle to 0 and still move by 1 degree...)

For the new measurement a possibility could be to start by an absorption measurement before switching to birefringences ones.

This would assure 100 % that the orientation of the sample during all measurements is the same.

Yuhang and Michael

We assembled the input mirror assembly for the OPO cavity as per 812. The only thing to mention here is the orientation of the input coupler convex surface (fig 4 and 5).

We then connected the interface electronics for controlling the Peltier, thermistor and piezo. The Peltier, thermistor and piezo each have two connecting wires that must be soldered onto whatever means we are using to connect them to their controllers. We soldered these onto a PCB connector. The Peltier and thermistor are connected to a DSub-9 cable going to the Thorlabs temperature controller (Peltier +ve pin4, -ve pin 5, thermistor +ve pin 2, -ve pin 3). The piezo wires were soldered to a LEMO F port.The connecting wires had to be soldered outside the cleanroom - the OPO cavity and input coupler assembly were placed in plastic zip lock bags and sealed so that only the wires were coming out (fig 6). This way they could be protected from fumes and dust.

The Peltier and thermistor were tested using the temperature controller. We saw that we could stabilise the resistance to ~10 kOhm. The piezo also made the expected high-pitched noise when it was actuated (~ 2 kHz test signal).

There might have been some issue with this measurement as after 35h it did not finished..

Today Yuhang kindly went to PCI to reset the parameters (sensitivity = 1 mV for both lockin amplifiers and input A for the one connected to the PC).

We restarted the measurement with HWP = 46 deg (ie incident polarization is p).

The measurement finished and I started with HWP = 91 deg (ie incident polarization s)

Analysis to follow.

Today I tried to check if I could improve the shielding of the PBS transmission photodiode but it seemed to be already fine like this.

Then, following Simon advices, I tweaked both QWP and HWP at the DC photodiode (s pol) minimum.

In the end, the s pol  is minized for HWP = 46 degrees.

I took some values of AC and DC in different configurations :

without laser

AC= 0.6 V
DC = 0.065 V with lockin = 0.2 uV (sensitivity 30uV)
DC = 0.076 V with lockin = 0.007 mV (sensitivity 1mV)

with laser (~30 mW ie HWP1 = 20 deg) & HWP = 46 deg

AC= 607 V
DC = 0.099 V with lockin = 0.01 uV (sensitivity 1mV)

with AZTEC sample :

AC= 378 V
DC = 0.326 V with lockin = 0.032 uV (sensitivity 30uV)

The AZTEC sample orientation is the same as for the absorption measurement (black dot at the top).

I used the AC (p pol) to find the AZTEC sample center and got Y_center = 327.72 mm and Y_center = 122.825 mm. I didn't tune the Z position which is now 53mm.

The measurement started with Median filter  = 1, average filter = 0, wait time = 500 ms, map radius = 20 mm and step size = 0.1 mm and HWP = 46 degrees (ie p pol maximum).

The measurement should last 4h30. But before leaving PCI room I touched by mistake the lockin sensitivity... so one or two points might be wrong...

I will start other polarizations over the holidays.

Marc, Matteo

Last Friday, we decided to slightly move the optical board hosting the birefringence readout part in order to be able to close the enclosure to reduce possible effects of scattered light.

The flipping mirror to switch to absorption measurement is not reinstalled but it should be straightforward. We checked that it is still possible to move the absorption imaging unit translation stage on all its range with this configuration. nm path but the sensor was too small.

We tried to use the photodiode for the 1310 nm path but the sensor seemed too small (1mm diameter vs 3.6 for the ones we installed). So we used a thorlabs power supply labelled 'broken' for our Thorlabs photodiode and it seemed to work fine (maybe just too noisy?)

In any case we plan to buy 2 new photodiode and 3 new powersupply.

We connected these 2 photodiodes to the lockin amplifier and could get signals.

With 2 optical density (2 and 3) there was no saturation.

We moved the HWP angle between the 2 lenses and confirmed a maximum and minimum for the photodiode in reflection of the PBS separated by 45 degrees.

_________

Today we started by changing the imaging unit PBS post by a more stable one and realigned the 2 photodiodes. This time, the photodiodes have a small angle to limit back-scattering.

Then, we installed walls on the birefringence board before putting the large enclosure and closing its 4 walls.

Then, we tuned the QWP angle so to be at a maximum on one of the 2 photodiode.

Then, we did a scan of the motorized HWP  with incident pump power around 30 mW) and results are reported in the attached figure.

min p pol = -0.16 mV

max p pol = 0.606 V

min s pol = 0.12 mV

max s pol = 0.588 V

where the offset without pump beam is already taken into account (p-pol = 0.6 mV, s-pol = 12 uV)

I guess we could try to add more shielding for the p pol photodiode (ie in transmission of the PBS).

Notes :

Be careful when acting on the QWP as its holder is not so stable !

Only use jog function for motorized HWP otherwise it will reset the current angle value to 0

You're welcome.

The input power is 30 mW and can indeed be tweaked with the HWP.

Also, the lockin amplifier settings have not been changed yet.

For the photodiode powersupply we will take the one used for the 1310 nm beam I think.

For the PSDs, one is now used in FC experiment so only one is available.

It seems that with the 100 mm focal length lens, the beam is small enough for the photodiode. And we can also tune their gain.

Thanks a lot for the report.

I have some questions. What is the input power of the beam and wouldn't it be possible to further reduce that power by using the HWP right after the fiber-output?
In any case, I remember that we had to use several ND filters as well...

Regarding the power supply of the PD, Can't you use the one from the absorption-PD? I think they are both from TL and the power supply should be the same.
By the way, was there a reason to reject the PSDs which we used in the past?

Following a discussion we had with Matteo and Simon I switch the position of the power meter and the birefringence readout with respect to the flipping mirror.

Meaning that the power meter is now in reflection and birefringence readout is now in transmission. The reason being to avoid possible polarizations issue at this reflection.

Because there is not enough space on the imaging unit, I added a small optical board that hosts all the birefringence readout part.

Just before it, I placed a 1" IR reflective mirror that sends the ir power to the power meter. This mirror is installed on a flipping mount (which was before holding a lens).

On the birefringence board I placed a optical density 3 and its reflection is sent to a vantablack beam-dump (not sure but seems so).

A 100 mm focal length lens coated for green and IR (it is coming from FC experiment) is mounted on a black tube linked to the PBS.

Then, 2 Thorlabs photodiode are looking at its reflection and transmission.

The trouble is that now there is only 1 power supply so we might have to switch one for a PSD.

Also, I tried to connect one photodiode to the lockin amplifier but it was saturating so I will have to add optical densities.

Yuhang and Michael

We completed assembly of the OPO crystal holder. We followed the intstructions from 812, although we were unsure of the orientation of the Peltier and tested it by applying a small (< 1 V) signal using a DC power supply. In this case, the cold side was with the text. The first time we assembled, it was placed the wrong way, so we had to flip the Peltier.

Figures 1-13 show the assembly of the OPO, though many of these are the same in substance as those presented in 812.

1: Incoupler mirror, looking at focus distance of ceiling light
2: Testing the hot/cold side of the Peltier (blank = hot)
3: Copper L placed on top of Peltier, with 0.1mm indium between the two
4: Black dot of OPO indicating the HR side
5: Indium not laying flush against the coppler L. We took off the Macor and made sure the indium was properly pressed against the copper
6: Exit hole of the OPO mount. The black dot is closer to this side
7: Other side of the OPO mount showing the plastic screws holding the Macor/copper L in place
8: Re-assembly of the OPO holder, placing the Peltier the correct way up (note the orientation of the wires)
9: Thermometers connected to the copper L with a mounting plate and screw
10: As above
11: Plastic holder with thermometers and correct Peltier orientation
12: Plastic screws holding the assembly
13: Exit hold of the OPO assembly again

Marc, Matteo

Today we first removed the PBS (it was taking too much place and prevented the installation of QWP and HWP).

We tweaked the last lens on the injection (L2) and recovered proper signals using the surface reference sample.

We placed the pinhole and tune its position at the waist (z=41mm) and plus/minus one Rayleigh range (ie Z = 36 and 46 mm). We checked that at these position all the pump power was properly transmitted (ie P_t = P_in = 28.8 mW)

We installed back the PBS and started to try to align it.

It was not possible to recover more than 22 mW. We checked that it was not due to misalignment by moving the pinhole that also gave 22 mW of transmitted power at maximum.

We simulated the effect of the PBS on waist size and position using Jammt : It moves the waist by ~ 10 cm. But this can be recovered by moving L2 by ~1 cm towards the imaging unit.

We did this change and could recover more than 27 mW at the extremum positions of the pinhole.

We then placed the razor blade (that cuts the beam in vertical) so that it is at the center of the pinhole holder.

We wanted to use the Translation_Stage_v3.vi to automatize the measurement but no values except the position changed...

By using the ..._v2.vi we got the beam waist = 33.39 um at z = 40.76 mm (see figure 1).

We reinstalled the surface reference sample and moved the translation stage to 40.8+1.5 = 42.3 mm (to compensate for the 3 mm thickness of this sample) and by tuning the horizontal and vertical screws of L2 we recovered the expected AC value (ie >0.4V).

We reinstalled the QWP and HWP, beam dump in reflection of the PBS and a small one instead of the steering mirror that catches the pump beam reflection from the sample (not enough space with the current holder) as in figure 2.

We did a Z scan of this sample and found R = 18.57 /W.

Either the alignment was the best since few months, either there are some polarizations effects that prevented to reach this value before I guess.

Finally, we started to reinstall the imaging unit part as in figure 3.

The steering mirror that was removed is now placed in front of the high-power power meter to allow for quick switch between absorption and birefringence measurement.

A 2" lens and the PBS inside the black tubing were also installed.

We still have to install ND filters and the 2 photodiodes on the readout path as well as proper walls.

Also, the motorized HWP is not yet connected to PC.

Yuhang and Michael

We measured again the RIN of the new Mephisto laser. The laser was sent to a tilted ND filter (prevents back reflection) -> 75mm focal length lens -> PD. The measurements were taken with the spectrum analyser taken out of the clean room.

The relative intensity noise and PD dark noise are shown in figures 1 and 2 (low freq span, high freq span). The units differ between the curves, though the RIN isn't recalibrated by much - the intensity noise was measured from the PD and then divided by the DC value of 0.89 V. For comparison, the KAGRA HP laser RIN shown in Aso-san's LVK presentation is given in figure 3. The new Mephisto laser at TAMA seems to have better RIN than the "Current" KAGRA laser, but not quite as good as the "New" laser.

Today I checked the 2 PBS available in PCI.

There is one with label (1064 nm PBS) so I decided to use this one.

I moved it to the mount with the good height but I'll need to print a new label for it.

I measured its transmission to be 166.0 uW with 166.4 uW of incident power.

Actually, it seems that the beam is too large for the small power-meter head as the power is supposed to be 30 mW there...

Anyway I started to tweak the alignment by checking the AC value of the surface reference sample.

Up to now I did not manage to recover proper alignment and it seems that the shape of the AC signal during Z scan is a bit strange (kind of similar to bulk sample...)

Hopefully I'll have a bit more time tomorrow to finish this alignment.

Note that to save time the pump laser is kept on with low power (30 mW)

I attached the measurement results which gives absorption = 267+/-59 ppm/cm with P_in = 3.745 W and P_t = 3.198W.

Yuhang and Michael

We did a quick measurement of the new incoupling mirror using the OPO replacement setup in ATC. The incoupling mirror was placed with the convex side facing forward in the mirror mount. The beam was reflected at a low angle (approximately 10 degrees), which is within the specified incident angle tolerance. Reflection and transmission were measured on both faces of the incoupler.

Using thorlabs power meter, we obtained:

Forward mount

Incident =   6.58 mW
Reflected =  6.07 mW (92.2%)
Transmitted = 0.537 mW (8.1%)

Reverse Mount

Incident = 6.64 mW
Reflected = 6.04 mW (91.0%)
Transmitted = 0.535 mW (8.1%)

The power meter is imprecise when being intermittently moved around between measurements - certain beam spot positions around the edge of the power meter seem to record more power (also noted during a previous measurement of OPO nonlinear gain in TAMA)

Very rough measurement obtained by focusing ceiling lights onto the table indicates the focal length to be approximately 15cm.

Today I installed the SHINKOSHA evaluation plate #7 on the translation stage so that the top side of the ingot is facing the laser sources with marking on the right side of the sample looking from the laser source side.

I moved the IU to 62.6 mm to compensate for the thickness.

I checked the X and Y centering using drop of the DC values to ~ 8mV which gave : X_center = 397.466 mm and Y_center = 120.629 mm.

I increased the pump power to ~3W and checked the Z centering at that position from the phase = 0 deg and got Z_center = 53.2 mm.

I also checked the top/ bottom and left/right Z centering over a 120 mm diameter area and found a tilt of this sample in both these directions to be ~ 0.8 mm.

Matteo confirmed that it was small enough to start absorption measurement.

The XY absorption measurement started over a circular area with diameter 120 mm and step size 250 um with P_t = 3.198 W and P_in = 3.745 W

Mainly two mechanisms cause zero baseline drift (ZBD): the birefringence of electro-optic crystal and the etalon effect formed by two parallel end facets of the EOM crystal [Z. Li, et al., Optics Letters, 41, 14, 2016]. The reduction of birefringence effect can be done by controlling modulation voltage or the crystal's temperature [K. Kokeyama, et al., J. Opt. Soc. Am. A 31, 81 (2014)].

To monitor ZBD, we can lock cavity on anti-resonance. In this situation, the PDH signal is very much insensitive to phase change. Therefore, the signal magnitude change comes mainly from sideband amplitude change, usually addressed as residual amplitude modulation (RAM). This amount of RAM exists all the time, including the situation when cavity is on resonance. The control loop makes error signal to be zero with such RAM present, which makes the cavity not exactly on resonance. Or we can say RAM introducing detuning for cavity. This is the effect we are interested in. Notably, this effect may appear also for the CCFC control loop, which should be investigated.

We used the method described in the last paragraph to monitor ZBD. Since this effect exists when cavity is on resonance, we use the slope of PDH signal at resonance to calibrate ZBD. In this way, we have the detuning influence cause by ZBD for detuning change. The attached figure (is not yet calibrated), after calibration, shows a ~4Hz detuning change around zero.