HWP birefringence measurement

I reinstalled the output cross-polarizer and also installed a HWP (WPQ-10640-2M from SigmaKoki) just after LC2.

I am now scanning the LCs voltages with various step to measure birefringence in 'electronic Soleil-Babinet' mode.

long measurement

The measurement started in elog 3295 finished.

It consists of about 6.3 millions couples of votlages applied to LC1 and LC2.

Figures 1 to 3 reports the resulting polarization states which is similar to previous measurement despite the 90 deg rotation of LC1.

Somehow we have an overall phase difference of 90 deg as before. I'll cross-check simulations.

When measurement finished, the LCs temperature was about 31degC. This indicates a stronge heating as the usual LC temperature is about 24 degC before heating for temperature control.

This seems to be related to the air conditioning failing noticed by Takahashi-san that could also explain the strange power fluctuations of the FDS laser reported in elogs 3306 and 3307.

Measured FIS of 5.4 dB at 25mW and 5.7dB at 40mW green power

8/1 [Chien-Ming and Hsun-Chung]

We measured around 5.4dB squeezing and 11.4dB antisqueezing at 25mW green pumped,
                             and 5.7dB squeezing and 16.7dB antisqueezing at 40mW green pumped.

Without green pump, OPO T=7.107 kOhm; BAB=192mV, p-pol PLL: 225MHz
At 25mW pump, OPO T=7.118 kOhm; BAB max=1.27V, min=80mV, p-pol PLL:165MHz
Nonlinear gain=6.61

At 40mW pump, OPO T=7.120 kOhm; BAB max=3.28V, min=40mV, p-pol PLL:130MHz
Nonlinear gain=17.08

The SHG power in front of MZ drops to 151.5mW today.
In addition, we found the LO power to HOM also drops to 1.68mW. We can't increase it by alignment, so we have to replace the OD0.3 ND filter before IRMC to OD0.1.
As a result, the LO power is restored to 1.94mW.
We suspect that the output power of the main laser may be continuously decaying, causing the SHG and LO power to drop!

Attached are two squeezing measurement at different pump powers. It is worth mentioning that when the HOM DC cable is too close to the oscilloscope, some noise peaks will appear obviously, especially on dark noise.
The units setting parameters of SR785 was also attached.

OPO temperature is not optimized resulting in poor squeezing measurement

7/31 [Chien-Ming and Hsun-Chung]

We investigate the reasons for the squeezing level decline on 28th July http://www2.nao.ac.jp/~gw-elog/osl/?r=3305.
Originally we suspected it was caused by poor cleaning of the optics, but today we found out that the phase-matching temperature of OPO is not at the optimal value.

On 7/28 we set OPO T=7.159 kOhm with 25mW green pump and T=7.184 kOhm with 40mW green respectively in order to lock the p-pol at 270MHz.
Today we check that the optimal value of OPO T should be set near 7.107 kOhm to achieve the maximum nonlinear gain.

OPOT=7.107k; 25mW green; BAB Max=1.26V; p-pol=33MHz*5=165MHz.
Without green; BAB=196mV-6mV(Background)=190mV; p-pol=49MHz*5=245MHz.
Therefore the nonlinear gain=6.63 at 25mW pump.

HOM visibility:
LO at AMC: TEM00=5.96v; hom=2.4mV+2.4mV; mode-matching: 99.92%
BAB at AMC: TEM00= 196mV; hom=6.4mV+1.8mV; mode-matching: 95.98%

SHG output power in front of MZ drops to 165.5mW even if the SHG temperature and alignment are optimized (194.9mW on 7/28)

We preset the SR785, however, the parameters are incorrect so the squeezing measurement is weird today.

Measure FIS at 25mW and 40mW after clean some optics

7/28 [Chien-Ming and Hsun-Chung]
At first, the SHG is 119.7mW in front of MZ.
We correct the HOM DC balanced from -10mV to 0mV by fine-tuning HOM BS Yaw.

Hsun-Chung bought some glass dropper bottles to hold acetone (buy from HANDS).
We cleaned some optics on the optical path of HOM and SQ and then checked the BAB alignment at AMC.
TEM00=206mV; hom=5.6mV+5.6mV; mode-matching:94.8%

In the beginning, the OPO T was 7.117k(p-pol is 270MHz) with pump 25mW, the BAB max=52mV, min=24mV.
After setting the OPO T to 7.159k(p-pol is 270MHz), the BAB max= 952mV, min= 104mV
At the 25mW green pump, we measured 4.47dB squeezing and 8.43dB anti-squeezing which is worse than yesterday's result.
Therefore, those optics may not be cleaned well.

Then we measured twice FIS at the 40mW pump, got 5.28dB SQ and 12.13dB Anti-SQ at first, and 5.29dB SQ and 11.65dB ASQ at second.
Before the second measurement, the MZ failed to lock again. The reason is that the output power of the SHG seems insufficient (100mW in front of MZ).
By setting the temperature of SHG from 3.158k to 3.123k, the green light in front of MZ becomes 194.9mW.
The BAB max=1.54V and min=152mV at 40mW pump and OPO T= 7.182k (p-pol is 270MHz).
Without green pump, the BAB=234mV at OPO T=7.121k (p-pol is still 270MHz)
Therefore, the nonlinear gain is 6.58 at the 40mW pump.

The attachment is the comparison of the measurement data for two days

FIS 5 dB squeezing measured

7/27 [Chien-Ming, Hsun-Chung]

Summary:

 We measured about 5 dB squeezing and 11 dB anti-squeezing at 25.1mW pump power.

 

 Current parameter settings:

DDS1	ch1	SHG & IRMC demod.	phase 140 deg.
DDS2	ch2	GRMC demod.	phase 200 deg.
DDS3	ch3	P-Pol PLL	54MHz (0 mW green) 38MHz (25 mW green)
DDS3	ch1	homodyne phase	SQ: 75 deg ASQ:140 deg

Details:

CC light is not coupled into the OPO at all, after realignment

CC: TEM00=19.8mV; hom= 4.6mV+2.2mV; mode-matching: 74.4%.

 

SHG output power dropped to half, so we realign the incident 1064nm again, and now the green power before MZ is 126.7mW.

When the green light is well coupled into OPO, the GRMC and MZ are not easy to be locked. Seems to be interference from the feedback light.

 

When put the LO flip mirror back, the LO light is not coupled into the 2nd HOM PD at all.(but it worked yesterday)

We have to adjust the lens position to achieve HOM DC balance.

 

 

The BAB nonlinear gain today without green and with 25.1mW green again: (this time we have considered the background by blocking the BAB.)

without green, Max=240mV-6mV(BG)= 234mV ; OPO T:7.117
with 25.1mW green, Max=1390mV (Min=96mV) ;OPO T:7.119

Pump power (mW)	0mW	25.1mW
OPO temperature (kOhm)	7.117	7.119
P-Pol PLL frequency (MHz)	270	190
BAB maximum (mV)	234	1390
nonlinear gain	1	5.94

cc1 cc2 can be locked smoothly.

cc1 error signal EPS1OUT Vpp=266mV.

cc2 error signal EPS1OUT drifts up and down over time in the range of 348mV, the Vpp is around 170mV.

The parameters we use for SR785 are:

128Hz~102.4kHz AC 
FFT1 LogMag Hanning

Compute Avgs: Yes; type: Exp./cont; # Avgs: 100 ; Display Avg: RMS
SAVE: Display to Disk

realigned green to OPO, measured nonlinear gain, and HOM visibility

[Chien-Ming, Hsun-Chung]

 

To check the alignment of green pump to OPO, we measure the nonlinear gain of BAB maximum in 1st HOM PD.

without green, Max=248mV (forgot to subtract the background); p-pol PLL 270MHz; OPO T:7.127

with 25.1mW green, Max=1540mV (Min=120mV) (also forgot to subtract the background) ; p-pol PLL 190MHz; OPO T:7.129

after two hours..

with 25.1mW green, Max=1200mV (Min=120mV) ;OPO T:7.129

I realign the green to OPO and adjust temp. of OPO

with 25.1mW green, Max=1580mV (Min=120mV) ;OPO T:7.123

without green, Max=248mV; OPO T:7.122
 

green power (mW)	0mW	25.1mW
OPO temperature (kOhm)	7.122	7.123
p pol PLL frequency (MHz)	270	190
BAB maximum (mV)	248*	1580*
nonlinear gain	1	6.37

*This value does not check and subtract the background caused by p-pol.

 

To optimize HOM visibility, we realign the OPO output BAB mode matching at AMC.

Before: TEM00=164mV; hom=34mV+8mV+8mV ; 76.6% mode-matching

After: TEM00=218mV; hom=8mV+8mV ; 93.2% mode-matching

 

Realign the IRMC mode matching by monitoring the T of IRMC servo
Before: TEM00=920mV; hom=48mV+48mV; 90.6% mode-matching

After: TEM00=960mV; hom=56mV; 94.5% mode-matching

 

IRMC PDH signal and SHG PDH signal are using the same DDS channel to demodulate, so readjust the DDS phase from 110 to 140 degrees to reach a compromise. (Used 160 degrees to reach the best SHG PDH signal yesterday)

 

LO mode matching at AMC

TEM00=6.64V; hom=16mV ; 99.76% mode-matching

 

GRMC failed to lock again, checking the PDH signal and reloading the DDS2 to change the phase of GRMC demod. from 165 degrees to 200 degrees (Used 100 degrees yesterday).

realigned BAB, p-pol, SHG, GRMC,and MZ

7/25

[Chien-Ming, Hsun-Chung, Marc, Michael]

We realign the P-pol and BAB by using the steering mirrors before PBS

p-pol :

Before: TEM00=1.78V; hom=460mV+100mV ; 76% mode-matching
After: TEM00=1.92V; hom=160mV+160mV ; 85.7% mode-matching

 

BAB :
Before:  TEM00=1.05V; hom=264mV+200mV+180mV+140mV+50mV=834mV ; 55.7% mode-matching

After:  TEM00=1.17V; hom=232mV+200mV+200mV+140mV+50mV=822mV ; 58.7% mode-matching

To check the green pump to OPO alignment, we use BAB to optimize the nonlinear gain.
Without green, we phase lock the p-pol at 270MHz and tune the OPO temp. from 7.147 to 7.123.

we obtain the maximum BAB output by putting the power meter before homodyne BS.

The value is 1.78V - 900mV(background) =  880mV (however, power meter is set at 532nm)

 

When we want to introduce the green pump, we failed to lock GRMC and MZ, also the SHG output is not good.

 

So I realign the SHG input 1064nm beam,
Before:TEM00=1.58V; hom=1.28V; 55.2% mode-matching
After:TEM00= 354mV; hom:10mV; 97.3% mode-matching

 

Then I check the PDH signal of SHG and optimize it by changing the phase from 110 degrees to 160 degrees.

I successfully locked SHG. The power in front of MZ is 132.6 mW.

 

Then I check the PDH signal of GRMC, it's quite small, the S/N=280mV/50mV ~ 280mV/100mV.

Optimizing this signal by adjusting the mirror at the GRMC input reflected beam and then reloading the DDS2 to change the phase of GRMC demod. from 165 degrees to 100 degrees. We obtain the S/N= 126mV/15mV.

Finally, we successfully locked GRMC after adjusting the parameters of the servo.

After locking the MZ, the pump power corresponding to the MZ offset is as follows:

 

MZ offset vs green pump power

MZ offset	Green pump power
4.0	15.7 mW
4.1	21.6 mW
4.2	27.0 mW
4.3	32.3 mW
4.4	37.8 mW
4.5	43.4 mW
4.6	49.0 mW
4.7	54.6 mW
4.8	59.6 mW
4.9	64.9 mW
5.0	70.1 mW

larger polarization states generation

I scanned LC1 and LC2 voltages from 0 to 25V with 0.02V step at 30degC.

Today I rotated LC1 by +90 deg to investigate the phase difference with respect to simulation.

I started measurement between 0 to 25V with 0.01V step at 30degC.

polarization states generation with smaller step

I tried to generate again polarization states but this time with 0.01V increment instead of the previously used 0.05V.

Because of TAMA visit I had to turn off the laser before the several days long measurement finished.

In any case, the results shows again good agreement with previous results and simulation and also higher polarization states coverage.

Comment to arbitrary polarization states generation (Click here to view original report: 3290)

This result can be compared with simulation were I assumed for now same retardance vs voltage and extinction ratio equals to 1.

The rotation and retardation are shown in figures 1 and 2.

The retardation agrees really well with measurement if we wrap the measured retardation to positive values (fig 3).

Forthe rotation, it seems we have some kind of 90deg offset. I'm wondering if it could be because the first LC is rotated by -45deg instead of +45deg.

realigned MZ and GRMC

[Chien-Ming, Hsun-Chung, Marc, Michael]

First we found out that the power just after SHG is a bit lower than usual (measured 216mW) so we might have to tune its temperature.

Then, we found that MZ and GRMC were quite misaligned (especially in pitch).

We placed a power meter in transmission of GRMC to realign them.

First, the power had some periodic increase so we reduced the MZ scan period from 10 to 0.

Also, realigning it helped. After realignment we got :

1st arm TEM00 2.54V ; hom = 0.18V : 93% mode-matching

2nd arm TEM00 308mV ; 3 hom each = 6.4mV  ; 94%

We tried to lock GRMC and MZ but the lock was not stable. So we had to increase the gains as follow:

increased GRMC gain to 3.1 from 1 ; increased MZ gain from 1 to 3

Finally, they locked and with MZ offset 4.54 we measured 44.7mW in transmission of GRMC in good agreement with the expected value.

arbitrary polarization states generation

After spending a lot of time generating polarization states and rotating the output polarizers I found out that this method is not too precise.

While the overall shape is similar to simulation, there are several points were the fitting is failing and gives polarization rotation 0 or 90 deg and retardation 45deg.

This is especially annoying as the measurement is far longer than expected (poor vi implementation limited measurement speed to 6Hz and need to rotate at least 36 times the ouput polarizer)...

In the end I removed the ouput polarizer and used the polarization camera readout.

THe measurement are agreeing now really well with simulations.

Note that in the simulation an arccos of the retardation limits the retardation to positive values only.

recovery of IRMC automatic lock

Marc, Pierre (remote)

Since the replacement of the EOM, we had difficulties to achieve automatic lock of IRMC.

This is mainly due to the fact that we're using 78MHz sidebands instead of 88MHz and had to change the PD in reflection of the IRMC for a non-resonant one.

First, the reflection signal of a scan of IRMC is different than before.

The base is at 1.38V and the TEM00 dip at 692mW while in the past the dip was negative.

This meant that we had to modify the pin P4 strap from pin 2-3 to pin 1-2.

Also, the threshold value was tweaked looking at this signal.

Then, the error signal was about 83mVpk while it was about 10 times more in the past.

Despite putting the servo gain to max I could not achieve lock (even though it was detected from the blinking LED).

I used the CCFC amplifier (32dB) and got an error signal about 2.2Vpk.

As the error signal slope and triangle scan slope are identical, the lock can be achieved with the switch INV.

I put a gain of 0.5 and could lock reliably the IRMC.

PLL control

Marc, Michael

I wanted to check some of the multiplexer output diagnostics for the PLLs but the BEAT signal (the electrical signal from the fiber PD) had disappeared. We roughly checked the coupling into the CC fiber PD using the power meter and it seems reasonable (ML 4.34 mW, CC 0.64 mW, 1.05 mW out). The fiber PD electrical output have a large DC offset. I noticed that the PPol signal was oscillating at 45 kHz which I thought was a bit strange.

I eventually plugged the CC BEAT signal into the spectrum analyser with a wide setting, and noticed that the large peak corresponding to what was at the level of the previous measured CC BEAT (-11 dBm) had drifted quite far away from its usual frequency setting and the PPol BEAT peak was sitting at the previously mentioned 45 kHz. Both aux laser temperatures were quite far from the values listed in the wiki, so I readjusted them to the proper values and saw the good BEAT signals (CC -11 dBm, PPol -7 dBm).

However, the CC PLL lock is still unstable. Using MUXOUT on the CC PLL board, I could see that the digital lock detection was not particularly great. Digital lock detection outputs 1 when the phase delay between 3 cycles is less than 15 ns, and 0 when not. I noticed the output was not continuously 1 (i.e. has a lot of phase noise). It remained locked for about 4-5 minutes. I still need to figure out what the other diagnostic functions do.

Comment to Generating arbitrary polarization states! (Click here to view original report: 3285)

The step voltage sub vi was fixed (misplaced connection) so I will spend the week end trying to generate and analyze 40 000 polarization states.

I'm scanning each LC voltage from 0 to 10V with 0.05V increment, at 30degC.

The output polarizer angle conversion is so that 298.25deg is our 0deg (ie cross polarizers configuration). The output polarizer will be rotated by 5deg increment.

Note that for tprevious measurement there is a -5deg offset with cross polarizer angle as it was assumed to be 293.25 deg.

PLL controls

I checked the CC PLL fast and slow control as described in 1205, with the only difference being that I used 21 MHz as the base signal. The purpose is to see how the PLL control loop responds to small modulations in a beat signal that is somewhat offset from the main LO.

For 1 MHz deviation, I saw DC offset ~ 50 mV (vs 10V for Yuhang) and no apparent AC signal. (fig 1)
For 1 kHz deviation, I saw DC offset ~ 50 mV and a somewhat messy signal with several frequency components. One is about 1.7 kHz (fig 2) and 100 Hz is highlighted in figure 3.

Seems like the fast loop is behaving a bit strangely

I also stared at the fast and slow loop oscilloscope signal for a few minutes each but didn't see much glitch behaviour. Probably not a conclusive test though.

Generating arbitrary polarization states!

[Marc, Shalika]

Labview modifications :

A new sub-vi has been created 'step voltage' that scan LC1 voltage from min to max voltage with user defined step. Once it reaches the maximum, the LC2 voltage is increased by the same step.

The time of each iteration was defined to have 1 data / step.

There is some little issue sometimes where Vlc2 > Vmax... Maybe du to our exit condition that does not take into account the finite numerical resolution we might have...

Controller issue

We found out that when the Kinessis software is started after Labview, the voltage controller of LC1 can not be used..

This is indicated in our vi front panel if the 'controller' LED is off.

This was the case for previous measurement and is now corrected.

First polarization state generation!!

Figures 1 and 2 are respectively measurement and simulation.

They agree quite well with each others!

We need to improve the fit by taking more measurement (was done every 10 deg rotation) and improve the simulation by taking the temperature and extinction ratio of LC1.

First trial of polarization states generation

The goal is to scan the voltages of LC1 and LC2 and monitor the generated polarization state for each voltages couple.

LC1 fast axis is at 45deg of input polarizer and LC2 fast axis is at 0 deg of input polarizer.

The polarization state is scanned by rotating the output polarizer rotation by 5 deg increment from 0 deg (cross polarizer configuration) to 180 deg.

Labview modifications :

In order to somewhat efficiently scan our voltage space parameters, we modified the 'Sawtooth Formula Voltage Theta' subvi to scan our parameter space.

We can apply 1 sawtooth to each LC voltage controller with a user defined phase shift and frequency ratio. Below we used a 11:12 frequency ratio with 0.5 Hz frequency applied to LC1 (11/12*0.5Hz for LC2).

They are each doing 10 periods and then the output polarizer is rotated by 5 deg.

Analysis :

The data are stored in 2 files : one from rotation angle of 0 to 90 deg and one from 95 to 180 deg.

All the data are combined, voltages are rounded to 0.1V and for every couples (V_LC1,V_LC2) if there are at least 25 rotation angles values, the data are further processed. More precisely, the normalized transmitted power is fitted by P(theta) = a/2 * ( 1 + cos (2*ellipticity) + cos (2 * (theta - azimuth))

In the fit, the initial guess of the azimuth is given by the rotation angle with maximum transmitted power.

You can see some results in the attached figure (in total we have 1161 couples that meet our requirements).

next steps

I would like to replace the sawtooth scan by a step function scan to avoid the 0.1V rounding.

It could be useful to optimize our Lissajous scanning coefficients

Polarizers and LC calibration

In order to characterize the polarization states generated by our LCs, we can rotate the output polarizer while applying different voltages to our LC.

As our motorized rotator is from Thorlabs, we had to switch the position of our input and output polarizers.

We estimated the extinction ratio of our polarizers to be about 25 000 each.

Then, we calibrated again each LCs as shown in fig 1 and 2 for LC1 and 3 and 4 for LC2.

Their respective fast axis direction is 45.74 deg and 69.00 deg.

From figures 2 and 4, the maximum retardation at these angles is below 6 nm offset from 532 nm which indicates a misalignment of about 0.01 deg from the fast axis from simulation.