[Shalika, Mitsuhashi-san]

This elog is about doing the ringdown measurement. In continuation to work in elog. 

Details:

1. The amplitude of TEM00 mode in transmitted signal is 37mV. The PD at transmitted beam is set with 20dB attenuator. The cavity was able to lock the TEM00 mode at 37mV for 12 minutes. The error signal(CH1--yellow) for the corresponding peak looks as shown in Fig 1. The demodulation phase was set to 185º.

2. The filter cut off frequency was 3 Hz and gain was 100. 

3. The laser temperature was 8510±20. The offset voltage varied from -0.1 to 0.1. 

4. The feedback signal(CH4--green) and error signal(CH1--yellow) during lock looks as in Fig 2.

5. The ringdown measurement was done by setting the trigger and switching off AOM. The signals look as shown in Fig 3,4,5. 

Next Step:

We will analyze the data to calculate the decay time. 

[Shalika, Mitsuhashi-san]

This elog is about us trying to improve lock time. In continuation to work in elog. 

Details:

1. Today we finalized our cavity mirror alignment. The TEM00 mode observed using PD at transmitted beam is of 10.4V ampitude and is as shown in Fig 1 during laser scan. The corresponding error signal looks is as shown in Fig 2. The demodulated signal's phase is set at 110º. All the higher order modes were less than 50% of TEM00 amplitude.

2. We found the laser temperature at which TEM00 mode appears. We set the laser temperature to 9100. 

3. We changed the offset voltage and around 0.7 to 0.8 V, the cavity was able to lock for 10 minutes(shown in Fig 3).The filter cut off frequency was 10 Hz and gain was 200. The amplitude of the locked signal was a bit less than the TEM00 mode amplitude. We are certain it was locked to TEM00 and was not locked to higher order mode, since no modes other than TEM00 were observed at an amplitude of more than 5V. To fix this we will change the filter cut off frequency and gain. 

Next Step:

We will try to change the filter properties and make the locked signal amplitude same as TEM00 mode amplitude(as in Fig 1).

[Shalika, Mitsuhashi-san]

This elog is about us trying to improve lock time. In continuation to work in elog. 

Details:

1. We tried tuning the cavity mirror alignment to increase the TEM00 mode. The amplitude of modes has improved (more because of the incident mentioned in point 2). Also, we tuned more to suppress other higher-order modes. Now the modes look as in CH2 in Fig 1. The error signal for the corresponding TEM00 mode is shown in CH1 in Fig 2. The scale for CH2 is 5V. We tried locking but it only locked to higher order modes as in Fig 3. 

2. On Tuesday evening by mistake I touched the first mirror in the optical setup outside the cavity. As a result, the transmitted beam was lost. I was able to fix the problem, and now the AOM diffraction efficiency is improved and is 57.3% (output: 6.59mW and incident:11.5mW). In the past, the efficiency was 38%. I made sure that there are no higher-order modes (shown in Fig 4)

Next Step:

We will try to tune the temperature of the laser to find the desired frequency at which TEM00 mode appears. Also, we will try to find the optimum offset voltage and keep improving the mirror alignment.

Marc with remote help of Matteo and Yuhang

We changed the ground of all DDS boards and the 500 MHz clock from the 'high quality ground' to 'ground' as in http://tamago.mtk.nao.ac.jp/tama/ifo/general_lib/circuits/000000_general/NIM_pin_connection.pdf

We tested that all components could be turned on and all applied voltages were correct.

We reinstalled the clock and DDS boards in the usual nim rack without issue.

DDS1 and DDS2 provide the expected signal (ie about +9/10dBm with internal amplifier and --8/9dBm without).

DDS3 have all outputs connected to internal amplifiers. Only the DAC0 provide the expected output. I checked that the output of the evaluation board is correct (-8dBm) so the issue is either in the internal amplifier or from loose connections.

[Marc, Mitsuhashi, Rishab, Shalika]

Summary of these past days activities as elog was down.

Preparation of optical table and rack

We moved the table away of TAMA AS vacuum chamber and installed a spare rack we found in TAMA north arm.

We setup Manuel's PC there with all appropriate softwares. It is now connected by Ethernet to the FC new DGS switch.

Optical setup and camera test

We installed a second f=50mm lens roughly 100 mm after the first one to have a collimated beam.

Then, we installed a 10:90 BS so to have about 6 mW in reflection.

We measured the beam diameter to be about 800 um and quite well collimated.

In the reflection, we installed a QWP and HWP mounted in motorized mounts.

Finally, we installed our polarization camera (PAX1000IR2) several tens of cms after to have some space for our future tests.

As expected from the datasheet, we can measure the polarization rotation or retardation with accuracy of 0.5 deg. This is same order of magnitude of our currents birefringence characterization setups (both from NAOJ or ICRR)!

We could confirm that the rotation of QWP and HWP yield the expected ellipticity and rotation changes respectively.

We then applied continuous rotation of QWP and HWP with a 5:6 Lissajous pattern.

Fig 1 shows a screenshot of the camera software during this measurement where you can see the polarization state, Poincare sphere, Stokes parameters and ellipticity/rotation angles.

For now, we can save several parameters in csv and in figure 2 you can see the time evolution of some of them.

Check of LC

Finally, we wanted to install our 2 LCs.

We found that one of them is likely broken as it can not be recognized from the temperature controller out of the box.

We are in contact with Thorlabs to solve this issue

[Marc, Mitsuhashi, Shalika]

We installed the surface reference sample and did several z scans to optimize the translation stage and imaging unit positions.

In the end we found the best calibration factor R = 14.37/W with z = 38mm and z_iu = 69.5 mm.

We installed the half inch sample with HR side facing the laser source and optimized the (x,y) position to maximize the transmission (x,y) = (327.4,121.8).

Finally, we measured absorption with 3 different inputs power and measured the HR surface absorption to be about 45 ppm.

Shalika, Mitsuhashi-san

This elog covers the aspects of

1. Attempting to lock the cavity. We were able to lock the cavity for 10s.  

Details:

After proper installation of the new mirrors, we observed the transmitted beam. We scanned the laser by providing a ramp temperature signal (see Fig 1). In the oscilloscope image the colors correspond to the following signals.

CH1-yellow--> Error Signal from mixer

CH2-blue-->Output from PD at Transmitted beam path

CH3-purple--> DC output from RF PD at FI

CH4-green--> Feedback signal from SR560

We then attempted maximizing the TEM00 mode by tuning the pitch and yaw of the cavity mirrors. Since we didn't have access to a camera sensitive to 1550 nm, we had to do the tuning intuitively. We tried maximizing the modes and at the end we were left with only one mode maximized, as in Fig 2. No mode was observed to be maximized at an amplitude more than this, and so we guessed that it could be the TEM00 mode. We then trying locking the cavity by providing the error signal as feedback to the laser. We were able to lock the cavity for 10s (See Fig 3)

How to lock the cavity (This is the approach we took and is also for anyone to refer who tries to attempt this in future for first time like us).

1.Make sure that you have tuned the mirrors to have reflected beam power (at the FI) to be maximum.

2.Observe the transmitted beam and Reflected beam using Photodiodes. The photodiode used to analyze the reflected beam is a RF PD. It has DC and AC output.

3.Scan the laser by providing a temperature ramp signal.

4.Initially you will see a lot of peaks corresponding to higher order modes in the transmitted beam (as in Fig 1). The zero-order beam is embedded among them, and the purpose is to tune the alignment of the mirror to maximize this and suppress other higher order modes as much as you can. Access to a camera can make this work easy, but in case if you don't have access, you may simply maximize all the beams. And at the end you will be left with only TEM00 mode maximized.

5.When you have obtained the most maximized beam (as in Fig 2), you may try to lock

6.Make the demodulation path using AC of PD, mixer and SR560.

         a.Connect the AC of PD to F terminal of mixer.

         b.Set the frequency of mixer same as EOM frequency using function generator (L terminal of mixer)

         c.Provide the demodulation phase shift to the mixer (calculated using 2*pi*L/Lambda, where L is optical length after EOM)

         d.The unfiltered mixer output from terminal I is the error signal. Filter the error signal and provide the output of filter is provided to another SR560

         e.The above output is coupled with a DC offset voltage to provide feedback to the laser.

7.Set the temperature of the laser at which you observed the TEM00 mode during the scan.

8.Tune the offset voltage to see if the cavity can lock.

9.When the cavity is locked you would see the transmitted beam PD signal maintain a constant value same as that around the TEM00 mode amplitude level (as in Fig 3).

10.The error and feedback signal will be varying as an attempt to maintain the lock.

11.To increase the lock time keep tuning the mirror alignment, laser temperature and the offset voltage.

What we will do next:

We will try tuning the alignment, offset voltage and laser temperature to improve lock time. 

Mitsuhashi, Shalika,

We installed the new cavity mirrors and the demodulate part.

• install the new cavity mirrors
The new cavity mirrors' ROC is the same as the previous one(ROC=50mm)

• install the demodulate part
We install the preamplifier(SR560 made by Stanford Reserch System), the local osillator(AFG1062 made by Tektronix), and the mixer(ZAD-1-1+ made by Mini-circuits) to demodulate the beam.
The setup of the local oscillator's phase was 85°.
The setup of the preamplifiler was as follows(The picture was attached).

[Marc, Mitsuhashi, Shalika]

We plan to use the 120 * 180 cm optical table down the stairs in TAMA to install the heterodyne WFS scheme and compare several birefringence measurements schemes.

First, we removed all the installed setups : the scatterometer optics have been placed in a box (all are for 633 nm) placed in storage room; the scattering calibration setup has been placed together with the other optics/ components of this setup; the small clean booth is now in storage room, and the u-shape optical breadboard was placed closed to the end mirror of TAMA IMC.

Then, we also brought several unused TAMA optics and mounts that are now stored in a plastic rack nearby the optical table.

We installed the laser source at the expected position (note that it starts to emit with 0.7A and we are currently using it with 0.8A that corresponds to about 72mW).We installed a QWP, HWP, a f=50mm lens and an old FI from the FDS experiment (from QIOPTIC in the FC wiki). After tuning the QWP and HWP angle we had about 93% transmissivity.

The plan will be to use the heterodyne WFS LO beam as a pick-off beam to compare the various birefringence measurement techniques while using the main laser beam path to start the cavity lock activities.

Nishino,Mitsuhashi,Shalika

What we did:
We modelated a lazer frequency to observe the resonance of the cryogenic cavity, and checked the reflected beam power.
As a result, we can't observed the resonance of the cavity at all.
The picture of a oscilloscope was attached when we modulated a lazer frequency. The blue line is the reflected beam power.


What we will do next:
We optimize the mirror angle and the lens position.

[Marc, Nishino]

We restarted the probe and pump laser sources.

When we tried to move the translation stage it tried to move to crazy position so we stopped the LabView motion and manually brought the translation stage close to the home positon.

Then we home the translation stage with Zaber.

After that, we could properly move the translation stage.

We tried to measure the surface reference sample at the previous good position but got lower calibration factor than expected (about 12.2/W).

We slightly changed the sample z position and the imaging unit position but could not really improve the calibration factor meaning that the alignment might have drifted.

I tried to restart the standalone and controls pc after the electrical shutdown but could not get any signal in medm.

I found that the timing signal generator was also resetted.

I turned off the standalone PC, recovered the correct timing signal parameters(  as from elog 1537 ie square wave at 65536 Hz, level 2.5 V, offset 1.25 V) then turned on the timing signal once the standalone pc was on.

After that, I could get the usual readout in medm screen so I restored the snapshot that was saved just before the shutdown.

Mitsuhashi, Shalika,

What we did:
We install the input mirror(Figure 1) anyway and checked that the reflected and the incident beam overlapped.
The reflected beam's power was 1.71mW.
We install the photo detector to get a error signal from cavity.

Now we didn't detected signals from the transmitted beam path, so we should optimaize the mirror's angle.

What we will do next:
We will optimaize the mirror angle.
We will checked whether the code for modulating laser frequency can run well or not.
 

erratum : the correct HR surface absorption should be about 34 ppm (eg peak at z = 32 mm) while the previous estimated value is due to interferences.

Mitsuhashi, Shalika,

What we did:
We install HWP before mirror and observed the reflected power with respect to the degree of rotation HWP. The result was attached.
The maximized power was 3.42V and the angle was 260°.

We install all instrument before the cavity anyway. 

What we will do next:
We will try to make the cavity and install a photo detecter to catch the error signal from the cavity.

[Marc,Shalika]

It is likely that the half inch substrate is fused silica (n=1.45) so we shifted the imaging unit accordingly (ie by 1.1mm instead of 1.3mm).

We repeated the surface absorption measurements with incident power  of the pump between 0.89W to 1.66 W.

Results are in figure 1 and we got 10.5 ppm absorption of the HR surface.

Shalika, Mitsuhashi-san,

This elog covers the following aspects:

1. Setting up EOM after LB1901C

2. Setting up lens LA1986C after EOM and beam fitting after it. 

3. Setting up Faraday Isolator after LA1986C.

(Details below)

1. a. In continuation of our setup, we installed a HWP (to optimize the polarisation of the beam entering the EOM). To find the desired angle, we set a PBS after the HWP and checked for the transmitted power. (The PBS was removed before installing EOM) The power was observed at various angles of rotation of HWP to find the optimum position (see image 1 for graph). The angle was set at 117.5° where the power of the beam was observed to be 3.91 mW. (See image 2 for setup)

  b. We installed the EOM at the minimum waist position from the lens (LB1901C) after the HWP. Although we needed to supply 6.177V for 1 rad phase shift, we could only supply 5 Vp-p. This was a limitation of the function generator. The observed beam power after EOM was 3.768 mW.

2. We then installed a mirror and set up a lens of f=125mm. To optimize beam propagation through Faraday Isolator, we observed the beam profile after the lens to find the minimum waist position. The minimum waist was found at 194.7 mm and 175.6 mm for major and minor radius respectively. (see graph 3 for details)

3. The Faraday Isolator(FI) was installed at the minimum waist obtained from 175 mm from lens. The power was observed after the FI to find its optimized location and was found to be 3.390 mW. For efficient reflection from the surface of the next mirror, we installed a HWP after the FI. (see setup in the image 4)

Next Step:

1. We will optimize the angle of HWP after FI.

2. We will install the last lens before the cavity. 

Shalika, Mitsuhashi-san.

This elog report covers the following aspects:

1. We observed that the beam after EOM was experiencing astigmatism and in order to make corrections we tried increasing the diffraction efficiency of the AOM. 

2. The desired operable voltage for the EOM was calculated.

(Details below)

1.   a. Initially we hadn't placed any HWP after the Faraday Isolator(FI). Since mirrors reflect one particular polarisation more efficiently than others. As a result in our case, the mirror was not reflecting efficiently. Yesterday, we placed a HWP after the FI and observed the reflected power with respect to the degree of rotation HWP (see Graph 1). The angle was set to 100° and the maximum power found was 11.50 mW.

      b. We then observed the diffraction power (We had chopped the zero order beam) of AOM with respect to the degree of HWP(placed just before AOM). See 2nd graph. This HWP was set at 100° too. The power of beam before AOM  was 11.5 mW. The AOM was aligned efficiently and the power of 1st order beam obtained was 4.38mW. The diffraction efficiency is now 38%. (The connector being used for the RF driver is correct but loose. The loose connection alters the diffraction power from 0.1 to 0.9 mW. We feel that the proper connector can remove this issue)

      c. Since we changed the alignment of AOM we had to do beam fitting after lens LB1901C (see Graph 3 and 4). This would help the beam to enter EOM efficiently, as we will place the EOM at the minimum waist position i.e 128mm from the lens. The energy filters used were, 3.0(attached to beam profier) and 1.0(placed after lens to avoid saturation). We also made sure that there was no zero order beam (see images 5 and 6). Before the adjustment the beam waist was at 91.4 mm and 142.9 mm for major radius and minor radius respectively. After doing the adjustments the beam waist is at 128.6 mm and 137.1 mm for major radius and minor radius respectively. 

(See Image 7 for experimental setup)

2. We calculated the input power for the EOM. Since 1W is the maximum RF power, the maximum voltage(peak-peak) that can be applied was found to be 20 V. The impedance was considered to be 50 ohms at the termination. For 1 rad phase shift, the required RF power is 19.8dBm. Therefore, Vp-p decided to be applied is 6.177 V. (Since we didn't have the datasheet for this EOM in the lab, we took this EOM (being the closest one) into consideration for calculation). 

Next Step:

We will place the EOM at the optimum position obtained after the results from the above beam fitting, and place the last lens before the cavity. We will then try to lock the cavity. 

Previous measurements of Shinkosha7 were taken before we updated our calibration procedure.

I performed again the birefringence measurements with the updated calibration.

First I reinstalled the 2 steering mirrors and realigned the IR beam. I measured vertical AOI = 0.003 deg and horizontal AOI = 0.000 deg.

I tuned the HWP and QWP to minimize the power in reflection of the readout PBS.

Then, I took 10mn measurements while injecting s then p polarization.

Fig 1 reports our calibration factors. Also we have an error of 4e-4 on the p and s polarization estimation.

Then, I installed the sample and took measurement from 0 deg input polarization angle (s polarization) up to 75 deg with 15 deg increment.

For some measurements, I was worried about saturation so I repeated such measurement with larger lockin amplifier range.

The measurements are reported from fig 2 to 7.

The birefringence parameters are shown in fig8. The wrapping of dn and theta is highly visible.

Actually, it is possible to unwrap dn as shown in fig9 but this assumes that all our degeneracy comes from delta n and not theta while in reality it is a combination of both.

I think that this is the reason why we have some 'peaks' in delta n that seem to be present only for some input polarization angle.