Mitsuhashi, Shalika-san

What we did:

1. We measured the beam propagation after AOM and LB1909-C. When we measured it, we used PBS, WHP and QWP.
The result was as fellows and attached.

2.We set EOM following the above results. The picture was attached. We put QWP and HWP before EOM to meet the requst of EOM about polarization.
We confirmed that the beam power after EOM was maximized, and measured the beam shape to check whether higher-order mode are there or not.The result was attached.
We thinked that higher-order mode are not there.
 

What we will do next:
We will adjust the design for the cavity following the above reults. And we will measure the beam propagation after EOM, and put a isolator to pick off the erorr signal from the cavity.

Using the razor blades, I checked the IR beam propagation.

Note that this measurement can not be done with I_laser = 7A as this requires to have the HWP that controls the beam power in a position where the beam is highly deforms..

This HWP should be replaced.

I found a waist size of 36 um that overlapped with the probe beam measurement of last July (this probe beam seems quite stable since Manuel measurements).

Using the last periscope mirror and the last lens on the pump beam path, I tuned the propagation direction to have about 2.72 deg incidence in the horizontal plane and about 0 deg in the vertical plane.

I installed the surface reference sample and after tuning both the translation stage and imagning unit z position I measured R = 14.01 /W with z = 33.7 mm and z_iu = 69.5mm.

I installed the 0.5 inch mirror with the curved side (HR coated) facing the laser sources.

I has 6.5mm thickness and I assumed it has a refractive index of 1.53 to tune the imaging unit position.

However, because this mirror is coated for 1550nm, I could see a lot of spurious beam due to the etalon effect (see fig 1)

I tried to tune the mirror position to maximize the DC and pump laser transmitted (ie I tried to center the beams on the mirror surface).

But even with 0.1 mm step, the spurious beams were present so I decided to not use too large power to not burn the 0.5inch to 1 inch plastic adapter.

I increased the power from 28mW up to 1W and I report the last 2 absorption measurements in figure 2.

It seems the coating absorption of the HR side is about 20 ppm (see the peak at about z=33mm).

If my assumption on the substrate refractive index is wrong, I expect the absorption to be even larger.

Mitsuhashi, Shalika-san

What we did:

1.We measured the RF work about frequency, and the result was as follows. and the picture was attached. And we change the Tuning Voltage according to the result.

2.We measured the beam profiling and try to fit the beam propagation after LB1909-C. The result was as follows, and the picture was attached.
We thinked the result was not good.

What we will do next:

When we measured the beam radius, we used ND filter(ND=3.0). We thinked that ND filter maked interfere, and it maked the result bad.So we will use HWP and PBS instead of ND filter.
The set up was attached.
 

Shalika

This elog report covers two aspects:

1. Measurement of the beam size and fitting of the beam to find the minimum waist position.

2. Optimizing the position of AOM to observe maximum power in the first-order beam.

(Details below) 1. Yesterday we (Mitsuhashi-san, and Shalika) observed that the unclean lens was causing issues with the beam after AOM. So, we cleaned the lens and had to perform beam fitting again. Today, I performed beam fitting. When I measured the beam size, I used ND(2.0) filter with the beam profiler. The result of it as follows and attached. The width of the error bar is 4σ.

2. The AOM was placed at the position obtained from beam fitting. The power of the beam observed before AOM was 11.44mW. The RF driver was tuned with 9.85V of supply for 80MHz output. The power of the beam after AOM was 11.26mW. The power of the 1st-order beam was found to be 3.81mW (33.83% of beam after AOM). 

Next Step:

We will perform beam fitting for lens position after AOM and see if it agrees with our design. 

Aso-san, Shalika-san, Mitsuhashi

When we did a experiment in ATC, we found that AOM didn't work.
Now we try to solve this.

In addition, I and Shalika-san need to resign a optical design for the cavity in ATC.
And now, we are making new designs.

Yuhang and Marc and Matteo

We found DDS boards made sound last week. Then we tried many different combinations of inserting and removing different DDS boards, but the sound remains.

Then what happened was that we started to check the voltage provided by the NIM rack. We found the drop of voltage when a certain DDS board is inserted. The details of which caused which (whether 6V or 12V) were not recorded. And we found some very strange values. So we stopped at that time.

After that, we tried to figure out where the problem is from. Is the problem from DDS board, or NIM rack, or 500MHz oscillator?

To check if the problem is from NIM rack, we tried to insert DDS boards and oscillator to another NIM rack or a new NIM rack. 

We found that 

1. When we insert DDS board separately to the NIM rack holding PLL boards, DDS board didn't make sound.

2. When we insert more than two DDS boards (or DDS with oscillator) to a new NIM rack, NIM rack starts to make sound.

Then we confirm the problem is from DDS or oscillator. In particular, we found that the oscillator doesn't provide 500MHz signal any more.

Now, the DDS system is left in TAMA with the problem not figured out yet.

Yuhang, Michael, Marc

After aligning back SHG, we worked on aligning back GRMC and OPO.

For GRMC, we noticed that the EOM which was used for GRMC and FC locking must be remained if we don't want to adjust mode matching. The influence of this EOM on mode matching is substantial. 

For OPO, we noticed that the beam is going through the two lenses at very low place but not the center. There is an iris on the beam path going to OPO, which is checked again to be centered well.

The green beam going to the filter cavity is still misaligned at the stage of AOM. However, this will require another RF source.

The new OPO could be replaced soon.

Yuhang and Marc

We found that the SHG is misaligned this morning. This misalignment cannot be corrected by only tilting the steering mirrors before SHG. This is a bit strange for us. The situation of the misalignment is shown in the attached figure.

It was found later that this is caused by: the small scan amplitude of the locking servo.

The scan amplitude is adjusted to be small so that the lock loop can be engaged around the TEM00 resonant peak. However, today, due to the drift of cavity length, the small scan range is located accidentally around the first order resonance peak. This created the confusion at the beginning.

Yuhang and Michael

We locked the SHG in the new setup and took some temporary power level references. The locking is not perfect - the SHG temperature was optimised (reading 3.140 on TED200C) to maximise the amount of transmitted green power, and by leaving a cursor level we could see that the lock was offset a bit from the maximum level of the transmission peak. This requires some adjusting of the custom servo filter (i.e. ask Pierre Prat...). Anyway, that's why this is only temporary.

Figure 1 shows the new SHG setup. We measured the following power levels:

Incident IR (measured before dichroic): 684.3 mW
Reflected green (measured before FI): 237 mW
Reflected IR (measured before ND2 on photodetector): ~ 87 mW.

Compared to previous measurements, it is almost exactly the same. A 1:1 efficiency calculation p_green/p_IR gives 0.347.

After replacing wedged crystal EOM with the old one, we had alignment and mode matching seriously messed up.

Now the alignment and mode matching have been recovered.

The higher order modes and TEM00 power are shown in the attached figures.

Now, the mode matching level is: 1.68/(1.68+0.036+0.052) = 95%

After the beam size issue is solved, we proceeded to recover the alignment of SHG. Since we touched several mirrors for the path from new EOM to SHG, we had to make sure several mirrors and lens are in relatively good position.

We made several mistakes since the reflected beam from the 2-inch BS is not going along the holes on the bench. To make sure this reflected beam is roughly going along the good path, we used two lens just after the BAB pick-off BS as reference. But we found that these two lens are roughly in the imaginary plane with each other. So we need to find other reference for the rough alignment of the 2-inch BS. These references were selected to be the two lenses in front of OPO. After that we fixed 2-inch BS and the following lens.

Since the alignment was completely messed up, we also set up a camera after for SHG transmission. For monitoring higher order modes, a power meter was set up in the reflection of SHG. Now we started to align back SHG.

Mitsuhashi

I measured the beam size and fitted beam propagation.
When I measured the beam size, I used ND(2.0) filter.
The result of it was as follows and attached.The width of error bar are 2σ.

Attached figure 1 is a characterization of laser current dependent beam size measurement. The measurement is done at a point shown in the attached figure 2.

There was two abrupt beam parameter change points. They coincides with the change of half wave plate before Faraday isolator, which is used to reduce laser power going to the beam profiler. It seems that there is also depdence of the measured beam size on this half wave plate. Or maybe the measurement depends on the laser power going to the beam profiler. A slightly more investigation would be better.

The half wate plate angle was originally 38 degrees. 

Yuhang and Marc

As reported in the last logbook 3084, where we found the beam size changed significantly. Fortunately, we have a reported beam parameter measurement four years ago (logbook 1079). Thus, to compare, we measured again the beam parameter (relative to the first hole after the 2-inch BS). The new measurement is shown in Fig.1. According to this new measurement, we made a comparison of beam waist positions on the scheme as Fig. 2. We note that although the beam waist position is measured to be very different, the beam waist size is the same as before.

Yuhang and Marc

It was verified in logbook 3081 that the main laser EOM (as shown in the attached figure) was introducing significant amount of phase noise for the squeezing measurement. However, a better way of reducing the modulation caused by EOM is to use mode cleaner to reduce it. For instance, the new EOM has modulation frequency of 88MHz, which will be filterted out with a factor of about 4000.

r1 = r2 = 0.992

fin = np.pi*np.sqrt(r1*r2)/(1-r1*r2)

l = 0.5844

FSR = c/l

spac = 88e6/FSR

1/(1+4*(fin/np.pi)**2*np.sin(np.pi*spac)**2) = 0.0002449

Thus the phase noise caused by EOM modulation should be removed to a negilible level.

On the other hand, the new wedged EOM will remove residual amplitude modulation. However, the old EOM doesn't have wedge so that the introduction of new EOM deflects the laser beam. This causes a substantial alignment change, although we expect small beam path length change. To handle the alignment change, we decide to put the wedge oriented in the horizontal direction. In this situation, the misalignment will happen without causing beam height change. In general, we would like to avoid beam height variation.

For the SHG/BAB path, we could correct alignment change by the 2-inch BS after EOM. For the LO/PLL path, we set up two one-inch mirror with small mirror mount to correct the beam tilt. Using this strategy, we started to align beam again to SHG and IRMC as the first step. In particular, we found that we resulted to have a very large beam in front of SHG by moving just the 2-inch BS to recover alignment. This is out of our expectation because we expect small beam path length change. However, now it seems to be not the situation. To solve this problem, we decided to measure beam parameter again after the 2 inch BS. This measurement will be reported in the next logbook since it's a slightly different topic.

The extracted optical losses and phase noise are 22.51+/-3.49 % and 8.88 +/- 55.25 mrad.

The error bar for phase noise is very large since points are scattered around the fitted curve. Either more precise measurement or more points would help to reduce the error bar.

Measurement conditions:

MZ 4.6, CC1 44.4 temp 7.165, sqz 110, asqz 160 

MZ 4.7, CC1 51.2 temp 7.172, sqz 125, asqz 180

MZ 4.4,                 temp 7.163, sqz 110, asqz 150

Measurement results:

sqz: 5.41, 6.34, 6.03

asqz: 10.84, 14.63, 16.64

Marc, Matteo, Yuhang

We found out that we were only rotating the PBS circular dump and not the PBS itself..

We removed the circular dump and could see some beam. After a little realignment of the LO back-reflection to the FI, we measured :

Pinc = 3.54 mW, 12.6 uW and 8.4 uW rejected by the first PBS respectively towards top and bottom, 3.5 mW rejected by the second PBS toward the top and 0.4 uW transmitted by the FI.

This is closer to what we expect, especially because the 8.4 uW and 3.5 mW measured have some error due to space constraints to place the power-meter.

With this measurement, we can estimate the FI isolation to be about 40 dB which is in agreement with the specsheet.

We reinstalled the ND filters on the LO path, realigned the LO into the AMC, removed the steering mirror we used to increase the back-scatter light of LO (ie recovered the usual situation) and measured 3nW incident on the FI. It means that the LO seed should be about 0.3 pW.

Then, we measured the FI transmission using the BAB. We optimized the PLL frequency to be at maximum OPO transmission and got 342 uW incident on FI and 328 uW transmitted.

Finally, we were also able to remove one titled HWP on the squeezing path towards the Homodyne because we were able to use the HWP just after the FI to remove the p pol as seen in the AMC scan.

Yuhang, Marc, Matteo

The main laser phase modulation introduces phase noise for squeezing measurement. This phase modulation is reduced from DDS setting by a factor of 4. Before and after this modulation reduction, we measured squeezing level. The result of squeezing measurement is shown in the attached figures.

Since there are peaks appearing in the squeezing measurement, to compare measuremet in a fair manner, I did an average of data from 9.6kHz to 16kHz for each measurement separately. We see that the squeezing level differs by 0.2dB, it seems that this gain of 0.2dB could come from the reduction of phase noise. To confirm this phase noise, we would make a squeezing measurement at a higher pump power situation (the current measurement used 40mW pump with 4.5 MZ offset). CC1 pk-pk was 41.2mW

Yuhang and Michael

This measurement is more for the purpose of the KAGRA filter cavity project. We would like to optimise the design of the length control for the filter cavity. This involves setting the parameters of the control loop (bandwidth, filters, RMS noise etc) as well as choosing a control scheme of one of the three available for frequency dependent squeezing (A+ Resonant Locking Field, AdV+ Subcarrier, TAMA CCFC).

The performance of the filter cavity length control is in principle limited by two noise types:

• backscattering - the length noise of the filter cavity causes fluctutations of incident coherent fields, most notably the residual interferometer dark port power that leaks through the squeezing path Faraday isolators. This light has length noise imprinted on it by the filter cavity which degrades squeezing at frequencies where mirror motion is dominant (~ < 10 Hz). Backscatter can be decreased by increasing the gain or bandwidth of the control loop. However, increasing the gain at low frequency causes issues with stability, and increasing the bandwidth causes issues with...
• sensing noise - primarily caused by phase (frequency) noise of the voltage controlled oscillator that generates the sensing field for filter cavity locking. The phase noise of the sensing light causes locking fluctuations of the filter cavity length, which in the case of sensing noise can't be suppressed by increasing the gain of the control loop. In the A+ case, the phase noise of the AOMs that generate the resonant locking field is quite severe and the reinjection of sensing noise becomes prominent at and above about 10 Hz.

Changing the control bandwidth will raise one of these noises and lower the other, and the "bucket" is of the order 10-30 Hz for ~ 100m filter cavities with km scale GW detectors. Thus, we would like to measure the frequency or phase noise of the TAMA PLL at low frequency, as a placeholder for the KAGRA filter cavity sensing control noise. The PLL generates a sine wave that is the frequency difference of the main laser and CC/p-pol. Obviously we would like to keep this difference as stable as possible. We can beat this sine wave with a different one in order to check the stability. When the frequency of the signal and LO is the same, provided that the LO is stable enough, the voltage coming out of the mixing between the local oscillator and PLL is proportional to the phase noise. The relevant equations for this principle of measurement are given in Yuhang's thesis, 4.25 to 4.31. By default, we are using 190 MHz for the CC PLL and 250 MHz for p-pol, but the digital system in TAMA can only generate sufficiently clean signals up to 100 MHz. So we lowered them to 50 MHz. 

The spectrum of the phase noise is related to the spectrum of the mixed voltage by:

PSD (rad/rtHz) = PSD (V/rtHz) / Apk^2

Where Apk^2 is the amplitude of the oscillation that occurs when we change the LO by a small amount.

For the actual measurement, I used the P-POL PLL MON channel as the signal and the CC PLL LO (DDS3 board, Channel 0) as the local oscillator. The level of the DDS3 Ch0 output at 50 MHz was found to be 7.61 dBm after removing the attenuator that is normally attached to it. The signal and LO were send to a small mixer. By setting DDS3 Ch0 to 50.0001 MHz, the Apk is found to be 0.083 V. Then, resetting it back to 50 MHz, the signal and LO are combined at the mixer to give the phase noise spectrum. Of course, you should make sure that the PLL has remained locked throughout the measurement :).

Figure 1 shows the spectrum of PLL phase noise. Figure 2 shows a previous measurement from Yuhang's thesis. In general seems mostly consistent but I think we have more noise in the tens of Hz band right now.