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.

Marc, Matteo

We brought a DC voltage supply from the elec shop to the FC clean room.

First, we used it with the 500 MHz clock after the first voltage regulator. We could see the on the spectrum analyzer the 500 MHz peak from all the output channels.

Then, we tried to use it with DDS boards and found that all the expected voltage were provided.

However, when connecting 1 DDS board to the nim rack, we could not get any voltage (about 0.2 V and fluctuating).

After some checks, we found out that the high quality ground of the nim rack is not working while with the ground we can get the +/-24V, +/-12V and +/-6V.

We found similar issue for the spare nim rack that was used to control TAMA BS and PR.

However, the high quality ground of the nim rack used for the QPD is working properly.

We connected the 3 DDS boards and the 500 MHz clock to this rack and got the 500 MHz signal as well as all expected voltages.

We also checked that all the boards installed on this rack do not use the high quality ground, so we will swap this nim rack with the one previously used for the DDS boards and the 500 MHz clock.

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.