Abe, Aritomi, Marc

Yesterday we removed the shinkosha evaluation plate and its holder from the translation stage.

We installed back the bulk reference sample and got R_bulk ~0.773 W/cm with

AC_bulkref = 0.0555 V;
DC_bulkref = 4.103 V;
P_in = 24.2e-3 W;
P_t = 11.7e-3 W;
abs_bulkref = 1.04;

Then, we brought the KASI sample inside the clean room and cleaned it with alcool while checking with the green light.

With this sample, the IU is now at z=43.1mm and the mirror centers are :

X_center = 327.5 mm

Y_center = 122.95 mm

Z_center = 62.5 mm (we had to increase the laser power to more than 1 W (~3W) to clearly see the phase change at the sample surfaces)

We added new X limit (lower vertical) on Zaber and started a circular map at Z_center with 2 cm radius.

Our initial guess of the absorption is abs~78 ppm/cm given by

AC = 2.3e-4 V;
DC = 4.242 V;
P_in = 3.24 W;
P_t = 2.77 W;
T_sample = P_t/P_in;
R_bulk = 0.7730 W/cm;
mat_correction=3.34;

abs = AC/(DC*P_in*sqrt(T_sample)*R_bulk)*1e6*mat_correction

Yuhang and Michael

We did some more inspection of the OPO replacement setup at ATC. Perhaps we will need some more low f lenses.

I recalculated the error signal and mode matching shown in 2469.

We can obtain a much larger error signal in reflection when the meniscus is used as the input mirror. Even with <10 mW incident power it should not be a problem. Here I use 4 mW incidence, 40 MHz modulation and 0.3 modulation depth to achieve ~160 mVpk error signal in the linear range. Figure 1 shows the error signals and power outside of the cavity. Figure 2 shows the scan of demodulation phase versus reflection and transmission photodetector signals. Figure 3 shows the circulating power of 160 mW, for 4 mW input.

I also show a beam profile with a more complete mode matching telescope into the OPO. I did a mode matching telescope calculation using JamMT and the database of Thorlabs lenses. We wish to obtain a beam waist of 20.66 µm within the OPO cavity using two lenses. Judging by the OPO setup in TAMA, it would be good to leave approximately 15cm OPO to lens and lens to lens.

In 2469 I used an f = 100mm lens placed at 175mm from the last preset optic on the ATC bench. The beam waist of 150µm is located within 1cm of the last preset optic on the ATC bench This creates another 150µm beam waist 175mm from the lens. The rationale for doing this was:

i) to have a couple of steering mirrors before this lens, so that the beam would be level at 76mm height when going through the FI/EOM/AOM. However, Yuhang suggested to simply move the 100mm lens to achieve beam levelling.

ii) to reduce the size of the beam waist, and make the beam less than 1/5th of the EOM aperture size for a reasonable distance. 

This time I used an f = 100mm lens placed 125mm from the last preset optic (as per photograph and measurement in 2439). The mode matching telescope, calculated by JamMT, is shown in figure 4. We should have enough space for the modulators between the f = 100mm and f = 40mm (90mm FI, HWP, 56mm EOM, 22mm AOM). Steering mirrors can fit in the space between the f = 40mm and f = 75mm, and then there should be a beam splitter between the f = 75mm and the OPO cavity.

Marc, Matteo, Yuhang

We used the green light to check possible dust on the mirror and took a picture after already using the air dust once.

The mirror is really dirty...

We then used again the air dust and could remove most of the large dust particles but not the smallest one (not visible on picture so no 'after picture').

We can still see the ethanol/acetone trace at the mirror edge so the mirror got dirty either during shipping or during the few weeks it stayed inside the clean room without cover.

We will clean it with first contact after the golden week.

Before the measurement we also moved the IU to z=68.6mm.

I'll add to the wiki the IU position for surface and bulk reference as well as TAMA size, 1cm shinkosha and KAGRA samples.

Abe, Marc, Matteo

Following the measurement of the absorption map at z=51.4 mm, we performed 2 others map in the XZ and YZ planes.

They correspond respectively to figure 1 and 2.

An important point to notice is that there seems to be a point absorber on the surface that distorts the absorption scale (up to 4000 ).

We decided to turn off the ir laser just in case.

Anyway looking at the histogram, it seems that the nominal absorption is around 400 ppm/cm (more precise analysis to come).

In figure 3 you can see a combination of the 3 maps where the colorbar scale has been limited to the maximum of the circular map (ie 1500 ppm/cm).

The absorption map and absorption distribution of the 1 cm thick shinkosha sample is attached to this entry.

It has a mean absorption of 355 +/- 127 ppm/cm

Abe, Marc, Matteo

This morning, we installed the 1cm thick SHINKOSHA sample.

We recentered the beam dump on the laser unit and covered back this unit.

Then, we increased the power to ~300 mW to check the sample center and inclinaison.

We checked its X,Y and Z centers :

X center = 398.195 mm

Y center = 121.5 mm

Z center = 51.4 mm

Then, we checked the z center at the left, right, bottom and top position of the sample with extremal positions separated by 13 cm.

We found a 0.8 mm vertical tilt of the mirror and a 0.4 mm horizontal tilt.

Finally, we increased the laser power to 3.46W and started the measurement of absorption map of this sample.

We also did a preliminary evaluation of the absorption as

AC = 3.6e-3 V;
DC = 4.71 V;
P_in = 3.46 W;
P_t = 2.94 W;
T_sample = P_t/P_in;
R_bulk =0.7414 W/cm;
mat_correction=3.34;

abs = AC/(DC*P_in*sqrt(T_sample)*R_bulk)*1e6*mat_correction ~1080 ppm/cm
 

Yuhang and Michael

We took some equipment from TAMA - camera + TV setup for when we will assemble the OPO cavity there, as well as some mounts to keep the beam height correct (should be 76mm).

The Faraday Isolator was placed and characterised. There was a scratch on the polariser window which reduced transmission somewhat, so the fine tuning of the translation stage on the FI mount was adjusted to avoid this.

The power of the Lightwave laser placed there does not seem very stable and drifts from minute to minute.

If the Faraday isolator is not well aligned along the beam axis, the beam becomes astigmatic.

Using the Thorlabs power meter, we measure the following:

• Initial transmission adjustment: 4.79 mW transmit, 5.03 mW incident
• Blocking adjustment: 0.25 µW transmit, 5.185 mW incident
• Placed back in transmitting mode: 4.991 mW transmit, 5.343 mW incident

This gives us 93.4% power transmission and 43.2 dB isolation (Thorlabs specifies 92% transmission and 35 dB isolation).

Abe, Aritomi, Marc

Today we scanned the bulk sample in z to find the sample center (at z=37.935mm)

Then we had to slightly move the power-meter to get the beam centered.

Finally, we could get R_bulk = 0.7414 with

AC_bulkref = 0.05762;
DC_bulkref = 4.615;
P_in = 23.0e-3;
ACDC = 0.01253;
P_t = 11.4e-3;

This is quite close to the previous value (0.78).

We used as a goal for this calibration 0.74 and stopped when we got this value...

We'll try little more tuning on Monday before starting the absorption measurement.

I estimated the level of error signal that we should obtain for locking an OPO cavity. I use similar values as those described in Yuhang/Aritomi's theses for the optical cavity, but will be working with a different modulator. We will lock this cavity using the EOM and then switch off the beam using the AOM in order to obtain a ring down measurement to characterise the losses of the new OPO.

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COMPONENTS:

Faraday Isolator  - Thorlabs IO-5-1064-VLP

• Aperture: 5 mm diameter
• Aperture heigh: 38mm
• Length: 90 mm
• Max CW power density: 25 W/mm^2 blocking, 100 W/mm^2 transmission

Electro-Optic Modulator - Newport New Focus 4003 Resonant EOM

• Resonant frequency: 40 MHz
• Modulation strength: 0.1-0.3 rad/V
• Aperture: 2mm aperture (ideally 0.4mm beam diameter)
• Aperture height: 14mm
• Length: 56mm
• Max CW power density: 4 W/mm^2
• Max RF power: 1W

Acousto-Optic Modulator - AA Optoelectronic MT110  IR 27 - use manual for MT110-A1.5-xx

• Aperture: ~4mm aperture, should be satisfied by passing through EOM properly
• Aperture height: 8mm
• Length: 22.4 m
• Max rise/fall time: 192 ns
• Max separation angle (0-1): 28.8 mrad
• Max CW power density: 10 W/mm^2

• Bandwidth: 150 MHz bandwidth
• Photocurrent - 0.6 A/W photocurrent at 1064nm
• Transimpedance gain - 5x10^3 V/A for 50 Ohm load, 1x10^4 V/A for high impedance load
• Noise - 2 mVrms nominal
• Detector size - 0.5 mm diameter

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SPECIFICATIONS OF OPO CAVITY:

• Input mirror: OPO curved surface, R = 0.99975
• End mirror: Meniscus, R = 0.92
• Length of cavity: 38mm
• Beam waist in OPO cavity: 20.66 µm
• Finesse: 75
• FWHM: 52.5 MHz
• FSR: 3.9 GHz
• No Gaussian beam parameters specified in Finesse modelling

--------

Given a laser input power of approximately 4 mW, I estimate that we will have an error signal with a peak to peak value of less than 10 mV within the linear range. This is extremely small compared to the noise value of the photodetector, so we should use a high power. Figure 1 shows the error signals from the reflection and transmission photodetectors, as well as the reflected and transmitted cavity power, using an input power of 40 mW. Each error signal plot shows the signals of I and Q phase which are spearated by 90 degrees. Figure 2 shows the optimisation of demodulation phase for the particular EOM settings (40 MHz, mod depth 0.3). Figure 3 shows the error signals again but with a different transmission demodulation phase. Perhaps it is better to have the Q phase part be flatter when the I phase is in the linear range.

Looking in the ELog, Eleonora seemed to have the low power issue in 997. This was later fixed by adjusting the mode matching telescope to increase the amount of p-pol into the OPO. Yuhang in 1010 found a simulated error signal reaching 120mVpk and 0.6 mW in transmission of the cavity, for an EOM at 87.6 MHz.

I am not entirely sure of the PPKTP damage threshold but Marc told me it may be an issue. For the modulation optics, the maximum intensity at 40 mW is 0.57 W/mm^2 at the waist. Inside the OPO cavity, the circulating power is ~5 mW with an intensity of 3.7 kW/mm^2.

I have also estimated the following lens setup, given that we want a beam at most 0.67 µm diameter through the EOM (after the FI), and 40 µm on the OPO. For the isolator and modulators, the beam should be less than 1 mm diameter for a range of more than 20 cm, and less than 0.4mm diameter when travelling through the 56mm EOM. The lens setup is shown in Figure 4 with approximate indications of the FI, EOM and AOM lengths.

Michael and Yuhang

Today we found we couldn't lock FC with GR. After checking error signal, I found the phase is not optimized. Then we reload DDS1 with the good phase, we saw good shape GR error signal. Checking from rampauto EPS2, the error signal pk-pk is 3.24V.

This DDS1 phase change is very strange. It seems to happen by itself without power off/on.

The frequency independent squeezing spectrum became clean due to IRPS improvement and lens flip(elog2445). This means back scattered noise becomes negligible.

To check back scattered noise for frequency dependent squeezing with the same homodyne (BHD) situation. I aligned filter cavity injection and reflection (injection is not optimized). With vacuum going to filter cavity and back to homodyne, I took noise spectrum as attached figure.

Comparing with elog2053 and elog2054, we can see back scattered noise became worse. The back scattered noise was checked twice, which shows change during these two measurement. The difference is that I switch on the oplev laser inside PR/BS chamber. However, oplev laser has different wavelength with infrared, which should not be an issue.

To further examine this situation, we will optimize injection to filter cavity or try to tilt more the lens. Some more investigation about oplev laser is necessary as well.

Abe, Aritomi, Marc

Today we reconnected the particle counter.

It seems that when noone is too close to it we have ~80 particles of size 0.3um and ~30 of size 0.5 um. -> Do we need to clean the cleanroom filters?

However, several larger particles are present when someone get close to it...

We will change cleansuits tomorrow.

Abe, Aritomi, Marc

Because the 2 lenses positions on the ir path has been changed quite a lot, today we decided to scan the imaging unit position.

Results are presented in figure 1.

It seems that now the IU position that maximizes the surface calibration value is Z(IU) = 68 mm.

After finding this new position, we aligned really carefully the pump & probe beams and got R_surf = 17.9 [1/W].

Then, we replaced the surface calibration sample by the bulk sample.

We got R_bulk = 0.67 [cm/W]. A reason of small R_bulk could be that we were not maximizing the DC signal at exactly the center of the sample (?)

The spectrum got from elog2456 is after the replacement of op-amp.

After op-amp replacement, I compared the PD spectrum again with simulation and old measurement. The comparison is shown in the attached figure.

We can see that the new modified PD is still a bit far from simulation.

Also note that because we didn't take many points around ir waist position and with quite large step size (0.05mm), the ir beam parameters might also not be so reliable in this configuration.

Indeed we did the measurement today and seems that we are finally realigned !

For the red, the measurement is totally distorded by a peak when the blade starts to cut the beam as shown in elog2450...

I tried to use different methods to fit (direct fit of the power profile, beam size from the 90% and 10% of the maximum power) without much success...

I'll try again tomorrow with better filtering of the data.

Abe, Aritomi, Marc

As reported in entry 2446, we could recover the expected ir beam parameter.

Today, we placed the surface reference sample on the translation stage.

After maximizing DC and AC, first calibration gave R_surf~15.

Then we moved back the imaging unit from 74mm to 70 mm and got R_surf~17.3 (AC_surfref = 0.3512,DC_surfref = 3.942,P_in = 23.37mW)

The results are presented in figure 1.

The new crossing point seems to be at z=39.25 mm. Also, the 2 lateral peaks are still slightly unbalanced...

I think tomorrow we can try to do few measurements of the surface reference while varying the imaging unit positions and then try to measure the bulk reference.

With the filter cavity aligned, I checked the newly modified TAMA PD signal to noise ratio.

According to elog 2414, the modified TAMA PD should have SNR 5 times better than the old design.

I measured PD noise when there was no light incident on these two PDs. The noise is mainly electronic noise at this moment. I also measured the signal with 40mW green power and a 50:50 BS pick-off mirror. To compare SNR easily, I multiply newly modified PD both signal and noise by a factor of 140. This makes both PDs electronic noise overlap.

As shown in the attached figure, the signal from the modified PD is larger than the old PD. SNR is roughly a factor of 2 larger (low frequency ~1.7, high frequency ~2.3). This measurement result is not consistent with 5 times SNR increase.

That sounds very good!

So it means that you have already the required beam-size (in IR), right?
The red-laser, however, seems to be a little small...

Anyway, do you plan to do a reference-sample measurement?