NAOJ GW Elog Logbook 3.2
Here is attached noise spectra of the filter cavity transmission and reflection. As for transmission spectra, data measured in two different frequency span were put together afterwards to retain good resolution in lower frequency range. There are spikes on IR transmission in the range above 10kHz which do not exist in reflection nor in green. We also examined IR beam before entering the cavity and confirmed that there were not such spikes (data not shown here).
We have verified that the spikes oberved in the IR transmission, in the region above 10 kHz are prensent even if there is no light impingin on the photodiode. They are likely be due to electronics.
This calibration factor is measured while sine wave noise frequency is 28kHz. I also did the measurement for different sine wave frequency. It is shown in picture 2. For this, I have a question. Is this frequency dependence related to the transfer function of our control servo?
From the picture 2, we can read the peak value of this noise. It is 45.7407uVrms. And according to the block flow, we can calibrate this value to error signal. The formula is Err_V = K(V/Hz) * S_Hz/(1- (f/f_0)^2).
K is 3.1e-3, S_Hz is V_RMS (V) * 100 * sqrt(2) * 2e6 Hz/V=12937Hz, f_0=1.45kHz. In this case Err_V=38.8Vrms. However we read from the spectrum, it is 1.6mVrms.
We also checked the situation if there is not noise injecting to the end mirror. It is shown in picture 4 and 5. By comparing them, I found the 20Hz disappear. But the noise level for the whole frequency band(from 1Hz to 50Hz) has decreased.
Conclusion:
1. The shaking of EM can cause precisely the same frequency noise in PZT. And also the harmonic peak.
2. In the aspect of error signal, the shaking of EM noise will be distributed from one frequency to the whole band. This distribution may come from filter.
3. By using the transfer function we created before, we cannot predict the behavior of error signal in the low-frequency band.
Next step:
To solve the problem of transfer function for low-frequency.
We measured the misalignment again by changing the driving frequency of AOM. The measurement shows it is around 0.168. And the standard deviation is 0.0165.(See attached picture 5) So the residual misalignment is 17% (+/-2%) .
Besides, we injected a sine-wave noise into the local control of optical lever. We choose the frequency of 20Hz, 200Hz and 500Hz. The amplitude is 1V. The noise is injected into EM (yaw and pitch). The 20Hz is appreciable on the transmission of green.(See attached picture 1)We are sure the noise is injecting to EM.(We can see it from the local control like picture2) And the frequency of the noise we are injecting is the same with the frequency we can see in the transmission signal of green light. At the same time, we measured the noise spectrum of error signal. We found it has no difference from the case without noise injecting.(For all the test, they have no difference. See picture 3 and 4)
This perturbation test may tell us that the angular to length coupling is low, so that its affect for error signal is below the present noise level.
I use the data to calculate the standard deviation of IR transmission and GREEN transmission. For green, it is 0.0286. For IR, it is 0.1235.
The result is also shown as attached figure. Next step is to figure out the reason of instability.
Participants: Eleonora, Matteo L., Raffaele, Tomura
In the past days we kept working on the characterization of the IR lock, with the main purpose of measuring the cavity losses and the decay time.
ROUND TRIP LOSSES
We observed that in good alignement condition (IR trasmission above 1.5 V), the fluctuations of the transmitted and reflected power were much less than what observed before.
In this condition we were able to measure a change in the cavity reflectivity when the cavity is resonant and when it is not and give a preliminar estimation of the round trip losses (RTL)
In Fig.1 the trasmission and the reflection of the IR are shown when a set of lock/unlocks of the cavity was done. The reflected light has been focused on a photodiode using a lens with f = 50 mm. Et the beginning of the measurement the IR light has been blocked to measure possible offsets of the photodiodes.
The technique used to switch from resoant to not resoant state was to suddenly change the driving frequency of the AOM of 5 kHz. By using the values of the reflected power in the two states (resonant and not resonant) as explained in detail in the attached pdf we estimated the RTL to be about 80 +/-12 ppm, corresponding to 0.26 ppm/m. The error is mainly do to the residual fluctuations of the refected power when the cavity is locked. The associated squeezing degradation is reported in FIg 2.
The presence of light not coupled in the cavity (mismatching/misalignement) normaly reduces the measured losses and has to be compensated in order to have a real estimation of them. In the previous calculation I assumed a mismatching of 15%.
[An idea of the impact of the mimastch compensation: assuming no mismatching the computed losses are 70 ppm while with 20% of mismatching they becomes 85 ppm. (Details about this can be found in my thesis at pag.101)]
According to the simulation we expected about 55 ppm of RTL (40 ppm from flatness, 10 ppm from rougness/point defects and 5 ppm from trasmission and absorpition). Note that losses from small angles scattering (between mrad and few degrees) have not been considered in this loss budget.
DECAY TIME and FINESSE
A preliminar estimation of the decay time has also been done. To do that we used different tecniques: bringing the cavity suddenly out of resonance (by stopping the lock with the servo or changing the AOM driving frequency) or cutting the light in input. The transmission and reflection in this 3 cases are reported in the second pdf attached)
A fit of the transmitted power for the first measurement shows a decay time of 0.0027 s, corresponding to a finesse of 4250 (Finesse = pi*FSR*decaytime ). See third figure attached.
Assuming the nominal reflectivity of the mirrors, this value is compatible with RTL of about 100 ppm.
A better analysis of the decay time, with an estimation of the error bars will be done soon.
Participants: Eleonora, Tomura
We conducted characterization of two auxiliary lasers and faraday isolators.
For two aux. lasers, beam dimension evolution along propagation were measured. Laser power during measurements were approx. 260 mW, which corresponds to 1.2A in laser current. Attached figures shows bean waist sizes and position. The origins of x-axes were set at aperture of laser housing. The waist positions were somewhat different from what expected.
The beam polarizaiton purity was confirmed with half/quarter waveplate. It was 96.7% and 99.9% for aux1 and aux2, respectively.
Beam transmission through FI was also measured using aux laser as a light source. Pictures of two FIs were attached.
For first FI (thorlabs), transmitted power was 213 mW out of 265 mW, which means 80.4%.
For second FI (Gsaenger), it was 79%.
Great!
Participants: Eleonora, Raffaele, Matteo L.
We have installed in the end bench a thorlab passband filter (FL1064-10) on the path of the IR beam after the harmonic beam splitter. This allowed us to get rid of the residual green light and finally to monitor the beaviour of the IR light when the cavity is locked.
By changing the driving frequency of the AOM installed on the green path we were able to induce a frequency shift between the the green and the IR frequency in order to have both resonant at the same time.
NOTE THAT: since the AOM is put on the green path, the change in the frequency which it induces is compensated by the servo with a change on the IR which is half of the frequency change in the AOM. This means that a shift of 1 MHz in the driving frequency of the AOM corresponds to a shift of 500 kHz in the frequency of the IR light.
In the following are reported some preliminary results that we were already able to obtain:
TEM00 resonance and FSR: The resonance frequency of the TEM00 has been found at about 109.366 MHz. (The standard driving frequency of the AOM is 110 MHz). We have verified that it occurs every time we shift the AOM frequency of 1 MHz, meaning that the FSR is 500 kHz, as expected.
Cavity finesse for IR: We did a rough estimatin of the IR linewidth by slightly changing the AOM frequency in order to scan the IR resonance. We drove the AOM in order to find the maximum in the transmission (about 1.5 V) and then we checked the frequency shift necessary to get half of the maximum. Repeating this procedure few times, we found a FWHM of about 110 Hz +/- 10. This corresponds to a finesse of about 4500 +/- 450, which is consistent with what we expect. The big error is due to a large fluctuation in the trasmitted power.
HOM resonance frequency: The first order modes have been found at a AOM frequency of 108.970 MHz (and FSR multiples). It means that the the difference in the resonance frequency betwen the fundamental and the first HOM is about 198 +/-1 kHz. This value is in very good agreement with what expected, considering the cavity g-factors computed by using the RoC values measured at LMA.
According to LMA measurement, we have R_in = 436.7 m and R_end = 445.1 m which correspond to a frequency shift given by
δν = (FSR/pi)* acos (sqrt (1-L/R_in)*(1-L/R_end) = 198.1 kHz
We also observed that different modes of the first order resonate at slightly different frequencies, which is likely to be an effect due to the astigmatism. More investigations have to be done.
Residual misalignment: by comparing the trasmitted power of the funtamental mode with that of the first HOM we estimated the misaligment to be between 20% and 25%.
Lock accuracy: Since we don't have an error signal for the IR lock, it is not easy to evaluate the lock accuracy. Anyway the fact that resoanance can be scanned by shifting the AOM frequency gives us a lower limit that is surely better than few tens of Hz, that is much better than the accuracy on the green lock, estimated to be about 100 Hz. This is compatible with the hypothesis proposed by Matteo B, according to which the IR pole (at much lower frequency than the green one) contributes to filter the high frequency laser noise, improving the lock accuracy.
Stability: Once put on resonace the IR seems to be stable as long as the lock is mantained, anyway we observed that when the cavity unlocks and relocks it is not always possible to get the IR resonant again. This is probably due to the fact that only half of the green locking points are also good locking points for the IR.
The attached picture shows the transmitted beam at resonance for the IR (top screen) and green (bottom screen)
Great!
Participants: Eleonora, Matteo L.
Last Thurday, while measuring the hight of some optical component on the bench, the green beam got a bit misaligned. When we realigned it, we observed that the trasmission when the cavity is locked is higher than usualy (1.5 V). The beam seemed still centered on the cavity mirrors and the IR is still well aligned.
We observed that the references of the PR and in particualr BS have slightly changed in yaw and we marked a new one.
At some point we had a problem of spikes with PR yaw error signal. We observed that this was the only one very far from zero. The spikes disappear as soon as we put it backto zero.
Participants: Tomura, Eleonora
As reported in entry 654, we had to move the yaw of SM2 dicroic mirror in PR chamber to avoid the IR reflection to touch the viewport side. Since after the last 1000 steps done, we were not able to get a good IR alignent anymore, yesterday we moved back SM2, hoping to find a good postion where we could have both a good alignement and the maximum of the power in reflection.
The input power is about 16 mW.
The situation is such that in the original position (before last thurday) the aligment can be very good but the beam hits the viewport side and the reflected power is very low (less than 1 mW). In order to have the maximum reflected power possible (about 12.5 mW) SM2 has to be moved so much and it is not possibile to recover a good alignent. (We remarked that in this case the cavity seems very mismatched, but we where not able to improve the situation by moving the last IR lens on the bench).
Today we tried to put back SM2 towards the original position and to fine-tune its position in order to have the best compromise between good alignement and high reflected power.
We did the followoing series of steps (velocity 500)
-300, -300, -300, -1000, -1000, -1000, +1000, +1000, +500, +300, -300, -300, +200, -100
In the end, we stopped in a position where we can have quite a good alignement (but not the best possible) and about 11 mW of reflected power. By looking with the camera it is not evident that the reflection is touching the viewport side.
We also moved the last IR lens back, from 20 mm to 17 mm.
In the attached files we recoreded the new reference for the green beam: the white cross corresponds to the orginal position and the black point corresponds to the maximum displacement done by SM2.
Considering the maximum reflection we could get (12.5 mW over 16 mW), we estimated the injection losses in vacuum to be about 22% (11% single pass) which is not much higher than we expected.
We measured the beam profiles of all the 3 lasers we have. See the plot
We aligned them at the cross point and maximized the signal using the surface reference sample.
Then we made a scan of the surface and the bulk reference samples.
Yesterday I did a very preliminary check of the AOM operation. After locking the filter cavity I have changed the driving frequency of the AOM (which is normaly set at 110 MHZ) and I have looked at the correction sent from the servo to the laser piezo.
In the attached picture you can see in yellow the trasmission of the filter cavity and in blue the laser correction (from PZT mon).
I changed the AOM driving frequency in steps of 100 kHz each time. Most of the time this change causes a very short unlock of the cavity, as it can be seen from the picture. Nothing can be noticed in the correction signal.
Since the piezo calibration is 2 MHz/V and the channel PZTmon has an attenuation of 100, a change of 100 kHz should result in a change of 2 mV in the correction, which is likely not visible. I tried to do bigger steps but the cavity loses easly the lock.
I observed that the correction is about a factor 3 bigger than that measured this summer. (See last picture of entry 475)
Participants: Raffaele, Eleonora
As reported in entry 649, while improving the IR aligment we realized that the power of the reflected beam was very small. After some investigations we were able to see with the IR camera that the reflected beam was hitting a side of the inner part of the viewport. in order to avoid this we decide to slightly change the positon of the diachroic mirror SM2 (where green and IR recombine) in order to change the way the beam passes through the faraday.
We moved the yaw picomotor of SM2 several times (velocity 500, direction +), trying to recover the IR aligment with the two steering mirrors on the bench after each picomotor movement.
We did the following series of steps:
10, 10, 100, 300, 300, 600, 600 (Thursday) and 600,1000 (Friday)
Before the last movement (1000 steps) we were able to have a quite good alignment and a reflection between 11 and 12 mW (the input was about 16 mW), nevertheless we were still able to see some IR scattered light inside the viewport. After the last movement (1000 steps) the scatterd light was no more there but we were not able to recover a good aligment.
Sometimes, while recovering the aligment we had the impression that the matching was worsened (more LG modes), so we have changed the postion of the last IR lens different times in a range from 7 mm (original postion) to 22 mm.
Note that since we moved SM2, one of the two green references (the reflection from SM2) has changed accordingly. See attached picture.
Participants: Manuel, Eleonora, Tomura
Today, after asking Takahashi-san, we removed most of the optics intalled on the optical table placed at the bottom of the stairs in TAMA central building. There were three lasers ( two 532 and one 1064) and many other optics which have been temporarily stored in a plastic box below the table. Currenty the table is being used to test the two auxiliary lasers and faraday isolators for the filter cavity experiment.
I attach some pictures taken to the table before we removed the optics.
Participants: Yuhang, Tomura, Eleonora
Today we kept working on the alignemet of the infrared beam:
1) We have centered the green beam on the end mirror. To do this we have made a scan in pitch and yaw with the BS in order to reach the edge of the mirror and see the green beam on the coils. We recorded the corresponent offsets and pick the intereidiate value as the good one.
2) We have investigated the origin of the bright halo visible on the camera for the IR trasmission (see pic1). We have looked insiede the end bench putting a camera in the viewport behind the end mirror (the one through which the cavity transmission passes) (See picture 2). The bright halo seems to come from the contour of the end mirror. Raffaele suggested that it may be the light diffused by the input mirror at small angle, which is trasmitted by the most external part of the end mirror. In fact the HR coating has not bean applied there and it is almost trasparent. The fact that we see it very bright could be due to the comparison with the very small transmissivity (few ppm) of the HR coating anywhere else on the mirror surface.
3) We tried to maximize the alignement of the IR beam by moving the steering mirrors on the bench. As usualy it was done after having aligned the cavity for the green. We could obtain quite a good alignement but we observed that in this condition the reflection from the faraday was not maximized anymore and it became actually very small (less the 1 mW, with 16 mW in input). This suggest that the axis of the faraday is not well aligned with that of the cavity and so an additional work to align them will be required. To my knowledge we have never payed attention to the faraday reflection while aligning the IR beam in the past, so it is likey that it was the case also before.
The NTC used seems to be SEMITEC 103JT-050.
According to the datasheet, the R25 = 10kΩ ± 1% and B = 3435K ± 1% (note that this value is correct only between 25deg to 85deg Celsius).
PARTICIPANTS: Yuhang, Matteo, Raffaele, Tomura, Eleonora
Finally today we managed to realign the IR beam.
First we aligned and locked the cavity with the green light and then we cut the green.
We checked the IR reflection from the invacuum faraday on the bench and we maximized it by moving the two last steering mirrors for the IR on the bench.
The laser power is set to the standard value (current: 1.2 A), the incoming IR beam just before the windows is about 16 mW and the maximum reflection we could find is 12.8 mW. Increasing the laser power (2 A) we have an incoming beam of about 33 mW and the maximum reflection is about 25.7 mW
We checked that the reflection was moving accordingly to the IM motion, so we were pretty sure that it was coming back from it.
After maximizing the reflected power the beam was good on the PR reference but still we couldn't see anything on the first target, except for a very dim scattered light.
Then we put down the first target and rised the second target where we found a very strange beam, showing some fringes. We observed that fringes were reduced and the shape changed when we misaligned the end mirror.
After that, we reinstall the IR camera in the end bench and we center it by using the residual transmission of the green (when it was locked). By changing the yaw of the BS of a small amount we were able to see IR flashes on the camera. Then we put the BS back in the initial position (the one that makes the green to flash) and we adjust the yaw of the two last IR steering mirrors on the bench in order to make the IR flashes appear again. In this condition we have both green and IR aligned in the cavity.
The IR aligment was not optimized: we were still able to see HOM flashing but it is not easy to fine tune the alignment by moving the steering mirror on the bench
We remarked that there is a very bright circle of IR light on the end camera around the flashes. It resembles the contour of the end mirror and it is not clear why we can see it. Since we suspect a mismatching we tried to move a bit the last IR lens on the bench but it didn't improve the situation. (We put it back to the previouse position.)
Lessons learned:
1) The reflected beam is a good reference for the alignment of the beam and it should be maximized as a good starting point.
2) The behaviour of the IR beam on the target when it is aligned is much diffent from that of the green
- we can't see anything on the first target, so we should probably not consider it in the future
- when the second target is up proobably same unpredictable reflections between the mirrors and the target produce a very strange beam.
More work will be done to understand the origin of the bright circle and to improve the alignment.
