NAOJ GW Elog Logbook 3.2
As Eleonora pointed out we used a wrong datasheet for the EOM (and also did some wrong calculations for the max beam size inside the EOM...)
Here is the good size range : between 300 and 80 um
We designed a new telescope (Fig1) as the following : f=125mm lens and 10cm after f=-25mm lens.
This should allow to have a beam size around 200um inside the EOM.
Question : For the green EOM, the astigmatism depended a lot on the lens position.
Is the astigmatism also that problematic? We will have quite a short beam path until the OPO.
Anyway, we found 2 trails on which we can translate the lenses borrowed from Manuel's experiment.
Fig 2 : beam size before the EOM
The beam didn't seem to be too much astigmatic after the EOM (posted soon)
You can find attached the complete data sheet for the 88 MHz EOM which I get from Quibig. Specs may be a bit different from that reported in the entry.
Yesterday, we measured the Green Mode-cleaner output beam. While measurement, I found the lock of green mode cleaner is more stable for half fringe. However, the lock we did last week is only for fringe higher than half fringe. If we lock it on half fringe we will have less power transmitted but more stable. However, I should say that even with half-fringe lock, the lock can be destoried by vibration. So maybe we should consider to put some rubber under the MZ to isolate the vibration.
Anyway, by locking the beam many times. I must say here that the measurement is performed with different locking condition. Although I tried to make the lock the same, I cannot make sure they are exactly the same. But the result is fine. I attached the figure here.
Last time I used the wrong set up of the beam profiler. This time we used a correct one. The result is attached. The beam is not a round shape in this measurement. And I found the beam is shaking while measurement.
I'm wondering if the increase of the phase noise at high frequency when the cavity is locked is due to the fact that, when the cavity is locked, the frequency changes of the master laser are very large ~ MHz.
Possible tests to check this hypothesis are to damp the mirrors (sending the PZT correction signal to the mirrors, upon filtering) or to excite the mirror oscillations, to artificially increase the laser frequency changes.
A related question: when we compute the residual RMS phase noise between the main laser and the auxiliary laser we integrate down to 100 Hz. Maybe the 1 Hz region is dominant with respect to the high frequency region, and thus we should solve in any case this problem of the main laser frequency changes in the Hz region.
Participants : Eleonora, Yuefan, Yuhang
Since last friday we started designing the EOM telescope.
EOM parameters:
Following the EOM datasheet (attached to this entry) the beam conditions inside the EOM are the following :
Max beam size defined by EOM aperture (3x3mm) : max beam radius = 425 um
Min beam size defined by max optical intensity (20W/mm^2) : min beam radius = 75 um (as the input power is around 350mW)
This requirements can be meet if we use a f=175 lens and place the EOM 10cm after it. (actually we first used the wrong value of max optical density first meaning that the beam is now more than 100um inside the EOM)
Issues :
Because of the Faraday Isolators, the beams after the two 98:2 are quite astigmatics (the datas will be added tomorrow morning).
The beam were also vertically tilted (3.1 mrad for the beam going to the EOM).
By using only 1 lens after the 98:2 we couldn't achieve better than 86% of transmission.
Possible solution and future work :
We then installed 2 steerings mirrors before the lens in order to correct the beam tilt. This means that the EOM path is now shifted 5cm away from the laser with respect to the nominal position.
It seems that there is enough space to use this solution (and to recombine the 2 beams we could then use 1 steering mirror and rotate the PBS).
Tomorrow we will installed EOM and characterized the output beam.
It should then be possible to use this solution to recombine the 2 beams.
You can find attached the complete data sheet for the 88 MHz EOM which I get from Quibig. Specs may be a bit different from that reported in the entry.
As Eleonora pointed out we used a wrong datasheet for the EOM (and also did some wrong calculations for the max beam size inside the EOM...)
Here is the good size range : between 300 and 80 um
We designed a new telescope (Fig1) as the following : f=125mm lens and 10cm after f=-25mm lens.
This should allow to have a beam size around 200um inside the EOM.
Question : For the green EOM, the astigmatism depended a lot on the lens position.
Is the astigmatism also that problematic? We will have quite a short beam path until the OPO.
Anyway, we found 2 trails on which we can translate the lenses borrowed from Manuel's experiment.
Fig 2 : beam size before the EOM
The beam didn't seem to be too much astigmatic after the EOM (posted soon)
At the begining, we used the wrong beam dimmsion, the initial beam DIAMETER is 2000um.
BUT, all the telescopes we designed are using RADIUS as 2000um.
Today, we realized this problem. I designed the telescope again. The EOM doesn't make a large difference. This design can be a fine reference.
Lesson: actually we have many chances to realized this problem, we checked every time after putting the lens. But everytime, we checked only the beam waist position. We never checked the beam waist size. So we didn't realize this problem. So next time we should check both of them carefully.
effect in the setup?
- When the loop is closed there is an oscillation at high frequency. Is the loop gain too high?
- Having the beams well focused within the photodiodes is important for this test.
Still looking for the sapphire calibration factor of ~3
I simulated the absorption signal for bulk reference and sapphire sample in two different conditions:
- pump size (waist radius) 40micron
- pump size (waist radius) 80micron
The absorption parameter used to simulate the sapphire absorption signal is 60ppm/cm.
To calculate the sapphire absorption from the simulated signal I applied the formula Abs = AC/DC / P / R
where the calibration factor is R = (AC/DC)_ref / P_ref / Abs_ref
with P = 10W and P_ref = 30mW.
I didn't apply the material correction because that's the final estimation of this simulation.
In the case of pump size 40micron, the absorption is 19.5ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 3.08
In the case of pump size 80micron, the absorption is 23.8ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 2.52
Conclusion:
decreasing the pump size results in a better estimation of the material correction (comparing it with the value of 3.34 given by the SPTS company),
But it is still far from the factor of ~3 discrepancy of my measurements.
If the discrepancy was all due to the pump size and the simulation were exact, the material correction in the case of 80micron pump size should have been about 1, instead of 2.52.
Comments:
- There is a strong approximation on the bulk reference material, which is schottglass#21 but in the simulation is silica (because I couldn't find the thermal properties of schottglass).
- The phase for the same material shouldn't change with the pump size. But the simulation gives different values of the phase. This may be due to a different optimal position of the Imaging Unit for different pump sizes, and I didn't optimize it for the new 40micron pump size.
Simulating the absorption of the surface reference, I optimized the Imagin Unit distances to have the maximum signal in the two cases, pump waist 40um and pump waist 80um. See the first plot, it shows the signal as a function of the distance d2 from the lens and the small sphere.
Using the optimum value of d2 in the two cases, I repeated the simulation of elog entry 905.
In the case of pump size 40micron, the absorption is 14.7ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 4.09
In the case of pump size 80micron, the absorption is 19.7ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 3.03
the probe size is still 180um in both cases, next step is to reduce it as well to be 3 times larger than the pump
I reduced the probe size as well, from 180um to 120um, to be 3 times larger than the pump (which is 40um), but the signal doesn't change much.
For the 7 of step, first thing is to demodulate this signal with the frequency of beat note. Then by chaning the phase of this demodulation signal, we can make the demodulation output close to zero. This is crucial for the measurement of phase noise with DC coupling.
Participaint: Eleonora and Yuhang
Following the procedure of Marco, we measured the phase noise again. The difference is now rampeauto doesn't have ramp-in port and one of the resistors is changed.
Before, we found the error noise spectrum is limited by the laser frequency noise after change.
The measurement of phase is performed in three different cases: rampeauto off, rampeauto on and lock on.
The result is shown in attached figure. However, we found the phase noise level is comparable with before while locking.
I'm wondering if the increase of the phase noise at high frequency when the cavity is locked is due to the fact that, when the cavity is locked, the frequency changes of the master laser are very large ~ MHz.
Possible tests to check this hypothesis are to damp the mirrors (sending the PZT correction signal to the mirrors, upon filtering) or to excite the mirror oscillations, to artificially increase the laser frequency changes.
A related question: when we compute the residual RMS phase noise between the main laser and the auxiliary laser we integrate down to 100 Hz. Maybe the 1 Hz region is dominant with respect to the high frequency region, and thus we should solve in any case this problem of the main laser frequency changes in the Hz region.
I replaced the PM100D power meter with another DET10N that I borrowed from Tanioka-kun, and I repeated the measurement.
This time the two signals at the oscilloscope look really similar. See the attached videos. (also the coherence on the spectrum analyzer is close to 1, sorry I didn't save the coherence data).
Then I closed the loop using the PD#1 in-loop and the PD#2 out-of-loop.
Then I exchanged them and closed the loop. See the two figures.
The control loop reduces a lot the noise in-loop but it doesn't really work for he out-of-loop PD (same situation when they are exchanged).
One possible reason could be the clipping noise, because I'm not sure how precisely the beam is focused inside the area of the PD.
Another possible reason could be the OD2 filter (that I'm putting after the laser to limit the power and avoid the PDs saturate). If I remove it, 40mW would imping on each PD. I'm not sure how safe it will be for the PD, and In order to avoid saturation, I will have to drastically reduce the load resistance.
Another way to reduce the power would be to enlarge the beams up to much more than the PD size (which is 1mm).
Participants : Yuefan, Eleonora
We prepare the set up to characterize the reflected beam from the FC. In the final configuration we will need to take a part of the reflected beam to send it to the quadrants for the AA.
At present the green FI reflects about 4.5 mW which are attenauted in order to send only 150 uW to the PD used for the filter cavity lock. (150 uW corresponds to a DC output of 88 mV )
Up to now we used a set of optical densities just placed on the bench to attenuate the power and today we changed them with two "mirror shaped" optical densities ( ND 1 and ND 0.5) which we coud directly screw on the last lens before the PD, making the setup more stable.
Since the beam height is only 3.8 cm, we prepere a periscope to be installed before the PD. The lower mirror of the telecope should be a BS which trasmit at least 3% to the PD and send the rest to the quadrants. Since we couldn't find any suitable mirror we could not perform the beam characterization with the FC cavity locked. We will do it as soon as we can get the mirror for the green.
Pic 1 and 2 show the periscope we assembled.
We remark that there won't be a lot of space to put quadrants and galvo in that area of the bench, so the desing has to be studied carefully.
Following Yuhang: Procedure to lock MC green (MCG) and MZ
- Lock SHG
- turn on high-voltage drivers of MZ and MCG
- Check alignment of MCG using a ramp (typical value 7.05Hz, 1Vpp) now we are using s-pol but anyway it should be the same procedure with p-pol
- if the alignment is fine, set the gain of standford to gain = 1. Use a simple lowpass pole at 3Hz on the standford.
- tune MCG PZT driver offset to go as close as possible to the TEM00 peak
- close the loop of MCG (put MCG correction in input of MCG driver)
- increase the gain of standford to 5
- tune the MZ high voltage offset. You should see max and min. Now we are locking a little higher than mid-fringe in order to get enough tranmitted power by MCG (1V on oscilloscope which corresponds to 12.5mW)
- On MZ servo check EP mon (Error Point monitor) and put it to 0 changing its offset
- NOTICE : WE NEED TO MAKE SURE THE GAIN IS HIGH ENOUGH (USUALLY 10) TO AVOID THE UNSTABILITY AT 600 HZ
- On MZ servo turn on lock and integrator
To measure MZ openloop TF we injected noise in the servo input ''add'' and take TF between EP mon and input. For a swept sine typical noise value is 50 mVpk
Notice: if you vibrate the bench too much, you will destory the lock of MZ.
We left the Mach-Zehnder and green mode cleaner locked and we found them still locked when we came back after about 2 hours.
The attached picure shows the transmitted power from the MC over a period of about 8 min (the longest that can be recorded with the oscilloscope) after two hours of lock.
The gain of the potentiometer of MZ was 9.9 and fluctuations seem to be of the order of 1.5%
The laser is polarized. Using a non-polarizing 50:50 cube beam splitter I separated the 1310nm probe in 2 beams. One beam is detected by the DET10N and the other by the PM100D integrating sphere.
Conditions:
laser current = 200mA,
power attenuated by a od2 nd filter,
power on each PD = about 250microW,
load resistance of DET10N = 7.5kOhm.
DC PM100D = 1.52 V
DC DET10N = 2.62 V
I recorded 2 videos of the oscilloscope measuring the two signals simultaneously.
Then I measured the coherence, and this time the signals are more coherent. See plot
then I closed the loop with the PM100D in-loop and then I closed the loop with the DET10N in-loop. See the plots.
Now the loops work better, but maybe its better to use another DET10N, because it looks less noisy.
I replaced the PM100D power meter with another DET10N that I borrowed from Tanioka-kun, and I repeated the measurement.
This time the two signals at the oscilloscope look really similar. See the attached videos. (also the coherence on the spectrum analyzer is close to 1, sorry I didn't save the coherence data).
Then I closed the loop using the PD#1 in-loop and the PD#2 out-of-loop.
Then I exchanged them and closed the loop. See the two figures.
The control loop reduces a lot the noise in-loop but it doesn't really work for he out-of-loop PD (same situation when they are exchanged).
One possible reason could be the clipping noise, because I'm not sure how precisely the beam is focused inside the area of the PD.
Another possible reason could be the OD2 filter (that I'm putting after the laser to limit the power and avoid the PDs saturate). If I remove it, 40mW would imping on each PD. I'm not sure how safe it will be for the PD, and In order to avoid saturation, I will have to drastically reduce the load resistance.
Another way to reduce the power would be to enlarge the beams up to much more than the PD size (which is 1mm).
effect in the setup?
- When the loop is closed there is an oscillation at high frequency. Is the loop gain too high?
- Having the beams well focused within the photodiodes is important for this test.
We made a measurement of the power stability after the green mode-cleaner with both mode-cleaner and MZ locked.
The mode-cleaner was locked with the Stanford preamplifier (first order low pass, cut off frequency 3 Hz, gain = 5). The unity gain frequency should be measured, but the lock was very stable.
The MZ was locked with the Emil board at a position of the frange intermediate between the half fringe and the bright fringe (bright fringe correspond to 1,3 V and MZ was locked at about 1 V, corresponding to 11.3 mW), using the transmisison signal after the mode-cleaner. The measurement of the unity gain frequency gave a 3 kHz ugf (see picture 2), compatible with the ugf measured by Emil. (gain of the potentimeter 9.9).
Figure 1 shows the power transmitted with no lock, then with the 3 kHz ugf lock and with a gain reduced (from the position 9.9 to the position 3.5, corresponding to a measured ugf less than 100 Hz). We remark that, during the transition between high gain and low gain the loop experiences an instability at about 600 Hz (as altready observed by Emil) and it is coming back to a stable position.
The power transmitted by the mode-cleaner was 11.3 mW (corresponding to about 1 V). The fluctuations of the transmitted power on several tens of seconds timescales have an amplitude less than 10 mV (which corresponds to 1%) with the high gain. With the reduced gain, the fluctuations increase by roughly 1.5 and they remain less than 2%.
We left the Mach-Zehnder and green mode cleaner locked and we found them still locked when we came back after about 2 hours.
The attached picure shows the transmitted power from the MC over a period of about 8 min (the longest that can be recorded with the oscilloscope) after two hours of lock.
The gain of the potentiometer of MZ was 9.9 and fluctuations seem to be of the order of 1.5%
I simulated the box that separates the DC and the AC before input in the lock-in amplifier.
I drew the box with a voltage generator to show that the transfer function is a simple low pass filter and a simple high pass filter.
A more realistic simulation is with a current generator from the PD.
Today we have continued the investigations about the mode-cleaner transmission.
First of all we have tried to carefully align the cavity. To do that we have blocked one of the beams of the MZ and turned the polarization of the incoming beam to have the "p" polarization (lower finesse). In this way the peaks are broader and more visible. In the final alignment, the power in the 01/10 modes was of the order of a few % (see figures 1 and 2).
Note that, since in a 3-mirrors triangluar cavity the 01 and 10 are at different resonance frequency, we can align the vertical and horizonal directions and check immediately the effect on the peaks.
We have also measured the visibility of the cavity using the reflected DC photodiode. Note that when the the two beams of the MZ are present, the photiode saturates and then we put an optical density OD1 in front of the photodiode.
Measurements with the cavity locked
We have made three measurements. All the powers are in mW
1) MZ blocked - "p" polarization
trans = 6, in = 7.6, ref = 1.2, end 0.03
--> transmitted power = 79%, reflected power = 16%, Losses = 5%
2) MZ blocked - "s" polarization
trans = 4.5, in = 7.6, ref = 1.5, end 0.26
--> transmitted power = 59%, reflected power = 20%, Losses = 21%
3) MZ unblocked - "s" polarizaton
trans = 11.7, in = 18, ref = 4.2, end 0.6
--> transmitted power = 65%, reflected power = 23%, Losses = 12%
Measurement with the ramp
In the three cases the cavity visibility (1-R_res/R_nores) ~ 20%, which is in rough agreement with the measurements with the cavity locked.
Conclusions
1) the measurement of the cavity visibility ~ 20% (and the reflected power at resonance) seems to exclude a problem of asymmetry in the in/out mirror reflectivities.
2) a transmission of the order of 80% for "p" polarization and 60% for "s" polarization is possible. The 80% transmission for "p" polarization is compatible with the lower finesse of the "p" polarization.
3) we still have some non understood losses. More investigations are needed.
Figure 1 and 2 shows the alignment conditions with "p" polarization (we see that the high-order modes are a few %).
Figure 3 shows the cavity visibility
Participants: Matteo B., Yuhang, Eleonora
In entry # 876 we reported a computation about the difference between the reflectivity of the input and output MC mirrors needed to explain the low transmission. The results in that is has to be of the order of 0.5%
In order to see if the low transmissivity of the green MC is due to such a difference, we have changed the input and output mirrors, hoping to find a combination with smaller difference.
The mirrors installed belong to a batch of 6 mirrors with nominal reflectivity 99.2%. (see pic in entry 850). Among them 2 have been used in the mode cleaner, other 2 have been used to extract pick off of the MC reflected and transmitted beam. One was still in the box and the last one I don't know.
Here the different combination we tired:
| in | out | tra pickoff | spare | transmissivity | |
| initial | 1 | 2 | 3 | 4 | 44% |
| 12/07 | 3 | 2 | 1 | 4 | 45% |
| 17/07 | 3 | 1 | 2 | 4 | 35% |
| 17/07 | 3 | 4 | 2 | 1 | 56% |
The best results have been found with the last configuration
INPUT POWER : 23 mW
TRASMITTED: 13 mW
REFLECTED: 7 mW
We added a lamba/ 2 to improve the polarization and we observed that we can get rid of the broad mode observed in the spectrum.
The alignment was good but can be further improved.
Note that between the second and third configuration I have reduce the modulation depth sent to the 15.2 MHz EOM of a factor 2 (selecting 1/2 scale in the windows 'channel control' of the DDS software). As expected it improves the transmissivity of few percents. The lock of the SHG doesn't seem affected by this change.