We measured the Finesse of green mode cleaner (while p-pol and s-pol)

For s-pol, the measurement is fine. But for p-pol, the main peak overlaps with sidebands, as you can see in the attached figure 2. I take only the central part of data to do fit to avoid the influence of sideband. The result is listed as following.

For s-pol, the Finesse is 300

For p-pol, the Finesse is 36.7

I opened 2 new sections on NAOJ wiki :

"Pictures" which can be useful for external people to know what space is available on the bench for example.

It isn't yet totally working but it has already 1 picture on it.

"Available optics and datasheets" could also be useful (e.g for future telescope design)

During the last week and today, we did a lot of investigation about how to put lens to achieve many limitations. Today, we finally found it really diffcult to design telescope with all the condition we have now. We talked with Matteo, we got an important conclusion. It is we can decrease laser low to 30mW.

However, after measured the beam dimension, we found the beam is not like the simulation result of Jammt. We guess this comes from that the inital beam is not very collimated. We decided to decrease power further more. So we decrease power to 18mW. Then we characterize beam again. The beam is really similar with before. See Fig.1

Then we put the EOM, the position is decided as shown in attached Fig.2. After put EOM, we characterized beam again. The Fig.2 is the characterization result.

According to this result. We tried to put another lens. We can recover the beam to beam waist as 940um. See attached Fig.3. This is a little different from what we need. By putting more lens, we can have good dimension of 1000um of waist. However, more lenses cost too much space.

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.

Participiant: Marc and Yuhang

We want to know how the beam will look like after putting the EOM. So we put EOM yesterday and checked the output of it today.

Before putting EOM: we found for the far field(more than 40cm), we can see clearly the astigmatism even with the card. See attached Fig.1

After putting EOM: we found the astigmatism disappear for the far field(the beam looks very perfect by card now, unfortunatly I didn't take picture). But we found it in near field. See attached Fig.2. You can see the waist position is really different. But for the far field, it looks fine.

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.

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.

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.

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 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

1. Lock SHG
2. turn on high-voltage drivers of MZ and MCG
3. 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
4. if the alignment is fine, set the gain of standford to gain = 1. Use a simple lowpass pole at 3Hz on the standford.
5. tune MCG PZT driver offset to go as close as possible to the TEM00 peak
6. close the loop of MCG (put MCG correction in input of MCG driver)
7. increase the gain of standford to 5
8. 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)
9. On MZ servo check EP mon (Error Point monitor)  and put it to 0 changing its offset
10. NOTICE : WE NEED TO MAKE SURE THE GAIN IS HIGH ENOUGH (USUALLY 10) TO AVOID THE UNSTABILITY AT 600 HZ
11. 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.