Last Friday we wanted to do this characterization again because it seems that our previous points were still inside the Rayleigh range of the beam (which should be few meters)

The solution is to add a lens and recover the beam parameters without the lens.

We faced few issues during this measurement :

Faraday Isolator

1. The Faraday Isolator output (facing the FC) is clipping probably higher order mode which are quite large w.r.t. the FI aperture
2. The FI PBS which reflects the beam toward the locking photodiode is a bit broken ( it is the cause of the "strange" behavior of the beam we could observe. However, with proper alignment it is possible to avoid hitting the broken part of the PBS and have a good beam ( meaning we can avoid to have black line on the beam)

For now it seems we can keep going on with this FI at least for the beam characterization.

It may be useful in the future to change this FI for a larger aperture one or at least change the broken PBS.

BS

1. We found out that the beam was shaking a lot ( ~40 urad rms ) while performing this measurement. This is due to the fact that the BS pitch correction is close to saturation (-10V) and some peaks lead to correction saturation.

The solution is to move the BS pitch picomotor. Matteo also recommends to follow the GEO procedure : Every ~2,4 weeks, use picomotors to put mirrors on good positions in order to avoid this kind of saturation.

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. 

Here are the data :

z = [ 0 1 2 3 4 5 6 15]*2.5+40 [number of holes]+position of first measurement

wW = [2170 2192 2181 2137 2239 2183 2214 2380]/2
wV = [ 2169 2207 2216 2211 2207 2202 2112 2114]/2

Since few days (weeks?) it is quite hot and humid in the 2nd floor of Tama.

If I remember correctly it wasn't the same last summer.

We also found few mosquitoes inside Tama and even a spider web in the "clean" booth.

This seems to indiquate that there are some ways from the outside air to enter Tama quite freely..

Today we did some progress on the EOM telescope.

As shown in Fig1, by using a f = 100mm lens at the output of the EOM, we can obtain a collimated beam roughly the size of the CC beam (Fig. 2).

We our many attempts to design the EOM telescope, we have placed at least 3 times a f = 100 mm lens before the EOM without noticing any huge difference on the astigmatism of the beam at the output of the EOM.

Therefore, we think that it can be useful to put the lens before the EOM of a "simple" post (its position is on a hole) and place the lens after the EOM on the rail.

This might be useful to match more precisely the beams of the CC and p-pol laser.

The next steps are :

install the 2 steerings mirrors and the lambda/2 on the p-pol path.

install the steering mirror and the pbs on the cc path to recombine the 2 beams.

Install a converging lens after the pbs to finely match the 2 beams.

Start the design of the OPO telescope with CC beam as reference. For this telescope, we plan to use a 30 cm long rail to be able to finely match the beams with the OPO

Participants : Eleonora, Marc, Yuefan, Yuhang

Since last week there were few attempts to characterize the green beam reflected by the FC.

One first task was to design a periscope able to extract the beam going inside the photodiode used to lock the FC.

As Matteo Tacca requested, this periscope was installed before the lens of the photodiode.

We couldn't find any green BS so we used a IR BS as the bottom mirror of the periscope.

We still need to use 2 optical density in order to reach power values similar to the previous configuration.

This let around 200 uW going inside the photodiode which is enough to lock

[ By the way one attempt let only 18 uW inside the photodiode and lock wasn't possible.]

Another difficulty was to avoid the clipping of the reflected beam at the FI output (face toward the FC) which needed to change a bit the alignment condition of the FC.

There are also some strange big features on this beam which might be investigate a bit.

We then installed a steering mirror at the output of the periscope in order to have enough space to characterized the reflected green beam.

The characterization is Fig.1 attached to this entry.

The first points seems to suggest that the beam is collimated around 2.2 mm.

However the last points are quite differents in size.

This might come from the fact that the FC unlocked several times during this measurement.

We started from a quite good alignment (1.2V transmission) to end with a poor one (0.8 V).

Paticipants : Yuhang

After some troubles to design the telescope, we finally found a convenient solution :

use 2 steerings mirrors to have the beam tranmitted by the 98:2 at the nominal height (75 mm)

Install a lambda/2 and replacing one steering mirror by a pbsw : power is now 14mW which is low enough to have the beam waist inside the EOM ( the ihgher limit is 18.1 mW)

We installed a beam dump to get rid of the spurious reflection of the (wedged) pbsw.

use one f=100mm lens to obtain a good beam size inside the EOM. It was installed on a rail in prevision for some EOM induced astigmatism.

The last picture shows the setup before installing the EOM 4 holes away of the lens on the rail.

The first figure is a fit of the beam after installing the lens.

(following is comparison of W and V profiles)

difference in waist position : 4mm

difference in waist size : 1.3 um

The second one is a fit after installing the EOM.

(following is comparison of W and V profiles)

difference in waist position : 2mm

difference in waist size : 2 um

The installation of the EOM changed the beam waists sizes by 2um and beam waists positions by 2 mm

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.