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. 

I order to reduce the size of the pump and keep the waist on the same position we should add a diverging lens before the converging one.

We designed a configuration on Jammt using a -100 lens and a +100 lens. See the attached image.

The new pump size will be 37.6um (radius).

A more precise design will be after a better characterization of the pump beam profile before the lenses

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  used ABCD matrix to calculate the beam parameter before the lens(the lens used to perform better beam measurement). The result is as following:

beam waist position: -3.2m (relative to Faraday Isolator)

beam waist size: 877.20um

I also attached the python code, if you are interested, please have a look.

Today we work on the recombination of the 2 auxiliary laser beams.

1. We put the rail after the EOM so that the lens before the EOM is on an usual post. The purpose of this change was to be able to match the CC and P-Pol beam sizes. After trying a lot of lenses before the EOM, we couldn't see any drastic changes on the beam shape so we guessed this EOM doesn't affect the beam shape as much as the green one. We now have 85% transmission of the EOM. The missing few % can come from dust inside the EOM making the alignment a bit more tricky.
2. We put 2 steering mirrors before the lens in order to save some space on the bench.
3. The second lens is set such as the beam is collimated with diameters 2150 and 2050 um close to the CC beam size (2100 um)
4. we put the lambda/2 on the rail in order to save place
5. We put a 2' PBS cube and checked that the 2 beams are superposed. It is still the case after the last steering mirror.

Today the 2 beams seem to be overlapped.

One issue is that we have some power loss on the p-pol beam (EOM path) :

This might be linked with some polarization troubles. We will look this hypothesis tomorrow.

The new optical path is attached to this entry and the optical scheme will soon be updated.

After some rough simulations, it seems that one f = 125mm lens 5 holes before the OPO can provide a beam waist of 40 um. This is close to the nominal 36 um.

Anyway, we will use this lens tomorrow to make sure the 2 beam parameters are similar.

In previous entry ( #934 ) we reported that the BS pitch was saturating causing a lot of residual motion of the beam.

Yesterday, we found out that the BS pitch correction was not saturating anymore (~ -6V) and the motion was at the order of 10 urad both for pitch and yaw.

This is still yet ~5 times higher than other mirror motions at the exception of EM pitch which has similar motion.

To do the characterization we installed a f = 100 mm lens

In order to be sure that we had a good beam shape we installed a CCD after the periscope.

When the cavity is locked, the beam is shaking a lot preventing to take much more points.

Good news is that the beam doesn't seems astigmatic.

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.