CC and p-pol beams

We slightly moved the positions of the last lens of the OPO telescope as well as the one of the EOM.

We obtain the 2 characterizations attached to this entry.

The nominal value of the beam inside the OPO should be 36 um so we are now within +/- 1.3 um of the nominal value.

The OPO telescope was adjusted so to have the mean beam radius of 36 um.

The x-axis "0" is the last hole of the rail.

The 2 beams are still superposed as we could see on the beam profiler.

Vacuum system recovery after the blackout

The vacuum system has been rebooted after the blackout of this Monday.
All the four pumping station are now back in operation and the vacuum level reached the 10^(-8) mbar level.
As a consequence of the blackout, all the three big gate valves along the arm were closed. After switching on the air compressor, the three gate valves opened.

Note: the penning at the South End station, tunnel side, measures 1.0e-7 TORR even if the other instrument (tower side) is deep in the 10^(-9) region. I tried to close the arm gate valve and the turbo pump gate valve on that side to check if the value was real or stuck and it seems that that penning cannot go lower than 1.0e-7 TORR.

1310nm probe noise investigation: change polarization

The output fiber of the laser is single mode, polarization maintaining.

The reason of the jumps could be related to the polarization or some modes hops. So I tried to rotate the polarization.

Until now the polarization of the beam exiting the fiber was P (I didn't consider this parameter when I assembled it).

I rotated the fiber collimator in its mount by 90deg, in order to have S-polarization.

In this configuration, the loop works better. See the plot

Since rotating the collimator caused the misalignment, I ordered beam splitters and a half wave plate to optimize the polarization without loosing the alignment every time (they should arrive within one week.)

The out-of-loop PD was on the waist of the probe (at the sample position). The final test will be to put the out-of-loop PD on the imaging unit, where it will sense only the central part of the beam.
If the jumps we see in the intensity are related to the modes, and therefore to the shape of the beam, then the loop will not work, because the in-loop PD will see the total power, and doesn't see the shape.

Tama in the storm

On behalf of Takahashi-san whose presence and help today was crucial !!

Participants: Takahashi, Marc, Yuhang, Manuel

Today from around 15h there was a really strong storm in Mitaka.

Issue 1: power shut down.

There was a power shut down (caused by a lightning) while we were doing experiment for few minutes.

We turned off everything but many parameters have to been checked as they have been reset. We didn't yet turn on laser or others electronic devices (We heard some strange sounds we need to investigate).

Takahashi-san was there to help with the compressed air leakage.

All the pumps were also turned off!

He closed the valves of the chambers and of the pipe.

When we arrived at the south end room it was quite hot and humid (similar situation as the other one). Takahashi-san turned the air conditioning on but this might show that it turned off by itself from time to time.

Issue 2: water in central room

Coming back from the end room we found out that a lot of water was leaking inside Tama on an electrical board (probably the general power).

We removed as much water as possible but this shows that there is a leak coming from there. The electric board seem not having any damages.

Issue 3: air conditioning in elec shop

Going inside the electrical shop we saw that one air conditioning system has fallen off the wall. The humidity didn't drain outside so a lot of water was on the floor. We removed 4 buckets of water.

Isse 4: air leakage

Takahashi-san also had the time to fix the air leakage by tightening the joints (without replacing them). When we were there, also other 2 joints in the central room started leaking air. We tightened them as well.

 

Finally, electricity seems to be back everywhere.

A careful check will be made tomorrow.

Takahashi-san said he will turn on the pumps / open the valves tomorrow.

Toward the OPO

This entry will summarize past working days (sorry for the delay)

New Optical scheme and wiki sections :

The new optical scheme (v 31) ".svg" has been updated in the wiki. The ".png" is attached to this entry.

The available lenses have also been added to the "Optics" section of the wiki.

EOM alignment and p-pol power :

The 2 steerings mirrors before the EOM have been remplaced by PBSW-1064 which should only transmitt 5% of p-pol each.

When doing this, we saw that the lens before the EOM was a bit tilted so we aligned this EOM path again.

We now have

before EOM	10.0 mW
after EOM	8.7mW
p-pol after EOM	100 um

Note that the power values where chosen to compute easily the % transmission.. We can increase a bit the power if needed. However to lock the OPO, it is planned to use a similar photodiode to the one used to lock the FC where the power is ~200 uW.

It seems that p-pol inside a modulator can lead to amplitude fluctuations. Is this power low enough to avoid them or should install an additional PBS before the EOM?

P-Pol and CC beam characterizations :

In order to characterize this 2 beams, we did as for the FC green reflection.

We added one f=100mm lens and check the beam profiles after it.

This 2 characterizations are attached to this entry.

The lens after the EOM was then moved a little in order to match more precisely the beam waists.

The 2 beams were aligned before the first sterring mirrors of the OPO telescope using 2 irises and checking the power transmitted.

The 2 beams are now quite well overlapped on the rail as we can see on the beam profiler.

OPO telescope :

Using the CC beam as our reference, we designed a telescope that is now placed between the 2 last steering mirrors before the OPO on a rail.

Our first simulation didn't match the data. It seems that we need to understand it a bit better. Please note that the value of the FC relfected green beam parameter before the lens is wrong!

Using Jammt for the moment we could recover the beam parameters before the lens and design the telescope.

It consists of 2 lenses as attached to this entry (sol2).

Before the storm we found a mean beam waist of 36.3 um quite close to the nominal one!

A better characterization of the 2 beams will be performed tomorrow.

1310nm probe noise investigation

The noise may come from the fiber.
I checked with a sound generator on my phone and put the speaker near the fiber. I see the peak at 4690Hz (random choice, but the same thing happen for other frequencies) on the spectrum (see pictures). Some acoustic noise is coupled to the fiber. So I fixed the fiber with some tape.

I put the OD2 filter to reduce the power, then a non-polarizing BS to send half of the beam on the in-loop PD.
I put the in-loop PD on a separate optical board standing on a periscope post because there was no space on the breadboard.
Before the in-loop PD, I put a f=50mm lens. The in-loop PD is mounted on an XY lens mount for a fine centering of the beam on the detector.
I put the out-of-loop PD on the translation stage and moves it to center the beam inside the PD.

Both PDs have a load of 7.5kOhm.
On the oscilloscope, the two signals (AC coupling) are very similar. Also, the coherence is high.
I closed the loop but the noise doesn't reduce on the out-of-loop PD.

I took the spectra with 400 peakhold averages, not linear averages, because I'm interested in reducing the nose peaks, the spikes due to the jumps.
I really need to understand where these "jumps" come from.

Comment to Preliminar design of the pump size reduction (Click here to view original report: 939)

I made a measurement of the pump beam profile to make sure of the parameters without the last converging lens.
Result:
waist = 500um
waist position = -1m

the axis origin is the converging lens mount.

Then I updated the Jammt design using these parameters for the initial beam.
The current converging lens is f=150mm, so Jammt gives a resulting waist of 67um at 165mm from the lens. See figure. 

I also made a measurement and fit after putting back the lens. the fit gives a waist of 70um at 171mm. Quite comparable with the jammt result.

then I updated the configuration to have a pump size of 70 - 75um (diameter).
See the diagram.

I need a diverging lens f=-125mm and a converging lens f=100mm

Comment to Simulation: change pump size (Click here to view original report: 905)

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. 

Preliminar design of the pump size reduction

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

Comment to Simulation: change pump size (Click here to view original report: 905)

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

Comment to Green beam reflected by the FC (Click here to view original report: 935)

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.

CC and P-Pol beam recombination

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) :

before EOM	After EOM	After 1st steering mirror	before lambda/2	After	PBS transmission	PBS reflection
13.5 mW	11.5 mW	10.1 mW	9.5 mW	9.36 mW	8.07 mW	1.12 mW

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.

Green beam reflected by the FC

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.

Comment to Green beam reflected of FC characterization (Click here to view original report: 929)

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.

Comment to The characterization of beam going out from Green Mode-cleaner (Click here to view original report: 911)

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. 

Comment to Green beam reflected of FC characterization (Click here to view original report: 929)

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

leakage in Tama (?)

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..

Few beams characterizations

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

 

Green beam reflected of FC characterization

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).

EOM telescope

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