Measurement behind P4 (planar mirror)

After turning on, one can see the 133.33MHz Onefive output power for ~40min with OD2 filter (~/20). So the real output power is ~57mW. A measurment over several days (15?) should come in few weeks.

The spectrum here has been taken one day after turning on the Onefive (see Fig. 133_spectrum_full and 133_spectrum). One can see the central wavelength of 1030.5 nm and a small peak at 1054nm (see Fig. 133_spectrum2).

1)  Note: The OneFive laser used for experimenting is the one for the  SBox

but, for now only this laser arrived from the company so we are doing tests (measuring the spectrometer and power ) on it inside the ThomX cavity clean room.

2)  on the RF - Analyzer the value of the laser repetition frequency is measured:

* 17/12/2020 (when first turning it on, the day before in the afternoon )  --->  133.330 700 MHz 

*  this day18/12/2020 (in the afternoon, after a full day to it being on)  ---->    133.330 840 MHz

they have a difference of 140 Hz this comes from normal thermal expansion inside the laser which is ok, as it changed over the course of a day of operating the laser.

3) the power meter is connected to the desktop in the ThomX cavity room and a TeamViewer application has been installed to observe the measurement over the period of several days mentioned (15?) remotely.

to access this you need to have an account on the application and allow your account to access it from the desktop.

for now, only Ronic and Manar has remote access. 

as the iris used to locate the FP-cavity axis have been removed before moving the table inside the Igloo, we have to find again this FP-cavity axis.

this morning with Viktor, we started to make the alignment of the FP-cavity with a red laser using only the "input window" iris mount built by Yann.
we used a 4 axis mount for the red laser, plus a 2 axis mount for the injection mirror (we didn't use the final injection mirrors).
the red laser is clearly visible in the transmission of the 3 "output mirrors" of the cavity.

1- we made a pre-alignment of the red laser using the reflection on the input window
=> one can see the beam at the output of all the 3 "output mirrors", but not centered on their respective windows.
2- we made a final alignment of the red laser to have roughly the beam going through the middle of all the 3 "output windows".
(rough alignment as we don't have the iris mounts for these windows, yet).
3- we put a second iris in the input path to fix the input beam axis relative to the FP cavity axis.
(the first iris is the one used on the "input window" iris mount).

this afternoon, we plan to replace the mirror used by the final injection mirrors of the cavity.
and then, use the Koheras laser to try to get some resonances.

This afternoon, we continued the alignment of the red laser.
we did it using the 2 final injection mirrors.
we still see a clear transmission after M2, a weak transmission after M3, and almost nothing after M4 due to the power loss going through the dielectric injection mirrors (which are not optimized for red wavelength).

we placed 2 new iris in the path before the injection mirrors to help the alignment of the Koheras with the periscope.
and we prepared different equipments to continue next time: scope, photodiode, beam profiler, power meter....
 

A continuation of the alignment process was done, there was change in it due to variation in temperature, 

it was done using the semiconducting laser, and we were able to obtain an output at M2.

2 references were placed before the alignment mirrors Ma and Mb , to fix the line when changing from semiconducting laser to CW "koheras"

Another reference was placed at the reflection line.

Then we changed to CW laser and placed a beamprofiler at the output of m3 trying to observe the cavity mode, but with no success

(there was a shaped observed which we thought of as the cavity mode, but it changed position when moving the alignment -- > not mode (the cavity mode only changes intensity with alignment mirrors, or disappears))

later a continuation will be done for the alignment using 2 beamprofilers

Note: a reference file of the mirror positions was saved on the command computer and a laptop dedicated to ThomX cavity is placed in the casmate

Note the direction of injection is     M1 - M2 - M3 - M4

                                                       P1 - S2 - S3 - P4

The injected beam is aligned at the center of the irises placed at the windows mounts of mirror M1 (Injection) and M2(spherical)

At Transmission of M2 :  in addition to centered beam, we observe diffraction which interferes in observing the beating at M2 output

(could be diffracted beam from the metal pipes inside or from the D-shaped mirror installed inside)

At Transmission of M3 : we observe a beam output could be part of TM00 mode (the shape is distorted !!)

suspicious reasons :

•  when we have a frequency sweep on the CW(Koheras) piezo; we observe it beating (when increasing the drive it is increasing in intensity)
• when we adjust the alignment mirrors; the beam doesn't change position and only its intensity changes

  continuation with the alignment and try to eliminate the diffraction and find the shape of the beam.

A manual change in the D-shaped mirror position to remove any possible effects from it.

The alignment is on hold until next week 17th - 18th  Jan

an Alignment attempt will be done when the ring part close to the FP cavity is opened.

***** Continuation of the alignment ---- FP Cavity Open --- ****

The cavity was put under outer pressure and was opened for the alignment

The alignment was done using CW koheras infrared laser and  the inside mirrors irises

we observed the beam output centered at S2, S3 and P4

transmission from S4 to P1 was aligned at the center of P1 iris and an outside reference was fixed, then P1 mirror was placed, and we aligned the reflection with the transmission.

....... After the interior alignment, the cavity windows were closed ....

A beam profiler was placed at P4 transmission -----> nothing observed even with a piezo drive on the CW infrared  laser

observed a beam output at S2 and S3, but the fundamental mode is not seen, or even a higher order mode (which we can't explain, as the beam is centered on the mirrors)

images show the output at S2 and S3

Yesterday, we did the alignment again using Iris and beam profilers to obtain a more precise result.
at the end of the alignment procedure, we successfully obtained the beating modes.

the geometrical alignment seems good (weak odd modes) but we could need to put a telescope on the Koheras line as the beam size seems quite different from the mode size (quite strong even modes).

without any improvement of the alignment using photodiodes, one obtains about 15-20% of coupling.

we put several Iris on the table:
- 2 Iris before the 2 alignment mirrors to fix the axis of the laser on these mirrors
- 1 Iris just before the cavity (we will add an additional one today) to fix the cavity axis
- 1 Iris in reflection of the cavity to fix the M1 orientation

this morning with Manar, we did a cavity mode axis change.

we checked the cavity mode centroid position on the beam profiler (placed behind P4) and changed it by roughly 1mm on the X-axis and by 500µm on the Y-axis.

for doing that, we played only on the S2 mirror while the cavity is locked and we slightly changed X and Y motors, step by step, and we realigned the laser beam with external injection mirrors when needed.

in the end, we were able to improve the transmission power by roughly 25% (810mV to 1020mV on the Transmission photodiode on the scope).

we did several Finesse measurements after that: 4950, 5190, 5100, 5120, 5200 => ~ 5100

compare to the previous Finesse value, around 4200, this is also roughly 25% better, in agreement with the transmission power increase.

conclusion: it seems the Finesse, thus the losses are almost the same "everywhere" on the mirrors => we need to clean them or replace them.

Updated beam image after change on the mode axis.

Taken after P4.

Aurélien, Manar, and I spent more than 2h trying to start the software communicating with the Alphanov amplifier controller.

each time, we had a problem with the software, asking to switch OFF and ON the controller before being able to switch the amplifier diodes ON.
we switched OFF and ON many times without any success.

in the end, Aurélien called Guillaume from Alphanov... and without changing anything, it worked... strange!

one possible problem could be the correct detection of Frep of the seed laser (OneFive).
as we didn't check the signal coming from the seed laser, it could be the reason... to be confirmed.

the present status for the controller is:
- the power connector (on the rear side) is ON
- the green relay (on the rear side) is ACTIVATED
- the key (on the front side) is OFF
- the emission button (on the front side) is OFF

the normal procedure to start the controller is:
- switch the front side key ON
- start the software (possible error msg asking to switch OFF and ON the power button: don't do that)
- switch the emission button on the front side (which is red) ON
- switch the preamplifier button ON
=> all the software LEDs should be green and the PD_PULSE window should indicate 133.33MHz
otherwise, try a RESET on the software and restart the procedure (and pray).

at the end of the day, we successfully switched ON the preamp and increased the Power adjustment around 20% to get something about 10W on the big PowerMeter placed at the output of the amplifier.
=> we need another day of practice to be more confident with the software!

Additional information related to the injected power into the amplifier fibers.

power = 4.45 mW (as shown from the software)

the minimum to inject into amplifier is 2 mW

This morning, with Manar and Aurélien, we measure the power directly at the output of the Alphanov amplifier with the 2 compressors CVBG (seeding laser at Frep = 133MHz)
(we had to remove the base plate where the deflector mirrors were mounted to put the large powermeter).

with the previously described procedure, it seems that the Alphanov software is now working and we were able to start immediately the amplifier.

the chiller temperature was set at 25°C but we saw in the Alphanov documentation that the chiller temperature was closer to 20°C.
we did several measurements at 25°C and 20°C and it doesn't change a lot the output power.
so, we set the temperature to 23°C to avoid condensation (if too cold).
we will ask Guillaume what is the best temperature for the chiller.

power measurements:
power ratio     =>   measured output power (with external powermeter, not with the software)
10%               =>               1W
20%               =>               9W
30%               =>               17.6W
40%               =>               26.4W
50%               =>               34.5W
60%               =>               42.5W
70%               =>               49.3W
80%               =>               56.6W
90%               =>               63.6W
100%             =>               ~70W (expected value, not measured)

we stopped the measurements at 90% as we observed a difference in the software between the expected values (previously recorded by Alphanov) and the present ones.
the present values are quite bigger than the ones measured by Alphanov.
Aurélien will call Guillaume to check if it is a problem or not.

answers from Guillaume:

- the chiller temperature has to be set to 25°C
- the power measured with the software can change a little from what Alphanov measured with their laser.
=> one can set power tolerances to 30% in the "caracterisation.csv" file.

Today: Manar, Ronic and Aurlien started the study of the beam profile of the alphanov amplifier at high power.

The setup shown in the image shows how the power is reduced by using Anti-reflective mirrors,

pick up 1 : Anti-reflective coating on both sides , pick up 2 : Anti-reflective coating on one side and High reflectivity on the other

using this method, we avoid saturation and damage to the beam profiler from the high power amplified laser

in addition, an OD3 filter is placed in front of the beam profiler. and a reflective mirror is placed close to deflect the reflection from the mirror(pick up 1) second surface.

The beam observed is relatively well shaped and fitted up to 50% of amplification is where the shape starts to deform a little and is not well-fitted by a Gaussian. (image attached shows the fit for 60% amplification)

The cause is yet to be determined, as it could only be related to the reflections that occur from the mirrors (pick up 1 and pick up 2)

*** Note be always careful at High Power :)

Adding the fitted beam in 1D in Horizontal and vertical axis.

for 10%   to  60%

Today a second reading of the beam profile was done , with Victor ,Ronic, Aurlien and Manar

an additional set-up was made using two 4 deg wedges instead of High reflecting (HR) and Anti-reflecting (AR) mirrors.

Two reading were taken:

• at the output of the compressor with a NE30A filter on the beam profiler
• at the output of the amplifier with NE30A + NE10A filter on beam profiler