ThomX has been locked with a new telescope while using compression CVBGs.
Coupling is ~50-55% and lock is stable.

Power up to ~50kW should follow up soon (stop when HOM are observed). We'll not go over ~100kW to not risk any breakdown due to the short pulse length (~2.5 ps).

Fabian and Ronic discussed LIDT (laser induced damage threshold) for Ta2O5 at 2.5 ps is => 1J/cm²
We have w > 2 mm ( => surface ~= 0.126 cm²) . At 100 kW it means 800 kW/cm². At 33.33 MHz it means 24 mJ/cm². With 2.5 ps, peak power is 10 GW/cm² (1.2 GW peak).

A power up has been performed on ThomX until 65kW intracavity power. We didn't see any modes but stored power was really unstable since ~60kW (see image tek00004.png).

Measurement report is shown in the table below. Gain seems to decrease against intracavity power.

As a conclusion, pulse length compression does not seems to bring any trouble in the PDH loop. But at relatively high intracavity power, power start to be really unstable even if we did not see any mode.
We can notice that we tried to align, change CEP but it had no important impact on the quality of the lock. See further experiments with Ronic and D-shape.

Last Thursday (20th of February), Loïc and Titouan realigned the stretcher CVBG to its nominal angle and they used an interferometric technique with a delay line and combining the two paths in a CCD to measure interferences.

After supressing the global shape of the superposed pulses, one measures the amplitude of the remaining fringes (standard deviation) each 50µm of the delay line (100µm of round-trip).
one gets the interferences pulse shape with a FWHM of 2.3 ps... (see curve)

The Koheras @0.5mW is directly connected with fibers FC/APC -> FC/PC to the Avantes optical spectrum analyzer.

The FWHM wavelength, measured with the Avantes software, is 0.126nm

Same measurement with Koheras @100mW and using fiber coupling lenses between Koheras and Avantes OSA.

The FWHM wavelength, measured with the Avantes software, is 0.116nm

Measurement of the optical spectrum with Avantes OSA before (1st picture) and after CVBG (2nd picture) with 2nd stage on the laser amplifier.
The 1st CVBG stretches the beam horizontally due to the default incident angle and the fact that different wavelength are reflected in the CVBG with different depth.
as a result, the optical spectrum is varying along the transversal axis of the beam.
The 2nd CVBG is injected with the opposiste incident angle and should compensate the stretch effect to get back a circular beam.
spatially, the beam is quite circular but one can still see a dependance between position in the beam and optical spectrum.

Then, it is difficult to show the "right" optical spectrum after CVBG (one could use a diffuser for that) but it is clear that the spectral width is quite the same (~ 2nm due to the CFBG at the input of the amplifier) before and after CVBG.

Yesterday, with Titouan, we made some measurements on pointing stability of the laser beam after amplifier and CVBG.
the amplifier beam go through a first telescope to be small enough and colimated before going to CVBG's, then go to CVBG's and then is sent through the FP-cavity telescope to the FP-cavity itself.
the total length is about 6m to mirror M1.
the surface of the mirror M1 is imaged with several wedges to a Basler CCD.
(x is for veritcal position and y is for horizontal position)

* the 1st plot shows the pointing stability at low power of amplifier without airflow and walking around during about 5 minutes extracted from the Basler CCD video.
when walking around or with airflow the pointing stabily is much worse.

* the 2nd plot shows the pointing stability vs amplifier current.
it is comparable (a litle bit worse) to the pointing stability at low power.
one clearly see the beam expanding in vertical direction and also in horizontal direction.
the effect could come from the CVBG telescope lenses which are standard lenses and not high power lenses.
the pointing effect could come also from the same effect if the lens is not perfectly centered.

we took also some picture of the CVBG at different power with the Thermal camera but we need to get the data from camera (old software not compatible with Windows 10)

After 3 months without human presence, finesse has been measured 3 times with average optimization: 
- 16150
- 16067
- 16172
Last measurement was on 6th of november 2019, finesse was 17000. So it didn't change or just slightly.

We used the last Onefive telescope (used with CVBGs). Coupling reached ~30% (see pink curve on image attached).
Cavity vacuum is ~1.2e-9 mbar and pneumatic valves were still openned after these 3 months.

After the Edge installation inside the cavity, the Finesse has been measured several times by modulation technique with an average around 16000:
(the scan speed is 50kHz in 10 seconds.)

So, it hasn't changed since the last measurement in June, just before the Edge installation.
The Edge position is normally the furthest from cavity beam (all rotating knobs are at 0 positions)

We used the last Onefive telescope (used with CVBGs). Coupling reached ~50% after alignment.
The cavity vacuum is ~7.2e-9 mbar as the cavity has been recently opened...

Coatings reflectivity curves and datas for HR Saphir, HR Suprasil and HR ULE.

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.

Picture of the installed mirrors inside the FP cavity.

To install an Iris instead of a mirror :

One has to remove manually the orange nuts and replace the mirror mount with the Iris mount.

https://atrium.in2p3.fr/0dded51a-1cb0-43e6-ae0d-626cd5db7078

a lot of schematics regarding the FP cavity are stored by the mechanic group on the Atrium repository.