Measurement behind P4 (planar mirror)

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

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...

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.

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)

This is a long time issue for the ThomX amplifier : on the 4 pumping diodes available, the 1st diode has a higher temperature than the others around 40°C (see the picture).
the problem comes from the TEC which is not activated (see diodes parameter files in execel format). the related software windows are shaded.

I phoned to Jerome to ask him if one can securely activate the TEC, and he answered "yes".
but once the TEC is activated by loading parameters and modifying the line of the TEC activation, the temperature does'nt fall down as it seems the TEC does'nt work properly.
maybe it is not connected properly ? to be checked...

the result is, as the diode temperature is too high, an alarm is fired and the diode is deactivated... impossible to increase the current.

one has to deactivate the TEC and shut down electrically the amplifier to go back to the starting point.

Conclusion :
- the TEC of the diode 1 does'nt work properly.
- one can't activate it otherwise an alarm is fired and the diode is disabled.
- one should check the TEC connections in the amplifier

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.

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

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

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)

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.

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).

CVBGs telescope is done with w ~= 1mm. 

ThomX telescope is done with w ~= 2.3mm in x and y.

Newt step is to lock with these short pulses (measured ~2.5ps with FWHM standard deviation interference method).

Yesterday, Loïc put the CVBG's back to their specified angles.

We 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 2.5 ps... (see curve)

from Fabian calculation, at 100kW, with w=2mm, the fluence on mirror should be around 0.05J/cm^2.

from this article (https://www.sciencedirect.com/science/article/pii/S0030402618313275), the damage threshold for SiO2/Ta2O5 multilayers should be around 4.8J/cm^2 @ 1030nm

Yesterday, Loic installed a telescope before CVBG's to reduce the spot size on CVBG's and reduced the incident angle 0.5-1° on them (on the datasheet the specified incident angle is 2.8°).

We 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 ps... (see curve)
we still see an 'after pulse'.

if the pulse is 1ps long and 100kW is stored in the cavity, it means 3GW peak for the whole beam... it is comparable with the damage threshold of the mirror !!!

We installed the 2 CVBG for compression after the amplifier.
We used an interferometric technique with a delay line and combining the two paths in a CCD to measure interferences... see interferences.avi video file
One can notice some misalignement at the end of the video.

After supressing the global shape of the superposed pulses, one measures the amplitude of the remaining fringes (peak-peak or standard deviation) each 250µm of the delay line (500µm of round-trip). one gets the interferences pulse shape with a FWHM of 6ps...
It seems that an 'after pulse' is visible in the interferences.

An other meausurement using a 2 photons photodiode will be used to confirm this measurement.

Onefive output power measured today. 3.52 mW with OD1 filter ==> ~35mW.

The Stretcher is the "D 24-02-II", it must be used in double path before the amplifier injection.

There are two compressors in single path, "D 25-14-I" and "D 25-14-II".

Eric calculations are attached below.

The pump voltage has been increased few days ago when looking for a modlock of the laser.

The 33MHz Onefive output power is now 3.58mW with OD1 filter ==> ~36mW.

There are 2µW @ 1% coupling fiber with OD1 filter ==> ~20µW ==> 2mW @99% coupling fiber (2mW to the amplifier).

The 30 Hz noise issue has been solved !

It came from translation stage of P1 and/or P4.

Fabian remembered that close to the mirror's mount translation stage end coarse, there is a mechanical instability. The mount kind of "lift up" because of the spring strength and could induce resonance.

Initial positions:

• P1: - 1 500 000 
• S2: + 1 400 000
• S3: + 1 400 000
• P4: - 1 500 000

The 30 Hz noise was removed while moving only P1 closer (then if we put back P1 to the initial position, the noise clearly appear again):

• P1: - 1 300 000 --> no 30Hz noise
• S2: + 1 400 000
• S3: + 1 400 000
• P4: - 1 500 000

The 30 Hz noise appeared again while moving P4 further:

• P1: - 1 300 000 
• S2: + 1 400 000
• S3: + 1 400 000
• P4: - 1 600 000 --> 30Hz noise

Final positions:

• P1: - 1 300 000 
• S2: + 1 400 000
• S3: + 1 400 000
• P4: - 1 500 000