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

The pneumatic valves are now plugged on the university air network.
The air valve is fully openned for the moment.
We still have to check if there is some minor leakage close to the pneumatic valve to improve the setup.
But it's not anymore an issue for the lock.

The 500 ms noise disapeared. We had to disable the "power save" mode of the smaract translation stage. AND turn OFF and ON the controler box.

since some days, we observe the cavity is difficult to lock and a strange 30Hz noise has appeared on the PZT signal which normally compensate the phase noise difference between the cavity and the laser.
to test if the problem could come from the laser, we changed the OneFive laser for the Koheras but we have exactly the same problem, thus we concluded that the problem come from the cavity or from the feedback.

today, we borrowed an accelerometer measurement setup to Julien Bonis to test if we can see a clear noise at 30Hz from the seismic noise.
we placed to accelerometer directly on the top of the cavity but the spectrum we obtained do not show a clear noise line at 30Hz, only a small excess of noise in this region...
nothing which clearly indicate the cause of our problem.

yesterday, we changed the feedback setup in changing the PDH box from n°2 to N°3 without any change in the 30Hz noise line.
today, we also changed the feedback setup by introducing and amplifier of 100 just after the PDH box.
if noise is coupled after the amp we should be more immunized from it... but nothing changed again.

still looking for the origin of this problem...

Beam Size Onefive added

The ouput power of the ThomX amplifier has been measured as a function of the 3rd stage pumps current (0A is 1st and 2nd stage turned on).

We measured the total power, the total power without the "donuts" pump signal dumped by an iris and the transmission of a dichroic mirror (for ref see image).

The spherical mirrors are at the closest position to get a FSR ~=33.3435 MHz. Planar mirrors are at the largest position.

The finesse has been measured about 17 000. 

Beam diameter (2*w) is ~= 4.6 mm x and y.

Yesterday, we've observed the first degeneracy on ThomX cavity at ~115 kW stored. The shape of the stored power on the oscilloscope is really close to what has been published in the D-shape mirror's paper ("tek0000" transmission in yellow, piezoelectric activator in green, error signal in blue).
This mode could be a 8.0 or 9.0. 
We have also observed a change of the angle of the mode (the pictures "degenere", "degenere2" and "degenere3" were taken by order). The only changing parameters are the alignement and the length of the cavity. 
The last thing we noticed is the shape of the mode which could not be an hermite-gauss mode (Bessel mode ?).

Before, the locking scheme was to produce an error signal with the PDH box, send its output to the electronic amplifier box and then to the laser EOM for the fast loop and to the Laselock input. The drawback was changing one loop gain, changes also the other loop gain.

Now, the locking scheme is the error signal coming from the PDH box is directly sent to the Laselock input and in a same time to the electronic amplifier box which is only connected to the laser EOM for the fast loop. then, one has the fast loop gain which can be changed with the electronic amplifier box, one has the slow loop gain which can be changed on the Laselock and if one needs to change global gain of both loops, one can move the PDH diffuser.

This scheme is currently used and locked the setup.