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).

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).

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.

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...

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.

Global locking almost clean : tranmission and reflection are quite noiseless except the peaks at every 500ms.

During the amplifier 2nd stage lock, we experienced 2 strange behaviors :

- short "unlocks" every 500ms exactly all the time, during the whole lock process

- some PZT (in green on the picture) shift exactly synchronized with these "unlocks" (see the red circle).
these shifts are triggered by some OneFive motor move (Frep adjustment).

We tried to disconnect software activity (use the disconnect button on the software), to disconnect USB cable, to disconnect power supply of the motor controller box but none of these actions had an effect
=> still short unlocks every 500ms...

The amplifier 2nd stage seems to have small fan for the power supply... we can try to disconnect it.

We tried to turn on the 3rd stage which uses several fans for cooling the pump diodes... thus, one can see immediatly a big increasing in the PZT signal which tries to compensate the vibrations and quicly one loose the lock.
we have to think on how to suppress these vibrations.

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.

Power during ~20h, power after turn on, spectrum image and excel datas, polarisation and pulse length.

Powermeter measurments were done with a /10 filter.. 3 mW means 30 mW

Important: Onefive does not automatically locks after being turned on.

Beam Size Onefive added

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.

1st lock today with the second stage of the amplifier.
~ 300mW of injected power (but which proportion of the pump and of the signal ? => One must use a spectrometer to determine this proportion)
MAYBE the real injected power of the signal is lower => maybe the Finesse is higher !

Coupling ~ 75%

1.2 mW measured in transmission => ~800W inside the FP cavity
=> Gain ~ 3500 => Finesse ~ 11k !!!

Last Finesse measurement was about 19k ! :-(

First of all, we used and optical spectrometer to determine which proportion of pump and signal is in the injected beam.
the wavelength is THE SAME for the 2nd stage for the beam coming from the core or coming from the clad !
thus, it is impossible to use a spectrometer to obtain this proportion.

Thus, we made the assumption the pump beam (Sp) is unpolarized in contrary to the core beam (Sc) which is almost linearly polarized (Scp, Scs)

with a half waveplate and a PBS, we can define 2 cases in rotating the half waveplate at the output of the PBS :
maximum power : Scp + Sp/2 = a1
minimum power : Scs + Sp/2 = a2
The sum of both power a1 and a2 : a1+a2 = (Scp+Scs) + Sp is the total power of the beam before PBS.
The subtraction of power a1 and a2 : a1-a2 = Scp - Scs is independant of the pump power which can vary for example in opening an iris which normally block the most part of the pump beam.

if the core beam is perfectly linear, Scs=0 => a1-a2=Scp and Sp=2*a2
We did 2 measurements with a1=227mW, a2=67mW => Scp=160mW (Sp=134mW)
and with a1'=277mW, a2'=107mW => Scp=170mW (Sp'=214mW with a wider opened iris)

if these measurements and assumptions are correct, the real power injected to the cavity was 170mW (instead of 300mW).
with a coupling of 75% => Gain ~ 6.3k => Finesse ~ 19.7k !

to be confirmed...

The Onefive spectrum has shifted a little bit to lower lambda. It's still enough for the experiment (3mW injection of ampli intead of 1mW needed).

Fiber injection is finished with the new ThomX stretching CVBG in double pass (+50% injection power compared to K-BOX CVBG in simple pass).

- Output Onefive: 27 mW
- After Onefive + lambda/2 + isolator: 25.8 mW
- After Onefive + lambda/2 + isolator + lambda/2 + PBS + CVBG*2 + PBS: 9.6 mW
- After injection into fiber and EOM (+all preceding): 3.2 mW
- Input 99% of the amplifier: 2.86 mW

> 660 mW on 1% photodiode DET100 @1Mohm

FSR is 33.347 MHz @1e-8 mbar

We've added a clamp in the 3rd stage to prevent the pump fiber from moving and tilting (it was tilting to the left).

The ThomX M2 has been measured between 1.5-1.6 for x and y

The circuit breaker on ThomX cavity drops @1.5-1.8kW when we turn on 1/2nd stage of the amplifier or 3rd stage of the amplifier.
Amplifiers are now connected to K-BOX UPS.

We are buying a clamp ammeter for further investigations.

The cavity is right now at 6.5e-8 mbar (turbo pumps on for 1 night and ionic pumps for 3h), primary pumps and turbo pumps are cut, ionic pump are on and the pressure level is still going down. 
Therefore there aren't any big leak in the cavity and we can expect a final vacuum level close to e-9 mbar.