the goal is to estimate what could be the frequency drift at 500MHz for the SMA100A: see phase noise datasheet in attachement

Sphi(f) = FFT ( Rphi(T) ) = FFT ( < Phi(t) Phi(t+T) > )

at low frequency, Sphi(f) ~ A / (f^n) = A*(2pi)^n / (i2pi*f)^n => Rphi(T) = A*(2pi)^n*T^(n-1) / (n-1)!

for the SMA100A : n ~ 2 and A =10^-7 at f0=1GHz with B22 option

=> Rphi(T) ~ 4e-6*T

This morning FP Laser was operating well locked at 30 KWatts in stable vay.

There are some fluctuations due tu pointing instabilities probably dues to temperature fluctuations in de bunker.

In the attacced picture are reported the lock parameters and signals.

By adjusting the position of laser caviti length the lock old all de morning.

Last monday with Victor we have cheked the stability of de pointing of the laser FP.

The climatisation was operating since 3 days in satble way. The laser pointing was very stable before to inejct in to the cavity (picture 1) and also for the reflexion from the cavity (picture 2).

That means thant it is necessary to have a stable climatisation operation.

Today, the power measured at the input of the amplifier (PD_IN on the LAL amplifier software) is 2.5mW instead of >3mW generally measured

The power coming from the NKT/Onefive Origami oscillator is still >37mW (measured directly with the powermeter at the laser output without OD2).

thus, the problem should come from:

- the strecher/fiber alignment.

- or maybe from a wavelength shift of the oscillator

bunker temperature @ 22.5°C

today, Sebastien Pitrel improved its python software to manually control the laser peltier.
we were able to make to some test. unfortunately, doing only one step change the 500MHz relative frequency by ~400Hz (see the attached plot from 480Hz to 100Hz which is the laser harmonic @500MHz compared to the RF reference @500MHz), which is equivalent to a round trip length variation of 8µm !!!
the present PZT's ranges are equivalent to 400nm of the round trip length. the laser PZT range could be extended by a factor 10 if one drives it using 100V instead of 10V actually, but it would not be enough to be able to use the Peltier ! :-(

if the Peltier changes the inox baseplate of the laser, the relative length change is 1e-5 /K which is equivalent to 100µm/K of the round trip length.
it means the internal temperature steps, done by the peltier, are around 0.08K.
maybe we could try to have smaller steps by using an external thermal setup ?

this morning I monitored the frequency shift between the FP-cavity frequency (the laser is locked on it) and the RF reference frequency @ 500MHz,
during the warm-up time, for 50kW stored in the cavity (the recording started ~ 9h20).

one observes on fig.1 a simple exponential behavior with 500Hz frequency shift @500MHz (equivalent to ~ 10µm) for 50kW stored. 
the time constant T of the exponential curve is 12.5 minutes and the stable region starts at 5T ~ 1h.

the small drop at the end of the curve ( ~ @10h30) could come from the external temperature which started to drift before (see fig.2)

the 2 files describe the specfications for the16  mirrors ordered (4 for ThomX + spare, 4 for SBOX + spare) and the measurements made by the LMA.

today, the Alphanov amplifier stopped because of the too low injected power : ~2.5mW (measured by the Alphanov software)
it was still working at ~2.7mW.
normally, the injected power is ~3.1mW (equivalent to 6.2mW measured directly at the fiber output).
I checked the power coming from the OneFive oscillator : it is still 36mW, identical to the power measured the first day we installed it.

by moving the "focus" knob of the mount, we saw this 3.1mW back but it was impossible to get it in a stable way.
so we think the "strecher box alignment" is still good and we decided to realign the Schafter-Kirchhoff fiber mount.

up to now, we lost all the coupling in the mount, then we improved the power at the fiber output to 4.2mW, then we lost everything again.
we will continue this afternoon.

this afternoon, I was able to go back to 5.4mW in the fiber output (3.2-3.3mW in the Alphanov software).
IN CASE OF SMALL LOSS OF POWER IN THE FIBER : DON'T TOUCH THE SCHAFTER-KIRCHHOFF MOUNT
it is too sensitive... open the Strecher box and do the alignment improvement with the 2 final (alignment) mirrors in the box.
even if it is mandatory to adjust the Schafter-Kirchhoff mount, you will have to finish the alignment with the 2 final (alignment) mirrors of the Strecher box.

Now, all the Schafter-Kirchhoff mount screws are well tighten !
So next time, use the Strecher box final mirrors to improve the injection into the fiber.

after the new fiber alignment, I was able to lock "as usual" the cavity to 47kW @30% of laser amplifier ratio.

we plan to use Electrically Tunable Lenses EL-10-30-C NIR to build the ThomX telescope but they need to be compatible with the powerful laser beam.

for CW regime, on the datasheet (see attached file), the optical damage threshold is 10 kW/cm².
with 70W laser power and 1mm² beam surface (which is much smaller than the real beam size), we are at 7kW/cm².

for pulsed regime, with Daniele, we can imagine that the inner material is water (n ~ 1.3 in the datasheet).
One found a paper about laser induced electric breakdown in water (see attached paper) which give a breakdown field of about 1e8 V/m (see Fig. 1 of the paper) for 7ns pulse width.
with 70W average power, 33MHz repetition rate, 10ps pulse width 1mm² beam surface, one has an intensity of ~ 2e11 W/m² (or 2.7 GW/cm²)
I = E² / 377 => E = 8.7e6 V/m which is also much smaller than the breakdown value.

from the fig. 4 of the paper, it seems the breakdown threshold increases a lot for shorter pulses from 30 GW/cm² (~10ns pulses) to ~150 GW/cm² (~10ps pulses)
so, one can expect a good behavior in pulsed regime too.

can anyone check the validity of these rough calculations ???

last week, we measured the transmission of a plan mirror from the same batch than the ThomX mirrors and we got ~ 1.75ppm instead of 3ppm which was the calculated value by the LMA.

previously, we put 1/3ppm = 333 333 as multiplication factor.
from today, we will put 1/1.75ppm ~ 570 000 as multiplication factor.

then, previously we got 50kW, now we should get 85.5kW instead in the same conditions.

FYI : to access special menus on the Alphanov software,

login : "Administrator"
pwd : no password

to modify parameters in these special menus :

login : "Alphanov"
pwd : "AE32HF56J"

the present situation is :

Guillaume asked me to disconnect the "Laser head" cable from the pump diode module, installed on the optical table.
on the pump diode module, the connector has several small pins and 3 larges pins : A1, A2, A3.
if the diode of the preamplifier (not the ones of the last stage) is alive, the impedance between A2 and A3 should be around 5 ohms and not HiZ.

I checked several days ago the impedance between all these 3 pins to avoid any error on the schematics.
the impedance is always HiZ.
then, Guillaume thinks the diode is dead.

we are waiting for further tests or procedure.

Guillaume and Julien Bour from Alphanov will come on wednesday 24/07 to have a first look and check the amplifier.

to prepare their intervention, I tested this morning the OneFive oscillator : it's still working at 33MHz.
and we have ~5mW in the LAL software without the EOM.

so, we are in "standard" conditions.

picture of the connectors used to carry signals between the amplifier pump module and the rack under the table.
DB13W3 standard has been used to carry high current on the 3 dedicated pins:

https://en.wikipedia.org/wiki/DB13W3

the connectors are male types on both sides of the cable, and then female on the pump module and rack : see the 2 pictures.

Yesterday Ronic and I worked on the high laser power measurements with the Vertical set-up of the Focus Tunable Lens. We went upto 38W@4 dpt at the begining and then also changed the focal power at 38W.

Observation:

1. We observed nice rings @38W 4dpt, maybe it hits some optics we have to check.

2. We also observed flactuations of power for two adjacent index of the beam profiler when we increase the laser power, which we need to understand.

3. We could see that the vertical deformation is not present for this set-up, we could remove the effect of gravity by this set-up.

On Friday 11th of October, due to the Yvette flooding, all the AC power of ThomX have been shut down.
After the shutdown, I had to restart the Onefive oscillator but the laser modelock was lost.
I had to do a step of 1mm at 10mm/s with the Smaract motor on the rep rate channel (CH1)

After the step, the laser was mode-locking again, and I put it back in the original position at a much lower speed, ~10µm/s, to prevent a new modelock loss.
the power detected by the laser amplifier software is still above 3mW! => OK

Surprisingly, the laser amplifier software is not able to detect the oscillator frequency.
hopefully, the internal photodiode of the oscillator is sent to the CH4 of the R&S scope (192.168.229.21)
and we measured a correct 30ns of period, corresponding to 33MHz.

Because all the safety procedures are not fulfilled, the laser amplifier is not granted to start.
Maybe it is the Reason for the lack of measurement => to be verified later
 

This morning, I verified the reason why the seeder frequency was not displayed on the amplifier software screen:

one needs to start the amplifier (even at 0%) to see the seeder frequency !!!
now, we have 33.371MHz as expected ! => OK

the last thing to do is to tune again the PDH demodulation frequency, as the generator has been shut off during the AC shut down.
 

After the new Yvette flooding of the last week and the consecutive AC-shut down, I restarted this morning the laser with the same procedure (step of 1mm at 10mm/s with the Smaract motor on the rep rate channel (CH1)).

now, we have again 33.371MHz and 3.1mW input power as expected ! => OK

the last thing to do is to tune again the PDH demodulation frequency, as the generator has been shut off during the AC shut down.

For the modulation/demodulation generator for the PDH signal,
after a power shutdown, on can :
1) restart the generator to get the correct parameters:
CH1:3Vrms on 50 ohms @ 8.4MHz (demod)
CH2: 100mVrms on 50 ohms @ 8.4MHz (modulation on EOM)

2) do an "Align phase" on the generator

3) put 270° on CH2 (0° on CH1) to get the max Error signal
and use a "positive" sign on the Laselock : cf https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/365

today, we were able to store more than 93kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio