Attached is the current setup in ThomX

Updated beam image after change on the mode axis.

Taken after P4.

this morning with Manar, we did a cavity mode axis change.

we checked the cavity mode centroid position on the beam profiler (placed behind P4) and changed it by roughly 1mm on the X-axis and by 500µm on the Y-axis.

for doing that, we played only on the S2 mirror while the cavity is locked and we slightly changed X and Y motors, step by step, and we realigned the laser beam with external injection mirrors when needed.

in the end, we were able to improve the transmission power by roughly 25% (810mV to 1020mV on the Transmission photodiode on the scope).

we did several Finesse measurements after that: 4950, 5190, 5100, 5120, 5200 => ~ 5100

compare to the previous Finesse value, around 4200, this is also roughly 25% better, in agreement with the transmission power increase.

conclusion: it seems the Finesse, thus the losses are almost the same "everywhere" on the mirrors => we need to clean them or replace them.

This morning, me and Ronic managed to obtain the FSR and image of the finesse which is to be analyzed.

Adding:

• Beam Image at P4 propagation, no lens is added.
• Image of the Finesse on Oscilloscope

more information to be included later

Will continue in the afternoon .

to measure the Finesse, instead of having 2x EOM for the PDH and for the modulation sweep, I simply used 2x generators coupled with DC-blocks to a T connector (SMA) screwed directly on the EOM input.

as the transmission signal is fluctuating, it is not easy to have a good fit of the Airy peak.

if I measure the width at half of the maximum of the peak, I found roughly 10kHz instead of the awaited 2kHz... :-(

one needs a better evaluation with a more stable transmission signal and also to be sure that the L-shape metal piece (used to remove high order modes) does not introduce some losses and then reduce the Finesse...

After changing the way of injecting modulation for PDH and modulation for FSR scanning (we split the modulations to 2 different EOMs), the locking is very stable and we can measure the Finesse.

FSR = 33.34MHz

hereafter, the transmission power during an FSR scan and its fit (sweep of 300kHz of FSR in 2s)

we took 5 acquisitions which give a Finesse of :

4236
4254
4400
4045
4177

=> roughly Finesse = 4200 ! far from the 17000 previously obtained........

one thing which could reduce the Finesse is the L-shape metal piece if it is slightly inserted in the FP-cavity mode path.
understanding where this L-shape effectively is is not easy... some pictures are attached.

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.

***** Continuation of the alignment ---- FP Cavity Open --- ****

The cavity was put under outer pressure and was opened for the alignment

The alignment was done using CW koheras infrared laser and  the inside mirrors irises

we observed the beam output centered at S2, S3 and P4

transmission from S4 to P1 was aligned at the center of P1 iris and an outside reference was fixed, then P1 mirror was placed, and we aligned the reflection with the transmission.

....... After the interior alignment, the cavity windows were closed ....

A beam profiler was placed at P4 transmission -----> nothing observed even with a piezo drive on the CW infrared  laser

observed a beam output at S2 and S3, but the fundamental mode is not seen, or even a higher order mode (which we can't explain, as the beam is centered on the mirrors)

images show the output at S2 and S3

Yesterday, we did the alignment again using Iris and beam profilers to obtain a more precise result.
at the end of the alignment procedure, we successfully obtained the beating modes.

the geometrical alignment seems good (weak odd modes) but we could need to put a telescope on the Koheras line as the beam size seems quite different from the mode size (quite strong even modes).

without any improvement of the alignment using photodiodes, one obtains about 15-20% of coupling.

we put several Iris on the table:
- 2 Iris before the 2 alignment mirrors to fix the axis of the laser on these mirrors
- 1 Iris just before the cavity (we will add an additional one today) to fix the cavity axis
- 1 Iris in reflection of the cavity to fix the M1 orientation

Installed and inputs:

• CW laser ; power = 101 mW
• PDH ;
  • at output
    • Low pass filter 50 ohm  DC-1.9 MHz
  • at input
    • freq = 8.4 MHz , Ampl = 1.2 V  , phase = 160 /
    • voltage =   ~ 6 V
• EOM ; freq = 8.4 MHz , Ampl = 100 mV , phase = 90
• Photodiodes
  • reflection
  • transmission
• beam profiler

Observed during this morning:

• coupling :   below 10% .... approximately ~ 8%
• Oscilloscope : 
  • yellow : transmission
  • purple : reflection
  • Blue : error signal
  • green : scan signal

Note : We observe a lot of higher order modes, and they are not occurring regularly.   

A schematic of the current setup is attached.

An image from the oscilloscope show a low coupling but clean error signal.

This afternoon, I did some alignment of the injection mirrors with a fast scan on the LaseLock to get a regular transmission photodiode signal.
the coupling increased to 10-15%

I got a first lock of the cavity only with the PZT.
there is some ringing on the error signal and the locking is quite noisy, which means the cavity LW seems to be more narrow than the Koheras LW, which is a good sign.
tomorrow, I will add an AOM to improve the locking.

I did some alignment after locking.
it was difficult because of the outside noise (engines producing loud and low-frequency noises).
the coupling is now about 20% (position saved in the injection motors software).
I added a half waveplate which has to be optimized.

This morning with Manar, we installed the AOM+RF amplifier and the associated fast feedback loop.
Now the locking with the Koheras is good with a coupling of 20%.
Tomorrow is dedicated to the measurement of the Finesse.... we will have to add the 2nd EOM.

We observed a very stable lock if the airflow is OFF.
when it is ON, the lock is much less stable... maybe a problem of optimization of the feedback... we will see that tomorrow if we have time enough.

We also observed a quite important sensititvity of the PZT voltage when slightly pushing on the housing with the finger: we clearly see the compensation on the PZT voltage.
I didn't calibrate this voltage but it seems to be an important fraction of 1µm... I would say around 100nm
 

Adding Oscilloscope images

• delocked + locked intervals showing the 20% coupling
• lock when the housing panels are closed + airflow on .
• lock when the housing panels are closed + air flow at lowest setting

an additional factor to the PZT voltage sensitivity is the housing panels, we see a decrease in the voltage when closing them.

Ending alignment series !!

As it can be seen on the first plot, even with a good locking (good reduction of "high" frequencies noise: we had better locking than on the picture) we still have very low frequency (~ 1Hz) fluctuations

these fluctuations prevent having a good measurement of the Finesse and they need to be understood.

they can come from fluctuations due to :

- input power
- input or feedback polarization
- phase noise
- alignment
- mode matching

1) input power:
we looked at the direct reflected power from the cavity without locking as an image of the input power.
=> we don't see these fluctuations

2) input polarization:
as there are many unconstrained fibers after the NKT (EOM/AOM) it could produce some polarization fluctuations.
we put a PBS and half and quarter waveplates in front of the reflected photodiode when the cavity is not locked to detect a change in the input polarization
=> we don't see these fluctuations

3) feedback polarization:
the beam on the PDH box is coming from a wedge which can change the relative gains between different polarizations.
we put half and quarter waveplates in the injection path to adapt the input polarization with the cavity mode polarization axis
and we put half and quarter waveplates and a PBS in front of the PDH box to select the right polarization for the feedback.
=> it didn't change the power fluctuations effect.

4) phase noise
we adjusted the feedback parameters (PID gains, AOM gain, locking offset, digital and analog low pass filters) to have a clean signal without high frequencies noise.
the transmission and coupling signals exhibit quite narrow lines at the millisecond level but we see 10-15% transmission change at the second level.
as the PID has a higher gain at low frequencies, one should not see more fluctuations at these frequencies.
or if it comes from external noise, one should see a correction signal on the PZT which is the image of these fluctuations => we don't see that.
we also stopped the cavity motors controllers without any effect on the transmission stability.

5) alignment
the alignment cannot change except if some vibrations are present, which should be seen also on the PZT correction signal => we don't see that.

6) mode matching
as the coupling is only 20% and the alignment has been already optimized, the mode matching is quite bad for sure.
could it be the source of the problem?
from experience, we know that a bad mode matching implies a bad locking but the reason is not clear.
=> to be discussed with Viktor: can we improve the mode matching with a simple lens?

.

The fluctuation problem has been solved.
It was simply the new scheme to inject 2 RF frequencies in a single EOM.
It maybe produces some standing waves in the EOM RF input and creates some phase noise.

we went back to the standard solution with 2x EOM and the problem vanished.
Now we have a very good lock and we can measure the Finesse.

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.

we did again the complete alignment procedure starting with iris and optimizing injection motors Ma and Mb, then installing mirror S2, S3, P4 and P1, optimizing thetaX and thetaY axis for each motor.

everything was fine until we installed P1. we tried to optimize thetaX and thetaY of P1 and we clearly observed a strange motion when doing that :
   - for thetaX axis, steps in one direction seem to have a different length from steps in reverse direction.
   - for thetaY axis, trying to move in one direction, makes sometime a motion in the reverse direction.
at this moment, it is difficult to say if the problem comes from the controller, the mirror mount or the motor itself.
if both axis are concerned on the same mirror (P1), maybe the problem comes from the mount... to be continued on Monday.

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.

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

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. 

FSR initiale : 33.34 MHz (S3 = -825000 et S4 = -825000)

Beam size initiale : wx=2.05mm et wy=2.15mm

Finesse initiale : 3400

FSR après éloignement des miroirs sphériques : 33.29 MHz (S3 ~= -1496000 et S4 ~= -1496000)

Beam size : wx=1.7mm et wy=1.85mm

Finesse : 3600

FSR après rapprochement des miroirs sphériques : 33.39 MHz (S3 ~= -250000 et S4 ~= -500000)

Beam size : wx = 2.1mm et wy=2.4mm

Fit de la caméra jamais au dessus de 90%. ~~80%.

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.

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

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.

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 !!!

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

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).