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.

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

This morning, with Ronic and Daniele, we locked the cavity again (The lock was relatively easy and stable during the reading of the finesse)

We took 2 reading of finesse

1st   : @ 33.2808 MHz (what the cavity length was at)           :  Finesse = 27 024

2nd :  @ 33.3382 MHz (adjusted to match the pulsed laser)  :  Finesse = 30 433

The difference between the 2 is the FSR, during the change we also did some alignment to counter the reduced transmitted power.

We took an image of the beam profile before and after changing FSR and there was ~ 70 um change in the beam position on the Horizontal axis of the beam.

Mail de Bruno Mercier du 19/11/2018 :

Une première cavité a été étuvée, la pression est actuellement de 
3.10-10 mbar, l' étuvage de la deuxième cavité vient de commencer et se 
finira en début de la semaine prochaine.

Bonne journée,
Bruno

on the August 30rd and 31th, a global power shutdown was forseen for ThomX.

to prevent damage on equipments, I switched off all of them (and disconnected power cables from the wall plugs).

yesterday, after the week-end, all the equipments have been restarted and the cavity as been locked properly.

the power coupled to the input fiber of the amplifier has decreased a little bit from 3.8mW to 3mW during summer.
=> we need perharps to do some alignment on the Schaftner-Krischoff mount.

Vacuum tests finished on 19 december 2018.

Spent last two days to discuss the importance of removing aluminium and baking wires before to insert mirrors for cleanliness issues. Finally we took pictures to support our demand.

Cavity should be ready to clean for the new term 2019

Le 19/12/2018 à 14:56, Bruno Mercier a écrit :

Bonjour Daniele, Loïc,

Nous sommes à votre disposition pour effectuer une entrée d'azote sur
la cavité.

Bruno

08/02/19 - End of the cleaning process.

we installed a measurement at the IP with the BPM for ion cleaning for electrons and we used a fast photodiode at the output of the FP-cavity and measured with a fast oscilloscope the phase jitter between the 2 signals.
in red, the BPM signal
in blue, the photodiode signal.

on the right of the picture, this is the trend of the phase difference measurement.
in yellow, this is the histogram of this phase difference.

one measured 35ps of FWHM which is equivalent to 15ps rms for a Gaussian distribution.

the phase between the 500Mhz ring RF oscillator and the electrons is measured on a BPM and is very stable after 2-3ms (<< 6ps, typically) !

in the same time, we can use the residual phase beating after our 500MHz mixer to estimate the phase noise coming from the laser part.
the rms noise of this signal is around 60mV for 1.25V peak (2.5 pk-pk) signal => ~ 50mrad rms

dt = dphi / 2pi * 2ns ~ 15 ps which is the noise measured between electrons and photons at the IP !!!

we clearly see that this rms noise level is correlated with the 20Hz oscillation observed on the FPC feedback signals.

BUT even when this rms phase noise varies, we DO NOT see any effect on the X-ray production at this time,
which means this is not yet a limiting factor.
this observation is confirmed by the X-ray production flux calculation for which the time jitter is "added" to the electron beam size parameter which seems to be dominant.