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...

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.

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 .

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?

.

Ending alignment series !!

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.

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
 

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.

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.

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

***** 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

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.

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.

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.

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.

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 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....
 

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.

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. 

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).