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?
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| Manar Amer wrote: |
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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.
| Manar Amer wrote: |
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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
| Manar Amer wrote: |
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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.
| Manar Amer wrote: |
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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
- 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.
| Manar Amer wrote: |
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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
| Manar Amer wrote: |
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***** 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
| Manar Amer wrote: |
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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.
| Manar Amer wrote: |
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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.
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