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Entry   Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics 
    Reply   Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics 20230907_182412.jpg20230907_182442.jpg
       Reply   Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics 
          Reply   Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics 
Message ID: 240     Entry time: Thu Sep 7 19:23:36 2023     In reply to: 237     Reply to this: 260
 Author: Ronic Chiche 
 Status: Fixed 
 Type: info 
 Category: lasers and optics | detectors and electronics 
 Location: ThomX igloo 
 Title: Fast feedback loop between laser and FP cavity 

today, after the several unsuccessful attempt yesterday to get an improvement in the lock,
we decided to "rebuild" the error signal electronics block by block and step by step :
the scheme is basically :
- DET36 photodiode
- followed by a 10MHz low pass filter to remove the laser frequency harmonics and keep only the modulation frequency at 8.4MHz.
- connected to a FEMTO HPVA AC-coupled 40dB gain amplifier with 50ohms in parallel on its input (which is connected to the photodiode).
- connected to a Minicircuit mixer which is also demodulated by the generator at 8.4MHz
- followed by a 1.9MHz low pass filter to remove image frequencies

when this signal is sent to the Laselock box, the lock of the cavity is possible but very noisy.
we need to put a large D values in the PID to maintain the lock at the price of oscillations and gain loss ! :-(

when this signal is connected also to the M250 video EOM amplifier (which is 50 ohms), but this amplifier is not used,
we suddenly got a much better lock (see the attached pictures), certainly due to the 50 ohms connected to the input of the Laselock system => to be verified.
one could have some noise current at the Laselock input which produces less noise at the output when the input impedance is 50 ohms, instead of the several kohms when the input is unloaded...

then, we were able to get a stable lock at ~40kW with 30% coupling and 30% of amplifier :
see this post with the same values : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/227
but now, the lock has been done WITH ALL the motors controllers ON !!! :-)))
now, we can try to improve the lock with the fast feedback loop.

and the only real problem is the mode degeneracy we need to block with the L-shape.

Ronic Chiche wrote:

today, we tried to lock the FP cavity with the Smaract motors ON (with option -LV).
we know the Smaract controllers produce some noise and the lock is very bad or impossible when the controller in ON (whatever the displacement mode is : closed or open loop, or piezo scan).
then we need to do a fast feedback loop on the EOM inside of the Onefive laser.
the problem is we cannot fill directly the error signal (~ 300mVpp) as the signal level is too low to produce some effect.

1- we tried to use the AC-coupled homemade amplifier alone but the output range is too low (+/- 3V)

2- we tried to use the M250 video amplifier for EOM with AC-coupling but the output range is still to low (+/-30V ? => to be confirmed)
but we saw an improvement in the locking.

3- we tried to combine AC-coupled homemade amplifier + M250 video amplifier for EOM with AC-coupling.
we are able to lock (~ 30% coupling) but the lock quality is very poor : we clearly see that we oscillate around the maximum of the Airy peak.
we tried several combination of the global gain (fast feedback + Laselock) using the diffuser, of the Laselock PID parameters but it seems we are not able to lock properly.

we measured a global delay of this double amplification stage of 80-90ns for the homemade amplifier and 150ns for the homemade+M250 amplifiers.
this delay is compatible with ~ 500kHz BW for the feedback => it seems it is not the reason...

we measured also the linearity of the homemade amp => there is ~ 30dB between a frequency and its 1st harmonic even at low signal...
the amp scheme is not very linear.
in comparison, the HV M250 amplifier exhibit > 80 dB of linearity !
we will try to remplace the homemade amp by a commercial FEMTO amplifier to imrpove the linearity and see if it improves also the lock.

we measured also a 100Hz AM modulation on the output signal of the homemade amp+M250.
=> we can try to work in differential (HV+ - HV-) to see if it helps to remove this modulation.

 

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