|
this morning, I tested the laser+amplifier @ 30% lock on the FP cavity with and without Smaract motors.
I recorded the PDH error signal during a lock:
- blue : with Smaract motors controller powered ON but motors are stopped
- yellow: with Smaract motors controller powered OFF
with Smaract motors controller powered ON and motors stopped, one can see a group of resonances around 10kHz (8 - 11 - 14kHz) which disappears when the controller is powered OFF.
one can see also a group of resonances around 25-30kHz for which some peaks desappear when the controller is OFF but most of them are still there... could it come from noise on the Onefive laser PZT ?
one can see also a noise reduction at low frequency with a corner frequency around 17kHz, which could be the Unity Gain Bandwidth of the feedback loop on the laser PZT (fast feedback loop on EOM was disconnected)
=> to be confirmed
*************************************************************************************************************************************************************
I was able to lock with a decent noise on transmission and reflection signals @ Pin=17W (30%) of input power and with a coupling ~ 20%.
I measured 31mW in transmission => Pcav ~ 10.3kW (T ~ 3ppm)
as T1=115 ppm and F=30000, the cavity gain is T1*(F/pi)^2 = 10.5k,
so, the FP cavity power should have been 17W * 20% * 10,5k = 35.7 kW !!! (maybe the formula is wrong if the coupling loss comes from the CEP detuning effect)
=> we have to check the incoming power and the formula !
so, maximum expected power in FP-cavity could be 70W * 100% * 10,5k * (10.3/35.7) = 210 kW !!! :-(
*************************************************************************************************************************************************************
I was able to redo the lock easily in remote in the control room (with the Smaract motors controller OFF).
| Ronic Chiche wrote: |
|
After removing the 2 generators from the optical table, the lock is much more stable and now, it is possible to lock on the main resonance with a poor CEP but with quite good stability.
the coupling is still very low ~ 5% for that CEP but if one improves it (CEP ->0), using the laser double-wedge motor, one clearly sees an improvement of the coupling... but at the cost of the lock stability.
the reason of the poor coupling is also because the laser amplifier is used at 0%, for which we know the part of the laser signal power, compared to the total power, is low.
(a part of the beam @1030nm is not propagating in the fiber core of the amp, and then, it cannot be coupled to the FP-cavity).
the fast lock loop on the EOM has been disabled for the moment.
it has to be installed back to improve the stability at a better CEP.
at present, the FP-cavity power is estimated at ~ 90W (~270µW in transmission of ~3ppm mirrors) for ~300mW of total power coming from the laser amp.
next steps :
- in Open Loop : check what is the best coupling we can observe for CEP=0 @ P ~ 10W (laser amp at ~ 25%)
- in Closed Loop : @ P ~ 10W => measure the best transmitted power after alignement/polarization/feedback adjust => ~ 3-10kW in the cavity ?
| Ronic Chiche wrote: |
|
today with Daniele, we locked easily (but with a noisy lock) on the secundary resonance and we tried to lock on the main resonance (with very low coupling ~10% which mean a CEP ~Pi)
the lock was possible but was very noisy.
I installed a fast loop using my small DC amplifier based on OP37 (max gain=100) modified to be AC coupled to avoid to amplify the PDH box offset.
the output votage swing of the OP37 is only ~10V. Thus, the effect of this fast loop on the lock stability is not visible !
Thus, I added the M250 Leysop HV amplifier (see attached documentation), which is able to drive an EOM with >5MHz bandwidth and ~250V swing, after my OP37 amplifier.
with this additionnal HV amplifier, now we can clearly see the effect of the EOM loop which improves the lock stability BUT, even with a poor CEP, the lock is very unstable on the main resonance.
it seems the optical phase noise is still too large and/or its BW too high to be completely compensated.
The next step is to try to remove all the possible noise sources from the optical table:
- the laptop placed on the ionic pump
- the 2 Rigol generators on the table surface
and switch off the controller of the Smaract laser cavity motors.
If it doesn't help, we can send the error signal to a spectrum analyzer to have a better view of the different harmonics involved in the residual phase noise.
could it remain some noise above the present PDH box BW (1.9MHz LP filter) ?
lastely, we can also make an optical phase noise measurement to check if the Alphanov amplifier does not add some noise.
| Ronic Chiche wrote: |
|
finding the right modulation/demodulation PDH phase is very difficult on the main resonance because the we get non stationnary signals with a lot of oscillations.
changing the phase, in this condition, does not really change the error signal.
Then, we moved on the first secundary resonance with less gain and less coupling.
Thus, the error signal is more similar to the theoretical PDH signal => one can adjust the modulation/demodulation PDH phase to get the maximum error signal.
then, we locked pretty easily on this first secondary resonance, with a coupling around some % when we adjust the CEP motor.
we tried to lock on the main resonance but it is too noisy and unstable.
it seems we really need high BW feedback.
I tried to add a fast analog loop on the laser intra-cavity EOM but without a clear effect.
the problem is the gain of this loop : it is difficult to produce a "high voltage" (above 10Vpp) on this EOM.
I put "my" amplifier but the voltage output is limited... commercial amplifiers will have the same issue.
we can add HV amplifiers but it takes place and it will add some noise on the signal.
A loop with an AOM could be easier to install and manage... but at the price of a loss of power before the laser amplifier...
| Ronic Chiche wrote: |
|
the last tries to lock the 33MHz + amplifier to the 30k Finesse FP-cavity were unsuccessful.
during a laser Frep scan using the Laselock, one observes that the main cavity resonance is not able to stay inside the PZT scan range from one scan to another (500ms-1s period)
is it the effect of a large and slow phase noise ?
some informations:
- The 33MHz laser came back at lab from repair on March 2018.
- it has been sent to Alphanov in May 2020.
- it failled and has been sent to NKT/OneFive for repair in September 2021
- it came back to lab from repair in June 2022.
- on post #92 (Feb. 2020), it seems that we already locked the 33MHz laser + CELIA amplifier to the ThomX FP-cavity.
- The PZT sensitivity for the 33MHz laser is given to 0,3Hz/V for Frep <=> 2.6MHz/V for optical frequency.
=> 10V on PZT is equivalent to 26MHz of optical frequency shift which is less than FSR !
- by comparison, the PZT sensitivity for the 133MHz laser is given to 3.9Hz/V for Frep <=> 8.5MHz/V for optical frequency.
- by comparison, the PZT sensitivity for the NKT CW laser is given 10pm/100V for Wavelength <=> 30MHz/V for optical frequency
- by comparison, the PZT sensitivity for the ThomX FP cavity (Z20H38x40C) is 4nm/V for length expansion => 8nm/V for round-trip expansion <=> 0.03Hz/V for FSR expansion <=> 260kHz/V for optical frequency !!!
the PZT expansion estimation is in attached file.
|
|
|
|
|