FP-cavity FSR shift, posted by Ronic Chiche at ThomX igloo about lasers and optics 
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today, to match the ring frequency at 500.25MHz (15th laser harmonic), we did a frequency shift on the laser using the Smaract motors (not too fast,
~2µm/s, to prevent laser modelock loss)
and we "followed" this shift on the FP-cavity using the FP-cavity motors. |
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Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics
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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). |
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Smaract controller remplacement, posted by Ronic Chiche at ThomX igloo about lasers and optics
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previously, we were using a spare Smaract MCS controller to drive the Onefive linear stages (without -LV -low vibration- option and
using Ethernet),
during the repair of the initial MCS controller with -LV option. |
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Onefive laser modelock and Smaract CEP controller, posted by Ronic Chiche at ThomX igloo about lasers and optics
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today, during an attempt in improving the CEP with the Smaract controller CH2, we lost the laser modelock.
at this point, we decided to reference the Smaract CH2 for CEP :
now, 0 mm is roughly the middle of the motor range. |
Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics
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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 : |
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Removing high order modes, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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this morning we locked the laser and the FP-cavity with ~ 40kW => some HOM appeared "randomly" depending on the power.
we tried to play manually on the L-shape arm :
- first, we moved horizontally to put the L-shape in the beam axis |
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Removing high order modes, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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here is the picture of the L-shape arm and the inside of the vessel.
this image corresponds to the "zero" position on each positionner.
Ronic |
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lock oscillateur 33MHz - synthé 500MHz, posted by Ronic Chiche at Optical room about detectors and electronics
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this afternoon, I installed the frequency detection scheme in the bunker :
- I changed the large reflected photodiode DET100 by a fast small DET10.
- a 50ohms splitter is connected to the photodiode : |
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33MHz oscillator + Alphanov amplifier lock, posted by Ronic Chiche at ThomX igloo about lasers and optics
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redoing the PDH error signal scheme with discrete components is more flexible and it is easier to check the signal/noise ratio.
now, we are able to get a quite clean lock only with the PZT correction (w/o fast feedback correction using the EOM) even with both motors controllers
ON (cavity and laser). |
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Removing high order modes, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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yesterday with Daniele, we tried again to move the arm horizontally and vertically from the "zero position" to have a better understanding
of the beahvior,
but it is roughly the same : |
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lock oscillateur 33MHz - synthé 500MHz, posted by Ronic Chiche at Optical room about detectors and electronics
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this afternoon, I tried to lock the FP-cavity on a local 500MHz synthesizer as a frequency reference
(it's faster to change the frequency from the sythesizer than from the FP cavity !!!)
I found some parameters on the PID of the Laselock to phase-lock the reference and the FP-cavity |
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lock oscillateur 33MHz - synthé 500MHz, posted by Ronic Chiche at Optical room about detectors and electronics
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this morning, I added a Thorlabs lowpass filter at 1kHz after the mixer and a resistor at the Laselock output to the FP-cavity PZT.
I set the PID with P=10 and I=D~0
in attachement, a plot of the 500MHz mixer output, before and during the lock. |
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work with HV voltage on the Laselock, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics
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This afternoon, I connected the HV output of the Laselock (instead of the standard 0-10V output) to the FP-cavity PZT to increase the locking range.
I didn't see any degradation of the FP-cavity/synthesizer lock.
Ronic |
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lock oscillateur 33MHz - synthé 500MHz, posted by Ronic Chiche at Optical room about detectors and electronics
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this afternoon, I firstly connected the HV output of the Laselock to the FP-cavity PZT : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/248
locking the FP-cavity on the local synthesizer works better with the improved HV output range.
the ~20Hz oscillation in the beating signal of the FP-cavity/synthesizer is still there. |
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lock oscillateur 33MHz - synthé 500MHz, posted by Ronic Chiche at Optical room about detectors and electronics
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this afternoon, we were able to lock the laser on the FP-cavity and the FP-cavity on the Ring reference oscillator in a same time from the control room.
the procedure when laser, cavity and reference oscillators are far in frequency, is :
1) start the amplifier to get the laser signal in the reference photodiode used to measure the 33MHz. |
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Fast feedback loop between laser and FP cavity, posted by Ronic Chiche at ThomX igloo about lasers and optics | detectors and electronics
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this afternoon, we tried to better understand how to drive properly the EOM to kill high frequency noise.
we locked to laser on the FP-cavity as usual.
then we injected a 0-10V square signal on the laser EOM @ 1KHz (with fast rise and fall times ~ 10ns) |
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!!! strange FP cavity behavior => impossible to lock !!!, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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suddenly, BEFORE switching the HV amplifier ON and BEFORE connecting it to the EOM (in order to explore the HV effect on the EOM for the fast feedback
loop),
we lost the lock between the laser and the FP-cavity: in attchement a plot a the lock with the "best" PID parameters. |
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drift calculation for the SMA100A, posted by Ronic Chiche at ThomX igloo about lasers and optics
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the goal is to estimate what could be the frequency drift at 500MHz for the SMA100A: see phase noise datasheet in attachement
Sphi(f) = FFT ( Rphi(T) ) = FFT ( < Phi(t) Phi(t+T) > )
at low frequency, Sphi(f) ~ A / (f^n) = A*(2pi)^n / (i2pi*f)^n => Rphi(T) = A*(2pi)^n*T^(n-1) / (n-1)! |
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!!! strange FP cavity behavior => impossible to lock !!!, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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this morning, we tried:
- to move a bit the arm of the L-shape off the beam axis (in case of it could have touch something and induce vibrations) => no effect
- to change the CEP to get an equivalent lower Finesse => weak improvement |
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!!! strange FP cavity behavior => impossible to lock !!!, posted by Ronic Chiche at ThomX igloo about mechanics | lasers and optics | detectors and electronics
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it could be also an optical unstability, as when the intra-cavity power increases, the radius of curvatures of the mirrors increases too due to thermal
effect, and then one could go in the instability region.
but if it was the case, by reducing the power in the FP-cavity, we would also reduce the thermal effect and then, we would come back in the stability |