long-term correlation, over 5-6 days, between the temperature measured in the bunker, outside of the housing (blue curve) and the temperature measured with a probe stuck on the laser housing, inside of the FP-cavity housing (green).
it's a perfect correlation with almost the same temperature scale : 1°C outisde the housing => 1°C of laser housing
thus, a stabilization of the temperature, inside of the housing, could help to reduce the frequency drifts of the laser.
| Ronic Chiche wrote: |
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today, I swapped the 500MHz RF reference oscillator for a Siglent 500MHz DDS oscillator.
see the attached plot : the beating frequency with the 500MHz laser harmonics produces the same behavior as before.
so, the oscillations should come from the laser temperature regulation.
| Ronic Chiche wrote: |
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the last thing we did with Daniele, is to start/stop the airflow on top of the housing (closed) to see a possible pressure effect on the laser frequency drift.
fig 1 : in green, the temperature measured with the probe stuck on the laser housing.
one can clearly see the 2 "start - wait ~10mins - stop" we did at 16h50 then at 17h30.
the air temperature blowed by the airflow is cooler than the housing temperature and we see the effect on the probe.
fig 2 : this is the laser frequency drift during the 1st airflow start/stop
the airflow has been turned on at 100 iterations and stopped at 500 iterations (~5 mins)
we don't see any correlation
fig 3 : this is again the laser frequency drift during the 2nd airflow start/stop
the airflow has been turned on at 1850 iterations and stopped at 2550 iterations (~10 mins)
we don't see any correlation
CONCLUSION : neither external temperature change or pressure variations can explain the 10-20min period oscillations observed on the laser frequency variations.
it can be either the laser temperature regulation or the RF reference oscillator temperature regulation (due to the oven of the quartz)
| Ronic Chiche wrote: |
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Now, I placed a temperature probe stuck on the laser housing itself.
fig. 1
one can compare the temperature measured at the surface of the laser housing and the beating frequency with the 500MHz reference oscillator
one sees a possible very long term correlation but there is no correlation at the minute level when we see the frequency oscillation after t=2000s.
the laser housing temperature seems not to induce directly a frequency variation.
fig 2 / 3
we applied 15W on the heating wire rolled around the FP-cavity flange.
in red, we see the temperature increasing on the probe rolled around the wire, reaching almost 30°C.
we heat the inside of the housing (airflow stopped) during more than 30 minutes
in green, we don't see any variation (even if one makes a zoom) of the temperature of the probe stuck on the laser housing.
in same time, on fig 3, one can see the frequency drift.
there is no correlation between the oscillations and the temperature.
CONCLUSION :
the laser frequency fluctuations does not seem to come from the outside temperature.
| Ronic Chiche wrote: |
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laser cavity :
when one decreases the laser motor position, the laser repetition rate increases (laser cavity length decreases).
=> +/- laser motor step => +/- laser cavity roundtrip length => -/+ laser repetition rate => -/+ laser harmonic @500MHz
=> +/- 100nm => +/- 200nm => -/+ 0.7Hz @33.33MHz => -/+ 10Hz @500MHz
here is the natural variation of the laser cavity frequency beating with RF @500MHz over 1h (~1.6s / iteration)
one can see some oscillations equivalent to ~1µm of roundtrip length with ~10 minutes period and maybe a slower drift or oscillation with ~2µm of roundtrip range over the hour.
I mention that I moved the laser "PZT" motor before taking the data : could it be the reason of the 10-20min oscillations ?
during the same time, here is a probe temperature curve (the probe in stuck on the end flange, close to X-hutch, of the FP-cavity, inside the housing... not close to the laser position).
the temperature variation range is ~2.5/100 °C which induces on inox (relative length thermal effect : 17e-6 /K) a length variation of 4µm of roundtrip (10m) which could be compatible to the laser cavity length variation measured.
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