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all the injection power in the chart have not been measured recently but during the Loic thesis period.
and these old measurements stopped at 5.5A of pump current.... so, the data at "8A" is a pure estimation.
about the last measurement :
it was made at 6A/8A/8A/8A for the 4 pump diodes of the amplifier (because 1st stage has a Peltier issue and we cannot check its temperature), so the average current is 7.5A instead of 8A.
and the linear scale between pump current and amplifier power is ~ 12W/A, then the estimated amplifier power for the last measurement is 76W instead of 87W
and the estimated gain is more 2658.
for this current, the amplifier works out of its nominal limits (temperature set at 25°C but measured at 30°C !!!) and the fans of the crate are making noise like hell.
so the last gain estimation should be treated very cautiously.
about the transmission and reflection signals behavior, one can write :
R + T + L = 1 => energy conservation for the cavity.
dR + dT + dL = 0 => dL = - (dR + dT)
if dX = Xfinal - Xinitial, dR and dT are < 0 on the last picture, then dL > 0.
it means that this picture seems to show that some losses are increasing from the beginning of the locking process.
several possibilities :
- we saw a strange D-shape effect on the large port of the cavity.
it seems that one of the D-shape mount/mirror is touching the intra-cavity beam producing some ghost effect on this large cavity port.
some cavity axis changing during the beginning of the lock could introduce some additionnal losses.
it can be easily tested by puting the D-shapes far from the beam.
- because of cavity axis changing at the beginning of the lock, the mirror losses are different.
but it is surprising that it is still going in the same direction... more losses at the end.
could be tested by slightly changing the optical axis of the cavity.
- "prior damage" behavior with a bump in the middle of the mirror due to thermal effect which introduces some losses at the end.
=> if it's the case, it's not a good behavior !!! :-(((
can be tested by looking at the wavefront phase in transmission.
- Non linear effect is the coatings.
but the field density seems not so much to produce this kind of effect
- A thermally induced change in the refractive index of the mirrors.
Daniele mentionned a relation between real and imaginary (related to absorption) parts of this refractive index which could explain that a reflectivity change could induce an absorption change.
| Xinyi Lu wrote: |
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These days, Ronic and I achieved 200kW inside the cavity and 70% coupling efficiency.
- By optimizing the telescope, the coupling reached 70% with iris fully open and maintained 60%-70% coupling at high power.
- The cavity mode went from 2.2mm,2.5mm (38kW) to finally 2.3mm,2.8mm (200kW) without changing a lot.
- Gradually raising the power while optimizing alignment, CEP, and locking, we got the following stable power:
| Amp current (A) |
Injection power (W) |
Circulating power (kW) |
Gain |
| 2 |
10 |
38 |
3800 |
| 2.3 |
14 |
50 |
3571 |
| 3 |
22 |
70 |
3181 |
| 4 |
35 |
115 |
3285 |
| 5 |
48 |
158 |
3292 |
| 8 |
87(Estimated) |
202 |
2322 |
- Next steps:
- Explain the strange drop phenomenon that occurs at high power, where both transmission and reflection drop, as in Fig. 2.
- Maintains a half-hour locking at 200kW. Now the temperature of the amplifier at 8A is over 40 degrees, which may be risky.
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