Yesterday, Ronic, Xing, Qili and I achieved a more stable 520kW power at 7.5A (71W injection) by optimizing the alignment and locking parameters. (Figure 1)
- The cavity can be stable locked when airflow is on. At 7.5A, the pump temperature is about 28℃. The chiller temperature didn't change, to the same 23 ℃ setting. We can try 8A later (75W injection) for a short time;
- Figure 2 demonstrates the cavity mode variation, wy/Pc ~ 1.7 mm/MW, half that of the OL paper (3.3 mm/MW). The thermal deformation of our device is much smaller.
- The experimental data are shown in Figure 3. Figure 4 shows the injection power vs circulating power.
- There are some tests that can be done at the moment. I'll update on the elog after discussing the necessity today. ^_^
| Xinyi Lu wrote: |
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Last week, we achieved a stable intracavity average power of 500kW, limited by amplifier power. The experimental data are shown in Figure 1.
- We measured the transmitted laser with a power meter in the windows behind M2 and M4 respectively, and the results were consistent, so the measurements were credible.
- There is only one transmitted laser spot behind both M2 and M4.
- We measured 10-minute locking data at different powers (Figure 2). 480 kW data was not optimized, and we will add 500 kW locking data later.
- We compared cavity modes at different powers (Figure 3). There are fluctuations because we only saved one data at one power. More data will be collected for averaging later.
- After finishing the high-power experiments, we will measure the finesse and the transmission of the mirrors used. As well as the pulse duration, spectrum, phase noise, and repetition rate of the laser.
| Xinyi Lu wrote: |
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Today, Ronic, Daniele, Aurélien and I measured the amplifier power and mirror transmission.
| Current (A) |
0 (2rd stage) |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
7.5 |
8 |
| Power (W) |
1 |
1.8 |
11.8 |
23.5 |
35.5 |
47 |
57.5 |
66.9 |
70.7 |
74.9 |
For transmission measurements, we used the same new mirrors as Sbox and ThomX, and installed an iris and a 2-inch mount to block the scattering laser.
The angle of incidence during the measurement was about 0.5°. We changed the angle and the measurements remained the same.
| Mirror Number |
PL-0898 |
PL-10978 |
| Nominal Value |
3 ppm |
115 ppm |
| Measured Value |
1.75 ppm |
113 ppm |
If the mirror being used also has a transmission of 1.75 ppm, the original 270kW is actually 463kW!!! The gain is 6549 and the finesse is 28585 (70% coupling).
We will do more tests to check it.
- Redo the experiment and check the spot behind the window at high power.
- Move the power meter to the plane mirror M2 window. It was previously behind the curved mirror M4 window.
- Compare locking curves, cavity mode sizes, and coupling efficiency at different powers.
- After finishing the high-power experiments, we will measure the finesse using CW laser and the transmission of the mirrors used.
| Xinyi Lu wrote: |
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Today, Ronic and I achieved 272kW inside the cavity at 7.5A. The coupling maintained 60%-70%.
| Amp current (A) |
Injection power (W) |
Circulating power (kW) |
Gain |
| 2 |
10 |
50 |
5000 |
| 3 |
22 |
105 |
4773 |
| 4 |
34 |
156 |
4588 |
| 5 |
47 |
210 |
4468 |
| 6 |
58(Estimated) |
250 |
4310 |
| 7.5 |
76(Estimated) |
272 |
3579 |
- Compared to yesterday's experiment, we moved the position of the D-shaped mirrors farther in two directions to make the higher-order modes just disappear.
- Possible reasons for higher gain: D-shaped mirrors position, high power and pump vacuum cleaned cavity mirrors so that improve the finesse.
- We didn't see the strange drops like yesterday (Figure 1). However, in the window behind the M3, we can see 3 spots correlating with the intracavity power, even though moving the D-shaped very far does not make them disappear, only weakens them. We don't know where they came from. When this round of experiments is over, we can open the cavity and observe the optical paths.
- Next steps:
- Repeat the experiment to ensure that the gain does not drop.
- Long-term measurement at maximum power when the amplifier temperature is safe.
- Measure the transmittance of the cavity mirrors and the amplifier power.
- Open the cavity and observe the optical paths and the mirror surface.
| Xinyi Lu wrote: |
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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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