Today, Ronic and I lock the cavity on the other polarization and achieved 50kW with 22W injection.
- After we optimize and lock the cavity in vertical polarization, we rotate the waveplate at the transmission to minimize the signal, then rotate the waveplate at the injection laser to maximize the signal. We lock the cavity in this horizontal polarization and optimize CEP and alignment. The results are: when injected at 10W, the circulating power in vertical polarization is 21.4kW and in horizontal polarization is 23.3kW. The coupling are both ~30%.
- The cavity reflection signal obtained in horizontal polarization is weaker than that in vertical polarization (1/10), so we usually lock on vertical polarization first.
- We then increase the power to see the change in coupling. From current 2A to 3A, coupling change from 30% to 40%. Finally, we obtained a circulating power of 50kW with 22W injection (3A current). In the initial stage of locking, high-order modes appear, but in stable locking, there is only fundamental mode and no mode degeneration. Although there are many fluctuations in transmission and reflection.
- Tomorrow, we will optimize the coupling and add removable stages under the telescope.
| Xinyi Lu wrote: |
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just to add some details :
about the S and P polarization frequency shift:
the PZT scan is 10Vpp at 3.1Hz => the slope is 62 V/s because of the triangle shape of the PZT scan.
so 100µs of separation of the 2 polarization is equivalent to 6.2mV on the PZT.
as the PZT sensitivity is 3.7Hz/V on Frep, the separation of the 2 polarization is equivalent to 23mHz on Frep.
△Frep/Frep = △v/v => △v = 41.8 kHz
about the possibility to separate S and P polarization states on secondary resonances:
the total spectral width is ~2nm which is equivalent to 565GHz and contain about 3500 laser harmonics at 160MHz.
the central spectral harmonic is roughly the number n0=1.82M, so with the first secondary resonance condition, the Frep/FSR detuning corresponds to n0*Frep = (n0+1) FSR
so (Frep - FSR) = FSR/n0 ~ 88Hz.
then if the central frequency, related to n0, is on an S-polarization resonance, the harmonics at (n0+475) will be on the P-polarization resonance and so on...
the conclusion is the power detected by a photodiode on secondary resonances are a mix of S and P polarizations (if the laser input beam is also a combination of S and P polarizations)
and we cannot make an observation of different peaks with different polarizations in transmission of the FP-cavity.
for that, it is mandatory to be on the main resonance with Frep = FSR (CEP~0) as condition of resonance.
| Xinyi Lu wrote: |
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These days, Ronic, Aurélien, Fatematuj and I have been doing some tests on polarization issue, trying to see if it is possible to obtain higher gain under other polarization conditions.
- We installed an additional half-wave plate + PBS + PD at the transmission. By rotating the waveplate of the injection laser, we can compare the resonance signals of single and full polarization. Figures 1 and 2 demonstrate this comparison. The yellow curve is full polarization and the green one is single polarization. The intensity ratio of the different polarizations is unstable in the open-loop state.
- Based on PZT scan frequency = 3.1 Hz, amplitude = 10 V, sensitivity = 3.7 Hz/V, time difference between two polarization peaks = 100 us, we can calculate △Frep = 10mHz and △v = 42kHz, which means the frequency variation between two polarizations. We see two polarizations only at the main resonance.
- By the way, we found two spots behind the M3 window (see Figure 3) and the power of both is related to the intra-cavity power. We moved the position of D-shaped mirror and the second spot became weaker and larger like mirror's edge. Maybe the D-shaped mirror is causing a part of laser to be reflected through the window, but it's unclear exactly how the optical path works.
Next steps:
- We will lock the cavity in different polarization and see if there is higher gain.
- We will move D-shaped to the maximum and see if the second spot disappears.
- We will check the coupling value and try to optimize the telescope using adjustable stages.
| Xinyi Lu wrote: |
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Today, Ronic and I optimized the locking at the amplifier current of 2 A and obtained ~ 21 kW inside the cavity.
- When all the iris open, the injected power is 10 W and the coupling is ~ 40%, corresponding to an effective gain of 2,100 and a full gain of 5,250. But the coupling may not be the true value because there is a large spot around the output beam.
- We have optimized the CEP, alignment, D-shaped mirrors and locking state. We optimized alignment after leaving the iris open and the inside power went from 14kW to 21kW.
- The transmission and reflection signals both have some same fluctuations, and they seem to come from the cavity. It's possible that the over-angled mirror mount could be the cause, but not sure. We will check in different power and see the stability of the signal.
- In addition, we found that the design values of the mirror incidence angles for the SBOX (3.359°, 5.900°) are different from the mirror ratings (1.146°). This may result in parameters such as reflection and transmission being different from the datasheet. It will also change the estimated maximum finesse, gain, and power inside the cavity. It might be better if the mirror parameters could be recalculated based on the actual angle of incidence.
| Xinyi Lu wrote: |
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- Today, Ronic and I locked at the amplifier current of 2 A and obtained ~60% coupling after optimizing the CEP (see Figure 1).
- The injected power is 10 W at 2 A. We measured only 14 kW inside the cavity, which corresponds to an effective gain of 1,400 and a full gain of 2,300. The cavity finesse is 23,000 and the normal gain should be around 6,200.
- We found fluctuations in transmission, possibly because of mode degradation. Tomorrow we will use D-shape mirrors to suppress high-order modes and optimize alignment and locking.
| Xinyi Lu wrote: |
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Today, Ronic and I installed the new telescope and locked the cavity.
- We locked at the amplifier current of 1 A and obtained 32% of coupling. (see Figure 1)
- The telescope was designed for a current of 2 A (output power ~10 W). To inject this power, we need to add some filters to devices.
- For CEP tuning, when we changed the AOM frequency while cavity locking, sometimes it caused unlock and power drops. It will be dangerous in high-power cases. So it's better to optimize the AOM frequency in low power and just tune the laser current in high power. Now the current variation range of the menhir laser is 750mA to 950mA.
| Xinyi Lu wrote: |
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- These days, Ronic, Fatematuj and I measured the beam parameters of the output of the third-stage amplifier.
- We used 2 wedges and reflection filters to reduce the intensity on the CCD.
- We measured multiple points at pump current of 2 A (output power ~10 W). The waist diameter of the output is w_x = 792.26 um, w_y=873.90 um.
- The next step is to design the telescope and improve the coupling efficiency.
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