Retry of the OPO nonlinear gain measurement in 2528, motivated by the idea that the uncertainty in the detuning fit (i.e. 2544, 2546, 2550) may be caused by uncertainty in the fit parameters (see 2544, 2549).
The previous measurement of OPO nonlinear gain was too imprecise in a way that user error had a lot of impact. In the previous process, we left the OPO unlocked while sending a sine wave to GRPS to modulate between amplification and deamplification. We then used the oscilloscope persist function and triggered the resonant peak to appear on the screen. The way this was intended to work is shown in figure 1, however, it seemed like the peak was fluctuating in the horizontal axis too much due to the cavity unlock, and obtaining the minimum value was quite imprecise. On top of that, the OPO temperature was not optimised for different green injection power to compensate for absorption. This of course causes an error in the co-resonance of p-pol/BAB and green.
This time I measured again with the OPO locked, and optimised the temperature and PLL frequency for each value of green injection power. With the cavity locked, the output transmission is just a sine wave due to GRPS modulation, going between amplification and deamplification. While there is less uncertainty in just reading off the values now, there is of course some user uncertainty in optimising the temperature and PLL frequency, especially because being too careful about it is very time consuming. Juding by the optimisation process, I would say that the temperature is easily optimised to about +/- 0.01 kOhm on the thermistor, with differences in this range amounting to about +/- 2-8 mV on the amplification reading on the oscilloscope, for values between 100-1000 mV.
The values were measured using a power meter in transmission of the OPO. There are a lot of green junk beams coming out of the OPO as well, so they were removed with a laser line filter to leave only the BAB transmission. I measured a calibration factor of 0.00451 mW/mV for the correspondance between power meter readout and oscilloscope voltage, but in retrospect this is unnecessary, since only the relative power matters, and the oscilloscope voltage is linear with power meter incident optical power. Aritomi also has a more accurate calibration in 2566.
Figure 2 shows the OPO nonlinear gain with simple curve fitting. I will further consider the error of the input parameters in a future update.
Good measurement! Now it is obvious that the OPO threshold is lower than before as I said.
I recorded the thermistor (temperature) values that I used for the OPO nonlinear gain measurement.
Initially I searched in increments of +/- 0.01 kOhm on the thermistor, and then checked a bit within the optimal range. Perhaps there is further room for optimisation when zooming in on the oscilloscope though. At 100 mV/div ranges on the oscilloscope, the difference in voltage for thermistor change under 0.01 kOhm was hard to distinguish.
In the table I give the temperature, oscilloscope reading as well as the range value on the power meter - using a range of 1.6 mW on the power meter gives 10x more voltage on the oscilloscope than a range of 16 mW.
| green power [mW] | 0 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 50 | 60 |
| MZ offset | 3.9 | 4.0 | 4.1 | 4.2 | 4.3 | 4.4 | 4.5 | 4.7 | 4.9 | |
|
Amplification reading [mV] (power meter range [mW]) |
456 (1.6) | 1140 (1.6) | 1710 (1.6) | 234 (16) | 308 (16) | 408 (16) | 578 (16) | 816 (16) | ||
| De-amplification reading [mV] (power meter range [mW]) | 456 (1.6) | 240 (1.6) | 216 (1.6) | 192 (1.6) | 178 (1.6) | 166 (1.6) | 160 (1.6) | 152 (1.6) | ||
| Thermistor value [kOhm] | 7.137 | 7.147 | 7.165 | 7.173 | 7.185 | 7.196 | 7.203 | 7.215 | 7.215 | 7.225 |
| p pol PLL frequency [MHz] | 190 | 180 | 195 | 190 | 200 | 205 | 200 | 205 | 185 | 185 |
| Nonlinear gain | 1 | 2.5 | 3.8 | 5.1 | 6.8 | 8.9 | 12.7 | 17.9 |