I modulated laser power to see how it causes frequency noise in the cavity, no sign of RIN induced frequency noise has been seen yet.
We have been thinking about how RIN will cause random absorption on the mirror surface and turn into thermo elastic/ thermo refractive noise in the cavity.
So we try to observe it, by modulating the laser power with an EAOM. I used white noise, generated by SR785 at 2.5V, as a source to modulate the intensity.
The power input to both cavities are 3mW. This corresponds to ~15mW total power before the BS (3 to RCAV, 12 to ACAV path).
1)The intensity changes as we can see from ACAV_trans_PD (fig1).
The lower curve is the power fluctuation of the laser without the modulation. The upper curve is the power fluctuation with the added white noise.
So we are sure that intensity does change from our modulation.
2)Then I measured the feedback signal to PZT and to AOM, with and without external white noise. These signals represent the frequency change of the laser.
However no change observed in both signals. I plotted only the feedback signal to VCO(to AOM), since this signal should be more sensitive to change than feedback to PZT, as the
beam to ACAV path is stabilized to the RCAV already.
3) There is no observed effect from RIN on beat measurement yet, see fig 3.
The laser power output is ~54 mW, so the maximum power we can get to each cavity equally is ~10 mW.
I will try to increase more power to the cavity, as the effect should be proportional to the input power
Actually, we don't need both cavities to have the same power in order to measure the feedback signal. I can try
1) lock the laser to RCAV only with power of 10mW or upto 40mW and measure the feedback signal to PZT
2) lock both cavities, have maximum power to ACAV (could be upto 12,13 mW, the efficiency is ~25%) and measure the feedback to VCO