Tara and I re-attempted to implement ISS using the SR560

Setup:

Like last time, we set up the SR560 as the gain, in between the PD measuring intensity fluctuations behind RCAV and the EAOM. Contrary to the previous setup, we omitted the amplifier after the gain, because it was not needed in achieving the necessary noise suppression. The following diagram illustrates the setup we used:

The gain of 10000 was adequate to suppress the intensity noise behind RCAV to about 10^-8 at 100Hz. We used AC Coupling and bandpass settings to get the necessary gain shape. The corner frequencies at .1Hz, 1kHz reduced noise at high frequencies and optimized the suppressed signal at both high and low frequencies. The RCAV input power was set to 1mW.

Data:

 Free and Suppressed RIN:

We measured both the free intensity noise and suppressed intensity noise behind RCAV, as well as the free and suppressed noise behind ACAV. Given that we were only using the ISS to actuate on RCAV, suppression of the intensity noise behind ACAV was not expected. We aimed to suppress the RCAV noise to about 10^-8[1/sqrt(Hz)], and with a gain of 10000, this was achieved. The results for both cavities are plotted below:

Transfer Function:

Dividing the free RIN noise by the suppressed RIN noise, subtracting one, and converting to dB, I calculated the the Transfer Function. The following plot shows this calculated TF as compared to the one we measured:

While the frequency ranges differ, the shapes are similar for the two curves.

The following plot shows the free and suppressed RIN, along with the measured TF shifted so that the suppressed noise is at zero dB:

Conclusion:

The results of this experiment vastly improve on the results from the last (see attached elog entry), as the ideal level of suppression was reached, and the TF's match up much more accurately. The actual ISS built will utilize an AOM to actuate on the ACAV fluctuations, so that both cavities intensity fluctuations will be corrected.