Eleonora and Yuhang

While I was doing this measurement, I checked the error signal on an oscilloscope. I found the error signal noise is smaller if I use gain of 10.

So I changed the gain of filter cavity locking to 10.

Following the method Matteo B and Eleonora, I characterized the filter cavity locking noise again. (Measuring sine-wave single frequency noise at a frequency higher than unity gain frequency (channel 'ramp mon' and channel 'eps1'). Calibration by considering ramp monitor factor of 100, PZT gain of 2e6 and filter cavity pole of 1+(f/f0)^2 with f0=1.45kHz )

• Take ramp mon signal, calibrate it to frequency. S_Hz = V_RMS (V) * 100 * sqrt(2) * 2e6 Hz/V = 6.477 Hz
• Take eps1 signal, compare it with ramp signal(already calibrate to frequency), to have the conversion factor from Hz to V. K(V/Hz) = Err_V/S_Hz*(1+ (f/f_0)^2) =   4.568e-3 V/Hz

Then we use this K to calibrate the measurement of error signal noise spectrum to Hz. The result is shown in the attached figure.

However, compared with the measurement of more than one year ago. This level is much lower(almost 50 times lower). We checked the calculation and measurement procedure. We couldn't find out what is the problem.