**Calibrating error signal to beat frequency;**
I injected 0 dBm RF sine wave into the beatbox and sweeped the frequency(just like we did in elog #6815).
This time, we have different whitening filters. I sweeped the frequency from 0 to 100 MHz in 200 sec.
The length of the delay line is ~1.5 m for COARSE.
**Y arm length;**
Here, I think we need some assumption. Let's assume wavelength of IR lamb_IR = 1064 nm and Y end green frequency is nu_g = 2*nu_IR.
There is a relation
dnu_g / nu_g = dL / L
So,
dnu_g / (dL/lamb_IR) = 2*nu_IR * lamb_IR / L = 2c/L
We know that dL/lamb_IR = 1/2 for difference in beat frequency between TEM00s. Therefore, slope of the dnu_g vs dL/lamb_IR plot gives us the arm length L(figure below, middle plot).
Error estimation is not done yet, but I think the COARSE_I_IN1 error signal to the beat frequency calibration has the largest error because it seems like the amplitude of sine wave changes ~10% day by day.
**Calibrating beat frequency to Y arm length change;**
I used L = 32.36 m (figure above, bottom plot).
dnu_g / dL = c / lamb_g / L = 1.74 MHz/m |