Michimura, Takano

We tried to measure the spectra of TMSX and TMSY poilarization monitor with PRFPMI. It seems that the monitor PDs for s-pol are saturated, but for p-pol seems to work.

PD saturation check

Figure1 shows the raw spectra and coherence of TMSX and TMSY polarization monitor. It is apparent that the spectra of X arm s-pol and Y arm s-pol are not sensitive to the power fluctuation. I checked the time series data as shown in Figure2, and found that the output of these two PDs are saturated.

Referring to the power measurement, the power injected to the polarization monitors and the corresponding output DC voltage are as listed below. I also estimated the power and output with PRFPMI so that the equivalent input power is 140 times larger than the previous measurement, single arm lock. In this calculation the response of the monitor PDs are taken from this post, 0.39 [A/W] and 4.75e4 [V/A] for 30dB gain setting.

From the calculation it is confirmed that with PRFPMI the s-pol monitor PDs should be saturated. To avoid the saturation, we need to reduce the gain setting of s-pol monitor PDs from 30dB to 10dB (4.75e3 [V/A]) or lower. When the input power is increased to 10 W, we also have to reduce the gain of p-pol monitors.

Polarization rotation angle

Using p-pol monitor signal, we estimated the polarization rotation angle.

• startTime=1408765218 # 1.1 W at IMC output, whitening off
• Calibration: almost same as the original post , but 130 time smaller (because of increased transmission power)

Figure3 (Figure4) and Figure5 (Figure6) shows the spectra of polarization monitors of TMSX (TMSY) in cnt/rtHz and the polarization rotation angle of TMSX (TMSY) in rad/rtHz. Now the sensitivity is roughly 100 times better than the previous measurement.