[Yuhang, Yaochin, Aritomi, Eleonora]

We wanted to check the gain of the quadrant PD by shining some light on it and look at the response of the RF channel with a spectrum analyzer. We assumed that amplitude fluctuations at that high frequencies are shot noise dominated and thus have a flat spectrum. It this way it would be possible to measure the gain TF of the PD and indentify its resonances.

We sent to one sector of the quadrant about 1.5 mW of green light in reflection from FC and we amplified the RF signal of 32dB. We couldnt' see any difference in the spectrum with respect to the case when we don't send any light, except for the light at the modulation frequency that we could not switch off (15 MHz, needed for SHG).

We decided to take the same measuremnt with a red laser diode, in the same configuration for differen power value. We found that the RF spectrum level increases with the power showing a peculiar shape.

We observed that the RF signal stop increasing already at about ~250uW, while DC seems linear (see first plot) 

Not that the responsivity of the sensor is similar for green and red (0.2 [A/W] wrt 0.25 [A/W])

In order to confirm that the laser diode has not any strange amplitude feature at high frequency, we tried it on a QUBIG PD and found a reasonable RF spectrum with a cut off at 100 MHz ( as reported by the data sheet)

Conclusion: something is wrong. The quadrant seems to repond too differently to green and red light. But the gain doesn't show any resonace. We will investigate more.