[Aritomi, Yuhang]

First we changed the position of laser illumination point by moving lens in three directions. However, the difference is still the same as before.

Then we measured some values as following.

The ratio of power is reasonable. But the ratio of modulation is not. So we guess the problem should between lens and oscilloscope.

The reason can be: incident angle resulting in different responsivity; lens problem; PD problem

We first tried to change the angle of incident beam on PD1 by doing the 'alignment' of BS and lens. Here alignment means makeing the modulation signal on PD small by BS and recover by lens. However, it doesn't make any difference.

Then we think PD should be fine. And the last check is for lens. However, there is no space to put power meter after the lens. So we take it off from the original position. And put it before the IRMC. The power before and after this lens is

The ratio of these two power is 0.784. If we make this ratio as 1, the modulation on PD1 should be 775mV. It will be almost the same with PD2. So it explains why we have so large unbalance in homodyne.

This lens is from Nowport lens box(for infrared), however, it seems a green coating.

Conclusion: the unbalance is from using of wrong lens. We have already ordered a proper lens. It should arrive soon.