Recently we were given the idea of sending the beam to the photodiode at Brewster angle. If we do so, ideally one particular polarization (parallel to the plane of incidence) will not reflect back. So if we send the beam polarized in this direction (or set the incidence plane such that these conditions are matched), we can minimize the reflection from PD significantly.
Sounded like a good idea, so I started reading about the InGaAs detector we have. Unfortunately, the datasheet of the C30642 detector we use does not mention either the fraction of In in InGaAs or the refractive index of it. So I went into the literature found these two papers:
Kim et al. Applied Physics Let Vol 81, 13 23 (2002) DOI: 10.1063/1.1509093
Adachi et al. Journal of Applied Physics 53, 5863 (1982); doi: 10.1063/1.331425
Using the empirical coefficients and functions from these paper, I calculated the refractive index for InGaAs for various fractions of x and the corresponding Brewster angles (Find Attached).
However, just after doing the analysis, we realized that doing this is not really possible. The Brewster angle is arctan(n2/n1) where n2 is the medium light is going into. This implies the Brewster angle would always be greater than 45 degrees and detector won't really absorb much light at this angle. So currently the conclusion is that this idea won't work.
However, there might be some error in our assumption of InGaAs as a transparent medium as the calculations do not take into account absorption of the photon at all. Attaching the python notebook too in case someone figures this out.