Coherent Detection With Double-Side Vertically Illuminated Photodiodes for Spatially Resolved Ranging Applications

We theoretically investigate and demonstrate coherent detection with double-side vertically illuminated photodiodes by injecting signal and local oscillator (LO) collinearly from opposite sides of the photodetector chip. This avoids optical combiners in front of the detector surface resulting in a 4.5 dB advantage over conventional coherent detection with 50-50 beam splitters to combine signal and LO. The heterodyne efficiency in the case of counter-propagating signal and LO exhibits a penalty of 1.5 dB at optimal absorption layer thickness compared to co-propagating signal and LO and has the same pronounced dependency on the relative propagation angle.A first proof of concept for using this dependency for spatially resolved coherent lidar systems is demonstrated.