Abstract
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.
Highlights
COHERENT light detection and ranging, like frequencymodulated-continuous-wave (FMCW)-lidar, is considered to have several advantages over direct detection approaches, like pulsed time-of-flight lidar [1]
In [7], we proposed a novel approach of spatially resolved coherent detection by illuminating the photodetector chips with signal and local oscillator (LO) from opposite sides for coherent detection
We experimentally confirm the in [7] proposed approach superseding optical beam splitters and delivering new degrees of freedom in the design of solid-state coherent lidar systems by making use of the strong intermediate frequency (IF) power dependency on the relative propagation angle between signal and LO
Summary
COHERENT light detection and ranging, like frequencymodulated-continuous-wave (FMCW)-lidar, is considered to have several advantages over direct detection approaches, like pulsed time-of-flight lidar [1]. FMCW-lidar systems are considered insensitive to background noise due to, e.g., solar glare, or other lidar systems [2] They provide the velocity information due to the detectable Doppler frequency shift [3]. Other FPA-based coherent receivers use 50-50 optical beam splitters to combine signal and LO and require accurate collinear superimposition of signal and LO and suffer from a fourfold reduction in signal strength [6], [1]. In [7], we proposed a novel approach of spatially resolved coherent detection by illuminating the photodetector chips with signal and LO from opposite sides for coherent detection. We experimentally confirm the in [7] proposed approach superseding optical beam splitters and delivering new degrees of freedom in the design of solid-state coherent lidar systems by making use of the strong intermediate frequency (IF) power dependency on the relative propagation angle between signal and LO
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