Coherent optical detection has gained enormous interest in the past decade with many applications spanning through Optical Communication, Data Center, and LiDAR. The enabling factors are the availability of large scale photonic integrated circuits (PIC) and digital signal processing (DSP). Coherent optical detection has been known since the early 1980s, but its implementation in large volume devices, with reliable and stable performances, has been possible only in the second decade of 2000s for the advent of Si-based PIC and DSP. In this chapter, we will consider different architectures of optical coherent detection, providing a quantitative comparison of performances and complexities. The focus is on the code-modulated (CM) LiDAR, where the transmitted optical field gains the benefits of the binary-coded dual polarization quadrature optical modulation.