Why coherent detection?
Optical sensors are broadly classified into two categories: direct and coherent detectors. Direct detectors are essentially square law devices that are sensitive to the intensity of the received electromagnetic signal. In contrast, coherent detection involves mixing the received signal with an optical local oscillator (LO) in a balanced photodiode to downconvert it to a suitable microwave intermediate frequency (IF). Balanced photodetection suppresses the laser intensity noise and improves the noise figure of the sensor. Coherent detection is a linear process that is sensitive to the amplitude, phase and polarization of the received signal. As a result, information embedded in the signal phase, such as Doppler shifts and vibration signatures, can be recovered easily.
