Direct-detection receivers employ a square-law device which produces an electrical
signal proportional to the intensity of the incident optical signal (e.g., a
photodiode)---the signal's power is measured directly. Since any optical phase information
is lost in the process, direct detection cannot be used to measure the Doppler frequency
shift of the radar echo, and direct detection is subject to thermal and dark current noises
in the receiver and to background light incident on the detector. Under certain conditions
of limited signal strength, direct detection therefore offers sensitivity inferior to that
of coherent detection.
However, direct detection has advantages over coherent detection when either source temporal
coherence or the spatial phase characteristics of the received signal cannot be strictly
controlled, or when complexity or cost are important design issues.
Coherent optical detection is analogous to the detection technique used for decades in
superheterodyne radio receivers, but is much more difficult to implement at optical
frequencies. The received field is beat against a local oscillator field of nearly the same
frequency, and the output signal is proportional to the received field strength. The
proportionality of the beat term to the local oscillator field strength provides essentially
noiseless pre-detection gain in the ideal case, so that thermal and dark-current noises
inherent to the photodetector and pre-amplifier are dwarfed by the quantum noise inherent in
the signal itself. Thus coherent detection techniques provide superior sensitivity to
direct detection under ideal conditions when signal strength is limited. Coherent detection
also makes possible measurement of the small Doppler frequency shifts associated with
velocities of interest for ground targets.
Coherent detection places strict requirements on the spectral purity of the source and
requires that the received signal and the local oscillator have spatial phase fronts which
are nearly perfectly aligned over the active area of the detector.
IRAR:
Coherent Detection
