Advantages and disadvantages of coherent detection

1. What are the advantages of coherent detection over direct detection optical receivers?
2. What is the difference between coherent and noncoherent detection?
3. What does coherent detection mean?
4. What is the difference between coherent and noncoherent demodulator?

What are the advantages of coherent detection over direct detection optical receivers?

Coherent detection can not only achieve higher sensitivity than direct detection but can significantly increase the spectral efficiency (encoding more bits on each symbol) as well because it uses phase, amplitude, and polarization of an optical carrier to carry information.

What is the difference between coherent and noncoherent detection?

Non-coherent means optical transmission system does not require coherent local oscillator light; Coherent means optical transmission system that uses local oscillator light for coherent detection.

What does coherent detection mean?

In coherent detection systems, the output of the receiver front-end (after high and low pass filtering) yields a signal proportional to the optical field down-converted to some appropriate IF, unlike direct detection, which produces a signal proportional to the optical intensity.

What is the difference between coherent and noncoherent demodulator?

Coherent systems need carrier phase information at the receiver and they use matched filters to detect and decide what data was sent , while noncoherent systems do not need carrier phase information and use methods like square law to recover the data.