We summarize the principle and experimental
setup of optical steganography based on amplified spontaneous
emission (ASE) noise. Using ASE noise as the signal
carrier, optical steganography effectively hides a stealth
channel in both the time domain and the frequency domain.
Coherent detection is used at the receiver of the
stealth channel. Because ASE noise has short coherence
length and random phase, it only interferes with itself
within a very short range. Coherent detection requires the
stealth transmitter and stealth receiver to precisely match
the optical delay,which generates a large key space for the
stealth channel. Several methods to further improve optical
steganography, signal to noise ratio, compatibility with
the public channel, and applications of the stealth channel
are also summarized in this review paper.