The semiconductor laser amplifier (Semiconductor Optical Amplifier, abbreviated as SOA) has the same amplification principle as that of a semiconductor laser, and it also uses the excitation phenomenon of transitions between energy levels for optical amplification. In order to increase the gain, people have removed the resonant cavity that constitutes the laser oscillation and are directly excited by the current to obtain an optical gain of more than 30dB (1000 times).
There are three types of semiconductor laser amplifiers. One is to use ordinary semiconductor lasers as optical amplifiers, called Fabry-Perot (F-P) semiconductor laser amplifiers (FPA); the other is to coat the two end faces of the F-P laser Anti-reflection film to obtain broadband, high output and low noise. This type of amplifier amplifies light during the travel of light, so it is called traveling wave optical amplifier (TWLA) and there is also an injection lock-in amplifier (IL-SOA), which is identical in structure to F-P-SLA. But it is biased above the threshold current. If weak single-mode light is injected into this amplifier, a high-power single-mode output will be obtained.