A master laser is a single-frequency laser which is used for, e.g., injection locking of one or several slave lasers. Sometimes, the term master laser is also used for a seed laser of a master oscillator power amplifier system.
A master laser often generates an optical power which is well below the output power of the complete laser system; the injection-seeded slave lasers may then be high-power lasers. The modest power requirements for the master laser make it possible to realize a very low level of laser noise, which may be more or less transferred to the whole system. Also, a larger range for wavelength tuning may be accomplished.