Noise properties of injection lasers due to reflected waves

The injection noise due to incoming reflected waves in a laser diode is analyzed, with an optical fiber considered as a typical example of a medium that causes reflection of the laser output waves. It is shown that the noise due to reflected waves should be regarded as two different phenomena, a double-cavity state and an external-light injection state, characterized by three relations among pulse length, reflection-point location, and the coherence state of the laser-diode emission. The noise in the double-cavity state is attributed to phase variation resulting from a variation in the equivalent length between the laser and the reflection point or in the optical fiber's effective refractive index, produced by mechanical vibration or temperature variation; the noise in the other state is ascribed to repetitions of locking and unlocking states due to random frequency variation or mode jumping. It is concluded that such noise can be reduced by increasing the laser-diode bias current to a level well above the threshold current and that the use of an optical isolator is apparently necessary to eliminate this noise.