Narrow spectral linewidth in the far-field zone of active-grating surface-emitting semiconductor laser-amplifier

The spectral linewidth properties of an active-grating semiconductor laser-amplifier have been investigated. The spectral linewidth of the amplified output in the far field zone is a factor of 1.4 to 0 narrower than that measured in the near field depending on oscillator power. The amplifier linewidth was found to match the free-running oscillator linewidth with sufficiently strong input coherent power. In particular, a narrow 2.1 MHz far-field linewidth was obtained at a CW output power of 200 mW. This is the first observation of different values of the spectral linewidth being measured in the near and far-field zones of a single-frequency laser-amplifier. The effect is related to the spectral and spatial filtering of properties of the active-grating output coupler.<>