Abstract

Semiconductor lasers with their relatively low-Q Fabry-Perot cavities typically have multimode spectra and poor frequency stability. Injection locking has proved useful in achieving single-mode operation under high-speed modulation and enabling pure amplitude or phase modulation. Compared with a conventional setup, where light is injected into the free-running dominant mode, side-mode injection locking permits us to extend the available range of device parameters such as lasing frequency, linewidth, or modulation bandwith. Also, spectral variation of laser characteristics, e.g., gain or linewidth enhancement factor, can be determined by scanning the injected longitudinal modes of the slave. In our analysis, we use multimode rate equations including a phase equation for the injected mode. The dynamic stability is investigated by considering small fluctuations around the stationary solutions. The results are then compared with a simple single-mode approximation. As an example, a typical index guided 1.54-µm InGaAsP laser is considered. A full multimode stability test confirms the single-mode result that detuning the lasing mode toward the shortwavelength side of the gain peak increases the resonance frequency v_r. At a pumping level 20% above the free-running threshold, the maximum enhancement of v_r occurs three mode spacings away from the central mode. An additional benefit is that optical injection enhances the field damping, thereby narrowing the spectral linewidth, increasing modal stability, and reducing the dynamic frequency chirp. We conclude that the side-mode injection locking may be useful for such uses as optical communication, fast switching, and ultrashort pulse generation.

© 1989 Optical Society of America