Methods for mode selection in injection lasers

External spectrally selective elements were introduced into the resonator of an injection semiconductor laser in order to improve the spectral composition of the emitted radiation, and, in particular, to increase the output power in the single-mode operation. The following variants were investigated experimentally: a) a composite resonator; b) a resonator with an interference filter; c) two optically coupled Fabry–Perot resonators of different length. An external selective element always affected the emission spectrum of a semiconductor laser. A single longitudinal mode was obtained when the pumping level exceeded the threshold by an amount larger than in ordinary injection lasers. The output power under single-mode conditions ranged up to 0.5 W (in the case of a composite resonator). An investigation was made of the possibility of using an external element for varying the emission wavelength within the gain band of the semiconductor.