Control circuits stabilize and maintain coupled-cavity lasers in a single longitudinal mode. The laser is electrically connected and mutually and optically coupled to a control means responsive to a light output signal from the laser for controlling the laser by generating a control signal to the laser to produce single longitudinal mode output. The single mode output is maintained even during high speed modulation in which the current or ambient conditions vary.

Two or more parallel waveguide lasers of different effective length are coupled together by their evanescent waves such that the composite structure functions as a single cavity having a longitudinal mode with resonances related to the resonances of the separate cavities. A very strong selection of the composite operating frequency, and suppression of most or all of the other longitudinal modes, with frequency tuning of the composite structure (both continuously and by mode hopping), can be accomplished by varying the relative currents supplied to each laser. Upon holding one cavity at one current level for a selected operating point and shifting the other, bistable operation can be achieved.

A semiconductor heterojunction optical loss modulator section (30), having an anti-reflection coating (52) on its optical output beam facet (32), is located with an opposite facet (31) optically coupled to, and closely spaced from, a semiconductor heterojunction laser section (20) having substantially the same cross-section structure as the loss modulator section (30). The laser section is operated CW in a single mode. In this way, the output beam (50) emanating from the laser section (20) and passing through the anti-reflection coating (52) can be coupled into an optical fiber (60) for transmission purposes. This output beam (50) is substantially single frequency and is intensity modulated, in a signal-pattern independent fashion, in accordance with an electrical signal applied to the loss modulator section (30).

A light transmitter comprising a cleaved-coupled cavity laser and stabilization means to maintain the laser in single longitudinal mode operation is described. '

An optical waveguide type wavelength filter having a Fabry-Perot resonator portion formed in the waveguide. The Fabry-Perot resonator portion has a length corresponding to the resonator length of the propagated light and formed by cutting at least two suitable gaps in the optical waveguide. By varying the index of refraction of the Fabry-Perot resonator portion by refractive index varying means, the resonator length L of the Fabry-Perot resonator portion is optically varied and, thereby, the filter characteristic of the Fabry-Perot resonator portion can be made variable.