The optical design of a laser resonator smooths out the intensity beam profile of the generated laser beam and prevents the formation of "hot-spots." The active laser resonator containing the active laser medium is optically coupled to a free space passive optical resonator to provide mixing and filtering of the spatial modes.

Radiation at the frequency of a preselected absorbing resonance is produced by use of split and recombined interfering optical paths, arranged to produce near white light interference effects, and a preselected absorption medium disposed in one path of the interfering configuration. By association of this configuration with a laser oscillator, the regeneration characteristic of the laser oscillation can be determined by a peak in the reflected or transmitted wave produced by the cooperation of the optical interference system and the absorption medium. Use with standing wave, ring, and gain-switched transient lasers all can produce radiation essentially confined to the narrow absorption profile of a selected gas. Improved methods of laser chemistry and remote and close range gas detection are made possible by the new system. The system is also useful as an extremely narrow filter for broader band light sources, or as an optical element for other purposes, such as for establishing high Q response for optical resonators of the standing wave or traveling wave, ring type.

Mode-locked pulsed output is induced in a laser by introducing a varying frequency dependent loss in the cavity. The varying, frequency dependent loss allows a large number of longitudinal modes to oscillate. In one of the preferred embodiments of the subject invention, the varying frequency dependent loss is generated by reciprocating one of the mirrors of the resonator. In this embodiment, the laser has a resonator with a primary light path defined by first and second end mirrors. A gain medium is located between the end mirrors. A beam splitter is provided for redirecting the light into a secondary light path. A third mirror functions to feed back the light into the primary light path. Pulse formation is induced by varying the path length of one of the two paths by reciprocating an end mirror. Moving the mirror creates varying interference effects when the light energy is recombined at the beam splitter. The mirror is reciprocated relatively slowly, yet ultra-short pulses are generated.

One end of semiconductor laser 1 is coated an antireflection film 1A. And, the external resonator is constructed with a diffraction grating 2 and an external reflective mirror 4. Additionally, a beam splitter 3 is provided between the diffraction grating 2 and the external reflective mirror 4. An output light from antireflection film 1A of semiconductor laser 1 is transformed to a parallel light by the convex lens 5A, and is supplied to the beam splitter 3. The parallel light 13A changes a light path thereof at beam splitter 3, and is then supplied to the external resonator. A wavelength of the parallel light 13A is determined by the diffraction grating 2, so that the light has a selected wavelength, and a phase which is matched with a phase condition of the external resonator. A portion of the resonated light is reflected by the beam splitter 3, and is fed back to the semiconductor laser 1 via convex lens 5A, as a reflected light. When the light is fed back, the semiconductor laser 1 outputs a light of narrow spectral line width. This light is outputted from another end, which is not coated with antireflection film 1A, of semiconductor laser 1, and is transformed to a parallel light by the convex lens 5B. This parallel light 13B forwards to a light isolator 6, is then collected at optical fiber 8 by the convex lens 7. The light which supplied at optical fiber 8 is output to an external apparatus.

A laser device incorporates a rectangular multi-mode beamsplitter waveguide connected at one end to a retro-reflecting mirror. The beamsplitter waveguide is connected at a second end to an output coupling waveguide and a reflection coupling waveguide. The reflection coupling waveguide is terminated by a second retro-reflecting mirror. Radiation produced within the device is reflected by the mirror and coupled to the two coupling waveguides in a manner such that a partially reflecting mirror is not required at an output to the device.