The lasing action in an unstable optical resonator is controlled as to frequency by limiting frequency selectivity to an axis-near beam. The control is provided preferably outside the cavity by means of active or passive elements. A passive element returns a frequency-controlled beam into the cavity, an active element injects a control beam at the desired frequency.

A gas laser device comprises a discharge space in which a laser gas is excited by electric discharge, the discharge space being in the form of a slab whose section perpendicular to a laser optical axis has a longer side and a shorter side; and laser resonator mirrors disposed at both ends of the discharge space, respectively. The laser resonant mirrors constitute a negative branch unstable resonator in a first dimension of longer side of the discharge space section, and a laser beam is obtained at one end of the longer side of the discharge space section.

High-power laser metalworking apparatus, especially one having a carbon dioxide laser. After the laser beam emerges from an optical resonator it is deflected by a mirror against a workpiece and focused on the surface thereon. An image-rotating means is disposed in the beam path after the deflection mirror, and has an odd number of fully reflective mirrors, but at least 3 such mirrors. The angle of rotation of the image-rotating means is electronically controllable in accordance with the direction of the path of the working movement. The image-rotating means and focusing system advantageously form a single component in which at least one of the mirrors of the image-rotating means focuses the laser beam onto the surface of the workpiece. Preferred is a laser having an unstable optical resonator with at least three fully reflective mirrors of cylindrical curvature, the mirrors whose planes of curvature are parallel to one another defining the optical resonator, and the third mirror, or every additional mirror, being concavely curved in a plane perpendicular with respect to the mirrors defining the resonator.

A low diffraction-feedback high-energy laser system includes an injection laser, a master oscillator power amplifier (MOPA), and means for aligning the injection laser to the MOPA. The alignment means includes a relay mirror and sensor array cooperative to center the injection laser externally of the cavity, and the alignment means further includes a relay mirror and cooperative optics and sensor to center the injection laser interiorly of the cavity. The master oscillator power amplifier preferably includes a convex and a concave cavity. reflector. In one embodiment, an intracavity laser separator is disclosed for providing alignment of the injection laser, and in a further embodiment an extracavity laser separator is disclosed for providing injection laser alignment.

A Coupled Multiple Output Resonator (CMOR) having two symmetric modules 10,20, is created by adding self-feedback (SFB) mirrors 104,106 aligned with apertures 100,102, in output scraper mirrors 14,24, respectively, to a conventional Multiple Output Resonator (MOR). The SFB mirrors re-inject a portion of the output beams 34,42 back into the resonator cavity as adjoint beams 108,110 which alter the modes of the system such that the CMOR only lases at desired modes where all the output beams 34,42 are in-phase. The CMOR provides substantially constant acceptable mode discrimination for more than 4 modules with only one SFB mirror per module and no additional external coupling paths between modules, thereby allowing the system to be scaled to any number of lasers without reduction of target intensity. Also, the SFB mirrors are small and are easily controlled to compensate for cavity mirror variations to maintain the desired mode discrimination.