Apparatus is provided which includes a laser oscillator for providing light along a first optical path, and a laser amplifier for amplifying light along a second optical path. Means for coupling light between the first and second optical paths is disposed at one end of the laser amplifier, and a nonlinear medium disposed at the other end thereof for intercepting light provided along the first and second optical paths. The laser oscillator provides a single-mode laser beam along the first optical path which is made incident upon the nonlinear medium from substantially opposite directions. A portion of the beam from the laser oscillator is coupled through the laser amplifier, which forms a probe beam that samples the laser amplifier system. This beam further impinges upon the nonlinear medium and interacts with the counter-propagating laser beams therein. A phase-conjugated beam is reflected from the nonlinear medium and is amplified by the amplifying medium providing a single-mode output beam of the laser amplifier system. A portion of the phase conjugated beam is reflected back into the oscillator and used as the counter-propagating pump beams for the phase conjugation process. This self-pumping feature allows for minimization of the pumping power required for the oscillator. The combination provides for a phase corrected, diffraction-limited beam at the output of the system. A second embodiment utilizes the amplifier medium to accomplish both amplification and phase conjugation therein.

In a laser beam modulation system, e.g. for pattern machining, in which a laser beam is modulated prior to power amplification, an acousto-optic modulator can provide a low beam extinction level when the diffracted beam is used, but the modulator causes an optical frequency change (Dopplar effect) taking the modulated beam outside the laser amplifier passband when both the source and amplifier are low pressure (1 torr) CO.sub.2 lasers. This is overcome by operating at least one of the lasers at an elevated gas pressure thus increasing the lasing amplification bandwidth. Thus a waveguide CO.sub.2 laser can be used as the amplifier but the power available is limited. Preferably a tunable waveguide CO.sub.2 laser source is used tuned to a frequency offset sufficient to accommodate the modulation beam and sidebands within the passband of a conventional low pressure high power CO.sub.2 amplifier laser.