Abstract

A typical approach to improve· the beam quality of a high-gain, short-pulse, large-Fresnel- number laser such as copper laser is to use an unstable resonator. However, in such an oscillator cavity, only the latter part of the output pulse reaches near-diffraction-limited beam quality since only a small number of cavity round trips (typically 3 or 4) can be established during the 50-70 ns copper laser pulse. For achieving a low divergence output for tire entire duration of the pulse, injection control techniques are usually applied.^1,2 The most commonly used phase-matching injection scheme is achieved by matching the injected wavefront to the injection-locked oscillator (ILO) divergent wavefront. By using a gated camera, we found that the ILO output beam using the phase-matching scheme was characterized by two independent modes which appeared at different times. The front part of the ILO output is actually a reamplified (master oscillator) MO injection beam with its pointing and collimation controlled by the injection beam. As soon as the ILO feedback process starts after the first cavity round trip, the ILO cavity quickly converts the MO modes into the ILO modes owing to the high mode-discrimination nature of the ILO unstable resonator (M = 15). This characteristic, accompanied by small injection misalignment, results in undesirable multiple far-field spots due to different pointing and collimation.

© 1993 Optical Society of America