Master oscillator with power amplifiers: performance of a two-element cw HF phased laser array

The experimental performance of a two-element phased array of multiline cw HF chemical lasers in the master oscillator with power amplifier (MOPA) configuration has been measured. The mutual coherence of the two amplified beams was inferred from measurements of the visibility of interference fringes obtained when the beams were overlapped in the near field. When the optical path difference for the two beams was minimized, multiline visibilities of 0.90 +/- 0.02 were measured. White light interferometry was used to equalize the optical path lengths. Spectral mismatch between the master oscillator output and the amplifier's preferred gain distribution affected neither the amplification factor nor the mutual coherence of the amplified beams. The effect of spatial coherence of the master oscillator beam on these near field measurements and the eventual requirement of far field measurements to precisely optimize path lengths are discussed. Spectra data, amplification factors, and mutual coherence measurements are shown, and the resulting phased array far field performance is presented.