Efficient Broadband Third Harmonic Frequency Conversion Via Angular Dispersion

In this paper we present experimental measurements and theoretical modeling of third harmonic (3(omega) ) conversion efficiency with optical bandwidth. Third harmonic conversion efficiency drops precipitously as the input bandwidth significantly exceeds the phase matching limitations of the conversion crystals. For Type I/Type II frequency tripling, conversion efficiency begins to decrease for bandwidths greater than approximately 60 GHz. However, conversion efficiency corresponding to monochromatic phase-matched beams can be recovered provided that the instantaneous propagation vectors are phase matched at all times. This is achieved by imposing angular spectral dispersion (ASD) on the input beam via a diffraction grating, with a dispersion such that the phase mismatch for each frequency is zero. Experiments were performed on the Optical Sciences Laser (OSL), a 1 - 100 J class laser at LLNL. These experiments used a 200 GHz bandwidth source produced by a multipassed electro-optic phase modulator. The spectrum produced was composed of discrete frequency components spaced at 3 GHz intervals. Angular dispersion was incorporated by the addition of a 1200 gr/mm diffraction grating oriented at the Littrow angle, and capable of rotation about the beam direction. Experiments were performed with a pulse length of 1-ns and a 1(omega) input intensity of approximately 4 GW/cm2 for near optimal dispersion for phase matching, 5.2 (mu) rad/GHz, with 0.1, 60, and 155 GHz bandwidth, as well as for partial dispersion compensation, 1.66 (mu) rad/GHz, with 155 GHz and 0.1 GHz bandwidth. The direction of dispersion was varied incrementally 360 degrees about the beam diameter. The addition of the grating to the beamline reduced the narrowband conversion efficiency by approximately 10%. Sufficient dispersion to allow nearly full phase-matching of all frequency components along the sensitive axis of the tripler allowed recovery of the narrow band conversion efficiency with bandwidth. However, even partial dispersion compensation was shown to significantly increase broadband 3(omega) conversion efficiency.