High-Frequency Bulk Phase Modulator for Broadband Smoothing by Spectral Dispersion on OMEGA

LLE Review, Volume 78 53 High laser-irradiation uniformity is an important requirement for successful direct-drive inertial confinement fusion (ICF). Direct-drive laser-irradiation uniformity is achieved on OMEGA for different ranges of spatial frequencies using smoothing by spectral dispersion (SSD), polarization smoothing with distributed polarization rotators (DPR’s), and multiple-beam overlap.1 SSD significantly improves irradiation uniformity by rapidly shifting the laser speckle pattern generated by distributed phase plates (DPP’s). A high-frequency electro-optic phase modulator produces a wavelength modulation that is subsequently converted by a diffraction grating into the angular deflection required to shift the speckle pattern. The low spatial frequency cutoff of the smoothing produced by SSD is determined by the maximum deflection of the beam. Extremely smooth, time-averaged intensity profiles are achieved on a time scale corresponding to the inverse bandwidth impressed by the phase modulator; thus, larger SSD bandwidths are desirable. Two-dimensional SSD (2-D SSD) extends the smoothing benefits of SSD by using two separate stages of bulk electro-optic phase modulators and gratings to deflect the laser speckle pattern in orthogonal directions.