Nanosecond pulsed laser energy and thermal field evolution during second harmonic generation in periodically poled LiNbO3 crystals

Abstract

Computational study of nanosecond pulse laser radiation in periodically poled LiNbO3 (PPLN) crystals reveals the complex spatio-temporal evolution of the 1.064μm fundamental harmonic (FH) and second harmonic (SH) energy fields with associated temperature fields, leading to the thermal dephasing and inhibition of second harmonic generation (SHG). The investigated range of the laser input power is W0=0.5–50W (with the pulse energy Q0=0.01–1mJ∕pulse and repetition rate of 50kHz). For input laser powers W0>10W the FH and SH energy fields are found to strongly couple with nonuniform temperature field, leading to significant thermal dephasing and SHG efficiency loss. Heat generation and temperature distributions also exhibit very significant nonuniformities along and across the laser beam, maximizing at the rear or inside the crystal, depending on the input power. However, conformal temperature tuning along the operating crystal inhibits these nonuniformities, and significantly enhances SHG efficiency under high input powers. For instance, selected PPLN conformal cooling parameters lead to the formation of a temperature-uniform quasi-phase-matching channel for a 300μm diameter laser beam providing a high SHG efficiency (≈64%) at 20W input power.