Phase-Matching Optimization and Walk-off Analysis in LBO Crystals

Abstract

Lithium Triborate (LBO) is a biaxial nonlinear optical crystal widely used for laser frequency conversion owing to its broad transparency range (160–2600 nm), high damage threshold, and favorable phase-matching properties. This study presents a comprehensive numerical model based on solving the Fresnel equation with modified Sellmeier coefficients to analyze spectral birefringence, critical phase-matching angles, walk-off, and OPO tuning in LBO. For Type-I SHG at 1.064 μm, phase matching is achieved in the XY plane at φ = 16.13° (∆n = 2.85×10⁻⁶) and in the XZ plane at θ = 30.99° (∆n = 2.34×10⁻⁵), with a global off-axis optimum at θ = 67.42°, φ = 25.59° (∆n = 1.02×10⁻⁷). For Type-II SHG, the XY plane yields phase matching at φ = 51.35° (∆n = 9.06×10⁻⁶), with an off-axis optimum at θ = 39.43°, φ = 64.41° (∆n = 6.15×10⁻⁸). Phase matching is not achievable in the YZ plane for either type. Walk-off peaks at ~0.95° near 0.8 μm in the XY plane and ~1.38° at 1.1 μm in the XZ plane. OPO tuning pumped at 532 nm demonstrates broad coverage, with Type-II extending the signal to 2.5 μm via azimuthal rotation in the XY plane.