Compensation for Self-Focusing of Picosecond Pulses in Nd:Glass by Using Cascaded Quadratic Nonlinearity

One of the obstacles in obtaining high power/energy laser sources is self-focusing, which stems from the nonlinear phase shift (B-integral) accumulated during beam propagation in Kerr media. Phase-mismatched second-harmonic generation may impose a nonlinear phase shift on the fundamental with controllable sign and magnitude, which can be used to compensate for self-focusing with a single-pass configuration. We have demonstrated such a possibility for picosecond pulses theoretically and experimentally, and both configurations of pre- and post-compensation by a β-barium borate crystal have been studied in detail. Cascaded second-order nonlinearity-based compensation for self-focusing may provide an auxiliary means to the conventional B-integral control techniques.