Picosecond Optical Pulse Generation by Nonlinear Mirror Mode-Locking: A Review

Abstract

Background/Objectives: This article provides comprehensive review on nonlinear mirror mode-locked laser which is compact, efficient, simplest and stable source of picosecond optical pulse and is of immense research interest. Methods/ Analysis: Starting with the basics of nonlinear mirror mode-locking technique, a chronological development of the subject is presented. Issues with the efficiency and stability of the nonlinear mode-locking are addressed in both theoretical and experimental perspective. Different schemes of nonlinear mirror mode-locking, reported till date, are illustrated and compared in reference to the obtained pulse durations, power scaling, self starting and self sustained efficient modelocking. Findings: Nonlinear mirror, comprising a second harmonic generating crystal and a dichroic output coupler, can mode-lock a laser due to its behaviour analogous to fast saturable absorber. Efficiency of the laser is increased with the diode pumping; however nonlinear mirror is observed prone to passive Q-switching instability. Simultaneously Q-switched and mode-locked operation gives a simple way of increasing the peak power but is of no use because of rapid fluctuations of pulse amplitude and repetition rate. Incorporation of an accousto-optic Q-switch in the laser cavity helps run the laser in actively Q-switched and passively mode-locked regime which provides enormous stability of the Q-switched and modelocked pulse envelops and the enhanced peak power becomes useful. Pure contentious wave mode-locking is obtained by incorporation of additional intensity dependent loss mechanism by way of introducing third harmonic generation in the laser cavity and it gives an inverse saturable nonlinear loss modulation. Inverse saturable nonlinear mirror produces efficient, self sustained and stable contentious wave mode-locked picosecond optical pulse train. Application /Improvement: Nonlinear mirror mode-locked lasers have wide application in micromachining, optical frequency conversion, pumping optical parametric oscillators etc. Propositions are made for getting improved nonlinear mirror mode-locking to get bandwidth limited pulse width.