External cavity operation with complete removal of sub-cavity effects for mode-locking injection lasers

Abstract

To date attempts to mode-lock semiconductor injection lasers have not produced clean transform-limited pulses. It was formerly thought (1) that dispersion was the principal problem preventing good mode-locking, but as pointed out by Ho(2) this effect does not become important in external cavity operated systems for pulse durations greater than 1ps. Within an external cavity operated injection laser system the effect of the sub-cavity formed by the laser facets shows up as deep Fabry-Perot modulation of the lasing spectrum similar to that seen in the amplified spontaneous emission (A.S.E.) spectrum of the isolated diode. While anti-reflection coating does not completely remove this effect polishing the facet at an angle to the lasing direction does. However, modern stripe geometry double heterostructure GaAlAs lasers have cleaved, not polished facets. Angle-polishing by chemical means of the multi-component structure is not possible and mechanical polishing introduces the risk of damage.