Improved 2^nd harmonic rise-time in a thermal-lens-dominated extended-cavity micro-laser

Abstract

Many commercially available compact, micro-cavity diode-pumped solid-state lasers achieve a stable resonator condition following the formation of an intra-cavity thermal lens. Typically, this thermal lens forms as a result of localized thermal expansion in the host material due to the very high optical power of the infrared pump beam. Additionally, most common DPSS host media, such as Nd:YAG and Nd:YVO4 exhibit high dn/dT, change in refractive index with temperature. Thermal lens formation time dominates stable resonator establishment and, thus, the rise-time of the intra-cavity-generated visible secondharmonic output light. Thermal lens formation time may range from 10's of microseconds to several milliseconds, depending on pump power, the host material dn/dT, and the volume and length of the host material. This paper describes a unique approach that improves the second-harmonic risetime -and, thus, the modulation bandwidth -for high-power VECSELs by several orders of magnitude. This approach, while typically used in some laser systems for other purposes, allows direct modulation of high-power VECSELs to greater than 100 megahertz with second-harmonic rise-time of a few nanoseconds. IntroductionHand-held, mobile projectors for the display of video content were first introduced as consumer products two years ago and have since received much attention as the next breakthrough mobile consumer device. These compact devices incorporate either High-Brightness LEDs or lowpower visible lasers as light sources. One approach to the design of the hand-held projector uses single, or dual, MEMS-based micromirror scanners to create a raster video image -similar to that of the common CRT display [1]. While producing a displayed image comparable in size and brightness to a notebook computer, the projector's micromirror scanner uses a silicon mirror about 1 millimeter in diameter. Thus, the optical system pupil is small and requires a very small source size. Laser sources are ideal for this application.Semiconductor, direct-emitting, directly-modulated lasers are readily available in Red (~630nm) and Blue (~460nm) wavelengths for video displays. However, the equivalent Green (~530nm) semiconductor lasers remain far from common availability. Laser developers have concentrated on 2 nd -harmonic Green lasers based on direct 2 nd -harmonic generation of semiconductor, or solid-state lasers. These 2 ndharmonic designs are far more complex than direct-emitting semiconductor lasers and typically cannot achieve the high modulation rates required for real-time video displays. Although, they usually produce better beam quality than their semiconductor counterparts.