Infrared dye lasers for the wavelength range 1–2 μm

Abstract

Dye lasers represent a standard source of coherent light in the visible spectral range, where the spectral bandwidth, the temporal characteristics of the laser emission and the power can be varied over many orders of magnitude. Laser dyes emitting in the visible frequently exhibit fluorescence quantum yields of 0.5 to 1 with lifetimes of the fluorescing $1 state in the nanosecond regime I-7.1-1. The excited fluorescent state is predominantly deactivated by radiative transitions to the ground state making dyes a favorable active material for a variety of laser systems. Laser action of dye molecules in the infrared wavelength region between 1 and 2 lam is more difficult to achieve. Compounds with a So-S1 transition in this range show very small fluorescence quantum yields of 10 -4 to 10 -5 , corresponding to lifetimes of the fluorescent state of several picoseconds [7.2-5]. Fast radiationless internal conversion to the S O state represents the dominating mechanism of depopulation of the S 1 state. Due to the rapid ground state recovery, infrared dyes have found application as saturable absorbers in modelocked and Q-switched solid-state lasers. In infrared dye lasers, highly judicious and efficient pumping schemes are necessary to obtain strong stimulated emission, in spite of the high radiationless transition rates. In recent years, different dye laser systems emitting intense pulses beyond 1 I.tm have been realized. In particular, picoand subpicosecond pulses tunable over a broad range have been generated with dye lasers pumped by short pulses from mode-locked Nd lasers. In this case, the duration of the pump pulses is close to the lifetime of the emitting S~ state of the molecules, resulting in a stable laser output of high power. In this paper, we present a review on infrared dye lasers working in the wavelength range from 1 to 2 lam. We first discuss briefly the photophysical properties of infrared dyes. The main part of the article is devoted to laser systems generating short infrared pulses of a duration from several hundreds of femtoseconds up to nanoseconds. Different laser configurations will be presented with emphasis on the production of ultrashort light pulses. Finally, nonlinear frequency conversion of the dye laser output to longer wavelengths in the midinfrared is reported.