Broadly tunable Cr 4+-doped solid-state lasers in the near infrared and visible

Abstract

This manuscript provides a comprehensive review of the characteristics of Cr 4+-doped solid-state lasers which produce broadly tunable coherent radiation in the near-infrared region of the electromagnetic spectrum between 1.13 and 1.63 μm. Cr 4+-doped gain media have several favorable characteristics including broad absorption bands that overlap with the operating wavelengths of several commercial pump lasers, a 4-level energy structure that allows continuous-wave as well as pulsed operation with low threshold pump powers, and the presence of broad amplification bands for the generation of ultrashort optical pulses. In addition, non-linear frequency conversion schemes can be readily employed to construct broadly tunable coherent visible sources based on Cr 4+-doped gain media. These favorable features make Cr 4+-doped lasers potentially important in many applications in optical communications, eye-safe imaging, medicine, and spectroscopy. After a description of the basic physical mechanisms that lead to broad emission in tunable solid-state lasers in general and Cr 4+-doped gain media in particular, the manuscript provides a detailed description of the operational characteristics of Cr 4+-doped lasers, including rate-equation analysis of power performance, role of thermal effects, and different mode-locking techniques for the generation of ultrashort optical pulses. Later sections focus on the work performed in the development and characterization of specific examples of Cr 4+-doped lasers. Particular attention is given to Cr 4+:forsterite and Cr 4+:YAG laser systems due to their superior power performance. Gain-switched, continuous-wave, and mode-locked operations of these lasers are described with a particular emphasis on recent developments. Studies carried out with other Cr 4+-doped lasers such as Cr 4+:Y 2SiO 5, Cr 4+-doped oxyapatite, Cr 4+-doped garnets, and Cr 4+:Ca 2GeO 4 are also described. Finally, use of non-linear conversion schemes in the generation of visible radiation with Cr 4+-doped lasers is discussed. In particular, experiments that have been performed with pulsed and continuous-wave Cr 4+:forsterite lasers to generate broadly tunable radiation in the 565–685 nm range of the electromagnetic spectrum are reviewed.