Compact tunable diode laser with diffraction limited 1000 mW in Littman/Metcalf configuration for cavity ring down spectroscopy

Abstract

High resolution spectroscopy of environmental and medical gases requires reliable, fast tunable laser light sources in the mid-infrared (MIR) wavelength regime between 3 and 5 &#956;m. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources. We report a new, very compact, alignment insensitive, robust, external cavity diode laser system in Littman/Metcalf configuration with an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The center wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally single mode with a mode-hop free tuning range of up to 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept can be realized within the wavelength regime between 750 and 1060 nm. We investigated this light source for high resolution spectroscopy in the field of Cavity Ring-Down Spectroscopy (CRDS). Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes difference-frequency generation in Periodically Poled Lithium Niobate (PPLN) crystals. At the wavelength of 3.3 &#956;m we were able to achieve a high-resolution absorption spectrum of water with four resolved isotopic H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements.