Mid-IR solid-state lasers for spectroscopy and metrology applications

Abstract

Summary form only given. Wide bandwidth coherent emission in the mid-infrared (mid-IR) region (2-10 μm) is of great interest in different fields, such as high-resolution molecular spectroscopy, frequency metrology, and remote sensing of environmental gases and pollutants in the atmosphere. In fact, in this spectral interval, the so called “fingerprint” region, inorganic and organic molecules show the strongest absorption features associated with fundamental ro-vibrational transitions. Among the different generation schemes, such as semiconductor lasers (quantum cascade lasers and lead salt compounds or gallium-antimonide) and nonlinear optical techniques (namely difference frequency and optical parametric generation), impurity doped crystalline lasers constitute another viable route for mid-IR generation [1]. In the last years, particular attention has been devoted to the development of Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> :ZnSe lasers due to excellent lasing and optical properties of this material, which represents the mid-IR analog of Ti:sapphire, and different operating regimes have been subsequently demonstrated [2].Here we report on two different and complementary approaches for the generation of highly-coherent radiation in the wavelength range from 2.1 to 2.6 μm based on the use of room-temperature Cr2+:ZnSe gain medium. In the first implementation, we demonstrate single-frequency Cr2+:ZnSe laser widely tunable in the spectral region from 2.12 to 2.58 μm. Using a compact unidirectional ring cavity configuration (see Fig. 1a), a maximum output power of 160 mW with an emission linewidth of ~100 kHz in 1-ms observation time is obtained. The low frequency and intensity noises allows the use of this oscillator in high-resolution and highprecision molecular spectroscopy in the mid-IR spectral. The second example consists of a frequency comb source with a maximum average output power up to 1.2 W, tunable in the 2.2-2.6 μm wavelength region, based on a Cr2+:ZnSe multipass solid-state amplifier seeded by the output of an actively-stabilized optical parametric oscillator, synchronously pumped by a commercial 250-MHz Er:fiber laser. The amplifier, based on an Er-fiberpumped Cr2+:ZnSe crystal in a 3-pass configuration (see Fig. 1b), boosts the output power per comb-mode up to 150 μW. Phase relationship between idler, signal, and pump waves is exploited to perform frequency comb stabilization in the whole 2.2-2.6 μm mid-infrared spectral region. This phase-locking, together with the available power level, makes the system an ideal tool for direct comb spectroscopy in the spectral region from 2 to 3 μm.