Compact mid-infrared trace gas sensor based on difference-frequency generation of two diode lasers in periodically poled LiNbO3

The development and characterization of a compact mid-infrared source for high-resolution spectroscopic detection of trace gases such as methane and water vapor at 3.3 μm in ambient air is reported. This source utilizes difference frequency generation (DFG) in a periodically poled LiNbO3 (PPLN) crystal pumped by two single-frequency diode lasers. A maximum DFG power of 1.6 μW at 3.6 μm was generated with a pump power of 61.4 mW at 832 nm and a signal power of 41.5 mW at 1083 nm incident on a 19-mmlong PPLN crystal, which corresponds to a conversion efficiency of 335 μW W−2 cm−1. PACS: 7.65; 42.60B; 42.80 The motivation for this work has been the need to develop a robust, compact and consumable-free sensor for the sensitive and selective detection of pollutants and environmentally important trace gas species in ambient air. To accomplish this, numerous optical and non-optical techniques have been developed. However, the advent of novel enabling technologies involving room-temperature diode lasers, efficient nonlinear optical materials, optical fibers and non-cryogenic infrared detectors has made it possible to investigate a new device architecture for laser-based DFG gas sensors that is particularly suited for the 3–5 μm spectral region. This approach allows access to considerably stronger fundamental ro-vibrational absorption lines rather than weaker overtone molecular transitions for convenient gas monitoring by means of direct absorption spectroscopy. The design issues of an infrared spectrometer based on DFG were considered previously in [1, 2]. Several groups have demonstrated DFG by mixing two laser diodes in AgGaS2 [3–5], which resulted in IR power levels of tens of nanowatts. The recent commercial availability of periodically poled LiNbO3 (PPLN) allows the use of a high nonlinear coefficient d33 = 24 pm/V [6, 7] and convenient DFG quasi-phase-matching (QPM) of diode-laser pump sources. 1 Experimental details and sensor optimization In a second-order nonlinear DFG process the two incident waves, customarily called pump (ωp) and signal (ωs), are frequency down-converted to generate the idler beam (ωi) in accordance to the relation ωi = ωp−ωs. The schematic layout of the DFG-based sensor is shown in Fig. 1. A distributed Bragg reflector (DBR) type diode laser (P = 50 mW) and Fabry–Pérot (FP) GaAlAs diode laser (P = 100 mW) are collimated and spatially overlapped via a dichroic beamsplitter. The center wavelength of the DBR diode laser (signal) is at 1083 nm whereas FP diode lasers (pump) are available with center wavelengths ranging from 810 nm to 865 nm and a typical gross tuning range of ±3 nm [8, 9]. This leads to an overall DFG spectral coverage from 3.2 μm to 4.3 μm, which corresponds to a region of optimum optical transmission of PPLN [10]. After passing through an anamorphic prism pair (3×), the beams are focused by a plano-convex lens of 38 mm focal length into a 19-mm-long PPLN crystal. The DFG beam Ge Filter HgCdTe Detector Parabolic Mirror Multipass Cell Optical Path Length 18 m