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or superluminescent diodes, can be used but have limited spectral resolution [2][3][4]. Methods based on correlation spectroscopy, COSPEC, provide a convenient method of detecting multiple gases but require a reference cell, at the same temperature and pressure as the target sample, for each species being detected [5,6]. The COSPEC technique, employing multi-mode lasers, has also been used for simultaneous detection of CO and CO 2 , thus demonstrating multi-species detection capability [7].The emerging spectroscopic techniques based on femtosecond frequency combs utilize the very broad spectrum of the light emitted by mode-locked lasers as the source for absorption spectroscopy. The modes are densely packed in frequency space and provide ultra-stable and precisely determined frequency markers [8]. The basic technique requires highly dispersive instruments to detect the absorption features of each species. Thus, spectral coverage or spectral resolution is compromised, and the systems are often complex, expensive, and unsuited to field applications. The fs-combs usually span the visible and near-infrared regions but, mid-infrared combs have been generated using difference frequency generating schemes or directly using quantum cascade lasers with parametric coupling by four-wave mixing processes in the gain media [9-12]. Alternatively, the multiple modes of Fabry-Perot-type QCL cavities can be used, and by use of heterodyne methods, the spectral information can be effectively studied in the radio frequency, RF, region [13]. Most manifestations so far reported used fixed frequency modes, and so resolution is constrained by the inter-mode frequency interval of the modes in the mid-infrared. The overall bandwidth will be limited by the bandwidth of the RF detectors. Scanning of the mid-infrared modes requires that the phase stability of the laser and the local oscillator laser be maintained during the scanning. Spectroscopic applications of scanned Abstract Multi-mode absorption spectroscopy of ammonia and methane at 3.3 μm has been demonstrated using a source of multi-mode mid-infrared radiation based on difference frequency generation. Multi-mode radiation at 1.56 μm from a diode-pumped Er:Yb:glass laser was mixed with a single-mode Nd:YAG laser at 1.06 μm in a periodically poled lithium niobate crystal to produce multimode radiation in the region of 3.3 μm. Detection, by direct multi-mode absorption, of NH 3 and CH 4 is reported for each species individually and also simultaneously in mixtures allowing measurements of partial pressures of each species.

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