JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 10, MAY 15, 2010 1435 Detection of CH in the Mid-IR Using Difference Frequency Generation With Tunable Diode Laser Spectroscopy Ian Armstrong, Walter Johnstone, Kevin Duffin, Michael Lengden, Arup Lal Chakraborty, and Keith Ruxton Abstract—This paper demonstrates detection of methane using tunable diode laser spectroscopy (TDLS) through difference fre- quency generation (DFG) in order to address fundamental rota- tional-vibrational absorption lines, located around 3404 nm. Di- rect detection confirms that wavelength referencing of recovered lineshapes, developed for Near infra-red (Near-IR) systems, has been successfully transferred to the presented Middle infra-red (Mid-IR) system. Traditional 1f and 2 f TDLS with WMS detection regimes are also functionally confirmed analogous to their Near-IR equivalents. Index Terms—Difference frequency generation, nonlinear optics, spectral analysis, tunable diode laser spectroscopy, wave- length measurement. I. INTRODUCTION T HE use of tunable diode laser spectroscopy (TDLS) for gas detection has been well documented [1]–[6] for sev- eral species of gas and has had considerable success, with de- ployment of systems for industrial applications. Most commer- cially viable field systems are based in the Near-IR where ef- ficient, high power DFB lasers and optical fibres are readily available due to the development of such components for op- tical communications networks. These near infra-red (Near-IR) gas sensing systems address overtone absorption lines of their target species which are generally much weaker than the funda- mental lines located in the middle infrared (Mid-IR). In order to increase sensitivities, TDLS systems that address the funda- mental absorption lines of a gas species can provide orders of magnitude improvement over the Near-IR overtone absorption equivalent. However, traditional laser sources in the Mid-IR are not as evolved as their Near-IR counterparts, producing lower powers and requiring more maintenance to provide stable single mode operation, with lead salt lasers also requiring cryogenic cooling to operate [6]. In order to avoid laser instability issues with lead salt lasers, attention has been paid to alternative technologies [7]–[9], including nonlinear wave-mixing techniques [10]–[12], Manuscript received October 30, 2009; revised January 28, 2010; accepted February 02, 2010. First published February 17, 2010; current version published April 30, 2010. The authors are with the University of Strathclyde, Centre for Microsystems and Photonics, Department of Electronic and Electrical Engineering, Glasgow G1 1XW, U.K. (e-mail: iarmstrong@eee.strath.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2010.2042789 to produce light in the region of interest. In these systems, com- mercial Near-IR lasers can be purchased “off-the-shelf” to act as the mixing sources, providing simple modulation schemes for the system at the expense of requiring more system components. In particular, difference frequency generation (DFG) systems using periodically poled lithium niobate (PPLN) as the non- linear mixing medium have been used to detect various species [13], [14]. This paper details the detection of methane (CH ) using the DFG process in PPLN to address absorption lines in the 3.4 m region while employing tunable diode laser spec- troscopy with wavelength modulation spectroscopy (WMS). Section II of this paper will introduce the basic principle of difference frequency generation and detail the implemen- tation of the experimental system used to carry out this work. Section III provides characterisation information, defining the range of wavelengths that the system is capable of addressing and the spectral bandwidth over which Mid-IR generation is possible for a single PPLN temperature setting. Parameters and methods central to the optimization of the presented TDLS detection schemes are also defined. Section IV presents the application of three TDLS modulation schemes (direct, 1f and 2 f detection) to the DFG system and their respective detection limits. The salient points of the paper are summarized in the final section. II. SYSTEM OVERVIEW A. Mixing Concept Difference frequency generation is a wave mixing process whereby energy is transferred between three interacting beams; the Pump (highest frequency, shortest wavelength, ), the Signal (intermediate frequency, wavelength, ) and the Idler (lowest frequency, longest wavelength, ). The mixing process is based upon the operator of the non- linear medium’s susceptibility tensor [15] which generates a second order nonlinear polarization state. This polariza- tion facilitates the transfer of energy between the interacting beams and gives rise to second harmonic , sum frequency and difference frequency components. Wave mixing can be carried out in two general schemes; birefringent phase matching (BPM) and quasi-phase matching (QPM). BPM requires the input beams to be orthogonally polarised and incident upon the nonlinear medium at spe- cific angles with respect to the crystalline structure, requiring 0733-8724/$26.00 © 2010 IEEE Authorized licensed use limited to: STRATHCLYDE UNIVERSITY LIBRARY. Downloaded on May 10,2010 at 15:53:28 UTC from IEEE Xplore. Restrictions apply.