Mid-Infrared Laser Applications in Spectroscopy

The vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in the mid-infrared spectral region (≈ 2–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection capability for trace gases. The focus time period of this chapter is the years 1996–2002 and we will discuss primarily CW mid-infrared laser spectroscopy. We shall not attempt to review the large number of diverse mid-infrared spectroscopic laser applications published to date. The scope of this chapter is rather to discuss recent developments of mid-infrared laser sources, with emphasis on established and new spectroscopic techniques and their applications for sensitive, selective, and quantitative trace gas detection. For example, laboratory based spectroscopic studies and chemical kinetics, which will also benefit from new laser source and technique developments, will not be considered. 1 Solid-State Mid-IR Spectroscopic Laser Sources Mid-infrared laser sources come in many different varieties. For their application to spectroscopic measurement, and specifically for quantitative measurements, the ideal source would have the following properties: (1) sufficient optical power to overcome inherent electronic detection noise and ensure high laser signal-to-noise ratios, (2) narrow linewidth to obtain high selectivity and sensitivity, (3) single longitudinal mode operation with low amplified spontaneous emission output for high selectivity and elimination of intermode competition noise; (4) ease of tailoring the inherent laser operating wavelength (design of gain material and/or cavity structure) to access the desired absorption region; (5) low source noise and low amplitude modulation; (6) high beam quality, i.e., small beam divergence, small astigmatism, and stable, predictable beam output direction, for optimum coupling into and through a gas sampling cell; (7) low temperature and current tuning rates to minimize wavelength jitter induced by controller noise; (8) rapid wavelength tunability for fast response and high data acquisition rates; (9) minimal susceptibility to changing environmental conditions of temperature, pressure, humidity, I. T. Sorokina, K.L. Vodopyanov (Eds.): Solid-State Mid-Infrared Laser Sources, Topics Appl. Phys. 89, 445–516 (2003) c © Springer-Verlag Berlin Heidelberg 2003 446 Frank K. Tittel et al. and vibrations; (10) no long term changes in laser wavelength and/or spatial output characteristics; (11) highly reliable performance for many years; and (12) compact and robust overall sensor package size. Of course it is challenging to realize all these idealized attributes in any one real world mid-infrared laser source, and they are listed here to serve as a general guideline. However, some of the attributes are more important than others for a given application to obtain the best possible measurement performance. In the sections that follow, we will elucidate wherever possible those important attributes necessary for achieving optimal performance employing the technique being discussed. It is common practice in infrared spectroscopy to express transition frequencies in inverse centimeters (cm−1), or wavenumbers, defined simply as the inverse of the transition wavelength in vacuum, ν = λ−1. Multiplying this quantity by c gives the frequency in hertz; thus 1 cm−1 is roughly 30 GHz. We shall use both units throughout this chapter, where appropriate. The spectral coverage of the most frequently employed tunable CW midinfrared sources is shown in Fig. 1. In general, one can distinguish between two classes of mid-IR laser sources. Class “A” includes sources which generate tunable mid-IR laser radiation directly from gain in gas discharge, semiconductors, rare-earth and transition-metal doped solid-state bulk materials or optical fibers. Class “B” laser sources are based on nonlinear optical parametric frequency conversion of near-infrared (≈ 0.9 to 2 μm) laser sources. 0.1 1 10 Antimonide III-V 8 9 7 6 5 4 3 2 Semiconductor Direct Solid-State Gas (Line Tunable) Frequency Conversion Based Sources Nd 3+ CO Laser Rotation-Vibrational OPSL Solid State and Fiber Laser / Amplifier DFB Diode Laser Yb 3+ Pr 3+ Er 3+ Tm 3+ / Ho 3+ Overtone Region II I Wavelength ( m) QC-Laser