Optical chemical sensors have been the focus of much research attention in recent years because of their importance in industrial, environmental and biomedical applications [1]. This class of sensors combines chemical and biological recognition with advances in optoelectronic technologies. The application of solgel materials to these sensors, especially in the form of thin films, has attracted considerable interest due to the ease of fabrication and design flexibility of the process. The nature of the sol-gel process lends itself very well to the deposition of thin films using a variety of techniques such as dip-coating, spin-coating and spraying. In many sensor applications, the sol-gel film is used to provide a micro-porous support matrix in which analyte-sensitive molecules are entrapped and into which smaller analyte species may diffuse and interact [2,3]. Sol-gel films have many advantages as support matrices over polymer supports, including, for example, strong adhesion, good mechanical strength as well as excellent optical transparency. The versatility of the process facilitates tailoring of the physico-chemical film properties to optimize sensor performance. For example, films can be designed which have optimum porosity while minimizing leaching of the indicator molecules. In this chapter, the versatility of the sol-gel process with regard to the design of films for specific optical chemical sensor applications is highlighted.

Optical chemical sensors.

Near- and mid-infrared laser-optical sensors for gas analysis

A review of recent advances in semiconductor laser based gas monitors

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.

Mid-Infrared Laser Applications in Spectroscopy

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

Antimonide semiconductor laser devices designed for continuous-wave
emission in the 3–4-µm spectral range have been investigated
with respect to spectroscopic applications. Representative data on
the mode structure, output power, noise characteristics, far-field
pattern, and modulation response are presented. Selected laser
devices have been applied for methane (CH4) and
formaldehyde (HCHO) measurements by use of a high-frequency
modulated diode laser spectrometer. From an Allan variance analysis
of experimental data a detection limit for HCHO of 120 pptv (where 1
pptv = 10-12 volume mixing ratio) with a 40-s
integration time and for CH4 of 2 ppbv (where 1 ppbv
= 10-9 volume mixing ratio) with 20-s integration time
were determined. The results show that, for selected gases, InAsSb
lasers can be an alternative to lead-salt diode lasers.

Application of antimonide lasers for gas sensing in the 3–4-µm range

Spectroscopic gas analyzers based on indium-phosphide, antimonide and lead-salt diode-lasers

Single-frequency InAsSb lasers emitting at 3.4 μm

Relaxation oscillations have been investigated in A3B5 narrow band-gap semiconductor lasers. Based on wideband intensity noise measurements, the relaxation oscillation frequency has been observed up to 2 GHz for a 2 mW cw single-frequency InAsSb laser at a 3.4 μm wavelength. Laser parameters that influence the bandwidth, including the photon lifetime τp, the differential gain A and the spontaneous recombination lifetime τs were calculated from experimental data obtained at 95 K. We found τp in the range 0.7–1.9 ps, A was estimated to be (4.7–7.3)×10−6 cm−3 s−1 and τs=2.5–6.9 ns. Relaxation oscillations occurred beyond the modulation bandwidth required for typical applications of antimonide lasers in spectroscopy.

Relaxation oscillations in single-frequency inassb narrow band-gap lasers

Tuning and spectral characteristics of 3 micrometers wavelength InAsSb lasers were considered in viewpoint of requirements of tunable diode laser absorption spectroscopy. The spectra of lasing, far-field patterns, long term stability, current and temperature tuning performance of CW lasers which are operated at 3.4-3.6 micrometers and designed for formaldehyde detection were investigated versus various ambient conditions. A specific characteristic for III-V compound lasers was observed: InAsSb lasers are tuned by direct current to the short wavelength side whereas mode jumps occurred towards longer wavelengths. The lasers showed a good current and temperature tuning. The tuning rate is determined by competition of the electronic and thermal mechanisms of tunability. The thermal shift of the band gap of the active area leads to an average mode tuning rate of about 40 GHz/K. The tuning characteristics were compared with mode jump values to calculate the whole spectral region covered by tunable laser emission. A good stability of the emitted wavelength during a long time period of operating and a lot of cooling-heating cycles could not be found, which can be explained by the higher stability of the III-V compound compared to lead-salts. The lasers were suitable for detection of ambient formaldehyde concentration level of 200 pptv at 1 Hz electronic bandwidth.

Andrei A. Popov

Papers

Application of antimonide lasers for gas sensing in the 3–4-µm range

Spectroscopic gas analyzers based on indium-phosphide, antimonide and lead-salt diode-lasers

Single-frequency InAsSb lasers emitting at 3.4 μm

Relaxation oscillations in single-frequency inassb narrow band-gap lasers

Tuning and spectral performance of mid-infrared InAsSb lasers