The fundamental description of the absorption of light by a gas through the Beer-Lambert law is introduced with the definitions given of the important parameters, such as line-strength, absorption cross-section and absorption coefficient. Broadening of gas absorption lines from Doppler effects and molecular collisions is explained in detail and the consequent absorption line-shape functions are presented in the form of Gaussian, Lorentzian or Voigt profiles. The extraction of information on the gas concentration, pressure or temperature from a measured line-shape is discussed, along with the practical issues and limitations. The origin and nature of the absorption lines arising from the excitation of rotational and vibrational states of gas molecules is reviewed with a particular interest in the overtone lines in the near-IR region. Examples of near-IR absorption lines from the HITRAN database for carbon monoxide, carbon dioxide, acetylene, methane, water, ammonia and hydrogen sulfide are presented so that the optical attenuation may be calculated in the design of a practical gas sensor system.

The state-of-the-art of mid-IR laser absorption spectroscopy is reviewed to take advantage of the stronger absorption lines. The properties of mid-IR diode lasers are discussed, including quantum well, inter-band cascade and quantum cascade lasers for gas sensing at wavelengths beyond two microns. As an alternative to diode lasers, mid-IR laser sources based on down-conversion from the near-IR are reviewed using either difference frequency generation or optical parametric oscillation and examples are given of their design as tuneable mid-IR CW sources or as mid-IR frequency combs. Examples of compact mid-IR laser combs formed from micro-resonators in silicon are also discussed. The important spectroscopic techniques of wavelength modulation spectroscopy, cavity-enhanced, evanescent-wave and dual-comb spectroscopy are all discussed in the context of the mid-IR with examples of the performance that can be attained. The performance and limitations of the most common mid-IR transmitting fibres and mid-IR detectors are also reviewed. Finally a comparison is given of the relative merits of gas absorption spectroscopy in the near-IR and mid-IR and where each has an important role to play.

Applications of near-IR fibre amplifiers and fibre lasers in gas spectroscopy are reviewed. Examples are given where fibre amplifiers may be employed to boost the optical power, for example, in photoacoustic spectroscopy or when splitting a single laser output over multiple fibre optic paths in tomographic imaging. The use of mode-locked fibre lasers for the generation of high-performance frequency combs is discussed and examples given of the state-of-the-art in compact, field-deployable erbium fibre laser combs. The method of dual comb spectroscopy is explained and illustrated with applications in the monitoring of atmospheric trace gases, pollution and exhaust emissions. Several techniques are considered for enhancing sensitivity by means of a high-finesse fibre laser cavity, such as by fibre ring-down spectroscopy or through use of the amplified spontaneous emission present within the laser cavity. Intra-cavity laser absorption spectroscopy, where the fibre laser’s spectral distribution is monitored during the transient period, is discussed in detail with examples given of its potential application for the simultaneous measurement of several gas species in various environments.

The fundamental principles which govern the operation and define the characteristics of rare earth-doped fibre amplifiers and lasers are discussed in detail.The important role of phonon interactions with the Stark energy levels of the 4f electron orbitals is explained and the McCumber relationship for the absorption and emission cross-sections is derived. Atomic and cavity rate equations for fibre amplifiers and lasers are derived from first principles, including the contributions from spontaneous and amplified spontaneous emission. The rate equations are used to model fibre lasers under the various conditions of operation that relate to possible applications in near-IR gas spectroscopy, such as for tuneable or multi-wavelength sources, frequency combs and intra-cavity laser absorption spectroscopy.Examples are given of the theoretical laser output when operating under steady-state, multi-wavelength, transient or mode-locked regimes.The principles of stimulated Raman scattering are also discussed for accessing near-IR absorption lines at longer wavelengths by extending, through the Stokes shift, the available wavelength range of operation with fibre amplifiers or lasers.

The principles of photoacoustic spectroscopy and the acoustic wave equation are introduced for describing the acoustic waves generated from a modulated heat source. Acoustic resonant cells for signal enhancement are considered in detail with a full mathematical description of the resonant modes. Analytical expressions are derived for the amplitude of the acoustic modes generated by excitation of a gas with a modulated DFB laser, describing the coupling of the harmonics from the wavelength and intensity modulation to the acoustic modes.Conditions for the selective excitation of longitudinal, azimuthal and radial modes by the laser beam are explained in relation to the overlap factor between the acoustic mode profile and the beam profile.Expressions are given for the Q-factor of the cell and how cell dimensions may be chosen to optimise the performance. Calibration and sensitivity issues are discussed with examples given of typical photoacoustic cells in bulk or miniaturised form and the expected signal output at the microphone. The technique of quartz-enhanced photoacoustic spectroscopy (QEPAS) is also briefly reviewed as an alternative to the use of photoacoustic cells.

The applications of near-IR spectroscopy with DFB lasers and fibre optic networks are reviewed. Since near-IR absorption lines are relatively weak, techniques to enhance the sensitivity are reviewed, including the use of multi-pass cells, ring-down spectroscopy and the various forms of cavity-enhanced spectroscopy. Examples of non-enhanced gas cellsconsidered include micro-optic cells for integration with optical fibre networks, evanescent-wave cells on silicon chips, and open-path free space propagation for atmospheric monitoring based on collection of scattered light or from a retro-reflector. The design of fibre optic sensor networks is discussed in detail particularly the use of spatial-division multiplexing for multi-point detection of gases over large areas. Throughout the chapter, a number of application areas are considered with examples given of near-IR systems employed for the detection of gas leaks from pipelines and storage facilities, characterisation of combustion processes, tomographic imaging of carbon dioxide in aero-engine exhaust emissions, imaging of hydrocarbons within internal combustion engines and atmospheric sensing of water vapour and greenhouse gases.

Based on Fourier analysis, a theoretical description is given of the harmonics arising from current modulation of a DFB laser with its wavelength scanned through a gas absorption line. It is shown that each harmonic consists of a primary component from the wavelength modulation and two secondary components arising from the mixing of the intensity and wavelength modulations, with additional components if the laser light-current characteristic is non-linear. The importance of the lock-in detection phase is discussed and the need for calibration-free, consistent operation in the face of possible drift of laser parameters with time or with aging. Two methods are examined for extraction of gas parameters, one based on the effect of gas absorption on the laser intensity modulation, with correction factors applied at high modulation indices, and the other based on measurement of the second harmonic signal normalised through the first harmonic. It is shown that both methods can give similar sensitivities, but the harmonic ratio method is much superior in noise performance at the expense of increased complexity in signal processing and uncertainty if the laser parameters are prone to drift.

A detailed account of the electronic, thermal and optical properties of DFB lasers is presented relevant to the requirements of wavelength modulation spectroscopy. An RC thermal model is first used to provide a simple analytical description of the thermal scanning and wavelength tuning properties of the laser and then a one-dimensional solution of the heat conduction equation is derived for a better description of thermal tuning. Perturbationanalysis of the laser rate equations is used to determine the effect of current modulation on the carrier, intensity and wavelength modulation, taking into account both spectral and spatial hole-burning effects. Theoretical relationships are derived for the overall tuning coefficient, combining both thermal and carrier contributions, with expressions given for its magnitude and phase as a function of the modulation frequency relative to the intensity modulation. This is important for the interpretation of the harmonic components that arise with wavelength modulation spectroscopy. It is shown how a fibre-optic ring resonator may be employed for the experimental determination of the tuning coefficient and for calibration of wavelength scans.