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.