The work presented in this thesis is concerned with development of spectroscopic detection methods based on absorption spectroscopy using semiconductor lasers, with particular ref- erence to the field of medical diagnostics through breath analysis. The first part of this thesis deals with the design and testing of a prototype analyser for simultaneous monitoring of the exchange gases 02, C02 and H20 in breath. The aim of this analyser is to provide information required to monitor respiration, with potential use in intensive care monitoring or during anaesthesia. The relatively high concentrations of these gases in breath and read- ily available diode laser sources make detection in the near-infrared (NIR) ideal. However, the relatively weakly absorbing A-band 02 transitions at 760 nm require the application of a sensitive spectroscopic method, cavity enhanced absorption spectroscopy (CEAS). In contrast, CO2 and H20 are monitored using direct single pass absorption spectroscopy, with transitions arising from the 21/1 + 1/3 band at 2 uu: and 1/1 + 1/3 band at 1.3 p,m, re- spectively. It has been demonstrated that these gases can be detected simultaneously over a short pathlength (2.74 - 4 cm) in the respiratory flow by combining various spectroscopic methodologies and real-time data analysis. This analyser is shown to offer a viable alter- native for monitoring respiration, exhibiting absolute detection limits of changes of 0.26 % O2, 0.02 % CO2 and 0.003 % H20 with a 10 ms time resolution, which are comparable to current mass spectrometry based methods, but without their inherent delays. Following this, investigations into the detection of the main gas constituents in breath in the NIR employing noise-reduction modulation based spectroscopic techniques, namely wavelength and frequency modulation (WMS and FMS respectively) are also reported. The described WMS studies on water at 1.37 ust: provide a demonstration of conventional WMS detection, as well as a "proof-of-principle" example of a relatively new approach to calibrat- ing the non-absolute information obtained from a WMS absorption signal. Typically WMS spectra are calibrated using mixtures of known gas concentrations or an absolute direct absorption spectrum where possible. In this work however, a self-calibrating method, the phasor decomposition method (PDM), is employed and the returned concentration from this ~alibration is compared to direct absorption measurement. From this, the calculated concentration using the PDM is found to differ by 9 % from the concentration value ob- tained by direct absorption, providing an alternative method of calibration for when direct absorption measurements are not possible. The use of FMS in the NIR is also demon- strated as a potential alternative to CEAS for monitoring O2 at 760 nm. FMS detection is performed on atmospherically broadened 02 and a time-normalised O!min(t) of 2.45x10-6 cm-1 s~ is obtained, which is two orders of magnitude less sensitive than the value of 1 O!min(t) = 2.35x10-8 cm "! S2 obtained with CEAS. This combined with the experimental I requirements of an FMS system, make its use for detection of 02 a less practicable option compared to CEAS for real-time breath analysis. The latter work in this thesis involves a change in focus to detection of trace gases in breath in the mid-infrared (MlR). The move of spectroscopic detection to the MlR exploits the larger absorption cross-sections available in this region, and to achieve this, a relatively new form of semiconductor laser, the quantum cascade laser (QCL) is used. The design of a continuous wave QCL spectrometer at 8 usx: and its operating characteristics are demon- strated and improvements in its performances are also discussed. This QCL system is then utilised to demonstrate the potential of monitoring species in breath, namely the narrow- band absorber methane and the broadband absorber acetone, taking into consideration the potential interference from other absorbing species in breath and the different spectroscopic characteristics exhibited by these molecules. Finally, the potential to further improve the sensitive detection of trace gases in breath in the MlR is also investigated with studies on the use of CEAS and multipass cells. In this work, the molecule of interest is the biomarker OGS, using transitions of the 2V2 band at 1031 cm-I, that are probed using a 10 usx: QCL. The application of CEAS in the MlR is not as well developed as in the NIR, and the experimental consequences of using optical cavities at these wavelengths, where equipment tends to be more limited, are investigated and sensitivities discussed in the context of other literature. The experimental procedure of optimising a cavity for CEAS using the off-axis alignment method is also studied in detail, as well as the addition ofWMS to further improve the signal quality. An effective absorption pathlength of ,....., 100 m was achieved in the cavity, with a bandwidth reduced O!min(BW) 1 of 1.7x10-7 cm"? HZ-2 using WMS CEAS achieved. With the poorer quality optics and limitations in equipment in the MlR for CEAS experiments, the use of a multipass cell, a 238 m Herriott cell, is also investigated as an alternative to the use of an optical cavity at 10 tui". Detection of OCS using direct absorption and WMS is demonstrated in the 1 Herriott cell, achieving O!min(BW) = 2.03xlO-8 cm "! HZ-2 using WMS. This shows an improvement in sensitivity compared to WMS CEAS, and also shows the potential for future work on biomarker detection, as it approaches the r-- ppb levels required for breath analysis.