Diode-laser absorption spectroscopy - applications in medicine and environmental monitoring

Abstract: A major part of this thesis work has been research in a medical project, where, together with industry and clinical partners, the aim was to develop a prototype system for non-invasive monitoring of premature babies based on the optical measurement technique gas in scattering media absorption spectroscopy (GASMAS). Part of the project was to develop a 3D-printed tissue phantom based on chest X-ray images of the thorax of a premature infant. Experiments were conducted on the phantom, where the oxygen and water vapor content in the lungs were evaluated for different light source- and detector positions. Furthermore, delivery of light with a diffusor attached to the surface of the model was compared with internal light delivery through an optical fiber. A pre-clinical study was performed on anesthetized newborn piglets with different inspired oxygen concentrations and induced lung complications using both dermal and internal light illumination. The measurements with internal illumination were performed by delivering the light through an optical fiber with a diffuse end, and the optical fiber positioned in the esophagus of the piglet. The inspired oxygen concentration was stepwise increased and decreased while measuring the oxygen and water vapor content using two diode lasers emitting at a wavelength of 764 nm and 820 nm, respectively. From the absorption at the two wavelengths an oxygen concentration could be evaluated. When investigating the oxygen content during induced lung complications, i.e., pneumothorax and atelectasis, different responses of the absorption and transmission were identified for the two cases. The project was concluded by a clinical study on healthy newborn infants, where the light was delivered through a diffuser placed in the armpit and the detector was positioned on the thorax of the infant. The initial results are promising; it was possible to detect oxygen absorption signals. However, the study also shows that the system needs further improvements in order to deliver strong enough signals for reliable oxygen diagnostics.In this thesis, work is also presented on atmospheric monitoring of CO2 using differential absorption Lidar (DIAL) based on a tunable, continuous-wave (CW) diode laser with an emission wavelength at 1573 nm. The diode laser was coupled to a fiber amplifier to achieve output powers about 1.3 W. The backscattered light was range resolved by arranging the laser beam, collection lens and a line array detector in Scheimpflug geometry, where focus is achieved along the laser beam. This technique is known as Scheimpflug Lidar. From the recorded DIAL curves, CO2 concentrations of around 400 ppm were evaluated. In addition, aerosol releases in the beam path were studied by observing backscattered light from two diode lasers, one emitting in the near-infrared (980 nm) and the other one in the short-wave infrared region (1550 nm).