This thesis describes the use of short external cavity (SXC) semiconductor diode lasers as single longitudinal mode (SM) tunable sources. A SXC forces a multimode diode laser to lase on a single longitudinal mode. Various laser types were investigated in SXC configurations using both planar and spherical external mirrors. The side mode suppression ratio (SMSR) and the SM tuning range were measured with respect to the positioning of the external cavity element. With a planar mirror as the SXC element, SMSR of —33 dB and SM tuning ranges of 1 nm (110% of a mode spacing) were obtained with inverted rib waveguide (IRW) lasers. For external cavity lengths of ~ 60 um the total continuous SM tuning range summed over all modes was found to be 72 cm^-1 or 12 nm. The use of a spherical mirror improved the results. A SXC laser consisting of a spherical mirror and an IRW laser had SMSR values of —37 dB and SM tuning ranges of 1.10 nm. Power and voltage characteristics of SM SXC lasers were also examined. It was found possible to use the laser voltage and electronic feedback to control the external cavity length for optimum SM output. The external differential quantum efficiency (DQE) was found to be wavelength dependent and may be explained by the wavelength dependence of the scattering/absorption loss. One aspect of the characteristic trend of the DQE with respect to wavelength is that it offers the possibility of determining the lasing wavelength of the SM without the use of a monochromator. / Thesis / Master of Engineering (ME)

Semiconductor diode lasers are successful tools in atomic spectroscopy. They are routinely used in frequency conversion applications to develop devices that access regions of the spectrum not directly available. This thesis describes the practical application of novel violet diode laser systems and their possible inclusion in spectroscopic systems. The design, assembly and successful operation of a doubly resonant optical parametric oscillator is described. There is discussion of the spectral behaviour of the device and the potential for pumping with a violet diode laser. Methods to adapt the output from the solitary diode devices are demonstrated with the use of microlensed diode lasers and extended cavity configurations. Details of the current tuning, linewidth and smooth tuning characteristics of a number of the lasers used are given. A commercial violet diode laser is used within an extended cavity to measure the hyperfine structure of atomic indium from a hollow cathode galvatron source at room temperature. Stabilisation of the diode laser to a line from the indium spectrum is attempted. The remainder of the thesis is concerned with the development of techniques to deliver clearer and more precise spectral information about trace species. Microlensed red and violet diode lasers are used to generate light at 254nm via sum frequency generation for the direct detection and modulation spectroscopy of mercury vapour, with microlensed lasers with modulation allowing more accurate discrimination between spectral features than direct absorption measurements. In addition Raman tweezers modulation spectroscopy is undertaken to investigate polymer microspheres and biological cell samples where the use of the modulation technique demonstrated improvements in the acquisition time and clarity of spectra through increased signal to noise and rejection of background fluorescence effects. A comparison between the direct and modulation techniques for all the systems indicates the greater sensitivity of the modulation technique.