In this thesis, strong optical feedback is utilized to realize broad-band wavelength tuning and to stabilize the frequency of a semiconductor diode laser in a grating-external-cavity (GEC) configuration. To reach the regime of strong optical feedback, the laser facet through which the feedback occurs has to be antireflection (AR) coated. Multi-layer AR coatings were designed using SiO2, Si3N4, SiOxNy, and a:Si for specific laser waveguide structures, and were fabricated by an electron cyclotron resonance, plasma enhanced, chemical vapor deposition (ECR-PECVD) system. The film thickness and refractive index were monitored by in situ ellipsometry during the deposition. This scheme permitted very low reflectivities, in the order of 5 x 10-4, to be readily and reproducibly obtained. The diode laser thus obtained was used in a strong feedback configuration. Light emitted from the coated facet was collimated and fed back onto the laser cavity after being reflected off a diffraction grating. The diffraction grating provides frequency selectivity, which is a desirable feature for obtaining a stable single longitudinal mode laser. The laser in this configuration oscillated in a single mode with a greater than 30 dB side mode suppression ratio and a wide tuning range. / Thesis / Master of Applied Science (MASc)

The purpose of this thesis is to make a broadband antireflection coating configuration known as a quarter-wave stack consisting of one layer of titania and of one layer of silica nanoparticles. We utilize much that is already known about silica nanoparticle deposition. The first objective of this thesis is deposition and characterization of titania nanoparticle films deposited on glass microscope slides by a technique known as Ionic Self-Assembled Multilayers or ISAM deposition. This technique takes advantage of the electrostatic attraction between oppositely charged materials and ideally results in a uniform nanoparticle film whose thickness and optical properties can be tightly controlled. Deposition of a quarter-wave stack based on ISAM deposition of silica and titania nanoparticles is significantly simpler and less expensive than alternative deposition methods. Initial attempts to deposit titania films were unsuccessful because of excess diffuse scattering due to inhomogeneities in the film. In order to reduce diffuse scattering, two approaches were considered. The first approach was to improve the deposition process itself by experimenting with different values of deposition parameters such as solution pH and solution molarity. The other approach focused on removing the large nanoparticle aggregates from the colloidal solutions of titania nanoparticles that were suspected to be responsible for rough film surfaces resulting in diffuse scattering. This approach was successful. In addition, evidence suggested that surface roughness contributed more to diffuse scattering than the bulk of the films. After minimizing diffuse scattering from titania nanoparticle films, we used known results from research on silica nanoparticle films to deposit quarter-wave stacks consisting of one layer of titania nanoparticles with high refractive index and one layer of silica nanoparticles with low refractive index. This contrast in refractive indices is a desirable characteristic of quarter-wave stacks. The thicknesses and refractive indices of the two layers in the quarter-wave stacks were measured by ellipsometry and compared to the nominal thicknesses of these layers. Finally, the reflectance was derived from a model of the quarter-wave stack and was compared to the measured reflectance. It was found that construction of a quarter-wave stack by ISAM is possible but that it will be necessary to acquire data from more experiments. / Ph. D.