Bragg-grating-based rare-earth-ion-doped channel waveguide lasers and their applications

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The research presented in this thesis concerns the investigation and development of Bragggrating-based integrated cavities for the rare-earth-ion-doped Al2O3 (aluminium oxide) waveguide platform, both from a theoretical and an experimental point of view, with the primary purpose of realizing narrow-linewidth, monolithic channel waveguide lasers. To determine the optimum design parameters and for understanding the operation principles of Bragg-grating-based rare-earth-ion-doped channel waveguide lasers, a mathematical model of such a laser is implemented. The mathematical description consists of laser rate equations, which describe the population dynamics of rare-earth ions, as well as coupled mode equations, which describe the operation of waveguide Bragg gratings. Making use of reactive co-sputtering from high purity metallic targets, rare-earth-iondoped Al2O3 waveguide layers are deposited onto thermally-oxidized silicon substrates, after which channel waveguides are etched using a chlorine-based reactive ion etching process. A high-resolution lithography technique, known as laser interference lithography is used to define the Bragg-grating structures, which are finally etched via reactive ion etching into a SiO2 cladding layer on top of the waveguides. Optimized waveguide and grating geometries allowed various rare-earth-ion-doped integrated channel waveguide lasers to be demonstrated. These include an erbium-doped Al2O3 distributed feedback channel waveguide laser having a linewidth of 1.7 kHz, as well as highly efficient ytterbium-doped Al2O3 distributed feedback and distributed Bragg reflector lasers with slope efficiencies as high as 67%. The use of two local phase shifts in a distributed-feedback structure enables the demonstration of a dual-wavelength distributed feedback channel waveguide laser. This device is used for the photonic generation of a microwave signal via the heterodyne detection of the two optical waves emitted by the laser. By varying the values of the respective phase shifts, various laser cavities producing microwave signals with frequencies ranging between 12.43 GHz and 23.2 GHz are demonstrated. The stability performance and narrow-linewidth of these free-running lasers show the great potential of using rare-earthion-doped monolithic waveguide lasers for the photonic generation of stable microwave signals in novel applications such as phased array antennas. Another innovative application of the photonic generation of stable microwave signals is the demonstration of an integrated intra-laser-cavity micro-particle optical sensor based on a dual-wavelength distributed-feedback channel waveguide laser. Real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 µm and 20 µm is achieved. This represents the typical size of many fungal and bacterial pathogens as well as a large variety of human cells. A limit of detection of ~ 500 nm is deduced. The sensing principle relies on measuring changes in the frequency difference between the two longitudinal laser modes as the evanescent field of the dual-wavelength laser interacts with micro-sized particles on the surface of the waveguide.