Fabrication and characterization of waveguide lasers operating in the infrared spectral range

Resumen

Since the first demonstration of flashlamp pumped ruby laser by Theodore H. Maiman in 1960, lasers have transformed almost every aspect of our lives. They had an estimated worldwide market worth more than $ 13 billion in 2018. Currently, lasers are being applied in various fields such as, astrophysics - for monitoring the velocities of astronomical objects, archaeology - for analysing ancient artefacts, military - for free space communications and target recognition in the battlefields, medicine - for implementing therapeutics and diagnostic procedures, surveillance - for direct remote sensing of greenhouse gases and harmful toxins, optical sensing – for a lab-on-a-chip integrated bio-medical and chemical analysis, lithography, and telecommunications. In recent years, a lot of attention has been given to the development of lasers emitting at ~2 µm. Such an emission corresponds to the absorption bands of a number of atmospheric molecules (H2O, CO2, N2O). Due to the strong water absorption band, such emission falls into the category of “eye-safe” lasers which makes them especially suitable for open-space applications, e.g. in range-finding (LIDAR systems), atmospheric sensing and wind mapping. Due to the moderate absorption of such lasers in plastic materials, they are suited for engraving, marking and welding of transparent plastics. In molecular spectroscopy, where mid-infrared sources are needed, 2 µm lasers are better suited for pumping the nonlinear crystals that are implemented for generating such longer wavelengths. Typically, laser emission at ∼2 μm is achieved using solid-state materials doped with the trivalent rare-earth thulium (Tm3+) or holmium (Ho3+) ions or a combination (co-doping) of them. Based on the type of gain medium they can be bulk, fiber, slab, thin-disk or (planar / channel) waveguide lasers. The unique advantage offered by a waveguide-based laser source is, its compactness as the optical modes are confined and index-guided in a small volume. This would lead to higher gain, lower laser threshold, small footprints, better heat dissipation and good beam quality. High power continuous wave and pulsed waveguide lasers operating ~ 2 μm are potentially suitable for making compact LIDAR devices operating on a surveillance-airplane or a satellite for directly monitoring and mapping atmospheric CO2. By taking advantage of the direct accesses (to the gain media) offered by surface waveguides, integration with nonlinear materials would lead to more compact devices having advanced functionalities. Specific functionalization (with bioreceptor molecules) or decoration (with plasmonic nanoparticles) of the deposited materials may lead towards active-biosensing and on-chip spectroscopic applications. Hence, the potential applications in environmental monitoring, security, and medicine, greatly motivate the development of such waveguide lasers. The goal of this thesis work was, the fabrication and characterization of compact and efficient waveguide lasers operating around 2 μm. To achieve this thulium and holmium doped monoclinic double tungstate crystalline materials were chosen as gain media, owing to their excellent spectroscopic properties. Their polarized laser emission as well as the high absorption and emission cross-sections, makes these gain media suited for making compact and monolithic waveguide lasers. The combinations of the top-seeded solution growth (TSSG), liquid phase epitaxy (LPE), diamond saw dicing and femtosecond direct laser writing (fs-DLW) methods were employed for fabricating and structuring the waveguides. Furthermore, different characterization techniques such as confocal microscopy, μ-Raman, and μ-luminescence mapping were implemented to assess the quality and suitability of the fabricated waveguides for lasing application. The saturable absorbers employed in the passive Q-switching operation included transition-metal-doped chalcogenide crystals (Cr2+:ZnSe or ZnS), few-layer transition metal dichalcogenide (MoS2) and carbon nanostructures such as graphene and single-walled carbon nanotubes deposited on a transparent substrate or directly onto the surface of the sample containing the waveguides. The fabrication of an active (Tm3+- doped) surface channel waveguide, by combining the LPE and the diamond saw dicing methods resulted in a record slope efficiency (82.6%) almost approaching the theoretical limit. Femtosecond laser written buried channel waveguides (with circular and hexagonal optical-lattice-like cladding), surface channel waveguides (with half-ring-shaped cladding) and Y-branch splitters (with rectangular cladding) were fabricated and studied in a monoclinic double tungstate crystalline material, paving the way towards advanced photonic structures such as a Mach – Zehnder interferometer for biosensing application. Fs-DLW Tm3+ waveguide laser capable of delivering a watt-level output power was demonstrated around 2 μm spectral range. In-band pumping of such Tm3+ waveguide laser resulted in a record slope efficiency (85.7%) and output power (1.37 W). Passive Q-switching of Tm3+ surface waveguide laser based on an evanescent field interaction led to stable operation with a high Q-switching conversion efficiency (approaching 90%) and a lower intensity instability in the pulse train (below 15%). In the pulsed operation regime, sub-100 ns pulses with MHz repetition frequency were demonstrated.