Channel Waveguide Lasers in Epitaxial Garnet Films

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Titel: Channel Waveguide Lasers in Epitaxial Garnet Films
Autor(en): Gerhardt, Reinald
Erstgutachter: Prof. Dr. Dötsch
Zweitgutachter: Prof. Dr. Dötsch
Prof. Dr. Betzler
Zusammenfassung: The subject of this thesis are channel waveguide lasers in epitaxial garnet films grown by liquid phase epitaxy. Results of this thesis include: The feasibility of a waveguide laser with erbium-doped YIG as the active material is discussed. Nonlinear rate equations that describe the behavior of this laser are formulated and solved numerically. The material parameters that are needed as the input are taken from literature. The simulations reveal that the performance of the laser depends critically on the magnitude of the upconversion parameter W22. A thorough analysis shows, however, that the values of W22 that are found in literature are most likely too small. The other parameter of importance is the background loss of the waveguide. Our results indicate that, unless the absorption losses of the LPE grown iron garnet films can be significantly reduced, the chances to realize such a device are only minimal. The mechanisms behind the absorption losses in LPE grown YIG films are investigated. During the LPE growth process, lead, platinum and other non-trivalent impurities are incorporated into the films in high concentrations. While platinum is tetravalent only, lead can be both di- and tetravalent. Depending on the ratio of the lead and the platinum concentrations, the near infrared absorption can be caused by three entirely different mechanisms. In case the tetravalent impurities are the majority, charge compensation is achieved by the formation of Fe2+ and the absorption is caused by crystal field transitions between the 5E and the 5T2 states of the Fe2+-ion. If the divalent impurities are in the majority, the absorption is due to crystal field transitions between the 5T2 and the 5E states of the Fe4-ion. In LPE-grown iron garnets, however, the impurity with the highest concentration is usually lead. We prove that lead performs self-compensation as it is incorporated in Pb2+/4+ pairs on neighboring lattice sites, and that the optical absorption is caused entirely by a charge transfer transition between the two neighboring lead ions. The electrical conductivity is measured in a temperature range from 300 to 800 K. It is demonstrated that in this temperature range the so-called impurity conduction is the predominant conduction mechanism in all types of films. Around room temperature, the conductivity shows an abnormal temperature behavior as the measured Bohr radius of the impurity ions almost doubles between 295 and 370 K. This can be explained by the fact that film and substrate have different temperature expansion coefficients. As the temperature increases, the stress in the film changes from tensile to compressive. When that happens, the {Pb2+} and the [Pb4+] ions counteract the strain in that they interchange their lattice sites. Also, Fe4+, if present, switches from the tetrahedral to the octahedral site which reduces the lattice constant of the film. This interpretation finds support in the results on the optical absorption, in the lattice mismatch, and in MCD spectra that were previously measured by Milani and Paroli. Based on the above findings, an easy way to eliminate the growth induced absorption in LPE iron garnet films is presented. By co-doping the films with other di- and tetravalent ions like Mg and Ge in high concentrations, the concentration of absorbing Pb2+/Pb4+ centers is greatly reduced due to the formation of non-absorbing Pb2+/Ge4+ and Ca2+/Pb4+ centers. As a result, absorption losses as low as 0.05 cm-1 at 1.3 µm wavelength have been achieved. In the second part of this thesis the fabrication of a channel waveguide laser in epitaxial Nd:GGG films is described. The films are grown by LPE on [111] oriented GGG substrates from a PbO/B2O3 flux. To increase the index contrast between film and substrate, Bi is added to the melt. Initially, the films showed strong brown discoloration and a high defect density. Luminescence lifetime measurements revealed severe quenching problems. The chemical analysis of the films by EPMA shows that platinum from the crucible enters the films but no lead from the flux. It is found that the platinum in GGG, similar to the lead in YIG, forms Pt2+/Pt4+ centers in order to achieve charge compensation. The quenching is caused by the charge transfer transition between the two platinum ions. To eliminate the quenching, MgO is added to the melt. As a result, the brown discoloration, the quenching and the growth defects disappear. From the planar film rib waveguides are fabricated using ion beam etching. The unclad waveguides are cut and the endfaces are polished to couple light in and out. Both endfaces are perpendicular to the waveguide within 0.1°. The losses are derived from the Fabry-Perot interferences. Losses as low as 0.2 dB/cm are achieved for both TE and TM modes. Laser oscillation starts at a launched pump power of 5 mW. The resonator is formed by the polished endfaces of the waveguide, no dielectric mirrors have been applied. Thus, the facet reflectivity is only about 10%. The launched pump power is measured precisely using a novel technique at which the luminescence intensity emitted through the surface is spatially resolved with a beam profile analyzer. Coupling efficiencies of over 90% are measured. The slope efficiency is found to be 48%.
Schlagworte: waveguide; solid state laser; rare earths; garnet
Erscheinungsdatum: 15-Feb-2002
Enthalten in den Sammlungen:FB04 - E-Dissertationen

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