High-order mode selection in Yb:YAG ceramic laser Yb:YAGセラミックレーザーにおける高次モード選択

博士論文

2010

This dissertation demonstrates an economical and less complicated laser schemeto generate high-order modes from an Yb:YAG ceramic laser cavity with uniqueproperties and hence varied applications. The intra-cavity mode selecting elementis a simple plano-convex glass lens and it has been shown to offer excellent modeselection in scalar Laguerre-Gauss modes, modes with near-diffraction free characteristics,and radial and azimuthally polarized modes from within the Yb:YAGcavity.The current study bridges two different regimes that have not been broughttogether so far; and the end-product is a quasi-three level laser system to realizecircular mode selection. In case of a laser resonator with intra-cavity lens, thestability conditions of the cavity in as defined by ray transfer matrix are enclosedbetween the focusing and imaging conditions of the lens at the resonator mirrors.In the presence of strong spherical aberration, the focal length of the lens variescontinuously from its axis to the periphery and is no more a defined point but aline. When such a lens is placed at focal length distance from the cavity mirror andshifted along the resonator axis to span across the line focus, the cavity conditionsprovide continuous selection in modes of increasing mode diameter at the outputcoupler because of the stability condition shifting to rings of increasing radiusalong the refracting surface of the lens, generating pure hollow scalar Laguerre-Gauss modes of increasing azimuthal orders as high as 28 (order limited only bythe diameter of the cavity optics). Generation of the whole lineage of Laguerre-Gauss modes from a single scheme has been reported by the proposed scheme forthe first time.The laser scheme with aberrated intra-cavity lens can also be set up suchthat the cavity conditions offer stability conditions for both focusing and imagingconditions simultaneously. Coupling of the cavity modes, enhanced by apertureguiding at the gain element and long cavity length, generates beams which exhibitnear-diffraction free properties in their central parts (maximum or minimum). Thecoupling of these two modes in the cavity has also been confirmed experimentally.Near-diffraction free properties associated with central parts of doughnut-like andmulti-ring modes are reported. The near-diffraction free properties of these beams(divergence of their central parts 10 times smaller than the diffraction limit thatis associated with the mode radius at the output and constant power confinedwithin the center) exist for over 40 m of propagation distance.When the intra-cavity lens in combination with a birefringent crystal (positiveor negative birefringence) inside the Yb:YAG ceramic laser cavity, the stabilityregion of the cavity between the imaging and focusing conditions of the lens issplit into two regions; one corresponding to the extraordinary ray and the otherto the ordinary ray. The splitting of the stability region allows mode selection inazimuthal and radially polarized modes. The transition point from azimuthal toradial polarization as the lens is shifted along the resonator axis was clearly distinguishablewhen the width of the stability region (enclosed between focusing andimaging conditions) matched the birefringence shift. Azimuthal and radial polarizeddoughnut modes were generated with high polarization purity. Polarizationextinction ratio as high as 100:1 in certain cases have been reported. Propagationfactor M2 was estimated to vary between 2-2.5 (theoretical value of M2 for doughnutLG?01 = 2). Apart from these doughnut modes, new polarization distributionpatterns wherein azimuthal and radial polarizations existed together in a singleoutput beam were generated for the first time. When the intra-cavity lens hadpronounced spherical aberrations, the cavity offered mode selection in high-orderazimuthal and radial polarized modes.The experimental work has brought into limelight the realization of certainhigh-order modes which were only theoretically investigated till date. High-ordermodes have wide applications related to angular momentum transfer, material processingand many more. Near-diffraction free beams generated from the schemecan be seen as a valuable replacement to Bessel beams which has only a very limitedpropagation distance. Radial and azimuthal polarized modes have extensiveapplications in the optical toolbox for micromanipulation and some of the newpolarization distributions are added tools for realizing new phenomena.