A polarised , brightness - enhanced Nd : Y Al O planar waveguide laser

A simple method for the production of a polarised Nd:Y Al O on Y Al O planar waveguide laser is reported. 3 5 12 3 5 12 A 50nm layer of Au deposited on the surface of an unclad, 8.3 μm thick core region has produced an improvement in the TE/TM polarisation extinction ratio from 7dB to 27dB. The M of the beam in the guided 2 plane has been reduced from 1.53 ±0.04 in an uncoated region of the waveguide to 1.17 ±0.01 in the Au coated region using optimum pump launch conditions. Laser devices based on a planar waveguide electromagnetic spectrum this requirement can be geometry hold out the prospect of a compact, fulfilled by a dielectric and a metal. Under these high-power visible light source by the combination conditions TE polarised light will not excite a of an active gain region and a non-linear surface plasmon mode and will, in our case, see waveguide. However, in order to pump effectively reduced propagation losses compared to the TM the non-linear region, a polarised output from the polarisation. waveguide laser is required. Nd:Y Al O 3 5 12 (Nd:YAG) provides an attractive choice for the If we consider an unpolarised multimode slab gain medium, however due to its isotropic nature, waveguide system which guides both TE and TM the laser output is unpolarised. In this letter we modes, coated with a thin metal film, then present a very simple technique for obtaining a operation of the coated waveguide can be linearly polarised, transverse electric field (TE) reasonably described in terms of coupled modes . output which when combined with recently The TM modes couple to the surface plasmon obtained diode-bar pumped planar waveguide mode. The strength of this coupling depends on results could offer a route to a high-power how closely phase-matched a given TM mode is to [1] polarised planar waveguide laser. The method the surface plasmon mode. This coupling of TM demonstrated also gives the significant benefit of polarised light out of the dielectric waveguide into improving the beam quality parameter (M ) of the the lossy surface plasmon mode increases the 2 output beam in the guided direction. This should losses of the TM modes leading to preferential TE allow brighter outputs from such a device than laser operation. In a multimode waveguide device, could otherwise be obtained. the evanescent part of the laser mode extends One straightforward mechanism for controlling than for the fundamental. Thus the TE polarised modal losses in waveguide structures is to coat the modes have increasing propagation losses as the waveguide with a thin metal film . The transverse mode number increases resulting in a device which [2] magnetic field (TM) modes of the waveguide has lowest propagation losses for the fundamental penetrate more deeply into the metal layer resulting mode in the TE polarisation. in higher losses for the TM modes compared to the TE modes. In addition, our calculations show that In order to verify this principle we carried out a surface plasmon mode should be excited for the modelling for an 8.3 μm planar waveguide with a TM modes resulting in enhanced coupling into the 50nm gold coating. The waveguide modelled was metallic film. A surface plasmon is a TM mode chosen to match a waveguide available for guided by the interface between two media whose subsequent experiments. This waveguide was dielectric constants have real parts of opposite fabricated by liquid phase (LPE) epitaxy and sign. In the visible and infra-red part of the consisted of an 8.3 μm thick core region of 1at% [3] further into a surface layer for higher order modes