Small Dimension Hollow-core Silicon Waveguides for Bragg Reflection Studies at 10·6 µm

Abstract

Abstract A Bragg-reflection approach to CO2 waveguide laser resonator design based on the use of wall gratings has a number of potential advantages. In particular, a move away from the need for bulky, accurately aligned, multidielectric-coated mirror resonators, could have a significant impact on both manufacturing time and overall size leading to a new generation of compact low-cost 10 µm waveguide lasers. Bragg reflection from wall gratings in hollow dielectric waveguides has been treated theoretically, the critical result of the analysis being that in order to attain sufficient feedback from a wall grating the waveguide wall separation should be of the order of 100 µm. Unfortunately, waveguide attenuation is proportional to the inverse cube of the waveguide wall separation, with the result that commonly used waveguide materials such as alumina, whilst having sufficiently low attenuation for the fabrication of 1·5–2·0 mm bore lasers [3], give prohibitively high losses at 100 µm bore sizes. In this paper, following a brief review of hollow-waveguide Bragg reflection phenomena the results of experimental and theoretical studies of novel hollow-core silicon waveguide structures having bore sizes 100–400 µm are described. The transmission characteristics of such waveguides have been studied as a function of doping density and plate separation. Power transmissions in excess of 50% have been achieved with a 100 µm wide, 5 cm long heavily doped silicon slab waveguide at 10·6 µm. Such waveguides incorporating grating structures will form the basis of further studies of Bragg reflection phenomena.