Weak-guidance-theory review of dispersion and birefringence management by laser inscription

A brief review of laser inscription of micro- and nanophotonic structures in transparent materials is provided in terms of a compact and convenient formalism based on the theory of weak optical waveguides, We derive physically instructive approximate expressions allowing propagation constants of laser-inscribed micro- and nanowaveguides to be calculated as functions of the transverse waveguide size, refractive index step, and dielectric properties of the host material, Based on this analysis, we demonstrate that dispersion engineering capabilities of laser micromachining techniques are limited by the smallness of the refractive index step typical of laser-inscribed structures. However, a laser inscription of waveguides in pre-formed micro- and nanostructures suggests a variety of interesting options for a fine dispersion and birefringence tuning of small-size waveguides and photonic wires. A graph is presented. Dispersion and loss of guided modes in a silica-core air-cladded waveguide with a core radius a = 5 µm modified by a laser-inscribed grating with an index contrast dn = 0.001 and a lattice constant A/l ˜ 0.54 µm (l is an integer): propagation constant of the fundamental mode in the waveguide without a grating ß_o (dashed line), the real parts of propagation constants of the forward (ß_f, solid line) and backward (ß_b, dash-dotted line) modes coupled by the laser-inscribed grating, the imaginary part of the propagation constants of strongly coupled guided modes a (dotted line). © 2008 by Astro Ltd.