Collimating lens pair for planar silica waveguides

Abstract

This thesis presents an original contribution to the field of planar silica optical waveguide devices. An integrated collimating lens pair is proposed which enables an optical signal to travel in free space between two opposing planar waveguides with minimal optical loss. Each lens in the lens pair consists of a convexly shaped front face to focus the optical beam in the horizontal plane, and a parabolically graded refractive index profile to focus the beam in the vertical plane. The lens pair has significant advantage over alternative means of collimation because the lens fabrication can be integrated with the planar silica waveguide fabrication. The lens pair has application in micro-optical systems which require free-space propagation such as planar waveguide MEMS optical switches and on-chip optical interconnects. This work provides a rigorous theoretical analysis of the lens pair. A design methodology is presented to determine the lens pair parameters required for zero optical power loss for any pre-set propagation distance (excluding reflection loss). The sensitivity of the lens pair to fabrication errors is investigated by using the Beam Propagation Method (BPM). The propagation of an optical beam is simulated under conditions of etch mask angular and lateral misalignment, deviation of the parabolically graded index lens from the desired refractive index profile, poor dimensional control of the planar silica waveguides, and non-vertical etching of the front face of the lens. A compensation scheme is proposed to minimise the optical loss caused by the non-vertical front face etch and the effectiveness of this technique is verified through further BPM simulation. The lens pair can be fabricated using Plasma Enhanced Chemical Vapour Deposition (PECVD) and Reactive Ion Etching (RIE) techniques, provided state of the art processing facilities are available. Preliminary PECVD and RIE process characterisation work has been done, however the complete fabrication of the device is left for future work.