Fabrication and optical characterisation of planar silica lens pair

Abstract

The fabrication and experimental results on the performance of a planar silica lens pair that facilitates low-loss optical beam propagation in free space between opposing planar silica waveguides/singlemode fibres are presented. Each lens uses the parabolic graded refractive index vertically and the convex curvature horizontally to focus the optical beam in free space. The lens pair, designed for 200 mm of free space propagation distance with ideal zero loss, is fabricated and measured for net loss of 1.6 dB which is 12.4 dB less than the coupling loss without a lens pair at 633 nm wavelength.Introduction: Silica optical waveguides constitute key elements for the fabrication of photonic circuits and optical MEMS devices. Their low index contrast enables mode matching and easy alignment to optical fibres, which has been crucial to their success in telecommunications applications. A planar lens pair which allows the low-loss transmission of light between two opposing waveguides has been theoretically analysed and simulated [1, 2] for intended application in optical crossconnect switches and optical interconnects. The lens pair uses a parabolic graded index silica slab to guide the optical beam in the vertical direction and a convex curvature front face, defined by a deep oxide etch, to guide the light in the horizontal direction. A single graded index lens [3] was used to focus the optical beam from a laser diode (LD).In this Letter, we focus on the fabrication and characterisation of a planar silica lens pair, designed for an ideal zero-loss free-space propagation distance of 200 mm. Furthermore, a set of identical lens pairs with free-space propagation distances less than and greater than 200 mm were fabricated. The measured losses at various free-space distances are compared with the theoretical results, showing good agreement.

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