Recent Developments in Direct-UV-Written Planar Waveguides, Gratings, Sensors, and Substrates

We present our recent developments in direct-UV-written integrated optical devices, based on applications in telecommunications, material characterisation, and optical sensing. The inherent advantages of this channel definition technique over traditional etching based approaches are reiterated and demonstrated in the production of small angle, low loss X-couplers with negligible polarisation dependence. Interfering two focussed UV spots also provides us with the capability to simultaneously define a channel waveguide and Bragg grating, opening new device and application opportunities. Such structures provide a method to quantify the photosensitivity effects observed in conventional multilayer substrates, and assess the core uniformity of a novel ‘flat-fibre’ format recently developed within the ORC. We will also discuss electrically tuned Bragg gratings via liquid crystal overlayers, displaying a dynamic range in excess of 100GHz for use in dynamic optical networks. Introduction Direct UV writing is planar lightwave circuit fabrication technique with enormous potential for the development of novel integrated optical devices and applications. Based on the refractive index change induced by a scanning UV beam, this technique allows channel waveguide structures to be literally ‘drawn’ into photosensitive materials without the need for photolithography, etching, or expensive cleanroom processing. Historically, the concept of UV-based waveguide definition was introduced by Chandross et al. in 1974 [1], who produced 4μm optical waveguides in doped polymer films with a 364nm laser. Referring to the process as ‘photolocking’, Chandross commented that the ‘smooth interfaces and low optical losses’ would provide a route to avoid the characteristic edge roughness induced by solvent development or sputtering. For integrated optical applications, direct UV writing in silica-on-silicon was first reported by Svalgaard et al. in 1994 who demonstrated buried single-mode channel waveguides in germaniumdoped silica films with a refractive index increase of 10 [2]. This was followed by work on practical devices such as directional couplers and power splitters in 1997 [3, 4] displaying losses of ~0.2dB. The first reported experimental work from the Smith group at the ORC of Southampton University was in 2002 when Gawith reported direct-UV-written buried channel waveguide lasers in direct-bonded glass [5]. This was quickly followed by Emmerson, who described a more advanced ‘Direct Grating Writing’ technique for the simultaneous definition of channel waveguides and Bragg gratings via the interference of two focussed UV spots (Fig.1) [6]. Since then, direct UV writing at the ORC has primarily developed towards the realisation of planar Bragggrating-based devices for real-world integrated optical applications, such as telecommunications and bio/chemical sensing. Presented here is an overview of some of our recent developments in direct UV writing, and the many emerging applications discovered from this versatile technology. Specific examples include X-couplers for dense optical networks, and a subsequent study on photosensitivity and local control of refractive index. The use of direct UV writing as a characterisation tool is discussed during the introduction of a novel ‘flat-fibre’ extended-length planar substrate material developed for distributed lab-on-a-chip applications. Also described is a new Bragg grating device that can be electrically tuned over more than four 25GHz-spaced wavelength division multiplexed (WDM) channels by means of index modification of a liquid crystal layer. Finally, we aim to highlight UV writing technologies that we believe may benefit other areas, such as bio/chemical sensing and the automotive industry. Fig: 1 Schematic diagram of simultaneous channel and grating definition via Direct Grating Writing Fig: 2 Schematic diagram of an X-coupler showing the UV induced refractive index structure. Note the raised index close to the overlap region (β = crossing angle; y=100μm) WB2