Optimal design of optical fibre-holding microclips with metamorphic development

Silicon- or silicon nitride-based microclips have recently been developed to hold optical fibres in micro-optical systems for high optical coupling efficiency. The fabrication process is simple, with one or two lithographic steps required on a thin film deposited on a silicon substrate. Single-mode optical fibres are inserted and held in position in V-shaped grooves etched in silicon substrates by the cantilever clips protruding from the edges of the V grooves. As the fibre core is, in general, either level with or above the silicon surface, the clips are deflected by the fibre and act as springs holding the fibres kinematically in place. Ideally, the mass and thickness of the film clips should be minimized. However, the design of the microclips is faced with apparent contradicting constraints: on the one hand, the clips need to be sufficiently flexible for fibre connections and disconnections; on the other hand, they need to be sufficiently stiff so that the clamping force exerted on the fibres is reliable. Additional problems that must be addressed in designing the clips include substrate spalling and interfacial debonding due to stress concentrations at the clip-substrate joint. In this work the optimal thickness, mass and shape of the fibre-holding clips are studied with an effective optimization procedure, called `metamorphic development' (MD). This procedure, coupled with the method of finite elements, aims at finding structural shapes and topologies of the microclips that minimize their structural compliance and weight, subject to stress and deflection constraints. It allows the clips to grow and degenerate from the simplest possible start-up geometry towards an optimum topological layout.