Planar lightwave circuits: it's all in the cladding

The top cladding layer in planar lightwave circuits (PLC) is more than an optical buffer. By variously doping, adjusting the thickness of, etching patterns in and annealing the cladding layers in waveguide devices, a wide range of sensors and photonic devices can be realized. The material properties of the cladding determine, for instance, the modal birefringence of the waveguides; knowledge and control of these properties can be harnessed to produce polarization-independent components. The fabrication of thermo-optically controlled switches and interferometers for tunable filtering and optical signal processing is possible through the creation of micro heaters on top of the cladding. The optimization of such components can benefit from engineering of the cladding, ranging from better planarization and thickness control, to selective etching to better confine the heat distribution and provide stress relief. In addition, the thermal properties of a given device can be radically enhanced by using a polymer layer as top cladding, which yields an order of magnitude increase in the temperature sensitivity, an invaluable enhancement that can be harnessed for phase-tunable waveguides or sensor structures. Long period gratings (LPGs) can be etched in the lower cladding to provide filtering, signal processing, or sensor functions. In a borophosphosilicate cladding, typically used in silica-on-silicon PLCs, control of the reflow properties through composition can be exploited to manufacture fillable microchannels that are monolithically integrated with solid-core devices, enabling a unique platform for sensing, signal processing, or nonlinear optics.