Lattice-Like Waveguides With Integrated Bragg Gratings in Planar Cyclic Olefin Copolymers

This contribution demonstrates femtosecond laser direct writing of lattice-like waveguides in planar cyclic olefin copolymer substrates. Based on numerical simulation and experimental near-field analysis, stable single-mode waveguiding around wavelengths of 1550 nm is demonstrated. The waveguiding mechanism is based on a hexagonal array of laser-induced, positive refractive index modification lines. Thus, the lateral extension of the guided mode can be adapted by varying the fabrication parameters and, in consequence, the resulting cross-sectional arrangement of the refractive index perturbations. With an optical attenuation of 2.2 dB·cm ^-1 around 1550 nm, the fabricated waveguides are well-suited for on-chip integrated photonic devices. Moreover, the waveguides can also be equipped with Bragg gratings to enable the application of the photonic platform as a sensing device. Depending on their length, the Bragg grating structures exhibit reflectivities of up to 99% and spectral widths down to 0.3 nm. The flexibility of the fabrication process and the sensing capabilities of the lattice-like waveguides with integrated Bragg gratings are underlined by an exemplary application study demonstrating a relative pressure sensor. For that, a photonic platform is micromilled to generate a 300 μm thick diaphragm and a reference pressure chamber. The strain introduced to the diaphragm by external pressure changes can then be quantified by the integrated photonic structures. This way, absolute pressure sensitivities of up to 38 pm·kPa ^-1 can be achieved in a relative pressure range from -60 to 100 kPa. The newly-developed lattice-like waveguides with integrated Bragg gratings are therefore well-suited for the realization of novel and adaptable photonic devices and sensors.