Bragg Waveguide Ultrasound Detectors

Polymer Bragg grating waveguides (BGWs) are demonstrated as ultrasound detectors. The device is fabricated by a direct electron beam lithography technique using an epoxy-based photoresist as the core material, with grating features fabricated on the side walls of the rib waveguide. The main motivation for this design is the linear geometry of the device, which can be used in a linear array, facilitating high-frequency ultrasound imaging. The fabricated BGW device has a cross-sectional area of 1.5 x 1.5 mu m and the grating length is 500 mu m. The optical resonance spectrum is measured and compared with a theoretical model. The BGW device is experimentally demonstrated for the detection of ultrasound waves emitted by a 25-MHz transducer. Detection sensitivity depends on optimal grating design for a steep resonance. The extension of a single-element BGW device to a linear array using optical wavelength division multiplexing is presented. The results demonstrate the potential use of BGW devices in highly compact array of optoacoustic detectors for high-sensitivity ultrasound detection and photoacoustic imaging.