Direct laser writing of a phase-shifted Bragg grating waveguide for ultrasound detection

We demonstrate a phase-shifted Bragg grating waveguide (PS-BGW) fabricated by a direct laser writing technique via two-photon polymerization as a high-frequency ultrasonic sensor. The PS-BGW device has a cross-sectional area of 1.5  μm×2  μm, and the grating length is about 100 μm. The optical resonant spectrum is investigated numerically and experimentally. The result shows that the fabricated device with a grating depth of 250 nm and a quarter wave phase lead to a 20-nm stopband and a 0.085-nm full-linewidth of the transmission notch, making it suitable for photonic sensing applications. Here, the PS-BGW device is demonstrated for the detection of acoustic waves, which can cause strain or deformation of the device and lead to a shift in the resonance wavelength. The sensitivity and the limitations of the device for ultrasound detection are also investigated.