Fiber-end suspended polymer micro-rod inscribed with Bragg grating for highly sensitive ultrasonic detection

A suspended polymer rod grating is fabricated on a fiber end for highly sensitive ultrasonic detection. Initially, the uniform polymer waveguide is prepared via the interconnection of holey fibers and the photopolymerization of an ultraviolet glue. A femtosecond laser point-by-point technique is then employed to form periodic grating structures inside the customized waveguide. A final uncovered micro-rod is achieved based on different corrosion resistances of the polymer waveguide and the fiber cladding. The polymer rod presents uniform morphology and controllable size with the support of the constructed air-hole microchannel. The self-alignment and the self-adhesion between the polymer waveguide and the fiber core contribute to the stable efficient optical coupling at the fiber-to-polymer joint. When applied to ultrasonic waves, the decreased size and low Young's modulus of the suspended rod provide benefits for the interaction between the polymer grating and the ultrasound strain. This sensor exhibits a noise equivalent pressure of 33 Pa and -10 dB bandwidth of 7.6 MHz. After packing with a waterproof adhesive, the polymer rod shows sufficient robustness for long-term operation. This Letter proposes a new, to the best of our knowledge, strategy for the fabrication of advanced polymer probes in multifunctional sensing.