Ultracompact twisted silica taper for 20  kHz to 94  MHz ultrasound sensing

An ultracompact twisted silica taper with an asymmetric structure is fabricated by fire-drawing two twisted single-mode fibers for broadband ultrasound sensing. A piezoelectric transducer (PZT), peaked at 3.7 MHz, is used as an ultrasound generator. A steel plate with a silica taper attached is adhered to the PZT and is used as the ultrasound propagation medium. The transmission spectrum of the silica taper has high contrast owing to multimode interference in this twisted silica structure. Specially, the taper waist length and waist diameter are optimized for the highest optical sensitivity with high contrast at high slope in the transmission spectrum. The ultrasound sensitivity is characterized by a different thickness of the steel plate from 0 to 2.36 mm to achieve the highest ultrasound frequency response. With the taper waist length of 5 mm, waist diameter of 5 µm, and steel thickness of 0 mm, a broadband ultrasound frequency of 20 kHz to 94.4 MHz can be detected at high harmonics of the PZT, verifying the high sensitivity of the compact twisted silica taper.