Ultracompact all-fiber self-transceiving ultrasonic probe with an enhanced working distance.

All-optical ultrasonic probes exhibit notable benefits in ultrasonic detection and imaging. Typically, two separate optical fibers are used for excitation and detection, yet limited research has explored the integration of both functionalities within a single fiber. In this Letter, to our knowledge, a new method for fabricating an all-fiber self-transceiving ultrasonic probe is proposed with a lateral dimension of less than 500 µm. Double cladding fiber (DCF) is spliced with a short segment of thin-diameter single-mode fiber (TDSMF), which is then embedded into a fiber bubble to form a Fabry-Perot cavity, and the bubble surface is coated with a composite material layer. The pulsed laser propagates through the inner cladding of DCF and leaks from the splicing point of DCF-TDSMF, inducing the material excitation for efficient ultrasound generation. The core-guided detection laser is directed to the TDSMF end, entering the bubble microcavity and inducing an optical interference for weak echo detection. The emitting functionality produces an ultrasound with a -6 dB bandwidth of 17.5 MHz and a peak frequency of 6.29 MHz, which is well-matched with the fiber microcavity's response frequency of 3.29 MHz. Through self-transceiving experiments, low-noise pulse-echo signals are captured at varying working distances of up to 3.78 cm. The proposed probe exhibits great potential in biomedical and industrial fields due to its all-fiber miniaturization and enhanced-distance detection capability.