High-Speed and Precise Ultrasonic Absolute Ranging Based on the Ultrafast Swept Laser

Ultrasonic absolute ranging is desirable for the applications in profilometry, robot obstacle avoidance, and resource exploration, whose performances are determined by the bandwidth, noise floor, and stability of the ultrasound detection system. Here, an ultrasonic absolute ranging system with a 20 kHz update rate and micrometer-level precision is proposed by optical ultrasound detection technology based on an ultrafast time-stretched swept laser. Benefiting from the high coherence and stability of the swept laser, the noise equivalent pressure reaches 100 mPa/√Hz order in the MHz region with a linear response degree of 0.9996. Different from utilizing the single-frequency laser source, the optical ultrasound detection with the swept laser need not require any servo-feedback setups to lock the linear work region. Besides, the maximum detection bandwidth can reach 50 MHz when the sweep rate of the laser is 100 MHz, which ensures a high-speed ultrasonic ranging system. A repetition-rate-tunable ultrasonic frequency comb with a pulse duration of about 0.8 μs and a carry frequency of 2 MHz is further induced to achieve the underwater absolute ranging. The ranging precision with an update frame of about 20 kHz reaches 10 μm in order. The Allan deviation shows that the highest precision reaches ∼2.5 μm at an average time of 6 ms. This work provides new insights for large-bandwidth and high-stability optical ultrasound detection as well as fast and high-precision underwater ranging.