The results show a measurement resolution within one-tenth of the wavelength of ultrasound and an accuracy better than 0.3 mm for targets at a distance up to 0.8 m. The repeatability error of TOF is less than 4 ns, and the theoretical resolution of TOF is 0.4 ns. The sound velocity is measured by the sound velocity profiler (SVP). Five targets and a test piece are used to evaluate the ranging performance. Transducers with a central frequency of 1 MHz and diameters of 20 mm and 28 mm are used in the experiment. It can be used to improve the accuracy and resolution of the time-of-flight (TOF) by a cross-correlation method. Unlike conventional methods, the ultrasound transmitted from the transducer is recorded as the reference signal when it first passes through the laser. The laser sensing provides a noncontact method for ultrasound detection based on acousto-optic diffraction. In this paper, we design a high-resolution ultrasonic ranging system that uses a thin laser beam as an ultrasonic sensor. Ultrasound has been proven to be a valid tool for ranging, especially in water. Our work could provide a promising and powerful tool for underwater distance measurement with high precision at long range, and open the door to a host of underwater applications related to the measurement of distance, e.g. The Allan deviation is 1.596 μm at 5 s averaging time, 481 nm at 100 s averaging time, and can achieve 261 nm with 540 s averaging time. The experimental results show the differences within ±4 μm at 4.4 m range, compared to the reference values.
Distances up to 4.4 m and displacements down to 5 μm underwater can be determined by virtue of the spectral phases. Due to the inherently dispersive characteristic of water, the measurement pulse will be chirped, and the spectrograms exhibit unstable oscillations, the spectral phase of which features a quadratic law. In this work, we describe a spectral interferometry-based method using a laser frequency comb at 518 nm (green light), which is capable of underwater distance measurement with high precision and accuracy. However, frequency comb-based underwater distance measurement has rarely been reported. Recent developments of optical frequency combs have given rise to revolutionary progress in metrology, spectroscopy, and optical distance measurement in air. acoustic pulses or incoherent optical pulses), the precision can only achieve centimeter level at best, which greatly obstructs the advancement of underwater science and technology. However, limited by the measurement sources (e.g. Underwater distance measurement has been playing an important role in the fields of underwater navigation, search and rescue, and bathymetric survey.