Noncontact detection of underwater ultrasound using laser light based on the self-coupling effect of a semiconductor laser

When a hydrophone with a vibrating membrane is placed in water, the ultrasonic waves to be detected diffract and reflect. Therefore, conventional hydrophones cannot accurately measure the sound field distribution. Another noncontact method for measuring the sound field distribution is the Schlieren method. However, this method requires meticulous optical axis adjustment using a Schlieren lens and a knife edge, and this method is not versatile. Therefore, a laser hydrophone, which uses the self-coupling effect of a semiconductor laser to detect ultrasonic waves without contact with the sound field, is developed, and the sound field is investigated. The optical system of the laser hydrophone is composed of only a few components. In addition, because ultrasonic waves can be detected using only a small amount of light, no optical axis adjustment is necessary. The frequency response of the laser hydrophone is flat. The upper limit of the detectable frequency is determined by the relationship between the large diameter of the laser beam and the frequency of the ultrasonic waves. The measured sound pressure distribution of the laser hydrophone qualitatively agreed with that of the simulation.