Submicrometer-Order Displacement Measurements Using an Air-Coupled Ultrasonic Transducer at Frequencies of 40 and 400 kHz

Noncontact air-coupled ultrasonic systems for measuring micrometer- or submicrometer-order displacements are newly constructed at frequencies of 40 and 400 kHz. The phase detection accuracy of the displacement measurements under several thousandths of ultrasonic wavelengths λ is markedly improved by the introduction of phase differences between the reference waves and the transmitting wave signal. To verify our system, the displacement of the reflector, which is translated by a high-precision mechanical stage, is measured using a single piezoelectric ceramic transducer in the reflection mode. Resolutions of displacement of better than 1 µm or 0.1 µm (λ/8500) using ultrasonic waves of 40 or 400 kHz in air, respectively, are achieved.