Nanometer-Order Resolution Displacement Measurement System by Air-Coupled Ultrasonic Wave Introducing Maximum Phase-Sensitivity Tuning

An improved system introducing an automatic tuning of the maximum phase sensitivity of ultrasonic signals is constructed for detecting displacements with a nanometer-order resolution. The phases of multiple-reference waves introduced in our previous system are controlled and appropriately set to tune the phase sensitivity. To evaluate our system, the displacement of a high-precision mechanical stage was measured using 40 kHz air-coupled ultrasonic waves. The resolution of the phase detection for our system is improved more than ten times our previous system. A resolution of 40 nm (λ/200,000) displacement has been achieved with a measurable displacement range of 8 mm (≃λ).