Ad hoc calibration of interferometric system for measuring nanometer-scale displacements induced by laser ultrasound

Abstract

Laser Ultrasound (LUS) is commonly used in many fields including thickness measurement and defect inspection. In a conventional LUS system, a piezo-based transducer (PZT) is generally used for detecting the ultrasound echo waves, which requires direct contact with a specimen and thus prolongs measurement time when any lateral scanning is necessary. We present a novel non-contact interferometric system based on a 3 × 3 optical fiber coupler. Even though the 3 × 3 interferometric system works stably at any operating point and allows quantitative measurements, it is generally known that careful calibration is necessary before main measurements. Experimentally, it was observed that the surface displacement, induced by the ultrasound wave of LUS, of a cornea phantom was so minute that averaging was necessary. In this study, we discovered that by using the multiple data sets acquired for averaging, we could obtain the system ad hoc characteristic ellipse without performing the conventional calibration process. Furthermore, by utilizing coherent average we could extract the displacement with a 0.14 nm sensitivity. We could also measure the thickness variation, induced by ocular pressure, of the cornea phantom with a resolution of 4.3 μm by measuring the time of a round trip of the ultrasound wave. This straightforward system, composed solely of a 3 × 3 coupler, is expected to promise a compact and efficient solution to diverse applications.