Synthetic Wavelength Vibrometry – Towards Versatile Vibration Imaging

Mechanical and acoustic actuators, as well as vibrations in machinery such as engines or trans-formers, often exhibit displacement amplitudes on the order of micrometers. For most single-shot-capable incoherent topography measurement methods, this lies close to their lower detec-tion threshold. Vibration measurements using interferometry, however, are more feasible for dis-placement amplitudes smaller than the wavelength. For larger amplitudes, many measurements per vibration period and camera frame rates much higher than the vibration frequencies are re-quired for unambiguous results. In this contribution, we introduce a novel vibrometry approach that exploits “Synthetic Waves” (SW) to address this challenge. A SW is the beat wave formed by two optical waves with closely spaced wavelengths. The complex-valued synthetic field can be computed from interferograms recorded at the two optical wavelengths. The resulting synthetic wavelength can be orders of magnitude larger than the optical carrier wavelengths and is freely tunable. This allows dynamic adaptation to applications with different displacement amplitudes. Using single-shot synthetic field acquisition, vibrations can be studied using conventional CMOS cameras at comparatively low frame rates. We present first SW-vibrometry measurements on optically rough surfaces and discuss funda-mental limits and trade-offs. Additionally, we show spatially resolved vibration measurements of objects hidden behind scattering media. 78 ngungen und Vibrometrie Ulrike Boehm