Laser-Doppler vibrometry with variable GHz heterodyne carrier via frequency-offset lock

摘要

The generation of a heterodyne carrier frequency via offset-lock in an optical phase-locked loop (OPLL) is a widespread technique in communication, spectroscopy and other fields. Commercial state-of-the-art laser-Doppler vibrometers (LDV) generate heterodyne frequency carrier by acoustooptic devices (Bragg cells) efficiently with the slow shear mode up to 409 MHz. Therefore, these LDVs are limited in measurement bandwidth and it is impossible to adjust the heterodyne carrier frequency to the optimal value in respect to the requested demodulation bandwidth. For RF-MEMS (radio-frequency microelectromechanical systems) testing, carrier frequencies in heterodyne LDVs have to exceed 1 GHz to enable the unambiguous reconstruction of the measured vibration, which is restricted by the conventional heterodyning techniques. Recently, we demonstrated a LDV microscope with the generation of a variable heterodyne carrier frequency up to 200 MHz by offset-lock in an OPLL with visible DBR semiconductor lasers. In this paper, we demonstrate the increase of heterodyne carrier frequency by conventional RF electronics up to 1.4 GHz and discuss the decisive OPLL parameters for the application of this technique to ultra-high-frequency laser-Doppler vibrometry. Our LDV microscope shows an (out-of-plane) vibration-amplitude sensitivity of less than 1 pm/Hz~(1/2) for vibration frequencies higher than 50 MHz, which enables the vibration measurement of most RF-MEMS. First measurements of resonances of a piezoelectric transducer are presented.