Nonlinear signal errors in homodyne laser Doppler vibrometry induced by strong second-order ghost reflections and their mitigation.

Yanlu Li,Roel Baets

doi:10.1364/oe.418439

Abstract

A variety of mechanisms can induce distortions in the output signals of a homodyne laser Doppler vibrometer (LDV). In this paper, the nonlinear LDV distortions caused by a strong second-order ghost reflection originating from lens flares are theoretically explained and analyzed. We propose a simple compensation method to mitigate this distortion. The performance and limitations of this method are also explained both in simulation and in experiment.

Highlights

Laser Doppler vibrometry (LDV) is a non-contact vibration measurement method with picometer or even sub-picometer displacement resolution over a large frequency range [1], by taking advantage of the Doppler effect of a coherent light beam generated from a laser source
We have developed several compact laser Doppler vibrometer (LDV) systems on a silicon-based photonic integrated circuit (PIC) [7,8]
The frequency shifter is usually an acousto-optic modulator (AOM), and it is placed in one arm of the optical interferometer in the LDV to ensure a constant frequency difference between the measurement and reference signals

Summary

Introduction

Laser Doppler vibrometry (LDV) is a non-contact vibration measurement method with picometer or even sub-picometer displacement resolution over a large frequency range [1], by taking advantage of the Doppler effect of a coherent light beam generated from a laser source. Since the high-order ghost reflections have twice or more times the Doppler frequency shift in the main reflection, the corresponding errors in LDV signals will behave differently from the nonlinear errors handled by Heydemann correction. They have developed several models to analyze the periodic nonlinearities originated from different orders of the ghost reflections Based on these models, they analyzed the impacts of the ghost reflections on ultra-precision displacement measurement interferometers. We will focus on the impacts of strong second-order ghost reflection on the homodyne PIC LDVs as well as on the compensation method to mitigate the problem. When the second-order ghost reflection is directed to the sensor as a result of the DUT movement, the strength of the major reflection is usually considerably reduced In this case, the LDV signal will be distorted by the second-order ghost reflection.

Working principle of the homodyne LDV and different distortion mechanism

Compensation algorithm for a strong second-order ghost reflection

Findings

Summary and discussion