Evaluation of laser Doppler vibrometer’s performance by a multimode laser and changeable reference arm length, equipped with an auto-focus system

In the present study, the theoretical framework underlying interferometric measurement using multimode lasers-forming the basis of laser vibrometry-is comprehensively described. A numerical simulation of the Doppler signal for the Laser Doppler Vibrometer (LDV) is conducted, and the technique for extracting vibration velocity via zero-crossing counting is detailed and subsequently verified through experimental assessment. In addition, two critical optical challenges in this field, namely the determination of the optimal object distances and laser focusing, are addressed. To overcome these issues, a novel set-up is proposed that incorporates systematic adjustments of the reference arm length along with active auto-focusing, facilitated by a laser rangefinder. Moreover, to validate the performance of the proposed device and facilitate comparison with conventional contact accelerometers, several frequency response function (FRF) tests were conducted under different conditions. The test cases included laser measurements when the accelerometer was not mounted on the plate, measurements obtained directly from the accelerometer, laser measurements when the beam was directed onto the accelerometer, and laser measurements when the beam was directed onto the plate. Comparative analysis of the data acquired via the accelerometer and the LDV confirmed the accuracy and reliability of the LDV set-up. Moreover, the measurements revealed that mass loading from the accelerometer caused detectable shifts in the natural frequencies of the 1-mm-thick galvanized plate-shifts that typically require a correction step often treated as modal analysis noise in the literature. This underscores the advantage of LDV's non-contact nature in preserving the structure's true vibrational characteristics of the sample and eliminates the need for such corrective procedures.