Laser Doppler vibrometer

About: Laser Doppler vibrometer is a research topic. Over the lifetime, 6319 publications have been published within this topic receiving 76068 citations.

Fast scanning cavity offset lock for laser frequency drift stabilization.

Abstract: We have implemented a compact setup for long-term laser frequency stabilization. Light from a stable reference laser and several slave lasers is coupled into a confocal Fabry–Perot resonator. By stabilizing the position of the transmission peaks of the slave lasers relative to successive peaks of the master laser as the length of the cavity is scanned over one free spectral range, the long-term stability of the master laser is transferred to the slave lasers. By using fast analog peak detection and low-latency microcontroller-based digital feedback, with a scanning frequency of 3 kHz, we obtain a feedback bandwidth of 380 Hz and a relative stability of better than 10 kHz at timescales longer than 1 s, a significant improvement on previous scanning-cavity stabilization systems.

Heterodyne laser-Doppler vibrometer with a slow-shear-mode Bragg cell for vibration measurements up to 1.2 GHz

Abstract: Several new applications for optical ultra-high frequency (UHF) measurements have been evolved during the last decade by advancements in ultra-sonic filters and actuators as well as by the progress in micro- and nanotechnology. These new applications require new testing methods. Laser-based, non-influencing optical testing is the best choice. In this paper we present a laser-Doppler vibrometer for vibration measurements at frequencies up to 1.2 GHz. The frequency-shifter in the heterodyne interferometer is a slow-shear-mode Bragg cell. The light source in the interferometer is a green DPSS (diode pumped solid state) laser. At this wavelength the highest possible frequency shift between zero and first diffraction order is a few MHz above 300 MHz for a slow shear-mode Bragg cell and, therefore, the highest possible bandwidth of the laser-Doppler vibrometer should usually be around 300 MHz. A new optical arrangement and a novel signal processing of the digitized photo-detector signal is employed to expand the bandwidth to 1.2 GHz. We describe the utilized techniques and present the characterization of the new ultra-high-frequency (UHF) vibrometer. An example measurement on a surface acoustic wave (SAW) resonator oscillating at 262 MHz is also demonstrated. The light-power of the measurement beam can be switched on rapidly by a trigger signal to avoid thermal influences on the sample.

Detection of Internal Holes in Additive Manufactured Ti-6Al-4V Part Using Laser Ultrasonic Testing

Abstract: For a non-contact, non-destructive quality evaluation, laser ultrasonic testing (LUT) has received increasing attention in complex manufacturing processes, such as additive manufacturing (AM). This work assessed the LUT method for the inspection of internal hole defects in additive manufactured Ti-6Al-4V part. A Q-switched pulsed laser was utilized to generate ultrasound waves on the top surface of a Ti-6Al-4V alloy part, and a laser Doppler vibrometer (LDV) was utilized to detect the ultrasound waves. Sub-millimeter (0.8 mm diameter) internal hole defect was successfully detected by using the established LUT system in pulse-echo mode. The method achieved a relatively high resolution, suggesting significant application prospects in the non-destructive evaluation of AM part. The relationship between the diameter of the hole defects and the amplitude of the laser-generated Rayleigh waves was studied. X-ray computed tomography (XCT) was conducted to validate the results obtained from the LUT system.

Visualization of solitary waves via laser Doppler vibrometry for heavy impurity identification in a granular chain

Abstract: We study the propagation of highly nonlinear solitary waves in a one-dimensional granular chain composed of homogeneous spherical particles that includes a heavy impurity. We experimentally investigate the transmission and backscattering behavior of solitary waves in the region of the impurity by using a laser Doppler vibrometer. To assess the sensitivity of solitary waves to various impurity masses, this non-contact measurement technique is complemented by a conventional contact measurement method based on an instrumented sensor particle. By leveraging these two schemes, we find that the travelling time and attenuation of backscattered solitary waves are highly sensitive to the location and mass of an inserted impurity. The experimental results are found to be in satisfactory agreement with the numerical results obtained from a discrete element model and the theoretical predictions based on nonlinear wave dynamics and classical contact theory. This study demonstrates that laser Doppler vibrometry can be an efficient tool to visualize highly nonlinear wave propagation in granular media. With a view towards potential applications, highly nonlinear solitary waves can be employed as nondestructive probing signals to identify heavy impurities embedded in ordered granular architectures. (Some figures may appear in colour only in the online journal)

Investigation of the propagation characteristics of excimer lasers using a Hartmann–Shack sensor

Abstract: For wave front distribution measurements of excimer laser radiation, a Hartmann–Shack wave front sensor with 40×30 subapertures and a field of view adapted to the lateral laser beam profile was utilized. The peak-to-valley wave front resolution is λ/12 and the dynamic range 100λ per subaperture at 248 nm. Time-averaged propagation characteristics were determined for several combinations of resonator configuration and laser medium from the ns down to the fs pulse length regime. Above that, time-resolved measurements were carried out on a KrF excimer laser, showing a reduction of the beam divergence during the laser pulse by a factor of 1.6. This transient behavior is discussed on the basis of a simple geometrical model. The theoretical results are in qualitative agreement with the experiment.