A frequency domain laser based ultrasonic system for time resolved measurement of broadband acoustic transients

Abstract

A high-sensitivity frequency domain laser based ultrasonic system is presented which uses a low power, amplitude modulated continuous wave (cw) laser source for acoustic wave generation. The acoustic signals are detected using a path stabilized Michelson interferometer coupled to a rf lock-in amplifier. The modulation frequency of the generation laser is scanned over the bandwidth of interest, and transient acoustic signals are reconstructed from the frequency domain data. The effects of measurement frequency resolution, bandwidth, and time domain aliasing on the reconstructed transient response are discussed. Experimental results on thin plates, where diffuse acoustic wave fields lasting several hundred microseconds are seen as a result of multiple reflections off of sample boundaries, demonstrate that the time domain signal can be unambiguously reconstructed through appropriate selection of frequency resolution. Time domain reconstructions of acoustic signals over a bandwidth of 200MHz demonstrate the utility of the approach for acoustic microscopy; individual acoustic transients can be detected with extremely high signal to noise ratio and time gated for analysis.

Measurement and Analysis of Narrow-Band Surface Acoustic Waves in Ceramic Environmental Barrier Coatings

“…The primary advantage of using a CW laser source over a pulsed laser source is that the bandwidth of the detection system can be reduced to match that of the excited acoustic waves, leading to a significant reduction in noise [7,8]. The experimental setup is shown in Figure 2.…”

A highly flexible pulsed photoacoustic setup based on a tunable laser source and external modulation

“…In cases where the peak optical power is limited rather than the total optical energy, the optimal tradeoff between SNR and spatial resolution can be achieved by using pulse compression. In this technique, which is well known in radar theory and also in ultrasonography and a few recent works in photoacoustics [6][7][8], the transmitted pulse has linear frequency modulation (LFM) of the form:…”

Spatial and temporal frequency domain laser-ultrasound applied in the direct measurement of dispersion relations of surface acoustic waves

“…8,9 Our technique-spatial and temporal modulated laser-ultrasound (STeMoLUS)-combines the previous methods, allowing an arbitrary frequency resolution in the measurement without the necessity of using scanning optomechanics. Moreover, the dispersion relation is obtained directly from the maxima of the amplitudefrequency scan, without the transformation techniques required for the evaluation of a spatio-temporal scans.…”

“…[5][6][7] Surface acoustic waves (SAWs) up to the GHz frequency range were excited and measured by amplifying the output of a harmonically modulated laser diode with a fiber amplifier. [8][9][10] Spatial modulation is usually achieved by interfering two (or more) excitation pulses on a sample surface; an approach known as the transient grating (TG) technique or impulsive stimulated thermal scattering (ISTS). 11,12 In this technique, monochromatic SAW and bulk waves are excited with the wavelength defined by the interference pattern.…”

“…low frequencies) and by the damping of the waves at high frequencies. 14 The method using harmonically modulated laser excitation and phase sensitive detection 8,9 requires highly accurate scanning of the excitation beam.…”