Location

Brunswick, ME

Start Date

1-1-1997 12:00 AM

Description

Laser generation of ultrasound and the subsequent detection of the ultrasonic waves using laser interferometry are areas of active research [1–6]. In earlier papers, the present authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress towards the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications. Bowers first reported [7] the use of a Sagnac-type interferometer for surface acoustic wave detection, and the present authors have previously reported [8 QNDE 95] a variant of that scheme. In this paper, we present an alternative lower noise system that uses low cost, long coherence He-Ne lasers that have better intensity noise characteristics than typically used laser diodes. A scheme for elimination of a parasitic interference utilizing a frequency shifting technique has been developed. The primary advantage of the Sagnac interferometer is that it is exactly path matched and as such requires no heterodyning or static path compensation for sensor stabilization. The Sagnac interferometer described below is suitable for the measurement of ultrasonic surface waves arising from laser- or PZT-generated sources or from acoustic emissions. The laser-based ultrasonics (LBU) system can be used to detect and characterize discrete defects such as cracks.

Book Title

Review of Progress in Quantitative Nondestructive Evaluation

Volume

16B

Chapter

Chapter 8: NDE Systems

Section

Ultrasonic

Pages

2053-2060

DOI

10.1007/978-1-4615-5947-4_268

Language

en

File Format

application/pdf

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Jan 1st, 12:00 AM

Frequency-Shifted Low-Noise Sagnac Sensor for Ultrasonic Measurements

Brunswick, ME

Laser generation of ultrasound and the subsequent detection of the ultrasonic waves using laser interferometry are areas of active research [1–6]. In earlier papers, the present authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress towards the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications. Bowers first reported [7] the use of a Sagnac-type interferometer for surface acoustic wave detection, and the present authors have previously reported [8 QNDE 95] a variant of that scheme. In this paper, we present an alternative lower noise system that uses low cost, long coherence He-Ne lasers that have better intensity noise characteristics than typically used laser diodes. A scheme for elimination of a parasitic interference utilizing a frequency shifting technique has been developed. The primary advantage of the Sagnac interferometer is that it is exactly path matched and as such requires no heterodyning or static path compensation for sensor stabilization. The Sagnac interferometer described below is suitable for the measurement of ultrasonic surface waves arising from laser- or PZT-generated sources or from acoustic emissions. The laser-based ultrasonics (LBU) system can be used to detect and characterize discrete defects such as cracks.