Fifth International Congress on Sound and Vibration December 15-18, 1997 Adelaide, South Australia Real-time Vibration Mode Imaging Using Photorefractive Holography

Advances in optics over the last two decades have led to the development of optical processing mechanisms in photorefractive materials that provide unique capabilities for intelligent sensing applications. These capabilities include adaptability to environmental effects, image correlation, and optical computing. This paper describes the utilization of photorefractivity for performing noncontacting optical vibration detection that is most useful for small peak amplitudes less than A/4n. Multi-wave mixing with synchronous detection allows measurement of both the vibration amplitude and phase of a vibrating surface directly as a function of the excitation frequency. Narrow bandwidth detection with flat frequency response can be achieved at frequencies above the photorefractive response (-100 Hz). A minimum detectable displacement amplitude of a few picometers has been demonstrated for a point measurement, with the possibility of further improvement. Full-field imaging of vibrating surfaces is performed in a manner that employs the adaptive properties of the photorefractive effect for real-time processing. The result is an output image intensity directly proportional to the vibration amplitude for small amplitudes, making this approach complementary to other electronic speckle interferometry methods. An all optical vibration measurement technique is demonstrated by employing laser thermoplastic heating for excitation. Measurements of a vibrating stainless steel plate are presented showing the capabilities of the photorefractive approach for vibrational spectral analysis. INTRODUCTION Vibrational motion is often used to measure or characterize material properties. Many optical techniques have been developed for noncontacting measurements; most of these methods have similar sensitivities and are based on coherent optical interferometry’>z. Adaptive interferometry, which uses the photorefractive effect in optically nonlinear materials, offers a potentially powerful method for real-time optical imaging processing and automatic correction for environmental effects.3’4 Photorefractivity employs optical excitation and transport of charge carriers to produce a hologram of an interference pattern inside the nonlinear optical material. The spatiallyand temporally-modulated charge carrier distribution is a direct measure of the phase information impressed onto the optical object beam by the vibrating surface. This hologram stores phase information from all the surface points on the vibrating specimen simultaneously. The hologram can be detected via diffraction of a readout beam off the photoinduced-volume grating. A method has been developed for vibration detection5’6 that employs the photorefractive effect in a synchronous detection manner.’ This method phase modulates the object and reference beams such that an alternating photorefractive grating at a fixed beat frequency is established within the material regardless of the specimen vibration frequency. The intensity of a readout beam scattered off the photoinduced grating directly measures the vibrational amplitude and phase. It can be used for spectral analysis with a response proportional to the Bessel function of order one, providing a linear output for small amplitudes. The method accommodates rough surfaces and exhibits a flat frequency response above the photorefractive cutoff frequency. A minimum detectable displacement of 2 picometers has been recorded, and further improvement is possible. This paper further describes the ability of this photorefractive method, when coupled with laser thermoplastic heating, to provide an all-optical vibration detection approach that is intrinsically calibrated and capable of real-time full-field imaging. Typically, nonphotorefractive interferometric methods do not image more than one surface point at a time. However, since the photorefractive process records a volume hologram of all vibrating surface points simultaneously, imaging can be readily performed. Both four-wave and two-wave mixing configurations have been employed for reading out the vibrationinduced phase grating image. Results are presented for vibration modes of a free, square stainless steel plate with diffusely reflecting surfaces that is driven piezoelectrically at one corner or by laser thermoplastic heating at points on its surface. PHOTOREFRACTIVE OPTICAL VIBRATION DETECTION Figure 1 shows the experimental setup for optical detection of a vibrating plate. A diode-pumped Nd:YAG laser source (532 nm, 200 mW) is split into object and reference beams. The excited vibrational modes of the plate phase modulate the object beam. The reference beam is phase modulated by an electro-optic modulator (EOM) at a fixed specified modulation depth. Detection Laser \ I 1 Lock-in Amplifier