Microvibrometer uses photorefractive crystals

Dec. 1, 2000
TOKYO—Scientists at the University of Tokyo and the University of Joensuu (Joensuu, Finland) have jointly developed a remote microvibrometer system that can measure vibration amplitudes below 10 nm.

Incorporating news from O plus E magazine, Tokyo

TOKYO—Scientists at the University of Tokyo and the University of Joensuu (Joensuu, Finland) have jointly developed a remote microvibrometer system that can measure vibration amplitudes below 10 nm. The system takes advantage of the photorefractive effect and can be used to measure any material that scatters light. In addition, the system is resistant to external noise, so it can be used in nonideal conditions such as those occurring at manufacturing sites.

In the photorefractive effect, light with a strong spatial distribution is incident upon a material, affecting the refractive index; the charge distribution within the material changes depending on how much light reaches the material, forming a gradient. The refractive index is changed by the internal electric field and electro-optical influences. The effect is not dependent on optical intensity, but rather relies only on the contrast between the light and dark regions. Sufficient change in the refractive index can be seen using a weak light source of only 1 mW/cm2.

Photorefractive crystal is affected by shifts in a speckle pattern induced by oscillation of an object. The resulting refractive-index changes provide a measure of transient phenomena such as vibration.
Click here to enlarge image

Light reflected from the object being measured is focused onto the photorefractive crystal using an adjustable aperture and lens (see figure). A speckle interference pattern forms inside the crystal due to scattering from the rough surface; a change in the double-refraction properties results from photorefractive effects. The speckle pattern vibrates at a frequency proportional to the frequency of the measured object. However, when the vibration is above a certain level, the photorefractive effect cannot keep up, so instead a constant refractive index distribution analogous to the time-averaged spectrum is recorded. When the speckle pattern moves relative to the static refractive index distribution in the crystal, the polarization of the light changes as a result of a spatial gap between the speckle pattern and the refractive index distribution (in other words, the ellipticity ratio of elliptically polarized light changes).

This method can be used to measure objects that are sensitive to high temperatures or otherwise easily damaged; the materials must not have mirrorlike surfaces. The photorefractive effect cuts out low-frequency noise so that only signals above a certain frequency are detected. Thus, the method is resistant to external noise—for example, fluctuations of the air.

This system has some restrictions with regard to the vibration amplitude and vibration frequency range. However, by changing the system parameters in an appropriate manner, it is possible to adjust the sensitivity of the system to match the particular sample. The range of possible vibration amplitudes that can be measured using this method is limited by the ratio of the average diameter of the spectrum to the vibration amplitude. If the amplitude is too small, the output is hidden by noise. If the vibration amplitude is too large, the linear relationship between the vibration of the object and the output signal starts to shift, and eventually becomes averaged. By manipulating the adjustable aperture in front of the crystal and changing the average diameter of the spectrum, it is possible to control the range of signals that can be detected. The vibration amplitude of the sample determines the range.

In addition, the lower cutoff frequency is determined by the time constant of the photorefractive effect. In this system, the output signal consists of the components where the photorefractive effect cannot keep up with the vibration of the measured sample. Therefore, it is not possible to detect slow vibrations where the photorefractive effect can keep up. This cutoff frequency can be adjusted by using different photorefractive materials depending on the vibration frequency of the object. The response time of the photorefractive effect is inversely proportional to the intensity of the light, so fine adjustments of the cutoff frequency can be made by adjusting the light intensity.

If this method is used commercially, it would be possible to make real-time thickness measurements of objects such as high-temperature rolled metal sheets. A pulsed laser would irradiate the front surface, creating an ultrasound pulse; the reflected pulse off the back surface would then be detected using this new system. The thickness of the sheet could be calculated by measuring the time delay. Defects inside solids such as concrete or metal blocks could be detected by measuring the reflected ultrasound waves from internal defects.

The researchers have so far verified the operation steps and evaluated the quality of the system. They are making plans to build a practical system and are developing joint research projects with industry.

Courtesy O plus E magazine, Tokyo

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