<title>Fiber specklegram sensing with phase-conjugation</title>

Abstract

A multimode fiber specklegram sensor using phase-conjugate mirror is presented. This sensing technique is based on the phase compensation of phase-conjugate signal, which is very sensitive to external perturbation. The preliminary experiments on displacement measurement are also provided. . INTRODUCTIONOptical fiber sensing is a most promising technology for many industrial observation problems. The basic advantages implicit in optical fiber sensing, such as immunity from electromagnetic interference, chemical resistance, and potentially high sensitivity, make this approach attractive'. A new class of optical fiber sensor, recently proposed by Wu et a12, is based on the specklegram from multimode fiber. Since the speckle distribution is attributed to interference among those guided modes in multimode fiber, the specklegram fiber sensor is of sensitivity as high as interferomethc sensors. Based on the correlation between speckle patterns, this kind of optical fiber sensor has been used to measure displacement, vibration and temperature35. Furthermore, the specklegram fiber sensor can also be used in multichannel sensing because of the high space-bandwidth product of multimode fibers.The high space-bandwidth product of multimode fiber has also been utilized to transmit spatial information.7'8. The key point in this technique is how to reconstruct the spatial information from distortion caused by modal coupling and dispersion. Yariv7 has proposed to retrieve the image using the wave-front-reversal property of phase-conjugate mirror. Such an image transmission technique requires the same phase distribution in forward and backward paths. Therefore, in Yariv's method two identical fiber must be found before this technique can be realized. To overcome that difficulty, Dunning and Lind8 proposed a single fiber architecture, and implemented reconstruction of image transmitting through fiber. Although it is apparently not a practical method because transmitted image can only be reconstructed in the input end of fiber, they successfully demonstrated the process of phase compensation using phase-conjugate mirror. We note that, in Dunning and Lind's architecture the degradation of the image will not be compensated completely when external perturbation exists, and that deviation from original pattern can be utilized to measure that external perturbation. In this Technical Note, a fiber specklegram sensor with phase-conjugate mirror is presented. We analyzed the effect of external perturbation on the reconstructed light field, and the preliminary experimental results of submicrometer displacement sensing are also provided. . PHOTOREFRACTIVE EFFECT AND PHASE CONJUGATIONPhotorefractive effect is a phenomenon in which the local index of refraction of a medium is changed by the illumination of a beam of light with spatial variation of the intensity9. This effect is attributed to the formation of the photoinduced space-charge field and formation of index grating via the linear electro-optic effect ( the Pockles effect). In comparing...