Abstract

Recently Wu et al.¹ have proposed an optical sensing technique using multimode fibers, called a fiber specklegram. In this paper we propose a system comprising an optical fiber coupled into a photorefractive crystal fiber to construct a compact sensor, which can be integrated into a large scale composite system. To physically simulate a photorefractive fiber, a LiNbO₃ waveguide with a cross-section of 1 mm² and a length of 10 mm is used. The specklegram is made by recording the interference pattern formed by the output speckle field and a plane wave both coupled into the waveguide. Since the readout from the specklegram, or speckle hologram, is sensitive to modal phase changes of the multimode fiber sensor, an application to detect acoustic disturbances on the fiber is feasible. We have shown that the proposed technique yields an efficient and compact sensing and measurement system that can be embedded into composite structures. We have also shown that by using a photorefractive fiber array, multiple sensors for different sensing applications can be synthesized, with little cross-talk among the sensors. Experimental demonstrations also verify its wavelength and angular multiplexing capabilities.

© 1992 Optical Society of America