Abstract

Most fiber-optic sensors and transducers use one of the classical modulation techniques: intensity, phase, or polarization modulation. The principal complication in intensity modulation results from the need for a high-stability optical-source intensity or reference channel. Furthermore, sensors based on phase or polarization modulation use single-mode or special fibers and require expensive techniques for detection. This paper presents a new technique for sensor applications that uses modal power distribution (MPD) modulation. It is a highly sensitive and economical technique and can be applied to multimode as well as to singlemode fibers. The principle of the MPD technique has been described in previous work and may be characterized as a smart structure.¹-² Measurements on MPD can be used to sense induced perturbations in optical fibers. This can be accomplished by scanning the far-field pattern at the fiber end with a charge-coupled device camera or an array of photodetectors. In our work, multimode fibers with a continuous perturbation in the cladding index are extensively studied. This perturbation is created by chemical etching of the cladding material. The etching process results in continuous decrease in the clad thickness, which is replaced by a modified cladding. The optical fiber is excited by a beam of light at 10° off the fiber axis. During the etching process, the modal power distribution, as well as the total throughput, are continuously measured and recorded. We show, based on our results, that the modal power distribution technique is much more sensitive than the intensity modulation technique and that it can be used in a variety of sensor applications.

© 1990 Optical Society of America