Microbending effects in singlemode optical fibers: investigation and novel applications

Abstract

Microbends are axial distortions on the optical fiber that have a spatial wavelength small enough to effect coupling between guided and radiation or cladding modes. The magnitude of this wavelength-dependent coupling is a function of the nature and the number of microdefonnations. Since these periodic perturbations lead to an attenuation in signal level, they are avoided in fiber-based communication systems. However, controlled induction and signal processing of microbending losses has led to the fabrication of novel optical fiber~based sensors, devices, and components. A systematic study of microbending effects in singlemode optical fibers is presented in this thesis. The theoretical analysis is based on the coupling between the fundamental LP01 mode to discrete cladding modes. An algorithm is developed to characterize optical attenuation as a function of the spatial period of the microbend defonnation. Optical attenuation peaks are described in terms of central wavelength, amplitude and spectral width. An excellent correlation is shown between the experimental results and the theoretical predictions, with nominal errors less than 2.5%. The algorithm developed may be used with any commercially available singlemode fiber, and any kind of microbend de former apparatus, provided the microbend defonnation function ⠱(z) is known accurately. Based on the above analysis, a wavelength-tunable fiber polarizer is proposed and demonstrated. The polarizer is fabricated by inducing a periodic perturbation on a high birefringence singlemode optical fiber. The fiber thus exhibits polarization· selective attenuation characteristics. The operating wavelength is shown to be tunable by changing the spatial period of the defonnation. A polarization extinction ratio of 25 dB is obtained with an attenuation of 1.3 dB, at an operating wave length of 1177 nm.