High speed plastic optical fiber for data communications

In this thesis, we report the results of our theoretical and experimental studies of high speed plastic optical fiber (POF). These low loss and high bandwidth POF have potentially important applications in rapidly expanding short distance communication systems.; With the perfection in fabrication techniques, the optical loss of POFs have dropped considerably. With the optical loss problem under control, the next obstacle to the success of POF is the limited bandwidth. The optical bandwidth in POF is limited mainly by intermodal dispersions, as our theoretical studies show. The large core diameter and large numerical aperture of POF results in millions of guiding modes, all traveling at their own characteristic speed. Such dispersion is most severe in step index (SI) POF, which is the only available POF. It has been shown that by making the refractive index a smoothly varying gradient, the intermodal dispersion can be greatly reduced. One of the main objectives of our study is to understand the relationship between refractive index profiles and optical bandwidth in POFs. The waveguiding problem is most conveniently solved by WKB approximations based on the scalar wave equations. Besides analyzing known refractive index profiles, we derived a new analytic form of refractive index profile which can potentially support very high bandwidth. A more practical profile, the stair case profile is optimized based on the knowledge we gain from our WKB results. Such a profile, which is much easier to fabricate, can be a good alternative to the gradient index profile.; Optical loss measurements are performed on both SI POF and GI POF with excellent results. The SI POF we measured at 650 nm has an optical loss of merely 110 dB/km, which is approaching its theoretical limit of 106 dB/km. The GI POF exhibits an optical loss of 158 dB/km at 650 nm, which is also very impressive. Another equally important parameter is the bandwidth, which we measured in time domain using a high speed optical pulse source and optical sampling oscilloscope. The GI POF shows an impressive 3 GHz-100 m bandwidth, which exceeds requirements of today's short range data communication systems.; However, as we show in our study, the 3 GHz-100 m bandwidth is still far below the potential of an optimal GI fiber. The origin of this limited bandwidth is the actual refractive index profile. With the refractive near field measurement that we developed for POF, we were able to perform highly accurate measurements of the refractive index profile of the first GI POF. Using the theory we developed, we identified the defects in the refractive index profile of the current GI POF. The main feature we found is that the gradient in the index is not formed properly near the core-cladding boundary. The theoretical model that we developed based on the measured profile quantitatively explained the reduced bandwidth observed from the ideal profile. By comparing the theoretical calculated bandwidth with the experimentally measured one, we also found strong evidence that the mode distribution is inhomogenous and that there exists mode coupling which can actually results in an increase in the actual bandwidth.; We have therefore developed experimental test beds and theoretical tools for studying high speed POF for the first time. Work is now underway to develop high speed POF for data communications here at Penn.