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Browse Prior Art Database

TAILORED EMITTER SPECTRUM FOR PLASTIC OPTICAL FIBER

IP.com Disclosure Number: IPCOM000006231D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2001-Dec-17
Document File: 5 page(s) / 280K

Publishing Venue

Motorola

Related People

Paige Holm: AUTHOR [+2]

Abstract

Throughout the evolution of fiber optic technol- ogy, optoelectronic component development has been driven by the characteristics of glass fiber for telecom- munications applications. These glass fibers typically have small core diameters (SM, 50um, or 62.5 urn) and low numerical apertures (0.1 - 0.2). Because of the low NAs (and graded index), multimode distortion is negligible when compared to the effects of material dispersion. The combination of low values of material dispersion and attenuation result in a high bandwidth- distance product for glass fibers.

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MOTOROLA INC. Technical Developments Volume 13 July 1991

TAILORED EMITTER SPECTRUM FOR PLASTIC OPTICAL FIBER

by Paige Holm and Bill Rhyne

INTRODUCTION

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  Throughout the evolution of fiber optic technol- ogy, optoelectronic component development has been driven by the characteristics of glass fiber for telecom- munications applications. These glass fibers typically have small core diameters (SM, 50um, or 62.5 urn) and low numerical apertures (0.1 - 0.2). Because of the low NAs (and graded index), multimode distortion is negligible when compared to the effects of material dispersion. The combination of low values of material dispersion and attenuation result in a high bandwidth- distance product for glass fibers.

  The characteristics of optoelectronic emitters, LEDs and lasers, have been designed to match the specifications of the glass fiber. The low NA and small diameter of the fiber core require a good emitter to have high radiance or brightness (flux per unit area per steradian). This means that the emission spot must be smaller than the fiber core and the device must operate at high current densities. Emitter wavelengths are chosen to coincide with attenuation or dispersion minimums which occur in the near IR. Narrow spec- tral widths (single mode lasers) are desired to mini- mize dispersion effects.

In recent years there has been growing interest in relatively inexpensive plastic fiber for fiber optic local

area networks (LANs) in applications including auto- motive, datacorn, video display, and medical sensing. Plastic fiber specifications differ greatly from those of glass fiber. Core diameters are much larger (-lmm), as are the NAs (-0.5 with step index). As a result, multimode distortion is dominant over the effects of material dispersion and, together with high attenuation values, severely limits the bandwidth-distance product for plastic fiber.

  To optimize the performance of plastic FO links, the emitter characteristics must be tailored to the spec- ifications of the fiber. This may force us to abandon some concepts widely applied to emitters for glass

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fibers. In particular, the much larger numerical aper- ture and multimode bandwidth-distance limitation of plastic fiber suggest ways of improving emitter design. Described below is a novel idea for increasing the overall efficiency of a plastic fiber link which involves expanding and tuning the emission spectrum of the emitter.

RELEVANT PLASTIC FIBER CHARACTERISTICS

Pulse Distortion The factors relating to pulse spreading for a popo- lar plastic fiber (Mitsubishi's Eska fiber) is shown in the table below.

Type of Magnitude of Dispersion Distortion Pulse Spread Slope

Chromatic (Ah = 20 nm) 6.6 ns/km 1.4 ns/(nm'-km)

Intermodal (NA = 0.5) 240 nskm None

  Chromatic distortion may include the effects of both material and waveguide dispersion (material dis- persion usually dominates at the wavelengths of inter- est) while the intermodal distortion arises from the fact that...