Jürgen Krauser

The use of polymer optical fibers for in-house-networks, advantages of 520 nm LED transmission systems

The use of polymer optical fibre (POF) will be an attractive solution for future broadband CPN. Present systems work with 650 nm sources. This paper propose the use of 520 nm InGaN LED and will show advantages and open problems. We present the present status of POF systems, based on 650 nm components. The ATM forum specifies a link with a capacity of 155 Mbps and 50 m reach. For a number of applications it would be helpful to improve this system in the direction of a longer reach and/or more system margin in the power budget.

Applications of Polymer Optical Fibers

The polymer optical fiber has been employed for many years now. In lighting technology it represents a widely used and recognized medium. The share of POF in sensor technology and data communications on the other hand is rather small. Up until now POF was predominately used in niche areas. At present a dramatic change is taking place especially in data communications. The digitalization of diverse entertainment media (music, language, video, pictures), a process which is practically completed, and the steadily growing amount of terminal equipment has led to a massive demand for reasonably priced, fast and reliable data connections in all areas of private and public life. In this chapter some possible uses for POF, as well as areas in which it is already being utilized, will be discussed. Those areas of use which lie outside the field of data communications and beyond the scope of this book will only be treated briefly. For these areas we refer to existing publications ([Wei98], [FOP97]).

Optical measuring methods

There are three main areas in which optical measuring methods are applied: 1. for manufacturing, control and product specification, 2. during and after installation, 3. for maintenance and for searching faults.

Development of the polymer optical fiber (POF)

The first POF were manufactured by DuPont as early as the late sixties. Due to the incomplete purification of the source materials used, attenuation was still in the vicinity of 1,000 dB/km. During the seventies it became possible to reduce losses nearly to the theoretical limit of approximately 125 dB/km at a wavelength of 650 nm. At that point in time glass fibers with losses significantly below 1 dB/km at 1,300 nm/1,550 nm were already available in large quantities and at low prices. Digital transmission systems with a high bit rate were then almost exclusively used in telecommunications for long-range transmissions. The field of local computer networks was dominated by copper cables (either twisted-pair or coaxial) that were completely satisfactory for the typical data rates of up to 10 Mbit/s commonly used then. There was hardly any demand for an optical medium for high data rates and small distances so that the development of the polymer optical fiber was slowed down for many years. A significant indicator for this is the fact that at the beginning of the nineties the company Hochst stopped manufacturing polymer fibers altogether.

The Reliability of POF

Polymer optical fibers, like other technological products, are subjected throughout the whole of their service life to a great many kinds of stress from the environment — mechanical, climatic, chemical, biological, and radiometric. As a result of these stress factors, physical and chemical alterations may arise in the materials used. These can in various ways have an effect on the functional behavior, suitability for use, and serviceable life expectancy (that is to say, the durability) of the POF. Environmental stress factors, then, have an influence that should not be underestimated on the quality and reliability of the fiber optic transmission system. When polymer optical fibers are used, it is thus imperative to understand and take into consideration the effects of environmental influences — particularly of industrial environmental influences - on those properties of POF which enable it to transmit optical signals.

Light propagation in optical fibers

Many of the properties of light such as interference, refraction, and polarization can be explained with the wave model. Others, such as the photo effect, show that light is not a continuous type of radiation, but is rather made up of very small particles called photons. These photons are elementary quanta and cannot be divided any further. The energy of a photon is expressed by the following equation: W = h · f, where W = the energy in Joule [J], h is Planck’s constant = 6.626 · 10−34 Js and f is the frequency of the light in [s−1]. The frequency of the radiation is calculated from c/λ, whereby c is the speed of light in vacuum = 2.99792458 ·108 m/s and λ, is the wavelength of light in [m].

Properties of polymer optical fibers

Like glass fibers, polymer optical fibers are characterized by a wide range of different parameters. In Chapter 1 we discussed the extent to which light propagation affects such essential properties as bandwidth and attenuation. The user will also be interested in other characteristics such as: Durability of the fiber Thermal resistance Sensitivity to bending Mechanical stability (stress limits for tensile and pressure stress, etc.) Radiation resistance (e.g. for applications in nuclear power plants) Resistance to chemical substances

Components for POF systems

An optical transmission system essentially consists of three components. The transmitter converts the electric sequence of signals into an optical one and inputs it into the optical transmission channel, in this case the polymer optical fibers. The transmission channel, which may contain further active or passive components in addition to the fibers, forwards the signal to the receiver. Here the signal is converted back into an electric signal that is then available for further processing. Usually, the goal is to make the electric signal received as similar to the starting signal as possible. Of course, the transmitter and the receiver play an important role in the process since they are primarily responsible for converting the signal (optical or electric voltage).

Lichtausbreitung in optischen Fasern

Viele Lichterscheinungen, wie z.B. Interferenz, Beugung, Polarisation, lassen sich im Wellenmodell erklaren, andere, wie z.B. der lichtelektrische Effekt, zeigen, das das Licht nicht wie kontinuierliche Strahlung wirkt, sondern einen Teilchencharakter zeigt. Diese Lichtquanten (Photonen) sind nicht weiter teilbar, sondern stellen eine elementare Grose dar. Die Energie W eines Photons wird durch W = h · f beschrieben, mit W: Energie in Joule [J], h: Plancksche Konstante = 6,626–10-34 Joulesekunden [Js] und f: Frequenz des Lichtes in [Hz]. Die Frequenz der Strahlung errechnet sich aus f =c/λ, mit: c = 2,99792458·108 m/s, Lichtgeschwindigkeit im Vakuum und λ Wellenlange des Lichtes in [m]. Wird die Energie in Elektronenvolt [eV] angegeben, erhalt man fur die Umrechnung: 1 eV = 1,602 · 10‒19 As · 1 V = 1,602 · 10‒19 J, 1 J = 6,25 · 1018 eV. Der Teilchencharakter des Lichtes tritt umso mehr in Erscheinung, je kurzwelliger die Strahlung bzw. je hoher die Frequenz wird.

Eigenschaften optischer Polymerfasern

Optische Polymerfasern zeichnen sich, ebenso wie Glasfasern, durch eine Vielzahl unterschiedlicher Parameter aus. In Kapitel 1 wurde bereits gezeigt, in welchem Mase die Lichtausbreitung solche wesentlichen Eigenschaften wie Bandbreite und Dampfung beeinflust. Daneben interessieren den Anwender weitere Kenngrosen, wie z.B.: Lebensdauer der Faser Temperatur- und Feuchtebestandigkeit Biegeempfindlichkeit mechanische Stabilitat (Zug- und Druckfestigkeit usw.) Strahlungsfestigkeit (z.B. fur den Einsatz in Kernkraftwerken) Bestandigkeit gegenuber Chemikalien Einigen dieser Punkte sind im weiteren Verlauf spezielle Kapitel gewidmet. Die Dampfungen unterschiedlicher Faservarianten werden in Kapitel 3 detailliert beschrieben. Dem Komplex der Zuverlassigkeit und Umweltbestandigkeit wird ebenfalls ein Kapitel gewidmet. Im folgenden Text sollen einige der ubrigen Eigenschaften genauer betrachtet werden. Dabei gilt der wesentliche Blick der Bandbreite von Stufenindexprofll-Polymerfasern. Diese Grose ist nicht nur einer der wichtigsten Parameter fur das Systemdesign, sondern auch mit am schwierigsten zu verstehen, bzw. exakt zu bestimmen, weswegen hier ausfuhrlich darauf eingegangen werden soll.