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Fiber Optic Communication Link Design DAFFODIL INTERNATIONAL UNIVERSITY JOURNAL OF SCIENCE AND TECHNOLOGY, VOLUME 3, ISSUE 2, JULY 2008 5 FIBER OPTIC COMMUNICATION LINK DESIGN Md. Masud Rana, and Md. Saiful Islam Department of Electronics and Communication Engineering Khulna University of Engineering and Technology, Khulna-9203, Bangladesh E-mail: [email protected] and [email protected] Abstract: In all-optical networks, traffic is carried attenuation and cost. For this reason they end-to-end in the optical domain, without any preferred optical fiber for network purposes as intermediate optical-electrical-optical conversion. transmission medium and over the high bit rate The promise of such networks is the elimination of a point to point communication link. In word, for significant amount of electronic equipment, as well higher speeds and longer distance as added capabilities, such as the ability to transport any type of data format through the communication the transmission link must be network. Fiber optic communications has been made based on optical fiber. Therefore, they growing at a phenomenal pace over the past twenty are used as a preferred transmission medium in years, so rapidly, in fact, that its impact is current communication systems. Ease of increasingly felt in nearly all aspects of communication in any country is the communications technology. The design of such a prerequisite for development. Communication system involves many aspects such as the type of is not just building roads, railway and bridges source to be used (LED, LASER), the kind of fiber to only. To foster and sustain all round be employed (multimode or single mode), and the advancement, a robust, efficient and detector (PIN or APD). This work emphasizes the widespread telecom infrastructure is as integral basic requirements and design approaches of an optical fiber communication link. For designing and essential component of development as any such system the main concern is the optimization of other. Research and development efforts have cost, maximum link length and stability of led to commercial realization of low cost, low performance characteristics. For a given bit rate, loss optical fibers. The optical fiber link length and typical bit error rate, the cost communication system must be in cost effective design is chosen here with software effective which are sensitive to the proper implementation. selections of source, optical fiber and detector. So, before going to implementation of optical Key Words: Bit Error Rate (BER), Light communication system it must take a hand in Amplification Stimulated Emission of Radiation the software analysis in the link design such (LASER), Power Budget, Time Budget, Graded Index (GI) Fiber, and Avalanche Photo Diode that it would be cost effective and meet the (APD). availability of the components. 1. INTRODUCTION 2. OPTICAL FIBER COMMUNICATION The information Age is in a stage of rapid Technologies used in wireless broadband growth. As quick as new technologies are systems are typically data-driven and require a introduced, users demand more capability, very high processing speed. So, before more speed, and greater flexibility. Users designing such a long–haul high speed link one demand more capabilities from every kind of must have knowledge about optical fiber electronic device: more processing power, communication system. An optical fiber is a more features, and better connectivity. Fiber dielectric waveguide made by ultra thin fiber of optic transmission medium is first emerging as glass that operates at optical frequencies i.e., in an alternative and strong competitor to coaxial which the signal is transmitted by the virtue of cables in telecommunication networks. The visible of light. To guide light, optical fiber communication engineers have always dreamt contains of two concentric layers called the of higher information bandwidth with low core and the cladding. A basic optical fiber communication system consists of an optical RANA ET AL: FIBER OPTIC COMMUNICATION LINK DESIGN 6 transmitter (Modulator, Channel coupler and processing circuitry and optical detector e.g., optical source e.g., LED, LASER), a PIN, APD). Fig. 1 shows a representative fiber transmission channel (optical fiber), and an optic communication link. optical receiver (optical amplifier, electronic Fig. 1 A typical optical fiber communication system In Fig. 1 an optical source such as a transmission the use of optical cable may not semiconductor LASER or LED is modulated be cost effective as compared to another by a signal. The output light is introduced into procedures. In that case one may prefer another the fiber link through a set of connectors or procedure for data transmission. One option for through a permanent fiber splice. If the link is a data transmission line is traditional metal long, the light intensity diminishes because of cable, such as a twisted-pair cable and a coaxial attenuation in the fiber, and the optical signal cable. It is suitable for many applications, but may need to be regenerated by an optical is not suitable where noise immunity is repeater. The optical repeater consists of an required or for long-distance data transmission. optical detector that converts the light into an Instead of metal cable, optical-fiber cable has electrical signal, then amplifies it by means of been developed to overcome these limitations. an electronic processing circuits and finally For digital audio applications, all-plastic fiber coverts the electrical signal into an optical is already well known and widely used for data signal by an optical source. An alternative to transmission lines. Fiber optic cable is more repeating the signal is to optically amplify the efficient for interconnecting cable TV or phone light by means of an optical amplifier. The companies that are consolidating with WDM combines the signal beam and a pump geographically adjacent companies. laser beam (at two different wavelengths) in an optical fiber amplifier. The fiber amplifier 3. FIBER LINK strengthens the signal beam using the power To provide the communication through optical from the pump laser beam without requiring fiber must need a procedure to design a usable optical/electronic and electronic/optical optical fiber link to meet the desired conversion. Finally, the optical signal arrives as specifications such as coupling losses, bit error its destination and the information is recovered rate, data rate, etc. The procedure is iterative, and converted back into its original format. that is certain assumptions are made and the The main objective of fiber optic cable is to design is carried out based on those support greater bandwidth and high assumptions. The design is not finished at that transmission rate. But, incase of short distance point; however the designer must verify that it DAFFODIL INTERNATIONAL UNIVERSITY JOURNAL OF SCIENCE AND TECHNOLOGY, VOLUME 3, ISSUE 2, JULY 2008 7 meets the objectives and represents economical design must be chosen in such a way that the and technical solutions. If not, it is required to losses (e.g., source to fiber coupling loss, fiber pass to another combination through procedure. to fiber coupling loss, fiber to detector coupling In particular, the assumptions need to be loss, dispersion, attenuation etc.) involved in inspected to determine if changes might those requirements should be maintained to a provide a simpler or cheaper alternative. minimum. The optical fiber link design simulator flow chart is given below Fig. 2[1]: Flow Chart of Link Design 3.1 Requirements in the Link Design Start The key system requirements needed in the link Specify design are: R, BER, L (a) Data or bit rate/bandwidth; (b) Bit error rate/signal to noise ratio and Select source, (c) Transmission distance or link length. Fiber, detector In the optical fiber communication link design the basic issues are: Evaluate Pr, for given R, BER (a) Attenuation which determines the power available at the photodetector input for given Determine Pm source power (known as power budget) and (b) Dispersion which determines the limiting NO data rate usable bandwidth (known as time All Is No Select different combinations budget) [2]. Pm > 4 dB? combinations tried? Yes 3.2 Power Budget Yes In a fiber-optic communication link, the Make time Repeater budget required allocation of available optical power (launched into a given fiber by a given source) among Stop various loss-producing mechanisms such as Is it No satisfactory? launch coupling loss, fiber attenuation, splice losses, and connector losses, in order to ensure Yes that adequate signal strength (optical power) is Evaluate max. available at the receiver. The amount of optical link length power launched into a given fiber by a given transmitter depends on the nature of its active List out source, fiber, detector optical source (LED or laser diode) and the type of fiber, including such parameters as core diameter and numerical aperture. Stop Manufacturers sometimes specify an optical Fig. 2 Flow chart showing the steps involved in a power budget only for a fiber that is optimum fiber optic link design for their equipment--or specify only that their equipment will operate over a given distance, The starting point for optical fiber without mentioning the fiber characteristics. communication link design is choosing the The purpose of the power budget is to ensure operating wavelength, the type of source (i.e., that enough power will reach the receiver to LED or LASER), and whether a single mode or maintain reliable performance during the entire multimode fiber is required. In a link design, system lifetime. Performance of the system is one usually knows (or estimates) the data rate evaluated by analyzing the link power budget required to meet the objectives. From this data of the system and the cost is kept minimum by rate and an estimate of link length, one chooses carefully selecting the system components from the wavelength, the type of source, and the a variety of available choices [2].
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