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 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 is in a stage of rapid Technologies used in 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 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 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 (, 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 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 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]. fiber type. Also, the requirements for the link RANA ET AL: FIBER OPTIC COMMUNICATION LINK DESIGN 8

In the preparation of link power budget, certain upgraded. With Pm ≤ 4dB, system will become parameters like required optical power level Pr less reliable. A typical losses involved in the at the receiver to meet the system requirements, calculation of power budget: The loss model coupling losses etc. are required. In any for a point to point optical fiber link is shown practical design, an allowance has to be made in Fig. 3 [2]. From the above figure optical for the degradation of components with ageing, power loss occurs due to replacement, variations due to temperature (a) Coupling losse (Lsf, Lff and Lfd) fluctuations, manufacturing spreads, imperfect (b) Connector loss repeatability on reconnection, field repairs, (c) Splice loss maintenance, and variations in drive conditions (d) Fiber attenuation and and so on. In an optical communications link, (e) Fiber bend loss power margin is the minimum optical power The typical values of Lsf, Lff and Lfd are about 2 that is required by the receiver for a specified dB, 0.5 dB and 0.2 dB respectively. But they level of performance. The amount of optical also vary with wave length [2]. The source to power launched into a given fiber by a given fiber coupling loss is largely dependent on the transmitter depends on the nature of its active numerical aperture, source, fiber size (core optical source (LED or laser diode) and the diameter/cladding diameter) and can be type of fiber, including such parameters as core calculated as follows which is listed in Table 1 diameter and numerical aperture. After [7]. computing various losses and fixing safety margin, power budget of the link is calculated Table 1 Coupling efficiencies calculation by the following equations [2]: Nonlambertian Lambertian emitter Power margin in dB, Pm = (Pt - Pr(min) - Lsf - emitter NLff - αL - Lfd)……………………………(1) Coupling Coupling Coupling Where, Pt= Source output power...…...dBm efficienc efficienc efficiency Fiber Pr (min) = Minimum receiver power….dB y (η) rs ≤ y (η) (η) rs > a Lsf = Source to fiber coupling loss……dB a rs < a L = Fiber to detector coupling loss… dB fd Step (m+1/2) L = Fiber to fiber coupling loss……..dB NA2 NA2(a/r )2 ff s index NA2 N=Number of splice = Integer part of [L/L0] 2 L = Fiber link length………………… km NA [1- 2 2 Not NA (a/rs) Not L0 = Factory unit length of fiber…... …km (2/(g+2)) assig g (g/(g+2)) assigned α = Attenuation coefficient of fiber... dB/km ( rs/a) ] ned αL = Fiber loss……………………… dB N = Total splice loss………………...dB α where, rs = radius of the source a = the fiber core radius g = the refractive index profile; m = a constant has a value larger than 1 (typically in the range 14 to 34).

Now the source to fiber coupling loss Lsf can be calculated (in dB) from the table (Table 1) as follows:

Lsf = -10 log10 η (dB)…………………….(2)

After specifications all the terms mentioned Fig. 3 Loss model of point to point optical fiber link above one can simply estimate the power (C; connector and S; splice) budget calculation. For example the power budget of a 0.85 µm lightwave system is listed A power margin Pm ≥ 4dB is acceptable in Table 2 [5]. otherwise some components need to be DAFFODIL INTERNATIONAL UNIVERSITY JOURNAL OF SCIENCE AND TECHNOLOGY, VOLUME 3, ISSUE 2, JULY 2008 9

Table 2 Power budget of a 0.85 µm light wave System time budget is considered to be system satisfactory if σsys does not exceed (1/4R); Quantity Symbol LASER LED where R is the data rate in Mbps.

Transmitter -13 P 0 dBm 3.4 Transmitter and Receiver Rise Time power t dBm The time taken for a signal to rise from silence Receiver -42 -42 P (min) to full intensity is called rise time. The rise sensitivity r dBm dBm time of transmitter primarily depends on the Power electronic components of driving circuit and P 6 dB 6 dB margin m electrical parasitic associated with the optical Available CL=-Lsf + source. It is few nanoseconds for LED based channel NLff + 36 dB 23 dB transmitter, but can be as short as 0.1 ns for a loss αL+ Lfd LASER based transmitter. Presuming an Maximum exponential rise and decay, σ is nearly equal L 9.7 km 6 km tx fiber length to half of the rise time [2]. The receiver rise time is determined as follows: 3. 3 Time Budget trx = 350/B (ns) ………...…………………(4) The objective of the time budget is to ensure where, B is the receiver bandwidth in MHz. that the system is able to operate properly at the desired data rate. Sometimes, it may happen 3.5 Dispersion that the total system is not able to operate at the Dispersion is the phenomenon that the phase desired data rate even if the bandwidth of the velocity of a wave depends on its frequency. individual system exceeds the data rate. The There are generally two sources of dispersion: root means square (RMS) width of pulses material dispersion, which comes from a propagating in nonlinear, dispersive fibers is frequency dependent response of a material to useful for predicting how far a pulse can travel waves and a modal dispersion that results from before it suffers significant distortion due to the the different transit lengths of different combined influence of the nonlinearity of the propagating modes in a multimode optical refractive index and the dispersive properties of fiber. Considering both the modal and material the fiber. This theory applies to pulses dispersions in fiber optic the rms pulse width of operating near the zero-dispersion wavelength the fiber σf is determined as follows: 2 2 where dispersion alone has a negligible σf = √(σ mod + σ mat) ………………………(5) influence, but where the combined influence of Where, σmod and σmat are the rms pulse widths nonlinear self-phase modulation and dispersion due to modal and material dispersion (and can produce a significant effect. It is usually expressed in ps/km-nm) of the fiber demonstrated that for an optical pulse respectively. For SM fiber, modal dispersion propagating along an optical fiber the rms pulse does not contribute and therefore, σf becomes width varies parabolically with distance, same as σmat. irrespective of initial pulse form and frequency The optical fiber link may consist of several chirp variation. Furthermore, the result is true concatenated sections and the fiber in each to arbitrary dispersive order and should prove a section may have different dispersion very useful tool in determining the characteristics. Further, there may be mode information-carrying capability of long- mixing at the splices and the connectors. As a distance optical-fiber transmission systems. If consequence, propagation delay associated the transmitter, fiber and receiver are with different modes tends to average out. In considered to have the rms pulse widths σtx , σf the absence of mode mixing, σmod for SI fiber is and σrx respectively , then rms pulse width of the [2]: overall system response will be [2]: σmod=(n1∆L)/(2√3c)……………….………(6) 2 2 2 σsys = √ (σtx + σf + σ rx ) …………….…..(3) Where, c is the velocity of light and the parameter ∆ depends on the core and cladding refraction indices. For GI fibers, delay time is a RANA ET AL: FIBER OPTIC COMMUNICATION LINK DESIGN 10 function of the refractive index profile (g). The 4 Simulation Results minimum intermodal rms pulse broadening For the combination of LASER-GI-APD with an optimum g is [2]: (Fig.4), it is clearly observed that both the 2 σmod=(n1∆ L)/(20√3c)…………………….(7) attenuation limited distance and dispersion The rms pulse width due to material dispersion limited distance are decreased with data rate is determined as follows: (Mbps). As the total dispersion is σmat = | Dmat |σλ L……………..…….……..(8) approximately zero and a significant amount of Where, σλ is the rms spectral width of the attenuation around the 1300nm operating source and Dmat is the dispersion parameter. wavelength which is a common phenomenon [4][6-7] so, the dispersion limited distance is 3.6 Link Length quiet high as compared to attenuation limited The design of fiber optic communication distance before their intersection point. For systems requires a clear understanding of the considering the optimum and reliable link limitations imposed by the loss, dispersion and design both the attenuation limited distance and nonlinearity of the fiber. Since fiber properties dispersion limited distance are equal at the are wavelength dependent, the choice of the desired data rate with suitable selection of operating wavelength is a major design issue. fiber, source and detector. From this curve When a particular set of components meets the (Fig.4) it is observed that both the attenuation design requirements, one would like to know limited maximum distance and dispersion the maximum distance up to which these limited maximum distance are equal (12km) at components could be used. Further, if the link a data rate of 60Mbps. This 60Mbps data rate is length is quite large, it will help in determining not desired for proposed link design however, the repeater location. The maximum link length GI fiber is expensive as compared to the SI is determined presuming that link is attenuation fiber. So, we ignore this combination as it is limited and there is no dispersion effect. And not suitable for the optimum and reliable link again when the link is dispersion limited and design. there is no attenuation effect. Minimum of the For the combination of LASER-SI-APD as two is taken as the maximum practicable link shown in Fig.5, It is observed that at optimum length. For dispersion limited link, maximum point (i.e. certainly the intersection point) the data rate that can be transmitted over an optical data rate is 45.65Mbps and the maximum link fiber system is given by [2]: length is 17km. As the detector APD is quiet R= (1/4 σsys)………………………………(9) expensive and this proposed link design is Where, R is data rate in Mbps. considered for 10km. So, again this The maximum allowable fiber dispersion will combination is not suitable for the optimum be: and reliable link design. 2 2 2 σal=√((1/4R) - σ tx -σ rx)ns……………...(10) Finally, it is described sharply that the Now we can easily determine fiber dispersion combination of LASER-SI-PIN as shown in per unit length as (σf/L). Therefore, the Fig.6 which clearly states that for the same data maximum link length under dispersion limited rate 45.65Mbps the desired maximum link condition is determined as [2]: length is 12km. And these optimum values are Ld (max) = (L*σal)/σf (km)………….(11) obtained at a lowest cost combination of fiber For comparison purposes we frequently want to and detector. But in the combination of calculate the maximum link distance for a LASER-GI-APD maximum link length is 10 system limited only by the fiber attenuation. Km which is lower than LASER-SI-PIN Therefore, the formula for maximum link combination. So, the lowest cost combination length under attenuation limited condition is: of source, fiber and detector for the broadband La (max) = [Pt (dBm) – Pr (dBm)]/α (dB/km) optical link design is LASER-SI-PIN. (km) ...... (12)

DAFFODIL INTERNATIONAL UNIVERSITY JOURNAL OF SCIENCE AND TECHNOLOGY, VOLUME 3, ISSUE 2, JULY 2008 11

5 Conclusion The optical reach of today’s ultra-long-haul system is clearly shorter than the length of the longest demands. We have looked at network design in today’s realistic backbone networks. The key system requirement for a fiber optic link design is power budget and time budget, in general. If these two basic issues are satisfied then easily calculate the maximum link length. These are tried to be satisfied in such a way that we have moderate data rates and moderate distance transmission. For this the link design reduces to the selection of commercially Fig. 4 Maximum transmission distance vs. data rate available transmitter and receiver modules. The for the combination of LASER-GI-APD user design is only in the selection of the power modules and in the design of the electronic interface circuitry. Depending upon these issues; the LED is tried to be selected first as an optical source because it is commercially available and low cost but for long distance link design (in which LED can not meet the key system requirement) LASER is used. The other components such as fiber (SI-SM, SI- MM and GI-MM), detector are also chosen.The results over a range of real networks have demonstrate that treating routing and wavelength assignment as separate steps can produce cost-eeffective, efficient designs. Finally we may conclude that for this Fig. 5 Maximum transmission distance vs. data rate proposed link design lowest cost combination for the combination of LASER-SI-APD of source-fiber and detector is LASER-SI (SM)-PIN. The design of a fiber optic communication system involves the optimization of a large number of parameters associated with transmitters, optical fibers, optical amplifiers and receivers. The aspects are too simple to provide the optimized values for all system parameters. Since economic considerations often play a more important role than technical consideration in the design of optical link. For this design we have made an approach that uses computer simulations and provides a much more realistic modeling of fiber optic communication systems. The computer aided design techniques are capable of optimizing the whole system and can provide the optimum values of various system Fig. 6 Maximum transmission distance vs. data rate parameters such that the design objectives are for the combination of LASER-SI-PIN met at a minimum cost combination.

RANA ET AL: FIBER OPTIC COMMUNICATION LINK DESIGN 12

References Applications”, Published by Van Nostrand Reinhold [1] J.H. Franz, V.K. Jain, “Optical Communications Company, First edition, 1987. Components and System”, Narosa Publication [10] Md. Masud Rana, Md. Saiful Islam, Sk. Shariful house, India, 2001. Islam , D.M. Motiur Rahaman, “Design [2] Jain, V.K., “An approach to optical fiber link Considerations of Fiber Optic Communication design”, Student Journal IETE, 29 (1988)2, 35-41. System for a Community Area Networks (CANs)”, [3] Jain, V.K. Gupta, H.M. “Optical fiber Proceedings National Conference on communication link design”, Journal of Communication and Information Security (NCCIS), Opt.Commun.6 (1985)2, 58-66. pp. 29, 24 November, 2007, Dhaka, Bangladesh. [4] F. Forghierhi, R. Tkach, A. Chraplyvy, and A. [11] Jane M. simmons,” Network Design in Realistic” Vengsarkan, “Dispersion- compensating fiber: Is All- Optical” Backbone Networks”, IEEE there merit in the figure of merit?,” Optical Fiber Communication Magazine, November 2006, Vol. 44. Communications Conference, Vol.2, 1996 pp 88-94. Technical Digest, (Washington DC), Optical [12] Victot P. Hubenko, Jr. Richard A. Rains, Robert F. Society of America, 1996. Mills, Rusty O. Baldwin, Barry E. Mullins, and [5] C. A. Szabo, I. Chlamtac, E. Bedo, “Design Michael R. Grimaila, ”Improving the Global Considerations for Broadband Community Area Information grid’s Performance through Satellite Networks,” Proceedings of the Proceedings of the Comunication Layer enhancements” IEEE 37th Annual Hawaii International Conference on Communication Magazine, November 2006, Vol. 44. System Sciences (HICSS'04), Vol. 5, pp.50123.2. pp 66-72. [6] W. Gambling, H. Matsumura, and C. Ragsdale, [13] X. Yang and B. Ramamurthy,” Dynamic Routing in “Zero total dispersion in graded-index single -mode Translucent WDM Optical Network: The fibers”, Electronics Letters, vol. 15, pp. 474-476, Intradomain Case”IEEE J. Lightwave Tech., 1979. vol. 23, No. 3, March 2005, pp. 955-71. [7] B.J. Ainslie and C.R. Day, “A review of single-mode [14] Juan C. Juarez, Anurag Dwivedi, A. Roger fibers with modified dispersion characteristics”, J. Hammons, Jr., Stenen D. Jones, Vijitha Light wave Technology, vol. LT-4, no. 8, pp. 967- Weerackody, and Robert A. Nichols, “ Free- 979, 1986. Space Optical Communications for Next- [8] W. Gambling, H. Matsumura, and C. Ragsdale, Generation Military Networks”, IEEE “Mode dispersion and profile dispersion in graded- Communication Magazine, November 2006, Vol. 44. index single mode fibers”, Microwave, Optics, and pp 46-51. Acoustics, vol. 3. pp. 239-246, 1979. [15] S. Griggs, “Optical and RF Combined Link [9] Donald G. Baker, “Monomode Fiber-Optic Design with Local Area and Long Haul Networks Experiment”, Invited paper, Proceding SPIE, Volume. 5820, 2005, pp. 75-82.