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Daffodil International University
Institutional Repository
DIU Journal of Science and Technology Volume 3,Issue 2,July 2008
Daffodil International University
http://hdl.handle.net/20.500.11948/
Downloaded from http://dspace.library.daffodilvarsity.edu.bd, Copyright Daffodil International University Library
Department of Electronics and Communication Engineering Khulna University of Engineering and Technology, Khulna-9203, Bangladesh E-mail: mamaraece28@yahoo.com and masudece@ece.kuet.ac.bd
Abstract: In all-optical networks, traffic is carried end-to-end in the optical domain, without any intermediate optical-electrical-optical conversion. The promise of such networks is the elimination of a significant amount of electronic equipment, as well as added capabilities, such as the ability to transport any type of data format through the network. Fiber optic communications has been growing at a phenomenal pace over the past twenty years, so rapidly, in fact, that its impact is increasingly felt in nearly all aspects of communications technology. The design of such a system involves many aspects such as the type of source to be used (LED, LASER), the kind of fiber to be employed (multimode or single mode), and the detector (PIN or APD). This work emphasizes the basic requirements and design approaches of an optical fiber communication link. For designing such system the main concern is the optimization of cost, maximum link length and stability of performance characteristics. For a given bit rate, link length and typical bit error rate, the cost effective design is chosen here with software implementation.
Key Words: Bit Error Rate (BER), Light Amplification Stimulated Emission of Radiation (LASER), Power Budget, Time Budget, Graded Index (GI) Fiber, and Avalanche Photo Diode (APD).
The information Age is in a stage of rapid growth. As quick as new technologies are introduced, users demand more capability, more speed, and greater flexibility. Users demand more capabilities from every kind of electronic device: more processing power, more features, and better connectivity. Fiber optic transmission medium is first emerging as an alternative and strong competitor to coaxial cables in telecommunication networks. The communication engineers have always dreamt of higher information bandwidth with low
attenuation and cost. For this reason they preferred optical fiber for network purposes as transmission medium and over the high bit rate point to point communication link. In word, for higher speeds and longer distance communication the transmission link must be made based on optical fiber. Therefore, they are used as a preferred transmission medium in current communication systems. Ease of communication in any country is the prerequisite for development. Communication is not just building roads, railway and bridges only. To foster and sustain all round advancement, a robust, efficient and widespread telecom infrastructure is as integral and essential component of development as any other. Research and development efforts have led to commercial realization of low cost, low loss optical fibers. The optical fiber communication system must be in cost effective which are sensitive to the proper selections of source, optical fiber and detector. So, before going to implementation of optical communication system it must take a hand in the software analysis in the link design such that it would be cost effective and meet the availability of the components.
Technologies used in wireless broadband systems are typically data-driven and require a very high processing speed. So, before designing such a long–haul high speed link one must have knowledge about optical fiber communication system. An optical fiber is a dielectric waveguide made by ultra thin fiber of glass that operates at optical frequencies i.e., in which the signal is transmitted by the virtue of visible of light. To guide light, optical fiber contains of two concentric layers called the core and the cladding. A basic optical fiber communication system consists of an optical
meets the objectives and represents economical and technical solutions. If not, it is required to pass to another combination through procedure. In particular, the assumptions need to be inspected to determine if changes might provide a simpler or cheaper alternative.
Flow Chart of Link Design
Start
Specify R, BER, L
Select source, Fiber, detector
Evaluate P (^) r, for given R, BER
Determine P (^) m
Is Pm > 4 dB?
Make time budget
Is it satisfactory?
Evaluate max. link length
List out source, fiber, detector
Stop
Repeater required
Select different combinations
Stop
All combinations tried? Yes
Yes
Yes
No
No
Fig. 2 Flow chart showing the steps involved in a fiber optic link design
The starting point for optical fiber communication link design is choosing the operating wavelength, the type of source (i.e., LED or LASER), and whether a single mode or multimode fiber is required. In a link design, one usually knows (or estimates) the data rate required to meet the objectives. From this data rate and an estimate of link length, one chooses the wavelength, the type of source, and the fiber type. Also, the requirements for the link
design must be chosen in such a way that the losses (e.g., source to fiber coupling loss, fiber to fiber coupling loss, fiber to detector coupling loss, dispersion, attenuation etc.) involved in those requirements should be maintained to a minimum. The optical fiber link design simulator flow chart is given below Fig. 2[1]:
The key system requirements needed in the link design are: (a) Data or bit rate/bandwidth; (b) Bit error rate/signal to noise ratio and (c) Transmission distance or link length. In the optical fiber communication link design the basic issues are: (a) Attenuation which determines the power available at the photodetector input for given source power (known as power budget) and (b) Dispersion which determines the limiting data rate usable bandwidth (known as time budget) [2].
3.2 Power Budget In a fiber-optic communication link, the allocation of available optical power (launched into a given fiber by a given source) among various loss-producing mechanisms such as launch coupling loss, fiber attenuation, splice losses, and connector losses, in order to ensure that adequate signal strength (optical power) is available at the receiver. The amount of optical power launched into a given fiber by a given transmitter depends on the nature of its active optical source (LED or laser diode) and the type of fiber, including such parameters as core diameter and numerical aperture. Manufacturers sometimes specify an optical power budget only for a fiber that is optimum for their equipment--or specify only that their equipment will operate over a given distance, without mentioning the fiber characteristics. The purpose of the power budget is to ensure that enough power will reach the receiver to maintain reliable performance during the entire system lifetime. Performance of the system is evaluated by analyzing the link power budget of the system and the cost is kept minimum by carefully selecting the system components from a variety of available choices [2].
NO
In the preparation of link power budget, certain parameters like required optical power level Pr at the receiver to meet the system requirements, coupling losses etc. are required. In any practical design, an allowance has to be made for the degradation of components with ageing, replacement, variations due to temperature fluctuations, manufacturing spreads, imperfect repeatability on reconnection, field repairs, maintenance, and variations in drive conditions and so on. In an optical communications link, power margin is the minimum optical power that is required by the receiver for a specified level of performance. The amount of optical power launched into a given fiber by a given transmitter depends on the nature of its active optical source (LED or laser diode) and the type of fiber, including such parameters as core diameter and numerical aperture. After computing various losses and fixing safety margin, power budget of the link is calculated by the following equations [2]: Power margin in dB, P (^) m = (P (^) t - P (^) r (min) - L (^) sf - NL (^) ff - αL - L (^) fd )……………………………(1) Where, P (^) t = Source output power...…...dBm P (^) r (min) = Minimum receiver power….dB L (^) sf = Source to fiber coupling loss……dB L (^) fd = Fiber to detector coupling loss… dB L (^) ff = Fiber to fiber coupling loss……..dB N=Number of splice = Integer part of [L/L 0 ] L = Fiber link length………………… km L 0 = Factory unit length of fiber…... …km α = Attenuation coefficient of fiber... dB/km αL = Fiber loss……………………… dB Nα = Total splice loss………………...dB
Fig. 3 Loss model of point to point optical fiber link (C; connector and S; splice)
A power margin P (^) m ≥ 4dB is acceptable otherwise some components need to be
upgraded. With P (^) m ≤ 4dB, system will become less reliable. A typical losses involved in the calculation of power budget: The loss model for a point to point optical fiber link is shown in Fig. 3 [2]. From the above figure optical power loss occurs due to (a) Coupling losse (L (^) sf , L (^) ff and Lfd ) (b) Connector loss (c) Splice loss (d) Fiber attenuation and (e) Fiber bend loss The typical values of L (^) sf , L (^) ff and Lfd are about 2 dB, 0.5 dB and 0.2 dB respectively. But they also vary with wave length [2]. The source to fiber coupling loss is largely dependent on the numerical aperture, source, fiber size (core diameter/cladding diameter) and can be calculated as follows which is listed in Table 1 [7].
Table 1 Coupling efficiencies calculation
Lambertian emitter Nonlambertianemitter
Coupling efficienc y (η) rs ≤ a
Coupling efficiency (η) rs > a
Fiber
Coupling efficienc y (η) r (^) s < a
NA^2 NA^2 (a/r (^) s )^2 indexStep (m+1/2)NA 2
NA^2 [1- (2/(g+2)) ( r (^) s /a) g^ ]
NA^2 (a/r (^) s )^2 (g/(g+2))
Not assig ned
Not assigned
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 L (^) sf can be calculated (in dB) from the table (Table 1) as follows: L (^) sf = -10 log 10 η (dB)…………………….(2)
After specifications all the terms mentioned above one can simply estimate the power budget calculation. For example the power budget of a 0.85 μm lightwave system is listed in Table 2 [5].
function of the refractive index profile (g). The minimum intermodal rms pulse broadening with an optimum g is [2]: σmod =(n 1 ∆^2 L)/(20√3c)…………………….(7) The rms pulse width due to material dispersion is determined as follows: σmat = | D (^) mat |σ (^) λ L……………..…….……..(8) Where, σ (^) λ is the rms spectral width of the source and Dmat is the dispersion parameter.
3.6 Link Length The design of fiber optic communication systems requires a clear understanding of the limitations imposed by the loss, dispersion and nonlinearity of the fiber. Since fiber properties are wavelength dependent, the choice of the operating wavelength is a major design issue. When a particular set of components meets the design requirements, one would like to know the maximum distance up to which these components could be used. Further, if the link length is quite large, it will help in determining the repeater location. The maximum link length is determined presuming that link is attenuation limited and there is no dispersion effect. And again when the link is dispersion limited and there is no attenuation effect. Minimum of the two is taken as the maximum practicable link length. For dispersion limited link, maximum data rate that can be transmitted over an optical fiber system is given by [2]: R= (1/4 σsys )………………………………(9) Where, R is data rate in Mbps. The maximum allowable fiber dispersion will be: σal =√((1/4R) 2 - σ (^2) tx - σ^2 rx )ns……………...(10) Now we can easily determine fiber dispersion per unit length as (σf /L). Therefore, the maximum link length under dispersion limited condition is determined as [2]: L (^) d (max) = (L*σal )/σf (km)………….(11) For comparison purposes we frequently want to calculate the maximum link distance for a system limited only by the fiber attenuation. Therefore, the formula for maximum link length under attenuation limited condition is: L (^) a (max) = [P (^) t (dBm) – P (^) r (dBm)]/α (dB/km) (km) ..........................................................(12)
For the combination of LASER-GI-APD (Fig.4), it is clearly observed that both the attenuation limited distance and dispersion limited distance are decreased with data rate (Mbps). As the total dispersion is approximately zero and a significant amount of attenuation around the 1300nm operating wavelength which is a common phenomenon [4][6-7] so, the dispersion limited distance is quiet high as compared to attenuation limited distance before their intersection point. For considering the optimum and reliable link design both the attenuation limited distance and dispersion limited distance are equal at the desired data rate with suitable selection of fiber, source and detector. From this curve (Fig.4) it is observed that both the attenuation limited maximum distance and dispersion limited maximum distance are equal (12km) at a data rate of 60Mbps. This 60Mbps data rate is not desired for proposed link design however, GI fiber is expensive as compared to the SI fiber. So, we ignore this combination as it is not suitable for the optimum and reliable link design. For the combination of LASER-SI-APD as shown in Fig.5, It is observed that at optimum point (i.e. certainly the intersection point) the data rate is 45.65Mbps and the maximum link length is 17km. As the detector APD is quiet expensive and this proposed link design is considered for 10km. So, again this combination is not suitable for the optimum and reliable link design. Finally, it is described sharply that the combination of LASER-SI-PIN as shown in Fig.6 which clearly states that for the same data rate 45.65Mbps the desired maximum link length is 12km. And these optimum values are obtained at a lowest cost combination of fiber and detector. But in the combination of LASER-GI-APD maximum link length is 10 Km which is lower than LASER-SI-PIN combination. So, the lowest cost combination of source, fiber and detector for the broadband optical link design is LASER-SI-PIN.
Fig. 4 Maximum transmission distance vs. data rate for the combination of LASER-GI-APD
Fig. 5 Maximum transmission distance vs. data rate for the combination of LASER-SI-APD
Fig. 6 Maximum transmission distance vs. data rate for the combination of LASER-SI-PIN
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 available transmitter and receiver modules. The 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 proposed link design lowest cost combination 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 parameters such that the design objectives are met at a minimum cost combination.
