International Conference on Advances in Electrical Engineering Title: Digital Fiber Optic Link Design
Md.Nazrul Islam, Asaduzzaman, Md.Rezwanul Haque, Rakesh Swarup Mandal, Md.Faruk Hossain
Department of Electrical and Electronic Engineering Rajshahi University of Engineering and Technology (RUET), Rajshahi-6204 E-mail: nazrul_eee06@yahoo.com, rezwanul.ruet@gmail.com, rakesh01912@yahoo.com, faruk94_ruet@yahoo.com
ABSTRACT Fiber optic communication is a method of transmitting information from one place to another place by sending light through an optical fiber. Before connecting the submarine cable in Bangladesh, the data rate was 155Mbps and after connecting the submarine cable the data rate is 10Gbps. So it 64 times higher than previous speed. In this work we have designed a digital system for high data rate RZ signal and long distance transmission at which wavelength varies (800-1600) nm, data rate varies (50-1000) Mbps, and distance varies from Zero (0) to 300 Km. From the results we will select perfect value of wavelength, data rate and distance for optimum rise time and power margin. KEY WORDS: Data rate, Wavelength, Signal Distance, Power margin. 1. INTRODUCTION Fiber optic communication is a method of transmitting information from one place to another by sending light through an optical fiber. The light transmitted forms an electromagnetic carrier wave that is modulated to carry information [1]. The fiber optic link system is similar in concept to any type of communication system. Information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier. The optical source provides the electrical-optical conversion maybe either a semiconductor laser or light emitting diode (LED)[1]. The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier. The detector may be Avalance photo detector or PIN[2]. A transmission link is a point to point line that has a transmitter on one end and a receiver on the other as shown in figure 1. Figure 1 shows a fiber optic communication system which have core to cladding diameter 50/125 and have information rate 30 MHz*km. Figure1: Generic optical fiber communication system 2. LINK DESIGN FOR THE BASIC OPTICAL LINK For digital fiber optic link design we have to calculate two parameter:1. Rise time budget 2. Power budget 2.1 Rise time budget: For the RZ (Return to Zero) code the pulse duration () and the repetition period (T) are both equal to 1/2R, Where R is the data rate. A reasonable estimate of the required rise time ts is that it to be no more than 35% of the pulse duration. The rise time must be limited to Ts=0.35/Rrz(1) The total rise time must be apportioned between the light source, the fiber and photo detector in the manner indicated by t2s=t2LS+t2F+t2Pd ts=( t2LS+t2F +t2Pd)1/2 ...(2) tLs= Light source rise time tPD= Photo detector rise time There is relationship between the fiber rise time and its pulse spread. The equation is given bellow tF=0.35/f3-db(electrical)=T The fiber rise time is given by T=LMt(3) Where Mt is the total dispersion including material dispersion and waveguide dispersion [1].
Figure 2 shows a binary signal is encoded with polar return to zero (RZ) code.
Figure 2 shows the attenuation or total loss in every window from this we see that the attenuation is very low in third window 2.4 Source selection Once the wavelength window has been selected, the next step would be to select the transmitter .The light-emitting diode (LED) is preferred over the laser diode as the optical source in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. But in terms of long distance, higher data rate, single mode system the laser diode is preferable for the following comparison: Table 1:-Comparison between PIN and APD [1] Single mode Property LED Laser Diode Spectral 20-100 <0.2 width(nm) Rise time(ns) 2-250 0.05-1 Modulation Bandwidth <300 6000 (MHz) Coupling Very low High efficiency Temperature Low High sensitivity Primary use Moderate path, Very long Moderate data path, Very rate high data rate 2.5. Fiber Selection Pulse spreading in Single mode fiber includes Material dispersion and Waveguide dispersion only. In case of multimode fiber Pulse spreading includes Material dispersion, Waveguide dispersion and modal distortion. For this reason an additional loss in multimode fiber. So for long distance transmission single mode fiber is suitable [1]. 2.6. Detector Two types of detector are used in fiber optic link design. 1. PIN 2. APD The PIN is cheaper, less sensitive to temperature; require lower reverse bias voltage than the APD. But PIN is not suitable for long distance because:Table 2:-Comparison between PIN and APD [1] Detector Rise Wavelength Responsivity Gain time (nm) (A/W) (ns) PIN 0.3 900-1700 0.6 1 APD 0.25 1000-1700 12 20 For this reason APD is suitable for long distance.
Figure 2: A binary signal is encoded with polar return to zero (RZ) code. 2.2 Power budget The purpose of the power budget is to ensure that enough power will reach the receiver to maintain reliable performance during the entire system life time. In the preparation of link power budget, certain parameter like required optical power level(pr) to meet the system requirements, coupling losses, connector losses, splicing losses etc After computing various losses and fixing safety margin, power budget of the link is calculated by the following equation. Power margin in dB, pm=pt+pr(min)-Lsf-Lfd- Lff-Lc-LsL.(4) Where, pt = Source output power (dB) Pr = Minimum received power (dB) Lsf= Source to fiber coupling loss (dB) Lfd= Fiber to detector coupling loss (dB) Lff= fiber to fiber coupling loss (dB) Ls= splicing loss (dB) = Attenuation (dB/km) L = fiber link length (km) A power margin pm 6 dB is acceptable otherwise some component need to be upgraded. With pm6 dB, system will become less reliable[1]. Link Design proceeds in the following steps:2.3 Window selection Due to lower attenuation loss over others window the third window is preferable for long distance transmission.
Figure 2: three basic windows of operation for optical fiber [1].
2.7. Detector sensitivity The detector sensitivity Vs data rate(Mb/s) are shown in the figure 3. From figure 3 we see that APD has higher sensitivity than PIN.
3. RESULT AND DISCUSSION From the above discussion the selected parameter:Window: third Source: Single mode LD Detector: APD Fiber: Single mode When data rate varies from 50 to 1000 Mbps then power margin varies with data rate as shown below:-
Figure 3:- detector sensitivity Vs data rate(Mb/s)[1]. 2.8. Material dispersion Variation of velocity with wavelength is known as dispersion. When velocity variation is caused by some property of material is known as material dispersion. From figure 4 we see that the material dispersion is minimum for 1350nm wave length. 2.9. Waveguide dispersion Waveguide dispersion occurs because the effective refractive index for any one mode varies with wavelength. From figure 5 we see that the wave guide dispersion is low for lowest wave length.
Figure 6:-graph between data rate vs power margin Figure 6 shows the power margin vs data rate. For a system minimum 6dB power margin is required for stable operation, for this reason we select 8dB power margin which corresponds 700Mbps data rate. This is our optimum data rate. When wavelength varies from 800 to 1600 nm then rise time varies with wavelength as shown below:-
Fig 4:-Material dispersion [1]
Figure 7:- graph between wavelength vs rise time From figure 7 we can see that optimum rise time obtain from 1350nm wavelength. But from figure 2 at 1350nm wavelength attenuation loss is very high. As a result power margin decreases for that wavelength. For this reason we have selected 1550nm wavelength at which the attenuation loss is comparatively less than 1350nm wavelength. The change of wavelength 1350-1550nm the rise time change only 0.01ns.
Figure 5:- Waveguide dispersion [1]
When distance varies from 0 to 300km then power margin varies as shown below:-
Figure 8:-graph between link length vs power margin From the above figure we see that for 8dB power margin the optimum link length is 120km. So that from the above figure and discussion we find maximum link length 120km, maximum data rate 700Mbps and wavelength 1550nm for optimum rise time and power margin. Now we will Design a digital fiber optic link that must transmit a 700Mbps RZ pulse train over a 120 km path with a bit error of 10-9 without using any repeater. For different combinations the results are shown below:Table 2: Output Combination Rise time Power Comment budget(ns) budget(dB) LED-SI-SM- 95.1267>0. Wrong PIN 5 design LED-SI-SM- 95.1266>0. Wrong APD 5 design LD-SI-SM- 0.4559<0.5 -3.1<6 Wrong PIN design LD-SI-SM- 0.4247<0.5 7.9>6 Correct APD design From this table we get our optimum combination for long distance is LD-SI-SM-APD. 4. CONCLUSION In conclusion, the outcome of this thesis has been achieved. It is a good learning experience for us. The whole designing and development process was passed through. The designing, developing and conducting of technical investigations and experiments of the thesis was tough but enjoyable as we have learn a lot more on fiber optics design. The optical transmission is a major innovation in the field
of telecommunications. This new technology in the communications field will depend on the economic viability of fiber systems compared to conventional system such as copper that require high cost[3]. Its An advance in very high speed digital application lines makes it necessary to develop long haul information superhighways capable of transmitting data at high bandwidth [4]. Conventional copper cables are stretched to their limits, and fiber optic transmission is the technology for the future communication systems. Optical fiber can transmit more information than copper wire. With the available wavelength spectrum of light divided into a series of parallel channels, thousand of signal can be transmitted along a single fiber. Transmission rates of the fiber optics has been increasingly increase since and a lot of big corporation is doing a lot of research and development work to even increase the bandwidth and transmission distance of the fiber optics. Feasibility is being demonstrated in many ongoing project and trials such as the National Broadband Network that is current being Install Island wide in Singapore as well as few other Asian countries [4]. REFERENCES [1] Joseph C.Palais, Fiber Optic Communications, Fifth Edition, Prentice Hall, 2005. [2] J. Wilson and J.Hawkes, Optoelectronics, An introduction, 3rd Edition, London, Prentice Hall Europe, 1998 [3] Transition Networks www.transition.com [4] The Fiber Optic Association www.thefoa.org
