NEW YORK CITY COLLEGE of TECHNOLOGY

Transcription

1 NEW YORK CITY COLLEGE of TECHNOLOGY THE CITY UNIVERSITY OF NEW YORK DEPARTMENT OF ELECTRICAL AND TELECOMMUNICATIONS ENGINEERING TECHNOLOGY Course : Prepared by: TCET 4102 Fiber-optic communications Module 4: Specs of MMF Professor Djafar K. Mynbaev Spring 2008 D. Mynbaev TCET 4102, Module 4, Spring

2 Module 4: Reading the data sheet of optical fibers Review of optical fiber Optical fiber as a transmission medium Modes in MMF Attenuation in MMF and SMF Modal dispersion in MMF Bandwidth (transmission capacity) of MMF Improvement of the MMF bandwidth Reading the data sheet General review Optical characteristics Textbook: Djafar K. Mynbaev and Lowell L. Scheiner, Fiber-Optic Communications Technology, Prentice Hall, 2001, ISBN Notes: The figure numbers in these modules are the same as in the textbook. New figures are not numbered. Always see examples in the textbook. Key words Transmission medium Modes in MMF Attenuation in MMF and SMF Modal dispersion in MMF Bandwidth (transmission capacity) of MMF Specifications Data sheet Optical characteristics D. Mynbaev TCET 4102, Module 4, Spring

3 Review of MMF Optical fiber Input pulse Output pulse As a transmission medium, optical fiber must deliver an information signal. Distortion of the signal results in the decrease of an amplitude caused by loss and specified by attenuation and the spread of a pulse caused by dispersion and specified by bandwidth. D. Mynbaev TCET 4102, Module 4, Spring

4 MMF basic structure Figure 3.1 Note: A core diameter of a singlemode fiber is typically between 8 and 10 µm. D. Mynbaev TCET 4102, Module 4, Spring

5 Modes in MMF Figure 3.10 D. Mynbaev TCET 4102, Module 4, Spring

6 Modes in MMF When entering a fiber core, continuous light travels within the core at distinctive, separate paths, as required by resonant conditions. We usually consider these paths as discrete beams called modes. The beam (mode) traveling along a fiber centerline is designated as a zero-order (fundamental) mode; the highest-order mode is termed as the critical mode. Between them, hundreds and even thousands of intermediate-order modes exist. These beams (modes, that is) propagate differently in step-index and garded-index MMFs, as shown in figure in Slide 15. The sense of the term multimode fiber should have become clear to you by now. D. Mynbaev TCET 4102, Module 4, Spring

7 Modes in MMF Cladding, diameter = 125 µm Core, diameter = 62.5 µm a) Continuous light Discrete light beams (modes) Continuous light b) How multimode fiber conducts light: a) Step-index MMF; b) graded-index MMF. D. Mynbaev TCET 4102, Module 4, Spring

8 Attenuation in MMF Attenuation vs. Distance in MMF Distance (Km) Typical result of a hands-on experiment in measuring attenuation in MMF, which proves the existence of modes. D. Mynbaev TCET 4102, Module 4, Spring

9 Attenuation in MMF To understand the discrepancy between theory and experiment, we need to refer to figure in Slide 14 that clearly shows that the zero-order mode and the critical mode travel different distances within the same fiber; therefore, they experience different losses. In deriving formula for attenuation, we assumed that the fiber length and the propagation distance were the same, but, in general, they are not. This assumption is true only for a fundamental mode. This is why attenuation in a singlemode fiber, where only the fundamental mode exists, should be a constant number. Since the highest-order modes in MMF travel the longest distances, they experience the greatest losses: thus, these modes will disappear after traveling over a short fiber length. Then, the next higher-order modes will disappear, then the next ones and the ones after that in succeeding order, the process continuing until only the lower-order modes remain. There are two consequences of this consideration. First, attenuation must change with a fiber length and the shorter the length, the higher the attenuation because more modes exist within a fiber. Secondly, as a fiber length increases, a steady situation, when only the lower-order modes remain, should be reached. Indeed, such a situation exists and is called equilibrium mode distribution (EMD). Under this condition, the total light beam will concentrate around the fiber axis. Starting from the point where EMD is achieved, the attenuation of an MMF becomes almost constant. The fact that attenuation in an MMF changes with respect to fiber length is a proof that modes do exist. Since we can t arrange for direct observation of modes within a fiber, this indirect observation proves the existence of modes in MMF. Manufacturers specify attenuation of their fibers at the lowest (that is, the best) value. In fact, when measuring attenuation, manufacturers take special measures to strip off the higher order modes and achieve EMD condition. Ironically, manufacturers call these figures maximum values. Typical maximum values of MMF attenuation are 2.5 db/km at 850 nm and 0.7 db/km at 1300 nm. D. Mynbaev TCET 4102, Module 4, Spring

10 Bandwidth of MMF Bandwidth Dispersion Modal Dispersion (MMF) Chromatic Dispersion (SMF and MMF) Nonlinear Effects (SMF) Polarizationmode Dispersion (SMF) Bandwidth of an optical fiber: general view D. Mynbaev TCET 4102, Module 4, Spring

11 Bandwidth of MMF Bandwidth (transmission capacity) of an MMF has three distinguish features. First, it determines by dispersion, which, in turn, includes several phenomena, as shown in Figure 6. Secondly, as specifications sheets show, manufacturers use the term bandwidth to describe bandwidth-length product, which is a true measure of transmission capacity of any transmission medium. Third, term bandwidth in optical communications also refers to a wavelength window, given in THz, within which an optical fiber can conduct light at an acceptable attenuation. D. Mynbaev TCET 4102, Module 4, Spring

12 Modal dispersion in MMF Bandwidth of a MMF is determined by modal dispersion. Modal dispersion results in a pulse spread, which, in turn, leads to restriction of MMF bandwidth. The difference in arrival time among modes within a pulse is referred to as differential mode delay (DMD). This parameter is a measure of modal dispersion in regards to the MMF bandwidth. Figure 3.11 D. Mynbaev TCET 4102, Module 4, Spring

13 Modal dispersion in MMF Optical fiber has entered the telecommunications world as a transmission medium that has promised unlimited bandwidth. Yet, network engineers found very quickly that the bandwidth of the first MMFs did not exceed by very much the bandwidth of a coaxial cable. This is because the total power of an input optical pulse is delivered inside the fiber in small amounts carried by individual modes. Since these modes travel different distances within a fiber, they arrive at the destination point at different times, which results in the spreading of an output pulse. This phenomenon is known as intermodal, or modal, dispersion. These explanations are visualized in Figure 7. It is modal dispersion that severely limits the bandwidth of an MMF because transmission capacity (bandwidth) is inversely proportional to pulse width. All other dispersion causes, shown in Figure 6, play minor roles in restricting the MMF bandwidth. Since modal dispersion results in a pulse spread, which, in turn, leads to restriction of MMF bandwidth, we can say that the bandwidth of a MMF is determined by modal dispersion. The difference in arrival time among modes within a pulse is referred to as differential mode delay (DMD). This parameter is a measure of modal dispersion in regards to the MMF bandwidth. D. Mynbaev TCET 4102, Module 4, Spring

14 Modal dispersion in MMF ???????????? High DMD = High ISI Dispersion in general and modal dispersion in particular appears as a spread of optical pulse in time domain. Since pulses become wider, the fewer number of them can be accommodated within a second; therefore, the fewer number of pulses per second can be transmitted. Eventually, modal dispersion results in sever intersymbol interference (ISI), which makes impossible the recovery of information. This is how dispersion restricts a bit rate. Actual dispersion process is presented in this figure. D. Mynbaev TCET 4102, Module 4, Spring

15 Modal dispersion in MMF a) b) c) d) Pulse spread in MMF caused by modal dispersion: a) Train of input pulses; b) The same rain after transmission over 100 meter; c) after 200 m; d) after 300 m. (Computer simulation using LinkSim by RSoft, Inc.; student Pedro Rojas, class TC 700, Spring 2004.) D. Mynbaev TCET 4102, Module 4, Spring

16 Bandwidth of MMF After studying the bandwidth theory, students have to perform experiments. In these exercises we rely on computer simulation by using specialized software package called LinkSim and developed by RSoft Corporation. Typical results of such an experiment is presented in Figure 9. Four graphs in Figure 9 show the transformation of pulses transmitted through a MMF over a fiber length. By analyzing the results of their experiments, students understand that presented theory is true and, indeed, optical pulses get spread within an MMF, as shown in Figures 7 and 8. Since we can t perform direct measurements of a pulse spread within an optical fiber, this indirect proof based on analysis of the experimental results is important concept that engineering-technology students must be familiar with. Now students comprehend the point that bandwidth of an MMF is very restricted and one of the fundamental concepts of telecommunications bandwidth-length product has to be applied. This concept states that there is a tradeoff between bandwidth, BW, and length, L. In other words, bandwidth-length product for a given transmission medium is a constant, as Formula 4 shows. This is why manufacturers specify MMF transmission capacity as a bandwidth-length product. BW (Hz) x L (km) = constant (4) Based on this study, student now are able to understand the sections of manufacturers specifications sheets Bandwidth or Transmission Characteristics. D. Mynbaev TCET 4102, Module 4, Spring

17 Improvement of MMF bandwidth Figure Dispersion in three types of optical fiber: a) step-index (SI) MMF, b) graded-index (GI) MMF and c) singlemode fiber (SMF). D. Mynbaev TCET 4102, Module 4, Spring

18 Improvement of MMF bandwidth LED VCSEL Launching light into an MMF from an LED (top) and VCSEL (bottom). D. Mynbaev TCET 4102, Module 4, Spring

19 Improvement of MMF bandwidth Fortunately, recent developments have brought about a new type of light source the vertical-cavity surface-emitting laser, VCSEL that combines the excellent characteristics of light produced by a laser with a low manufacturing cost. VCSEL generates a well-directed beam in contrast to the widespread beam produced by a traditional LED, which results in exciting fewer modes within an MMF without changing the fiber s mechanical dimensions, as shown in Figure 11. At the same time, a VCSEL costs about $25, which is slightly higher than the cost of LED but much lower than the cost of a long-distance laser. The VCSEL radiates at 850 nm, the wavelength at which all short-reach fiber-optic links operate. Analyzing figure in Slide 28, you can easily comprehend the terms Overfilled and Laser in Minimum Bandwidth Specifications that manufacturers routinely use. (See Appendix.) D. Mynbaev TCET 4102, Module 4, Spring

20 Reading the MMF s data sheet The statement of problem Imagine you are a newly hired technologist, fresh from a college with bachelor degree in telecommunications technology. Your supervisor assigns you to choose an MMF vendor for installation a local area network a new project your company is involved in. Surely, you go to the Internet, find several available manufacturers and try to compare specifications of their optical fibers. You certainly remember from your college course in optical communications that you need to look first at two main characteristics of an optical fiber: attenuation and bandwidth. You try to understand data on these characteristics presented in the manufacturers specifications sheets and you get confused. Why is attenuation given with a remark Maximum Value? Is attenuation a constant number or varies below its maximum value? Why is bandwidth specified in MHz-km units while you have learned that bandwidth is a range of frequencies and, therefore, must be measured in hertz? Why does one manufacturer specifies overfilled bandwidth while the other refers to legacy performance? You will certainly have many questions of this nature when you start your professional work. To better prepare our graduates for their professional career, we must rely on using and discussion the industrial documentation. This module gives the example of such an approach. D. Mynbaev TCET 4102, Module 4, Spring

21 Reading the MMF s data sheet See Corning InfiniCor 50µm Optical Fibers specifications in Appendix 1. See OFS LaserWave 500/300 Fibers specifications in Appendix 2. D. Mynbaev TCET 4102, Module 4, Spring

22 Reading the MMF s data sheet Fiber specifications: Physical characteristics. (See Appendix 2.) Discuss such parameters as diameters of a core, cladding and coating. Discuss tolerances (margins) given by manufacturer for these diameters. Stress importance of these tolerances for splicing and connections. D. Mynbaev TCET 4102, Module 4, Spring

23 Attenuation (db/km) Reading the MMF s data sheet 1380 nm 1300 nm Wavelength (nm) Fiber specifications: Optical characteristics - attenuation. (See Appendix 2.) Manufacturers specify attenuation of their fibers at the lowest (that is, the best) value. In fact, when measuring attenuation, manufacturers take special measures to strip off the higher order modes and achieve EMD condition. Ironically, manufacturers call these figures maximum values. Typical maximum values of MMF attenuation are 2.5 db/km at 850 nm and 0.7 db/km at 1300 nm. Attenuation at 1380 nm minus attenuation at 1300 nm 1.0 db/km. Discuss spectral attenuation. (See Appendix 1.) D. Mynbaev TCET 4102, Module 4, Spring

24 Reading the MMF s data sheet Fiber specifications: Transmissions characteristics. (See Appendix 2.) Minimum bandwidth specifications (MHz-km) Bandwidth-length product. See also Applications support examples where Ethernet applications is given in distances. Now you comprehend the point that bandwidth of an MMF is very restricted and one of the fundamental concepts of telecommunications bandwidth-length product has to be applied. This concept states that there is a tradeoff between bandwidth, BW, and length, L. In other words, bandwidthlength product for a given transmission medium is a constant. This is why manufacturers specify MMF transmission capacity as a bandwidth-length product. BW (Hz) x L (km) = constant Based on this study, student now are able to understand the sections of manufacturers specifications sheets Bandwidth [3]or Transmission Characteristics (Appendix). LED Laser effective modal bandwidth (EMB) and overfilled bandwidth. VCSEL D. Mynbaev TCET 4102, Module 4, Spring

25 Reading the MMF s data sheet It is modal dispersion that severely limits the bandwidth of an MMF because transmission capacity (bandwidth) is inversely proportional to pulse width. All other dispersion causes, shown in Figure 6, play minor roles in restricting the MMF bandwidth. Since modal dispersion results in a pulse spread, which, in turn, leads to restriction of MMF bandwidth, we can say that the bandwidth of a MMF is determined by modal dispersion. The difference in arrival time among modes within a pulse is referred to as differential mode delay (DMD). This parameter is a measure of modal dispersion in regards to the MMF bandwidth. See DMD specifications and explanations in Appendix ???????????? High DMD = High ISI D. Mynbaev TCET 4102, Module 4, Spring

26 Module 4: Assignments See reading assignment and homework problems in the course s outline. After study this module you must be able to: Discuss the main areas of applications of multimode optical fibers (MMFs). Explain importance of tolerances that manufacturers give at mechanical dimensions, such as diameters, of MMFs. Explain why attenuation of a MMF is different at 850 nm and 1300 nm. Explain the meaning of the following specification: Attenuation at 1380 nm minus attenuation at 1300 nm 1.0 db/km. Describe the recent developments in manufacturing optical fibers regarding absorption peaks in fiber attenuation. Discuss spectral attenuation of multimode fibers. Calculate bandwidth for a given MMF if the transmission length is known. Calculate transmission length for a given MMF if the bandwidth is known. Explain the meaning of laser EMB and overfilled bandwidth specifications and explain why these specifications are different. Explain why in application-support specifications a manufacturer specifies a distance as a measure of transmission capacity. (See Appendix 2.) D. Mynbaev TCET 4102, Module 4, Spring