Fiber Optic Communication Systems. Unit-05: Types of Fibers.

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1 Unit-05: Types of Fibers Department of Telecommunication, MUET UET Jamshoro 1

2 Optical Fiber Department of Telecommunication, MUET UET Jamshoro 2

3 Single mode Step Index fiber Most widely used in long haul high speed links Offer transmission bandwidths Lowest losses No modal dispersion Have upgrade capability (for future wide-bandwidth services using faster optical Tx/Rx) Anticipated lifetime of more than 20 years Only one electromagnetic mode is allowed to propagate For single mode condition, the V-Number (Normalized Frequency) < Cut-off V 2 a( NA) V V Department of Telecommunication, MUET UET Jamshoro 3 c

4 Single mode Step Index fiber A small-core optical fiber through which only one mode will propagate. The typical diameter is about 3.5 x 10-4 inches or 9 microns. Step-index Fiber: Fiber that has a uniform index of refraction throughout the core that is a step below the index of refraction in the cladding. Department of Telecommunication, MUET UET Jamshoro 4

5 Single mode Step Index fiber The central region, the core, has greater refractive index than the outer region, the cladding. The fiber has cylindrical symmetry. We use the coordinates r, φ, z to represent any point in the fiber. Cladding is normally much thicker than shown. Department of Telecommunication, MUET UET Jamshoro 5

6 Single mode Step Index fiber Department of Telecommunication, MUET UET Jamshoro 6

7 Single mode Step Index fiber The optical fiber with a core of radius a and a constant refractive index n 1 and a cladding of slightly lower refractive index n 2 is known as step index fiber. n r = ቊ n 1 n 2 for r < a for r > a There are two basic types of single mode step-index fibers: matched clad (MC) and depressed clad (DC). Matched cladding means that the fiber cladding consists of a single homogeneous layer of dielectric material. Depressed cladding means that the fiber cladding consists of two regions: the inner and outer cladding regions. Department of Telecommunication, MUET UET Jamshoro 7

8 Single mode Step Index fiber Matched Cladding Core of radius a and RI n 1 Cladding of RI n2 where n 1 > n 2 Depressed Cladding Core of radius a and RI n 1 Inner cladding having RI n 2 where n 1 > n 2 Outer cladding having RI n 3 where n 1 > n 3 > n 2 Department of Telecommunication, MUET UET Jamshoro 8

9 SMF cutoff wavelength A wavelength above which a guided mode of a waveguide ceases to exist. So shorter the wavelength, greater number of modes can be guided through fiber. Single mode fiber cutoff wavelength is the smallest operating wavelength when single mode fibers propagate only the fundamental mode. At this wavelength, the 2nd-order mode becomes lossy and radiates out of the fiber core. As the operating wavelength becomes longer than the cutoff wavelength, the fundamental mode becomes increasingly lossy. Department of Telecommunication, MUET UET Jamshoro 9

10 SMF cutoff wavelength Department of Telecommunication, MUET UET Jamshoro 10

11 SMF cutoff wavelength Normalized propagation constant b vs. V-number for a step index fiber for various LP modes S.O. Kasap, Optoelectronics (Prentice Hall) Department of Telecommunication, MUET UET Jamshoro 11

12 Single Mode Step-index fiber A SM step-index fiber has low attenuation low intermodal dispersion (broadening of transmitted light pulse), as only one mode is transmitted, and high bandwidth properties. Present applications for single mode fibers include Long-haul, high-speed telecommunication systems. Future applications include single mode fibers for sensor systems. Department of Telecommunication, MUET UET Jamshoro 12

13 Multi-mode fiber Multimode (MM) Fiber: An optical fiber that has a core large enough to propagate more than one mode of light. The typical diameter is about 2.5 x 10-3 inches or 62.5 microns. Multimode Step-Index Fiber Fiber has a uniform index of refraction throughout the core that is a step below the index of refraction in the cladding Allows more than one mode of light. Department of Telecommunication, MUET UET Jamshoro 13

14 Multi-mode fiber Multimode Graded-Index Fiber A multimode graded-index fiber has a core of radius (a). Unlike step-index fibers, the value of the refractive index of the core (n1) varies according to the radial distance (r). The value of n1 decreases as the distance (r) from the center of the fiber increases. Department of Telecommunication, MUET UET Jamshoro 14

15 Multi-mode step-index fiber Multimode step index fibers allow the propagation of a finite number of guided modes along the channel. A multimode step-index fiber has a core of radius a and a constant refractive index n 1. A cladding of slightly lower refractive index n 2 surrounds the core. The difference in the core and cladding refractive index is the parameter Δ, given by: n n n1 Department of Telecommunication, MUET UET Jamshoro 15

16 Multi-mode step-index fiber In a typical MM step-index fiber, there are hundreds of propagating modes. Most modes in multimode step-index fibers propagate far from cut-off wavelength λ c. Modes away from the λ c concentrate most of their light energy into the fiber core. Since most modes propagate far from cutoff, the majority of light propagates in the fiber core. Modes close to λ c have a greater percentage of their light energy propagate in the cladding. Therefore, in multimode step-index fibers, cladding properties such as cladding diameter, have limited effect on mode (light) propagation. Department of Telecommunication, MUET UET Jamshoro 16

17 Multi-mode step-index fiber The total number of guided modes or mode volume Ms for a step index fiber is related to the normalized frequency, V, by the approximate expression: 2 V M s 2 This allows an estimate of the number of guided modes propagating in a particular multimode step index fiber. Only for large number of modes (V >> 2.4), the number of modes can be given by V 2 /2. Under this condition, the ratio between power travelling in the cladding and in the core is given by P cladding P total Department of Telecommunication, MUET UET Jamshoro M

18 Multi-mode step-index fiber Multimode step-index fibers have relatively large core diameters and large numerical apertures. A large core size and a large numerical aperture make it easier to couple light from LED into the fiber. Large number of modes possible Each mode travels at a different velocity Used in short links, mostly with LED sources Unfortunately, multimode step-index fibers have limited bandwidth capabilities. Dispersion, mainly modal dispersion, limits the bandwidth or information-carrying capacity of the fiber. Short-haul, limited bandwidth, low-cost applications typically use multimode step-index fibers. Department of Telecommunication, MUET UET Jamshoro 18

19 Multi-mode graded-index fiber Core refractive index gradually changes towards the cladding The light ray gradually bends and the TIR happens at different points The rays that travel longer distance also travel faster Offer less modal dispersion compared to Step Index MMF Department of Telecommunication, MUET UET Jamshoro 19

20 Multi-mode graded-index fiber In multimode (MM) graded-index fiber with core of radius a, unlike step-index fibers, the value of the refractive index of the core n1 varies according to the radial distance r i.e., such fibers do not have constant refractive index in the core. The value of n 1 decreases as the distance r from the center of the fiber increases. The index variation may be represented as: n r = n 1 1 r/a α 1 2 r < a core n = n 2 r a cladding where Δ is the relative refractive index difference and α is core radius. Department of Telecommunication, MUET UET Jamshoro 20

21 TIR in multi-mode graded-index fiber n decreases step by step from one layer to next upper layer; very thin layers. Continuous decrease in n gives a ray path changing continuously. (a) A ray in thinly stratified medium becomes refracted as it passes from one layer to the next upper layer with lower n and eventually its angle satisfies TIR. (b) In a medium where n decreases continuously the path of the ray bends continuously. Department of Telecommunication, MUET UET Jamshoro 21

22 Multi-mode graded-index fiber Multimode graded-index fibers exhibit far less intermodal dispersion than multimode step index fivers due to their refractive index profile. This results in the transmission bandwidths which may be orders of magnitude greater than multimode step index fiber bandwidths. Department of Telecommunication, MUET UET Jamshoro 22

23 Multi-mode graded-index fiber The total number of guided modes or mode volume Mg for a graded-index fiber is related to the normalized frequency, V, by the approximate expression: M g 2 V 2 2 Hence, for a parabolic refractive index profile core (α= 2), 2 V M g 4 Department of Telecommunication, MUET UET Jamshoro 23

24 Multi-mode graded-index fiber Step-index Multimode fiber Graded-indexed Multimode fiber Step-indexed Single mode fiber Department of Telecommunication, MUET UET Jamshoro 24

25 Typical dimensions Department of Telecommunication, MUET UET Jamshoro 25

26 Applications of Optical Fiber Department of Telecommunication, MUET UET Jamshoro 26

27 Applications of Optical Fiber Applications Most present day applications involving multimode fiber use graded-index fibers. LAN Advantages In most applications, a multimode graded-index fiber with a core and cladding size of 62.5/125 μm offers the best combination of the following properties: Relatively high source-to-fiber coupling efficiency Low loss Department of Telecommunication, MUET UET Jamshoro 27

28 Applications of Optical Fiber Low sensitivity to microbending and macrobending High bandwidth Expansion capability Disadvantages In LAN type environment, macrobends and microbands losses are hard to predict. Cable tension, bends, and local tie-downs increase macrobend and microbend losses. Department of Telecommunication, MUET UET Jamshoro 28

29 Summary Department of Telecommunication, MUET UET Jamshoro 29