EEE440

1. EEE440 Modern Communication Systems Optical Fibre Communication Systems

2. Optical Fibre • composed of two concentric layers: the core and the cladding • have different indices of refraction with the core having n1 and the cladding n2. • an additional coating around the cladding called the jacket usually consists of one or more layers of polymer to protect the core and cladding from shocks • the cladding guides the light along the core by using the method of total internal reflection

3. Optical Fibre • Fiber optic cable sizes are usually expressed by first giving the core size followed by the cladding size. • 50/125 indicates a core diameter of 50 microns and a cladding diameter of 125 microns • The larger the core the more light can be coupled into it • However, larger diameter cores may actually allow too much light in and may cause receiver saturation problems. • The 8/125 cable, is often found when a fiber optic data link operates with single-mode propagation. • The 62.5/125 cable, is often found in a fiber optic data link that operates with multi-mode propagation.

4. Optical Fibre • light propagates down the fiber optic cable and is confined to the core • the core and cladding have different indices of refraction with the index of the core, n1, always being greater than the index of the cladding, n2 • Light can be guided down the fiber optic cable if it enters at less than the critical angle • This angle is fixed by the indices of refraction of the core and cladding and is given by the formula: • Qc = arc cosine (n2 /n1). • The critical angle is measured from the cylindrical axis of the core. if n1 = 1.446 and n2= 1.430 then the critical angle is 8.53 degrees.

5. Optical Fibre • If the light ray strikes the core-to-cladding interface at an angle greater than the critical angle then it is reflected back into the core. • Since the angle of incidence is always equal to the angle of reflection the reflected light will again be reflected • The light ray will then continue this bouncing path down the length of the fiber optic cable. • If the light ray strikes the core-to-cladding interface at an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with propagation distance.

6. Optical Fibre • When light wave is guided down a fiber optic cable it exhibits certain modes • Mode can be one of two types, multi-mode or single-mode • the number of modes that exist depend upon the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section

7. Optical Fibre • For the multi-mode propagation with a step index refractive index profile • the diameter of the core is fairly large relative to the cladding • a sharp discontinuity in the index of refraction between core and cladding. • when light enters the fiber optic cable on the right it propagates down toward the left in multiple rays or multiple modes. • the lowest order mode travels straight down the center. • The higher modes represented by rays, bounce back and forth, going down the cable to the left. • The higher the mode the more bounces per unit distance down to the left.

8. Optical Fibre • the output pulse is significantly attenuated relative to the input pulse and suffers significant time dispersion. • The higher order modes, the bouncing rays, tend to leak into the cladding as they propagate down the fiber optic cable. • They lose some of their energy into heat and results in an attenuated output signal. • The bouncing rays and the lowest order mode, traveling down the center axis, are all traversing paths of different lengths from input to output. • Consequently, they do not all reach the right end of the fiber optic cable at the same time. • When the output pulse is constructed from these separate ray components the result is time dispersion.

9. Optical Fibre • Fiber optic cable that exhibits multi-mode propagation with a step index profile is thereby characterized as having higher attenuation and more time dispersion than the other propagation modes. • However, it is also the least costly • It is especially attractive for link lengths up to 5 km. • Usually, it has a core diameter that ranges from 100 microns to 970 microns. • It can be fabricated either from glass or plastic.

10. Optical Fibre • For the single-mode propagation with a step index refractive index profile • the diameter of the core is fairly small relative to the cladding. • Typically, the cladding is ten times thicker than the core. • When light enters the fiber optic cable on the right it propagates down toward the left in just a single ray • This lowest order mode is confined to a thin cylinder around the axis of the core. • All energy is confined to this single, lowest order, mode

11. Optical Fibre • The higher order modes are absent. • Consequently, there is no energy lost to heat by having these modes leak into the cladding. • Attenuation is not significant. • Also, since the input signal is confined to a single ray path, that of the lowest order mode, there is little time dispersion • Less time dispersion means higher bandwidth and this is in the 50 to 100 GHz/ km range.

12. Optical Fibre • However, it is the most costly for local applications • It has been used more with Wide Area Networks than with local data communications. • It is attractive for link lengths go all the way up to 100 km. • The core diameter for this type of fiber optic cable is exceedingly small ranging from 5 microns to 10 microns. The standard cladding diameter is 125 microns. • Single-mode fiber optic cable is fabricated from glass. • Because of the small thickness of the core, plastic cannot be used to fabricate single-mode fiber optic cable

13. Optical Fibre • For multi-mode propagation with a graded index refractive index profile • the variation of the index of refraction is gradual • the core is much larger than in the single-mode step index case • Multi-mode propagation exists with a graded index. • However, the paths of the higher order modes are somewhat confined • The attenuation through them due to leakage is more limited than with a step index. • The time dispersion is more limited than with a step index • Fiber optic cable that exhibits multi-mode propagation with a graded index profile is thereby characterized as having attenuation and time dispersion properties somewhere between the other two.

14. Optical Fibre • Likewise its cost is somewhere between the other two. • Popular graded index fiber optic cables have core diameters of 50, 62.5 and 85 microns. • They have a cladding diameter of 125 microns - the same as single-mode fiber optic cables. • This type of fiber optic cable is extremely popular in local data communications applications. • In particular, the 62.5/125 fiber optic cable is the most popular and most widely used in these applications. • Glass is generally used to fabricate multi-mode graded index fiber optic cable. However, there has been some work at fabricating it with plastic.

15. Fibre Attenuation • Mainly due to absorption, scattering and radiative loss. • Important in determining the maximum distance between transmitter and receiver • The more light that can be coupled into the core the more light will reach the receiver and the lower the BER. • The lower the attenuation in propagating down the core the more light reaches the receiver and the lower the BER. • The less time dispersion realized in propagating down the core the faster the signaling rate and the higher the end-to-end data rate

16. Fibre attenuation • Optical losses of a fiber are usually expressed in decibels per kilometer (dB/km). • The expression is called the fiber’s attenuation coefficient α and the expression is • where P(z) is the optical power at a position z from the origin, P(0) is the power at the origin. • For a given fiber, these losses are wavelength-dependent • The value of the attenuation factor depends greatly on the fiber material and the manufacturing tolerances

17. Fibre Attenuation • Attenuation varies with the wavelength • The three principal windows of operation correspond to wavelength regions where attenuation is low and matched to the ability of a transmitter to generate light efficiently and a receiver to carry out detection.

18. Fibre Attenuation • Absorption is due to the fibre material whereas scattering is due to the structural imperfection in the optical waveguide • Intrinsic Material Absorption • caused by interaction of the propagating lightwave with one more more major components of glass that constitute the fiber’s material composition. • These looses represent a fundamental minimum to the attainable loss and can be overcome only by changing the fiber material. • An example of such an interaction is the infrared absorption band of Silica. • However, in the wavelength regions of interest to optical communication (0.8-0.9um and 1.2-1.5um), infrared absorption tails make negligible contributions. • Extrinsic Impurity Ions Absorption • Extrinsic impurity ions absorption is caused by the presence of minute quantity of metallic ions (such as Fe2+, Cu2+, Cr3+) and the OH- ion from water dissolved in glass.

19. . Fibre Attenuation

20. Fibre Attenuation

21. Fibre attenuation A 30km long optical fibre that has attenuation coefficient of 0.8dB/km at 1300nm wavelength. What is the optical output power if 200 microwatt of optical power is launched into the fibre?

22. Fibre link budget • Short haul point-to-point link • Long haul multistage point-to-point link with amplifier