Optical Fiber Communications

1. Optical Fiber Communications

2. Fiber Optics • Fiber optics uses light to send information (data). • More formally, fiber optics is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers. • Light frequencies used in fiber optic systems are 100,000 to 400,000 GHz.

3. Brief History of Fiber Optics • In 1880, Alexander Graham Bell experimented with an apparatus he called a photophone. • The photophone was a device constructed from mirrors and selenium detectors that transmitted sound waves over a beam of light.

4. In 1930, John Logie Baird, an English scientist and Clarence W. Hansell, an American scientist, was granted patents for scanning and transmitting television images through uncoated cables.

5. In 1951, Abraham C.S. van Heel of Holland and Harold H. Hopkins and Narinder S. Kapany of England experimented with light transmission through bundles of fibers. Their studies led to the development of the flexible fiberscope, which used extensively in the medical field.

6. In 1956, Kapany coined the termed “fiber optics”.

7. In 1958, Charles H. Townes, an American, and Arthur L. Schawlow, a Canadian, wrote a paper describing how it was possible to use stimulated emission for amplifying light waves (laser) as well as microwaves (maser).

8. In 1960, Theodore H. Maiman, a scientist built the first optical maser.

9. In 1967, Charles K. Kao and George A. Bockham proposed using cladded fiber cables.

11. FIBER OPTIC DATA LINKS • To convert an electrical input signal to an optical signal • To send the optical signal over an optical fiber • To convert the optical signal back to an electrical signal

12. Optical Transmitter Voltage-to-current Converter Source-to-fiber interface Light Source A/D Interface Input Optical Fiber Fiber-to-light detector interface Current-to-current converter Light Detector A/D Interface Output Optical Receiver Fiber Optic Data Link

13. Fiber Optic Cable • The cable consists of one or more glass fibers, which act as waveguides for the optical signal. Fiber optic cable is similar to electrical cable in its construction, but provides special protection for the optical fiber within. For systems requiring transmission over distances of many kilometers, or where two or more fiber optic cables must be joined together, an optical splice is commonly used.

14. The Optical Receiver • The receiver converts the optical signal back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.

15. The Optical Transmitter • The transmitter converts an electrical analog or digital signal into a corresponding optical signal. The source of the optical signal can be either a light emitting diode, or a solid- state laser diode. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers

16. Types of Optical Fiber • Plastic core and cladding • Glass core with plastic cladding (PCS) • Glass core and glass cladding (SCS)

17. Cladding Cladding Modes of Propagation • Single mode – there is only one path for light to take down the cable • Multimode – if there is more than one path

18. Index Profiles A graphical representation of the value of the refractive index across the fiber • Step-index fiber – it has a central core with a uniform refractive index. The core is surrounded by an outside cladding with a uniform refractive index less than that of the central core • Grade-index fiber – has no cladding, and the refractive index of the core is nonuniform; it is highest at the center and decreases gradually toward the outer edge

19. Optical Fiber Configuration • Single-Mode Step-Index Fiber – has a central core that is sufficiently small so that there is essentially one path that light takes as it propagates down the cable • Multimode Step-Index Fiber – similar to the single-mode configuration except that the core is much larger. This type of fiber has a large light-to-fiber aperture, and consequently, allows more light to enter the cable. • Multimode Graded-Index – it is characterized by a central core that has a refractive index that is non-uniform. Light is propagated down this type of fiber through refraction.

20. Single-Mode Step-Index Fiber Advantages: • There is minimum dispersion. Because all rays propagating down the fiber take approximately the same path, they take approximately the same amount of time to travel down the cable. • Because of the high accuracy in reproducing transmitted pulses at the receive end, larger bandwidths and higher information transmission rates are possible with single- mode step-index fibers than with other types of fiber. Disadvantages: • Because the central core is very small, it is difficult to couple light into and out of this type of fiber. The source-to-fiber aperture is the smallest of all the fiber types. • A highly directive light source such as laser is required. • It is expensive and difficult to manufacture.

21. Multimode Step-Index Fiber Advantages: • Inexpensive and easy to manufacture. • It is easy to couple light into and out; they have a relatively high large source-to-fiber aperture. Disadvantages: • Light rays take many different paths down the fiber, which results in large differences in their propagation times. Because of this, rays traveling down this type of fiber have a tendency to spread out. • The bandwidth and rate of information transfer possible with this type of cable are less than the other types.

22. Index Profile Single Mode Step Index Multimode Step Index Multimode Graded Index

24. Acceptance Angle & Acceptance Cone • The acceptance angle (or the acceptance cone half angle) defines the maximum angle in which external light rays may strike the air/fiber interface and still propagate down the fiber with a response that is no greater than 10 dB down from the peak value. Rotating the acceptance angle around the fiber axis describes the acceptance cone of the fiber input.

25. Optical Fiber Acceptance Cone Acceptance Angle MaximumAcceptanceAngle=

26. Numerical Aperture For a step-index fiber: NA = Sin (Acceptance Angle) And NA = For a Graded-Index: NA = sin (Critical Angle) The acceptance angle of a fiber is expressed in terms of numerical aperture. The numerical aperture (NA) is defined as the sine of one half of the acceptance angle of the fiber. It is a figure of merit that is used to describe the light-gathering or light-collecting ability of the optical fiber. The larger the magnitude of NA, the greater the amount of light accepted by the fiber from the external light source. Typical NA values are 0.1 to 0.4 which correspond to acceptance angles of 11 degrees to 46 degrees. Optical fibers will only transmit light that enters at an angle that is equal to or less than the acceptance angle for the particular fiber.

27. Attenuation in Optical Fibers L = the length of fiber in kilometers Therefore the unit of attenuation is expressed as dB/km

28. Losses in the Optical Fiber • Absorption Losses • Material or Rayleigh Scattering Losses • Chromatic or Wavelength Dispersion • Radiation Losses • Modal Dispersion • Coupling Losses

29. Absorption Losses • Absorption loss in an optical fiber is analogous to power dissipation in copper cables; impurities in the fiber absorb the light and convert it to heat. • Absorption in optical fibers is explained by three factors: • Imperfections in the atomic structure of the fiber material • The intrinsic or basic fiber-material properties • The extrinsic (presence of impurities) fiber-material properties

30. Absorption • Essentially, there are three factors that contribute to the absorption losses in optical fibers: • ultraviolet absorption, • infrared absorption, • ion resonance absorption.

31. Ultraviolet Absorption • Is caused by valence electrons in the silica material from which fibers are manufactured. • Light ionizes the valence electrons into conduction. The ionization is equivalent to a loss in the total light field and, consequently contributes to the transmission losses of the fiber.

32. Infrared Absorption • Is a result of photons of light that are absorbed by the atoms of the glass core molecules. • The absorbed photons are converted to random mechanical vibrations typical of heating.

33. Ion Resonance Absorption • Is caused by OH- ions in the material. • The source of the OH- ions is water molecules that have been trapped in the glass during the manufacturing process. • Ion absorption is also caused by iron, copper, and chromium molecules.

34. Material or Rayleigh Scattering Losses • This type of losses in the fiber is caused by submicroscopic irregularities developed in the fiber during the manufacturing process. • When light rays are propagating down a fiber strike one of these impurities, they are diffracted. • Diffraction causes the light to disperse or spread out in many directions. Some of the diffracted light continues down the fiber and some of it escapes through the cladding. • The light rays that escape represent a loss in the light power. This is called Rayleigh scattering loss.

35. Chromatic or Wavelength Dispersion • Chromatic dispersion is caused by light sources that emits light spontaneously such as the LED. • Each wavelength within the composite light signal travels at a different velocity. Thus arriving at the receiver end at different times. • This results in a distorted signal; the distortion is called chromatic distortion. • Chromatic distortion can be eliminated by using monochromatic light sources such as the injection laser diode (ILD).

36. Radiation Losses • Radiation losses are caused by small bends and kinks in the fiber. • Essentially, there are two types of bends: • Microbends and constant-radius bends. • Microbending occurs as a result of differences in the thermal contraction rates between the core and cladding material. A microbend represents a discontinuity in the fiber where Rayleigh scattering can occur. • Constant-radius bends occur where fibers are bent during handling or installation.

37. Modal Dispersion • Modal dispersion or pulse spreading is caused by the difference in the propagation times of light rays that take different paths down a fiber. • Obviously, modal dispersion can occur only in multimode fibers. It can be reduced considerably by using graded-index fibers and almost entirely eliminated by single-mode step-index fibers.

38. Coupling Losses • Coupling losses can occur in any of the following three types of optical junctions: light source-to-fiber connections, fiber-to-fiber connections, and fiber-to-photodetector connections. Junction losses are most often caused by one of the following alignment problems: lateral misalignment, gap misalignment, angular misalignment, and imperfect surface finishes.

39. Loss Loss Axial displacement Angular displacement Loss Loss Gap displacement Surface Finish Coupling Losses

41. Light Sources • There are two devices commonly used to generate light for fiber optic communications systems: light-emitting diodes (LEDs) and injection laser diodes (ILDs). Both devices have advantages and disadvantages and the selection of one device over the other is determined by system economic and performance requirements.

42. Light-Emitting Diode (LED) • Simply a P-N junction diode • Made from a semiconductor material such as aluminum-gallium arsenide (AlGaAs) or gallium-arsenide-phosphide (GaAsP) • Emits light by spontaneous emission: light is emitted as a result of the recombination of electrons and holes

43. Light-Emitting Diode (LED) • The simplest LED structures are homojunction, epitaxially grown, or single-diffused devices. • Epitaxially grown LEDs are generally constructed of silicon-doped gallium arsenide. A typical wavelength of light emitted is 940 nm, and a typical output power is approximately 3 mW at 100 mA of forward current. • Planar diffused (homojunction) LEDs output approximately 500 microwatts at a wavelength of 900 nm.

44. Light-Emitting Diode (LED) • The primary disadvantage of homojunction LEDs is the nondirectionality of their light emission, which makes them a poor choice as a light source for fiber optic systems. • The planar heterojunction LED is quite similar to the epitaxially grown LED except that the geometry is designed such that the forward current is concentrated to a very small area of the active layer.

45. Light-Emitting Diode (LED) Advantages of heterojunction LED over the homojunction type: • The increase in current density generates a more brilliant light spot. • The smaller emitting area makes it easier to couple its emitted light into a fiber. • The small effective area has a smaller capacitance, which allows the planar heterojunction LED to be used at higher speeds.

46. Light Emission n-type substrate n-epitaxial layer p-epitaxial layer Planar heterojunction LED Homojunction LED structure: silicon-doped-gallium arsenide Light-Emitting Diode (LED)

47. Emitted light rays The Burrus etched-well LED • For the more practical application such as telecommunications, data rates in excess of 100 Mbps are required. The Burrus etched-well LED emits light in many directions. The etched well helps concentrate the emitted light to a very small area. These devices are more efficient than the standard surface emitters and they allow more power to be coupled into the optical fiber, but they are also more difficult to manufacture and more expensive.

48. Edge-Emitting Diode • These LEDs emit a more directional light pattern than do the surface-emitting LEDs. The light is emitted from an active stripe and forms an elliptical beam. Surface-emitting LEDs are more commonly used than edge emitters because they emit more light. However, the coupling losses with surface emitters are greater and they have narrower bandwidths.

49. Injection Laser Diode (ILD) Advantages of ILDs: • Because ILDs have a more direct radiation pattern, it is easier to couple their light into an optical fiber. This reduces the coupling losses and allows smaller fibers to be used. • The radiant output power from an ILD is greater than that for an LED. A typical output power for an ILD is 5 mW (7 dBm) and 0.5 mW (-3 dBm) for LEDs. This allows ILDs to provide a higher drive power and to be used for systems that operate over longer distances. • ILDs can be used at higher bit rates than can LEDs. • ILDs generate monochromatic light, which reduces chromatic or wavelength dispersion.

50. Injection Laser Diode (ILD) Disadvantages of ILDs: • ILDs are typically on the order of 10 times more expensive than LEDs. • Because ILDs operate at higher powers, they typically have a much shorter lifetime than LEDs. • ILDs are more temperature dependent than LEDs.