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Chapter 8: Optical Fibers and Components

Chapter 8: Optical Fibers and Components

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Chapter 8: Optical Fibers and Components

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  1. Chapter 8:Optical Fibers and Components TOPICS • WDM optical networks • Light transmitted through an optical fiber • Types of optical fibers • Impairments • Components: Lasers, optical amplifiers, couplers, OXCs Connection-Oriented Networks

  2. WDM optical networks 1 A point-to-point connection 1 Tx Rx … … optical fiber optical fiber W Pre- amplifier In-line amplification Power amplifier W Tx Rx Wavelength multiplexer Wavelength demultiplexer Connection-Oriented Networks

  3. Mesh network Ring 4 Ring 1 Ring 2 Ring 3 An example of an optical network Connection-Oriented Networks

  4. How light is transmitted through an optical fiber Waves and electrical fields Wave Electric field Source Connection-Oriented Networks

  5. An optical fiber Cladding Core Cladding • Core and cladding Cladding Cladding Core Core n1 n1 Refractive index Refractive index n2 n2 Radial distance Radial distance • a) Step-index fiber b) graded-index fiber Connection-Oriented Networks

  6. Refraction and reflection of a light ray Refracted ray f n2 n1  r Incident ray Reflected ray Connection-Oriented Networks

  7. Angle of launching a ray into the fiber Cladding Cladding Core Core  r l Cladding Cladding Cladding Optical transmitter Core Cladding Connection-Oriented Networks

  8. Multi-mode and single-mode fibers • Core/diameter of a multi-mode fiber: • 50/125 m, • 62.5/125 m, • 100/140 m • Core/diameter of single-mode fiber • 9 or 10 / 125 m Connection-Oriented Networks

  9. Electric fields Cladding A Core 2 1 Cladding B Connection-Oriented Networks

  10. Electric field amplitudes for various fiber modes Cladding Core Cladding m=0 m=1 m=2 Connection-Oriented Networks

  11. Propagation of modes Cladding Cladding a) step-index fiber Cladding Cladding b) Graded-index fiber Connection-Oriented Networks

  12. Cladding Cladding Single-mode fiber Connection-Oriented Networks

  13. Impairments • The transmission of light through an optical fiber is subjected to optical effects, known as impairments. • There are: • linear impairments, and • non-linear impairments. Connection-Oriented Networks

  14. Linear impairments • These impairments are called linear because their effect is proportional to the length of the fiber. • Attenuation: • Attenuation is the decrease of the optical power along the length of the fiber. • Dispersion • Dispersion is the distortion of the shape of a pulse. Connection-Oriented Networks

  15. Attenuation 2.5 2.0 1.5 Attenuation, dB 1.0 0.5 800 1000 1200 1400 1600 1800 Wavelength, nm Connection-Oriented Networks

  16. Attenuation in Fiber • Attenuation • P(L) = 10-AL/10P(0) • Where P(0) optical power at transmitter, • P(L) power at distance L Km, and • A = attenuation constant of the fiber • Received Power must be greater or equal to • receiver sensitivity Pr • Lmax = 10/A log10(P(0)/P(r)) Connection-Oriented Networks

  17. Dispersion • Dispersion is due to a number of reasons, such as • modal dispersion, • chromatic dispersion, • polarization mode dispersion. Connection-Oriented Networks

  18. Time Modal dispersion • In multi-mode fibers some modes travel a longer distance to get to the end of the fiber than others • In view of this, the modes have different delays, which causes a spreading of the output pulse Power Power Power Time Time Connection-Oriented Networks

  19. Chromatic dispersion • It is due to the fact that the refractive index of silica is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays. • These delays cause spreading in the duration of the output pulse. Connection-Oriented Networks

  20. Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber. • Dispersion compensating fiber introduces attenuation of about 0.5 dB/km. Connection-Oriented Networks

  21. Polarization mode dispersion (PMD) • It is due to the fact that the core of the fiber is not perfectly round. • In an ideal circularly symmetric fiber the light gets polarized and it travels along two polarization planes which have the same speed. • When the core of the fiber is not round, the light traveling along the two planes may travel at different speeds. • This difference in speed will cause the pulse to break. Connection-Oriented Networks

  22. Non-linear impairments • They are due to the dependency of the refractive index on the intensity of the applied electrical field. The most important non-linear effects in this category are: self-phase modulation and four-wave mixing. • Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering. Connection-Oriented Networks

  23. Types of fibers • Multi-mode fibers: They are used in LANs and more recently in 1 Gigabit Ethernet and 10 Gigabit Ethernet. • Single-mode fiber is used for long-distance telephony, CATV, and packet-switched networks. • Plastic optical fibers (POF) Connection-Oriented Networks

  24. Single-mode fibers: • Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm. • Non-zero dispersion fiber (NZDF):This fiber has zero dispersion near 1450 nm. Connection-Oriented Networks

  25. Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm. • Low water peak fiber (LWPF): The peak in the attenuation curve at 1385 nm is known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region. Connection-Oriented Networks

  26. Plastic optical fibers (POF) • Single-mode and multi-mode fibers have a high cost and they require a skilled technician to install them. • POFs on the other hand, are very low-cost and they can be easily installed by an untrained person. • The core has a very large diameter, and it is about 96% of the diameter of the cladding. • Plastic optic fibers find use in digital home appliance interfaces, home networks, and cars Connection-Oriented Networks

  27. Components • Lasers • Photo-detectors and optical receivers • Optical amplifiers • The 2x2 coupler • Optical cross connects (OXC) Connection-Oriented Networks

  28. Light amplification by stimulated emission of radiation (Laser) • A laser is a device that produces a very strong and concentrated beam. • It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material. • The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity. Connection-Oriented Networks

  29. Left facet Right facet Fabry-Perot resonator cavity. It consists of two partially reflecting parallel flat mirrors, known as facets, which create an optical feedback that causes the cavity to oscillate. Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected. Connection-Oriented Networks

  30. Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 to 200 GHz. • A single wavelength can be selected by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths. Connection-Oriented Networks

  31. Tunable lasers • Tunable lasers are important to optical networks • Also, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths. • Several different types of tunable lasers exist, varying from slow tunability to fast tunability. Connection-Oriented Networks

  32. Modulation • Modulation is the addition of information on a light stream • This can be realized using the on-off keying (OOK) scheme, whereby the light stream is turned on or off depending whether we want to modulate a 1 or a 0. Connection-Oriented Networks

  33. WDM and dense WDM (DWDM) • WDM or dense WDM (DWDM) are terms used interchangeably. • DWDM refers to the wavelength spacing proposed in the ITU-T G.692 standard in the 1550 nm window (which has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier operates.) • The ITU-T grid is not always followed, since there are many proprietary solutions. Connection-Oriented Networks

  34. The ITU-T DWDM grid Connection-Oriented Networks

  35. Photo-detectors and optical receivers • The WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver. • Each receiver consists of a • photodetector, • an amplifier, and • signal-processing circuit. Connection-Oriented Networks

  36. Optical amplifiers • The optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected. • Optical amplification is used to restore the strength of the signal Connection-Oriented Networks

  37. 1 1 Tx Rx … … Amplifiers: power amplifiers, in-line amplifiers, pre-amplifiers optical fiber optical fiber W Pre- amplifier In-line amplification Power amplifier W Tx Rx Wavelength multiplexer Wavelength demultiplexer Connection-Oriented Networks

  38. 1R, 2R, 3R • Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply • 1R (re-amplification), or • 2R (re-amplification and re-shaping) or • 3R (re-amplification, re-shaping, and re-timing) and then converting the regenerated signal back into the optical domain. Connection-Oriented Networks

  39. Amplification and Regeneration Connection-Oriented Networks

  40. Coupler Signal to be amplified 1550 nm Erbium-doped fiber Isolator Isolator Laser 850 nm The Erbium-doped fiber amplifier (EDFA) Connection-Oriented Networks

  41. Coupler Coupler Signal to be amplified 1550 nm Erbium-doped fiber Isolator Isolator Laser 850 nm Laser 850 nm Two-stage EDFA Connection-Oriented Networks

  42. The 2x2 coupler The 2x2 coupler is a basic device in optical networks, and it can be constructed in variety of different ways. A common construction is the fused-fiber coupler. Fiber 1 Input 1 Output 1 Input 2 Output 2 Fiber 2 Tapered region Coupling region Tapered region Connection-Oriented Networks

  43. 3-dB coupler A 2x2 coupler is called a 3-dB coupler when the optical power of an input light applied to, say input 1 of fiber 1, is evenly divided between output 1 and output 2. Connection-Oriented Networks

  44. If we only launch a light to the one of the two inputs of a 3-dB coupler, say input 1, then the coupler acts as a splitter. • If we launch a light to input 1 and a light to input 2 of a 3-dB coupler, then the two lights will be coupled together and the resulting light will be evenly divided between outputs 1 and 2. • In the above case, if we ignore output 2, the 3-dB coupler acts as a combiner. Connection-Oriented Networks

  45. A banyan network of 3-dB couplers 128 1 2 128 3 128 4 128 5 128 6 128 7 128 128 8 Connection-Oriented Networks

  46. Output fibers CPU Input fibers 1 1 W W Fiber 1 Fiber 1 … … 1 1 W W Switchfabric Fiber N Fiber N Optical cross connects (OXCs) Connection-Oriented Networks

  47. OXC (cont’d) • Optical cross-connects Wavelength Router OXC WDM link To & from other nodes To & from other nodes GMPLS Plane UNI Access Station IP router Tx Rx Connection-Oriented Networks Local Add Local Drop

  48. OXC: switching fabric • Switching fabric MEMS: one mirror per output Input WL λ1 to output 1 OXC Output 1 2 3 4 Connection-Oriented Networks

  49. OXC: switching fabric (cont’d) • Switching fabric MEMS: one mirror per output Input WL λ1 to output 4 OXC Output 1 2 3 4 Connection-Oriented Networks

  50. OXC functionality • It switches optically all the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers. • For instance, it can switch the optical signal on incoming wavelength i of input fiber k to the outgoing wavelength i of output fiber m. Connection-Oriented Networks