1 / 78

Optical Communications Semester 2/2005

Optical Communications Semester 2/2005. Lecture 1 Introduction. What is lightwave technology?. Lightwave technology uses light as the primary medium to carry information . The light often is guided through optical fibers (fiberoptic technology).

emmett
Download Presentation

Optical Communications Semester 2/2005

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Optical CommunicationsSemester 2/2005 Lecture 1 Introduction

  2. What is lightwave technology? • Lightwave technology uses light as the primary medium to carry information. • The light often is guided through optical fibers (fiberoptic technology). • Most applications useinvisible (infrared) light. (HP)

  3. Why lightwave technology? • Most cost-effective way to move huge amounts of information (voice, data) quickly and reliably. • Light is insensitive to electrical interference. • Fiberoptic cables have less weight and consume less space than equivalent electrical links. (HP)

  4. Use Of Lightwave Technology • Majority applications: • Telephone networks • Data communication systems • Cable TV distribution • Niche applications: • Optical sensors • Medical equipment

  5. Basic Fiber-Optic System • Transmitter (laser diode or LED). • Fiber-optic cable. • Receiver (PIN diode or avalanche photodiode). • Most fiber systems are digital but analog is also used.

  6. Basic Link Design Transmitter Connector Cable Splice Cable Receiver

  7. Terminal Equipment Amplifier Unit Amplifier Unit Terminal Equipment Regenerator Unit Amplifier Unit Typical Long-haul System Two pairs of single-mode fiber Amplifier spans: 30 to 120 km Regenerator spans: 50 to 600 km Terminal spans: up to 600 km (without regenerators) up to 9000 km (with regenerators)

  8. Typical Regenerator Unit Pulse re-shaping & re-timing Modulation & bit rate dependent! Telemetry & Remote Control Power Supply

  9. Typical Amplifier Unit Optical Amplifiers Modulation & bit rate independent! Telemetry & Remote Control Power Supply

  10. How “fast” is fiber optics? (Light travels in fibers at about 2/3 the speed of light, but so do electrical signals in wire!) • Copper wire (twisted pair) up to ~ 100 Mb/sec (short distances) • 1,500 phone calls • 2 TV channels • 2 Bibles/sec • Coaxial cable (also copper) Up to ~1 Gb/sec (short distances) • 15,000 phone calls • 20 TV channels (> 200 with “data compression”) • 20 bibles/second • Optical Fiber up to 50 Tb/s (50,000 Gb/s) (long distances) • 0.78 billion phone calls • 1 million TV channels • 1 million Bibles/second

  11. Port 1 DWDM Port 2 Port 3 Port 4 COMMON Company Types • Component Manufacturers • Lasers/LEDs, photodetectors, couplers, multiplexers, isolators, fibers, connectors • Subsystem Manufacturers • Transmitters, receivers, amplifiers (EDFA), repeaters • System Manufacturers • Point-to-point, SONET/SDH, WDM • Installers & Service Providers • Link signature, fault location

  12. Physical Basics LW Technology

  13. Conduction band Bandgap Valence band The Carrier - Light Particles Waves Rays n0 n1 n0 Absorption Emission Interference Refraction Reflection

  14. Light Properties - Wavelength Field Strength Distance Wavelength : distance to complete one sine wave 1000 pm (picometer) = 1 nm (nanometer) 1000 m = 1 mm (millimeter) 1000 nm (nanometer) = 1 m (micrometer) 1000 mm = 1 m (meter)

  15. 1 km 1 nm 1 pm 1 mm 1 m 1 Mm Electromagnetic Spectrum Shortwave Radio Visible Light Infrared Light AM Broadcast FM Radio/TV Ultraviolet Ultrasonic X-Rays Sonic Radar 1 MHz 1 YHz 1 ZHz 1 THz 1 GHz 1 kHz Frequency Wavelength c = f • l • n c: Speed of light ( 2.9979 m/µs ) f: Frequency l: Wavelength n: Refractive index (vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to 1.48)

  16. LW Transmission Bands 193 229 353 461 THz Frequency Near Infrared UV Wavelength (vacuum) 1.0 0.6 1.8 1.6 1.4 1.2 0.8 0.4 0.2 µm HeNe Lasers 633 nm Longhaul Telecom Regional Telecom Local Area Networks 1550 nm CD Players 780 nm 1310 nm 850 nm

  17. Wavelength and “Color” Names Ultra-violet* *not visible to human eyes Infra-red* blue green red • Wavelength (and “color”) can be controlled by type and amount of “dopants” (alloy materials) used to make the P and N sides of the light emitting diode. • Light emitting diodes (LEDs) with visible light output are used for indicator lights, etc. • LEDs with infra-red output used as electro-optic (EO) converters for step or graded index fibers • Construction of two parallel semi-reflecting surfaces on the diode with proper spacing relative to desired wavelength produces enhancement of one wavelength, yielding almost monochromatic LASER radiation (laser diode -- LD), used for single-mode fiber • Proper efficient coupling of light into the fiber core is a major design consideration as well (not discussed here) 400nm 500nm 600nm 700nm 850nm 1300nm 1550nm 850, 1300 and 1550 nm are local minima in the fiber transmission spectrum, wavelengths often used for fiber systems.

  18. Optical Power • Power (P): • Transmitter: typ. -6 to +17 dBm (0.25 to 50 mW) • Receiver: typ. -3 to -35 dBm (500 down to 0.3 µW) • Optical Amplifier: typ. +3 to +20 dBm (2 to 100 mW) • Laser safety • International standard: IEC 825-1 • United States (FDA): 21 CFR 1040.10 • Both standards consider class I safe under reasonable forseeable conditions of operation (e.g., without using optical instruments, such as lenses or microscopes)

  19. Snell’s “Law” • Demonstration with glass of water no=1/co=oo :vacuum (or air) n1=1/c1=1o :lower index medium n2=1/c2=2o :higher index medium Snell’s “law”: n2•Sin(D) = n1•Sin(F) Incident ray power is partly in reflected ray, partly in refracted ray. Angle of Reflected Ray R Angle of Refraction F R=D and Sin(R)=Sin(D) Line perpendicular to interface at point where ray intersects interface. Angle of Incident Ray D Material with lower dielectric constant , faster wave speed, c1, smaller index n1. Material with higher dielectric constant , slower wave speed, c2, larger index n2.

  20. Total Internal Reflection • When angle of incidence is beyond B, ~100% of optical power is reflected internally • some sources measure angle from the perpendicular line rather than from the interface, so inequality is stated differently • When you (or a fish) go under a smooth water surface (e.g., a swimming pool), you can see up to the air only inside of a circle. Outside that circle, you see only reflections from the surface. B Location of your (underwater) eye

  21. What is an optical fiber? • It’s basically, a highly transparent “light pipe” High index Core Input Light Low index cladding “Total internal reflection” up to many kilometers

  22. The Logarithmic Scale dB = 10 • log10 (P1 / P0) dBm = 10 • log10 (P / 1 mW) 0 dB = 1 + 0.1 dB = 1.023 (+2.3%) + 3 dB = 2 + 5 dB = 3 + 10 dB = 10 -3 dB = 0.5 -10 dB = 0.1 -20 dB = 0.01 -30 dB = 0.001 0 dBm = 1 mW 3 dBm = 2 mW 5 dBm = 3 mW 10 dBm = 10 mW 20 dBm = 100 mW -3 dBm = 0.5 mW -10 dBm = 100 W -30 dBm = 1W -60 dBm = 1 nW

  23. Interference • Incoherent light adds up optical power • Coherent light adds electromagnetic fields • Zero phase shift:constructive interference • 180º phase shift: destructive interference + = + =

  24. 1 1/e CL Coherence • Coherent lightPhotons have fixed phase relationship (laser light) • Incoherent lightPhotons with random phase(sun, light bulb) • Coherence length (CL)Average distance over which photons lose their phase relationship

  25. Pi Pr Reflections • Reflections: root cause for many problemsReturn loss definition: • RL = 10 * log P incident P reflected

  26. Polarization • Most lasers are highly polarized • Degree of polarization (DOP):DOP = P polarized / P total • State of polarization (SOP):describes the orientationand rotation of thepolarized light SOP: linear vertical z y SOP: linear horizontal x

  27. Brief quantum description of gain process

  28. Optical Resonator

  29. Focusing to overcome diffraction

  30. Why use Guided Waves?

  31. Optical Waveguides

  32. Optical Waveguide Properties

  33. Waveguide Principles ➤ Waves propagating in a waveguide are called MODES ➤ Perpendicular Polarised Wave ➤ Electric Field Transverse to the direction of Propagation (TE MODE) ➤ Parallel Polarised Wave ➤ Electric Field Parallel to the direction of Propagation (TM MODE)

  34. A History of Fiber Optic Technology

  35. The Nineteenth Century • John Tyndall, 1870 • water and light experiment • demonstrated light used internal reflection to follow a specific path • William Wheeling, 1880 • “piping light” patent • never took off • Alexander Graham Bell, 1880 • optical voice transmission system • called a photophone • free light space carried voice 200 meters • Fiber-scope, 1950’s Light

  36. core cladding The Twentieth Century • Glass coated fibers developed to reduce optical loss • Inner fiber - core • Glass coating - cladding • Development of laser technology was important to fiber optics • Large amounts of light in a tiny spot needed • 1960, ruby and helium-neon laser developed • 1962, semiconductor laser introduced - most popular type of laser in fiber optics

  37. The Twentieth Century (continued) • 1966, Charles Kao and Charles Hockman proposed optical fiber could be used to transmit laser light if attenuation could be kept under 20dB/km (optical fiber loss at the time was over 1,000dB/km) • 1970, Researchers at Corning developed a glass fiber with less than a 20dB/km loss • Attenuation depends on the wavelength of light

  38. • Late 1970s, early 1980s: – Second-generation technology » Sources/receivers: visible and near-IR (600 to 920 nm) » Fibers: individual multi-modefiber • Mid -1980s to present:: – Third generation technology » Sources/receivers: near-IR (1300, 1550 nm) » Fibers: individual single-mode fibers • Present: – Fourth generationtechnology » 1550 nm operation touse fiberamplifiers » Several wavelengths per fiber(WDM) – Wavelengthaddressable networks The Twentieth Century /Present

  39. Real World Applications • Military • 1970’s, Fiber optic telephone link installed aboard the U.S.S. Little Rock • 1976, Air Force developed Airborne Light Fiber Technology (ALOF) • Commercial • 1977, AT&T and GTE installed the first fiber optic telephone system • Fiber optic telephone networks are common today • Research continues to increase the capabilities of fiber optic transmission

  40. The Future • Fiber Optics have immense potential bandwidth (over 1 teraHertz, 1012 Hz) • Fiber optics is predicted to bring broadband services to the home • interactive video • interactive banking and shopping • distance learning • security and surveillance • high-speed data communication • digitized video

  41. Immunity from Electromagnetic (EM) Radiation and Lightning Lighter Weight Higher Bandwidth Better Signal Quality Lower Cost Easily Upgraded Ease of Installation Advantages of Fiber Optics

  42. Advantages of Fiber Optics Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibers are: • Less expensive • Higher carrying capacity • Less signal degradation. • Less interference • Low power losses • Safer • Lightweight • Flexible • HIGHER SPEED COMMUNICATIONS

  43. Why Not Fibers? • Lack of bandwidth demand–HDTV requires high bandwidth • Lack of standards»Telecomm industry»Computer industry • Radiation darkening–Depends on dose, exposure, glass materials, impurity types and levels–Clears with time

  44. Fiber Optic Components - Fiber • Extremely thin strands of ultra-pure glass • Three main regions • center: core (9 to 100 microns) • middle: cladding (125 or 140 microns) • outside: coating or buffer (250, 500 and 900 microns)

  45. Fiber Structure • Core and cladding are both transparent, usually glass, sometimes plastic. • Core has higher index of refraction. • Light propagates down the core, reflecting from cladding.

  46. Fiber Communication

  47. Fiber Optic Components - Light Emitters • Two types • Light-emitting diodes (LED’s) • Surface-emitting (SLED): difficult to focus, low cost • Edge-emitting (ELED): easier to focus, faster • Laser Diodes (LD’s) • narrow beam • fastest

  48. Communications Diode Laser & Modulator Modulator Laser p-InP/InGaAs Current Blocking Layers n-InP substrate Grating Frequency Stability~10-5 Lifetime >> 25 years InGaAsP Multiquantum Well Layers Maximum modulation speed ~ 40 GHz ( 25 psec ber bit) (hard to do) - but fibers can carry more information than this

  49. Lasers are us Laser light and LED light compared • LED are an extended source; light appears as many independent light modes • each small element of the LED is spatially incoherent • minimum focused size is an image of the LED and this is much larger than the core of a single-mode fibre and hence coupling efficiency is poor • Multimode fibre is normally used with LED Multi-mode fibre Ever Ready • Ideal laser light is a single ordered light beam • It is spatially and temporally coherent • Laser light can be focused to a very small spot Single-mode fibre

  50. Fiber Optic Components - Detectors • Two types • Avalanche photodiode • internal gain • more expensive • extensive support electronics required • PIN photodiode • very economical • does not require additional support circuitry • used more often

More Related