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Modern Devices: Chapter 11 – Optical Couplers including Optical Fibers

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1 Modern Devices: Chapter 11 – Optical Couplers including Optical Fibers
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Chapter 11 – Optical Couplers including Optical Fibers Modern Devices: The Simple Physics of Sophisticated Technology by Charles L. Joseph and Santiago Bernal

2 Solid Fiber Optic Cables
Fiber core Cladding Fiber Coating (Light-blocking) Stiffening & strength member Solid Fiber Optic Cables Protective Outer Jacket Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Figure 11.1 The anatomy of a single-fiber (Left) and multi-fiber (Right) cable.

3 Ray too steep for the bend
Fiber Optic Wave Guide Reflected Ray Absorbed Refracted Ray  > C C Ray too steep for the bend ( < C only at bend) Figure 11.2 A cross section of an optical fiber, showing the responses to various light rays entering the fiber and traveling down the central fiber. Typical optical fibers are cylindrical glass or acrylic rod-in-collar designs, making use of the principle of total internal reflection. The collar has to have an optical index, n, smaller than that of the rod. A fiber will loose significant amounts of signal and perhaps be damaged permanently if it is bent too sharply. There is also a maximum acceptance angle (a cone of light rays) that will be guided down the fiber. Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc.

4 Classes of Fiber Optic Wave Guides
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Classes of Fiber Optic Wave Guides Figure 11.3 A few types of optical fibers. A multimode fiber (top) has a large acceptance angle, while a single-mode fiber (middle) has a small cone of acceptance. A gradient fiber (bottom) has an index of refraction that decreases continuously and smoothly as a function of fiber radius. Multimode Fiber (Diameter > 10) Single Mode Fiber (Diameter ≤ 10) Gradient Fiber (index of refraction, n(r) ) The diameter of a rod-in-core architecture is of critical importance. When the diameter is approximately ten times or less than the wavelength of light being sent down the fiber, then the acceptance angle, , is small and it is a single-mode fiber. If the diameter is very large compared to the wavelength of light, then  is large and it is a multimode optical fiber. A gradient fiber produces a continuous turning of a light ray as it traverses down the fiber, causing the ray to curve back towards the central part of the fiber.

5 Dispersion of Bits Traveling Down a Fiber
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Figure 11.4 The dispersion of a three-bit data stream as it travels down an optical fiber. (Still recognizable) ? ? ? 0,1 discrimination level Pulse Dispersion Dispersion of Bits Traveling Down a Fiber A bit stream of light pulses traveling down a fiber experience dispersion. I.e., a pulse of light spreads out and is attenuated as it propagates. After the 1,0,1 sequence has traveled some distance down the fiber, it is still recognizable, but has begun to blend with adjacent bits. If the sequence is allow to proceed further down the cable, the blending with adjacent bits continues and at some point the electronics cannot distinguish which bits are above threshold (i.e. a "1") from those just below (i.e. a "0"), especially in the presence of noise.

6 Hollow-Core Fiber Optics
Sapphire capillary tube (n = 10.6 ) 250-1,000 m Hollow Core (Air, n = ) Silver Silver iodide (AgI) Glass capillary tube Protective jacket m Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Figure 11.5 Cross sections of two hollow-core fiber optics (also known as a capillary tube fibers). The left one is a conventional step optical fiber using total internal reflection. The right capillary uses reflection, which is efficient at infrared (IR) wavelengths. Hollow-Core Fiber Optics The telecomm. industry uses three near infrared (near-IR) windows with wavelength centers of 850, 1310, & nm. Intense light sources used over long distances can melt optics or solid fibers without sufficient cooling. A hollow waveguide has an advantage over a solid-core fiber because there is no optical boundary or change in the index of refraction as the light enters the fiber.

7 Modern Devices: The Simple Physics of Sophisticated Technology
Copyright © John Wiley and Sons, Inc. Fig Some of the physical mechanisms responsible for attenuation at a joint. Finish & Dirt Core Mismatch Coaxiality Axial Run-out End Angle Both hollow-core and solid fiber optical cables can be fabricated up to a few hundred meters in length. For several kilometer distances, the ends of continuous optical fibers have to be mated to each other, known as joints. There are inherent physical imperfections when two fibers are joined as shown in the figure to the right. Imperfections in mating produce small fractional signal losses. Whenever light crosses a surface boundary, some of the beam intensity is lost to reflection and absorption with the remainder being transmitted. Fiber Optical Joints

8 for information branching
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Equivalent Physical Splice Optical Coupler Symbol Coupler Symbol & 1 termination Beams from left or below is spit to right and below Beams from right or below is spit to left and below Figure 11.7 An example optical coupler for separating and combining signals with a fiber optic cable. Bottom left: "circuit" symbols of fiber optics. Top: physical splice corresponding to optical coupler. Bottom center and bottom right: informational flow paths through a coupler. Fiber Optic Coupler for information branching

9 Long-Distance Repeaters
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Figure 11.8 The multi-step process that occurs in the repeater system. Original Bit- Stream Signal Arriving Signal (after a long distance) Reamplifying the raw Signal Pulse Reshaping + Pulse Retiming Long-Distance Repeaters Cumulative attenuations, dispersions, and other changes to profiles of the pulses require active signal restoration, including reamplification, pulse shaping, and pulse timing. A segment of the original bit-stream signal arrives at a remote distant location (second plot from the top) with pulse profiles that have been attenuated, spread (dispersed), and the individual arrival times slightly shifted compared to the original bits (dotted line overlays). The repeater station could convert the optical signal into an electronic signal, process the bit stream, and then convert it back to an optical signal. This approach is very expensive and introduces significant delays. It is cost effective, faster, and more reliable to use a repeater constructed completely of photonic components.

10 Long-Distance Multiplexed Signals
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Long-Distance Multiplexed Signals Figure 11.9 A basic long-distance photonic communication system using multiple channels. Fiber Optic Cables (over long [many km] distances) Optical Amplifiers Joint Optical Couplers MUX Laser 1 Laser 2 Laser N Laser 3 Laser 4 Laser 5 E1 E2 E3 E4 E5 EN Input Electrical Data Signals Electro-optical Modulators Individual Output Electrical Signals DeMUX

11 Long-Distance Signal Transmission
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Attenuation Losses Long-Distance Signal Transmission Sout/Sinputin dB = 20 log (Sout/Sinput) Table 11.1 Attenuation values from the Cisco Webpages Engineers measure signals and signal losses in decibels, a logarithmic scale defined by the equation below since signal losses are multiplicative. The decibel scale enables a numerical conversion from a multiplicative calculation to an additive calculation just as 103  101  105 = 10(3+1+5) = Table 11.1 has the attenuations per length of fiber along with the discrete losses per joint and per connector for the devices used by Cisco Corporation. Thus, one can simply count all of the connectors, joints, and lengths of fiber, to calculate a total attenuation by adding all of the individual losses. Wavelength (nm) Attenuation per fiber length Attenuation per connector Attenuation per joint 1310 -0.38 dB/km (  0.957) -0.6 dB (  0.933) -0.1 dB (  0.989) 1550 -0.22 dB/km (  0.975) -0.35 dB (  0.961) -0.05 dB (  0.994)

12 Long-Distance Signal Transmission
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Figure Plot of signal losses through a long optical fiber, including reamplification Distance  Power in Signal  (logarithmic) Reamplification Joint Coupler Losses transmitter output Connector The power of the signal decreases steadily as function of cable length with small discrete jumps at the joints and connectors. The process, expressed mathematically in the previous slide, is provided here graphically. Attenuation Losses Long-Distance Signal Transmission

13 Optical Coupler / Optical Isolator
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Optical Coupler / Optical Isolator Figure A simplified circuitry of an optical isolator, an optical coupler used to separate physically two electronic packages carrying data signals. Input Output Amplified DC Voltage Coupler / Isolator An optical coupler is used in electronics to prevent a voltage spike or a short to ground in one device from disrupting the operations of a more central electronics package. These optical couplers are also known as opto-isolators, optocoupler, or optical isolator. The actual isolator is contained in the dashed-line box, consisting of a photoemitting diode and photodiode detector.

14 Intro Physics Flashback FB11.1 – Total Internal Reflection
Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Intro Physics Flashback FB11.1 – Total Internal Reflection Auxiliary Material Figure FB11.1 For a light beam coming from a material with a higher index of refraction incident into to a material with a lower index of refraction, total internal reflection occurs when the incident angle 1 becomes greater than the critical angle, C. 1 2 2 = 90° 1 = C Critical Angle Total Reflection In general, whenever a light beam encounters an interface between two materials with different indices of refraction, part of it is transmitted and part of it is reflected. Total internal reflection occurs whenever the beam encounters a surface having a lower index of refraction and the incident angle is greater than the critical angle. This physical property is invaluable for waveguides, binoculars, periscopes, and other applications.


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