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.