Optical Fiber Communications 1 Fiber Optics Fiber optics

Optical Fiber Communications 1

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. 2

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. 3

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. 4

In 1951, Abraham C. S. van Heel of Holland 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. 5

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

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). 7

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

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

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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 11

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

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. 13

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. 14

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 15

Types of Optical Fiber 1. Plastic core and cladding 2. Glass core with plastic cladding (PCS) 3. Glass core and glass cladding (SCS) 16

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

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 18

Optical Fiber Configuration 1. 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 2. 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. 3. 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. 19

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. 20

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. 21

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

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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 d. B down from the peak value. Rotating the acceptance angle around the fiber axis describes the acceptance cone of the fiber input. 24

Maximum Acceptance Angle = Acceptance Cone Optical Fiber Acceptance Angle 25

Numerical Aperture For a step index fiber: And NA = Sin (Acceptance Angle) 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 26 acceptance angle for the particular fiber.

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

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

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 29

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

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. 31

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. 32

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. 33

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. 34

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). 35

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 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. 36

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. 37

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. 38

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

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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. 41

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

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 m. W at 100 m. A of forward current. § Planar diffused (homojunction) LEDs output approximately 500 microwatts at a wavelength of 900 nm. 43

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. 44

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. 45

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

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. Emitted light rays 47

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. 48

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 m. W (7 d. Bm) and 0. 5 m. W ( 3 d. Bm) 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. 49

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. 50

Light Detectors § There are two devices that are commonly used to detect light energy in fiber optic communications receivers: PIN (p type intrinsic n type) diodes and APD (avalanche photodiodes). 51

PIN Diode § Assignment 52

Avalanche Photodiode § Assignment 53

Basic Cable Design § The two basic cable designs are the loose tube cable and tight buffered cable ( either a single fiber or a multi fiber). § Loose tube cable, used in the majority of outside plant installations in North America, and tight buffered cable, primarily used inside buildings. 54

Basic Cable Design § The modular design of loose tube cables typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. Loose tube cables can be all dielectric or optionally armored. The modular buffer tube design permits easy drop off of groups of fibers at intermediate points, without interfering with other protected buffer tubes being routed to other locations. The loose tube design also helps in the identification and administration of fibers in the system. 55

Basic Cable Design § Single fiber tight buffered cables are used as pigtails, patch cords and jumpers to terminate loose tube cables directly into optoelectronics transmitters, receivers and other active and passive components. § Multi fiber tight buffered cables also are available and are used primarily for alternative routing and handling flexibility and ease within buildings. 56

Loose Tube Cable § In a loose tube cable design, color coded plastic buffer tubes house and protect optical fibers. A gel filling compound impedes water penetration. Excess fiber length (relative to buffer tube length) insulates fibers from stresses of installation and environmental loading. Buffer tubes are stranded around a dielectric or steel central member, which serves as an anti buckling element. § The cable core, typically surrounded by aramid yarn, is the primary tensile strength member. The outer polyethylene jacket is extruded over the core. If armoring is required, a corrugated steel tape is formed around a single jacketed cable with an additional jacket extruded over the armor. Coated Fiber. Outer Jacket. Steel Tape Armor Inner Jacket Aramid Strength Member. Binder. Interstitial Filling. Central Member § (Steel Wire or Dielectric) Interstitial Filling. Loose Tube Cable § Loose tube cables typically are used for outside plant installation in aerial, duct and direct buried applications. 57

Loose Tube Cable Outer Jacket Steel Tape Armor Interstitial Filling Inner Jacket Central Member (Steel Wire or Dielectric) Aramid Strength Member Interstitial Filling Binder Coated Fiber Loose Tube Cable 58

Tight Buffered Cable § With tight buffered cable designs, the buffering material § § § is in direct contact with the fiber. This design is suited for "jumper cables" which connect outside plant cables to terminal equipment, and also for linking various devices in a premises network. Multi fiber, tight buffered cables often are used for intra building, risers, general building and plenum applications. The tight buffered design provides a rugged cable structure to protect individual fibers during handling, routing and cable connection. Yarn strength members keep the tensile load away from the fiber. As with loose tube cables, optical specifications for tight buffered cables also should include the maximum performance of all fibers over the operating temperature range and life of the cable. Averages should not be acceptable. 59

Tight Buffered Cable PVC Jacket (Non Plenum) or Fluoride Co Polymer Jacket (Plenum) Aramid Strength Member Glass Fiber Coating Thermoplastic Overcoating Buffer or Tight buffered Cable 60

Optical Fiber Connectors § Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers. These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices. In operation, the connector centers the small fiber so that its light gathering core lies directly over and in line with the light source (or other fiber) to tolerances of a few ten thousandths of an inch. Since the core size of common 50 micron fiber is only 0. 002 inches, the need for such extreme tolerances is obvious. § There are many different types of optical connectors in use today. The SMA connector, which was first developed before the invention of single mode fiber, was the most popular type of connector until recently. 61

Fiber Connectors 62

Optical Splices § While optical connectors can be used to connect fiber optic cables together, there are other methods that result in much lower loss splices. Two of the most common and popular are the mechanical splice and the fusion splice. Both are capable of splice losses in the range of 0. 15 d. B (3%) to 0. 1 d. B (2%). § In a mechanical splice, the ends of two pieces of fiber are cleaned and stripped, then carefully butted together and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remained aligned. § A fusion splice, by contrast, involves actually melting (fusing) together the ends of two pieces of fiber. The result is a continuous fiber without a break. Fusion splices require special expensive splicing equipment but can be performed very quickly, so the cost becomes reasonable if done in quantity. As fusion splices are fragile, mechanical devices are usually employed to protect them. 63

Designing Optical Fiber Systems The following step by step procedure should be followed when designing any system. § Determine the correct optical transmitter and receiver combination § § § based upon the signal to be transmitted (Analog, Digital, Audio, Video, RS 232, RS 422, RS 485, etc. ). Determine the operating power available (AC, DC, etc. ). Determine the special modifications (if any) necessary (Impedances, Bandwidths, Special Connectors, Special Fiber Size, etc. ). Calculate the total optical loss (in d. B) in the system by adding the cable loss, splice loss, and connector loss. These parameters should be available from the manufacturer of the electronics and fiber. Compare the loss figure obtained with the allowable optical loss budget of the receiver. Be certain to add a safety margin factor of at least 3 d. B to the entire system. Check that the fiber bandwidth is adequate to pass the signal 64 desired.

BASIC TYPES OF OPTICAL FIBER CABLE 1. 2. 3. 4. 5. 6. 7. 8. 9. Breakout Cable Interconnect Cable Loose Tube Cable Low Smoke – Zero Halogen Cable LXE Light Guide Express Entry Cable Light Pack Cable Indoor/Outdoor Loose Tube Cable Tactical/Military Cable TEMPEST Cable Description 65

Breakout Cable § Breakout cables are designed with all dielectric § § § construction to insure EMI immunity. These cables are obtainable in a wide range of fiber counts and can be used for routing within buildings, in riser shafts, and under computer room floors. The Breakout design enables the individual routing, or "fanning", of individual fibers for termination and maintenance. In addition to the standard duty 2. 4 mm subunit design, a 2. 9 mm heavy duty and a 2. 0 mm light duty design are also available. 66

Interconnect Cable § Cable for interconnecting equipment is available in single § § mode and multimode fiber sizes and its all dielectric construction provides EMI immunity. Available in one and two fiber designs, these cables are optimized for ease of connectorization and use as "jumpers" for intra building distribution. Its small diameter and bend radius provide easy installation in constrained areas. This cable can be ordered for plenum or riser environments. Products include single fiber cable, two fiber. Zipcord, and two fiber. DIB Cable. Uncabled fiber, coated only with a thermoplastic buffer, is also available for pigtail applications with inside equipment. 67

Loose Tube Cable § Loose tube cables are for general purpose § § § outdoor use. The loose tube design provides stable and highly reliable transmission parameters for a variety of applications. The design also permits significant improvements in the density of fibers contained in a given cable diameter while allowing flexibility to suit many system designs. These cables are suitable for outdoor duct, aerial, and direct buried installations, and for indoor use when installed in accordance with NEC Article 770. 68

Features § Different fiber types available within a cable § § § (hybrid construction). Lowest losses at long distances, for use in duct aerial, and direct buried applications. Wide range of fiber counts (up to 216). Available with single mode and multimode fiber types. All dielectric or steel central member. Loose Tube Cable is also available with armored construction for added protection. 69

Low Smoke – Zero Halogen Cable § Halex RTMis a low smoke, zero halogen fiber optic cable, designed to replace standard polyethylene jacketed fiber optic cables in environments where public safety is of great concern. § In addition to having low smoke properties, Halex Rcable meets the NEC requirements for risers, passes all U. S. flame requirements for UL 1666 and UL 1581, and is OFNR listed up to 156 fibers. 70

LXE Light Guide Express Entry Cable § The LXE (Lightguide Express Entry) sheath system is § § designed with the loop distribution market in mind, where express entry (accessing fibers in the middle of a cable span) is a common practice. The LXE sheath system achieves a 600 pound (2670 N) tensile rating through the use of linearly applied strength members placed 180 degrees opposite each other. High density polyethylene (HDPE) is used for the cable jacket to provide both faster installation, through a lower coefficient of friction, and optimum cable core protection in hostile environments. 71

Features § Strength members in cable sheath (not in cable core). § Non metallic cable core. 72

Light Pack Cable § Lightpack Cable consists of fiber "bundles" held § § § together with color coded yarn binders. Cable can hold up to 144 fibers and still maintain a large clearance in the core tube. A water blockingcompound, specifically designed for LIGHTPACK Cable, adds extra flexibility, protects the fiber and virtually eliminates microbending losses. Lightpack cable is compact size, rugged design, contains a high density polyethylene sheath and has a high strength to weightratio. 73

Indoor/Outdoor Loose Tube Cable § The RLT Series of loose tube fiber optic cables is designed for installation both outdoors and indoors in areas required by the (NEC) to be riser rated Type OFNR. They meet or exceed Article 770 of the NEC and UL Subject 1666 (Type OFNR). They also meet CSA C 22. 2 No. 232 M 1988 Type OFN FT 4. § All of the RLT products utilize a proprietary Chroma. Tek 3 jacketing system that is designed for resistance to moisture, sunlight and flame for use both indoors and outdoors. These cables are loose tube, gel filled constructions for excellent resistance to moisture. They are available with single mode or multimode fibers with up to a maximum of 72 fibers. 74

Indoor/Outdoor Loose Tube Cable § Because these outdoor cables are riser rated, they eliminate the need for a separate point of demarcation, i. e. , splicing to a riser rated cable within 50 feet of the point where the outdoor cable enters the building as required by the NEC. These cables may be run through risers directly to a convenient network hub or splicing closet for interconnection to the electro optical hardware or other horizontal distribution cables as desired. § No extra splice or termination hardware is required at the entrance to the facility, and cable management is made easier by the use of just one cable. This installation ease is especially useful in Campus type installations where buildings are interconnected with outdoor fiber optic cables. 75

Tactical/Military Cable § Tactical cable utilizes a tight buffer configuration in an all dielectric construction. § The tight buffer design offers increased ruggedness, ease of handling and connectorization. § The absence of metallic components decreases the possibility of detection and minimizes system problems associated with electromagnetic interference. 76

Features § Proven compatibility with existing ruggedized connectors. § Lightweight and flexible: no anti buckling elements required. § Available in connectorized cable assemblies. § Available with 50, 62. 5 and 100 micron multimode fibers, as well as single mode and radiation hardened fibers. 77

TEMPEST Cable Description § For use where secure communications are a major consideration, and Tempest requirements must be met. The Tempest rated cable is available in a variety of cable constructions. § Tempest relates to government requirements for shielding communications equipment and environments. § One common application is the use of fiber optic cable in conjunction with RF shielded enclosures. These enclosures have been specially constructed to suppress the emission of RF signals, and must meet the Transient Electro magnet, Pulse Emanation Standard (TEMPEST). 78

Cont. § For a system to be TEMPEST qualified, it must be tested in accordance with MIL STD 285, and it must also meet the requirements stated in NSA 65 6. All elements of the system, individually and combined, must meet the TEMPEST standard. § In the case of fiber optics, the "system" consists of the cable (which is dielectric and non conductive), and the tube through which the cable passes. 79