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Fiber Optic Technology - Part One - Fiber Optics Theory

Fiber optic networking is now the standard for high speed data transfers and backbone cabling. While fiber does require more expensive transmission equipment, the extra expense is well worth the gains in speed and efficiency that fiber optic brings. Installing a fiber network will help you transmit more info over longer distances. It also provides added security, and is resistant to electrical interference, which is a major issue with traditional copper cabling and components.


In this first article in our new series on Fiber Optic Technology, we explore the Fiber Optics Theory; how it is made; how it works; and the different types of fiber.

Fiber Optics is a technology that works by sending signals down hair thin strands of glass fiber (and sometimes plastic fiber.). A fiber cable consists of a bundle of these glass threads, each of which is capable of transmitting messages by way of light waves. This is not a new technology and is in fact a concept that is over a century old, however it has only been used commercially for the last 40 years, with the first installation in 1976 in Chicago. 

By the 1980s, fiber networks connected major cities around the world and by the mid-80s, fiber was replacing all the telco copper, microwave and satellite links. In the 90s, CATV discovered fiber and used it first to enhance the reliability of their networks. Along the way, they discovered they could offer phone and Internet service on that same fiber which greatly enlarged their markets. Now, even fiber to the home is cost effective. Computers and LANs started using fiber about the same time as the telcos. Industrial links were among the first as the noise immunity of fiber and its distance capability make it ideal for the factory floor. Mainframe storage networks came next, the predecessors of today's fiber storage area networks – or SANs.

​Fiber optics has several advantages over traditional communications lines:

  • Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data.
  • Fiber optic cables are less susceptible than metal cables to interference. Because they are made from glass they do not suffer from induced EMF's​.
  • Fiber optic cables are much thinner and lighter than metal wires. This makes them a logical choice for aircraft and situations where weight is a critical factor.
  • Data can be transmitted digitally (the natural form for computer data) rather than analogically​.


Some myths regarding fiber: 

Light from the fiber will harm your eyes:
Fiber optic sources, especially LEDs used with multimode fiber are generally too low in power to cause any eye damage. Some laser transmitters used in telecom and CATV systems have very high power and they could be harmful, so it is better to be safe than sorry. It is advised to never look into the end of the fiber as it is infrared light, so your eyes cannot see it under any circumstances. ​

Fiber is extremely hard to work with:
Fiber is no harder to install, splice or terminate than copper wire. It takes some training, practice and patience, but so does copper.

Fiber is fragile:
Fragile? What is a fiberglass boat reinforced with?

Fiber is expensive:
Today, fiber is cheaper than kite string or fishing line. Connectors are getting cheaper too. And all the while, copper components are getting more expensive as they try to keep up with fiber for new high bit rate networks. And a good fiber test set is under US$1000 while a copper tester will run US$6000 or more.


Optical fiber for telecommunications consists of three components: core, cladding & coatingThe core is the central region of an optical fiber through which light is transmitted. 

In general, the telecommunications industry uses sizes from 8.3 micrometers (µm) to 62.5 micrometers. The standard telecommunications core sizes in use today are 8.3 µm (single-mode), 50 µm (multimode), and 62.5 µm (multimode). (Single-mode and multimode will be discussed shortly.)

The cladding is a glass sheath that surrounds the core, which acts like a mirror, reflecting light back into the core. The cladding itself is covered with a plastic coating and strength material when appropriate. The diameter of the cladding surrounding each of the cores is typically 125 µm.

Light in the core travels slightly slower than light in the cladding and this property tends to keep any light sent into the core from one end of the fiber from leaking out, until it reaches the far end. The core and cladding are manufactured together as a single piece of silica glass with slightly different compositions, and cannot be separated from one another. The glass does not have a hole in the core, but is completely solid throughout.

The third section of an optical fiber is the outer protective coating. This coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber. During the installation process, this coating is stripped away from the cladding to allow proper termination to an optical transmission system. 

​How is the fiber made?

  • Create the pre-form

  • Take glass tube

  • Inject germanium and silicon dioxide

  • Fuse internal gasses into a glass

  • Draw optical fiber from the pre-form

  • Attach coating layer while drawing

  • Roll drawn fiber off onto rolls

  • Test fiber 

Multimode & singlemode fiber are the two types of fiber in common use. Both fibers are 125 microns in outside diameter - a micron is one-millionth of a meter and 125 microns is 0.005 inches - a bit larger than the typical human hair. Multimode fiber has light travelling in the core in many rays, called modes. It has a bigger core (almost always 62.5 microns, but also 50 microns) and is used with LED sources at wavelengths of 850 and 1300 nm (see below) for slower local area networks (LANs) and lasers at 850 and 1310 nm for networks running at gigabits per second or more.

Multimode optical fiber is mostly used for communication over shorter distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters - more than sufficient for the majority of premises applications.

Singlemode fiber has a much smaller core, only about 9 microns, so that the light travels in only one ray. It is used for telephony and CATV with laser sources at 1300 and 1550 nm. Plastic Optical Fiber (POF) is large core (about 1mm) fiber that can only be used for short, low speed networks. Unlike multimode optical fibers, singlemode fibers do not exhibit dispersion resulting from multiple spatial modes. Singlemode fibers are also better at retaining the fidelity of each light pulse over long distances than multimode fibers. For these reasons, singlemode fibers can have a higher bandwidth than multimode fibers. Equipment for singlemode fiber is more expensive than equipment for multimode optical fiber, but the singlemode fiber itself is usually cheaper in bulk.

The three most widely used fiber types, all made from pure glass:

Multimode fiber with core/cladding sizes of 50/125 and 62.5/125 microns and singlemode or 9micron. 50/125 is often referred to as "laser rated" fiber for it's higher bandwidth capacity.

50/125 comes in 2 types - OM2 and OM3 and now also OM4; whereas 62.5 micron is classified as OM1. 'OM' stands for 'Optical Mode'.


OM1 

This is very much the optical fiber that has been specified and we have been using for the past number of years. This can be either 62.5 or 50 micron optical fiber. The use for this type of cable is for legacy application support and short run Gigabit networks. It is limited in its bandwidth capabilities.

OM2 

This is either 62.5 or 50 micron fiber with Overfilled Launch Bandwidth of 500MHz/Km for both. 

The applications will be legacy application support and Gigabit networks up to 500 meters.

OM3 / OM4

This is essentially the newer laser optimized fiber with refractive index profile optimized for laser light insertion @ 850nm. The applications can be legacy network support, but it is targeted at gigabit and 10G transmissions up to 300 meters (OM3) and 550 meters (OM4). VCSEL = vertical-cavity surface-emitting laser.

What is Fiber Bandwidth? 

The amount of information that a system can carry such that each pulse of light is distinguishable by the receiver. System bandwidth is measured by frequency in MHz or GHz. In general, when we say that a system has bandwidth of 2000 MHz, it means that 2000 million pulses of light per second will travel down the fiber and each will be distinguishable by the receiver. 


Refraction of Light 

Refraction, or the bending of light as it passes from one material into another, is a key component in fiber optic transmissions.  It is this principle that causes an object in water to look like it is bent. 

How much the light ray changes its direction, depends on the refractive index of the mediums. 

Refractive Index

Refractive index is the speed of light in a vacuum (abbreviated c, c=299,792.458 km/second) divided by the speed of light in a material (abbreviated v). Refractive index measures how much a material refracts light. 

Refractive index of a material, abbreviated as n, is defined as n = c / v:

n = refractive index / c = speed of light in a vacuum (299,792.458 km/second) /
v = speed of light in a material


Snell's Law

In 1621, a Dutch physicist named Willebrord Snell derived the relationship between the different angles of light as it passes from one transparent medium to another. When light passes from one transparent material to another, it bends according to Snell's Law which is defined as:

n1sin(θ1) = n2sin(θ2):
n1 is the refractive index of the medium the light is leaving
θ1 is the incident angle between the light beam and the normal (normal is 90° to the interface between two materials)
n2 is the refractive index of the material the light is entering
θ2 is the refractive angle between the light ray and the normal

Note:

For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 = 0° regardless of the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent. The above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of Snell's Law shows that the same ray paths are applicable in opposite direction.
Total Internal Reflection

When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling across an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal. This has an interesting implication: at some angle, known as the critical angle θc, light traveling from a higher refractive index medium to a lower refractive index medium will be refracted at 90°; in other words, refracted along the interface.

​If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection. For any angle of incidence larger than the critical angle, Snell's Law will not be able to be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1, which is not possible. In that case all the light is totally reflected off the interface, obeying the law of reflection.

Optical Fiber Mode

As described earlier, an optical fiber guides light waves in distinct patterns called modes. Mode describes the distribution of light energy across the fiber. The precise patterns depend on the wavelength of light transmitted and on the variation in refractive index that shapes the core. In essence, the variations in refractive index create boundary conditions that shape how light waves travel through the fiber, like the walls of a tunnel affect how sounds echo inside.

We can take a look at large-core step-index fibers. Light rays enter the fiber at a range of angles, and rays at different angles can all stably travel down the length of the fiber as long as they hit the core-cladding interface at an angle larger than critical angle. These rays are different modes.

Fibers that carry more than one mode at a specific light wavelength are called multimode fibers. Some fibers have very small diameter core that they can carry only one mode which travels as a straight line at the center of the core. These fibers are singlemode fibers. This is illustrated in the picture above.

Optical Fiber Index Profile

​Index profile is the refractive index distribution across the core and the cladding of a fiber. Some optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include power-law index profiles and parabolic index profiles.

Numerical Aperture (NA)

Three points which are important to appreciate:

  • The optic fiber is solid, there is no hole through the middle.
  • The buffer and the jacket are only for mechanical protection.
  • The light is transmitted through the core but to a small extent, it travels in the cladding and so the optical clarity of the cladding is still important.

Multimode optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber.

The half-angle of this cone is called the acceptance angle, θmax. Variations in refractive index create boundary conditions that shape how light waves travel through the fiber. 

It is like how the walls of a tunnel affect how the sound echoes inside.

​So why does the light enter the cladding? If the angle of incidence is greater than the critical angle, the light ray is reflected back into the first material by the process of refraction (TIR). 

A cladding that was opaque would prevent the ray from being propagated along the fiber since the light would not be able to pass through the cladding.

Fiber Attenuation

The attenuation of the optical fiber is a result of the combination of two factors, absorption and scattering. The absorption is caused by the absorption of the light and conversion to heat by molecules in the glass. Primary absorbers are residual OH- and dopants used to modify the refractive index of the glass. This absorption occurs at discrete wavelengths, determined by the elements absorbing the light. The OH- absorption is predominant, and occurs most strongly around 1000 nm, 1400 nm and above 1600 nm.

The largest cause of attenuation is scattering. Scattering occurs when light collides with individual atoms in the glass and is anisotropic. Light that is scattered at angles outside the numerical aperture of the fiber will be absorbed into the cladding or transmitted back toward the source. Scattering is also a function of wavelength, proportional to the inverse fourth power of the wavelength of the light. Thus if you double the wavelength of the light, you reduce the scattering losses by 24 or 16 times. Therefore for long distance transmission, it is advantageous to use the longest practical wavelength for minimal attenuation and maximum distance between repeaters. Together, absorption and scattering produce the attenuation curve for a typical glass optical fiber as shown.

Material Absorption and Rayleigh Scattering are two components of Intrinsic Attenuation.

Material Absorption

Rayleigh Scattering 

Extrinsic attenuation can be caused by two external mechanisms: Macro-Bending and Micro-Bending. A bend on an optical fiber places a strain on the fiber along the region that is bent and then the bending strain affects the refractive index and the critical angle of the light ray in that specific area. Light travelling in the core may then refract out, resulting in a loss. This loss is generally reversible after bends are corrected. Micro-bending is caused by imperfections in the cylindrical geometry of fiber during the manufacturing. It is rarely reversible.

Macro-Bending

Micro-Bending

Fiber Optic Transmission
Fiber optic transmission systems all work similar to the diagram shown below. They consist of a transmitter on one end of a fiber and a receiver on the other end. Fiber optic transmission systems all consist of a transmitter which takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light is ultimately coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment.

Most systems use a "transceiver" which includes both transmission and receiver in a single module. Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter.
Fiber Optic Link Performance

​The ability of any fiber optic system to transmit data ultimately depends on the optical power at the receiver as shown above, which shows the data link bit error rate as a function of optical power at the receiver.

BER is the inverse of signal-to-noise ratio, e.g. high BER means poor signal to noise ratio. Either too little or too much power will cause high bit error rates. Too much power, and the receiver amplifier saturates, too little and noise becomes a problem as it interferes with the signal. This receiver power depends on two basic factors: how much power is launched into the fiber by the transmitter and how much is lost by attenuation in the optical fiber cable plant that connects the transmitter and receiver.

Fiber Optic Data links can be either analog or digital in nature, although most are digital. Both have some common critical parameters and some major differences. For both, the optical loss margin or power budget is most important. This is determined by connecting the link up with an adjustable attenuator in the cable plant and varying the loss between transmitter and receiver until one can generate the curve shown above.

Analog datalinks will be tested for signal to noise ratio to determine link margin, while digital links use bit error rate as a measure of performance. Both links require testing over the full bandwidth specified for operation, but most data links are now specified for a specific network application, like AM CATV or RGB color monitors for analog links and SONET, Ethernet or Fiber Channel for digital links.
Fiber Optic Link Power Budget

The optical power budget of the link is determined by two factors, the sensitivity of the receiver, which is determined in the bit error rate curve and the output power of the transmitter into the fiber. The minimum power level that produces an acceptable bit error rate determines the sensitivity of the receiver. The power from the transmitter coupled into the optical fiber determines the transmitted power. The difference between these two power levels determines the loss margin (power budget) of the link.

High speed links like gigabit or 10gigabit Ethernet LANs on multimode fiber have derating factors for the bandwidth of fiber. Older 62.5/125 OM1 fiber will generally operate only on shorter links while links on 50/125 OM3 laser-optimized fiber will go the longest distance. Even long distance singlemode fiber links may have limitations caused by chromatic or polarization-mode dispersion. 

If the link is designed to operate at differing bit rates, it is necessary to generate the performance curve for each bit-rate. Since the total power in the signal is a function of pulse width and pulse width will vary with bit-rate (higher bit-rates means shorter pulses), the receiver sensitivity will degrade at higher bit-rates.

​Fiber optics has become widely used in telecommunications because of its enormous bandwidth and distance advantages over copper wires. The application for fiber in telephony is simply connecting switches over fiber optic links. Commercial systems today carry more phone conversations over a single pair of fibers than could be carried over thousands of copper pairs. Material costs, installation and splicing labor and reliability are all in fiber's favor - not to mention space considerations. In major cities today, insufficient space exists in current conduit to provide communications needs over copper wire.

​Telecom systems operate at bit rates up to 10 gigabits per second and many links use WDM - Wavelength Division Multiplexing - to put several channels of signals over one fiber.

Wave Division Multiplexing

​How does WDM work? It is easy to understand WDM. Consider the fact that you can see many different colors of light - red, green, yellow, blue, etc. all at once. The colors are transmitted through the air together and may mix, but they can be easily separated using a simple device like a prism, just like we separate the "white" light from the sun into a spectrum of colors with the prism.

The input end of a WDM system is really quite simple. It is a simple coupler that combines all the inputs into one output fiber. These have been available for many years, offering 2, 4, 8, 16, 32 or even 64 inputs. It is the demultiplexer that is the difficult component to make. The demultiplexer takes the input fiber and collimates the light into a narrow, parallel beam of light. It shines on a grating (a mirror like device that works like a prism, similar to the data side of a CD) which separates the light into the different wavelengths by sending them off at different angles. Optics capture each wavelength and focuses it into a fiber, creating separate outputs for each separate wavelength of light. 

Current systems offer from 4 to 32 channels of wavelengths. The higher numbers of wavelengths has led to the name Dense Wavelength Division Multiplexing or DWDM. The technical requirement is only that the lasers be of very specific wavelengths and the wavelengths are very stable, and the DWDM demultiplexers are capable of distinguishing each wavelength without crosstalk.

CATV Technology

The reason fiber is used in CATV networks is that the fiber pays for itself in enhanced reliability. 

Applications in CATV were slow until the development of the AM analog systems. By simply converting the signal from electrical to optical, the advantages of fiber optics, especially reliability, became cost effective. Now CATV has adopted a network architecture that overbuilds the normal coax network with fiber optic links. 

The Hybrid-Fiber-Coax (HFC) network lets the CATV provider have a two-way connection to the subscriber that allows them to offer broadband Internet connections at a low cost.
Local Area Networks - LANs

Fiber penetration in LANs is very high in long distance or high bitrate backbones in large LANs, connecting local hubs or routers, but still very low in connections to the desktop. When simply replacing copper cables, fiber is more expensive in a LANs, but using a centralized fiber architecture makes fiber more cost effective than copper in most cases. Rapidly declining costs of the installed fiber optic cable plant and adapter electronics combined with needs for higher bandwidth at the desktop are also making fiber to the desk more viable.

• Every LAN offers a fiber option or can be converted with "media converters".
• LAN backbones today are predominately fiber.
• Fiber to the desk has not been popular due to cost of electronics, but offers greater bandwidth and lower power consumption at higher speeds - and reduces the need for constant upgrades in cable types.

Other Applications of Fiber Optics
  • ​Communications (cellular/wireless/PCS antennas); and wireless LAN antennas
  • Security (closed-circuit TV; Intrusion sensors
  • Surveillance proofing
  • Building management
  • ​Traffic control
  • Process control
  • Utility network management
  • Sensors (high voltage/current; chemicals)
  • Hazardous environments

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