Definition
Fiber-optic communications is based on the principle that light in a glass medium can carry more information over longer distances than electrical signals can carry in a copper or coaxial medium. The purity of today's glass fiber, combined with improved system electronics, enables fiber to transmit digitized light signals well beyond 100 km (60 miles) without amplification. With few transmission losses, low interference, and high bandwidth potential, optical fiber is an almost ideal transmission medium.Overview
The advantages provided by optical fiber systems are the result of a continuous stream of product innovations and process improvements. As the requirements and emerging opportunities of optical fiber systems are better understood, fiber is improved to address them. This tutorial provides an extensive overview of the history, construction, operation, and benefits of optical fiber, with particular emphasis on outside vapor deposition (OVD) process.
An important principle in physics became the theoretical foundation for optical fiber communications: light in a glass medium can carry more information over longer distances than electrical or radio frequency (RF) signals can carry in a copper, coaxial or wireless medium.
The first challenge undertaken by scientists was to develop a glass so pure that one percent of the light would be retained at the end of one kilometer (km), the existing unrepeatered transmission distance for copper-based telephone systems. In terms of attenuation, this one-percent of light retention translated to 20 decibels per kilometer (dB/km) of glass material.
Glass researchers all over the world worked on the challenge in the 1960s, but the breakthrough came in 1970, when Corning Incorporated scientists Drs. Robert Maurer, Donald Keck, and Peter Schultz created a fiber with a measured attenuation of less than 20 dB per km. It was the purest glass ever made.
The three scientists’ work is recognized as the discovery that led the way to the commercialization of optical fiber technology. Since then, the technology has advanced tremendously in terms of performance, quality, consistency, and applications.
Working closely with customers has made it possible for scientists to understand what modifications are required, to improve the product accordingly through design and manufacturing, and to develop industry-wide standards for fiber.
The commitment to optical fiber technology has spanned more than 30 years and continues today with the endeavor to determine how fiber is currently used and how it can meet the challenges of future applications. As a result of research and development efforts to improve fiber, a high level of glass purity has been achieved. Today, fiber’s optical performance is approaching the theoretical limits of silica-based glass materials. This purity, combined with improved system electronics, enables fiber to transmit digitized light signals hundreds of kilometers without amplification. When compared with early attenuation levels of 20 dB per km, today’s achievable levels of less than 0.35 dB per km at 1310 nanometers (nm) and 0.25 dB per km at 1550 nm, testify to the incredible drive for improvement.
The operation of an optical fiber is based on the principle of total internal reflection. Light reflects (bounces back) or refracts (alters its direction while penetrating a different medium), depending on the angle at which it strikes a surface.
One way of thinking about this concept is to envision a person looking at a lake. By looking down at a steep angle, the person will see fish, rocks, vegetation, or whatever is below the surface of the water (in a somewhat distorted location due to refraction), assuming that the water is relatively clear and calm. However, by casting a glance farther out, thus making the angle of sight less steep, the individual is likely to see a reflection of trees or other objects on an opposite shore. Because air and water have different indices of refraction, the angle at which a person looks into or across the water influences the image seen.
This principle is at the heart of how optical fiber works. Controlling the angle at which the light waves are transmitted makes it possible to control how efficiently they reach their destination. Lightwaves are guided through the core of the optical fiber in much the same way that radio frequency (RF) signals are guided through coaxial cable. The lightwaves are guided to the other end of the fiber by being reflected within the core.
The composition of the cladding glass relative to the core glass determines the fiber’s ability to reflect light. That reflection is usually caused by creating a higher refractive index in the core of the glass than in the surrounding cladding glass, creating a “waveguide.” The refractive index of the core is increased by slightly modifying the composition of the core glass, generally by adding small amounts of a dopant. Alternatively, the waveguide can be created by reducing the refractive index of the cladding using different dopants.
The Design of Fiber
Core, Cladding, and Coating
An optical fiber consists of two different types of highly pure, solid glass, composed to form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the cladding. In most cases, the protective coating is a dual layer composition.
Figure 1. Core, Cladding, and Coating
A protective coating is applied to the glass fiber as the final step in the manufacturing process. This coating protects the glass from dust and scratches that can affect fiber strength. This protective coating can be comprised of two layers: a soft inner layer that cushions the fiber and allows the coating to be stripped from the glass mechanically and a harder outer layer that protects the fiber during handling, particularly the cabling, installation, and termination processes.
Single-Mode and Multimode Fibers
There are two general categories of optical fiber: single-mode and multimode (see Figure 2).
Figure 2. Single-Mode and Multimode Fibers
Multimode fiber was the first type of fiber to be commercialized. It has a much larger core than single-mode fiber, allowing hundreds of modes of light to propagate through the fiber simultaneously. Additionally, the larger core diameter of multimode fiber facilitates the use of lower-cost optical transmitters (such as light emitting diodes [LEDs] or vertical cavity surface emitting lasers [VCSELs]) and connectors.
Single-mode fiber, on the other hand, has a much smaller core that allows only one mode of light at a time to propagate through the core. While it might appear that multimode fibers have higher capacity, in fact the opposite is true. Singlemode fibers are designed to maintain spatial and spectral integrity of each optical signal over longer distances, allowing more information to be transmitted.
Its tremendous information-carrying capacity and low intrinsic loss have made single-mode fiber the ideal transmission medium for a multitude of applications. Single-mode fiber is typically used for longer-distance and higher-bandwidth applications (see Figure 3). Multimode fiber is used primarily in systems with short transmission distances (under 2 km), such as premises communications, private data networks, and parallel optic applications.
Optical Fiber Sizes
The international standard for outer cladding diameter of most single-mode optical fibers is 125 microns (µm) for the glass and 245 µm for the coating. This standard is important because it ensures compatibility among connectors, splices, and tools used throughout the industry.
Standard single-mode fibers are manufactured with a small core size, approximately 8 to 10 µm in diameter. Multimode fibers have core sizes of 50 to 62.5 µm in diameter.
Figure 3. Optical Fiber Sizes
Basic OVD optical fiber manufacturing consists of three steps: laydown, consolidation, and draw.
Laydown
In the laydown step, a soot preform is made from ultrapure vapors as they travel through a traversing burner and react in the flame to form fine soot particles of silica and germania (see Figure 4).
Figure 4. OVD Laydown Process
The OVD process is distinguished by the method of depositing the soot. These particles are deposited on the surface of a rotating target rod. The core material is deposited first, followed by the pure silica cladding. As both core and cladding raw materials are vapor-deposited, the entire preform becomes totally synthetic and extremely pure.
Consolidation
When deposition is complete, the bait rod is removed from the center of the porous preform, and the preform is placed into a consolidation furnace. During the consolidation process, the water vapor is removed from the preform. This high-temperature consolidation step sinters the preform into a solid, dense, and transparent glass.
The Draw
The finished glass preform is then placed on a draw tower and drawn into one continuous strand of glass fiber (see Figure 5).
Figure 5. Optical Fiber Drawing Process
First, the glass blank is lowered into the top of the draw furnace. The tip of the blank is heated until a piece of molten glass, called a gob, begins to fall from the blank—much like hot taffy. As the glob falls it pulls behind it a thin strand of glass, the beginning of an optical fiber.
The gob is cut off, and the fine fiber strand is threaded into a computer-controlled tractor assembly and drawn. Then, as the diameter is monitored, the assembly speeds up or slows down to precisely control the size of the fiber’s diameter.
The fiber progresses through a diameter sensor that measures the diameter hundreds of times per second to ensure specified outside diameter. Next, the inner and outer primary coatings are applied and cured, using ultraviolet lamps. At the bottom of the draw, the fiber is wound on spools for further processing.
Fiber from these spools is proof-tested and then measured for performance of relevant optical and geometrical parameters. Each fiber has a unique identification number that can be traced to all relevant manufacturing data (including raw materials and manufacturing equipment). Each fiber reel is then placed into protective shipping containers and prepared for shipment to customers worldwide.
Fiber produced using the OVD process is purely synthetic, exhibits enhanced reliability, and allows for precise geometrical and optical consistency. The OVD process produces a very consistent “matched-clad” fiber.
OVD fibers are made of a core and cladding glass, each with slightly different compositions. The manufacturing process provides the relationship between these two glasses. A matched-clad, single-mode fiber design allows for a consistent fiber (see Figure 6).
Figure 6. Index Profile of a Matched-Clad Fiber Design
The OVD process produces well-controlled fiber profiles and geometry, both of which lead to a more consistent fiber. Fiber-to-fiber consistency is especially important when fibers from different manufacturing periods are joined, through splicing and connectorization, to form an optical system.
Depressed-Clad Fiber Profile
The modified chemical vapor deposition (MCVD) process produces what is called depressed-clad fiber because of the shape of its refractive index profile.
Figure 7. Index Profile of a Depressed-Clad Fiber Design
Depressed-clad fibers are made with two different cladding glasses that form an inner and an outer cladding region. The outer cladding consists of a glass from a substrate tube that is generally purchased from an outside supplier, as opposed to the OVD method, where all of the glass is made synthetically within the fiber manufacturer’s control. The inner cladding region adjacent to the fiber core has an index of refraction that is lower than that of pure silica, while the outer cladding has an index equal to that of pure silica. Hence, the index of the glass adjacent to the core is depressed.
Questions of Strength
One common misconception about optical fiber is that it must be fragile because it is made of glass. In fact, research, theoretical analysis, and practical experience prove that the opposite is true. While traditional bulk glass is brittle, the ultrapure glass of optical fibers exhibits both high tensile strength and extreme durability.
How strong is fiber? Figures like 600 or 800 thousand pounds per square inch are often cited, far more than copper’s capability of 100 pounds per square inch. That figure refers to the ultimate tensile strength of fiber produced today. Fiber’s real, rather than theoretical, strength is 2 million pounds per square inch.
ABCs of Fiber Strength
The depth of inherent microscopic flaws on its surface determines the actual strength of optical fiber. These microscopic flaws exist in any fiber. As in a length of chain, the weakest link (or, in fiber’s case, the deepest flaw) determines the ultimate strength of the entire length of the chain. The flaws are distributed along the fiber length – the larger the flaw, the more distance between them along the fiber.
Many fiber manufacturers tensile-load, or proof-test, fibers after production. This process eliminates proof-test size flaws and larger, thereby ensuring that the flaws of most concern are removed and creating a minimum design strength for the fiber.
Life Expectancy
Fiber is designed and manufactured to provide a lifetime of service, provided it is cabled and installed according to recommended procedures. Life expectancy can be extrapolated from many tests. These test results, along with theoretical analysis, support the prediction of long service life. Environmental issues are also important to consider when evaluating a fiber’s mechanical and reliability performance.
Bending Parameters
Optical fiber and cable are easy to install because it is lightweight, small in size, and flexible. Nevertheless, precautions are needed to avoid tight bends, which may cause loss of light or premature fiber failure.
Experience and testing show that bare fiber can be safely looped with bend diameters as small as two to three inches, depending on allowable optical loss. Splice trays and other fiber-handling equipment, such as racks, are designed to prevent fiber-installation errors such as this.
As greater volumes of fiber in higher fiber-count cables are installed, system engineers are becoming increasingly conscious of the impact of splicing on their systems. Splice yields and system losses have a profound impact on the quality of system performance and the cost of installation.
Glass geometry, the physical dimensions of an optical fiber, has been shown to be a primary contributor to splice loss and splice yield in the field. Early on, one company recognized the benefit provided by tightly controlled fiber geometry and has steadily invested in continuous improvement in this area. The manufacturing process helps engineers reduce systems costs and support the industry’s low maximum splice-loss requirement, typically at around 0.1 dB.
Fiber that exhibits tightly controlled geometry tolerances will not only be easier and faster to splice but will also reduce the need for testing by ensuring predictable, high-quality splice performance. This is particularly true when fibers are spliced by passive, mechanical, or fusion techniques for both single fibers and fiber ribbons. In addition, tight geometry tolerances lead to the additional benefit of flexibility in equipment choice.
The benefits of tighter geometry tolerances can be significant. In today’s fiber-intensive architectures, it is estimated that splicing and testing can account for more than 30 percent of the total labor costs of system installation.
Fiber Geometry Parameters
The three fiber geometry parameters that have the greatest impact on splicing performance include the following:
- cladding diameter—the outside diameter of the cladding glass region.
- core/clad concentricity (or core-to-cladding offset)—how well the core is centered in the cladding glass region
- fiber curl—the amount of curvature over a fixed length of fiber
These parameters are determined and controlled during the fiber-manufacturing process. As fiber is cut and spliced according to system needs, it is important to be able to count on consistent geometry along the entire length of the fiber and between fibers and not to rely solely on measurements made.
Cladding Diameter
The cladding diameter tolerance controls the outer diameter of the fiber, with tighter tolerances ensuring that fibers are almost exactly the same size. During splicing, inconsistent cladding diameters can cause cores to misalign where the fibers join, leading to higher splice losses. The drawing process controls cladding diameter tolerance, and depending on the manufacturer’s skill level, can be very tightly controlled.
Core/Clad Concentricity
Tighter core/clad concentricity tolerances help ensure that the fiber core is centered in relation to the cladding. This reduces the chance of ending up with cores that do not match up precisely when two fibers are spliced together. A core that is precisely centered in the fiber yields lower-loss splices more often.
Core/clad concentricity is determined during the first stages of the manufacturing process, when the fiber design and resulting characteristics are created. During these laydown and consolidation processes, the dopant chemicals that make up the fiber must be deposited with precise control and symmetry to maintain consistent core/clad concentricity performance throughout the entire length of fiber.
Fiber Curl
Fiber curl is the inherent curvature along a specific length of optical fiber that is exhibited to some degree by all fibers. It is a result of thermal stresses that occur during the manufacturing process. Therefore, these factors must be rigorously monitored and controlled during fiber manufacture. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be misaligned during splicing, thereby impacting splice loss.
Some mass fusion splicers use fixed v-grooves for fiber alignment, where the effect of fiber curl is most noticeable.
Figure 8. Cladding Diameter, Core/Clad Concentricity, and Fiber Curl
Single-Mode Fiber Performance Characteristics
The key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter.Optical fiber performance parameters can vary significantly among fibers from different manufacturers in ways that can affect your system's performance. It is important to understand how to specify the fiber that best meets system requirements.
Attenuation
Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km).Optical fiber offers superior performance over other transmission media because it combines high bandwidth with low attenuation. This allows signals to be transmitted over longer distances while using fewer regenerators or amplifiers, thus reducing cost and improving signal reliability.
Attenuation of an optical signal varies as a function of wavelength (see Figure 9). Attenuation is very low, as compared to other transmission media (i.e., copper, coaxial cable, etc.), with a typical value of 0.35 dB/km at 1300 nm for standard single-mode fiber. Attenuation at 1550 nm is even lower, with a typical value of 0.25 dB/km. This gives an optical signal, transmitted through fiber, the ability to travel more than 100 km without regeneration or amplification.
Attenuation is caused by several different factors, but primarily scattering and absorption. The scattering of light from molecular level irregularities in the glass structure leads to the general shape of the attenuation curve (see Figure 9). Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. It is these water ions that cause the “water peak” region on the attenuation curve, typically around 1383 nm. The removal of water ions is of particular interest to fiber manufacturers as this “water peak” region has a broadening effect and contributes to attenuation loss for nearby wavelengths. Some manufacturers now offer low water peak single-mode fibers, which offer additional bandwidth and flexibility compared with standard single-mode fibers. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.
Figure 9. Typical Attenuation vs. Wavelength
Dispersion
Dispersion is the time distortion of an optical signal that results from the time of flight differences of different components of that signal, typically resulting in pulse broadening (see Figure 10). In digital transmission, dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO).
Figure 10. Impact of Dispersion
Dispersion vs. Wavelength
Single-mode fiber dispersion varies with wavelength and is controlled by fiber design (see Figure 11). The wavelength at which dispersion equals zero is called the zero-dispersion wavelength (λ º ). This is the wavelength at which fiber has its maximum information-carrying capacity. For standard single-mode fibers, this is in the region of 1310 nm. The units for dispersion are also shown in Figure 11.
Figure 11. Typical Dispersion vs. Wavelength Curve
Chromatic dispersion consists of two kinds of dispersion. Material dispersion refers to the pulse spreading caused by the specific composition of the glass. Waveguide dispersion results from the light traveling in both the core and the inner cladding glasses at the same time but at slightly different speeds. The two types can be balanced to produce a wavelength of zero dispersion anywhere within the 1310 nm to 1650 nm operating window.
Transmission in the 1550 nm Window
Optical fibers also can be manufactured to have low dispersion wavelength in the 1550-nm region, which is also the point where silica-based fibers have inherently minimal attenuation. These fibers are referred to as dispersion-shifted fibers and are used in long-distance applications with high bit rates. For applications utilizing multiple wavelengths, it is undesirable to have the zero dispersion point within the operating wavelength range and fibers known as non-zero dispersion-shifted fiber (NZ-DSF) are most applicable. NZ-DSF fibers with large effective areas are used to obtain greate capacity transmission over longer distance than would be possible with standard single-mode fibers. These fibers are able to take advantage of the optical amplifier technology available in the 1530 to 1600+ nm operating window while mitigating nonlinear effect, as that can be troublesome at higher power levels.For applications such as the interconnectino of headends, delivery of programming to remote node sites, high-speed communication networks, and regional and metropolitan rings (used primarily for competitive access applications), NZDSF fiber can improve system reliability, increase capacity, and lower system costs.
Mode-Field Diameter
Mode-field diameter (MFD) describes the size of the light-carrying portion of the fiber. For single-mode fibers, this region includes the fiber core as well as a small portion of the surrounding cladding glass. MFD is an important parameter for determining a fiber’s resistance to bend-induced loss and can affect splice loss as well. MFD, rather than core diameter, is the functional parameter that determines optical performance when a fiber is coupled to a light source, connectorized, spliced, or bent. It is a function of wavelength, core diameter, and the refractive-index difference between the core and the cladding. These last two are fiber design and manufacturing parameters.Cutoff Wavelength
Cutoff wavelength is the wavelength above which a single-mode fiber supports and propagates only one mode of light. An optical fiber that is single-moded at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength.The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment and the longer the fiber, the lower the effective cutoff wavelength. Or the smaller the bend radius of a loop of the fiber is, the lower the effective cutoff wavelength will be. If a fiber is bent in a loop, the cutoff is lowered. The cutoff wavelength of a fiber is reduced when it is cabled. The reduction is predictable, enough so that fiber manufacturers can specify a maximum “cable cutoff wavelength” for the fiber.
Environmental Performance
While cable design and construction play a key role in environmental performance, optimum system performance requires the user to specify fiber that will operate without undue loss from microbending.Microbends are small-scale perturbations along the fiber axis, the amplitude of which are on the order of microns. These distortions can cause light to leak out of a fiber. Microbending may be induced at very cold temperatures because the glass has a different coefficient of thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable become more rigid on the glass to cause microbends. Coating and cabling materials are selected by manufacturers to minimize loss due to microbending.
Specification Examples of Uncabled Fiber
To ensure that a cabled fiber provides the best performance for a specific application, it is important to work with an optical fiber cable supplier to specify the fiber parameters just reviewed as well as the geometric characteristics that provide the consistency necessary for acceptable splicing and connectorizing.Splicers and Connectors
As optical fiber moves closer to the customer, where cable lengths are shorter and cables have higher fiber counts, the need for joining fibers becomes greater. Splicing and connectorizing play a critical role both in the cost of installation and in system performance.The object of splicing and connectorizing is to precisely match the core of one optical fiber with that of another in order to produce a smooth junction through which light signals can continue without alteration or interruption. There are two ways that fibers are joined:
- splices, which form permanent connections between fibers in the system
- connectors, which provide remateable connections, typically at termination points
Fusion Splicing
Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by creating a homogenous joint between the two fiber ends. The fibers are melted or fused together by heating the fiber ends, typically using an electric arc. Fusion splices provide a high-quality joint with the lowest loss (in the range of 0.01 dB to 0.10 dB for single-mode fibers) and are practically nonreflective.Mechanical Splicing
Mechanical splicing is an alternative method of making a permanent connection between fibers. In the past, the disadvantages of mechanical splicing have been slightly higher losses, less-reliable performance, and a cost associated with each splice. However, advances in the technology have significantly improved performance. System operators typically use mechanical splicing for emergency restoration because it is fast, inexpensive, and easy. (Mechanical splice losses typically range from 0.05.0.2 dB for single-mode fiber.)Connectors
Connectors are used in applications where flexibility is required in routing an optical signal from lasers to receivers, wherever reconfiguration is necessary, and in terminating cables. These remateable connections simplify system reconfigurations to meet changing customer requirements.Acronyms Guide
CNR - carrier-to-noise ratio
CSO - composite second-order distortion
IVD - inside vapor deposition
MCVD - modified chemical vapor deposition
MFD - mode-field diameter
NZDSF - nonzero dispersion-shifted fiber
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