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Fiber Optics 101 The Basics

Source: David R. Goff. Fiber Optic Video Transmission, 1st ed. Focal Press: Woburn, Massachusetts, 2003

and other private writings.

Optical Fiber


Optical fibers are extremely thin strands of ultra-pure glass designed to transmit light from a transmitter to a receiver. These light signals represent electrical signals that include video, audio, or data information in any combination. Figure 1 shows the general cross-section of an optical fiber. The fiber consists of three main regions. The center of the fiber is the core. This region actually carries the light. It ranges in diameter from 9 microns (μm) to 100 microns in the most commonly used fibers. Surrounding the core is a region called the cladding. This part of the fiber confines the light in the core. The cladding typically has a diameter of 125 microns or 140 microns. A key design feature of all optical fibers is that the refractive index of the core is higher than the refractive index of the cladding. Both the core and cladding are usually doped glass materials. Other fiber types incorporate quartz or pure fused silica and plastic, but these are not used in mainstream high-performance applications. The outer region of the optical fiber is called the coating or buffer. The buffer, typically a plastic material, provides protection and preserves the strength of the glass fiber. Typical diameters for the buffer are 250 microns, 500 microns, and 900 microns.

Cross-section of an Optical Fiber

Figure 1 - Cross-Section of a Typical Optical Fiber


The development of glass-coated glass fibers was motivated by the optical loss experienced when using uncoated glass fibers. The inner fiber, or core, was used to transmit the light, while the glass coating, or cladding, prevented the light from leaking out of the core by reflecting the light within the boundaries of the core. The cladding is designed so that it causes the light to be angled back into the core as illustrated in Figure 2 and thus carried along the length of the fiber.

Optical Fiber with Cladding

Figure 2 - Light Guided Along a Bent Optical Fiber by Total Internal Reflection

Why is fiber important? Because it provides a private pipeline that can carry huge amounts of data. Alternatives are over-the-air broadcast or hard-wired copper wires carrying electrons. Figure 3 demonstrates the three basic schemes that can be used for transmitting (or sending) information from one point to another.

The first scheme is metallic transmission which uses a copper wire or coaxial cable to carry a modulated electrical signal containing information. This is the method used to carry signals to a “wired” telephone. This method allows for a limitless number of private channels (assuming you have that much copper cable), but each channel has limited information and distance capability due to the inherent characteristics of copper cable. The second scheme for moving information between two locations is free-space transmission. This is how cell phone, radio signals and over-the-air TV signals are received. Free-space transmission has the advantage of providing very large bandwidth capability as well as long distance capability, but it does not provide a private channel. Also, the free-space spectrum is a finite, limited, and costly commodity. Free space cannot provide the millions of high-speed communication channels required by tomorrow’s information age. The last scheme is waveguide transmission. This describes optical fiber transmission. A waveguide (optical fiber) confines the electromagnetic radiation (light) and moves it along a prescribed path. Optical fiber offers the best of both metallic and free-space transmission. It has the key advantage of metallic transmission, the ability to carry a signal from point A to point B without cluttering the limited free-space electromagnetic spectrum; however, fiber does not have the disadvantage of metallic transmission: very limited bandwidth and data rate.

Transmission Schemes

Figure 3 - Three Basic Schemes for Sending Information from One Point to Another


In the growing world of improved communication, fiber optics offers a method of transmitting information (or data) that allows for clearer, faster, more efficient communications than copper. A fiber optic transmission system (FOTS) holds many advantages over a copper wire system. For example, while a simple two-strand wire can carry a low-speed signal over a long distance, it cannot send high-speed signals very far. Coaxial cables can better handle high-speed signals but still only over a relatively short distance.

Fiber optics uses light rather than electrical signals to carry information. Light seen by humans has a range of wavelengths from 0.4 μm or 400 nm (deep blue) to 0.7 μm or 700 nm (deep red). The optical wavelengths used in fiber optics communications are usually in the infrared region (greater than 0.7 μm or 700 nm wavelength.) Fiber optics wavelengths for most communication applications range from 0.85 μm or 850 nm to 1.625 μm or 1625 nm. The wavelength region near 1.55 μm or 1550 nm is the most popular for high-performance systems because the optical attenuation is lowest in that region. Figure 4 shows the near -infrared region used by fiber optics. The shaded regions listed at the “Third Window” and “Fourth Window” are the most useful for modern FOTS.

Wavelengths Used for Fiber Optic Communications

Figure 4 - Near-Infrared Wavelengths Used for Fiber Optic Communications


In the late 1990's, the telecommunications industry (e.g. the phone company) faced tremendous challenges due to the ever growing amounts of data being generated by the rapidly expanding Internet. In the late 90's, the total load generated by Internet traffic matched and then exceeded the load generated by voice calls. Today the load from Internet traffic is five to ten times as large as the load generated by voice calls. The dominance of the load generated by Internet over voice is shown in Figure 5.

Growth of Voice and Data Communications

Figure 5 - Voice & Data Growth

Fiber optics offered the only practical means of meeting this soaring demand. The telecommunications industry already had a great deal of fiber in the ground, but most of it was already being used and the cost and time to lay additional fiber was prohibitive in many cases. The telecommunications industry needed a new solution that would allow them to economically keep up with demand. The companies that could provide these new solutions could be assured of a very large amount of business providing upgraded systems.

The capacity of commercial optical fiber communications systems has increased about as fast as Moore’s Law for integrated circuits, doubling every two years, as shown in Fig. 6. The recent rate of increase for experimental systems has been even faster.

Transmission Capacity of Optical Fiber Over Time

Figure 6 - Capacity of Optical Fiber Over Time

Three problems needed to be addressed by the next generation of fiber optic transport systems (FOTS) for the telecommunications industry. First, the data rate had to be increased. In the mid to late 1990's, the fastest data rate being used commercially was 2.5 Gb/s, also referred to as OC-48. This needed to be increased to handle the increasing need. The usual speed increase for SONET was a factor of four, so the next logical data rate was 10 Gb/s or OC-192. Second, it was highly desirable to put more than one channel of information onto a single fiber. This would reduce or eliminate the need to deploy more fiber. Third, the maximum distance that a FOTS could serve needed to increase dramatically. This would reduce the overall cost of the data transmission link by eliminating or reducing the need for costly signal regeneration facilities along the length of the transmission path.

The gap between the unfilled growing needs of the telecommunications industry and the growing demands for more bandwidth created a huge opportunity for fiber optics to make a breakthrough in performance and cost. Several pieces of new fiber optic technology including DWDM (Dense Wavelength Division Multiplexing), EDFA’s (Erbium-Doped Fiber Amplifiers), Solitons, Dispersion Compensators, FEC coding and many more were rushed out of the hands of researchers and rapidly converted into next-generation FOTS that provided more than 100 times the capacity on one fiber compared to existing hardware and at the same time substantially increased the maximum allowable transmission distances.

In many ways, the telecommunications industry was adopting much of the technology that had been deployed in submarine fiber optic systems for years. Submarine fiber optic systems, which must run for thousands of miles underwater, have always been on the leading edge in terms of the largest possible capacity and the longest possible transmission distance on a single fiber as well as the highest possible reliability. Until the Internet created a need to dramatically increase the capacity of terrestrial systems, most of the more exotic technologies associated with leading edge submarine fiber optic systems had been largely ignored by the terrestrial telecommunications industry. In the late 1990's, many large telecommunications equipment suppliers recognized the huge financial opportunity that was looming for greatly expanded capacity and distance on the fibers that they already had deployed. However, most of the new systems that were being developed tended to be evolutionary, rather than revolutionary. The new systems that many of the telecommunication equipment suppliers conceived would increase the capacity by a factor of eight or perhaps sixteen and may double the maximum distance to a few hundred kilometers.

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