FIBER OPTICS Fiber optics is a unique transmission medium. It has some unique advantages over conventional communication media, such as copper wire, microwave or coaxial cables. The major advantage is its high transmission capacity, i.e. optical fibers can carry information at high data rates over very long distance. Since fibers are made of a dielectric material, they are immune to radiated and conducted interference. It is nearly impossible to tap an optical fiber, therefore fiber-optic transmission is very secure. Optical fiber is small and light weight, which is an evident issue whenever weight and bulk are a practical concern, e.g. in aircraft, ships and automobiles. Advantages of Fiber Optics High Transmission Capacity Immunity Security Low weight The Electromagnetic Spectrum Electromagnetic radiation can be measured in several ways, as the length of a wave, as the oscillation frequency of an electromagnetic field, or as the energy of a photon. When referring to electromagnetic radiation above the microwave region, by convention, wavelength (rather than frequency) is used. The figure shows the entire electromagnetic spectrum from sonic frequencies to cosmic rays. The term "light" is commonly used to refer to visible light which occupies a tiny portion of the spectrum from 391 to 770 nm. However, because of the transmission properties of optical fibers, lightwave systems use radiation with wavelengths belong to infrared (IR) portion of the electromagnetic spectrum but the term lightwave is commonly used when referring to them. Understanding lightwave technology can be made very easy when comparing lightwave to microwave and just realize that frequencies are 4 to 5 decades higher than microwave frequencies and wavelengths fall into the micrometer region rather than are millimeters. Therefore, in many cases we have to deal with dimensions that are close to length of infrared lightwaves. The light used as carrier in optical transmission systems is an electromagnetic wave with a wavelength around 1 urn and an oscillation frequency of about 300 THz.
Common Fiber Optic Wavelengths 660 nm Best Transmission Characteristics of Plastic 820/850 nm First generation of semicon component to emit infrared light 1300 / 1310 nm and 1550 nm Best transmission characteristic of glass The typical fiber-optic wavelengths are 850 nm, 1300 nm and 1500 nm, all being located in the near infrared range of the electromagnetic spectrum. These 3 wavelengths result from the attenuation characteristics of glass as well as from the availability of semi-conductor type sources and receivers. They are referred to as the three wavelength windows in fiber optics. 660 nm is the preferred wavelength for light transmission in plastic fibers, which are used in low cost links, where distances are small (<100 m) and bandwidth requirements are modest. (The Hewlett-Packard Components Group offers versatile links based on 660 nm technology and 1 nm diameter plastic fiber cable.). Modulation AM (Intensity Modulation) FM (Coherent) The majority of today's fiber-optic transmission systems use intensity modulation. Intensity modulation is very similar to amplitude modulation (AM) in that the optical power is varied in accordance with a modulating signal. For digital systems, intensity modulation means the lightwave carrier is switched on and off or high and low as controlled by a digital data streams. For analog systems, intensity modulation means that the intensity level of the carrier is controlled by an analog signal. Digital modulation, however, is the standard technology in today's lightwave applications. Rather than modulating the intensity (amplitude) of the light, one can also modulate the light's frequency, similar to FM modulation in conventional electronic communications. Indeed this transmission scheme, called coherent transmission, is in development and it will be the biggest challenge in lightwave technology commercially available for practical use. A Fiber Optic Communication Link The figure shows the typical layout of a fiber-optic transmission system. The electrical input signal is converted into a lightwave signal. The light is launched into an optical fiber, which guides the light into the receiver, where it is reconverted into an electrical output signal.
In the following overview we will have a quick look at the components involved in such a system. Light Sources LED (Light Emitting Diode) - Scatterre, Incoherent light - Easy to use - May be modulated at hundreds of Megahertz Laser Diodes - Narrow beam, coherent light - More powerful than LED, more Expensive but shorter life - Needs controlled environment - Can be modulated at highest frequency up to 10 GHz The light sources in today's fiber-optic communications systems are semiconductor LED's (light emitting diodes) and laser diodes. These devices exhibit remarkable differences in those characteristics that influence the transmission capacity. LEDs produce scattered, incoherent light like that of the sun or an electric light bulb. The lightwaves of incoherent light are of varied frequency, amplitude and phase. Lasers are more powerful. They emit a straight and narrow beam of coherent light - meaning the waves are of the same frequency, phase and amplitude. Another advantage is that lasers can be modulated at much higher rates than LEDs. Lasers, however, have their drawbacks. Their sensitivity to temperature changes requires a controlled environment. Lasers can be irritated by backreflected light and they are more expensive and have a shorter lifetime than LEDs. The Medium Fibers are made of a core and a cladding. The core has a slightly higher index of refraction. Light is guided by total reflection at the boundary between core and cladding
The medium used in fiber-optic transmission is glass or plastic. The optical behavior of any transparent material is described by its index of refraction which is the ratio of the speed of light in vacuum and in the material. It is always equal to or greater than 1. Light is refracted (bent) or totally reflected at the boundary between two media with different index of refraction. Whether light is reflected or only bend depends on the angle at which light strikes the boundary. The Receiver PIN Diodes - Least Expensive - Simple circuitry - Very linear APD (Avalanche Photo Diode) - Highest sensitivity - Needs controlled environment - Needs high bias voltage The receivers most commonly used in today's fiber-optic communications are PIN Diodes and APDs (Avalanche Photo Diodes). Only some low-cost, low speed systems use phototransistors. PIN diodes are widely used because of their high speed and good sensitivity. Avalanche photodiodes get high sensitivity from internal multiplication of light-generated electrons. They are suitable for long distance transmission. To make the internal amplification (avalanche effect) happens, APDs require a very high bias voltage. Similar to lasers, APDs are also sensitive to temperature changes, therefore APDs need more complex control circuitries. Fiber Attenuation Scattering is the main cause for fiber attenuation Glass fibers have minimum attenuation at 1550nm.
Attenuation of lightwaves in fibers occurs because of scattering, absorption and bending phenomena. Scattering, the dominant effect, is caused by microscopic non-uniformity of the refractive index of glass. The scattering phenomenon in optical fiber is called Rayleigh scattering after its discoverer. Longer wavelengths are affected less by scattering which explains the general shape (decreasing) of the shown attenuation curve. Absorption is caused by impurities in the fiber. Water molecules cause the absorption peak at approximately 1400 nm. Beyond 1600 nm, the silica (present in the quartz material of the fiber) starts to absorb light causing the attenuation to increase again. Microscopic imperfections (microbending) in the geometry of the fiber, for instance, changes of the core diameter due to stress or bubbles, may occur during the manufacturing process and are the third reason for fiber attenuation. Step Index Multimode Fiber Common SI - Multimode Fibers Fiber Type Core Cladding NA Attenuation. 1mm Plastic 0.98 mm 1.00 mm 0.5 0.3 db/m @ 660 nm 100/140 Silica Glass 100 um 140um 0.2-0.3 5db/km @ 850 nm There are various types of optical fibers, each with different physical characteristics and different methods of guiding light. Step index multimode fibers are the simplest fibers and were the first to find practical use. When light travels through the fiber, it is reflected at the core-cladding boundary, thus being propagated along the core. Looking at the index profile you can see how the index of refraction changes from core to cladding as a step function. The term "multimode" refers to the existence of more than one possible path (mode) of lightwave propagation. Two common step index multimode fibers and their typical characterization as listed in the table. Note that the attenuation of plastic fibers is so high that it is specified in dbs per meter rather than dbs per kilometer. Core and cladding dimensions are essential physical parameters of a fiber, therefore the type description of glass fibers usually refers to core and cladding diameters, e.g. 100 / 140 SI MM stands for a step index multimode fiber with 100 um core diameter and 140 um cladding diameter. Graded Index Multimode Fiber
Common GI-Multimode Fibers Fiber Type NA Attenuation Modal Dispersion BW X Distance. 62.5/125 Glass 0.27 0.7 db/m @ 1300 nm 0.4-0.9 ns / km 500-1200 MHz x km 50/125 Glass 0.21 0.5 db/km @ 1300 nm 0.3-0.9 ns / km 500-1500 MHz x km A second type of multimode fiber is the graded index (GI) fiber. It has been designed to minimize the modal dispersion effect and therefore increase the transmission bandwidth. In graded index fibers, the index of refraction gradually changes from the center of the core to the outer perimeter. Therefore, the light rays are gradually bent back toward the core instead of being reflected. As speed of light is a function of the refractive index, i.e. it is faster the lower the refractive index, the modes taking the longer way through the other regions of the core are faster and catch up with the modes travelling right in the core. Such fibers have slightly less core diameter and NA as compared to step index multimode fibers. The 50/125 GI multimode fiber is the current standard multimode fiber in short distance, medium data rate systems. The 62.5/125 fiber is gaining importance, as it is being considered to become the standard fiber for FDDI systems. Single-mode Fiber Fiber Type NA Attenuation Multi-mode Dispersion 9/125 Glass 0.1 0.35 db/km @ 1300 nm - 0.25 db/km @ 1550 nm - Single-mode fibers have a step index design, but the core has such a small diameter (10 um) that only one mode can propagate. This eliminates the multimode dispersion problem. The small diameter core, however, makes single-mode fibers difficult to handle, i.e. coupling light from sources or joining fibers. Nevertheless, single-mode fibers have become the de facto standard for telecommunications. They are widely used even for applications where their high transmission bandwidth is not so important because today single-mode fibers have become cheaper than graded index fibers, due to the high demand.
To really understand the light guiding mechanism of single-mode fibers one would need to go into waveguide theory, which is beyond the scope of this tutorial. Cables Major Types of Environments: Underground Intrabuilding Submarine Aerial outdoor connection Instrumentation Different environment require different cable design Fibers must be packaged in cables to protect them from mechanical damage and environmental degradation. Each fiber manufacturer has his own fiber-optic cable design, and cables are designed for particular environments. Thus, the choice of a cable design depends on the environment where it is to be installed. Some cables carry only a single fiber, other cable constructions may combine more than 100 fibers in one cable. With all the protection material, it is obvious that the final cable diameter must exceed the small diameter of a single fiber by order of magnitude. Fiber optic cables are available in typical lengths of 2 km to 4km. For longer routes, it becomes necessary to join many cables. Fiber joining techniques will be discussed in the next chapter. Basic Considerations Connectors Non Permanent Indoor usage Simple to use Easy to reconfigure Join to fiber cable to terminal device Splices Permanent Outdoor Usage Easier to seal hermetically Less Expensive Join Fiber cables in trunk lines Basically, there are two techniques to join fibers, either using connectors or splices. receiver. Connectors are used to make temporary connections between two fiber ends or between a fiber end and a transmitter or Splices permanently weld, glue or otherwise bond together the ends of two fibers. As a simple rule of thumb, think of splices as being used in long-distance, high-capacity fiber-optic systems, while connectors are used on shorter-distance, lower capacity systems. Alternatively, splices are used to join segments of cable that run over long distances, while connectors join short segments of cable and terminal devices. Another viewpoint is to think of splices as being used outdoors and connectors, indoors.
Loss Offset and separation are the main contributors to loss in fiber connections. You can imagine what precision is needed to join two hair thin fibers without losing too much light. If the fiber cores are not exactly aligned, light will escape, causing signal attenuation. For example, a 10% offset of the two fiber cores causes a loss of 0.6 db (13%). For a single-mode fiber with a 10 um core, just such a loss occurs if the fibers are offset by only 1 um! Any variation in the internal fiber geometry (elliptical or off-center core) can also cause a mismatch even if the fiber seems to be perfectly aligned viewing it from the outside. Normal manufacturing tolerances of the fiber geometry can cause up to 0.5 db loss. Not only geometrical mismatch, but also index mismatch can cause loss in an optical connection. If the two fiber ends are separated by an air gap, then light is reflected at the two glass surfaces because of the mismatch of the refractive index of glass (n = 1.5) and air (n = 1.0). Light, what is reflected can't be transmitted, therefore reflections cause an additional loss Fiber End Face Preparation A good perpendicular cleave is essential for a good fiber connection
One of the first steps that must be followed before fibers are connected or spliced to each other is to properly prepare the fiber end faces. In order not to have light deflected or scattered at the joint, the fiber ends must be flat, perpendicular to the fiber axis, and smooth. Precise mechanical strippers are used to remove plastic coatings from the fiber. Extreme care must be taken to avoid nicking the fiber. After the fiber end has been carefully cleaned, it can be cleaved. This is done by scribing the fiber with a diamond knife and then bending it over a curved form or applying tension to the end. The fiber will break perpendicular at the point where the scratch was made with the diamond knife. Cleaving is a delicate process and needs some skill. A good fiber preparation is essential for a good fiber connection, especially when fibers shall be spliced. Splices Lowest loss ( <0.1 db) Strongest fiber connection No reflections Less expensive equipment Easier to perform Some removable, and re-usable Two different splicing techniques have arisen during the evolution of optical fiber technology. The most common type of splice is the fusion splice, formed by welding the ends of two optical fibers together. It is performed with a specialized instrument called a fusion splicer, which includes mounting stages and precision micrometers to handle the fiber. A fused fiber junction has a mechanical strength similar to that of an unspliced fiber. Fusion splicing is intended only for all-glass fibers with their plastic coatings removed; it is not intended for plastic fibers. Fusion splices cause no reflections. Mechanical splices join two fiber ends by either clamping them within a structure or gluing them together. Losses of mechanical splices tend to be higher than those of fusion splices, but they are easier to perform and do not require expensive equipment. Some types of mechanical splices can even be removed.