Optical Fiber – Optical Transmission Modes Advantages and Disadvantages of Optical Fiber

In case of coaxial cables and twisted cable the maximum signal frequency, and hence the information rate that can be transmitted using a solid conductor is limited. Optical fiber differs from both these transmission media in that it carries the transmitted information in the’ form of a fluctuating beam of light in a glass fiber rather than as an electrical signal on a wire. This type of transmission has become strong support for digital network owing to its high capacity and other factors favorable for digital communication.

Optical fiber consists of thin glass fibers of a very narrow strand of glass called the core and can carry information at frequencies in the visible light spectrum and beyond. Light pulses move easily down the fiber because of a principle known as total internal reflection. According to total internal reflection, when the angle of incidence exceeds a critical value, light cannot get out of the glass instead, the light bounces back in. This principle when applied to the construction of the fiber strand, it is possible to transmit information down fiber in the form of light pulses. The basic optical fiber has a buffer coating preventing it from any damage during the manufacturing process. It is then enclosed in a central PVC loose tube, which allows the fiber to flex and bend, particularly when going around comers or when being pulled through conduits. The PVC tube is a braided using yarn material, which absorbs most of the strain, put on the fiber during installation. Finally, a PVC outer jacket seals the cable and prevents moisture from entering. A core, which is surrounded by a concentric layer of glass called the cladding. The typical dimension of core and cladding diameter are 62.5 microns 125 microns (1 micron=10-6 meters) respectively; Cladding is protected using a protective coating consisting of plastic, it is called the Jacket.

The optical fiber cables may be available in various variants such as basic types, direct buried, armored, rodent resistant cable with steel outer jacket, colour-coded, multi-fiber cable etc depending upon the applications.

Fiber optic transmission systems are opto-electric in nature. In other words, a combination of optical and electrical electromagnetic energy is involved. The signal originates as an electrical signal, which is translated into an optical signal which subsequently is reconverted into an electrical signal at the receiving end. Thin glass fiber is very clear and designed to reflect light internally for efficient transmission carries light with encoded data. Plastic jacket allows fiber to bend (some!) without breaking. Light Emitting Diode (LED) or laser injects light into fiber for transmission. Light sensitive receiver at the other end translates light back into data.

The optical fiber consists of a number of substructures. ID this case, a core made of glass, which carries most of the light, is surrounded by a cladding made of glass with lower refractive index. This bends the light and confines it to, the core. The core is surrounded by a substrate layer (in some fibers) of glass, which does not carry light, but adds to the diameter and strength of the fiber. A primary buffer coating and a secondary buffer coating to provide mechanical protection cover all these.             The light pulse travels down the center core of the glass fiber. Surrounding the inner core is a layer of glass cladding, with a slightly different-refractive index. The cladding serves to reflect the light waves back into the inner core. Surrounding the cladding is a layer of protective plastic coating that seals the cable and provides mechanical protection. Typically, multiple fibers are housed in a single sheath, which may be heavily armored.

There are two basic types of fibers used today and many different types of Fiber Optic Cable. These are Single Mode (SM) and Multi-Mode (MM). Single mode is more expensive but more efficient than multi-mode. Single mode fiber is generally used where the distances to be covered are greater. These come in a variety of configurations determined by a variety of factors and light propagates along the optical fiber core in one of the following ways as given below depending on the type and width of core material used.

Optical Transmission Modes

There are following types of transmission modes’ used in optical fiber. They are:

1. a)Step Index
2. b)Graded Index
3. c)Single Mode.
   

Multi-mode Fiber

In the case of a multi-mode fiber, the core diameter is relatively large compared to a wavelength of light. Core diameter ranges from 50 micrometers (pm) to 1,000 pm, compared to the wavelength of light of about 1 pm. It means that light can propagate through the fiber in many different ray paths, or modes, hence the name multimode.

Multi-mode fiber is less expensive to produce and inferior in performance because of the larger diameter of the inner core. When the light rays travel down the fiber, they spread out due to a phenomenon known as modal dispersion. Although reflected back into the inner core by the cladding, they travel different distances and, therefore, arrive at different times. The received signal thus has a wider pulse width than the input signal with a corresponding decrease in the speed of transmission. As a result, multimode fiber is relegated to applications involving relatively short distances and lower speeds of transmission, for example, LANs and campus environments.

Two basic types of multi-mode fibers exist. The simpler and older type is a “step index” fiber, where the index of refraction (the ability of a material to bend light) is the same all across the core of the fiber and the second one is graded index fiber with varying index of refraction across the core.

Step Index Multi-mode Fiber

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This causes some rays to take a longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.

With all these different ray paths or modes of propagation different rays travel different distances, and take different amounts of time to transit the length of a fiber. This being the case, if a short pulse of light is injected into a fiber, the various rays emanating from that pulse will arrive at the other end of the fiber at different times, and the output pulse will be of longer duration than the input pulse. This phenomenon is called modal dispersion (pulse spreading), and limits the number of pulses per second that can be transmitted down a fiber and still be recognizable as separate pulses at the other end. This, therefore, limits the bit rate or bandwidth of a multi-mode fiber. For step index fibers, where in no effort is made to compensate for modal dispersion, the bandwidth is typically 20 to 30 MHz over a length of one kilometer of fiber, expressed as “MHz =km”,

Graded Index Multi-mode Fiber

In the case of a graded index multi-mode fiber, the index of refraction across the core is gradually changed from a maximum at the center to a minimum near the edges, hence the name graded index. This design takes advantage of the phenomenon that light travels faster in a low-index-~f-refraction material than a high-index material. If a short pulse of light is launched into the graded, index fiber, it may spread some during its transit of the fiber, but much less than in the case of a step index fiber. Therefore, dispersion can be reduced using a core material that has a variable refractive index. In such multi-mode graded index fiber light is refracted by an increasing amount as it moves away from the core. This has the effect of narrowing the pulse width of the received signal compared with stepped index fiber, allowing a corresponding increase in the speed of transmission. These, therefore, can support a much higher bit rate or bandwidth. Typical bandwidths of graded index fibers range from 100 MHz-km to well over 1 GHz-km. The actual bandwidth depends on how well a particular fiber’s index profile minimizes modal dispersion, and on the wavelength of light launched into the fiber.

Monomode/ Single-Mode Fiber

This has a thinner inner core. In this case, the core diameter of about 9 pm is much closer in size to the wavelength of light being propagated, about 1.3 pm. This limits the light transmission to a single ray or mode of light to propagate down the core of the fiber .All the multiple-mode or multi-mode effects described above are eliminated. However; one pulse-spreading mechanism remains. Just as in the multi-mode fibers, different wavelengths of light travel at different speeds, causing short pulses of light injected into the fiber to spread as they travel. This phenomenon is called “chromatic dispersion”.

It performs better than does multi-mode fiber over longer distances at higher transmission rates. Due to reduced core diameter all the emitted light propagates along a single path. Consequently, the received signal is of a comparable width to the input signal. Although more costly, mono mode fiber is used to advantage in long haul and especially in high bandwidth applications. Single-mode fibers have the very broadest bandwidth, lowest cost and lowest attenuation of any available optical fiber. Therefore, they are universally used in long-distance telephony and cable television applications.

Based upon the above discussion, a fiber may have LED (Light Emitting Diode) as light source of 850 nanometers wavelength, 3:5 dB/km attenuation (loses 3.5 dB of signal per kilometer), typically 62.5/125 (core diameter/cladding diameter) for indoor application. It is known that a multi-mode fiber can run many light sources and monomode (single mode) has a single light source. In case of outdoor applications a fiber cable may have Laser as light source of 1170 nanometers wavelength, 1 dB/ km attenuation (loses I dB of signal per kilometer).

Advantages of Optical Fiber

1. Noise resistance: It is immune to electromagnetic interference and crosstalk and external light, the only possible interference, is blocked from the channel by the outer jacket.

2. Less signal Attenuation: It has transmission distance significantly greater than that of other guided media.

3. Higher bandwidth: Currently, data rates and bandwidth utilization over fiber optic cable are limited not by the medium but by the signal generation and reception technology even though it offers a large bandwidth compared to other media. Larger bandwidth offers larger capacity and faster transmission rate.

4. High security: Using fiber optic cables prevents the emanation of radiation and therefore, radiation-containing signal becomes difficult to tap. This makes fiber cable secure against signal leakage and interference.

5. Free from electrical problems: It does not require electrical ground loop preventing it from short circuit as light waves are being used the carrier of data signal. It is also safe in combustible areas (no arching) and offers immunity to lightning and electrical discharges.

6. Less number of repeaters: A repeater used to strengthen a signal is always required during the Course of signal transmission. Compared to copper media, it requires less number of repeaters.

7. Physical structure: It has small size, lightweight, flexibility, high strength, potential high temperature operation and no electrical hazard when cut or damaged.

Disadvantages of Optical Fiber

1. Cost-The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive. As this transmission medium becomes more popular and in demand, economies of scale will decrease the cost of installation and profits will increase.

3. Installation/Maintenance-It is difficult to splice. Special equipment and expertise are required to splice and instal the cables.
4. Fragility-It has limited physical arc of cable, if it is bent too much it will break. Physical vibration will show up as signal noise.