MODERN TRENDS IN OPTICAL COMMUNICATION (FIBER OPTIC COMMUNICATION) CONTENTS ? Introduction ? Advantages of Fiber Optics ? Optical Transmitters ? The Optical Fiber ? Launching the Light ? Losses in Optical Fiber ? Optical Fiber Bandwidth ? Fiber Optic Cable Construction ? Other Types of Fibers ? Optical Receivers Introduction Our current “age of technology” is the result of many brilliant inventions and discoveries, but it is our ability to transmit information, and the media we use to do it, that is perhaps most responsible for its evolution .
Progressing from the copper wire of a century ago to today’s fiber optic cable, our increasing ability to transmit more information, more quickly and over longer distances has expanded the boundaries of our technological development in all areas. Today’s low-loss glass fiber optic cable offers almost unlimited bandwidth and unique advantages over all previously developed transmission media. The basic point-to-point fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver.
The Optical Transmitter: The transmitter converts an electrical analog or digital signal into a corresponding optical signal. The source of the optical signal can be either a light emitting diode, or a solid state laser diode. The most popular wavelengths of operation for optical transmitters are 850, 1310, or 1550 nanometers. Most Fiberlink® transmission equipment manufactured by Communications Specialties operates at wavelengths of 850 or 1310nm. [pic] The Optical Fiber | |Launching the Light | |the transmitter has converted the electrical input signal into whatever form of modulated light is desired, the light must be “launched” into the optical fiber. | |As previously mentioned, there are two methods whereby light is coupled into a fiber. One is by pigtailing. The other is by placing the fiber’s tip in very close proximity to | |an LED or LD.
When the proximity type of coupling is employed, the amount of light that will enter the fiber is a function of one of four factors: the intensity of the LED or | |LD, the area of the light emitting surface, the acceptance angle of the fiber, and the losses due to reflections and scattering. Following is a short discussion on each: | |Intensity: The intensity of an LED or LD is a function of its design and is usually specified in terms of total power output at a particular drive current. Sometimes, this | |figure is given as actual power that is delivered into a particular type of fiber.
All other factors being equal, more power provided by an LED or LD translates to more power | |”launched” into the fiber. | |Area: The amount of light “launched” into a fiber is a function of the area of the light emitting surface compared to the area of the light accepting core of the fiber. The | |smaller this ratio is, the more light that is “launched” into the fiber. | |Acceptance Angle: The acceptance angle of a fiber is expressed in terms of numeric aperture. The numerical aperture (NA) is defined as the sine of one half of the acceptance | |angle of the fiber.
Typical NA values are 0. 1 to 0. 4 which correspond to acceptance angles of 11 degrees to 46 degrees. Optical fibers will only transmit light that enters at | |an angle that is equal to or less than the acceptance angle for the particular fiber. | |Other Losses: Other than opaque obstructions on the surface of a fiber, there is always a loss due to reflection from the entrance and exit surface of any fiber. This loss is | |called the Fresnell Loss and is equal to about 4% for each transition between air and glass.
There are special coupling gels that can be applied between glass surfaces to | |reduce this loss when necessary. | |The Optical Fiber-Losses in Optical Fiber | |Other than the losses exhibited when coupling LEDs or LDs into a fiber, there are losses that occur as the light travels through the actual fiber. | |The core of an optical fiber is made of ultra-pure low-loss glass. Considering that light has to pass through thousands of feet or more of fiber core, the purity of the glass | |must be extremely high.
To appreciate the purity of this glass, consider the glass in common windowpanes. We think of windowpanes as “clear,” allowing light to pass freely | |through, but this is because they are only 1/16 to ? inch thick. In contrast to this clear appearance, the edges of a broken windowpane look green and almost opaque. In this | |case, the light is passing edgewise into the glass, through several inches. Just imagine how little light would be able to pass through a thousand feet of window glass! |Most general purpose optical fiber exhibits losses of 4 to 6 dB per km (a 60% to 75% loss per km) at a wavelength of 850nm. When the wavelength is changed to 1300nm, the loss | |drops to about 3 to 4 dB (50% to 60%) per km. At 1550nm, it is even lower. Premium fibers are available with loss figures of 3 dB (50%) per km at 850nm and 1 dB (20%) per km at| |1300nm. Losses of 0. 5 dB (10%) per km at 1550 nm are not uncommon. These losses are primarily the result of random scattering of light and absorption by actual impurities | |within the glass. |Another source of loss within the fiber is due to excessive bending, which causes some of the light to leave the core area of the fiber. The smaller the bend radius, the | |greater the loss. Because of this, bends along a fiber optic cable should have a turning radius of at least an inch. | |The Optical Fiber: Optical Fiber Bandwidth | |All of the above attenuation factors result in simple attenuation that is independent of bandwidth. In other words, a 3 dB loss means that 50% of the light will be lost whether| |it is being modulated at 10 Hz or 100 MHz. |There is an actual bandwidth limitation of optical fiber however, and this is measured in MHz per km. The easiest way to understand why this loss occurs is to refer to Figure 6| | | |[pic] | | In short, the less modes, the higher the bandwidth of the fiber. The way that the number of modes is reduced is by making the core of the fiber as small as possible. |Single-mode fiber, with a core measuring only 8 to 10 microns in diameter, has a much higher bandwidth because it allows only a few modes of light to propagate along its core. | |Fibers with a wider core diameter, such as 50 and 62. 5 microns, allow many more modes to propagate and are therefore referred to as “multimode” fibers. | |Typical bandwidths for common fibers range from a few MHz per km for very large core fibers, to hundreds of MHz per km for standard multimode fiber, to thousands of MHz per km | |for single-mode fibers.
And as the length of fiber increases, its bandwidth will decrease proportionally. For example, a fiber cable that can support 500 MHz bandwidth at a | |distance of one kilometer will only be able to support 250 MHz at 2 kilometers and 100 MHz at 5 kilometers. | |Because single-mode fiber has such a high inherent bandwidth, the “bandwidth reduction as a function of length” factor is not a real issue of concern when using this type of | |fiber. However, it is a consideration hen using multimode fiber, as its maximum bandwidth often falls within the range of the signals most often used in point-to-point | |transmission systems. | |The Optical Fiber: Fiber Optics Cable Construction | |[pic] |Fiber optic cable comes in all sizes and shapes. Like coaxial | | |cable, its actual construction is a function of its intended | | |application.
It also has a similar “feel” and appearance. | | |Figure 7 is a sketch of a typical fiber optic cable. | | |The basic optical fiber is provided with a buffer coating which| | |is mainly used for protection during the manufacturing process. | |This fiber is then enclosed in a central PVC loose tube which | | |allows the fiber to flex and bend, particularly when going | | |around corners or when being pulled through conduits.
Around | | |the loose tube is a braided Kevlar yarn strength member 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. |Basic optical fiber is ideal for most inter-building applications where extreme ruggedness is not required. In addition to the “basic” variety, it is also available for just | |about any application, including direct buried, armored, rodent resistant cable with steel outer jacket, and UL approved plenum grade cable. Color-coded, multi-fiber cable is | |also available. | |The Optical Fiber: Other Types of Fibers | |Two additional types of fiber – very large core diameter silica fiber and fiber made completely of plastic – are normally not employed for data transmission. |Silica fiber it typically used in applications involving high power lasers and sensors, such as medical laser-surgery. | |All-plastic fiber is useful for very short data links within equipment because it may be used with relatively inexpensive LEDs. An isolation system for use as part of a high | |voltage power supply would be a typical example of an application for plastic fiber. | |The Optical Fiber: Optical Connectors | |Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers.
These connectors are similar to their electrical | |counterparts in function and outward appearance but are actually high precision devices. In operation, the connector centers the small fiber so that its light gathering core | |lies directly over and in line with the light source (or other fiber) to tolerances of a few ten thousandths of an inch. Since the core size of common 50 micron fiber is only | |0. 002 inches, the need for such extreme tolerances is obvious. | |There are many different types of optical connectors in use today.
The SMA connector, which was first developed before the invention of single-mode fiber, was the most popular | |type of connector until recently. | |[pic] | |The most popular type of multimode connector in use today is the ST connector. Initially developed by AT for telecommunications purposes, this connector uses a twist lock | |type of design. A typical mated pair of ST connectors will exhibit less than 1 dB (20%) of loss and does not require alignment sleeves or other similar devices.
The inclusion | |of an “anti-rotation tab” assures that every time the connectors are mated, the fibers always return to the same rotational position assuring constant, uniform performance. | |ST connectors are available for both multimode and single-mode fibers, the primary difference being the overall tolerances. Note that | |multimode ST connectors will only perform properly with multimode fibers. More expensive single-mode ST connectors will perform properly with both single-mode and multimode | |fibers. | |The installation procedure for the ST connector is not difficult and can be easily mastered by any system installer.
Figure 9 shows some of the major features of the typical ST| |connector. | |[pic] | Optical Receivers The basic optical receiver converts the modulated light coming from the optical fiber back into a replica of the original signal applied to the transmitter. The detector of this modulated light is usually a photodiode of either the PIN or the Avalanche type. This detector is mounted in a connector similar to the one used for the LED or LD.
Photodiodes usually have a large sensitive detecting area that can be several hundred microns in diameter. This relaxes the need for special precautions in centering the fiber in the receiving connector and makes the “alignment” concern much less critical than it is in optical transmitters. Since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal amplifiers. Because of this, optical receivers can be easily overloaded. For this reason, it is important only to the size fiber specified for use with a given system.
If, for example, a transmitter/receiver pair designed for use with single-mode fiber were used with multimode fiber, the large amount of light present at the output of the fiber (due to over-coupling at the light source) would overload the receiver and cause a severely distorted output signal. Similarly, if a transmitter/receiver pair designed for use with multimode fiber were used with single-mode fiber, not enough light would reach the receiver, resulting in either an excessively noisy output signal or no signal at all. The only time any sort of receiver “mismatching” might be considered is hen there is so much excessive loss in the fiber that the extra 5 to 15 dB of light coupled into a multimode fiber by a single-mode light source is the only chance to achieve proper operation. However, this is an extreme case and is not normally recommended. As in the case of transmitters, optical receivers are available in both analog and digital versions. Both types usually employ an analog preamplifier stage, followed by either an analog or digital output stage (depending on the type of receiver). Figure 10 is a functional diagram of a simple analog optical receiver.
The first stage is an operational amplifier connected as a current-to-voltage converter. This stage takes the tiny current from the photodiode and converts it into a voltage, usually in the millivolt range. The next stage is a simple operational voltage amplifier. Here the signal is raised to the desired output level. Designing A Fiber Optic System When designing a fiber optic system, there are many factors that must be considered – all of which contribute to the final goal of ensuring that enough light reaches the receiver. Without the right amount of light, the entire system will not operate properly.
Figure 12 identifies many of these factors and considerations. [pic] Additional stages are often added to both analog and digital receivers to provide drivers for coaxial cables, protocol converters or a host of other functions in efforts to reproduce the original signal as accurately as possible. It is important to note that while fiber optic cable is immune to all forms of interference, the electronic receiver is not. Because of this, normal precautions, such as shielding and grounding, should be taken when using fiber optic electronic components. Optical Receivers
The basic optical receiver converts the modulated light coming from the optical fiber back into a replica of the original signal applied to the transmitter. The detector of this modulated light is usually a photodiode of either the PIN or the Avalanche type. This detector is mounted in a connector similar to the one used for the LED or LD. Photodiodes usually have a large sensitive detecting area that can be several hundred microns in diameter. This relaxes the need for special precautions in centering the fiber in the receiving connector and makes the “alignment” concern much less critical than it is in optical transmitters.
Since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal amplifiers. Because of this, optical receivers can be easily overloaded. For this reason, it is important only to the size fiber specified for use with a given system. If, for example, a transmitter/receiver pair designed for use with single-mode fiber were used with multimode fiber, the large amount of light present at the output of the fiber (due to over-coupling at the light source) would overload the receiver and cause a severely distorted output signal.
Similarly, if a transmitter/receiver pair designed for use with multimode fiber were used with single-mode fiber, not enough light would reach the receiver, resulting in either an excessively noisy output signal or no signal at all. The only time any sort of receiver “mismatching” might be considered is when there is so much excessive loss in the fiber that the extra 5 to 15 dB of light coupled into a multimode fiber by a single-mode light source is the only chance to achieve proper operation. However, this is an extreme case and is not normally recommended.
As in the case of transmitters, optical receivers are available in both analog and digital versions. Both types usually employ an analog preamplifier stage, followed by either an analog or digital output stage (depending on the type of receiver). Figure 10 is a functional diagram of a simple analog optical receiver. The first stage is an operational amplifier connected as a current-to-voltage converter. This stage takes the tiny current from the photodiode and converts it into a voltage, usually in the millivolt range. The next stage is a simple operational voltage amplifier.
Here the signal is raised to the desired output level. [pic] | Whether loose-buffer or tight-buffer, the actual glass fiber used in any fiber optic cable only comes in one of two basic types, multimode fiber for use over short to| | |moderate transmission distances (up to about 10 Km) and single-mode fiber for use over distances that are generally greater than 10 Km. Communications grade multimode| | |fiber normally comes in two sizes, 50 micron core and 62. 5 micron core, the latter being the size most commonly available.
The outer diameter of both is 125 microns | | |and both use the same connector size. Single-mode fiber comes in only one size, 8-10 microns for the core diameter and 125 microns for the outer diameter. Connectors| | |for single-mode fiber are not the same as those designed for multimode fiber but can look the same as we will soon discuss. | | |[pic] | | |Figure 3 is a drawing of the construction of two types of optical fiber, step index and graded index. | | | | |Step index fiber has a core of ultra-pure glass surrounded by a cladding layer of standard glass with a higher refractive index. This causes light traveling | | |within the fiber to continually “bounce” between the walls of the core much like a ball bouncing through a pipe. Graded index fiber on the other hand operates by | | |refracting (or bending) light continually toward the center of the fiber like a long lens. In a graded index fiber the entire fiber is made of ultra-pure glass. In | | |both types of fiber however, the light is effectively trapped and does not normally exit except at the far end. | |Losses in an optical fiber are the result of absorption and impurities within the glass as well as mechanical strains that bend the fiber at an angle that is so sharp | | |that light is actually able to “leak out” through the cladding region. Losses are also dependent on the wavelength of the light employed in a system since the degree | | |of light absorption by glass varies for different wavelengths. At 850 nanometers, the wavelength most commonly used in short-range transmission systems, typical fiber| | |has a loss of 4 to 5 dB per kilometer of length.
At 1300 nanometers this loss drops to under 3 dB per kilometer and at 1550 nanometers, the loss is a dB or so. The | | |last two wavelengths are therefore obviously used for longer transmission distances. | | |The losses described above are independent of the frequency or data rate of the signals being transmitted. There is another loss factor however that is frequency (and| | |wavelength) related and is due to the fact that light can have many paths through the fiber. Figure 4 shows the mechanism of this loss through step-index fiber. | |[pic] | | |A light path straighter through a fiber is shorter than a light path with maximum “bouncing”. This means that for a fast rise-time pulse of light, some paths will | | |result in light reaching the end of the fiber sooner than through other paths. This causes a smearing or spreading effect on the output rise-time of the light pulse | | |which limits the maximum speed of light changes that the fiber will allow. Since data is usually transmitted by pulses of light, this in essence limits the maximum | | |data rate of the fiber. The spreading effect for a fiber is expressed in terms of MHz per kilometer.
Standard 62. 5 micron core multimode fiber usually has a | | |bandwidth limitation of 160 MHz per kilometer at 850 nanometers and 500 MHz per kilometer at 1300 nanometers due to its large core size compared to the wavelength of | | |the propagated light. Single mode fiber, because of its very small 8 micron core diameter has a bandwidth of thousands of MHz per kilometer at 1300 nanometers. For | | |most low frequency applications however, the loss of light due to absorption will limit the transmission distance rather than the pulse spreading effect. | |Optical Connectors | | |Since the tiny core of an optical fiber is what transmits the actual light, it is imperative that the fiber be properly aligned with emitters in transmitters, | | |photo-detectors in receivers and adjacent fibers in splices. This is the function of the optical connector. Because of the small sizes of fibers, the optical | | |connector is usually a high precision device with tolerances on the order of fractions of a thousandth of an inch. | |Although there are many different styles available the most common optical cable connector in current use is the ST type shown in figure 5. The connector consists of | | |a precision pin that houses the actual fiber, a spring-loaded mechanism that presses the pin against a similar pin in a mating connector (or electro-optic device) and | | |a method of securing and strain-relieving the outer jacket of the fiber optic cable. ST connectors are available for both multimode and single-mode fibers. The main | | |difference between the two is the precision of the central pin.
Since this difference is not readily noticeable, care must be taken to use the correct connector. | | |While single-mode connectors will work properly with multimode emitters and detectors, connectors intended for use with multimode fiber such as the ST type will not | | |work well (or at all) in a single-mode system. | | |[pic] | | CONCLUSION Thus we can find that fiber communication is used in every where in every place in communication systems. So optical communication plays a vital role in optic systems