• Advantages of Fiber Optics

  

• The Light
• Losses in Optical Fiber
• Optical Fiber Bandwidth
• Fiber Optic Cable Construction 
• Other Types of Fibers
• Optical Connectors
• Optical Splices

The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 2 shows a graphic representation of these two basic schemes.

The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). In a fiber optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light emitting region in order to couple as much light as possible into the fiber. In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus "every last drop" of light onto the fiber and in other cases, a fiber is "pigtailed" directly onto the actual surface of the emitter.

LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input and stable in terms of light output versus ambient operating temperature. LDs, on the other hand, have very small light emitting surfaces and can couple many times more power to the fiber than LEDs. LDs are also linear in terms of light output versus electrical current input, but unlike LEDs, they are not stable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability. In addition, their added cost makes them primarily useful for applications that require the transmission of signals over long distances.

LEDs and LDs operate in the infrared portion of the electromagnetic spectrum so that their light output is usually invisible to the human eye. Their operating wavelengths are chosen to be compatible with the lowest transmission loss wavelengths of glass fibers and highest sensitivity ranges of photodiodes. The most common wavelengths in use today are 850 nanometers, 1300 nanometers, and 1550 nanometers. Both LEDs and LDs are available in all three wavelengths.
 

LEDs and LDs, as previously stated, are modulated in one of two ways; on and off, or linearly. Figure 3 shows simplified circuitry to achieve either method with an LED or LD. As can be seen from

Figure 3A, a transistor is used to switch the LED or LD on and off in step with an input digital signal. This signal can be converted from almost any digital format by the appropriate circuitry, into the correct base drive for the transistor

Linear modulation of an LED or LD is accomplished by the operational amplifier circuit of figure 3B. The inverting input is used to supply the modulating drive to the LED or LD while the non-inverting input supplies a DC bias reference. Once again temperature stabilization circuitry for the LD has been omitted from this example for simplicity.

Digital on/off modulation of an LED or LD can take a number of forms. The simplest, as we have already seen, is light-on for a logic "1", and light-off for a logic "0". Two other common forms are pulse width modulation and pulse rate modulation. In the former, a constant stream of pulses is produced with one width signifying a logic "1" and another width, a logic "0". In the latter, the pulses are all of the same width but the pulse rate changes to differentiate between logic "1" and logic "0".

The equivalent operating frequency of light, which is, after all, electromagnetic radiation, is extremely high – on the order of 1,000,000 GHz. The output bandwidth of the light produced by LEDs and Laser diodes is quite wide. Unfortunately today’s technology does not allow this bandwidth to be selectively used in the way that conventional radio frequency transmissions are utilized. Rather, the entire optical bandwidth is turned on and off in the same way that early "spark transmitters" (in the infancy of radio), turned wide portions of the RF spectrum on and off. However, with time, researchers will overcome this obstacle and "coherent transmissions", as they are called, will become the direction in which the fiber optic field progresses.

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.

Losses in Optical 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 ¼ 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.

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

.

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 when using multimode fiber, as its maximum bandwidth often falls within the range of the signals most often used in point-to-point transmission systems.

Fiber Optics Cable Construction

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.

Other Types of Fibers

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.

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. Figure 8 shows an exploded view of the parts of this connector.

The most popular type of multimode connector in use today is the ST connector. Initially developed by AT&T 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.

Optical Splices

In a mechanical splice, the ends of two pieces of fiber are cleaned and stripped, then carefully butted together and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remained aligned.

A fusion splice, by contrast, involves actually melting (fusing) together the ends of two pieces of fiber. The result is a continuous fiber without a break. Fusion splices require special expensive splicing equipment but can be performed very quickly, so the cost becomes reasonable if done in quantity. As fusion splices are fragile, mechanical devices are usually employed to protect them.