Light Sources in Objective-C

Printer QR Code ISO/IEC18004 in Objective-C Light Sources

As mentioned earlier in this chapter two basic types of devices are used , to convert electronic signals into light signals for fiber-optic transmission: light-emitting diodes (LEDs) and lasers In this section we will examine each device separately

The light-emitting diode (LED) represents a semiconductor device that emits incoherent optical radiation in the form of electromagnetic waves

when biased in the forward direction Optical radiation in the form of light is referred to as incoherent as it radiates in all directions from the surface of an LED This is illustrated in Figure 42, which shows how light generated by an LED flows in many directions Although we will discuss LED operation in detail in the next subsection, we can note several interesting general operational characteristics of an LED-based optical transmission system: (1) because of the dispersion of light, we need to use a thicker core fiber; (2) because of light dispersion, it is extremely important to attempt to couple the LED as best as possible to the fiber to minimize light loss; and (3) because of the incoherent optical radiation, the transmission distance associated with LEDs is considerably less than that obtainable with lasers Nevertheless, LEDs are relatively inexpensive and serve well for short transmission distances This explains why most early fiber-optic transmission systems were based on the use of LEDs as a light source Operating at a wavelength of 850 nm for use with multimode optical fiber early LED-based , systems were capable of transmitting at rates of up to 140 Mbits/s Unfortunately such systems required repeaters every 10 to 15 km With the , development of low-cost lasers and the migration to single-mode fiber by most communications carriers, the use of LEDs is now relegated primarily for LAN and other short-distance applications Today the upper throughput limit of LED-generated signals is approximately 300 Mbits/s In comparison, lasers routinely operate at 10 Gbits/s and in field trials now operate at 40 Gbits/s Now that we have a general appreciation for LEDs versus lasers, let s examine how the former operates

Light can be generated in several ways The familiar lightbulb represents incandescent light since optical radiation results from the temperature applied to a filament In comparison, an LED represents light generated by the electronic excitation of a material, a term referred to as luminescence To understand how semiconductor chips, including LEDs and lasers, operate, let s review some basic physics, including the structure of atoms and the p-n semiconductor junction, to see how a semiconductor becomes capable of generating light THE COMPOSITION OF AN ATOM Figure 43 illustrates the composition of an atom An atom consists of an inner nucleus surrounded by electrons that circle the nucleus That circle is referred to as a shell Each shell has a maximum number of electrons, ranging from two in the inner shell (K) to 32 in the outer shell (N) The outer shell is referred to as the valence shell or valence band, and represents the shell that is involved in chemical bonding, which permits compounds to be developed The ability of an electron to flow as current depends on the molecular structure of a material, including its valence shell Specifically an , electron must break its bond to an atom to obtain the ability to flow as current When it breaks its bond, the electron is said to be in the conductive band CONDUCTORS The degree of movement of an atom from a valence band to the conductive band determines whether the material is a good or poor conductor For example, copper gold, and silver have only one , valence electron This means that those elements require a minimal amount of energy for an electron to break its band Thus, this explains why those materials are relatively good conductors THE p-n JUNCTION In the world of electronics the semiconductor positive-negative (p-n) junction represents the basic material used to develop LEDs and lasers Because p-n junction operation is easily described in terms of the use of silicon material, let s continue our discussion of atoms by focusing on the silicon atom A silicon atom has four valence electrons Those electrons form the bonds that hold silicon atoms together, which in turn forms the crystalline structure of silica The silicon bonds are also referred to as cova-