print barcode printer c# Light Sources and Their Development in Software

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Operation of an optical ber communication system starts at the light transmitter, which injects the optical signal into the ber This light
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Fiber-Optic Technology in Cable Systems
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source has to be modulated with the message signal The stronger or more intense the light source becomes, the stronger the received signal Light energy traveling through the optical waveguide carries the message traf c to the receiving end The light source at the transmitting end has to be modulated with the message-carrying signal The light sources used in current systems are either a solid-state LED or a solid-state laser diode LEDs are often used for LANs carrying multimode data and provide satisfactory operation for a variety of applications The solid-state laser diode is used mainly in single-mode applications, requiring communication distances of 10 to 50 km This is the type used in cable television systems There are two basic types of laser diodes: the FabryPerot and the distributed feed back (DFB) The characteristics of these diodes differ, as shown in their characteristic curves in Figure 4-10 Since the laser bandwidth is important in the calculation of the effects of dispersion in ber-optic systems, it may not seem important to discuss LED light sources given that the solid-state laser diode is the optical transmitter of choice for cable television systems Many buildings, however, are already wired using multimode ber In many instances, apartment buildings as well as commercial of ce buildings contain unused optical ber Such ber could be used to connect services to subscribers because many of the distances are short compared to ordinary cable systems beroptic plant Small multimode optical detectors could feed the RF signal spectrum to a mini-distribution coax system or directly into a converter
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Figure 4-10 Laser spectrums
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4121 Solid-state LEDs were used in many early type data communication optical- ber systems Much was learned about these light sources and many improvements were made LEDs had several problems associated with their use as optical transmitters The rst was speed An LED cannot be modulated at rates (speed) as high as a laser Also, LEDs do not have as high an optical output as lasers do Because speed and intensity are important factors for light sources acting as transmitters, lasers are the best choice Generally, LEDs have a much wider spectral width than lasers, and this increases the effects of modal dispersion in multimode systems Dispersion causes the stretching of short pulse signals in time, which severely limits the pulse or data rate if it becomes excessive LEDs are made to operate in the 850-nm and 1300-nm regions, which are in the lower operating loss regions for glass optical ber 4122 The solid-state laser diodes used in most single-mode, longdistance ber-optic systems can be made to operate in all the low loss optical windows of most bers The 1300-nm and 1500-nm regions are used in cable television single-mode ber-optical systems Both lightemitting diodes and laser diodes are directly modulated by control of the drive current Both have operating characteristics that vary with temperature LEDs in general can operate within the range of 25 C to 125 C Lasers are somewhat more temperature sensitive Therefore, cooling using a Peltier cooler may be required in some applications When dealing with laser diodes in cable television systems operating in the usual headend air-conditioned environment, proper heat sinking is adequate A typical operating curve of optical output versus drive current is shown in Figure 4-11 An examination of this gure indicates that the operating region is the linear portion of the curves for both the LED and the laser Notice that the temperature characteristics are different and the LED is operational over a wide temperature range The extreme linearity of the laser diode and the high optical power output are important factors that make the laser a superior transmitting device A laser diode light source is available as a single device or as a packaged unit mounted in a heat-sink enclosure Most laser diodes in cable television applications appear in system optical transmitters that include the associated stabilization and drive circuits The RF signal, consisting of the band or bands of cable television channels, is connected to the input The optical transmitter converts this signal to a modulated light signal that is coupled to the ber-optic transmission cable The transmitter out-
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