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This chapter is from the book

This chapter is from the book

Light Communication

Light has historically been used as a means of communication and continues to be used to communicate. In electrical systems today, the information is sent by superimposition onto a sinusoidally varying wave called the carrier. This is called modulation. The amount of information is related to frequency. The higher the frequency, the more information sent. The keyword is bandwidth. Bandwidth must increase. Bandwidth was increased through transmission at higher frequencies. The result of this increase was the formation of higher frequency spectra such as radar, radio, television, and microwaves. Towards the end of the 20th century, the demand for more communication services increased exponentially. The skyrocketing sales of personal computers placed additional needs for increased bandwidth on the communications market. In addition, developed countries now produce more information than anything else. So telecommunications has become a giant—a monster that must be fed. There is no turning back. Everyone wants more capacity!

This leads us to the basic forms of signal—analog and digital. Comparing the two, analog varies amplitude, frequency, and phase to send information, whereas digital sends a 1 or a 0. The analog signal is susceptible to noise. The digital is not so susceptible. The downside is that digital demands more bandwidth. It takes a whole lot more bandwidth to send a simple digital "one" than an analog message. That is where fiber optics enters the picture. Before we get to fiber optics, let's cite a limit to blindly increasing bandwidth. There is the Shannon-Hartley theorem that states

(1.25)

where C is the information capacity, BW is the bandwidth in Hz, and S/N is the signal-to-noise power ratio. So we see that the higher the bandwidth, the greater the capacity. Let's try some numbers.

A copper wire can carry a 1 MHz signal. A coax can carry a 100 MHz signal. A fiber optic can carry a 1000 THz signal. Now if bandwidth is, say, 10% of the carrier, the bandwidth increases appreciably from copper to fiber optics. This is summarized in Table 1-2. Notice that the bandwidth increases from 105 for a copper wire to 1014 Hz for a fiber.

Table 1-2 Summary of Bandwidths and Channels

Cable

Carrier Frequency [Hz]

Bandwidth [Hz]

Channels

Copper wire

106

105

--

Coax

108

107

13,000

Microwaves

1010

109

20,000

Fiber optic

1015

1014

300,000


In addition the channels are shown in the last column. The number of channels increased from 13,000 channels for a coaxial cable to 300,000 two-way voice channels for a fiber. It should be clear that fiber optics cables have a distinct advantage over other types of wiring because of increased information transfer. There are other advantages as well, but the high-speed communication possibilities make fiber the medium of choice.

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