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In the modern information age, transmission of high-density intelligible signals across distances ranging from a few to thousands of kilometers is best performed by making the signals ride on an electromagnetic wave carrier. The signals are inscribed on the carrier by modulating its amplitude, intensity, frequency, or phase in proportion to the corresponding characteristics of the signal. The higher the frequency of the carrier, the greater its information-carrying capacity becomes. The bandwidth of a carrier is at most a few percent of its center frequency. Thus, if we compare frequencies of light waves (∼100 THz) with those of microwaves (1 ∼ 10 GHz) as a carrier, and take 1% of the carrier center frequency as the typical modulation bandwidth that is usable in practice, it becomes immediately evident that transmission of 1-THz bandwidth information is, in principle, possible with an optical carrier, in contrast to a maximum of 0.1 GHz that is feasible with microwaves. Since one telephone channel transmission requires a bandwidth of ∼4 kHz, one can, in theory, pack 250 million telephone channels into one optical carrier.2 In fact, transmission of more than 10-Tb/s optical pulses via one optical fiber cable has been already demonstrated in the laboratory. Fiber optics from the point of view of telecommunication is now almost taken for granted in view of its wide-ranging application as the most suitable singular transmission media for voice, video, and data signals. The initial revolution in this field centered on achieving optical transparency in terms of exploiting the low-loss and low-dispersion transmission wavelength windows of high-silica optical fibers. The development of broadband optical fiber amplifiers, which led to the birth of the era of wavelength division multiplexing (WDM) technology in the mid-1990s, ushered in the next revolution in fiber optics. WDM technology, more precisely known as dense WDM (DWDM) technology involving simultaneous transmission of at least four wavelengths at the 1550-nm low-loss wavelength window through one single-mode fiber has indeed resulted in an enormous increase in available bandwidth for high-speed telecommunication and data transfer.
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