wavelength

Signals in electrical and radio networks are defined in terms of frequency and described in terms of cycles per second (cps) traditionally, and Hertz (Hz) in a contemporary context. Once the frequency of the electromagnetic signals exceeds the extremely high frequency (EHF) of level of 300 GHz and crosses into the infrared light (IR) range of the optical spectrum, however, Hz no longer has relevance as either a bandwidth measurement or a signal descriptor. The numbers are just too large and difficult to express.

Rather, wavelength is used in the optical domain to describe the nature of the signal. By way of illustration, consider that the upper range of an analog voice channel is 4 kHz.The velocity of propagation of all electromagnetic energy in a vacuum is roughly that of the speed of light, or 300,000 kilometers per second.

The velocity of electromagnetic energy through a copper wire is approximately 60–80 percent of that, or 180,000–240,000 kilometers per second.Therefore, at a frequency of 4 kHz (4,000 cycles per second) and assuming that the speed of the electrical current through the twisted pair copper circuit is 200,000 kilometers per second, each cycle is 50 kilometers in length over a typical single-channel, voice grade analog local loop. See the mathematical equation below:

200,000,000 km/s = 50,000 m

4,000 cycles/s

Now consider that an infrared optical signal in a fiber optic cable at a commonly used wavelength of 1550

nm (.000001550 meters) has a nominal frequency of 193 THz (193,548,387,096,774 Hz). So, the same voice conversation over a typical long haul fiber optic system is carried by an optical signal that has a wavelength of .000001550 meters, not considering the Vp. In consideration of the fact that adjacent signals can be spaced at intervals of 200 GHz, or 1.6 nm (at 1550 nm), it is fairly obvious that it is a lot easier to discuss optical signals in terms of wavelength than frequency (Hz). See the math below:

300,000 km/s

= .000001550 m

193,548,387,096,774 cycles/s

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