**1.2. Light and electromagnetic spectrum**

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object has a different meaning, and is instead the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequen‐ cy) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous. Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing interactions, as ways to study and characterize matter. In addition, radiation from various parts of the spectrum has found many other uses for communications and manufacturing (see electromagnetic radiation for more applications).

**Figure 1.** The electromagnetic spectrum.

The electromagnetic spectrum as demonstrated in Fig. 1, can be expressed in term of wave‐ length, frequency, or energy. Wavelength (λ), frequency (ν) are related by the expression [3]. The higher the frequency, the higher the energy.

$$
\lambda = \frac{c}{\nu} \tag{1}
$$

Where *c* is the speed of light (2.998 × 10<sup>8</sup> m / s). The energy of the various components of the electromagnetic spectrum is given by the expression

$$E = \hbar \nu \tag{2}$$

Where *h* is Planck`s constant = 6.63×10-<sup>34</sup> Joule seconds. The units of wavelength are meters with the terms microns (denoted μm and equal to 10-<sup>6</sup> m) and nanometers (10-<sup>9</sup> m) being used just as frequently. Frequency is measured in Hertz (Hz), with one Hertz being equal to one cycle of one cycle of sinusoidal wave per second. A commonly used unit of energy is the electron-volt.

There are several transmission windows that are nearly transparent (attenuation < 0.2 dB/km), between 780 nm and 1600 nm wavelength range. These windows are located around several specific center wavelengths:

**•** 850 nm

**1.2. Light and electromagnetic spectrum**

162 Contemporary Issues in Wireless Communications

applications).

**Figure 1.** The electromagnetic spectrum.

length, frequency, or energy. Wavelength (λ), frequency (

The higher the frequency, the higher the energy.

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object has a different meaning, and is instead the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequen‐ cy) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous. Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing interactions, as ways to study and characterize matter. In addition, radiation from various parts of the spectrum has found many other uses for communications and manufacturing (see electromagnetic radiation for more

The electromagnetic spectrum as demonstrated in Fig. 1, can be expressed in term of wave‐

λ = c

ν

ν

) are related by the expression [3].

(1)

Characterized by low attenuation, the 850 nm window is very suitable for FSO operation. In addition, reliable, high-performance, and inexpensive transmitter and detector components are generally available and commonly used in today's service provider networks and trans‐ mission equipment. Highly sensitive silicon avalanche photo diode (APD) detector technology and advanced vertical cavity surface emitting laser (VCSEL) technology can be used for operation in this atmospheric window [4].

**•** 1060 nm

The 1060 nm transmission window shows extremely low attenuation values. However, transmission components to build FSO system in this wavelength range are very limited and are typically bulky (e.g. YdYAG solid state lasers). Because this window is not specially used in telecommunications systems, high-grade transmission components are rare. Semiconductor lasers especially tuned to the nearby 980 nm wavelength (980 nm pump lasers for fiber amplifiers) are commercially available. However, the 980 nm wavelength range experiences atmospheric attenuation of several dB/km even under clear weather conditions.

**•** 1250 nm

The 1250 nm transmission window offers low attenuation, but transmitters operating in this wavelength range are rare. Lower power telecommunications grade lasers operating typically between 1280-1310 nm are commercially available. However, atmospheric attenuation increases drastically at 1290 nm, making this wavelength only marginally suitable for free space transmission.

**•** 1550 nm

The 1550 nm band is well suited for free space transmission due to its low attenuation, as well as the proliferation of high-quality transmitter and detector components. Components include very high-speed semiconductor laser technology suitable for WDM operation as well as amplifiers (EDFA, SOA) used to boost transmission power. Because of the attenuation properties and component availability at this range, development of WDM free space optical systems is feasible.
