**2. Optical wireless transceiver design**

FSO contains three components: transmitter, free space transmitted channel line of sight, and receiver. Transmitter is considered as an optical source 1-laser diode (LD) or 2-light emitting diode (2-LED) to transmit of optical radiation through the atmosphere follows the Beer-Lamberts's law as indicated in subsection 3.6 Eq. 34.

FSO link is demonstrated as in Fig. 5. The selection of a laser source for FSO applications depends on various factors. It is important that the transmission wavelength is correlated with one of the atmospheric windows. As noted earlier, good atmospheric windows are around 850 nm and 1550 nm in the shorter IR wavelength range. In the longer IR spectral range, some wavelength windows are present between 3–5 micrometers (especially 3.5–3.6 micrometers) and 8–14 micrometers [5]. However, the availability of suitable light sources in these longer wavelength ranges is pretty limited at the present moment. In addition, most sources need low temperature cooling, which limits their use in commercial telecommunication applications. Other factors that impact the use of a specific light source include the following:


eye do not lead to large magnification of power densities. Therefore, much greater power is required to cause damage. The relevance of this is that lasers deployed in FSO that utilize wavelengths greater than 1,400 nm are allowed to be approximately 100 times as powerful as FSO equipment operating at 850 nm and still be considered eye safe. This would be the "killer app" of FSO except that the photo diode receiver technologies suffer reduced sensitivity at greater than 1,400 nm, giving back a substantial portion of the gain. Also, lasers that operate at such wavelengths are more costly and less available. Nevertheless, at least one FSO manufacturer has overcome these obstacles and currently offers equipment deploying

With respect to infrared radiation, the absorption coefficient at the front part of the eye is much higher for longer wavelength (> 1,400 nm) than for shorter wavelength. As such, damage from the ultraviolet radiation of sunlight is more likely than from long wavelength infrared. Eye response also differs within the range that penetrates the eyeball (400 nm—1,400 nm) because the eye has a natural aversion response that makes it turn away from a bright visible light, a response that is not triggered by an (invisible) infrared wavelength longer than 0.7 *μ*m. Infrared light can also damage the surface of the eye, although the damage threshold is higher than that

High-power laser pulses pose dangers different from those of lower-power continuous beams. A single high-power pulse lasting less than a microsecond can cause permanent damage if it enters the eye. A low-power beam presents danger only for longer-term exposure. Distance

FSO contains three components: transmitter, free space transmitted channel line of sight, and receiver. Transmitter is considered as an optical source 1-laser diode (LD) or 2-light emitting diode (2-LED) to transmit of optical radiation through the atmosphere follows the Beer-

FSO link is demonstrated as in Fig. 5. The selection of a laser source for FSO applications depends on various factors. It is important that the transmission wavelength is correlated with one of the atmospheric windows. As noted earlier, good atmospheric windows are around 850 nm and 1550 nm in the shorter IR wavelength range. In the longer IR spectral range, some wavelength windows are present between 3–5 micrometers (especially 3.5–3.6 micrometers) and 8–14 micrometers [5]. However, the availability of suitable light sources in these longer wavelength ranges is pretty limited at the present moment. In addition, most sources need low temperature cooling, which limits their use in commercial telecommunication applications.

Other factors that impact the use of a specific light source include the following:

reduces laser power density, thus decreasing the potential for eye hazards.

**2. Optical wireless transceiver design**

Lamberts's law as indicated in subsection 3.6 Eq. 34.

**•** Price and availability of commercial components

**•** Transmission power

**•** Lifetime

multiple 1,550 nm lasers.

170 Contemporary Issues in Wireless Communications

for ultraviolet light.


**Figure 5.** Block diagram of an optical wireless link showing the front end of an optical transmitter and receiver [7].

Electrical input is a network traffic into pulses of invisible light representing 1`s and 0`s. The transmitter, which consists of two part main parts: an interface circuit and source driver circuit, converts the input signal to an optical signal suitable for transmission. The drive circuit of the transmitter transforms the electrical signal to an optical signal by varying the current follow through the light source. Transmitter function is to project the carefully aimed light pulses into the air. This optical light source can be of two types:


The information signal modulates the field generated by the optical source. The modulated optical field then propagates through a free-space path before arriving at the receiver. In the receiver side, transmitted data realizes inverse operations i.e., photo detector converts the optical signal back into an electrical form as indicated in previous figure. In other words, a receiver at the other end of the link collects the light using lenses and/or mirrors. Received signal converted back into fiber or cooper and connected to the network. Reverse direction data transported the same way (full duplex). We can see, anything that can be done in fiber can be done with FSO.

Equation (5) illustrates the data rate of FSO system:

$$\text{Data Rate} \left[ \overset{\text{bits}}{\text{sec}} \right]\_{\text{sec}} = \frac{1}{\text{\textdegree}} P\_r \left[ \overset{\text{photons}}{\text{sec}} \right] \tag{5}$$

Where Pr is a received power, and η is a received power sensitivity of the receiver [photons/ bit].

Small angles – divergence angle and spot size between transmitter and receiver are presented in Fig. 6.

$$1^{\circ} \approx 17 \text{ mræd} \to 1 \text{ mræd} \approx 0.0573^{\circ}$$

*θ* is a divergence angle between transmitter and receiver FSO units.

**Figure 6.** Small angles – divergence and spot size between transmitter and receiver.

The geometric path loss for an FSO link depends on the beam-width of the optical transmitter, the path length (*L*), and the divergence angle (*θ*). Transmitter and receiver aperture diameters are quantifiable parameters, and are usually specified by manufacturer. Table (1) illustrates the relation of divergence in (mrad), range in (km), and spot diameter in (inches or feet).


**Table 1.** The divergence, range, and spot diameter.
