ucial Free-Space Optics (FSO) INTERNET PoP Figure 9-6 WAN Gateway Example of MAN environment. CUSTOMERS FIBER MULTIPLE FLOORS CUSTOMERS factor when utilizing the FSO system, a high-speed microwave radio backup link also needs to be installed. In order to understand how FSO net- work elements work in detail, it is beneficial to describe the design aspect of an optical transceiver. The design of an optical transceiver can be categorized into two types: reflective optical design and directive optical design. In the case of the reflective optical design, the goal is to couple incident light onto the core of a multimode fiber that is connected to a switch. Each transceiver consists of an individual receiver aperture as well as four individual transmitting apertures. The receiver aperture is usually connected to a multimode fiber with a FC-PC jumper. The four individual transmitting apertures are linked to single-mode fiber jumpers, while an equal length of fiber is used to provide the optical signal to each transmitting aperture. In addition, two 785-nm lasers are used as alignment beacons and are integrated into the transceiver housing for initial alignment purposes. Figure 9-7 illustrates the design of an optical transceiver.4 The second optical transceiver design utilizes a convergent lens to focus laser beams onto a light-sensing element; thus, it is called a directive opti- cal design. The transmitter portion of this design is composed of the fiber- optic interface, interface circuitry, laser diode, and beam collimator. The fiber-optic interface is an optical transceiver module operating with data rates from 100 to 622 Mbps using multimode fiber with distances up to Chapter 9 xmtr apertures Figure 9-7 Design of an optical incoming alignment beacons transceiver. Receive aperture, multi-mode fiber incoming FC-PC Secondary mirror Primary mirror 1,100 meters. The optical interface output is coupled to the laser diode dri- ver circuitry. The laser diode driver circuitry provides the threshold bias and modulation curre