| General Design Specifications Performance Specification for Optical Links Subsystem Parameter Requirement Wave Length 1310 NM or 1550 NM Fiber Optic Cable Loss £.4 dB KM @ 1310 NM Laser Output Power Typically 3-10 dBm V Carrier-to-Noise Ratio (C/N) 51 dB Composite Second Order Distortion (CSO) 65 dB Composite Triple Beat Distortion (CTB) 65 dB Cross Modulations Distortion (X-MOD) 63 dB Frequency Range 5 – 750 MHz Bandwidth (MHz) 750 MHz Input & Output Impedance 75 OHMS Component Isolation ³ 35 dB Fiber Dispersion @ 1310 NM Many of the performance specifications relevant to optical links are identical to those of standard coaxial distribution systems, such as carrier-to-noise ratio (C/N), composite second order distortion (CSO), composite triple beat distortion (CTB), and cross-modulations distortion (X-mod), Frequency flatness and RF input return loss also are important characteristics of optical receivers. Using the previous specifications, coaxial links can be treated just as any component in a system design with predictable contributions to system noise, distortion and flatness. One of the fundamental characteristics of an optical link is the output power of the laser. A laser with higher output power can be used to improve link C/N, drive more optical receivers or transmit the signals over a greater distance. Choosing a transmitter with the optimum optical output power is an important consideration in designing with fiber optics. Although the performance parameters for fiber-optic links (as mentioned earlier) are familiar to the system designer, there are several effects specific to fiber that must be kept in mind. The most important of these effects are outlined here. Because optical fiber is an amorphous substance, light passing through it is subject to scattering. Therefore, a small portion of the forward traveling optical signal is reflected back to the laser. A portion of this backward traveling light is then re-reflected back toward the receiver. The twice-reflected light then appears at the optical receiver, where it interferes with the main optical signal. This interference appears as noise at the optical receiver, resulting in a degradation of the link C/N. The amount of degradation depends on many different parameters, the most important of which is the length of the fiber. Therefore, an optical link with a short fiber and a lot of optical splitting will have a higher C/N than a link with a long fiber and little or no optical splitting, even if the optical loss is the same for each case. Therefore, the system designer must consider the fiber length in the design to prevent over specifying performance for short links or under specifying it for long links. The unavoidable scattering is not the only source of optical reflections in a link. Reflections also are caused by optical connectors, splices, receivers and splitters. Minimizing the reflections from these devices is crucial to ensure optimum link performance. Excessive reflections can severely degrade the noise and distortion performance of optical links. These problems, however, can easily be avoided by following a few simple guidelines. The use of high return loss angle-polished optical connectors such as FC/APC or SC/APC prevents reflection problems that can occur with flat-polished connectors. The use of fusion splices instead of mechanical splices also prevents problems caused by reflections. Finally, most optical receivers and single-mode splitters have sufficiently low reflections that problems do not arise. |