Selection of Components in the Design of Optical Fiber Communication Systems

A basic fiber-optic communication system is very simple: an LED transmitter converts an electrical signal into an optical signal and couples it into a transmission optical fiber. The optical signal travels through the optical fiber to an optical receiver, which restores the received optical signal to its original electrical signal output.

·Optical cable selection : Generally, silica glass optical fiber is used for long-distance communication links due to its low loss and high bandwidth. For example, Ethernet and FDDI standards specify the use of multimode 62.5/125μm silica glass optical fiber. These thin-core optical fibers require high-precision connectors to reduce coupling losses. For industrial applications, low-cost optical cables and connectors are required. Therefore, 1mm POF (Plymer Optical Fibers) and 200μm HCS (Hard Clad Silica) optical fibers are the best choices, both of which are step-index multimode optical fibers.

The typical loss value of 1mm POF is 0.2dB/m at 650nm wavelength, while the typical loss value of 200μm HCS fiber at 650mm wavelength is only 8dB/km, and even less at 820nm wavelength. The core of HCS fiber is quartz glass, and the cladding is a patented high-strength polymer, which not only increases the strength of the fiber, but also prevents moisture and pollution. The outer sheath is 2.2mm polyvinyl chloride. HCS fiber can operate in a temperature range of -40℃ to +85℃, and the installation temperature range is -20℃ to +85℃, meeting system requirements in terms of performance and price.

· Selection of operating wavelength : The design of optical fiber communication system must consider the impact of optical fiber loss and dispersion on the system. Since both loss and dispersion are related to the operating wavelength of the system, the selection of operating wavelength becomes a major issue in system design.

Taking into account the system requirements and the selected optical fiber, the 820nm wavelength can reduce the HCS optical fiber loss to 6dB/km, while also minimizing the dispersion.

· Choice of light source : At 820m wavelength, LED is the best light source to choose. Compared with semiconductor laser, LED driving circuit is simple and low cost.

In summary, the optical cable uses 200μm HCS optical fiber, and the optical transceiver uses HP's 820nm wavelength HFBR-0400 series. The HFBR-14X2/HFBR-24X2 in this series can reach a rate of 5MBd within a distance of 1500m, and the operating temperature range is -40℃～+85℃. There are various port models such as ST, SMA, SC and FC to choose from. HFBR-14XZ uses 820nm wavelength AlGaAs LED, and HFBR-24XZ integrates an IC chip including PIN photodetector, DC amplifier and collector open output Schottky transistor, and its output can be directly connected to popular TTL and CMOS integrated circuits.