Optical Wave Division Multiplexing System

Author: Ji Yuefeng
Publisher:
Publish Date: 2000-06-01
Features:
Fragment: With the advent of the information age, communication services have been growing rapidly year by year. To adapt to the continuous growth of transmission capacity in communication networks and meet the requirements of network interactivity and flexibility, various multiplexing technologies have emerged. In digital optical communication, in addition to the well-known Time Division Multiplexing (TDM) technology, other multiplexing technologies such as Optical Time Division Multiplexing (OTDM), Optical Wavelength Division Multiplexing (WDM), Optical Frequency Division Multiplexing (FDM), and Microwave Subcarrier Multiplexing (SCM) have also appeared. The emergence of these multiplexing technologies has significantly improved the transmission efficiency of communication networks.
1.1.1 Definition of WDM
Optical Wavelength Division Multiplexing (WDM: Wavelength Division Multiplexing) is a technology that enables the simultaneous transmission of multiple wavelength optical signals over a single fiber. The basic principle is that at the sending end, different wavelength optical signals are combined (multiplexed) and coupled into the same fiber in the optical cable for transmission. At the receiving end, the combined wavelength optical signals are separated (demultiplexed) and further processed to recover the original signals, which are then sent to different terminals. Therefore, this technology is referred to as optical wavelength division multiplexing, abbreviated as WDM. Due to the current immaturity of some optical devices and technologies, it is still challenging to achieve very dense optical frequency division multiplexing. In this context, the Wavelength Division Multiplexing with smaller channel spacing within the same window is called Dense Wavelength Division Multiplexing (DWDM: Dense Wavelength Division Multiplexing). Currently, such systems operate within the 1550 nm wavelength range (see Figure 1.1), simultaneously using 8, 16, or more wavelengths on a pair of fibers (or a single fiber), where the spacing between each wavelength is 1.6 nm, 0.8 nm, or even smaller, corresponding to approximately 200 GHz, 100 GHz, or narrower bandwidths. The sharp peaks in the loss spectrum shown in Figure 1.1 are caused by OH roots in the fiber. If these can be eliminated, the low-loss characteristics of the fiber can be fully utilized in the 1280–1620 nm band (known as full-wave fiber), allowing the available wavelength range of the WDM system to reach about 340 nm, significantly increasing transmission capacity. The 1525–1565 nm band is generally referred to as the C band, which is currently used in systems, while the bands under research and development are the L band (1570–1620 nm) and S band (1400 nm). In general system applications, the channel wavelengths used are equally spaced, i.e., k×0.8 nm, where k is a positive integer. Previously, technicians were accustomed to using WDM and DWDM to distinguish between simple multiplexing at 1310/1550 nm or dense multiplexing within the 1550 nm wavelength range. However, in the telecommunications industry, DWDM technology is now widely used. The 1310/1550 nm multiplexing, which falls outside the range of EDFA, is only used in specialized applications. Therefore, this book will use the broader term WDM to describe DWDM technology below.