In the field of modern communications, wavelength division multiplexing (WDM) and optical transport network (OTN) technologies are like the mainstays that support the high-speed transmission of massive amounts of information. WDM technology cleverly uses different wavelengths of light to simultaneously transmit multiple optical signals in the same optical fiber, greatly increasing the transmission capacity of optical fibers, just like a highway divided into multiple lanes to allow different vehicles (optical signals) to run in parallel. Among them, coarse wavelength division multiplexing (CWDM) has a wider wavelength interval, usually 20 nanometers, covering the 1270-1610 nanometer range, which can provide a smaller number of channels and is suitable for short-distance, low-cost transmission scenarios, just like ordinary roads within the city, meeting daily short-distance commuting needs; dense wavelength division multiplexing (DWDM) is in the 1530-1565 nanometer (C band) and 1565-1625 nanometer (L band) windows, and the wavelength interval is refined to 0.8 nanometers or even smaller, like a carefully planned urban expressway, which can fit more “lanes” in the same optical fiber to achieve ultra-high-speed, long-distance transmission, and is the main force of the backbone network.
OTN technology is optimized and upgraded on the basis of WDM, and it builds a complete “management system” for optical signal transmission. It not only has powerful overhead management functions, but can also monitor the performance, faults and other information of optical channels in real time, just like equipping each vehicle with an accurate positioning and status monitor; it also has flexible cross-connection capabilities, and can flexibly allocate services carried by different wavelength channels according to demand, just like the intelligent dispatching system of a transportation hub, allowing data to flow to where it is most needed, ensuring efficient and reliable information transmission, and is widely used in key areas such as long-distance backbone networks and the core layer of metropolitan area networks.
Analysis of the key requirements of WDM/OTN transmission on optical cables
1.Low loss characteristics: ensure long-distance signal transmission
Taking G.652 single-mode optical fiber as an example, in the 1550nm wavelength window, its loss can be as low as about 0.2dB/km, which means that the power of the optical signal only decreases slightly for every 1 km it transmits.
2.Dispersion characteristics: allowing optical signals to be precisely focused
G.652 fiber has a small dispersion near 1310nm and is suitable for low-speed, short-distance transmission; however, when transmitting at a high speed at a wavelength of 1550nm, the dispersion problem becomes prominent, and an additional dispersion compensation module is required to “correct” the signal, which is very expensive. G.653 dispersion-shifted fiber shifts the zero dispersion wavelength to 1550nm. The original intention was to optimize single-wavelength long-distance transmission, but it ran into trouble in WDM multi-channel scenarios because nonlinear effects such as four-wave mixing caused by zero dispersion caused “crosstalk” between channels. G.655 non-zero dispersion fiber is a clever compromise, maintaining a certain dispersion in the 1550nm window, which can not only suppress nonlinearity but also ensure the orderly transmission of multi-wavelength signals, becoming a “capable player” favored by WDM/OTN systems.
Nonlinear effects: maintaining “order” in signal transmission
When optical signals are transmitted in parallel at high power and multiple wavelengths in WDM/OTN systems, nonlinear effects are likely to emerge. They are caused by the nonlinear relationship between light intensity and fiber refractive index, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM). These effects will broaden the signal spectrum, transfer energy between channels, generate additional noise, and seriously disrupt the transmission “order”.
Characteristics of various optical cables
G.651 optical cable–50/125µm Graded-Index Multimode Fiber for FTTH Systems
G.652 optical cable–Standard Single-Mode Fiber for CWDM Systems
G.653 optical cable–Dispersion-Shifted Single-mode Optical Fiber for Long Haul Transmission
G.654 optical cable–Cut-off Shifted Single-mode Fiber for Long Haul Submarine & Terrestrial Networks
G.655 optical cable–Legacy Long Haul Single-mode Fiber for CWDM System
G.656 optical cable–Non-zero Dispersion Fiber for CWDM and DWDM System
G.657 optical cable–Bend-insensitive Single-mode Fiber for FTTH Systems
WDM/OTN has different performance in these optical fibers, so it is necessary to lay appropriate optical cables according to actual needs.
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