WDM is a technique in fiber optic transmission that enables the use of multiple light wavelengths (or colors) to send data over the same medium. Two or more colors of light can travel on one fiber and several signals can be transmitted in an optical waveguide at differing wavelengths.
Early fiber optic transmission systems put information onto strands of glass through simple pulses of light. A light was flashed on and off to represent digital ones and zeros. The actual light could be of almost any wavelength—from roughly 670 nanometers to 1550 nanometers.
WDM is a technique in fiber optic transmission for using multiple light wavelengths to send data over the same medium.
During the 1980s, fiber optic data communications modems used low-cost LEDs to put near-infrared pulses onto low-cost fiber. As the need for information increased, so did the need for bandwidth. Early SONET systems used 1310 nanometer lasers to deliver 155 Mb/s data streams over very long distances.
But this capacity was quickly exhausted. Advances in optoelectronic components allowed the design of systems that simultaneously transmitted multiple wavelengths of light over a single fiber. Multiple high-bit rate data streams of 2.5 Gb/s, 10 Gb/s and, more recently, 40 Gb/s, 100 Gb/s, and 200 Gb/s could be multiplexed through divisions of several wavelengths. Thus, WDM was born.
There are two types of WDM today:
Coarse WDM (CWDM): WDM systems with fewer than eight active wavelengths per fiber. CWDM is defined by wavelengths. DWDM (see below) is defined in terms of frequencies. DWDM’s tighter wavelength spacing fits more channels onto a single fiber, but cost more to implement and operate.
CWDM is for short-range communications, so it employs wide-range frequencies with wavelengths spread far apart. Standardized channel spacing permits room for wavelength drift as lasers heat up and cool down during operation. CWDM is a compact and cost-effective option when spectral efficiency is not an important requirement.
Dense WDM (DWDM): DWDM is for systems with more than eight active wavelengths per fiber. DWDM dices spectrum finely, fitting 40-plus channels into the same frequency range used for two CWDM channels.
DWDM is designed for long-haul transmission, with wavelengths packed tightly together. Vendors have found various techniques for cramming 40, 88, 96, or 120 wavelengths of fixed spacing into a fiber. When boosted by Erbium Doped-Fiber Amplifiers (EDFAs)—a performance enhancer for high-speed communications—these systems can work over thousands of kilometers. For robust operation of a system with densely packed channels, high-precision filters are required to peel away a specific wavelength without interfering with neighboring wavelengths. DWDM systems must also use precision lasers that operate at a constant temperature to keep channels on target.
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