In the early times, fiber optic signal transmission is one wavelength in one fiber glass. People finally found a way, which is known the widely used WDM(Wavelength Division Multiplexing) technology nowadays, to enable various kinds of fiber optic light to transmit in a single fiber glass in different wavelength.
In early WDM systems, there were two IR channels per fiber. At the destination, the IR channels were demultiplexed by a dichroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be demultiplexed using cascaded dichroic filters, giving rise to coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM). In CWDM, there are usually eight different IR channels, but there can be up to 18. In DWDM multiplexer, there can be dozens. Because each IR channel carries its own set of multiplexed RF signals, it is theoretically possible to transmit combined data on a single fiber at a total effective speed of several hundred gigabits per second (Gbps).
The first WDM systems were two-channel systems that used 1310nm and 1550nm wavelengths. Shortly afterwards came multi-channel systems that used the 1550nm region – where the fiber attenuation is lowest. These systems used temperature stabilized lasers to provide the needed channels count. Because of the high costs involved, DWDM was only economical for long-haul applications. Therefore, most optical systems vendors competed at providing the highest channel count and the longest distances.
The need for WDM solutions in the metro region became stronger and a new alternative technology emerged. Transmode was in the forefront in introducing a solution based on less expensive transmitters without temperature stabilization and where the wavelengths were more separated in the spectrum, CWDM. Another Transmode solution is based on a patented low-loss DWDM architecture on single-fiber configurations where the expensive Optical Amplifiers can be omitted.
WDM has revolutionized the cost per bit of transport. Thanks to WDM, fiber networks can carry multiple Terabits of data per second over thousands of kilometers – at cost points unimaginable less than a decade ago. WDM technology has the advantages of high capacity, long reach distance and ease of use. WDM is now recognized as the Layer 1 transport technology in all tiers of the network. It offers low-cost transport for all applications and services, scales easily in terms of capacity and reach and provides rapid protection against any fiber plant failure. Fully transparent to any bitrate and protocol, WDM is the natural integration layer for modern networks. This allows networks to become more manageable, operate more efficiently and transport considerably higher bandwidth for high-volume data transmission.
WDM is similar to frequency-division multiplexing (FDM). But instead of taking place at radio frequencies (RF), WDM is done in the IR portion of the electromagnetic spectrum. Each IR channel carries several RF signals combined by means of FDM or time-division multiplexing (TDM). Each multiplexed IR channel is separated, or demultiplexed, into the original signals at the destination. Using FDM or TDM in each IR channel in combination with WDM or several IR channels, data in different formats and at different speeds can be transmitted simultaneously on a single fiber.
The WDM structure looks similar like a common optical fiber coupler, but they are different because optical couplers divide the same wavelength fiber optic signals by different ratios such as 1:99, while WDM divide the two different wavelength fiber optic light as shown on the above picture.
The use of WDM technology can multiply the effective bandwidth of a fiber optic communication system, but its cost must continue to fall or using multiple fibers bundled into a cable.