What if you could double the bandwidth of your outside plant fiber optic network easily, or even multiply the bandwidth by 10, 20 or more? In high-speed links, it’s a common practice and accomplished using wavelength division multiplexing (WDM).
WDM works because light of different wavelengths does not mix in the fiber, just like it does not mix in the air. If you look at an object with parts that are red and others that are blue, you see those colors distinctly because they travel through the air from the object to your eyes independently. In a fiber, two or more wavelengths can be transmitted simultaneously to multiply the bandwidth of the fiber link.
WDM was first used with multimode fiber in the earliest days of fiber optics, using 850 and 1,310 nanometer (nm) links on multimode fiber, sometimes carrying two signals in the same direction or allowing bidirectional transmission on one fiber.
When used in the same direction, filters remove one wavelength to prevent crosstalk between the two wavelength channels.
When used in bidirectional transmission on a single fiber, the two different wavelengths prevent crosstalk between the channels.
WDM in single-mode fiber for high-speed, long-distance networks took off in the mid-1990s when the distributed feedback (DFB) laser, was developed. DFB lasers had a very stable wavelength and much narrower spectral width than current lasers (called Fabry-Perot lasers) to allow spacing laser wavelengths very close together in a narrow wavelength range.
International standards were developed for two types of WDM: CWDM (“coarse” WDM) and DWDM (“dense” WDM).
Current standard systems offer up to 128 wavelength channels in two versions over the wavelength range of approximately 1,270–1,600 nm. CWDM lasers are spaced 20 nm apart in the wavelength range of 1,270–1,600 nm. DWDM lasers are spaced 0.8 nm apart in the range of 1,530–1,625 nm. (The letters O, E, S, C, L and U refer to specific wavelength “bands” defined in international standards.)
The input of a WDM system is quite simple. It is a coupler that combines all the inputs into one output fiber. These have been available for many years, offering 2, 4, 8, 16, 32 or even 64 inputs.
The demultiplexer is the difficult component to make because takes the input fiber with all the wavelengths, separates out each wavelength and focuses it into a fiber, creating separate outputs for each wavelength of light. The lasers must be very specific wavelengths, and the wavelengths must be very stable, since the DWDM demultiplexers must be capable of distinguishing each wavelength without crosstalk.
Today, many OSP links already use WDM since it is a much cheaper way to expand fiber capacity than installing more fibers. As a bonus, the wavelength range used for DWDM is appropriate for fiber amplifiers to be used as repeaters.
Another widely used application of WDM is fiber to the home (FTTH) using a passive optical network (PON.)
The FTTH PON uses one wavelength downstream (1,490 nm) and another upstream (1,310 nm) through the PON splitter to allow bidirectional links for multiple users over only one single-mode fiber.