Wavelength Division Multiplexing (WDM) Working and Applications

Wavelength division multiplexing (WDM) is a technique modulating various data streams, i.e. optical carrier signals of varying wavelengths in terms of colours of laser light onto a single optical fiber. Wavelength division multiplexing WDM is similar to frequency-division multiplexing (FDM) but referencing the wavelength of light to the frequency of light. WDM is done in the IR portion of the electromagnetic spectrum instead of taking place at radio frequencies (RF). Each IR channel carries several RF signals combined with frequency-division multiplexing (FDM) or time-division multiplexing (TDM). Each multiplexed infrared channel is separated or demultiplexed into the original signals at final point. Data in different formats and at different speeds can be transmitted simultaneously on a single fiber by using FDM or TDM in each IR channel in combination with WDM. It allows network capacity to be gradually and cost effectively increased.

What is Wavelength Division Multiplexing?

WDM enables bi-directional communication and multiplies signal capacity. Each laser beam is modulated by separate set of signals. Since wavelength and frequency have an inverse relationship (shorter wavelength means higher frequency), the WDM and FDM both contains the same technology in them. At the receiving end, Wavelength-sensitive filters, IR analog of visible-light color filters are used.The first WDM technique was conceptualized in the early 1970s. Later, Wave division multiplexing (WDM) systems were able to handle 160 signals that will expand a 10 Gbit/second system with a single fiber optic pair of conductors to more than 1.6 Tbit/second (i.e. 1,600 Gbit/s). 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.

Wavelength division multiplexing systems can combine signals with multiplexing and split them apart with a demultiplexer. WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber by using WDM and optical amplifiers. These two devices work as drop multiplexer (ADM), i.e. simultaneously adding light beams while dropping other light beams and rerouting them to other destinations and devices and this type of filtering of light beams were made possible with e talons, devices called Fabry-Perot interferometers using thin-film-coated optical glass.

In general, WDM systems use single-mode optical fiber (SMF) in which only a single ray of light having a core diameter of 9 millionths of a meter (9 µm). Other systems with multi-mode fiber cables (MM Fiber) which are also called as premises cableshave core diameters of about 50 µm. Present modern systems can handle up to128 signals and can expand a basic 9.6 Gbps fibre system to a capacity of over 1000 Gbps. It is mostly used for optical fiber communications to transmit data in several channels with slight variation in wavelengths. WDM can increase the total bit rate of point-to-point systems.

Uses of Wavelength Division Multiplexing:

• WDM multiply the effective bandwidth of a fiber optic communications system
• A fiber optic repeater device called the erbium amplifier can make WDM a cost-effective and it is the long-term solution.
• This reduces the cost and increases the capacity of the cable to carry data.
• Wavelength Division Multiplexing (WDM) uses multiple wavelengths (colors of light) to transport signals over a single fiber.
• It uses light of different colours to create a number of signal paths.
• It uses Optical prisms to separate the different colours at the receiving end and optical prisms does not require power source.
• These systems used temperature stabilized lasers to provide the needed channels count.

WDM systems are divided according to wavelengths – WDM (CWDM) and dense WDM (DWDM). CWDM operates with 8 channels (i.e., 8 fiber optic cables) which is called as the “C-Band” or “erbium window” with wavelengths about 1550 nm (nanometers or billionths of a meter, i.e. 1550 x 10-9 meters). DWDM also operates in the C-Band but with 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. Mostly WDM systems are operated on single-mode fiber optical cableshaving a core diameter of 9 µm. Wavelength division multiplexing is a technique where optical signals with different wavelengths are combined, transmitted and separated.

Each colour obtained from the prism is capable of carrying 10Gbps to 40Gbps. A 16 colour solution, based on 10Gbps per colour, yields a total network capacity of 160Gbps. Each colour can come off the network at multiple nodes and all these nodes are terminated in one or more data centres by allowing for resilient routing between circuits and also for ‘on ramp’ services.

As shown in figure, wavelength division multiplexing in optical fiber, the input signals are assigned a wave length that are combined on one fiber for transmission and separated before receiving.

Dense Wavelength-division Multiplexing (DWDM) :

Dense Wavelength Division Multiplexing (DWDM) is a technology that allows multiple signals simultaneously that are to be transmitted on a single fiber at different wavelengths and it is also an optical multiplexing technology used to increase bandwidth over existing fiber networks. Due to the wide amplification bandwidth of erbium-doped fiber amplifiers, all channels can often be amplified in a single device. DWDM systems feature high channel count and longer reach.

In this technology, another fiber is not required and because of DWDM, single fibers have been able to transmit data at up to400 GB/s of speed. This technology offers excellent performance characteristics including narrow channel separation and wide channel bandpass in the range of frequencies which are passed through a filter.

What is the difference between CWDM and DWDM?

1. CWDM means Coarse Wavelength Division Multiplexing

• CWDM is defined by wavelengths
• CWDM is short-range communications.
• It uses wide-range frequencies and spreads wavelengths

           DWDM means Dense Wavelength Division Multiplexing.

• DWDM is defined in terms of frequencies.
• DWDM is designed for long transmissions where wavelengths are packed tightly.

Dense Wavelength Division Multiplexing (DWDM) is a technique or technology for transmission of huge information or data over long distances.

Thus, the technology of sending signals through different wavelengths of light into fibers is nothing but wave length division multiplexing in fiber optic communication. In this, multiple optical carrier signals are multiplexed on a single optical fiber using different wavelengths of laser light to different signals.Comment below to know more on WDM and clarify your doubts.

Photo Credits:

• Wavelength Division Multiplexing (WDM)by ytimg
• CWDM and DWDM by packetlight
• difference between CWDM and DWDM by wordpress