Principles of Wavelength Division Multiplexing (WDM) Technology

WDM technology is an advanced optical fiber communication technology, known as wavelength division multiplexing. It involves transmitting light of different rates mixed together within a single optical fiber, where the digital signals carried by these light signals of different wavelengths can be of the same rate and format, or different rates and data formats.

At the receiving end, these combined signals of different wavelengths are separated using a demultiplexer and further processed to restore the original signals, which are then sent to different terminals. Hence, this technology is called optical wavelength division multiplexing, abbreviated as optical WDM.

Here, a single optical fiber can be likened to a "multi-lane" highway. Traditional TDM systems only utilize one lane of this road, and increasing the bit rate is akin to speeding up on that lane to increase the transport capacity per unit time. Using Dense Wavelength Division Multiplexing (DWDM) technology is akin to utilizing the unused lanes on this highway to tap into the vast, untapped transmission capacity of the optical fiber.

WDM technology is of great significance for network expansion and upgrade, developing broadband services, harnessing the bandwidth capability of optical fibers, and achieving ultra-high-speed communication.

 

1. Basic Composition of WDM Systems

The basic composition of WDM systems mainly includes two types: dual-fiber unidirectional transmission and single-fiber bidirectional transmission. Unidirectional WDM involves all optical channels being transmitted in the same direction through a single optical fiber. At the transmitting end, modulated optical signals with different wavelengths, each carrying various information, are combined using an optical multiplexer and transmitted unidirectionally through one optical fiber. Since each signal is carried by light of a different wavelength, they do not mix. At the receiving end, an optical demultiplexer separates the optical signals of different wavelengths, completing the transmission of multiple optical signals, while the reverse direction is transmitted through another optical fiber.

Bidirectional WDM means that optical channels transmit simultaneously in two different directions on a single optical fiber, with the wavelengths used being separated to achieve full-duplex communication.

A WDM system typically comprises four components: an optical transmitter, an optical repeater amplifier, an optical receiver, and an optical supervisory channel.

1.1. Optical Transmitter:

As the core equipment of the WDM system, at the transmitting end, it first converts the optical signals output from terminal equipment into signals with stable specific wavelengths using an optical transponder, then synthesizes multi-channel optical signals using a multiplexer, and amplifies the output through an optical power amplifier.

1.2. Optical Repeater Amplifier:

After long-distance (80~120km) optical fiber transmission, the optical signal needs to be amplified. In WDM systems, gain flattening technology must be used to ensure that the Erbium-Doped Fiber Amplifier (EDFA) provides the same amplification gain for optical signals of different wavelengths and that the gain competition of optical channels does not affect transmission performance.

1.3. Optical Receiver:

At the receiving end, the main channel signal, which has been attenuated by transmission, is amplified by an optical preamplifier. A demultiplexer is used to separate the optical channel of a specific wavelength from the main channel optical signal. The receiver must meet the requirements for parameters such as optical signal sensitivity and overload power, and must be able to withstand signals with certain optical noise.

1.4. Optical Supervisory Channel:

The optical supervisory channel is used for monitoring WDM optical transmission systems. The ITU-T recommends using a wavelength of 1510nm with a capacity of 2Mbit/s. It can still operate normally with a high receiving sensitivity (better than -48dBm) at low rates. However, it must be removed from the optical path before the EDFA and added to the optical path after the EDFA.

Throughout the entire WDM system, the optical multiplexer and demultiplexer are the key components of WDM technology, and their performance is decisive for the transmission quality of the system. A device that combines signals of different light source wavelengths and outputs them through a single transmission optical fiber is called a wavelength division multiplexer.

Conversely, a device that decomposes multi-wavelength signals arriving from the same transmission optical fiber into individual wavelengths for output is called a demultiplexer. In principle, this device is bidirectional, meaning that if the output and input ends of the demultiplexer are reversed, it becomes a multiplexer. Performance indicators for optical wavelength division multiplexers include insertion loss and crosstalk, with requirements for low loss and frequency offset, insertion loss below 1.0~2.5dB, low channel crosstalk, high isolation, and minimal interference between signals of different wavelengths.

 

2. Advantages of WDM Systems:

 

2.1  Ultra-large capacity and ultra-long distance transmission:

Currently, ordinary optical fibers can transmit over a wide bandwidth, but their utilization rate is still very low. Using Dense Wavelength Division Multiplexing (DWDM) technology can increase the transmission capacity of a single optical fiber by several times, dozens of times, or even hundreds of times compared to single-wavelength transmission. The highest capacity optical fiber transmission system currently is 3.2Tbit/s.

2.2  Transparent data transmission:

Since the DWDM system multiplexes and demultiplexes based on different optical wavelengths and is independent of signal rate and electrical modulation method, it is "transparent" to data. The WDM system performs transparent transmission; for "service" layer signals, each optical wavelength channel in the WDM system acts like a "virtual" optical fiber.

2.3  High flexibility, economy, and reliability in network composition:

The new communication network formed using WDM technology is simpler in structure and more hierarchical compared to networks composed of traditional electrical time-division multiplexing technology. The scheduling of various services can be achieved by adjusting the wavelength of the corresponding optical signal. The resulting flexibility, economy, and reliability of the network are evident.