DWDM Tutorial: Basics of Dense Wavelength Division Multiplexing

This tutorial covers the fundamentals of DWDM (Dense Wavelength Division Multiplexing), including the DWDM transmitter and receiver. We’ll also delve into optical fiber basics, optical amplifiers (EDFA), and other essential system components.

DWDM is essentially an optical multiplexing technique. It allows us to combine multiple discrete transport channels, each using a different wavelength, and transmit them over a single optical fiber. For example, if we multiplex 32 STM-1 signals, the optical fiber capacity increases from 2.5 Gbps to a remarkable 80 Gbps.

The figure below illustrates the transmitter and receiver components within a DWDM system. Information signals, represented as binary data, are converted into corresponding light wavelengths. These wavelengths are then multiplexed using couplers and multiplexer devices. An optical isolator is included to minimize back reflection. The signal is amplified using an EDFA before transmission over a single optical fiber cable. This amplification is crucial to overcome path loss, especially when signals need to travel 50-100 km. An optical circulator, combined with a Fiber Bragg Grating (FBG), allows for the selective addition or removal of specific channels.

WDM (Wavelength Division Multiplexing) is used when combining 1550nm signals with 1310nm signals. At the receiver, demultiplexing separates the 1310nm signal from the combined wavelengths. The DWDM demultiplexer at the receiver separates all the 1550nm signals.

Optic Fiber

DWDM systems primarily operate in the C-band (1530 to 1565 nm) due to the availability of optical amplifiers at 1550nm and lower attenuation at this wavelength. However, conventional fibers are not suitable at these wavelengths due to higher dispersion. Therefore, dispersion-compensating fibers or zero-dispersion fibers (as per ITU G.653) are used. ITU G.655 compliant fiber is commonly employed due to its lower dispersion and ability to support high-speed transmission over long distances. This level of dispersion also helps suppress nonlinearity, such as four-wave mixing.

Optical cables adhering to ITU-G.652 standards are generally used for access networks, while ITU G.655 cables are preferred for backbone networks. Many systems use a combination of both types.

Optical Amplifier (EDFA)

In a DWDM system, a post-amplifier boosts the signal at the transmitter, while a pre-amplifier strengthens the signal at the receiver. EDFAs (Erbium-Doped Fiber Amplifiers) are employed as in-line amplifiers to prevent signal degradation.

EDFAs are primarily used to amplify signals within the 1530nm to 1570nm spectrum. They offer the advantage of regenerating the signal without converting it back to an electrical form.

EDFAs utilize a laser source with a wavelength of either 980 nm or 1480 nm, and can achieve amplification levels of approximately 30dB.