Dispersion Compensation Solution for Long-Haul DWDM System
Updated at Jul 10th 20201 min read
In recent years, DWDM (dense wavelength division multiplexing) technology has made tremendous progress on transmission capacity, which is defined by optical signal modulation method and transmission bandwidth. As transmission speeds rise, implementing effective chromatic dispersion management becomes crucial to maintain signal integrity. This paper explores the types of dispersion and introduces solutions for long-haul DWDM systems.
Dispersion in DWDM Optical Transmission System
Dispersion is referred to as the broadening of light pulses and is a consequence of the physical properties of the transmission medium. When optical signals are transmitted over optical links, different wavelength components of the optical signals will generally experience different propagation times due to the fact that an optical fiber has different effective refractive indices for different wavelengths, which results in a pulse distortion and broadening. This phenomenon is referred to as dispersion. Dispersion reduces the effective bandwidth and escalates the error rate due to an increasing intersymbol interference, which has become a critical factor limiting the quality of signal transmission over optical links.
Types of Dispersion
There are mainly three kinds of dispersion in fiber optical transmission, Polarization Mode Dispersion (PMD), Chromatic Dispersion (CD) and Inter modal Dispersion.
Polarization Mode Dispersion (PMD): PMD is a form of dispersion where optical pulses are spread because optical signals in different phase states are transmitted at different speeds due to the random birefringence of optical fibers.
Chromatic Dispersion (CD): CD is a form of dispersion where optical pulses are spread because different wavelengths are transmitted at different speeds in optical fibers and the periods for different wavelengths to traverse the same distance are different. Chromatic dispersion is the most significant impairment that should be considered in fiber transmission systems.
Intermodal Dispersion: Intermodal dispersion is a phenomenon in multimode optical fiber that occurs between different light modes (i.e., different propagation paths). Since there are multiple propagation modes in optical fiber, each mode has a different propagation speed, resulting in different times for them to arrive at the destination. The time delay between the low-order mode (shorter propagation path) and the high-order mode (longer propagation path) causes pulse expansion and signal distortion, thus affecting the quality of data transmission.
Dispersion Compensation Solution
Dispersion in fiber optic communications causes time broadening of the signal, which causes distortion and difficulty in decoding the signal, especially in long-distance transmission. In high-capacity transmission systems such as dense wavelength division multiplexing (DWDM), dispersion compensation technology is a key means to ensure signal quality and system performance. The following are several common dispersion compensation methods:
Dispersion Compensation Module (DCM): Dispersion compensation modules compensate for the accumulated dispersion in single-mode fiber, and the dispersion coefficient is used to characterize the dispersion value. The dispersion value of conventional single-mode fiber is about +16~17ps/(nm*km) at 1550nm. To properly manage this problem, DCM uses a special type of dispersion compensating fiber in the module, whose dispersion coefficient is negative, ranging from -30 to -300ps/(nm*km). DCM can be used to compensate for dispersion at data rates from 10Mbps to 40Gbps in ultra-long-distance coherent networks such as SDH/SONET, CATV, and DWDM systems. These modules can achieve dispersion values from -10 to -2100ps/nm at 1550nm wavelength and provide advanced compensation for standard single-mode fiber (SMF28 - G.652 fiber) throughout the C-band while maintaining extremely low and flat insertion loss and low latency. Currently, FS mainly provides DCF-based DCM (in figure 1 ),When used in conjunction with EDFA and OEO, DCM provides a simple, reliable and cost-effective long-distance transmission solution, allowing signals to be transmitted farther without regeneration.
Dispersion Compensating Fiber (DCF): By adding negative dispersion fiber to conventional optical fiber, the dispersion in the optical fiber is effectively compensated to ensure that the total dispersion of the entire optical fiber line is close to zero, thereby improving the performance of high-speed, high-capacity and long-distance communications. Common compensation methods include pre-compensation, post-compensation and symmetrical compensation, which are widely used in the upgrade of 1310nm optical fiber links, especially at 1550nm, to achieve better dispersion compensation and improve signal quality.
Fiber Bragg Grating (FBG): Through the periodic modulation of the refractive index in the optical fiber, a passive device with reflection characteristics for a specific wavelength is formed. This reflective property can significantly reduce the dispersion effect in long-distance transmission. FBG not only has the characteristics of low insertion loss and cost-effectiveness, but also can be seamlessly integrated with other optical devices. Its functions are not limited to dispersion compensation, but also include applications such as laser wavelength stabilization, narrowband wavelength division multiplexing filtering and sensing. Due to its small size and stable performance, FBG is particularly suitable for use in high-density, high-reliability optical network systems.
Electronic dispersion compensation (EDC): The electrical signal form of the optical signal is processed by an electronic filter to eliminate the distortion caused by dispersion. EDC mainly uses a transverse filter to delay and weight the input signal so that it automatically adjusts the filter parameters while restoring the signal integrity. EDC can be used in single-mode fiber and multimode fiber systems, and plays an important role in reducing the cost of single-mode fiber transmitters and extending the transmission distance of multimode fiber. This method is often integrated into high-speed receiver chips and optimized in coordination with other functional modules. It is an indispensable part of modern optical communication systems.
Dispersion compensation technology plays a vital role in optical fiber communications. Different compensation methods have their own characteristics and can effectively improve the quality of signal transmission and the overall performance of the system. Choosing the right compensation technology is crucial to ensuring the stability and efficiency of long-distance, high-speed optical communication systems.

Figure 1: DCF-Based Passive Dispersion Compensation Module
Dispersion Compensation Module Use Case Study
As an example, the cumulative dispersion for a 10km length of fiber will be +160~170ps/nm, so in order to compensate for that amount of dispersion, a DCM module will be added to the link with a specific and calculated fiber length aimed at reducing the total dispersion close to 0ps/(nm*km).
As seen below, the signal begins to disperse as soon as it leaves the source, which means it can not be recognized at the receiver side. But with a DCM module, the lost signals can be compensated before the receiver can effectively neutralize the fiber’s dispersion, which eventually guarantees normal working and ensures reliable long-haul transmission.

Figure 2: Use Case of Dispersion Compensation Module
Typically, a DCM module provides a fixed amount of dispersion (e.g. normal dispersion in the 1.6-μm spectral region), although tunable dispersion modules are also available. A module can easily be inserted into a fiber-optic link because it has fiber connectors for the input and output. The insertion losses may be compensated with a fiber amplifier (e.g. an EDFA in a 1.5-μm telecom system). A dispersion compensation module is often placed between two fiber amplifiers in a long-haul DWDM system.
FS Dispersion Compensation Module Products
FS offers a wide range of dispersion compensation module products, including FMT Series, M6200 Series and MS8100 Series. The FS dispersion compensation module is specially designed to correct optical signal distortion caused by dispersion, providing a wide range of compensation functions with low and flat insertion loss to ensure signal integrity over long distances. When used with EDFA and OEO, DCM can significantly extend signal transmission range without the need for signal regeneration, making it ideal for long-distance DWDM systems.
Additionally, FS offers Tunable Dispersion Compensation Modules (TDCM) customized solutions to meet specific network requirements. The module supports a variety of adjustable parameters such as compensation length, connector type, and is suitable for dispersion compensation in high-speed transmission systems. It can accurately manage the residual dispersion after segmented fixed light compensation, providing a flexible and accurate solution for dispersion compensation. The module ensures the independence, transparency, security and reliability of optical signal transmission and ensures stable communication of the system. It is especially suitable for high-speed and long-distance WDM transmission systems.

Figure 3: FS DCM&TDCM
Conclusion
In optical fiber communications, dispersion compensation technology is a key factor in ensuring signal quality and system performance. Different methods such as dispersion compensation modules, dispersion compensating fibers, fiber Bragg gratings, and electronic dispersion compensation can effectively deal with different types of dispersion problems. The continuous advancement and adoption of dispersion compensation technology is essential to ensure reliable, high-quality, and uninterrupted optical signal transmission in modern communication networks, especially in long-distance DWDM systems and other high-capacity transmission environments.