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The critical role of testing for chromatic dispersion in fiber characterization

Speed and accuracy are two key requirements of fiber optic networks carrying mission-critical communications, whether connecting communities or spanning continents. Subscribers require faster FTTH links and access to 5G mobile connectivity for telehealth, autonomous vehicles, video conferencing, and other bandwidth-intensive applications. With the massive proliferation of data centers and fast-moving advancements in AI and machine learning, coupled with faster network speeds and feeds to 800G and beyond, full fiber characterization becomes critical.  

According to the ITU-T G.650.3 standard, Optical Time Domain Reflectometer (OTDR), Optical Loss Test Set (OLTS), and chromatic dispersion (CD) and polarization mode dispersion (PMD) testing is required to perform full fiber characterization and ensure high network performance. Managing CD is therefore key to ensuring the integrity and efficiency of transmission. 

While CD is a natural characteristic of all optical fibers whereby there is expansion of the broadband spectrum pulse over long distance, it becomes problematic for fibers carrying data rates over 10 Gbps, per ITU-T G.650.3. Chromatic dispersion can significantly impact signal quality, especially in high-speed communication systems, and testing is the key to managing this challenge. 

What is Chromatic Dispersion? 

When pulses of light comprising various wavelengths travelling through an optical fiber are subject to CD, the spreading out of light can lead to overlapping pulses, distortion, and ultimately, degradation of the transmitted signal. There are two forms of chromatic dispersion: material dispersion and waveguide dispersion.  

Material dispersion is a factor inherent in all fiber types, causing different wavelengths to travel at varying speeds, with the end result that wavelengths arrive at the far end transceiver at different times.  

Waveguide dispersion results from the waveguide structure of the fiber, with the optical signal travelling in both the core and cladding of the fiber which have different refractive indices. That causes mode field diameter to vary, and results in variation in the signal velocity per wavelength.    

Retaining some level of CD is important to avoid other non-linear effects from happening, so zero CD is not desirable. But managing CD to acceptable levels to avoid negative impact on the integrity of signal and the quality of service is imperative. 

What is the impact of fiber types on CD ?

While as mentioned earlier, chromatic dispersion is a natural characteristic inherent in any optical fiber, fiber type plays a crucial role in managing CD. Network operators can choose “natural” dispersion fiber, or fibers where the dispersion curve has been shifted to reduce impact of CD within certain wavelength ranges.  

The most frequently deployed optical fiber in today’s networks is standard ITU-T G.652 optical fiber with natural dispersion. ITU-T G-653 zero dispersion-shifted fibers do not support DWDM transmission, and G.655 non zero dispersion-shifted fibers have lower CD but are optimized for long distance and are more expensive. 

Ultimately, it’s important for operators to know what type of fiber is present in their network. If most of the fiber is standard G.652 but some is another fiber type, then without visibility to chromatic dispersion in all links, quality of service could be jeopardized. 

How to address the challenges of CD? 

In networks where the type of fiber is known and transceivers can be aligned accordingly, CD testing may seem less critical. However, in heterogeneous networks where mixed fiber types are prevalent, comprehensive fiber characterization and testing become imperative.  

Using random transceivers on unknown fiber types can lead to errors and service interruptions. Non-linear effects can occur when long-reach coherent transceivers and amplifiers are deployed on dispersion-shifted fiber, for example.  

EXFO’s FTBx-570 single-ended dispersion analyzer emerges as a game-changer in this scenario. This innovative solution is the only such device in the industry providing fast, single-ended CD and PMD testing as part of a network operator’s full fiber characterization routine.  

With support for network speeds up to 800G and beyond, the FTBx-570 ensures service integrity across metro, core, DCI, and cellular/mobile xhaul networks. Its single-ended testing capability empowers technicians to efficiently characterize multiple optical links in less than 30 seconds from a single location, reducing deployment time and operational costs.  

Looking to the future

Chromatic dispersion remains a challenge that must be addressed to ensure the reliability and efficiency of high-speed communications systems. Fiber characterization and testing are key to addressing the complexities of chromatic dispersion, providing technicians and engineers with the insight needed to design, deploy, and maintain the infrastructure that carries the world’s mission-critical communications. As networks evolve and expand, EXFO will continue to innovate and bring solutions to market to stay a step ahead to support the adoption of advanced technologies.  

Stay tuned for our blog about PMD! 

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