When making a decision about the type of fiber optic cable that is best suited to their needs, communications engineers face many complex choices. Should the fiber be single-mode or multimode? If it is a multimode fiber, is it a step-index fiber or a graded-index fiber? These are just two of the many questions that need to be answered when choosing the best fiber solution.
In many cases, the best approach is to understand the specific use case or application that the fiber will be used for, and then determine the cable that best fits that use case. How far is the distance required? How important are speed, delay, etc.? Knowing the answers to these questions helps guide the decision-making process.
1. When to choose a graded-index fiber
A graded-index fiber is an optical fiber in which the refractive index of the core decreases as the radial distance (distance from the center of the core) increases. According to the International Telecommunications Union (ITU) standards, this fiber is usually classified as G.651.1.
Graded index fibers provide the best solution in many use cases. Like most other multimode fibers, the gradient index is also best suited for short-distance applications.
Compared with step-index multimode fiber, graded-index fiber provides lower modal dispersion, allowing it to handle higher transmission bandwidth.
2. Use graded-index fiber to reduce modal dispersion
Modal dispersion is the result of multiple modes passing through different paths in the same fiber. When light propagates through multiple modes, some signals are received faster than others. This causes the signal to become wider and the bandwidth to become smaller. Modal dispersion limits the bandwidth of the signal to tens of megahertz. km, that is, when the fiber length is 1km, the signal bandwidth cannot exceed tens of MHz without confusion.
As mentioned above, the multimode fiber can be a graded index or a step-index. Gradient index has many advantages over step-index multimode fibers, including a significant reduction in modal dispersion. By using fibers with a smaller core size, the modal dispersion is further reduced. For example, the 50um in OM3 and OM4 fibers commonly used today are used to transmit data above 10G within a range of hundreds of meters.
3. Suitable for graded-index fiber
Graded-index fiber has the ability to provide high-level bandwidth across short to medium distances. For various use cases, graded-index fiber is the best choice.
- Residential and commercial buildings: Higher bandwidth is achieved through hierarchical index fiber, which is deployed in various buildings, such as offices, apartments or similar larger structures.
- Datacenter: Although today’s data center also uses single-mode fiber, hierarchical indexed multi-mode fiber (especially OM3 and OM4) is used across data centers in a large number of high-bandwidth applications.
- Large ships: Since communication networks are important everywhere, even at sea, ships are an excellent opportunity to use graded-index fiber, because it provides a reliable bandwidth, and gradient index fiber helps the signal to propagate farther.
- Local area network and small campus: In buildings or campuses that have their own local area network, this kind of optical fiber can provide better bandwidth and distance, especially between large buildings around the campus.
