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Singlemode fiber and multimode fiber

2022-09-05
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There are two types of fiber optic cabling - multimode and singlemode. As most of you probably know, multimode cabling has shorter lengths than singlemode cabling, so singlemode is suitable for outdoor long-haul fiber optic applications, while multimode is the primary choice for data center and interior building applications.

 

However, due to the inherent high bandwidth capabilities of singlemode fiber, its popularity for shorter distance applications is also increasing, and more and more technicians are faced with the problem of installing both singlemode and multimode fiber. But we found that not everyone understands the technical differences between these two fiber types. Let's take a closer look next.

 

Singlemode fiber and multimode fiber

What is the pattern?

In the field of fiber optic data transmission, the term "mode" is used to describe how an optical signal propagates within the fiber's glass core—that is, a mode is the path that light travels. So, in single-mode light, light travels along one path; in multimode fiber, light travels in multiple paths.

 

Think of it this way: as you descend from the top of a water slide, you're bouncing and sliding down between the side flaps of the slide. Each angle in the glide process is a pattern. Light propagates in the core of a multimode fiber in the same way as described above. Light hits the glass at an angle and is reflected back while propagating along the length of the fiber core. Why doesn't the light come out of the fiber core? First, the light hits the glass at a small angle, so that the glass reflects the light like a mirror. Second, there is a layer of cladding on the outside of the core. To keep the light inside the core, the cladding has different refractive index properties, a technical parameter that determines how much light is reflected or refracted as it strikes the material.

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In single-mode fiber, by contrast, light travels in a straight line, and because the core size of single-mode fiber is smaller (about one-tenth the size of a multimode fiber core), the light does not bounce.

 

Bandwidth Limit Latency

Why does single-mode fiber support higher bandwidth and longer distances? Sending light in a single mode eliminates differential mode delay (DMD), which is a major factor limiting the bandwidth of multimode fiber.

 

When propagating in multiple modes in a multimode fiber, some light travels along the center of the fiber, while other light travels along paths close to the core cladding. Propagating modes at the outer edges are called higher-order modes, and those propagating near the center of the core are called lower-order modes. Higher-order and lower-order modes have different propagation velocities, and DMD is the difference in propagation time.

 

The smaller the DMD, the less spread of the light pulse over time and the higher the bandwidth. The greater the time difference between the pulses, the receiver may not be able to distinguish the pulses correctly. DMD is directly related to distance - it increases with fiber length. This is why multimode fiber has much shorter distance requirements than single mode fiber, which can be up to 500 meters long, while single mode fiber can be up to 10 kilometers in length.

 

Fiber defects are also a contributor to DMD, and fiber manufacturers have mastered limiting DMD by carefully optimizing the fiber's refractive index profile. Mode refraction not only occurs at the junction between the core and the cladding, but multimode fibers use a graded index profile where the index of refraction changes continuously from the center of the core to the boundary between the core and the cladding. This creates a parabolic (i.e., symmetrical curve) path that causes lower-order modes to travel slower over shorter distances near the fiber core, while higher-order modes travel faster over longer distances closer to the edge of the fiber. This minimizes the time delay between pulses, resulting in lower DMD and higher bandwidth.

 

more significant difference

The more significant differences between multimode and singlemode include cable color, light source, and testing. Singlemode is almost always yellow, while multimode is usually light green. Different types of multimode may also have different colors - OM3 is almost all light green, OM4 multimode is sometimes in a pink color called Erika Violet to help distinguish it from OM3, and the latest generation Mode fiber OM5 is gray-green.

 

Another key difference is the cost associated with the light source and transmission equipment. Single-mode fiber requires a laser source with a narrow spectral width, so the cost of the receiver is higher. Compared with multimode fiber optic cable, the price of single mode fiber optic cable itself is lower, but the cost of single mode fiber optic receiver is 1.5 to 4 times that of multimode receiver.

 

For multimode and singlemode test methods, it is important to understand that the two fiber types cannot be mixed and the access line must match the type of fiber being tested. Testing multimode fiber also requires an Encircled Flux (EF) test to indicate how much light is being injected into the cable under test. EF testing limits the number of firing patterns to reduce variability and achieve accurate, repeatable test results.

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