Multimode Step Index

Multimode Step Index

Based on refractive index distribution, multimode fiber (MMF) tin can be classified into 2 categories; graded-index fiber and step-index cobweb. Graded-index and footstep-index fiber have different operating principles and they are considered for different networking scenarios. Ahead in this mail service, we are going to discuss the differences betwixt the two types of multimode fibers and their applications besides.



Step-Index Blazon Multimode Fiber (MMF)

In this type of fiber, a uniform index of refraction is maintained within the core. At the core-cladding interface, the refractive index value almost zeroes resulting in a abrupt reflection of calorie-free beams. This poor refractive index at the core-cladding interface is a manifestation of cladding`s lower refractive index. Step-alphabetize multimode fibers operate on the principle of full reflection and allow traveling of low-cal across the cobweb/cadre axis in a zigzag design.

Low-cal entering the stride-index multimode fiber at different incidence angles volition pass through different paths ensuring effective multimode operation. Each light beam travels at the same speed across the fiber however the time it takes to exit depends on the angle at which information technology entered. Beams inbound at steeper angles will counter more than reflection resulting in exiting later than the light beams entered at less steep angles.

Modal Dispersion in Step-Index MMF

Every bit we accept discussed above, in multimode fiber rays of calorie-free are introduces along the fiber axis with unlike angles. Rays inbound with a shallower bending take a relatively direct path to reach the end of the fiber. On the other manus, rays introduce with sharp angles take more time to leave the fiber because of undergoing more than reflection spells off the cladding/core boundaries. In multimode fibers, this distortion mechanism is represented by a miracle called Modal Dispersion or intermodal distortion.

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In fiber optics-based, digital communications light pulses are used for the manual of signals through the fiber. Modal dispersion causes the signals or pulses to spread in time because optical signals generated at dissimilar modes come with different propagation velocities. In multimode fibers the magnitude of pulse spread out depends upon the number of modes being transmitted. The available bandwidth of the step-index multimode cobweb gets compromised if information technology is subjected to a higher number of modes. Modal dispersion tends to downgrade the functioning of an optical fiber-based communication system considering in example of extensive dispersion overlapping of pulses cannot be prevented resulting in alleviated fleck mistake charge per unit (BER) readings.

Stride-Index Multimode Cobweb Applications

It is quite evident from the earlier discussion that, pace-alphabetize multimode fibers operate with higher dispersion. Cobweb dispersion does non only affect the information manual capability of the cobweb but it also limits the maximum effective communication distance. Considering of these limitations, the stride-index multimode fiber is generally considered for low-speed and brusk-distance applications only. This type of fiber is more suited to applications where college power densities are required such equally industrial and medical power delivery. For data transmission and telecom graded-index fiber is preferred.


Graded-Index Type Multimode Fiber

In this type of fiber college refractive index is maintained at the core axis and information technology decreases gradually as the radial distance (measured from the core centre) increases. In short, the refractive index inside a graded-index fiber subtract away from its center and at the edge of its core refractive index value becomes nearly equal to the refractive index value of the cladding. This gradual alter in refractive index value results in internal refraction instead of reflection. Thus, the light rays curve towards the fiber centrality every bit they move through low refractive index cobweb. In graded-alphabetize type multimode fiber we see almost zero full internal reflection because light beams are forced dorsum to the core axis earlier they strike the cladding. Following are a few notable properties of graded-index multimode cobweb;

  • Depression loss
  • High bandwidth
  • Expansion capability
  • Low sensitivity to macro-bending
  • Low sensitivity to micro-bending
  • Supports longer distances
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Modal Dispersion in Graded-Index MMF

Modal dispersion is a mutual phenomenon related to multimode fiber applications where dissimilar light modes enter the fiber simultaneously but exit asynchronously. In graded-alphabetize multimode fibers, the effect of modal dispersion can significantly be reduced because the speed of calorie-free inside the core varies with the refractive index. Light traveling away from the core of the fiber improves in speed as the refractive alphabetize decreases gradually. This difference in speed compensates for the longer times taken by the light beams that traveled through longer paths. Every bit a result, in graded-index multimode fibers, the manual fourth dimension of various modes remains almost identical resulting in a pregnant reduction of the model dispersion and making the fiber capable of operating at a higher bandwidth.


The effigy above is showing that;

  • Two low-cal rays (fashion A and mode B) entered the fiber with dissimilar angles of incidence
  • Both low-cal rays left the fiber at the same fourth dimension, approximately.
  • Mode A traveled through a longer path and covered more than distance.
  • Way B traveled through a shorter road relatively.
  • The longer distance covered by mode A got compensated by its college average speed
  • Depression refractive alphabetize regions of the fiber enabled the way A to reach better speeds
  • Way B remained closer to the fiber axis and, that’s why it traveled at a lesser speed

Graded-Index Multimode Fiber Applications

Graded-Index MMF has evolved as a default choice for multimode optical fiber applications. Today, this type of fiber is beingness widely utilized in information transmission applications. This type of fiber is considered for high-speed (34 Mb/s to 140 Mb/s) and medium-distance (10km-20km) applications. Step-alphabetize fibers cannot handle that higher bandwidth and altitude because of extensive model dispersion of internal baloney.

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Which type of Multimode Cobweb is Preferred?

About present-day applications use graded-alphabetize when it comes to the preferred blazon of multimode fiber. Unlike manufacturers are producing different graded-alphabetize cobweb grades. Here, information technology is of import to understand that core and cladding size is two of the near important design parameters. Cables with different cadre and cladding size combinations are being manufactured today. However, the 62.5/125 fiber cablevision offers the all-time operation and can be considered for a variety of applications. On the other hand step-index, multimode fiber is considered for brusk-range, low-bandwidth applications simply. In the following chart, you can see a quick comparing between the two types of multimode fibers.



  • Step-index fiber and graded-index fiber are 2 types of multimode fibers
  • Graded-index fiber offers superior transmission quality, higher-bandwidth, and longer altitude
  • Higher modal dispersion limits the bandwidth of the step-index fiber
  • In modern advice applications, graded-index fiber is preferred.
  • Pace-index fiber is cheaper but it cannot offering higher bandwidth nor longer distances

Multimode Step Index