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Multimode Fiber (Multi Mode Fiber)

Multimode Fiber

Fibers that carry more than one mode are called multimode fibers. There are two types of multimode fibers. One type is step-index multimode fiber and the other type is graded-index multimode fiber.

The following illustration shows the differences between these two types of multimode fibers on refractive index profile and how they guide light.

Multimode Fiber Index Profile

Index Profile Difference Between Step-Index Multimode Fiber and Graded-Index Multimode Fiber

Multimode Fiber Transmission
Light Transmission in a Step-Index Multimode Fiber and a Graded-Index Multimode Fiber

Step-index multimode fibers are mostly used for imaging and illumination. Graded-index multimode fibers are used for data communications and networks carrying signals moderate distances - typically no more than a couple of kilometers.

Modal-Dispersion and Limit on Step-Index Multimode Fibers’ Bandwidth

Take a look at the illustration for a step-index multimode fiber. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles (which reflect many more times off the core/cladding boundaries as they travel the length of the fiber). The arrival of different modes of the light at different times is called Modal Dispersion.

Multimode Fiber Modal Dispersion

Modal Dispersion is also called modal distortion, multimode dispersion, intermodal distortion, intermodal dispersion, and intermodal delay distortion.

Digital communications use light pulse to transmit signal down the length of the fiber, as we explain in the fiber optic network tutorial. Modal dispersion causes pulses to spread out as they travel along the fiber, the more modes the fiber transmits, the more pulses spread out. This significantly limits the bandwidth of step-index multimode fibers.

For example, a typical step-index multimode fiber with a 50 µm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km.

 

Graded-Index Multimode Fibers Solves the Problem of Modal Dispersion

Graded-index fiber’s refractive index decreases gradually away from its center, finally dropping to the same value as the cladding at the edge of the core. The change in refractive index causes refraction, instead of total internal reflection, which bends light rays back toward the fiber axis as they pass through layers with lower refractive index. No total internal reflection happens because refraction bends light rays back into the fiber axis before they reach the cladding boundary.

Graded-Index Multimode Fiber Compensates Modal Dispersion

Different light modes in a graded-index multimode fiber still follow different lengths along the fiber, as in step-index multimode fiber.  However their speeds differ because the speed of guided light changes with fiber core’s refractive index.

So the farther the light goes from the center of the fiber, the faster its speed. So the speed difference compensate for the longer paths followed by the light rays that go farthest from the center of the fiber. This equalizing of transit times of different modes greatly reduces modal dispersion.

The bandwidth of a typical off-the-shelf graded-index multimode fiber with a 50 µm core may approach 1 GHz·km or more. Multimode graded-index fibers having bandwidths approaching 3 GHz·km have been produced.

But please note that modal dispersion may be considerably reduced, but never completely eliminated.

Laser Optimized Multimode (Multi Mode) Optical Fibers from Corning

Index Profile Type

Corning Fiber Type

Core Diameter
(
mm)

Cladding Diameter
(
mm)

Attenuation
(dB/km)

Bandwidth
(MHz*km)
850nm/1300nm

Working with Laser Type

Optimized Data Rate Over Distance

@850nm

@1300nm

Graded-Index Multimode Fiber

Infinicor 300

62.5

125

2.9

0.6

200/500

850nm VCSEL or 1300 FP Laser

1Gb/s over 300m at 850nm
1Gb/s over 550m at 1300nm

Infinicor CL 1000

62.5

125

2.9

0.6

200/500

850nm VCSEL or 1300 FP Laser

1Gb/s over 500m at 850nm
1Gb/s over 1000m at 1300nm

Infinicor 600

50

125

2.4

0.7

510 @850nm

850nm VCSEL

1Gb/s over 600m at 850nm

Infinicor SXi

50

125

2.4

0.7

850 @850nm

850nm VCSEL

1Gb/s over 750m @850nm
10Gb/s over 150m @850nm

Infinicor SX+

50

125

2.4

0.7

2000 @850nm

850nm VCSEL

1Gb/s over 1000m @850nm
10Gb/s over 300m @850nm

Infinicor eSX+

50

125

2.4

0.7

4700 @850nm

850nm VCSEL

1Gb/s over 1100m @850nm
10Gb/s over 550m @850nm





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