Graded Index Fiber vs. Step Index Fiber

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Multimode fiber Profiles: Step Index vs. Graded.  What are the difference, advantages, and applications in Industry.

Graded Index Fiber vs. Step Index Fiber

Slideshow Tutorial Transcript

  1. Graded Index Fiber vs. Step Index Fiber: Why use graded index fiber?
  2. There are mainly two types of multimode fibers: Step Index and Graded Index fibers • The core on a step index fiber has the same refractive index throughout. • The refractive index on a graded index fiber gradually changes from the center of the core, to where the core meets the cladding. MM Fiber Profiles: Step Index vs Graded
  3. Step Index vs Graded Step Index Fiber Shows a clear distinction between the core and cladding. Graded Index Fiber Shows a gradual change from the core to the cladding.
  4. Issue with Step Index Fiber • Light rays travel somewhat uncontrolled inside a step index fiber. • The blue ray travels directly down the center of the fiber, meaning it travels a shorter distance than the red, green or black ray. • Rays that travel a longer distance will take more time to get to the end of the fiber than rays that travel a shorter distance. • The result is that some of the signal arrives before the rest of the signal. The signal becomes smeared. • This is bad for communication.
  5. Advantage of Graded Index Fiber • Light rays are controlled well within a graded index fiber. • Just as with the step index fiber, certain rays will travel shorter distances than other rays. • However, in a graded index profile fiber the rays will reach the end of the fiber at the same time… • How does this happen?
  6. The speed of light depends on the refractive index of whatever the light is traveling in. – When traveling in space (air, refractive index is 1.0) light travels at 300,000km/s – When traveling in glass, where the refractive index is 1.5, it travels at 200,000km/s – Changing the refractive index changes the speed of light!!! Since the refractive index of the core in a step index fiber is the same, the light rays will all travel the same speed. (Rays that travel a shorter distance will arrive sooner than rays that travel a longer distance.) However, the refractive index of the core in a graded index fiber changes, which means light rays inside the core will travel a different speeds. Optics of Step and Graded Index Fiber
  7. Graded Index Fiber • In a graded index fiber, the refractive index at the center of the fiber is the highest. This means that the light traveling at the center of the fiber travels slower. • Rays that don’t travel in the center will travel faster. As the rays move farther from center, they will travel faster. • The result is that all of the rays in the core will arrive at the end of the fiber at the same time. This is ideal for communication.
  8. Long distance applications • Higher Cost • Less prone to attenuation • Over longer distances, the benefits of the graded fiber are worth the added cost • Short distance applications • Lower cost • More prone to attenuation • Over shorter distances, the disadvantages of step fiber are outweighed by the lower cost of deployment Step vs Graded in the Industry Step Graded
  9. OP1021 Launch Condition Analyzer – Nearfield Scanner profiles the optical power density of the fiber – Farfield Scanner produces plots to determine numerical aperture (NA) and mode field diameter (MFD) – Measure the profile of Step Index and Graded Index fiber
  10. OP940 Insertion Loss & Return Loss.  Fastest IL & RL testing. 3 seconds, using the front panel in Dual ILRL mode – Simultaneous, dual wavelength IL & RL – Easy front panel maintenance – It can be expanded using an external optical switch or the switch can be integrated for multifiber connector testing.

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