Graded index vs. step index

Why use graded index fiber?

1. Graded Index vs. Step Index:
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 fiberare worth the added
cost
•Short distance applications
•Lower cost
•More prone to attenuation
•Over shorter distances, the disadvantages
of stepfiber are outweighed by the lower
cost of deployment
Step vs Graded in the Industry
StepGraded

9. OP1021
Launch Condition Analyzer
üNearfield Scanner profiles the optical power
density of the fiber
üFarfieldScanner produces plots to determine
numerical aperture (NA)and mode field
diameter (MFD)
üMeasure the profile of Step Index and Graded
Index fiber

10. OP940
Insertion Loss & Return Loss
üFastest IL & RL testing§3 seconds, using the front panel in Dual ILRL mode
üSimultaneous, dual wavelength IL & RL
üEasy front panel maintenance
üIt can be expanded using an external optical
switch or the switch can be integrated for
multifiber connector testing.

11. www.OptoTest.com
1.805.987.1700