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In the more common graded index multimode fiber the light rays are also guided down the fiber in multiple pathways.
But unlike step index fiber, a graded index core contains many layers of glass,
each with a lower index of refraction as you go outward from the axis.
The effect of this grading is that the light rays are speeded up in the outer layers,
to match those rays going the shorter pathway directly down the axis.
Fiber can be identified by the type of paths that the light rays, or modes, travel within the fiber core.
Most OTDRs have an internal ultra physical contact (UPC) spherical polish,
but some reflection-sensitive systems use the angled physical contact (APC) polish.
The result is that a graded index fiber equalizes the propagation times of the various modes
so that data can be sent over a much longer distance and at higher rates before light pulses
start to overlap and become less distinguishable at the receiver end.
There are two basic types of fiber: multimode and single-mode.
Multimode fiber cores may be either step index or graded index.
Step index multimode fiber derives its name from the sharp step like difference in the refractive index of the core and cladding.
For installed spans, linking the OTDR to the span under test requires a hybrid patch cord.
There are two methods of terminating a fiber.
The first method uses a bare-fiber adapter, whichconsists of a plug body that grips the fiber to be
tested. The design of the bare-fiber adapter is such that the fiber can pass completely through the
adapter body and damage the optical port. Because of this, the barefiber adapter must never be
connected directly to the OTDR.Instead, the adapter should be used with a short patch cord and mating
adapter sleeve to isolate damage from the OTDR’s port.
The second method uses a pigtail with a reusable mechanical splice, which allows easy mating of
the fibers to be tested with the OTDR. The Norland reusable mechanical splice has been used for
decades for testing bare fibers.
It features a glass body that internally holds and aligns two fibers.
The splice is filledwith a refractive index matching fluid to reduce reflections.
To make the connection, strip and cleave both of the fibers to be tested,
then insert and center both into the mechanical splice to complete the termination.
