Page 137 - Optical Communications Essentials

Connectors and Splices

Connectors and Splices 127

more serious loss occurs when one inadvertently tries to couple light from a
multimode to a single-mode fiber. For example, if one connects a 62.5-µm mul-
timode fiber to a 9-µm single-mode fiber, the loss from the area mismatch will
be 17dB, or almost 98 percent of the light.

Numerical aperture mismatches. In the case of multimode fibers, if the diame-
ters and the index profiles of two coupled fibers are identical but their axial
numerical apertures are different, then the joint loss from this effect is

  NA  2
 −10 log R for NA NA
L (NA) =    NA   R E
F
E
 (8.9)
 0 for NA NA E
R
Core index mismatches. If the fiber diameters and the axial numerical aper-
tures are the same but the core refractive-index profiles differ in the joined
fibers, then the joint loss is

 α ( α + 2) for α α
R
E

α
L () =  −10 log α ( α + 2) R E (8.10)
R
E
F
 α
 0 for α E
R
This relationship comes about because for α R α E the number of modes that the
receiving fiber can support is less than the number of modes in the emitting fiber.
If α E α R , then the receiving fiber captures all modes from the emitting fiber.
8.4. End-Face Preparation
One of the first steps that must be followed before fibers are connected or
spliced is to prepare the fiber end faces properly. In order not to have light
deflected or scattered at the joint, the fiber ends must be flat and smooth and
must have the proper angle relative to the axis (either perpendicular or at an 8°
angle). Common end preparation techniques include a grinding and polishing
method and a controlled-fracture procedure.

8.4.1. Grinding and polishing
Conventional grinding and polishing techniques can produce a very smooth
surface. Normally this is done in a controlled environment such as a laboratory
or a factory, but polishing machines also are available for use in the field to
attach connectors and to perform emergency repairs. The procedure employed
is to use successively finer abrasives to polish the fiber end face. The end face is
polished with each successive abrasive until the scratches created by the previ-
ous abrasive are replaced by the finer scratches of the present abrasive.

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