FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

left. The ends of the two fibers are brought together
wittmut necessarily eliminating lateral displacement.
Upon heating, the fiber melts and surface tensions tend
to align the fibers as shown. The graph on the right
side of Fig. 3.12 shows connector loss as a function
of lateral displacement both before and after heating.
The results prior to fusing is a theoretical curve with
experimental reaults superimposed. As can be seen a
significant reduction in loss is realized as a reault
of fusion splicing.
One method of connecting an optical fiber to
an encased laser diode is shown in Fig. 3.13. The laser
prevent damage to the mirrored faces of the laser, a
alight separation must be maintained. For butt coupling,
utilizing an index-matching liquid and no additional
optical elementa, it is us~i to couple less
than 5% of the emitted light into the fiber. From 10
to 20% coupling of the light would be considered excellent.
If lenaes are used to focus the light into the
fiber it is possible for 70% or more of the light to
be coupled into the fiber and trapped. The manner in
which such a lens can be formed from the core of the
fiber is shown in Fig. 3.15. In this case the core
OPTICAL SPOT
EMITTING SOURCE
/ /1 I n
Fig. 3.15
Fabrication of an integral elliptical lens
at the end of an optical fiber.
Fig. 3.13
SOURCE PACKAGE
A laser-to-optical-fiber connection.
surface is displaced from the fiber end because of the
covering that protects the laser face. The resulting
displacement between the fiber and laser source causes
a great deal of the light to miss the fiber due to
spreading. A lens can be used to collect the light and
focus it into the core. The problem is illustrated in
Fig. 3.14. At the top, the spreading (divergent) angle
from the laser and the acceptance angle into the fiber
are ahown. The laser emission is such that light
apreads out in a 20° to 40° cone, while the acceptance
angle (cone) for the fiber is 10° to 14”. Thus, even
if the fiber is brought into actual contact (butt
coupled) with the laser face aa shown at the bottom of
Fig. 3.14, a large portion of the light being emitted
will mlaa the core or go into the core at such an angle
that it will be trans~tted through the core-cladding
interface, into the cladding, and lost. In order to
glass haa a lower glass transition (softening) temperature
than the cladding. Thus, the end of the fiber can
be heated and preasure applied to force a portion of
the core to bulge out aa shown, forming a lens. A
variety of other techniques for connecting lasers to
fibers have been developed and described in technical
literature. These should be reviewed In the event
such an Interconnection is required.
A mechanical device used to align and hold a
fiber and laser is ahown in Fig 3.16. The fiber is
positioned on one anvil in a V-groove and epoxied in
place. The laser sets on a second anvil that also aerves
as a heat aink. The two structures are brought together
and the smooth faces are butted, aligned, and
epoxied. A sleeve is placed over the connector. In
this way a rather small fiber pigtailed laser can be
formed.
ELECTRICAL LEAD
LASER
@ ,oo--
ER
EMISSION PROFILEOF
ALASERIN THE PLANE
PERPENDICULAR TOTHE
JUNCTION
LASER PELLET
+
a.
E:
ACCEPTANCE CONE FORA
TYPICAL STEP-INDEX FIBER
FIBER
LIGHT LAUNCHED ATAN ANGLE GREATER
THANaWILLBE LOST
CUTAWAYA
Fig. 3.16
3.1.1 References
A pigtailed anvil for laser-to-optical
ber connection.
1. Fiberoptic Technology, p. 115, Dec. 1981.
fi-
Fig. 3.14
Loas of optical power in a pigtail connection
between a laser and an optical fiber.
The angle = is equal to or less than the
critical angle.
2-5
3.2 FIBEROPTIC COUPLERS
It is often necessary to divide the beam emitted
from a laser and insert it into two or more fibera.