FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

CHAPTER 3
FIBEROPTIC COMPONENT INTERCONNECTION
Optical power loss (attenuation) has been
drastically reduced in optical fibers since 1970. A
power loss of 0.2 dB per kilometer has been achieved
and the prospect is good for another order of magnitude
improvement to 0.02 dB per kilometer. If this occurs,
approximately 50 kilometers of fiber would exhibit only
a 1 dB leas. One consequenceof this progress inreducing
attenuation in fiber is the increased importance of
the attenuation associated with component-to-fiber,
fiber-to-fiber, and fiber-to-component interconnections.
Little is accomplished if 0.02 dB per kilometer
attenuation is achieved in optical fibers and at the
same time a number of Interconnections are required,
each resulting in an appreciable fraction of a dB loss.
In the case of fiberoptic sensora, where much shorter
lengths of optical fiber are utilized than in communication
systems, such as several hundred meters of fiber
or less per sensor, and where the fiber used in most
cases is not chosen for low 10SS, problems with interconnections
may be less important although connection
insertion losses can still be a large portion of the
total loss in a fiberoptic sensor. Interconnections,
especially singlemode fiber interconnections, are still
required and therefore of importance. The current
state of their development and manufacture will be considered
in the following discussions.
3.1 ‘FIBEROPTIC CONNECTORS AND SPLICES
Some of the uses of interconnections in the
fabrication of fiberoptic sensors include joining
sources and detectors to fiber , splitting the output of
a source (especially laser diodes) among a number of
sensors, beam splitting and combining of light in interferometers,
and providing fiber-to-fiber interconnections.
All interconnections must be designed taking
reflection and consequent insertion losses into account,
with the aim of minimizing the insertion losses.
into which light is being introduced will be designated
the “sink” fiber.
In connectors and splices, power losses fall
into two general classes: intrinsic and extrinsic.
Intrinsic losses are due to variations or imperfections
in the fiber that occur during the manufacturing process
and are not mechanically or externally correctable.
Extrinsic losses are those that occur after the manufacturing
process and are mechanically or externally
correctable, such as incorrect finishing of the fiber
end-surfaces or incorrect mechanical mating of fibers.
Some of these effects are shown in Fig. 3.1. Only the
x
INTRINSIC
(I:C
CORE AREA MISMATCH
NUMERICAL APERTURE MISMATCH
PROFILE MISMATCH
EXTRINSIC
~ ~d
-c
END SEPARATION
+ p’-
ANGULAR MISALIGNMENT
-L /
t
LATERAL OFFSET
Fig. 3.1 Some causes of intrinsic and extrinsic
power losses in optical fiber interconnections.
Interconnections can be grouped into three
classes, namely (1) connectors (remountable interconnections
between fibers or between a fiber and some
component, such’as a source, a detector, or an integrated
chip), (2) splices, (fusion joints or permanent
joints between two fibers or a fiber and some optical
component, and (3) couplers (connections that redistribute
energy between two or more fibers). In the case
of singlemode fibers, splices are relatively easy to
form. Splices and connectors with less than one tenth
dB insertion loss per splice can be achieved. Also,
in the case of singlemode couplers, especially simple
four-port couplers having two input ports and two output
ports, losses of less than a dB have been achieved.
Multimode connectors and couplers are now commerically
available and their singlemode counterparts are just
beginning to become available also. In the case of
multimode connectors, the average loss is about 1 or 2
dB. Goals are set for less than 0.5 dB. For purposes
of discussion, the fiber from which light is emerging
will be designated the ‘source” fiber while the fiber
3-1
fiber cores are shown in these sketches. Intrinsic
effects are shown on the left of Fig. 3.1. If the
core areas of the source and sink fibers are not the
same, the mismatch can result in a power loss. Differences
in numerical aperture (NA) between the two fibers
can also result in losses. For the case of graded
index fibers, discussed earlier, a refractive index
profile mismatch can lead to intrinsic losses. Losses
occur only when directing light from a fiber of larger
core or NA into a fiber of smaller core or NA. In
these cases some of the light from the core of the
source fiber will not be trapped in the core of the
sink fiber. For the reverse, small-to-large core or
NA, losses due to the mismatch do not occur.
Examples of causes of extrinsic losses are
shown in the right column of Fig. 3.1. If the light
input to a sink fiber or output from a source fiber
diverges, such as at cone angles of 15° to 20”, a core
separation will allow some of the light emanating from
the core of the source fiber to miss the core of the