The notes gives the reader a very simplified approach on fiber connector and coupler.I hope students will find it very easy to fallow.

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4. FIBER COUPLERS AND CONNECTORS
INTRODUCTION:
1.Jointing and termination are the two common requirements of any optical fiber link.
2.Number of joints depends upon :
a. A continuous length of fiber cable manufacturable.
b. Length of cable conveniently installable as continuous section of an optical fiber link.
3.Repeater spacing is also an important parameter.
Ex: A fully operational 2.49bits/sec system operating within a 100km repeater spacing has
recently been reported
4. Two major categories of fiber joints:
a. Fiber splices: Semi permanent or permanent joints’. Find major uses, analogs to electrical
soldered joints
b. Demountable fiber connectors or simple connectors: Removable joints which allow easy,
fast ,manual coupling and uncoupling of fibers, analogs to electrical plug and socket.
5. Fiber couplers: “Branching devices that split all the light from a main fiber into two or more
fibers
6.Optical loss associated with the connection is a crucial aspect
7.Fiber alignment and joint loss: Optical losses are encountered at fiber to fiber interface even
when the two jointed fiber ends are smooth and perpendicular to fiber axes and the two fiber
axes are perfectly aligned
FRESNEL REFLECTION:
“A small part of light propagating, may be reflected back into tx fiber causing attenuation at the
joint this phenomenon is known as FRESNEL REFLECTION”
Fresnel reflection formula gives an estimation of partial reflection of light txed through the
interface
R= (n1-n) 2
/ (n1+n) 2

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n1=R.I of fiber core
n=R.I of medium between the two jointed fibers
 Misalignment of the jointed fibers is potentially greater source of loss
 Loss in decibels due to Fresnel reflection is given by, Loss(fres) = -10log(1-R)
3. Geometrical and optical parameters of the two jointed optical fibers will contributes to
total attenuation.
4. Thus Inherent connection problems when jointing two fibers are:
a. different core and/or cladding diameters
b. different NA and/or RI differences
c. different RI profiles
d. fiber faults
INTRINSIC JOINT LOSSES: “Losses caused by above factors together with those of Fresnel
reflection”.
1.Quality of fiber alignment is again a problem
2.Possible types of misalignment (which may occur when jointing two compatible optical
fibers) are shown in figure below:

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(a) Occurs due to separation between two fibers
(b) Occurs due to offset between fiber axis
(c) Occurs due to angle between the core axis
Figure below shows loss occurring due to displacement and angle of separation between
two fibers.
FIBER SPLICES: “Permanent joint formed between two individual optical fibers (in field or
factory)”.
1.Divided into two broad categories (depending upon splicing technique):
a. Fusion splicing (or welding)
b. Mechanical splicing
2.Requirement for splicing: Fiber should have smooth and square end faces.

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a. Fusion splicing: “Accomplished by applying localized heating (as shown in fig) ex: By a
flame or an electric arc”,at the interface between two butted joints , prealigned fiber ends
causing them to soften and fuse.
b. Mechanical splicing:
In mechanical splicing, “Fiber are held in alignment by mechanical means (ex: using tubes
around fiber ends)
1.Prepared ends are permanently bounded into a rigid alignment tube (as shown in fig)
Ex: Glass or ceramic capillary

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As can be seen from figure above:
a. Prepared ends are first butted together in a V-shape groove
b.V-shaped channel can be either a ground cover plate silicon plastic or metal substrate.The
loss that may occure depends up on the size of fiber core and position of core relative to
center of fiber

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Single Mode Fiber Joints: Losses in single mode fiber joint can become a serious problem.
Expression for losses in single-mode fiber is deduced as fallows.
Loss(T1) due to lateral offset(y) [in the absence of angular misalignment] is,
T1=2.17(y/w)2
dB
w= normalized spot size of fundamental mode.
Ex: The normalized spot size for the fundamental mode (LP)01 mode (HE mode) is:
W= a ( 0.65+1.62 v
− 3/2
+2.88 v
− 6
)/ 2
−3/2
W=spot size (in micro meter)
a=fiber cone radius
v=normalized frequency for fiber
y=lateral offset
1.Insertion loss (Ta) caused by an angular misalignment (Ta) at a joint in a single mode fiber.
Ta=2.17(wn1v)/(aNA)2dB
N1 = fiber core RI , NA if fiber
Ta=angular misalignment (in radian)
Tot insertion loss =T1+Ta
Ex: If v=2.4, n1=1.46, a=4mm, NA=0.1, estimate total insertion loss, take lateral
misalignment=1mm , angular misalignment = 1 degree.
Ta=0.71dB Take ϴ =П/180

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Lensing schemes for coupling improvement:According to fundamental energy and radiance
conservation principle (or law of brightness), max power can be coupled into fiber-core, from
source, if source emitting area greater than fiber core area,results in loss of optical power.
Therefore to improve source to fiber power coupling efficiency micro lens are used. These
lenses are responsible for magnifying emitting area of source. Figure below shows some of the
possible lensing scheme

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Fiber connectors: Fiber Connectors are removable joints, used for simple channel-to-channel
connections and also from channel to multichannel connections. It is usually applied in harsh
military environment.
Some of the principle requirements of a good connectors design are:
1.Low coupling loss: The connectors assembly must maintain strict alignment tolerances to
assure low mating losses.
2.Interchangeability:Connectors of the same type must be compatible from one manufacture to
another.
3.Ease of assembly:A service technician should readily be able to install the connector in a field
environment also connection losses should be insensitive to the assemblying skills.
4.Low environmental sensitivity: Effect of temperature, dust and moisture should be minimal
5.Low cost and reliability: The connector must have precision and low cost.
6.Ease of connection: The connection should be fairly simple to mate and demate.
Connector design types: Most commonly used designs are: 1.Twist_on or snap_on
2.screw_on
Basic coupling mechanisms used in these connectors belong to either of the two classes:
1.Butt joint
2.Expended beam
1.But joint class:In this class of connector a guide ring and sleeve length are responsible for
connecting two fibers.They are classified as shown below along with relevant diagram.
Straight sleeve: Here the length of the sleeve and guide ring on the ferrules determine the end
separation of the fibers ,as shown in figure.
Tapered sleeve: Biconical connectors uses a tapered sleeve to accept and guide tapered
ferrules,as shown in figure.

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Expended beam fiber connector:This employs lenses on the ends of the fibers the lenses
either collimate the light emerging from the transmitting fiber or focuses the expanded beam
onto the core of the receiving fiber. The fiber to lens distance is equal to the focal length of the
lens.