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15 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Week 8

Optical fiber communications

1

Bahria University

Fiber Connectors


Demountable fiber connectors are more difficult to
achieve than optical fiber splices
.



This is
because they must maintain similar tolerance
requirements to splices in order to
couple light
between fibers efficiently, but they must accomplish
it in a removable fashion
.


Also, the connector design must allow for repeated
connection

and
disconnection

without problems
of
fiber alignment
, which may lead to degradation in
the performance of
the transmission
line at the joint.

2

Bahria University

Conti…


There are a large number of
demountable single
-
fiber connectors
, both
commercially available
and
under development, which have insertion losses in
the
range 0.2 to 3 dB
.


Fiber connectors may be separated into two broad
categories
:

1.
Butt
-
jointed connectors

2.
E
xpanded
beam
connectors




Bahria University

3

Conti….


Butt
-
jointed connectors
rely upon alignment of the
two
prepared
fiber ends in close proximity (butted) to each
other so that the fiber core
axes coincide.



Expanded beam connectors
utilize interposed optics at
the joint (i.e. lenses)
in order
to expand the beam from
the transmitting fiber end before reducing it again to a
size compatible
with the receiving fiber end
.



Butt jointed connectors are the most widely used
connector type and a substantial number have been
reported, Now we will discuss the both connectors types
one by one.

Bahria University

4

Types of the Butt Joint Connectors


There are various types of the Butt joint connectors,
we will discuss some of them in detail that are as
follows:

1.
Cylindrical Ferrule connectors

2.
Biconical Ferrule connectors

3.
Double Eccentric connectors

4.
Duplex and multiple fiber connectors

Bahria University

5

Cylindrical Ferrule connector


The basic ferrule connector (sometimes referred to
as a concentric sleeve connector
), which
is perhaps
the simplest optical fiber connector
design shown in
the figure below:

Bahria University

6

Conti….


The two fibers to be connected are permanently
bonded (with epoxy resin)
in metal
plugs known as
ferrules
which have an accurately drilled central hole
in their
end faces
where the stripped (of buffer
coating) fiber is located
.


Within the connector
two ferrules are
placed in an
alignment sleeve
which,
( Each ferrule is for one
fiber) using
accurately machined
components, allows
the fiber ends to be butt jointed. The
ferrules are
held in place via a retaining
mechanism.

Bahria University

7

Alignment Issue


The fiber alignment accuracy
of the basic ferrule
connector is largely dependent
upon the
ferrule hole
into which the fiber is inserted
.


The ceramic materials
possess outstanding
thermal,
mechanical
and

chemical resistance
characteristics
in
comparison with
metals and
plastics.


In
addition, unlike metal and plastic components,
the
ceramic
ferrule material is harder than the optical
fiber and is therefore unaffected by
the grinding
and
polishing
process.


Bahria University

8

Average Losses


Typical average losses for multimode graded index
fiber (i.e.
core/cladding: 50/125
μm
) and single
-
mode fiber (i.e. core/cladding: 9/125
μm
) with the
precision ceramic
ferrule connector are 0.2 and 0.3
dB
respectively
.


Bahria University

9

Duplex and multiple fiber connector


A number of
duplex fiber connector
designs were developed
in order to provide two
-
way communication, but few have
found widespread use to date.


The
media interface connector
plug shown in Figure below
was part of a duplex fiber connector which was developed to
meet the American National Standards Institute (ANSI)
specification for use within
optical fiber LANs.


This connector plug will mate directly with
connectorized

optical LAN components (i.e. transmitters and receivers)

Bahria University

10

Multifiber connection


Multiple
-
fiber connection is obviously advantageous when
interconnecting a large number of fibers. The cylindrical
ferrule connectors can be assembled in housings to form
multiple
-
fiber configurations.


In addition to assembling a number of single
-
fiber
connectors to form a multiple
-
fiber connector, other
examples of multiple
-
fiber connectors have been explored.


Silicon chip arrays were suggested for the jointing of fiber
ribbon cable, which utilizes V
-
grooved silicon chips.

Bahria University

11

Conti….


In this connector,
ribbon fibers
were mounted and bonded
into the V
-
grooves in order to form a plug together with
precision metal guiding rods and coil springs.


Connection is then accomplished by butt jointing the two
pairs of guiding rods in the
slitted

sleeves located in the
adaptor.

Bahria University

12

Commercial used Connectors


Connectors are for both
single mode
and

multimode fibers
.


Multimode optical connectors are generally used within data
communications

(i.e. LANs), transport (i.e. automobiles and aircraft) and
with specific test instruments, while
single
-
mode fiber connectors
are
employed extensively in optical fiber telecommunication systems.


Bahria University

13

Conti….

Bahria University

14


Expanded Beam Connectors


An alternative to connection via direct
butt joints
between optical
fibers is offered by the principle of the
expanded beam.


Fiber connection utilizing this principle is illustrated in Figure below,
which shows a connector consisting of two lenses for collimating
and refocusing the light from one fiber into the other.


The use of these interposed optics makes the achievement of
lateral alignment much less critical than with a butt
-
jointed fiber
connector.

Bahria University

15

Conti…


A similar configuration has been used for
single
-
mode fiber
connection in which the lenses have a
2.5 mm
diameter but
with
antireflection
-
coated lenses
, average losses around 0.7
dB were obtained using single
-
mode.


Bahria University

16

Alternative Arrangement


Figure below shows an expanded beam connector which employs a
molded spherical Lens
.


The fiber is positioned approximately at the focal length of the lens in
order to obtain a collimated beam and
hence minimize lens
-
to
-
lens
longitudinal misalignment effects.



A lens alignment sleeve is used to minimize the effects of angular
misalignment which, together with a ferrule, grommet, spring and
external housing, provides the complete connector structure.



The repeatability of this relatively straightforward lens design was
found to be good, incurring losses of around 0.7 dB.

Bahria University

17

Design Problem





On White Board

Bahria University

18

GRIN Rod Lenses


An
alternative lens geometry

to facilitate efficient beam
expansion and collimation within expanded beam connectors
is that of the graded index (GRIN) rod lens.

Arrangement:


The GRIN
-
rod lens, which arose from developments on graded
index fiber waveguides, comprises a
cylindrical glass rod
typically 0.2 to 2 mm in diameter which exhibits a
parabolic
refractive index profile

with a maximum at the axis similar to
graded index fiber.


Light propagation through the lens is determined by the lens
dimensions and,
because
refractive index is a wavelength
-
dependent
parameter, by the wavelength of the light.



Bahria University

19

Conti….


The GRIN
-
rod lens can produce a
collimated output beam
with a divergent angle α
of between
1
°

and 5
°

from a light
source situated on, or near to, the opposite lens face, as
illustrated in Figure below:






Conversely, it can focus an incoming light beam through a
small area located at the center of the opposite lens face.


Bahria University

20

Conti….


Ray propagation through the GRIN
-
rod lens medium is
approximately governed by the paraxial ray equation:




Furthermore, the refractive index at
r from the optical
axis in
a gradient index medium, may be expressed as:




By using both equations: ( Assuming n = n1 )


Bahria University

21

Source Coupling


Coupling from the light source to the fiber can be very
inefficient, the coupling efficiency is defined as:



=Pf/Ps


Where Pf is the power in the fiber and Ps is the power
emitted by the source.


Several mechanism contribute to the inefficiency produced
due to the source coupling:

1.
Reflection Loss

2.
Area mismatch loss

3.
Numerical Aperture loss

Bahria University

22

Reflection Loss


If an air gap exists between the emitting surface and the fiber
then power
is reflected

at the boundary, that is called the
Fresnel reflection we have already discussed.


If the index matching fluid has been filled between the fiber
and source or if the fiber and source has close enough (in
contact) then this loss will disappear.


4 % of the light will be reflected back towards the source of
light.

Bahria University

23

Area Mismatch loss


If the source area is larger than the fiber core
then some of the power will lost.


The reduction in the efficiency is the ratio of
the core area to the source area (Ac/As).


If the source is smaller than the core then this
loss will disappear.

Bahria University

24

NA Loss


As we know that light incident at the angles larger than the
waveguide’s acceptance angle is not efficiently transmitted.


When coupling light from the source into the fiber, the losses
owing to this effect can be very significant.


The efficiency for this phenomenon is as follows:






As the NA of the fiber is higher this loss will be lower. Like for
NA= 0.24 this loss will be 12.4 while for the NA=0.48 this will
be 6.4 dB.


Bahria University

25

Coupling Efficiency of the Graded
Index Fiber


Coupling efficiency
of the GRIN fiber is lesser as compare to
the SI fiber, because here source is selected according to the
NA of the core axis that is larger as compare ( moving to
cladding interface) other axis
.


Therefore Power is less efficiently coupled into GRIN than into
SI fiber, the coupling efficiency will be:




This equation holds if the LED’s emitting surface has the
same size as the fiber core.


Bahria University

26

Conti….


But if the
emitting surface is smaller
, then most of the light
will be collected over fiber axis where NA is larger, and over all
collection efficiency will be increased then efficiency will be:




Where
ae
, radius of the emitting surface and
af

is the radius
of the fiber core.


Edge emitting and LASER diodes can improve the coupling
efficiency if these are used with the Lenses.


As we know the coupling inefficiency in multimode fibers is
the wide angular distribution( spread) of the light the best
way to reduce this angular spread is to use Lens.


Bahria University

27

Summary


The efficiency of the couplers depends on the
radiation pattern and the NA of the fiber.


Surface emitting LEDs has loss more than 12
dB but that loss can be improved by using
lens.


Laser diodes and the edge emitting LEDs has
narrower radiation pattern than the
surface
emitting LEDs.

Bahria University

28