UNIT II

Fiber Couplers and Connectors

 Optical fiber connector

 An optical fiber connector is a flexible device that

connects fiber cables requiring a quick connection
and disconnection.

 Optical fibers terminate fiber-optic connections to

fiber equipment or join two fiber connections
without splicing.
 Basic Fiber Optic Link

Transmitter Connector Cable

Splice Cable
Receiver
 OPTICAL FIBER JOINTS
 Technical requirement for both jointing & termination
of transmission media.

 Number of Joints or Connections

 Link length between repeaters
 Continuous length of fiber
 Length of fiber cable practically or conveniently
installed as continuous length

 Repeaters Spacing (A continuously increasing parameter)

 Ranges from  40-60 km at 400 Mbits/s
 100 km at 2.4 Gb/s
 300 km at 1.7-10 Gb/s using
SMDSFs
FIBER JOINTS
• Source- Fiber
• Fiber- Fiber
• Fiber- Detector

 Manufacturers Electro-optical devices

(Sources
supply and Detectors) with fiber optic pigtail
to facilitate direct fiber-fiber connection

 IMPORTANT ASPECT IS FIBER-TO- FIBER CONNECTION

WITH LOW LOSS AND MINIMUM DISTORTION
Two major categories of fiber joints
 FIBER SPLICES: Permanent or Semi-permanent
joints
 Soldering

 FIBER CONNECTORS: Demountable or Removable joints

 Plugs or Sockets

 FIBER COUPLERS: Branching devices

 Splitters or Combiners
 Importance in Networks
 Fiber Alignment
 In any fiber optic communication system, in order to
increase fiber length there is need to joint the length of
fiber.

 The interconnection of fiber causes some loss of optical

power. Different techniques are used to interconnect
fibers.

 A permanent joint of cable is referred to as splice and a

temporary joint can be done with the connector.
 The fraction of energy coupled from one fiber to other
proportional to common mode volume Mcommon . The
fiber-to-fiber coupling efficiency is given as
𝑀 𝑐𝑜𝑚𝑚𝑜𝑛
ƞ𝐹=
𝑀𝐸

where,
ME is number of modes in fiber which launches power
into next fiber.
The fiber-to-fiber coupling loss LF is given as
LF = -10log ηF
 Fiber Alignment

LOSS MECHANISMS AT JOINTS:

1. Fresnel Reflection

 Optical Loss encountered at the interfaces (Even

when two fiber ends are smooth, perpendicular to
fiber axes and perfectly aligned)
 A small proportion of light may be reflected
back into transmitting fiber causing attenuation
at the joint.
 Fresnel Reflection
 Return Loss or reflection loss
 The return loss RL is a measure of the portion of light
that is reflected back to the source at the junction.

 In optics (particularly in fiberoptics) a loss that takes

place at discontinuities of refractive index, especially
at an air-glass interface such as a fiber end face.

 At those interfaces, a fraction of the optical signal is

reflected back toward the source. This reflection
phenomenon is also called "Fresnel reflection loss,"
or simply "Fresnel loss."
 Reflection Loss

 Occurs due to step changes in refractive index

at jointed interface
Glass – Air - Glass
Fraction of light reflected at a single interface:

( )
2
𝑛1 − 𝑛
𝑟=
𝑛1 +𝑛
Where,
n1 : R.I. of core,
n : R.I. of interfacing medium ( = 1 for air)

Loss in decibel due to FR at single interface

LossFres = -10 log10(1-r)

 Can be reduced to a very low level using index
matching fluid
in the gap between jointed fibers.
2. Deviation in Geometrical & Optical Parameters
 All light from one fiber is not transmitted to another fiber;
Because of mismatch of mechanical dimension

Three major cases :

a) Core mismatch
b) NA mismatch
c) Index Profile
Intrinsic Losses:
Losses due to:
• Fresnel Reflection
• Deviation in Geometrical & Optical parameters

 Minimized using fibers manufactured with lowest

tolerance i.e.(same fiber)
Extrinsic Losses:
 Losses due to some imperfection in splicing
 Caused by Misalignment

Mechanical Misalignment
 The diameter of fiber is few micrometer hence the
microscopic alignment is required.
 If the radiation cone of emitting fiber does not match
the acceptance cone of receiving fiber, radiation loss
takes place.
 The magnitude of radiation loss depends on the degree
of misalignment.
 Different types of mechanical misalignments are

1. Lateral misalignment

2. Longitudinal misalignment

3. Angular misalignment
1. Lateral misalignment:
 Lateral or axial misalignment occurs when the axes
of two fibers are separated by distance ‘d’.

2. Longitudinal misalignment:
 Longitudinal misalignment occurs when fibers have
same axes but their end faces are separated by
distance ‘S’.

3. Angular misalignment:
 Angular misalignment occurs when fiber axes and
fiber end faces are no longer parallel.
 There is an angle ‘θ’ between fiber end faces.
 The axial or lateral misalignment is most common in
practice causing considerable power loss.
 The optical power coupled is proportional to common
area of two fiber cores.
 The axial offset reduces the common core area of two
fiber end faces as shown in Fig.
(a) Insertion loss due to lateral and longitudinal misalignment for a 50 µm core
diameter graded index (GI) fiber.
(b) Insertion loss due to angular misalignment for joints in two Multi mode step
index fiber (MMSI) fibers with NA of 0.22 and 0.3.
 Fiber Splices
 A permanent or semi permanent connection between
two individual optical fibers is known as fiber splice.

 And the process of joining two fibers is called as

splicing.

 Typically, a splice is used outside the buildings and

connectors are used to join the cables within the
buildings.

 Splices offer lower attenuation and lower back

reflection than connectors and are less expensive.
Types of Splicing
There are two main types of splicing
i) Fusion splicing
ii) Mechanical splicing / V groove
 Fusion Splicing or Welding

 Fusion splicing involves butting two cleaned fiber end

faces and heating them until they melt together or fuse.

 Fusion splicing is normally done with a fusion splicer

that controls the alignment of the two fibers to keep
losses as low as 0.05 dB.

 Fiber ends are first pre aligned and butted together

under a microscope with micromanipulators.

 The butted joint is heated with electric arc or laser pulse

to melt the fiber ends so can be bonded together.
Fusion Splicers
 Drawback: Fiber get weakened near splice (30%)
 Fiber fracture occurs near the heat-affected zone adjacent
to
the fused joint.
 Splice be packaged to reduce tensile loading
Protection of Joints

Protection Sleeves for spliced

fibers

Fiber joint enclosures

Underground fiber splice tray

Mechanical Splicing / V Groove

 Mechanical splices join two fibers together by

clamping them with a structure or by epoxying the
fibers together.

 Mechanical splices may have a slightly higher loss

and back reflection.

 These can be reduced by inserting index matching gel.

 V groove mechanical splicing provides a temporary

joint i.e., fibers can be disassembled if required.
 Mechanical Splicing
 Uses accurately produced rigid alignment tubes into which
the prepared fiber ends are permanently bonded.

 Techniques for tube splicing of optical fibers:

(a) Snug Tube Splice
(b) Loose Tube Splice; Square Cross section
Capillary
 Comparison of Two Approaches

•Snug Tube Splices Loose Tube Splices

• Exhibits problems with • Avoids the critical
capillary tolerance
requirements tolerance requirements.

• Losses  up to 0.5 dB with • Losses  0.1 dB with loose

Snug tube splice (ceramic tube splice using MMGI
capillaries) using MMGI fibers.
and SM fibers.
Groove Splices
 Use of grooves to secure the fibers to be jointed
 better alignment to the prepared fiber ends.

V-groove splices

 Insertion losses  0.1 dB using jigs for producing V-groove splice.

 Spring Groove Splice
 Utilizes a bracket containing two cylindrical pins, which serve
as an alignment guide for two prepared fibers.
 An elastic element (a spring) used to press the fibers into groove
and maintain alignment of fiber ends.

Mean Losses  0.05 dB with

MMGI Fibers.

 Practically used in
Italy.

Springroove Splice : (a) Expanded overview

(b) Cross-section Schematic
 Connectors
Connectors are mechanisms or techniques used to join an
optical fiber to another fiber or to a fiber optic component.

Three different types of connectors are used for connecting

fiber optic cables.

1. Subscriber Channel (SC) connector

2. Straight Tip (ST) connector
3. MT-RJ connector
Subscriber Channel (SC) connector:

SC connectors are general purpose connections. It has push-

pull type locking system.
Straight Tip (ST) connector

ST connectors are most suited for networking devices.

It is more reliable than SC connector.
ST connector has bayonet type locking system.
MT-RJ connector is similar to RJ45 connector.
Principles of Good Connector Design
1. Low coupling loss.
2. Inter-changeability – No variation is loss whenever a
connector is applied to a fiber.
3. Ease of assembly.
4. Low environmental sensitivity.
5. Low cost – The connector should be in expensive
also the tooling required for fitting.
6. Reliable operation.
7. Ease of connection.
8. Repeatability – Connection and reconnection many
times without an increase in loss.
 Connectors use variety of techniques for coupling
such as screw on, bayonet-mount, push-pull
configurations, butt joint and expanded beam fiber
connectors.
 Fiber is epoxied into precision hole and ferrules are
used for each fiber.
 The fibers are secured in a precision alignment sleeve.
 Butt joints are used for single mode as well as for
multimode fiber systems.
Two commonly used butt-joint alignment designs are:
1. Straight-Sleeve.
2. Tapered-Sleeve/Biconical.

Straight-Sleeve
In straight sleeve mechanism, the length of the sleeve
and guided ferrules determines the end separation of
two fibers.
Tapered-Sleeve/Biconical

 In tapered sleeve or biconical connector mechanism, a

tapered sleeve is used to accommodate tapered
ferrules.
 The fiber end separations are determined by sleeve
length and guide rings.
Connector Return Loss

 At the connection point of optical link low

reflectance levels are desired since the optical
reflections can be source of unwanted feed back into
the laser cavity.

 Due to this unwanted feedback the optical frequency

response may degrade, also it generates internal noise
within the source affecting overall system
performance.
The return loss for the index-matched gap region is given
by,
{ [
𝑅 𝐿 =−10 𝑙𝑜𝑔 2 𝑅 1 −𝑐𝑜𝑠 (
4𝜋𝑛 𝑑
𝜆
1
)]}
Where,
d is the separation between fiber ends.
n1 is index-matching material.
R is reflectivity constant.
 FIBER CONNECTORS
 Demountable fiber connectors
 More difficult to achieve than fiber splices

 Must maintain similar tolerance requirements, but in a removable fashion.

 Must allow for repeated connection and disconnection without problems

for fiber alignment - without degradation in performance.

 Must protect the fiber ends from damage – due to handling

 Must be insensitive to environmental factors ( e.g. moisture & dust)

 Must cope with tensile load on the cable and can be fitted with relative
ease.

 Should ideally be a low cost component,

 Three Major Parts:
 Fiber Termination : protects and locates the fiber ends

 Fiber end Alignment : provide optimum optical

coupling

 Outer shell : maintains the connection and fiber

alignment, protects the fiber ends from the environment and
provides adequate strength at the joint.

 Losses in the range 0.2 to 0.3

dB
A. Butt Jointed Connectors

 Alignment of two prepared fiber ends in close

proximity (butted) to each other so that the fiber axes
coincide.
B. Expanded-Beam Connectors
 Utilize interposed optics at the joint in order to expand
the beam from the transmitting fiber end before reducing
it again to a size compatible with the receiving fiber end.
Cylindrical Ferrule Connector
 Glass Ferrules with central drilled
hole
 Concentric alignment sleeve

• Preparation of fiber
ends before fixing
the ferrules

• Insertion Losses  1 to 2
dB with MMSIF
• Watch jewel for close
end approach and
tolerance requirement

Ferrule Connectors: (a) structure of a basic ferrule connector;

(b) structure of a watch jewel connector ferrule.
Ceramic Capillary Ferrules
 Ferrules made from ceramic material
 End preparation after fixing ceramic ferrules

 Outstanding
• Thermal,
• Mechani
cal
• Chemica
l
 Average Losses
Resistan
 0.2cedB with MMGI
ST series multimode fiber connector using  0.3 dB with SMF
ceramic capillary ferrules.
 Commonly Used Connectors

FC Connectors ST Connectors SC Connector

 DIN Connectors MTRJ Connector
(Spring loaded free-floating
Zirconia ceramic ferrule)

SMA
Connector

Biconic Connectors D4 Connectors

Biconical Connectors
 Widely used as part of jumper cable
 Fiber end faces polished after plug attachment

Cross-section of biconical connector

 Mean insertion losses  0.21 dB with connectors of

50m diameter GI fibers.
Double Eccentric Connector
 Does not rely on a concentric sleeve approach
 Consists of two eccentric cylinders within outer plug.

 An active assembly adjustable,

allowing close alignment of
fiber ends
 Operation performed under
inspection microscope or peak
optical adjustment.
Connector Structure

 Mean insertion loss  0.48 dB with MMGIFs reduces

to
2. dB with index matching gel.
 Also used with SMFs giving losses 0.46 dB.
Duplex Fiber Connector
 Developed to provide two way communications
 Uses ferrules of different types

 Mostly used in LANs

 Commercially

available for use in

FDDI  loss of
0.6 dB.
Media interface plug with
DFC
Multiple Fiber Connectors
 Utilizes V grooved Silicon chips for mounting

 Metal guiding rods and

coil springs for precise
alignment

 Average Losses
•  0.8 dB with MMFs
• Reduced to 0.4
dB using index
matching fluids

(a) Fiber ribbon connector (b) SM Ten

fiber connector.
 EXPANDED BEAM CONNECTORS
 Collimating and refocusing the light from one fiber into
the other.

Principle of Operation

 Very attractive for multi-fiber connections and edge

connections for PCBs
Lens Coupled Expanded beam connectors
 Utilize spherical micro-lenses ( 50 m ) for beam expansion
and reduction

(a) Two Micro lenses connector (b) Moulded plastic lens connector

 Average Loss  1 dB, reduced to 0.7 dB with AR

coating
 GRIN-rod Lenses
 An alternative lens geometry to facilitate efficient
beam expansion and collimation

 Arose from development of GI fiber waveguides

 A cylindrical glass rod 0.5 to 2 mm in diameter with parabolic

refractive index profile.

 Light propagation is determined by the lens dimension and wavelength

of the light.

 Produce a collimated output beam with divergent angle of 1o to 5o from
light source.onto the opposite face of lens
GRIN-rod Lenses

• Ray propagation determined by paraxial ray equation

d 2r 1

dn dz 2

n dr
 Solution is
Various fractional pitch GRIN-rod lenses

 0.25, 0.23, 0.29

etc.
 SELFOC from
Sheet
NipponGlass Co. Ltd.
 Losses  1 dB

 Average Losses
 0.2 dB with MMGI
 0.3 dB with SMF
Fiber Reels, Connectors & Patch cords

Adapters

Connectors Patch cords

Fiber Splicing and Connectorization kits