Unit4
10EC72
Chapter overview:
For long distance transmission, optical link required repeaters, couplers, connectors, Splice
Example:
A transatlantic optical fiber system operating over a distance of 6000 km employs 120 optical repeaters with a 50 km spacing and is capable of carrying 32 wavelength division multiplexed channels each at a transmission rate of 10 Gbit s−1
Issues with link design:
Fiber misalignment and joint loss
Single mode fiber joints Fiber splices
Coupling Efficiency
A measure of optical power emitted from the source
that will be coupled to a fiber.
Where,
=Power coupled to the fiber
= Power emitted from the source
𝜂 =
𝑃
𝐹𝑃
𝑆Fiber alignment and joint loss
A permanent bond is generally referred to as Splice whereas demountable joint is known as connector
Three fundamental types of misalignments:
Lateral : Axes are separated by distance ‘d’
Axial misalignment
Axes are offset separated by ‘d’ at the common junction.
Common area between the circles,
Loss due to Fresnel reflection
Reflection of light at the Glass-air-Glass interface
Magnitude of this reflection is given by
Where, r is the fraction of the light reflected at a single interface,
n1 is the refractive index of the fiber core and n is the refractive index of the medium between the two jointed fibers (i.e. for air n = 1)
Loss due to Fresnel reflection is given by:
Fresnel loss can be eliminated using Index-matching fluid.
Extrinsic joint losses
Different core and/or cladding diameters;
Different numerical apertures and/or relative refractive index differences;
Different refractive index profiles;
Fiber faults
Fresnel loss due to reflection.
Intrinsic joint loss
Insertion loss characteristics for jointed optical fibers with various types of misalignment
(a) insertion loss due to lateral and
longitudinal misalignment for a graded index fiber of 50 μm core diameter
(b) insertion loss due to angular misalignment for joints in two multimode step index fibers with numerical apertures of 0.22 and 0.3.
Single-mode fiber joints
In single mode fiber, mode distribution is nearly
Gaussian in nature.
Loss due to lateral offset is given by:
Where,
= Normalized spot size =
Loss due to angular misalignment θ is given by
Total loss,
𝑇1 = 2.17 𝑑 𝑤𝑛
𝑤𝑛 𝑤0
2 𝑤𝑛 = 𝑎 0.65 + 1.62𝑉
−32
+ 2.88𝑉−6 2
Single mode fiber joint loss (contd..)
If we neglect all extrinsic misalignments (lateral,
angular, longitudinal, Fresnel loss) then the loss due to
MFD differences can be expressed as
and are the spot size of the transmitting and
receiving fiber respectively.
Fiber Splices
(A permanent or semi-permanent joint of two optical fiber)
Splicing types
A permanent or semi-permanent joint formed between two optical fiber in factory or field is called splice
Avg. splice loss is 0.1-0.2 dB per splice
Splice Types
Fusion Splice Mechanical Splice
(Permanent Joint using flame or electric arc)
Tube Splice Groove splice
Fusion Splicing
Self alignment problem after splicing
Possible drawback of fusion splicing is
Fiber may weaken at the joint point
Mechanical Splicing
Tube Splicing:
Loss
0.1-0.5 dB
Transparent adhesive is inserted before connecting
two fibers.
Elastic tube splicing
Used for splicing single mode fiber as lateral misalignment is a serious factor in this case.
Groove Splicing
Used to secure the fiber joints
Simple example is V-groove, where the fiber ends are
inserted. Substrate is V-shaped.
Fiber Connectors
(Used at Tx and Rx in a removable fashion)
Desirable features of Fiber Connectors
Similar tolerance requirements. Must accomplish in a removable fashion.
Repeated connection and disconnection without problems in fiber alignments.
Protect the fiber end faces from damage and environmental factors.
Low cost component.
Compatible with all manufacturer fibers.
In short,
Low coupling loss Interchangeability
Ease of assembly
Low environmental sensitivity
Low cost and reliable construction
Connector types
Connector loss is typically 0.2 to 3 dB per connector. Connector Types
Butt-joint Connectors Expanded beam connectors
Butt-joint Connectors
Employ a metal, ceramic or molted plastic ferrule for each fiber and a precision sleeve into which the ferrule is fit.
Used for both single and multimode fiber.
Expanded-beam Connectors
Employs lenses on the ends of the fibers.
Light is either collimated or focused from transmitting
to receiving fibers.
Fiber to lens distance is equal to the focal length of the
lens.
Fiber Coupler
(Distributes light from a main fiber into one or more branch fibers) Also known as Directional Coupler
Basic features
Used excessively in LAN, WAN and telecommunication areas
.
Optical fiber couplers are often passive devices in which the power transfer takes place either
(a) through the fiber core cross-section by butt jointing the fibers or by using
some form of imaging optics between the fibers (core interaction type); or
Coupler Types
Multiport optical couplers are sub-divided into three broad categories.
Four port couplers are also known as 2×2 couplers.
Coupler Types
Coupler types:
Three-Four port couplers
Lateral offset method: Light from one fiber can be coupled to another fiber based on the degree of overlap.
Semitransparent mirror method: Incorporate a beam splitter element between the fibers.
Four-port couplers
Most common method of manufacturing couplers is fused biconical tapper technique (FBT).
Fibers are generally twisted together and spot fuse under tension so that the fused section is elongated to form a bi-conical tapper structure.
Exercise for you….
Star Couplers
Star couplers can couple power from 1 to N or M to N connections.
Mainly two types:
WDM coupler
Mainly it uses GRIN-rod lens to direct the incoming ray.
