OPTICAL COUPLER/SPLITTER

Prepared by : -Yogesh Aggarwal

892640

What exactly is Optical Coupler ? Device having one or more inputs &

several outputs. Light enter from one input can appear

on one or more output. The output thus obtained can vary

according to power distribution which in turn depends upon wavelength and polarisation

Pretty simple, isn’t it ??

Need for fiber couplers

For combining optical signals For dividing optical signals from one

branch to several branches Can be used as the optical switches Very useful for distribution systems

including data buses, LAN, computer networks and many more . . .

How power transfer can take place ? Two types: - a) Core interaction type

b) Surface interaction type

Core interaction type

Surface interaction type

Different types of couplers

Three Port Splitter Four port Coupler

Three Port Combiner

N ports

M or N Ports

Star Coupler

Methods of fabricating Couplers A wide variety techniques for fabrication have been

developed.

FBT is most widely used.

Fiber Couplers

Lateral offset technique

Fused Biconical Taper (FBT)

Mirror based beam splitters

Semi-transparent mirror method Micro-lens and mirror

method

Lateral Offset method

Light from input fiber is coupled to the output fibers according to the degree of overlapping of fiber end faces.

Suited for use with SMI fiber. Not all the input power is coupled to output.

Semi-Transparent Mirror Method

The input power is splitting in any desired ratio between the reflected and transmitted beam according to the property of intervening mirror

Can be used for MM and SM fiber.

Semi-transparent mirror

GRIN rod lens based micro-optic coupler Comprises of 2 quarter pitched lenses

with semi-transparent mirror between them.

Light from F1 collimate in the first lens before they are incident on the mirror. A portion of the incident beam is reflected back and coupled to F2 while the transmitted light is focused in the second lens and coupled to fiber F3

Fused Biconical Taper Coupler

The most common method of manufacturing couplers.

The fibers are twisted together and then spot fused under tension such that the fused section is elongated to form biconical taper structure.

Suited for the use with MM & SM fiber.

Steps to make it FBT Twist two bare fibers around each other Heat the junction until the glass softens and the

cladding fuses Pull the fiber ends apart, forming a taper in the

fused portion of the coupler

How Operation takes place? In Multimode Fibers: - Coupling occurs because higher ordered modes no

longer strike the core-cladding interface because of the bending occurring in the interaction length.

Lower ordered modes do not travel near critical angle and so propagate to port 2.

In Single Mode Fibers: -

The taper brings the two cores closer to each other, and reduce the fiber core diameter.

Hence, it lowers the normalized frequency, V

Losses in FBT

Excess Loss: -

10 log10 (P1/P3+P2) Insertion Loss: -

10 log10 (P1/P2) Crosstalk: -

10 log10 (P4/P1) Split Ratio: -

[P3/(P3+P2)]*100%

Star Coupler It is SM & MM fiber coupler with more than 4 ports.

The FBT technology is used to fabricate star couplers, which relies on the coupling of the higher order modes between the different fibers.

Fabrication of Star Coupler

Losses For Star Coupler

Splitting Loss : -

10 log10 N(dB)

Excess Loss: -

10 log10[Pi/Σ Pj] (dB) (Summation from 1 to N)

Insertion Loss: -

Same as FBT.

WDM Couplers The coupler which enables the light from two or

more optical sources of different optical wavelength to be launched in parallel into a single optical fiber.

Optical Mux couples light while Demux separate the different carriers.

A WDM system can transmit voice, video and data on separate wavelengths.

Multiplexer Demultiplexerλ 1,λ2,λ3, λ4

λ1

λ2

λ3

λ4

λ1

λ2

λ3

λ4

Any Questions?