Directional Coupler Based on Twin-Core Fiber

Bilal A. Alvi, Amber Israr, and Muhammad Aamir

Electrical Engineering Department, Institute of Business Management, Karachi,

Sir Syed University of Engineering & Technology, Karachi, Pakistan

Agenda

• Introduction

• Background

• Twin Core Optical Fiber

• Calculation of Antisymmetric Mode Cutoff

• Experimental Set up

• Conclusion

Introduction

• Optical communications uses a fiber basically as an information carrying line between electronic component.

• There is steady progress towards optical manipulation of signal.

• Perhaps fiber couplers are the simplest and the most important devices in fiber communication applications.

Directional Coupler • The Coupler allows an incoming

signal to be split between 2 out put fibers.

• Rely on Evanescent wave coupling

– Two waveguides close together

– Evanescent field of first waveguide “feels” second waveguide

– Gradual coupling of light between both waveguides

– Power transfer ratio P1/P2 depends on the core spacing and on the interaction length

Application

• Power Divider

• Passive filters

• Wavelength multiplexers/Demultiplexers

• Combiner

• Fiber sensors,

• Fiber Amplifier

• Switches

Coupler Construction Techniques • Several techniques have been developed to

construct couplers: – Polished cladding couplers

• Advantages: Wavelength-tunable

• Disadvantages: Unstable

– Fusion elongation techniques • Mechanically stable

• Difficult to handle

– Chemical etching • Handling more intangible, and high losses

– Multi-cored fibers • core to core spacing controlled, free from undesirable

birefringence

• Difficult to couple light

Twin core optical Fiber

• Twin core fiber consist of a pair of identical cores embedded in cladding with slight lower index, ncl, that of the core, nco.

• The center to center spacing is d.

2a 2a

d

Advantages

• Geometrical Dimension

– Control of core to core spacing

• Undesirable generation of birefringence can be avoided

• No reference fiber required for interferometry applications

• Eliminate many optical devices in optical set up.

Twin core Fiber Coupling Process • The optical coupling in twin core fiber was analyzed in terms of

linear combination of symmetric and antisymmetric modes . • Figure below illustrate the Geometry of twin-core fiber. • Two straight circular cores of same diameter, r, are embedded in

same cladding • The center to center spacing is d. • Refractive index profile n(r).

Processes in Isolation

• In isolation each core behaves as single mode fiber and field can be constructed from the scalar wave equation.

• There is no narrowing of the cores so light is still well guided.

• In case of twin core, the symmetry of composite wave guide doubles the number of modes.

• Thus two fundamental solution of the scalar wave equation required.

• These are the symmetric and antisymmetric superposition of the field in the two cores in isolation. Ψ+ =Ψ1 + Ψ2 : Ψ- =Ψ1 – Ψ2

• Where Ψ1 and Ψ2 are the fundamental

solution for each fiber in solution, referred to common axes

• Due to the unequal propagation constant of two modes, the fields propagating down the system develop a relative phase difference with the distance.

• Just as with the normal modes of a circular waveguide, Ψ+ is not cut off and Ψ- has finite cut off.

• Beyond the cut off Ψ- ,, it becomes radiative mode and is attenuated.

Ψ+

Ψ-

Calculation of Cutoff Conditions • A variation calculation was used to calculate the cut-off

condition for the antisymmetric mode. • The eigen-value problem for the twin-core structure

shown in above Figure has been solved by variation of the trial function

where r is the variational parameter, Es is the symmetric mode and EA is the anti-symmetric mode.

• The variation solution in the weak guiding approximation was performed with analytic evaluation of the infinite integrals and a Monte Carlo evaluation of the integral of

∆ (n2) = nco2 - ncl

2 • In this approximation, the result of bringing the cores close

together can be expressed in reduced form by plotting dispersion curves of the reduced propagation constant,

U =a √nco2ω2/c2 – β2,

• Against reduced frequency (fiber parameter), V = a ω/c √∆(n2) , • for different values of the reduced core spacing D = d/a

Dispersion curve U vs V

Experimental Results

• Twin-cored fibers were prepared by the rod and tube method from laboratory Pyrex cladding (ncl = 1.474) and Schott ZKN7 zinc crown optical glass core (nco = 1.506).

• Core diameter is 1.5µm • Core separation is 6µm • The core diameters were such as to satisfy the

single mode condition for each core considered separately, that is to say that the fiber parameter, V, was less than 2.405

•

Phase Contrast Image

Near field Image

• Figure shows, when the cores of the fiber were equally excited with a HeNe laser.

• The cores of twin core were sufficiently well separated for their fundamental modes to be phase mismatched so that significant coupling between cores was avoided.

Far Field Image

• Figure shows, far field expected sine-squared-theta fringes.

Bi-conical Taper

• The next step was the introduction of bi-conical taper in a fiber with well separated cores, the waist diameter in the taper being chosen to ensure cutoff of the antisymmetric mode.

• The technique used for making a taper was that of “heating and pulling”.

• Taper waist “25 µm” and taper length “3.5 cm”

• The bi-conical taper provides 3dB splitting between the two cores of the symmetric part of the input excitation.

Experimental Set-up

• A schematic diagram of the experimental set up is shown in figure in next slide

• System consist of

– Light source

– Beam coupling system

– Fiber

– Recording system

Laser Beam Delivery System

Piezoelectric Device

Focusing Object

Oscilloscope

Camera Twin Core Fiber

Experimental Set up

Coupler’s Operation

• Three important section

– An entrance down-tapering conical section

– A central section

– An exit up-tapering conical section

Coupler’s Operation

Entrance down-tapering section

• The transmission through a entrance fiber is due to the fundamental core mode.

• While propagating down the taper, the wave encounters a gradually reduction of d, the tail of the isolated guide modes begin to overlap

• Which split the fundamental mode into two modes, Symmetric and antisymmetric mode

• Interference take place between the two modes as they propagate with different phase velocity through down taper section and result in power transfer between the core.

• When these modes are in phase the energy in core 1 • When these modes are out of phase the energy in core 2.

A central section

• Taper diameter is less and the cores are very thin and almost touching each other.

• Their ratio d/a is equal to 3 and V parameter less than about 1.

• The lowest order antisymmetric mode of this composite structure cut off as shown in dispersion curve.

• While symmetric super mode propagate through the centre region

Exit up-tapering conical section

• As the light propagates beyond the point of minimum cross section of the taper, it eventually encounters the up-taper, where the size of the core progressively increases.

• In this region the V parameter of the core is again approximately equal to 1.

50-50 splitting

Conclusion

• The experimental results reported, firm evidence for the feasibility of fabricating in-fiber splitting (and combining) components for single fiber interferometers .

• The inclusion of reference and measured paths within a single fiber offers the prospect of fiber interferometry with much reduced sensitivity to temperature induced noise and drift.

• Ultimately it may even be possible to incorporate fluorescent source and detector sections within a single fiber and so achieve true intelligent measurement systems within a self-contained fiber structure.

Acknowledgment

• Prof. Dr. Bhawani Shankar Chowdhry for his technical and moral support

• Mr. Shah Jahan Karim President of Institute of Business Management for providing financial assistance

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