BRILLOUIN CHARACTERIZATION OFOPTICAL MICROFIBERS

Name: Kazi Tasneem FarhanProgramme: M. Eng. Sc.Faculty: FOE

Registration Date: 1st February 2013Supervisor: Assoc. Prof. Dr. Zulfadzli YusoffCo-supervisor: Siti Azlida Ibrahim Ghazali

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Content

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INTRODUCTIONBackgroundMotivationProblem StatementResearch Objective

LITERATURE REVIEWNonlinear fiber opticsMicrofibersStimulated Brillouin scattering

RESEARCH METHODOLOGYIntroductionFabrication of MicrofibersBrillouin Effect Characterization

Brillouin ScatteringBrillouin Lasing

Spatial Characterization of Brillouin Effects in MicrofibersEXPERIMENTAL RESULTS & DISCUSSION

IntroductionMicrofiber Shape ProfileInsertion Loss MeasurementBrillouin ScatteringBrillouin LasingSpatial Characterization of Brillouin Effects in Microfibers

CONCLUSION AND FUTURE RECOMMENDATIONS

ConclusionFuture Recommendations

INTRODUCTION

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Introduction - Background

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Light has been a medium of communication for many centuries.

The entire world relies on the exchange of information in bulk over long distances.

The ability of silica (single-mode) fibers to transmit large amount of information makes it an excellent choice for the communications field.

At around 1974 fiber optics came into the world of communication and since then it has experienced a growth in transmission capacity by 10 times every four year.

Introduction - Background

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The light travelling through the fiber experiences scattering.

Two types of scattering: Linear scattering and Nonlinear scattering

When nonlinear scattering occurs, the fiber does not produce a linear output to power changes at the input.

Stimulated Brillouin scattering is one type of nonlinear scattering. The incident light interacts with sound wave in the fiber and scatters backward with downshifted frequency. The downshift is equal to the acoustic velocity.

Introduction - Motivation

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There is always a technological drive in making devices compact, cheaper and greener.

One of the many focuses of technological advancements are nonlinear optics.

Microfibers show great potential to be used as a nonlinear medium for nonlinear devices. They show nonlinear properties equivalent to very long lengths of fibers (~kms) just in a short length of microfiber (~cms).

Introduction – Problem Statement

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Among the nonlinear effects in optical fibers, Brillouin scattering is the easiest to observe as it has the lowest threshold power.

Characterization of Brillouin scattering in various long microfibers has not been reported yet.

Introduction – Research Objectives

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To design and fabricate uniform microfibers of different lengths and diameters from different types of fibers.

To characterize and compare Brillouin effects in the fabricated microfibers through three different approach (scattering, lasing and spatial measurement).

LITERATURE REVIEW

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Nonlinearity is an effect of high intensity light (laser) travelling through an optical fiber which will alter the properties of the medium.

Anharmonic movement of electrons when a field is appliedP(t) = ϵ0[χ(1)E1(t)+ χ(2) E3(t)+ χ(3) E3(t)+· ·

·] The third order susceptibility χ(3) leads to the

nonlinear effect such as self-phase modulation and Brillouin scattering.

Literature Review – Nonlinear Fiber Optics

Literature Review – Microfibers

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Figure: The segments of a microfiber

Figure : First Images shows the light guided in the core. Second image shows the light guided outside the core but within the cladding. Third image shows the light guided outside the cladding and the air acting as the cladding

Literature Review – Stimulated Brillouin scattering

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Figure : Graphical representation of Stimulated Brillouin scattering process

Light travelling through a medium interacts with the acoustic phonons within the optical waveguide.

The scattered light (ωS Stokes) travelling opposite to the incident beam (ωL) and is downshifted in frequency by an amount equal to the acoustic frequency (Ω).

RESEARCH METHODOLOGY

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Research Methodology - Introduction

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Design and fabricate microfibers of different shapes and sizes, from different kinds of fibers Microfiber profiles Insertion loss measurement

Brillouin effect characterization of the microfibers fabricated Brillouin scattering Brillouin lasing Spatial characterization of Brillouin effects

Research Methodology - Fabrication of Microfibers

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Figure : Complete microfiber fabrication rig

Research Methodology - Fabrication of Microfibers

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Figure : The full mechanism length of microfiber

Where radius rω is the final waist diameter of the taper, ro is the original radius of the fiber, LT is the total length of the microfiber, LH is the heat zone, LE is the extended length

Research Methodology - Fabrication of Microfibers

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Figure : Shape profile and simulated profile of a microfiber with a waist length LT of 20cm and waist diameter rω of 30 µm

Figure : Matlab simulation of desired Microfiber

Research Methodology – Fabrication of Microfibers

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Insertion loss measurement

Figure : Insertion loss measurement setup

Research Methodology – Brillouin Scattering

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Brillouin scattering

Figure : Brillouin scattering measurement setup

Research Methodology – Brillouin Lasing

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Figure : Brillouin lasing measurement setup

Figure : Setup for Spatial Characterization of Brillouin Effects

Research Methodology – Spatial Characterization of Brillouin Effects

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EXPERIMENTAL RESULTS & DISCUSSION

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Experimental Results & Discussion - Introduction

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Microfiber shape profile Insertion loss measurement Brillouin scattering

Observation in three types of microfibers of different lengths and waist diameters.

Brillouin lasing Comparison between SMF microfiber and

chalcogenide microfiber. Spatial characterization of Brillouin effects

Characterization of Brillouin effects along the length of a microfiber.

Figure : The shape profile for all microfibers produced

Experimental Results & Discussion - Microfiber Shape Profile

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Experimental Results & Discussion - Microfiber Shape Profile

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The fabricated microfibers profile follow shape of the simulated profile within ±5%.

The “ripples” in the plot is due to errors in image processing.

The microfiber with 3µm waist is hard to profile due to too much movement.

Experimental Results & Discussion - Insertion Loss Measurement

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Figure : Power performance all the microfibers produced of different shapes and sizes. The power profile for microfibers of different shapes and sizes for different fiber material composition is also showed.

Experimental Results & Discussion – Insertion Loss Measurement

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Insertion loss is made up of loss in the tapered loss as well as transmission loss in the uniform waist.

The transmission loss is not significantly different with the various length or diameter. The insertion losses are mainly due to the tapered loss.

Ge-doped have lower loss than SMF Microfibers

Experimental Results & Discussion - Brillouin scattering

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Microfibers with same lengths but different diameters

Figure : Brillouin stokes from SMF, Ge-doped and Ga-doped microfibers with same length but different diameter of 3 µm, 5 µm and 10 µm

Experimental Results & Discussion - Brillouin Scattering

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Microfibers with same lengths but different diameters

Figure : The combined plot for of Brillouin stoke for increasing waist diameter of SMF, Ge-doped and Ga-doped Microfibers with fixed length

Experimental Results & Discussion - Brillouin scattering

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Peak power of Brillouin stoke decreases for increasing waist diameter of SMF, Ge-doped and Ga-doped Microfibers with fixed length.

Nonlinear coefficient (γ) is defined by

where Aeff is he effective mode area and n2 is the refractive index.

As waist diameter increases so does Aeff , thus nonlinearity decreases so does Brillouin stoke.

Ga-doped microfibers shows the highest stoke power. Second is Ge-doped and lastly SMF.

Experimental Results & Discussion - Brillouin scattering

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Microfiber of same diameter but different lengths

Figure : Brillouin stokes from SMF, Ge-doped and Ga-doped Microfibers with same diameter but different lengths of 10 cm, 20 cm and 30 cm.

Experimental Results & Discussion - Brillouin scattering

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Microfiber of same diameter but different lengths

Figure : The combined plot for of Brillouin stoke for increasing length ofSMF, Ge-doped and Ga-doped Microfibers with fixed waist diameter.

Experimental Results & Discussion - Brillouin scattering

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The increase in peak power of Brillouin stoke for increasing length of SMF, Ge-doped and Ga-doped Microfibers with fixed waist diameter.

The value of the Brillouin threshold is represented with good approximation using the equation below

where Leff is he effective mode area and n2 is the refractive index

Ga-doped microfibers shows the highest stoke power. Second is Ge-doped and lastly SMF.

Experimental Results & Discussion - Brillouin Lasing

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1.90940000000000E+14 1.90960000000000E+14

-80

-70

-60

-50

-40

-30

-20

-10

0Frequency (THz)

Pow

er (

dB)

1.90940000000000E+14 1.90960000000000E+14

-80

-70

-60

-50

-40

-30

-20

-10

0Frequency (THz)

Pow

er (

dB)

Figure : BEFL using SMF Microfiber with waist 1µm and length 13cm

Figure : BEFL using Chalcogenide Microfiber with waist 1µm and length 13cm

10.68 GHz 7.05 GHz

Experimental Results & Discussion - Brillouin Lasing

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BEFL using SMF microfiber of length 13cm and waist 1 µm.

BEFL using chalcogenide microfiber of length 13cm and waist 1 µm.

The generated Brillouin gain is not sufficient to overcome the cavity loss hence no lasing was observed.

The difference in frequency shift is due to the different material used in the two fibers.

Experimental Results & Discussion - Spatial Characterization of Brillouin Effects

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A sample of 20 cm length and 3.07 µm waist diameter to perform a Brillouin gain measurement spatially.

Figure : Profile of microfiber used for spatial resolution measurement

Experimental Results & Discussion - Spatial Characterization of Brillouin Effects

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Figure : Brillouin gain along the length of the microfiber

Pigtail attached to the microfiber2.5cm uniform waist

5cm transitions

Zoom over 12.5 cm of the microfiber

Experimental Results & Discussion - Spatial Characterization of Brillouin Effects

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The sample shape profile used can be seen to match the simulated shape profile quite effectively. The sample has a diameter maintained around 3.07 µm.

The transitions of the microfiber show a Brillouin gain of ~ 11.1 GHz which is like 5cm region around the waist.

The uniform waist of 2.5cm shows two frequencies. One at 11.1 GHz and another at 10.85 GHz.

The 10.85GHz frequency according to the experiment is also found to be 3 times in higher magnitude than the 11.1 GHz.

Experimental Results & Discussion - Spatial Characterization of Brillouin Effects

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Figure : Brillouin Gain Pattern and Strength for Microfibers with wait 3 µm to 4 µm

Figure : Brillouin Gain Simulation for Microfiber with waist diameter of 3.07 µm

Experimental Results & Discussion - Spatial Characterization of Brillouin Effects

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The initial simulation done by our collaborator Jean Charles at Femto-ST shows the frequency around the waist changes from 10.85 GHz to 11.1 GHz.

This full acoustic spectrum was published in Nature Photonics (J. C. Beugnot, S. Lebrun, G. Pauliat, H. Maillote, V. Laude, T. Sylvestre, Brillouin light scattering from surface acoustic waves in a

subwavelength-diameter optical fiber, Nature Communications October 2014 ).

Our observations match to the simulated results shown in the publication.

CONCLUSION AND FUTURE RECOMMENDATIONS

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Conclusion and Future Recommendations - Conclusion

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Insertion loss is made up of loss in the tapered loss and is not significantly different with the various length or diameter.

Brillouin stoke gain decreases as the diameter increases. Aeff increases causing reduction in nonlinearity.

Brillouin stoke increases as the length of the Microfiber increase. Leff increases causing reduction in Pth thus increases

nonlinearity. Ge-doped and Ga-doped have stronger Brillouin gain

than SMF. But Ga-doped Microfiber has a more stronger response to Brillouin gain than Ge-doped.

Conclusion and Future Recommendations - Conclusion

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No lasing observed in microfibers because the Brillouin gain is not sufficient to overcome the cavity loss.

Brillouin gain shifts to11.1 GHz around the uniform waist.

The uniform waist of 2.5 cm shows two different types frequency.

Experimental results are similar to the numerical simulation reported in Nature photonics.

The 10.85GHz frequency according to the experiment is also found to be 3 times in higher magnitude than the 11.1 GHz.

Conclusion and Future Recommendations – Future Recommendations

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Ge-doped and Ga-doped fiber show great possibility in nonlinear application due to their better show of performance in generating Brillouin gain.

Longer lengths of microfibers may be used for lasing.

One other interesting thing to be looked into deeply is the generation of multiple high frequencies in the uniform waist of the Microfibers.

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THANK YOUQ & A