This document presents a thesis on characterizing Brillouin effects in optical microfibers. The thesis was conducted by Kazi Tasneem Farhan for their M. Eng. Sc. program under the supervision of Assoc. Prof. Dr. Zulfadzli Yusoff. The research objectives were to fabricate uniform microfibers of different lengths and diameters and characterize Brillouin scattering, lasing, and spatial effects. Various microfibers were fabricated and their profiles, insertion losses, and Brillouin responses were measured and analyzed. Spatial characterization showed Brillouin gain shifts along the microfiber length. The experimental results agreed with previous numerical simulations. Future work could focus on optimizing Ge-doped microfibers for

1. BRILLOUIN CHARACTERIZATION
OF
OPTICAL MICROFIBERS
Name: Kazi Tasneem Farhan
Programme: M. Eng. Sc.
Faculty: FOE
Registration Date: 1st February 2013
Supervisor: Assoc. Prof. Dr. Zulfadzli Yusoff
Co-supervisor: Siti Azlida Ibrahim Ghazali
MMU FORC 17/11/2015

2. Content
MMU FORC 17/11/2015
Background
Motivation
Problem Statement
Research Objective
Nonlinear fiber optics
Microfibers
Stimulated Brillouin
scattering
Introduction
Fabrication of Microfibers
Brillouin Effect
Characterization
Brillouin Scattering
Brillouin Lasing
Spatial Characterization of
Brillouin Effects in Microfibers
Introduction
Microfiber Shape Profile
Insertion Loss Measurement
Brillouin Scattering
Brillouin Lasing
Spatial Characterization of
Brillouin Effects in
Microfibers
Conclusion
Future Recommendations

3. INTRODUCTION
MMU FORC 17/11/2015

4. 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.

5. Introduction - Background
MMU FORC 17/11/2015
 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.

6. Introduction - Motivation
MMU FORC 17/11/2015
 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).

7. Introduction – Problem Statement
MMU FORC 17/11/2015
 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.

8. Introduction – Research Objectives
MMU FORC 17/11/2015
 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).

9. LITERATURE REVIEW
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10. MMU FORC 17/11/2015
 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
applied
P(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

11. Literature Review – Microfibers
MMU FORC 17/11/2015
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

12. Literature Review – Stimulated Brillouin
scattering
MMU FORC 17/11/2015
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).

13. RESEARCH METHODOLOGY
MMU FORC 17/11/2015

14. 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

15. Research Methodology - Fabrication of
Microfibers
MMU FORC 17/11/2015
Figure : Complete microfiber fabrication
rig

16. Research Methodology - Fabrication of
Microfibers
MMU FORC 17/11/2015
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

17. Research Methodology - Fabrication of
Microfibers
MMU FORC 17/11/2015
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

18. Research Methodology – Fabrication of
Microfibers
MMU FORC 17/11/2015
 Insertion loss measurement
Figure : Insertion loss measurement setup

19. Research Methodology – Brillouin
Scattering
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 Brillouin scattering
Figure : Brillouin scattering measurement setup

20. Research Methodology – Brillouin
Lasing
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Figure : Brillouin lasing measurement setup

21. Figure : Setup for Spatial Characterization of Brillouin Effects
Research Methodology – Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015

22. EXPERIMENTAL RESULTS & DISCUSSION
MMU FORC 17/11/2015

23. 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.

24. Figure : The shape profile for all microfibers produced
Experimental Results & Discussion -
Microfiber Shape Profile
MMU FORC 17/11/2015

25. Experimental Results & Discussion -
Microfiber Shape Profile
MMU FORC 17/11/2015
 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.

26. Experimental Results & Discussion -
Insertion Loss Measurement
MMU FORC 17/11/2015
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.

27. Experimental Results & Discussion –
Insertion Loss Measurement
MMU FORC 17/11/2015
 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

28. Experimental Results & Discussion -
Brillouin scattering
MMU FORC 17/11/2015
 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

29. Experimental Results & Discussion -
Brillouin Scattering
MMU FORC 17/11/2015
 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

30. Experimental Results & Discussion -
Brillouin scattering
MMU FORC 17/11/2015
 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.

31. Experimental Results & Discussion -
Brillouin scattering
MMU FORC 17/11/2015
 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.

32. Experimental Results & Discussion -
Brillouin scattering
MMU FORC 17/11/2015
 Microfiber of same diameter but different lengths
Figure : The combined plot for of Brillouin stoke for increasing
length of
SMF, Ge-doped and Ga-doped Microfibers with fixed waist
diameter.

33. Experimental Results & Discussion -
Brillouin scattering
MMU FORC 17/11/2015
 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.

34. Experimental Results & Discussion -
Brillouin Lasing
MMU FORC 17/11/2015
-80
-70
-60
-50
-40
-30
-20
-10
0
1.90940000000000E+14 1.90960000000000E+14
Power(dB)
Frequency (THz)
-80
-70
-60
-50
-40
-30
-20
-10
0
1.90940000000000E+14 1.90960000000000E+14
Power(dB)
Frequency (THz)
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

35. Experimental Results & Discussion -
Brillouin Lasing
MMU FORC 17/11/2015
 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.

36. Experimental Results & Discussion - Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015
 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

37. Experimental Results & Discussion - Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015
Figure : Brillouin gain along the length of the
microfiber
Pigtail attached to the microfiber
2.5cm uniform waist
5cm transitions
Zoom over 12.5 cm of the microfiber

38. Experimental Results & Discussion - Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015
 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.

39. Experimental Results & Discussion - Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015
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

40. Experimental Results & Discussion - Spatial
Characterization of Brillouin Effects
MMU FORC 17/11/2015
 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.

41. CONCLUSION AND FUTURE
RECOMMENDATIONS
MMU FORC 17/11/2015

42. Conclusion and Future Recommendations -
Conclusion
MMU FORC 17/11/2015
 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.

43. Conclusion and Future Recommendations -
Conclusion
MMU FORC 17/11/2015
 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.

44. Conclusion and Future Recommendations –
Future Recommendations
MMU FORC 17/11/2015
 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.

45. MMU FORC 17/11/2015
THANK YOU
Q & A