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Electrostriction in As2Se3-PMMA Microtapers

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Date

2019-10-30

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Université d'Ottawa / University of Ottawa

Abstract

Electrostriction is the tendency of materials to acquire a fluctuation in density in the presence of coherent electro-magnetic fields. It leads to a change in the refractive index of the material, which can be explained in the language of nonlinear optics as a consequence of third-order optical nonlinearities (known as Kerr nonlinearities). Such phenomenon is observed when intense beams, such as laser light, travel in a medium, e.g. an optical fiber, and the spontaneous scattering of the laser field from the thermally excited acoustic waves occurs. This effect, known as spontaneous Brillouin scattering, can become quite significant, giving rise to an interaction between the incident and backward reflected field (or the Stokes field), through the means of electrostriction. Such interaction can be classified under two categories of stimulated processes: (1) stimulated Brillouin scattering (SBS), in which two incident optical fields interact with longitudinal acoustic waves, and (2) guided acoustic wave Brillouin scattering (GAWBS), in which two incident optical fields interact with transverse acoustic waves. In case of SBS in optical fibers, the scattered light propagates backwards with a downshifted frequency of the order of GHz, while in case of GAWBS the scattered light propagates forward with multiple frequency shifts of hundred of MHz relative to the frequency of the pump laser. In this thesis, I demonstrate simultaneous generation of SBS and GAWBS from electrostriction of optical waves in a 60 cm As2Se3-Poly(methyl methacrylate) (or As2Se3-PMMA) microtaper waveguide. I show that the GAWBS in the microtaper couples with SBS through a complex energy transfer between weak Stokes and Anti-Stokes (AS) continuous waves in the presence of a high power pulsed pump wave. This results in an amplification of Stokes wave at 7.4 GHz, which is present in addition to a standard strong Stokes peak at 7.62 GHz and a secondary peak at 7.8 GHz that are contributed by SBS for a 2 micron As2Se3 core radius. The additional peak arises due to modulation of the optical fiber by GAWBS at 211 MHz generated by the pump. Such strong coupling of forward and backward Brillouin scattering due to large acoustic impedance between the core and cladding in such compact, highly nonlinear fibers plays a vital role in simultaneously sensing longitudinal and transverse strain within the core as well as its surroundings. I also report high Brillouin frequency shift of 0.08 +- 0.02 MHz/Nmm^(-1) in a 60 cm As2Se3-PMMA hybrid microtaper with 2 micron As2Se3 core and 100 micron PMMA cladding diameters under transverse load. such shifts are in agreement with a numerical value of 0.06 +- 0.01 MHz/Nmm^(-1), obtained through finite element analysis of SBS under the influence of contact stress from the loading fixture. Further numerical analysis show that uncertainties in the Brillouin frequency shift are result of birefringence that is 14 times stronger than what is found in standard silica fibers. Lastly, I present the characterized Brillouin profile of a Dual-Core As2Se3-PMMA microtaper with 2 micron As2Se3 core and 100 micron PMMA cladding diameters. I demonstrate a Brillouin peak difference of 14 MHz between an even or odd optical supermode, observed in a Brillouin Optical Time-Domain Analysis (BOTDA) measurement as well as with numerical simulations. This allows for definition of a novel transverse stress sensor that determines the magnitude as well as the orientation of a transverse load applied to the fiber. This system exhibits a linear relation between the net load applied to the fiber and the average Brillouin peak frequency shifts of the even and odd eigenmodes.

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Keywords

Fiber Optics, Brillouin Scattering, Chalcogenide Fibers, Finite-Element Analysis

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