
U41 30 min 
ICMAT13A0276 Invited
A Revised Optical and Acoustical Wave Equation Formalism for Stimulated Brillioun Scattering
Valeri KOVALEV^{1,2}^{#}^{+} ^{1}Lebedev Physical Institute, Russian Federation, ^{2}HeriotWatt University, United Kingdom ^{#}Corresponding author: V.Kovalev@hw.ac.uk ^{+}PresenterA diversity of application areas for the phenomenon of stimulated Brillouin scattering (SBS), together with a growing variety of SBS interaction regimes and media suitable for its realization, stimulate continuing interest in the phenomenon. These studies obviously require an adequate theoretical framework. Physically, SBS is the reflection of incident (pump) radiation by the acoustic wave that is excited in an optically transparent medium through electrostriction. The acoustic wave in SBS serves as the Bragg grating travelling with the velocity of sound, and resulting in a Doppler shift of the reflected (scattered) light. It was noted in [1] that, since the period of such a grating is roughly half of the pump radiation wavelength, excitation of an acoustic wave in a medium makes it macroscopically inhomogeneous. In spite of this, the currently prevalent theoretical formalism of SBS uses the optical wave equation (OWE) for the scattered field obtained in [2] in the approximation of a macroscopically homogeneous medium; equations for the pump field and the material response are respectively fitted. I shall present (i) a revised formalism of SBS which is based on an OWE which explicitly accounts for the medium’s macroscopic inhomogeneity, (ii) a corrected OWE for the pump field and (iii) a consequently reexamined description of the material response. The new formalism opens up, firstly, opportunities for a more adequate description of already observed features of SBS and predictions of new effects and, secondly, gives grounds for revealing and rejecting certain speculative claims. References: [1]. V.I.Kovalev, R.G.Harrison. Phys. Lett. A 374, 2297 (2010). [2]. J.A.Armstrong, N.Bloembergen, et al. Phys. Rev. 127 1918 (1962).

U42 15 min 
ICMAT13A1969 Invited
Fiber Laser Beam Quality Improvement with Builtin Lens
Vivek DIXIT^{1}, Eng Huat KHOO^{2}^{+}, Zhengtong LIU^{1}, Chun Yong NGO^{3}, Liying HONG^{4}, Ruifen WU^{4}, Jing Hua TENG^{3}, Erping LI^{5}^{#} ^{1}Institute of High Performance Computing, Singapore, ^{2}EP, Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore, ^{3}Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), Singapore, ^{4}DSO National Laboratories, Singapore, ^{5}Electronics and Photonics, Institute of High Performance Computing, Singapore ^{#}Corresponding author: eplee@ihpc.astar.edu.sg ^{+}PresenterFiber
lasers are widely used to perform precise mechanical operations used for
industrial applications, such as cutting, sintering, engraving and welding. The
precision of the mechanical process is largely dependent on the beam quality of
the fiber laser, as determined by their M^{2} parameter. A smaller M^{2}
value indicates better beam quality, which could be used to improve performance
in industrial applications. To improve the precision of mechanical process, we
have designed dielectricpillars based lenses to convert the measured laser beam
profile into a Gaussian beam and improve M^{2} of the laser beam. The
inhouse code based on diffractive optics has been used to calculate the
required lens structure to transform the measured laser beam profile into a
Gaussian shape with near unity M^{2}. The procedure is based on the
generalized coordinate transformation of the measured beam profile to that of a
Gaussian shape. Lens structure is calculated using diffractive optics based
calculation, by solving the general form of diffraction equation. This converts
the measured beam profile into Gaussian shape. The builtin lens has a
dimension less than 1mm and is thus extremely difficult to fabricate by
conventional methods using either grinding and polishing, or diamond turning.
The proposed lens curvature is then converted to an array of dielectric pillars
with subwavelength dimensions using effective medium and filling fraction
theory. The dielectric nanopillar array is being fabricated on a quartz
substrate using Ebeam lithography and plasma etching. Experiment has shown
that the nanopillar array can withstand fibre laser power.

U43 15 min 
ICMAT13A2146 Contributed
Analysis of Transverse Modes Propagation in Nonadiabatic Tapered Fibers
Siti Azlida IBRAHIM^{1}^{#}^{+}, Mohd. Adzir MAHDI^{2} ^{1}Faculty of Engineering, Multimedia University, Malaysia, ^{2}Department of Computer and Communication Systems Engineering, Faculty of Engineering,, Universiti Putra Malaysia, Malaysia ^{#}Corresponding author: azlida@mmu.edu.my ^{+}PresenterThis paper presents
simulation results on propagation characteristics of all possible excited modes
in a nonadiabatic tapered step index singlemode fiber with 8 µm core
diameter. The power evolution of each mode along the propagation direction was
simulated using the Beam Propagation Method. Four modes were observed to be
supported in the tapered SMF with symmetrical downtaper and uptaper length of
15mm and waist length of 10mm. The waist diameters used were 15 µm and 5 µm.
The power of mode LP_{02 }is
seen to be the most dominant mode at the waist region. As the waist diameter
was reduced, the power of all modes experience larger oscillation with higher
oscillation frequency at the waist region.

U44 15 min 
ICMAT13A1875 Contributed
Experimental Verification of a Mathematical Model for the Draw Process of Microstructured Optical Fibers
Roman KOSTECKI^{1}^{#}^{+}, Heike EBENDORFFHEIDEPRIEM^{1}, Erik SCHARTNER^{1}, Peter HENRY^{1}, Tanya MONRO^{1} ^{1}Institute for Photonics and Advanced Sensing, School of Chemistry and Physics, University of Adelaide, Australia ^{#}Corresponding author: roman.kostecki@adelaide.edu.au ^{+}PresenterThe highly versatile class of microstructured optical fiber (MOF) can provide a broad range of unique and tailorable optical properties, made possible by the presence of the characteristic longitudinal air holes. Whatever material is used or configuration of holes desired, the final fabricated MOF geometry is affected by a large set of conditions, including material viscosity, the effects of surface tension, and the pressure difference of the holes and atmosphere. The efficient and accurate fabrication of MOFs requires a practical understanding of the `draw process’. Our results, when applied to suspendedcore and exposedcore MOF fabrication, experimentally validate the Fitt, et al. fluidmechanics model for describing the draw process. In recent work by Kostecki, et al., the silica `exposedcore’ MOF, in which the central lightguiding core is directly accessible to the outside environment, was published for the first time. The fabrication of exposedcore fibers is challenging due to their asymmetric cross section, where the method involved modification of established procedures and the practical application of the Fitt model. By understanding the draw process, both in terms of the draw tower temperature profile and establishing a method to define the geometry, it was found that the Fitt model could be used to predict the parameters necessary for the chosen structure. This model provided a practical framework that contributed to the efficient and accurate fabrication of the desired MOF geometry by predicting the fiber draw conditions.

U45 15 min 
ICMAT13A3146 Contributed
Fouriertransformbased ‘Regularization’ Approach to Timedomain Spectroscopy Data Analysis
Valeri LIGATCHEV^{1}^{#}^{+} ^{1}Material Science and Engineering, Insititute of High Performance Computing, Singapore ^{#}Corresponding author: valeri@ihpc.astar.edu.sg ^{+}PresenterTimedomain spectroscopy (TDS) is widely used at analysis of data of emission and absorption measurements on optical fibers due to its higher sensitivity and accuracy as compared to usual frequency domain approach. Traditional modeling approach for such analysis is based on decomposition of the measured TDS signal into sum of Fourier components (harmonics), dumped due to different kinds of the energy loss [1]. Analytical Fourier transform in complex frequency domain provides spectral representation of the measured signal in such a case. Generic problem of the harmonic decomposition approach consists in the fact that there is no unique combination of magnitudes and phases of those harmonics, as well as of corresponding dumping factors, which are known a priory. Moreover, possible effects of the measurements errors (‘noise’) are ignored at standard harmonic decomposition. Therefore, separation of physically meaningful information (i.e. parameters of factual harmonic oscillators in the studied system) from the artifacts (noise) becomes one of the key problems at TDS data analysis and interpretation. For that reason, ‘sensitivity’ of the modeling approach and its general applicability for TDS data analysis are obviously limited. Appropriate approach to numerical analysis of TDS data could be based on socalled ‘regularization’ procedures [2]. Within framework of this approach, ‘common sense’ criteria are routinely implemented in order to separate physically meaningful and stochastic (noise) contributions to the system response based on its spectral representation. Practical example of Fouriertransformbased ‘regularization’ algorithm is described in Ref. [3]. Implication of similar ‘regularization’ approach to TDS data analysis is discussed in the presentation. 1. M.C. Nuss, J. Orenstein, In Millimeter and Submillimeter Wave Spectroscopy of Solids; Ed. G. Gruner, SpringerVerlag: Berlin, 1998; Vol. 74. 2. A.N. Tikhonov and V.A. Arsenin, Solution of Illposed Problems, Winston & Sons, Washington, 1977. 3. V. Ligatchev, S.K. Chin, ECS Transactions 25, 19 (2009).

U46 30 min 
ICMAT13A3616 Invited
High Resolution Optical Spectrum Characterization Using Optical Spectrum Stitching Technique Incorporating with Optical Channel Estimation
Zhaohui LI^{1}^{#}^{+} ^{1}Institute of Photonics Technology, Jinan University, China ^{#}Corresponding author: li_zhaohui@hotmail.com ^{+}PresenterA novel method based on the optical spectrum stitching technique incorporating with optical channel estimation is proposed to realize the highresolution and large measurement range optical component characterization, including amplitude, phase responses and polarization sensitivity property. Two kinds of fiber Bragg grating based FabryPerot cavity with ultrafine structures have been characterized. By using 1024point fast Fourier transform and a narrow linewidth wavelengthtunable laser source, the achieved frequency resolution and measurement range of this method are ~ 10 MHz and 250 GHz respectively.

U47 15 min 
ICMAT13A2445 Contributed
Allfiber Optical Computer for Solving Npcomplete Decision Problems
Kan WU^{1}^{#}^{+}, Javier GARCIA DE ABAJO^{2}, Cesare SOCI^{3}, Perry Ping SHUM^{4}, Nikolay I. ZHELUDEV^{5} ^{1}Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore, ^{2}Institute of Chemistry  Physics Rocasolano (IQFR)National Research Council (CSIC), Spain, ^{3}Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, ^{4}Nanyang Technological University, Singapore, ^{5}Centre for Disruptive Photonic Technologies, Nanyang Technological University & University of Southampoton, Singapore ^{#}Corresponding author: wuka0002@e.ntu.edu.sg ^{+}PresenterA class of diverse mathematical complexity problems such as the travelling salesman problem of looking for a shortest possible route on a map are often referred as NPcomplete problems. No efficient algorithms have been identified to tackle these interlinked problems on a conventional computer. Unconventional approaches such as quantum and DNA computing are seen as a way to attack them. We demonstrate an allfiber optical computer for solving one of the most difficult complexity problems, the Hamiltonian path challenge of finding if a map can be traveled in a unidirectional way that each town is visited exactly once. To solve it we implement the map as a network of optical fiber (roads) of noncommensurate length that connect nodes (towns) and probe the network with a short optical pulse. Each node (town) introduces a unique delay. The decision on the existence of a Hamiltonian path can be made by monitoring the delay of the returning pulse.:A positive answer is given if a pulse with delay equal to the sum of each town’s delay, meaning that this pulse has visited all the towns exactly once. With singlemode fiber networks representing a map of five towns and operating at a telecom wavelength we performed a proofofprinciple Hamiltonian path tests that took only a few tens of nanoseconds. We argue that current fiber technology shall allow interrogating graphs of hundreds of nodes and providing a simpletoimplement alternative to a quantum computer approach.

