Proceedings Volume 9507

Micro-structured and Specialty Optical Fibres IV

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Proceedings Volume 9507

Micro-structured and Specialty Optical Fibres IV

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Volume Details

Date Published: 22 May 2015
Contents: 8 Sessions, 25 Papers, 0 Presentations
Conference: SPIE Optics + Optoelectronics 2015
Volume Number: 9507

Table of Contents

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Table of Contents

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  • Front Matter: Volume 9507
  • Fibres Tailored for Applications
  • Modelling and Analysis of Speciality Fibre and Components
  • Sensors and Devices based on Speciality Fibres
  • Fibre Design, Processing and Fabrication
  • Polymer Optical Fibre Based Sensors and Devices
  • Testing and Characterisation Methods Applied to Special Fibre Types
  • Poster Session
Front Matter: Volume 9507
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Front Matter: Volume 9507
This PDF file contains the front matter associated with SPIE Proceedings Volume 9507, including the Title Page, Copyright information, Table of Contents, Authors, Introduction (if any), and Conference Committee listing.
Fibres Tailored for Applications
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Development of optical fibers for mid-infrared sensing: state of the art and recent achievements
Chalcogenide glass fibers are matchless devices to collect mi-infrared signal. Depending on the spectroscopic strategy, different kind of optical fibers have been developed during the past 10 years. The first fibers have been fabricated from selenide glasses to implement Fiber Evanescent Wave Spectroscopy (FEWS). It is an efficient way to collect optical spectra in situ, in real time and even, in the future, in vivo. Thanks to selenide glass fibers, it is possible to record such spectra on the mid-infrared range from 2 to 11 μm. This working window gives access to the fundamental vibration band of most of biological molecules and numerous multi-disciplinary works have been led in biology and medicine.

New glasses, only based on tellurium, have been recently developed, initially in the frame of the Darwin mission led by the European Space Agency (ESA). These glasses transmit light further toward the farinfrared and permit to reach the absorption band of CO2 located at 15 μm as requested by the ESA. Moreover, these telluride glass fiber are also very interesting for FEWS and medical application. Indeed, they give access to the mid-infrared signal of biomolecules beyond 11 μm, where classical selenide glass fibers are blind. Alternatively, in order to fight against global warning, some optical fibers have been developed for the monitoring of the CO2 stored into geological storage area underground. These fibers were doped with Dy3+ which emits a broad fluorescent band embedding the CO2 absorption band at 4.3 μm. thus, these fibers are used both to transmit light and as secondary sources in the mid-infrared.

To conclude, original microstructurated fibers have also been used for mid-infrared sensing. They exhibit a nice sensitivity compared to classical chalcogenide glass fibers.
Special optical fibers doped with nanocrystalline holmium-yttrium titanates (HoxY1-x)2Ti2O7 for fiber-lasers
Jan Mrázek, Ivan Kašík, Jan Boháček, et al.
The paper deals with the preparation and characterization of the silica optical fibers doped with nanocrystalline holmium-yttrium titanates (HoxY1-x)2Ti2O7 with optimized luminescence properties. The sol-gel approach was employed to prepare colloidal solution of (HoxY1-x)2Ti2O7 precursors. The concentration of Ho3+ ions in the compounds was varied up to x=0.4. Prepared sols were calcined at 1000 °C forming xerogels which were characterized by X-ray diffraction to confirm their structure. The xerogels were analyzed by the mean of steady-state luminescence technique to optimize the concentration of Ho3+ ions in the compound. The most intensive emission at 2050 nm was observed for the compound (Ho5Y95)2Ti2O7. Sol of the corresponding composition was soaked into the porous silica frit deposed inside the silica substrate tube which was collapsed into preform and drawn into optical fiber. Single mode optical fiber with the core diameter 12 μm and outer diameter 125 μm was prepared. Numerical aperture of prepared fiber was 0.16. The concentration of Ho3+ ions in the fiber core was 0.03 at %. Background attenuation of prepared fiber at 850 nm was smaller than 0.5 dB⋅m-1.
Spectral broadening in low OH content and dispersion-managed tellurite fibres for compact mid IR sources
C. Strutynski, J. Picot-Clémente, F. Amrani, et al.
We report the manufacturing and characterization of Tellurite micro structured fibres (MOFs) with low OH content. The different purification processes used during the fabrication of the TeO2 - ZnO - Na2O glass allowed us to reduce the hydroxyl compounds concentration down to 1ppm mass. A suspended core MOF was drawn from this material and then pumped by nanojoule-level femtosecond pulses at 1.7μm, its zero dispersion wavelength (ZDW), and well above it at 2.5μm. We show the related supercontinuum (SC) generated in the two distinct dispersion regimes of the waveguide. Moreover, the SC spanning was extended in both visible and mid-IR regions (between 600nm up to 3300nm) by the taperisation of the previously tested MOFs.
Silica optical fibers with high oxygen excess in the core: a new type of radiation-resistant fiber
Pavel F. Kashaykin, Alexander L. Tomashuk, Mikhail Yu. Salgansky, et al.
The technology, initial properties, and the value of radiation-induced attenuation (RIA) of light in the optical communication spectral range ~1.1–1.7 μm are discussed of the novel MCVD-produced undoped-silica-core F-dopedsilica- cladding fibers, of which the core is synthesized in high O2 excess (HOE) conditions (HOE-fibers). The RIA mechanisms are analyzed and compared in the HOE-fibers and in the F-doped-silica-core fibers previously commonly considered as the most radiation-resistant. The measured RIA values in the HOE-fibers and the literature data on the RIA in the commercial radiation-resistant F-doped-silica-core fibers of Fujikura are compared at λ=1.31 and 1.55 μm. Based on this consideration, the HOE-fibers are argued to be potentially superior to the F-doped-silica-core fibers as to radiation resistance especially at long wavelengths (in particular, at λ~1.55 μm). It is also argued that the fiber drawing tension reduction can further lower RIA in the HOE-fibers. A direct experimental comparison of RIA under γ-radiation from a 60Co-source at a dose rate of 8.7 Gy/s up to a dose of 94 kGy is carried out in two HOE-fibers and a commercial radiation-resistant fiber of European make. RIA in the HOE-fibers is found to be many times lower than that in the commercial fiber throughout the optical communication spectral range ~1.1–1.7 μm.
Low bending loss square-core optical fiber for optical communication
P. F. Liu, J. Y. Sung, C. W. Chow, et al.
In this work, we propose and demonstrate a single-mode square-core optical fiber for optical communications. In the proposed square-core single-mode-fiber (SC-SMF), high bandwidth-distance product and low bending loss can be achieved. In this paper, first of all, we discuss the single mode condition of the SC-SMF theoretically and numerically. Then, we discuss the fabrication of the SC-SMF. We also characterize the performances of the proposed SC-SMF, such as the bending loss, and compare it with the standard single mode fiber (SSMF). A 10 Gb/s transmission experiment using 200 m, 500 m and 1 km SC-SMFs are performed. Negligible power penalty is observed.
Modelling and Analysis of Speciality Fibre and Components
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Highly nonlinear chalcogenide suspended-core fibers for applications in the mid-infrared
Enrico Coscelli, Federica Poli, Jianfeng Li, et al.
Due to their unique dispersion and nonlinear properties, chalcogenide suspended-core fibers, characterized by a few micrometer-sized core suspended between large air-holes by few small glass struts, are excellent candidates for mid-infrared applications. In the present study the influence of the main cross-section characteristics of the chalcogenide suspended-core fibers on the dispersion curve and on the position of the zero-dispersion wavelength has been thoroughly analyzed with a full-vector modal solver based on the finite element. In particular, the design of suspended-core fibers made of both As2S3 and As2Se3 has been optimized to obtain dispersion properties suitable for the supercontinuum generation in the mid-infrared.
Microfluidic flow and heat transfer and their influence upon optical modes in microstructure fibres
E. Davies, P. Christodoulides, G. Florides, et al.
Using finite element analysis (FEA), a model has been constructed to predict the thermo-fluidic and optical properties of a microstructure optical fibre (MOF). The properties under study include external temperature, input water velocity and optical fibre geometry. Under laminar flow the steady-state temperature is dependent on the water channel radius while independent of the input velocity. A critical channel radius is observed below which the steady-state temperature of the water channel is constant, while above, the temperature decreases. The MOF has been found capable of supporting multiple modes whose response to temperature was dominated by the thermo-optic coefficient of glass, despite the larger thermo-optic coefficient of water. This is attributed to the majority of the light being confined within the glass, which increased with increasing external temperature due to a larger difference in the refractive index between the glass core and the water channel.
Large mode area aperiodic fiber designs for robust singlemode emission under high thermal load
Romain Dauliat, Enrico Coscelli, Federica Poli, et al.
In this paper, we investigate the potential of various large mode area fibers under thermal load, that is the state-of-the-art air-silica large pitch fibers, as well as the recently devised symmetry-reduced photonic crystal fiber and aperiodic all-solid by carefully considering the degrees of freedom offered all along the fiber fabrication. This work aims to discuss the mode filtering ability of these structures in regard to the power scaling and to confirm their potential for robust singlemode operation at high power level. Structural principles contributing to improve their performances such as the impact of air holes / solid inclusions size will be presented. We also intend to establish that the range of average absorbed/output power for which a robust singlemode operation is available can be shifted to fulfill user requests in term of power range.
Investigation of optical thin films printed on the surface of facets of photonic crystal fibers
M. Lucki, S. Kraus, R. Zeleny, et al.
Optical fibres are widely used in various applications as a medium for optical signals or optical transfer. This transport can be realized on long distance, compared to free space optics, which significantly extends reach of applications. Free space optics and fibre optics are combined in practice to yield the maximum performance of individual components forming a particular system. In such cases, light coupling from free space into fibres is required and it is frequently implemented with the use of lenses. An optical signal coupled into a fibre may also need certain modifications of spectral and spatial properties to allow its propagation down the fibre or reduce the amount of power carried in. The above requirement has been fulfilled by modifying surface of facets of photonic crystal fibres. By extrusion of a certain amount of host material from the surface, it is possible to obtain a structure resembling a thin film or an opaque layer for certain wavelengths. Several different structures of photonic crystal fibres and materials are considered to show influence of such thin-film on signal properties. This investigation is carried out in context of abilities of ablation of material from surfaces of photonic crystal fibres. Only certain shapes and geometrical arrangements can be considered. One of the goals is to specify, which of them are key for potential modification of spectral characteristics of photonic crystal fibres. The printed structures could potentially work like a thin-film ablation. Rigorous and versatile finite difference method has been employed to model propagation of light, its incidence onto a surface of the photonic crystal fibre, and subsequent propagation down the fibre. The simulations are carried on small pieces of photonic crystal fibres, with the length of tens of micrometres, due to well-known demands of the simulation technique on computational resources. Nevertheless, such a simplification is valid, since the structure is longitudinally uniform beyond the thin-film layer. However, this is aspect is not covered in the presented paper and it is our ongoing effort. Finally, the goal is to verify if the investigated structures can work as a slot waveguide.
Propagation of laser pulse with a few cycles in layered medium with time-dependent dielectric permittivity
Vyacheslav A. Trofimov, Elena S. Komarova, Eugeniy V. Pedan, et al.
Using computer simulation, we investigate a propagation of laser pulse with a few cycles in a linear layered medium with dielectric permittivity modulated in time. This means that we consider, so called, temporal photonic crystal. Such kind of crystals is produced, for example, by action of another laser beams propagating perpendicular to a propagation direction for a laser beam under consideration. Therefore, such photonic crystal may be induced for various ranges of frequencies, from optical range of frequencies to infrared range of frequencies, and may exist during time of the laser beam action. The process under consideration is described by 1D Maxwell’s equations. We investigated light energy localization in such time-dependent structure in dependence of both absolute phase of pulse, and duration of dielectric permittivity increasing. We believe that a considering scheme of laser pulse interaction with medium can be used in processing of data by optical method, for example.
Sensors and Devices based on Speciality Fibres
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Speciality optical fibres for astronomy
S. C. Ellis, J. Bland-Hawthorn
Astrophotonics is a rapidly developing area of research which applies photonic technology to astronomical instrumentation. Such technology has the capability of significantly improving the sensitivity, calibration and stability of astronomical instruments, or indeed providing novel capabilities which are not possible using classical optics. We review the development and application of speciality fibres for astronomy, including multi-mode to single-mode converters, notch filters and frequency combs.In particular we focus on our development of instruments designed to filter atmospheric emission lines to enable much deeper spectroscopic observations in the near-infrared. These instruments employ two novel photonic technologies. First, we have developed complex aperiodic fibre Bragg gratings which filter over 100 irregularly spaced wavelengths in a single device, covering a bandwidth of over 200 nm. However, astronomical instruments require highly multi-mode fibres to enable sufficient coupling into the fibre, since atmospheric turbulence heavily distorts the wavefront. But photonic technologies such as fibre Bragg gratings, require single mode fibres. This problem is solved by the photonic lantern, which enables efficient coupling from a multi-mode fibre to an array of single-mode fibres and vice versa. We present the results of laboratory tests of these technologies and of on-sky experiments made using the first instruments to deploy these technologies on a telescope. These tests show that the fibre Bragg gratings suppress the night sky background by a factor of 9. Current instruments are limited by thermal and detector emission. Planned instruments should improve the background suppression even further, by optimising the design of the spectrograph for the properties of the photonic components. Finally we review ongoing research in astrophotonics, including multi-moded multicore fibre Bragg gratings, which enable multiple gratings to be written into the same device simultaneously, femtosecond direct-write photonic lanterns and Bragg gratings, and complex notch filters and frequency combs using microring resonators, and plans for future astrophotonic instrumentation.
Nanostructured tapered optical fibers for particle trapping
Mark Daly, Viet Giang Truong, Síle Nic Chormaic
Optical micro- and nanofibers have recently gained popularity as tools in quantum engineering using laser-cooled, neutral atoms. In particular, atoms can be trapped around such optical fibers, and photons coupled into the fibers from the surrounding atoms could be used to transfer quantum state information within the system. It has also been demonstrated that such fibers can be used to manipulate and trap silica and polystyrene particles in the 1-3 μm range. We recently proposed using a focused ion beam nanostructured tapered optical fiber for improved atom trapping geometries1. Here, we present details on the design and fabrication of these nanostructured optical fibers and their integration into particle trapping platforms for the demonstration of submicron particle trapping. The optical fibers are tapered to approximately 1-2 μm waist diameters, using a custom-built, heat-and-pull fiber rig, prior to processing using a focused ion beam. Slots of about 300 nm in width and 10-20 μm in length are milled right though the waist regions of the tapered optical fibers. Details on the fabrication steeps necessary to ensure high optical transmission though the fiber post processing are included. Fiber transmissions of over 80% over a broad range of wavelengths, in the 700-11100 nm range, are attainable. We also present simulation results on the impact of varying the slot parameters on the trap depths achievable and milling multiple traps within a single tapered fiber. This work demonstrates even further the functionality of optical micro- and nanofibers as trapping devices across a range of regimes.
Specialty fibers for high power lasers and amplifiers
This paper reviews our recent work on novel large-mode area fibers for high power lasers and amplifiers. An ultra-low- NA fiber and single-trench fiber have been proposed for mode area scaling of the fundamental mode. In case of single-trench fiber design, resonant coupling of the higher order modes has been exploited to achieve effective single mode operation in fiber with large effective mode area. Our proposed fiber designs are easy to fabricate using conventional low-loss fiber fabrication techniques, and moreover, being all solid structure, they ensure easy cleaving and splicing. A monolithic and compact high power fiber laser/amplifier device with a good output beam quality can be achieved using Single-trench fiber design.
Fibre Design, Processing and Fabrication
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Soft glass photonic crystal fibres and their applications
Most of the research work related to photonic crystal fibres has to date been focused on silica based fibres. Only in the recent years has there been a fraction of research devoted to fibres based on soft glasses, since some of them offer interesting properties as significantly higher nonlinearity than silica glass and wide transparency in the infrared range. On the other hand, attenuation in those glasses is usually one or more orders of magnitude higher that in silica glass, which limits their application area due to limited length of the fibres, which can be practically used. We report on the development of single-mode photonic crystal fibres made of highly nonlinear lead-bismuth-gallate glass with a zero dispersion wavelength at 1460 nm and flat anomalous dispersion. A two-octave spanning supercontinuum in the range 700–3000 nm was generated in 2 cm of the fibre. In contrast to the silica glass, various oxide based soft glasses with large refractive index difference can jointly undergo multiple thermal processing steps without degradation. The use of two soft glasses gives additional degrees of freedom in the design of photonic crystal fibres. As a result, highly nonlinear fibres with unique dispersion characteristics can be obtained. Soft glass allow also development of fibres with complex subwavelength refractive index distribution inside core of the fibre. A highly birefringent fibre with anisotropic core composed of subwavelength glass layers ordered in a rectangular structure was developed and is demonstrated
Fabrication of three dimensional microstructure fiber
Ying Luo, Jie Ma, Zhe Chen, et al.
A method of fabricating three dimensional (3D) microstructured fiber is presented. Polystyrene (PS) microspheres were coated around the surface of a micro-fiber through isothermal heating evaporation induced self-assembly method. Scanning electron microscopy (SEM) image shows that the colloidal crystal has continuous, uniform, and well-ordered face-centered cubic (FCC) structure, with [111] crystallographic direction normal to the surface of micro-fiber. This micro-fiber with three-dimensional photonic crystals structure is very useful in the applications of micro-fiber sensors or filters.
Investigation of passive and active silica-tin oxide nanostructured optical fibers fabricated by "inverse dip-coating" and "powder in tube" method based on the chemical sol-gel process and laser emission
G. Granger, C. Restoin, P. Roy, et al.
This paper presents a study of original nanostructured optical fibers based on the SiO2-SnO2-(Yb3+) system. Two different processes have been developed and compared: the sol-gel chemical method associated to the “inverse dip-coating” (IDC) and the “powder in tube” (PIT). The microstructural and optical properties of the fibers are studied according to the concentration of SnO2. X-Ray Diffraction as well as Transmission Electron Microscopy studies show that the SnO2 crystallizes into the cassiterite phase as nanoparticles with a diameter ranging from 4 to 50 nm as a function of tin oxide concentration. A comparative study highlights a better conservation of SnO2 into the fiber core with the PIT approach according to the refractive index profile and energy dispersive X-Ray spectrometry measurement. The attenuation evaluated by the classic cut-back method gives respectively values higher than 3 dB/m and 0.2 dB/m in the visible (VIS) and infrared (IR) ranges for the PIT fibers whereas background losses reach 0.5 dB/m in the VIS range for IDC fibers. The introduction of ytterbium ions into the core of PIT fibers, directly in the first chemical step, leads to a laser emission (between 1050 and 1100 nm) according to the fiber length under 850 nm wavelength pumping. Luminescence studies have demonstrated the influence of the tin oxide on the rare earth optical properties especially by the modification of the absorption (850 to 1000 nm) and emission (950 to 1100 nm) by discretization of the bands, as well as on the IR emission lifetime evaluated to 10 μs.
Polymer Optical Fibre Based Sensors and Devices
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Bragg grating inscription in CYTOP polymer optical fibre using a femtosecond laser
A. Lacraz, M. Polis, A. Theodosiou, et al.
We report on the inscription of fibre Bragg gratings (FBGs) in CYTOP (cyclic transparent optical polymer) optical fibres. A femtosecond laser beam, operating in the visible wavelength range, is focussed into the core of the fibre for direct inscription of FBGs. The fibre is moved under the focussed beam by a nanometre-resolution air-bearing stage for maximal inscription precision. The grating plane dimensions (measured with bright field microscopy) are typically 30μm × 30μm × 1μm (line by line grating) or 10μm×1μm×1μm (point by point grating) and centred in the core of the fibre for optimal grating efficiency. The FBGs have a typical reflectivity of 70%, a bandwidth of 0.25nm and an index change of ~10-4. The FBG operate in the C-band, where CYTOP offers key advantages over poly (methyl methacrylate) optical fibres, having a significantly lower optical loss in the important near infra-red (NIR) optical communications window, with a theoretical loss of ~0.3dB/km at 1550nm. Additionally, CYTOP has a far lower affinity for water absorption and a core mode refractive index that coincides with the aqueous index regime. These properties offer several unique opportunities for polymer optical fibre sensing at NIR wavelengths, such as compatibility with existing optical networks, the potential for optical fibre sensor multiplexing and suitability for bio-sensing. We have investigated the temperature response of the grating: a linear positive shift of ~ +40pm/K has been measured with little difference between the heating and cooling response. The strain response of the FBG has also been studied with a linear shift of ~ +1.3pm/μɛ measured over a few hundreds of μɛ. We also demonstrated compatibility with a commercial Bragg grating demodulator.
Optimisation of polymer optical fibre based interferometric sensors
A numerical model for studying the performance of polymer optical fibre-based interferometric sensors is presented. The strain sensitivity of Fabry-Perot and two-beam interferometric sensors is investigated by varying the physical and optical properties corresponding to frequently used wavelengths. The developed model was used to identify the regimes in which these devices offer enhanced performance over their silica counterparts when used for stress sensing.
High performance liquid level monitoring system based on polymer fiber Bragg gratings embedded in silicone rubber diaphragms
Liquid-level sensing technologies have attracted great prominence, because such measurements are essential to industrial applications, such as fuel storage, flood warning and in the biochemical industry. Traditional liquid level sensors are based on electromechanical techniques; however they suffer from intrinsic safety concerns in explosive environments. In recent years, given that optical fiber sensors have lots of well-established advantages such as high accuracy, costeffectiveness, compact size, and ease of multiplexing, several optical fiber liquid level sensors have been investigated which are based on different operating principles such as side-polishing the cladding and a portion of core, using a spiral side-emitting optical fiber or using silica fiber gratings. The present work proposes a novel and highly sensitive liquid level sensor making use of polymer optical fiber Bragg gratings (POFBGs). The key elements of the system are a set of POFBGs embedded in silicone rubber diaphragms. This is a new development building on the idea of determining liquid level by measuring the pressure at the bottom of a liquid container, however it has a number of critical advantages. The system features several FBG-based pressure sensors as described above placed at different depths. Any sensor above the surface of the liquid will read the same ambient pressure. Sensors below the surface of the liquid will read pressures that increase linearly with depth. The position of the liquid surface can therefore be approximately identified as lying between the first sensor to read an above-ambient pressure and the next higher sensor. This level of precision would not in general be sufficient for most liquid level monitoring applications; however a much more precise determination of liquid level can be made by linear regression to the pressure readings from the sub-surface sensors. There are numerous advantages to this multi-sensor approach. First, the use of linear regression using multiple sensors is inherently more accurate than using a single pressure reading to estimate depth. Second, common mode temperature induced wavelength shifts in the individual sensors are automatically compensated. Thirdly, temperature induced changes in the sensor pressure sensitivity are also compensated. Fourthly, the approach provides the possibility to detect and compensate for malfunctioning sensors. Finally, the system is immune to changes in the density of the monitored fluid and even to changes in the effective force of gravity, as might be obtained in an aerospace application. The performance of an individual sensor was characterized and displays a sensitivity (54 pm/cm), enhanced by more than a factor of 2 when compared to a sensor head configuration based on a silica FBG published in the literature, resulting from the much lower elastic modulus of POF. Furthermore, the temperature/humidity behavior and measurement resolution were also studied in detail. The proposed configuration also displays a highly linear response, high resolution and good repeatability. The results suggest the new configuration can be a useful tool in many different applications, such as aircraft fuel monitoring, and biochemical and environmental sensing, where accuracy and stability are fundamental.
Testing and Characterisation Methods Applied to Special Fibre Types
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High-precision confocal reflection measurement for two dimensional refractive index mapping of optical fibers
Philippe Raisin, Jonas Scheuner, Valerio Romano, et al.
We introduce a new fiber-optical approach for reflection based refractive index mapping. Our approach leads to improved stability and reliability over existing free-space confocal instruments and significantly cuts alignment efforts and reduces the number of components needed. Other than properly cleaved fiber end-faces, this setup requires no additional sample preparation. The instrument is calibrated by means of a set of samples with known refractive indices. The index steps of commercially available fibers are measured accurately down to < 10-3. The precision limit of the instrument is currently of the order of 10-4.
Characterization of double-clad thulium-doped fiber with increased quantum conversion efficiency
Jan Aubrecht, Jakub Cajzl, Pavel Peterka, et al.
In this paper we present experimental results of characterization of the experimentally prepared thulium-doped optical fibers in double-clad hexagonal fiber geometry for cladding optical pumping at a wavelength of 793 nanometers. The fiber was fabricated by the modified chemical vapor deposition and solution doping method and coated with polymer with lower refractive index than silica. The fiber was characterized in views of its refractive index profiles, thulium ions concentration, spectral absorptions, fluorescence lifetime, and performance in fiber laser.
Transmission of red-laser radiation by using Bragg fibers with air cores
This paper presents experimental results on transmission characteristics of a Bragg fiber with an air core at a wavelength of 632 nm. The results are compared with those recently reported for the same fiber but at a wavelength of 1064 nm. The cladding of the fiber consists of three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index. A diameter of the air core is of about 50 μm. A theoretical modelling of Bragg fibers was carried out from which band gap characteristics of the fiber cladding were determined for the refractive-index contrast of the high- and low-index layers on a level of 0.03 and wavelength of 632 nm and 1064 nm.

Preforms of the Bragg fiber in the form of tubes were prepared by the MCVD method. Germanium dioxide and phosphorous pentoxide were used as silica dopants for the high-index layers. The low-index layers were fabricated of silica slightly doped with phosphorous pentoxide. The last layer applied was the high-index one. Bragg fibers were drawn from the tubes under controlled temperatures around 2000 °C in order to obtain the fibers with designed dimensions of Bragg claddings and air cores. Results of characterization of prepared fibers with optical microscopy are presented in the paper. The transmittance and attenuation of the fibers at 632 nm were measured with a continuous-wave He-Ne laser as a light source. Spatial distributions of output beams from the fibers were also determined.
Light-guidance in step-index fibers with non-circular shaped core
Jan Heimann, Philipp Raithel, Tim Tobisch, et al.
Step-index multimode fibers with non-circular core are becoming popular and offer several interesting new light-guiding properties, which can be studied with an emphasis on selective modes or ray patterns. Observing the far-field profile at the fiber output, ray-conversion can be determined, taking the azimuthal angle defined in respect to an adjusted reference into account. Using the inverse far-field method at one laser wavelength, the numerical aperture (NA) of these specialty fibers is determined as a function of excitation condition, such as input angle and rotation around the fiber axis. Further angledependent analysis of ring-shaped far-field pattern at the fiber output resulted in significant differences between the fibers with non-circular core shapes and classic circular fibers. In addition, with squared core fibers, the near- and farfield pattern at their respective fiber output were varying due to rotating the fiber around the fiber axis at their input side. Especially, a disturbance of the azimuthally homogenous ring shape was observed using input angles close to the numerical aperture of the fiber.
Poster Session
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Nanoimprint lithography using TiO2-SiO2 ultraviolet curable materials
Ultraviolet nanoimprint lithography has great potential for commercial device applications that are closest to production such as optical gratings, planar waveguides, photonic crystals, semiconductor, displays, solar cell panel, sensors, highbrightness LEDs, OLEDs, and optical data storage. I report and demonstrate the newly TiO2-SiO2 ultraviolet curable materials with 20-25 wt% ratio of high titanium for CF4/O2 etch selectivity using nanoimprint lithography process. The multiple structured three-dimensional micro- and nanolines patterns were observed to be successfully patterned over the large areas. The effect of titanium concentration on CF4/O2 etch selectivity with pattern transferring carbon layer imprinting time was investigated. CF4/O2 etching rate of the TiO2-SiO2 ultraviolet curable material was approximately 3.8 times lower than that of the referenced SiO2 sol-gel ultraviolet curable material. The TiO2-SiO2 ultraviolet curable material with high titanium concentration has been proved to be versatile in advanced nanofabrication.