This PDF file contains the front matter associated with SPIE Proceedings Volume 9128, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We investigate theoretically and experimentally the possibility of electrostatic actuation of nanomechanical optical fibers with integrated electrodes. The fiber has two optically guiding cores suspended in air by thin flexible membranes. This fiber structure allows for control of the optical properties via nanometer-range mechanical core movements. The electrostatic actuation of the fiber is generated by electrically charged electrodes embedded in the fiber cladding. Fiber designs with one to four electrodes are analyzed and, in particular, a quadrupole geometry is shown to allow for all-fiber optical switching in a 10cm fiber with an operating voltage of 25 - 30V. A multi-material fiber draw technique is demonstrated to fabricate a fiber with well-defined dual core structure in the middle and four continuous metal electrodes in the cladding. The fabricated fiber is analyzed and compared with the modeled requirements for electrostatic actuation.

Micro Optical Fibre Biosensors (MOFBs) are emerging as one of the most sensitive bio-detection system technologies which do not require of labelling or amplification of the analyte. In these devices, a short region of the fibre core is exposed to the external environment so that the evanescent field can interact with biological species such as cells, proteins, and DNA. In order to increase the sensitivity and selectivity, MOFBs are often used in combination with other optical transduction mechanisms such as changes in refractive index, absorption, fluorescence and surface plasmon resonance. In this work we present the full characteristics, analysis and design of a MOFBs for Flavin and Porphyrin detection.

We report in this communication on the investigation of novel triple-clad Very-Large-Mode-Area (VLMA) largepitch- fibers demonstrating numerically a strengthening of the fiber singlemodeness. The practical feasibility of the proposed fiber designs is ensured by taking into account the specifications of the resorted manufacturing technique. After a brief discussion about the mechanism of modal filtering into state-of-the-art air-silica leaky structures, we will present a novel kind of VLMA fibers. Here, the standard 6-fold symmetry as well as the periodic photonic crystal cladding are removed in profit to a totally aperiodic inner cladding microstructuration. This aperiodicity enables to exacerbate the singlemode robustness by maximizing the high-order-modes (HOMs) leakage out of the gain region in favour of the Gaussian fundamental mode. The behaviour of two different aperiodic LPFs is reported in regard to the refractive index of the inclusions, the doped core diameter and the operating wavelength. Thus, the scalability of these novel structures is evidenced. Finally, the manufacturability challenge is estimated by taking into account small index-mismatch between the active core and the background material.

Innovative Photonic Crystal Fibers (PCF) with optimized air-hole matrix, designed to break the C_6ν symmetry of the inner cladding while preserving their feasibility through the well-established stack-and-draw technique, are presented. The possibility to provide stable SM guiding at λ = 2 μm with core diameter up to 80 μm and a coupled pump power exceeding 300 W is analyzed by means of a full-vector modal solver based on the finiteelement method with embedded thermal model, to account for the effects of heating on the mode confinement. Simulation results have shown this approach is effective in providing modal discrimination, allowing selective amplification of the sole fundamental mode due to the delocalization of the high-order modes with mirrorsymmetric field distributions. Effective suppression of the high-order modes under a heat load of 340 W/m, while keeping an effective area exceeding 2500 μm² has been demonstrated.

Silica-based multimode fibers with a step-index refractive-index profile are commonly used for light transportation from 185 nm (DUV) up to 2300nm (NIR). Core diameters of such mono fibers range from 100 to 600 μm and their clad-tocore- ratios (CCR) typically are 1.1 and larger. If bundles are required for applications in the UV- and VIS-region, fibers with smaller core diameter and thinner cladding thicknesses are desired to reduce coupling losses, as their light acceptance surface area is then relatively larger. However, using these bundles at higher wavelengths, e.g. in the NIR-region, change of light-guiding properties can be observed. In fiber-optic light delivery systems, the transmission including light acceptance and guidance can be described by the concept of pupil apodization. However in fiber characterization, the numerical aperture (NA) of specialty fibers is an useful key parameter, which will be determined using the inverse far-field method at two separate laser wavelengths with focused light excitation. With parallel light, skew rays/modes must be taken into account. In addition, the spectral fiber attenuation with Uniform Mode Distribution (UMD) illumination and mode-selective illumination will be discussed and compared with experimental NA results. As expected, the ratio of cladding thickness and wavelength is the most important parameter on the light-guiding properties in short-length applications with increasing wavelength.

Supercontinuum (SC) generation in six core hexagonal lattice photonic crystal fiber (PCF) made of As₂Se₃-based chalcogenide glass was numerically investigated. The fiber was excited by 50 fs input pulse with hyperbolic secant field profile with tunability range of 2.5-5.5 μm. We obtained a flat-top normal dispersion profile when choosing Λ=1 μm and d=0.9 μm. By pumping near the zero dispersion wavelength, a high coherent SC spanning over 4 μm was generated with a launched pulse energy of E= 100 pJ in only 1 cm fiber length. The generated SC is therefore ideal for applications in ultrafast science, metrology, coherent control, non-destructive testing, spectroscopy, and optical coherence tomography in the mid-infrared region.

The application of shear stress sensors in structural health monitoring remains limited because current sensors are either difficult to implement, they feature a low measurement resolution or the interrogation of the output signal is complex. We propose to use fiber Bragg grating-based sensors fabricated in dedicated highly birefringent microstructured optical fibers. When embedded in a host material, the orientation angle of the fiber should be chosen such that their polarization axes are aligned parallel with the direction of maximum shear stress when the host is mechanically loaded. We present experimental results of sensors embedded in the adhesive layer of single lap and double lap structural joints. These tests demonstrate that when the joints are tension loaded, the embedded sensors have a shear stress sensitivity of around 60 pm/MPa. We study the influence of the adhesive material on the sensor response, as well as the influence of sensor orientation and location in the bond line. Finally, we demonstrate the minimal thermal cross-sensitivity of the shear stress sensitivity of this sensor.

Microstructured optical fibers (MOF) sometimes also referred to as photonic crystal fibers (PCF) have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the microstructure geometry (e.g. distribution and size of the air-holes) fiber properties can be significantly modified to better fit specific applications. In this manuscript we present a novel fiber design with three large air-holes neighboring the core and report on how the air-hole diameter influences the effective refractive index strain sensitivity. As direct measurement of the effective refractive index change may be complex and challenging, we propose to use fiber Bragg gratings (FBG) in our sensing set up. The Bragg wavelength is a function of the effective refractive index, hence the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Furthermore we also include an analysis of the fibers temperature sensitivity.

Controlling the water content within a product has long been required in the chemical processing, agriculture, food storage, paper manufacturing, semiconductor, pharmaceutical and fuel industries. The limitations of water content measurement as an indicator of safety and quality are attributed to differences in the strength with which water associates with other components in the product. Water activity indicates how tightly water is “bound,” structurally or chemically, in products. Water absorption introduces changes in the volume and refractive index of poly(methyl methacrylate) PMMA. Therefore for a grating made in PMMA based optical fiber, its wavelength is an indicator of water absorption and PMMA thus can be used as a water activity sensor. In this work we have investigated the performance of a PMMA based optical fiber grating as a water activity sensor in sugar solution, saline solution and Jet A-1 aviation fuel. Samples of sugar solution with sugar concentration from 0 to 8%, saline solution with concentration from 0 to 22%, and dried (10ppm), ambient (39ppm) and wet (68ppm) aviation fuels were used in experiments. The corresponding water activities are measured as 1.0 to 0.99 for sugar solution, 1.0 to 0.86 for saline solution, and 0.15, 0.57 and 1.0 for the aviation fuel samples. The water content in the measured samples ranges from 100% (pure water) to 10 ppm (dried aviation fuel). The PMMA based optical fiber grating exhibits good sensitivity and consistent response, and Bragg wavelength shifts as large as 3.4 nm when the sensor is transferred from dry fuel to wet fuel.

A focused CO₂ laser beam has been previously used to successfully fabricate both symmetric and asymmetric long period fiber gratings which have been used for a variety of sensing applications. However fabrication by a CO₂ laser beam demands a time consuming laser scanning process which increases the difficulty and cost of fabrication. In this paper a fibre sensor based on a fibre heterostructure with a simple configuration consisting of a series of periodical tapers in a photonic crystal fibre (PCF) sandwiched between two singlemode fibres is proposed and investigated experimentally. The tapers are periodically fabricated along the PCF section using a CO₂ laser beam. The proposed fibre heterostructure can be used for strain sensing by measuring the wavelength blueshift of the multimode interference dip of the transmission spectrum as a function of strain. An average stain sensitivity of -68.4 pm/μ ε has been experimentally achieved over a microstrain range from 0 to 100 μ ε. Assuming in practice that the sensor is interrrogated with a ratiometric power measurement system, then the strain resolution is estimated to be better than 1.18×10^-2 microstrain. The mechanisms for refractive index modulation periodically tapered PCF under tensile strain measurements are complex but may be regarded as a combination of stress-relaxation and refractive index perturbations over the length of the tapered PCF induced by strain and by tapering. The proposed fibre strain sensor has the advantage of low temperature sensitivity (average 8.4 pm/°C) and an experimental demonstration of this reduced sensitivity is also presented. The proposed strain sensor benefits from simplicity of fabrication and achieves a competitive sensitivity compared with other existing fibre-optic sensors.

In this paper, we report the fabrication and characterization of a new concept of optical fibers whose cladding is composed of palladium particles embedded into the silica glass cladding. Since conventional fiber processes are not suitable for such realizations, we developed an original process based on powder technology to prepare our specific preforms. Step, graded index and photonic crystal optical fibers with original shapes were realized. The use of high purity powders as raw materials combined to a specific preforms heat treatment allowed the fabrication of resistant and long length metal-cladding optical fibers. Microstructured Pd-SiO2 composite cladding optical fibers with single-mode behavior and optical losses lower than 2 dB/m at 1530 nm were characterized. Hydrogen-induced attenuation sensitivity of these fibers at the 1245 nm wavelength was demonstrated after long H2 exposure. Dehydrogenation kinetics calculations and experiments were studied.

Transverse light propagation through the air holed cladding of microstructured optical fibers (MOFs) has recently attracted interest owing to its importance for several applications such as fiber grating writing, particle trapping in hollow core fibers, all optical switch devices, etc. The air holes in the MOF cladding region impede efficient delivery of transversely propagating light to the core region when the fiber is illuminated from the outside. To overcome this problem we designed MOFs with a gradient air holed cladding structure that will actually focus transversely propagating light to the fiber core. We used photonic crystal Mikaelian lenses (PCML) as focusing gradient media in the holey cladding. First, we considered and compared several PCML types with varying air hole diameter and air hole pitch, in both hexagonal and rectangular lattices. We studied the design rules of those structures and we carried out a frequency response analysis to identify the most efficient operational regions of the PCMLs. The polarization properties of the considered structures were investigated as well. Finally, we analyzed the transverse coupling properties of the MOF equipped with such a PCML structure in the cladding and we obtained a fiber that clearly focuses transversely incident light to its core region with a normalized focal intensity up to 18, whilst exhibiting good guiding properties.

Visual access to physically inaccessible parts has become the forefront of research and development in medical diagnostics tools and procedures. Flexible and thin endoscopes with fiber bundle as an image conduit serves this purpose. However, when the light passes through the core of the fiberlet, it is blocked by the inter fiberlet gap. This structural limitation creates special honeycomb like pattern overlaying the image captured with the image fiber assisted probes, known as the comb structure or fiber pixelation. It obstructs the perception of the original image sacrificing resolution and contrast and inhibits the use of object recognition and tracking algorithms. Generally, comb structure removal or depixelation methods are employed to remove honeycomb pattern from an image. In the recent past, several depixelation techniques have been proposed albeit using different set of pixilated images by different researchers. It is quite difficult to make a comparison of their performances based on such images, as they adopt different images for different particular framework of their study. In this context, a basic database of such images is the need of the hour to meet the growing diagnostic needs in the medical and industrial arena. This paper in this context proposes and details a Comb Structure Affected Image database (CSAI) to meet the objective. Images are generated considering the image fiber specifications and the characteristics at different targeted optical imaging modalities delineated by resolution scales. The proposed database is designed to have a set of synthetically generated pixelated images of test patterns of different scales, sizes and shapes.

The Bragg wavelength of a PMMA based fiber grating is determined by the effective core index and the grating pitch, which, in temperature sensing, depend on the thermo-optic and thermal expansion coefficients of PMMA. These two coefficients are a function of surrounding temperature and humidity. Amorphous polymers including PMMA exhibit a certain degree of anisotropic thermal expansion. The anisotropic nature of expansion mainly depends on the polymer processing history. The expansion coefficient is believed to be lower in the direction of the molecular orientation than in the direction perpendicular to the draw direction. Such anisotropic behavior of polymers can be expected in drawn PMMA based optical fiber, and will lead to a reduced thermal expansion coefficient and larger temperature sensitivity than would be the case were the fiber to be isotropic. Extensive work has been carried out to identify these factors. The temperature responses of gratings have been measured at different relative humidity. Gratings fabricated on annealed and non-annealed PMMA optical fibers are used to compare the sensitivity performance as annealing is considered to be able to mitigate the anisotropic effect in PMMA optical fiber. Furthermore an experiment has been designed to eliminate the thermal expansion contribution to the grating wavelength change, leading to increased temperature sensitivity and improved response linearity.

Fiber Bragg grating (FBG) writing in PMMA microstructured Polymer Optical Fibers (mPOFs) using the UV Phase Mask technique is a time consuming process requiring about 40 minutes to inscribe a grating in an undoped fiber. Here we demonstrate the FBG inscription with the writing times shorter than 10 min. By careful alligning and increasing the beam intensity in the core of the fiber, writing times as short as 6 minutes and 50 second were achieved. The FBGs were written in a 125 μm PMMA mPOF having 3-rings of holes, the reflection peaks were centred at 632.6 nm and have a reflectivity as high as 26 dB. We also demonstrate how the writing dynamics depends on the intensity of the writing beam.

In this paper we report, for the first time to our knowledge, an increase of the photosensitivity of a microstructured polymer optical fibre (mPOF) made of undoped PMMA due to applied strain during the fabrication of the gratings. In the work, fibre Bragg gratings (FBGs) have been fabricated in undoped PMMA mPOFs with a hexagonal structure of three rings in the inner cladding. Two sets of FBGs were inscribed at two different resonant wavelengths (827 nm and 1562 nm) at different strains using an UV He-Cd laser at 325 nm focused by a lens and scanned over the fibre. We observed an increase of the reflection of the fibre Bragg gratings when the fabrication strain is higher. The photosensitivity mechanism is discussed in the paper along with the chemical reactions that could underlie the mechanism. Furthermore, the resolution limit of the material was investigated.

Step-index polymer optical fiber Bragg gratings (POFBGs) and microstructured polymer optical fiber Bragg gratings (mPOFBGs) present several attractive features, especially for sensing purposes. In comparison to FBGs written in silica fibers, they are more sensitive to temperature and pressure because of the larger thermo-optic coefficient and smaller Young’s modulus of polymer materials. (M)POFBGs are most often photowritten in poly(methylmethacrylate) (PMMA) materials using a continuous-wave 325 nm HeCd laser. For the first time to the best of our knowledge, we study photoinduced birefringence effects in (m)POFBGs. To achieve this, highly reflective gratings were inscribed with the phase mask technique. They were then monitored in transmission with polarized light. For this, (m)POF sections a few cm in length containing the gratings were glued to angled silica fibers. Polarization dependent loss (PDL) and differential group delay (DGD) were computed from the Jones matrix eigenanalysis using an optical vector analyser. Maximum values exceeding several dB and a few picoseconds were obtained for the PDL and DGD, respectively. The response to lateral force was finally investigated. As it induces birefringence in addition to the photo-induced one, an increase of the PDL and DGD values were noticed.

A detailed investigation regarding the applicability of the stationary phase point method in higher-order dispersion measurement is presented. The cases of dominant second-, joint second- and third-, dominant third- and dominant fourthorder dispersion were studied using simulations. We have come to the conclusion that the stationary phase point method is suitable for dispersion retrieval when the group-delay dispersion or the third-order dispersion term is the dominant, however, it became inaccurate in the presence of dominant fourth-order dispersion as the stationary phase point spread. The results obtained by simulation were compared with measurements performed on a second-order reduced, third-order dominant fiber sample. We found good agreement between simulations and the measurements.

We discuss both theoretical and experimental aspects of modal discrimination phenomenon that takes place in largemode- area photonic crystal fibers. A few special fiber designs providing efficient higher-order mode filtering were implemented and investigated. First adaptation had the core comprised of 7 elements (instead of 1) with a view to reduce the pitch, since smaller pitches correspond to lower bend-induced losses. That measure aided to realize a series of fibers with a 35-75 μm core diameter propagating only the fundamental mode within a wide spectral range due to embedded leakage channels for the higher-order mode which losses were rated to be above 1 dB/m. Second variation included the fiber with circularly distributed air holes surrounding a core of 30-50 μm in diameter. Circular geometrical configuration enabled leakage losses of the higher-order mode to be 120 times larger than leakage losses of the fundamental mode. Third adaptation had the alternation of large and small air holes (C_6V symmetry converted to C_3V symmetry) resulting in partial or complete delocalization of the higher-order mode power outward a core region. Fourth design represented the regular triangular-lattice structure with a core of 35-60 μm in diameter shifted from its usual location in the center of the lattice. The main idea consisted in provoking an enhancement of the higher-order mode discrimination, as higher-order mode has a larger field near to the air-hole silica interfaces compared to fundamental mode. Those fibers demonstrated distinguished bending resistance properties, since could be exploited with a bending radius of 2-3 centimeters.

Nonlinear photonic crystal fibers with small effective mode area allow to control chromatic dispersion in the near-infrared region. In this paper the chromatic dispersion is controlled entirely by structural parameters and the influence of each structural parameter is examined and described in detail. Understanding of the influence not only permits fiber design and dispersion tailoring, but also predicts the potential manufacturing tolerances. As a consequence, the fiber structural parameters are modified to found the balance between the operating bandwidth and the high negative dispersion parameter. We found that the limit value for the dispersion parameter is of about −1600 ps•nm^-1•km^-1 at 1550 nm whereas the desired dispersion slope is achieved over the 120 nm wide band. We predict that the negative dispersion parameter cannot be higher in the small effective mode area photonic crystal fibers operating over the bandwidth larger than the one considered in our paper. The results are calculated by the full-vectorial finite difference frequency domain method.

This paper proposes a measurement of nonlinear refractive index in the course of multi wavelength technique. We have generated a multi wavelengths formation by utilising a photonic crystal fibre (PCF) which mismatches zero dispersion wavelength from transmission wavelength at 1550 nm. We provide an experimental set-up in generating the multi wavelength phenomenon. A fibre ring laser configuration consists of erbium doped fibre amplifier (EDFA) set up and arrangement of FBGs is described. Encouraging results obtained from the set up proves the relations of signals generated through FBGs and new wavelengths. These findings shows, multi wavelengths able to present valuable inputs in determination of nonlinear refractive index parameter.

The production of special fibers relies on new methods and materials to incorporate new functionalities into optical fibers by virtues of dopants and structure. In particular, the granulated silica method allows to rapidly produce active fibers with high dopant content and with virtually any microstructure. The implementation of this production method requires a multitude of process steps at various temperatures and temperature gradients that can significantly influence the optical properties of the produced preforms and fibers. To better understand and optimize the processes of active material production and fiber drawing parameters we have done a thorough analysis of microstructure, phase development, crystallinity and chemical mapping of active fiber cores produced by a combination of sol-gel process and granulated silica method with and without employment of a CO₂ laser treatment. The microstructure of fibers have been analyzed with a diverse suite of techniques in Transmission Electron Microscopy (TEM), revealing formation of various silica polymorphs and distribution of active elements (i.e. Yb and P) into the core structure. Our results show the presence of another polymorph of silica with low crystallinity dispersed in the main amorphous polymorph (i.e. quartz). We conclude that in spite of importance of homogeneous distribution of Yb and P into the core, the formation of various silica polymorphs resulting from materials processing has to be considered.

In this paper, a novel design of tellurite photonic crystal fiber (PCF) is presented and analyzed. The proposed design is analyzed using full-vectorial finite-difference method (FVFDM). The analyzed parameters are the birefringence, effective mode area, nonlinearity and dispersion for the two fundamental polarized modes. The effects of the structure geometrical parameters on the modal properties are studied in detail. The numerical analysis reveals that the proposed design has high birefringence of 0.10568 and high nonlinearity of 4784 W^-1 Km^-1 and 4030 W^-1 Km^-1 for the quasi transverse magnetic (TM) and quasi transverse electric (TE) modes, respectively at the operating wavelength of 1.55 μm with low losses for the two fundamental polarized modes. Also, the dispersion of the reported design can be tailored to achieve flat and zero dispersion at the desired wavelength.