This pdf file contains the front matter associated with SPIE Proceedings Volume 7839, including Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Two factors are the keys to the rapid commercial development of fiber lasers in the last decade. The first is the technology development of specialty fibers, especially double-clad rare-earth-doped silica-based optical fibers and the second is the development of high power pump diodes. I will take this opportunity to review our work in the development of specialty fibers and their applications in fiber lasers. I will try to cover three areas of development, mode-area scaling using leakage channel fibers, highly ytterbium-doped low-photo-darkening silica-based fibers which can be closely index-matched to silica, and silica suspended-core fibers for highly efficient super-continuum generations.

We review our recent progress in the design and fabrication of lead silicate glass fibers with high nonlinearity and tailored near-zero dispersion at telecommunication wavelengths. We have explored a range of different fiber structures, including suspended core fibers, holey fibers, all-solid microstructured and conventional W-type profiled fibers. The optical properties of the fabricated fibers are assessed both experimentally and through accurate numerical simulations. The relative merits of each fiber design are discussed and the significant potential of lead silicate highly nonlinear fibers for all-optical signal processing at telecommunication wavelengths is shown through reference to a number of key experimental demonstrators.

Fluoride glasses are very unique materials that transmit light continually from the UV to mid-infrared (0.3 to 9 μm) without any absorption peaks, and can be drawn into high quality optical fibers. They have been discovered at Rennes University in the mid-seventies, and have experienced an extraordinary and intensive development for more than 25 years for their outstanding optical properties. They have been first intensively developed for long haut telecommunication applications due to their ultra low theoretical optical loss (0.01 to 0.001 dB/km). After many years of intensive research, unfortunately, this goal has not been reached yet and remains a challenge. In the late nineties, the research activities around fluorides glasses and fibers have slowed down and only a few laboratories continue to have some ongoing activity focusing mainly on applications such as fiber lasers, Supercontinuum, spectroscopy and laser power delivery....

As both a waveguide and a gas/liquid transmission cell, photonic crystal fiber (PCF) allows synergistic integration of optics and microfluidics to form an unconventional optofluidic platform with long interaction path. In this paper, we report our strategy to achieve surface-enhanced Raman scattering (SERS) PCF optofluidics by polyelectrolyte-mediated immobilization of Ag nanoparticles (NPs) inside the fiber air channels. Through forward propagating Raman measurements and hyperspectral Raman imaging, we demonstrate the realization of SERS-active PCF optofluidics with accumulative Raman signal gain along the entire fiber length using both solid-core PCF (SC PCF) and hollow-core PCF (HC PCF). By numerical simulation and Raman measurements, we show that suspended-core PCF (SP PCF) consisting of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust platform of the three SC PCF microstructures investigated for evanescent-field SERS spectroscopy.

Microstructured optical fibers offer different possibilities for infiltration with unconventional fiber materials. By this way the propagation properties of the guided light can be modified in a very flexible way and new functionality in sensing or modulation can be introduced in optical fiber structures.

Photonic crystal fibers have been the subject of several studies for potential application in areas such as sensing, nonlinear optics, telecommunication and nanophotonics. Many applications are enabled by the possibility of selectively inserting gases, liquids, polymers and colloids into the internal microstructure, which results in efficient interaction with the guided light, allowing for the development of, e.g., sensitive chemical sensors also, the insertion of materials can be exploited to modify waveguide characteristics such as modal field distributions, the nonlinear coefficient and the chromatic dispersion. Experimentally, the insertion of liquids is particularly straightforward and enables many of the envisaged studies. However, evaporation is an important limiting issue, which ultimately prevents the realization long-term practical applications. Also, in some cases contact of the liquid with the external environment may degrade its properties. To address these issues, we experimentally demonstrate a new technique to selectively seal a liquid-filled hole of a photonic crystal fiber. The characteristics of the sealed fibers remained stable for at least a few weeks. Two experiments were, then, carried out to demonstrate the potential of the technique. In the first experiment, a water-core photonic crystal fiber was used for supercontinuum generation, with the generated spectrum not showing degradation over time. In the second experiment, a colloid of CdSe nanoparticles was inserted into the core of a fiber and stable photoluminescence was observed.

We review recent work on evaluating the performance of a simple porphyrin-based acid sensor using structured fibre technology. Specifically, the same sensor in a multimode liquid core is compared to that in a sol-gel coated structured optical fibre. General implications for fibre chemical sensing are discussed.

A new class of photonic crystal fiber, namely side-hole PCF, is analyzed and demonstrated. The presence of massive holes surrounding a microstructured cladding allows the realization of devices based on PCFs with integrated electrodes and fibers with high pressure sensitivity. The side holes can also be used to produce ultrahigh birefringence fibers based on squeezed-lattice structures and tunable single-polarization single-mode polymeric microstructured fibers.

We address the bandgap effect and the thermo-optical response of high-index liquid crystal (LC) infiltrated in photonic crystal fibers (PCF) and in hybrid photonic crystal fibers (HPCF). The PCF and HPCF consist of solid-core microstructured optical fibers with hexagonal lattice of air-holes or holes filled with LC. The HPCF is built from the PCF design by changing its cladding microstructure only in a horizontal central line by including large holes filled with high-index material. The HPCF supports propagating optical modes by two physical effects: the modified total internal reflection (mTIR) and the photonic bandgap (PBG). Nevertheless conventional PCF propagates light by the mTIR effect if holes are filled with low refractive index material or by the bandgap effect if the microstructure of holes is filled with high refractive-index material. The presence of a line of holes with high-index LC determines that low-loss optical propagation only occurs on the bandgap condition. The considered nematic liquid crystal E7 is an anisotropic uniaxial media with large thermo-optic coefficient; consequently temperature changes cause remarkable shifts in the transmission spectrums allowing thermal tunability of the bandgaps. Photonic bandgap guidance and thermally induced changes in the transmission spectrum were numerically investigated by using a computational program based on the beam propagation method.

We report the fabrication and optical characterization of the first hybrid chalcogenide-polymer microtapers. With this material combination, the chalcogenide wire induces a large Kerr effect whereas the polymer coating provides sufficient mechanical robustness and flexibility to the assembly for normal handling as well as limiting the evanescent interaction with the environment. A few centimeters of such a nonlinear microtaper can replace kilometers of highly nonlinear silica fiber. Becoming a building block in itself, the hybrid microtaper can also be used to assemble new highly nonlinear components such as nonlinear couplers.

We report on mode interferometers built with photonic crystal fibers (PCFs) and optical micro/nano fibers (MNFs). This type of mode interferometers exploit the beating between two modes, are very compact and highly stable over time for which they are suitable for a myriad of sensing applications. Moreover, their fabrication is simple since it can be carried out by means of cleaving and splicing or tapering techniques. The transmission spectrum of these interferometers typically exhibits truly sinusoidal interference patterns which simplifies their analysis. PCF-based mode interferometers may have niche applications since they are capable of operating at extreme temperatures (up to 1000ºC). To make these interferometers functional and competitive, our group has placed emphasis on the design of the PCF microstructure, minimizing the insertion losses, and on the elaboration of ad-hoc packaging for both harsh environment and biosensing applications. MNF-based interferometers, on the other hand, are extremely compact, require minimal amount of sample and can be combined with microfluidics for which they may be adequate for refractometric or biosensing applications. Adequate protection of the MNFs and ad-hoc microfluidics are being implemented to make MNF interferometers practical.

Optical microfibers decorated with PdAu nanoparticles are proposed for fast hydrogen sensing. The microfibers were obtained by simply tapering conventional telecommunications fiber down to dimensions comparable to the wavelength of the guided light. A few millimeters of the microfiber were coated with a PdAu layer in island form by depositing the layer at low evaporation rate (0.1 Å/s). Then the islands were grown with a thermal annealing process until composite nanoparticles were formed. The PdAu nanoparticles deposited on the optical microfibers experience optical and physical changes when they exposed to hydrogen. This gives rise to reversible transmission changes with an unusual pulsed like behavior which is attributed to scattering of the guided light. The devices are promising for detecting low concentrations of hydrogen (up to 8%) at room temperature with response and recovery times on the order of seconds.

Rare-earth doped fiber lasers and fiber amplifiers are highly attractive due to their efficiency, compactness, and, particularly, for their potential to various applications including communication systems, biomedical equipment, materials processing, LIDAR, and fiber-optic sensing. At the heart of these devices is the active fiber - most commonly based on silica host glass. However, the ability to dope silica glass fibers with high concentrations of erbium is limited due to clustering and nonlinear up-conversion - both of which degrade the efficiency of the gain fiber. Over many years, we have focused on developing highly doped phosphate glass fibers. The erbium concentrations can reach 4-5% weight erbium concentration without any negative effect to the optical gain. As a result, highly erbium doped phosphate glass fibers can produce large gain per unit length (typically 5 dB/cm) [1, 2]. This characteristic is a key enabler for a variety of optical devices that can make use of high optical gain in a short length - most notably high power single frequency fiber lasers and short length fiber amplifiers. In this presentation, we focus on two applications of the highly doped phosphate fiber. One is a high power, narrow linewidth single frequency fiber laser. The second is a fiber amplifier for coherent LIDAR applications capable of power scaling transform limited pulses without deleterious nonlinear effects. Both are examples of how this type of active fiber can lead to unique and superior performance.

A novel room-temperature multi-wavelength erbium-doped fiber ring laser is proposed. In this laser, four-wave mixing (FWM) is used to mitigate mode competition, achieving stable room-temperature multi-wavelength operation. Active mode-locking is incorporated to enhance the FWM effect, improving the amplitude flattening and spectral bandwidth of the multi-wavelength output. Stable simultaneous 42 wavelength lasing operation with 0.55 nm wavelength spacing within the 3 dB spectral band is experimentally demonstrated.

An all fiber based compact phase-locking fiber laser design is proposed by using a Talbot mirror fiber device (TMFD). We numerically demonstrate that periodically placed fibers lasers can be phased together using a large mode area (LMA) fiber element. The LMA fiber with a partial reflector at one end acts as a Talbot mirror, when the input beam to the LMA fiber travels one round trip in the LMA, it will self image back at the fibers' input.

The fabrication of a polarization-maintaining version of a large-mode-area multi-clad fiber design with high Yb concentrations and a robust output beam represents a significant challenge due to the high risk of cracking of the doped silica multi-clad next to the core during the drilling procedure. A new preform fabrication approach permitting the realization of a large first-clad fiber featuring a high birefringence, while preserving the preform integrity is presented. The birefringence was improved by locating the stress-applying-parts in the first-clad region and by increasing their boron content. The preform and fiber fabrication will be presented as well as the fiber performances in a pulsed amplifier configuration.

In the recent past we have studied the granulated silica method as a versatile and cost effective way of fiber preform production. We have used the sol-gel technology combined with a laser-assisted remelting step to produce high homogeneity Rare Earth or Transition Metal - activated microsized particles for the fiber core. For the fiber cladding pure or index-raised granulated Silica has been employed. Silica glass tubes, appropriately filled with these granular materials, are then drawn to fibers, eventually after an optional quality enhancing vitrification step. The process offers a high degree of compositional flexibility with respect to dopants; it further facilitates to achieve high concentrations even in cases when several dopants are used. By this "rapid preform production" technique, that is also ideally suited for the preparation of microstructured optical fibers, several fibers ranging from broadband emitters, PCFs and large mode area fibers have been produced and will be presented here.

We investigate the inherent gain flattening characteristics of an EDFA based on a highly asymmetric dual-core photonic crystal fiber for operation in the C-band. The gain flattening was achieved by exploiting the strong optical power coupling between the two cores like that in a directional coupler at the phase matching wavelength (λ _Ρ), which is designed to be around 1533 nm. The inner core is partially doped with erbium. The fiber refractive index profile parameters were so tailored such that a large fraction of the composite guided power flips from the un-doped outer core to the inner erbium-doped core at wavelengths greater than λ _Ρ. Thus the guided power at relatively longer wavelengths gets amplified more as compared to that at shorter wavelengths in the C-band. This phenomenon resulted in an effective flattening of the gain spectrum. Optimization of the design has led to an estimated median gain of ~ 21.2 dB with gain excursion within ± 1.25 dB within the C-band (1532-1562 nm). Results of this work should be of importance for realizing relatively inexpensive (due to cost saving on gain flattening filter head) and efficient EDFAs suitable for potential deployment in transparent wide area and metro networks.

The bending characteristics of all-solid photonic bandgap fibers (AS-PBGFs) are investigated for aiming to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for Yb-doped fiber applications. Through detailed numerical simulations, the impacts of the order of photonic bandgap (PBG) on the bending performance are evaluated and the limitation of core-size enlargement due to bending loss (BL) increase in the AS-PBGFs with a one-cell core structure is pointed out. In addition, it is found that the AS-PBGFs having a seven-cell core can achieve sufficient differential BL between the fundamental mode (FM) and the higher-order modes (HOMs) and a much larger effective area limit as compared with previously-reported index-guiding LMA fibers, by taking into account practical constraints.

Air-core photonic bandgap fibers offer many unique properties and are critical to many emerging applications. A notable property is the high nonlinear threshold which is the key for applications at high peak powers. The strong interaction of light and air is also essential for a number of emerging applications, especially those based on nonlinear interactions and spectroscopy. For many of those applications, much wider transmission bandwidths are desired to accommodate a wider tuning range or a large number of optical wavelengths involved. All demonstrated air-core photonic bandgap fibers so far have a cladding of hexagonal lattice. The densely packed geometry of hexagonal stacking does not allow large nodes in the cladding, which are essential for a further increase of photonic bandgaps. On the other hand, a photonic cladding with a square lattice can potentially provide much larger nodes and consequently wider bandgap. In this work, the potentials of much wider bandgap with square lattice cladding are theoretically studied.

The results of development of photonic crystal (microstructured, holey) fibers with large (12-35 μm) core are presented. Firstly, large core was comprised by substitution of 7 central holes in the triangular cladding lattice for the glass rod. In another method the core was shifted from the central position by one step of a lattice. In the third type of presented fibers so-called circular core was surrounded by 3 rings of holes; each of rings has contained 12 holes. Listed methods ensure improvement of the fundamental mode confinement in the fiber core. The investigations of modal consistence and resistance of the fundamental mode to bend are provided. Varying the dimensions of holes in the cladding it is possible to make the attenuation of higher order mode considerably stronger than the fundamental one. The fiber supports the single-mode regime, differs from conventional microstructured fibers by high resistance to bend and possesses an expanded operating spectral area.

Chalcogenide glasses are known for their large transparency in the mid-infrared and their high refractive index (>2). They present also a high non-linear coefficient (n₂), 100 to 1000 times larger than for silica, depending on the composition. An original way to obtain single-mode fibers is to design microstructured optical fibers (MOFs). These fibers present unique optical properties thanks to the high degree of freedom in the design of their geometrical structure. A classical method to realize MOFs is the stack-and-draw technique. However, with chalcogenide glasses, that technique induces optical losses at the interfaces in the stack of capillaries. In consequence, we have developed a new casting method to fabricate the chalcogenide preform. This method permits to obtain optical losses around 1 dB/m at 1.55 μm and 0.3 dB/m in the mid-IR region. Various chalcogenide microstructured fibers working in the IR range were prepared in order to take advantage of the non-linear properties of these glasses and of the original MOF properties. For example, fibers with small effective mode area (A_eff < 10 μm²) have been realized to exacerbate the non-linear optical properties. Such fibers will find applications for signal regeneration in telecom, and for the generation of supercontinuum sources. On the contrary, for military applications in the 3-5 and 8-12 μm windows, large effective mode area and single mode fibers have been designed to permit the propagation of high-power gaussian laser beams.

The potential of four-wave mixing conversion in As2S3 step-index and microstructured fibers for light generation in the 3-5 μm wavelength range is assessed using the Fully Vectorial Effective Index Method to obtain the dispersion curves of the microstructured fibers. We demonstrate that it is theoretically possible to obtain a signal wavelength located at 3.05 μm from a pump at 2.31 μm, with sufficiently broad linewidth to perform efficient frequency conversion. Thereby, we highlight the superiority of microstructure over classical step-index structure for efficient mid-IR generation through four-wave mixing process.

We present a new computational scheme to design supercontinuum spectra "à la carte" by means of Genetic Algorithms. Due to the potentially large amount of computations required by this strategy, the deployment of these heuristic algorithms is performed using distributed computing in the form of a Grid platform. The optimization procedure is automated within the Grid platform and permits escalation to large computational Grids. Some examples of designed supercontinua are given and potential applications for the design of future photonic devices are briefly described.

We numerically investigate supercontinuum (SC) generation in fibers with all-normal group velocity dispersion (GVD) under femtosecond pumping, including photonic crystal fibers (PCF), optical nanofibers and suspended core PCF. It is shown that all-normal dispersion (ANDi) fibers are ideally suited to generate extremely flat and more than octave spanning SC spectra which are highly coherent over the entire bandwidth. Due to the suppression of soliton fission in the normal GVD regime, the SC spectra are mainly generated by self-phase modulation and optical wave breaking dynamics, resulting in smooth spectral profiles without significant fine structure. A single pulse is maintained in the time domain, which can be externally compressed to the few-cycle regime. We present specific design examples of ANDi PCF for pumping at 1080 nm and extend the concept to optical nanofibers for deep ultraviolet (UV) SC generation at 400 nm pump wavelength as well as tapered suspended core PCF for visible and near UV SC generation at 465 nm and 530 nm pump wavelength. First "proof of principle" experiments confirm the basic findings of the numerical simulations and show the feasibility of the proposed SC generation scheme.

Fibre optic modal interferometry has been around for long as a sensing concept. Initially mainly supported on the utilization of standard Hi-Bi fibres associated to polarimetric modal interferometry, later this sensing approach evolved to modal interference based on spatial modes propagating in the core, and on spatial modes propagating in the core and in the cladding, with coupling performed by fibre devices such as long period gratings and tapers. More recently the outcome of Photonic Crystal Fibres (PCF) originated a burst of activity around the concept of modal interferometry for sensing. The reasons for that viewed in a historic perspective are presented in this work.

UV-induced Bragg gratings are written into the three concentric GeO₂-doped rings of an Yb³⁺-doped-core Cantor fractal photonic-bandgap fibre. These rings can support several modes and the effective indices of these modes are derived experimentally from the grating peaks. They are found to be in excellent agreement with numerical simulation.

We describe a new optical fiber coating, comprising layers of UV-curable silicone and high-temperature acrylate, with and without hermetic carbon. Optical and mechanical properties of graded index 50/125 μm multimode fibers drawn with the new coating are examined. The new coatings display superior thermal stability in comparison with conventional dual acrylate coatings.

Low index polymer claddings were developed and tested for use with silica core fibers. Polymers with varying indices of refraction were developed, so that numerical apertures useful for multiple applications were produced. High transmission over a wide wavelength range was obtained, both for films and for clad fibers. A refractive index as low as 1.363 was achieved, which results in a numerical aperture of 0.50 (at 852 nm) when used in cladding silica cores. Results for fibers clad with 1.373 index material under high temperatures (150 °C) show that worst case change in loss was within 0.084 dB, even over a time frame of 6400 hours.

Manufacturing processes for different types of hermetically coated fibers are described. Optical and mechanical properties of metal and carbon coated fibers are compared. Prospects of application of both types of hermetically coated fibers in special applications are discussed.

In this work it is investigated the strain and temperature sensing characteristics of modal interferometers supported by two Bragg fibers with different cross-section cladding geometries. It is shown that the sensitivity to these measurands is different for the two fibers, which turns feasible the conception of several sensing configurations based on the combination of these two fiber types for simultaneous measurement of strain and temperature.

A design methodology is presented for the enhancement of the Brillouin scattering properties of optical fiber for distributed sensing applications. Performance, such as sensitivity or resolution, of a distributed sensing system may be improved through the use of a fiber whose Brillouin gain spectrum is optimized for a desired application. Analogous to the design of a refractive index profile, the acoustic velocity profile may be tailored in order to realize the Brillouin spectrum. However, key to the implementation of the velocity profile is the selection of the appropriate compositional profiles. Design examples and dopant data are also presented and discussed.

An experimental setup and a method to obtain the Brillouin scattering spectrum (BSS) out of optical fibers are proposed. The setup is described and experimentally validated by developing the measurement of the Brillouin spectral distribution of a birefringent microstructuted optical fiber. The setup here proposed is based on a Brillouin ring cavity that uses the fiber under test as the active medium. The measurements are obtained in base band by beating the Stokes wave with a reference wave that is taken from the optical pump. The data can be obtained with high resolution frequency.

High power, high-energy, rare-earth-doped fiber laser sources are now considered a suitable option for a number of applications in Medicine, Telecom, LIDAR and industrial applications. Regarding the generation of high-energy pulses, there is a compromise between gain volume and M2 value or beam parameter product for the generation of pulses with useful energy content, for the aforementioned applications and good pulse shape and even a Gaussian output. In this paper, we will present a set of large mode area fiber designs along with full optical characterization data will be presented. Results on energy per pulse from 4 to 20KHz repetition rate and up to 1.3mJ energy for a 350um inner cladding LMA fiber will be discussed among other fiber designs. All fibres were made from proprietary procedures and will be employed in micromachining applications such as metal deposition in applications where laser-assisted cold spraying processes are used. A full review of our LMA fibre designs will be presented.

The guidance of the fundamental mode of microstructured fibers with a Ge-doped core can be cut off by filling the holes with a material, liquid in our case, that increases the refractive index in the air-holes up to a value between the refractive index values of the Ge-doped region and the silica. A section of such a liquid-filled fiber defines a short-pass filter widely tunable by adjusting slightly the refractive index values. Thus, a small change of temperature or a small strain of the fiber can be used to adjust the transmittance of the filter. Alternatively, temperature changes and strain could be determined by measuring the cutoff wavelength. A simple measurement of the power transmitted through a short section of liquid-filled fiber can be used to monitor temperature and strain, enabling the measurement of fast transients such as mechanical vibrations. Several Y-shaped fibers with a Ge-doped core were fabricated. This microstructure with only three big holes surrounding the core makes straightforward the filling of the fiber with liquids. The experimental characterization includes the measurement of the cutoff wavelength of a number of devices with different lengths, different microstructure geometries and different refractive index liquids. The cutoff wavelength and the transmission were measured as a function of temperature and strain. A simple theoretical simulation permits to explain the experimental results.

We demonstrate a highly-sensitive fiber optic sensor based on polarization mode beating (PMB) techniques for measuring changes in the optical pathlength (i.e., length and refractive index) of a laser cavity. This technique employs the two independent orthogonal modes from within the fiber laser to measure relative phase changes. By heterodyning the modes it is possible to obtain a beating signal and perturb only one of the modes so frequency changes can be measured. This results in the elimination of common intra-cavity noise and sensitivity enhancement. Frequency changes of the PMB signal are evaluated as a function of displacement, intra-cavity pressure and air density and monitored in real-time with high precision and accuracy. The high sensitivity and narrow laser linewidth show a potential application for ultra-sensitive biological measurements, chemical sensing and explosive detection.

In this work, a multiwavelength Raman fiber laser based on a highly birefringent photonic crystal fiber is presented. A laser resonator is formed when the Raman amplification with cooperative Rayleigh scattering in a dispersion compensating fiber is used as a distributed mirror and combined with a highly birefringent photonic crystal fiber loop mirror. The multiwavelength Raman fiber laser presents 11 stable channels per nm with a peak power of ~1.5mW. Stable multiwavelength lasing at room temperature is achieved due to the low sensitivity to temperature and environmental noise of the highly birefringent photonic crystal fiber based fiber loop mirror.

We propose a kind of novel super-lattice photonic crystal fibers (SL-PCFs) with uniform air holes formed basic cell structure, which can achieve high birefringence or large negative dispersion. Using the uniform air holes in the PCF has the advantage of minimizing the structural distortion during fabrication while forming a complex-structure cross-section. An effectively-elliptical-hole SL-PCF with high birefringence is proposed and investigated, which has the similar birefringence and confinement loss properties as the previously reported elliptical-hole PCF. We also propose a large negative dispersion SL-PCF, which has a huge potential for the application of dispersion compensation in the optical fiber communication system.

We prepare optrodes of fiber optic plastic with sol-gel technique. Suitable concentration of carbone nanotubes (CNTs), phenol red, bromophenol blue and cresol red, design optrodes with fiber optic plastic. The surface charge of silica and the refractive index, which play an important roll on the fiber, modifies the conditions of light propagation into the plastic optical fiber. We use the transmittance to measure the pH of a solution or fluid in a range between 3 and 9.

A three-wavelength ytterbium-doped fiber laser based on a long period grating induced mechanically in a twisted holey fiber is proposed and demonstrated. The long period grating is inserted into the laser cavity to introduce inhomogeneous loss in order to obtain up to three output laser wavelengths at room temperature. The lasing wavelengths are localized at 1081.5 nm, 1090.5 nm, and 1100.7 nm with an average wavelength separation of 9.6 nm which can be slightly modified by changing the twist rate of the holey fiber into the laser cavity.

In this work, it is described the fabrication and characterization of optical fiber refractometers based on lossy-mode resonances (LMR) originated by deposition of different thin-film coatings around the optical fiber core. Two devices with different coating materials are compared: one coated with conducting tin doped indium oxide (ITO) coatings and the other one coated with semiconducting indium oxide. The response of these devices is characterized and compared as a function of the external refractive index. The sensitivity obtained for indium oxide based refractometers resulted 39% higher than that of ITO based ones when the resonance is located in the same spectral region. This behaviour is attributed to the spectral characteristics of indium oxide, which allow an earlier generation of the resonance thanks to its higher refractive index as well as permitting the accomplishment of LMR conditions in a wider spectral range. Moreover, these devices are an adequate platform for the development of a wide variety of sensors by the addition of the suitable layer onto the transparent oxide coating.

The viability of an all-optical fibre optical thermocoupler for remote sensing of ultra-high temperatures, independent of electronics, is explored. Simple packaging of regenerated fibre Bragg gratings (FBGs) within silica capillaries were shown to protect the fibre structure sufficiently to allow the temperature measurement of a furnace element to >1500 °C; a temperature at which the regenerated FBG, which was written in relatively soft boron-codoped germano silicate glass, was shown to decay rapidly.

In this work, the transverse stress response of fiber Bragg gratings in large-mode-area micro-structured Panda-type fibers is numerically investigated. The grating wavelength shifts are studied as a function of external load and loading angle. Simulation results indicate that introducing air holes in the fiber reduces the nonlinear response of the grating to transverse stress. Furthermore, it is feasible to achieve the same sensitivity of gratings written in conventional Panda fibers, focusing the stress into the fiber core.

The integration of rare-earth doped optical fibers as part of fiber-based systems in space implies the development of waveguides tolerant to the radiation levels associated with the space missions. We report the spatial distribution, the photoluminescence (PL) properties of color centers and the related changes induced by X-rays radiation at different doses (50, 500 and 1000 krad) for two different prototypes of Er-doped optical fibers. Each sample (in the version pristine, X-irradiated and H2 loaded prior to radiation exposure) was characterized by confocal microscopy luminescence (CML) measurements in Visible range with Visible (488 nm) or UV (325 nm) laser light excitation. The set of tested fibers allowed us to obtain information on the radiation responses of the silica-based host matrix and on the transitions between the energy states of rare-earth ions. Under Vis-excitation, the luminescence spectrum of the core revealed the typical emission pattern of Er³⁺ ions, with an increase of the emission intensity around 520 nm due to the radiation treatment; whereas no spectroscopic change induced by radiation was observed when a particular sensitizing element is added to the core composition or when the fiber was previously H₂-loaded. The PL-core spectra under UV-excitation showed the behavior of the ODC, typical of the silica-based host matrix. For these spectra, addition of the sensitizing element annihilates the depressions that characterize the profile of ODC emission and that are due to the Er³⁺ ions absorption.

An index-guiding photonic crystal fiber with one hole adjacent to the solid core filled with liquid is theoretically and experimentally investigated. The use of an index-matched liquid induces efficient and short-range coupling from the solid core to the fluidic channel and back, which can be used to optically probe and/or pump the inserted liquid. Experimentally, the use of a micropipette allowed for selective filling the desired air hole. By varying the filled fiber length, the periodicity of the observed spectral modulation could be tuned. One fiber sample was then filled with a CdSe quantum dot colloid; pumping via the solid core resulted in photoluminescence that partially exited the fiber via the solid core, showing the filled waveguide's potential to offer distributed quantum dot pumping and an efficient collection method for the generated spontaneous emission.

We study the transmission of light through different lengths of Hollow-core bandgap fiber. We demonstrate 95% transmission of 5 picosecond pulses at 1064nm through fiber lengths of 1m, but only 77% transmission through longer lengths of 10m. This variation is not consistent with the measured attenuation of the "fundamental" low-loss mode of the fiber as being below 20dB/km in this spectral range, because the light transmitted through the short fiber not exclusively in the fundamental fiber mode. We conclude that great care is required to understand coupling efficiencies using short fiber lengths.

An analytic method based on Mueller matrix formalism and the Poincaré sphere is used to evaluate the birefringence dispersion in hexagonal photonic fibers with circular air-holes when the hexagonal symmetry is modified. This evaluation has been performed in the wavelength range from 1520 to 1570 nm, using monochromatic signals.

Heat treatments are important methods to provide safe foods. Conventional heat treatments involve the application of steam and recently microwave treatments have been studied and applied as they are considered as fast, clean and efficient. Optical fiber sensing is an excellent tool to measure the temperature during microwave treatments. This paper shows the application of optical fiber temperature sensing during the heat treatment of different foods such as vegetables (jalapeño pepper and cilantro), cheese and ostrich meat. Reaching the target temperature, important bacteria were inactivated: Salmonella, Listeria and Escherichia coli. Thus, the use of optical fiber sensors has resulted be a useful way to develop protocols to inactivate microorganisms and to propose new methods for food processing.

An all fibre Mach-Zehnder interferometric configuration based on a suspended twin-core fibre is described. Due to the birefringence of the fibre cores, two interferometers were obtained by illuminating the fibre with polarized light. Applying strain, curvature and temperature to the sensing head, different sensitivities were observed, which permits the use of the matrix method to discriminate these three measurands.

In this work, the birefringence response of two commercial single-mode erbium-doped fibers to right and left twist is reported. The evolution of the output polarization state for a variable twist (0 to ±8π) was mapped on the Poincaré sphere and analyzed considering the specific variation of each one of the Stokes parameters. These fibers were studied over the wavelength range from 1520 to 1570 nm. We found that the contributions of photo-elastic effect and torsion introduced during the fiber fabrication modify twist induced birefringence in erbium-doped fibers.

This paper presents results of experimental and theoretical studies of light transmission through optical fibers with disorder generated in its germanium-doped core via UV radiation transmitted through a diffuser. The experimental results on transmission of the radiation of 543 nm wavelength demonstrate the presence of the disorder in the core of the optical fiber - beyond a certain characteristic length, the transmitted power is observed to be distributed over all modes of the fiber. A theoretical model based on coupled mode theory is developed. An analytical expression for the mixing length is obtained and agrees well with the experiment. For long sections of disordered fiber, the experimentally measured distribution of the near-field intensity at the output surface of the fiber is well described by the Rayleigh negative exponential function. This suggests a statistically uniform distribution of the transmitted power over all modes, that agrees with the prediction of the theoretical model. The reported technique provides an easy way to fabricate different configurations of controlled disorder in optical fibers suitable for such applications as random fiber lasers.

We present an optical fiber liquid-level sensor which employs an array of plastic optical fibers coupled to a single semicylindrical refractometric detection element of plastic. The sensor measures the level of liquids in a discrete way and also is capable of discriminating between different liquids (such as gasoline and water) in a tank or reservoir.

We demonstrate an all-fiber polarization rotator optically activated based on polymer-azobenzene complexes via guest-host system. The azo compound (Disperse Red 1) was incorporated in two polymeric matrices (PMMA and PDMS), which were used as coatings for tapered optical fibers and fused tapered couplers. These devices were then incorporated in a fiber ring laser cavity and the optically induced birefringence was monitored via changes in the polarization state of the fiber laser output and emission spectra. We present experimental results showing that this polymer coated devices could provide a simple means for developing optically controlled polarization switches.

We present an experimental work of an erbium-doped optical fiber operating in the superluminescent regime. Experimental results for different pump power levels and fiber length show that the theoretical and numerical model could render useful information for predicting parameters such as total output power, spectral bandwidth and optimum fiber length to achieve the superluminescent regime. These types of sources could have direct application in wavelength multiplexed arrangements of fiber sensors, fiber gyroscopes or in general, in any sensors in which a broad wavelength and stable light source is required.

The critical power level provides an objective tool for the determination of the maximum power available in a fiber laser based on physical parameters as: core diameter, temperature, and absorption and emission cross section for both the pump and laser wavelengths. This work presents a theoretical study of critical power levels when Ytterbium-doped fibers are exposed to changes of temperature. We found that critical power curves extend their wavelength dependence, ranging from 1 μm to 1.2 μm when fibers were heated up 300°K. Also we found that critical power values are large than those obtained in conditions of room temperature. While low critical powers were obtained at lower temperatures (around 77°K) with a reduced interval of wavelengths from 1 μm to 1.1 μm.

Experimental studies of titania nanotubes (TiO₂) and tungsten oxide nanospheres (WO3) as devices of saturable absorption for a fiber laser in ring configuration to optical short-pulse generation are presented. A deposition technique, based on optical pressure radiation generated from a coherent source at 1550 nm is used to deposit the nanostructured materials. Since this nanomaterials can be deposited directly on the optical fiber, this proposal results very interesting for applications of saturable absorbers. Experimental results, by using nanotubes TiO₂ and nanospheres deposited on the fiber as a saturable absorption device, show that the TiO₂ nanotubes exhibit better saturable absorption properties than WO₃ nanospheres, generating pulses with a wavelength of 1550 nm, frequency of 10 MHz, temporally width of 4.5 ps and an output power of 1 mW.

In this paper we show numerically how a Gires-Tournois etalons (DGTE) is used to flatten the gain spectrum of an erbium-doped fiber amplifier. A broadband amplifier with uniform gain over 40-nm with a residual gain <2-dB is presented.

A novel all-fiber Multimode Interference (MMI) liquid level sensor is proposed and demonstrated. We show that MMI effects can be effectively applied for multiplexed liquid level sensing, and by selecting an adequate fiber, discrete and continuous level sensing is feasible. Using a standard 105/125 multimode fiber a simple discrete level sensor was fabricated, which can also can also discriminate the refractive index of the liquid. When a specialty fiber, know as No- Core fiber is used, both continuous and discrete level sensing can be achieved. We can also modify at will the continuous level range by increasing the No-core fiber by the appropriate length, while retaining the ability to determine the refractive index of the liquid during the level measurement. The MMI liquid level sensors are not only inexpensive, but their fabrication is quite simple.

We propose a single-mode optical fiber sensor for characterization of physical and chemical properties of liquids. The sensor is based on monitoring changes in the back-reflected signal from the interface between the fiber end-face and the liquid sample. Changes in the reflection spectrum are registered while dipping the cleaved end of an optical fiber into liquid samples and different spectral variations are observed owing as a consequence of characteristic properties, such as surface tension, viscosity and refractive index, among others. We present results obtained for different liquids (distilled water, methanol, glycerin, silicone, mineral oil) showing the feasibility of this approach for developing a simple fiber optic liquid analyzer.

We present a simple and inexpensive way to incorporate nanostructures in optical fibers based on optically driven transport of nanoparticles. The technique has been previously used to incorporate carbon nanotubes in fiber laser systems and relies on the deposition of nanostructures driven by the optical radiation propagating in the fiber. We demonstrate that this technique allows for incorporating graphite nanotubes and nanoparticles on the tip of the fibers and fiber connectors. Flow visualization techniques and thermal analysis of the deposition process show experimental evidence of the thermophoretic and pressure gradients involved in the incorporation of nanostructures onto optical fibers. Furthermore, we demonstrate the formation of micron sized bubbles on the tip of the fibers coated with nanostructures and show that this technique could render useful for developing micro bubble generators and mixers for micron sized structures.

Yb₂O₃ doped high yttrium alumino-silicate nano-particles based D- and pentagonal (P-) shaped optical fibers having core diameter around 30.0-35.0 μm have been obtained through the conventional modified chemical vapour deposition (MCVD) process and solution doping (SD) technique using thin wall tubes of CSA, around 58-60 mm², followed by enlargement of the inner deposition surface area under suitable pressurization. The process parameters at the different stages of preform fabrication have been optimized in order to obtain the uniform distributions of Al, Yb, and Y along the core diameter. Nano-particles were synthesized under soaking of the porous phospho-silicate core layer in a solution of optimum strength of ytterbium chloride, yttrium chloride, and aluminum chloride. The size of nano-particles was maintained within 5-10 nm under doping of 0.2 mol.% of fluorine. The EDX data reveal that the nano-particles are rich in yttria-alumino-silicate phase and dispersed uniformly. The novelty of this technique involves the direct synthesis of rare-earth doped phase-separated nano-particles within the large core of optical fiber. The critical fabrication parameters of the process along with the nano-structuration results and spectroscopic properties are highlighted.

We present an experimental study of light polarization on solid core microstructured optical fibers type (Large Mode Area) LMA-16, LMA-20 and LMA-25. These fibers were partially filled with one micron diameter polyethylene spheres or carbon nanotubes diluted in distilled H₂O by capillarity. Polarization characterization was realized on these Photonic Crystal fibers with air filled holes and partially particle filled holes using a He-Ne 633nm central wavelength laser at 10mW. The achieved results were compared using graphical data of every test collected at different exposition dates where the fiber was exposed to particle sedimentation. Results depicted that short time sedimentation of these particles does not change the light polarization leaving from the fiber except for the phase of the beam. On the other hand, longer time particle exposition visibly changes the light phase measured at the end of the photonic crystal fiber with repeatable results.

We demonstrate pulsed operation in a fiber laser based on a polarization switched three-mirror resonator. An electrooptic polarization switch is incorporated within the external cavity feedback thus allowing for polarization rotation of the beam returning to the resonator. Time and frequency domain analysis show that external polarization switching provides a frequency shifted feedback effect leading to a pulsed operation regime. Pulse rates five times higher than those used for driving the switch can be obtained owing to the population dynamics of the fiber laser.

In this work we experimentally demonstrated a wavelength-tunable fiber laser by the use of a Er:Yb double-clad fiber placed in a Fabry-Perot cavity formed by two Dichroic Mirrors (DM). We used a 976 nm wavelength pump laser. The maximal output power from this laser that we can introduce to the fiber was 20 W. We used a Diffractive Grating (DG) with 600 l/mm in our laser cavity to get wavelength tunability. The zero order reflection from the DG is the output coupler and the first order diffracted beam is reflected back into the fiber using a DM. With this configuration we can tune the wavelength from 1535 to 1567 nm by rotating a DM at a rate of 0.02 degrees/nm. With our configuration wide tunability was possible only when we used fiber lengths between 2.5 and 3.6 m. The maximal output power was 850 mW using 3.6 m of fiber length. We believe that a Q-switched extension of this source can be used in nonlinear optics applications, such as Terahertz generation.

In this work we study experimentally a novel passively mode-locked erbium-doped figure-eight fiber laser based on a polarization-imbalanced Nonlinear Optical Loop Mirror (NOLM). The NOLM operation strongly depends on the polarization state at its input. In this experiment, the input polarization state is set to linear, and its orientation is controlled through a half-wave retarder plate. The variation of the input polarization angle allows adjusting the NOLM switching power over a wide range. In this work we show experimentally that this adjustment makes it possible to tune the spectral bandwidth and the temporal properties of the generated pulses over a wide range.

We experimentally investigate optical switching based on stimulated Raman scattering in optical fibers. The experimental setup consists of two fiber stages connected in series with a spectral filter rejecting a signal inserted between them. When both pump and signal are launched to the input, the pump is saturated because of the signal amplification in the first stage; the amplified signal is rejected by the filter, so that only the low-power pump enters the second stage and no signal pulses appear at the output. When pump only enters at the input, it passes through the first stage without saturation, enters the second stage and amplifies the signal entering this stage; strong signal pulses appear at the output. We used 2-ns pulses at 1528 nm as the pump and a cw 1620-nm diode as the signal source for the first as well as for the second stages. In fibers with anomalous dispersion pump saturation was affected by modulation instability. We found that the contrast (the ratio of energies) may be improved using fibers with normal and anomalous dispersion connected in series in the first stage provided that the ratio between the lengths of the fibers with normal and anomalous dispersion is appropriately selected. The best achieved contrast was 15 dB at peak pump power of 6 W.

We present the temperature response of a mechanically-induced long-period fiber grating (MLPFG) made in photonic crystal fiber (PCF) with and without the coating polymer. In both cases, we found a wavelength shift to shorter wavelengths and a critical decrease of the attenuation peaks. A maximum wavelength shift of 6 nm at 1060 nm was obtained when the temperature changed from 20 to 80 °C in PCF without the polymer. Whereas, the depth of the attenuation peaks were dramatically reduced from 12 to almost 2 dB at 1060 nm when the temperature increase from 20 to 100 °C in both experiments. These results are important to consider when MLPFG are applied in a medium with room temperature variation.

Assuming that the imaginary part of the propagation constant of leaky modes is much smaller that its real part, we describe a procedure for the calculation of the effective refractive index of multilayer optical fibers. In our method there is no need to look for roots in the complex plane, since the differential equations for the transverse modal distribution are expressed in terms of the absolute value of the propagation constant.

This paper presents a 3-Axis enhanced alignment system for optical fiber. The arrangement uses in one of its axis a vision recognition system which employs Canny edge detector and Phase correlation. The other two-axis are aligned by controlling a couple stepper motors through displacement algorithms. The setup uses a commercial multimodal transmitter and receiver, multimode fiber, a digital microscope, three stepping motors and software. This automatic system takes an alignment time of 20 seconds and up to 70% of coupling power efficiency.

A sensor instrument able to measuring the thickness of different semitransparent objects with a resolution of one micron is described. This is based on a fiber optic reflectometer and a laser autofocus system and permit to measuring the thickness of thin surfaces such as semiconductor films, plastic materials and semitransparent objects. The response time for the measuring was roughly 2 sec and the thickness results were compared with a digital mechanical micrometer and both are in good agreement.

We show an approach to a tunable multi-wavelength fiber laser. The beam bending steel technique has been applied for our purpose. Variations on the relative distance between the fiber and the steel beam demonstrate alterations on important laser characteristics like, output power, wavelength shift and wavelength spacing.

A comparative analysis of ultraviolet light absorption is presented for distinct photosensitive optical fibers. Fibers are irradiated by ultraviolet light, coming from a pulsed Nd:YAG Laser (90 mJ, 5-7 ns) at 266 nm. Absorption is analyzed from ultraviolet to infrared region and it is compared for different photosensitive optical fibers. The final goal of this work is to identify absorption spectral regions, which would be useful in order to improve fiber grating printing.