This PDF file contains the front matter associated with SPIE Proceedings Volume 11355 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

We report on the inscription of a long period grating (LPG) in a multimode cyclic transparent optical polymer (CYTOP) fibre using the plane-by-plane femtosecond laser inscription method. The LPG was inscribed in the centre of the fibre core and tailored for operation at C-band wavelength range. The CYTOP-LPG sensitivity was characterised in transmission for relative humidity and temperature. The humidity measurements performed are the first for a POF-LPG, whereas the temperature sensitivity is significantly higher than reported in other works. In addition, dynamic mechanical measurements were performed comparing the mechanical characteristics of the laser exposed sections of the polymer fibre, where the LPG was inscribed, with the unexposed regions.

A significant amount of public funds in Cyprus are allocated for civil infrastructure construction and maintenance. As technology progresses, coupled with the need to increase the general safety of structures and citizens, the civil infrastructure is required to become “smart” by incorporating sensing technologies. An essential parameter to assess the health condition in concrete structures is the internal moisture which is linked with the concrete deterioration mechanisms, such as carbonation, frost, corrosion and crack formation and can be measured indirectly by monitoring the relative humidity (RH). This paper is a review of the latest results regarding the project “ACOReS”. During the research project, we developed a novel monitoring system based on polymer optical fibre sensors embedded in concrete structures. The proposed polymer sensors used to measure essential quantities in concretes, such as relative humidity, temperature and strain for estimating the health condition of the building structures.

The radiation of pulsed laser systems can generate changes in various materials. On the one hand, these modifications can be used for a variety of applications i.e. laser welding, cutting and many more [1]. The precision and quality depends on the material and laser parameters. On the other hand, material changes are not always desired in other applications. When using optical materials such as optical fibers as a light guide or as a sensor, laser-induced damage effects inside the fiber are to be prevented to ensure constant light guidance and the reliable monitoring of a desired parameter. Therefore, investigations for quality assurance need to be performed. For this reason, this work investigates laserinduced damage in polymer optical fibers (POF) using a nanosecond pulsed laser system at a wavelength of 532 nm. The impact of different laser and fiber parameters on the long-term degradation behavior is observed. In addition, the overall degradation behavior as well as the knowledge gained by analyzing the damage morphology and distribution will be used to obtain a better understanding of the damage mechanisms.

In this paper, we report a highly effective relative humidity (RH) sensor implemented on graphene oxide (GO) coated long period grating (LPG). The GO nanocolloides bonded onto a cylindrical fibre cladding enables the LPG with strong evanescent waves to absorb more water molecules increasing its RH sensitivity. In an LPG, the phase matching condition occurs when a forward propagating core mode is coupled with the co-propagating lower order cladding modes generating evanescent waves to interact with the surrounding medium. This unique effect of LPGs can be more enhanced with multilayer GO deposition. There is an expansion of GO film with the absorption of more water molecules as RH increases. The absorption of water molecules on GO coating increases the conducting carrier (holes) density on it, thus decreasing the refractive index of GO film. The combined effect of increasing evanescent waves and modulated refractive index makes the GO coated LPGs as effective RH sensors. Our recently achieved results have shown the RH sensitivity of the GO coated LPG is about 0.01 dB/%RH. We have also investigated the effect on GO layer thickness, showing thicker layer increases the RH response of the LPG cladding mode resonances in lower wavelength region.

We present new results regarding a novel melt spinning process for the continuous production of polymer optical fibers (POF) with a graded index profile. The fabrication process comprises a conventional melt spinning process of PMMA and a conclusive abrupt cooling of the monofilament. During the quenching, the outer part of the fiber cools down faster than the inner part, which leads to a density gradient within the fiber due to the time difference for thermal expansion along the fiber radius. This results in a radial density and by that in a refractive-index profile. The density of the POFs is determined with a pycnometer. Moreover, the structural properties of the fibers are investigated by small-angle X-ray scattering (SAXS). Lastly, the resulting optical parameters such as refractive-index profile, fiber attenuation and scattering properties, are studied and related to the manufacturing parameters. The measurements of the density and refractive index show that the refractive-index profile of a PMMA fiber can be strongly influenced in the outer sector of the fiber, but the influence decreases radially inwards. The SAXS measurements indicate different polymer chain structures within the fiber form.

Propagation time through standard optical fibres changes with temperature at a rate of 40 ps/km/K. This can pose significant challenges in many diverse application areas of optical fibres in physics and engineering. Primary examples lie in applications in which very precise timing signals need to be disseminated for synchronization purposes in large experimental infrastructures such as synchrotrons, linear particle accelerators, large telescope arrays, and in phase arrayed antennae. A value of 40 ps/km/K equates to a phase temperature sensitivity of about 48 rad/m/K. This can adversely affect many applications relying on fibre interferometers (e.g. fibre optic sensors, quantum-optics, interferometric measurement techniques, and so on), in which maintaining stable interference would require temperature stabilization below mK level. Similarly, a few key optical metrology applications require the dissemination of optical signals at a precise frequency, for example to compare distant ultra-precise clocks (e.g., national standard clocks) with a precision (fractional stability) at/below the 10-18 level. Such a level of precision is easily compromised by thermally-induced changes in optical path length (temperature drift) with time that unavoidably result in a Doppler frequency shift. Here, we review our recent results in which we show why and how Hollow-Core Fibres (HCF) are significantly better than solid-core fibres in terms of their sensitivity of propagation time and accumulated phase change to temperature and thus are a better alternative to standard fibres in the above-mentioned fields.

The paper describes the polymer-salt method of neodymium-doped aluminum yttrium garnet (YAG:Nd) crystals formation inside the channels of a microstructured silica fiber preform. The crystals formation was performed through the impregnation of inner surfaces of the channels by aqueous solutions of thermally decomposable salts (yttrium nitrate, aluminum nitrate, neodymium chloride) and an organic polymer with subsequent processes of drying and thermal treatment at the temperature of 1100°C. The composite structure prepared was drawn into the fiber at the temperature of 2000°C. The X-ray diffraction analysis revealed the formation of YAG:Nd crystals from 25 nm to 37 nm in size in the silica glass matrix of the fiber. Measurement of the attenuation spectral dependence confirmed the presence of optical signal absorption bands inherent to Nd³⁺ ions. The shape of the nanocrystals luminescence spectrum is characteristic to the YAG:Nd with a peak at the wavelength of 1064 nm.

Efficient high-power 2 μm fiber laser sources play a growing role in direct pumping of nonlinear crystals for frequency conversion into to mid-IR. There is an ongoing progress in fiber development and cavity improvement achieving outstanding laser performance for an efficient optical parametric generation. In this paper, we report on the investigation and characterization of a polarization-maintaining (PM) Thulium-Holmium-codoped triple-clad fiber (THTF). First fundamental studies were performed in a continuous-wave (CW) regime and showed highly promising results as a high power pump source for frequency conversion into the mid-IR. The paper focuses on first fundamental studies and the comparison of laser setups based on a 4.1 m and a 7 m active fiber length. Using the 7 m fiber, the THTF laser delivered an output power of 181 W. The laser had a degree of polarization of 98 %, a slope efficiency of 34.1 %, an optical-to-optical efficiency of 30% and a linewidth of 250 pm centered at 2050 nm. The laser output performance is compared with previous data of a THTF laser with a 4.1 m long fiber.¹

Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to maintain overlap with the core. This shaping is traditionally undertaken via diamond, or ultrasonic, milling of the fibre preform into an octagon or hexagon, leaving a rough surface which is subsequently fire polished before drawing into a fiber. We report on our developments of an alternative approach for shaping the inner-cladding using a 10.6µm pulsed CO2 laser to machine the fibre preform. This process is shown to allow fabrication of N sided polygon shaped fiber claddings as well as novel cladding structures, which include concave and convex surfaces, as well as core to cladding area ratio adjustment. Processing speed is significantly increased whilst maintaining improved surface qualities that remove the requirement of further fire polishing prior to fibre drawing. We will discuss recent developments of novel cladding geometries such as Reuleaux polygon cladding shapes that allow use of automated x-y profilometers on the draw tower whilst maintaining a shaped cladding for pump scrambling.

The high-power fiber lasers rely on the use of double-clad active fibers with noncircular symmetry of their inner-cladding cross-section. Therefore, the optical fiber preforms had to be shaped before fiber drawing. A new technique of preform-shaping by a CO2 laser is now available along with the conventional mechanical-based grinding. This innovative technique retains the advantages of enabling to produce complex inner-cladding shapes that not easily achievable by a conventional grinding technique. However, one of the drawbacks of the CO2 laser-based preform-shaping is weak of hydroxyl OH-groups reduction during the ablation process. The water is often penetrating into the preform surface via the oxyhydrogen flame during preform manufacturing. The thermophysical nature of the CO2 laser ablation process causes further diffusion of the OH-ions deeper towards the preform center during shaping. The diffused OH-groups in the glass material cause high attenuation at some wavelengths which are associated with the overtones of the fundamental OH absorption peaks. Unfortunately, some of these peaks lay rather close to the commonly used laser pumping wavelengths. This should be considered when designing a double-clad fiber laser as well as when selecting the preform-shaping technique. In this work, we will present a new method of mitigation of the water penetration into the optical fiber preform when a CO2 laser preform-shaping technique is applied. This method includes an optical fiber preform etching procedures prior to the preform laser shaping and to the fiber drawing. The acquired data helps also to predict the thickness of the layer that should be removed from the preform surface. The knowledge of the thickness of the optimal layers is of great benefit for the advanced estimation of the inner-cladding attenuation, an important parameter of double-clad fibers intended for high-power fiber lasers.

Holmium-doped silica-based optical fibers belong to intensively studied materials for fiber laser sources operating around 2.1 μm. In this contribution, we deal with silica-based optical fibers doped with holmium and aluminum oxide. The fibers were prepared by the modified chemical vapor deposition method combined either with a solution doping or a nanoparticle doping. A large set of fibers with various dopant concentrations was characterized related to their fluorescence lifetime, laser threshold and slope efficiency. The best-performance fibers exhibited a fluorescence lifetime longer than 1 ms, a laser threshold under 200 mW and a slope efficiency around 80%. These characteristics are discussed regarding the doping method and dopant concentrations.

We present a model for a Monte-Carlo simulation of polymer optical fiber fabrication based on a novel method using heat treatment after melt spinning. The polymer is modeled using three-dimensional bond-fluctuation with the Leonard-Johns potential for non-bonded interactions of different polymers alongside the bond, bend and torsion potential for the bonded interactions within the same polymer. The studied fabrication parameters were different cooling rates, pressure and the polymer chain length. Their influence is investigated on properties such as the radius of gyration R_G, physical and optical density ρ and n, and the isothermal compressibility, which will be affected e.g. by the phase-transition temperature T_g. They influence both mechanical and optical properties of the produced fiber.

The photonic nanojet (PNJ) was first reported by Chen et al.in 2004 through finite-difference time domain modeling of cylindrical structures under plan wave illumination. PNJ is a propagative beam with a full width at half maximum (FWHM) slightly smaller than a half wavelength. The power density can be more than 200 times higher than the one of the incident wave. This concentration can be achieved using a transparent dielectric microcylinder with a wavelength-scale radius. Gustav Mie obtained the exact solution of Maxwell’s equations for a dielectric microsphere in 1908. Applying this method, PNJ in the 3D case of a dielectric sphere was first reported by Li and Lecler. The photonic nanojet appears as a narrow and elongated spot with a high intensity electromagnetic field. The key parameters of PNJs, which are their FWHM, focal distance, decay length, and light intensity maximum, have been the subject of extensive theoretical and experimental studies. An in-depth understanding of photonic nanojet is nevertheless needed to fully exploit the potential of microspheres as optical super-lens. The original properties of PNJs make them ideal candidate in a wide range of applications, including nonlinear absorption enhancement, single-molecule fluorescence measurement, laser surgery, super-resolved microscopy, manipulation and detection of single sub-100 nm nanoparticles and biomolecules and laser sub-micro etching. Recently, the possibility to obtain a PNJ using an optical fiber with a shaped tip has opened new interests in the challenging field of direct laser subwavelength micromachining. To start with, the fiber is easier to move than the microsphere. Furthermore, the sphere is corrupted after the first irradiation due to their contact with the sample. On the opposite, the fiber tips have no contact with the processed surface and are not altered by the removed material. PNJs have already demonstrated the ability to reduce the laser etching size using shaped optical fiber tips. With a shaped optical fiber tips, the PNJ phenomenon is only due to the fundamental mode of the fiber. However, a standard single-mode fiber has three drawbacks for PNJ generation. (1) The small core diameter (<10 μm) makes it difficult to couple high power in the single mode fiber. (2) Given the small diameter of the core compared to the one of the optical cladding, the core does not see the curvature of the shaped tip; only the cladding is shaped. (3) The distance between the tip and the irradiated sample, in the magnitude order of the core diameter, is too short for industrial systems. We show how single mode fiber and especially large mode area (LMA) fiber can achieve the same process with 8 times less power, maintaining a reasonable working distance which may open new possibilities for direct laser micro-and nano-processing.

The confinement of high frequency acoustic waves inside a suspended core fibre (SCF) is numerically investigated for the first time. A 500 μm long acoustic cavity, based on a four-hole SCF, is designed, simulated and evaluated by using the finite element method. The cavity is acoustically excited in the frequency range of 50 - 56 MHz and the induced displacements are integrated along the fibre. A standard single mode fibre is simulated under the same conditions for comparison. The results show strong Lamb acoustic modes oscillating in the silica bridges and overlapping in the SCF core at the resonance of 52.84 MHz. The induced displacement achieves a maximum in the core centre decaying to an almost null value in the cladding. The acoustic wave concentration in the SCF core is 13 times higher compared to the standard fibre, indicating a promising solution to overcome the frequency limitation of the current all-fibre acousto-optic devices. The modulation efficiency is increased without reducing the fibre diameter, making the devices more stable, fast and suitable to modulate all-fibre lasers.

Space division multiplexing (SDM) technique is proposed to augment performance of short-reach optical transmission systems by utilizing 8-core multicore fiber (MCF) that typically degrades at high data rates for higher order modulation. Intercore crosstalk (XT) and higher order modulation format are the most challenging impairments of SDM based interconnection technology for efficient usage of MCF as OI. To satisfy the capacity requirements of emerging heterogeneous and bandwidth-intensive short reach communication links a frequency interleaving scheme is applied to MCF OI transmission systems. The negative effects of spectral overlap and intercore XT is reduced by shifting channel frequencies between adjacent cores. To combat explosive growth in data volume requirement of next generation highperformance short-reach OIs transmission higher order quadrature phase shift keying (QPSK) modulation in index profiled MCF system is proposed. Digital signal processing such as multiple input multiple output (MIMO) equalization are used to evaluate the symbol error probability in QPSK modulated SDM system.

This Conference Presentation, “Bioresorbable phosphate glass microstructured optical fiber for simultaneous light and drug delivery,” was recorded for the SPIE Photonics Europe 2020 Digital Forum.

Optogenetics is known as a technique that genetically targets specific neurons to express light sensitive channel proteins and provides the capability to stimulate the central nervous system with millisecond precision: Compared to deep brain stimulation, which affects bigger regions of the brain and thereby making it impossible to precisely stimulate individual cells, optogenetics offers the possibility of targeted neuron stimulation without affecting surrounding neurons. Numerous optogenetic devices have been developed for light delivery and simultaneous electrophysiological recordings. Most of them consist of an optical fiber and separate micro electrodes attached to the optical fiber. Even such small electrodes can induce tissue damage. In order to reduce the tissue damage, we present a fiber-based optogenetic light-delivery device with microelectrodes coated on the surface of the optical fiber. Our work uses metalized optical fiber, in order to deliver the light and simultaneously record the induced modulation (electrophysiological signals) with minimum tissue damage. Light delivery is done from the optical fiber tip, while microelectrodes are fabricated from the metal thin-film. These coating serves as the microelectrodes are used for the recording of electrophysiological signals. In this work, we present the details of electrode micro-structuring on the optical fiber using sputtering process. We also present and comment on the results of resistance measurements of the sputtered electrodes. Currently, we use copper for electrode fabrication, later we foresee to utilize gold as a biocompatible electrode material.

Biocompatible optical waveguides receive increasing attention owing to their application potential in the biomedical field. Our focus is on the fabrication and characterization of a new step-index biodegradable polymer optical fiber (bioPOF) using two commercial polyesters: poly(D,L-lactic-co-glycolic acid) (PDLGA) and poly(D,L-lactic acid) (PDLLA). Both polymers are regulated by US FDA, which allows projecting future clinical use of fibers made from these materials. We manufactured three preforms and we subsequently drew optical fibers with a standard heat-draw tower. We describe the chemical properties of the materials throughout the whole production chain from polymer granulates to preforms and then to optical fibers. We look into to the influence of the processing on the molecular weight and thermal characteristics of the polymers. Our step-index bioPOF with an outer diameter of 1000 ± 50 μm and a core of around 570 ±30 μm features record low attenuation of 0.26 dB∕cm at 950 nm for step index bioPOF, and a numerical aperture of 0.163. Immersion in phosphate-buffered saline (PBS) leads to hydrolytic degradation of the bioPOFs over a period of 3 months, accompanied with a 91% molecular weight loss. From the degradation study results, we anticipate that our bioPOFs can be used for biophotonic applications requiring deep tissue light delivery, such as photodynamic therapy.

We consider the sensing characteristics of a compact all-in-fiber Mach-Zehnder interferometer (MZI) inscribed using a femtosecond laser. The structure was created in the cladding of a single-mode optical fiber close to the cladding-air interface (<5μm gap) to deviate light from the core and encourage evanescent field interaction with the fiber’s surroundings. This compact device features a refractive index sensitivity beyond 5000 nm/RIU in aqueous solutions. Two fiber Bragg gratings (FBGs) were also manufactured, one in the pristine fiber core and the other in the cladding MZI and are monitored for sensing purposes. We used the plane-by plane (Pl-by-Pl) fabrication method, ensuring reliability and repeatability in the sensor development, as all gratings and MZI were inscribed with the same femtosecond laser parameters. We focus on the device response to changes in temperature, strain, bend, surrounding refractive index and relative humidity. By combining the compound sensor with a core FBG we produced a device capable of measuring multiple parameters using the same demodulation equipment, whilst simultaneously enhancing and individually separating each measurand of interest.

Grating-based Fiber Optic Sensors (FOSs), i.e. relying on Bragg Gratings (BGs), Long Period Gratings (LPGs), Tilted Fiber BGs (TFBGs), have seen a popularity in recent years for sensing applications, however, most of these are inscribed on Single-Mode Fibers (SMFs). Multi-Mode Fibers (MMF), on the other hand, offer new and different properties in grating design and performance characteristics compared to SMFs, since the spectral response may be tuned by core size, refractive index profile, numerical aperture, and mode coupling characteristics of the gratings. Also, MMFs can be readily coupled with inexpensive light sources and other optical components due to their large core and, thus, gratings in MMFs are preferred to yield lower cost systems. Moreover, in terms of sensing region, MMFs have a greater mode field surrounding the fiber when compared with SMFs, due to the larger core diameters of MMFs and, thus, even greater mode fields can be accessed with a smaller reduction of the fiber diameter which would have better mechanical robustness, when compared with gratings inscribed in SMFs. In this talk we present our latest research in BG structures inscribed in multi-mode optical polymer and glass fibers.

Due to circumstances beyond the presenters' control, a better quality audio recording was not possible.

A bismuth-doped fiber amplifier (BDFA) operating between 1650 nm and 1700 nm will be presented. This wavelength region is particularly interesting due to potential application is laser-based methane detection. However, typical output power from laser diodes operating in this spectral region is only between 5 and 15 mW which may limit sensitivity and/or detection range in some spectroscopic systems. Application of fiber amplifiers could help to overcome these limitations. BDFA presented in this paper provides output powers up to 80 mW at 1651 nm and 100 mW at 1687 nm. We analyze the noise at the output of the amplifier and demonstrate its application to photothermal spectroscopy of methane near 1651 nm.

Selective liquid filling of photonic crystal fibers opens up possibilities for tailoring their guidance properties or adding new functionalities. Among other techniques, 3D printing on optical fiber tips using two-photon polymerization has been applied for selective infiltration of individual air holes in photonic crystal fibers with liquids. However, in existing techniques care should be taken during the post-print photoresist development in order to avoid penetration of the developer solution into the air channels intended for filling. This limits the applicability of those methods. The technique proposed in this paper ensures that contamination of the air holes with the developer solvent is prevented. We apply two-photon polymerization lithography followed by an injection-cure-cleave procedure while omitting the post-exposure development. Selective filling of two fiber types is demonstrated. The first is a birefringent fiber with two rows of 3.6 μm air holes and one row of 0.9 μm holes in between. Another PCF has a hexagonal arrangement of 1.4 μm air holes. Our approach allows repeated selective filling to realize the infiltration of more than one liquid. Optofluidic fiber devices filled with one or more liquids have potential applications in the nonlinear optical domain and in the field of fiber sensing.

Gas sensors are crucial instruments for different industries and air quality monitoring. Micro-electro-mechanical system MEMS technology was proved as a solution for reducing the cost and size of spectrometers. Building gas cells with same technology enables the whole integration of the whole sensor. We present an HWG, as an example for integarted gas cell, which is fabricated on the silicon wafer using the MEMS technology. The HWG length is 2 cm long. The insertion loss of the HWG was measured. Carbon dioxide from exhaling and butane were measured using the HWG in conjunction with the MEMS based spectrometer. This proves the applicability of such HWG for portable gas sensors.

We show that such modes can be reflected from the optical fiber facet. Due to reflection, a locking effect occurs near the optical fiber facet for the whispering gallery modes with an axial component of the wave vector. Effective locking occurs due to the destructive interference of the waves coming from the excitation source and the waves reflected from the fiber facet. We obtained an analytical expression for determining the resonant wavelengths. We also experimentally characterized the reflectance of the fiber edge by studying dynamics of an optical pulse propagating towards the edge and reflecting from it.

The high-power operation of fiber lasers was enabled mainly by the invention of cladding pumping within a double-clad fiber structure. Various cross-sectional shapes of double-clad fibers as well as unconventional coiling methods have been investigated both experimentally and theoretically in order to enhance the absorption of the multimode-pump. With enhanced pump absorption efficiency, the double-clad fiber of shorter length can be used in the fiber devices and in such a way the unwanted effects of background losses and nonlinear effects can be mitigated. In this paper we report on numerical modelling of optical pump absorption in double-clad octagonal active fiber of different fiber geometry and layouts. Namely we investigate the effect of the bending radii, twist rate of the fiber, doped core area (holmium is considered in this as a doping ion) and pump beam shape. The numerical model is based on FEM-BPM method. The optimized geometries and layouts shall finally result in a highly efficient laser of small footprint without the need of water cooling with great potential for application with low power consumption, tightly limited space and weight requirements. Optimized design will also minimize risk of damage of the fiber during operation of the fiber laser.

In this paper, we experimentally investigate the response of CYTOP-based multimode fiber interferometer to variations of the laser’s wavelength. We analyze the interferometric signals of the polymer CYTOP fiber with core diameter of 50 μm for lengths of 1 to 20 meters, using a commercially available FBG interrogator operating for wavelengths from 1510-1590 nm. The analysis was implemented using the averaged characteristics approach. In addition, the results of this work were compared with those of obtained using a silica-based multimode fiber interferometer.

Spin-dependent effects on vortex light beams propagating in an inhomogeneous medium are demonstrated by solving the full three-component field Maxwell equations using the perturbation analysis. It is found that the hybrid Laguerre–Gauss modes with polarization-orbital angular momentum (OAM) entanglement are the vector solutions of the Maxwell equations in a graded-index medium. Focusing of linearly and circularly polarized vortex light beams in a cylindrical graded-index fiber is investigated. It is shown that the vortex light beam undergoes an additional transverse force acting differently on circular polarized beams with opposite handedness. The wave shape variation with distance taking into account the spin-orbit and nonparaxial effects is analyzed. Effect of long-term periodical revival of wave packets due to mode interference in a graded-index cylindrical optical fiber is demonstrated.

In this paper, a method for interrogation of Fibre Bragg Grating (FBG) and single mode-multimode-single mode (SMS) fiber for simultaneous measurement of temperature and strain using is proposed and demonstrated. A half etched FBG is deployed in the optical circuitry for sensitivity enhancement. The sensor exhibits the temperature and strain sensitivity of 20.2 pm/°C and 1.91 pm/με over the range of 20-200 °C and 100-2020 με respectively. The resolution for temperature and strain measurement is ±0.5 °C and ±12 με. The experimental results show that the sensor is able to measure strain and temperature simultaneously by sensitivity matrix with additional advantages such as simple structure, compact size, ease of fabrication, and low cost.

FBG in polymer optical fibers (POFs) is a promising technology for a wide range of sensing applications due to a lower Young’s modulus and a large range of applying strain. Furthermore, POFs have several properties which make them attractive for biosensing applications such as nonbrittle nature, flexibility in bending and biocompatibility. Chirped Fiber Bragg gratings (CFBGs), which are characterized by a nonuniform modulation of the refractive index show a broad reflection spectrum, enabling shortlength distributed sensing. The combining benefits of POF and CFBGs is attractive for biomedical applications. Here, we present a novel method to obtain CFBG in POF with a postprocess uniform POF FBG by using resin.

There are several techniques available for fabrication of phase-shifted gratings in single mode fibers. Yet, very few studies have examined inscription of such gratings in photonic crystal fibers (PCFs). In this paper, we report what we believe to be the first demonstration of the phase-shifted grating inscription in PCFs using a phase mask and a beam stop. The grating inscription is demonstrated for three hexagonal lattice PCFs with different air-filling fractions. The transmission spectra of the fabricated gratings reveal phase shift resonance peaks with a -3 dB bandwidth between 40 and 80 pm which is up to 4 times narrower than of the resonances of uniform Bragg gratings inscribed in the same fibers.

The granulated silica method for preform and fibre production offers a high versatility with respect to material composition and shape. Based on this composition flexibility along with the possibility to introduce dopants and co-dopants at high concentration levels, the refractive indexes of the passive and active fibre areas can be tailored in a wide range. In particular fibres with an inverted refractive index step (npassive>nactive) can be realized. A local lack of population inversion in Ytterbium-doped standard step-index fibres leads to reabsorption of the Ytterbium emission. In order to avoid this reabsorption, an “inversed” guiding fibre structure is proposed, where the light generating area and the guiding area are physically separated. This can be realized by inverting the refractive index step (npassive>nactive). By using the granulated silica method, the refractive index of both areas can be influenced and tailored in order to realize such an “inversed” guiding fibre structure. Within this research, the first results for two different fibre designs featuring the “inverse” guiding fibre structure are presented: 1) Guiding cladding structure: Al-passively doped cladding and Yb/Al-actively doped core with npassive,clad>nactive,core. The light will be generated in the active core but refracted into the passive cladding and guided by it (due to total internal reflection between the passive cladding and the surrounding air). 2) Guiding core structure: Al-passively doped core and Yb/Al-actively doped cladding with npassive,core>nactive,clad. The light will be generated in the active cladding but refracted into the passive core and only guided by it (due to total internal reflection between the passive core and active cladding). Here, the active area is based on the granulated silica method while for the passive core a sapphire rod has been used.

Water and oil detection as well as oil (or water) in water (or oil) estimation play important roles in the oil processing industry due to advantages such as reduction of the residence time of oil in oil-water separation units (or three-phase oil separators), increasing of the oil purity and avoiding environmental issues regarding the disposal of water contaminated with oil. In addition, it also enables the detection of water contamination in fuels with important impacts on the environment and oil quality. Advantages such as lightweight, multiplexing capabilities, water affinity and electromagnetic fields immunity make polymer optical fibers (POFs) an interesting option for detecting water and oil contamination. This paper presents the development of a POF sensor for oil, water and oil in water detection using chirped fiber Bragg gratings (CFBGs). The grating structure was inscribed in cyclic transparent optical polymer (CYTOP) fibers through the plane-by-plane method using a femtosecond laser. The CFBG is submerged in the liquids (water, mineral oil and their mixture) in order to verify the ability of such sensor of performing temperature independent measurements, which is accomplished by analyzing not only the central reflected wavelength, but also the full width half-maximum. The results indicate the capability of the proposed sensing approach of detecting oil and water. Furthermore, oil contamination as low as 0.1% (in volume) in water was detected using the CFBGs inscribed in CYTOP fibers. Therefore, the results depicted in this work can result in reliable systems for oil or water quality monitoring, which finds important application in oilfields and process industry.