Distributed measurement of mode group effective refractive index difference in a few mode optical fibers.

The possibility to perform distributed measurements of the effective refractive index difference between distinct modes in few mode optical fibers is demonstrated using phase sensitive optical time domain reflectometry. Effective refractive index differences between LP02, LP21a and LP21b modes are measured with for a spatial resolution of 24m.

Sensitivity tailoring of an all-fiber bend sensor based on a dual-core fiber unbalanced Michelson interferometer.

In this work, a compact all-fiber bend sensor based on a dual-core microstructured optical fiber has been manufactured and characterized. The sensor relies on the unbalanced Michelson interferometric technique realized by attaching a piece of silica fiber to one of the fiber cores acting as the unbalancing element. Three probes with different lengths of the unbalancing element have been experimentally tested for sensitivity tailoring analysis. Additionally, a theoretical model has been developed for comparison and verification of the results. Good linear behavior of the spectral shift with bend has been measured and it has been proven that the sensitivity of the sensor depends on the length of the unbalancing element and the orientation of the cores. Higher sensitivities are achieved for shorter lengths of the unbalancing element and orientation of the core axis parallel to the bend direction. The highest sensitivity reported is 9.97 pm/µm for the case of 34 µm of unbalancing element and orientation of 0 degrees.

High-resolution distributed shape sensing using phase-sensitive optical time-domain reflectometry and multicore fibers.

In this paper, a highly-sensitive distributed shape sensor based on a multicore fiber (MCF) and phase-sensitive optical time-domain reflectometry (φ-OTDR) is proposed and experimentally demonstrated. The implemented system features a high strain sensitivity (down to ∼0.3 µÉ›) over a 24 m-long MCF with a spatial resolution of 10 cm. The results demonstrate good repeatability of the relative fiber curvature and bend orientation measurements. Changes in the fiber shape are successfully retrieved, showing detectable displacements of the free moving fiber end as small as 50 µm over a 60 cm-long fiber. In addition, the proposed technique overcomes cross-sensitivity issues between strain and temperature. To the best of our knowledge, the results presented in this work provide the first demonstration of distributed shape sensing based on φ-OTDR using MCFs. This high-sensitivity technique proves to be a promising approach for a wide range of new applications such as dynamic, long distance and three-dimensional distributed shape sensing.

Analysis of phase sensitivity to longitudinal strain in microstructured optical fibers.

We investigate the influence of air holes on phase sensitivity in microstructured optical fibers to longitudinal strain. According to the numerical simulations performed, large air holes in close proximity to a fiber core introduce significant compression stress to the core, which results in an increase in the effective refractive index sensitivity to longitudinal strain. The theoretical investigation is verified by an experiment performed on four fibers drawn from the same preform and differentiated by air hole diameter. We show that introducing properly designed air holes can lead to a considerable increase in normalized effective refractive index sensitivity to axial strain from -0.21 ε-1 (for traditional single mode fiber) to -0.14 ε-1.

Cross talk analysis in multicore optical fibers by supermode theory.

We discuss the theoretical aspects of core-to-core power transfer in multicore fibers relying on supermode theory. Based on a dual core fiber model, we investigate the consequences of this approach, such as the influence of initial excitation conditions on cross talk. Supermode interpretation of power coupling proves to be intuitive and thus may lead to new concepts of multicore fiber-based devices. As a conclusion, we propose a definition of a uniform cross talk parameter that describes multicore fiber design.

Study on the Sensing Coating of the Optical Fibre CO2 Sensor.

Optical fibre carbon dioxide (CO2) sensors are reported in this article. The principle of operation of the sensors relies on the absorption of light transmitted through the fibre by a silica gel coating containing active dyes, including methyl red, thymol blue and phenol red. Stability of the sensor has been investigated for the first time for an absorption based CO2 optical fiber sensor. Influence of the silica gel coating thickness on the sensitivity and response time has also been studied. The impact of temperature and humidity on the sensor performance has been examined too. Response times of reported sensors are very short and reach 2-3 s, whereas the sensitivity of the sensor ranges from 3 to 10 for different coating thicknesses. Reported parameters make the sensor suitable for indoor and industrial use.

Experimental study of dispersion characteristics for a series of microstructured fibers for customized supercontinuum generation.

We demonstrate an experimental study of the chromatic dispersion properties for a series of microstructured fibers (MSFs) dedicated for a supercontinuum generation. With white-light interferometry application we analyze experimentally how the small variations of structural parameters, i.e. an air-hole diameter and a lattice constant, influence dispersion characteristics in different groups of MSFs. Our study provides useful information on how to design the fiber which is less sensitive to the fabrication imperfections. Moreover those investigations are the initial step to the development of the customized or tunable supercontinuum light sources based on MSFs with slightly changed structural parameters which can generate light with a different spectrum range, adapted to a proper application.

Fiber Bragg grating inscription in few-mode highly birefringent microstructured fiber.

In this Letter, we present the technology of fiber Bragg grating (FBG) inscription in highly birefringent (HB) few-mode microstructured fibers (MSFs) with two different (nanosecond and femtosecond) lasers in a Talbot interferometer setup. The spectral characteristics of FBGs written in the core region of the investigated fiber, with particular modes represented by dual peaks, are presented and discussed. Furthermore, we calculate the fundamental fiber parameters (mode effective refractive index and phase modal birefringence) from the spectral characteristics and show very good agreement with the performed numerical fiber characterization. We expect the results of our experiments to be very useful in future development of FBG sensors based on novel HB MSFs, with enhanced strain sensitivity of higher-order modes.

Highly birefringent dual-mode microstructured fiber with enhanced polarimetric strain sensitivity of the second order mode.

We present the results of theoretical and experimental characterization of a designed and manufactured dual-mode highly birefringent microstructured fiber. We also demonstrate the measured values of polarimetric temperature and strain sensitivity of both the fundamental and second order modes. As the mode field of the second order mode has a strong interaction with the fiber air holes, we observed a significant (over two orders of magnitude) increase in the polarimetric strain sensitivity of this mode in comparison to the fundamental mode. The enhanced strain sensitivity together with the low temperature sensitivity makes our fiber very attractive for application as extremely sensitive temperature independent strain transducers.

Large-mode-area photonic crystal fiber with double lattice constant structure and low bending loss.

We report on a bendable photonic crystal fiber for short pulse high power fiber laser applications. This fiber uses a double lattice structure and enables single mode operation with a very large mode area that reaches 1454 µm² when the fiber is kept straight and 655 µm² in the fiber bent around a 10 cm radius. Single mode operation is enforced by the very large bending loss in excess of 50 dB/m experienced by the higher order modes, whilst bending loss for the fundamental mode is smaller than 0.01 dB/m. We outline the principles of our fiber design and we explore the guiding properties of the fiber.

Extremely large-mode-area photonic crystal fibre with low bending loss.

We report on the design of a novel flexible very large mode area photonic crystal fibre for short pulse high peak power fibre laser and beam delivery applications. This fibre has an extremely large mode area exceeding 2500 microm(2) when kept straight and over 1000 microm(2) when bent over a 10 cm radius at a wavelength of 1064 nm. In addition our fibre exhibits very small fundamental mode bending loss below 10(-2) dB/m. The large difference between the propagation loss levels of fundamental and higher order modes forces efficient single-mode guidance in the fibre core while bent. This allows using the fibre to build compact high power laser systems. The paper further explores the major features of this fibre including: the dependence of the mode field area on the fibre core shape, the influence of the bending radius and of the bending direction as well as the impact of manufacturing tolerances on the fibre specifications.

Highly birefringent microstructured fibers with enhanced sensitivity to hydrostatic pressure.

We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 microm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.

Point-by-point fiber Bragg grating inscription in free-standing step-index and photonic crystal fibers using near-IR femtosecond laser.

We report what we believe to be the first highly symmetric first-order IR femtosecond laser fiber Bragg gratings within the telecommunications C band in free-standing optical fiber, fabricated with a relatively low NA lens and without use of oil immersion techniques. This grating features the smallest dimensions for a point-by-point fiber grating reported so far (to our knowledge). This achievement paves the way to rapid mass manufacturing of highly efficient and stable Bragg gratings using ultrafast lasers in any type of fiber. Mastering this femtosecond grating inscription technique also allowed the fabrication of the first Bragg gratings with direct near-IR femtosecond inscription in photonic crystal fibers, and without the use of techniques that rely on the compensation of the holey structure.

Modular coherent photonic-aided payload receiver for communications satellites.

Ubiquitous satellite communications are in a leading position for bridging the digital divide. Fulfilling such a mission will require satellite services on par with fibre services, both in bandwidth and cost. Achieving such a performance requires a new generation of communications payloads powered by large-scale processors, enabling a dynamic allocation of hundreds of beams with a total capacity beyond 1 Tbit s-1. The fact that the scale of the processor is proportional to the wavelength of its signals has made photonics a key technology for its implementation. However, one last challenge hinders the introduction of photonics: while large-scale processors demand a modular implementation, coherency among signals must be preserved using simple methods. Here, we demonstrate a coherent photonic-aided receiver meeting such demands. This work shows that a modular and coherent photonic-aided payload is feasible, making way to an extensive introduction of photonics in next generation communications satellites.

Design and tolerance analysis of a low bending loss hole-assisted fiber using statistical design methodology.

We present the design of a low bending loss hole-assisted fiber for a 180?-bend fiber socket application, including a tolerance analysis for manufacturability. To this aim, we make use of statistical design methodology, combined with a fully vectorial mode solver. Two resulting designs are presented and their performance in terms of bending loss, coupling loss to Corning SMF-28 standard telecom fiber, and cut-off wavelength is calculated.

Dual-core all-fiber integrated immunosensor for detection of protein antigens.

An optical fiber interferometric microprobe for detection of specific proteins is presented in this paper. The microprobe is an all-fiber device, which is based on Michelson interferometer configuration, which allows for detection of protein antigens in an analyzed solution thanks to antibodies immobilized on the sensor surface. The interferometer is made of dual core fiber and has a precisely formed arm length difference, achieved by splicing a fragment of polarization maintaining fiber to one of the cores. An all-fiber configuration of the sensor decreases substantially cross-sensitivities to temperature and deformation in comparison to other optical fiber interferometers. Reported sensor has a sensing region on the tip of the interferometer and therefore may be used for point measurements in medicine. The immunosensor and optical measuring system are designed to utilize the most common broadband light sources that operate at a central wavelength of 1.55 µm. The results show that it is possible to detect a protein antigen present in a solution by using an all-fiber interferometer coated with specific antibodies. The resulting peak shift can reaches 0.6 nm, which is sufficient to be measured by an optical spectrum analyzer or a spectrometer. A model allowing for estimation of the value of lower limit of detection for such sensors has been elaborated. The elaborated detection system may act as a framework for detection of various antigens and thus it can find future applications in medical diagnostics.

Enhanced cross phase modulation instability in birefringent photonic crystal fibers in the anomalous dispersion regime.

We study Cross Phase Modulational Instability (CPMI) -a particular form of vector modulational instability- in the anomalous dispersion regime in highly birefringent, strongly dispersive optical fibers. When the pump power is high, the detuning of the Scalar Modulational Instability (SMI) is comparable to the detuning of the CPMI. The gain of the CPMI -which is usually much smaller than the gain of the SMI-, is then strongly enhanced and becomes much larger than the gain of the SMI. This theoretical prediction is well verified experimentally using small core photonic crystal fibers.

New Methods of Enhancing the Thermal Durability of Silica Optical Fibers.

Microstructured optical fibers can be precisely tailored for many different applications, out of which sensing has been found to be particularly interesting. However, placing silica optical fiber sensors in harsh environments results in their quick destruction as a result of the hydrolysis process. In this paper, the degradation mechanism of bare and metal-coated optical fibers at high temperatures under longitudinal strain has been determined by detailed analysis of the thermal behavior of silica and metals, like copper and nickel. We furthermore propose a novel method of enhancing the lifetime of optical fibers by the deposition of electroless nickel-phosphorous alloy in a low-temperature chemical process. The best results were obtained for a coating comprising an inner layer of copper and outer layer of low phosphorous nickel. Lifetime values obtained during the annealing experiments were extrapolated to other temperatures by a dedicated model elaborated by the authors. The estimated copper-coated optical fiber lifetime under cycled longitudinal strain reached 31 h at 450 °C.

Experimental and theoretical investigations of birefringent holey fibers with a triple defect.

We have manufactured and characterized a birefringent holey fiber of a new construction. The birefringence in this fiber is induced by the highly elliptical shape of the core, which consists of a triple defect in a hexagonal structure. Using a hybrid edge-nodal finite-element method, we calculated the spectral dependence of phase and group modal birefringence for spatial modes E11 and E21 in idealized and in real fiber, whose geometry we determined by using a scanning-electron microscope. Results of our calculations show that technological imperfections significantly affect the fiber's birefringence. Normalized cutoff wavelengths for higher-order modes relative to the filling factor were also determined for the idealized structure. We observed a significant disagreement between theoretical and experimental values of cutoff wavelengths, which was attributed to high confinement losses near the cutoff condition. We also measured the spectral dependence of the phase and the group modal birefringence for spatial modes E11 and E21. The measured parameters showed good agreement with the results of modeling.