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.

Long-distance distributed pressure sensing based on frequency-scanned phase-sensitive optical time-domain reflectometry.

In this paper, a long-distance distributed pressure sensing system based on a special fiber and using frequency-scanned phase-sensitive optical time-domain reflectometry is proposed. The fiber shows high pressure sensitivity (159 MHz/bar) and low loss (3 dB/km) owing to its simple structure made of two large air holes in the cladding. The pressure response of the two orthogonal polarization axes of the fiber is explored distinctively. Distributed pressure sensing over a long sensing range (720 m) and high spatial resolution (5 cm) is demonstrated, resulting in 14,400 resolved sensing points with uncertainty on pressure of 0.49 bar. Discrimination between the temperature/strain and pressure responses is demonstrated, taking advantage of the different pressure and temperature sensitivities of the two polarization axes. In addition, the temperature response of the fiber is studied and the simulation results show the possibility of scaling the temperature sensitivity by adjusting the size of the core. The sensing distance limit due to crosstalk between the polarization axes is also discussed.

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.

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.

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.

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.