This article presents a technique to reconstruct the shape of a flexible instrument in three dimensional Euclidean space based on data from Fiber Bragg Gratings (FBG) that are inscribed in multi-core fibers. Its main contributions are the application of several multi-core fibers with FBGs as shape sensor for medical instruments and a thorough presentation of the reconstruction technique. The data from the FBG sensors is first converted to strain measurements, which is then used to calculate the curvature and torsion of the fibers. The shape of the instrument is reconstructed using Frenet-Serret equations in conjunction with the calculated curvature and torsion of the instrument. The reconstruction technique is validated with a catheter sensorized with 4 multi-core fibers that have FBG sensors. The catheter is placed in 8 different configurations and the reconstruction is compared to the ground truth. The maximum reconstruction error among all the configurations is found to be 1.05 mm. The results show that shape sensing for flexible medical instruments is feasible with FBG sensors in multicore fibers.
Multicore optical fibers are especially attractive for the fabrication of curvature and shape sensors due to the spatial distribution of the different cores. Fiber Bragg gratings have been used in the past for the implementation of these sensors, however, despite their inherent properties, they have a very limited sensitivity. In this paper, we study the use of long period gratings (LPGs) for the implementation of a directional curvature sensor. We inscribed a set of three different LPGs in a seven core optical fiber using a selective inscription technique. We inscribed a single LPG in the external cores and an array of three LPGs in the central core. We have characterized the proposed sensor for strain, torsion, and curvature magnitude and direction. The proposed sensor shows a linear response for curvature magnitudes from 0 m-1 to 1.77 m-1 with a maximum curvature sensitivity of-4.85 nm/m-1 and shows a near sinusoidal behavior in all the cores with curvature directions from 0º to 360º. The sensor shows a good insensitivity to strain. The torsion in the MCF can be detected and measured using the maximum attenuation of the LPGs in the external cores.
We have inscribed a tilted fiber Bragg grating (TFBG) in selected cores of a multicore optical fiber. The presence of the TFBG permits to couple light from the incident-guided mode to the cladding modes and to the neighbor cores, and this interaction can be used for optical sensing. We measured different magnitudes: strain, curvature magnitude and direction, and external refractive index. The curvature results show a linear dependence of the maximum crosstalk with the curvature magnitude with a sensitivity of 2.5 dB/m-1 as the curvature magnitude increases and at the same time a wavelength shift of 70 pm/m-1. Changes in the external refractive index gradually vanish the cladding modes resonances and the crosstalk between the different cores, obtaining a reduction of the 90% of the optical spectra integral area for refractive indexes between 1.398 and 1.474.
We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3sample configurations with different time delay properties are implemented over the same 60meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP 01 mode into the LP 02 , permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.
Multicore optical fibers are of great interest in the optical sensing field. Their core diversity and spatial distribution enable the development of sensing mechanisms that are not possible in single-core fibers. In this paper we study the use of the inter-core crosstalk phenomena for the implementation of a surrounding refractive index (SRI) sensor. The selective inscription of a tilted fiber Bragg grating (TFBG) intentionally increases the inter-core crosstalk between the inscribed cores and makes it sensitive to the SRI. With this technique we simplify the measurement of the SRI and improve the identification and tracking of the excited cladding modes, as compared with the analysis of the transmission spectrum of a TFBG in single-core fibers. The proposed device is also sensitive to temperature. Temperature is obtained from the crosstalk wavelength shift with a measured sensitivity of 9.75pm/ºC. The SRI is obtained from the measurement of the crosstalk optical power. For increasing SRIs the cladding modes gradually fade, reducing the crosstalk optical power. We observed that the higher the tilt, the higher the sensor sensitivity. For a 7º TFBG the SRI sensitivity obtained is -74.2 dB/RIU from 1.31 to 1.39 and -250.8 dB/RIU from 1.39 to 1.44.
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