Article

# Fiber Bragg gratings in hole-assisted multicore fiber for space division multiplexing

Authors:
• Karol Stępień DT
• Polish Center of Photonics and Fibers
Article

# Fiber Bragg gratings in hole-assisted multicore fiber for space division multiplexing

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## Abstract

In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and temperature sensitivities of all of the Bragg peaks is also included.

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... Several types of multiplexing techniques have been proposed for FBG based multipoint/quasi-distributed sensing networks, including time division multiplexing (TDM) [11], wavelength division multiplexing (WDM) [12], a combination of TDM and WDM [13], space division multiplexing (SPD) [14], etc. Among these multiplexing techniques, WDM based quasi-distributed sensing interrogation scheme is the most preferable because of its high speed. ...
... It can be seen from Eqs. (14) and (15), that slow-light sensitivity depends on the product of delay and peak transmissivity at Bragg wavelength which is relevant "figure of merit (FoM)" of slow-light FBG sensors. ...
Article
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In this paper, we theoretically analyze the slow-light -phase-shifted fiber Bragg grating (-FBG) and its applications for single and multipoint/quasi-distributed sensing. Coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modeling of slow-light -FBG. The impact of slow-light FBG parameters, such as grating length (L), index change (∆n), and loss coefficient (α) on the spectral properties of -FBG along with strain and thermal sensitivities are presented. Simulation results show that for the optimum grating parameters L = 50 mm, ∆n = 1.510-4, and α = 0.10 m-1, the proposed slow-light -FBG is characterized with a peak transmissivity of 0.424, the maximum delay of 31.95 ns, strain sensitivity of 8.380 με-1, and temperature sensitivity of 91.064 °C-1. The strain and temperature sensitivity of proposed slow-light -FBG is highest as compared to the slow-light sensitivity of apodized FBGs reported in the literature. The proposed grating have the overall full-width at half maximum (FWHM) of 0.2245 nm, and the FWHM of the Bragg wavelength peak transmissivity is 0.0798 pm. The optimized slow-light -FBG is used for quasi-distributed sensing applications. For the five-stage strain quasi-distributed sensing network, a high strain dynamic range of value 1469 με is obtained for sensors wavelength spacing as small as 2 nm. In the case of temperature quasi-distributed sensing network, the obtained dynamic range is 133 °C. For measurement system with a sufficiently wide spectral range, the -FBGs wavelength grid can be broadened which results in substantial increase of dynamic range of the system. The additional advantage of slow-light -FBG in quasi-distributed sensing networks over the conventional apodized FBG sensing networks is that slow-light peaks are free from the side-lobes.
... Several types of multiplexing techniques have been proposed for FBG based multipoint/quasi-distributed sensing networks, including time division multiplexing (TDM) [11], wavelength division multiplexing (WDM) [12], a combination of TDM and WDM [13], space division multiplexing (SPD) [14], etc. Among these multiplexing techniques, WDM based quasi-distributed sensing interrogation scheme is the most preferable because of its high speed. ...
... It can be seen from Eqs. (14) and (15), that slow-light sensitivity depends on the product of delay and peak transmissivity at Bragg wavelength which is relevant "figure of merit (FoM)" of slow-light FBG sensors. ...
Article
Full-text available
In this paper, we theoretically analyze the slow-light-phase-shifted fiber Bragg grating (-FBG) and its applications for single and multipoint/quasi-distributed sensing. Coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modeling of slow-light-FBG. The impact of slow-light FBG parameters, such as grating length (L), index change (n), and loss coefficient (˛) on the spectral properties of-FBG along with strain and thermal sensitivities are presented. Simulation results show that for the optimum grating parameters L = 50 mm, n = 1.5×10 −4 , and ˛ = 0.10 m-1 , the proposed slow-light-FBG is characterized with a peak transmissivity of 0.424, the maximum delay of 31.95 ns, strain sensitivity of 8.380-1 , and temperature sensitivity of 91.064 • C-1. The strain and temperature sensitivity of proposed slow-light-FBG is the highest as compared to the slow-light sensitivity of apodized FBGs reported in the literature. The proposed grating have the overall full-width at half maximum (FWHM) of 0.2245 nm, and the FWHM of the Bragg wavelength peak transmissivity is of 0.0798 pm. The optimized slow-light-FBG is used for quasi-distributed sensing applications. For the five-stage strain quasi-distributed sensing network, a high strain dynamic range of value 1469 is obtained for sensors wavelength spacing as small as 2 nm. In the case of temperature of quasi-distributed sensing network, the obtained dynamic range is of 133 • C. For measurement system with a sufficiently wide spectral range, the-FBGs wavelength grid can be broadened which results in substantial increase of dynamic range of the system.
... Here, it is worth mentioning that writing gratings in MCF turns out to be more complicated than in SMF, due to the geometric difference in spatial position between cores of the MCF, which adds some technical challenges in fabrication, e.g., due to the lens effect of the fiber, the grating strength may have variations between cores, and it may also lead to different transmission/reflection profiles of the FBGs [39,62,76,[88][89][90][91]. Currently, the most widely used way to write gratings in MCF is to use UV light irradiation, and this method will normally bring in gratings in all cores of the MCF. ...
Article
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In recent years, multicore fiber (MCF) has attracted increasing interest for sensing applications, due to its unique fiber structure of multiple parallel cores in a single fiber cladding, which offers a flexible configurable platform to establish diverse functional fiber devices for sensing applications. So far, a variety of discrete fiber sensors using MCF have been developed, among which one of the major categories is the MCF grating sensors. The most distinct characteristic of MCF that differs from the normal single mode fibers is that the off-center cores of a MCF are sensitive to bending, which is caused by the bending induced tangential strain in off-center waveguides through either compression or stretching. The bending sensitivity has been widely developed for bending/curvature sensing or measuring physical parameters that are associated with bending. In this paper, we review the research progress on MCF-based fiber grating sensors. MCF-based diverse fiber grating sensors will be introduced, whose working principles will be discussed, and various types of applications of the MCF grating sensors will be summarized. Finally, the challenges and prospects of MCF grating for sensing applications will be presented.
... The multiplexing technology uses the same interrogation system to query the measurement information of multiple sensors, which not only greatly simplifies the complexity of the system, but also ensures the measurement accuracy and reliability of the system [4]. The multiplexing technologies that have been developed are mainly time division multiplexing (TDM), frequency division multiplexing (FDM), wavelength division multiplexing (WDM), code division multiplexing (CDM), and space division multiplexing (SDM) [5][6][7][8]. ...
Article
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Optical fiber sensor networks (OFSNs) provide powerful tools for large-scale buildings or long-distance sensing, and they can realize distributed or quasi-distributed measurement of temperature, strain, and other physical quantities. This article provides some optical fiber sensor network technologies based on the white light interference technology. We discuss the key issues in the fiber white light interference network, including the topology structure of white light interferometric fiber sensor network, the node connection components, and evaluation of the maximum number of sensors in the network. A final comment about further development prospects of fiber sensor network is presented.
... The latter is particularly useful in large civil engineering structures and in oil and gas exploration and distribution systems. In order to reduce the system cost and multiplex a large number of FBGs for multipoint measurements, many multiplexing schemes have been developed, including optical time-domain reflector (OTDR) [7], optical frequency-domain reflector (OFDR) [8], space-division multiplexing (SDM) [9], wavelength-division multiplexing (WDM) time-division multiplexing (TDM) and combinations thereof [10,11]. However, except for a recent few demonstrations [12][13][14], FBG multiplexing usually has capacities limited to a few or tens of gratings. ...
Article
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We demonstrate interrogation of a large-capacity sensor array with nearly identical weak fiber Bragg gratings (FBGs) based on frequencyshifted interferometry (FSI). In contrast to time-division multiplexing, FSI uses continuous-wave light and therefore requires no pulse modulation or high-speed detection/acquisition. FSI utilizes a frequency shifter in the Sagnac interferometer to encode sensor location information into the relative phase between the clock-wise and counter-clockwise propagating lightwaves. Sixty-five weak FBGs with reflectivities in the range of -31 ∼-34 dB and with near identical peak reflection wavelengths around 1555 nm at room temperature were interrogated simultaneously. Temperature sensing was conducted and the average measurement accuracy of the peak wavelengths was ± 3.9 pm, corresponding to a temperature resolution of ± 0.4 °C. Our theoretical analysis taking into account of detector noise, fiber loss, and sensor cross-talk noise shows that there exists an optimal reflectivity that maximizes multiplexing capacity. The multiplexing capacity can reach 3000 with the corresponding sensing range of 30 km, when the peak reflectivity of each grating is -40 dB, the sensor separation 10 m and the source power 14 mW. Experimental results and theoretical analysis reveal that FSI has distinct cost and speed advantages in multiplexing large-scale FBG networks.
... Today a diversity of MCF fibres from different manufacturers is available on the market. I order to avoid cross-talk between cores different methods can be used to increase the signal to noise ratio [4]. Such multicore fibres have also become attractive for multiplexed fibre sensor systems. ...
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Fibre Bragg gratings in multicore optical fibres are attractive sensing elements for multiplexed measurements (e.g. shape sensing). In order to achieve optimized uniformity in the grating inscription, a setup with control of orientation was applied.
... The fiber Bragg grating (FBG) sensor, in particular, is widely employed to sense strain, temperature or pressure changes by monitoring the shifting of the reflected optical carrier in the wavelength. The FBG-based sensor systems have many advantages, such as electromagnetic interference immunity, lightweight, compact size, easy fabrication, broad wavelength-tunability, excellent multiplexing capability, etc. Numbers of large-scale FBG sensor networks have been experimentally demonstrated based on timedivision multiplexing (TDM), wavelength-division multiplexing (WDM) and spatial-division multiplexing (SDM) techniques [11][12][13][14][15][16][17][18][19][20][21]. Nevertheless, as the amount of sensing information is increased, maintaining the resilience of optical fiber sensor networks becomes a challenge because the probability of failure in connecting fibers is increased and the amount of affected sensing traffic is generally greater when a fiber failure occurs. ...
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An all-passive optical fiber sensor network is proposed based on a novel single-line bidirectional optical add-drop multiplexer (SBOADM). By reasonably employing fiber Bragg gratings (FBGs) and optical circulators (OCs) to compose the self-developed SBOADM, single-line bidirectional transmission can be easily achieved by the SBOADM without the assistance of a power supply or optical switch. When the SBOADM is employed to bridge a hybrid tree-based and ring-based optical fiber sensor network, self-healing functionality can be easily embedded into the network. Once an interruption occurs in the network, the only thing that the system maintainer needs to do is to adjust one optical switch pre-installed in the remote node of the sensor network. That is to say, it will reduce the system's complexity to achieve self-healing functionality to immediately recover the connection of optical signals as soon as the optical fiber interruption occurs. The advancement of the self-developed SBOADM is experimentally demonstrated, and the self-healing functionality of the proposed all-passive optical fiber sensor network is simulated. Great results show that the proposal can easily overcome any one fiber-link failure without adjusting the network deploying setting or employing complex control management.
... expand the sensing range, and thus various multiplexing FBGs techniques have been proposed and demonstrated [4]- [6]. In particular, wavelength division multiplexing (WDM) [5], time domain division multiplexing (TDM) [6], frequency domain multiplexing (FDM) [7] and their combinations [8] are most widely adopted. ...
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The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragg grating wavelength. Since the spatial and temporal coherence properties of the femtosecond laser source differ strongly from those of conventional laser sources, specific limits and tolerances in the interferometric setup have to be considered. Such limits are investigated on the basis of an analytical ray tracing model. The results are applied to tolerance measurements of fiber Bragg grating reflections recorded with a DUV sub-picosecond laser source at 262 nm. Additionally we demonstrate the wavelength versatility of the two-beam interferometer setup for femtosecond inscription over a 40 nm wavelength band. Inscription experiments in Al/Yb doped silica glasses are demonstrated as a prove for the access to non-photosensitive fibers.
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The inscription of fiber Bragg gratings during the drawing process is a very useful method to realize sensor arrays with high numbers of gratings and excellent mechanical strength and also type II gratings with high temperature stability. Results of single pulse grating arrays with numbers up to 100 and definite wavelengths and positions for sensor applications were achieved at 1550 nm and 830 nm using new photosensitive fibers developed in IPHT. Single pulse type I gratings at 1550 nm with more than 30% reflectivity were shown first time to our knowledge. The mechanical strength of this fiber with an Ormocer coating with those single pulse gratings is the same like standard telecom fibers. Weibull plots of fiber tests will be shown. At 830 nm we reached more than 10% reflectivity with single pulse writing during the fiber drawing in photosensitive fibers with less than 16 dB/km transmission loss. These gratings are useful for stress and vibration sensing applications. Type II gratings with reflectivity near 100% and smooth spectral shape and spectral width of about 1 nm are temperature stable up to 1200 K for short time. They are also realized in the fiber drawing process. These gratings are useful for temperature sensor applications.
Article
We present KrF excimer laser-induced dynamics of Bragg grating growths in GeO2 doped microstructured optical fibers. The studied fibers all have 6 rings of airholes in a hexagonal lattice and a GeO2 doped region in the center of the microstructure. We compare the growth rates of fiber Bragg gratings in the different microstructured fibers with UV grating inscription. The influence of the doping level, the airhole filling factor, the airhole pitch distance and the fiber orientation are investigated. We expand the range of microstructured optical fibers in which Bragg gratings can be inscribed, achieving reflection strengths that are useable for FBG-based sensing applications, even for doped regions with GeO2 concentrations as low as 1.36 mol% and 0.45 mol%.
Article
Photosensitivity in optical fibres properties of Fibre Bragg gratings inscribing Bragg gratings in optical fibres Fibre Bragg grating theory applications of Bragg gratings in communications Fibre Bragg grating sensors impact of Fibre Bragg gratings.
Article
We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.
Article
The feature of a multicore fiber with one-ring structure is theoretically analyzed and experimentally demonstrated. The one-ring structure overcomes the issues of the hexagonal close-pack structure. The possibility of 10-core fiber with Aeff of 110 μm<sup>2</sup> and 12-core fiber with Aeff of 80 μm<sup>2</sup> is theoretically presented. The fabricated 12-core fibers based on the simulation results realized Aeff of 80 μm<sup>2</sup> and crosstalk less than -40 dB at 1550 nm after 100-km propagation. The MCF with the number of core larger than seven and the small crosstalk was demonstrated for the first time.
Article
We report on a photonic crystal fiber with a large mode area designed for compact high power fiber lasers and amplifiers. The fiber suppresses higher order modes when bent around a 10-cm radius and enables single mode operation in small footprint laser and amplifier architectures. We experimentally confirm the peculiar bending properties of this fiber in its passive version, by reporting on the measurement results of fundamental mode loss in bent and straight fibers, and of the influence of the bending plane orientation on this fiber loss.
Article
A review of optical fiber sensing demonstrations based on photonic crystal fibers is presented. The text is organized in five main sections: the first three deal with sensing approaches relying on fiber Bragg gratings, long-period gratings and interferometric structures; the fourth one reports applications of these fibers for gas and liquid sensing; finally, the last section focuses on the exploitation of nonlinear effects in photonic crystal fibers for sensing. A brief review about splicing with photonic crystal fibers is also included.
Article
We present several applications of microstructured optical fibers and study their modal characteristics by using Bragg gratings inscribed into photosensitive core regions designed into the air-silica microstructure. The unique characteristics revealed in these studies enable a number of functionalities including tunability and enhanced nonlinearity that provide a platform for fiber device applications. We discuss experimental and numerical tools that allow characterization of the modes of the fibers.
Article
We report a compact two-dimensional accelerometer based upon a simple fiber cantilever constructed from a short length of multicore optical fiber. Two-axis measurement is demonstrated up to 3 kHz. Differential measurement between fiber Bragg gratings written in the multicore fiber provides temperature-insensitive measurements.
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