Four-core Fiber Research Articles

Multifunctional fiber-optic sensor, based on helix structure and fiber Bragg gratings, for shape sensing

In order to realize the simultaneous measurement of directional torsion and vector bending, a multifunctional fiber-optic sensor, based on helical-twisted-structure-assisted fiber Bragg gratings (HTSA-FBGs) on a triangular four-core fiber (TFCF), is proposed. The helical-twisted region (HTR) was fabricated using CO2 laser beam. FBGs were prepared in the HTR of the TFCF cores through the defocusing phase mask method. The sensing mechanism of directional torsion and vector bending was analyzed. The influence of temperature and strain can be eliminated by subtracting the wavelength change of the central core HTSA-FBG from the wavelength change of the side core HTSA-FBGs, so shape accuracy can be improved. The torsion sensitivity can be changed by adjusting the pitch of the HTR.

High-Sensitivity Bending Sensor Based on Supermode Interference in Coupled Four-Core Sapphire-Derived Fiber

A Mach-Zehnder interferometer (MZI) sensor based on a four-core sapphire-derived fiber (FSDF) for bending sensing is proposed and demonstrated. The MZI is formed by splicing a coupled FSDF between two single-mode fibers, delivering supermode interference among four LP01-like supermodes (LP01s modes) and eight LP11-like supermodes (LP11s modes). Benefiting from the different response of the field distribution of each supermode to curvature changes, MZI sensors with a variety of bending properties can be fabricated based on the interference of different supermodes in the FSDF. The experimental results show that the sensitivity of the FSDF-MZI sensor I based on the interference between the LP01s and the LP11s modes is about 4.5 dB/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in the curvature range from 0.4 to 2.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . The FSDF-MZI sensor II based on the interference of two LP11s modes can realize bending sensing in ± Y directions. The sensitivity of FSDF-MZI sensor II in the curvature range of -10.0-5.0 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is about -24.526 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . It suggests that the proposed FSDF-MZI sensor has potential application in the field of structure health detection.

Reflective fiber-optic magnetic field sensor based on a magnetic-fluid-filled capillary probe structure

A novel reflective magnetic field sensor is presented and experimentally demonstrated. The sensing probe is fabricated by splicing a section of uncoated four-core fiber (FCF) between two sections of no-core fibers (NCFs), and the end face of the last NCF is coated by silver. The sensor structure is sealed in a magnetic fluid (MF)-filled capillary, so that the NCF–FCF–NCF magnetic field sensing probe structure is obtained. The NCF is used to excite the high-order modes of the FCF’s cladding. Combined with the adjustable refractive index of MF, the sensor is sensitive to the magnetic field and temperature. Experimental results indicate that the presented sensor can measure the low magnetic field and high magnetic field. In the low magnetic field ranges of 0–10 mT and 10–18 mT, the wavelength near 1548 nm varies linearly with the magnetic field intensity, and the sensitivities are −7.43 pm mT−1 and −60 pm mT−1, respectively. The sensitivity of the interference intensity is −0.30782 dB mT−1 in the low magnetic field intensity range of 0–18 mT. The sensor under low magnetic field is also affected by temperature. In the temperature range of 20 °C–70 °C, the temperature sensitivity of the wavelength shift around 1548 nm is 62.33 pm °C−1, and the corresponding sensitivities of the interference intensity are −0.03888 dB °C−1 and −0.02076 dB °C−1 in the range of 30 °C–50 °C and 50 °C–70 °C, respectively. However, the effect of temperature on the sensor in the high magnetic field (18–36 mT) can be ignored. Within the high magnetic field intensity range of 26–36 mT, the sensitivity of the interference intensity near 1598 nm is −1.20993 dB mT−1. The dual-parameter matrix is used to eliminate the cross-sensitivity of temperature in the low magnetic field. This sensor, with the advantages of easy fabrication, high sensitivity and low cost, has the potential application in the magnetic field monitoring.

Temperature-insensitive intensity-modulation liquid refractive index sensor based on fiber-optic Michelson probe structure

Abstract A novel liquid refractive index sensor based on the connected single-mode fiber (SMF), no-core fiber (NCF), four-core fiber (FCF), and silver mirror (SM) to form an SMF–NCF–FCF–SM Michelson probe structure is proposed and fabricated. The change of light field in the probe structure has been simulated by the light-beam propagation method. The theoretical results show that light is excited in the NCF and couples into the cores and cladding of FCF at the junction of NCF and FCF. The interference fringes are generated between the cladding modes and core modes of FCF. The sensitivities of the probe in NaCl, sucrose, and glycerol are 171.75 dB/RIU, 121.41 dB/RIU, and 207.50 dB/RIU, respectively. The temperature sensitivity is 0.05 nm/°C, and the intensity change of temperature (≤0.046 dB/°C) is very small and has little effect on the liquid refractive index. Thus, the cross-sensitivity of temperature for the liquid refractive index can be removed. The proposed probe structure has the advantages of easy fabrication, good stability, and linear response, having potential application in the liquid refractive index monitoring environments.

Theoretical Study of Four-Core Five-Mode Microstructured Optical Fiber with Low Crosstalk and Large Mode Field Area

Theoretical Study of Four-Core Five-Mode Microstructured Optical Fiber with Low Crosstalk and Large Mode Field Area

Sensitivity optimization of symmetric multi-core fiber strain sensor based on mode-coupling theory

In this paper, a method to optimize the sensitivity of multi-core fiber (MCF) strain sensors by properly designing fiber structure is investigated from theory, simulation and experiments. The mode-coupling between cores is tuned by changing fiber structure parameters, which will result in a change in sensitivity of such MCF strain sensor. Thus it is possible to obtain an optimal sensitivity in advance by reasonably designing the fiber structure parameters. Taking a symmetrical four-core fiber as an example, numerical simulation has been implemented and the results show that the highest strain sensitivity of −11.85 nm/mε can be obtained when the core diameter, the core-to-core pitch, the refractive index difference between core and cladding are set to 4.53 µm, 18.1 µm and 0.0108, respectively. The method was verified through experiments and the experimental results are very close to the simulated results. The proposed MCF strain sensor has the advantages of simple structure, low cost and high sensitivity, which makes it have high research value in commercial sensing applications.

Heat Load Influence on Supermodes in Yb-Doped Four-Core Fibers

Multicore fibers have been recently proposed as a possible solution to overcome the problems which affect fiber-based lasers and amplifiers when their output power increases, especially the severe power limitations imposed by thermal effects, responsible of the detrimental phenomenon of mode instability. In this article the influence of the heat load generated in the Yb-doped cores of four-core fibers on the guiding properties of the supermodes is analyzed. Effective area and single-mode regime are calculated for fibers with different core-to-core separations, in order to identify the most convenient optical coupling regime among the cores in the presence of strong thermal effects. Moreover, through the comparison with dual-core fibers when the same total heat load is generated, useful guidelines for the choice of core number in multicore fiber cross-section are provided. Results show that there is a trade-off between effective area and single-mode regime, because four-core fibers provide the largest effective area, but they become multimode when thermal effects are significant, especially if the cores are optically coupled. When the distance among the cores is large enough, multicore fibers demonstrate to be less affected by the generated heat load, regardless of the number of the doped cores in the transverse section.

Ultra-broadband and highly sensitive surface plasmon resonance sensor based on four-core photonic crystal fibers

An ultra-broadband surface plasmon resonance (SPR) sensor with high sensitivity adopting four-core photonic crystal fiber is designed. The impacts of fiber structure parameters on the sensing performance are theoretically studied in detail. The investigation results demonstrate that, the average wavelength sensitivity of our proposed sensor is up to 1.135 × 104 nm/refractive index unit (RIU) within the ultra-broad wavelength range of 1.548–2.683 μm when the refractive index (RI) of analyte varies in a large range of 1.423–1.523. Furthermore, the maximum amplitude sensitivity of 550 RIU−1 is achieved if RI changes from 1.423 to 1.433. Therefore, the proposed sensor can work both in the telecommunication window, and the mid-infrared wavelength regime.

Selective fiber Bragg grating inscription in four-core fiber for two-dimension vector bending sensing.

The paper presents selective fiber Bragg grating (FBG) inscription in four-core fiber based on a phase mask scanning method. The inscription factors are systematically investigated, which involves fiber core position and focused laser beam size in fiber, etc. Several specific inscriptions (including individual, dual and all inscriptions) are demonstrated. Two orthogonally positioned cores are selectively inscribed and applied to two-dimension vector bending measurement. The measured bending sensitivities of two FBGs range from -54.3 pm/m-1 to 52.2 pm/m-1 and -53.7 pm/m-1 to 52.8 pm/m-1, respectively. More importantly, it has been revealed that their sensitivities versus bending direction follow regular cosinoidal and sinusoidal distribution. The direction and amplitude of the vector bending can be recovered using measured central wavelength shifts of those two FBGs.

Strain and temperature sensor based on fiber Bragg grating cascaded bi-tapered four-core fiber Mach–Zehnder interferometer

A novel strain and temperature sensor based on the fiber Bragg grating (FBG) cascaded two-tapered four-core fiber (FCF) Mach–Zehnder interference (MZI) is proposed and fabricated. The two optical fiber tapers, acting respectively as a beam splitter and a coupler, are able to produce interference phenomena, so the high interference extinction ratio (above 20 dB) can be obtained. The attenuation peak at 1562 nm and the FBG center wavelength at 1550 nm in the transmission spectra are, respectively, selected to represent MZI and FBG as the monitoring wavelengths. The results indicate that the sensitivities of MZI to strain and temperature are −1.83 pm μϵ−1 and 46.93 pm C−1, respectively, and those of FBG to strain and temperature are 1.26 pm μϵ−1 and 10.58 pm C−1, respectively. Both the FBG and MZI exhibit insensitivity to the refractive index. The cascaded sensor has good linearity and is applied to measure the strain and temperature simultaneously.

GAWBS phase noise characteristics in multi-core fibers for digital coherent transmission.

We describe the guided acoustic-wave Brillouin scattering (GAWBS) phase noise characteristics in multi-core fibers (MCFs) used for a digital coherent optical fiber transmission both experimentally and analytically. We first describe the GAWBS phase noise in an uncoupled four-core fiber with a 125 µm cladding and compare the phase noise spectrum with that of a standard single-mode fiber (SSMF). We found that, unlike SSMF where the R0,m mode is dominant, off-center cores in MCF are affected by higher-order TRn,m modes. We then report measurement results for GAWBS phase noise in a 19-core fiber with a 240 µm cladding. The results indicate that the cores exhibit different spectral profiles depending on their distance from the center of the fiber, but the amount of phase noise generated in each core is almost identical. These results provide a useful insight into the space division multiplexing transmission impairments in digital coherent transmissions using MCF.

Dynamic In-Line Routing Between Distant Cores of a Multi-Core Fiber

Dynamic and coherent coupling between distant cores in a commercially available optical four-core fiber is demonstrated. Wavelength-selective excitation of cladding modes is experimentally realized by an acousto-optic mode coupler by which higher-order cladding modes are visualized and compared with simulations. By comparing the coupling conditions to a global cladding mode, the relative propagation constants of the local core modes are precisely measured at an accuracy level down to 10−6. Core-cladding-core three-mode coupling operation dynamically switches optical signals between two arbitrary distant cores in the four-core fiber and achieves an inter-core switching efficiency of 55.4%.

High fringe visibility Mach-Zehnder interferometric sensor based on a Four-Core fiber

An in-line optical fiber Mach-Zehnder interferometer (MZI) sensor with high fringe visibility is reported. The sensor is formed by splicing a short section of four-core fiber (FCF) between two coupling sections. The coupling section is fabricated by cascading very short segment of few mode fiber, multimode fiber (MMF), and large core diameter MMF together. A maximum fringe visibility of approximately 30 dB is obtained in the transmission spectrum. The temperature and curvature sensing response were also experimentally investigated. The temperature sensitivities are 61 pm/°C and 49 pm/°C for dip A and dip B ranging from 30 °C to 80 °C, respectively. The curvature sensitivities for dip A and dip B are 20.77 nm/m−1 and 12.89 nm/m−1 in the range of 0.2983 m−1 to 0.7916 m−1, as well as 9.99 nm/m−1 and 5.38 nm/m−1 from the range of 0.7916 m−1 to 1.039 m−1, respectively. The proposed MZI sensor has the advantage of easy fabrication, compactness, higher fringe visibility, and low cost.

Feasibility Demonstration of Spatial Channel Networking Using SDM/WDM Hierarchical Approach for Peta-b/s Optical Transport

This article reports on a proof of concept demonstration of a recently proposed spatial channel network (SCN) that was conducted over an SCN testbed that comprises low-loss hierarchical optical cross-connect (HOXC) prototypes and four-core multicore fiber links. The HOXC prototypes used in the testbed are based on sub-matrix-switches and core selective switches both implemented with commercially available discrete optical switches. Using these two types of HOXC prototypes, the spatial channel networking including spatial bypassing, spatial add/drop and spectral grooming, spatial-lane change, and spatial-channel (SCh) protection is successfully demonstrated for SChs carrying 100-Gb/s–900-Gb/s optical channels.

Multi-parameter sensor based on the fiber Bragg grating combined with triangular-lattice four-core fiber

A novel multi-parameter (refractive index, temperature and curvature) sensor based on the splicing-point tapered FBG-SMF-FCF-SMF (FBG, fiber Bragg grating; SMF, single-mode fiber; FCF, four-core fiber) Mach-Zehnder interferometer (MZI) is proposed and fabricated. Two waist-enlarged tapers formed at the fusion points between SMFs and FCF play the roles of the beam splitter and coupler, respectively. FBG is used to monitor temperature change and to compensate for temperature shift in the measurements of refractive index and curvature. The results show that the extinction ratio of transmission spectrum can reach at 27.22 dB. The sensitivities of the sensor to the external refractive index and curvature are 116.396 nm/RIU and 15.57 pm/m−1, respectively. The temperature sensitivities of MZI and FBG are respectively 67.31 pm/℃ and 8.73 pm/℃ in the range of 0-80℃. All the three-parameter measurements present good linear relationship. The sensor has the advantages of simple structure, easy process and low cost, which can be used in multi-parameter detection.

PANDA Type Four-Core Fiber With the Efficient Use of Stress Rods

We design a panda type four-core fiber with a comparable optical property of the polarization maintenance single mode fiber (PM-SMF) and can support 8 non-polarization-degenerate eigenmodes with a birefringence of approximate 3.24 × 10 -4 over the wavelength range from 1288 to 1630 nm. In particular, only four stress rods are efficiently used to achieve the PM characteristics for each core, in comparison with traditional design of panda type PM-SMF. Meanwhile, we identify that the optimal configuration of stress rods can not only enhance the birefringence, but also further suppress the crosstalk (XT) between two primary eigenstate of polarization (ESOP) arising in adjacent cores, although the corresponding XT at secondary ESOP remains unchanged. Moreover, our numerical results show that both chromatic dispersion (CD) together with mode field diameter (MFD) and the bending sensitivity are consistent among four cores. We believe that such specialty multi-core fiber is ideal for compact and cost-effective fiber-optic gyroscope (FOG) application, besides the potential in the MIMO-free spatial division multiplexing (SDM) interconnection.

All-Fiber Highly Sensitive Bragg Grating Bend Sensor.

In this paper, we demonstrated a novel, all-fiber highly sensitive bend sensor based on a four-core fiber rod with a diameter of 2.1 mm. We observed a high resolution of the sensor at a level of 3.6 × 10−3 m−1. Such a sensor design can be used in harsh environments due to the relatively small size and all-fiber configuration, containing no adhesive, nor welded joints.

All-fiber acoustic particle velocity sensor based on graphene-coated four-core fiber.

A graphene-coated four-core fiber (FCF)-based acoustic particle velocity (APV) sensor for the detection of acoustic location is proposed and experimentally demonstrated. A graphene film is coated on the cross section of the FCF, which can be heated through a 532-nm laser. Due to the antisymmetric temperature distribution of the graphene induced by the APV, the reflection power of different fiber cores in the FCF is modulated, and the acoustic source direction can also be located based on the vector characteristic of the APV. The experimental results show that the figure-of-eight directional response of the graphene-coated FCF with a directional sensitivity of 0.56mV/deg is achieved. The all-fiber vector acoustic sensor possesses the potential for applications in structural health monitoring, partial discharges, public security, etc.

Ultra-Low Crosstalk 125-μm-Cladding Four-Hole Four-Core Fibers Fabricated by the Over-Cladding Bundled Rods Method

We realized a four-core fiber with 125 μm-cladding and four-holes using a novel fabrication technique, namely the over-cladding bundled rods (OBR) method. Four air holes were successfully introduced by using the OBR method without the need for a drilling process, and this enabled a 10 dB/km improvement in the crosstalk characteristic. As a result, we realized four-core fibers with a 125 μm cladding diameter and operating in both the single mode and 2 linear polarized (LP) modes in the C + L band were realized while maintaining an inter-core crosstalk of less than −30 dB/100 km. The OBR method is a promising technique for effectively realizing a multi-core fiber with ultra-low crosstalk and a high spatial density.

400-Gb/s DMT-SDM Transmission Based on Membrane DML-Array-on-Silicon

We report on space-division multiplexing (SDM) transmissions of up to 400 Gb/s over a homogeneous four-core fiber using discrete multitone (DMT) modulation for intra-datacenter applications and 200/400 GbE links. The transmission system is enabled by a compact, SDM-channel scalable, multi-core fiber (MCF) pluggable, and energy-efficient SDM transmitter composed of a 4-channel 1.3-μm membrane directly modulated laser (DML) array-on-silicon integrated with a fiber-bundle type fan-in with loss of less than 1 dB and negligible optical and electrical crosstalk (XT). By simultaneously modulating all four lasers, 200-Gb/s (50-Gb/s/channel) and 400-Gb/s (100-Gb/s/channel) are achieved over a 425-m MCF link for KP4 and soft-decision forward error correction, respectively, with a power reduction of ~6× compared with conventional DML transmitters. Additionally, numerical simulations based on a detailed rate-equations model predict the feasibility of the system for MCF transmissions up to 10 km even with moderate levels of XT.