Two-core Fiber Research Articles

Large Range Curvature Measurement Using FBGs in Two-Core Fiber with Protective Coating

In this work, we propose a fiber Bragg grating (FBG)-based sensor for curvature measurements. Two gratings are inscribed through the protective coating in a specialty optical fiber using focused femtosecond laser pulses and point-by-point direct writing technology. One grating is inscribed on the central core adjacent to an air channel, while the other is inscribed on the eccentric core. The bending characteristics of the two-core fiber strongly depend on the bending direction due to the asymmetry of the fiber cores. A bending sensitivity of 58 pm/m−1 is achieved by the FBG in the eccentric fiber core over the curvature range of 0–50 m−1 . Temperature and humidity cross-sensitivity could be significantly reduced by analyzing the differences in peak shifts between the two gratings. The sensor features a large sensing range and good robustness due to the presence of its protective buffer coating, which makes it a good candidate for curvature sensing in engineering fields.

MZI-based high intensity responsive sandwiched multi-layer fiber optic humidity sensor

This paper presents a multi-layer optical fiber humidity sensor based on dual-beam Mach-Zehnder interference (MZI) for optical intensity sensitivity. The sensor is fabricated using a large-scale axial offset fusion technique between a multimode fiber (MMF) and an offset hole two-core fiber (OHTC), making it suitable for agricultural and health monitoring applications. The 27 μm axial offset enables tilted light incidence, enhancing environmental sensitivity. With a single-cycle Graphene oxide (GO) coating of approximately 22 nm, the MMF-OHTC structure achieves high humidity sensitivity (0.39 dB/%RH) over the 25 %-80 % RH range with excellent linearity (R2≈0.99). The sensor exhibits stable, rapid response (2.52 s/0.46 s) and low temperature cross-sensitivity (0.06 %RH/°C). When combined with a hollow-core fiber Fabry-Perot interferometer (HCF-FPI), the system allows zero cross-talk simultaneous humidity and temperature measurements, expanding the potential applications of multi-layered structures in multifunctional detection systems.

Temperature Sensor Based on Side-Polished Two-Core Fiber Michelson Interferometer

Temperature Sensor Based on Side-Polished Two-Core Fiber Michelson Interferometer

Directional Torsion Sensor Based on a Two-Core Fiber with a Helical Structure

A fiber-optic torsion sensor based on a helical two-core fiber (HTCF) is proposed and experimentally demonstrated for simultaneously measuring torsion angle and torsion direction. The sensor consists of a segment of HTCF and two single-mode fibers (SMFs) forming a Mach-Zehnder interferometer (MZI). The helical structure is implemented by pre-twisting a 1 cm long two-core fiber (TCF). The performance of the sensor with pre-twisted angles of 180°, 360°, and 540° is experimentally analyzed. The results show that the sensor can realize the angular measurement and effectively distinguish the torsion direction. It is worth noting that the sensor has maximum sensitivity when the pre-twist angle is 180 degrees. The obtained wavelength sensitivities of torsion and temperature are 0.242 nm/(rad/m) and 32 pm/°C, respectively. The sensor has the advantages of easy fabrication, low cost, compact structure, and high sensitivity, which is expected to yield potential applications in fields where both torsion angle and direction measurements are required.

All-optical vector magnetic field sensor based on a side-polished two-core fiber Michelson interferometer.

A magnetic field sensor based on a side-polished two-core fiber (SPTCF)-based Michelson interferometer (MI) has been developed and demonstrated. The magnetic field sensor is composed of a standard single mode fiber (SMF) and a section of tapered TCF. By side-polishing a segment of the TCF, the effective index of the exposed core can be made sensitive to the environmental refractive index (RI). To evaluate its performance, a magnetic fluid is used to cover the polished region with a magnetic field sensitive material, where the sensor then measures the magnetic field intensity by sensing the RI change of the magnetic fluid through the evanescent field in the polished core. The SPTCF MI device developed allows for vector magnetic field sensing because of its asymmetric structure, with its highest directional sensitivity being 55.2 pm/degree. Experimental results obtained show that when the magnetic field is parallel to the side-polished plane, a sensitivity of 1.262 nm/mT can be achieved, operating over the magnetic flux density region of 0-5 mT and over a temperature range of 20∼85 °C, where the device is minimally affected by temperature changes. The sensor is well suited to a variety of potential applications given its low cost, strong anti-interference ability, simple structure and high stability.

Analysis of intercore crosstalk of WDM channels around zero-dispersion wavelength in homogeneous multicore fibers

We present a numerical analysis of the average intercore crosstalk (IC-XT) of wavelength-division multiplexed (WDM) optical channels in a homogeneous two-core fiber system. This analysis is performed considering cores with zero-dispersion wavelengths at 1550 nm. In the analysis, we consider 11 WDM channels spaced 100 GHz apart transmitted in three different schemes, one centered at 1510 nm with negative dispersion D = − 3.5 ps / nm · km, one centered at 1550 nm with D = 0, and one centered at 1590 nm with D = + 3.5 ps / nm · km. This selection allows for the observation of how the IC-XT of WDM channels is modified using positive, zero, and negative dispersion parameters. To analyze more realistic scenarios of IC-XT in multicore fibers, we considered random bending and twisting perturbations along the fiber. In addition, we considered fiber nonlinearities such as four-wave mixing (FWM) among WDM channels. The results show that FWM produces a power transfer among the transmitted WDM channels that depends on the dispersion parameter D at core 1, and this effect is transferred to the average crosstalk of the WDM channels at core 2. Therefore, the average IC-XT of WDM channels can be modified in a controlled way by selecting an adequate dispersion parameter D in combination with FWM nonlinearity. These results provide valuable information for understanding the wavelength dependence of the average IC-XT of homogeneous multichannel MCF systems working around a zero-dispersion wavelength.

Effects of ellipticity angle on soliton solutions and modulation instability spectra in two-core birefringent optical fibers

Effects of ellipticity angle on soliton solutions and modulation instability spectra in two-core birefringent optical fibers

Solitary wave solutions in two-Core optical fibers with coupling-coefficient dispersion and Kerr nonlinearity

In this paper, we studies chirped solitary waves in two-Core optical fibers with coupling-coefficient dispersion and intermodal dispersion. To construct chirp soliton, the couple of nonlinear Schrödinger equation which describing the pulses propagation along the two-core fiber have been reduced to one equivalent equation. By adopting the traveling-waves hypothesis, the exact analytical solutions of the GNSE were obtained by using three relevant mathematical methods namely the auxiliary equation method, the modified auxiliary equation method and the Sine-Gordon expansion approach. Lastly, the behavior of the chirped like-soliton solutions were discussed and some contours of the plot evolution of the bright and dark solitons are obtained.

Survey of third- and fourth-order dispersions including ellipticity angle in birefringent fibers on W-shaped soliton solutions and modulation instability analysis

In this work, the modified Sardar sub-equation method is used to construct optical soliton solutions to the coupled nonlinear Schrodinger equation (CNLSE) having third-order and fourth-order dispersions describing short-pulse propagation in two-core fibers. Several solutions are determined including W-shaped bright, dark soliton solutions, singular soliton solutions, periodic function solutions and the combined complex soliton solutions. It should be noted that this integration scheme is very powerful mathematical tools for obtaining exact optical soliton solutions of nonlinear evolution equations. Under suitable values for the physical parameters, some representative wave structures are graphically displayed. In addition, the linear stability technic is used to analyze the modulation gain spectra in the birefringence associated with an ellipticity angle. In our knowledge, these obtained results are new in the context of nonlinear birefringent optical fibers.

Crosstalk Characteristics of WDM Channels in Quasi-Homogeneous Multicore Fibers

The relative average inter-core crosstalk (IC-XT) among wavelength-division multiplexed (WDM) optical channels in a quasi-homogeneous two-core fiber system exhibiting slight differences on the propagation constant, inverse group-velocity, and the group-velocity dispersion (GVD) between cores is numerically investigated. This analysis is performed using a theoretical formalism that includes fiber nonlinearities in the presence of random perturbations. The results show oscillations on the average IC-XT of WDM channels that mainly depend on the walk-off between cores. These oscillations arise superimposed to the linear wavelength-dependence of the IC-XT caused by the core-coupling coefficient. As transmission power increases, the nonlinearities do not reduce the presence of these oscillations, but slightly modify their amplitudes and periods. These results provide valuable information to fully understand the wavelength dependence of the average IC-XT of real multi-channel MCF systems, especially those of very short reach.

All-Fiber Active Tractor Beam Generator and its Application

We propose and demonstrate an all-fiber probe to generate an active tractor beam by changing the gradients of optical intensity distributions. We grind the tip of a seven-core fiber (SCF) into a truncated hexagonal pyramid shape to integrate three equivalent two-core fiber optical tweezers in an SCF. Three optical tweezers produce three trapping locations along the fiber main axis. By adjusting and controlling the incident laser power in each core, we may perform multiple functions, including microparticles optical trapping, long-distance attraction, bidirectional transportation, and axial-direction position controllable adjustment. The proposed all-fiber active tractor beam generator is convenient to integrate and low cost. The all-fiber probe has the compatibility of optical tweezers and optical tractor beams. The increased effective range of optical pull broadens the scope of feasible optical trapping and will pave the way towards more efficient light-powered miniature machines, tools, and applications.

D型对称双芯光子晶体光纤双谐振峰折射率传感器

D型对称双芯光子晶体光纤双谐振峰折射率传感器

X-typed curvilinear transport of strongly absorbing particle in a dual-beam fiber optical trap.

We propose and demonstrate a novel approach to transport a strongly absorbing particle in an X-typed trajectory reciprocally in pure liquid glycerol based on a dual-beam optical fiber trap. We perform the X-typed light field by integrating a glass microsphere on the tip of a two-core fiber. The motion of the absorbing particle in pure liquid glycerol is dominated by the Δα-type photophoretic forces (FΔα). The incident laser power determines the direction of FΔα. Therefore, we may perform the reciprocating transport of the absorbing particle by changing and controlling the laser power. It is simple to manufacture the fiber probe and convenient to operate the transport of the microparticle. Our research expands the applications of absorbing particles in targeted drug delivery, biological sampling, and optically mediated particle clearing.

Two-core fiber based mode coupler for single-mode excitation in a two-mode fiber for quasi-single-mode operation

Abstract Quasi-single mode operation in few-mode fibers is a promising technique for extending the Kerr nonlinearity threshold of standard single mode fibers. Under this operation the design goals of few-mode fibers used for mode division multiplexing can be relaxed. Based on the Min Δ n eff , we calculated the maximum core radius of two-mode and four-mode fibers for quasi-single-mode operation, and is found to be 8 μm and 9.7 μm respectively. We also propose the design of a two-core fiber coupler, based on thermally diffused core technique, for effective excitation of the fundamental mode alone in the above two-mode fiber for the entire C-band. The robustness of the proposed design is studied by varying the design parameters and observing their sensitivity to coupling efficiency and extinction ratios and are found to be well above −0.5 dB and 25 dB respectively.

Two-core fiber-based mode converter and mode demultiplexer

Multicore fibers and few-mode fibers are potential candidates for achieving high-capacity space division multiplexing and mode division multiplexing (MDM) systems. Mode converters are an essential part of any MDM for effectively launching light into the desired mode. Fiber-based mode converters will be more useful than bulk converters because of their compatibility and ease of use. In this contribution, a two-core fiber-based mode converter design is proposed that can effectively convert the fundamental LP01 mode into the LP11 mode by mode coupling. This device can also be used as a demultiplexer in the reverse operation. Such a mode converter can be fabricated by heating a piece of two-core fiber with the thermally diffused core technique. This leads to the diffusion of dopants into the cladding, resulting in an expanded core giving power coupling between the cores. The conversion efficiency of this converter is −0.5  dB at the optimum value. This all-fiber mode converter ensures an extinction ratio of more than 30 dB over 45 nm wavelength, which covers the entire C band. The robustness of the proposed design is analyzed by evaluating the effect of changes in design parameters on the conversion efficiency and extinction ratios.

Random Raman fiber laser based on a twin-core fiber with FBGs inscribed by femtosecond radiation.

Narrowband Raman lasing in a polarization-maintaining two-core fiber (TCF) is demonstrated. Femtosecond point-by-point inscription of fiber Bragg gratings (FBGs) in individual cores produces a half-open cavity with random distributed feedback. The laser linewidth in the cavity with a single FBG inscribed in one core of the TCF reduced by ∼2 times with respect to the cavity with a fiber loop mirror. It is shown that the inscription of two FBGs in different cores leads to the formation of a Michelson-type interferometer, leading to the modulation of generation spectra near threshold. This technique offers new possibilities for spectral filtering or multi-wavelength generation.

Multispot launching for single-mode excitation in multimode fibers for high-speed applications.

Several launching methods have been employed in multimode fibers (MMFs) to reduce the intermodal dispersion. Selective excitation of the fundamental mode alone in an MMF can eliminate intermodal dispersion completely, provided the mode coupling along the fiber is minimum. In this paper, we present the design of a two-core fiber taper that results in the generation of a mode-field diameter-matched twin-spot beam, which can effectively excite only the fundamental mode (LP01) in an MMF. We also propose a new launching method, viz. dual twin-spot launching, which excites the LP01 mode alone in an MMF for a wide range of input beam spot sizes as compared to the other existing methods. The design of a four-core fiber taper for generating the proposed dual twin-spot beam is also presented. Misalignment tolerance of the dual twin-spot beam launching is compared with mode-field diameter-matched center launch (MFDM-CL) and mode-field diameter-matched twin-spot launch (MFDM-TSL). We show that the dual twin-spot launching method has higher alignment tolerances.

Modulation Instabilities in Asymmetric Fiber Coupler

We study modulation instability (MI) in an asymmetric two-core fiber coupler. The analysis is performed for the MI gain and MI bandwidth. Effects of the parameters, input power and nonlinear coefficients on the MI are investigated. The results show that input power and nonlinear coefficients of two cores influence the MI gain and MI bandwidth. When nonlinear coefficients of two cores are both negative, better modulation stability can be obtained than other situations. In particular, if the negative nonlinear coefficients are equal, the modulation instabilities will not appear.

Stimulated Raman Scattering and Four-Wave Mixing Effects on Crosstalk of Multicore Fibers

The numerical analysis of inter-core crosstalk in multicore fibers (MCFs) based on a novel theoretical approach that includes stimulated Raman scattering (SRS) and four-wave mixing (FWM) nonlinearities in the presence of random perturbations is presented. The coupled propagation equations can be employed to numerically analyze wavelength-division multiplexed systems with homogenous and heterogeneous cores. Propagation of ten optical channels, 200-GHz spaced, along 10 km of a homogeneous two-core fiber shows that the wavelength dependence of the relative average inter-core crosstalk induced by SRS and FWM is comparable with the one produced by only considering the wavelength dependence of the linear coupling coefficient, thus allowing tuning of the overall effect. Our formalism represents a valuable tool in the design of future multi-channel MCF systems.

Impact of linear coupling on nonlinear phase noise in two-core fibers

We study the effect of linear coupling on nonlinear phase noise in two-core fibers (TCFs). Considering the elementary TCF switching process, we demonstrate reduction of nonlinear phase noise in the propagation of optically amplified pulses through TCFs. Our results show that the reduction occurs just when the first maximum transfer of optical power is carried out between the TCF cores.