Twin-core Fiber Research Articles

Wavelength-tunable mode-locked fiber laser based on a bending strain-controlled filter

Wavelength-tunable mode-locked fiber lasers are highly versatile and find applications in pump–probe measurement, optical communications, optical parametric oscillation, among others. Herein, we introduce a novel approach for tuning carbon nanotube-based passively mode-locked fiber laser using a bending-based mechanism. Our method involves a bending strain controlled multimode interference all-fiber filter. This filter consists of a hybrid structure built from a cascaded seven-core fiber (SCF)-twin-hole twin-core fiber (THDCF)-SCF. It achieves a continuous wavelength tuning range of 25 nm from 1558 nm to 1583 nm with a tuning efficiency of −3.38 nm/m−1. By increasing the pump power, the dual-wavelength locked modes are also achieved at specific curvature. Owing to advantages of simple structure, low cost, and high bending sensitivity, our tunable filter shows promise for efficient wavelength tuning in ultrafast fiber laser.

High FOM PCF-SPR refractive index sensor based on MgF2-Au double-layer films.

A simple twin-core D-shape photonic crystal fiber sensor based on surface plasmon resonance (SPR) is designed for the measurement of refractive indices (RI). The twin-core D-shape structure enhances the SPR effect, and the M g F 2-Au dual-layer film narrows the linewidth in the loss spectrum, consequently improving both the sensitivity and figure of merit (FOM). The properties of the sensor are analyzed by the finite element method. In the RI range of 1.32-1.42, the maximum wavelength sensitivity, FOM, and resolution are 62,000nm/RIU, 1281R I U -1, and 1.61×10-6, respectively.

A MachA-Zehnder interferometer salinity sensor based on a twin-core fiber with C-type microcavity

A MachA-Zehnder interferometer salinity sensor based on a twin-core fiber with C-type microcavity

Butterfly-shaped and dromion-like optical similaritons in an asymmetric twin-core fiber amplifier

Butterfly-shaped and dromion-like optical waves in a tapered graded-index waveguide (GRIN) with an external source are reported for the first time, to our knowledge. More pertinently, we obtain these waves both analytically and numerically in a generalized nonlinear Schrödinger equation (GNLSE), which describes self-similar wave propagation in GRIN with variable group-velocity dispersion (GVD), nonlinearity, gain, and source. The proposed GNLSE appertains to the study of similariton propagation through asymmetric twin-core fiber amplifiers. Dromion-like structures, which have generally been investigated in the (2+1) or higher dimensional systems, are reported in the (1+1) dimensional GNLSE with an external source. Herein, we introduce the concept of soliton management when the variable group-velocity dispersion and Kerr nonlinearity functions are suggested. For example, when the GVD parameter is perturbed, we observe the emergence of vibration of dromion-like structures. Then the dromion-like structure is transformed into oscillation by the modulation instability of the modified coefficient of the Gaussian GVD function, exhibiting interference based on two dromion-like structures. Additionally, the phenomenon of unbreakable PT symmetry of these nonlinear waves has been demonstrated for three explicit examples.

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

A vector bending fiber sensor based on core-by-core inscribed fiber Bragg gratings in a twin-core fiber has been proposed and experimentally demonstrated. An in-fiber integrated vector bending sensor is realized by using the thermal diffusion technique to fabricate the coupler. The characteristics of the coupler fabricated by thermal diffusion are simulated and experimented. By inscribing fiber Bragg gratings with different reflection wavelengths in the two cores of a symmetrical twin-core fiber, the curvature sensitivity can be enhanced by tracking the wavelength difference between the fiber Bragg gratings of the two cores. The measured bending sensitivity of the fiber Bragg grating ranges from −161.6 pm/m−1 to +165.5 pm/m−1. The differential sensitivity of the two cores is twice that of a conventional single grating, and the temperature-induced crosstalk is also reduced. The bending sensor proposed in this paper has the advantages of high integration, enhancing the sensitivity and two-dimensional orientation recognizability, and reducing temperature crosstalk, which can be a promising candidate for structural health monitoring or wearable artificial electronics applications.

Simultaneous measurement of axial strain and temperature based on a twin-core single-hole fiber with the optical Vernier effect.

An ultrasensitive optical fiber sensor based on the optical Vernier effect is proposed for the simultaneous measurement of axial strain and temperature. The sensor structure comprises two cascaded Mach-Zehnder interferometers (MZIs) with different free space ranges. The single MZI is built up by fusion splicing a segment of ∼3 mm twin-core single-hole fiber (TCSHF) between two pieces of ∼5 mm none core fibers (NCF). When acting separately, each MZI can respond linearly to the axial strain change with a sensitivity of ∼ 0.6 pm/µε and temperature with a sensitivity of ∼34 pm/°C. When the two MZIs are cascaded in series, the sensitivities are amplified about 30 times because of the optical Vernier effect. Experimental results demonstrate that the cascaded structure exhibits a high axial strain sensitivity of ∼ 17 pm/µε in the range of 0 to 2000 µε and temperature sensitivity of ∼1.16 nm/°C in the range of 30 to 70 °C. Moreover, the cascaded structure can simultaneously measure the axial strain and temperature change in the acceptable error ranges.

Thermal diffusion coupling mechanism and its application of discrete waveguide

For discrete optical systems integrated into optical fibers, the optical fields of the individual waveguides are coupled and correlated with each other. This paper studies how to adjust the refractive index of discrete waveguides by thermal diffusion, so as to enhance the coupling between discrete waveguides, and also constructs the discrete waveguide thermally diffused model and the thermally diffused coupling model of twin-core and three-core fibers. The multicore fiber is heated different times by a hydrogen-oxygen flame, and the outgoing light field at the end face of the optical fiber is monitored at the same time. Then, the three-dimensional refractive index measurement results of the thermally diffused multicore fiber verify the feasibility of thermal diffusion technology to change the refractive index of discrete waveguides for coupling. Thermal diffusion technology can be used to fabricate multicore fiber couplers. By combining multicore fiber and core-by-core inscribed fiber Bragg grating technology and by using thermal diffusion technology, the single-channel sensing measurement can be realized. The method of changing the refractive index of discrete waveguides through thermal diffusion has the advantages of high integration, high stability, and mass fabrication. The research on the thermal diffusion of discrete waveguides can improve the application potential of multicore fiber sensing systems, and promote the broad application of discrete waveguide structure optical fiber in the fields of optical communication, optical sensing, biomedicine, and artificial intelligence.

High Sensitivity Balloon-Like Sensor Based on Twin-Core and Twin-Hole Fiber

An all-fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. A high sensitivity balloon-like mode interferometer is fabricated with a twin-core and twin-hole fiber (TCTHF). The variations in ambient temperature and RI cause changes in phase differences between the modes, leading to shifts in the interference spectrum. The two resonance dip wavelength shifts within the spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI. The obtained maximum sensing sensitivities are 0.051 nm/°C and 423.168 nm/RIU. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its high sensitivity, low cost, and small size.

Twin-core fiber sensor integrated in laser cavity

In this work, we report on a twin-core fiber sensor system that provides improved spectral efficiency, allows for multiplexing and gives low level of crosstalk. Pieces of the referred strongly coupled multicore fiber are used as sensors in a laser cavity incorporating a pulsed semiconductor optical amplifier (SOA). Each sensor has its unique cavity length and can be addressed individually by electrically matching the periodic gating of the SOA to the sensor’s cavity roundtrip time. The interrogator acts as a laser and provides a narrow spectrum with high signal-to-noise ratio. Furthermore, it allows distinguishing the response of individual sensors even in the case of overlapping spectra. Potentially, the number of interrogated sensors can be increased significantly, which is an appealing feature for multipoint sensing.

Study on the temperature characteristic of tapered twin-core fiber working at dispersion turning point

We present a study on the spectral characteristics and ultrahigh sensitivity near the dispersion turning point (DTP) of tapered twin-core fiber (TCF). The tapered TCF with the DTP can be fabricated by the one-step taper method by the flame taper machine. Numerical simulations reveal that DTP exists when the group effective refractive index difference between the HE11 mode and the HE21 mode equals zero, where ultrahigh sensitivities can be achieved. The position of the DTP is dependent on the diameter of the taper waist and the refractive index of the external environment. We experimentally demonstrate the temperature characteristic of tapered TCF with DTP in the air, and its maximum sensitivity is − 994 pm/ºC. Finally, by coating polydimethylsiloxane (PDMS) material with the high refractive index and the high thermal-optical coefficient on the tapered TCF, not only the DTP can be achieved in large diameter, but also ultra-high temperature sensitivity can be obtained. The measured temperature sensitivity of the tapered TCF coated with PDMS is as high as 36.71 nm/ºC.

Refractive index sensing of twin-core fiber based on Mach Zehnder interference and SPR effect

The sensing characteristics of twin-core fiber for refractive index (RI) detection are presented and experimentally investigated. In this work, for the first time, we demonstrate the manufacturing method of optical fiber sensor based on Mach Zehnder interference and surface plasmon resonance. Mode interference and resonance coupling effect are thoroughly realized with the misalignment fusion and silver mirror reaction. It is found that the sensitivity of the proposed sensor is 3020 nm RIU−1 in the RI ranging from 1.3333 to 1.3804, which is much higher than that of the conventional fiber sensor. Moreover, experimental data indicate that this sensor has the advantages of easy fabrication, good repeatability and good stability. According to these characteristics, we introduce one specific application of pregnancy detection to validate the application of the designed sensor in biomedical field.

Refractive index and temperature dual parameter sensor based on a twin-core photonic crystal fiber

A twin-core photonic crystal fiber sensor is proposed for measuring liquid refractive index (RI) and temperature simultaneously. The air holes of the sensor are arranged in a hexagonal pattern, and two planes are introduced by polishing in the cladding. On one side of the plane, the gold film is deposited for RI measurement, and on the other side, the gold film and polydimethylsiloxane are deposited for temperature measurement. We analyzed its sensing characteristics by using the finite element method. The numerical results show that the two channels for measuring RI and temperature have no mutual interference and the arrangement reduces the complexity of the sensing measurement. The maximum spectral sensitivity of the sensor is 20 000 nm/RIU and 9.2 nm °C−1, respectively, when the liquid RI is in the range of 1.36–1.42 and the temperature is in the range of 0 °C–50 °C. The results also show the sensing accuracy was not very sensitive to the change of structural parameters, which makes the sensor very easy to fabricate. Our work is very helpful for implementation of a high sensitivity, easy fabrication and real-time multi-parameter surface plasmon resonance sensor.

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

In-Fiber Thermally Diffused Coupler and Fiber Bragg Grating Inscribed in Twin-Core Fiber for Sensitivity-Enhanced Vector Bending Sensing

Simulation of highly sensitive temperature sensing based on a selectively ethanol-filled twin-core photonic crystal fiber

A selectively ethanol-filled twin-core photonic crystal fiber (TC-PCF) sensing scheme is proposed and theoretically investigated for highly sensitive temperature sensing. Different filling cases and fiber structure were simulated to investigate the mode coupling of two solid fiber cores of the TC-PCF. Our calculation for the sensing performance shows that with properly designed fiber structure and filling configuration, a high temperature sensitivity of −16.7 nm/°C could be achieved in a range from 20 to 50 °C.

Modulation instabilities in twin-core fibers with self-steepening effects

Effects of self-steepening (SS) on modulation instabilities (MIs) are systematically studied in twin-core optical fibers with the asymmetric continuous wave (CW) state. In anomalous dispersion, there is one MI band. SS can reduce the instability gain significantly, shrink the MI band dramatically, and move the instability band to low frequency. MI never vanishes for any incident total powers above a required minimum total power, being contrary to the case in single core fibers that MI disappears at total powers above a threshold value in relation to SS. In normal dispersion, there are two MI bands. With SS, the high frequency band vanishes for incident total powers above a threshold value, and the low frequency band enhances with gain increasing and moves to the lower frequency. We also discover that SS can significantly suppress (enhances) the high frequency band that appears due to coupling coefficient dispersion and fourth-order dispersion (Raman scattering).

Darboux transformation, generalized Darboux transformation and vector breather solutions for a coupled variable-coefficient cubic-quintic nonlinear Schrödinger system in a non-Kerr medium, twin-core nonlinear optical fiber or waveguide

Non-Kerr media, twin-core nonlinear optical fibers and waveguides are widely applied in optical fiber communications. In order to model the effects of quintic nonlinearity for the ultrashort optical pulse propagation in a non-Kerr medium, twin-core nonlinear optical fiber or waveguide, a coupled variable-coefficient cubic-quintic nonlinear Schrödinger system is investigated in this paper. For the two components of the electromagnetic fields, on the basis of the existing Lax pair, we take a gauge transformation to convert a Lax pair into an Ablowitz-Kaup-Newell-Segur system. Then we acquire a system, which is equal to the original system. The N-fold Darboux transformation is derived based on our Lax pair, where N is a positive integer. Applying Taylor series, we derive the N-fold generalized Darboux transformation. The second- and third-order vector breather solutions are obtained via the generalized Darboux transformation method. All our results depend on the variable coefficients of the original system. Our results might be of some use in a non-Kerr medium, twin-core nonlinear optical fiber or waveguide system.

Performance improvement of a plasmonic tapered twin–core fiber optical tweezers

In this paper, a plasmonic tapered twin-core fiber optical tweezers is proposed. Numerical simulations are carried out to fully investigate its performance. Strong resonance behavior is observed at λ = 14.7 µm. It is shown that in the proposed structure, high values of trapping force can be achieved due to intrinsic field enhancement of plasmonic effect. Moreover, it is observed that our proposed structure can trap and induce force to the particles as small as 0.01 µm radius size.

A Twin-Core and Dual-Hole Fiber Design and Fabrication

We have proposed and demonstrated novel twin-hole and dual-core optical fiber. We used the normal MCVD technology to fabricate a single core fiber preform with a thin cladding. Then, two small holes were drilled in the center and on the side of the pure silica rod, the fabricated single-core fiber preform was inserted into the two holes, and then the combined preform was heated until three parts were fused. Finally, two larger holes were drilled close the center core and twin-core and dual-hole are vertically distributed. In order to retain the shape of air hole, we filled nitrogen gas in the two holes to against the ambient pressure, which made the holes collapse during the fiber drawing. The dual-hole microstructure may be used as micro-containers to inject modulation medium and aside the center core, and to induce accumulated interaction between flow material and evanescent field of the center core, therefore, it can be used as a modulation function fiber to fabricate a very compact in-fiber integrated device.

Dual-demodulation large-scope high-sensitivity refractive index sensor based on twin-core PCF.

In this paper, a refractive index (RI) sensor based on the twin-core photonic crystal fiber (TC-PCF) is presented. Introducing the rectangular array in the core area makes the PCF possible to obtain high birefringence and low confinement loss over the wavelength range from 0.6 µm to 1.7 µm. Therefore, the core region can enhance the interaction between the core mode and the filling material. We studied theoretically the evolution characteristics of the birefringence and operating wavelength corresponding to the strongest polarization point under the condition of filling the rectangular array with RI matching fluid range from 1.33 to 1.41. Simulation results reveal that the proposed TC-PCF has opposite evolutions of change rates between the B and wavelength, and the maximum RI sensing sensitivities of 1.809×10−2 B/RIU and 8 700 nm/RIU at low and high RI infill are obtained respectively, which means that the TC-PCF features of dual-parameter demodulation for the RI sensing can maintain a high refractive index sensing sensitivity within a large scope of RI ranging from 1.33 to 1.41. Compared with the results of single-parameter demodulation, it is an optimized method to improve the sensitivity of low refractive index sensors, which has great application potency in the field of biochemical sensing and detection.

Two core optical fibers coupled nonlinear model in the framework of Hausdorff fractal derivative

In this article, two core optical fibers coupled nonlinear model in the sense of Hausdorff fractal derivative is established for the construction of bright, dark, and exponential novel solitons. The model built in terms of the fractal derivative is ideal for propagating the pulse in asymmetric twin-core fibers. Using the variational method, computed novel bright, dark, and exponential solitons are achieved. For the production of soliton solutions, the constraint conditions are defined. Under the choice of appropriate values of physical parameters, the wide range of newly obtained bright, dark, and exponential solitons is represented graphically in 3D, 2D, and contour graphs.