Dual-core Optical Fiber Research Articles

Endoscopic sensing of distal lung physiology

The alveolar space forms the distal end of the respiratory tract where chemoreceptor driven gas exchange processes occur. In healthy humans, the physiological state within the alveoli is tightly regulated by normal homeostatic mechanisms. However, pulmonary abnormalities such as chronic obstructive pulmonary disease may induce significant perturbation of the homeostatic baselines of physiology as well as cause host tissue damage. Therefore, physiological parameters (pH, glucose, oxygen tension) within the alveolar space provide a key biomarker of innate defence. Here, we discuss an endoscope-deployable fibre-optic optrode for sensing pH in the alveolar space. In order to circumvent the unwanted Raman signal generated within the fibre, the optrode consists of a custom asymmetric dual-core optical fibre designed for spatially separated optical pump delivery and SERS signal collection. pH sensing is achieved using the surface enhanced Raman spectroscopy (SERS) signal generated from functionalised gold nanoshell sensors. We show a ∼ 100-fold increase in SERS signal-to-fibre background ratio and demonstrate multiple site pH sensing in the alveoli of an ex vivo ovine lung model with a measurement accuracy of ± 0.07 pH unit.

Narrowband Raman fibre laser based on a dual-core optical fibre with FBGs inscribed by femtosecond radiation

The characteristics of Raman lasers based on a dual-core optical fibre are presented. Fibre Bragg gratings (FBGs), selectively inscribed in chosen cores by femtosecond radiation, are used as cavity mirrors. The laser configuration is optimised with respect to the output FBG reflectivity. The possibility of additional spectral filtering, based on the formation of a Michelson interferometer in the laser cavity by inscribing a pair of FBGs in different cores, is demonstrated.

Dark, bright and other optical solitons to the decoupled nonlinear Schrödinger equation arising in dual-core optical fibers

The dynamical systems of soliton propagation through optical fibers for trans-continental and trans-oceanic distances is one of the most interesting areas of study. Optical solitons are restrained electromagnetic waves that stretch in nonlinear dispersive media and allow the intensity to remain unchanged due to the balance between dispersion and nonlinearity effects. In this study, we successfully acquire dark, bright, combined dark–bright, singular and combined singular soliton solutions to the decoupled nonlinear Schrodinger equation arising in dual-core optical fibers by using the extended sinh-Gordon equation expansion method. The constraint conditions for the existence of valid soliton solutions are given. We discuss how change in parameters affect the solitons transmission. We present the 2D, 3D and the contour graphs to some of the reported solutions.

Impact of higher order dispersion and nonlinearities on modulational instability in a dual-core optical fiber

In this investigation, we examine theoretically modulational instability (MI) in a dual-core optical fiber with higher order dispersion as well as higher order nonlinear effects. For dual-core or multicore optical fiber the investigation of MI analysis extends with parameters like linear coupling coefficient and coupling coefficient dispersion using extended coupled nonlinear Schrodinger equation. We analyze the impact of these parameters on MI gain spectrum at anomalous and normal dispersion regime along with higher order parameters. Our results show that the combined effect of higher order dispersion like fourth order dispersion and nonlinear parameter like self-steepening effect due to cubic–quintic nonlinearity, stimulated Raman response which is inevitable for the study of ultrashort pulse propagation and generation of the soliton. The study reveals that the frequency selection of MI gain is primarily controlled by the dispersion parameters while nonlinear parameters restrict the intensity and bandwidth of the MI gain. Though most of the distinguishable effects are observed in anomalous dispersion regime due to the presence of higher order parameters decent gain is also visible in the normal dispersion regime.

Dual-Core Optical Fiber as Beam Splitter With Arbitrary, Tunable Polarization-Dependent Transfer Function

We present the design of a microstructured dual-core optical fiber with integrated electrodes and filled with liquid crystals. The dual-core structure acts as a directional coupler whose properties depend on the liquid crystal alignment. We show that with four electrodes and two separate driving voltages below 30 V on the electrodes, the beam-splitting properties of the fiber can be controlled independently and continuously for the two polarization components, thus allowing for the realization of any arbitrary 2 × 2 transfer function, such as tunable polarizers, polarization-dependent attenuators, or polarization-independent beam splitting.

Endoscopic sensing of alveolar pH.

Previously unobtainable measurements of alveolar pH were obtained using an endoscope-deployable optrode. The pH sensing was achieved using functionalized gold nanoshell sensors and surface enhanced Raman spectroscopy (SERS). The optrode consisted of an asymmetric dual-core optical fiber designed for spatially separating the optical pump delivery and signal collection, in order to circumvent the unwanted Raman signal generated within the fiber. Using this approach, we demonstrate a ~100-fold increase in SERS signal-to-fiber background ratio, and demonstrate multiple site pH sensing with a measurement accuracy of ± 0.07 pH units in the respiratory acini of an ex vivo ovine lung model. We also demonstrate that alveolar pH changes in response to ventilation.

Design of a low cost and high sensitivity sensor for curve measurement

In the dual-core optical fiber, the two beams are launched into two cores simultaneously, the solution of coupled mode equations is derived. It is shown that the slight change of the refractive index can affect the coupling between the cores, which causes the change of power of two cores. Based on this idea, the low-cost, high-sensitivity curve sensor based on the dual-core optical fiber is proposed. Numerical simulations with a full vectorial beam propagation method demonstrate that it allows highly accurate curve angle detection. The sensitivity of the curve sensor can reach 1.1405mW/deg. Moreover, by optimizing the pitch and choosing the different propagation direction and the output power of light source, we can obtain the higher sensitivity.

Non-linear switching based on dual-core non-linear optical fiber couplers with XPM and Raman intrapulse applied to femtosecond pulse propagation

In this work, we investigated the optical switching process for three shapes of femtosecond pulses (soliton, Gaussian and super-Gaussian) propagating inside a symmetrical dual-core non-linear directional coupler by simulating their propagation via the coupled non-linear Schrödinger equations. In all simulations, we considered the dispersive effects of second and third order, besides the self-phase modulation and self-steepening non-linear effects. We studied three scenarios for each of the three pulse shapes under investigation. In the first scenario, we added only cross-phase modulation (XPM); in the second approach, we added only Raman scattering; in the third one, we combined both. The study was performed for distinct polarization modes and for different values of the Raman factor, with power range varying from 1 to 300 W. We noted that the XPM non-linear effect results in a decrease in the critical power threshold, whereas the Raman scattering causes an increase. For the first scenario (only XPM effect), the critical power threshold reduced from 113.72 to 104.69 W for the soliton pulse, from 111.49 to 100.77 W for the Gaussian and from 92.79 to 80.47 W for the Super-Gaussian pulse shape. For the second scenario (only Raman scattering), the critical power increased for a Raman factor varying from 1 to 10 fs, and the three pulse shapes reached thresholds above 150 W from a 5 fs factor, reaching more than 200 W for the super-Gaussian pulse as the Raman factor increased. For the third scenario (with both effects combined), we highlight that for a fixed XPM factor of 2, the critical power remained unchanged with the variation of the Raman factor. Hence, we observed that the Super-Gaussian pulse reached lower values for critical power when compared to the other pulse shapes.

Study of the Effect of the Coupling in a Dispersion-managed Dual Core Optical Fiber Using the Collective Variables Approach

This paper highlights the interaction between two gaussian pulses propagating inside a dual core optical fiber by the mean of collective variables (CVs) approach. The main result is that energies of the signals being propagated in such fiber with linear coupling always end up being compensated whatever their amplitudes at the entry. It also appears convergence or divergence of the temporal positions of the neighboring solitary waves.

Design of dual-core optical fibers with NEMS functionality

An optical fiber with nano-electromechanical functionality is presented. The fiber exhibits a suspended dual-core structure that allows for control of the optical properties via nanometer-range mechanical movements. We investigate electrostatic actuation achieved by applying a voltage to specially designed electrodes integrated in the cladding. Numerical and analytical calculations are preformed to optimize the fiber and electrode design. Based on this geometry an all-fiber optical switch is investigated; we find that optical switching of light between the two cores can be achieved in a 10 cm fiber with an operating voltage of 35 V.

A low transmission loss THz polarization splitter based on dual-core optical fiber

We propose a design of a low loss terahertz polarization splitter based on a dual-core terahertz fiber with crossed dielectric strips in the fiber cross section. Low transmission loss is realized by extending the mode field to the air holes adjacent to the solid material. An 11.4-cm-long terahertz polarization splitter is obtained with the extinction ratio better than −15dB and a bandwidth of 16μm.

A theoretical investigation of soliton induced supercontinuum generation in liquid core photonic crystal fiber and dual core optical fiber

The supercontinuum generation (SCG) in liquid core photonic crystal fiber (LCPCF) with versatile nonlinear response and the spectral broadening in dual core optical fiber is presented. The analysis is presented in two phase, phase I deals with the SCG in LCPCF with the effect of saturable nonlinearity and re-orientational nonlinearity. We identify and discuss the generic nature of the saturable nonlinearity and reorientational nonlinearity in the SCG, using suitable model. For the physical explanation, modulational instability and soliton fission techniques is implemented to investigate the impact of saturable nonlinear response and slow nonlinear response, respectively. It is observed that the saturable nonlinearity inevitably suppresses the MI and the subsequent SCG. On the other hand, the re-orientational nonlinearity contributes to the slow nonlinear response in addition to the conventional fast response due to the electronic contribution. The phase II features the exclusive investigation of the spectral broadening in the dual core optical fiber.

Interplay between relaxation of nonlinear response and coupling coefficient dispersion in the instability spectra of dual core optical fiber

The modulational instability of a dual core optical fiber under the combined effect of coupling coefficient dispersion (CCD) and relaxation of nonlinear response is presented. The effect of wavelength dependence of coupling coefficient is effectively included in the pair of generalized linearly coupled nonlinear Schrödinger equation (CNLSE). In order to account for the finite response time, a time dependent nonlinear response is incorporated in the system of CNLSE using the Debye relaxation model. The results from the linear stability analysis show that in anomalous dispersion regime for any finite relaxation time, the nonlinearity becomes complex and there exist two unstable modes. In the normal dispersion regime, a single unstable band at higher detuning frequency is observed which is identified as the Raman band. In symmetric case the CCD is found to be immaterial, whereas in asymmetric case the CCD is found to be crucial and brings new spectral bands. A critical value of CCD is predicted where the dynamics evolve dramatically in different manners. Thus in this paper, the MI dynamics of a dual core optical fiber is fully explored and the interplay between CCD and relaxation is clearly highlighted.

Nanomechanical optical fiber

Optical fibers are an excellent transmission medium for light and underpin the infrastructure of the Internet, but generally after fabrication their optical properties cannot be easily modified. Here, we explore the concept of nanomechanical optical fibers where, in addition to the fiber transmission capability, the internal core structure of the fiber can also be controlled through sub-micron mechanical movements. The nanomechanical functionality of such fibers is demonstrated in the form of dual core optical fibers, in which the cores are independently suspended within the fiber. The movement-based optical change is large compared with traditional electro-optical effects and we show that optical switching of light from one core to the other is achieved through moving one core by just 8 nm.

Characterization of a novel dual-core elliptical hollow optical fiber with wavelength decreasing differential group delay

The modal distribution of a novel elliptical hollow optical fiber is experimentally and numerically characterized. The fiber has a central elliptical air hole surrounded by a germanosilicate lanceolate ring core. Experiments reveal that the fiber behaves like a dual core waveguide and it is found that the differential group delay of each core decreases with wavelength with a PMD coefficient slope of ~10(-2) ps/m/THz. Experimental results are also compared with numerical modeling based on scanning electron microscopy images.

Design of dual-core resonant leaky optical fibre based gain flattening filters for erbium doped fibre amplifiers

We propose optical fibre based filters employing dual-core resonant leaky structure for gain equalization of erbium doped fibre amplifier (EDFA). Spectral loss variation of the structure has been utilized to suppress gain peak and, thus, flatten overall gain profile in the C-band. We show 15.7 dB flat gain with ± 1.6 dB ripple in the wavelength range from 1525 nm to 1555 nm using a single filter and 18±0.7 dB gain using two cascaded filters.

Simulation of the modulation instability in dual-core optical fiber based on complex Ginzburg-Landau equation

Instability of the continuous-wave states in a dual-core optical fiber system is investigated based on the complex Ginzburg-Landau equation. The dual-core optical fiber system consists of an active nonlinear dispersive core and a linear passive core. The modulation instability (MI) conditions are found from linearized equations for small perturbations. Simulations of the full system demonstrate that the development of the MI in the former regime leads to the establishment of a regular or chaotic array of pulses if the MI saturates, or a chain of well-separated peaks with continuously growing amplitudes if the instability does not saturate. It indicates that the peak value of multiple return-to-zero (RZ) pulses or a single RZ pulse will be amplified and the RZ pulse sources emerge in the optical fiber in the anomalous group velocity dispersion regime. This research can be used as the resource of the optical fiber telecommunications and will be useful for the study of the fibre optics and physics and also in some other fields.

Studies on dual-core-bidirectional optical fiber chaotic secure communication system

A dual-core-bidirectional optical fiber chaotic secure communication system is proposed based on coupling-feedback synchronization of chaotic injected multi-quantum well (MQW) laser systems with fiber channels for optical fiber secure communications. A physical model of the system is built using dual-core optical fiber connecting the transmitter and receiver. Chaotic synchronization was theoretically and numerically studied and we found theoretically that self-phase modulation in the fiber would limit the chaotic synchronization. The nonlinear phase shift, power limit and propagation length are formulated. Chaotic synchronization of two long-distance fibers was numerically achieved, and synchronization time varying with fiber propagation length was numerically analysed. Optical fiber chaotic communications and their synchronization errors were numerically simulated in long-distance dual-core-bidirectiond optical fiber chaotic secure communications with an analog signal of modulation frequency 06 GHz, a digital signal of modulation rate 06 bit/s and a bit rate of 005 Gbit/s of chaos keying shift, respectively. The system bandwidth is also numerically analysed. The system has a good security capability of application in dual-core-bidirectional fiber secure communications.

The Hamiltonian perturbation approach of two interacting nonlinear waves or solitary pulses in an optical coupler

The traveling-wave solutions dynamics of two linearly coupled nonlinear Schrödinger equations, describing the wave propagation in a dual-core optical fiber, is studied in the framework of the Hamiltonian perturbation theory. The surface of section of the phase space for each wave is obtained as a contour plot of an approximate invariant of the system and the phase space distortions due to the interaction are analyzed. Depending on the magnitude of the coupling strength, the parametric resonance involved leads to weak, strong and chaotic interactions. The coupling strength threshold above which the interaction between two nonlinear waves of given amplitude becomes chaotic is also estimated. On the other hand, the interaction of a solitary pulse with a nonlinear periodic wave is studied using the Melnikov method for homoclinic orbits. This interaction causes stochastization of the homoclinic orbit, which corresponds to the solitary pulse and the separatrix splitting as well as the width of the stochastic layer near this orbit is given in terms of the Melnikov function. The latter is applied in the case of two interacting solitary pulses as well.

Gain flattening by using dual-core fiber in erbium-doped fiber amplifier

We show that a dual-core optical fiber in which one core is doped with erbium and the other core is undoped can be used as an optical amplifier with very flat gain over the wavelength range 1520 nm to 1560 nm. The gain variation over this range is less than 0.7 db. The noise floor is less than 4 dB.