Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber （SMF） with high and steady coupling efficiency is crucial for fiber- based free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler （AFC） which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent （SPGD） algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80/3tad. The compensation bandwidth is 35 Hz at sine-jitter of 15 ~rad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error （MSE） of coupling efficiency drops from 7.43% to 1.75%.

Design of a Tapered Lens for Semiconductor Laser to Single-Mode Fiber Coupling

The theoretical investigation of the coupling efficiency of a laser diode to a single mode fiber via a hemispherical lens on the tip of the tapered fiber in the presence of possible transverse offset and angular mismatch is reported.Without the misalignment,coupling efficiency increases with the decreasing of taper length.With the misalignment,this relation is that the coupling efficiency decreases with each kind of offset.

Single-mode antiresonant terahertz fiber based on mode coupling between core and cladding

Based on the index-induced mode coupling between the higher-order mode in core and the fundamental mode in cladding tubes,the single-mode operation can be realized in any antiresonant fibers(ARFs)when satisfying that the area ratio of cladding tube and core is about 0.46:1,and this area ratio also should be modified according to the shape and the number of cladding tubes.In the ARF with nodal core boundary,the mode in core also can couple with the mode in the wall of core boundary,which can further enhance the suppression of high-order mode.Accordingly,an ARF with conjoint semi-elliptical cladding tubes realizes a loss of higher-order mode larger than 30 dB/m;simultaneously,a loss of fundamental mode loss less than 0.4 dB/m.

Derivation of General Formula for Coupling Loss of Single-mode Fiber Collimators with Gradient-index Rod Lenses

Gradient-index rod lens (GRIN-lens) whose pitch is ordinary value with bevel ferrule coupling system is analyzed, an equivalent method which can be used to analyze this system is put forward, and a general formula for determining the coupling loss with axes mismatch, radial mismatch, and angular mismatch is derived by use of the Gaussian field approximation and mode-field coupling theory. The experimental results are in good agreement with the theory prediction. It indicates that these formulas are suitable to analyze the gradient-index rod lens coupling system with pigtail fiber.

High efficiency of Brillouin scattering behavior in single-mode Ge–As–Se–Te fibers at 2μm

The Brillouin characteristics of step-index Ge–As–Se–Te(GAST)fibers at 2μm are designed and simulated on the basis of optical and acoustic properties.The refractive indexes of Ge_(20)As_(20)Se_(45)Te_(15)glass and Ge_(20)As_(20)Se_(43)Te_(17)glass serving as fiber core and cladding are 3.20 and 3.18 at 2μm,and their acoustic velocities are 2200 m/s and 2300 m/s,respectively.Numerical results indicate that the stimulated Brillouin scattering(SBS)efficiency is 248 m^(-1)·W^(-1),and the Brillouin threshold power is 66 m W when the core diameter of the 2-m-long GAST fiber is 4μm at 2-μm wavelength.The optic–acoustic coupling factor,the Brillouin frequency shift,and the Brillouin gain coefficient are 0.98,7.02 GHz,and 3.81×10^(-9)m/W,respectively.The SBS effect of GAST fibers simulated for the first time provides a new promising approach to selecting gain medium based on 2-μm-wavelength fiber laser.

Effect of Silane Coupling Agent Concentration on Interfacial Properties of Basalt Fiber Reinforced Composites

The purpose of this study is to investigate the effect of the concentration of silane coupling solution on the tensile strength of basalt fiber and the interfacial properties of basalt fiber reinforced polymer composites.The surface treatment of basalt fibers was carried out using an aqueous alcohol solution method.Basalt fibers were subjected to surface treatment with 3-Methacryloxypropyl trimethoxy silane at 0.5 wt.%,1 wt.%,2 wt.%,4 wt.%and 10 wt.%.The basalt monofilament tensile tests were carried out to investigate the variation in strength with the concentration of the silane coupling agent.The microdroplet test was performed to examine the effect of the concentration of the silane coupling agent on interfacial strength of basalt reinforced polymer composites.The film was formed on the surface of the basalt fiber treated silane coupling agent solution.The tensile strength of basalt fiber increased because the damaged fiber surface was repaired by the firm of silane coupling agent.The firm was effective in not only the surface protection of basalt fiber but also the improvement on the interfacial strength of fiber-matrix interface.However,the surface treatment using the high concentration silane coupling agent solution has an adverse effect on the mechanical properties of the composite materials,because of causing the degradation of the interfacial strength of the composite materials.

The entanglement evolution of two coupling two-level atoms interacting with a single-mode vacuum field in multiphoton Tavis-Cummings model

Using multipohton Tavis-Cummings model,the entanglement evolution of two coupling two-level atoms in Bell states interacting with a single-mode vacuum field is investigated by using negativity.The influences of coupling constants between atoms,the atomic initial states and the photon number of transition on the entanglement evolution of two coupling two-level atoms are discussed.The results obtained using the numerical method show that the entanglement of two atoms is related with coupling constants between atoms,the atomic initial states and the photon number of transition.The two-atom entanglement state will forever stay in the maximum entanglement state when the initial state is β11〉.When the initial state of two atoms is β01〉,the entanglement of two atoms displays periodic oscillation behavior.And its oscillation period decreases with increasing of coupling constant between atoms or the photon number of transition.On the other hand,when the initial state is β00〉 or β10〉,the entanglement of two atoms displays quasiperiodic oscillation behavior and its oscillation period decreases with increasing of coupling constant between atoms or the photon number of transition.

Fiber Coupling of Laser Diode Bar to M ultimode Fiber Array

A piece of multimode optical fiber with a low num er ical aperture (NA) is used as an inexpensive microlens to collimate the output r adiation of a laser diode bar in the high numerical aperture (NA) direction.The emissions of the laser diode bar are coupled into multimode fiber array.The radi ation from individual ones of emitter regions is optically coupled into individu al ones of fiber array.Total coupling efficiency and fiber output power are 75% and 15W,respectively.

High Brightness,High Power Density Fiber Coupling of High Power Laser Diode

The output radiation from the 100μm×1μm aperture of a high power Laser Diode (LD) is efficiently coupled into a 50μm multimode optical fiber.The fiber output of the high power LD with high brightness and high power density is achieved.The power density is up to 3 6×104W/cm2 and the coupling efficiency is 70%.The extreme divergence and the astigmatism of high power LDs require the optics with complex lens structures and high performance.A double-curved lens with two crossed cylindrical lenses structured on both sides of the glass substrate is used in the coupling system.

Coupling characteristics of high birefringence dual-core As_2S_3 rectangular lattice photonic crystal fiber

A type of As2S3 chalcogenide glass mid-infrared dual-core photonic crystal fiber has been proposed. The dualcore photonic crystal fiber （PCF） consists of two asymmetric cores. The high polarization property and the coupling characteristics have been studied by using the finite dement method and mode coupling theory. Numerical results show that the birefringence at wavelength λ = 10 μm is up to 0.01386 and the coupling length can reach wavelength = 5 μm, 261 μm and 271.44 μm for x-polarized mode and y-polarized mode, respectively. It demonstrates that a 6.786-ram-long fiber can exhibit an extinction ratio of better than -10 dB and a bandwidth of 180 nm.

Wavelength dependent loss of splice of single-mode fibers

After reviewing three different definitions of mode field diameter of single-mode fibers, coupled efficiency calculation methods associated with lateral offset, longitude separation and wavelength, the effects produced by them, and the influences of splicing defects were discussed in detail. The regularities of the effects were studied according to the first order derivation of couple efficiency formula, and a simplified formula for couple efficiency calculation was presented under the circumstance of slight misalignment, with respect to wavelength, 2, and in a good agreement with the theoretical model. The simplified formula provides a new but simple approach to evaluate wavelength dependent couple efficiency of single-mode fibers. Theoretical analyses and numerical calculations show that, when those defects exist, the wavelength produces additional effects on the couple loss that growth of wavelength causes an increase on the couple efficiency for the lateral offset or longitude separation whereas lessens the couple efficiency due to angular misalignment or mode fields mismatching, and that the wavelength degrades the couple efficiency distinctly when λ≥2.5 μm whereas it distorts the couple slightly in range of λ≤2λ≤2 μm.

Suppression of Cladding Mode Coupling by B/Ge Codoped Photosensitive Fiber With Photosensitive and Depressed Inner Cladding

Excess loss on the short wavelength side of the Bragg resonant wavelength caused by cladding mode coupling limits wide use of grating in the fiber communication system, especially in densed wavelength division multiplexing (DWDM) system. A novel photosensitive fiber design that have depressed cladding and photosensitive inner cladding in the same fiber is proposed, which can suppress cladding mode coupling greatly. Using MCVD method B/Ge codoped fiber with depressed cladding was fabriceted out, which was also doped in boron and germanium and had the photosensitivity. Finally, the transmission spectrum of written grating in this fiber by phase mask method verified its larger photosensitivity and greatly suppression of cladding mode coupling.

Characteristics of High-power GaAs Laser Beams and Their Coupling with Fibers

The beams of 980nm high-power LDs are analyzed, and the reasons that aspect ratio of LD beams is high are explained. It is certified by the test that cylindrical lens can efficiently compress the perpendicular divergence angle of the beam. Some typical and popular lensed fibers were compared and analyzed according to coupling characteristics. The factors which affect the coupling efficiency and tolerance of the wedged-shaped GRIN tipped lensed fiber are pointed out, and some methods to reduce the coupling loss of the lensed fibers are proposed finally.

Calculation of the Coupling Coefficient of Twin-Core Fiber Based on the Supermode Theory with Finite Element Method

Currently, coupled mode theory (CMT) is widely used for calculating the coupling coefficient of twin-core fibers (TCFs) that are used in a broad range of important applications. This approach is highly accurate for scenarios with weak coupling between the cores but shows significant errors in the strong coupling scenarios, necessitating the use of a more accurate method for coupling coefficient calculations. Therefore, in this work, we calculate the coupling coefficients of TCFs using the supermode theory with finite element method (FEM) that has higher accuracy than CMT, particularly for the strong coupling TCF. To investigate the origin of the differences between the results obtained by these two methods, the modal field distributions of the supermodes of TCF are simulated and analyzed in detail.

DISTRIBUTED OPTIC－FIBER FORCE SENSOR BASED ON MODE COUPLING EFFECTS IN HIGH－BIREFRINGENT FIBER

An optic-fiber sensor system for measuring distributed forces is presented. The sensor system chooses high-birefringent fiber as sensing e-iement. Based on coupling effects of polarization modes in high-birefringent fiber, the distribution of the force points along the sensing fiber can be measured by detecting mode couplings in the fiber which are caused by external force disturbance. The location and magnitude of the measured forces are determined by a heterodyne interferometry and the technique of optic path scanning. The spatial resolution of the sensor system is better than 0. 15m for a 50m testing fiber.

Realization of a broadband terahertz wavelength-selective coupling based on five-core fibers

We propose a novel kind of wavelength-selective coupling for the terahertz range based on solid five-core fiber （FCF）. The performances of coupling, propagation characteristics, and confinement loss properties are numerically investigated by using a full vector beam propagation method （BPM）. Simulation results show that it is possible to realize a broad- band wavelength-selective coupling. The coupling length can reach 1.913 cm, and the confinement loss is better than 1.965 ×10^-4 cm^-1. Furthermore, a parameter, the power difference, is defined, and it numerically demonstrates the working performance of the wavelength-selective coupler; that is, when the power difference is better than - 15 dB, the frequency located in the range of 0.76 THz-100 THz is separated relatively well from the frequency of 0.3 THz. Finally, the effect of the structural parameter on the working performance of the coupler is also investigated. We show that the performance optimization is possible by appropriately tuning the core diameter, and the tunabilities of frequency and bandwidth are possible by appropriately tuning the pitch. The wavelength-selective coupler is of potential application for optical fiber sensing and communication in terahertz wavelength division multiplexer fields.

Theoretical analysis of the mode coupling induced by heat of large-pitch micro-structured fibers

In this paper,a theoretical model to analyze the mode coupling induced by heat,when the fiber amplifier works at high power configuration,is proposed.The model mainly takes into consideration the mode field change due to the thermally induced refractive index change and the coupling between modes.A method to predict the largest average output power of fiber is also proposed according to the mode coupling theory.The largest average output power of a large pitch fiber with a core diameter of 190 μm and an available pulse energy of 100 mJ is predicted to be 540 W,which is the highest in large mode field fibers.

FAST EVALUATION OF THE CHARACTERISTICS OF SINGLE-MODE OPTICAL FIBERS

This paper describes a method for fast engineering evaluation of the transmission characteristicsof single-mode optical fibers.A versatile microcomputer program is presented which can be utilized to ana-lyze mode field characteristics of single-mode fibers with arbitrary refractive index profiles.Our computationshows that the mathematical algorithm and the corresponding programs developeod in this paper are rela-tively simple and accurate enough for the engineenring design of single-mode fibers for optical communica-tion systems.

Single-polarization single-mode hollow-core negative curvature fiber with nested U-type cladding elements

Hollow-core negative curvature fibers(HC-NCFs)have become one of the research hotspots in the field of optical fiber because of their potential applications in the data and energy transmissions.In this work,a new kind of single-polarization single-mode HC-NCF with nested U-type cladding elements is proposed.To achieve the single-polarization single-mode transmission,we use two different silica tubes in thickness,which satisfy the resonance and anti-resonance conditions on the U-type cladding elements and the cladding tubes,respectively.Besides,the elliptical elements are introduced to achieve good single-mode performance.By studying the influences of the structure parameters on the propagation characteristics,the optimized structure parameters are obtained.The simulation results show that when the wavelength is fixed at 1550 nm,the single-polarization single-mode transmission is achieved,with the polarization extinction ratio of 25749 and minimum high-order mode extinction ratio of 174.Furthermore,the confinement loss is only 0.0015 dB/m.