Showing papers on "Equilibrium mode distribution published in 2014"

Controlling mode instability in a 500 W ytterbium-doped fiber laser

Abstract: In this paper mode instability in a 500 W ytterbium-doped fiber laser is experimentally examined by changing the pumping wavelength, spectral bandwidth of signal light, active fiber temperature and coiling radius. The magnitude of power transfer from the fundamental mode to the higher order mode due to mode instability is measured as a criterion for its incident. The experiments show that the coiling radius of the first few tens of centimeters of the active fiber plays a significant role in controlling mode instability, and shifting the pumping wavelength from 976 to 973 nm can mitigate mode instability.

Measurement of mode coupling distribution along a few-mode fiber using a synchronous multi-channel OTDR.

Abstract: We describe the nondestructive measurement of mode coupling along a few-mode fiber using a synchronous multi-channel optical time-domain reflectometer (OTDR). By installing a few-mode fiber (FMF) coupler made with a phase mask method, we excite the LP01 mode in an FMF under the test as an input mode, and then we detect backward Rayleigh scattered LP11a or LP11b modes, which were generated as a result of the mode coupling through the coupler. The mode coupling distribution between the LP01 and LP11a,b modes along the test FMF was successfully measured with a 10-m spatial resolution by obtaining the ratio between the backscattered LP01 mode and LP11a or LP11b. The value of the mode coupling obtained with the present method agreed well with that obtained with the conventional transmission method.

Strip-slot waveguide mode converter based on symmetric multimode interference.

Abstract: Optical mode mismatch makes coupling between strip and slot waveguides a tough issue in integrated photonics. This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI). Distinct from previous reported converters which gradually convert the mode through sharp tips, the proposed solution makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode. A converter based on this mechanism is able to convert light from a TE-polarized fundamental mode of a strip waveguide to that of a slot waveguide, and vice versa. Strip-slot waveguide coupling though this mode converter has a measured efficiency of 97% (−0.13 dB), and the dimensions are as small as 1.24×6 μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.

Modeling of Mode Coupling in Multimode Fibers With Respect to Bandwidth and Loss

Abstract: A fast, accurate and simple field coupling model is presented which is capable of describing mode coupling effects due to bends and splices in multimode fibers with parabolic index profile as well as the coupling losses induced by this process. This model is validated numerically by comparing the results to the well-known coupled amplitude theory model yielding the same relative bandwidth increase behavior as long as the coupling losses are the same. It is shown, that the number of discrete segments used in this model can be reduced considerably as long as the coupling losses are kept constant. The effect of mode coupling on the differential group delay, mode dependent loss, bandwidth gain and impulse response width reduction are analyzed. It is shown that the relative bandwidth gain induced in MMF links induced by the coupling process is independent of fiber parameters or number of guided modes; it can be fully characterized by coupling induced losses. The model is compared to well-known results given by power coupling models and a good agreement is observed for high steady state loss values.

Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material

Abstract: High-index-contrast (HIC) waveguide such as Si and Si3N4 has small mode size enabling compact integration. However, the coupling loss with single mode fiber is also remarkable owning to the mode mismatching. Therefore, mode size converter, as the interface between HIC waveguide and optical fiber, takes an important role in the field of integrated optics. The material with refractive index (RI) between HIC waveguide and optical fiber can be used as a bridge to reduce the mode mismatching loss. In this letter, we employ silicon oxynitride (SiON) with RI about 1.50 as the intermediate material and optimize the structure of the SiON waveguide to match with cleaved single mode fiber and HIC waveguide separately. Combined with inverse taper and suspended structure, the mismatching loss is reduced and the dependence to the dimension of the structure is also released. The coupling loss is 1.2 and 1.4 dB/facet for TE and TM mode, respectively, with 3 dB alignment tolerance of ± 3.5 μm for Si(3)N(4) waveguide with just 200 nm-wide tip. While for Si waveguide, a critical dimension of 150 nm is applied due to the higher index contrast than Si(3)N(4) waveguide. Similar alignment tolerance is realized with coupling loss about 1.8 and 2.1 dB/facet for TE and TM mode. The polarization dependence loss (PDL) for both platforms is within 0.5 dB.

Comparative study of light propagation and single-mode operation in large-mode area fibers designed for 2-μm laser applications

Abstract: Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S 2 imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1 dB) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600 μm 2 ) beam delivery.

Mode conversion based on forward stimulated Brillouin scattering in a hybrid phononic-photonic waveguide

Abstract: We propose a scheme for on-chip all optical mode conversion based on forward stimulated Brillouin scattering in a hybrid phononic-photonic waveguide. To describe the mode conversion the theoretical model of the FSBS is established by taking into account the radiation pressure and the electrostriction force simultaneously. The numerical simulation is carried out for the mode conversion from the fundamental mode E11x to the higher-order mode E21x. The results indicate that the mode conversion efficiency is affected by the waveguide length and the input pump light power, and the highest efficiency can reach upto 88% by considering the influence of optical and acoustic absorption losses in the hybrid waveguide. Additionally, the conversion bandwidth with approximate 12.5 THz can be achieved in 1550nm communication band. This mode converter on-chip is a promising device in the integrated optical systems, which can effectively increase the capacity of silicon data busses for on-chip optical interconnections.

Large mode area modified clad leakage channel fibers with low bending and higher differential losses

Abstract: In this work, a simple design for all-solid large mode area leakage channel fibers with asymmetric cladding has been proposed to achieve effectively single mode operation through low index doped rods of two different diameters for high power fiber lasers and amplifier applications. Through numerical simulations, carried out by a full-vectorial finite-element method based solver, the modal properties of the proposed design have been investigated and the results have confirmed that the proposed fiber exhibits low fundamental mode bending loss and a very high differential ratio between the fundamental and higher order modes at practically allowable bending radii of 5–15 cm. The effective mode area of the fundamental mode at 1064 nm is 1974.2 μm2 for the straight fiber and shrinks when the fiber is bent at different radii. We have also numerically investigated the mode area enhancement under bent conditions by introducing a microsized center doped capillary in the fiber core.

Mode-hopping suppression of external cavity diode laser by mode matching

Abstract: Mode hopping is a critical performance limitation of external cavity diode lasers (ECDLs) We developed a method to suppress the mode hops by shifting the internal cavity longitudinal mode spectrum to match the other mode spectra A model of longitudinal mode structure based on Littman–Metcalf configuration has been deduced for analyzing the characteristics of external cavity mode hop and internal cavity mode hop The best mode matching point appears at the peak of output power when tuning the ECDL Both the short-term and long-term wavelength stability were investigated in different laser diode current values while the tuning element maintains a fixed position The experimental results demonstrated the effectiveness of the proposed mode matching methods for improving the mode stability and reducing the sensitivity to environment change

Mode coupling in low NA plastic optical fibers

Abstract: Using the power flow equation and experimental measurements, investigated in this article is the state of mode coupling in low NA (0.3) step-index plastic optical fibers for laser beam launch. Results show the coupling length L c at which the equilibrium mode distribution is achieved and the length z s of the fiber required for achieving the steady-state mode distribution. Numerical results obtained using the power flow equation agree well with experimental measurements.

Variable optical attenuator and dynamic mode group equalizer for few mode fibers

Abstract: Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to −28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber’s two supported mode group (LP01 and LP11). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes’ distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.

High-power Mode Converter of Overmoded Bent Circular Waveguide

Abstract: Based on the mode coupling theory, a TE01—TM11 mode converter was designed and optimal results were obtained. In this paper, bandwidth of mode converter with axis exponential structure is the largest, and mode converter with axis sinusoidal structure is the most compact in the case of the center frequency is 28GHz and the waveguide radius is 16mm. If the bending angle between input port and output port of mode converter was demanded for 90°, mode converter with axis parabola structure had the characteristics of compact structure, higher mode purity and larger relative bandwidth. Meanwhile, we found that the converter could be also used as TE11 mode transition with transmission efficiency above 99%. But due to bending angle of this structure was restricted, its center frequency and waveguide radius had the obvious corresponding relation.

Mode Shape Variation for a Low-Mode Number Flexible Cylinder Subject to Vortex-Induced Vibrations

Abstract: The excitation of two low-mode number, flexible cylinders in uniform-flow is investigated to determine effects of structural mode shape on vortex-induced vibrations Experiments are performed in a re-circulating flow channel and in a small flow visualization tank using object tracking and digital particle image velocimetry (DPIV) to measure the excitation of the cylinder, to estimate forces acting on the structure, and to observe the wake of the structure under the observed body motions Previous research has focused on understanding the effect of in-line to cross-flow natural frequency ratio on the excitation of the structure in an attempt to model the excitation of multiple structural modes on long, flexible bodies The current research investigates the impact of structural mode shape on this relationship by holding the in-line to cross-flow natural frequency constant and attempting to excite a specific structural mode shape It is found that the combination of an odd mode shape excited in the cross-flow direction with an even mode shape in the in-line direction results in an incompatible synchronization condition, where the dominant forcing frequency in-line may experience a frequency equal to the cross-flow forcing frequency, a condition only observed in rigid cylinder experiments when the natural frequency ratio is less than one This is consistent with the first mode being excited in both in-line and cross-flow directions, however this leads to an asymmetric wake The wake is observed using DPIV on a rigid cylinder with forced motions equivalent to the flexible body A case of mode switching is also observed where the even in-line mode exhibits an excitation at twice the cross-flow frequency; however the spatial mode shape in-line appears similar to the first structural mode shape It is hypothesized that this situation is possible due to variation in the effective added mass along the length of the cylinderCopyright © 2014 by ASME

3 Modes transmission using hybrid separation with high mode selectivity and low losses spatial mode multiplexer

Abstract: We present a new spatial multiplexer based on a succession of transverse phases that limits modal crosstalk and injection loss. We demonstrate a three mode transmission experiment in 20km-long fiber using optical and digital mode separation.

Mode Beating Analysis by Sample Stretching and Wavelength Sweeping in a Few-Mode Fiber

Abstract: We analyze the beating of coherently copropagating modes in a few-mode optical fiber using two complementary techniques, which involve respectively stretching the sample under examination and sweeping the wavelength for different polarizations of the incoming light. We show that analyzing the spectral composition of the intensity fluctuations allows for a direct measurement of the difference between the mode propagation constants and between their group delays, and we observe the lifting of the mode degeneracy, most possibly induced by residual anisotropies and birefringence. The outlined methodology can be used for the modal characterization of few-mode fibers employed in sensing and in spatially-multiplexed communications.

Specific mode output from multimode fiber oscillators by designing rare earth doping profiles

Abstract: Based on rate equations, a theoretical model of a fiber oscillator with a multimode gain fiber was built. We studied the effect of the rare earth doping profile in the core on the output characteristics of the multimode fiber oscillator. The results indicated that a pure fundamental mode can be obtained by partly doping the core of the large mode area (LMA) ytterbium doped fiber (YDF) in the fiber laser. Furthermore, a sole specific high-order mode can also be implemented by tailoring the rare earth doping profile according to the simulations. The mode coupling effect was also taken into account in the model. In spite of the mode coupling effect, the specific mode was able to dominate in the output of the fiber laser by utilizing the designed LMA YDF.

Mode Coupling Coefficient Measurement of a Two-Mode Fiber

Abstract: We experimentally investigate the mode coupling of a two-mode fiber based on the bending method. The mode coupling coefficient between LP 01 and LP 11 modes of the two-mode fiber wound on a bobbin was an order of 2×10−3 km−1.

High-Dimensional Stokes-Space Analysis for Monitoring Fast Change of Mode Dispersion in Few-Mode Fibers

Abstract: We adopt high-dimensional Stokes-space analysis to measure the time evolution of the mode dispersion in few-mode fibers. One advantage of the method is that the laser phase noise is intrinsically cancelled.

Fiber amplifiers under thermal loads leading to transverse mode instability

Abstract: Transverse mode instability (TMI) in rare-earth doped fiber amplifiers operating above an average power threshold is caused by intermodal stimulated thermal Rayleigh scattering due to quantum defect heating. We investigate thermally induced longitudinal waveguide perturbations causing power transfer from the fundamental mode (FM) to the higher order mode (HOM) by a nonlinear gain, which depends on the FM-HOM frequency shift and position along the fiber. We take temperature and mode profile evolution along the fiber into consideration to engineer fiber designs with increased TMI threshold and operation stability at higher average powers.

New possibilities of higher-order mode filtering in large-mode-area photonic crystal fibers

Abstract: We discuss both theoretical and experimental aspects of modal discrimination phenomenon that takes place in largemode- area photonic crystal fibers. A few special fiber designs providing efficient higher-order mode filtering were implemented and investigated. First adaptation had the core comprised of 7 elements (instead of 1) with a view to reduce the pitch, since smaller pitches correspond to lower bend-induced losses. That measure aided to realize a series of fibers with a 35-75 μm core diameter propagating only the fundamental mode within a wide spectral range due to embedded leakage channels for the higher-order mode which losses were rated to be above 1 dB/m. Second variation included the fiber with circularly distributed air holes surrounding a core of 30-50 μm in diameter. Circular geometrical configuration enabled leakage losses of the higher-order mode to be 120 times larger than leakage losses of the fundamental mode. Third adaptation had the alternation of large and small air holes (C 6V symmetry converted to C 3V symmetry) resulting in partial or complete delocalization of the higher-order mode power outward a core region. Fourth design represented the regular triangular-lattice structure with a core of 35-60 μm in diameter shifted from its usual location in the center of the lattice. The main idea consisted in provoking an enhancement of the higher-order mode discrimination, as higher-order mode has a larger field near to the air-hole silica interfaces compared to fundamental mode. Those fibers demonstrated distinguished bending resistance properties, since could be exploited with a bending radius of 2-3 centimeters.

Twist effects in multi-mode fibers

Abstract: The recent interest in spatial division multiplexing as a tool to increase optical fiber communication link capacity has recently revived the interest in multi-mode optical fibers. Mode coupling within such fibers is a critical aspect of propagation, for it poses a limit to system capacity. In this paper we analyze fiber twist as a possible cause of mode coupling. It is shown that in a twisted fiber the LP modes undergo a rigid rotation proportional to the twist applied to the fiber. This rotation involves both mode power distribution and mode polarization, generalizing the well known polarization rotation (optical activity) that occurs in twisted single mode fibers. The results of this analysis may enable a more accurate modeling of propagation in randomly coupled multi- and few-mode fibers.

Influence of mode competition on beam quality of fiber amplifier

Abstract: Theoretical and experimental studies of the influence of the mode competition on the output beam quality of fiber amplifiers are presented. Rate equations and modal decomposition method are used in the theoretical model. In the experiment, the output beam-quality factor of a fiber amplifier, which is based on a Yb-doped double-clad large mode area fiber as a function of the seed beam quality and the pump power of the amplifier, is measured. The experimental results are consistent with the theoretical analysis.

Equilibrium mode distribution in w-type glass optical fibers

Abstract: Power flow equation is used to calculate equilibrium mode distribution in W-type glass optical fibers. It has been shown how the coupling length for achieving the equilibrium mode distribution in W-type glass optical fibers varies with the depth and width of the intermediate layer and coupling strength for different widths of launch beam distribution. W-type optical fibers have shown effectiveness in reducing modal dispersion and bending loss.

Effective Mode Field Diameter Definition and Splice Loss Estimation of LP 11 Mode in Few Mode Fibers

Abstract: We propose a definition of the effective mode field diameter (MFD) of LP 11 mode in few mode fibers (FMFs). The splice loss between LP 11 modes is successfully estimated by the MFD and a correction factor.

One-way electromagnetic mode at the surface of a magnetized gyromagnetic medium

Abstract: The interface of magnetized gyromagnetic medium and air can sustain a surface electromagnetic mode that propagates only in one direction. This nonreciprocal phenomenon originates from the breakage of the time-reversal symmetry of the system by the application of an external DC magnetic field. Accordingly, a one-way waveguide structure is proposed, the guiding mode of which not only travels in a single direction but also is robust against boundary imperfections. The unique properties of this one-way waveguide are demonstrated through mode analysis as well as numerical simulation.

Selective mode conversion using phase plates for 10-mode fiber

Abstract: We investigate mode conversion using phase plates for 10-mode fibers. Crosstalk from undesirable modes is unavoidable for mode conversion to LP₁₁ or LP₁₂ due to imperfection of phase-plate-based mode conversion, while it can be suppressed for LP₀₂, LP₂₁, and LP₃₁.

Transverse mode propagation studies in a thulium-doped fiber laser

Abstract: In this paper, we apply a transverse mode propagation model of thulium-doped fiber laser based on propagation equations. Based on this model the behaviors of transverse mode propagation are demonstrated and individual transverse mode power distributions of output are simulated numerically under various conditions including bending radius, fiber length, and pump power. Furthermore, the output power and beam quality characteristic for the thulium-doped fiber laser are investigated experimentally, and the experimental results agree well with the numerical results.

Dynamic Mode Group Equalization Filter and Variable Optical Attenuator for Few Mode Fibers

Abstract: Variable optical attenuation for few-mode fibers is presented, utilizing an amplitude spatial light modulator. -28dB uniform attenuation and 10dB differential mode equalization is demonstrated, potentially gain-balancing mode dependent amplification

Mode coupling in few-mode large-mode-area fibers

Abstract: We present an experimental study on mode coupling characteristics of few-mode large-mode-area (LMA) fibers, which are widely used in high power fiber lasers. The modal power allocation is measured by modal decomposition of the nearfield intensity profile of the output beam. Cut-back measurements are carried out with commonly-used fibers with different fiber geometries. The evolution of the modal power content due to mode coupling is presented. The influence of the fiber geometry on mode coupling is discussed.