Showing papers on "Equilibrium mode distribution published in 2006"

Light propagation with ultralarge modal areas in optical fibers.

Abstract: We demonstrate robust single-transverse-mode light propagation in higher-order modes of a fiber, with effective area Aeff ranging from 2100 to 3200 μm2. These modes are accessed using long-period fiber gratings that enable higher-order-mode excitation over a bandwidth of 94 mm with greater than 99% of the light in the desired mode. The fiber is designed such that the effective index separation between modes is always large, hence minimizing in-fiber mode mixing and enabling light propagation over lengths as large as 12 m, with bends down to 4.5 cm radii. The modal stability increases with mode order, suggesting that Aeff of this platform is substantially scalable.

Bend-resistant design of conventional and microstructure fibers with very large mode area

Abstract: Achieving very large mode area is a key goal in current research on microstructure and solid fibers for high power amplifiers and lasers. One particular design regime of recent interest has effective area over 1000 square microns and has effectively-single-mode operation ensured by bend losses of the higher-order modes. Simulations show that these fibers are extremely prone to bend-induced distortion and reduction in mode area. The calculated area reduction would significantly impact nonlinear impairments for bend radii relevant to any reasonable spooled package, and can be over 50 percent for bend radii tighter than 10cm. The parabolic-profile design has a natural immunity to bend-induced mode distortion and contraction, and shows superior performance in simulated fair comparisons with other fiber families, including microstructure fibers.

Silicon-based highly-efficient fiber-to-waveguide coupler for high index contrast systems

Abstract: A coupler to efficiently transfer broadband light from a single-mode optical fiber to a single-mode high-index contrast waveguide has been fabricated on a silicon substrate. We utilized a novel coupling scheme, with a vertically asymmetric design consisting of a stepwise parabolic graded index profile combined with a horizontal taper, to simultaneously confine light in both directions. Coupling efficiency has been measured as a function of the device dimensions. The optimal coupling efficiency is achieved for structures whose length equals the focal distance of the graded index and whose input width is close to the mode field diameter of the fiber. The fabricated structure is compact, robust and highly efficient, with an insertion loss of 2.2dB at 1550nm. The coupler exhibits less than 1dB variation in coupling efficiency in the measured spectral range from 1520nmto1620nm. The lowest insertion loss of 1.9dB is measured at 1540nm. The coupler design offers highly efficient coupling for single mode waveguid...

Selective excitation of the LP11 mode in step index fiber using a phase mask

Abstract: We present a novel mode selective coupling technique for step index fiber. This technique utilizes phase matching for excitation of higher-order modes while suppressing the fundamental mode. Using this technique, a phase element is fabricated and tested to demonstrate the high coupling efficiency to the LP11 mode. In addition, we derive an analytical expression of the coupling efficiency of the LP11 using a single

Antiresonant reflecting optical waveguide microstructured fibers revisited: a new analysis based on leaky mode coupling

Abstract: Using two different modal methods, the multipole method and the more recent fast Fourier factorization method, we exhibit and explain a core mode transition induced by avoided crossing between a core localized leaky mode and an high-index cylinder leaky mode in anti-resonant reflecting optical waveguide microstructured optical fibers (ARROW MOFs). Due to its wavelength selectivity and to the leaky nature of the involved modes, this transition doesn't seem to have already been described in detail and analyzed as done in this work in spite of several already published studies on core mode dispersion properties. The main properties of this transition are also described. We also revisite the already mentionned cut-off phenomena limiting the transmission band in ARROW MOFs in terms of mode coupling between the core mode and one or several high- index cylinder modes.

Fiber acousto-optic mode coupling between the higher-order modes with adjacent azimuthal numbers

Abstract: It is pointed out theoretically that fiber acousto-optic mode coupling can occur between the higher-order modes with adjacent azimuthal numbers. Selective fiber mode conversion from LP01 to LP11, and then to LP21 or LP02 is demonstrated experimentally in a few-mode fiber by using a method of cascaded acousto-optic mode coupling.

Mode coupling in strained and unstrained step-index glass optical fibers.

Abstract: By using the power flow equation, we have examined the state of mode coupling in strained and unstrained step-index glass optical fibers. Strained fibers show stronger mode coupling than their unstrained counterparts of the same type. As a result, the coupling length where equilibrium mode distribution is achieved and the length of fiber required for achieving the steady-state mode distribution are shorter for strained than for unstrained fibers.

High-order modes suppression in large mode area active fibers by controlling the radial distribution of the rare earth dopant

Abstract: Many high power fiber laser applications require doped fibers having large mode area but still working in the single mode regime. The most common techniques to keep a large mode area fiber in the single mode regime are to reduce the core numerical aperture, to strip the high order modes by coiling the fiber, to launch only a single transverse mode, or to use photonic crystal fibers. All these methods have limits and disadvantages. In this paper we demonstrate by simulation the effectiveness of another method to suppress the high order modes in large mode area active fibers by optimizing the rare earth dopant concentration across the core while keeping the step index structure of the core of the fiber. This method was not previously employed because the traditional doped fiber manufacturing technologies do not have the required capability to radially control the dopant concentration. However, Direct Nanoparticle Deposition (DND) can be used to manufacture large mode area fibers having any radial distribution of active element concentration and any refractive index profile. Thus, DND fibers can be designed to benefit from this high order mode suppression technique. The simulation results presented in this paper have been obtained using Liekki Application Designer v3.1, a software simulator for fiber lasers and amplifiers.

Variable Optical Mode Generator in a Multimode Waveguide

Abstract: A variable optical mode generator in an integrated-optic multimode waveguide is proposed and fabricated. It consists of a power divider, controllable parallel phase retarders, and a mode combiner. In this device, a desired output mode is generated by controlling the relative phase differences among the arms. The operation of device has been confirmed by observation of the output mode patterns at a wavelength of 1.55 mum

Influence of bending on power distribution in step-index plastic optical fibers and the calculation of bending loss.

Abstract: A means of calculating optical power distribution in bent multimode optical fibers is proposed. It employs the power-flow equation approximated by the Fokker-Planck equation that is solved by the explicit finite-difference method. Conceptually important steps of this procedure include (i) dividing the full length of the bent optical fiber into a finite number of short, straight segments; (ii) solving the power equation for each segment sequentially to find its output distribution; and (iii) expressing that output distribution in rotated coordinates of the subsequent segment along the curved fiber to determine the input distribution for that subsequent segment and thus enable the calculation of the power flow and output distribution for it. The segment length and bend-induced perturbation of output angles are determined by geometric optics. The resulting power distributions are given at different cross sections along the curved fiber axis. They vary with the radius of fiber curvature and launch conditions. Results are compared to those for straight fiber. Bending loss is calculated as well.

Method of High Order Mode Excitation for Multimode Intrusion Detection

Abstract: Intrusion detection for a multimode fiber optic cable uses a light signal launched into the fiber through the low ratio leg of a tap coupler to establish a narrow spectral width, under-filled non-uniform mode field power distribution in the cable. A small portion of the higher order signal modes at the remote end is monitored for transient changes in the mode field power distribution which are characteristic of fiber intrusion to activate an alarm. The active signal of a multimode optical fiber is monitored for both signal degradation and transient power disturbance patterns that could indicate fiber damage or physical intrusion. A translator can be provided in an existing optical fiber system in which the data signals are translated in wavelength and/or launch conditions to optimize the monitoring signals in an otherwise non-optimized system.

Analysis of the effects of mode coupling on the bandwidth characteristics of step‐index plastic optical fiber

Abstract: The mode coupling of SI-POF is analyzed by observing the far-field radiation patterns. The results show that no obvious mode coupling occurs at a transmission distance of 5 m. While a transmission distance of 10–30 m occurs, strong mode coupling and the equilibrium mode distribution is established after 30 m. In order to clarify the mode-coupling effect on the bandwidth of step-index plastic optical fiber (SI-POF), the bandwidths of SI-POF samples were measured under different launch conditions and compared with theoretical values that neglect mode coupling. It is found that although mode coupling contributes to enhancing the bandwidth of the fiber as it averages out the total propagation delays, the energy transfer between the neighboring modes will excite high-order modes during the propagation, which functions to increase the effective numerical aperture (NA) of the incident light and will decrease the bandwidth of the fiber. The net effect of mode coupling on the bandwidths of SI-POFs at different launch conditions indicates the competition between the two effects. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 432–435, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21372

Mode splitter based on triple-core waveguide.

Abstract: The mode index and mode field of a triple-core waveguide are simulated by the compact supercell method and the FEM method. The coupling property of LP(11) mode is analyzed, for the first time to our knowledge, which is similar to that of LP(01) mode except the different coupling length. Both LP(01) and LP(11) modes can be separated based on their different coupling lengths in the waveguide. When both modes are put into the central port, they will come out on different ports by optimizing the coupler length, and the mode extinction ratio can approach 30dB.

Bend resistance of large-mode-area higher-order-mode fibers

Abstract: Higher-order-mode fibers with very large mode area are shown to be naturally resistant to bend induced mode distortion, an important limitation on amplifier fibers. Simulations and measurements display resistance of these fibers to bending.

Apparatus for propagating optical radiation

Abstract: Apparatus for propagating optical radiation in a first optical mode (1) having a first spatial mode shape (8), and a second optical mode (2) having a second spatial mode shape (9), which apparatus comprises an optical path (3), mode transforming means (5), and propagating means (6), wherein the mode transforming means (5) transforms at least a portion of the first optical mode (1) to the second optical mode (2), the propagating means (6) is configured such that in use at least some of the optical radiation propagates along the optical path (3) more than once, and the apparatus is characterised in that the first spatial mode shape (8) is different from the second spatial mode shape (9).

Characteristic evaluation method, device and program of single mode optical fiber

Abstract: PROBLEM TO BE SOLVED: To provide a characteristic evaluation device of a single mode optical fiber capable of evaluating nondestructively and highly accurately a distribution characteristic in the longitudinal direction between connected optical fibers, in an optical fiber transmission path constituted of a plurality of single mode optical fibers. SOLUTION: This device has a back-scattering light intensity measuring device 11 for measuring back-scattering light intensity S (λ, z) on a position z of the single mode optical fiber 13 on both ends, when pulse light having the wavelength λ is allowed to enter the single mode optical fiber 13 having an optional length L; and a back-scattering light intensity waveform analyzer 12 for operating a specific refractive index difference Δ(z) on an optional position z of the single mode optical fiber 13 by using a normalization structure inconsistency loss component In (λ, z) on the position z of the single mode optical fiber 13 operated from the back-scattering light intensity S (λ, z), and a mode field diameter 2W (λ, z 0 ) and the specific refractive index difference Δ(z 0 ) at its optional reference point z 0 . COPYRIGHT: (C)2008,JPO&INPIT

Mode Exciting Properties of Photonic Crystal Fibres with the Optical Field Incident from a Single Mode Fibre

Abstract: We numerically investigate the mode exciting properties of photonic crystal fibres by using the beam propagation method when the optical field is input from a traditional single mode fibre. The results show that both the excited mode spectrum and the coupling-efficiency of each excited mode depend on the normalized pitch ?/? and the normalized hole-size ?/?. Furthermore, we obtain the boundary profile of the optimizing coupling-efficiency for the excited fundamental mode: the boundary (?/?)* is linear to the boundary (d/?)*. All of these will pave the way for smoothing applications of photonic-crystal fibres, such as splicing and designing photonic-crystal-fibre functional devices.

Evanescent Wave Coupling Between Optical Fiber and Planar Optical Waveguide

Abstract: Transverse coupling between single mode fiber and single mode planar waveguide is analyzed for the first time-to the authors' knowledge - by an Improved Coupled Mode Theory (ICMT). The coupling is established via the evanescent part of the modal field propagating in the fiber which penetrates the slab waveguide. The main parameters that characterize the coupling process are calculated. A comparison between the results of the ICMT and those obtained by the usual coupled mode theory reveals an improvement in the accuracy by almost twenty percent.

Modular interference characteristic of two-mode fiber

Abstract: The modular interference characteristics of circular-core and elliptical-core two-mode fibers are investigated in theory. The intensity distribution and figure of two-lobe mode patterns are evaluated and simulated quantitatively for different phase difference change between LP01 and LP11^(even) mode. The interference mode patterns of elliptical-core and circular-core two-mode fibers are compared, the result shows that the two-lobe interference patterns of the two-mode fibers generate energy exchange and oscillation, and the difference is that the interference mode patterns of circular-core two-mode fiber are almost elliptical, while the interference mode pattern of elliptical-core two-mode fiber is approximately circular on condition that proper selection of the ellipticity. Their two-dimensional (2D) profile determines the choice of the core shape of the information pick-up fiber.

A novel opinion about the coupling loss between single- mode fibers and waveguides

Abstract: A detailed analysis is presented, about the coupling loss dependence on the waveguide mode eccentricity and symmetry between single-mode fibers and waveguides. A new conclusion is obtained that more symmetric waveguide mode leads to lower coupling loss, but less eccentric waveguide mode does not always mean low coupling loss, which depends on the waveguide mode depth.

Amplification in hollow core photonic crystal fibers

Abstract: Hollow core photonic crystal fiber (HCPCF) amplifiers, in which Er 3+ - or Yb 3+ - doped glass acts as the gain medium, are proposed as a means of achieving high power pulse amplification. Double-clad configurations are identified which capture multimode pump light up to an NA of around 0.33. The nonlinear and breakdown properties of a HC-PCF amplifier with a mode area of approximately 50μm 2 are predicted to be comparable to those of a solid core fiber amplifier with a mode area of 1000μm 2 . Mode competition effects within the HC-PCF amplifier strongly degrade the output signal unless the net gains of the unwanted guided modes are below that of the signal mode. This can be achieved if the ratio of amplifier gain to scattering loss is larger for the signal mode than any of the undesired guided modes. Assuming loss is dominated by hole interface roughness scattering, and an even doping profile produces the gain, the ratios for the unwanted guided modes of a typical HCPCF geometry are calculated to be similar to that for the signal carrying mode. The mode competition also places a lower bound on the active fiber length, typically implying a longer length is required than in a solid core fiber amplifier. This adversely affects the device efficiency due to scattering loss of the pump field incurred at the air/glass interfaces. To achieve a clean mode output and acceptable efficiency, alternative designs for the HC-PCF will need to be developed.

Optical waveguide, optical device and optical communication device

Abstract: There is provided an optical device and an optical waveguide composed of a photonic crystal in which two optical waveguide modes that are orthogonal to a light propagation direction can be used, whereby design latitude is increased. In the optical waveguide device composed of a photonic crystal, in a dispersion relationship of the photonic crystal, light is propagated using a refractive index guide mode that is a minimum frequency optical waveguide mode. Alternatively, two optical waveguide modes that are orthogonal to light propagation direction are used, a linear defect waveguide mode is used for the first optical waveguide mode; and light is propagated in the second light guide mode by using a refractive index guide mode that is a minimum frequency optical waveguide mode in a dispersion relationship of the photonic crystal. Alternatively, in a dispersion relationship of the photonic crystal, light is propagated in two optical waveguide modes that are orthogonal to a light propagation direction using a refractive index guide mode that is a minimum frequency optical waveguide mode.

Large mode area fibers by using conversion to higher order modes

Abstract: The specification describes an optical fiber device wherein a lower order mode (LOM) i.e. LP 01 is converted to a higher order mode (HOM) i.e. LP 02 prior to entering the gain section. The gain section is a few mode fiber that supports the HOM. The output from the gain section, i.e. the HOM, may be utilized as is, or converted back to the LOM. With suitable design of the few mode fiber in the gain section of the device, the effective area, Aeff, may be greater than 1600 μm 2 The large mode separation in the gain section reduces mode coupling, allowing greater design freedom and reducing the bend sensitivity of the optical fiber. The fiber designs suitable for the gain section of the devices comprise a core, inner cladding and a trench (down doped region). The inner cladding and the trench regions govern the properties of the higher order mode (LP 02 ).

Pulse propagation effects at high bit rates

Abstract: The constant drive to increase bit rate per channel faces significant challenges due to linear and nonlinear propagation effects. Shorter pulses increase the sensitivity to dispersion effects including higher order, polarization mode dispersion and fiber nonlinearity.

Diffraction model of a semiconductor amplifier

Abstract: A three-dimensional diffraction model of a semiconductor heterostructure amplifier is developed. The model describes the field propagation and optical mode establishment taking into account the amplification kinetics within the framework of the diffusion equation for current carriers in a quantum well. The results of application of this model are presented for an amplifier with an asymmetric broad waveguide and an antiwaveguide structure in the lateral direction that does not confine the field. It is shown that the length of establishment of the fundamental mode for such a structure is comparable with the length over which a weak signal is enhanced by two orders of magnitude. The balance between the gain and loss for the steady-state mode as a function of the waveguide insert width is analysed. The effect of this width on the properties of the fundamental optical mode, including the far-field intensity distribution, is studied.

Research and Application of Elliptical Core Dual-Mode Fiber

Abstract: The transverse field pattern and the power distribution of modes in circular core fiber and in elliptical core fiber are compared,it is pointed out that,when the ellipticity of an elliptical core fiber is 1e23,the first two low order LP_(01) mode and even LP_(11) mode are transmitted,and the second higher order mode is(odd) LP_(11) mode,while e23,the second higher order mode is EH_(11) modeThe formation of LP_(11) mode is discussedCombined with the expressions for circular core dual-mode fiber,the inter-mode beatlength of elliptical core fibers is deduced,and the relation of cut-off frequency V_c of even LP_(11) mode and ellipticity is givenBased on these,the dual-mode working parameters of the chosen elliptical fiber are ascertainedFinally, the application of elliptical core dual-mode fiber in sensor field is introduced briefly

High-effective excitation of graded-index waveguide mode by use of diffractive microrelief on waveguide surface

Abstract: In the present paper it is suggested to use micro-relief on the waveguide's end face for effective excitation of selected mode. The strategy of the search of mode with intensity distribution close to the illuminating beam intensity distribution is suggested to be used. Corresponding numerical procedure is described. Computer simulation results as well as some experimental results are presented.

Origin of Shape Dependence in Nano-Aperture Transmission

Abstract: Transmission through nano-apertures was shown to be strongly dependent on the shape. Here we identify the shape dependence in mode propagation and coupling processes. We conclude that the waveguide mode plays the most influential role.

Active Mode Locking: Quantum Oscillator vs. Classical Coupled Oscillators

Abstract: Mode expansion approach is proposed to simulate the active mode locked laser. In this method, electric fields of optical modes in the laser cavity are treated as free classical oscillators. The optical modulator provides the coupling among the modes or oscillators. It is found that the eigenvalue of this coupled system is corresponding to the threshold optical gain and normal mode or eigenfunction is corresponding to optical field of each mode. The simulation results obtained by mode expansion method agree with those calculated by the well- known Master equation, which provides an analytical solution with a function similar to the quantum harmonic oscillator. However, since the proposed method focuses on the individual mode and its coupling with the other modes instead of mode profile in general, it gives more information than Master equation. To show the capability of the new method, several applications are explored where Master equation fails to solve the problems. It is believed that the proposed method will help design the devices such as optical pulse generator, multi-wavelength laser and therefore will have great application in optical signal generation and processing.