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Showing papers on "Equilibrium mode distribution published in 2008"


Journal ArticleDOI
TL;DR: In this paper, the authors describe the physics and properties of a novel optical fiber that would be attractive for building high-power fiber lasers and amplifiers, and demonstrate that for applications requiring meter-length fibers, signal stability actually increases with mode order.
Abstract: This paper describes the physics and properties of a novel optical fiber that would be attractive for building high-power fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses - a capability that is practically nonexistent in conventional fibers. Particularly interesting is the tact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.

175 citations


Journal ArticleDOI
TL;DR: A bend insensitive single mode optical fiber with a low-index trench with spot-size definitions and their optimization technique that preserves single mode properties while avoiding bending loss at a 5 mm of bending radius.
Abstract: We have designed a bend insensitive single mode optical fiber with a low-index trench using spot-size definitions and their optimization technique. The bending loss at a 5 mm of bending radius was negligible, while single mode properties were intact.

29 citations


Journal ArticleDOI
TL;DR: A polarization dependent mode amplification is found that is in good agreement with recent experimental results and the derivation of the modal powers and modal polarization states is found.
Abstract: We model the transverse mode interaction in a large-mode-area fiber amplifier by solving the Fresnel wave equation including local gain saturation. In order to calculate the electric field distribution we apply a finite difference beam propagation method, which is followed by the derivation of the modal powers and modal polarization states. A polarization dependent mode amplification is found that is in good agreement with recent experimental results.

18 citations


Patent
21 Nov 2008
TL;DR: In this paper, the authors describe optical devices and related methods where a multiple mode input in a Higher Order Mode (an HOM) optical fiber is converted by a complex mode transformer to produce a fundamental mode output in an optical medium with an E-field that is substantially different than that exiting the HOM optical fiber.
Abstract: Described are optical devices and related methods wherein a multiple mode input in a Higher Order Mode (an HOM) optical fiber is converted by a complex mode transformer to produce a fundamental mode output in an optical medium with an E-field that is substantially different than that exiting the HOM optical fiber. The medium is preferably a Large Mode Area (LMA) optical fiber, or free space. The mode transformer may be a series of refractive index perturbations created either by photo-induced gratings or by gratings formed by physical deformations of the optical fiber.

18 citations


Journal ArticleDOI
TL;DR: In this paper, a novel mode converter which is realized by mode coupling between a large core and a small core in an asymmetric dual-core photonic crystal fiber is proposed, where the mode indexes of the individual cores are index matched to produce effective mode coupling.
Abstract: A novel mode converter which is realized by mode coupling between a large core and a small core in an asymmetric dual-core photonic crystal fibre is proposed in this paper. The mode indexes of the individual cores are index matched to produce effective mode coupling. Numerical investigations demonstrate that an LP01 to LP02 mode converter with a bandwidth of 14 nm for the loss of 1 dB and polarization-dependent loss lower than 0.14 dB can be realized in a fibre with a length of 12.7 mm.

17 citations


Journal ArticleDOI
TL;DR: In this paper, the nonlinear waveguide equation is studied quasi-analytically for insights into the effect of waveguide designs, and the critical power for nonlinear self-focus is independent of the waveguide parameters.
Abstract: The nonlinear waveguide equation is studied quasi-analytically for insights into the effect of waveguide designs Equations governing stationary mode and transient process are successfully derived A number of new understandings of the nonlinear process at high peak power in optical waveguides are developed which will be critical for fiber laser research and developments at extremely high peak powers The first key finding is that the critical power for nonlinear self-focus is independent of waveguide parameters The second key finding is that the nonlinear guided stationary mode is a stable solution of a nonlinear waveguide below the critical power for nonlinear self-focus and has a reduced mode size dependent on the optical power and V value of the waveguide The third key finding is that the transient process scale with Rayleigh range similar to those in bulk media and are adiabatic, ie, the optical power remains in the fundamental mode The fourth key finding is that a larger V value increases the transient process and self-focus distance, and the self-focus distance is proportional to square root of the optical power and independent of V value at higher power levels, ie, it becomes more like a bulk medium at peak powers above ten times critical power levels B integrals are calculated for various amplifiers, taking into account the impact of the gradual collapse of beam size along the amplifier

13 citations


Proceedings ArticleDOI
TL;DR: In this paper, a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter is presented.
Abstract: Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with claddingmodes), which is undesirable both in terms ofmode area and beamquality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP 01 mode and degrading the beam quality. However, maximum transmission of the LP 01 mode is often required in order to have the best possible beam quality (minimal M 2 ). Launching in an LMA fiber with a mode field adapter (MFA) 1 provides an excellent way of ensuring maximum LP 01 coupling, but preservation of this mode requires highmodal stability in the output fiber. Small cladding, low N.A. LMA fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.

10 citations


Patent
21 Nov 2008
TL;DR: In this article, a surface-emitting laser has an active layer between a first distributed Bragg reflector and a second distributed BR reflector, which can oscillate in a single longitudinal mode, suppressing longitudinal mode hopping.
Abstract: A surface-emitting laser has an active layer between a first distributed Bragg reflector and a second distributed Bragg reflector. The first distributed Bragg reflector is formed so as to have a resonant mode and a first longitudinal mode different from the resonant mode included in the reflectivity stop band and a second longitudinal mode different from the resonant mode and the first longitudinal mode excluded from the reflectivity stop band. Oscillation is suppressed in the first longitudinal mode and in the second longitudinal mode. As a result, the surface-emitting laser can oscillate in a single longitudinal mode, suppressing longitudinal mode hopping.

10 citations


Journal ArticleDOI
TL;DR: In this paper, a competitive coexistence between a drift mode and a flute mode has been investigated experimentally in a bounded cylindrical electron cyclotron resonance (ECR) plasma.
Abstract: A competitive coexistence between a drift mode and a flute mode has been investigated experimentally in a bounded cylindrical electron cyclotron resonance (ECR) plasma. This competitive cycle was repeated during a whole continuous discharge. Competitive phenomena appeared in plasmas when the density scale length was close to the critical value: the drift mode is dominant and the flute mode is suppressed just below a critical value, but above this value, the flute mode grows abruptly and the drift mode is suppressed. In this phase, the density gradient increases temporally. A small crash takes place when the density scale length reaches a limit value, and then the density profile goes back to the previous states and thus the drift mode is excited and the flute mode is suppressed. The direct nonlinear coupling between the drift mode and the flute mode is weaker than the process of the second harmonic generation of each mode, suggesting that the growth and decay of them are closely related to the modification of the radial density profile. (Some figures in this article are in colour only in the electronic version)

7 citations


Journal ArticleDOI
TL;DR: As a function of the coupling coefficient in step index (SI) plastic optical fibers, the coupling length for achieving the equilibrium mode distribution is determined by a simple algebraic equation and graph, separately for light-emitting diode (LED) and laser launch-beam distributions.
Abstract: As a function of the coupling coefficient in step-index (SI) plastic optical fibers (POFs), the coupling length for achieving the equilibrium mode distribution is determined by a simple algebraic equation and graph, separately for light-emitting diode (LED) and laser launch-beam distributions. One benefit of the proposed function is that it simplifies the determination of the coupling length by eliminating the need to numerically solve the power flow equation (or equivalent) repeatedly for all new values of the coupling coefficient.

5 citations


Proceedings ArticleDOI
TL;DR: In this paper, the authors present the first experimental demonstration of strong coupling between the core modes in multi-core fibers (MCF) regardless of large spacing (~28μm) between them.
Abstract: We present the first experimental demonstration of strong coupling between the core modes in multi-core fibers (MCF) regardless of large spacing (~28μm) between them. The effect is very sensitive to bending of the fiber and is observed in the MCF laser as well as in the probe beam schemes. We explain the observed effect by a mechanism of the mode coupling based on their indirect interaction inside the fiber via intermediate cladding mode, analogues to the Bragg mode. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.

Proceedings ArticleDOI
01 Sep 2008
TL;DR: In this paper, the sought fields in terms of mode expansion with mode amplitudes being functions of time and longitudinal coordinate were presented, and a very good convergence of such a mode series was verified by direct FDTD calculation.
Abstract: The paper considers pulse propagation in a transverse inhomogeneous waveguide supporting a quasi-TEM wave. The calculation is based on presenting the sought fields in terms of mode expansion with mode amplitudes being functions of time and longitudinal coordinate (Mode Basis Method). In case of transverse inhomogeneous waveguide there is coupling between the modes. Meanwhile a very good convergence of such a mode series is shown here. The obtained solution was verified by direct FDTD calculation.

Patent
26 Feb 2008
TL;DR: In this article, an optical fiber-based light source that generates light via third-harmonic (TH) generation is disclosed, which includes a pump light source providing pump light having a fundamental mode of wavelength λp.
Abstract: An optical-fiber-based light source that generates light via third-harmonic (TH) generation is disclosed. The light source includes a pump light source that provides pump light having a fundamental mode of wavelength λp. The light source also includes an optical fiber optically coupled to the pump light source. The optical fiber's core refractive index profile has (i) a central region with a refractive index value Δ1, (ii) a first annular region immediately surrounding the central region and having a refractive index value Δ2, and (iii) a second annular region immediately surrounding the first annular region and having a refractive index value Δ3. The optical fiber satisfies the conditions Δ2 1.2%. The pump light produces TH-generated light having a higher-order mode of wavelength λTH = (1/3) λρ and that is outputted at the optical fiber output end. The pump light fundamental mode overlaps the TH-generated-light higher order mode over the length of the optical fiber to provide a conversion efficiency of 1% or greater.

Proceedings ArticleDOI
05 Nov 2008
TL;DR: In this article, the authors describe an experimental and theoretical study of the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide for the propagation of broadband THz pulses and demonstrate undistorted THz pulse propagation via the TE1 mode.
Abstract: We describe an experimental and theoretical study of the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide for the propagation of broadband THz pulses, and demonstrate undistorted THz pulse propagation via the TE1 mode. This demonstration overcomes two fundamental problems associated with the low-frequency cutoff of this mode, the group velocity dispersion and the spectral filtering, which have prevented the use of this mode for THz wave guiding in the past. We also observe a remarkable counter-intuitive property of the TE1 mode: its attenuation decreases with increasing frequency. This phenomenon has not been observed with any other THz waveguide to date, and can enable extremely low-loss propagation. In addition, we find that it is possible to achieve almost 100% coupling to the TE1 mode from an input Gaussian beam. These results favor the use of the TE1 mode for the efficient guiding of THz pulses.

Patent
19 Nov 2008
TL;DR: In this paper, a 1xN multimode optical power splitter was proposed, where a mode-disturbing area is formed by an input straight waveguide, and a light-splitting area was formed by N output straight waveguides or N output S-shaped waveguiders.
Abstract: The invention discloses a 1xN multimode optical power splitter. A mode-disturbing area is formed by an input straight waveguide, and a light-splitting area is formed by N output straight waveguides or N output S-shaped waveguides. The N output straight waveguides are distributed symmetrically along the center line of the input straight waveguide, each output straight waveguide and the center line of the input straight waveguide have an angle theta which becomes larger sequentially from the middle to both sides, and the angle theta is between 0 and 25 degrees. The N output S-shaped waveguides are distributed symmetrically along the center line of the input straight waveguide, the distance L from each output S-shaped waveguide to the centre line of the input straight waveguide becomes larger sequentially from the middle to both sides, and the distance L is between 0 and 2.2mm. The mode-disturbing area has the function of making input optical signals reach the equilibrium mode distribution, and the light-splitting area has the function of distributing the optical signals reaching equilibrium mode distribution to each output terminal to output. The optical power splitter overcomes the problem of mode noise of the multimode optical power splitter, and has the advantages of compact structure, low insertion loss, good light-splitting evenness, loose assembly tolerance and insensitiveness to wavelength.

Proceedings ArticleDOI
25 Nov 2008
TL;DR: In this paper, the authors investigated the origin of mode coupling in graded index polymer optical fiber (GI-POF) and waveguide and found that slight fluctuation of the refractive index profile can cause large mode coupling.
Abstract: We investigate the origin of mode coupling in graded index polymer optical fiber (GI-POF) and waveguide, and find out that slight fluctuation of the refractive index profile can cause large mode coupling

01 Jan 2008
TL;DR: In this paper, the coupling length for achieving the equilibrium mode distribution is determined by a simple algebraic equation and graph, separately for light-emitting diode LED and laser launchbeam distributions.
Abstract: lexandar Djordjevich vetislav Savovic* ity University of Hong Kong 3 Tat Chee Avenue owloon, Hong Kong hina Abstract. As a function of the coupling coefficient in step-index SI plastic optical fibers POFs , the coupling length for achieving the equilibrium mode distribution is determined by a simple algebraic equation and graph, separately for light-emitting diode LED and laser launchbeam distributions. One benefit of the proposed function is that it simplifies the determination of the coupling length by eliminating the need to numerically solve the power flow equation or equivalent repeatedly for all new values of the coupling coefficient. © 2008 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.3049858

Proceedings ArticleDOI
21 Apr 2008
TL;DR: In this article, a new simulation method for optical pulse distortion in macro-bends of single-mode optical fiber with coating layer is presented, showing that the pulse distortion can make bit errors for optical communication systems with high bit rate.
Abstract: A new simulation method for optical pulse distortion in macro-bends of single-mode optical fiber with coating layer is presented. The pulse distortion in macro-bends of standard single mode fibers are investigated theoretically, showing that the pulse distortion in macro-bends can make bit errors for optical communication systems with high bit rate. The simulation technique is validated by comparison with exact solution for macro-bend losses of standard single mode fiber.

01 Jan 2008
TL;DR: In this article, a broad set of diagnostics is employed: the Mirnov coils show clearly the footprints of all electromagnetic modes with finite perturbation at the plasma edge and allow for a reliable mode number identification.
Abstract: Introduction and experimental features In many ICRF heated discharges (hydrogen minority heating) at ASDEX-Upgrade a variety of fast particle driven modes can be observed. In order to determine their nature and their possible impact on fast particle transport, a broad set of diagnostics is employed: the Mirnov coils show clearly the footprints of all electromagnetic modes with finite perturbation at the plasma edge and allow for a reliable mode number identification. The soft x-ray cameras deliver valuable information about the radial mode position. Finally, the energy and the pitch angle of expelled particles can be directly measured by the fast-ion-loss detector (FILD) diagnostic allowing to reconstruct possible resonance conditions and the mode amplitude evolution. During an IRCF power ramp, the toroidal Alfven Eigenmodes (TAE) with typical mode numbers n = 4...7 at typically 200 280 kHz appear first, indicating that they are the least damped modes under these experimental conditions. At higher heating power another electromagnetic mode at about 70 110kHz is observed, a so-called ’BAE’ [1, 2] or ’sierpes’ [3] mode. At ASDEX-Upgrade this mode seems to be closely connected with the appearance of a q = 1 surface and therefore with sawtooth oscillations (see Fig. 2). Its mode numbers are n = 4 with a dominant m = 4 component and it is located radially at ρ pol � 0.2 0.4 (SXR reconstruction with the MHD-IC code [4]). It has been demonstrated that this mode is non-Alfvenic since neither B-field ramps nor density changes influence the mode frequency. Furthermore, the appearance of the ’BAE’ mode enhances the FILD-losses induced by the TAE mode [3] suggesting a possible ’channelling’ effect between the ’BAE’ and the TAE. As in earlier publications [2], no clear scaling with only the ion sound speed or only the dia

Proceedings ArticleDOI
TL;DR: In this paper, critical power, nonlinear guided stationary mode and transient dynamics in nonlinear waveguides are studied under Gaussian mode approximation, critical power for nonlinear self-focus is derived analytically and is found to be independent of waveguide parameters.
Abstract: Critical power, nonlinear guided stationary mode and transient dynamics in nonlinear waveguides are studied analytically. Under Gaussian mode approximation, critical power for nonlinear self-focus is derived analytically and is found to be independent of waveguide parameters. Nonlinear guided stationary mode is found to be the stable solution of nonlinear waveguide below critical power for nonlinear self-focus and has a reduced mode size dependent on optical power and V value of the waveguide. Equation governing transient dynamics of the nonlinear waveguide modes is also found. Transient dynamics scale with Rayleigh Raleigh range similar to that in bulk media. Mode is found to evolve adiabatically towards the local stationary mode in a fiber below self-focus limit. Mode will collapse to a singular point at a self-focus distance at and above critical power. Larger V value increases transient and self-focus distance. The self-focus distance becomes proportional to square root of optical power at higher power level and independent of V value. B integral are calculated for various amplifiers considering the impact of gradual collapse of beam size along the amplifier.

Proceedings ArticleDOI
01 Dec 2008
TL;DR: In this article, the authors report a theoretical characterization of a commercial large mode area with a single mode photonic crystal fiber, i.e., the LMA-15 fiber produced by Crystal Fibre A/S.
Abstract: We report a theoretical characterization of a commercial large mode area endlessly single mode photonic crystal fiber, ie the LMA-15 fiber produced by Crystal Fibre A/S For the study, we employed a finite element leaky mode solver developed in earlier work Based on the computed confinement loss of the modes, we found that as opposed to the claim of the factory, the fiber is obviously multi-moded in the visible light regime, even for a rather long fiber length Additionally, we also computed the dispersion of the fundamental mode which agrees nicely to the measured results given by the factory's datasheet

Proceedings ArticleDOI
24 Jun 2008
TL;DR: In this article, a non-reciprocal waveguide grating with a spurious non-coupling mode is proposed, in which the spurious mode is prohibited from propagating through the output waveguide, in a process mediated by a third mode.
Abstract: A non-reciprocal waveguide grating is one in which the modal coupling strength is not equal in both directions, in stark opposition to the case of the standard index grating, which is necessarily reciprocal. In all previous designs of non-reciprocal gratings, the desired coupling mode is accompanied by a spurious non-coupling mode, which is detrimental to some applications. In this work, we introduce a variation on the design of the non-reciprocal waveguide grating in which the spurious mode is prohibited from propagating through the output waveguide, in a process mediated by a third mode. An additional benefit of this design is that it provides the ability to achieve any desired power splitting ratio in the allowed coupling direction, independent of the total power in the device.

Proceedings ArticleDOI
26 Sep 2008
TL;DR: In this article, the authors used the offset launch and the near field imaging technique to analyse the modal content of large mode area (LMA) fibres in different launch conditions.
Abstract: We used the offset launch and the near field imaging technique to analyse the modal content of large mode area (LMA) fibres in different launch conditions. The coiling technique was applied to suppress higher order mode contents in order to estimate the modal power distribution and to verify the loss factor for the single modes. Based on this analysis, an optimised coiling for LMA fibres will be developed, to suppress the higher order mode contents while maintaining the highest possible fundamental mode content.