Showing papers on "Equilibrium mode distribution published in 2013"

Controlling mode instabilities by dynamic mode excitation with an acousto-optic deflector

Abstract: We demonstrate an approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold. Both the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold. The physical working principle is discussed at the light of the recently published theoretical explanations of mode instabilities.

Selective excitation of LP01 mode in multimode fiber using solid-core photonic crystal fiber

Abstract: To address the overwhelming bandwidth increase in premise backbones, an attractive alternative for selective mode excitation in multimode fiber (MMF) using solid-core photonic crystal fiber (PCF) is presented. The power coupling efficiency, differential mode delay, and bit-error rate performance of several structural designs of solid-core PCF waveguides are investigated for the selective excitation of mode LP01 in a MMF. The achieved coupling efficiency into mode LP01 is above 90% for PCF profiles with seven rings.

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems

Abstract: We study the surface plasmon modes in a silver double-nanowire system by employing the eigenmode analysis approach based on the finite element method. Calculated dispersion relations, surface charge distributions, field patterns and propagation lengths of ten lowest energy plasmon modes in the system are presented. These ten modes are categorized into three groups because they are found to originate from the monopole-monopole, dipole-dipole and quadrupole-quadrupole hybridizations between the two wires, respectively. Interestingly, in addition to the well studied gap mode (mode 1), the other mode from group 1 which is a symmetrically coupled charge mode (mode 2) is found to have a larger group velocity and a longer propagation length than mode 1, suggesting mode 2 to be another potential signal transporter for plasmonic circuits. Scenarios to efficiently excite (inject) group 1 modes in the two-wire system and also to convert mode 2 (mode 1) to mode 1 (mode 2) are demonstrated by numerical simulations.

Understanding the contribution of mode area and slow light to the effective Kerr nonlinearity of waveguides.

Abstract: We resolve the ambiguity in existing definitions of the effective area of a waveguide mode that have been reported in the literature by examining which definition leads to an accurate evaluation of the effective Kerr nonlinearity. We show that the effective nonlinear coefficient of a waveguide mode can be written as the product of a suitable average of the nonlinear coefficients of the waveguide’s constituent materials, the mode’s group velocity and a new suitably defined effective mode area. None of these parameters on their own completely describe the strength of the nonlinear effects of a waveguide.

Thermally induced mode distortion and its limit to power scaling of fiber lasers

Abstract: A general model is proposed to describe thermal-induced mode distortion in the step-index fiber (SIF) high power lasers. Two normalized parameters in the model are able to determine the mode characteristic in the heated SIFs completely. Shrinking of the mode fields and excitation of the high-order modes by the thermal-optic effect are investigated. A simplified power amplification model is used to describe the output power redistribution under various guiding modes. The results suggest that fiber with large mode area is more sensitive on the thermally induced mode distortion and hence is disadvantaged in keeping the beam quality in high power operation. The model is further applied to improve the power scaling analysis of Yb-doped fiber lasers. Here the thermal effect is considered to couple with the optical damage and the stimulated Raman scattering dynamically, whereas direct constraint from the thermal lens is relaxed. The resulting maximal output power is from 67kW to 97kW, depending on power fraction of the fundamental mode.

Mode instability thresholds of fiber amplifiers

Abstract: We show by detailed numerical modeling that stimulated thermal Rayleigh scattering can account for the modal instability observed in high power fiber amplifiers. Our model illustrates how the instability threshold power can be maximized by eliminating amplitude and phase modulation of the signal seed light and the pump light and by careful injection of the signal seed light. We also illustrate the influence of photodarkening and mode specific loss.

Nonlinear optics in the LP 02 higher-order mode of a fiber

Abstract: The distinct disperion properties of higher-order modes in optical fibers permit the nonlinear generation of radiation deeper into the ultraviolet than is possible with the fundamental mode. This is exploited using adiabatic, broadband mode convertors to couple light efficiently from an input fundamental mode and also to return the generated light to an output fundamental mode over a broad spectral range. For example, we generate visible and UV supercontinuum light in the LP02 mode of a photonic crystal fiber from sub-ns pulses with a wavelength of 532 nm.

Mode field adaptation between single-mode fiber and large mode area fiber by thermally expanded core technique

Abstract: The main source of light transmission loss between single-mode fiber (SMF) and large mode area (LMA) fiber is from the mode field mismatch. Thermally expanded core (TEC) technique was introduced to realize mode field adaptation between them. The SMF was heated by a H2–O2 flame to increase the mode field diameter (MFD) which can result in a mode field match with LMA. Both numerical calculation and experiment results show it is workable to realize mode field adaptation and get a low transmission loss by this method. For HI1060 (d=5 μm, NA=0.14) and LMA-GDF-15/130(d=15 μm, NA=0.08), a low-loss with 0.4 dB was obtained for SMF–LMA–SMF from 6.4 dB with TEC processing experimentally.

Bending Orientation Insensitive Large Mode Area Photonic Crystal Fiber With Triangular Core

Abstract: In view of the feasibility of fabrication and application, a bending orientation insensitive large mode area photonic crystal fiber with triangular core is proposed. This fiber structure breaks the limit of bend orientation angle and realizes single mode operation under any bending orientation angle at the bending radius of 30 cm. The mode field area of the fundamental mode at the wavelength of 1.064 μm achieves 1386 μm2 at the straight state and 1154 μm2 at a bending radius of 30 cm, respectively. The decrement of the mode field area at the bent state is only 16.85% compared to that at the straight state. The large mode area at the bent state, small decrement of the mode field area, and low sensitivity of bending orientation make the fiber of great potential in compact high power fiber lasers.

Higher-order mode fiber enables high energy chirped-pulse amplification

Abstract: Energy scaling of femtosecond fiber lasers has been constrained by nonlinear impairments and optical fiber damage. Reducing the optical irradiance inside the fiber by increasing mode size lowers these effects. Using an erbium-doped higher-order mode fiber with 6000 µm2 effective area and output fundamental mode re-conversion, we show a breakthrough in pulse energy from a monolithic fiber chirped pulse amplification system using higher-order mode propagation generating 300 µJ pulses with duration 600 MW at 1.55 µm. The erbium-doped HOM fiber has both a record large effective mode area and excellent mode stability, even when coiled to reasonable diameter. This demonstration proves efficacy of a new path for high energy monolithic fiber-optic femtosecond laser systems.

Circular hybrid plasmonic waveguide with ultra-long propagation distance

Abstract: We propose a novel plasmonic waveguide structure, which is referred to as a circular hybrid plasmonic waveguide (HPW) and consists of a metal wire covered with low- and high-index dielectric layers. The circular HPW exhibits two distinctly different modes, namely, the strongly localized mode and the extremely low-loss mode. Our numerical calculation demonstrates that the strongly localized mode exhibits 10-4 order scale in normalized mode area and can be performed even in tens of nanometer sizes of waveguide geometry. In the extremely low-loss mode, the HPW exhibits ultra-long propagation distance of more than 103μm that can be achieved by forming the dipole-like hybrid mode and properly adjusting the radius of the metal wire. It is also shown that, even with this long-range propagation, the mode area of the dipole-like hybrid mode can be maintained at subwavelength scale. The simultaneous achievement of a small mode area and ultra-long propagation distance contributes to the ultra-high propagation distance to mode size ratio of the waveguide. The HPW results are very helpful for plasmonic device applications in the fields of low-threshold nanolasers, ultrafast modulators, and optical switches.

An Analytic Connector Loss Model for Step-Index Polymer Optical Fiber Links

Abstract: The authors report on an analytic connector loss model for step-index polymer optical fibers (SI-POF). The model describes the non-linear dependencies for combinations of the most important intrinsic and extrinsic influence parameters like Fresnel reflections, mismatches in numerical aperture (NA) and core diameters, as well as lateral, longitudinal and angular offsets. As connector losses strongly depend on the mode distribution, expressions for the mode-dependent coupling efficiencies for the various influence parameters are derived. This way, the model can handle any rotationally symmetric mode distribution including the uniform mode distribution (UMD) and the equilibrium mode distribution (EMD) in the steady-state. It is shown that for step-index fibers, lateral and longitudinal offsets can be handled in a similar way, which reduces the overall effort for the mathematical description. For the typical case of identical fibers, the authors also derive easy-to-apply approximations for a combination of lateral and longitudinal offsets under UMD and EMD conditions. The results are in good agreement with ray-tracing simulations and are evaluated for the cases of identical fibers and the worst case parameter constellation for the SI-POF in the IEC A4a.2 fiber class.

High power optical fibers

Abstract: Photonic bandgap fibers are described that can be solid across the core and clad and have a large core diameter with little loss in the fundamental mode. In addition, the mode loss of the higher order modes can be much greater than that of the fundamental mode, providing high power fibers with high effective mode area. Excellent single mode output can be obtained from the fibers in length scale close to what is required for fiber laser and amplifiers.

Mitigation of mode instabilities by dynamic excitation of fiber modes

Abstract: By dynamically varying the power content of the excited fiber modes of the main amplifier of a fiber-based MOPA system at high average output power levels, it was possible to mitigate mode instabilities to a large extent. In order to achieve the excitation variation, we used an acousto-optic deflector in front of the Yb-doped rod-type fiber. Therewith, it was possible to significantly increase both the average and the instantaneous minimum power content of the fundamental mode. This, consequently, led to a substantial improvement of the beam quality and pointing stability at power levels well beyond the threshold of mode instabilities.

Method and device for reducing mode instability in an optical waveguide

Abstract: The invention relates to a method and corresponding devices for reducing mode instability in an optical waveguide (1), a light signal becoming unstable in the optical waveguide (1) beyond an output power threshold and energy being transformed from a basic mode into higher order modes. The invention proposes a reduction in temperature variation (2) along the optical waveguide (1) and/or a reduction in changes in the optical waveguide (1) that are caused by spatial temperature variation as a result of mode interference.

Mode interference and radiation leakage in a tapered parallel plate waveguide for terahertz waves

Abstract: To exploit tapered parallel plate waveguides for broadband terahertz (THz) spectroscopy, the impact of the waveguide geometry on transmission of terahertz pulses is investigated experimentally. We find that the approximation of single transverse electro-magnetic mode propagation is insufficient for describing the observed behavior. The TE02 mode plays a particularly important role. The mode composition, however, can be controlled by the gap between the waveguide plates, which affects the main loss mechanism, radiation leakage, and group velocity for the TE02 mode. Balancing the waveguide loss and coupling efficiencies results in an optimal gap for the tapered waveguide.

Transition Mode of Two Parallel Flags in Uniform Flow

Abstract: The coupled flapping of two side-by-side identical flags in uniform flow is observed experimentally and numerically. Besides the early reported stable, in-phase, and out-of-phase modes, a transition mode between the in-phase and out-of-phase modes is newly presented. Essentially different from the other modes, the flapping in this transition mode is no longer a single cycle motion. Especially, the FFT analysis of the displacement-time curve indicates that there are multiple peaks of frequency in this mode, of which the smaller value is comparable to the frequency of the in-phase mode and the larger one is close to that of the out-of-phase mode. Changing of the weights of different single cycle motions may explain the mode transition.

Influence of sinusoidal modulation on mode competition and signal distortion in multimode InGaAsP lasers

Abstract: This paper investigates influence of large-signal modulation on mode oscillation in InGaAsP/InP lasers. The mode competition dynamics under modulation are examined in terms of the temporal trajectories of the total output and the strongest modes as well as their Fourier spectra. The mode coupling induced by the strong spectral gain suppression is evaluated by both the correlation coefficients among the strongest modes and their signal distortion. We show that small and moderate modulations at the multimode hopping of the laser modulate the mode hopping along with modulation of each mode. The mode coupling is characterized by anti-correlation among the modes like as the non-modulated laser. Under modulation with the resonance frequency, the increase in the modulation depth changes mode coupling from anti-correlation to positive correlation and then to complete coupling that correspond to emission of periodic pulses. The mode coupling is characterized by mode competition distortion, which measures the amount of power carried by the mode signal at the frequency of multimode-hopping.

Mode coupling and field distribution in sub-mm permanently bent single mode optical fibers

Abstract: A simple mechanism useful to tailor the field profile in single mode optical fibers is proposed. It involves the local and permanent bend of the fiber with bending radius of few hundred micrometers. The permanent bend is obtained by local thermal treatment using the electric arc discharge method. The resultant sub-mm bend leads to a significant power coupling from the fundamental core mode to guided cladding modes. The order and number of the excited cladding modes depend on the geometrical features of the bent region, like angle and curvature rate. Also, since the light propagated by guided cladding modes interacts with the surrounding medium by evanescent wave, the field distribution can be additionally tailored by changing the refractive index of the medium surrounding the straight fiber region after the permanent bend. Here, the effects of permanent bend on the guided mode in single mode fiber is first theoretically introduced and then widely experimentally investigated. Finally, preliminary results of light re-coupling from excited cladding modes to the core mode using long period fiber gratings are reported opening new avenues for light manipulation within optical fibers.

Wide-band dual-mode operation of multi-core fibers with air-hole structure

Abstract: A novel multi-core dual-mode fiber consisting of 11 cores with eight air holes for wide-band dual-mode operation is proposed in this paper. Modal properties, operation range and effective area affected by core-to-core distance, core-to-pitch ratio, relative index difference and center core diameter are investigated systematically. For 11-core dual-mode fiber with air-hole structure, simulation shows that it can support both a strict dual-mode operation (propagation in HE11 mode and HE21 mode only) and low bending losses (less than 0.8 dB/m at R>0.20 m), meanwhile maintaining a large effective area up to 1120.33 μm2. With a proper design of structural parameters the effective area of the 11-core dual-mode could be even higher. It has great advantages over conventional few-mode fibers, multi-core fibers and step-index single-mode fibers in terms of less mode coupling, larger effective area and lower bending loss.

Single Mode Operation of Substrate Integrated Non-Radiative Dielectric Waveguide and an Excitation Scheme of ${\rm LSE}_{11}$ Mode

Abstract: It is shown that a substrate integrated non-radiative dielectric (SINRD) waveguide supports only the fundamental longitudinal-section-electric mode (LSE11) over a frequency band determined by the radius of air holes. The single mode operation eliminates the need of a mode suppressor which is a potential advantage of using LSE11 mode compared to the use of the other fundamental mode, namely the longitudinal-section-magnetic mode (LSM11). The conditions for obtaining the maximum single mode operation bandwidth are investigated. An excitation scheme of the LSE11 mode using a rectangular waveguide is presented and the measurement results of a fabricated back-to-back transition at 94 GHz are provided.

Highly efficient 90μm core rod fiber amplifier delivering >300W without beam instabilities

Abstract: Summary form only given. Ultrafast ps and fs laser systems using ROD fiber amplifiers delivering megawatts of peak power and several hundreds of watts in average power have attracted significant academic and industrial interest in recent years. However, the power scaling has reached a temporary limit caused by thermal effects, which has set a limit of average power scaling of these amplifiers due to so called modal instabilities (MIs) [1]. New types of fiber designs with very large cores have been suggested to increase the average power limit [2, 3]. One way of achieving this, is using resonant structures [3], however it is often speculated that these structures are sensitive to temperature causing unstable beam quality under high thermal load (high power extraction pr unit length).A 25-ps mode-locked linearly polarized seed source at 1032 nm (40MHz rep rate, 16 W average power) is used to seed a distributed mode filtering ROD fiber with 90μm core diameter. The ROD fiber shows excellent power conversion efficiency as shown in Fig. 1 with almost linear slope and the MI threshold level is observed at 314 W of average output power corresponding to ~320kW of peak power. Optical to optical conversion efficiency is 69% and the mode quality is stable below the MI threshold. No non-linear effects are observed and the ASE is suppressed by ~35dB, shown in Fig.1. The experiments are modeled using a semi-analytical approach [4, 5], where a nonlinear coupling constant that depends on the thermo-optical effect is determined. χ is combined with FEM-calculated mode distributions to predict the onset of modal instabilities for this particular fiber design.We demonstrate that a ROD fiber design having a resonant structure can deliver efficient and linear amplification to high average output power without suffering from thermal load induced beam quality degradation. In addition, we evaluate the influence of the noise of the seed source on the MI threshold level. We model the MI threshold for this fiber and show good agreement between measurements and simulations, when the MIs are seeded by system dependent noise.

Optical waveguide, mode scrambler and mode conditioner for controlling mode power distribution

Abstract: The present disclosure relates to a multimode optical waveguide comprising a cladding and a core. The core of the multimode optical waveguide has a polygonal cross-section. The core forms a coil spun around the longitudinal axis of the cladding. The multimode optical waveguide may be used to realize a mode scrambler and a mode conditioner.

Femtosecond pulse inscription of a selective mode filter in large mode area fibers

Abstract: We present a selective mode filter inscribed with ultrashort pulses directly into a few mode large mode area (LMA) fiber. The mode filter consists of two refractive index modifications alongside the fiber core in the cladding. The refractive index modifications, which were of approximately the same order of magnitude as the refractive index difference between core and cladding have been inscribed by nonlinear absorption of femtosecond laser pulses (800 nm wavelength, 120 fs pulse duration). If light is guided in the core, it will interact with the inscribed modifications causing modes to be coupled out of the core. In order to characterize the mode filter, we used a femtosecond inscribed fiber Bragg grating (FBG), which acts as a wavelength and therefore mode selective element in the LMA fiber. Since each mode has different Bragg reflection wavelengths, an FBG in a multimode fiber will exhibit multiple Bragg reflection peaks. In our experiments, we first inscribed the FBG using the phase mask scanning technique. Then the mode filter was inscribed. The reflection spectrum of the FBG was measured in situ during the inscription process using a supercontinuum source. The reflectivities of the LP 01 and LP 11 modes show a dependency on the length of the mode filter. Two stages of the filter were obtained: one, in which the LP 11 mode was reduced by 60% and one where the LP 01 mode was reduced by 80%. The other mode respectively showed almost no losses. In conclusion, we could selectively filter either the fundamental or higher order modes.

Influence of width of launch beam distribution on equilibrium mode distribution in W-type glass optical fibers

Abstract: Power flow equation is used to calculate spatial transients and equilibrium mode distribution in W-type glass optical fibers (doubly clad fibers with three layers). A numerical solution has been obtained by the explicit finite difference method. For the first time, we have 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. Such characterization of these fibers is consistent with their manifested effectiveness in reducing modal dispersion and bending loss.

Theoretical and experimental study of 37-core waveguides with large mode area

Abstract: The evanescently coupled multicore waveguide lattice composed of 37 linear type I cores hexagonally arranged has been theoretically studied and fabricated by low-repetition-rate femtosecond laser inscription of bulk fused silica. The effects of the single core's numerical apertures (NAs) and spacing on the mode characteristics of the 37-core waveguide were calculated by the finite-element method. It was found that the mode field areas of the fundamental mode LP₀₁ with 5 μm spacing of different NAs were all larger than 577 μm², which was confirmed by the experiments. The measured near-field mode profiles for different writing conditions and different spacing also showed that the waveguide supported both a single mode (LP₀₁) and two modes (LP₀₁ and LP₁₁). The multicore waveguide, according to our study, is particularly interesting for mode converters.

Optical fiber and fiber laser device using same

Abstract: An optical fiber (10) propagates light of prescribed wavelengths in at least LP01 mode and LP02 mode and is characterized by the Young's modulus of at least part of a core range (11c), which is more to the peripheral side than a range (11b) in which the intensity of the light in the LP01 mode is greater than the intensity of the light in the LP02 mode, among the ranges (11a, 11c) in which the intensity of the light of the LP02 mode is greater than the intensity of the light of the LP01 mode being smaller than the Young's modulus of the range (11b) in which the intensity of the light of the LP01 mode is greater than the intensity of the light in the LP02 mode.