Showing papers on "Equilibrium mode distribution published in 2012"

Physical origin of mode instabilities in high-power fiber laser systems.

Abstract: Mode instabilities, ie the rapid fluctuations of the output beam of an optical fiber that occur after a certain output power threshold is reached, have quickly become one of the most limiting effects for the further power scaling of fiber laser systems Even though much work has been done over the last year, the exact origin of the temporal dynamics of this phenomenon is not fully understood yet In this paper we show that the origin of mode instabilities can be explained by taking into account the interplay between the temporal evolution of the three-dimensional temperature profile inside of the active fiber and the related waveguide changes that it produces via the thermo-optical effect In particular it is proposed that non-adiabatic waveguide changes play an important role in allowing energy transfer from the fundamental mode into the higher order mode As it is discussed in the paper, this description of mode instabilities can explain many of the experimental observations reported to date

Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss

Abstract: A transmission fiber for mode division multiplexing supporting LP01 and LP11 modes, with low differential group delay, low mode coupling, and low loss for both modes is presented. Spatially and spectrally resolved mode imaging (S2 imaging) is used for characterization.

Thermally induced mode coupling in rare-earth doped fiber amplifiers.

Abstract: We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.

Distributed mode filtering rod fiber amplifier delivering 292W with improved mode stability

Abstract: We demonstrate a high power fiber (85 μm core) amplifier delivering up to 292 Watts of average output power using a mode-locked 30 ps source at 1032 nm. Utilizing a single mode distributed mode filter bandgap rod fiber, we demonstrate 44% power improvement before the threshold-like onset of mode instabilities by operating the rod fiber in a leaky waveguide regime. We investigate the guiding dynamics of the rod fiber and report a distinct bandgap blue-shifting as function of increased signal power level. Furthermore, we theoretically analyze the guiding dynamics of the DMF rod fiber and explain the bandgap blue-shifting with thermally induced refractive index change of the refractive index profile.

Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems.

Abstract: Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.

Effective mode area and its optimization in silicon-nanocrystal waveguides

Abstract: We revisit the problem of the optimization of a silicon-nanocrystal (Si-NC) waveguide, aiming to attain the maximum field confinement inside its nonlinear core and to ensure optimal waveguide performance for a given mode power. Using a Si-NC/SiO2 slot waveguide as an example, we show that the common definition of the effective mode area may lead to significant errors in estimation of optical intensity governing the nonlinear optical response and, as a result, to poor strength evaluation of the associated nonlinear effects. A simple and physically meaningful definition of the effective mode area is given to relate the total mode power to the average field intensity inside the nonlinear region and is employed to study the optimal parameters of Si-NC slot waveguides.

Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers.

Abstract: We demonstrate scaling of the effective area of higher-order mode, Er-doped fiber amplifiers Two Er-doped higher-order mode fibers, one with 3800 μm2 Aeff in the LP0,11 mode, and one with 6000 μm2 effective area in the LP0,14 mode, are demonstrated Output beam profiles show clean higher order modes, and S2 imaging measurements show low extraneous higher order mode content CW and pulsed amplifier experiments are reported Nanosecond pulses are amplified to 05 mJ pulse energy with 05 MW peak power

Mode coupling effects in multi-mode fibers

Abstract: In mode-division-multiplexed systems using coherent detection, strong mode coupling is beneficial. Mode coupling reduces modal dispersion, minimizing signal processing complexity. In combination with modal dispersion, mode coupling creates frequency diversity, mitigating the mode-dependent gain of optical amplifiers.

Weakly-Guiding Mode-Selective Fiber Couplers

Abstract: This paper presents analytical descriptions of the behavior of weakly-guiding/weakly-coupled mode-selective fiber couplers, based on coupled-mode theory. Mode-selective couplers rely on the phase-matching of a higher-order mode in one fiber with the fundamental mode of a second, closely-positioned fiber. Their behavior is shown to be highly-dependent on the principle and azimuthal mode numbers of the higher-order mode, as well as its spatial-orientation. Zero coupling is shown to be possible for an asymmetric higher-order mode even when there is perfect phase-matching. The theory is also numerically simulated, and could assist in the future design of efficient mode-selective couplers for a wide range of optical communications and sensor systems.

Beam quality degradation of a single-frequency Yb-doped photonic crystal fiber amplifier with low mode instability threshold power

Abstract: A current limit in power scaling of Yb-doped fiber amplifiers is the sudden onset of mode instabilities. We investigated this effect on a single-frequency Yb-doped photonic crystal fiber amplifier with a low mode instability threshold power. By measuring the overlap of the fiber output beam with the fundamental mode of an external cavity to be about 95%, we could exclude significant modal power transfer below a sharp power threshold. Furthermore, we directly measured the frequency resolved intensity noise spectra. No fluctuations in the overall output power were observed, but for the modal content different oscillation regimes were identified.

Mode evolution in long tapered fibers with high tapering ratio

Abstract: We have experimentally studied fundamental mode propagation in few meters long, adiabatically tapered step-index fibers with high numerical aperture, core diameter up to 117 μm (V = 38) and tapering ratio up to 18. The single fundamental mode propagation was confirmed by several techniques that reveal no signature of higher-order mode excitation. It can be, therefore, concluded that adiabatic tapering is a powerful method for selective excitation of the fundamental mode in highly multimode large-mode-area fibers. Annular near field distortion observed for large output core diameters was attributed to built-in stress due to thermal expansion mismatch between core and cladding materials. The mechanical stress could be avoided by an appropriate technique of fiber preform fabrication and drawing, which would prevent the mode field deformation and lead to reliable diffraction-limited fundamental mode guiding for very large core diameters.

Reception of Dual-Spatial-Mode CO-OFDM Signal Over a Two-Mode Fiber

Abstract: We demonstrate the use of mechanical grating based mode converters to achieve two forms of dual-spatial-mode transmission: LP01 and LP11, and dual LP01 modes. High modal extinction ratio (>; 20 dB) is shown for the mode converter within a 10-nm wavelength range. We first present 107-Gb/s coherent optical OFDM (CO-OFDM) transmission over a 4.5-km two-mode fiber using LP01 and LP11 modes where the mode separation is performed optically. We then show 58.8-Gb/s CO-OFDM transmission using dual LP11 modes where the mode separation is achieved via 4 × 4 electronic MIMO processing.

Spot-based mode coupler for mode-multiplexed transmission in few-mode fiber

Abstract: We present designs for a low-loss mode coupler based on multiple Gaussian spots that can selectively address spatial and polarization modes of a few-mode fiber. In particular we show a design optimized for a few-mode fiber with 12 spatial and polarization modes (LP 01 , the twofold degenerate LP 11 and LP 21 , and LP 02 ), with a mode dependent loss < 1dB and coupler insertion loss < 3dB.

Mode conversion in rectangular-core optical fibers.

Abstract: Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fiber’s transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.

Mode locking phenomena observed near the stability boundary of the ideal interchange mode of LHD

Abstract: This paper describes the first observation of mode locking phenomena in the Large Helical Device. In the experiments, a rotation of the m/n = 1/1 mode slowed down and stopped when the plasma approached the stability boundary of the ideal interchange mode. The minor collapse was caused by the growth of the mode just after mode locking. The mode before the locking has a radial structure with no inversion around the resonance, which implies that the mode has an interchange-type structure at least before the rotation stops. The location of the mode after the locking depends on that of the error or residual field in reduced error field operations.

Uv-green converting fiber laser using active tapers

Abstract: Nonlinear optical systems include fiber amplifiers using tapered waveguides such as optical fibers that permit multimode propagation but produce amplification and oscillation in a fundamental mode. The tapered waveguides generally are provided with an active dopant that is pumped with an optical pump source such as one or more semiconductor lasers. The active waveguide taper is selected to taper from a single or few mode section to a multimode section, and a seed beam in a fundamental mode is provided to a section of the waveguide taper associated with a smaller optical mode. An amplified beam exits the waveguide taper at a section associated with a larger optical mode. The amplified beam is directed to nonlinear conversion optics such as one or more nonlinear crystals to produce high peak power and high beam quality converted light using second or third harmonic generation, or other nonlinear processes.

Coupled-Mode Analysis for Chiral Fiber Long-Period Gratings Using Local Mode Approach

Abstract: Starting from the conventional coupled-mode theory and using the mode transformation method, a unified local mode approach is presented to analyze the chiral fiber long-period gratings (CLPGs) formed by twisting a high-birefringence fiber as an example of a chiral fiber with double-helix symmetry. Similarly, a helical-core fiber, as an example of single-helix CLPGs, is analyzed by using the ideal mode approach. The coupled-mode equations for both the cases are transformed and reduced to two-mode equations. The derivation is simple, and the results are explicit in concept. These results reveal the scheme of polarization selectivity of mode coupling, and for the first time, the necessary condition for achieving the polarization selection is given. The analysis indicates that conventional coupled-mode theory is efficient for studying chiral fiber grating, and more importantly, it is better to be understood. A numerical simulation has been done on a twisted conventional Panda fiber with a beat length of 2.5 mm. It has been shown that in a right-handed twisted structure, a total power transfer of a right-handed circular polarization light is obtained with a grating pitch of 0.382 mm and a grating length of 64.9 mm at the wavelength of 1.55 μm . On the other hand, through the simulation on single-helix CLPG with an eccentric core, it has been shown that a total power transfer of the core mode to LP11 mode is obtained with a grating pitch of 0.723 mm and a grating length of 10.46 mm at the wavelength of 1.55 μm.

Comparison of higher-order mode suppression and Q-switched laser performance in thulium-doped large mode area and photonic crystal fibers

Abstract: We report the influence of higher order modes (HOMs) in large mode fibers operation in Q-switched oscillator configurations at ~2 μm wavelength. S2 measurements confirm guiding of LP11 and LP02 fiber modes in a large mode area (LMA) step-index fiber, whereas a prototype photonic crystal fiber (PCF) provides nearly single-mode performance with a small portion of light in the LP11 mode. The difference in HOM content leads to a significant difference in Q-switched oscillator performance. In the step-index fiber, the percentage of cladding light increases by 20% to >40% with increasing pulse energy to ~250 µJ. We accredit this degradation to saturation of the gain in the fundamental mode leading to more light generated in the HOMs, which is eventually converted into cladding light. No such degradation is seen in PCF laser system for >400 µJ energies.

Distributed Mode Filtering Rod Fiber Amplifier With Improved Mode Stability

Abstract: We report 216W of average output power from a photonic crystal rod fiber amplifier. We demonstrate almost 50% power improvement of the mode onset instability by operating the rod fiber in a leaky guiding regime.

Impact of mode coupling on the mode-dependent loss tolerance in few-mode fiber transmission

Abstract: In this work the impact of mode-dependent loss (MDL) from optical amplifiers in few-mode fibers with weak and strong mode coupling is analyzed. For a 409-Gbit/s 3MDM-DP-QPSK system it is shown that strong mode coupling reduces the impact of MDL in a similar manner polarization-dependent loss is reduced by polarization-mode dispersion.

Dynamic Behavior of Mode Partition Noise in Multimode Fiber Links

Abstract: In this paper we investigate and quantify the dynamic behavior of the mode partition noise variance. We found that the mode partition noise variance is largest at the bit boundaries and depends on the slope (edge transition times) of the signal in links with one propagating fiber mode. In multimode fibers we show similar dependence, although the mode partition noise is reduced because fiber mode groups mix the chromatically dispersed laser modes responsible for mode partition noise.

Influence of the angle-dependence of mode coupling on optical power distribution in step-index plastic optical fibers

Abstract: Determination of the power distribution in step index plastic optical fibers by the power flow equation has been reported in the literature both with and without the simplifying assumption of constant coupling that is independent of the angle of light propagation The need for this assumption is evaluated in this paper Results with the angle-dependent coupling coefficient are compared to those derived under the simplifying assumption of constant coupling Benchmarked to values measured experimentally, this comparison covered the coupling lengths Lc (denoting where the equilibrium mode distribution is achieved in the fiber) and lengths zs (for achieving the steady-state distribution) Results differ slightly but only for longer fiber lengths, thus largely vindicating the simplifying assumption of constant coupling

Thermal tuning the optical cavity for 3 mode interaction studies using a CO 2 laser

Abstract: Three mode interactions could induce parametric instability in advanced gravitational wave detectors with high optical power circulating in the cavities. One of the conditions for parametric instability to occur is when the cavity frequency difference between fundamental mode and the high order mode matches the test mass acoustic mode frequency. The optical mode spacing is a function of cavity g-factor (radius of curvature). At the Gingin High Optical Power Facility, we have an 80 meter optical cavity particularly designed for studying high optical power effects in advanced gravitational wave detectors such as parametric instabilities. Here we present the recent results of thermal tuning the cavity g-factor by heating the test mass surface with a CO2 laser to investigate the 3-mode interactions. Observation of test mass thermal noise peaks above 160 kHz enhanced by 3 mode interaction is presented.

A Method for Gyrotron Beam Coupling into a Corrugated Waveguide

Abstract: In the radio-frequency (rf) power transmission system of an electron cyclotron heating and current drive (EC H&CD) system, the gyrotron power should couple with the fundamental mode of the corrugated waveguide (HE11 mode) because unwanted higher-order modes affect the beam radiation characteristics, which is a problem in the quasi-optical launcher design. To achieve high HE11 mode purity, a beam coupling method that measures the transmission mode in the waveguide was examined using a 170-GHz high-power gyrotron for the first time. In beam coupling, the offset and tilt angle of the input beam at the waveguide inlet were minimized by controlling the angles of the mirrors in the matching optical unit (MOU) to minimize unwanted LP11 modes in the waveguide. The rf field profile in free space after 1.3 m of the waveguide from the MOU was measured, and the transmission mode content was analyzed. According to the analyzed mode content, the HE11 mode content was optimized by remote adjustment of the mirror...

Propagation and excitation of the higher‐order E x11 mode in an insulated image guide

Abstract: The use of E mode in an insulated image guide is proposed. This mode has much lower conductor loss than the fundamental, E mode. The electric field of the mode is primarily parallel to the metal ground-plane; therefore, the mode can be efficiently excited with a compact dipole structure oriented parallel to the ground-plane. When the insulating layer is of low permittivity and relatively thick, the E mode exhibits the highest effective permittivity (largest propagation constant) of all modes. This reduces unwanted mode coupling at discontinuities. Two example structures, a straight line and a double-bended line are measured at frequencies around 8 GHz. The excitation of the waveguide causes insertion loss of less than 0.4 dB, and the radiation loss of the bends and the waveguide itself appears to be negligible. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:179–181, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26450

Stabilized Large Mode Area in Tapered Photonic Crystal Fiber for Stable Coupling

Abstract: A rigorous modal solution approach based on the numerically efficient finite element method (FEM) has been used to design a tapered photonic crystal fiber with a large mode area that could be efficiently coupled to an optical fiber. Here, for the first time, we report that the expanded mode area can be stabilized against possible fabrication tolerances by introducing a secondary surrounding waveguide with larger air holes in the outer ring. A full-vectorial -field approach is employed to obtain mode field areas along the tapered section, and the Least Squares Boundary Residual (LSBR) method is used to obtain the coupling coefficients to a butt-coupled fiber.

On the thermal origin of mode instabilities in high power fiber lasers

Abstract: Detailed simulations on the temperature profile inside of a large mode area fiber in high power operation with mode interference are presented. These simulations show that the pump power is not homogeneously absorbed along the fiber, which in turn gives rise to an oscillating temperature profile along the fiber. This longitudinal temperature profile creates an index grating with the right period to transfer energy between the interfering modes. Two cases are analyzed: mode beating between the fundamental mode and a radially anti-symmetric mode and mode beating between the fundamental mode and a radially symmetric mode.

General analysis of the mode interaction in multimode active fiber

Abstract: General analysis of the mode interaction in multimode fiber is presented in this paper. By taking local gain into consideration, the general coupled mode equations in multimode active fiber are deduced in the model and the effect of various factors can be analyzed based on the general coupled mode equations. Analytical expression of the beam quality factor is deduced for the optical field emerging from the multimode active fiber. The evolution of the mode power and M2 factor along the fiber are analyzed by numerical evaluations.

CGH-based real-time analysis of fiber Bragg gratings in few mode LMA fibers

Abstract: We report on a new approach for experimentally analyzing transversal mode coupling in few mode “ber Bragggratings (FBGs). Utilizing a spatial light modulator (SLM) enables real-time modulation of the incident spatiallight distribution coupled into the “ber. Thus, individual transversal “ber modes can be excited, allowing foreort-less mode switching and mode rotation. Simultaneously, the transmitted and re”ected modes are analyzed.We use this setup to demonstrate LP mode discrimination by asymmetric FBGs which favor re”ection of adesignated LP 11 mode orientation for few mode large mode area “bers.Keywords: “ber Bragg gratings, “ber mode excitation, mode coupling 1. INTRODUCTION Owing to their outstanding thermo-optical properties “ber concepts have become more and more important forhigh power laser designs in recent years. 1 In addition to their high brilliance, they facilitate compact and robustresonator designs. However, in high power “ber laser systems the tight power con“nement to the small “ber coreregion gives rise to nonlinear eects like self-phase modulation and Raman or Brillouin scattering, hamperingpower scaling. Besides photonics crystal bandgap (PCF) “bers,