Showing papers on "Equilibrium mode distribution published in 2010"

Extremely large-mode-area photonic crystal fibre with low bending loss

Abstract: We report on the design of a novel flexible very large mode area photonic crystal fibre for short pulse high peak power fibre laser and beam delivery applications. This fibre has an extremely large mode area exceeding 2500 µm2 when kept straight and over 1000 µm2 when bent over a 10 cm radius at a wavelength of 1064 nm. In addition our fibre exhibits very small fundamental mode bending loss below 10−2 dB/m. The large difference between the propagation loss levels of fundamental and higher order modes forces efficient single-mode guidance in the fibre core while bent. This allows using the fibre to build compact high power laser systems. The paper further explores the major features of this fibre including: the dependence of the mode field area on the fibre core shape, the influence of the bending radius and of the bending direction as well as the impact of manufacturing tolerances on the fibre specifications.

Measurement of RF Transmission Mode in ITER Relevant EC H&CD Transmission Line

Abstract: The preliminary results of mode measurement in the ITER relevant 40 m long transmission test line composed from 63.5 mm diameter corrugated waveguides and miter bends are presented. The field patterns were measured by taking temperature profiles on a paper screen placed in front of the waveguide end using an infrared imaging camera. The complex electric field at the waveguide end was retrieved from the measured temperature profiles. As a result, the transmission power includes 87% of HE11 mode and 6% of LP11 odd (HE21+TE01) mode and small ratios of other modes. The mode content had small dependence on length of the transmission line. This result indicated that the higher order mode content generated at the input of the transmission line is conserved and propagated through the transmission line. This suggests that the initial RF coupling to the waveguide is critical since it affects the launcher efficiency.

Wave guide that attenuates evanescent light of higher order TM mode

Abstract: A waveguide has a core through which laser light can propagate in a TM mode, that has a rectangular cross section perpendicular to a propagative direction of the laser light, and through which the laser light can propagate in a fundamental mode in which only one portion exists on the cross section of the core where a light intensity of the laser light becomes maximal, and a higher order mode in which two or more portions exist where the light intensity becomes maximal, a clad surrounding the core, and a light absorbing element in the clad, and wherein a distance between the light absorbing element and the core is shorter than a penetration length of evanescent light in the higher order mode, but is longer than a penetration length of evanescent light in the fundamental mode.

Stable coherent MIMO transport over few mode fiber enabled by an adiabatic mode splitter

Abstract: Simulations confirm stable MIMO transport of three channels over a two mode fiber — even in the presence of fiber bending induced mode coupling — enabled by a new adiabatic mode splitter output coupler.

Design of a single-polarization single-mode photonic crystal fiber with a near-Gaussian mode field and wide bandwidth

Abstract: Single-polarization single-mode (SPSM) fiber can efficiently eliminate polarization mode coupling, polarization mode dispersion, and polarization-dependent loss. Up to now, most single-polarization fibers have been designed based on form birefringence, which would result in a non-Gaussian field distribution and a small effective mode field area. In this paper, a novel structure of SPSM photonic crystal fibers based on the resonant coupling phenomena is proposed and analyzed by using a full-vector finite-element method with a second-order transparent boundary condition. From the numerical results it is confirmed that this fiber has a near-Gaussian mode field within the wavelength range from 1.46 to 2.2μm, where only one polarized mode exists effectively, and the mode field area is about 79μm2 at the wavelength of 1.55μm, matching that of the conventional single-mode fiber.

Modal Dispersion Mitigation in Standard Single-Mode Fibers at 850 nm With Fiber Mode Filters

Abstract: We investigate the possibility of using a fiber which supports only the fundamental LP01 mode at 850 nm as a mode filter to overcome modal dispersion in standard single-mode fiber links that employ either multitransverse or single-transverse-mode vertical-cavity surface-emitting lasers. The individual power penalties due to modal noise and modal crosstalk effects in the link are investigated.

A novel cylindrical TE2,1 mode converter

Abstract: A novel, compact arrangement for Ka-band mode couplers, which convert a rectangular waveguide TE1,0 to cylindrical waveguide TE2,1 mode, has been designed, constructed, and tested The design features a set of longitudinal slots, positioned in regions of negligible current flow for the TE2,1 mode, allowing its propagation to be preferentially favored, by suppression of the fundamental TE1,1 mode Numerical simulations and experimental measurements display good agreement, showing transmission of the intended TE2,1 mode at levels of better than −5 dB, from a frequency of ∼375 to 41 GHz Subsequent farfield measurements confirm the presence of the TE2,1 mode, demonstrating good agreement when compared with analytical expectations Such a device would be an ideal candidate for an application where mode purity, bandwidth, and ease of construction are of primary importance and where the transmission efficiency is of limited concern

Mode characteristics of metallically coated square microcavity connected with an output waveguide

Abstract: Mode characteristics of a square microcavity with an output waveguide on the middle of one side, laterally confined by an insulating layer SiO2 and a p-electrode metal Au, are investigated by two-dimensional finite-difference time-domain technique. The mode quality (Q) factors versus the width of the output waveguide are calculated for Fabry–Perot type and whispering-gallery type modes in the square cavity. Mode coupling between the confined modes in the square cavity and the guided modes in the output waveguide determines the mode Q factors, which is greatly influenced by the symmetry behaviors of the modes. Fabry–Perot type modes can also have high Q factors due to the high reflectivity of the Au layer for the vertical incident mode light rays. For the square cavity with side length 4 μm and refractive index 3.2, the mode Q factors of the Fabry–Perot type modes can reach 104 at the mode wavelength of 1.5 μm as the output waveguide width is 0.4 μm.

Ridge waveguide serial interferometers

Abstract: In a ridge waveguide serial interferometer mode conversion is induced by a first mode conversion section, a phase difference between modes is introduced by propagation over a length of waveguide and optical interference is produced following further mode conversion induced in a second mode conversion section. The first mode conversion section has a first radius of curvature, which is equal to a second radius of curvature of the second mode conversion section. The ridge waveguide interferometer advantageously provides an equal phase dependency as a function of temperature.

Modeling of large flattened mode area fiber lasers

Abstract: Potentialities of independent tailoring the index and gain profiles in fiber laser aiming to achieve a strong modal discrimination are theoretically examined. It is demonstrated by numerical simulations existence of fiber amplifier constructions which have the flattened fundamental mode profile in the gain region. It is shown that the fundamental mode retain the largest modal gain in comparison with modal gains of higher-order-modes for any depletion of the gain by the fundamental mode. The particular design is presented with the flattened fundamental mode area 6360 μm squared.

Frequency range dependent TE10 to TE20 mode converter

Abstract: We have reported that single-mode propagation is available for a metallic waveguide with dielectric rods arrayed at the center of the waveguide in the frequency under twice the cutoff frequency region using the TE10 mode, and in the frequency over twice the cutoff frequency region using the TE20 mode. In this investigation, a mode converter is proposed, which passes through a TE10 mode for low frequency range and converts TE10 to TE20 mode for the high frequency range. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 169–171, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24886

Field distribution and mode interaction in twin-core fiber

Abstract: A new direct method for the measurement of the longitudinal power distribution in multicore fibers using the luminescence of active ions is proposed. The method is employed in the study of the effect of curvature of twin-core fiber on the interaction of modes that propagate in different cores. The experimental distributions are in qualitative agreement with the model of mode interaction in curved fiber with allowance for the photoelastic effect. The results are used to determine the length of the mode interaction and the difference of the core refractive indices.

Improvement of Coiling Mode to Suppress Higher-Order-Modes by Considering Mode Coupling for Large-Mode-Area Fiber Laser

Abstract: Mode coupling, bend distortion and bend loss are important considerations in large mode area (LMA) fiber laser. Both mode coupling and mode distortion were studied in the step-index fiber in theory. The dependence of mode power on propagation distance, effective bend radius was calculated with the bend radius changing suddenly and gradually. Taking mode coupling into account, we designed a new cone-shaped coiling device for beam quality improvement. These results and design are of benefit to the applications of coiling fiber to improve beam quality for LMA fiber laser.

Simple suppression technique for higher-order mode amplification in bent large mode area triple-cladding fibers

Abstract: A novel technique to suppress the higher-order mode in bent LMA triple-cladding fibers is proposed. It is shown that the higher-order mode can be coupled to a cladding mode with only an adjustment of the bending diameter.

Analysis of mode characteristics for hexagonal resonator lasers

Abstract: Mode characteristics of hexagonal resonators are numerically simulated by the two-dimensional (2-D) finite-difference time-domain (FDTD) technique. For the hexagonal resonator with the side length of 3 μm and the refractive index of 3.2, the mode quality (Q) factors decrease from 104 to 102 as the resonant wavelength increases from 1.1 to 3.2 μm. The modes have a relatively high Q factor as their even and odd states have different mode wavelengths and different Q factors, i.e., they are accidentally degenerate modes. The azimuthal mode numbers obtained from mode field distribution for the accidentally degenerate modes satisfy the mode number relation derived from the symmetry characteristics for the split of double-degenerate modes. Furthermore, the numerical results indicate that an output waveguide parallel to one of the sides is the better scheme for directional emission. For the hexagonal resonator with 3 m side length and 0.3 μm wide output waveguide, the Q factor of 5.05×103 and output efficiency of 33% are obtained at the mode wavelength of 1.29 μm.

Light Propagation in a Circular Double Clad Fiber Using Coupled Mode Method

Abstract: For better understanding the light propagation in a double clad fiber(DCF),the perfect circular double clad fiber is numerically investigated by using the coupled mode method with translating the DCF into a single mode fiber(SMF)and an annular core fiber(ACF).The coupling coefficients between the LP01 mode of the SMF and the guided modes of the ACF are calculated.The results show that the LP0n modes of the ACF have greater coupling coefficients than the other guided modes,and for the LP0n modes the coupling coefficient of the high-order mode is larger than that of the low-order mode.The field distribution in the DCF core calculated by the coupled mode equations shows a quasi-periodic distribution along the DCF.The average period of the field distribution increases with the increasing of wavelength for different DCF parameters and the average normalized power of the field distribution is closely related to the chosen parameters of the DCF.

Zero-dispersion wavelength and mode field diameter managements of ZDSF by optimization technique

Abstract: A proposal for the new graded index single mode optical fiber applicable in zero-dispersion communication is presented in this paper. This method is capable to set the zero-dispersion wavelength and mode field diameter in the predefined desired values. The systematic design of the mode field diameter is suggested in this paper for the first time. The dispersion management is done by optimizing the structural parameters of the cladding layers with the aid of fitness function in the Genetic algorithm technique. The special assumption for the electrical field distribution of the core region is responsible to mode field diameter adjustment.

Numerical analysis of the propagation of light in a disordered waveguide system

Abstract: The propagation properties of light in a disordered waveguide system composed of randomly different cores in size are presented by numerically solving the coupled mode equation. The propagation constants of the modes are assumed to be a random variable. When one of cores is illuminated at the input end of the waveguide system the coherent part of the amplitude of light decreases exponentially with increasing distance and at large distance only the incoherent part propagates. The incoherent part is concentrated into a narrow region near the illuminated core in the cross section of the waveguide system and the distribution does not almost change along the waveguide axis.

Phase rematch on overmoded waveguide mode converter for BWO

Abstract: The overmoded transmission line is often adopted by high-power millimeter wave system, so the influence of various distortion has to be evaluated. The distortion of waveguide can be utilized if we know its influence to the modes. So, the questions of mode coupling should be studied which arising from waveguide distortion. For the output mode of Backward wave oscillator and Virtual cathode oscillator is often TM01 mode and TM0n mixed modes. The TM0n-TM01 mode converter can adopt gradual radius changes of the waveguide, and TM01 mode radiate outside only converter to TE11 mode, so the thorough study of the structure and characteristic of TM01-TE11 mode converter is needed. The size of a waveguide is large under a high-power situation, thus it is necessary to take account more waves and their mutual coupling as well as the attenuation of circular waveguide and the option of coupling mode etc. Heavy considerations have been given to the influence of multimode factor, backward wave, ohmic losses carried from metal wall, modes choice and phase rematch.

Theoretical analysis of light transmission in a perfectly circular double clad fiber using coupled mode method

Abstract: Light propagation in a perfect circular double clad fiber (DCF) is numerically investigated by using the coupled mode method with translating the DCF into a single mode fiber (SMF) and an annular core fiber (ACF). The results show that for the coupling coefficients between the LP 01 mode of the SMF and the guided modes (LP 0n modes) of the ACF, the high-order mode has a larger coupling coefficient than the low-order mode for the ACF. By using the coupled mode equations the field distribution in the DCF core is calculated, the results show that the power shows a quasi-periodic distribution along the DCF core and its average period increases with increasing wavelength for different DCF parameters. The simulation results agree with the light ray propagation in the DCF.

Fabrication of large flattened mode optical fiber for high power laser

Abstract: Large flattened mode optical fiber with raised index ring around the outer edge of the fiber core has been fabricated through modified chemical vapour deposition process to raise the threshold for non-linear interaction in high power laser fiber. The conversion of the fundamental mode shape from a Bessel function to a top hat function, enhances the effective area of the core intersected by the mode without increasing the physical size of the core. The shape of the fundamental mode is observed to be strongly dependent on the width of the raised index ring from the modal analysis. Suitable fiber parameters have also been estimated through the modal field analysis. Fabrication process steps have been optimized to achieve the desired fiber parameters. Modal field distribution, transmission properties and bending loss of the fabricated fibers have been characterized.

Fiber based ultrashort pulse laser systems at ultrahigh average power levels

Abstract: We report on the generation of 830 W compressed average power at 78 MHz pulse repetition frequency and 640 fs pulse duration. We discuss further power scaling including the issue of transversal spatial hole burning. Therefore, we describe a low-nonlinearity fiber design capable of producing fundamental mode radiation at ultra high average powers from short length (range of 1m) and large mode field diameter (>50μm) fibers. In conventional large mode area fiber most of the core is typically uniformly doped. As a consequence gain factors for the fundamental mode and the next higher order modes are comparable. Furthermore, the fundamental mode extracts inversion only in the central part of the core according to its intensity profile, leading at high pump and signal power levels to high and unused inversion density with a strong overlap with higher order transversal modes. In experiments this leads to a threshold-like onset of mode instability, originating from mode competition. Finally, this effect avoids further power scaling. The presented fiber features an optimized doping profile to prefer the amplification of the fundamental mode. In addition non-extracted inversion is minimized avoiding the issue of transversal spatial hole burning. As a consequence ultrafast fiber laser systems with novel performance are in reach, i.e. systems delivering simultaneously >1GW peak power and >1kW average power. In a first iteration a ROD-type fiber with 60μm MFD and 1.7m length was used in a CPA system to produce pump power limited 355 W of average power at 1 MHz.