Showing papers on "Equilibrium mode distribution published in 1999"

Tapered fiber laser

Abstract: A tapered fiber laser having a multi-mode section, a single-mode section, and either a tapered section or fundamental mode matching junction therebetween. The multi-mode section has a large core to directly receive pump light from a broad stripe laser or diode bar, and a length preferably longer than the absorption length of the pump light (so optical amplification occurs predominantly in the multi-mode section). Doping levels can be increased to reduce the multi-mode length. The taper angle is sufficiently small to produce adiabatic compression of the fundamental mode from the multi-mode to single-mode sections, and acts as a cutoff filter favoring lasing of the fundamental mode within the multi-mode section. Alternately, the step junction may have a mode field diameter matched to the lowest-order mode, with laser light output via the single-mode section. The invention can be applied to waveguides (particularly those having an aspect ratio corresponding to a broad stripe laser source), doped with ytterbium or neodymium ions, and is particularly advantageous as a pump source for an erbium-doped fiber amplifier (EDFA).

Tapered polymer single-mode waveguides for mode transformation

Abstract: This paper presents a tapered polymer waveguide structure for coupling light between optical waveguides with differing geometries. Optical fibers, lasers, and other photonic integrated circuit components can be coupled with tapered waveguides. The polymer waveguide performs a mode transformation between different mode shapes and sizes. For example, the mode transformation can be from an elliptical laser diode mode to that of a circular optical fiber mode. The input and output of a tapered waveguide structure are analyzed, for the case of laser to fiber coupling, in order to determine the effect of misalignments on the coupling efficiency. Adiabaticity in waveguide propagation is discussed. The fabrication of our polymer waveguides is also described.

Mode selection in high power cladding pumped fibre lasers with tapered section

Abstract: Summary form only given. Cladding pumped fibre lasers are undoubtedly becoming the most attractive source of high power radiation in the near IR region. The small overlap between the pump and the rare-earth-doped core leads to a small pump absorption, e.g., 1 dB/m. Larger core and absorption is often desirable, especially if (transversally) single-mode operation can be maintained. Thus, single-mode operation following selective excitation of the fundamental mode of large-core multimode fiber amplifiers was recently demonstrated in short fibres. In this paper, we use a simple fiber taper inside a laser cavity with a multimode gain fibre to suppress higher-order modes.

Helicon modes in a cylindrical plasma source

Abstract: The axial wavelengths and azimuthal mode structures of a cylindrical helicon plasma source have been investigated over a broad range of input conditions (pressure 1-16microbar, field 0-450 G, rf power 0-2500 W @ 13.56 MHz) in an argon plasma. Several distinct modes of operation, separated by discontinuous jumps, have been identified: an electrostatic mode, an m = 0 wave mode and three m = 1 helicon wave modes. Two different types of mode jump have also been identified. A cavity mode transition has been identified in which a mode jump occurs when the wavelength of standing waves governed by the length of the plasma vessel leads to a node in the wave field at the centre of the antenna. A radial mode transition has also been identified in which the favourable condition that the axial wavelength should be equal to twice the length of the antenna for efficient coupling of the rf power is maintained at high powers by the plasma making a transition from the first to the second radial mode. A procedure based on these observations is suggested for predicting where mode transitions are likely to occur within the parameter space of a helicon source.

Intermodal dispersion and mode coupling in perfluorinated graded-index plastic optical fiber

Abstract: Graded-index multimode perfluorinated plastic optical fibers typically exhibit bandwidths much greater than would be expected from their index profiles. To resolve this discrepancy, we have conducted the first measurements of differential mode delay in such fibers. These measurements show intermodal dispersion that increases as the square root of fiber length, implying strong mode coupling in these fibers. Significant power transfer between modes occurs at lengths less than 20 m, so that mode coupling results in improved bandwidth on length scales relevant for local area networks. The observed coupling arises from extrinsic nonuniformities of the waveguide.

Optical devices based on energy transfer between different modes in optical waveguide

Abstract: Optical waveguide devices (100) based on optical coupling of different modes (110, 130) by using a mediating mode (120). Such waveguide devices (100) generally have at least to coupling regions (104, 106). The first coupling region (104) couples an output mode and the mediating mode, and the second one (106) couples the mediating mode and an input mode that copropagates with the output mode. Under proper conditions, the energy in the input mode can be completely transferred to the output mode, without energy loss to the mediating mode (120).

Long haul single mode waveguide

Abstract: Disclosed is a single mode optical waveguide fiber having a segmented core. The relative indexes, the refractive index profiles and the radii of the segments are chosen to provide waveguide fiber properties advantageously used in severe environments, such as, undersea cables. The segmented core waveguide fiber has a negative total dispersion over the operating window of about 1530 nm to 1570 nm, which serves to eliminate soliton formation. The key properties of dispersion zero, cut off wavelength, attenuation, and bend resistance fall within desired ranges. The waveguide also features a low polarization mode dispersion.

Spliced optical fiber coupler

Abstract: In general, the mode field pattern of a single-mode polarization-maintaining fiber is symmetric. There are preferred axes for the polarization states, but the intensity pattern emerging from the fiber is rotationally symmetric. If a short piece of multi-mode fiber is spliced onto the end of a polarization-maintaining single-mode fiber, it is possible to change the apparent shape of the mode field. The use of a fusion splicer affects the stress pattern in polarization-preserving fiber, and introduces asymmetry into the shape of the fiber mode field. If the spliced fiber is cleaved near the splice, and the asymmetry of the fiber mode field is matched to the asymmetry of a laser diode beam, then the laser beam is coupled efficiently into the fiber without the use of additional beam-shaping optics. The asymmetric beam from the laser is brought to a focus, which is also asymmetric. By matching the asymmetries of the focused spot and the distorted mode field (caused by the addition of the short piece of multi-mode fiber), good coupling is achieved into the fiber. Once the light has entered the spliced fiber and propagated past the splice, the mode field returns to its original symmetric pattern and the light propagates in a single mode, as it would in an unspoiled, single-mode polarization-maintaining fiber. The output from the unspoiled end is still symmetric, and there is little attenuation caused by the splice.

Theoretical analysis of optimal quasi-phase matched second harmonic generation waveguide structure in LiTaO3 substrates

Abstract: Based on the QPM structure and the coupled-mode theory, the maximum SHG conversion efficiency of QPM-SHG waveguides in LiTaO3 substrates is analyzed by optimizing the mode confinement and the mode overlap factors. For TM00(ω)-to-TM00(2ω) conversion with a non-square shaped domain inversion QPM structure, the mode confinement and mode overlap factor are dependent not only on the optical field distribution for the fundamental and the SH waves in the waveguide, but also on the domain inversion shape. With the assumption that the refractive index profile of an annealed, proton exchanged LiTaO3 channel waveguide is an exponential decay function in the depth direction, and a Gaussian function in the width direction, the analytical expression of the optical field distribution for the lowest order mode can be obtained. By considering both a non-Gaussian field distribution and a non-square shaped domain inversion structure, the theoretical values for achieving the optimal QPM-SHG waveguide structure are determined.

Radially non uniform and azimuthally asymmetric optical waveguide fiber

Abstract: Disclosed are a single mode waveguide fiber and a multimode waveguide fiber which have a radially non-uniform and azimuthally asymmetric core, and methods of making such a waveguide fiber. This asymmetry provides additional degrees of freedom for use in forming a waveguide having particular performance characteristics.

Radially varying and azimuthally asymmetric optical waveguide fiber

Abstract: Disclosed is a single mode waveguide fiber and a method of making a single mode or multimode waveguide fiber which has an azimuthally and radially asymmetric core. This asymmetry provides additional degrees of freedom for use in forming a waveguide having particular performance characteristics.

Nondestructive measurement method of individual mode loss for waveguides

Abstract: The device concerns measuring the propagation and bending losses that take place in the individual mode of the multi-mode waveguide. The method applied is a new one comprising butt-couple and prism-couple together with phase-modulation method. Electrodes are made on the waveguide, and applied with voltage. The electro-optical effect is used to modulate the effective length of the waveguide in forming a Fabry-Perot etalon. From here, the contrast of the butt-couple's output and the loss can be measured. At the same time, as the prism-couple is able to separate the multiple modes individually, and measure the contrast and loss of each mode through every output mode which are then compared with the butt-couple, each mode's propagation and bending losses values of the multi-mode waveguide can be derived.

Long haul single mode waveguide fiber

Abstract: Disclosed is a single mode optical waveguide fiber having a segmented core. The relative indexes, the refractive index profiles and the radii of the segments are chosen to provide waveguide fiber properties advantageously used in severe environments, such as, undersea cables. The segmented core waveguide fiber has a negative total dispersion over the operating window, which serves to eliminate soliton formation. The key properties of dispersion zero, total dispersion cut off wavelength, attenuation, and effective area fall within desired ranges. The waveguide also features a low polarization mode dispersion.

Experimental study of a slot antenna excited by a TE011 mode coaxial cavity

Abstract: A slot antenna is developed to excite the high harmonic waveguide mode for generating large-area plasmas. This antenna consists of a TE011 mode coaxial cavity with the axial slots positioned on equal interval on the inner wall. The waves radiated from those slots can excite the high harmonic mode in the central area. With the azimuthal symmetric wave field of the TE011 mode, the number of the slots can be chosen to match the field pattern of the high harmonic mode. In this report, the dispersion relation of the coaxial waveguide, the coupling scheme and the mode competition of the cavity are studied. A method has been successfully developed to suppress the TE121 mode which is the most competing mode to the TE011 mode.

High order mode waves excited in GIS

Abstract: UHF signal measurement based on the high order mode waves, is an important tool for the partial discharge detection in GIS (gas insulated substation). In this paper, the high order mode wave excitation and propagation are discussed. From experimental results, some properties are derived: the velocity of the high order mode waves (TE and TM modes) is less than the TEM mode velocity; the high order mode wave in GIS is dependent on the GIS geometry and on the source of excitation; and the resonant cavity is important for the high order mode signal detection.

Common mode current prediction from normal mode voltage of printed circuit board with limited width ground pattern

Abstract: Radiated emission due to common mode current is usually dominant in total emission. The common mode current is generated when the ground width of the microstrip structure is narrow. A procedure is proposed to predict common mode current from normal mode voltage at a point where cross-sectional structure is varying. As a result, the predicted spectrum and distribution of common mode current agree well with measured data.

Experimental study on a rf-exctted slab waveguide co 2 laser with the far field intensity distribution of a single suppressed peak

Abstract: In this paper,we have studied experimentally the properties of the mode structure and modes evolution of a slab waveguide CO 2 laser with a far field intensity distribution of a single spatially sup-pressed peak. It has been found that the laser mode is a new single mode of a slab waveguide CO 2 laser,which has a different mode structure from the known mode of slab waveguide gas laser.

The study of five-layered directional coupler for the mode transformation

Abstract: A simple tapered directional coupler with a five-layered structure is employed for mode conversion between a single-order and higher-order modes. As a result, we confirmed that the first several modes in the tapered waveguide shift to single-order mode with high conversion efficiency by use of computer simulation, and observed the streak patterns of mode propagation experimentally.

New type of single mode optical fibres

Abstract: The new algorithm of single-mode fibres synthesis for given values of phase and group speeds of propagation mode on fixed normalized frequency is proposed and realized. New types of single-mode graded dielectric waveguides with an extended band of single-mode regime and increased cross sections of core are received. The dependence between bandwidth of single-mode regime and values of phase and group speed of propagating mode for fixed frequency is established. The dispersion characteristics and distribution of propagating mode field in synthesized waveguides are calculated.

Imperfect nonlinear optical fiber-propagation characteristic

Abstract: Numerical analysis of the imperfect nonlinear optical fiber using the coupled mode theory and the split-step Fourier method, is described. The nonlinear optical fiber is divided into a large number of appropriate cylindrical segments. In the analysis of light propagation between two segments of various diameters, the coupled mode theory is used. Propagation of a pulse, in the cylindrical nonlinear segment, is analyzed using the split-step Fourier method. Some results of computation of the real nonlinear optical fibers are also presented.

Single-end continuous-wave polarization mode dispersion measurement in long single-mode fibers

Abstract: Single-end measurement of statistical properties of polarization mode dispersion parameters was carried out by means of a continuous wave scheme based on the analysis of the Fresnel reflection at fiber far-end