Showing papers on "Radiation mode published in 2005"

Stress-induced single-polarization single-transverse mode photonic crystal fiber with low nonlinearity

Abstract: We report on the design of a single-polarization single-transverse mode large mode area photonic crystal fiber. By including index-matched stress applying elements in the photonic cladding an ultra-broadband single polarization window is obtained while a large mode field area of ~700 microm(2) is maintained. Based on that design, an Yb-doped double-clad photonic crystal fiber is realized that combines low nonlinearity and single polarization properties. A first result of the high power operation using this fiber is demonstrated.

Superradiant and stimulated-superradiant emission in prebunched electron-beam radiators. I. Formulation

Abstract: A formulation for the characterization of superradiant and stimulated-superradiant radiative emission from bunched electron beams is presented. The radiation is characterized in terms of power and spectral power per radiation mode, which provide a measure of the useful spatially coherent radiation power and spectral power emitted by a radiation source. When the bunched electron beam emits superradiantly, these parameters scale like the square of the number of electrons, orders of magnitude more than spontaneous emission. The formulation applies to emission from single bunches, a finite number of bunches in a macropulse, or periodic bunching. It can be employed on any kind of $e$-beam radiation scheme. Specific analytic expressions are derived for coherent synchrotron radiation and prebunched free-electron laser, providing a basis for comparing and understanding their connection.

Integrated planar composite coupling structures for bi-directional light beam transformation between a small mode size waveguide and a large mode size waveguide

Abstract: Composite optical waveguide structures or mode transformers and their methods of fabrication and integration are disclosed, wherein the structures or mode transformers are capable of bi-directional light beam transformation between a small mode size waveguide and a large mode size waveguide. One aspect of the present invention is directed to an optical mode transformer comprising a waveguide core having a high refractive index contrast between the waveguide core and the cladding, the optical mode transformer being configured such that the waveguide core has a taper wherein a thickness of the waveguide core tapers down to a critical thickness value, the critical thickness value being defined as a thickness value below which a significant portion of the energy of a light beam penetrates into the cladding layers surrounding the taper structure thereby enlarging the small mode size. This primary tapered core structure may be present in either a vertical or horizontal direction and may be combined with further up taper or down taper structures in the directions transverse to the primary taper direction. Another aspect of the present invention is directed to a non-cylindrical graduated refractive index (GRID) lens structure. The non-cylindrical GRIN structure has a graded refractive index having a maximum value at its core and a minimum value at its outer edges. The grading of the refractive index is provided in a either the vertical or horizontal directions and may have either a fixed refractive index or a graded refractive index in the transverse directions. Yet another aspect of the present invention is directed to composite optical mode transformers that are combinations of the taper waveguide structures and the non-cylindrical graduated refractive index structures. Yet another aspect of the present invention is the further integration of the mode transformers with V-grooves for multiple input/output fibers and alignment platform for multiple input/output photonic chips or devices.

Passive mode-locking by use of waveguide arrays.

Abstract: A novel mode-locking technique is presented in which the intensity-dependent spatial coupling dynamics of a waveguide array is used to achieve temporal mode-locking in a passive optical fiber laser. By use of the discrete, nearest-neighbor spatial coupling of the waveguide array, low-intensity light can be transferred to the neighboring waveguides and ejected (attenuated) from the laser cavity. In contrast, higher-intensity light is self-focused in the waveguide and remains largely unaffected. Numerical studies of this pulse-shaping mechanism (intensity discrimination) show that using current waveguide arrays and standard optical fiber technology produces stable and robust mode-locked soliton-like pulses.

Demonstration of mode conversion using anti-symmetric waveguide Bragg gratings.

Abstract: We experimentally demonstrate a novel grating which only produces reflection with mode conversion in a two-mode waveguide. That characteristic can improve the performance of optical devices that currently use tilted Bragg gratings to provide the mode conversion. Tilted Bragg gratings produce also reflections without mode conversion which increases noise and crosstalk of the optical device.

SBS threshold for single mode and multimode GRIN fibers in an all fiber configuration

Abstract: We investigate stimulated Brillouin scattering (SBS) threshold in single mode and multimode fibers in an all fiber network. The pump is a single mode fiber pigtail attached to a diode. We find the theory and experiment agree for both single mode and multimode GRIN fibers. We modify the bulk SBS threshold equation for use with fibers by properly accounting for mode sizes and modal dispersion.

Nonlinear mode-coupling for passive mode-locking: application of waveguide arrays, dual-core fibers, and/or fiber arrays

Abstract: A detailed analysis of mode-locking is presented in which the nonlinear mode-coupling behavior in a waveguide array, dual-core fiber, and/or fiber array is used to achieve stable and robust passive modelocking. By using the discrete, nearest-neighbor spatial coupling of these nonlinear mode-coupling devices, low-intensity light can be transferred to the neighboring waveguides and ejected (attenuated) from the laser cavity. In contrast, higher intensity light is self-focused in the launch waveguide and remains largely unaffected. This nonlinear effect, which is a discrete Kerr lens effect, leads to the temporal intensity discrimination required in the laser cavity for mode-locking. Numerical studies of this pulse shaping mechanism show that using current waveguide arrays, fiber-arrays, or dual-core fibers in conjunction with standard optical fiber technology, stable and robustmode-locked soliton-like pulses are produced.

Optical coupler devices, methods of their production and use

Abstract: The present invention relates in general to coupling of light from one or more input waveguides to an output waveguide or output section of a waveguide having other physical dimensions and/or optical properties than the input waveguide or waveguides. The invention relates to an optical component in the form of a photonic crystal fibre for coupling light from one component/system with a given numerical aperture to another component/system with another numerical aperture. The invention further relates to methods of producing the optical component, and articles comprising the optical component, and to the use of the optical component. The invention further relates to an optical component comprising a bundle of input fibres that are tapered and fused together to form an input coupler e.g. for coupling light from several light sources into a single waveguide. The invention still further relates to the control of the spatial extension of a guided mode (e.g. a mode-field diameter) of an optical beam in an optical fibre. The invention relates to a tapered longitudinally extending optical waveguide having a relatively larger cross­section that over a certain longitudinal distance is tapered down to a relatively smaller cross section wherein the spatial extent of the guided mode is substantially constant or expanding from the relatively larger to the relatively smaller waveguide cross section. The invention may e.g. be useful in applications such as fibre lasers or amplifiers, where light must be coupled efficiently from pump sources to a double clad fibre.

Leakage of the fundamental mode in photonic crystal fiber tapers.

Abstract: We report detailed measurements of the optical properties of tapered photonic crystal fibers (PCFs). We observe a striking long-wavelength loss as the fiber diameter is reduced, despite the minimal airhole collapse along the taper. We associate this loss with a transition of the fundamental core mode as the fiber dimensions contract: At wavelengths shorter than this transition wavelength, the core mode is strongly confined in the fiber microstructure, whereas at longer wavelengths the mode expands beyond the microstructure and couples out to higher-order modes. These experimental results are discussed in the context of the so-called fundamental mode cutoff described by Kuhlmey et al. [Opt. Express10, 1285 (2002) ], which apply to PCFs with a finite microstructure.

Microcavity confinement based on an anomalous zero group-velocity waveguide mode

Abstract: We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a reflective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately pi/k0, where k0 is the longitudinal wave vector for which the group velocity is zero.

Field modeling of circular microresonators by film mode matching

Abstract: The film mode matching method for rigorous vectorial field analysis of circular microresonators and bent optical waveguides is described in detail. Immittance formulation is used to assure high numerical stability. Problems with the evaluation of cylindrical functions of complex order in the transition region and their cross products have been satisfactorily solved, which makes the method reliable and accurate. Results of two commercial mode solvers are used for benchmarking. The importance of vectorial modeling of microresonators is demonstrated by a numerical example.

Coherent light source and optical device

Abstract: High power output can be easily obtained from a wide stripe laser, however, since its transverse mode is a multimode and an efficiency of coupling with a single mode waveguide and a single mode fiber is low, there have been problems in application to high-coherence devices. An oscillation mode of a semiconductor laser can be fixed to a single mode by feeding back beams projected from the wide stripe semiconductor laser to an active layer of the semiconductor laser, after having the beams permeate a mode transducer and a wavelength selecting filter.

Compact wavelength monitoring by lateral outcoupling in wedged photonic crystal multimode waveguides

Abstract: A device concept for laterally extracting selected wavelengths from an optical signal traveling along a waveguide, for operation in metropolitan area networks, is presented. The signal on the fundamental mode of a multimode photonic crystal waveguide is coupled to a higher-order mode, at a center frequency that spatially depends on the slowly varying guide parameters. The device is compact, intrinsically fault tolerant, and can split any desired fraction of the signal for monitoring purpose. Characterizations by the internal light source technique validate the optical concept whereas an integrated device with four photodiodes qualifies its potential with respect to real-world applications.

Large flattened mode tuned cladding photonic crystal fiber laser and amplifier

Abstract: A modified photonic crystal fiber yielding a higher peak power for a given maximum intensity. The multi-mode signal core has a depressed index of refraction that pushes the mode distribution to the core edges while a pattern of larger air holes is used to flatten the mode distribution. The core is further surrounded by tuned cladding elements defined by a pattern of smaller air holes that cause loss in all of the core modes except the fundamental while maintaining robust guiding of the fundamental mode.

Gaussian-mode analysis of waveguide-enhanced Kerr-type nonlinearity of optical fibers and photonic wires

Abstract: The Gaussian approximation of waveguide modes is used to analyze a Kerr-type nonlinearity of an optical fiber. This analysis gives physical insights into the behavior of the optimal transverse size of a waveguide providing the maximum enhancement of nonlinear-optical processes as a function of the core-cladding refractive-index step and the radiation wavelength. We show that, for the maximum enhancement of nonlinear-optical processes in a fiber where only the core is nonlinear, the effective mode area needs to be slightly larger than its minimum value. In the case of fused-silica fibers, the maximum enhancement of nonlinear-optical processes is achieved with fiber core diameters that are less than the radiation wavelength.

Temperature sensitivity of a long-period waveguide grating in a channel waveguide

Abstract: We present a simulation of the temperature sensitivity of the resonance wavelength of a long-period waveguide grating (LPWG). We find that the temperature sensitivity of an LPWG in a channel waveguide is proportional to a modal dispersion factor that depends sensitively on the size of the core of the waveguide. Measurements with experimental polymer LPWGs agree well with the simulation results. The dimension of the waveguide core is an effective parameter for the control of the thermal characteristics of an LPWG in a channel waveguide.

Apparatus, method, and computer program product for testing waveguided display system and components

Abstract: An apparatus and method for a waveguided switching matrix. A method of operating the switching matrix including a plurality of arranged waveguides each having an associated influencer structure for independently influencing an amplitude-effecting attribute of radiation propagating through a corresponding waveguide wherein the attribute includes a first mode for an “OFF” propagation mode with an exit amplitude substantially extinguished level and a second mode for an “ON” propagation mode with the exit amplitude at a substantially fully illuminated level, includes: a) establishing an “OFF” characteristic for the amplitude-effecting attribute to set the first mode; b) setting an “ON” characteristic for the amplitude-effecting attribute that does not match the second mode and establishes an intermediate propagation mode between the OFF propagation mode and the ON propagation mode; and c) adjusting a second attribute of radiation propagating through the waveguide so that the exit amplitude in the intermediate propagation mode substantially equals the fully illuminated level.

A refractometric sensor using index-sensitive mode resonance between single-mode fiber and thin film amorphous silicon waveguide

Abstract: A waveguide structure consisting of a side-polished single-mode polarization maintaining (PM) fiber and amorphous silicon film on top was applied for “in situ” control of the amorphous silicon deposition process. The changes in the fiber output intensities resulting from the film thickness growing were measured. The interaction of the fiber mode and TE0 and TE1 planar waveguide (PWG) modes was investigated. The applicability of the structure as a high sensitive refractometric sensor element was demonstrated.

System and Tapered Waveguide for Improving Light Coupling Efficiency Between Optical Fibers and Integrated Planar Waveguides and Method of Manufacturing Same

Abstract: A tapered waveguide optical mode transformer (20) includes a tapered core formed on a planar substrate structure (16). To vertically taper the core (21), steps (22) are etched into the top surface of the core. The steps have depths and lengths along the optical axis of tapered waveguide that are selected to transform the optical mode characteristics of a desired optical fiber to the optical mode characteristics of a desired planar waveguide. The core can also be tapered horizontally to form a 2-D tapered waveguide. The tapered waveguide can be integrally included in planar lightwave circuits (PLCs) to reduce light coupling losses between optical fibers and the PLC waveguides.

Optical fiber transmission system with increased effective modal bandwidth transmission

Abstract: A multi-mode optical fiber link is described. The multi-mode optical fiber link includes a first spatial mode converter that is coupled to a first single mode optical fiber. The first spatial mode converter conditions a modal profile of an optical signal propagating from the single mode optical fiber to the first spatial mode converter. A multi-mode optical fiber is coupled to the first spatial mode converter. A second spatial mode converter is coupled to an output of the multi-mode optical fiber and to a second single mode optical fiber. The second spatial mode converter reduces a number of optical modes in the optical signal. Both the first and the second spatial mode converters increase an effective modal bandwidth of the optical signal.

Optical fiber sensor based on retro-reflective fiber Bragg gratings

Abstract: A retro-reflective sensor for sensing mechanical, chemical or temperature related information, is disclosed The sensor is formed of an optical waveguide suitable for use in-situ in a high temperature environment having a Bragg grating written into a core region thereof with short-pulsed electromagnetic radiation, said optical waveguide having a glass transition temperature substantially higher than that of silica Preferably the sensor is written into a length of sapphire fiber or within a zirconium waveguide Preferably the pulse duration of the short pulsed electromagnetic radiation is less than 500 picoseconds

Radio-frequency common-mode noise propagation model for power-line cable

Abstract: Electromagnetic-interference (EMI) radiation from a power-line communications (PLC) network has been a major concern for the widespread use of broadband PLC technology. It is also well known that the dominant radiation mode of the PLC network is common mode (CM) by nature. Therefore, for electromagnetic-compatibility planning purposes, knowledge of the CM noise propagation path of the power line in the frequency range of 1 to 30 MHz is essential to provide insight of EMI radiation emitted by the power line. Based on a two-current-probe measurement approach, the CM noise propagation model for a three-wire power-line cable can be derived and represented by an equivalent two-wire CM transmission line. The equivalent CM noise propagation model allows us to predict the CM noise current on the power line with reasonable accuracy. The model will serve as a valuable tool in the future to identify effective ways to suppress EMI radiation from the PLC network.

Differential sensitivity characteristics of tilted fiber Bragg grating sensors

Abstract: Fiber Bragg gratings with grating planes tilted at small angles relative to the fiber axis couple light both to backward propagating core modes and cladding modes The resonant wavelengths for these mode couplings depend differentially on external perturbations Using the core mode back reflection resonance as a reference wavelength, the relative shift of the cladding mode resonances can be used to selectively measure perturbations affecting the region outside the cladding independently of temperature We have measured a relative wavelength shift lower than 04 pm/degree in conventional single mode fiber while the sensitivity to external changes in refractive index can be larger than 300 pm per % index change Experimental results on the bending selective sensitivity (relative to uniform axial strain) are also reported

Waveguide mode converter based on two-dimensional photonic crystals.

Abstract: We propose and numerically analyze a novel mode converter based on two-dimensional photonic crystal waveguides with square arrays of cylindrical dielectric rods in air. The mode converter uses small perturbation defects to decouple various modes in the multimode waveguide, thereby permitting propagation of only one mode at any given frequency, which permits one-to-one mode conversion without exciting unwanted modes. The mode converter can efficiently convert a TM0 mode supported in a single-mode photonic crystal waveguide into a TM2 mode supported in the multimode waveguide that is laterally coupled to the single-mode waveguide section for a wide wavelength range. Influences of different sizes and positions of perturbation rods on the band structure of the multimode waveguide are studied.

Waveguide substrate and high-frequency circuit module

Abstract: A waveguide substrate has an improved signal conversion characteristic to a cavity waveguide because it has a structure that is less affected by a manufacturing errors occurring at a portion leading the signal to the cavity waveguide. The waveguide substrate has a converting part (3B) provided at a position shutting off an end of the waveguide (3A), waveguide shutting-off conducting posts (33c) penetrating a dielectric plate (31) to electrically conduct between conductor layers (32a,32b) on both surfaces of the dielectric plate, and two slit-like regions (34a,34b), in which no conductor layer is formed, arranged in parallel one upstream and one downstream, respectively, with respect to a direction of propagation of a high-frequency signal from the waveguide toward the waveguide shutting-off conducting posts.

Stratified waveguide grating coupler for normal fiber incidence.

Abstract: We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that ?72% coupling efficiency is possible for fiber (core size of 8.3??m and ?=0.36%) to slab waveguide (1.2-?m core and ?=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.

Adiabatic mode multiplexer for evanescent- coupling-insensitive optical switching

Abstract: A novel adiabatic mode multiplexer enables a 2 x 2 optical switch whose operation does not depend on accumulated phase due to evanescent coupling between waveguides. The adiabatic mode multiplexer has a negligible insertion loss over C+L bands and modal cross talk better than -40 dB for any polarization state. Mode multiplexing is achieved by adiabatic transition from the fundamental mode of the single-mode waveguide to the higher mode of the multimode waveguide. Experimental measurement results for a device realized in silica-on-silicon technology are presented. By directly measuring the nonadiabatic transition probability, we show that the adiabatic mode multiplexer operates in the Landau-Zener regime.