Showing papers on "Equilibrium mode distribution published in 2009"

An investigation of the lowest-order transverse-electric (TE_1) mode of the parallel-plate waveguide for THz pulse propagation

Abstract: We experimentally and theoretically investigate the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide (PPWG) for the propagation of broadband THz pulses. We demonstrate undistorted THz pulse propagation via the single TE1 mode, solving the group-velocity-dispersion and spectral-filtering problems caused by the mode's low-frequency cutoff. We observe a remarkable counterintuitive property of the TE1 mode: its attenuation decreases with increasing frequency for all frequencies above cutoff. This phenomenon has not been observed with any other THz waveguide to date, and it can enable extremely low-loss propagation. We present a physical interpretation of this frequency-dependent behavior using a simple plane-wave description of the TE1 mode propagation. We also find that it is possible to achieve almost 100% coupling to the TE1 mode from a focused free-space Gaussian beam. In addition, using the above plane-wave analysis, we show how to mitigate the diffraction losses inherent to long path-length PPWGs via the use of transverse-concave plates.

Dispersion Relation and Loss of Subwavelength Confined Mode of Metal-Dielectric-Gap Optical Waveguides

Abstract: We have investigated the dispersion relation of a novel metal-dielectric-gap optical waveguide. This structure confines the optical field strongly in the gap region between metals and dielectric materials, and its size can be reduced to less than the wavelength of the transmitted light. In addition, the propagation length of light extends much greater than that of the surface plasmon modes on metal surfaces. We show that this mode of propagation has a cut-off at zero wavenumber, and that it is hollow-waveguide-like for small wavenumbers, while it approaches a surface-plasmon-like mode for large wavenumbers. A typical propagation length at around the communication wavelength is 10-20 mum, and optical fields are confined into an approximately 100 times 200 nm2 cross section.

Gain Filtering for Single-Spatial-Mode Operation of Large-Mode-Area Fiber Amplifiers

Abstract: Gain filtering of higher order modes in large-mode-area fibers is an extremely robust method for providing diffraction-limited performance regardless of core diameter or input beam quality. Analytic calculations demonstrate that reducing the diameter of the gain dopants compared to the waveguide diameter produces differential gain that is higher for the fundamental mode than all other fiber modes at all saturation levels. Matching the gain dopant to the mode profile is not as beneficial as a simple step profile since the primary mechanism of gain filtering is to deny gain toward the edge of the waveguide where most of the higher order mode power is contained. Numerical simulations of multikilowatt fiber amplifiers with up to 100-mum-diameter cores show that gain filtering is extremely robust, providing 99% of the output power in the fundamental mode output with only 90% of the seed power in the fundamental mode. Even with poor seed launch with 50% of the power in the fundamental mode, gain filtering can provide up to 90% of the output power in the fundamental mode.

Diffraction-Limited Fundamental Mode Operation of Core-Pumped Very-Large-Mode-Area Er Fiber Amplifiers

Abstract: Diffraction-limited fundamental mode amplification is demonstrated in Er-doped fibers with mode areas ranging from 900 to 1800 mum2. The amplifiers are core-pumped with Raman fiber lasers with both signal and pump selectively launched into the fundamental mode. This scheme results in differential gain for the fundamental mode and stabilizes it against mode mixing caused by perturbations in the core. Gains that are greater than 30 dB are demonstrated from a single stage without significant amplified spontaneous emission. The low nonlinearity of the large mode areas enables amplification to high peak powers without resorting to unconventional microstructured or higher order mode fibers.

Statistics of normal mode amplitudes in an ocean with random sound-speed perturbations: cross-mode coherence and mean intensity.

Abstract: In this paper Creamer’s [(1996). J. Acoust. Soc. Am. 99, 2825–2838] transport equation for the mode amplitude coherence matrix resulting from coupled mode propagation through random fields of internal waves is examined in more detail. It is shown that the mode energy equations are approximately independent of the cross mode coherences, and that cross mode coherences and mode energy can evolve over very similar range scales. The decay of cross mode coherence depends on the relative mode phase randomization caused by coupling and adiabatic effects, each of which can be quantified by the theory. This behavior has a dramatic effect on the acoustic field second moments like mean intensity. Comparing estimates of the coherence matrix and mean intensity from Monte Carlo simulation, and the transport equations, good agreement is demonstrated for a 100-Hz deep-water example.

Influence of Mode Loss on the Feasibility of Grating-Assisted Optical Fiber Surface Plasmon Resonance Refractive Index Sensors

Abstract: In this paper, the influence of mode loss on the feasibility of grating-assisted optical fiber surface plasmon resonance (SPR) refractive index (RI) sensors is investigated. The loss of surface plasmon polarition (SPP) mode plays a key role in the design and implementation of such sensors. It is demonstrated through simulation that the grating length should be smaller than or comparable with the propagation length of SPP mode in order to achieve effective coupling. The loss of SPP mode is the severe limiting factor for the implementation of the grating-assisted SPR-RI sensors. More generally, in order to achieve effective mode coupling with the help of waveguide grating, the grating length is bounded by the shortest propagation length of the modes in lossy waveguides.

Characterization of a photonic crystal fiber mode converter using low coherence interferometry.

Abstract: The relative group delay of the different modes present in an all-fiber LP11 mode converter at a central wavelength of 750 nm is observed using low coherence interferometric imaging. We have simultaneously measured the relative group delay and computed the intensity and the phase distribution of the modes emitted from the mode converter end face using a Fourier technique, providing unequivocal identification of the modes involved.

Comparison of amplification in large area fibers using cladding-pump and fundamental-mode core-pump schemes.

Abstract: We compare the amplification of various modes in large mode area fibers when the pump is coupled into the fundamental core mode versus the standard cladding-pump scheme. Our simulations show that pumping in the LP01 core mode results in differential gain for the fundamental signal mode which suppresses the higher order modes and amplified spontaneous emission compared to the cladding pump scheme. This differential gain effect is predicted to increase with core size and may provide a path to scale fiber mode area to several thousand square micron.

An investigation of the mode characteristics of SOI submicron rib waveguides using the film mode matching method

Abstract: The mode characteristics of SOI (silicon-on-insulator) submicron rib waveguides are very different from those of micrometer-sized ones. Using the full-vector film mode matching method, we propose a simple criterion to determine whether a waveguide mode is guided or not. The single-mode condition for deep-etched waveguides is obtained using this criterion. We also obtain the inherent TM mode leakage and sharp cancelation effects due to TE–TM mode coupling in shallow-etched rib waveguides from numerical simulations, which agree well with the analytical results based on total internal reflection and interference theories.

Systems and techniques for generating cylindrical vector beams

Abstract: A technique is described for generating a cylindrically polarized beam, e.g., a radially or azimuthally polarized beam. An input optical fiber is provided that supports signal propagation in a fundamental LP 01 mode. A mode converter device converts the fundamental LP 01 mode into a higher-order LP 11 mode output that includes a linear combination of modes, including cylindrically polarized TM 01 and TE 01 modes and mixed HE 21 (even) and HE 21 (odd) modes. The LP 11 mode output propagates through a connected phase-engineered fiber having a refractive index profile that includes a steep refractive index step proximate to a peak amplitude of a mode intensity profile of the LP 11 mode, such that at least one cylindrically polarized mode has an effective refractive index that is sufficiently separated from those of the mixed modes to allow for coupling to the at least one cylindrically polarized mode with minimal cross-coupling.

Resonator fiber optic gyroscope (rfog) with reduced bias error from high order spatial modes

Abstract: Multiple resonator fiber optic gyroscope (RFOG) configurations comprising one or more mode filters inside the resonator are adopted to effectively suppress unwanted high order spatial modes which can be a significant source of gyro bias errors. The resonator comprises at least a loop fiber, either two or more in/out coupling elements, and connectors that link elements into a circulating loop. Directional elements may be used to separate output light from input light in some of the embodiments. In all embodiments, mode filters are placed in the resonator to guarantee that the light reaching the photodetector is filtered by at least one mode filter in the resonator at least once. The mode filters may contain both spatial mode filters (such as single mode fibers or waveguides) and polarization mode filters (such as polarizing elements) so that both spatial and polarization mode filtering can be implemented simultaneously.

Calculation of the coupling coefficient in step index glass optical fibers

Abstract: A method for calculating the coupling coefficient in step-index multimode optical fibers is verified for glass fibers by comparison to published data and to an analytical solution for the steady-state mode distribution. The coefficient that the method calculates is used to determine the state of mode coupling along the fiber, including the coupling length for achieving the equilibrium mode distribution when measurement of fiber characteristics (such as linear attenuation or bandwidth) becomes meaningful.

Intermediate nonlinear regimes of line-tied g mode and ballooning instability

Abstract: A theoretical framework has been developed to describe the nonlinear regimes of line-tied g modes in slab geometry and ballooning instabilities in toroidal configurations. Recent experimental observation and numerical simulations demonstrate a persistence of ballooning-like filamentary structures well into the nonlinear stage of edge localized mode (ELM) activity in H-mode plasmas. Our theory is based on an expansion using two small scale lengths, the mode displacement across magnetic flux surfaces and the mode width in the most rapidly varying direction, both normalized by the equilibrium scale length. When the mode displacement across the magnetic flux surface is much less than the mode width in the most rapidly varying direction, the mode is in the linear regime. When the mode displacement grows to the order of the mode width in the rapidly varying direction, the plasma remains incompressible to lowest order, and the Cowley–Artun regime is obtained. The detonation regime, where the nonlinear growth of the mode could be finite-time singular, is accessible when the system is sufficiently close to marginal stability. At higher levels of nonlinearity, the system evolves to the intermediate nonlinear regime, when the mode displacement across the magnetic flux surface becomes comparable to the mode width in the same direction. During this phase, the nonlinear growth of the mode in the parallel and perpendicular directions are coupled, and sound wave physics contributes to nonlinear stability. The governing equations for the line-tied g mode and the ballooning instability in the intermediate nonlinear regime have been derived. A remarkable feature of the nonlinear equations is that solutions of the associated local linear mode equations continue to be valid solutions into the intermediate nonlinear regime in a Lagrangian reference frame. This property has been confirmed in the full ideal MHD simulations of both the line-tied g mode in a shearless slab and the ballooning instability in a tokamak, and may help explain the growth and persistence of the filamentary structures observed in ELM experiments well into the nonlinear phase.

Equilibrium Mode Distribution and Steady State Distribution in Step-Index Glass Optical Fibers

Abstract: Using the power flow equation, we have examined mode coupling in a step-index multimode glass optical fiber. As a result, the coupling length at which the equilibrium mode distribution is achieved and the length of fiber required for achieving the steady-state mode distribution are obtained. These lengths are much longer for glass fiber than they are for plastic optical fibers. Our results are in good agreement with experimental results reported earlier.

Systems and technique for generating cylindrical vector beam

Abstract: PROBLEM TO BE SOLVED: To provide a system and a technique for generating cylindrical vector beams regarding an optical fiber device. SOLUTION: An input optical fiber coping with signal propagation in a fundamental LP 01 mode, and a mode converter device for converting the fundamental LP 01 mode into a higher-order LP 11 mode output that includes a linear combination of modes, including cylindrically polarized TM 01 and TE 01 modes and mixed HE 21 (even) and HE 21(odd) modes, are included. The LP 11 mode output propagates through a connected phase-engineered fiber index step proximate to a peak amplitude of a mode intensity profile of the LP 11 mode, such that at least one cylindrically polarized mode has an effective refractive index that is sufficiently separated from those of the mixed modes in order that coupling to the at least one cylindrically polarized mode is made possible with minimal cross-coupling. COPYRIGHT: (C)2009,JPO&INPIT

Non-birefringent cross-slot waveguide

Abstract: Silicon photonics is a rapidly advancing technology [1]. Silicon based slot-waveguides for high optical mode confinement of quasi-TE (vertical slot) and quasi-TM (horizontal slot) mode have been demonstrated independently [2,3]. These structures are highly birefringent, which limits their usability in photonic devices. We propose a non-birefringent structure based on slot waveguides. To present the idea, a symmetric cross-slot waveguide with horizontal and vertical slots with widths, w sh =w sv =60 nm, in a silicon dioxide environment (n SiO2 =1.46) is shown in Fig. 1a. The material of the rail is amorphous silicon (n a-Si =3.58) with height, w h =300 nm, and width, w r =120 nm. Also shown is the intensity distribution of the quasi-TE mode in the structure, simulated with the Film Mode Matching (FMM) method [4]. As expected, the quasi-TM mode has the same effective index as the TE mode and its mode field is confined into the horizontal slot.

Non-linear optical device

Abstract: A non-linear optical device includes a frequency-conversion waveguide and first and second input waveguides. The longitudinal axes of the input waveguides are inclined to that of the frequency-conversion waveguide such a first transverse mode is excited in the latter at the input frequency in operation of the device. The frequency-conversion waveguide supports a second transverse mode at an output frequency of the device, such that the phase velocity of the second transverse mode at the output frequency is substantially equal to that of the first transverse mode at the input frequency, thus providing phase-matching by balancing the effects of chromatic and modal dispersion.

Modeling of the polarization mode dispersion in the single mode optical fiber links

Abstract: The transmission high-speed links requires the control of the phenomenon of polarization mode dispersion because it can limit the bandwidth of the transmitted signal mainly for long distances. This article presents two used methods of modeling in order to calculate the PMD in the single mode optical fiber links and evaluate its influence on the propagated signal. The results obtained in the modeling have been compared with experimental results in order to validate the proposed methods of modeling.

Comparison of Methods for Calculating Coupling Length in Step Index Optical Fibers

Abstract: A recently reported function for calculation of the coupling length at which the equilibrium mode distribution is achieved in step-index plastic optical fibers is compared to a long established calculation method and to experimental findings. The recent function, while simpler to apply as it eliminates the need to numerically solve the power flow equation repeatedly for every case, is also more accurate for high numerical aperture (0.45 to 0.5) plastic optical fibers.

Mode competition in steady-state optical waveguide amplifiers

Abstract: A theory of monochromatic light propagation in a fiber amplifier is developed. It is shown that competition between coherent gain-guided modes is qualitatively different from interaction between incoherent beams. Spatial beating between modes limits the gain of the mode with lower input power even if the corresponding modal gain coefficient is higher. Depending on the input beam parameters, the amplifier output can be dominated by either a single guided mode or their combination. The theory is illustrated by analyzing the gain behavior of a single planar waveguide and arrays of two and three narrow planar waveguides.

Bandgap modes in a coupled waveguide array

Abstract: This work examines a waveguide array that consists of ten Nb2O5/SiO2 double layers and supports a 0.63-μm surface wave. The deposition of a Nb2O5 capping layer on top of the waveguide array enables a marked increase in the wave field intensity on its surface. The efficiency of surface-wave excitation in the Kretschmann configuration can be optimised by adjusting the number of double layers. We analyse the behaviour of the Bragg mode in relation to the thickness of the layer exposed to air and the transition of this mode from the second allowed band to the first through the bandgap of the system. In addition, the conventional leaky mode converts to a surface mode and then to a guided mode.

High power, strictly single transverse mode ytterbium-doped large mode area fiber amplifier

Abstract: The paper reports a high power, strictly single transverse mode ytterbium-doped large mode area fiber amplifier with backward pump scheme that produces near diffraction-limited, 2-ns shaped pulses with energies of 38µJ and peak powers in excess of 39kW at a repetition rate of 1Hz.

Study of complex mode converter for 3mm-wave complex cavity Gyrotron

Abstract: A complex mode converter consisting of waveguide and quasi-optical mode converters is proposed to utilize high mode-transforming efficiency of the former and compact size of the latter. Radiation patterns of quasi-optical mode converter with TE 0n modes as its feeds are got by numerical calculation method based on the theory of vector diffraction integral. These patterns show that the radiation pattern is more similar to the wanted Gauss pattern when the feed takes the mode TE 01 . So the conclusion is that the complex mode converter, transforming TE 0n mode to TE 01 mode by waveguide converter and then TE01 mode to good Gauss pattern, is helpful to hold higher converting efficiency and smaller size than waveguide or quasi-optical one alone.

Active fibre design with flattened fundamental mode profile

Abstract: Single spatial mode operation of a fiber laser leads to perfect output beam quality, which is important for industry. Increasing of the effective mode area allows for achieving higher laser power at the wave field intensity lower than the limit imposed by non-linear effects. However, the large mode area (LMA) fiber lasers are open to higher order mode (HOM) appearance. Single mode operation was demonstrated for core diameter up to 40 µm in conventional double-clad fibers [1], and up to 100 µm in photonic crystal fiber amplifiers [2]. Recently, the leaky channel fiber amplifier with a core diameter up to 53 µm was demonstrated [3]. The single mode operation in all these constructions is due to the highest modal gain and smallest loss of the fundamental mode (FM). Generally, the higher small signal modal gain is insufficient condition for stable single mode operation at high powers. The spatially non-uniform gain saturation leads to HOM excitation. This effect can be eliminated if the FM has flat top profile in the gain region [4]. Flattened FM constructions of cylinder and ribbon fibers were suggested in [5] as possessing higher efficiency for inversion consumption.