Fundamental Mode Propagation Research Articles

Oversized microstrip line as differential guided-wave structure

An oversized microstrip line as a differential guided-wave structure is proposed. For most guided-wave structures such as microstrip line and waveguide, the signal transmission will be in the form of fundamental mode propagation, where higher-order mode propagations should be avoided. In this reported work, the first higher-order propagation mode (TE10) of a microstrip line is excited. Its guide-wave characteristic was simulated and measured. The oversized microstrip line supporting TE10 propagation mode is proposed for the microwave and millimetre-wave differential circuit and system designs.

Single-mode propagation in triangular tube lattice hollow-core terahertz fibers

The mode properties of hollow-core fibers with a cladding formed by a periodic arrangement of dielectric tubes in a triangular lattice for THz applications are numerically investigated. The fiber supports a high number of modes. Effectively single-mode operation can be obtain by reducing the core size, but at the expense of high fundamental mode propagation loss. Single-mode propagation can be obtained by exciting the fiber with a linear polarized gaussian beam with proper spot size.

Diffraction limited amplification of picosecond pulses in 1170μm^2 effective area erbium fiber

Robust fundamental mode propagation and amplification of picosecond pulses at 1.56 microm wavelength is demonstrated in a core-pumped Er fiber with 1170 microm2 effective area. Record peak power exceeding 120 kW, and 67 nJ pulse energy are achieved before the onset of pulse breakup. A small increase in input pulse energy results in a temporal collapse of the pulse center to 58 fs duration, with peak powers approaching 200 kW.

Optical poling in germanium-doped microstructured optical fiber for visible supercontinuum generation

We report visible supercontinuum generation initiated by the second harmonic generation obtained in a germanium-doped microstructured optical fiber after optical poling processing. The visible spectral broadening is due to a cross-phase modulation effect between the generated IR solitons and the second harmonic (532 nm) of the 1064 nm pump wave. The 400-650 nm white-light emission is obtained on the fundamental propagation mode of the fiber.

NOVEL SLAB-COUPLED LiNbO3 WAVEGUIDE FOR NONLINEAR OPTICAL APPLICATIONS

ABSTRACT A new slab-coupled optical waveguide (SCOW) using lithium niobate (LiNbO3) crystals were designed and experimentally demonstrated. The new LiNbO3SCOW structure composes of a main guiding core and a coupled leaky slab laterally confinement by two etched drains. The SCOW supports the propagation of the single spatial mode (having positive propagation constant), and leaks all high-order modes into the slab (imaginary propagation constants). Significance of such SCOWs will be the availability of a large and circular core to support the fundamental mode propagation, which could largely reduce the possibility of waveguide damage because of high optical power density, and reduce coupling losses to adjacent components including high power near infrared (NIR) diode lasers. Such SCOWs can find wide applications in nonlinear optics.

Laser-assisted fabrication of new slab-coupled lithium niobate optical waveguide

Recently a new type of lithium niobate waveguide was suggested for potential nonlinear optic applications. The waveguide consists of a uniform large core and a leaky coupled slab for realizing a lateral optical confinement to support the fundamental spatial mode propagation. Inside the waveguide, the slab layer is required to have a refractive index slightly lower than that of the core, but higher than that of the substrate. Lithium niobate doped with magnesium oxides shows an increased refractive index that is dependent on the dopant's concentration. Therefore, in order to fabricate such waveguides, the pulsed laser deposition approach was used to study the growth of such composition-modified lithium niobate as the slab layer. The as-grown films were characterized on its expitaxy, structure, and optical performance, via X-ray diffraction analysis, optical guiding experiment, etc.

Leakage channel optical fibers with large effective area

Leakage channel fibers, where few air holes form a core, can be precisely engineered to create large leakage loss for higher-order modes, while maintaining negligible transmission loss for the fundamental mode. This unique property can be used for designing optical fibers with large effective area, which supports robust fundamental mode propagation. The large air holes in the design also enable the optical fibers to be bend resistant. The principles of design and operation regime are outlined, demonstrating the potential of this approach for optical fibers that propagate a fundamental mode in core diameter exceeding 100 μm. Performance of a fabricated passive leakage channel fiber, an ytterbium-doped double-clad leakage channel fiber, and an ytterbium-doped polarization-maintaining double-clad leakage channel fiber are also discussed.

Bandwidth-enhanced multimode fiber

By designing multiple single-mode cores across the multimode fiber core area, this new multimode fiber can support only the fundamental mode or fewer low-order modes propagation to some extent. Therefore, the modal dispersion of the multimode fiber has been suppressed greatly, and the bandwidth is enhanced 6.5 times.

Bound modes of holey slab waveguides

The conventional leaky mode description of so-called fundamental mode propagation along single-mode holey fibers can be explained physically in terms of a dominant bound mode, guided predominantly by the central core-hole structure, together with a number of lower-amplitude cladding-like bound modes. A simple slab model is used to demonstrate the concept.

Determination of single mode condition in dielectric rib waveguide with large cross section by finite element analysis

The single mode condition in large cross section rib waveguides is of great interest because almost every kind of active and passive integrated optoelectronic device or sensor is designed to sustain only the fundamental mode of propagation for better matching with optical fibers. In this paper we present a criterion to determine the single mode condition for a large cross section rib waveguides, by comparison between the numerical solutions found with Neumann boundary conditions and Dirichlet boundaries conditions applied when solving the eigenvalues problem.

Demonstration of fundamental mode only propagation in highly multimode fibre for high power EDFAs

The use of short lengths of large core phosphate glass fibre, doped with high concentrations of Er or Er:Yb represents an attractive route to achieving high power erbium doped fibre amplifiers (EDFAs) and lasers (EDFLs). With the aim of investigating the potential of achieving diffraction limited output from such large core fibres, we present experimental results of fundamental mode propagation through a 20cm length of passive 300μm core multimode fibre when the input is a well-aligned Gaussian beam. Through careful control of fibre geometry, input beam parameters and alignment, we measured an output M2 of 1.1±0.05. The fibre had a numerical aperture of 0.389, implying a V number of 236.8. To our knowledge, this is the largest core fibre through which diffraction limited fundamental mode propagation has been demonstrated. Although the results presented here relate to undoped fibre, they do provide the practical basis for a new generation of EDFAs and EDFLs.

Curved quantum waveguides in uniform magnetic fields

A theoretical study of a planar electronic waveguide with a uniformly curved section in the perpendicular homogeneous magnetic field $\mathbf{B}$ is presented within the envelope function approximation. Utilizing analytical solutions in each part of the waveguide, exact expressions are derived for the scattering and reflection matrices and for the transcendental equation defining bound-state energies. It is shown that in the magnetic field a propagation threshold in the continuously curved channel is always smaller than its counterpart for the straight arm which means that bound states in the uniform magnetic field always exist. Their energies do not depend on the direction of the field, and at high magnetic intensities they approach the lowest Landau level. For the transport in the fundamental mode an interaction of a quasibound level split off from the higher-lying threshold as a result of the bend, with its degenerate continuum counterpart, causes a dip in the transmission. In the magnetic field, contrary to the field-free case, conductance in the minimum ${G}_{\mathit{min}}$, generally, ceases to be zero. It is shown that growing magnetic fields cause ${G}_{\mathit{min}}$ to saturate to $2{e}^{2}∕h$ which means that a quasibound level formed as a result of the bend is completely dissolved by the increasing $\mathbf{B}$; however, this transformation is very different for the different bend angles and radii. In particular, quasibound states of the fundamental propagation mode survive stronger fields for the smaller bend angles which is explained by the larger total magnetic flux through the curved section where these levels are formed. Since a magnetic length ${l}_{B}={(\ensuremath{\hbar}∕eB)}^{1∕2}$ is inversely proportional to the square root of $B$, states for the waveguide with a smaller radius also survive stronger fields, and their asymptotic approach to the dissolution possesses nonmonotonic ${G}_{\mathit{min}}$ dependence on the magnetic field with minimum conductance again reaching zero for some special values of $B$. Vortex structure of the currents flowing in the waveguide near the resonance is strongly affected by the field. In particular, small magnetic intensities change zero-field vortices in the straight arms into the magnetic antivortices which correspond to the interacting with each other surface currents flowing along opposite walls of the channel. Increasing the magnetic field suppresses the formation of the vortices pushing currents to the outer (inner) walls in the straight (bent) section. For fields larger than the saturation magnetic intensity, the only consequence of the bend is a strong surface current near the convex wall of the bend, and the electronic flow along the junctions between straight and curved parts.

Compact integrated star coupler on silicon-on-insulator

Asymmetrical rib waveguide bends on silicon-on-insulator were applied to the design of a compact integrated 39 × 39 star coupler. With bend radius of 500 μm, the effective chip size of the coupler is nearly 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The average excess loss as low as 1.3 dB was achieved at wavelength 1.55 μm. This bend configuration is not single-mode, but by appropriate design, the fundamental mode propagation can be achieved. It can be used in arrayed waveguide grating devices based on SOI.

Modeling gain-medium diffraction in super-Gaussian coupled unstable laser cavities

The diffractive effects of a single laser rod in an unstable super-Gaussian coupled cavity are modeled for a range of cavity configurations, with an intracavity, zero-thickness aperture. After fundamental mode propagation through a maximally flat output coupler, beam quality (M2) and far-field power loss values are related. Beam quality is most sensitive to cavity magnification and aperture Fresnel number, both correlated to the aperture-equivalent Fresnel number. In contrast, variation of M2 with aperture position is sufficiently conservative to predict the intensity profile of a solid-state laser with a typical gain length, in good agreement with experimental data.

Intermode conversion in a near-field optical fiber probe

The propagation of light in metal-coated fiber probes used in scanning near-field optical microscopy is studied computationally by means of the cross-section method. In contrast to previous work focusing on the behavior of the HE11 mode in the vicinity of the end aperture of the probe, we trace the propagation of the incident fundamental mode of the optical fiber through the tapered probe. The transformation of the fiber mode into the probe modes, intermode conversion, and modes cutoff are consistently calculated. In particular, it is shown that in the conventional gradual tapered probe only a few percent of the input power are transformed into the HE11 mode which has the smallest cutoff radius. In addition, an easily manufacturable modification of the shape of the probe’s tip that can provide a tenfold enhancement in optical near-field intensity has been proposed.

Design of Ti:LiNbO3 Single Mode Waveguides for 0.83μ/m

A channel waveguide is the basic structure of any integrated optical device for Opto-eletronic applications. The equations governing a channel guide are quite complex and are not available in closed form [1,2]. The design aspect of channel guides following Marcatilli method has been reported by K R Suresh et al [3]. In this paper we have applied a different procedure to design the channel waveguides using effective index method and ray approximations which appreciably simplify the calculations. From the published experimental data on Ti diffusion in LiNb03, temperature dependent diffusion constants are extracted. Diffused Ti concentration profiles and corresponding index profiles have been worked out using error functions in depth and width directions. A set of values of sputtered Ti film thickness and guide width for the permissible index difference to support fundamental mode propagation has been computed. Simple steps are described, with examples, to calculate the propagation constant and the mode angle from eigen value equations.

Highly efficient full-vectorial integral equation solution for the bound, leaky, and complex modes of dielectric waveguides

A full-vectorial contour integral equation analysis of the natural modes of dielectric waveguides (DW) of arbitrary cross section is presented. The Galerkin method, together with the Analytical Regularization procedure, is applied to discretizing and solving the eigenvalue problem. This ensures the fast convergence and superior accuracy of the numerical algorithms. The waveguide cross section is characterized by a parametrical curve defining its contour, with a limited curvature at each point. This avoids the singularity points at corner regions and provides accurate results, even for waveguides with virtually sharp corners. Both fundamental and higher order mode propagation characteristics are studied in the bound, leaky, and complex regimes. Numerical results consistent with other theories and experimental data are presented for a wide range of practical dielectric waveguides that demonstrate the efficiency, accuracy, and versatility of the method developed. Finally, the technique is applied to model a fused fiber coupler.

Temperature gradients and flow-meter performance

A quantitative treatment of the influence of radial temperature gradients on flow-meter performance is described for the first time. It is shown that large flow-meter errors can be induced due to temperature gradients in the case of fundamental mode propagation (several percentage error values are possible using typical parameter values for water as medium at low flow values corresponding to laminar parabolic flow profiles). It is also found that decreasing the cylinder radius or ultrasound frequency are effective ways to diminish the influence of temperature gradients on flow-meter performance besides the obvious choice of reducing temperature gradients by thermal insulation of the flow-meter tubes and connecting tubes.

A DISCRETE APPROACH FOR ANALYSIS OF SOUND TRANSMISSION IN PIPES COUPLED WITH COMPACT COMMUNICATING DEVICES

A new discrete method is presented for acoustic modelling of pipes that are coupled by compact devices. This is based on an approximate source analogy that allows the communicating devices to be modelled as continuous two-port elements. One of the areas in which this method finds application is the acoustic analysis of mufflers with multiple perforated pipes. For this case, the present formulation assumes fundamental mode propagation for simplicity. Also presented in the paper are applications in which perforate holes are modelled as continuous visco-thermal pipes, the results of which are compared with the lumped parameter modelling of perforate impedance. In general, the present method is computationally simpler and more versatile than the more commonly known distributed parameter method.

High-performance transitions for overmoded operation of elliptical waveguides

Overmoded elliptical waveguide systems are shown to provide considerable lower insertion loss compared with standard types using the frequency band with exclusive TE/sub c11/ fundamental mode propagation. The considerations for the overmoded waveguide operation are outlined. A novel transition design based on transformer steps with curved cross sections and containing means for higher order mode control is established for interfacing the oversized elliptical cross section with the standard rectangular waveguide ports. The accurate determination of the transition structure maintaining high performance demands is obtained by a suitable computer-aided-design procedure. The overmoded waveguide approach, including the transition design, is proven by computed and experimental results for a millimeter-wave design.