Mode transformation in very oversized circular waveguides

Very oversized waveguides used in FIR lasers have an unexpected electromagnetic behavior. For high frequencies, situated between IR and FIR, the classical equations are no longer valid.     

Matching of a Gaussian beam into hollow oversized circular waveguides

A Gaussian beam entering an hollow oversized circular waveguide of dielectric or low-conductivity wall material is considered: a simple formula giving the optimal beam charac-teristics for the lowest losses in the guide is derived from an optical theory. This optimization depends on the geometry and the wall material of the guide. The formula is valid for small beam diameters at the guide entrance and for low losses inside the guide, which are conditions corresponding to usual cases. Physical interpretations are given. Comparisons with some experimental results show a good agreement with the theory. These results are of interest for waveguides lasers and waveguide transmission lines.     

Field configurations and propagation constants of modes in hollow rectangular dielectric waveguides

The field configurations and propagation constants of linearly polarized low-loss modes in hollow dielectric waveguides with rectangular cross section are computed analytically under the assumption that the dimensions of the rectangular cross section are much larger than the free-space wavelength. The attenuation constant is then equal to the sum of the corresponding TE- and TM-mode attenuation constants in parallel-plate waveguides.     

Modes and attenuation constants in circular hollow waveguides withsmall core diameters for the infrared

The mode structures and attenuation constants in circular hollow waveguides are evaluated numerically based on the exact characteristic equations. Mode properties, which are strongly dependent on waveguide materials, and attenuation constants are discussed when the core diameter becomes small. Special modes are also analyzed in oversized metallic waveguides which approach very familiar modes in perfectly conducting cylindrical waveguides when the core diameter becomes small     

Optical propagation of the HE11 mode and Gaussian beams in hollow circular waveguides

The propagation of the HE₁₁ mode and Gaussian beams in hollow oversized circular waveguides is analyzed using optical theories. Different types of waveguides are considered : hollow dielectric or conducting waveguides, dielectric-lined waveguides, corrugated waveguides. General formulas are derived which give the power transmission through these different guides. The best wall materials and structures are determined from a comparison of the waveguide transmissions, at the infrared and millimeter wavelengths. The question of the coupling between the HE₁₁ mode and Gaussian beams is discussed and from a review of coupling coefficients derived before, an optimum value is pointed out. The problem of matching a Gaussian beam into circular waveguides in order to achieve the maximum power transmission is analyzed. These results are of interest for infrared lasers or waveguide applications and for Electron Cyclotron Wave (ECW) systems at the millimeter wavelength.     

Infrared surface waves in circular hollow waveguides with smallcore diameters

An asymptotic form of the characteristic equation that describes wave propagation at near-infrared wavelengths in small core hollow circular waveguides is developed. Analytic solutions for the transverse and axial propagation constants are obtained. These demonstrate the transition of the TE₁₁ and TM₀₁ modes to surface waves as the guide radius is increased to values much greater than at cutoff. Relative power density distributions illustrating these mode transitions are shown     

High-power TE11 and TM11 circular polarizers in oversized waveguides at 70 GHz

Elliptical deformation of oversized, smooth-wall circular waveguides can produce choosable elliptical or circular polarization from a linearly polarized TE11 or TM11 mode used as intermediate linearly polarized modes in TEO1 to HE11 mode conversion sequences in electron cyclotron resonance heating (ECRH) of magnetically confined thermonuclear fusion plasmas with high-power gyrotrons. Mode coupling in elliptically distorted overmoded circular waveguides has been studied theoretically and experimentally in order to optimize TE11 (and TM11) polarizers (I.D.=27.79 mm) for the 1 MW/70 GHz long-pulse (3s) ECRH system on the Garching Stellarator W VII-AS. Coupling coefficients for ellipticity coupling of non-degenerate modes are given (coupled-mode differential equations formalism). The polarization converters essentially consist of smooth-wall circular waveguides which are gradually squeezed. A sine-squared function of the length coordinate is used to get an almost elliptical crosssection in the middle and circular cross sections at both ends. Arbitrary elliptical polarization states can be generated introducing an extremely low level (<<1%) of undesired spurious modes. Well defined differential phase characteristics have been achieved.     

Waveguide-loss evaluation in circular hollow waveguides and its ray-optical treatment

A ray-optical representation of the attenuation constants in circular hollow waveguides characterized by a surface impedance and admittance has been derived from the characteristic equation based on the boundary-value problems. It is shown that in the first order of approximation, the attenuation constants of all modes including hybrid modes can be derived as if the corresponding rays were meridional. The effect of the Goos-Hänchen shift is also considered, which is significantly different from that in an intuitive formula given in circular dielectric waveguides.     

Phase constants and scattering electromagnetic properties of circular dielectric waveguides

A variational method, based on the telegraphist's equations, is used to calculate the dispersion and scattering characteristics of circular dielectric waveguides which arc enclosed by perfectly conducting walls of rectangular cross-section. Our propagation solutions are compared with those obtained by other authors and the convergence of the method is evaluated. A procedure which combines this variational method with the modal matching technique was implemented in order to analyse multiple discontinuities in cyclindrical dielectric waveguides. To verify the usefulness and accuracy of the procedure, theoretical and experimental results were obtained for different dielectric guiding structures, connected by means of abrupt and gradual transitions. The experimental values show a good agreement with the theoretical ones.     

Mode content analysis from intensity measurements in a few cross sections of oversized waveguides

A method of mode content analysis in oversized waveguides basing on intensity measurements in a few cross sections is suggested. The method is based on iterative synthesis of phase fronts of the radiation measured in a few cross sections of an oversized waveguide. The phase reconstruction makes it possible to calculate field distribution as well as the mode content at any place of the waveguide. The method was tested in experiments with circular and rectangular waveguides with impedance corrugation of the walls [8].     

Mode content analysis from intensity measurements in a few cross sections of oversized waveguides

A method of mode content analysis in oversized waveguides basing on intensity measurements in a few cross sections is suggested. The method is based on iterative synthesis of phase fronts of the radiation measured in a few cross sections of an oversized waveguide. The phase reconstruction makes it possible to calculate field distribution as well as the mode content at any place of the waveguide. The method was tested in experiments with circular and rectangular waveguides with impedance corrugation of the walls [8].     

Coupling between gaussian and guided modes at the input and output of oversized circular waveguides

The coupling between a gaussian TEMoo bearn with HE 1m modes for dielectric waveguides and TE1m and TM1m modes for metallic waveguides is analytically studied. An optimization method allows to know HE11 or TE11 modes create gaussian TEMmm bearns at the output of the waveguide.     

Effect of wall thickness on radiation from circular waveguides

An experimental study shows that the thickness of a circular waveguide wall greatly affects its radiation characteristics. The exact solution of Weinstein for infinitely thin walls appears in practice to be valid only when the wall thickness is less than 0.01 ?.     

Pump beam propagation in circular waveguides of optically pumped far-infrared lasers

A theory is presented describing the infrared pump beam propagation in circular waveguides of OPFIRLs. Based upon a ray representation of the Gaussian beam, the overall transport of pump power into the gas by nonlinear absorption and into the wall of the waveguide by partial reflections can be balanced. The theory was used to study the typical pumping conditions in OPFIRL and the main results are presented.     

Iterative calculation of complex propagation constants of modes inmultilayer planar waveguides

A Newton-Raphson complex root-finding algorithm, based on the thin-film transfer matrix analysis, is described. This second-order convergent algorithm, which requires only O(n) calculations per iteration, was presented to find the complex propagation constants of any mode in an n-layer planar waveguide. The number of calculations per iteration is reduced below those required by competitive schemes through application of the Cauchy-Riemann relations. The method demonstrates higher accuracy and improved efficiency over other iterative techniques, making this approach highly suitable for the analysis and design of planar structures consisting of a large number of layers     

Integrated hollow waveguides with arch-shaped cores

An optical waveguide is described that has a hollow arch-shaped core. Optical confinement for this structure is based on the antiresonant reflecting optical waveguide principle. The waveguides are built on a silicon substrate using a sacrificial etch technique with reflowed photoresist serving as the sacrificial material and producing the core's arch shape. Investigations of fabrication parameters are reported that allow for predicting a final arch-shaped geometry based on initial photoresist width and thickness. Optical mode guiding is demonstrated in an arch-shaped waveguide with a liquid core.     

High-power millimetre-wave mode converters in overmoded circular waveguides using periodic wall perturbations

This work reports on measurements and calculations (coupled mode equations) on the conversion of circular electric TE_0n gyrotron mode compositions (TE₀₁ to TE₀₄) at 28 and 70 GHz to the linearly polarized TE₁₁ mode by means of mode converter systems using periodic waveguide wall perturbations. Mode transducers with axisymmetric radius perturbations transform the TE_0n gyrotron mode mixture to the more convenient TE₀₁ mode for long-distance transmission through overmoded waveguides. Proper matching of the phase differences between the TE_0n modes and of lengths and perturbation amplitudes of the several converter sections is required. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TE₀₁ mode into the TE₁₁ mode which produces an almost linearly polarized millimetre-wave beam needed for efficient electron cyclotron resonance heating (ECRH) of plasmas in thermonuclear fusion devices. The experimentally determined TE_0n -to-TE₀₁ conversion efficiency is (98 ± 1)% at 28 and 70 GHz (99% predicted) while the TE₀₁-to-TE,, converter has a (96 ± 2)% conversion efficiency at 28 GHz (95% predicted) and (94 ± 2)% at 70 GHz (93% predicted) with ohmic losses included in each case. This paper also presents theoretical and experimental results on the two-step TE₁₆-to-TE₁₂-to-TE₁₁ mode conversion at 28 GHz by means of two periodically rippled-wall mode converters. The conversion efficiencies achieved are almost 92% and 95%, respectively. Similar converters might be used for transformation of the output modes of future high-frequency TE_1n gyromonotrons or 10 GHz gyro-klystron amplifiers into the TE₁₁ mode, which in turn can then be transformed by circumferentially corrugated or dielectrically coated mode transducers into the perfectly linearly polarized quasi-optical HE₁₁ hybrid mode. The efficiency of periodically modulated wall mode-converters can be considerably improved by proper re-matching of the phase difference between the two converted modes within the converter.     

Numerical method for the calculation of mode fields and propagation constants in optical waveguides

The scalar wave equation for optical waveguides with arbitrary refractive index profiles is solved using a numerically stable integration method. The mode fields and the corresponding propagation constants are determined iteratively. Results for waveguides based on different well-known fabrication techniques are shown and the design of a new sensitive integrated optical switch is proposed.     

Self-switching in hollow waveguides with a Kerrlike nonlinearpermittivity

The dispersion relations, stability, and excitation of the stationary nonlinear transverse electric (TE) waves guided by a nonlinear hollow waveguide are examined numerically. This waveguide consists of a self-focusing nonlinear film bounded by two identical linear claddings of the higher refractive index. Since a minimum power for guidance of the stationary waves always exists, the nonlinear hollow waveguide acts as a lower threshold device. A stability analysis using the beam propagation method shows that all the stationary TE_n modes, including the asymmetric TE₀ mode, are stable on the positively sloped branch (dP/dβ>0) of the nonlinear dispersion curve. The lower threshold device using the nonlinear hollow waveguide exhibits very sharp power-switching characteristics, and the required device length is fairly short     

Measurement of propagation constants related to material properties in high-bandwidth optical fibers

The material contribution to group index and material dispersion were measured in high-bandwidth graded-index optical fibers. A shuttle-pulse technique provided measurements of group index with precisions and accuracies of 0.1 and 0.2 percent using 5 m lengths of optical fiber. Material dispersion in fibers was measured over the0.8-0.9 mum wavelength region using different wavelength, short-pulse laser diodes. The influence of material dispersion on fiber bandwidth measurements was evaluated for laser diode sources. Limitations arising from source linewidth were experimentally determined from measurements on a fiber with high microbending enhanced bandwidth.