Segmented Waveguide Research Articles

Evanescent-mode applicators (EMA) for superficial and subcutaneous hyperthermia.

Evanescent-mode waveguide aperture applicators are proposed for hyperthermic treatments of superficial and subcutaneous tissues. They consist of air-filled waveguide segments which are working below the cutoff frequency, and therefore support only evanescent transverse modes. These are excited by radiators of suitable symmetry and configuration to produce modal heating fields of selected cross-section. This field is emerging from the waveguide active aperture and enters the tissue to be heated through an air gap on non-critical length. These devices work in a very large range of frequencies, and are extremely simple to manufacture even with a variety of cross-section size and shape, on account of their air-filled feature. This enables to obtain good heating field flexibility with improved penetration. Their operation is safe and practical also on irregular and curved tissue surfaces.

Simultaneous generation of UV and visible light in segmented KTP waveguides

We report on simultaneous generation of ultraviolet, blue, and green radiation in segmented KTP waveguides using two infrared input frequencies and combinations of second-harmonic and sum-frequency generation.

Distributed-Bragg-reflection properties of segmented KTP waveguides

Segmented KTP waveguides designed for quasi-phase-matched frequency doubling have a periodic variation in the refractive index that acts as a distributed Bragg reflector (DBR). We have measured reflectivity spectra for both TM and TE modes in various segmented KTP waveguides. A reflectivity as high as 70% was observed in a periodically poled KTP waveguide. Reflection bandwidths were typically 0.03–0.06 nm, depending on the length of the sample. The measured temperature tuning rate of the DBR wavelength was 0.017 nm/°C.

Modes of periodically segmented waveguides

Modal properties of periodically segmented waveguides (PSWs) are analyzed by using the beam-propagation method and coupled-mode theory. The modal effective indices in a PSW are shown to be equal to those of a continuous waveguide with an averaged Delta n. As a result, guided modes spread out, particularly those that are not strongly guided. The radiation loss is shown to oscillate with most of the waveguide parameters. In practical terms, this loss can be made negligible even for long periods. The results can be applied to the design of waveguide devices such as two-dimensional mode tapers and frequency-selective couplers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

27 MHz hybrid evanescent-mode applicators (HEMA) with flexible heating field for deep and safe subcutaneous hyperthermia.

A new class of low-frequency electromagnetic applicators for hyperthermic treatment of superficial and subcutaneous tissues is described. These applicators employ an air-filled waveguide segment which is operating below the cut-off frequency, the evanescent modes of which are energized by suitable exciters to produce model field components. Direct radiators are integrated into the waveguide to generate additional direct field components. All field components may be combined in different power level ratio, phase, and orientation, to provide a composite heating field exhibiting a large variety of field sizes, shapes, and penetration features. The composite field emerging from the waveguide aperture propagates within the tissue to be heated through a non-critical air-gap. These versatile heating devices appear of potential interest to heat a variety of deep and localized subcutaneous tissues to therapeutic temperatures without injury to access fat layers of substantial thickness.

Linear propagation characteristics of periodically segmented waveguides

Periodically segmented planar waveguides are modeled by using the rigorous modal method of diffraction gratings. The radiation loss, effective index, and electric-field strength distribution are analyzed for different segment duty cycles. It is shown that except for the low radiation loss and possible presence of stop bands, a segmented waveguide behaves much like a uniform waveguide. The theoretical calculation confirms the low radiation loss observed experimentally.

Detection of ferroelectric domain reversal in KTiOPO4 waveguides

Segmented optical waveguides in KTiOPO4 are fabricated by ion exchange of Rb/Ba for K using molten nitrate salts. The waveguides have been utilized to generate efficient second-harmonic generation (SHG) of blue light. Four techniques are described which demonstrate that the ion exchange can produce domain reversal in these waveguides, and quasi-phase matching is utilized to obtain blue light. The experiments involve surface SHG, electrostatic toning, selective etching, and piezoelectric measurements.

Sum-frequency generation in segmented KTP waveguides

Segmented KTP waveguides have been used to produce efficient type I sum-frequency generation. Two Ti-sapphire lasers, tunable between 730 and 1070 nm were used as pump sources and up-converted radiation near four wavelengths between 390 and 480 nm was obtained. More than 2 mW of blue light was generated with 100 mW at each of the two fundamental wavelengths, which is 2–3 orders of magnitude higher output power than previously reported for guided-wave sum-frequency generation. Tunability up to 3 nm was achieved for the up-converted light and an increased efficiency measured at the degeneracy point.

Comment on ‘‘Mechanism for efficient blue second-harmonic generation in periodically segmented waveguides’’

Comment on ‘‘Mechanism for efficient blue second-harmonic generation in periodically segmented waveguides’’

Segmented proton exchange waveguide on LiNbO3 for SHG using simultaneous modulation of the indices and nonlinear coefficients

ABSTRACTWith proton exchange on LiNbO3 or LiTaO3 it is possible to fabricate waveguides presenting simultaneous modulation of the refractive indices and nonlinear coefficients. This allows us to realize a new kind of Quasi Phase Matching, and we will describe the first experimental results.

Mechanism for efficient blue second-harmonic generation in periodically segmented waveguides

We present a theoretical investigation of the mechanisms responsible for the recently reported efficient type I blue second-harmonic generation in periodically segmented KTiOPO4:Rb,Tl waveguides. In these guides, grating-assisted phase matching of second-harmonic light between 390 and 480 nm has been achieved. 5-mm-long guides give output powers on the order of 1 mW at around 425 nm with fundamental powers in the 50–100 mW range. We show that such high efficiencies can be expected from strongly perturbed guides through terms representing the modulation of phase mismatch and mode size in addition to the nonlinear susceptibility and refractive index modulations.

Efficient type I blue second-harmonic generation in periodically segmented KTiOPO4 waveguides

We report type I phase-matched blue second-harmonic generation from periodically segmented channel ion-exchanged waveguides in KTiOPO4 with output wavelengths from 0.38 to 0.48 μm and efficiencies exceeding 50%/W cm2. Evidence is presented suggesting that these efficiencies result from ferroelectric domain reversals induced by waveguide fabrication. Waveguide structure, wavelength, and output power characteristics are presented.

Optimization of strongly guiding semiconductor rib waveguide Y-junctions

The three-dimensional split-step fast Fourier transform beam propagation method is used to demonstrate that losses associated with a strongly guiding semiconductor rib waveguide Y-junction can be substantially reduced by adding an optimized length of an initial straight broad waveguide segment. It is shown that the resulting transmission is relatively insensitive to the exact waveguide dimensions and in particular to the geometry of the Y-junction vertex.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Balanced phase matching in segmented KTiOPO4 waveguides

A new phase matching technique is described which is based on phase mismatch balancing in crystal or waveguide segments that are short compared to their respective coherence lengths. The technique is demonstrated experimentally in a mixed bulk crystal/waveguide structure in KTiOPO4, resulting in a second-harmonic generation conversion efficiency of 15%/W/cm2 at 1.064 μm, close to the theoretical maximum. We show that this technique can significantly broaden processing latitude for fabricating practical nonlinear optical waveguide devices.

Analysis of single-mode optical Y-junction by the bounded step and bend approximation

An approximate method for analyzing planar optical Y-junctions is presented. The junction structure is replaced by a discrete succession of bounded waveguide segments, and the presence of waveguide bends is assumed. Using this approximation, single-mode symmetrical Y-junctions operated as power dividers are investigated numerically. It is shown that strong mode conversion occurs between the guided and radiation modes of the local normal mode. A perturbation analysis shows that reflected fields are negligible. Reliable values of the transmitted power are obtained. >

Phase-conjugation suppression of the phase noise during propagation of giant laser pulses in an optical fiber

An experimental study was made of the suppression of phase noise during propagation of giant ruby laser pulses in short (up to 0.1 m) multimode fiber waveguides. A segment of a fiber waveguide, in which stimulated Brillouin scattering took place, acted as a mirror performing phase conjugation. The dependences of the parameters of the corrected signal on the lengths of both waveguide segments were determined.

Scattering of a dipole field by two coaxial segments of finite length circular waveguide

Scattering of a dipole field by two coaxial segments of finite length circular waveguide

Passive Ti:LiNbO3 channel waveguide TE-TM mode splitter

A passive compact TE-TM mode splitter has been demonstrated using Ti:LiNbO3 channel waveguides. Its operation is based on the interference between the two guided modes of a double-mode waveguide segment connecting single-mode input and output guides. A device made using one set of design parameters has splitting ratios of 12 dB for both polarizations. Another device made using different parameters has a splitting ratio of 17 dB for TM modes. The devices are capable of low-loss, broadband operation.

Fiber-optic communication line with multimode waveguides for data transfer over distances up to 8 km

A description is given of a fiber-optic communication line with multimode waveguides up to 8 km long. An experimental investigation was made of the influence of the waveguide parameters, conditions for their excitation, and of joints of the various segments of the waveguides on the attenuation and pass band of the whole line. The characteristic mode coupling length of the investigated waveguides (Lc=0.9±0.1 km) was determined together with the parameters representing the dependence of the pulse broadening on the waveguide length in the linear and squareroot parts of this dependence. The pass band of the 8-km line was 6.5 MHz and it agreed well with the value calculated from the parameters of the individual waveguide segments.