Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator

Single-mode X couplers and three-dimensional waveguides are fabricated in transparent glasses by use of an unamplified femtosecond laser generating energies of up to 100 nJ. Changing fabrication parameters such as power and scanning speed permits creation of waveguides with a wide range of structures and refractive-index difference. Optical coherence tomography shows large refractive-index changes of up to ~10(-2) in the waveguides; these changes are consistent with guided mode analysis.     

Single-mode optical waveguides fabricated in LiNbO3 by multienergy C implantations

Single-mode waveguides in LiNbO₃ are demonstrated by use of prism coupling method. The waveguides are fabricated by three different energies and single energy C²⁺ implantations at the equal total doses, respectively. Dark modes and propagation loss are measured by use of prism coupling and moving fiber methods, respectively. Damages produced by implantation are measured by RBS/channeling technique. The waveguide structures are investigated in two different implantation cases. The results of analysis indicate that the heavy ion-implanted waveguides are still defined by synergetic characteristics from both the raised-index region and the low-index barrier. The broadened barrier from multienergy implantation is demonstrated to play a significant role in reducing propagation loss.     

Comparative Studies of Er3☎ Ions in LiNbO3 Waveguides Produced by Different Methods

We compare the optical properties of Er^3? in LiNbO₃ waveguides produced by different techniques and find by confocal luminescence microscopy characteristic differences in the excitation and emission transitions. Besides a small redistribution among the regular defect sites, essentially no direct Er^3?—Ti^4? interaction can be observed in Ti^4? diffused waveguides. However a significant shift in transition energies is found, which increases with Ti^4?concentration. Based on earlier results we associate this shift with a decrease in the intrinsic electric field. In addition to a similar shift, we find in Zn^2? diffused waveguides new defect types which suggest a change in direct environment. Finally, in annealed proton exchange waveguides we find no interaction effects at all.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Anomalous scattering,transport, and spatial distribution of X-ray fluorescence at the exit of polycapillary structures

The angular distribution of the X-ray intensity at the exit of microchannel plates at grazing incidence of monochromatic radiation on the walls of microcapillaries has been investigated. The angles and energies of the primary radiation quanta at which the synchrotron beam excites X-ray fluorescence propagating inside polycapillary structures have been determined. The angular dependences of the intensity distribution of X-rays transmitted through the microcapillaries have been studied theoretically and experimentally for energies corresponding to the region of anomalous dispersion near the L _2,3 absorption edges of silicon. The propagation of waves in hollow polycapillary waveguides, the excitation of X-ray fluorescence, and the X-ray diffraction at the exit of microchannel plates have been modeled mathematically. The mathematical model takes into account the presence of a transition layer on the microchannel surface.     

Trapped modes in finite quantum waveguides

The eigenstates of an electron in an infinite quantum waveguide (e.g., a bent strip or a twisted tube) are often trapped or localized in a bounded region that prohibits the electron transmission through the waveguide at the corresponding energies. We revisit this statement for resonators with long but finite branches that we call ??finite waveguides??. Although the Laplace operator in bounded domains has no continuous spectrum and all eigenfunctions have finite L ₂ norm, the trapping of an eigenfunction can be understood as its exponential decay inside the branches. We describe a general variational formalism for detecting trapped modes in such resonators. For finite waveguides with general cylindrical branches, we obtain a sufficient condition which determines the minimal length of branches for getting a trapped eigenmode. Varying the branch lengths may switch certain eigenmodes from non-trapped to trapped or, equivalently, the waveguide state from conducting to insulating. These concepts are illustrated for several typical waveguides (L-shape, bent strip, crossing of two strips, etc.). We conclude that the well-established theory of trapping in infinite waveguides may be incomplete and require further development for applications to finite-size microscopic quantum devices.     

Amplification of frequency-modulated soliton-like pulses in inhomogeneous optical waveguides with normal dispersion

The possibility of effective amplification of self-similar frequency-modulated pulses (FMPs) in longitudinally inhomogeneous active optical waveguides is studied. Peculiarities of the dynamics of parabolic pulses with a constant frequency modulation rate are considered. An optimal profile of variation of the group velocity dispersion was obtained in correspondence with optimal amplification of a similariton-like pulse. The use of FMPs in amplifying and longitudinally inhomogeneous optical waveguides with a correspondingly matched profile of normal dispersion of group velocities is shown to be capable of providing for an amplification of subpicosecond pulses up to energies above 1 nJ.     

Femtosecond laser direct writing of embedded optical waveguides in aluminosilicate glass

Using tightly focused femtosecond laser pulses to irradiate lines in aluminosilicate glass, embedded lines with increased refractive index, which function as optical waveguides were observed. The pulse energy (4.5–11.2 μJ) and writing speed (50–700 μm/s) were shown to affect the resultant optical properties of the waveguides such as the magnitude of refractive index change, core diameter and propagation mode. At pulse energies above 5 μJ, two types of structures were observed, namely an inhomogeneous void-like structure and a cross-sectional crack-like structure. These structures were found to affect significantly the resultant waveguiding properties of the irradiated lines. Using pulse energy of 5 μJ or below, single mode waveguides were fabricated. Raman spectroscopy showed that the fs laser pulses generated structural changes to the aluminosilicate glass. The fabrication of a 1×4 splitter was also demonstrated. PACS 42.62.-b; 42.82.-m; 81.05.Kf     

An integrated optical TE-TM mode splitter

A hybrid technology developed for the deposition in planar waveguides of a multilayer which controls the guided optical power by interference has been tested using a TE-TM mode splitter as an example. The preparation of waveguides which exhibit good edge properties, the deposition of the multilayer and the mutual alignment of the waveguides by better than λ/5 are described. The ratio of the transmitted TE power to the reflected TM power has been measured to be 0.9±0.1, which agrees well with the 1.0 yielded by theory. Distortion due to mode conversion, TE reflectance and TM transmittance is of the order of 1%.     

Small-bore fluorocarbon polymer-coated silver hollow glass waveguides for Er:YAG laser light

Several types of fluorocarbon polymer (FCP)-coated silver hollow glass waveguides have been fabricated for Er:YAG laser delivery by using the improved wet chemical technique and dynamic coating procedure. The straight losses of 2 m long 700 μmØ and 540 μmØ waveguides are 0.4 and 1.0 dB, respectively. The transmission losses of these waveguides are below 1.5 dB even when the waveguides are bent to 180° with the bending radius of either 20 or 15 cm. The waveguides with the small diameters of 320 and 200 μm have also been developed for clinical treatment, which exhibit low enough transmission losses for Er:YAG laser light.     

Periodic poling of optical waveguides produced by swift-heavy-ion irradiation in LiNbO<Subscript>3</Subscript>

The generation of periodically poled structures in waveguides prepared by swift-heavy-ion (SHI) irradiation, i.e. in the electronic stopping power regime, has been achieved following two different strategies. In one of them we have prepared bulk PPLN samples by an applied electrical field, followed by irradiation with F ions at 22 MeV. After the ion irradiation, a waveguide showing a high optical confinement is obtained, preserving the original PPLN structure. The second strategy consisted of electric periodic poling of previously fabricated swift-ion-irradiated waveguides. To our knowledge this method has not been, so far, successful for conventional implanted waveguides. The successful fabrication of PPLN structures on novel waveguides prepared by SHI irradiation offers a promising potential for nonlinear integrated optical devices.     

Proposal for a 4-channel all optical demultiplexer using 12-fold photonic quasicrystal

In this work, 12-fold photonic quasicrystal (PQC) with cross section equals to 138 μm² has been used to design a 4-channel optical demultiplexer. The size of structure promises its applications in optical integrated circuits (OICs) and also, wavelength division multiplexing (WDM) communication devices. Finite difference time domain (FDTD) method has been employed in order to investigate the structure's band gap and output waveforms of each channel. Four channels, with spacing less than 1 nm and cross-talk level better than ? 2.8 dB have been separated by introducing defects in L-shaped and line defect waveguides (LDWs) in the crystal's structure. It has been shown that, L-shaped waveguides (LWs) are quite more frequency selective than line defect waveguides. Also, it has been found that the exact tuning of the central wavelength of each channel is possible by making use of defects with different radiuses and sites in the waveguides.     

Increase of refractive index induced by absorbing centres

An analytical expression of the complex permittivity is derived for absorbing centres featuring inhomogeneous absorption-line broadening. Such an expression gives the dispersion law of the real part of the permittivity when the imaginary part has a Gaussian lineshape. Our mathematical approach starts from an overlap integral of Lorentzian-type dielectric susceptibilities weighted by a Gaussian probability distribution of the resonance absorption energies. The analytical solution found is consistent with the Kramers–Kronig relation. We demonstrate that, like in the case of homogeneous absorption-line broadening, the refractive index increases at photon energies lower than the resonance absorption energy also for inhomogeneous absorption-line broadening; if the absorbing centres emit Stokes-shifted radiation, such an increase can be exploited for passive and active waveguiding applications. An example is reported regarding active waveguides based on colour centres in a lithium fluoride crystal.     

Optical waveguide writing inside Foturan glass with femtosecond laser pulses

We investigated the femtosecond laser writing of optical waveguides inside Foturan glass at various pulse energies and focusing depths. An optimal waveguide fabricated solely by femtosecond laser irradiation showed a refractive index modulation of ∼1.7×10^-3 and a minimum transmission loss of ∼0.80 dB/cm. This type of waveguide had lower transmission loss than those fabricated by a hybrid process of femtosecond laser exposure and following thermal treatment. An optical splitter was also fabricated at high pulse energy. PACS 42.65.Re; 42.82.Et; 42.70.Gi     

一种分析三维楔脊形光波导与光纤耦合的方法

和直波导的不同在于，三维楔脊形波导中的光场都是不稳定的.分别对波导-光纤耦合、光纤-波导耦合两种情况，将三维楔脊形波导等效为多段短的直波导，利用束传播法同时对三维楔脊形波导内不稳定的入射、反射场，及它们在传播方向上的偏导进行处理，再用自由空间辐射模法计算透射率和反射率.以SOI楔脊形波导和光纤的耦合为例，验证了该方法的可行性. 关键词： 自由空间辐射模法 束传播法 耦合 透射 楔脊形光波导 绝缘体上硅(SOI)     

Development and test of an L-band magnetron

An L-band relativistic magnetron has been designed, built, and tested at approximately 1.1 GHz. The authors have obtained a peak RMS power of 2.4 GW in one waveguide, and a total of 3.6 GW from two waveguides. The overall efficiency reached 9%, compared with the input power. In contrast with previous S- and X-band magnetron results, the L-band magnetron impedance does not collapse at the end of the pulse, and microwave generation tends to last as long as high voltage is applied. This implies that large total energies can be produced by applying long high-voltage pulses     

Nonlinear optical waveguides generated in lithium niobate by swift-ion irradiation at ultralow fluences

A novel method to produce optical waveguides is demonstrated for lithium niobate (LiNbO(3)). It is based on electronic excitation damage by swift ions, i.e., with energies at approximately 1 MeV/amu or above. The new technique uses high-energy medium-mass ions, such as Cl, with electronic stopping powers above the threshold value for amorphization (5-6 keV/nm), reaching the maximum value a few micrometers inside the crystal. At the ultralow fluence regime (10(12)-10(13) cm(-2)) an effective nanostructured medium is obtained that behaves as an optical waveguide where light propagates transversally to the amorphous nanotracks created by every single impact. The method implies a reduction of 4 orders of magnitude with respect to He implantation. The optical waveguides present reasonable losses (~10 dB/cm) and significant second-harmonic generation (SHG) and electro-optic (EO) responses (>50% bulk) for the lowest fluences.     

Losses in diffused optical waveguides

It has been shown that the attenuation characteristics of TE modes in diffused waveguides are different from those of a conventional slab. A simple semiempirical analysis is performed on the base of a ray-optics picture assuming a parabolic index distribution. Analytical expressions for surface scattering and volume loss are derived. The analysis precisely describes the modal loss behaviour of Ag⁺ ion-exchanged waveguides in glass (new results) and Nb diffused waveguides in LiTaO₃ (data reported by other authors).     

Transmission through metallic array slits curved with perpendicular waveguides

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with perpendicular waveguides has been explored. Two-dimensional electromagnetic fields inside and near the metal film are simulated by the finite-difference time-domain (FDTD) method. The perpendicular waveguides in the middle of slits affect the modes of slits in two different manners: commonly, the extended length of the waveguides leads to the increase of the resonant wavelength; shifting the waveguides location along the vertical slits, red or blue shift of resonant wavelengths can be achieved, however to some confirmed length of the perpendicular waveguides, there is the only one resonant wavelength. In addition, a new explanation model is proposed in which the resonance is ascribed to three reasons: the Fabry-Perot cavity theory, the surface current flow, and the surface charges. All the calculated results are well explained by our proposed model.     

Power handling capability of dielectric-coated,metallic, hollow waveguides for CO2 laser light

A theoretical analysis has been carried out to evaluate the power handling capability of hollow waveguides for CO₂ laser light. Dielectric-coated metallic waveguides are shown to transmit much higher powers than conventional metallic waveguides.