Low-loss Structure Research Articles

Multistage Polymeric Lens Structures Integrated into Silica Waveguides

A waveguide lens, composed of multistage polymer-filled thin grooves in a silica planar lightwave circuit (PLC) is proposed and a low-loss structure has been designed. A waveguide lens in a silica slab waveguide has been fabricated using reactive ion etching (RIE) and formed by filling with polymer. Both an imagding optical system and a Fourier-transform optical system can be configured in a PLC using a waveguide lens. It renders the PLC functional and its design flexible. To obtain a shorter focal length with a low insertion loss, it is more effective to use a multistage lens structure. An imaging optical system and a Fourier-transform optical system with a focal length of less than 1000 µm were fabricated in silica waveguides using a multistage lens structure. The lens imaging waveguides incorporate a 16–24-stage lens, with insertion losses of 4–7 dB. A 4 ×4 optical coupler, using a Fourier-transform optical system, utilizes a 6-stage lens with losses of 2–4 dB.

A 1 × 4 polarization and wavelength independent optical power splitter based on a novel wide-angle low-loss Y-junction

A 1 × 4 polarization and wavelength independent optical power splitter is reported. This device is based on a novel wide-angle low-loss Y-junction structure which can give a theoretical TE junction excess loss of 0.26 dB at a branching angle of 16°. To the best of our knowledge, it is so far the lowest reported loss at such a large angle. The detailed design of the device and its fabrication are described. Our experimental results show the measured TE excess loss to be 1.2 dB and TM excess loss 1.8 dB for the whole splitter over the wavelength between 1.47 μm and 1.57 μm.

Low conversion loss and high LO-RF isolation 94-GHz active down converter

We report a low conversion loss and high local oscillator (LO)-to-RF isolation 94-GHz monolithic-microwave integrated-circuit (MMIC) active down converter using 0.1-/spl mu/m InGaAs/InAlAs/GaAs metamorphic high electron-mobility transistor (MHEMT). The fabricated MMIC active down converter employs a one-stage MHEMT amplifier in the RF port of the active down converter, thereby amplifying the RF signal and improving the LO-to-RF isolation by using an inherent S/sub 12/ isolation characteristic. The fabricated MMIC active down converter shows an excellent conversion loss of 6.7 dB at an LO power of 10 dBm and high LO-to-RF isolations of 21 /spl plusmn/ 0.5 dB in a frequency range from 93.7 to 94.3 GHz. High dc and RF performances of the MHEMT used for the active down converter are due to the optimized epitaxial and device structure, and a maximum transconductance of 760 mS/mm, a current gain cutoff frequency of 195 GHz, and a maximum oscillation frequency of 391 GHz were measured. A active down-converter module is assembled by mounting the active down-converter chip on a jig with low-loss transition structure between the coplanar waveguide and waveguide. The fabricated active down-converter module shows a good conversion loss of 10.9 dB and a very high LO-to-RF isolation of 27.5 dB at 94.03 GHz.

Low-loss optical coupling structure between two ends of silica glass optical fibers by inserting TeO 2 melt

Less than 1.5 dB of insertion loss was realized in an optical coupling structure in which two TEC (thermal-diffusion expanded core) fibers are spliced via quenched TeO 2 melt whose length was 0.5 mm. The quenched melt seems to be free of precipitates because they would bring about larger loss if present. The loss due to imperfect optical coupling between the fibers is estimated to be about 1 dB, which can be reduced by introducing some refractive index modulation into the present structure.

Three cascade solar cells with graded band-gap layer on the base of GaAs-AlGaAs heterosystem : Hrayshat, E. S. et al. Solar Energy Materials & Solar Cells, 2002, 73, (3), 281–286

As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cell layers and substrate, the last 15 years have seen large improvements in III–V compound multi-junction (MJ) solar cells. In this paper, present status of R&D program for super-high-efficiency MJ cells in the New Sunshine Project in Japan is presented. InGaP/InGaAs/Ge monolithic cascade 3-junction cells with newly recorded efficiency of 31.7% at AM1.5 (1-sun) were achieved on Ge substrates, in addition to InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%. Future prospects for realizing super-high-efficiency and low-cost MJ solar cells are also discussed.

Electronic, dielectric, and optical properties of individual composite silver halide microcrystals using the EELS and LMTO-ASA techniques

The low-loss fine structure in electron energy-loss spectra (EELS) of individual composite high-aspect-ratio tabular $\mathrm{Ag}X(X=\mathrm{B}\mathrm{r},\mathrm{}\mathrm{I})$ microcrystals between 4 and 26 eV was investigated by cryo-EELS and energy-filtering transmission electron microscopy. Local dielectric permittivity, refractive index, and absorption coefficient have been determined using Kramers-Kronig relationships. Quantum-mechanical calculations of the AgBr band structure and dielectric permittivity by the linear muffin-tin orbital method in the atomic spheres approximation have been used to assign the structure attributed to exciton peaks. Experimental local dielectric parameters and joint optical density of states were found to be in fair general agreement with calculated dielectric properties and densities of states.

Characterisation and modelling of coplanar transmission lines on SiC substrates

The paper investigates the characteristics of microwave transmission in coplanar waveguides (CPWs) fabricated on various silicon carbide substrates. Propagation constant and characteristic impedance measurements were performed at frequencies from 0.1 to 10 GHz. Different propagation modes are observed as a function of frequency and substrate resistivity. In the case of the low-loss structure, a quasi-TEM equivalent circuit model was developed from the available process parameters, taking into account the effects of electromagnetic fields in CPW structures.

III–V compound multi-junction solar cells: present and future

As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cell layers and substrate, the last 15 years have seen large improvements in III–V compound multi-junction (MJ) solar cells. In this paper, present status of R&D program for super-high-efficiency MJ cells in the New Sunshine Project in Japan is presented. InGaP/InGaAs/Ge monolithic cascade 3-junction cells with newly recorded efficiency of 31.7% at AM1.5 (1-sun) were achieved on Ge substrates, in addition to InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%. Future prospects for realizing super-high-efficiency and low-cost MJ solar cells are also discussed.

Second-harmonic generation using an <italic>a</italic> axis <inline-formula><roman>Nd:MgO:LiNbO</roman><sub><roman>3</roman></sub></inline-formula> single crystal fiber with Mg-ion indiffused cladding

We report the implementation of a low-loss core-cladding waveguide structure of an a axis Nd:MgO:LiNbO3 single-crystal fiber with a step refractive index profile and the demonstration of a frequency- doubled laser made from such a cladded crystal fiber. The laser-heated pedestal-grown a axis single-crystal fiber, with an elliptical crosssectional area of 200x 150?m, is further shown with a Mg ion indiffusion process. Electron probe microanalysis is used to measure the Mg ion concentration distribution in the Mg-diffused layer. After several trials, diffusion parameters suitable for achieving a core-cladding waveguide structure with a step refractive index profile are obtained. The optical and structural properties of the cladded crystal fibers are also characterized. Room-temperature second-harmonic generation with such a cladded crystal fiber is demonstrated. At room temperature, cw green laser output with a power of 10 ?W at the wavelength of 0.532 ?m is achieved. The origin of the relatively low conversion efficiency is discussed.

Structural and analytical characterization of Ag(Br,I) nanocrystals by CRYO-AEM techniques

The morphology, crystalline and electronic structures and elemental compositions of AgX (X = Br,I) nanocrystals of 20–185 nm in diameter have been evaluated by cryo-energy-filtering transmission electron microscopy (EFTEM)/electron spectroscopic diffraction (ESD)/ electron energy-loss spectroscopy (EELS) and cryo-energy-dispersive X-ray (EDX) analysis in the scanning transmission (STEM) mode. A low-loss fine structure in EEL spectra between 4 and 26 eV was attributed to excitons and plasmons possibly superimposed with interband transitions and many-electron effects. Contrast tuning under energy filtering was used to image electron excitations in the AgX nanoparticles. Intensity oscillations caused by size-confined coupling of surface and volume losses were observed. The real and imaginary parts, ε 1 and ε 2, of dielectric permittivity were determined using Kramers-Kronig relations. The exciton peaks were assigned based on calculations of electronic band structure of AgBr. Combined silver and halide distributions were obtained by electron spectroscopic imaging (three-window method) and by STEM/EDX including high-magnification X-ray mapping of the individual nanocrystals.

Combined characterization of composite tabular silver halide microcrystals by cryo-EFTEM/EELS and cryo-STEM/EDX techniques.

The combination of cryo-energy filtering transmission electron microscopy (EFTEM)/electron spectroscopic diffraction (ESD)/electron energy-loss spectroscopy (EELS) and cryo-energy-dispersive X-ray (EDX) analysis in the scanning transmission (STEM) and scanning (SEM) modes was applied for the characterization of composite tabular Ag(Br,I) microcrystals. A low-loss fine structure in EEL spectra between 4 and 26 eV was attributed to excitons and plasmons possibly superimposed with interband transitions and many-electron effects. The contrast tuning under the energy-filtering in the low-loss region was used to image the crystal morphology, defect structure (random dislocations and ¿111¿ stacking faults) and bend and edge contours as well as electron excitations in the microcrystals. Sharp extra reflections at commensurate positions in between the main Bragg reflections and diffuse honeycomb contours in ESD patterns of the microcrystals taken near the [111] zone were assigned to the number of defects in the shell region parallel to the grain edges and polyhedral clusters of interstitial silver cations, respectively. The imaginary part of the energy-loss function, Im (-1/epsilon), and the real and imaginary parts, epsilon1 and epsilon2, of the dielectric permittivity were determined by means of a Kramers-Kronig analysis. An assignment of exciton peaks based on calculations of electronic band structure of silver bromide is proposed. Inner-shell excitation bands of silver halide were detected in line with EDX-analyses. The energy-loss near-edge structure (ELNES) of the AgM4,5-edge governed by spin-orbital splitting between the 3d3/2- and 3d5/2-states has been evaluated. Combined silver and halide distributions were obtained by a three-window method (EFTEM) and by EDX/STEM including area mapping and line profiling of iodide.

LOW TRANSIMPEDANCE-FLUCTUATION DESIGN FOR 10-GHz Si-BIPOLAR PREAMPLIFIER IN 10-Gb/s OPTICAL TRANSMISSION SYSTEMS

Design considerations concerning Si preamplifiers for 10-Gb/s optical transmission systems are discussed. A preamplifier with 300-Ω transimpedance was designed by focusing on attaining low transimpedance fluctuation in the frequency response despite bias variation caused by the photo current. To ensure good design accuracy, we optimized the current density of the transistor and the open-loop voltage gain using measured results to obtain the desired bandwidth. We also developed a low-loss pad structure that has U-grooves to improve the bandwidth. The U-grooves increase the pad parasitic resistance and reduce signal-loss from pad to Si substrate. A preamplifier IC fabricated using a Si bipolar technology with fT of 35-GHz provided a bandwidth of 10.2 GHz, transimpedance fluctuation within 0.5 dB, an input dynamic range of up to 1.6 mA p-p, and a low averaged input noise current density of 11.5 [Formula: see text].

A novel low-loss slow-wave microstrip structure

A low-loss slow-wave microstrip line using a periodic structure in the ground plane is presented. The periodic structure is realized with metal pads etched in the ground plane connected by thin lines to form a distributed LC network. The slow-wave factor is demonstrated to be 1.2-2.4 times larger than that of conventional microstrip lines over a wide frequency range. Due to the unique design of the structure, low insertion loss comparable to conventional microstrips has been achieved. The proposed structure is easier to fabricate than other slow-wave devices which require multilayer substrates or very fine features.

Subcellular Composition Based On Light-Element EELS

Recent work on biological electron energy-loss spectroscopy (EELS) in the analytical electron microscope has focused on measuring concentrations of the important elements, calcium and phosphorus. Analysis of the light elements, for which EELS was originally thought to be best suited, has so far not yielded much useful information from biological specimens. Whereas low-loss fine structure has been analyzed successfully to measure distributions of water and protein in frozen-hydrated cells, core-edge fine structure cannot be recorded at sufficiently low dose to obtain useful chemical information. However, the atomic ratios of the light elements (carbon, nitrogen and oxygen) obtained from EELS are less dependent on electron dose. Together with sulfur (in proteins) and phosphorus (in nucleic acids, phospholipids and phosphates) these ratios can help determine the types and proportions of compounds that are present at the subcellular level. Here we re-examine the potential for making these kinds of measurements.

Analysis of Cellular Organelles and Macromolecular Assemblies by Electron Energy Loss Spectroscopy

Recent advances in electron energy loss spectroscopy (EELS) have significantly extended the range of applications for biological microanalysis. For example, EELS can now detect physiological concentrations of the important element calcium in rapidly frozen cells with a sensitivity greater than that achievable by energy-dispersive x-ray spectroscopy (EDXS). It can also detect small numbers of phosphorus atoms bound to macromolecular assemblies, and measure water distributions in frozen hydrated tissue. Here we discuss some of these developments in the context of detection limits and mapping techniques in the scanning transmission electron microscope (STEM) and energy-filtering transmission electron microscope (EFTEM).The useful information about elemental composition in EELS of biological specimens generally resides in weak core-edge signals corresponding to atomic concentrations in the 10−5−10−3(1–100 mmol/kg dry weight) range. For example, the Ca L2,3signal/background ratio is typically only 10−3and it is necessary to measure differences in signal that are only 104 of the background. Changes in low-loss fine structure corresponding to varying chemical composition are also very subtle; for example, detection of a 3% change in water content requires reliable measurement of a 0.1 eV shift in the low-loss intensity maximum. To extract such information requires efficient parallel detection of the energy loss spectrum and a high-brightness source to provide a sufficient number of incident electrons. The dedicated STEM is particularly well-suited for analyzing low concentrations of biological elements. If desired, the probe current can be reduced into the picoampere range for low-dose, high-resolution imaging prior to elemental analysis. The STEM’S field-emission source can then be used to deliver a current approaching 10 nA into a ~10 nm diameter probe. High electron flux conditions are ideal for spectrum-imaging applications where adequate counting statistics must be achieved within a limited pixel dwell time. The cold field-emission source of the STEM has the additional advantage of providing electrons with a narrow energy spread of <0.5 eV which is important in fine structure studies.

Investigation of low-loss spectra and near-edge fine structure of polymers by peels

Transmission electron microscopy has changed from a purely imaging method to an analytical method. This has been facilitated particularly by equipping electron microscopes with energy filters and with parallel electron energy loss spectrometers (PEELS). Because of their relatively high energy resolution (1 to 2 eV) they provide information not only on the elements present but also on the type of bonds between the molecular groups. Polymers are radiation sensitive and the molecular bonds change as the spectrum is being recorded. This can be observed with PEEL spectrometers that are able to record spectra with high sensitivity and in rapid succession.A PEEL spectrum can be divided into a low loss range and an inner shell loss range of higher energy. The low loss spectra of polymers always show a broad peak at about 22 eV and a further peak at 7 eV, if aromatic groups are present, as is the case with PS (Fig. 1). In the course of exposure, the intensity of this peak decreases, a sign that the benzene ring is destroyed by the radiation (Fig. 2).

Some effects of anisotropy on planar antiresonant reflecting optical waveguides

In this paper, propagation characteristics of some planar antiresonant reflecting optical waveguides (ARROW's) comprised of anisotropic media are studied using an integral equation approach. The integral equation method is rigorous and general, with the added advantage that multiple layers of crystalline material with arbitrary anisotropy can be accommodated in a straightforward manner. The integral equation method is applied to study basic propagation characteristics of the ARROW structure where one or more dielectric layers are allowed to be anisotropic. Practically, the presence of anisotropy may be unintentional, due to material fabrication or processing techniques, or it may be intentionally utilized to allow integration of anisotropy-based devices and waveguiding structures on a single semiconducting substrate. Propagation characteristics and field distributions are shown for a uniaxially anisotropic ARROW where the material's optic axis is rotated in each of the three principal geometrical planes of the structure. It Is found that even moderately large levels of anisotropy do not significantly affect the propagation characteristics of the ARROW if either the optic axis of the material is aligned with one of the geometrical axes of the waveguide, or if the optic axis is rotated in the equatorial plane. In these cases, pure TE/sub 0/ modes can propagate, resulting in a low-loss structure. In the event of misalignment between the geometrical axes and the material's optic axis in the transverse or polar planes, the influeuce of even small levels of anisotropy is quite pronounced. In this case, pure TE/sub 0/ modes do not exist, and attenuation loss increases significantly due to the hybrid nature of the fundamental mode.

Low-loss intracavity AlAs/AlGaAs saturable Bragg reflector for femtosecond mode locking in solid-state lasers

We introduce a new low-loss semiconductor structure for femtosecond intracavity mode locking in low-gain solidstate lasers. This monolithic device can be engineered to exhibit specif ic saturation characteristics desirable for mode locking solid-state lasers. Self-starting 90-fs pulses are obtained with Ti:sapphire and diode-pumped Cr:LiSAF lasers. We discuss mode-locking mechanisms in quantum-well passively mode-locked solid-state lasers.

Analysis of microwave characteristics of coplanar traveling wave electrodes on III-V semiconductor with an n/sup +/ doped layer

The microwave characteristics of coplanar traveling wave electrodes on III-V semiconductor with an n/sup +/ doped layer have been studied by using the method of lines. Such a structure is analyzed by using even mode and odd mode of microwave. The behaviors of n/sup +/ doped layer have been studied carefully and found that at certain doping concentration region the n/sup +/ doped layer behaves like a perfect conductor for electric field and like an insulator for magnetic field. The relations between microwave characteristics of the odd mode and structural dimensions of the modulator have been analyzed in order to find a low loss and velocity-matched structure. A bandwidth as large as 37 GHz is predicted for the traveling wave type directional coupler modulator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A design consideration for optical power dividers composed of three coupled waveguides

In the present paper, we propose a design consideration for optical power dividers utilizing the distributed coupling among three adjacent slab waveguides. This type of power divider can easily be fabricated by means of the process of ion-exchange. However, being large in scale is a serious problem. We clarify that the size of the device can be remarkably reduced by introducing nonuniformly distributed coupling which is caused by step-like gap variations between the waveguides. In order to obtain wide separations at the output end, we also introduce the low-loss bent structure which is designed by the method of suppression of the optical wave undulation along the waveguide. The numerical results show that low-loss wide separations of optical waves are possible and over a 35% reduction in device length can be achieved. The numerical analysis is based on the two-dimensional finite difference method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>