Improved coupling efficiency using Δn-controlled polymerwaveguides with two-dimensional spot-size transformation

A new type of polymer waveguide for two-dimensional spot-size transformation is reported, with which coupling efficiency of waveguides with 4- and 8-μm core diameters was reduced by ≈1.6 dB. The length needed for spot-size transformation was less than 1 mm along the direction of propagation. In this polymer waveguide, refractive index difference between core and cladding (Δn) is properly controlled along the direction of propagation while the core cross section is kept uniform     

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 Δn. 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     

On the figures of merit for electroabsorption waveguide modulators

ΓΔα/F and Δα/α₀ (where Δα is the absorption change, α₀ is the residual absorption, F is the applied electric field, and Γ is the optical confinement factor in the waveguide) have been separately proposed as the relevant figure of merit for electroabsorption waveguide modulators. Using a quantitative and systematic argument, the authors show that they are both necessary and important to the total performance of the modulator     

Uniplanar monopulse antenna based on odd/even mode excitation ofcoplanar line

A uniplanar microstrip antenna, finding application in monopulse radars, is proposed. Two series-fed slot arrays are fed by a coplanar waveguide transmission line excited either in its coplanar mode, or coupled slot line mode, which provide the difference (Δ) and sum (Δ) patterns, respectively     

Theoretical design optimization of multiple-quantum-wellelectroabsorption waveguide modulators

Optical on-off modulators require low insertion loss, high contrast ratio (CR), small drive power and large bandwidth or bit-rate. A systematic approach to optimize the total performance of these modulators based on the quantum-confined Stark effect is presented here. The approach consists of minimizing the power/bandwidth ratio while satisfying a given CR and insertion loss. Our design consists of a large-core multimode passive waveguide with a thin buried active layer. The passive waveguide is designed to yield a high coupling efficiency to conventional single-mode fibers. The quantum well material structure is designed to maximize Δα/ΔF², while maintaining a sufficiently large Δα/α₀, where Δα is the absorption change, α₀ is the residual absorption at zero bias, and ΔF is the swing of the applied electric field. Our theoretical model shows that i) wider quantum wells give larger Δα/ΔF², and ii) the bandwidth/power ratio as high as 4 GHz/mW can be achieved simultaneously with small insertion loss, For example, with a drive voltage of 3 V, an RC limited bandwidth as high as 60 GHz is predicted, while a contrast ratio of 20 dB and a total insertion loss of 4.5 dB may also be obtained     

Use of a microwave cavity for sensing dielectric properties ofarbitrarily shaped biological objects

A rectangular waveguide resonator operating in the H₁₀₅ mode at 3.2 GHz is used in determining the change in resonant frequency, ΔF, and the Q factor of the cavity, ΔT , when measured with and without single corn kernels of various shapes and dimensions. By measuring those variables for a kernel oriented in two positions differing by a 90° rotation with respect to the maximum E-field vector, the average values of ΔF and ΔT are found to be independent of shape. The ratio ΔF/ΔT is independent of size and is a function of the material properties (ε'-1)/ε". This function is shown to be related to the material density, moisture content, or other characteristics when all other properties except the one selected remain unchanged     

A composite optical waveguide-based polarimetric interferometer forchemical and biological sensing applications

A new polarimetric interferometer has been developed on the basis of the phase difference between transverse electric (TE)₀ and transverse magnetic (TM)₀ modes in a composite optical waveguide (OWG). The composite OWG consists of a single-mode potassium ion-exchanged planar waveguide overlaid with a high-index thin film that has two tapered ends and supports only the TE₀ mode. Applying tapered velocity coupling theory, we found that the TE₀ and TM₀ modes coexisting in the potassium ion-exchanged layer were separated in the thin film region of the composite OWG: the TE₀ mode was coupled into the thin film while the TM₀ mode was confined in the potassium ion-exchanged layer. Interference occurs between TE- and TM-polarized output components when a single output beam is passed through a 45°-polarized analyzer. The phase difference φ between both orthogonal output components is very sensitive to the superstrate index n_c in the thin film region. Our experimental results indicate that a slight change of Δn_c=3.71×10^-6 results in the phase-difference variation of Δφ=1° for a 5-mm-long TiO ₂/K⁺ composite OWG with a 34-nm-thick TiO₂ film. Such a simple polarimetric interferometer can be applied to chemical or biological sensors by modifying the upper film surface of the composite OWG with a chemically or biologically active substance     

High-Power, Highly Efficient Second-Harmonic Generation in a Periodically Poled ${hbox {MgO}}{:}{hbox {LiNbO}}_{3}$ Planar Waveguide

We designed a single-pass quasi-phase-matched second-harmonic generation (SHG) device with a planar waveguide; the device comprised a Y-cut 5 mol% MgO-doped LiNbO₃ (MgO:LiNbO₃)crystal core that was 3 mum thick and SiO₂ cladding. The waveguide provided a high coupling efficiency of 95% between an incident Gaussian beam and the fundamental guided mode of a fundamental wave; it also provided high electric-field confinement in the case of both the fundamental and SHG waves in the core. Thus, a high overlap between nonlinear polarization and an SHG-guided mode was attained. The bonding of the device with the waveguide side positioned downward to a heat sink provided a large heat radiation area when pumping with a near-collimated Gaussian beam, which reduced the temperature rise and its gradient along the waveguide to minimize the phase mismatch. We demonstrated the green light generation of 1.6 W with 40% conversion efficiency using a 7-mm-long sample and 1.2-W SHG with 60% efficiency using an 18-mm-long sample.     

介质平面光波导TE0模模场分布的高斯近似

基于介质平面光波导端面无受限衍射场光束的光束传输因子的特点,阐明光波导TE0模模场分布采用高斯分布近似表达的合理性。基于场分布间的匹配效率计算公式,提出采用等效匹配效率方法确定用于高斯近似表达的等效模场半宽度,给出基于光波导芯层半宽度和归一化芯层驻波参量和归一化包层倏逝波参量表达的光波导等效模场半宽度的函数表达式,给出高斯近似分布与光波导本征场分布的匹配效率,阐明采用等效匹配效率方法确定等效模场半宽度的合理性。采用求解方程组的方法,给出基于光波导芯层半宽度和归一化频率表达的光波导等效模场半宽度的拟合函数表达式,并基于拟合引起的误差分析阐明了拟合函数表达式的精确性。     

非线性包层矩形波导基模的均匀芯区场

通过对芯片为线性、介质包层为非线性自聚焦Kerr介质的埋入型矩形波导的理论分析和数值计算表明：当模折射等于芯片折射率时,E00^y模芯区的场呈现等振幅的均匀分布四周包层的场则急剧衰减;在模折射率分别小于和大于芯片折射率时,E00^y模芯区场分布分别成为上凸和下凹的曲面。     

Embedded channel polyimide waveguide fabrication by direct electronbeam writing method

A new embedded channel polyimide waveguide fabrication process by a direct electron beam writing method (DEBWM) is described. The new technique uses an electron-beam induced effect to directly alter the refractive indices of the two-layer polyimide. Both the core and the lower cladding have been fabricated at the same time in two-layer polyimides using electron beam with 25 keV energy. The obtained embedded channel waveguide was made of two kinds of polyimides, one for lower cladding and one for core and other claddings. Guide losses are 0.3 dB/cm for both TE and TM polarized incident lights and guiding mode is single-mode for TE. The optical properties of the waveguide and a relationship between the doses of electron beam and optical losses or loss dependence on wavelength are also mentioned     

Small-size silicon-oxynitride AWG demultiplexer operating around725 nm

We present experimental results of very compact, high Δn, five-channel arrayed waveguide gratings (AWGs) with 5 nm-channel-spacing fabricated in the silicon-oxynitride material system. The on-chip insertion loss is about 5 dB and the crosstalk is -30 dB around 725 nm wavelength. The uniformity of the waveguide layers is better than ±1%,     

Stability of a light beam in a beam waveguide

Long-distance, high-capacity transmission using a light beam in a beam waveguide is an attractive project for optical communications. However, the transmission of energy in the form of a beam gives rise to a new problem, that is, the stability of a sharp beam. In this paper, the stability problems of a Gaussian beam in a beam waveguide are presented. The deflection and the spot-size variation of a beam in the waveguide with known imperfections, such as transverse displacements of lenses, variation in focal length, and so on, are studied. If the transverse displacements of the lenses are periodic in space, instability of the beam path may occur. The unstable condition is derived. The variation of the spot size at the lenses is obtained in terms of small perturbations of the ideal waveguide. If the incident beam from a laser is not matched to the mode of the waveguide, the spot size at the lenses varies periodically. Statistical results for the models of a waveguide with random lenses, having random transverse position, random spacing, and random focal length, are also given.     

Electric-field-induced refractive index changes in InGaAs-InAlAsasymmetric coupled quantum wells

Two kinds of InGaAs-InAlAs asymmetric coupled quantum wells (QW's) were numerically analyzed in an effort to enhance change in the refractive index (Δn) at longer wavelengths where absorption coefficient is small. The analysis reveals the operating voltage, Δn, chirp parameter, Δn/Δk, (Δk: change in the extinction coefficient) and figure of merit, Δn/α, (α: absorption coefficient) can be improved by these two kinds of the QW's. This improvement is caused by an abrupt electric-field-induced increase or decrease in the oscillator strength for the lowest heavy hole exciton     

Elimination of polarization sensitivity in silica-based wavelengthdivision multiplexer using a polyimide half waveplate

This paper proposes a low-loss technique for eliminating polarization sensitivity in a silica-based planar lightwave circuit (PLC) which uses a polarization mode converter formed at the center of the circuit. This converter consists of a waveguide gap housing a polyimide half waveplate. The excess loss of the converter was drastically reduced to 0.26 dB with a Δ=0.75% waveguide by employing an 18 μm-wide waveguide gap and a 14.5 μm-thick polyimide half waveplate. A polarization mode conversion crosstalk of -37 dB was achieved at 1.55 μm. Using this converter, we successfully eliminated the polarization sensitivity in some silica-based PLC-type wavelength division multiplexers. The converter is also insensitive to temperature and offers long term stability     

Integrated-optic ring resonators with two stacked layers of silicawaveguide on Si

A ring resonator with a radius of 1.5 mm is demonstrated using a stack structure consisting of two layers of GeO₂-doped silica waveguide with different refractive index differences (Δ). One is a high-Δ waveguide layer with a Δ of 2% for a small-radius ring and the other is a waveguide layer with a Δ of 0.75% for good fiber coupling. The waveguides and a flattened middle-cladding layer are stacked on a Si substrate. The ring resonator exhibited a wide free spectral range (FSR) of 21.6±0.1 GHz and a high finesse of 110±10     

Theoretical and experimental studies of a spot-size transformerwith integrated waveguide for polarization insensitive opticalamplifiers

The simple fabrication technology of an active waveguide for polarization insensitive optical amplifiers, integrated with tapers and a passive waveguide for fiber coupling, is presented. The spot size transformation along the taper, yielding a mode expansion factor of more than 9 in horizontal direction and about 2 in vertical direction, is experimentally verified and the results are compared to theoretical computations. Using a cleaved single mode fiber, the coupling loss is as low as 2.6 dB with -1-dB positional tolerances of 2.2 μm and 1.5 μm in vertical and horizontal directions. Besides relaxing on fiber coupling, the large spot size transformation in horizontal direction allows for a drastic reduction of the reflection coefficient to less than 10^-3 at a facet tilted by 4° with respect to the waveguide axis     

Optical fiber up-tapers with index perturbations-performanceanalysis

The performance of step index up-tapers subjected to certain index perturbations in investigated by computer simulation. Concentric symmetric and asymmetric index profile perturbations as well as localized spherical, called regions bubbles, having a different index are studied. It is found that some low-valued perturbations of small scale size (less than a wavelength) do not alter the optical fiber performance. However, the amount of beam expansion in the up-taper may be ultimately limited by such perturbations. Excessive magnification in the presence of perturbations can cause a distortion of the quasi-Gaussian shape and symmetry of the pure step-index fundamental mode as well as increase the excess loss and degrade the lateral and axial displacement tolerance improvements of the up-tapers. For example, an index variation of Δn≈10^-5 in conjunction with a perturbation scale size of 40 μm can be detrimental. Bubble-type perturbations could also cause similar problems. A single 12.8-μm-diameter bubble with a relative index of -0.02, embedded in a 128-μm-diameter step index core with relative index of 0.5%, produced a 0.3-dB excess loss     

Design and demonstration of polarization-insensitive Mach-Zehnderswitch using a lattice-matched InGaAlAs/InAlAs MQW and deep-etchedhigh-mesa waveguide structure

The authors have designed and demonstrated a 2×2 Mach-Zehnder switch in view of polarization independence as well as low propagation loss (α) and absorption change (Δα). To obtain polarization-insensitive refractive index change (Δn), a lattice-matched InGaAlAs-InAlAs multiple quantum-well (MQW) with a large detuning wavelength was used. Moreover, to reduce the insertion loss difference between polarizations, we applied a multimode-interferometer 3-dB coupler and a deep-etched high-mesa waveguide structure. This switch, therefore, can provide polarization-independent operation about both driving voltage and insertion loss, which is indispensable to practical optical switching applications. We also paid attention to Δα suppression when we decided the value of wavelength detuning and the length of the phase shift region. We also investigated the wavelength dependence of the switch. Within 1530-1560 nm, which is the erbium-doped fiber amplifier (EDFA) gain band, polarization independence in the driving voltage and the crosstalk was maintained. This result shows that the switch is also applicable in wavelength division multiplexing (WDM) applications     

Propagation-mode-controlled fabrication of self-written waveguide in photosensitive polyimide for single-mode operation

We have developed a new fabrication method of single-mode self-written waveguide by controlling the propagation mode in an optical fiber. This method is very appropriate for repeatable fabrication of the single-mode self-written waveguide. Since a Gaussian-like near-field pattern is required for the fabrication of a tiny and uniform waveguide core, the propagation mode in a conventional optical communication fiber was controlled by coupling with an optical fiber having 3-/spl mu/m core, which shows a single-mode operation at visible wavelength region. Single-mode propagation at optical communication wavelength was confirmed for the fabricated self-written waveguide. The evaluated core diameter of the self-written waveguide was /spl sim/9.5 /spl mu/m.