Nadir Dagli

An assessment of finite difference beam propagation method

A finite-difference beam propagation method (FD-BPM) is outlined and assessed in comparison with a conventional beam propagation method (FFT-BPM) which uses fast Fourier transformation. In the comparative study three straight waveguides with different index profiles that are frequently encountered in integrated optics are utilized. Using both methods normalized effective index values of the eigenmodes of these waveguides are calculated and compared with the exact values obtained from analytical expressions. As a further accuracy criterion, the power loss due to numerical errors when an eigenmode of a waveguide is excited is evaluated. Based on this comparison the accuracy, computational efficiency, and stability of the FD-BPM are assessed. >

Wide-bandwidth lasers and modulators for RF photonics

In this paper, the basic principle of operation, design issues, limitations, recent developments and emerging research trends on wide-bandwidth lasers and modulators for radio-frequency photonic applications are reviewed. The topics covered are wide-bandwidth lasers, lumped and traveling-wave electroabsorption modulators, traveling-wave LiNbO/sub 3/, GaAs, and polymer modulators.

A compact silicon-on-insulator polarization splitter

A compact directional coupler-based polarization splitter is designed and realized using silicon-on-insulator (SOI) waveguides. Even though silicon does not have any material birefringence, the high index contrast obtained in the SOI platform and reduced waveguide dimensions makes it possible to induce significant birefringence. Polarization splitting is achieved by making use of this geometry-induced birefringence. In this work, we demonstrate polarization splitting in devices as short as 120 /spl mu/m. Even smaller devices can be made using submicron-thick Si waveguides.

Modal reflection of quarter-wave mirrors in vertical-cavity lasers

Very high plane-wave reflection coefficients can be obtained with practical semiconductor quarter-wave mirrors, but for beams of finite width, the reflection coefficient of a mirror with no lateral guiding and hence the finesse of cavities that use such structures will be limited by diffraction loss. The authors analytically and numerically study the modal reflection of practical semiconductor quarter-wave mirrors. They introduce a quantity called the diffraction range of a quarter-wave mirror as a means of exact analytical comparison between infinite lossless mirrors (and approximate comparison for finite mirrors) in the Fresnel diffraction limit. The exact modal reflection coefficient for an arbitrary incident mode pattern is determined by vector plane-wave decomposition. The modal reflection coefficients of two representative semiconductor quarter-wave mirrors used in vertical cavity laser technology, AlAs/GaAs and InGaAsP/InP, are studied. >

Analysis of Z-invariant and Z-variant semiconductor rib waveguides by explicit finite difference beam propagation method with nonuniform mesh configuration

An efficient and simple explicit finite difference beam propagation method (EFD-BPM) incorporating nonuniform mesh is described. The criteria for stability are developed, and it is shown that this algorithm is power conserving when the stability criteria are met. EFD-BPM is applied to the analysis of single and coupled semiconductor rib waveguides and its accuracy is confirmed by comparing the results with the reported results. Nonuniform mesh is found to improve the efficiency of the method significantly for the analysis of weakly guiding waveguide structures. Several coupled rib waveguide structures with curved input and output branching sections are analyzed using both three-dimensional EFD-BPM and two-dimensional finite difference BPM combined with effective index approximation. >

Experimental analysis of transmission line parameters in high-speed GaAs digital circuit interconnects

Transmission line properties of typical high-speed interconnects were experimentally investigated by fabricating and characterizing coplanar strips on semi-insulating GaAs substrates. The strips have thicknesses of about 2500 AA or 5000 AA and widths of 4, 6, or 8 mu m so as to be representative of on-chip interconnects in high-speed GaAs digital circuits. Measurements are carried out up to 18 GHz, and the pertinent line parameters, such as resistance, capacitance per unit length, and characteristic impedance, are extracted using the measured S-parameters. The measurement results confirm the quasi-TEM properties of such interconnects. In all cases, the measured distributed capacitance and inductance are sensitive to frequency whereas the resistance is found to increase as much as 38% for the widest and thickest conductors. >

Reflection properties of coupled-ring reflectors

The reflection properties of a coupled-ring reflector (CRR) are analyzed using the transfer matrix method. The CRR is composed of two coupled rings that are coupled to a bus waveguide. Depending on the combination of the cross-coupling ratios of the couplers, the reflection spectrum shape changes greatly. In the lossless case, the reflection spectrum has four peaks when the ring-bus coupling ratio is small. When the ring-bus coupling ratio is more than half and the ring-ring coupling is very small, the four peaks broaden, forming a single peak with still considerably narrow reflection spectrum, which makes the CRR attractive in applications requiring compact reflectors such as for single-mode laser diodes. The dependence of the propagation and insertion losses of the couplers on the reflection spectrum is also investigated, and the design guidelines are presented.

High-Q silicon-on-insulator optical rib waveguide racetrack resonators

In this work, detailed design and realization of high quality factor (Q) racetrack resonators based on silicon-on-insulator rib waveguides are presented. Aiming to achieve critical coupling, suitable waveguide geometry is determined after extensive numerical studies of bending loss. The final design is obtained after coupling factor calculations and estimation of propagation loss. Resonators with quality factors (Q) as high as 119000 has been achieved, the highest Q value for resonators based on silicon-on-insulator rib waveguides to date with extinction ratios as large as 12 dB.

Finite-element analysis of valence band structure and optical properties of quantum-wire arrays on vicinal substrates

The valence subband dispersion of quantum-wire arrays grown on vicinal substrates in GaAs-Al/sub x/Ga/sub 1-x/As material system is calculated using a finite-element method with periodic boundary conditions. The variational functional for the Luttinger-Kohn Hamiltonian is derived using the integration by parts with proper boundary conditions. The validity of this method is confirmed by calculating subband structure of quantum wells and rectangular quantum wires. Along with the electronic band structure, a detailed study of gain in the quantum-wire arrays with rectangular and serpentine shapes is presented, including the effect of coupling between wires and polarization dependence of the momentum matrix element. Finally, these results are compared to those of quantum wells. >

Low-power thermooptical tuning of SOI resonator switch

A wavelength selective optical switch is developed based on a high-Q racetrack resonator making use of the large thermooptic coefficient of silicon. The racetrack resonator was fabricated using a silicon-on-insulator (SOI) single-mode rib waveguide. The resonator shows a high Q factor of 38 000 with spectral sidelobes of 11 dB down and can be thermooptically scanned over its full free-spectral range applying only 57 mW of electrical power. A low power of 17 mW is enough to tune the device from resonance to off-resonance state. The device functions as a wavelength selective optical switch with a 3-dB cutoff frequency of 210 kHz.