Design of strip-loaded optical waveguides for low-loss coupling tooptical fibers

Mismatch between the fundamental mode of a waveguide and that of an attached optical fiber may cause a significant loss of power in a guided-wave structure. These losses can be minimized, however, by proper shaping of the waveguide mode through judicious choice of dimensions and refractive indexes. It is shown by computer modeling and qualitative arguments that coupling losses between standard optical fibers (5-10-μm modal diameter) and strip-loaded waveguides constructed of GaAs/AlGaAs may be reduced to less than 10% at each facet. A wavelength of 0.85 μm is used as a specific example, with modal shapes explored systematically for a series of different waveguide structures     

Vector finite element solution of saturable nonlinear strip-loaded optical waveguides

A numerical solution is given for nonlinear optical waveguides with power confinement in both transverse dimensions. Self-consistent solutions are obtained by using an accurate vector-finite-element formulation along with the penalty technique. Numerical results for the first quasi-transverse-magnetic power-dependent mode are presented for strip-loaded waveguides with saturable self-focusing media. The variations with total power are illustrated for the modal index and for the fraction of the total power carried by different regions, showing interesting abrupt power switching for realistic geometries. It is shown that the switching effect is maintained in the presence of saturation and over a range of two-transverse-dimensions geometries. This switching effect can be controlled with variation of some of the parameters of the guide.<>     

Modal analysis of strip-loaded diffused optical waveguides by a variational method

The propagating modes supported by strip-loaded three-dimensional diffused optical waveguides are analyzed in detail theoretically by using a variational method. Some typical numerical examples of the propagation constants and the field distributions are illustrated. The dependence of propagation characteristics on the geometrical parameters of the loading dielectric strip is discussed.     

Design of strip-loaded weak-guiding multimode interferencestructure for an optical router

We introduce a new multimode interference structure that can be used as an integrated optical router. This structure is based on the existence of a first cross state in weakly guiding multimode waveguides. Such a cross image, not expected by the self-imaging technique, is tested by rigorous modal analysis as well as the nonmodal beam propagation method. We show that the quality of this image could be enhanced by strip loading the multimode section of the structure. Making use of this image, we propose a new optical router based on the large nonlinearity associated with the blockaded reservoir and quantum-well electron transfer structure     

Low-loss GaAs/GaAlAs strip-loaded waveguides with high coupling efficiency to single-mode fibres

We report measurements of low-loss MOVPE-grown GaAlAs/GaAs strip-loaded waveguides with high coupling efficiency to single-mode fibres. A propagation loss of 2 dB/cm and a coupling efficiency of 70% has been achieved for a p+-i-n+-structure waveguide. A propagation loss of 0.7 dB/cm and a coupling efficiency of 61% has also been achieved for an n?-i-n?-structure waveguide.     

Material dispersion zero in infra-red waveguides

The operation of very low loss, long optical fibres implies use of a wavelength near the material dispersion zero where d2n/d?2 = 0. Using a technique developed by Wemple, this wavelength is estimated for a large group of oxides and halides. These data can be used to locate potential infra-red waveguide materials.     

Design and fabrication of silicon photonic crystal optical waveguides

We have designed and fabricated waveguides that incorporate two-dimensional (2-D) photonic crystal geometry for lateral confinement of light, and total internal reflection for vertical confinement. Both square and triangular photonic crystal lattices were analyzed. A three-dimensional (3-D) finite-difference time-domain (FDTD) analysis was used to find design parameters of the photonic crystal and to calculate dispersion relations for the guided modes in the waveguide structure. We have developed a new fabrication technique to define these waveguides into silicon-on-insulator material. The waveguides are suspended in air in order to improve confinement in the vertical direction and symmetry properties of the structure. High-resolution fabrication allowed us to include different types of bends and optical cavities within the waveguides.     

Equivalent optical waveguides

We develop a comparison method which identifies a waveguide with properties that are well known and are very similar to the one under investigation. It has a very simple mathematical basis which allows a trivial derivation of the moment method for circular cross-sectional fibers includingW-fibers, and an extension to noncircular optical waveguides. We first use the scalar approximation, and then extend to the full vector theory which accounts for polarization. Our concern is with fundamental mode propagation constants and higher mode cutoffs.     

Sputtered-glass optical waveguides

Research into low-loss optical-frequency planar waveguides has produced a technique for controlling the propagation characteristics during the sputter-deposition process. The basis of the technique is discussed, and some results are given.     

Sandwich-ribbon optical waveguides

The preparation of composite-glass-ribbon optical waveguides is described, of which the guiding layer is a thin rectangular filament of lead glass, supported by a soda-lime-glass substrate. Single-mode and multimode guides have been made, and their mode-transmission properties observed.     

Low-loss polymeric optical waveguides fabricated with deuteratedpolyfluoromethacrylate

Deuterated polyfluoromethacrylate which has high transparency, low birefringence and good processability was newly synthesized for use as optical waveguide materials, and both single-mode and multimode optical waveguides were fabricated using the polymer. The propagation loss and waveguide birefringence of the single-mode waveguides were as low as 0.10 dB/cm and -5.5×10^-6 at 1.31 μm, respectively. The propagation losses of the multimode waveguides were less than 0.02 dB/cm at both 0.68 and 0.83 μm, and 0.07 dB/cm at 1.31 μm     

Field-realigned nematic-liquid-crystal optical waveguides

An external electric field exerts volume torque on the nematic liquid crystal due to the anisotropic molecular polarizability and tends to reorient the molecular axes. Phase-velocity tuning in these materials is thus possible. Eigenmodes of thin-film liquid-crystal waveguides are analyzed by a numerical method. A three-layer model of the waveguides is found to be in good agreement with the measured data, thus providing a simplified model for conceptual thinking as well as for computation.     

Mode-propagation in optical waveguides

The modal fields and propagation coefficients of a 4-region optical waveguide with diffused boundaries are determined using conventional perturbation theory.     

Transmission characteristics of optical waveguides with multiquantum-well structures

The fundamental modes in an optical waveguide with multiquantum-well structure are analysed by the matrix method. It is found that the separation between the propagation constants of the TE and TM waves is relatively large and that the equivalent planar slab guide approximation is available only for TE waves.     

Integration of high-speed optical taps with InP waveguides

Optical taps are key elements for optical signal processing. The authors demonstrate the high-speed operation of an InGaAs p-i-n photodetector on InP waveguides. For a 300- mu m-long tap that samples 19% of the light propagating in a 7- mu m-wide waveguide, the response time is 50 ps at a 1 V reverse bias. The taps require a simple epitaxial growth and can readily be integrated with electrooptic modulators and other optoelectronic devices for multigigahertz optical signal processing.<>     

Strip-loaded diffused optical waveguides

A strip-loaded diffused optical waveguide consists of a one-dimensional diffused guide with a narrow loading strip of dielectric deposited on the surface. Two-dimensional modes can exist in the diffused material, confined in depth by the diffusion and in width by the loading strip. The propagation constants of these two-dimensional modes are calculated. The requirements for the index of refraction and width of the loading strip are discussed along with the rate of decay of electric-field amplitude away from the strip. A numerical example typical of Ti-diffused LiNbO3is given.     

Phase coherence of optical waveguides

The effective refractive index of real waveguides is not constant, but fluctuates as a result of variations in composition and waveguide dimensions. Consequently, the accumulated phase during propagation has a component that undergoes a random walk and whose mean square increases with length 〈Δφ²〉=2L/L_coh. These phase fluctuations result in wavelength fluctuations in Mach-Zehnder interferometers, especially in interferometers of low order. By measuring these fluctuations for Mach-Zehnder interferometers of different order, we have verified the above relation and determined that L_coh≈27 m for our phosphorus-doped core silica on silicon waveguides