Coupled Channel Waveguides Research Articles

Full-vectorial analysis of high-index-contrast coupled channel waveguides

Coupling of two identical channel waveguides separated by an air gap is analyzed. The coupled structure exhibits a strong refractive index contrast in both the transverse and longitudinal dimensions, which necessitates the use of a full-vectorial model. The 3D full-vectorial bidirectional method-of-lines beam propagation is utilized for this purpose. The effect of the transverse and longitudinal displacements on the modal reflectivity and modal transmissivity of the fundamental TE-like and TM-like modes is reported. Numerical results are presented for both the full-vectorial model and the approximate semivectorial model. A significant difference between the predictions of these two models is seen.

Formation and light guiding properties of dark solitons in one-dimensional waveguide arrays

We report on the formation of dark discrete solitons in a nonlinear periodic system consisting of evanescently coupled channel waveguides in defocusing lithium niobate. Localized nonlinear dark modes displaying a phase jump in the center that is located either on-channel (mode A) or in-between channels (mode B) are formed, which is to our knowledge the first experimental observation of mode B. By numerical simulations we find that the saturable nature of the nonlinearity is responsible for the improved stability of mode B. The ability of the induced refractive index structures to guide light of a low-power probe beam is demonstrated.

Nonlinear beam interactions in 1D discrete Kerr systems

The interaction between parallel beams in one-dimensional discrete Kerr systems has been investigated using arrays of coupled channel waveguides. The experiments were performed in AlGaAs waveguides at 1550 nm which corresponds to photon energies just below one half the semiconductor's bandgap. The input intensity and relative input phase between the input beams was varied and the output intensity patterns were recorded. Observed was behavior ranging from a linear response, to soliton interactions between moderately and then strongly localized spatial solitons. Finally the influence of multiphoton absorption and asymmetric beam inputs on these interactions was investigated at very high intensities.

Discrete modulational instability in periodically poled lithium niobate waveguide arrays

Parametric gain associated with discrete modulational instability due to the second order nonlinearity chi(2)(-2omega;omega,omega) was investigated experimentally in periodically poled lithium niobate arrays of weakly coupled channel waveguides for conditions of both positive and negative phase-mismatch for second harmonic generation.

Observation of discrete quadratic solitons.

Discrete solitons with two frequency components mutually locked by a quadratic nonlinearity have been observed for the first time. Optical experiments have been performed in arrays of coupled channel waveguides with tunable cascaded quadratic nonlinearity. The tunability was the prerequisite that soliton species with different topology could be identified in the same array. Moreover, soliton stability has been experimentally probed. Good agreement with theoretical predictions was found.

Light propagation in an array of rectilinear and curvilinear coupled channel waveguides

Light transmission through an array of tunnel-coupled channel waveguides is studied in detail. It is shown that a discrete structure of the light-transmitting medium leads to a modification of the law of light refraction. It is also shown that a new type of total internal reflection of light inside the structure manifests itself in an inhomogeneous array of coupled waveguides. The process of light propagation in an array of identical channel waveguides, which are arranged equidistantly along concentric circles, is investigated. It is established that such a waveguide array is equivalent to an inhomogeneous system of coupled waveguides. The mode excitation process in an array of ring channel waveguides located on cylindrical and conical surfaces is considered.

Coupling between photonic crystal waveguides

A calculation procedure for evaluation of the coupling length of two parallel coupled channel waveguides in a planar photonic crystal is proposed. The first step of the calculation is evaluation of the band structure of a photonic crystal containing two coupled linear defects. The eigenvalue corresponding to eigenstates localized in the linear defect (the waveguide) is split due to the coupling. This splitting is treated within the coupled-mode theory that yields a simple relation between the splitting and the coupling length. The MIT photonic bands code is used to evaluate the coupling between channel waveguides in silicon./sup 1/ These calculations show that in contrast to the finite-difference time-domain approach, the method is effective for three-dimensional light propagation.

Vertically coupled InGaAsP/InP buried rib waveguide filter

We report the first demonstration of narrowband (<17 Å full width at half maximum) wavelength selective grating-assisted coupling between vertically stacked buried rib InGaAsP/InP channel waveguides. These vertically coupled channel waveguides form the basis both of integrable filters essential to photonic circuits such as amplifier/filter and demultiplexer/detector circuits as well as a general approach to vertical integration of photonic circuits.

Nonlinear transfer of light in coupled Rb:KTP waveguides

An investigation was made of the propagation of subnanosecond light pulses through a system of coupled channel waveguides formed in a potassium titanyl phosphate (KTP) crystal. An increase in the input signal to 5 kW resulted in a nonlinear transfer of 40% of the optical power from one channel to another.

Coupled mode formulation for directional couplers with longitudinal perturbation

A grating-assisted directional coupler is investigated using an improved coupled-mode formulation for multi-waveguide systems with longitudinal perturbation. This approach is capable of handling three-dimensional structures as well as a complex index of refraction and nonisotropic media. The case of two coupled channel waveguides is closely examined, and a numerical analysis is carried out for several cases of slab structures. >

Rigorous finite-difference analysis of coupled channel waveguides with arbitrarily varying index profile

A rigorous finite-difference formulation for the hybrid-mode analysis of coupled diffused dielectric channel waveguides is presented. The method includes the two-dimensional continuous index profile variations directly in the finite-difference form of coupled equations and avoids the shortcomings inherent in the usual staircase approximations. The formulation in terms of the wave equation for the transverse components of the magnetic field leads to an eigenvalue problem where the nonphysical, spurious modes do not appear. The analysis includes the complete set of hybrid-modes, takes mode-conversion effects and complex waves into account, if they exist, and allows the calculation of dielectric channel waveguides with large index difference levels. Dispersion characteristic examples are calculated for coupled structures suitable for optical integrated circuits. The theory verified by comparison with results available from other methods. >

Optical waveguide directional coupler measurements using a microcomputer-assisted TV camera system

Characterization of optical waveguide directional couplers using a microcomputer-assisted TV camera system is reported. A fluorescent technique is employed to visualize wave propagation in the optical waveguides, which exhibit small scattering. The method is simple and reproducible, and can be used to estimate waveguide losses and evaluate various optical devices such as waveguide lenses and waveguide interferometers on substrates. As an example of its application, parallel and nonparallel dual-chamber directional couplers and parallel triple-channel directional couplers are measured. Power transfer between the coupled channel waveguides is observed clearly, and coupling coefficients of the couplers are obtained nondestructively. The measured performance of the couplers is in good agreement with that predicted by coupled-mode analysis. >

Coupled GaAs multiple-quantum-well channel waveguides including quadratic electrooptic effect

Two coupled channel waveguides using GaAs multiple quantum wells are investigated using an improved coupled-mode theory for anisotropic waveguides and the effective index method. The quadratic electrooptic effect in the multiquantum-well structures is taken into consideration. It is shown that only a moderate electric field for multiquantum-well waveguides is necessary to achieve the optical power switching compared with that required for a bulk GaAs waveguide. >

Model of a nonlinear directional coupler in gallium arsenide

We have calculated the response of a nonlinear directional coupler fabricated in an AlGaAs/GaAs structure using a realistic model for the optical nonlinearities. The model takes into account carrier-density-dependent index of refraction and absorption, and lateral carrier diffusion in each of the two coupled channel waveguides. Our results differ considerably from those obtained using a Kerr model for the optical nonlinearities.

Investigation of an electrooptic modulator formed from coupled channel diffused waveguides in LiNbO3

A calculation was made of the characteristics of an electrooptic modulator utilizing coupled channel waveguides in a Z-cut LiNbO3 crystal. Various parameters of the waveguides and electrode system were assumed in the calculation. A modulator was made from coupled channel waveguides with a control voltage of 17 V (λ =1.06 μ) and a depth of modulation 90%. The frequency characteristic of the modulator was determined. A method was developed for the determination of the waveguide coupling coefficient by measuring the relative efficiency of second harmonic generation.