M. Cada

Optical bistability in semiconductor periodic structures

A theoretical demonstration of optical bistability in periodic layered media, particularly in long-period GaAs/AlAs superlattices is presented. The proposed structure consists of a periodic multilayer system, as opposed to previously demonstrated nonlinear bistable devices which employed Fabry-Perot etalons. The optical resonance effect which is essential for bistable devices is, in this case, induced by a refractive index modulation. The nonlinear active medium is distributed in the whole structure rather than placed between the two mirrors of a Fabry-Perot cavity. It is shown by a complete calculation of wave propagation in the periodic nonlinear medium that a multiple valued feature appears in the structures nonlinear reflectivity spectrum. The input/output characteristics of the structure exhibit bistable hysteresis similar to that of a nonlinear Fabry-Perot etalon. >

An analysis of a planar optical directional coupler with a lossless Kerr-like coupling medium

The nonlinear all-optically controlled coupling characteristics are studied analytically and numerically for two planar optical waveguides coupled through a lossless Kerr-like slab. The exact dispersion equation for this waveguiding structure is derived in terms of Jacobian elliptic functions. The optical intensity-dependent coupling length is described as a combination of symmetriclike and antisymmetriclike modes of propagation, derived from the nonlinear combination of the independent modes of the structure. A simple analytical formula is derived for the critical power, defined as the power above which the antisymmetric mode disappears and complete energy transfer between the two waveguides is no longer possible. The analysis provides insight into the complicated nature of the waveguiding structure behavior, and it shows one possible approach to solving difficult propagation problems in nonlinear optical waveguides of this or similar configuration. >

Intrinsic modes in tapered optical waveguides

Intrinsic modes are recently developed source-free field solutions in homogeneous wedge-shaped layers with penetrable boundaries. Expressed as spectral integrals over a continuum of plane waves, they can be computed to chart the continuous progress of a propagating wave field as it passes from its well-trapped regime in the wider section to the radiating regime through local cutoff in the narrower part. Intrinsic modes are explored here for application to tapered optical waveguides embedded within different exterior media. Two-dimensional electromagnetic TGE (transverse electric) and TM (transverse magnetic) field solutions are presented and evaluated numerically for small as well as large taper angles and various material parameters. The results reveal the effect of asymmetry in the exterior media on the intrinsic mode shapes, and the control that can be exercised thereby to transfer the optical energy from the taper to the surrounding environment. >

Exact analytic solutions to the nonlinear wave equation for a saturable Kerr-like medium: modes of nonlinear optical waveguides and couplers

Exact analytic solutions to the nonlinear wave equation for a lossless saturable Kerr-like medium are found. The nonlinearity is an intensity-dependent refractive index which saturates according to the model n/sup 2/=n/sub 0//sup 2//spl plusmn/a|E|/sup 2//(1+b|E|/sup 2/). The solutions to the wave equation are used to determine the TE power-dependent dispersion relations, and therefore the guided modes, of a single waveguide structure and of a directional coupler whose coupling medium is the nonlinear saturable material. Although the interaction between the sinh- and cosh-like modes of the coupler, and therefore the coupling length, is not treated in this work, it is shown that the critical power, defined in the literature to be the level of input power above which 100% of the power cannot be switched between the guides of a coupler, is a mathematical misinterpretation in both saturable media as well as in nonsaturable Kerr media. The absence of a critical power in single waveguide structures is also demonstrated. Parameters of GaAs-based waveguides and of a GaAs-GaAlAs MQW coupling medium are used in the numerical analysis. >

Bistable characteristics and all-optical set-reset operations of 1.55-μm two-segment strained multiquantum-well DFB lasers

Bistable characteristics and all-optical set-reset operations in 1.55-/spl mu/m two-segment InGaAsP-InP strained multiquantum-well (MQW) DFB lasers were studied. An extinction ratio as high as 20 dB with a lasing output power of 4 mW was obtained, partially due to the dispersion effect of strained MQW DFB structures. The detailed transient dynamics of the optical set-reset operations were observed for the first time, with optical injection from a single-mode laser, implying a potential for high-speed applications. A switch-on time in subnanosecond region and a switch-off time of 2.5 ns were measured using input pulses with a peak power of 500 /spl mu/W.<<ETX>>

A substantially improved treatment of intrinsic modes in tapered optical waveguides

The intrinsic modes are synthesized by self-consistent multiple reflected plane wave spectra, which furnish a spectral integral that satisfies exactly the wave equation and boundary conditions in the tapered environment away from the apex. For comparison to the local modes, and for numerical implementation so far, the spectral integrand has been approximated in a form suitable for small taper angles. This approximation introduces errors for larger tapers. It is shown that, by an alternative treatment of the plane wave spectra, the exact integrand can be restructured so that the integral is readily computable, with convergence improved for larger tapers. From this new form, it is verified by numerical tests that the previous errors are removed, and that an intrinsic mode is indeed a self-contained field solution which is decoupled from the other intrinsic modes. >

Optical harmonic mixers

Mixing of optical signals in a waveguide via the second-order nonlinear effect in semiconductors was studied theoretically and experimentally. The InP-based material system was the main focus of the investigations. Both the up-conversion leading to second-harmonic generation and the down-conversion leading to optical rectification were analyzed and experimentally verified. Experiments in nonlinear optical waveguide structures at 1.55 /spl mu/m wavelength have shown the existence of both the quadratic second-harmonic signals as well as the linearly dependent optical rectification signals. Quantitative estimates confirmed a good agreement between the theory and the experiment. Some interesting applications for high-speed optical data processing required in transparent optical telecommunication systems have been identified. Most attractive functions proposed to be implemented with these types of devices are, for example, flexible-to-bit-rate all-optical clock extraction in a transparent high-performance optical telecommunication network or accurate and fast measurement of optical frequency for optical synchronization needs in wavelength-division-multiplexing systems. >

Analysis and design of combined distributed-feedback/Fabry-Perot structures for surface-emitting semiconductor lasers

We propose combined distributed-feedback/Fabry-Perot (DFB/FP) structures for surface-emitting semiconductor lasers. The analysis is based on coupled-wave equations modified for surface-emitting lasers. The proposed structures, which exhibit enhanced resonance due to a matching between the gain and field distributions resulting in a reduced threshold compared with simple FP structures, are formed by placing the DFB structure between two DBR mirrors of an FP resonant cavity and introducing phase layers between the DFB region and the mirrors. It was found that the periodic-gain structures are a special case of the combined DFB/FP structures in which the index coupling effect is assumed to be negligible due to a small fill factor or a small refractive-index difference. The effect of complex (gain and index) coupling on the design and the threshold characteristics of the structures is clearly illustrated. Some important design considerations that were neglected in the previous papers are addressed.