Shyh Wang

Studies of the Use of Gyrotropic and Anisotropic Materials for Mode Conversion in Thin‐Film Optical‐Waveguide Applications

Wave propagation in thin‐film optical waveguides using gyrotropic or anisotropic material as substrate is studied. At the film‐substrate boundary, the relation between the incident and reflected fields can be expressed in terms of a scattering matrix with matrix elements representing the TE‐to‐TE and TM‐to‐TM reflection coefficients and the TE‐to‐TM and TM‐to‐TE conversion factors. The property of the scattering matrix is examined, and the condition for continuous and complete mode conversion is derived. Various schemes to achieve complete mode conversion are proposed and the performance characteristics, including the physical dimension and the tolerances in the incident angle and film thickness, for several mode‐converting structures are computed and compared. Once the practicability of complete mode conversion is established, it is possible to study the feasibility of practical devices. Various schemes for the realization of such devices as the gyrator, the isolator, the optical switch, and the nondestr...

Ti-diffused LiNbO 3 branched-waveguide modulators: Performance and design

We report here experimental studies of an amplitude modulator using Ti-diffused LiNbO 3 branched waveguides. In this paper we present experimental evidence in support of three problem areas connected with the improvement of performance of the device. Solution of the problems has involved the development of a new in-diffusion process to eliminate any background radiation, introduction of a new electrode arrangement which consists of extra electrodes at the branching section to control the splitting action, and finally performing measurements at a longer wavelength ( \lambda_{0} = 1.15 \mu ) to overcome the limitations of available photomasks.

Optical pulse compression by nonlinear coupling

Nonlinear coupled waveguide is studied for optical pulse compression, utilizing the power‐dependent performance of such waveguides caused by the intensity‐dependent refractive index in the region of interaction. Theoretical calculations predict that the pulse compression to less than 1/5 of the original pulse width is achievable with a loss of 3 dB.

Proposal of periodic layered waveguide structures for distributed lasers

Here new periodic structures are proposed for distributed lasers, utilizing the thickness variation introduced in a multilayer thin‐film optical waveguide. Wave propagation in such structures is analyzed. Expressions are derived for the amount of distributed feedback and possible scattering losses. From the coupled‐wave equations, the laser‐oscillation conditions are set up. Computational results for the GaAs–GaxAl1−xAs and dye lasers are presented. The applicability of the proposed scheme for distributed feedback to semiconductor, dye, and solid‐state lasers is discussed.

Semiconductor injection lasers with a circular resonator

Outstanding lasing characteristics of a new GaAs‐(GaAl)As ring laser is reported. The basic structure of the new laser includes a circular resonator and straight output branching waveguide. The possibility to eliminate the need for feedback reflectors should make it attractive for integrated optics applications.

Growth characteristics of GaAs‐Ga1−xAlxAs structures fabricated by liquid‐phase epitaxy over preferentially etched channels

In this paper, we report our studies of the liquid‐phase epitaxy of GaAs and Ga1−xAlxAs single‐ and double‐layered structures over preferentially etched channels in GaAs substrates. Results obtained indicate that various optical waveguide structures providing lateral optical confinement can be fabricated by this etch‐and‐fill technique. Further, it is found that the filling of the channels is dictated by surface tension of the melt rather than by preferential growth. This growth characteristic lessens the dependence of the final profile of the grown layer on the initial etched profile and makes the etch‐and‐fill technique particularly suitable for the fabrication of optical bends.

Surface‐emitting laser diode with vertical GaAs/GaAlAs quarter‐wavelength multilayers and lateral buried heterostructure

Threshold current of 2 mA at room temperature cw operation is realized in a vertical distributed feedback surface‐emitting laser diode with lateral buried heterostructure (LBH). In this LBH structure, the vertical distributed feedback active region (AlGaAs/GaAs multilayer) is entirely surrounded with n‐ and p‐type AlGaAs cladding layers for minority‐carrier confinement. The far‐field angle is 7°. The beam shape is nearly circular. However, the lasing spectrum is broad (2–3 nm) compared with the conventional edge‐emitting laser. Major differences between the surface‐emitting laser diode presented here and the conventional edge‐emitting laser diode are discussed.

Simulation studies of bifurcation and chaos in semiconductor lasers

We apply the phase portrait analysis for photon density and carrier density to analyze simulation results obtained from the rate equations for semiconductor lasers. On the two‐dimensional bifurcation diagram of modulation depth and modulation frequency, there are six regions: periodic pulse, continuous oscillation wave, chaos, period doubling of single period, period multiplying of single period, and period doubling of two period. The region of chaos is located below the region of periodic pulse. By properly choosing the modulation frequency, chaotic behavior can be avoided. A normalized two‐dimensional bifurcation diagram defining the periodic pulse region is presented for the purpose of locating the suitable region for modulation of a semiconductor laser.

Cross‐talk problems in optical directional couplers

Cross‐talk in optical directional couplers is usually considered to be limited only by the processing technology. We show that unequal coupling of an input wave to the even and the odd mode of the coupling region gives rise to another kind of cross‐talk. The extinction ratio is well approximated by the overlap integral of the fields in the two waveguides. For a given coupling length, the higher the index step a waveguide has, the less the cross‐talk is. For straight couplers without bends one way to eliminate the cross‐talk is to offset the input wave relative to the coupled guides with the penalty of power loss. A three‐waveguide coupler has a different cross‐talk problem of comparable magnitude.

Raman study of an epitaxial GaAs layer on a Si [100] substrate

A bevel has been etched in a GaAs epitaxial film grown on a Si substrate, so that the Raman spectrum of the GaAs layers can be measured as a function of distance from the GaAs/Si interface. The amount of strain and disorder in the GaAs film has been estimated from the GaAs longitudinal optical phonon line shape and frequency. Both the strain and the amount of disorder were found to decrease with increase in the distance from the interface.