W. S. C. Chang

Bragg coupling efficiency for guided acoustooptic interaction in GaAs

The guided acoustooptical interaction on the (100) plane of GaAs is investigated as a function of waveguide thickness type of mode, acoustic frequency, direction of acoustic wave propagation, and substrate refractive index Our calculated results indicate that best efficiency is obtained for TE(0) ? TE(0) optical modes and for acoustic surface wave propagating along the ?01l? direction at a waveguide thickness close to cut-off Under these conditions, approximately 75 mW of acoustic power is required for 100% diffraction. For a transducer aperture giving 50 Omega of radiation resistance, the rf bandwidth of the diffraction is limited essentially by the frequency bandwidth of the interdigital transducer. A comparison of the calculated results with experimental data at 1.06-microm optical wavelength is also given.

Fresnel lens in a thin‐film waveguide

The theoretical intensity profile of a phase shift and absorption Fresnel lens in a thin‐film waveguide has been calculated. An experimental lens has been constructed with F=5 and a focal length=4 mm using CeO deposited on a BaO waveguide. A 3‐dB spot size (full width half‐maximum) of 3 μm was obtained compared to a diffraction‐limited spot size of 2 μm from theory. Efficiency is approximately 25% and a variation of less than 3 dB in peak intensity is observed over incident angles of up to ±15° from normal.

Analysis of holographic thin film grating coupler.

The holographic grating coupler was used experimentally by H. Kogelnik and T. P. Sosnowski to obtain high coupling efficiency in thin film waveguides. This paper presents a perturbation analysis of that grating coupler. The calculated results agree very well with the experimental data. The analysis indicates that the maximum efficiency will depend on the effective length of the evanescent tail of the guided wave mode in the gelatin. The longer the evanescent tail is, the higher will be the excitation efficiency; the maximum possible efficiency is, of course, 81%. The maximum efficiency does not depend significantly on the refractive index of the grating, or the slant angle of the grooves, or the thickness of the gelatin (provided that the thickness is larger than the length of the evanescent tail).

Convolution using guided acousto‐optical interaction in As2S3 waveguides

Convolution of two acoustic surface waves (ASW) has been obtained by the acousto‐optical diffraction of an optical wave in an As2S3 waveguide. A simple theory is presented to show the convolution process. Experimentally, very high acousto‐optical diffraction efficiency (93% at 3 mW of acoustic power) has been observed in the As2S3 waveguide. A triangular pulse was observed on the oscilloscope through an optical heterodyne detector when two rectangular ASW pulses were convolved with each other.

A new method for the efficient interconnection of high‐index planar waveguides to low‐index transitional waveguides

An efficient method for interconnecting two planar optical waveguides on the same substrate is described. Efficiencies better than 80% have been obtained in coupling from high‐index GaAs and LiNbO3 waveguides to low‐index AZ 1350 and polyurethane waveguides. This method is important for the coupling of high‐index waveguides to optical fibers.

GaAs optical waveguide structures at 10.6-microm wavelength.

A comparison of GaAs/n(+)GaAs, GaAs/GaAsP, and GaAs/AlGaAs waveguide structures is presented. Their fabrication processes and their transmission properties at 10.6-microm wavelength are described. The loss due to the free carrier absorption of the substrate is analyzed. Experimentally, an attenuation rate of 2 dB/cm in a single mode (~7 microm) GaAs/GaAsp waveguide with a maximum dimension of the order of 7 cm has been achieved.

New method for exposing mammalian cells to intense laser radiation using the evanescent fields created in optical waveguides.

A new technique using optical waveguides has been developed for studying the effects of moderate to high power laser radiation on cells [1]. A monolayer of attached cells covering a wide area on the waveguide surface is exposed to the evanescent tail produced by the propagating waveguide mode. The optical radiation-cell interaction is limited to involve only the high power density and intense electric field strength associated with the evanescent tail. It is well established that extremely high power densities (105 to 108 watts/ cm2)are obtained in the waveguide when a waveguide mode is excited by a moderate power (1–2 watts), continuous wave (CW) laser [2–3]. These optical power densities create intense electric fields at optical frequencies ranging from 103 to 105 volts/cm within the waveguide. The evanescent tail of such a waveguide mode extends into the medium above the waveguide and creates an intense electric field at the interface between the waveguide and the medium. By adjusting the input laser power and waveguide parameters, the electromagnetic field intensity and distance of penetration into the cell of the evanescent tail can be varied over a wide range. The importance of intense electromagnetic field induced kinetic effects and photochemical reactions in producing lethal effects in cells has been stressed by previous investigators [4–6]. This report describes the reduced survival measured for monolayers of EMT-6 mammary sarcoma cells exposed to the evanescent tail created in a waveguide.

GaAs waveguide detectors for 1.06 μm

GaAs electroabsorption avalanche photodiodes in n‐n+ GaAs waveguides have been used to detect the 1.06‐μm radiation from a Nd : YAG laser with a responsivity of 125 A/W and an internal quantum efficiency of 23%. The response at this wavelength is due to a combination of band‐to‐band and defect‐to‐band electroabsorption.

Radiation‐induced effects in GaAs thin‐film optical (10.6 μm) waveguides

Two types of GaAs thin‐film optical waveguide structures operating at 10.6 μm were examined before and after exposure to neutron and γ irradiation. The attenuation rate of the GaAs/n+‐GaAs structure was particularly sensitive to neutron irradiation of 1013 cm−2 and exhibited postirradiation annealing at 150 °C. This is in contrast to the relative neutron irradiation insensitivity of a GaAs/GaAs1−xPx/n+‐GaAs structure. The effect of γ radiation is less pronounced for both structures. The radiation‐induced changes are discussed in terms of free‐carrier absorption, index of refraction, scattering centers, and absorption by complexes.