Eric Gung-Hwa Lean

Optical Guided Wave Mode Conversion by an Acoustic Surface Wave

The experimental observation of efficient (55%) mode conversion of thin‐film optical guided waves by a collinear interaction with a surface acoustic wave is reported. The effects of waveguide dispersion and finite geometry are discussed.

Thin-film acoustooptic devices

Many thin-film acoustooptic interaction experiments for Bragg modulators and deflectors, for mode converters, and for fast switches have been demonstrated. Due to the high-power density, long interaction length, and controllable dispersion associated with guided-wave structures, thin-film acoustooptic devices have proved to have advantages over their bulk wave counterparts. In this paper, thin-film acoustooptic devices are discussed in terms of 1) the unique features in theoretical calculations which arise from the use of guided-wave structures, 2) the possible device configurations which may improve efficiency and speed, 3) the thin-film material and fabrication problems, 4) the transducer efficiency bandwidth trade-off, and 5) a comparison with bulk acoustooptic devices and thin-film electrooptic devices. Recent experimental results for thin-film acoustooptic modulators and convolvers are included.

Optical probing of surface acoustic waves

Light diffraction by surface acoustic waves is used in studying the propagation characteristics of Rayleigh waves on y-cut z-oriented LiNbO 3 . In reflection, only the surface deformation contributes to the diffracted light. In transmission, the photoelastic effect, which is incident beam polarization dependent, must also be considered. Such optical probing has been used to study the Fresnel diffraction radiation pattern of an interdigital transducer, to measure beam steering effects related to transducer misalignment on an anisotropic substrate, and to measure reflection and transmission coefficients of an electrically loaded transducer. Attenuation measurements have been made over frequencies as high as 3.5 GHz. The power dependence of attenuation and harmonic generation is also examined.

Nondestructive Testing of Thin Films by Harmonic Generation of Dispersive Rayleigh Waves

A new and sensitive measurement technique of determining elastic properties of a thin film on a substrate based on harmonic generation of dispersive Rayleigh waves is demonstrated. The technique has a sensitivity of detecting a change of 0.001% in velocity due to the loading of the thin film. A damaged layer of about 20 A on a mechanically polished LiNbO3 substrate is detectable.

OPTICAL PROBING OF ACOUSTIC SURFACE‐WAVE HARMONIC GENERATION

The growth of nonlinearly generated harmonics of acoustic surface waves in LiNbO3 substrates has been observed directly by an optical probe. The attenuation of acoustic surface waves as a function of the level of input power and loss of energy to the nonlinearly generated harmonics is measured. The large nonlinear coupling to the high harmonics of surface acoustic waves, which is an encouraging sign for parametric amplification, may impose some limitations on linear devices at high power levels.

ACOUSTIC SURFACE WAVE MIXING ON α‐QUARTZ

Experimental demonstration of the sum and difference frequency generation of surface acoustic waves on α‐quartz is reported. Individually calibrated interdigital transducers were used to generate and detect the waves. The absolute level of the nonlinearly generated acoustic power was measured for various interaction lengths and found to agree with a theory based on nonlinear coupled amplitude equations.

Thin Film Acousto-Optic Devices

In this paper, thin film acousto-optic devices are discussed in terms of (1) the unique features in theoretical calculations which arise from the use of guided wave structures, (2) the possible device configurations which may improve efficiency and speed, (3) the thin film material and fabrication problems, (4) the transducer efficiency bandwidth trade-off, and (5) a comparison with bulk acousto-optic devices and thin film electro-optic devices. Recent experimental results for thin film acousto-optic modulators and convolvers are included.

III Interaction of Light and Acoustic Surface Waves

Publisher Summary This chapter describes the interaction of light and acoustic surface waves. It discusses two major areas in the interaction of light and acoustic surface waves. One is the light diffraction by acoustic surface waves and the other is the interactions of optical guided waves and acoustic surface waves. The study of light diffraction by acoustic surface waves results in a convenient optical probing technique to study the propagation characteristics of acoustic surface waves that are needed for the design and utilization of acoustic surface wave devices. The interest in the interaction of light and acoustic surface waves began after the recent advance of acoustic surface technology. In designing acoustic surface wave devices, it is necessary to have a detailed knowledge of the propagation characteristics of these waves on any isotropic or anisotropic substrate. Optical probing of acoustic surface waves based on the light diffraction by acoustic surface waves has become a convenient tool in studying and utilizing these waves.

Method and apparatus for acoustic scanning using bulk wave scattering of bulk waves by an acoustic grating

Electronically focused and automatically scanning acoustic bulk waves are produced by scattering chirp acoustic bulk waves at grazing incidence from an acoustic grating. The same apparatus may be used in reverse to convert diverging acoustic bulk waves produced by a focused beam into chirp acoustic bulk waves. A pulse compression filter then converts the chirp waves into signals describing the source.

Large‐angle acoustic‐beam steering in acoustically anisotropic crystal

We have demonstrated a unique scheme for electronically steering an acoustic beam over an angle large than 70° in a TeO2 crystal. The idea is based on the principle that the acoustic energy flow direction is always normal to the slowness curve in an acoustically anisotropic crystal. In the plane normal to the c axis of a TeO2 crystal and around the (110) axis, the shear wave acoustical energy flow directions can have a swing of more than 70° [from the (110) axis] by changing the wave‐vector direction only 3° from the (110) axis. Using a grating to diffract a shear wave propagation direction around the (110) axis, we have observed the wide‐angle acoustic‐beam steering effect by a laser probe.