Acoustooptic Bragg deflection in LiNbO3Ti-diffused waveguides

A LiNbO3acoustooptic Bragg deflector utilizing a Ti-diffused optical waveguide and acoustic surface waves, which requires only 50 mW of electrical drive power to obtain 70-percent deflection, has been demonstrated.     

Approximate calculation of leaky-mode loss coefficients for Ti-diffused LiNbO3waveguides

A hybrid-mode model is developed which allows analytical calculation of the leaky-mode loss coefficient in Ti-diffused LiNbO3waveguides. Field representations correspond to the approximation of geometrical optics. Hybrid-mode effective indexes are approximated utilizing the known anisotropy of LiNbO3. ForX- orY-cut waveguides the leaky mode is shown to be an extraordinary-like hybrid mode with a small ordinary field component which arises from extraordinary to ordinary power conversion at the abrupt index boundary. This ordinary index component is not bound by the graded index boundary, giving rise to the power loss. The theoretical dependence of the loss coefficient on propagation direction is in agreement with experiment.     

Strip-loaded diffused optical waveguides

A strip-loaded diffused optical waveguide consists of a one-dimensional diffused guide with a narrow loading strip of dielectric deposited on the surface. Two-dimensional modes can exist in the diffused material, confined in depth by the diffusion and in width by the loading strip. The propagation constants of these two-dimensional modes are calculated. The requirements for the index of refraction and width of the loading strip are discussed along with the rate of decay of electric-field amplitude away from the strip. A numerical example typical of Ti-diffused LiNbO3is given.     

Integrated optic electrooptic modulator electrode analysis

Integrated optical (IO) switches and modulators are based on the electrooptic effect obtained by applying a voltage between electrodes fabricated on the surface of a crystal (for example, LiNbO3). In this paper, we calculate the change in the optical propagation constant in single modeZ-cut LiNbO3:Ti-diffused waveguides for a variety of electrode configurations. These calculations are compared to experimental data from IO devices. The calculations predict the observed variations in switching voltage with wavelength and electrode separation.     

Optical switching device using leaky surface wave

The decay constants of leaky surface waves in an indiffused LiNbO3Y-cut plane propagation can be controlled by electro-optic effect. According to our calculation, we can obtain about 20 dB/ cm optical switching devices for applied electric field of 3 V/μm. Our experiments show that optical switching of 14 dB/cm can be obtained for 300 V/150 μm.     

Optical surface wave mode converters and modulators utilizing static strain-optic effects

Periodic perturbations of dielectric waveguides can be obtained by an evaporated SiO2thin film grating through the static strain-optic effect. These waveguides are applied to TE-TM mode converters and modulators in Ti-diffused LiNbO3waveguides. Using a coupling length of 3 mm in an optical surface waveguide, we have demonstrated a TE-TM mode conversion efficiency of 80 percent and a TE-TM mode modulator with 100 percent modulation with an applied field of 2 V/μm. A theoretical analysis of a reflector for a semiconductor laser waveguide is also described.     

Minimum-mode-size low-loss Ti: LiNbO3channel waveguides for efficient modulator operation at 1.3 µm

Efficient operation of Ti-diffused LiNbO3modulators and switches requires small, tightly guiding channel waveguide modes which effectively overlap the applied modulating field. This in turn requires Ti:LiNbO3channel waveguides with minimum mode size. The small-mode-size waveguides are obtained inz-cut crystals by optimizing the fabrication parameters to achieve maximum index change in the wave-guiding region and the optimum diffusion depth and width. For 6 h diffusion at 1025°C, and Ti strip thickness of 800 Å and strip width of 4 μm, the TM mode has a1/eintensity full width of 3.9 μm and full depth of 2.8 μm. This is by far the smallest mode size atlambda = 1.32 mum presented to date for Ti:LiNbO3waveguides. Since these waveguides have small mode size, largeDelta n, and low propagation loss (0.2 dB/cm) atlambda = 1.32 mum, it should be possible to fabricate modulators and switches with small voltage-length products and optical circuits that incorporate small bending radii curves, thus decreasing the total device length.     

Crosstalk in Ti:LiNbO3directional coupler switches caused by Ti concentration fluctuations

We have observed random fluctuations in the local Ti concentration profiles of Ti-diffused LiNbO3waveguides. Calculations show that these variations explain the degraded switch extinction ratio (i.e., crosstalk) and other nonideal characteristics observed in LiNbO3optical switches. We also demonstrate by calculation a new device configuration for eliminating this source of crosstalk.     

Characteristics of guided and leaky waves on multilayer thin-filmstructures with planar material gratings

This paper presents the characteristics of guided (surface) waves and leaky waves on multilayer structures with planar implanted periodic dielectric blocks. A three-dimensional (3-D) integral-equation formulation in conjunction with the method of moments (MoM's) is used to find the propagation constants of the surface-wave and leaky-wave modes. The analysis deals with layered structures with irregular implants. Photonic band-gaps of both guided waves and leaky waves for rectangular air-implants are identified. Anisotropic properties of the surface waves and leaky waves are investigated. The design of leaky-wave antennas with the information of mode characteristics is discussed. The analysis is validated through the comparison with a low-frequency effective-medium approach and results for linear gratings     

Radiation from cylindrical leaky waves

Formulas are derived for the far-infrared radiation pattern of cylindrical leaky waves propagating on a planar surface. The formulas can be used to predict the radiation pattern of a general class of leaky-wave antennas, consisting of a finite-size source which excites a radially propagating leaky wave on some planar surface. Leaky-wave antennas consisting of antenna elements embedded in dielectric layers (microstrip elements) fall into this category. Using the equivalence principle, formulas are derived for both transverse electric (TE) and transverse magnetic (TM) leaky waves with arbitrary propagation constants. The formulas allow for radiation from cylindrical apertures of arbitrary size, so that the effect of truncating the supporting planar surface with an absorbing material can be determined. Particular attention is devoted to the case of a leaky wave for which the real and imaginary parts of the complex propagation constant are equal, since this type of wave has been shown to be responsible for broadside radiation in certain leaky-wave antennas comprised of dielectric layers     

Leaky-wave propagation and radiation for a narrow-beammultiple-layer dielectric structure

Previous work has demonstrated that very narrow beam radiation patterns can be obtained from a simple source embedded within multiple dielectric layers of appropriate thicknesses above a ground plane. The configuration consists of dielectric layers having permittivities ϵ₁ and ϵ₂ stacked in an alternating arrangement, with ϵ₂>ϵ₁. This narrow-beam effect can be attributed to weakly attenuated leaky waves that exist on the structure. Simple asymptotic formulas for the propagation and attenuation constants are derived. The formulas show how the beamwidth varies with the number of layers and the material constants. The exact radiation pattern is compared with the leaky-wave pattern for a specific case to demonstrate the role of the leaky waves in determining the total pattern     

Theoretical and experimental studies on active Y-junctions in optical waveguides

Theoretical and experimental results are presented on single-modeY-junction switching devices in Ti-diffused LiNbO3waveguides. The operation of these devices relies on a redistribution, by means of the elcctrooptic effect, of the power in the input waveguide before it is transmitted to the output waveguides of theY-junction. Measured performance is shown to be predicted satisfactorily by a theoretical model of aY-junction in homogeneous, planar waveguide. Transmission losses of less than 1 dB and extinction ratios of about 15 dB have been obtained in a device with a junction half-angle of 0.57°.     

Theoretical and experimental study of superhigh electromechanicalcoupling surface acoustic wave propagation in KNbO3 singlecrystal

The propagation characteristics of surface acoustic waves (SAWs) and piezoelectric leaky surface waves in single crystal KNbO₃ were investigated theoretically and experimentally. The results show that the electromechanical coupling coefficient of the surface wave propagating along the X-axis of the rotated Y-cut plane is very large, K ²=0.53, compared to K² of 0.055 for LiNbO₃      

Optical integrated circuit for a fiber laser doppler velocimeter

In this paper, we propose and demonstrate a new optical integrated circuit (optical IC) pigtailed with a polarization-maintaining fiber for the laser Doppler velocimeter. In this optical IC, a waveguide frequency shifter, a TE/TM mode converter, and a mode splitter are integrated inZ-propagation Ti-diffused LiNbO3as well as a waveguide interferometer. Since the optical IC is as long as 32 mm, the waveguide patterning has been made with<0.2-mum accuracy by a laser-beam lithographic system recently developed. Using the fabricated optical IC, the velocity of a moving object has been successfully heterodyne-detected with the signal-to-noise ratio of 25 dB.     

More investigations of leakage and nonleakage conductor-backedcoplanar waveguide

Through rigorous full-wave analysis, the effective dielectric constant, normalized attenuation constant, characteristic impedance, and radiation pattern of two types of conductor-backed coplanar waveguides are obtained. The analytic results show that the leakage effect is not only controlled by the thickness and dielectric constant of the substrate but also by the slot width. The leakage power transforms to a radiation space wave and a surface wave, the transverse electric field diagrams in the substrate and the far-zone radiation pattern verify the leakage phenomenon. The first structure has the maximum radiation intensity in the endfire direction, while the second one which has more leaky waves radiates into the air, is better served as a radiating device. For both structures under the nonleakage condition, the characteristic impedance is sensitive to the change of the strip width but not the slot width. Using these properties, the nonleaky and leaky circuits can exist on the same circuit board by choosing appropriate circuit dimensions     

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 LiNbO3. 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.     

Propagation and radiation characteristics of the tape helix with aconducting core and dielectric substrate

An equation is presented to describe the dispersion characteristics of a tape helix separated from a concentric conducting core by a dielectric substrate. Numerical solutions of the complex-valued propagation constant can be used to obtain the complete k-β diagram of the helical geometry. The k-β diagram provides information regarding the real-valued solutions for β that represent true axial propagation, as well as the complex-valued solutions that represent surface waves and leaky waves. Several different helix geometries are considered. Under the assumption that these same solutions exist on a finite-length structure, the radiation pattern can be obtained for a given mode of propagation     

The grating array: A new surface acoustic wave transducer

A novel surface acoustic wave transducer, called the grating array, is described. Experimental results obtained at 9.2 MHz on PZT and 290 MHz on LiNbO3are presented which demonstrate the capability of this transducer to efficiently generate and detect Rayleigh waves on piezoelectric materials.     

Electronically variable time delay in a LiNbO3SAW delay line

The time delay of surface acoustic waves (SAW's) in 128° rotated Y-cut, X-propagating LiNbO3is found to vary linearly with a biasing electric field. Fractional changes in time delay greater than 9 × 10^-4have been obtained in a delay line fabricated using this material.     

Generation and correlation of quadriphase-coded signals using acoustic surface waves

The use of acoustic surface waves to generate and correlate a 4-phase 15-bit generalized Barker sequence is described. Experimental results using quartz and LiNbO3as substrates are reported.