Kevin J. Gamble

Simulation of short LSAW transducers including electrode mass loading and finite finger resistance

The theory for the 2-D numerical analysis of acoustic wave generation from finite length leaky surface acoustic wave (LSAW) transducer structures is presented. The mass loading of the electrodes is incorporated through the use of the finite element method (FEM). The substrate is modeled using both analytical and numerical means. The advantages of this simulation are twofold. First, it is capable of extracting the individual bulk wave conductances from the overall conductance of a given device. At large distances from the transducer, the angular distribution of power radiated relative to the substrate surface can then be calculated for each of the three possible bulk wave polarizations. The second advantage of the simulation is that the effect of finite electrode resistance is included through the use of a series equivalent resistance for each electrode in the structure. Once the resistance for each electrode in the structure has been determined, the overall effect on the device admittance is modeled by applying a constrained minimization process to the electrical boundary conditions of the transducer. To conclude the paper, the simulation will be compared against the experimental admittance of a 37-finger uniform transducer with a metallization ratio of 0.5 on 42/spl deg/ LiTaO/sub 3/. The agreement between theory and experiment is excellent.

Simulation of short LSAW transducers including electrode massloading and finite finger resistance

The theory for a 2-D numerical analysis of acoustic wave generation from finite length LSAW transducer structures is presented. The massloading of the electrodes is incorporated through the use of the Finite Element Method (FEM). The substrate is modeled using both analytical and numerical means. The simulation is capable of extracting the individual bulk wave conductances from the overall conductance of a given device. Once the bulk wave conductances are calculated, the angular distribution of power radiated relative to the substrate surface can then be found. The simulation also includes the effect of finite electrode resistance through the use of a series equivalent resistance for each electrode in the structure. The paper will conclude by presenting simulation results for two separate freestanding transducers and a short transducer combined with a grating. The substrate materials used are 42/sup 0/ LiTaO/sub 3/ and 64/sup 0/ LiNbO/sub 3/. The agreement between theory and experiment is shown.

Results of two-dimensional time-evolved phase screen computer simulations

This paper presents a 2D computer simulation of observed intensity and phase behind a time evolved phase screen. Both spatial and temporal statistics of the observed intensity is compared to theoretical predictions. In particular, the intensity statistics as a function of detector position within the propagated laser beam are investigated. The computer simulation program was written using the C-programming language running on a SUN SPARC-5 workstation.

Observation for nonlinear harmonic generation of bulk modes in SAW devices

Nonlinear generation of harmonics is measured at wafer level on Temperature Compensated Surface Acoustic Wave (TC-SAW) Resonators on lithium niobate. The obtained results are different from those published for resonators on lithium tantalate. For second harmonic, strong peaks looking like multiple resonant modes are found. The frequency difference between the successive peaks increases when the substrate thickness decreases. Results also show that the level of the signal depends on the roughness of the back side of the substrate. Due to these findings, it was assumed that this signal is due to nonlinear generation of a bulk mode. This assumption is confirmed experimentally by scanning the back side with an interferometer. Possible nonlinear mechanisms explaining these results are discussed.

Bulk wave excitation from finite length SAW transducers including massloading

A analysis of bulk wave generation from finite length transducers on YZ-LiNbO3 is presented. The effect of the metallic electrodes is taken into account using the Finite Element Method. The substrate is modeled using both analytical and numerical means. The simulation results suggest that thick electrodes (gold or aluminum) increase the electrical coupling of a uniform SAW transducer to the shear vertical bulk wave at approximately twice the SAW center frequency. Experimental data in the literature appears to support this assertion.

Computer simulation of a multiple-aperture coherent laser radar

This paper presents the construction of a 2D multiple aperture coherent laser radar simulation that is capable of including the effects of the time evolution of speckle on the laser radar output. Every portion of a laser radar system is modeled in software, including quarter and half wave plates, beamsplitters (polarizing and non-polarizing), the detector, the laser source, and all necessary lenses. Free space propagation is implemented using the Rayleigh- Sommerfeld integral for both orthogonal polarizations. Atmospheric turbulence is also included in the simulation and is modeled using time correlated Kolmogorov phase screens. The simulation itself can be configured to simulate both monostatic and bistatic systems. The simulation allows the user to specify component level parameters such as extinction ratios for polarizing beam splitters, detector sizes and shapes. orientation of the slow axis for quarter/half wave plates and other components used in the system. This is useful from a standpoint of being a tool in the design of a multiple aperture laser radar system.

Pyramid framework for image reconstruction from nonimaged laser speckle

A multiresolution approach for image reconstruction from the magnitude of its Fourier transform has been developed and implemented by employing the concept of pyramid sampling. In this approach several iterations of the error reduction algorithm are preformed at each level of the pyramid using a coarse-to-fine strategy, resulting in improved convergence and reduced computational cost.

Observation of Nonlinear Harmonic Generation of Bulk Modes in SAW Devices

Nonlinear generation of harmonics is measured on the wafer level on temperature compensated surface-acoustic-wave resonators on lithium niobate. The second-harmonic response features strong peaks looking like multiple resonant modes. The product of the frequency difference between the successive peaks to the substrate thickness is found to remain almost constant. The results also show that the level of the harmonic signal strongly depends on the roughness of the back side of the substrate. Due to these findings, it was assumed that the signal is due to nonlinear generation of a bulk mode. This assumption is confirmed experimentally by measuring the second-harmonic wave field on the back side of the substrate with an interferometer. Possible nonlinear mechanisms explaining these results are discussed.

Automated COM parameter extraction for SiO 2 /LiNbO 3 and SiO 2 /LiTaO 3 substrates

In this paper automated COM parameter extraction approach is described. COM parameters (velocity, reflectivity, coupling, propagation loss) are extracted from measurements of a set of test structures. Measurement data are validated prior to the parameter extraction. Frequency dependence of the velocity and propagation loss is taken into account. The method is applied to both SAW and LSAW substrates. Good agreement between measured device responses, and responses simulated using extracted COM parameters is shown for several examples of the devices built on SiO2/LiNbO3 and SiO2/LiTaO3 substrates.

Experimental Verification and Theory of CNR Gain for an Eight Element Multiple Aperture Coherent Laser Receiver

The detection and processing of laser communication signals are drastically affected by the fading induced onto these signals by atmospheric turbulence. One method of reducing this fading is to use an array of detectors in which each of the detector outputs are added together coherently. This requires measuring the phase difference between each of the receivers and co-phasing each of the detector outputs. This paper presents experimental verification at the Innovative Science and Technology Experimentation Facility over an outdoor range of a 1.06 micron eight element coherent receiver used to mitigate the effects of fading. The system is composed of a 60 mw Nd:Yag laser used as the transmitter and a 27 MHz AO modulator used to frequency shift the transmitted beam. The receiver is composed of eight 1 cm lenses launching the eight received optical signals into eight signal mode optical fibers. Phase compensation between each of the eight receivers is accomplished using single mode fibers wrapped around PZT cylinders that are controlled by phase compensating electronics. The carrier-to-noise (CNR) ratio was measured on a single channel and was then compared with the CNR obtained from the coherent sum of the eight channels. The improvement of the CNR for the coherent sum as compared to a single channel was then compared against theoretical predictions.