John P. Powers

Integrated Optical Sigma-Delta Modulators.

Abstract : Modern avionics equipment, such as super resolution direction-finding systems, now require resolutions on the order of 20 to 22 bits. Oversampled analog-to-digital converter architectures offer a means of exchanging resolution in time for that in amplitude and represent an attractive approach to implementing precision converters without the need for complex precision analog circuits. Using oversampling techniques based on sigma-delta modulation, a convenient tradeoff exists between sampling rate and resolution. One of the major advantages of integrated optics is the capability to efficiently couple wideband signals into the optical domain. Typically, sigma-delta processors require simple and relatively low-precision analog components and thus are well suited to integrated optical implementations. This thesis reviews the current sigma-delta methodology, the advantages of optical integrated circuits and presents the design of a second-order, integrated optical sigma-delta modulator. Simulation results for both a first and second order architecture are presented by evaluating the transfer characteristics numerically. Design parameters such as limit cycles are quantified and explained. Performance issues and future efforts are also considered. (AN)

Fiber-lattice accumulator design considerations for optical ΣΔ analog-to-digital converters

Integrated optical sigma-delta (??) analog-to-digital converters (ADCs) use a pulsed laser to oversample an input signal at two Mach-Zehnder interferometers. A fiber-lattice accumulator is embedded within a feedback loop around a single-bit quantizer to spectrally shape the quantization noise to fall outside the signal band of interest. Decimation filtering is applied to the quantizer output to construct the input signal with high resolution. Applications of integrated optical ?? ADCs include digitizing wideband radio-frequency signals directly at an antenna (digital antenna). In this paper, a novel fiber-lattice accumulator design is presented, and a coherent simulation of an integrated optical first-order, single-bit ?? ADC is reported. The accumulator leakage resulting from a mismatch in the optical circuit parameters is quantified. A time-domain analysis is presented, and the simulation results from an all-electronic ?? ADC are presented for comparison. A frequency-domain analysis of a ten times oversampling (n = 4 bits) simulation is used to compare the dynamic performance parameters, including the spurious-free dynamic range, signal-to-noise-plus-distortion ratio, and effective number of bits. The formation of image frequencies when the accumulator is overloaded (i.e., the optical amplifier gain is too large) is also investigated.

Computer Simulation of Linear Acoustic Diffraction

Computer-aided acoustical imaging systems and computer simulations of other acoustic imaging techniques frequently require simulation of linear acoustic diffraction of large complex-valued data arrays. Computation efficiency requires the use of fast Fourier transform techniques. This paper compares two Fourier transform formulations of the propagation problem: the Fresnel integral and the spatial frequency domain approach. The following features are compared: restrictions on maximum and minimum propagation distances, sample sizes and number of samples required, adaptability to image processing techniques, and computational requirements.

Propagation of Transient Acoustic Waves in Lossy and Lossless Media

This paper presents a computationally efficient technique for calculating the transient wave from a planar source within a medium. The spatial and temporal excitation are known at the input plane. The technique is applicable to lossless media, to media with a loss coefficient that is linear in frequency, and to media with a loss coefficient that is quadratic in frequency. The technique is computationally efficient in that it relies on FFT algorithms for the calculation rather than integral solutions requiring geometrical interpretation. In this method, we find the Green’s function that solves the applicable wave equation and that meets the required boundary conditions in the source plane. This Green’s function is then used in a form of the Kirchhoff integral that applies to transient wave propagation and we find the response to a time-domain impulse excitation. The solution is then expressed in the spatial frequency domain where a linear systems interpretation provides a physically intuitive interpretation of the results. The propagation is seen to be represent a time-varying spatial filter that increasingly attenuates the higher spatial frequencies as time goes on. Unlike the continuous wave case, the filter is neither band-limited nor a pure phase filter. The particular form of the spatial filter depends on the medium assumed and on the baffle conditions. The solutions for the impedance-matched baffle and the resilient baffle can be expressed in terms of the solution for the rigid baffle case. Several examples of calculated fields will be given.

Automatic particle sizing from rocket motor holograms

Solid-propellant rocket fuels frequently have spherical aluminum pellets added to improve combustion efficiency and to improve thrust performance. In order to model the combustion process, information is required about the size of the particles as they lift off of the propellant surface. We have used optical pulsed-laser holography to record an approximate 2.5 cm X 2.5 cm X 2.5 cm volume at the propellant surface in a test rocket motor. A pulsed ruby laser, combined with a laser line filter to remove the flame light, records the hologram. A diffuser is used to remove the phase gradients introduced by the thermal effects in the flame. The scene is reconstructed with a krypton laser, viewed with an optical microscope, and captured on videotape. The recorded scene is digitized and analyzed with digital image processing techniques on a personal computer equipped with a video memory board and an image processing subroutine library. The image processing techniques developed reduce the speckle in the scene, apply a threshold to differentiate the particles from the background, and locate and size the particles. Statistical analysis of the sizing results provides a histogram representation of the particles size distribution. Particle resolution down to 10 micrometers has been achieved with this technique.

Spatial Filtering Considerations in Bragg Diffraction Imaging

Many of the techniques and effects observed in images from systems using Bragg diffraction imaging can be explained using concepts of optical spatial filtering. This is possible because the Bragg diffraction phenomenom can be thought of as an interaction of a plane-wave of light with a plane-wave of sound to produce a plane-wave of diffracted light. Changing the incident light causes a change in the diffracted light. Hence, the input light field of the imaging system plays an analogous role to the filter transparency of an optical spatial filtering system. The plane-wave components of the diffracted light that form the image of the sound field can be selectively changed by modification of the incident light field. When the imaging method is analyzed as a plane-wave: plane-wave interaction, simple explanations can be given for such diverse effects as the observed astigmatic resolution, the dependence of the resolution on the semi-apex angle of the incident light wedge, dark field imaging, and reflection imaging.

High-Resolution Acoustic Arrays Using Optimum Symmetrical-Number-System Processing

We have presented the concept of designing an acoustic array and its signal processing based on the properties of the Optimum Symmetrical Number System (OSNS). The folding properties of the OSNS can be applied to the folding properties of the electronic phase detectors in the array processing system. Table 3 summarizes the relations between the OSNS assumptions and the array system properties. The number of moduli, N, determines the number of elements (N+1) and the number of processing channels. It also contributes to the total number of comparators required. The product of the moduli determines the dynamic range of the system, M. This property determines the element spacing and the angular resolution. Generally, for a given resolution, one can tradeoff the overall length of the array against the number of channels (and the number of comparators).

Preliminary Results of Computer Aided Acoustic Imaging

Preliminary results from a computer aided coherent acoustic imaging system are given. The system uses a raster scanned sensor to digitize samples of 1 MHz acoustic fields (both amplitude and phase) sampled at half-wavelength intervals. After transferring the data to computer memory three different types of displays have been used to display and process the data fields as images. The display devices include a black and white video system, a color graphics terminal, and a high resolution color image processing system. The advantages and disadvantages of each display system are discussed along with some of the most useful image processing operations for the acoustic images obtained. These operations include image zoom (i.e., magnification), histogram analysis, dynamic range manipulation, and other interactive capabilities.

Secure optical communications using wavelength tunable optical filters

This report presents several conceptual designs for all-optical implementations to enhance the security of optical communications links. The method takes advantage of technology which uses the wavelength selective filtering properties of electronically tunable high finesse Fabry-Perot interferometers (FPIs). For the application under consideration the FPI would physically separate from a broad band (e.g., >20 nm) optical source, such as superluminescent LED, a narrow band (e.g., <0.1 nm) of wavelengths and the latter would be modulated with the information signal of interest. The modulated optical signal is combined with the unmodulated part of the broadband signal prior to transmission. The decoding process at the receiver would require an identical FPI tuned to the same narrow band of wavelengths in order to selectively extract the modulated narrowband signal carrying the information transmitted. Robustness of the security is enhanced by the electronic tunability of the FPIs which could potentially lead to a wavelength-hopped transmission and tracking communication system.

Experimental Comparison of Measured Ultrasound Pressure Fields with Theoretical Prediction

We have presented a technique in the past [1–3] to calculate the acoustic potential and pressure from knowledge of the excitation spatial and temporal velocity function. In this paper we wish to present a comparison of a predicted field from using the technique with experimentally measured data.