Near-field acoustical imaging with boundary arrays

Good range resolution can be achieved with a shorter pulse. But on the other hand, shorter pulses need more peak power. The shorter the pulse gets, the more should its peak power be increased so that enough energy is packed into the pulse. High Peak Power makes the design of transmitters and receivers more difficult since the components used to construct these must be able to withstand the peak power. One way to overcome the problem of high peak power is to convert the short pulse into a long one and using some form of modulation to increase the bandwidth of the long pulse so that the range resolution is not compromised. This technique is called Pulse Compression and is used widely in Radar applications where high peak power is undesirable. This paper introduces a new radar pulse compression system, using a type of Linear Frequency Modulation (LFM) signal which lacks components around zero frequency. A core structure of a pulse processor is similar to a standard matched filter, modified to suit the application. Flexibility in the design permits the use of multiple code types and lengths needed, which eliminate ambiguities, caused by high pulse repetition frequencies. The device used in this application is a field programmable gate array (FPGA) which has distinct advantages compared to other application specific integrated circuits (ASIC) for the purposes of this work. The FPGA provides flexibility, for example, full reconfiguration in milli-seconds and permits a complete single chip solution.

Simulation of B-scan images from two-dimensional transducer arrays: Part I--Methods and quantitative contrast measurements.

Calibration measurements play a crucial role in improving the accuracy of both qualitative and quantitative microwave imaging. Ideally, in 3-D near-field imaging, a calibration measurement should be performed at each desired range (or depth) position, which can be very time-consuming. An analytical prediction of the range behavior of the resolvent kernel of scattering can reduce the calibration effort to a single measurement at a reference range position. Range-translation (or range-migration) analytical expressions are already widely used in far-zone radar and acoustic imaging; however, their accuracy deteriorates significantly in near-field scenarios. Here, we propose a range-migration technique for near-field microwave imaging with monostatic and bistatic measurement configurations. From a single measurement of the system point-spread function (PSF), the PSF magnitude and phase are accurately predicted at any desired range position. The proposed migration is performed in real space; however, it can also be applied with Fourier-domain (or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k$</tex-math> </inline-formula> -space) inversion methods. Here, it is applied with quantitative microwave holography in simulation-based and experimental examples, which validate its performance and illustrate its limitations.

The article discusses the problem of synthesis of complex broadband signals in the spectral area. It is shown that the formation of complex broadband signals in the spectral area allows to determine the classes of signals that have good resolution in range and speed. Mathematical models of uncertainty functions for polyharmonic and strip signals are presented.

Motivated by dual use of OFDM signal format for communications and radar in ever-worsening Electromagnetic (EM) coexistence environments, this paper deals with transmit waveform design problem considering multiple design objectives. Spectral nulling is a typical way for friendly coexistence with narrow band systems. However, a Non-Contiguous Orthogonal Frequency-Division Multiplexing (NC-OFDM) waveform generated by turning off the interfering sub-carriers does not lead to satisfactory results. In this paper a convex optimization based waveform design framework is used to achieve deep spectral nulling while retaining low waveform autocorrelation side lobes and good range resolution. Because of dual use of the waveform, the data blocks to transmit are either unknown or chosen from a known dataset. Optimal sub-carrier weights are obtained for given transmission data blocks. In addition, waveform design for unknown data blocks are discussed and examined.

Advances in millimeter wave solid-state power amplifier technology over the past decade have led to the development of a new generation of compact, lightweight cloud and precipitation radars. Solid-state power amplifiers (SSPAs) have lower peak power than traditional vacuum tube transmitters, but generally are capable of operating at much higher duty cycle. Taking advantage of the high duty cycle of SSPAs usually requires some signal processing and/or pulse compression technique to achieve similar sensitivity at comparable range resolution. FMCW (frequency modulated continuous wave) radars take full advantage of SSPAs capable of CW (continuous wave) operation, but the usual requirement for separate transmit and receive antennas is often impractical. Radars that combine a pulse compression waveform with a short pulse offset in frequency are now commonly used to operate at duty cycles as high as 10–20%. This paper reviews the latest solid-state millimeter wave radar designs, benefits, and limitations and describes operating modes and signal processing techniques that are especially well suited for single antenna operation. High PRF (pulse repetition frequency) operation using coherent power (single lag covariance) provides automatic noise cancellation without requiring explicit noise sampling. A novel phase coding technique employing Chu-codes with quadratically-increasing phase can be used to separate ambiguous signals when running at MHz-level PRFs. Weather data measured using the quadratic phase coding (QPC) technique with a solid-state Ka-band radar running at 3.1 MHz PRF is presented. This data was gathered at a transmitter duty cycle of approximately 50 percent and exhibits a 20 dB sensitivity improvement when compared to conventional short pulse data gathered at the same range resolution.

We describe an approach to construct a pulsed signal which, when propagating in an obscuring, discrete-scatterer medium (such as clouds, fog, dust or other aerosols) experiences reduced attenuation. The approach can thus find possible applications in communication or high resolution target detection. In the proposed scheme, the transmitted signal consists of a coherent train of short wide-band pulses emitted at chirped (linearly varying) time intervals. As the energy of a single pulse which has traveled a large distance in the medium is almost entirely concentrated in the precursor-type structures associated with its leading and trailing edges, the increase of the signal total energy is achieved by using trains with a large number of pulses. The (down-)range resolution is controlled by the chirp bandwidth characterizing the distribution of individual pulses in the train (that bandwidth may be sufficiently large to achieve a resolution typical of millimeter-wave radar), while the high cross-range resolution can be attained in analogy to the usual synthetic-aperture imaging. Processing of the received signal involves filtering associated with the chirp characteristics, analogous to that in the conventional matched-filter techniques. It is also shown that the proposed approach provides flexibility in choosing the chirped-train center frequency, e.g., to maximize the amount of energy passing through a selected atmospheric absorption window or to maximize the signal energy carried to a desired penetration depth.

A underwater acoustic imaging system capable of working in extremely turbid conditions and producing high resolution dimensioned images has been developed. It is suitable for inspecting pipelines and offshore structures. A large development program has been carried out in Australia as a collaborative effort between a number of organisations to overcome the traditional problems with underwater imaging. The system uses megahertz frequency sound, sparse array technology, one-bit digitisation and chirp illumination. Acoustic three dimensional images with range and orthogonal dimensions have been obtained from a prototype underwater acoustic imaging system and examples are presented. Resolutions of millimeters have been obtained. This high resolution, large image (103x103x103 voxels) system is a new-to-the-world capability. This paper is a step in diffusing the new technology so that underwater acoustic imaging may become an innovation in offshore inspection and measurement. Introduction A new technology has been demonstrated for generating three dimensional near video quality images using underwater acoustics. There are two technologies for producing focused acoustic images, lenses and beam forming from twodimensional arrays. The lens has been demonstrated for two dimensional images by Belcher (1998) using a onedimensional array in the focal plane. The lens system places an array of acoustic sensors at the focal plane of the system and avoids the processing load of beam forming. Erikson (1997) discusses a more capable two-dimensional focal plane array. Beam formed images rely on the ability of the signals from an appropriately spaced array of acoustic sensors to be delayed in time and added to make the reflected pressure signals in phase for a particular voxel. For a large imaging volume this can become a very large processing load. The Underwater Acoustic Imaging technology is founded on five elements. Sparse arrays, one-bit digitisation, chirp ensonification, economical acoustic tiles and correlation flattening. While all of these elements have been used in some context before, it took the implementation of them all in a single system to produce a potential innovation. The sonar equation sets limits to performance that are determined by the source level and the system noise. While the source level is felt to be limited by the cavitation pressure, even before that magnitude the acoustic signal is nonlinear. Non linear acoustic signals distort during propagation and make replicate correction more difficult. System noise limits the range at which an object of given target strength can be seen above this noise. Arrays do not focus exactly on a single voxel but allow acoustic energy from adjacent voxels to leak into the picture. Replicate correlation of the received signals with the chirp ensonification, which provides focusing in the range direction, does the same. The clutter from elements of the target limits the size of objects that can be viewed clearly by restricting the contrast. Technology For an active acoustic imaging system such as the present, the acoustic pressure on the face of the receive array depends on the source level, the range of the target, its target strength and the absorption and scattering of water.

We demonstrate the coupling of a scattering near-field scanning optical microscope combined with a Fourier transform infrared spectrometer. The set-up operates using either the near-field thermal emission from the sample itself, which is proportional to the electromagnetic local density of states, or with an external infrared synchrotron source, which is broadband and highly brilliant. We perform imaging and spectroscopy measurements with sub-wavelength spatial resolution in the mid-infrared range on surfaces made of silicon carbide and gold and demonstrate the capabilities of the two configurations for super-resolved near-field mid-infrared hyperspectral imaging and that the simple use of a properly chosen bandpass filter on the detector allows one to image the spatial distribution of materials with sub-wavelength resolution by studying the contrast in the near-field images.

Acoustic imaging is a technology for visualizing sound fields, which can be used in fault diagnosis of power equipment and gas leak detection. A spiral microphone array with a large number of microphones is generally used in ultrasonic acoustic imaging, but its application is limited due to the high cost. This paper addresses the issue of the high cost for acoustic imaging microphone arrays. On one hand, an adaptability function suitable for acoustic imaging applications is designed, and the genetic algorithm is used for sparse array arrangement to reduce the cost. On the other hand, the beam forming algorithm is also improved by using a time-varying smoothing coefficient to update the cross-power spectral matrix, further enhancing the performance. Experimental results show that by using the algorithms proposed in this paper, a sparse array can achieve acoustic imaging performance close to that of a large array, the cost of usage is significantly reduced.

Photocurable three-dimensional (3D) printing is a stepwise layer-by-layer fabrication process widely used in the manufacture of highly specialized objects. Current 3D printing techniques are easily implemented; however, the build rate is slow and the surface quality is less than ideal. Holographic 3D display (3DHD) technology makes it possible to reform planar wavefronts into a 3D intensity distribution, which appears as a 3D image in space. This paper examined the application of holographic imaging technology to 3D printing based on photocurable polymers. The proposed system uses a 3DHD diffractive optics system based on a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM), wherein a 3D layered image is created in the optical near field, based on a computer-generated hologram (CGH) optimized using the iterative angular spectrum algorithm (IASA) and a circular IASA. From a single CGH, multiple 2D sliced images are created in space to form a 3D optical image used to initiate the photopolymerization of photocurable resin to form 3D objects. In experiments, the proposed 3D printing system was used to create five polymer objects with a maximum axial length of 25 mm and minimum feature width of 149 μm. The phase-only CGH reformed the incident light into a distribution of optical intensity with high diffraction efficiency suitable for photocuring. Despite limitations pertaining to fabrication area and axial complexity in this initial study, the proposed method demonstrated high light efficiency, high resolution in the lateral direction, rapid fabrication, and good object continuity.

The Acoustic Imaging System (AIS) was developed by the Naval Ocean Systems Center (NOSC), San Diego, for use aboard deep diving Navy submersibles and also as a research tool. The operational specifications of the AIS were chosen to lie between those of closed circuit television (short range, high resolution) and most sonars (long range, low resolution) to provide moderate range (5–100 feet) and moderate resolution (0.3 degree) (figure 1) for classification of objects. Holography was selected as the most promising technique for this underwater acoustic imaging application over focused or beamformed techniques (figure 2). The primary reasons for selecting holography were for (1) compactness and lack of any cable-snagging protrusions, (2) failure tolerance, and (3) simplicity of hardware. Submersible pilots are generally very particular about external equipment getting entangled in cables. Since holography requires no lens, only the hydrophone array and its immediate processing electronics need to be in the water. Failure tolerance comes from the fact that the most unreliable components in an acoustic imaging system appear to be the hydrophone elements and their associated circuits. In holography, these components are in the Fourier transform domain. Hence, any errors or failures in them contribute only to the overall noise background of the reconstructed image. In focused systems, these components are in the image plane, and failures give rise to holes in the image.

Because active sonar systems are typically peakpower limited, and because sonar targets can often appear at speeds within the doppler spread of reverberation, pulses having large time-bandwidth (TW) products are called for. The resultant high resolution in range and doppler creates a potential problem because of multipath. Multipath is caused by multiple reflections from the two rough surfaces -- the ocean surface and the ocean bottom -- from which signals are reflected, refraction, eand by multiple reflections from the target itself. Due to motions of the echo ranging ship, the target, and the ocean surface, the multipath exhibits doppler spreading as well as time spreading. A model is postulated for the overall spreading of the signal seen at the receiver in which the magnitudes of time spreading and doppler spreading are represented by L and B. Detection performance of signals with large pulse length (T) and bandwidth (W) is derived as function of WL and BT. Performance is measured in terms of input signal level required to produce specified probabilities of detection and false alarm at the processor output. Two basic signal codes are considered -- random modulation in amplitude and/or phase (RM) and linear frequency modulation (LFM). Detection performance of a single correlator, tuned to one range and doppler, is given. Next, a processor is postulated consisting of a multiplicity of correlators covering the entire expected range and doppler spread of the signal, the outputs of which are combined incoherently.

Three-dimensionally assembled TES X-ray microcalorimeter arrays may be utilized for three purposes: (1) to obtain wide X-ray energy coverage of TES microcalorimeters, (2) to distinguish charged particle events from X-ray events, (3) to reconstruct Compton-scattering geometry for hard X-ray Compton cameras. We have designed and fabricated three-dimensionally assembled array of the minimum format i.e. 2 × 2 × 2 array in order to obtain a good energy resolution in a wide energy range of 0.5–20 keV and a high maximum counting rate of 2000 cps for energy dispersive X-ray spectrometer (EDS) system for a transmission electron microscope (TEM). Although we could not obtain required energy resolution because of a problem in the refrigerator system, we confirmed the operation of the three-dimensional array.

We present the tests performed by the SIDDHARTA-2 collaboration at the $$\textrm{DA}\Phi \textrm{NE}$$ collider with a quasi-hemispherical CdZnTe detector. The very good room-temperature energy resolution and efficiency in a wide energy range show that this detector technology is ideal for studying radiative transitions in intermediate and heavy mass kaonic atoms. The CdZnTe detector was installed for the first time in an accelerator environment to perform tests on the background rejection capabilities, which were achieved by exploiting the SIDDHARTA-2 Luminosity Monitor. A spectrum with an $$^{241}\textrm{Am}$$ source has been acquired, with beams circulating in the main rings, and peak resolutions of 6% at 60 keV and of 2.2% at 511 keV have been achieved. The background suppression factor, which turned out to be of the order of $$\simeq 10^{5-6}$$ , opens the possibility to plan for future kaonic atom measurements with CdZnTe detectors.