Development and analysis of a photon-counting three-dimensional imaging laser detection and ranging (LADAR) system

In this paper, a photon-counting three-dimensional imaging laser detection and ranging (LADAR) system that uses a Geiger-mode avalanche photodiode (GAPD) of relatively short dead time (45 ns) is described. A passively Q-switched microchip laser is used as a laser source and a compact peripheral component interconnect system, which includes a time-to-digital converter (TDC), is set up for fast signal processing. The combination of a GAPD with short dead time and a TDC with a multistop function enables the system to operate in a single-hit or a multihit mode during the acquisition of time-of-flight data. The software for the three-dimensional visualization and an algorithm for the removal of noise are developed. For the photon-counting LADAR system, we establish a theoretical model of target-detection and false-alarm probabilities in both the single-hit and multihit modes with a Poisson statistic; this model provides the prediction of the performance of the system and a technique for the acquisition of a noise image with a GAPD. Both the noise image and the three-dimensional image of a scene acquired by the photon-counting LADAR system during the day are presented.     

Flash light detection and ranging range accuracy limits for returns from single opaque surfaces via Cramer-Rao bounds

A Cramer-Rao lower bound on the range accuracy obtainable by a Flash light detection and ranging (LADAR) system receiving a return from a single surface in the instantaneous field of view of each detector is developed and verified with experimental data. The bound is compared to the performance of a new algorithm and that of a matched filter receiver by using both simulated and measured LADAR data. The simulated data are used to show that the estimator is nearly unbiased and efficient for systems that match the negative paraboloid model used in its derivation. It is found that the achievable range accuracy for the LADAR system and for the target geometry used to collect the measured data is of the order of 2.5 in. while the bound predicts a range accuracy limit of approximately 0.6 in.     

应用于脉冲TOF成像LADAR系统的高性能CMOS全差分放大器设计

本文设计了一种应用于脉冲飞行时间(TOF)成像激光雷达探测系统的高带宽、低噪声全差分放大器(FDMA)。该芯片采用多级级联结构和有源电感技术,增大电路带宽和减少芯片面积,并且通过使用失调隔离技术,增强了各增益级对工艺偏差的鲁棒性。在输出级电路中,为使全差分放大器具有更强的驱动能力,采用了宽带放大器和输出缓冲器级联结构做为输出。同时,为了满足激光雷达系统的实际需求,采用复用失调隔离电路的方式,实现了级间带通滤波来限制放大器的适用带宽。采用CMSC的CMOS工艺进行了FDMA流片。测试结果表明,该芯片具有730.6 MHz的-3 dB带宽,在使用带通滤波器优化后的开环增益为23.5 dB,等效输入噪声密度为2.7 nV/sqrt(Hz),有效地降低了系统噪声。芯片采用3.3 V电源供电,功耗为102.3 mW,整体面积为0.25 mmc×0.25 mm。作为激光雷达全系统集成芯片中的一部分,较好地满足系统指标要求。     

Spectral LADAR: active range-resolved three-dimensional imaging spectroscopy

We present the concept and experimental results for Spectral LADAR, an augmented LADAR imager combining three-dimensional (3D) time-of-flight ranging with active multispectral sensing in the shortwave infrared (1080-1620 nm). The demonstrated technique is based on a nanosecond regime pulsed supercontinuum transmitter and spectrally multiplexed receiver that computes a high-resolution range value for each of 25 spectral bands. A low frame-rate prototype unit is described. Results demonstrating 3D imaging and material type classification of objects, especially those obscured by camouflage, are shown at effective stand-off ranges exceeding 40 m. These capabilities and the highly eye safe wavelengths at which the system operates make it suitable for applications in military imaging and robotic perception.     

Acoustically steered and rotated true-time-delay generator based on wavelength-division multiplexing

The acoustically steered and rotated (ASTRO) true-time-delay (TTD) generator, capable of two-dimensional (2D) multiple beam generation without causing extraneous beams, is described. It is based on wavelength-division multiplexing to encode and decode various linear chirp time delays with wavelengths. An array of equally spaced light stripes with linear chirp time delays among them is formed and is subsequently rotated to a desired angle by use of our nonmechanical acousto-optic dove prism, believed to be novel. Our architecture reduces hardware complexity significantly by avoiding redundancy in conventional TTD generators. Experimental results to prove the concept of 2D multiple beam generation are shown. Also, an extension of the system for bidirectional communications is proposed. Various technical issues for practical applications are discussed.     

LADAR detection statistics in the presence of pointing errors

The probability of detection of optically rough targets with pulsed LADAR systems that use direct detection is considered. It is assumed that the LADAR operates under conditions of both unintentional pointing offset bias (i.e., bore-sight error) andjitter. Under these conditions the probabilities of detection of targets in both the near field and the far field of the collecting aperture (i.e., for resolved, partially resolved, and unresolved targets) and for both large and small photoelectron counts are derived, and in many cases of practical interest accurate, elementary analytic approximations that are useful for parametric system studies are obtained. A number of technical references are appended, in which some of the key results are derived. In particular, an interesting new mathematical result involving the complementary incomplete gamma function and an analytic expression for the probability distribution function of a signal photoelectron count obeying BoseEinstein statistics (such as that arising from unresolved targets) immersed in Poisson noise is derived.     

Intrinsic fiber-optic ultrasonic sensor array using multiplexed two-wave mixing interferometry

An intrinsic multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves by an array of fiber-optic sensors. The phase-modulated signals from each sensor are demodulated by use of an adaptive two-wave mixing setup. The light from each sensing fiber in the array is mixed with a reference beam in a single photorefractive crystal (PRC), and the output beams from the PRC are imaged onto separate photodetectors to create a multiplexed two-wave mixing (MTWM) system. The sensing fibers are embedded in graphite-epoxy composite panels, and detection of both acoustic emission and ultrasonic signals in these materials is demonstrated. The intrinsic MTWM system is an effective tool for the simultaneous demodulation of signals from a large fiber sensor array. Also, the adaptive nature of the MTWM setup obviates the need for active stabilization against ambient noise.     

Simple algorithms for sonar imaging and bathymetry with a linear swept-frequency (chirp) source

This article describes a digital implementation of time-delay sonar imaging for linear swept-frequency (chirp) sources. The technique, based on phase-shifting of match-filtered individual transducer echoes, is a direct equivalent of analog phased-array processing. Computing and sampling costs are minimized by sparse population of the receiver aperture, using a separate transmission array with controlled beamwidth to suppress grating lobes. Synthetic-aperture processing uses essentially the same computation, and is carried out for a chirp signal without frequency decomposition. When different arrays are used for transmission and reception, azimuthal resolution depends on transmission array length, but maximum movement between pings without aliasing is set by the length of the receiver array, so high resolution is compatible with fast survey. This article describes how a closely related algorithm can be used for swath bathymetric depth estimation, and presents simulation results using a random surface scatterer model. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 359–376, 1997     

Generating nonlinear concentration gradients in microfluidic devices for cell studies

We describe a microfluidic device for generating nonlinear (exponential and sigmoidal) concentration gradients, coupled with a microwell array for cell storage and analysis. The device has two inputs for coflowing multiple aqueous solutions, a main coflow channel and an asymmetrical grid of fluidic channels that allows the two solutions to combine at intersection points without fully mixing. Due to this asymmetry and diffusion of the two species in the coflow channel, varying amounts of the two solutions enter each fluidic path. This induces exponential and sigmoidal concentration gradients at low and high flow rates, respectively, making the microfluidic device versatile. A key feature of this design is that it is space-saving, as it does not require multiplexing or a separate array of mixing channels. Furthermore, the gradient structure can be utilized in concert with cell experiments, to expose cells captured in microwells to various concentrations of soluble factors. We demonstrate the utility of this design to assess the viability of fibroblast cells in response to a range of hydrogen peroxide (H(2)O(2)) concentrations.     

Parameterization of an acousto-optic programmable dispersive filter for closed-loop learning experiments

A representation of the search space in optical pulse shaping problems employing an acousto-optic programmable dispersive filter (AOPDF) is presented for use in closed-loop learning experiments where the optimal spectral phase function to some control problem is determined by an iterative learning algorithm. The representation allows the algorithm to select a value for the optical chirp at each frequency control point such that only acoustic grating functions which preserve the spectrum of the shaped pulses are tested. The limits of this space with respect to the rate of applied optical chirp, optical bandwidth and acoustic power are examined and tested through diffraction efficiency studies performed using a commercial AOPDF. The main benefits of this representation are the elimination of undesirable frequency mixing effects, reduction of diffraction efficiency variation between arbitrary pulse shapes and faster convergence of the evolutionary algorithm.     

Thermal spikes in Ag/Fe and Cu/Fe ion beam mixing

Ion beam mixing has been studied since 1980, and since then a lot of experimental and theoretical work has been done and knowledge has been gathered. Nevertheless, there are still many fundamental aspects that need to be clarified and with that aim many experiments need to be performed. Copper and iron are miscible in the liquid state, while silver and iron are not. However, both systems are thermally immiscible in the solid state. In order to have an insight into the importance of mixing within thermal spikes during ion beam irradiation, we deposited Cu/Fe and Ag/Fe bilayers onto Si substrates and irradiated them at room temperature with 2 MeV Cu and 2.5 MeV Au ions. A combination of Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) was used to analyze the atomic transport at the interface and the morphology changes of the samples. From the element profiles at the interface we conclude a mixing efficiency, which is indeed larger than the prediction of the ballistic model in the Cu/Fe system and smaller in the Ag/Fe system. Since ballistic mixing is expected in any case, we argue that demixing and phase separation in the Ag/Fe system occur in the thermal spike phase of the cascade as a consequence of the positive heat of mixing. Further mixing does occur in the thermal spike in the Cu/Fe system and they remain mixed even at the solid state because of the high cooling rate. In addition, ion irradiation induces a large surface roughening of the Ag and Cu top layers as proven by AFM. This effect is important for the correct interpretation of the results. Furthermore, this recrystallization affects also the interface, producing a rough interface, that appears in the RBS spectra as an atomic ‘diffusion’ at the interface.     

Concentration measurement of refrigerant/refrigeration oil mixture by refractive index

Refrigeration oil having good miscibility with refrigerant is generally used in refrigeration units. Precise measurement of the mixing ratio of refrigerant to refrigeration oil is required for a sufficient understanding of the refrigeration cycle. In this paper, refractive index is chosen as a property which indicates the mixing concentration of the refrigerant/oil mixture. A laser displacement sensor is used to detect a change of optical path which changes according to the refractive index of test medium. The refractive indices of pure refrigerant, pure refrigeration oil and refrigerant/refrigeration oil mixture are measured with several combination of refrigerant/oil. It is found that the difference of refractive index between the refrigerant and the oil is sufficient for the measurement of the mixing concentration of refrigerant/oil mixture, and that the refractive index of the refrigerant/oil mixture changes almost linearly according to the mixing concentration. These data will be utilized for development of an in situ sensor in refrigerant compressors.     

Directional synthetic aperture flow imaging

A method for flow estimation using synthetic aperture imaging and focusing along the flow direction is presented. The method can find the correct velocity magnitude for any flow angle, and full color flow images can be measured using only 32 to 128 pulse emissions. The approach uses spherical wave emissions with a number of defocused elements and a linear frequency-modulated pulse (chirp) to improve the signal-to-noise ratio. The received signals are dynamically focused along the flow direction and these signals are used in a cross-correlation estimator for finding the velocity magnitude. The flow angle is manually determined from the B-mode image. The approach can be used for both tissue and blood velocity determination. The approach was investigated using both simulations and a flow system with a laminar flow. The flow profile was measured with a commercial 7.5 MHz linear array transducer. A plastic tube with an internal diameter of 17 mm was used with an EcoWatt 1 pump generating a laminar, stationary flow. The velocity profile was measured for flow angles of 90 and 60 degrees. The RASMUS research scanner was used for acquiring radio frequency (RF) data from 128 elements of the array, using 8 emissions with 11 elements in each emission. A 20-micros chirp was used during emission. The RF data were subsequently beamformed off-line and stationary echo canceling was performed. The 60-degree flow with a peak velocity of 0.15 m/s was determined using 16 groups of 8 emissions, and the relative standard deviation was 0.36% (0.65 mm/s). Using the same setup for purely transverse flow gave a standard deviation of 1.2% (2.1 mm/s). Variation of the different parameters revealed the sensitivity to number of lines, angle deviations, length of correlation interval, and sampling interval. An in vivo image of the carotid artery and jugular vein of a healthy 29-year-old volunteer was acquired. A full color flow image using only 128 emissions could be made with a high-velocity precision.     

A 100-MHz ultrasound imaging system for dermatologic and ophthalmologic diagnostics

A major design problem concerning high-frequency broad-band ultrasound imaging systems is caused by the strong dispersive attenuation of the tissue, which gives rise to images with inhomogeneous resolution and poor signal to noise ratio (SNR). To address the noise problem, strongly focused transducers with high energy density in a narrow focal region are utilized, which also provide more isotropic images due to improved lateral resolution. To account for the short depth of the focal area two suitable imaging conceptions are used: 1) synthetic aperture concept and 2) B/D-scan concept. To avoid the inhomogeneity of the images, different transmitter signals for each depth are applied, which are pseudoinversely prefiltered according to the transfer function of the tissue. To gain signal energy required for inverse filtering, a pulse compression technique with nonlinearly frequency modulated chirp signals is utilized. These procedures have been implemented in an ultrasound imaging system, which has been developed in the authors' laboratory for eye and skin examinations, It can be used with transducers in a frequency range from 20 to 250 MHz.     

激光雷达反遮蔽探测技术研究

为了探索利用激光雷达探测遮蔽目标,构建了遮蔽目标激光雷达回波模型,取得了遮蔽目标激光雷达回波波形的仿真数据.采用Levenberg-Marguardt算法对回波波形非线性拟合,实现了对回波波形的高斯分解,获取了高斯回波分量的个数、位置、半宽和幅度等参数.通过对各个通道分离子波的时间匹配,得到了各个子目标在不同通道的回波...     

Range separation performance and optimal pulse-width prediction of a three-dimensional flash laser detection and ranging using the Cramer-Rao bound

This paper derives the Cramer-Rao bound (CRB) on range separation estimation of two point sources interrogated by a three-dimensional flash laser detection and ranging (LADAR) system. An unbiased range separation estimator is also derived to compare against the bound. Additionally, the CRB can be expressed as a function of two LADAR design parameters (range sampling and transmitted pulse width), which can be selected in order to optimize the expected range resolution between two point sources. Given several range sampling capabilities, the CRB and simulation show agreement that there is an optimal pulse width where a shorter pulse width would increase estimation variance due to undersampling of the pulse and a longer pulse width would degrade the resolving capability. Finally, the optimal pulse-width concept is extended to more complex targets and a normalized pulse definition.     

Hazardous industrial gases identified using a novel polymer/MWNT composite resistance sensor array

Hazardous industrial chemical gases pose a significant threat to the environment and human life. Therefore, there is an urgent need to develop a reliable sensor for identifying these hazardous gases. In this work, a silicon wafer microelectrode substrate for a resistance sensor was fabricated using the semiconductor manufacturing process. Conductive carbon nanotubes were then mixed with six different polymers with different chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. This array was then utilized to identify three hazardous gases at different temperatures. Experimental results for six polymers for chemical gases, such as tetrahydrofuran (THF), chloroform (CHCl₃) and methanol (MeOH) at different temperatures, indicate that the variation in sensitivity resistance increased when the sensing temperature increased. The poly(ethylene adipate)/MWNT sensing film had high sensitivity, excellent selectivity, and good reproducibility in detecting all chemical agent vapors. Additionally, this study utilized a bar chart and statistical methods in principal component analysis to identify gases with the polymer/MWNT sensor.     

Development of a broadband picosecond infrared spectrometer and its incorporation into an existing ultrafast time-resolved resonance Raman, UV/visible, and fluorescence spectroscopic apparatus

We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, approximately 150 cm(-1) FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of deltaOD approximately 10(-4) - 10(-5) with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.     

Observation of contrast agent response to chirp insonation with a simultaneous optical-acoustical system

Rayleigh-Plesset analysis, ultra-high speed photography, and single bubble acoustical recordings previously were applied independently to characterize the radial oscillation and resulting echoes from a microbubble in response to an ultrasonic pulse. In addition, high-speed photography has shown that microbubbles are destroyed over a single pulse or pulse train by diffusion and fragmentation. In order to develop a single model to characterize microbubble echoes based on oscillatory and destructive characteristics, an optical-acoustical system was developed to simultaneously record the optical image and backscattered echo from each microbubble. Combined observation provides the opportunity to compare predictions for oscillation and echoes with experimental results and identify discrepancies due to diffusion or fragmentation. Optimization of agents and insonating pulse parameters may be facilitated with this system. The mean correlation of the predicted and experimental radius-time curves and echoes exceeds 0.7 for the parameters studied here. An important application of this new system is to record and analyze microbubble response to a long pulse in which diffusion is shown to occur over the pulse duration. The microbubble response to an increasing or decreasing chirp is evaluated using this new tool. For chirp insonation beginning with the lower center frequency, low-frequency modulation of the oscillation envelope was obvious. However, low-frequency modulation was not observed in the radial oscillation produced by decreasing chirp insonation. Comparison of the echoes from similar sized microbubbles following increasing and decreasing chirp insonation demonstrated that the echoes were not time-reversed replicas. Using a transmission pressure of 620 kPa, the -6 dB echo length was 0.9 and 1.1 micros for increasing and decreasing chirp insonation, respectively (P = 0.02). The mean power in the low-frequency portion of the echoes was 8 (mV)2 and 13 (mV)2 for increasing and decreasing chirp insonation, respectively (P = 0.01).     

Antioxidant efficacy of chitosan/graphene functionalized superparamagnetic iron oxide nanoparticles

The antioxidant potential of superparamagnetic iron oxide nanoparticles functionalized with chitosan and graphene were examined in the present work. Coprecipitation technique was followed for the synthesis of iron oxide nanoparticles. Graphene-iron oxide nanocomposites were synthesized by mechanical mixing followed by the heat treatment at moderate temperature. The chitosan coated iron oxide nanoparticles were prepared by dispersing nanoparticles in chitosan solution. The nanoparticles/nanocomposites were characterized using XRD, SEM, TEM and HAADF-STEM for phase structure, morphology and elemental analysis. The superparamagnetic behavior of nanoparticles/nanocomposites were confirmed by magnetic measurements using vibrating sample magnetometry. Antioxidant efficacy of these nanoparticles/nanocomposites were investigated in terms of free radical scavenging and reducing potential using an array of in vitro assay system. Ferric reducing antioxidant power (FRAP) and 2,2′-diphenyl-1-picrylhydrazyl (DPPH) were used for the antioxidant capacity. The investigation suggests that the graphene improves the antiradical response of iron oxide nanoparticles at higher concentration which is almost comparable to the ascorbic acid used as standard.