Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography

We present theoretical analysis and experimental verification of the signal to noise ratio (SNR) of a common-path interferometer-based optical coherence tomography (OCT) system. Based on fully integrated all-fiber implementation of a common-path time-domain OCT system, we derived the SNR of the system including the effect of beat noise, which turns out to be twice as large as the excess noise term. We verified the theoretical SNR through a series of experiments, utilizing both controlled phantom and biological samples such as a rat brain with tumor and a frog retina. The results showed that the source power and the reference reflectivity can be easily controlled to optimize the SNR of OCT imaging. We have also analyzed the effect of the fiber delays and the offset in the fiber autocorrelator of the common-path OCT system on the overall SNR.     

相关MIMO信道下一种特征波束成型方案的性能研究

提出了相关多输入多输出信道下的一种空时分组码、空分复用与波束成型相结合的新方案．该方案利用了发射分集和空分复用所带来的分集和复用增益，并基于接收机反馈的信道相关矩阵设计了一种波束成型器．仿真结果表明，这种方案的性能较传统的方案在高信噪比时能获得较大的改善．     

Ultra-Deep Bone Diagnostics with Fat–Skin Overlayers Using New Pulsed Photothermal Radar

The constraints imposed by the laser safety (maximum permissible exposure) ceiling on pump laser energy and the strong attenuation of thermal-wave signals in tissues significantly limit the photothermally active depth in most biological specimens to a level which is normally insufficient for practical applications (a few mm below the skin surface). A theoretical approach for improvement of the signal-to-noise ratio (SNR), minimizing the static (dc) component of the photothermal (PT) signal and making use of the PT radiometric nonlinearity has been introduced. At low frequencies fixed-pulse-width chirps of large peak power were found to be superior to all other equal energy modalities, with an SNR improvement by up to two orders of magnitude. Compared to radar peak delay and amplitude, the long-delayed radar output amplitude is found to be more sensitive to subsurface conditions. Two-dimensional spatial plots of this parameter depicting the back-surface conditions of bones with and without fat tissue overlayers are presented. Pulsed-chirp radar thermography has been demonstrated to monitor the degree of demineralization in goat rib bone with a substantial SNR and spatial resolution that is not practicable with harmonic radars of the same energy density.     

Superresolution by use of code division multiplexing

Traditional methods for superresolution have sacrificed field of view for resolution. These methods multiplexed different parts of signals' spectrum on different carriers, and thus managed to transfer a wider range of frequency, in a manner that is similar to frequency division multiplexing in classical communication. We propose code division multiplexing for such an application, which has been shown to have superior capabilities. To enable such mutiplexing we propose a unique setup that creates an incoherent cosine transform of the image. A theoretical analysis of the setup is obtained and later compared with the empirical results.     

Influence of temperature on the sensitivity and signal-to-noise ratio of a fiber-optic surface-plasmon resonance sensor

We have theoretically analyzed the influence of temperature on the performance of a fiber-optic sensor based on surface-plasmon resonance (SPR). The performance of the sensor has been evaluated in terms of its sensitivity and signal-to-noise ratio (SNR). The theoretical model for temperature dependence includes the thermo-optic effect in the fiber core and sensing layer, and phonon-electron scattering along with electron-electron scattering in the metal layer. The effect of temperature on the SNR and the sensitivity of the sensor with two different metals has been compared. The same comparison is carried out for the sensing layers with positive and negative thermo-optic coefficients. The theoretical model has been analyzed for both the nonremote and remote sensing cases. This detailed analysis of temperature-dependent SNR and sensitivity leads to achieving the best possible performance from a fiber-optic SPR sensor against the temperature variation.     

Measurements of the thermal diffusivity of aluminum using frequency-scanned, transient, and rate window photothermal radiometry. Theory and experiment

The thermal diffusivity of various types of aluminum has been measured, using a completely noncontact experimental configuration based on infrared photothermal radiometry. Photothermal response transients, conventional frequency scans, and pulse duration- or repetition rate-scanned rate windows have been investigated. It has been shown that the conventional frequency scan is not suitable for measurements of aluminum with a short thermal transport time such as foils, due to an extremely degraded signal-to-noise ratio (SNR). Also, it has been found that the conventional frequency scan method is less sensitive to the actual value of thermal diffusivity than the rate-window scan. The rate-window method, furthermore, gives superior SNR especially for thin metals and yields excellent agreement between the theory and the data. An advantage of the pulse duration-scanned rate window mode is that it does not require knowledge of the instrumental transfer function as an input. The transient response gives the worst SNR but is best for the physical interpretation of the photothermal signals. In addition, it has been shown that the infrared photothermal radiometric transmission mode is less sensitive to surface roughness than the reflection mode and, therefore, is preferable for thermal diffusivity measurements of aluminum and of good thermal conductors, in general.     

Performance of spatially multiplexed MC-CDM with zero-forcing unified successive interference cancellation detection

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.     

Theoretical evaluation of a four-band fiber-optic radiometer

A theoretical simulation of a four-band fiber-optic radiometric technique is presented. This is a technique for remote, noncontact temperature measurement of a sample near room temperature, under conditions of unknown emissivity and ambient temperature. A realistic setup of a broadband IR detector, a set of three filters, an IR fiber, and a MATLAB software package for the calculations, is simulated in two steps: a calibration process and a measurement process. The results of the simulation show the limitations and advantages of the four-band radiometric technique and show the expected resolution of the sample temperature and emissivity and of the ambient temperature measurement. The theoretical resolution of the sample temperature measured by the four-band radiometric setup comes close to the resolution achieved in an equivalent single-band radiometric setup. The four-band method has an additional advantage of making it possible to calculate values of emissivity and ambient temperature.     

Influence of skew rays on the sensitivity and signal-to-noise ratio of a fiber-optic surface-plasmon-resonance sensor: a theoretical study

We have theoretically analyzed the influence of skew rays on the performance of a fiber-optic sensor based on surface plasmon resonance. The performance of the sensor has been evaluated in terms of its sensitivity and signal-to-noise ratio (SNR). The theoretical model for skewness dependence includes the material dispersion in fiber cores and metal layers, simultaneous excitation of skew rays, and meridional rays in the fiber core along with all guided rays launching from a collimated light source. The effect of skew rays on the SNR and the sensitivity of the sensor with two different metals has been compared. The same comparison is carried out for the different values of design parameters such as numerical aperture, fiber core diameter, and the length of the surface-plasmon-resonance (SPR) active sensing region. This detailed analysis for the effect of skewness on the SNR and the sensitivity of the sensor leads us to achieve the best possible performance from a fiber-optic SPR sensor against the skewness in the optical fiber.     

Multispectral imaging axicons

Large-aperture linear diffractive axicons are optical devices providing achromatic nondiffracting beams with an extended depth of focus when illuminated by white light sources. Annular apertures introduce chromatic foci separation, making chromatic imaging possible despite important radiometric losses. Recently, a new type of diffractive axicon has been introduced, by multiplexing concentric annular axicons with appropriate sizes and periods, called a multiple annular linear diffractive axicon (MALDA). This new family of conical optics combines multiple annular axicons in different ways to optimize color foci recombination, separation, or interleaving. We present different types of MALDA, give an experimental illustration of the use of these devices, and describe the manufacturing issues related to their fabrication to provide color imaging systems with long focal depths and good diffraction efficiency. Application to multispectral image analysis is discussed.     

Statistical adaptive modulation with TCOI-Tx

If a channel fades fast and the feedback rate of a wireless communication system is low, exploiting long-term channel statistics at the transmitter is more often practical than exploiting instantaneous channel state information. Statistical adaptive modulation scheme based on tap correlation information at the transmitter assuming an orthogonal frequency division multiplexing system with fast mobiles has been proposed. The proposed scheme improves the system performance compared with uniform bit loading scheme. It has been shown that at low SNR, statistical adaptive modulation applied to a channel with high tap correlation results in better performance than that applied to a channel without tap correlation     

Theory and experiments of a three-cavity wavelength-selective photodetector

We propose a novel wavelength-selective photodetector with three subcavities, i.e., a filtering cavity, a spacer cavity, and an absorption cavity, for obtaining a narrow spectral response linewidth and a high quantum efficiency simultaneously. A theoretical prediction has been made that a less than 1-nm linewidth and a quantum efficiency as high as 90% are possible. We discuss the effects of the key factors on the performance of this type of photodetector that has been designed and fabricated. A spectral response linewidth of approximately 1.4 nm (FWHM) and an external quantum efficiency higher than 50% have been achieved experimentally. Such devices are promising for wavelength-division multiplexing applications.     

Method of hologram multiplexing by use of a fiber bundle with rotary movement

A simple method of rotating an optical fiber bundle is presented to apply the random-pattern referencing scheme to hologram multiplexing. In the theoretical study, a dependence of the diffraction efficiency on the number of spatial frequencies in the reference pattern is estimated with the Bragg diffraction theory. In the experiment, hologram multiplexing is performed in which 30 holograms are recorded in a LiNbO3 crystal and are read out by rotating a fiber bundle. The result shows that this simple approach enables us to perform the hologram multiplexing and also contributes to the building of a compact optical setup.     

基于微处理器与现场可编程模拟阵列(FPAA)的数字音频均衡器设计

由于分立器件与运放构建的模拟电感音频均衡器,因受分立器件本身性能影响,存在许多不利因素,使得该音频均衡器在竞争中处于劣势;鉴于纯数字音频均衡器性能相对稳定但价位较高,其应用集中于高档场合而难以普及。本文提出了一种新的音频均衡器设计方法,采用摩托罗拉Anadigm子公司的现场可编程模拟阵列产品AN221E04和Freescale低成本微处理器GT60构建的高集成、高精度、高信噪比、高抗干扰能力的模拟可编程结构的数字音频均衡器系统。该系统软件仿真性能优良,容易达到预期设计效果。该设计方法证明可编程产品的应用将大大提高模拟领域的设计性能。     

Radiometric order preserving method to display wide-dynamic images for imagery photointerpretation

Wide-dynamic numerical images are increasingly frequent in professional environments, military photointerpretation, and x-ray or magnetic resonance medical imagery. However, a dynamic compression process is necessary to exploit such images without incessant image manipulation. A wealth of efficient methods has been developed to tackle this problem on aesthetic grounds. We argue that professional imagery interpretation needs preservation of the original radiometric order. We develop a measure of image efficiency use of the 8-bit radiometric channel and find that it correlates nicely with subjective appraisal. The image underscores a radiometric order-preserving process to reach a standard radiometric efficiency. Lost information is then reintroduced by addition of an edge image devoid of artifacts, with an automatic weighting ensuring a natural-looking image.     

Comparing Reliability-Redundancy Tradeoffs for Two von Neumann Multiplexing Architectures

Nanoelectronic systems are anticipated to be highly susceptible to computation and communication noise. Interestingly, von Neumann addressed the issue of computation in the presence of noisy gates in 1952 and developed a technique called multiplexing. He proposed multiplexing architectures based on two universal logic functions, nand and maj. Generalized combinatorial models to analyze such multiplexing architectures were proposed by von Neumann and extended later by others. In this work, we describe an automated method for computing the effects of noise in both the computational and interconnect hardware of multiplexing-based nanosystems-a method employing a probabilistic model checking tool and extending previous modeling efforts, which only considered gate noise. This method is compared with a recently proposed automation methodology based on probabilistic transfer matrices and used to compute and compare the reliability of individual nand and maj multiplexing systems, both in the presence of gate and interconnect noise. Such a comparative study of nand and maj multiplexing is needed to provide quantitative guidelines for choosing one of the multiplexing schemes. The maximum device failure probabilities that can be accommodated by multiplexing-based fault-tolerant nanosystems are also computed by this method and compared with theoretical results from the literature. This paper provides a framework that can capture probabilistically quantified fault models and provide quick reliability evaluation of multiplexing architectures     

矢量水听器在水声通信系统中的应用

将矢量水听器应用于水声通信系统,利用矢量水听器的指向性屏蔽强干扰。建立了基于矢量水听器的多载波正交频分复用(OFDM)系统模型,对各向同性噪声场中矢量水听器的增益进行了推导,为抑制相干干扰提供了新的解决方案,并进行了仿真验证。仿真研究表明,该方案能够有效地增强系统的抗各向同性干扰的能力,达到提高信噪比的目的,降低了误码率,增加了系统的稳健性。湖上试验验证了该方法的有效性,表明基于矢量水听器的水声通信系统可为实现高速水声通信提供一种有效的途径。     

Measuring absolute infrared spectral radiance with correlated visible photons: technique verification and measurement uncertainty

An experimental system in which correlated photons for radiometric measurements were used has been set up at the National Institute of Standards and Technology. We use visible-IR pairs of correlated photons produced by means of optical parametric downconversion to measure the radiance of a high-temperature IR source at 3.415 and 4.772 mum in an intrinsically absolute manner (i.e., without requiring any externally calibrated radiometric standard). To our knowledge, this is the only radiometric method with which one measures radiance directly, instead of using radiant power and aperture geometry measurements to deduce radiance indirectly. This technique has an additional unusual characteristic: It allows absolute radiometric measurements of IR radiation to be made with high-quality visible detectors. We compare measurements made with this technique with radiance measurements made with conventional means tied to existing radiometric standards. These comparisons show an average agreement to within ~3% between the two methods. The results demonstrate an accuracy consistent with the estimated uncertainty of the currentmeasurements. This is the first time to our knowledge that this method has been used to provide absolute radiance measurements of a source that has been calibrated conventionally, revealing unexpected systematic effects and allowing estimates of the ultimate accuracy of this method. In addition, these measurements are further into the IR than any previous measurements of this process and have produced the highest thermally stimulated downconversion signal yet seen.     

Shift multiplexing with spherical reference waves

Shift multiplexing is a holographic storage method particularly suitable for the implementation of holographic disks. We characterize the performance of shift-multiplexed memories by using a spherical wave as the reference beam. We derive the shift selectivity, the cross talk, the exposure schedule, and the storage density of the method. We give experimental results to verify the theoretical predictions.     

极化码在OFDM水声通信中的应用研究

极化码(Polar code)因其高可靠性、实用的线性编、译码复杂度和理论上唯一可达香农极限等特点,成为信道编码领域新的研究热点.其编、译码方法的研究扩展至多种信道类型和应用领域,但在水声信道中的理论证明和应用研究相对较少且滞后.针对具有显著多途、多普勒扩散和有限带宽等复杂特性的水声信道,文章提出了与之相匹配的极化码信...