Photon-counting chirped amplitude modulation lidar system using superconducting nanowire single-photon detector at 1550-nm wavelength

We demonstrate a photon-counting chirped amplitude modulation(CAM) light detection and ranging(lidar) system incorporating a superconducting nanowire single-photon detector(SNSPD) and operated at a wavelength of 1550 nm.The distance accuracy of the lidar system was determined by the CAM bandwidth and signal-to-noise ratio(SNR) of an intermediate frequency(IF) signal. Owing to a short dead time(10 ns) and negligible dark count rate(70 Hz) of the SNSPD, the obtained IF signal attained an SNR of 42 d B and the direct distance accuracy was improved to 3 mm when the modulation bandwidth of the CAM signal was 240 MHz and the modulation period was 1 ms.     

光子计数模式下的目标探测与成像

当光微弱到以单个光子发射时,成像系统只有利用光子计数模式才能探测到单光子信息。采用基于碰撞电离效应的全固态雪崩光电二极管作为探测元件,构成微光环境下的光子计数成像实验系统。该系统的硬件主要由雪崩光电二极管构成的单光子计数器、计算机、微光照度计、2维电控导轨、控制器、暗箱等组成。控制器的软件在Altera公司Quartus环境下设计,主要完成导轨运动的控制;上位机软件采用VC++编程实现系统的数据采集处理、系统功能控制和光子计数图像显示等。该系统为全固态结构,工作电压小于35 V,暗计数率小于4 Hz。所建光子计数成像系统在10－5 lx微光环境下实现了目标的探测成像。     

光子计数模式下的目标探测与成像

 当光微弱到以单个光子发射时,成像系统只有利用光子计数模式才能探测到单光子信息。采用基于碰撞电离效应的全固态雪崩光电二极管作为探测元件,构成微光环境下的光子计数成像实验系统。该系统的硬件主要由雪崩光电二极管构成的单光子计数器、计算机、微光照度计、2维电控导轨、控制器、暗箱等组成。控制器的软件在Altera公司Quartus环境下设计,主要完成导轨运动的控制;上位机软件采用VC++编程实现系统的数据采集处理、系统功能控制和光子计数图像显示等。该系统为全固态结构,工作电压小于35 V,暗计数率小于4 Hz。所建光子计数成像系统在10－5 lx微光环境下实现了目标的探测成像。     

Detection of quasitrapezoidal frequency and amplitude modulation

It has been proposed that the detection of frequency modulation (FM) of sinusoidal carriers can be mediated by two mechanisms; a place mechanism based on FM-induced amplitude modulation (AM) in the excitation pattern, and a temporal mechanism based on phase locking in the auditory nerve. The temporal mechanism appears to be "sluggish" and does not play a role for FM rates above about 10 Hz. It also does not play a role for high carrier frequencies (above about 5 kHz). This experiment provided a further test of the hypothesis that the effectiveness of the temporal mechanism depends upon the time spent close to frequency extremes during the modulation cycle. Psychometric functions for the detection of AM and FM were measured for two carrier frequencies, 1 and 6 kHz. The modulation waveform was quasitrapezoidal. Within each modulation period, P, a time Tss was spent at each extreme of frequency or amplitude. The transitions between the extremes, with duration Ttrans had the form of a half-cycle of a cosine function. The modulation rate was 2, 5, 10, or 20 Hz, giving values of P of 500, 200, 100, and 50 ms. TSS varied from 0 ms (sinusoidal modulation) up to 160, 80, 40, or 20 ms, for rates of 2, 5, 10, and 20 Hz, respectively. The detectability of AM was not greatly affected by modulation rate or by the value of TSS, except for a slight improvement with increasing TSS for the lowest modulation rates; this was true for both carrier frequencies. For FM of the 6-kHz carrier, the pattern of results was similar to that found for AM, which is consistent with an excitation-pattern model of FM detection. For FM of the 1-kHz carrier, performance improved markedly with increasing TSS, especially for the lower FM rates; there was no change in performance with TSS for the 20-Hz modulation rate. The results are consistent with the idea that detection of FM of a 1-kHz carrier is partly mediated by a sluggish temporal mechanism. That mechanism benefits from greater time spent at frequency extremes of the modulation cycle for rates up to 10 Hz.     

The intensity detection of single-photon detectors based on photon counting probability density statistics

Single-photon detectors possess the ultra-high sensitivity, but they cannot directly respond to signal intensity. Conventional methods adopt sampling gates with fixed width and count the triggered number of sampling gates, which is capable of obtaining photon counting probability to estimate the echo signal intensity. In this paper, we not only count the number of triggered sampling gates, but also record the triggered time position of photon counting pulses. The photon counting probability density distribution is obtained through the statistics of a series of the triggered time positions. Then Minimum Variance Unbiased Estimation(MVUE) method is used to estimate the echo signal intensity. Compared with conventional methods, this method can improve the estimation accuracy of echo signal intensity due to the acquisition of more detected information. Finally, a proof-of-principle laboratory system is established. The estimation accuracy of echo signal intensity is discussed and a high accuracy intensity image is acquired under low-light level environments.     

Detection of combined frequency and amplitude modulation.

This article is concerned with the detection of mixed modulation (MM), i.e., simultaneously occurring amplitude modulation (AM) and frequency modulation (FM). In experiment 1, an adaptive two-alternative forced-choice task was used to determine thresholds for detecting AM alone. Then, thresholds for detecting FM were determined for stimuli which had a fixed amount of AM in the signal interval only. The amount of AM was always less than the threshold for detecting AM alone. The FM thresholds depended significantly on the magnitude of the coexisting AM. For low modulation rates (4, 16, and 64 Hz), the FM thresholds did not depend significantly on the relative phase of modulation for the FM and AM. For a high modulation rate (256 Hz) strong effects of modulator phase were observed. These phase effects are as predicted by the model proposed by Hartmann and Hnath [Acustica 50, 297-312 (1982)], which assumes that detection of modulation at modulation frequencies higher than the critical modulation frequency is based on detection of the lower sideband in the modulated signal's spectrum. In the second experiment, psychometric functions were measured for the detection of AM alone and FM alone, using modulation rates of 4 and 16 Hz. Results showed that, for each type of modulation, d' is approximately a linear function of the square of the modulation index. Application of this finding to the results of experiment 1 suggested that, at low modulation rates, FM and AM are not detected by completely independent mechanisms. In the third experiment, psychometric functions were again measured for the detection of AM alone and FM alone, using a 10-Hz modulation rate. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. Significant effects of relative modulator phase were found when detectability was relatively high. These effects were not correctly predicted by either a single-band excitation-pattern model or a multiple-band excitation-pattern model. However, the detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently.     

Estimation of photon counting statistics with imperfect detectors

The study on photon counting statistics is of fundamental importance in quantum optics. We theoretically analyzed the imperfect detection of an arbitrary quantum state. We derived photon counting formulae for six typical quantum states(i.e.,Fock, coherent, squeeze-vacuum, thermal, odd and even coherent states) with finite quantum efficiencies and dark counts based on multiple on/off detector arrays. We applied the formulae to the simulation of multiphoton number detections and obtained both the simulated and ideal photon number distributions of each state. A comparison between the results by using the fidelity and relative entropy was carried out to evaluate the detection scheme and help select detectors for different quantum states.     

Detection and rate discrimination of amplitude modulation in electrical hearing

Three experiments were designed to examine temporal envelope processing by cochlear implant (CI) listeners. In experiment 1, the hypothesis that listeners' modulation sensitivity would in part determine their ability to discriminate between temporal modulation rates was examined. Temporal modulation transfer functions (TMTFs) obtained in an amplitude modulation detection (AMD) task were compared to threshold functions obtained in an amplitude modulation rate discrimination (AMRD) task. Statistically significant nonlinear correlations were observed between the two measures. In experiment 2, results of loudness-balancing showed small increases in the loudness of modulated over unmodulated stimuli beyond a modulation depth of 16%. Results of experiment 3 indicated small but statistically significant effects of level-roving on the overall gain of the TMTF, but no impact of level-roving on the average shape of the TMTF across subjects. This suggested that level-roving simply increased the task difficulty for most listeners, but did not indicate increased use of intensity cues under more challenging conditions. Data obtained with one subject, however, suggested that the most sensitive listeners may derive some benefit from intensity cues in these tasks. Overall, results indicated that intensity cues did not play an important role in temporal envelope processing by the average CI listener.     

采用Si薄膜的光子计数成像系统性能的研究

在陶瓷基底上利用电子束蒸镀方法制备了Si薄膜,用作感应读出方式光子计数成像系统的电荷感应层,并研究了薄膜的结构特征和表面形态.X射线衍射(XRD)测试和场发射扫描电子显微镜(FESEM)图像表明,沉积的Si薄膜为无定形态,由于陶瓷晶界的存在,薄膜较粗糙.搭建了相应的实验系统,对比了采用不同厚度Si薄膜时系统的空间分辨率、计数率、脉冲高度分布曲线等,发现薄膜的厚度对探测器的计数率影响较大.此外,实验还对比了采用相同电阻值的Si薄膜和常用的Ge薄膜时系统的性能.研究表明,采用Si薄膜时系统的畸变较小、计数率高     

Optical time domain reflectometry — a performance comparison of the analogue and photon counting techniques

This paper analyses and compares the performances of direct-detection Optical Time Domain Reflectometry systems using (a) photon counting and (b) analogue front-ends. Simple system models and a range equation have been developed to aid comparison. It is shown that the relative performances depend upon the number of signal averages. But, in the limit of the maximum range there is little to choose between the two for the state of the art systems.     

Discrimination of quantum states with linear optics and continuous photon counting

We investigate a systematic method of implementation of given measurements with linear optics and continuous measurement. We revisit the so-called Dolinar receiver, which is a quasi-physical model attaining the minimum error discrimination of binary coherent states, and show that it can be derived without heuristic assumptions. Our approach also can be applied to various kinds of measurements.     

Detailed calibration of the PI-LCX:1300 high performance single photon counting hard x-ray CCD camera

X-ray charge-coupled-device(CCD) camera working in single photon counting mode is a type of x-ray spectrometer with high-sensitivity and superior signal-to-noise performance. In this study, two single photon counting CCD cameras with the same mode(model: PI-LCX: 1300) are calibrated with quasi-monochromatic x-rays from radioactive sources and a conventional x-ray tube. The details of the CCD response to x-rays are analyzed by using a computer program of multi-pixel analyzing and event-distinguishing capability. The detection efficiency, energy resolution, fraction of multi-pixel events each as a function of x-ray energy, and consistence of two CCD cameras are obtained. The calibrated detection efficiency is consistent with the detection efficiency from Monte Carlo calculations with XOP program. When the multi-pixel event analysis is applied, the CCDs may be used to measure x-rays up to 60 ke V with good energy resolution(E /?E ≈ 100 at60 ke V). The difference in detection efficiency between two CCD cameras is small(5.6% at 5.89 ke V), but the difference in fraction of the single pixel event between them is much larger(25% at 8.04 ke V). The obtained small relative error of detection efficiency(2.4% at 5.89 ke V) makes the high accurate measurement of x-ray yield possible in the laser plasma interaction studies. Based on the discrete calibration results, the calculated detection efficiency with XOP can be used for the whole range of 5 ke V–30 ke V.     

光子计数法测量微光像增强器背景噪声分布特性

针对微光像增强器噪声特性,理论上分析了微光像增强器背景光子噪声特点。设计了采用光子计数法及使用类针孔微弱光照度计对微光像增强器的背景噪声进行测量的方案。在三代微光像增强器信噪比测量装置中,采用类针孔微弱光照度计对微光像增强器无光照时荧光屏上Ф0.2 mm光斑的照度进行了测量实验。对于空间特性,以荧光屏中心点为圆心的同心圆上照度值不均匀性小于1%;对于时间特性,在微光像增强器工作大约270 min后,荧光屏中心点照度值会达到稳定状态。以各个空间点和时间点照度值为基础分析了无光照时像增强器背景噪声在时间和空间的分布特性,为评价微光像增强器的噪声特性提供了数据支持。     

Formulation of nuclear polarization correction with the Feynman photon propagator

The leading order process of the nuclear polarization (NP) is formulated with the Feynman photon propagator. The present treatment incorporates the whole contributions due to the Coulomb and transverse interactions as well as their interference in the Coulomb gauge. An explicit formula is given in the point-nucleus limit for hydrogen-like heavy ions. The numerical method is also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluating the performance improvement of differential phase-shift keying signals by amplitude regeneration and phase-noise suppression

Theoretical bit error rates of the differential phase-shift keying format with various regeneration schemes is presented. A comparison with ideal regeneration reveals that averaging the phase noise of adjacent bits is found to eliminate most penalties that are induced by phase noise.     

Influence of second-order correction to Rayleigh scattering on squared field amplitude squeezing and photon antibunching in the two-level thermal Jaynes-Cummings model

We have investigated the influence of the second-order correction to Rayleigh scattering on intensity squeezing and photon antibunching in the standard two-level thermal Jaynes-Cummings model, thereby extending our earlier work [Phys. Rev. A 56 (1997) 796].     

Generation of pure electrical quadrature amplitude modulation with photonic vector modulator

A photonic vector modulator architecture for generating pure quadrature amplitude modulation (QAM) signals is presented. An electrical quadrature-modulated signal at microwave-millimeter-wave frequencies is generated from its corresponding baseband in-phase (I) and quadrature (Q) components. In the proposed scheme, no electrical devices apart from the electrical tone oscillator are needed in the generation process. In addition, the purity of the generated signal is increased, and the hardware requirements are reduced when compared with previously proposed architectures so a highly compact low-cost architecture can be implemented. A pure 1.25 Gbit/s 4-QAM signal has been experimentally generated at a 42 GHz carrier frequency.     

Detection of a signal with unknown amplitude and duration

We synthesize a maximum-likely algorithm for detecting an arbitrary-shaped signal with unknown duration and amplitude, which is observed against the background of additive Gaussian white noise. The synthesized algorithm is analyzed on the assumption of sufficiently large signal-to-noise ratios. Asymptotically accurate expressions for the detection-error probabilities are found. The efficiency of the synthesized algorithm is estimated by numerical simulation and the applicability limits of the asymptotic expressions are determined.     

Modeling the encoding structure and spatial resolution of photon counting imagers with Vernier anode readout

We present the spatial resolution estimation methods for a photon counting system with a Vernier anode.A limiting resolution model is provided according to discussions of surface encoding structure and quantized noise. The limiting resolution of a Vernier anode is revealed to be significantly higher than that of a microchannel plate. The relationship between the actual spatial resolution and equivalent noise charge of a detector is established by noise analysis and photon position reconstruction. The theoretical results are demonstrated to be in good agreement with the experimental results for a 1.2 mm pitch Vernier anode.