Compensation of chromatic dispersion-induced power fading using optimised chirped fibre Bragg grating for millimetre-wave radio-over-fibre system

Compensation of fibre dispersion-induced power fading in an externally modulated sub- carrier multiplexed radio-over-fibre transmission link using the chirped fibre Bragg grating (CFBG) has been investigated. The results show that periodic power fading caused by fibre chromatic dispersion is significantly reduced and the optimum transmission distance can be increased by proper design of grating parameters and optimum selection of apodisation profile. The investigation also reveals that carrier-to-noise ratio and bit-error rate of 156 Mbps DPSK signal are also significantly improved using the CFBG with asymmetric apodisation profile as the dispersion compensator.     

Dispersive spectral-domain two-beam interference analysed by a fibre-optic spectrometer

Abstract

The effect of a fibre-optic spectrometer on analysed spectral interference of two beams from a white-light source is studied theoretically and experimentally, including the effect of dispersion in an interferometer. First, the spectral interference law is expressed analytically under the condition of a Gaussian response function of a fibre-optic spectrometer, and then second, the theoretical analysis is accompanied by three experiments employing a fibre-optic spectrometer and a Michelson interferometer with different amounts of dispersion. Within one experiment the interference fringes are resolved over a wide spectral range and within two experiments the interference fringes are resolved only in a narrow spectral range around a wavelength at which the group optical path difference between interfering beams is zero. Knowing dispersion in the interferometer and the bandpass of the spectrometer, the positions of the interferometer mirror in the corresponding range are determined and good agreement between the recorded spectral interferograms and the theoretical ones is found.     

Broadband radio-frequency spectrum analysis in spectral-hole-burning media

We demonstrate a 20 GHz spectrum analyzer with 1 MHz resolution and >40 dB dynamic range using spectral-hole-burning (SHB) crystals, which are cryogenically cooled crystal hosts lightly doped with rare-earth ions. We modulate a rf signal onto an optical carrier using an electro-optic intensity modulator to produce a signal beam modulated with upper and lower rf sidebands. Illuminating SHB crystals with modulated beams excites only those ions resonant with corresponding modulation frequencies, leaving holes in the crystal's absorption profile that mimic the modulation power spectrum and persist for up to 10 ms. We determine the spectral hole locations by probing the crystal with a chirped laser and detecting the transmitted intensity. The transmitted intensity is a blurred-out copy of the power spectrum of the original illumination as mapped into a time-varying signal. Scaling the time series associated with the transmitted intensity by the instantaneous chirp rate yields the modulated beam's rf power spectrum. The homogeneous linewidth of the rare-earth ions, which can be <100 kHz at cryogenic temperatures, limits the fundamental spectral resolution, while the medium's inhomogeneous linewidth, which can be >20 GHz, determines the spectral bandwidth.     

Spectrum-preserving Amplification of Ultra-broadband Optical Pulses

Abstract

An amplifier scheme which allows control of the spectrum of the amplified pulse is proposed. The spectral components of the initial pulse are sent into laterally shifted regions of the amplifying medium, which enables control of the gain of particular frequency components. By the use of a properly formed pump, spectral narrowing during amplification can be avoided. A ‘spectrum preserving’ amplification of ultra-broadband (f.w.h.m. of about 30 nm) nanosecond pulses is demonstrated.     

单矢量传感器DEMON谱检测技术研究

水声目标辐射噪声的包络谱中包含调制线谱(DEMON),不同类型的水声目标其DEMON谱不同,研究单矢量传感器利用DEMON谱检测目标信号的能力。采用文中定义的解析声强流方法,将单矢量传感器拾取的同一点的多信号源的振速矢量叠加统一描述在解析声强流的相位中,目标方位可以通过计算解析声强流的相位得到。仿真计算结果表明,这一方法对不同基频调制的宽带信号的检测能力可达-20dB,方位分辨能力在信噪比为-15dB时约为10°。对湖试数据的处理结果已证明本方法的可行性。     

一种降低D类功率放大器开关噪声的设计

传统D类功率放大器因特有的开关噪声对水下电子设备的信号接收、通信控制和信号传输等电信号产生很大的干扰,限制了D类功率放大器在水下电子设备中的广泛应用针对这一现象,首先阐明了Σ-Δ调制的D类功率放大器降低开关噪声的原理,然后对传统调制方式和Σ-Δ调制方式的D类功率放大器进行原理分析,并在Simulink软件中进行仿真对比。仿真结果表明,传统D类功率放大器在开关频率处的开关噪声能量高,Σ-Δ调制的D类功率放大器的开关噪声能量分散在一定的带宽内,并且开关噪声能量峰值低于传统D类功率放大器。     

Spectral superresolution with ultrashort optical pulses

A superresolution technique for the measurement of transmission, reflection, and absorption spectra is proposed. An ultrashort laser pulse is propagated in a dispersive element and then periodically phase modulated. The temporal modulation is transformed into periodic spectral modulation, for which the number of harmonics, 2M+1, is determined by the modulation index. The modulated pulse is transmitted through (reflected from) the sample to be tested and measured by a spectrometer. By performing 2M+1 measurements for 2M+1 delays between the dispersed pulse and modulation signal, one can restore the spectral response of the sample with superresolution after simple processing. We numerically demonstrate the measurement of the transmission spectrum of an ultranarrow optical filter with a minimum feature of 0.43 pm by an optical spectrum analyzer with a 10 pm resolution. A twentyfold enhancement of the resolution is achieved in the presence of noise with a level of 0.1%. The advantage of the system is its full reconfigurability.     

On measurement noise in the European TWSTFT network

Two-way satellite time and frequency transfer (TWSTFT) using geostationary telecommunication satellites is widely used in the timing community today and has also been chosen as the primary means to effect synchronization of elements of the ground segment of the European satellite navigation system Galileo. We investigated the link performance in a multistation network based on operational parameters such as the number of simultaneously transmitting stations, transmit and receive power, and chip rates of the pseudorandom noise modulation of the transmitted signals. Our work revealed that TWSTFT through a "quiet" transponder channel (2 stations transmitting only) leads to a measurement noise, expressed by the 1 pps jitter, reduced by a factor of 1.4 compared with a busy transponder carrying signals of 12 stations. The frequency transfer capability expressed by the Allan deviation is reduced at short averaging times by the same amount. At averaging times of >1 d, no such reduction could be observed, which points to the fact that other noise sources dominate at such averaging times. We also found that higher transmit power increases the carrier-to-noise density ratio at the receive station and thus entails lower jitter but causes interference with other station's signals. In addition, the use of lower chip rates, which could be accommodated by a reduced assigned bandwidth on the satellite transponder, is not recommended. The 1 pps jitter would go up by a factor of 2.5 when going from 2.5 MCh/s to 1 MCh/s. The 2 Galileo precise timing facilities (PTFs) can be included in the currently operated network of 12 stations in Europe and all requirements on the TWSTFT performance can be met, provided that suitable ground equipment will be installed in the Galileo ground segment.     

Frequency stabilization of an external cavity diode laser to molecular iodine at 657.483 nm

The saturation spectrum of the P(84) 5-5 transition of 127I2 at 657.483 nm is obtained with the third-harmonic demodulation method using an external cavity diode laser. The laser frequency is modulated by modulating the diode current instead of modulating the cavity length with a piezoelectric transducer (PZT). Current modulation allows a modulation frequency that is higher than PZT modulation. The signal-to-noise ratio of 1000 is better than previous results presented in the literature. The laser is frequency stabilized to the hyperfine component o of the P(84) 5-5 transition with a frequency stability of better than 10 kHz (2.2 x 10(-11) relative stability).     

滚动轴承的实调制细化包络谱诊断方法研究

滚动轴承振动信号中载波频率较高而调制的故障频率较低,若直接采用Hilbert变换进行包络谱分析,往往由于谱分辨率较低而无法得到正确的结果。该文在Hilbert变换求包络谱之前增加了一个实调制移频过程,将原始信号的载波频率及调制频率(故障特征频率)移至低频段,然后再重新采样变为低频实信号,再经过Hilbert变换,得到了一种新的实调制细化包络频谱。在滚动轴承的故障诊断试验中进行了实际应用,结果表明:该方法不但可以提高包络谱的频率分辨率,而且计算效率较高,具有一定的实际应用价值。     

Statistical properties of the noise output current

Summary The response of an electronic system to the photons emitted by a radioactive source (noise source) is exponential. Some statistical properties of the system output current are investigated for Poisson-distributed decay. It is found that the power spectral density for the noise is approximately proportional to the reciprocal of the frequency square (²). Due to the monotonic, decreasing nature of the noise power spectrum, it is found that the higher the signal frequency, the larger is the signal-to-noise ratio for such a system.     

40 Gbits/s all-optical clock recovery for degraded signals using an amplified feedback laser

All-optical clock recovery for the return-to-zero modulation format is demonstrated experimentally at 40 Gbits/s by using an amplified feedback laser. A 40 GHz optical clock with a root-mean-square (rms) timing jitter of 130 fs and a carrier-to-noise ratio of 42 dB is obtained. Also, a 40 GHz optical clock with timing jitter of 137 fs is directly recovered from pseudo-non-return-to-zero signals degraded by polarization-mode dispersion (PMD). No preprocessing stage to enhance the clock tone is used. The rms timing jitter of the recovered clock is investigated for different values of input power and for varying amounts of waveform distortion due to PMD.     

Mechanical-thermal noise in MEMS gyroscopes

We derive expressions for the effect of mechanical thermal noise on a vibrational microelectromechanical system gyroscope, including the angle of random walk, the noise equivalent rotation rate, and the spectral density of the noise component of the rate measurement. We explicitly calculate and compare the output signal due to rotation and the output due to noise. We avoid several ambiguities in the literature concerning bandwidth and correctly observe a factor of two reduction in noise power due to synchronous demodulation. We use stochastic averaging to obtain an approximate "slow" system that clarifies the effect of thermal noise and shows the effect of frequency mismatch between the drive and sense axes. We compute the noise equivalent rate for both open-loop and force-to-rebalance operation of the gyroscope.     

Acoustooptical processor for radio signals with discrete frequency modulation

A study is made of a processor for pulsed radio signals with discrete frequency modulation, where the frequency varies discontinuously within a pulse and takes on N different values. The spectral components are separated with an acoustooptic spectrum analyzer whose spectral plane is imaged onto a linear charged-coupled photodetector by means of a planar (one-dimensional) spatial converter made of optical fibers. In the imaging, the spectral components of the signal injected into the fiber are transposed by a special arrangement (permutation) of the fibers. The charge-coupled detector, which operates in the time-delay and accumulation mode, detects the spectral components and sums them with the required delay, and thereby compresses the signal and separates it from the noise. The compression factor is limited by the value of N and can be several hundred. Results of experimental investigations of a prototype of the processor are presented. Pis’ma Zh. Tekh. Fiz. 23, 12–19 (January 26, 1997)     

基于改进EMD方法与1 1/2谱的DEMON谱提取方法

噪声的包络调制检测(Detection of Envelope Modulation on Noise, DEMON)谱分析技术已被广泛应用于特征提取领域,但经典 DEMON 谱提取中高频信号频段的选取会影响 DEMON 谱的提取效果。针对这一问题,文中首先运用经验模态分解(Empirical Mode Decomposition, EMD)方法获得一系列固有模态函数(Intrinsic Mode Function, IMF),依据各阶模态函数与原信号的相关程度,筛选出更具代表性的几阶固有模态函数进行解调,再对解调的结果运用1 1/2维谱分析方法进行谱分析以抑制高斯噪声,通过这种方法获得的 DEMON 谱信噪比优于传统方法。实测湖试数据分析结果表明,该改进方法可以有效地进行特征提取,结果优于经典 DEMON 谱分析方法;该改进方法具有一定的实用性,有利于进行后续目标分类识别。     

信号相位匹配算法的语音降噪及性能评估

针对信号相位匹配算法能够在信号估计中准确提取信号频率成分的优点,将三子阵信号相位匹配算法应用于提取强背景噪声中的语音信号,同时提出了基于平均段内功率谱密度距离的语音质量客观评价方法,将主观平均意见分（MOS）、信噪比和平均分段功率谱密度距离作为指标,分别对三子阵信号相位匹配法、谱减法和最小均方误差估计法这三种算法的处理结果进行客观评价并对算法的降噪性能做了对比。仿真结果表明,三子阵相位匹配算法能够达到强背景噪声环境下语音降噪的目的;同时基于平均段内功率谱密度距离的语音质量客观评价结果与主观评价结果相符合,该评价方法具有一定的可行性。     

Enhancing time-stamp counter phase modulation measurements

This paper demonstrates that digital signal processing techniques can enhance the quality of phase modulation measurements produced by a time-stamp (phase digitizing) frequency counter. A typical time-stamp counter utilizes a digital divider to reduce signal frequency to the desired sample rate. Unfortunately, division also reduces phase modulation to the point where useful information may be obscured by counter measurement uncertainty (jitter). An analogy between an analog-to-digital converter (ADC) and a time-stamp counter predicts that the counter induced modulation can be modeled as random noise which is white in phase. The noise magnitude is directly related to the instrument's resolution specification. Fourier analysis, subject to some restrictions, can compute the power spectra of phase or frequency modulation, revealing even low level responses. A number of techniques can be used to reduce the amount of counter induced noise that appears on time domain plots of phase and frequency modulation. Experimental data, generated by a prototype counter, illustrates the type of results that can be expected from Fourier analysis and various noise reduction techniques     

The effect of surface on the power spectrum of the Barkhausen noise in ferromagnetic materials

The Barkhausen noise power spectrum was investigated on laminated polycrystalline toroids of Fe and 4% SiFe as a function of sample thickness; that is, of surface-to-volume ratio. Upon decreasing the thickness strong variations of spectrum shape are observed-the cut-off frequency increases and the spectral density drops in the very-low-frequency range. On the other hand, only negligible variations of static magnetic properties are observed. According to a recent theory, the results are explained assuming that a change of thickness directly affects the number of elementary pulses clustering into a large Barkhausen discontinuity. The calculated variations of the spectrum intensity and cut-off frequency are in good agreement with the experimental results.     

The Distributed Feedback of Counter-propagating Waves in a Periodically Modulated Medium with Relaxing Cubic Nonlinearity

Abstract

An interaction of two counter-propagating waves in a periodically modulated medium with relaxing cubic nonlinearity is considered. On the basis of analytical solutions of the equations under the weak distributed feedback (DFB) approximation, we show that an increase in the reflectivity of ‘nonlinear’ DFB-structures takes place due to phase mismatching between the radiation and the light-induced grating. We carry out a numerical simulation of the obtained equations for arbitrary values of the coupling coefficient, Bragg detuning and incident pulse intensity. We also analysed the dynamics of the regular temporal modulation of initially continuous wave radiation caused by the ‘nonlinear’ grating.