Intrinsic noise of semiconductor lasers in optical communication systems

The upper limit of the achievable signal-to-noise ratio in optical communication systems is determined by the intrinsic laser noise due to quantum fluctuations inside the laser cavity. This achievable signal-to-noise ratio depends on the way in which different lasing modes are detected; wavelength filtering and material dispersion may yield a significant deterioration of the signal-to-noise ratio. It is concluded from theoretical calculations and from measurements on V-groove lasers that a d.c. signal-to-noise ratio of about 70 dB may be achieved for a noise bandwidth of 10 MHz.     

色噪声间关联的周期调制对单模激光随机共振的影响

讨论色噪声驱动的单模激光系统在噪声间关联程度受时间周期调制情况下的随机共振.用线性化近似的方法计算了光强功率谱及信噪比.具体讨论色噪声情况下信噪比R受噪声强度D,Q,时间周期调制频率Ω_λ以及噪声自关联时间τ₁,τ₂和噪声间关联程度λ的影响.发现信噪比随噪声强度的变化呈单峰共振,信噪比随时间周期调制频率的变化呈周期性共振,而信噪比随 关键词： 色噪声 时间周期调制 噪声间关联程度 周期性随机共振     

Characteristics of fourier phase holograms recorded on photopolymers

The results of the investigation of recording Fourier phase holograms on a self-developing photopolymer photosensitive in the range λ=400–515 nm are presented. It has been found that, due to the transient energy transfer between the beams, noise gratings are recorded, and a corresponding sharp reduction in the signal-to-noise ratio occurs, while the diffraction efficiency of the hologram as a whole remains relatively high (above 50%). It has been found that the noise-grating recording can be substantially suppressed by increasing the intensity of the reference beam relative to the intensity of the object beam. In this way, the signal-to-noise ratio has been considerably improved for Fourier holograms of binary phase masks: at a reference to object beam intensity ratio R=26, Fourier phase holograms are recorded with a diffraction efficiency η=15% and signal-to-noise ratio N=20 dB.     

Stochastic resonance characteristic analysis of new potential function under Levy noise and bearing fault detection

Based on the output saturation of classcial bistable stochastic resonance (CBSR), a new type of piecewise nonlinear bistable stochastic resonance (PNBSR) system is constructed. The mean signal-to-noise ratio gain is regarded as an index to measure the stochastic resonance phenomenon. The laws for the resonant output of piecewise nonlinear bistable system governed by l, c, a, b and D of Levy noise are explored under different characteristic index α and symmetry parameter β of Levy noise. The results show that the output of PNBSR system has increased 4?dB by comparing with the output signal-to-noise ratio of CBSR system. And the stochastic resonance phenomenon can be induced by adjusting the piecewise nonlinear system's parameters under any α or β of Levy noise. The interval of the parameters of system which induces good stochastic resonance is roughly the same. And the output signal waveform of resonance is very similar to the input signal waveform, which has some reference value for the signal recovery. Moreover, we can find the good stochastic resonance interval of the system parameters do not change with D of Levy noise under the different noise intensity D of Levy noise. On the basis of this, adjusting the intensity amplification factor D of Levy noise, which induces good stochastic resonance, and the interval does not change with α or β. At last, the piecewise nonlinear bistable system is applied to detect bearing fault signals, which achieves better performance compared with the classical bistable system.     

Performance of Optical OFDM Receiver in the Presence of System Impairments

Abstract

Recently, there is an increasing interest in using orthogonal frequency division multiplexing schemes in advanced optical communication systems to compensate fiber dispersion. This article presents a comprehensive theoretical analysis to treat the optical orthogonal frequency division multiplexing scheme as a special case of optical subcarrier multiplexing system. An analytical expression is derived to calculate the laser power required to achieve a specific level of signal-to-noise ratio, as a function of various system and noise parameters. Simulations show that the presence of laser relative intensity noise may cause a signal-to-noise ratio floor, where the bit error rate cannot be improved further even when the laser power increases dramatically.     

Quantization noise in binary holograms

The signal-to-noise (S/N) ratio in the reconstructed image from a binary hologram has been quantitatively related to the amplitude and phase quantization levels. The S/N ratio increases monotonically with increasing number of quantization levels. This observation is further supported by experimental results.     

A new and improved apodization function for resolution enhancement in NMR spectroscopy

In a variety of fields of spectroscopy, many apodization functions are presently available that can be used to multiply a time-domain signal, so that after Fourier transformation the resolution will be improved, in the sense that the linewidths will be reduced in the frequency domain. Until recently, these functions increased either the resolution or the signal-to-noise ratio (S/N), and always one at the expense of the other. Recently a function that may be used to increase either resolution, S/N, or both simultaneously was introduced. A mathematical explanation of how the function operates is presented, along with an improved version of the original form. Results are reported that demonstrate an increase in SIN of up to a factor of two, while simultaneously increasing resolution. Also presented are some problems that may arise from the use of this function, as well as a comparison with the commonly used Gaussian apodization.     

Thermal noise limit in measuring the gravitational constant G using the angular acceleration method and the dynamic deflection method

A general comparison is made between two methods of measuring the gravitational constant G. The angular acceleration method can avoid the anelasticity effect since the torsion fiber is not twisted. The dynamic deflection method is similar in principle but it does not use feedback, therefore a major noise introduced by the feedback control system in the angular acceleration method can be avoided. Both methods have their advantages and can be performed with the same device. Based on different expressions of G, we have expressed the signal-to-noise ratio and calculated the thermal noise limit for both methods. In order to get a lower thermal noise limit, the dynamic deflection method should avoid resonance.     

Minimizing error in the logarithmic decrement method through uncertainty propagation

Logarithmic decrement is a well known approach for estimating the damping ratio from a time history. This paper uses uncertainty propagation to unveil how uncertainties in different quantities can impact damping ratio estimates. More specifically, the trade-off between the number of periods N between the first and final peaks and measurement noise is addressed. The insights gained from this analysis are condensed into a single graphic or guide which provides the ideal number of periods N to use for a common range of measurement uncertainties and damping ratios. The guide is then applied to experimental data derived from image processing to estimate the damping ratio.     

Quantum state engineering in ion-traps via adiabatic passage

We propose two relatively robust schemes to generate entangled W states of three (or generally N) ions in ion trap systems by using adiabatic passage technique and appropriately designed ion-field couplings in a single step. In the first scheme, we apply the N-pod fractional stimulated Raman adiabatic passage (F-STIRAP) technique to generate W state of N ions using two Gaussian laser pulses. We also show that the W state of N ? 1 ions can be created via a simple N-pod standard STIRAP by two laser pulses. In the second scheme, we generate the entangled state of N ions via ??-pulse technique by a single laser pulse. We also study the population transfer of the system by numerical solutions of the master equation, considering the effect of decoherence channels due to laser intensity fluctuations and dissipation in the phonon modes.     

噪声间关联程度的时间周期调制对单模激光随机共振的影响

用线性化近似的方法研究了噪声间关联程度λ受时间周期性调制的单模激光增益模型的光强功率谱及信噪比.具体讨论信噪比R受加法噪声强度D,乘法噪声强度Q及噪声间关联程度λ的影响,以及受周期调制频率Ω_λ,输入信号频率Ω的影响.发现了一些新颖的现象. 关键词： 时间周期调制频率 噪声间关联程度 随机共振 信噪比     

Bragg diffraction of light in a restricted homogeneous system of tunnel-coupled waveguides

A system of tunnel-coupled rectilinear waveguides is studied. The dependence of the number of modes in this system on the number of waveguides N, the distance between the waveguides, and the number of modes in a separate waveguide is considered. It is shown that the modes of the m = N and N + 1 orders in such a system are Bragg modes; i.e., the angle between the direction of their propagation and the system axis is close to the Bragg angle. The effective refractive indices n ^* of these modes change stepwise. The step size Δn ^* is found to be dependent on the distance s between the waveguides and on the number of modes in a separate waveguide. A system of single-mode waveguides with the number of guided modes M = 34 < N = 50 is studied experimentally. It is shown that the Bragg modes of the system lie among the leakage modes of the system and have rather low losses. It is demonstrated that the localization of the Bragg modes among the leakage modes may be favorable for their selection upon light generation.     

单模激光系统随机共振的模拟研究

用数值模拟方法对具有调制信号的单模激光系统进行了研究, 分析了在阈值附近和远离阈值两种工作方式下的输出信噪比、输出信号功率、 背景噪声功率,并与线性化近似结果进行了对比. 研究发现,激光在阈值附近工作时,输出信噪比会随着量子 噪声的增大出现随机共振现象,这一现象不要求泵噪声与量子噪 声存在关联,但噪声之间的关联会进一步增大信噪比;在远离阈值时, 只有噪声间存在关联时才会发生随机共振现象.研究还发现噪声也 会影响输出信号的功率.     

脉冲信号被噪声调制的单模激光随机共振

建立了一个脉冲信号受噪声调制的新型的单模激光随机共振模型, 采用线性化近似的方法计算了相应的光强关联函数和系统的输出信噪比, 并详细讨论了相应的随机共振现象. 研究结果表明: 由于噪声调制脉冲信号, 使得单模激光表现出崭新的随机共振现象, 即通过改变脉冲信号周期T来实现抑制或优化输出信噪比的目的.     

一种提高激光告警器性能的自适应方法

激光告警是激光对抗的重要前提和基本手段，如何在复杂背景噪声下识别来袭激光信号，进而准确报警是激光告警面临的首要问题。该文分析了激光告警设备检测概率与信噪比的关系，从关系式可以看出，为了提高激光告警设备的检测概率，需要增加激光信号的信噪比。提出了一种最大信噪比的自适应算法，进行了算法分析，并利用矩阵变换作出了算法简化，从而保证了系统信噪比的最佳化。最后对最佳权向量的求解这个问题，给出了适用于工程化的最佳权向量求解的简单迭代算法。     

Signal-to-noise enhancement techniques for quantum cascade absorption spectrometers employing optimal filtering and other approaches

Optical feedback to the laser source in tunable diode laser spectroscopy (TDLS) is known to create intensity modulation noise due to elatoning and optical feedback (i.e. multiplicative technical noise) that usually limits spectral signal-to-noise (S/N). The large technical noise often limits absorption spectroscopy to noise floors 100-fold greater than the Poisson shot noise limit due to fluctuations in the laser intensity. The high output powers generated from quantum cascade (QC) lasers, along with their high gain, makes these injection laser systems especially susceptible to technical noise. In this article we discuss a method of using optimal filtering to reduce technical noise. We have observed S/N enhancements ranging from ∼20% to a factor of ∼50. The degree to which optimal filtering enhances S/N depends on the similarity between the Fourier components of the technical noise and those of the signal, with lower S/N enhancements observed for more similar Fourier decompositions of the signal and technical noise. We also examine the linearity of optimal filtered spectra in both time and intensity. This was accomplished by creating a synthetic spectrum for the species being studied (CH₄, N₂O, CO₂ and H₂O in ambient air) utilizing line positions and linewidths with an assumed Voigt profile from a commercial database (HITRAN). Agreement better than 0.036% in wavenumber and 1.64% in intensity (up to a 260-fold intensity ratio employed) was observed. Our results suggest that rapid ex post facto digital optimal filtering can be used to enhance S/N for routine trace gas detection. Received: 1 April 2002 / Revised version: 7 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-509/376-6066, E-mail: robert.disselkamp@pnl.gov     

Reduced quantum noise in waveguide coherent Raman scattering spectroscopy

Waveguide regimes of nonlinear-optical interactions allow a reduction of quantum noise in spectra of coherent Raman scattering (CRS). We will find the optimal waveguide length providing the maximum quantum limit for the signal-to-noise ratio in waveguide CRS and assess this ratio for laser intensities right below the optical breakdown threshold. This analysis shows that the quantum-limit signal-to-noise ratio of coherent femtosecond CRS spectroscopy of the gas phase can be increased in the waveguide regime by more than four orders of magnitude relative to the regime of tight focusing.     

Stochastic resonance in a gain--noise model of a single-mode laser driven by pump noise and quantum noise with cross-correlation between real and imaginary parts under direct signal modulation

Stochastic resonance (SR) is studied in a gain--noise model of a single-mode laser driven by a coloured pump noise and a quantum noise with cross-correlation between real and imaginary parts under a direct signal modulation. By using a linear approximation method, we find that the SR appears during the variation of signal-to-noise ratio (SNR) separately with the pump noise self-correlation time \tau , the noise correlation coefficient between the real part and the imaginary part of the quantum noise \lambda_q , the attenuation coefficient \gamma and the deterministic steady-state intensity I_0 . In addition, it is found that the SR can be characterized not only by the dependence of SNR on the noise variables of \tau and \lambda_q, but also by the dependence of SNR on the laser system variables of \gamma and I₀. Thus our investigation extends the characteristic quantity of SR proposed before.     

Population and current noise in semiconductor laser junctions

The population noise in a semiconductor laser is calculated by means of the quantum mechanical Langevin method. The resulting population noise is given by 〈δ N _c ² 〉=(T _c/2) (rate in+rate out)+K(¯n), whereN _c is the total number of electrons in the conduction band in the active region,T _c is a relaxation time. The first expression is the usual shot noise term. The transition rates are the sum of the rates due to the light field, the pumping and the spontaneous emission. The last termK(¯ n) is caused by the light field fluctuations;¯n is the mean number of photons in the laser mode.K(¯ n) consists of two parts: a) The main part is proportional to the intensity noise of the light field, which increases below but near threshold and gets constant above threshold. b) There is a second term due to the fact that parts of the fluctuations of the population and of the light field are correlated. — The noise spectrumS _I(ω) of the junction currentI is calculated for low frequencies. Beyond the usual shot noise termS _I(0)=2eI, additional noise is found in and above the threshold region, a) mainly because of the fluctuations of the light field in the laser mode and b) to a small amount, because the absorption processes due to the laser photons weaken the forward current, which is carried by emission processes, while the absorption noise adds to the emission noise.     

Dynamics of a horizontal saccadic oculomotor system with colored noise

Intermittent nystagmus is a special kind of nystagmus with an irregular attack time. Its pathogenesis remains unclear. Recently, nonlinear dynamic methods used to explore the mechanisms responsible for intermittent nystagmus have received increased attention. The current study examines the dynamic properties of the bistable horizontal saccadic oculomotor system driven by colored noise. The most probable amplitude, stationary probability density of response, and signal-to-noise ratio curves with different parameters are obtained through stochastic numerical simulation. Then, the stochastic P bifurcation and coherent resonance phenomenon of the saccadic oculomotor system are analyzed. Results indicate that inhibition strength, noise intensity, or correlation time could induce stochastic P bifurcation, which may explain the development of intermittent nystagmus. Further, it is found that intermittent nystagmus can be suppressed by increasing inhibition strength, and that correlation time and noise intensity can lead to coherence resonance.