Absorption line profile recovery based on residual amplitude modulation and first harmonic integration methods in photoacoustic gas sensing

Two methods for recovery of gas absorption line profiles are presented in this paper using photoacoustic spectroscopy and tunable diode laser spectroscopy (TDLS) with wavelength modulation (WM). A theoretical analysis based on Fourier coefficients is given in order to describe the various components that arise under simultaneous intensity and frequency modulation. The first method makes use of the residual amplitude modulation (RAM) signal which is always present in current modulation of distributed feedback (DFB) tunable diode lasers. The second method involves integration of a near-pure first harmonic derivative signal, separated from other distorting components by appropriate choice of the lock-in detection phase in the case of low modulation index. Good agreement is obtained with both methods between the experimental results and the theoretical simulation for the P17 absorption line of acetylene at 1535.39 nm but the second method gives a much improved accuracy and signal-to-noise ratio in lineshape recovery with photoacoustic spectroscopy.     

Influence of the light intensity on the operation of the self-oscillation photoacoustic setup

In the traditional approach to photoacoustic measurements, the light source modulation frequency is fixed, or a resonance tracking mechanism is applied, that adjusts the modulation frequency to the instantaneous value of the resonance frequency of the cell, especially in the case of high Q-factor cells. However, photoacoustic measurements can also be based on a novel method in which the light intensity is modulated by the photoacoustic signal induced in the cell, hence forming a photoacoustic oscillator. Frequency of such an oscillator and shape of the resulting photoacoustic signal depend mainly on the resonance properties of the cell, but also on the power of the light irradiating the investigated sample. This paper describes the influence of the intensity of light used for exciting the photoacoustic signal on the operation of such a photoacoustic generator.     

光声池中微弱光声信号检测

大气中的污染源气体含量很少, 用光声光谱对其进行监测得到的光声信号极其微弱. 本文首先分析微弱信号产生机理, 在分析Holmes Duffing方程的基础上, 提出了适合光声池微弱信号检测的变尺度差分方法. 该方法通过对信号进行尺度变换, 再做差分来检测微弱信号. 理论分析和实验表明, 变尺度差分方法能很好地抑制系统相空间的共模噪声, 而且能很好地凸显混沌状态临界值. 变尺度差分方法测出的信号相对误差都小于5%, 说明其可以用于较高频率、 相位和频率都未知的微弱光声信号幅值检测. 关键词： 光声光谱 微弱信号 幅值 Duffing     

Photoacoustic signal generation based on self-oscillation method

In conventional photoacoustic experiments a signal generated by photoacoustic phenomenon is produced by a forced stimulation of the investigated sample by electromagnetic radiation energy (light) which intensity is modulated at a user selected frequency. Resultant photoacoustic signal has the same frequency as the light modulation frequency. The paper presents a new technique of photoacoustic signal generation, based on self-oscillations resulting from a loopback in the signal path consisting of a modulated light source, a photoacoustic chamber, a microphone, and an amplifier. Signal from the amplifier output was used to drive the light source. Gain of the amplifier was chosen in order to obtain total loop gain of the signal path greater than one and positive feedback operation. On contrary to conventional photoacoustic methods, frequency of the photoacoustic signal in the presented self-oscillation method is determined mainly by acoustic properties of the photoacoustic cell, which in the experiments was corresponding to the resonance frequency of the cell. Taking into consideration that resonance frequency of a photoacoustic cell depends on the speed of sound, which is affected by properties of the fluid filling the cell, the method based on self-oscillations can be applied e.g. to quantitative analysis of the investigated substance.     

基于二次偏振调制的变频测距方法与系统实现

由于鉴相精度限制、电路等引入的附加相移干扰等因素,传统相位测距技术精度的提高受到了限制.采用二次偏振调制技术对相位测距技术进行了改进.利用二次偏振调制方法能够直接在相位调制器上对两次调制信号的相位差进行解调,大幅度简化了系统的复杂程度.采用变频方法替代传统的鉴相方法,从而系统的测量精度不再受鉴相问题的困扰.从理论上得到系统输出光强与调制频率成正余弦关系,并进行了实验验证.基于变频测距的实验中,系统频率的稳定度优于10-6,测量精度可以达到±10.6μm(被测距离为4.5 m).并对一段长200 m的光纤进行了实际测量,得到了清晰的调制频率与系统输出光强的曲线.     

Novel iteration-free blind phase noise estimation for coherent optical OFDM

We propose a novel iteration-free blind phase noise estimation scheme for coherent optical orthogonal frequency division multiplexing （CO-OFDM） systems. In the new algorithm, the cost function is selected as the similar expression with real and imaginary parts as that in the modified constant modulus algorithm, and the new cost function is derived under some assumptions, where it is infinitely approximated by the sine and cosine functions. By means of the analytical formula of the cost function, the initial coarse common phase error can be obtained with only some samples, where the algorithm avoids computational complexity of conventional blind phase noise compensation scheme. In CO-OFDM systems with high-order modulation format （32 quadrature amplitude modulation） and narrow linewidth lasers, it is proved by the simulation results that the nhase noise can be effectively compensated with the proposed blind estimation method.     

频率变化的光场对双J-C模型中原子-原子纠缠的调控

本文研究了频率随时间变化的光场对双J-C模型中原子--原子纠缠的动力学调控, 主要讨论了光场频率随时间作正弦变化和脉冲变化两种典型情况下, 原子--原子纠缠度随时间的演化特性. 当光场频率随时间作正弦变化时, 原子--原子纠缠度演化的周期、振幅与光场频率调制的振幅有关, 并随着调制振幅的增强而减小. 光场频率的正弦调制和脉冲调制均能使光场与原子的相互作用模式在共振和非共振之间发生变化, 直接影响原子--原子纠缠度的演化规律. 通过光场频率的调制可以实现原子--原子纠缠度的提高与稳定, 避免ESD现象的出现, 从而达到动态调控原子--原子纠缠的目的.     

基于FPGA与Matlab的数字正交解调器的设计

为有效提取测控系统输入信号的幅度和相位信息,设计了基于FPGA与Matlab的信号数参考文献6文献类型、文献出处、年份字正交解调器。在Matlab/Simulink环境中产生一路调幅信号,并在此环境下利用5个直接I型的4阶FIR滤波器节搭建了20阶FIR滤波器;利用FPGA查表法实现数控振荡器(NCO),并控制1路调幅信号与正交的正、余弦信号分别进行数字混频处理;对经FPGA数字混频处理后的两路倍频分量和基频分量信号进行滤波处理,经处理后的信号在FPGA的控制下进行相加处理。最后在硬件平台上进行了仿真测试实验,验证了该方案的正确性和可行性。     

光声光谱技术在多组分气体浓度探测中的应用

CO和CH_4气体作为判断变压器运行状态的故障气体,对其浓度的探测在变压器维护中具有重要意义.为了准确探测变压器运行过程中产生的CH_4和CO气体浓度,本文利用光声光谱技术,设计了一套基于宽带光源的多组分气体探测系统,和共振型光声系统相比,该系统中所用的非共振型光声池体积小,易加工,池内各处信号强度相同,降低了对声学信号探测器的安装要求.系统的性能通过对CO和CH_4气体的探测进行评估.首先,从理论上分析了信号强度与调制频率呈反比,然后根据宽带光声系统在不同调制频率下的响应,确定系统的最佳调制频率为22 Hz.在最佳调制频率下,根据温度与待测气体光声信号的关系,对光声信号进行温度补偿,消除温度变化对光声信号的影响,进一步提高了系统的稳定性.最后,通过不同浓度的CH_4和CO气体对系统进行标定.实验表明,温度补偿前后,光声信号随温度的漂移分别为0.023 23V/℃和8.383 48×10~(-5) V/℃,通过对不同浓度CH_4和CO气体的探测,系统的线性度分别达到0.995和0.998 4.在一个大气压下,积分时间为1s时,宽带光声探测系统对CO和CH_4气体的探测极限浓度能够达到1μL/L.该系统成本低,线性度好,探测灵敏度符合国标对变压器维护过程中CO和CH_4气体的探测要求.     

Perception of amplitude and frequency modulated signals (mixed modulation)

This article discusses the detection of mixed modulation, i.e., simultaneous amplitude and frequency modulation (MM). The investigations have incorporated both a sine wave modulating signal and an irregular modulating signal, a very narrow noise band, of a specified center frequency. The results revealed that for a sinusoidal low-frequency modulating signal, amplitude and frequency changes that were separately subthreshold could be detected by listeners in mixed modulation. This indicates summation of sensations caused by simultaneous AM and FM modulation. This effect was not observed in the case of the irregular modulating signal. A hypothesis is advanced that the perception of modulated signals is governed by two mechanisms, viz., temporal and spectral. The operation of the two mechanisms depends mainly on the modulating frequency. The type of modulation does not play any significant role in this case.     

Measurement of the frequency modulation transfer function of a laser using a Mach–Zehnder interferometer

A technique is presented for determining the frequency modulation transfer function of a laser. The method is based on a Mach–Zehnder interferometer, with a significant difference in the optical path lengths of the two arms. A frequency-modulated laser beam incident on the interferometer produces a phase-modulated photocurrent signal with an effective modulation index that is related to the amplitude of the optical frequency modulation. Techniques for determining both the amplitude and the phase of the optical frequency modulation from the photocurrent signal are described.     

光纤气体传感器解调方法的研究

利用可调谐激光光谱技术并结合二次谐波检测的波长调制方法对气体浓度进行检测,大大提高了检测灵敏度.然而光的传输和电路自身延时都会产生未知的相位延时,含有气体浓度信息的被测信号和二倍频参考信号的相位差变化严重影响二次谐波信号测量结果.本文设计出一种4路乘法锁相解调电路,设计中分别用正弦信号和余弦信号对被测信号进行解调,经积分电路后得到两路分别与相位差的正弦值和余弦值相关的解调信号.将两路解调信号平方,再通过加法器相加后消除相位差,得到完全与相位差无关的幅值信号.实验证明当被测信号相位在0°—90°变化时,解调 关键词： 可调谐半导体激光吸收光谱 波长调制 解调电路 相位差     

信号光的调制对基于光纤光参量放大光脉冲源的改善

在理论上从四波混频的强度耦合方程出发,给出了基于光纤光参量放大(FOPA)光脉冲的光场表达式,并进一步分析了信号光被相位调制或强度调制后,光脉冲的频率啁啾和强度演化.结论指出：若信号光被强度调制,对所生成脉冲宽度无明显的影响,但可以提高消光比;若信号光被相位调制,所生成的脉冲具有更大的线性正啁啾,可以在相同的抽运功率条件下得到比无相位调制时更窄的脉冲.实验上给出了10 GHz工作速率下的结果,其结果与理论分析符合得很好.通过信号光的相位调制,在05 W平均抽运功率条件下得到了消光比22 dB,脉宽为5 关键词： 光纤参量放大 四波混频 光脉冲源 频率啁啾     

散斑干涉相位条纹图的频域滤波处理

为了解决在数字散斑干涉技术测量时,散斑干涉相位条纹图像中大量噪声对相位解包裹结果和精度产生严重影响的问题,介绍了一种条纹正余弦分解和频域低通滤波结合的方法,实现了散斑干涉相位条纹图的高精度滤波。该方法的基本思路是在对相位图像进行滤波处理前,先将相位图通过正余弦函数进行映射转换成两幅图,分别经过频域滤波,然后再合成为相位图。这种分解频域滤波方法可以在滤波的同时,有效保留相位跳变信息。实验结果表明：与传统的图像降噪方法相比,该方法能够在保留图像“尖峰”信息的基础上,较好地滤除图像中的散斑噪声,方法简单有效,有效解决了传统滤波方法应用在相位条纹图中,相图灰度信息丢失10%~40%的问题。     

基于Labview的快循环扫频信号相位差测量算法实现

为了实现对中国散裂中子源(CSNS)/快循环同步加速器(RCS)射频系统中高频加速电压和束流两个同步变化的快循环扫频信号之间相位差的精确测量,需要选择合适的信号采集方案与相位差测量算法。介绍了常用的同频信号相位差测量算法,讨论了不同相位差测量算法的特点与适用性;详细描述了快速傅里叶变换(FFT)交叉谱法与加余弦窗FFT插值法两种相位差测量算法的原理。为了研究这两种算法在扫频信号测量中的适用性,利用NI的虚拟仪器技术以及Labview图形化编程技术,搭建扫频信号相位差测量系统,对这两种相位差测量算法进行仿真模拟。仿真结果表明,加余弦窗FFT插值算法可以满足快循环扫频信号相位差测量精度的要求。     

基于共振型高灵敏度光声光谱技术探测痕量乙炔气体浓度

乙炔气体作为判断变压器运行状态的一种故障气体,其浓度的高低反映了变压器的运行状况,因此对其浓度的探测在变压器的维护中具有重要意义。为了准确探测变压器运行过程中产生的乙炔气体浓度,为变压器的维护提供技术参数,针对基于DFB激光器的共振型光声光谱技术痕量乙炔气体检测技术开展研究,对传统的光声光谱探测系统进行改进。根据光声光谱技术的理论可知,光声信号的强度与入射激光的功率成正比,所以在光声池的出射窗口采用一个平面反射镜将红外光再次反射到光声池中以增加入射光功率,增强光声信号强度,进一步提高了光声系统的探测灵敏度。通过一定浓度的乙炔气体在不同调制频率和不同调制深度下光声信号强度的变化,确定光声探测系统的最佳调制频率和最佳调制深度为767 Hz和0.3 mV。利用不同浓度乙炔气体对系统进行标定,然后采用最小二乘法对光声信号与气体浓度进行拟合,二者具有很好的线性度。通过Allan方差计算可知,系统在平均时间达到200 s时,能够达到最低探测极限浓度。实验表明,在一个大气压下,积分时间为10 ms时,改进后的共振型光声光谱探测系统对乙炔气体的最低探测极限浓度达到了0.3 μL·L-1。还将小波去噪技术引入到低浓度下乙炔气体的光声信号处理中,有效消除了低浓度气体光声信号中的噪声,提高了信噪比。设计的共振型光声光谱探测系统操作简单,最低探测浓度符合国标中对变压器维护过程中对乙炔气体的探测需求,在变压器维护领域具有广阔的应用前景。     

Measurement of modulation index and half-wave voltage of an electro-optical phase modulator with a dispersion-based phase filter

In this paper, a novel method based on the use of an optical phase filter is proposed to evaluate the modulation index, as well as the half-wave voltage of an optical phase modulator. When a phase modulated signal propagates along the length of a dispersive fiber, the fiber dispersion acts as an optical phase filter and spectrally alters the relative phasing of the phase modulated signal, causing the phase modulation to become intensity modulation, which allows the swept frequency measurement of phase modulators by using a vector network analyzer. The modulation index and half-wave voltage as a function of the modulation frequency are experimentally measured for a commercial phase modulator, which agree well with the results obtained using the widely accepted optical spectrum analysis method. The proposed method requires only a length of dispersive fiber and works without any small-signal approximation, which is applicable for different driving levels and operating wavelengths with the same setup.     

Amplitude-modulation-free optoelectronic frequency control of laser diodes

A novel method is described for fast frequency modulation or frequency control of diode lasers that avoids problems associated with bias current modulation, namely, amplitude modulation and thermal phase delays. The method is based on amplitude-modulated, noninterfering control light with a wavelength near the transparency region of the laser diode, which specifically modifies the spectral gain profile to yield a constant gain but a controllable refractive index at the lasing wavelength. This permits amplitude-modulation-free frequency modulation at modulation frequencies up to the relaxation oscillation frequency. A phase lock between the emissions of two extended-cavity diode lasers that could not be achieved with bias current modulation was achieved by this method.     

Instantaneous frequency measurement based on transversal microwave filters with high resolution

We propose a novel photonic technique for microwave frequency measurement based on transversal microwave filters with high resolution. Two parallel microwave filters with sine and cosine frequency responses are obtained by cross gain modulation in a single semiconductor optical amplifier, which introduces two different frequency responses to achieve an amplitude comparison function. We also demonstrate a proof- of-concept experiment. The measurement error is less than ±0.04 GHz for the first band range of 0–3.45 GHz and less than ±0.03 GHz for the second band range of 3.45–5.8 GHz. Our scheme is found to be capable of being extended for larger frequency range measurements using a shorter fiber length.     

包含掺铒光纤和微结构光纤的光纤环镜光开关

设计了一种低开关功率的全光开关.将掺铒光纤和微结构光纤引入Sagnac环镜中,信号光在泵浦光作用下经过掺铒光纤被放大,破坏了环镜的平衡,利用交叉相位调制效应使两束反向传输的信号光产生非线性相移,实现了光开关效应.理论分析表明:信号光经过掺铒光纤后,增益越大微结构光纤的非线性系数越高,开关功率越低,并且环镜信号光的透射率随两束反向传输信号光的相移差成余弦变化.仿真得到开关功率约为26.73 mW,与理论分析一致.