Recovery of spectral features readout with frequency-chirped laser fields

A data-processing technique is proposed for use with conventional frequency-chirped absorption spectroscopy to ensure accurate mapping of spectral features into time-domain signatures with arbitrarily fast readout chirp rates. This technique recovers the spectrum from a signal that is distorted owing to the fast chirp rate and therefore facilitates fast measurement of the spectral features over a broad spectral range with high resolution. Both numerical simulations and experimental results are presented.     

线性调频信号激励过阻尼双稳系统的随机共振现象研究

线性调频信号是工程中常见的一种信号, 由于其为非周期信号, 无法以频域信噪比作为衡量其是否产生随机共振的测量手段, 故鲜有文献研究以线性调频信号为激励信号的随机共振现象. 本文利用线性调频信号在最优分数阶Fourier变换域上的能量聚集性, 首次提出以最优分数阶Fourier变换域上定义的信噪比作为测量手段, 研究了线性调频信号叠加高斯白噪声激励过阻尼双稳系统的随机共振现象, 且发现了以线性调频信号为激励信号时产生的新现象, 即随着信号频率的增大, 随机共振将逐渐减弱, 并给出了合理的解释.仿真的结果与理论分析一致, 验证了本文所提出方法的有效性. 关键词： 线性调频信号 分数阶Fourier变换 随机共振     

啁啾匹配和同步抖动对光参量啁啾脉冲放大的影响

 通过研究光参量啁啾脉冲放大过程转换效率随泵浦光啁啾参数的变化,以及信号光和泵浦光波长的对应关系,确立了信号光和泵浦光同步时实现线性啁啾匹配的方法。以磷酸二氢钾作为非线性晶体,研究了信号光和泵浦光之间的线性啁啾匹配和时间同步抖动对光参量啁啾脉冲放大后信号光脉冲波形和转换效率的影响。结果表明：在时间同步的情况下,线性啁啾匹配时的信号光脉冲波形很对称,转换效率也较高;在同步抖动程度一定的情况下,不同泵浦光啁啾参数时,信号光的增益及放大后的脉冲波形差异较大;在线性啁啾匹配程度一定时,信号光和泵浦光时间同步抖动越严重,放大信号光的脉冲波形变化越大,信号光增益越小,且转换效率也越低。     

A self-consistent analysis of semiconductor laser rate equations for system simulation purpose

A procedure for extraction of the rate-equation parameters of semiconductor laser diodes is presented. Using small signal measurements, we have formulated a complete set of equations, which can be resolved analytically to obtain a solution for a combination of parameters that constitute a modified rate-equation model and is presented here for the first time. The proposed method does not necessitate at, or below threshold measurements, which require high sensitivity from the measuring equipment. Furthermore, without the need for any optimization, the solution to the rate equations is completely self-consistent since the parameters are linked together through functional relationships. With this new method, the determination of all the necessary rate-equation parameters to carry out large signal simulation is exclusive of the measurements for signal chirp, and as such computation of the chirp parameter is not included here. Comparison between large signal measurements and simulations from the computed parameters show very good agreement for a range of above threshold bias currents.     

Characteristics of the Wavelength Converted Signal in Wavelength Conversion Based on Cross-Gain Modulation

An analytical theory describing the frequency chirp characteristics of the wavelength converted signal in wavelength conversion based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) is derived. By computation, we can see that low input pump power, high probe power, small line-width enhancement factor and low unsaturated single-pass gain of the SOA are favorable for reducing the frequency chirp of the wavelength converted signal. The analytical results have significance in understanding and designing wavelength converters.     

Chirp ionosonde as a gauge for impulse characteristic and transfer function of the radio channel

When the ionosphere is sounded by a chirp signal, the information source is the amplitude and phase modulation of the signal due to its propagation in the ionospheric radio channel. We propose the method of received-signal demodulation which allows us to use the chirp ionosonde as a gauge for impulse characteristic and transfer function of the ionospheric radio channel. Operation of the chirp ionosonde is numerically simulated and estimates of intrinsic accuracy in measuring the mentioned characteristics are obtained.     

Theoretical and Experimental Studies of the Correlation Characteristics of Signals Reflected by a Rotating Propeller

Acoustical Physics - Abstract—The paper studies the features of the cross-correlation function of a chirp signal reflected from a rotating propeller with an emitted replica signal. In the...     

Mach-Zehnder电光波导调制器啁啾的研究

在光纤通信系统中,调制器的啁啾参数是衡量调制器性能的重要参数之一.为了深入研究铌酸锂M-Z结构外调制的机理,导出了Mach-Zehnder结构外调制方式调制器的啁啾参数表达式,研究表明该结构调制器频率啁啾不为零,而且调制器的直流偏置漂移对啁啾的影响很小.详细讨论了影响啁啾参数的因素,分析了直流消光比、外加信号以及直流偏置漂移对器件啁啾参数的影响.分析结果表明,利用负啁啾可以改善系统的色散特性.     

Chirp excitation of ultrasonic guided waves

Most ultrasonic guided wave methods require tone burst excitations to achieve some degree of mode purity while maintaining temporal resolution. In addition, it is often desirable to acquire data using multiple frequencies, particularly during method development when the best frequency for a specific application is not known. However, this process is inconvenient and time-consuming, particularly if extensive signal averaging at each excitation frequency is required to achieve a satisfactory signal-to-noise ratio. Both acquisition time and data storage requirements may be prohibitive if responses from many narrowband tone burst excitations are measured. Here chirp excitations are utilized to address the need to both test at multiple frequencies and achieve a high signal-to-noise ratio to minimize acquisition time. A broadband chirp is used to acquire data at a wide range of frequencies, and deconvolution is applied to extract multiple narrowband responses. After optimizing the frequency and duration of the desired tone burst excitation, a long-time narrowband chirp is used as the actual excitation, and the desired tone burst response is similarly extracted during post-processing. Results are shown that demonstrate the efficacy of both broadband and narrowband chirp excitations.     

Spatial chirp in Ti:sapphire multipass amplifier

The spatial chirp generated in the Ti:sapphire multipass amplifier is numerically investigated based on the onedimensional(1D) and two-dimensional(2D) Frantz–Nodvik equations. The simulation indicates that the spatial chirp is induced by the spatially inhomogeneous gain, and it can be almost eliminated by utilization of proper beam profiles and spot sizes of the signal and pump pulses, for example, the pump pulse has a top-hatted beam profile and the signal pulse has a super-Gaussian beam profile with a relatively larger spot size. In this way, a clear understanding of spatial chirp mechanisms in the Ti:sapphire multipass amplifier is proposed, therefore we can effectively almost eliminate the spatial chirp and improve the beam quality of a high-power Ti:sapphire chirped pulse amplifier system.     

半导体激光器型全光波长转换器的啁啾分析

利用速率方程分析了转换信号光的频率啁啾,考虑到了外界注入信号光的放大.转换过程中,信号光的注入将会导致了光腔内载流子数发生变化,进而引起激光器输出频率的改变,产生啁啾.研究发现,较大的激光器注入电流可以有效地抑制频率啁啾,较高的注入信号光功率则可以在保证高转换信号消光比的同时抑制频率啁啾.     

Theoretical analysis of all-optical 2R regeneration in a SOA-based Mach–Zehnder interferometer

All-optical 2R regeneration in a semiconductor optical amplifier-based Mach–Zehnder interferometer is theoretically analyzed. It is shown that the chirp of the regenerated signal is positive, while the input signal has no chirp. The peak extinction ratio (ER) of the regenerated signal is associated with the input wavelength and there exists an input wavelength corresponding to the largest ER of the output signal. The output peak ER is also found influenced by the input power, with lower input power getting higher peak ER.     

Coded tissue harmonic imaging with nonlinear chirp signals

Coded tissue harmonic imaging with pulse inversion (CTHI-PI) based on a linear chirp signal can improve the signal-to-noise ratio with minimizing the peak range sidelobe level (PRSL), which is the main advantage over CTHI with bandpass filtering (CTHI-BF). However, the CTHI-PI technique could suffer from motion artifacts due to decreasing frame rate caused by two firings of opposite phase signals for each scanline. In this paper, a new CTHI method based on a nonlinear chirp signal (CTHI-NC) is presented, which can improve the separation of fundamental and harmonic components without sacrificing frame rate. The nonlinear chirp signal is designed to minimize the PRSL value by optimizing its frequency sweep rate and time duration. The performance of the CTHI-NC method was evaluated by measuring the PRSL and mainlobe width after compression. From the in vitro experiments, the CTHI-NC provided the PRSL of −40.6 dB and the mainlobe width of 2.1 μs for the transmit quadratic nonlinear chirp signal with the center frequency of 2.1 MHz, the fractional bandwidth at −6 dB of 0.6 and the time duration of 15 μs. These results indicate that the proposed method could be used for improving frame rates in CTHI while providing comparable image quality to CTHI-PI.     

Development of tools for real-time betatron tune measurements at the Nuclotron

A betatron tune measurement system was developed and tested at the Nuclotron. A white noise and chirp signals were used for transverse beam motion excitation. A custom FlexRIO digitizer module was developed which provides excitation signal generation for kicker electrodes and real-time signal acquisition from pickup electrodes. A high resolution FFT algorithm was implemented inside a NI PXI FPGA module, connected to digitizer. The measurement system is integrated with the NICA control system based on the TANGO Controls. Results and tests performed with the Nuclotron beam are presented.     

基于半导体光放大器中交叉增益调制效应的波长转换啁啾特性的分析

对基于半导体光放大器中交叉增益调制效应的波长转换后信号的啁啾特性进行了分析。研究了输入信号光和参考光和功率、波长、转换速率、信号光消光比以及半导体光放大器的偏置电流对转换后信号啁啾的影响。     

Modeling the recorded spectrum and reconstructing the transfer function of the wideband ionospheric HF radio channel in the case of chirp sounding

We model theoretically the received spectrum in the case of sounding of the ionospheric HF radio channel by a chirp signal. It is shown that the result of processing of an individual time sample of the received signal is equivalent to the sounding of the radio channel by a complex narrow-band pulsed signal such that the group delays of its propagation modes determine the maxima in the received spectrum. We analyze the quadrature components of realizations of the received signal at the intermediate frequency at the bandpass-filter output in the receiving channel of the chirp ionosonde. The results of our analysis show the possibility of reconstructing the transfer function of a HF radio channel in the sounding-frequency band for the delay range determined by the characteristics of the intermediate-frequency bandpass filter. We propose a method for reconstructing the transfer function of the ionospheric radio channel, which involves supplementing the circuit of primary processing of the signal by a corrective digital filter with specified amplitude-frequency and phase-frequency characteristics. The proposed method can be used for all operating regimes of the chirp ionosonde in the case of digital recording and processing of signals. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 5, pp. 387–395, May 2007.     

Short-time fractional Fourier methods for the time-frequency representation of chirp signals

The fractional Fourier transform (FrFT) provides a valuable tool for the analysis of linear chirp signals. This paper develops two short-time FrFT variants which are suited to the analysis of multicomponent and nonlinear chirp signals. Outputs have similar properties to the short-time Fourier transform (STFT) but show improved time-frequency resolution. The FrFT is a parameterized transform with parameter, a, related to chirp rate. The two short-time implementations differ in how the value of a is chosen. In the first, a global optimization procedure selects one value of a with reference to the entire signal. In the second, a values are selected independently for each windowed section. Comparative variance measures based on the Gaussian function are given and are shown to be consistent with the uncertainty principle in fractional domains. For appropriately chosen FrFT orders, the derived fractional domain uncertainty relationship is minimized for Gaussian windowed linear chirp signals. The two short-time FrFT algorithms have complementary strengths demonstrated by time-frequency representations for a multicomponent bat chirp, a highly nonlinear quadratic chirp, and an output pulse from a finite-difference sonar model with dispersive change. These representations illustrate the improvements obtained in using FrFT based algorithms compared to the STFT.     

光学线性调频步进信号合成与高分辨测距演示

利用分立激光器合成大带宽线性调频步进信号(FSCS)激光信号源以实现高分辨测距。依据FSCS信号测距原理,数值计算了激光器各参数对合成FSCS信号的影响及其误差容限。计算表明,在各激光器参数误差中,起始频率和起始时间误差是引起合成FSCS信号测距分辨力恶化的最主要因素。另外,利用一台中心波长1550nm、线宽为150kHz的线性调波长激光器,通过光学斩波方式,模拟产生了合成FSCS信号,进行了高分辨力的测距演示。理论分析与实验结果表明,采用分立激光器合成高分辨力测距FSCS信号是可行的。     

Experimental evaluation of a non-linear coded excitation method for contrast imaging

Previously, we have shown that for a single bubble, using chirps as the excitation signal improves both the linear and the non-linear response. Computer simulations of randomly distributed contrast agent bubbles show an increase of 10-13 dB in response when comparing pulse excitations with chirp excitations that have equal bandwidths and peak amplitudes. Second harmonic compression of simulated bubble echoes with chirp excitation shows low side-lobe levels and limited loss of axial resolution when compared to pulse excitation. Experimental results from water tank measurements with SonoVue contrast agent are in agreement with computer simulations showing increased signal-to-noise ratio and an increase of approximately 12 dB at the second harmonic when comparing pulse and chirp excitation.     

基于线性光放大器的交叉相位调制波长转换啁啾特性研究

建立了基于线性光放大器(LOA)的交叉相位调制波长转换模型,模型考虑了纵向载流子密度变化引起的有源区折射率的改变、端面反射、相位调制、放大的自发辐射噪声和横向垂直光场(VCL)的增益钳制作用。利用分段方法模拟计算了布拉格反射镜的反射率、偏置电流、信号光和探测光的功率和波长、信号速率与转换后信号啁啾的关系。结果表明:LOA具有增益钳制作用,能提高转换后信号的性能;增大布拉格反射镜的反射率可以降低转换后信号的啁啾;合理控制输入信号光和探测光的强度,调整干涉仪两臂LOA偏置电流的强度,可以获得啁啾较小的转换信号。