Timing interference to speech in altered listening conditions

A theory is outlined that explains the disruption that occurs when auditory feedback is altered. The key part of the theory is that the number of, and relationship between, inputs to a timekeeper, operative during speech control, affects speech performance. The effects of alteration to auditory feedback depend on the extra input provided to the timekeeper. Different disruption is predicted for auditory feedback that is out of synchrony with other speech activity (e.g., delayed auditory feedback, DAF) compared with synchronous forms of altered feedback (e.g., frequency shifted feedback, FSF). Stimulus manipulations that can be made synchronously with speech are predicted to cause equivalent disruption to the synchronous form of altered feedback. Three experiments are reported. In all of them, subjects repeated a syllable at a fixed rate (Wing and Kristofferson, 1973). Overall timing variance was decomposed into the variance of a timekeeper (Cv) and the variance of a motor process (Mv). Experiment 1 validated Wing and Kristofferson's method for estimating Cv in a speech task by showing that only this variance component increased when subjects repeated syllables at different rates. Experiment 2 showed DAF increased Cv compared with when no altered sound occurred (experiment 1) and compared with FSF. In experiment 3, sections of the subject's output sequence were increased in amplitude. Subjects just heard this sound in one condition and made a duration decision about it in a second condition. When no response was made, results were like those with FSF. When a response was made, Cv increased at longer repetition periods. The findings that the principal effect of DAF, a duration decision and repetition period is on Cv whereas synchronous alterations that do not require a decision (amplitude increased sections where no response was made and FSF) do not affect Cv, support the hypothesis that the timekeeping process is affected by synchronized and asynchronized inputs in different ways.     

Heterodyne beatings between frequency-shifted feedback lasers

Frequency-shifted feedback (FSF) lasers are potential candidates for long distance telemetry due to the appearance of beatings in the noise spectrum at the output of a homodyne interferometer: the frequencies of these beatings vary linearly with the path delay. In this Letter we demonstrate that these beatings also occur in the heterodyne mixing of two identical, but distinct, FSF lasers. This phenomenon is explained by the passive cavity model and is exploited to characterize the time-spectrum properties of FSF lasers. Consequences on telemetry with FSF lasers are presented.     

Controlling the frequency of the frequency-shifted feedback fiber laser using injection-seeding technique

In this paper we describe the stabilization of the optical frequency comb (OFC) generated by the frequency-shifted feedback (FSF) erbium fiber laser. Frequency of the comb is controlled using the distributed feedback laser diode seeded into the cavity of the FSF laser. When the DFB diode is stabilized to the molecular absorption line of hydrogen cyanide frequency stabilized OFC is obtained.     

Narrow-linewidth chirped frequency comb from a frequency-shifted feedback Ti:sapphire laser seeded by a phase-modulated single-frequency fiber laser

Frequency-shifted feedback (FSF) lasers have emerged as powerful tools for precision distance metrology. At the output of a Michelson interferometer, the detected rf spectra of the FSF laser light contain a length-dependent heterodyne beat signal whose linewidth ultimately limits the achievable accuracy of length measurements. Here, we demonstrate a narrow-linewidth chirped frequency comb from an FSF Ti:sapphire ring laser seeded by a phase-modulated, ultra-low-phase-noise, single-frequency fiber laser. We experimentally investigate the influence of the seed laser linewidth on the resulting width and shape of the length-dependent rf beat signal. An ultranarrow heterodyne beat linewidth of <20 Hz is observed.     

Experimental characterization of an Yb-doped fiber ring laser with frequency-shifted feedback

We present experimental characteristics of an Yb³⁺-doped fiber ring laser operating with frequency-shifted feedback (FSF) through an acousto-optic modulator (AOM) and seeded by both a stationary continuous-wave (CW) laser and spontaneous emission. We show the spectrum and output characteristics for operations with several effective gain bandwidths, as established by Fabry-Perot etalons inside the cavity. Observation using a high finesse Fabry-Perot interferometer shows that, as expected from earlier work, although the spectrum of the FSF laser without seeding is continuous, when seeded by a CW-laser the spectrum consists of a comb of discrete modes, each offset from the seed by an integer number of AOM frequency shifts. The experimental results are in excellent quantitative agreement with the theory developed earlier [L. Yatsenko, B.W. Shore, K. Bergmann, Opt. Commun. 236 (2004) 183].     

An all-fiber frequency-shifted feedback laser for optical ranging; signal variation with distance

A recent paper [L.P. Yatsenko et al., Opt. Commun. 242 (2004) 581] provided a first-principles prediction for the optical ranging signals obtained when using a frequency-shifted feedback (FSF) laser system, seeded by a phase-modulated laser. Such a system has many useful advantages over other alternative FSF laser techniques. We report here experimental verification of that theory, specifically the variation of the amplitude modulation signal with both distance and modulation index of the seed laser. We describe the operation of an all-fiber FSF laser that uses an Er³⁺-doped active fiber as the gain medium. To improve the signal and minimize the noise we seed the FSF laser with a phase-modulated (PM) laser; the measurement of distance derives from a measurement of amplitude modulation within a narrow frequency interval. We demonstrate that the resulting system is capable of fast and precise measurements. With the bandwidth limitations of our current system we achieved an accuracy better than 0.1 mm. Although measurements based on interferometry offer the potential for much greater accuracy under carefully controlled conditions, the present method does not suffer from the presence of a material-dependent phase shift at the surface of the measured object.     

Spectral dynamics of an all solid-state frequency-shifted feedback laser

A frequency-shifted feedback (FSF) laser has multi frequency components with simultaneous linear chirping in its oscillation spectrum even though the gain medium is homogeneously broadened. The spectral intensity and its bandwidth were analyzed by the proposed spectral dynamics model based on the regenerative amplification of the continuously chirped comb components with phase shift according to each round trip in the cavity. The spectral intensity peaks at an instantaneous frequency which depends on the net gain in the cavity and the gain bandwidth of atomic transition. The oscillation bandwidth is defined as the product of the saturation-broadened bandwidth and the total resonant modes contributing to the FSF operation.     

Coherence in the output spectrum of frequency shifted feedback lasers

We present a detailed theoretical analysis, using correlation functions, of the coherence properties of the output from a frequency shifted feedback (FSF) laser seeded simultaneously by an external seed laser and by spontaneous emission (SE). We show that the output of a FSF laser is a cyclostationary process, for which the second-order correlation function is not stationary, but periodic. However, a period-averaged correlation function can be used to analyze the optical spectrum. From the fourth-order correlation function of the output of a Michelson interferometer we obtain the essential characteristics of the radio-frequency (RF) spectrum, needed for describing the use of the FSF laser for optical-ranging metrology. We show that, even for a FSF laser seeded by SE, the RF spectrum comprises a sequence of doublets, whose separation gives directly a measure of the length difference between the interferometer arms. This doublet structure is a result of the correlation of interference terms of individual components of the cyclostationary stochastic process. It is not seen in the optical spectrum of the FSF laser but is observable in the RF spectrum. We analyze the competition between SE and continuous wave (CW) seeding to obtain an analytical expression for the ratio of power in the discrete CW signal to the background continuum spectrum from SE. We show that, unlike mode competition in conventional lasers, where there occurs exponential selectivity, here there is a balance between the two fields; the power in the fields is related linearly, rather than exponentially, to the control parameters.     

Positive and negative feedback effects of four-wave mixing in photorefractive crystals with two active centres

Based on a two-level model of optical transitions, the problem of positive and negative feedback effects during simultaneous reading and writing of dynamic holograms in photorefractive crystals has been solved analytically. The diffraction efficiency and the gain coefficient are shown to be nonlinear functions of the readout beam intensity and are nonreciprocal with respect to the readout from the two input ports. The readout beam intensity value that affects a previously recorded grating is shown to depend on microscopic parameters such as the cross-section of photoionization, the recombination coefficient and the concentration of photorefractive centres. An analysis of the amplitude of the overall shared grating has shown that with increasing readout beam intensity the holographic grating thickness decreases. In this case, under negative feedback the grating amplitude decreases, while with positive feedback it increases. The possibility is shown of achieving (under positive feedback) a maximum value of diffraction efficiency and complete energy transfer in a finite thickness of the photorefractive crystal, in contrast to the infinite thickness required in the standard two-beam coupling case.     

基于液晶/聚合物光栅的高转化效率有机半导体激光器

采用有机半导体发光材料聚[2-甲氧基-5-(2-乙基己氧基)-1,4-苯乙炔]作为增益介质,低官能度光敏单体制备的液晶/聚合物光栅作为外部反馈谐振腔,制备出参数可独立控制的分离式结构的有机半导体激光器.液晶/聚合物光栅中液晶分子的取向影响光栅折射率调制量,从而影响光栅的反馈能力,最终影响激光器出射激光的性能.通过研究发现决定液晶分子取向的主要有两种与光栅周期有关的作用力,利用这一原理制备不同周期的光栅,光栅周期小于450 nm时,相分离出的液晶分子取向由光栅矢量方向变为光栅沟槽方向,此时光栅的折射率调制量增加,光反馈能力增强.采用周期为395 nm的液晶/聚合物光栅制备二级布拉格散射的有机半导体激光器,相较于大周期光栅(593 nm)制备的激光器,激光阈值由0.70μJ/pulse降低至0.18μJ/pulse,转化效率由2.5%提高到6.4%,且出射激光垂直于基板表面发射,有利于后续的处理及应用.     

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.     

分布反馈量子级联激光器光栅反馈特性北大核心CSCD

为探究Bragg光栅结构对TM模反馈特性的影响,利用有限时域差分法对具有TM模的分布反馈(DFB)量子级联激光器Bragg光栅结构进行仿真研究。重点分析了侧向耦合光栅的光学特性以及光栅侧壁倾角对光栅反射谱、损耗的影响及原因,并探究了光栅刻蚀深度及占空比与TM模的耦合系数、损耗的关系。结果表明有效折射率是影响Bragg波长的主要因素,而光限制因子是不同周期的侧向耦合光栅结构耦合系数产生巨大差别的原因,当光栅侧壁倾角90°时镜面损耗最小。光栅周期、占空比、刻蚀深度与耦合系数的关系表明:这些参数不仅影响光栅的相对介电常数,也会对光限制因子产生作用,从而影响耦合系数的大小;耦合系数与刻蚀深度具有正比关系,大周期光栅耦合系数随占空比的变化率较小。对光栅光反馈特性的理论研究有利于提升对DFB量子级联激光器的认识,促进激光器性能的提升和发展。     

Analog asymmetrization of profile of thin dynamic holographic grating in liquid-crystal matrix transparency

The recording of dynamic holographic grating in an electrically addressed matrix liquid-crystal spatial light modulator, which asymmetrizes the grating profile due to the application of optical feedback (nonlinear or adaptive interferometer) with television closing of feedback loop, has been studied.     

增益半导体光放大器的特性

为满足半导体光放大器（SOA）在光纤到户FTTH系统接入网中的广泛应用,提出了基于光纤光栅外腔反馈型GC-SOA结构的全光增益机制,窄线宽激光光源经可变衰减器、隔离器和光纤光栅注入到SOA中,SOA的输出光经隔离器和光纤光栅送至光谱分析仪,通过光纤光栅反馈输入SOA形成钳制激光。对GC-SOA的阈值特性、增益特性及开关特性进行分析,结果表明:当注入电流小于GC-SOA的阈值电流时,增益随注入电流的增加而增加;当注入电流大于GC-SOA的阈值电流后,其增益不再随注入电流的变化而变化,实现了SOA的增益稳定,使SOA的饱和输出功率得到了提高。     

光子晶体理论应用于光纤布拉格光栅的研究

将光纤布拉格光栅近似为一维光子晶体，研究光栅的能带结构和光学传输特性。利用平面波展开法对光子晶体进行理论研究，将研究方法移植到光纤布拉格光栅上，并对其进行数值分析，得到光栅的传输特性。研究表明，光栅反射谱有多个反射峰，峰值中心频率是基频的整数倍，而且峰值处存在光子带隙，反射峰带宽对应光子带隙宽度，中心频率和带宽随光栅长度、高介电常量材料占空比、调制深度而变化。结果与传统的耦合模理论一致。     

光栅光谱仪光路结构的设计

为提高光栅光谱仪的波长扫描精度,设计了一套可对光栅转动情况进行实时反馈的系统结构,实现了仪器在高速和高精度扫描上的统一。在对反馈结构进行设计时,应用光栅尺测位移技术对正弦丝杆上的滑块进行位移反馈,由计算机对其进行接收和处理。对比实验显示,仪器本身波长扫描精度为 0.7 nm,加入反馈电路进行反馈后波长扫描精度可达 0.15 nm。结果表明,加入对光栅转动情况的反馈结构后,可使光栅光谱仪满足高精度扫描要求。     

Circular grating DFB and DBR semiconductor lasers: threshold current analysis

The threshold current is analyzed for distributed feedback (DFB) and distributed Bragg reflector (DBR) semiconductor lasers with circular gratings. It is shown that in circular grating DFB lasers, the threshold current becomes minimum at a certain cavity radius, while in circular grating DBR lasers it increases monotonically as the active region radius increases.