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.     

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.     

Chirped-frequency generation in a translated-grating-type frequency-shifted feedback laser

The properties of a frequency-shifted feedback (FSF) laser using a translated grating as the frequency-shifting element are investigated. FSF operation is attained by feedback of the first-order diffracted light from a grating coupler, which is translated in a direction perpendicular to the grating normal. A diode-pumped Nd:YVO(4) gain medium is used. Chirped-frequency components are periodically generated with a chirp rate of 1.8 THz / s. The unique spectral characteristics of this device are demonstrated with a Michelson interferometer and a heterodyne experiment.     

Beatings in the Spin Echo Signal in Media Having a Doublet Structure of the Spectrum

We have obtained an analytical expression for the twopulse echo signal arising in nonresonant excitation of a condensed medium with a nonuniformly broadened spectrum of doublet structure by two pulses of an electromagnetic field of different durations and of the same amplitude. It is shown that in the case where the differences between the carrying frequency of exciting pulses and the central frequencies of the spectral lines differ considerably, beatings are formed in the signal of the twopulse echo. The rate of decay of the amplitude of these beatings is found to depend only on the time of reversible and irreversible relaxation, and their circular frequency is determined by the shift of the spectral lines relative to each other. A technique for separate determination of the inhomogeneous width of the lines and time of the irreversible transverse relaxation is suggested which is based on the analysis of the twopulse echo signal beatings.     

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.     

A New Phenomenon—Amplitude‐Modulated Free Oscillations (Beatings) in Loaded,Highly Oriented Fibers from Semicrystalline Polymers

A new phenomenon in highly oriented semicrystalline polymers has been found—the occurrence of amplitude‐modulated oscillations (or beatings). The beatings appeared for certain external conditions. A preliminary interpretation of the phenomenon is given at the supermolecular level.     

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.     

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.     

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.     

Mechanical model of carbon dioxide vibrational spectrum

Classical dynamics methods have been used to study the nonlinear vibrations of a CO₂ molecule. Consideration includes not only the anharmonicity valence angle, which enables one to explain the Fermi resonance, but also the physical nonlinearity of the force field (stiffness and softness of springs). In the farthest neighbor approximation (with regard to oxygen–oxygen interaction), a set of nonlinear differential equations in the Lagrangian form has been derived. Their analytical solution has been derived using the method of invariant normalization. The occurrence of a strange attractor has been discovered by numerical simulation. Recommendations for the selection of initial conditions are given that take into account the possibility of regular beatings that change into to chaotic beatings.     

Spin torque on magnetic domain walls exerted by supercurrents

We calculate the spin density, spin currents and spin torque due to a spin polarized current on a magnetic domain wall juxtaposed to or inserted in a conventional superconductor. The superconductor is part of a heterostructure of the type NSN or FSF. In general, the spin torque exerted on the domain wall is weaker with respect to a normal metal. However, there are regimes where the torque is enhanced with respect to the normal metal. In these regimes the motion of the domain wall is therefore more efficient. A notable case is the passing of an unpolarized current which leads to a finite torque in the case of the superconductor.     

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].     

Focal fat deposition at liver MRI with gadobenate dimeglumine and gadoxetic acid: Quantitative and qualitative analysis

Objective

To evaluate the image findings of focal fat deposition (FFD) in the liver on gadobenate dimeglumine (Gd-BOPTA)- and gadoxetic acid (Gd-EOB-DTPA)-enhanced MRI, particularly during the hepatobiliary phase (HBP), and the relationship between relative enhancement (RE) and fat signal fraction (FSF) of FFD.

Subjects and Methods

Twenty-one patients with 27 FFDs (mean diameter, 21.9 mm), which showed low signal intensity on opposed-phase compared with in-phase MRI, were retrospectively evaluated. RE of the liver (RE_liver) and FFD (RE_FFD) and liver-to-lesion contrast-to-noise ratio (CNR) of FFD were measured on dynamic phases and HBP images with fat-saturated in-phase gradient-echo sequence. The FSF of each FFD was measured on in- and opposed-phase dual gradient-echo images. We qualitatively analyzed imaging findings of FFDs, including signal intensity, shape, margin, and homogeneity on HBP images, and enhancement pattern during dynamic phases. The correlations between RE_FFD and FSF and between CNR and FSF on HBP images were evaluated using Pearson’s correlation tests and a simple linear regression model.

Results

There were no significant differences between RE_FFD and RE_liver in dynamic phases and HBP, regardless of contrast agents (p ≥ 0.075). On HBP images, CNR (p = 0.008) but not RE_FFD (p = 0.122) was significantly correlated with FSF of FFDs (mean FSF, 19%). On HBP images, 21 of the 27 (77.8%) FFDs were hypointense, and 17 (63%) were homogeneous. Of the 21 hypointense FFDs, 12 (57.1%) had an ovoid shape and 11 (52.4%) were well margined. Although the 27 FFDs showed various enhancement patterns, 17 (63%) showed no enhancement.

Conclusion

Most FFDs appeared as hypointense lesions on Gd-BOPTA- and Gd-EOB-DTPA-enhanced MRI during HBP, with various enhancement patterns during dynamic contrast-enhanced phases. RE_FFD on HBP images was not significantly correlated with FSF of low grade FFDs.     

高能激光技术进展与面临的挑战

文章分析和比较了几种高能激光器技术发展现状和面临的挑战,以及各自可能适用的范围.特别对二极管抽运固体激光器和光纤激光器的几种束相干合成方法和光束质量评价方法进行了分析和讨论.二极管抽运固体激光器和光纤激光器近十年来异军突起,被人们看作是继化学激光器后的第二代高能激光器.光纤激光器进展要比二极管抽运固体激光器滞后5到10年.对几十万瓦以上的激光器来说,在相当长的时间内仍然要靠化学激光器.     

Synchronization of mutually coupled chaotic lasers in the presence of a shutter

Two mutually coupled chaotic diode lasers exhibit stable isochronal synchronization in the presence of self-feedback. When the mutual communication between the lasers is discontinued by a shutter and the two uncoupled lasers are subject to self-feedback only, the desynchronization time is found to scale as Adtau, where Ad>1 and tau corresponds to the optical distance between the lasers. Prior to synchronization, when the two lasers are uncorrelated and the shutter between them is opened, the synchronization time is found to be much shorter, though still proportional to tau. As a consequence of these results, the synchronization is not significantly altered if the shutter is opened or closed faster than the desynchronization time. Experiments in which the coupling between two chaotic-synchronized diode lasers is modulated with an electro-optic shutter are found to be consistent with the results of numerical simulations.     

新一代遥测网络系统及其传输组网方式研究分析

新一代遥测网络系统(TmNS)包括试验对象端(TAS)和地面站端(GSS)两大部分,在对这两部分的功能结构,包括系统组成,各组成单元功能关系进行研究的基础上,依据飞行试验遥测传输需求,对新一代遥测网络系统的传输与组网方式进行研究分析。