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Showing papers on "Chirp published in 1994"


Journal ArticleDOI
TL;DR: Based on quantitative comparison, the chirp scaling algorithm provides image quality equal to or better than the precision range/Doppler processor, as defined by the system bandwidth.
Abstract: A space-variant interpolation is required to compensate for the migration of signal energy through range resolution cells when processing synthetic aperture radar (SAR) data, using either the classical range/Doppler (R/D) algorithm or related frequency domain techniques. In general, interpolation requires significant computation time, and leads to loss of image quality, especially in the complex image. The new chirp scaling algorithm avoids interpolation, yet performs range cell migration correction accurately. The algorithm requires only complex multiplies and Fourier transforms to implement, is inherently phase preserving, and is suitable for wide-swath, large-beamwidth, and large-squint applications. This paper describes the chirp scaling algorithm, summarizes simulation results, presents imagery processed with the algorithm, and reviews quantitative measures of its performance. Based on quantitative comparison, the chirp scaling algorithm provides image quality equal to or better than the precision range/Doppler processor. Over the range of parameters tested, image quality results approach the theoretical limit, as defined by the system bandwidth. >

897 citations


Journal ArticleDOI
TL;DR: In this paper, the frequency-resolved optical gating (FROG) trace is used to measure the intensity and phase of an individual, arbitrary, ultrashort laser pulse, with the most common geometries being polarization gate, self-diffraction and second-harmonic generation.
Abstract: We recently introduced frequency-resolved optical gating (FROG), a technique for measuring the intensity and phase of an individual, arbitrary, ultrashort laser pulse. FROG can use almost any instantaneous optical nonlinearity, with the most common geometries being polarization gate, self-diffraction, and second-harmonic generation. The experimentally generated FROG trace is intuitive, visually appealing, and can yield quantitative information about the pulse parameters (such as temporal and spectral width and chirp). However, the qualitative and the quantitative features of the FROG trace depend strongly on the geometry used. We compare the FROG traces for several common ultrashort pulses for these three common geometries and, where possible, develop scaling rules that allow one to obtain quantitative information about the pulse directly from the experimental FROG trace. We illuminate the important features of the various FROG traces for transform-limited, linearly chirped, self-phase modulated, and nonlinearly chirped pulses, pulses with simultaneous linear chirp and self-phase modulation, and pulses with simultaneous linear chirp and cubic phase distortion, as well as double pulses, pulses with phase jumps, and pulses with complex intensity and phase substructure.

182 citations


Journal ArticleDOI
TL;DR: In this paper, the authors proposed two methods to improve the overall efficiency of an ultrasonic pulse-echo system using nonlinear frequency modulation and square wave chirps derived from sine type chirp signals.
Abstract: Chirp pulse compression is a signal correlation technique that uses frequency modulated pulses as transmitted signals. Usually, signals with linear frequency modulation are applied. They can be generated rather easily, but their spectra are not totally matched to the transfer function of ultrasonic systems. In pulse-echo mode operation, with signal duration and consequently the time-bandwidth product being critical parameters, waveforms should be applied which make full use of the available power and bandwidth resources. We report here two methods to improve the overall efficiency of an ultrasonic pulse-echo system. Transmitter signals with constant amplitude level and nonlinear frequency modulation can be generated in such a way that they are spectrally matched to the system. A formula for the calculation of such a matched nonlinear chirp signal is presented. This modulation scheme also leads to a side-lobe level reduction of the compressed pulses. The application of square wave chirps derived from sine type chirps yields an additional gain of echo signal amplitude. Moreover, the complexity of the signal generation hardware is reduced. The methods are illustrated by an example. >

176 citations


Journal ArticleDOI
TL;DR: The power levels suggest that erbium-doped fiber lasers may replace bulk solid-state lasers, such as the color-center laser, for some applications.
Abstract: We report a self-starting stretched-pulse polarization additive-pulse mode-locked erbium-doped fiber ring laser with high output power. By using the light normally absorbed by the intracavity polarizer as the output, we obtained pulses with energies greater than 0.5 nJ at a repetition rate of 48 MHz. External chirp compensation was used to shorten the highly chirped output pulses to durations of less than 100 fs. The power levels suggest that erbium-doped fiber lasers may replace bulk solid-state lasers, such as the color-center laser, for some applications.

154 citations


Journal ArticleDOI
TL;DR: A new method for introducing a chirp on a fiber Bragg grating through the use of a controllable temperature gradient device, which allows the linear chirP conditions to be totally adjustable in a nondestructive repeatable manner.
Abstract: We propose a new method for introducing a chirp on a fiber Bragg grating. Uniformly written fiber Bragg gratings are linearly chirped through the use of a controllable temperature gradient device. This device allows the linear chirp conditions to be totally adjustable in a nondestructive repeatable manner. These conditions can be selected with great precision and are very stable, once defined. To characterize the resulting chirped gratings, we have measured their dispersion. Dispersion of the order of 200 ps/nm have been obtained for 1-cm-long fiber Bragg gratings with temperature gradients near 20 degrees C.

139 citations


Journal ArticleDOI
TL;DR: In this paper, a survey of SAW sensor systems is given, where three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal processing systems.
Abstract: A survey is given of SAW sensor systems. Three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing systems. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance frequency, which are described quantitatively. To show the high resolution of active sensor systems, an electronic spirit level (based on SAW resonators) is described. New SAW chirp sensors allow the dispersive interdigital transducer or reflector geometry to be adjusted to the sensor system resolution and measurement range. In passive systems the sensor device is connected to an antenna. The SAW is excited by an interrogating radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensor's information. The time and frequency response of all sensor categories and the possibilities of temperature compensation are discussed.

123 citations


Journal ArticleDOI
TL;DR: In this article, the influence of light injection on the reduction of the dynamical linewidth broadening of directly current-modulated semiconductor lasers at high bit rates is theoretically investigated and experimentally verified for 10 Gb/s NRZ pseudorandom modulation with a large current swing of 40 mA pp.
Abstract: The influence of strong light injection on the reduction of the dynamical linewidth broadening of directly current-modulated semiconductor lasers at high bit rates is theoretically investigated and experimentally verified for 10 Gb/s NRZ pseudorandom modulation with a large current swing of 40 mA pp. Significant chirp reduction and single-mode operation are observed for bulk DFB, quantum well DFB lasers at 10 Gb/s and a weakly coupled bulk DFB laser at 8 Gb/s, so that an improvement of the transmission performance using standard monomode fibers in the 1.55 /spl mu/m low-loss wavelength region can be achieved for all these laser types, where dispersion otherwise causes severe penalties for long-haul transmission. The properties of injection-locked bulk DFB and quantum well DFB lasers with respect to high bit rate modulation have been systematically studied by the use of the rate equation formalism. A dynamically stable locking range of more than 30 GHz under modulation has been found for both laser types with injection ratios higher than 0.5. >

122 citations


Journal ArticleDOI
TL;DR: A strain gradient in an optical fibre Bragg grating is realized by mounting the grating in a yielding, soft medium (eg glue) such that tension applied to one end will be gradually relieved over a distance.
Abstract: A strain gradient in an optical fibre Bragg grating is realised by mounting the grating in a yielding, soft, medium (eg glue) such that tension applied to one end will be gradually relieved over a distance This strain gradient results in a grating chirp that can be dynamically controlled by varying the applied tension

114 citations


Journal ArticleDOI
TL;DR: In this paper, a Mach Zehnder interferometer was used as an optical discriminator to measure the time-resolved frequency chirp of an optical source, which was then used as a discriminator for measuring the optical signal.
Abstract: A Mach Zehnder (MZ) interferometer has been used as an optical discriminator to measure the time-resolved frequency chirp of an optical source.

112 citations


Patent
21 Dec 1994
TL;DR: In this article, an acousto-optic deflector is used to deflect a beam of light illuminating a spot on a surface and varying the size of the spot, without changing any system components.
Abstract: An apparatus for both deflecting a beam of light illuminating a spot on a surface and varying the size of the spot, electronically, without changing any system components. The apparatus includes an acousto-optic deflector driven with a linear FM signal produced by a chirp signal generator. The linear FM signal is characterized with a dispersion rate, and the chirp signal generator includes a chirp dispersion selector to vary the dispersion rate. A beam of collimated light passes through the acousto-optic deflector and appropriate focusing optics image the beam onto a spot in a nominal focal plane. The chirp dispersion selector sets the dispersion rate in accord to a nominal rate, resulting in the beam illuminating a spot in the focal plane. Generally, the focal plane coincides with a wafer surface, of the type having periodic and non-periodic features on it. The spot size may be varied from that of a diffraction limited spot to a spot whose maximum size is system dependent. The spot size varies as a result of changing the dispersion rate of the chirp signal. The spot size may vary as it is scanned, or may remain fixed during the inspection of a wafer. In this manner, inspection by periodic feature comparison may be implemented when it proves advantageous. Alternatively, a larger spot may be obtained when periodic feature comparison would provide no benefit, and spatial filtering would achieve an enhanced signal/background.

96 citations


Journal ArticleDOI
TL;DR: In this paper, the principle of operation and characteristics of all-fiber devices for dispersion compensation based on chirped distributed resonant coupling are reviewed and a numerical simulation of an optical fiber link is evaluated.
Abstract: We review the principle of operation and characteristics of all-fiber devices for dispersion compensation based on chirped distributed resonant coupling. These devices are the chirped Bragg grating, the chirped intermodal coupler, and the tapered two-dissimilar-core fiber. We discuss the use of a figure of merit to characterize equalizing filters. Based on a specific example, filter responses are computed and their performance is evaluated with a numerical simulation of an optical fiber link. Finally, design constraints are discussed. >

Journal ArticleDOI
TL;DR: Dispersion compensation is demonstrated experimentally by pulse compression with the use of chirped optical fiber Bragg gratings by the Gaussian intensity profile of the 240-nm wavelength beam used for holographic sidewriting of the grating.
Abstract: Dispersion compensation is demonstrated experimentally by pulse compression with the use of chirped optical fiber Bragg gratings. The gratings chirp is self-induced by the Gaussian intensity profile of the 240-nm wavelength beam used for holographic sidewriting of the grating. Chirped pulses generated by a 1.55-microm gain-switched distributed-feedback laser with an initial pulse duration of 21 ps and a spectral width of 0.7 nm are compressed to 13 ps, in good agreement with theory.

Journal ArticleDOI
TL;DR: Experimental filtering results for a Gaussian input function, which is modulated by an additive chirp noise, are shown and excellent agreement between experiments and computer simulations is achieved.
Abstract: In the Wigner domain of a one-dimensional function, a certain chirp term represents a rotated line delta function. On the other hand, a fractional Fourier transform (FRT) can be associated with a rotation of the Wigner-distribution function by an angle connected with the FRT order. Thus with the FRT tool a chirp and a delta function can be transformed one into the other. Taking the chirp as additive noise, the FRT is used for filtering the line delta function in the appropriate fractional Fourier domain. Experimental filtering results for a Gaussian input function, which is modulated by an additive chirp noise, are shown. Excellent agreement between experiments and computer simulations is achieved.

Journal ArticleDOI
TL;DR: In this paper, a large-signal dynamic model capable of modeling the transient behavior of the output power and wavelength of multielectrode DFB lasers is described, which is a modified form of the transfer matrix method resulting in a timedependent implementation of this technique.
Abstract: A large-signal dynamic model capable of modeling the transient behavior of the output power and wavelength of multielectrode DFB lasers is described here. The key feature of the model is the use of a modified form of the transfer matrix method resulting in a time-dependent implementation of this technique. Other features are the inclusion of longitudinal spatial hole burning and nonlinear gain in the model. The versatility of the model is demonstrated in an analysis of the response of a two-electrode DFB laser under large-signal direct current modulation which illustrates the important role played by longitudinal spatial hole burning. The limited use of wavelength tunability in controlling chirp is also demonstrated. However, a scheme to improve the damping mechanism through nonuniform excitation called backbiasing is proposed. Finally, wavelength switching is demonstrated using the model. >

Journal ArticleDOI
TL;DR: It is argued that although limitations exist, pulse compression techniques can still be used to improve resolution or increase penetrational depth and the real advantage over conventional short-pulse imaging comes from the possibility that these improvements can be accomplished without increasing the peak intensity of the interrogated pulse above any threshold levels set by possible bio-effect considerations.
Abstract: Pulse compression techniques that are capable of producing a large signal-to-noise (SNR) enhancement, have been used successfully in many different fields. For medical applications, frequency-dependent attenuation in soft tissue can limit the usefulness of this method. In the paper, this issue is examined through model-simulation studies. Frequency-modulation (FM) chirp, considered in the study, is just one form of pulse coding technique. Pulse propagation effects in soft tissue are modelled as a linear zero phase filter. A method to perform simulations and estimate the effective time-bandwidth product K is outlined. K describes the SNR enhancement attainable under limitations imposed by the soft-tissue medium. An effective time-bandwidth product is evaluated as a function of soft-tissue linear attenuation coefficient αo, scatterer depth z and the bandwidth of the interrogating FM pulse, under realistic conditions. Results indicate that, under certain conditions, K can be significantly lower than its expected value in a non-attenuating medium. It is argued that although limitations exist, pulse compression techniques can still be used to improve resolution or increase penetrational depth. The real advantage over conventional short-pulse imaging comes from the possibility that these improvements can be accomplished without increasing the peak intensity of the interrogating pulse above any threshold levels set by possible bio-effect considerations.

Journal ArticleDOI
F. Dorgeuille1, F. Devaux1
TL;DR: In this article, the effective chirp parameter of a multiple-quantum-well (MQW) electroabsorption modulator was measured with accuracy for several operating wavelengths in the 1.5 /spl mu/m window.
Abstract: The chirp parameter of a multiple-quantum-well (MQW) electroabsorption modulator was measured with accuracy for several operating wavelengths in the 1.5 /spl mu/m window. It varied strongly with applied bias. Effective chirp parameter, defined as the ratio of phase change to transmission change between modulator on and off states, is about zero, or even negative. However, experimental transmission length on standard fiber at 10 Gb/s NRZ is much smaller than what is expected for such a low chirp parameter. It is demonstrated that the effective chirp parameter should not be computed from changes between on and off states, but from the average of the chirp parameter values in a 3 dB region of the most transparent states of the modulator. This simple rule allows us to predict transmission performances based on measurements of the chirp parameter, and can be used to optimize optical components without actually experimenting on a transmission system. The effective chirp parameter of the MQW electroabsorption modulator is found positive. This should be intrinsic to red-shift electroabsorption effects, such as the quantum confined Stark effect. >

Journal ArticleDOI
TL;DR: In this paper, computer simulations are presented which yield lower dispersion penalties for mixed FM and AM compared to pure AM, and a laser modulated at a bit rate of 10 Gbit/s, with a peak to peak frequency chirp of 3 GHz and an extinction ratio of 4:1 does operate in the desired regime.
Abstract: The dispersion penalty increases with the spectral width of the modulated optical source. Pure AM leads to a two sided optical spectrum. Mixed FM and AM, produced by a directly modulated low-chirp laser diode, can lead to the partial suppression of one of those sidebands. Results of computer simulations are presented which yield lower dispersion penalties for mixed FM and AM compared to pure AM. A laser modulated at a bit rate of 10 Gbit/s, with a peak to peak frequency chirp of 3 GHz and an extinction ratio of 4:1 does operate in the desired regime. 180 km of fiber with a chromatic dispersion of 17 ps/km/spl middot/nm lead to 2-dB dispersion penalty at those operating conditions. >

Journal ArticleDOI
TL;DR: In this paper, a continuous adjustable chirp of an optical fibre Bragg grating was produced by straining the fibre using a cantilever beam of nonuniform cross-section.
Abstract: Continuously adjustable chirp of an optical fibre Bragg grating was produced by straining the fibre using a cantilever beam of nonuniform cross-section. A uniform shift in wavelength is also produced which can be eliminated by temperature control.

Journal ArticleDOI
TL;DR: In this article, the effects of the format of the modulating signal and the splitting ratio of the input Y-branch waveguide on the chirp characteristics of a Mach-Zehnder intensity modulator were examined.
Abstract: The system performance is assessed for 10 Gb/s lightwave systems that use a III-V semiconductor Mach-Zehnder intensity modulator and operate at a wavelength of 1.55 /spl mu/m with standard single-mode optical fiber. The modulator is modeled by an equivalent circuit and an experimentally derived functional dependence of the attenuation and phase constants on applied voltage. The influence of the format of the modulating signal and the splitting ratio of the input Y-branch waveguide on the chirp characteristics of the modulator is examined based upon calculation of the receiver sensitivity. Optimum system performance is obtained for an asymmetric splitting ratio. >

Journal ArticleDOI
TL;DR: In this paper, a chirped fiber grating was used to equalise the dispersion of 10 and 20 Gbit/s data over 80 and 160 km standard fiber at 1557 nm.
Abstract: A novel chirped fibre grating has been used to equalise the dispersion of 10 and 20 Gbit/s data over 80 and 160 km standard fibre at 1557 nm. Unlike earlier fixed value dispersion equalisers, the grating chirp is adjustable, enabling the same device to be used for different fibre lengths and bit rates.

Patent
29 Sep 1994
TL;DR: In this article, a method for controlling and compensating for high order frequency chirp in a fast tuned laser is presented, which allows for the generation of shorter bandwidth-limited optical pulses with spectra containing the complete tuning range of the tunable laser.
Abstract: A method of, and apparatus for controlling and compensating for high order frequency chirp in a fast tuned laser. Optical means in the form of chirped Bragg gratings, preferrably in-fiber gratings, are used to compensate for one or more orders of the frequency chirp. Alternatively, or in combination with the optical compensation, an electrical compensation scheme employs integrators in one or more parallel compensation channels to create a composite compensation signal to compensate for specific order nonlinearities. The present invention allows for the generation of shorter bandwidth-limited optical pulses with spectra containing the complete tuning range of the tunable laser.

Journal ArticleDOI
TL;DR: In this article, a comparison of binary and duobinary reception and achieved an improvement in the chromatic dispersion limited transmission distance using Duobinary transmission was made, and a 3.8 dB dispersion penalty was observed at 160 km using negative chirped binary transmission coupled to a receiver with a duobbinary filter and decision circuit.
Abstract: Chromatic dispersion penalties at 10 Gb/s and 1550 nm wavelength depend on the transmission bandwidth, not just the baseband information bandwidth. Duobinary transmission is known to reduce the transmission bandwidth relative to that of binary transmission. Using binary transmission, with both negative and zero chirp modulators, we present the first comparison of binary and duobinary reception and achieve an improvement in the chromatic dispersion limited transmission distance using duobinary reception. A 3.8 dB dispersion penalty is observed at 160 km using negative chirped binary transmission coupled to a receiver with a duobinary filter and decision circuit. >

Journal ArticleDOI
TL;DR: In this paper, a strained multiquantum well electroabsorption modulator without pre-chirping of the laser source is presented for 10 Gbit/s over 100 km of standard fiber at 1540 nm.
Abstract: Transmission of 10 Gbit/s over 100 km of standard fibre at 1540 nm is reported for a strained multiquantum well electroabsorption modulator without prechirping of the laser source. The device has been optimised for chirp performance using computer simulations and is fully compatible for monolithic integration with a distributed feedback laser.

Journal ArticleDOI
TL;DR: The generation of 30-W peak-power 250-fs-wide pedestal-free optical pulses at 1.54 microm by use of a chirpcompensated optically amplified semiconductor laser followed by simultaneous soliton compression and shaping within a nonlinear optical loop mirror is reported.
Abstract: We report the generation of 30-W peak-power 250-fs-wide pedestal-free optical pulses at 1.54 μm by use of a chirp-compensated optically amplified semiconductor laser followed by simultaneous soliton compression and shaping within a nonlinear optical loop mirror. A weak controllable attenuation is inserted within the loop to permit precise control of the pulse shape. Pedestal-free pulses are obtained after optimization of both the fiber length and the power balance between the counterpropagating pulses within the loop. The corresponding spectrum reveals a twofold split structure that is due to a residual chirp from self-phase modulation.

Journal ArticleDOI
TL;DR: It is shown numerically and analytically that the self-focusing process can be controlled by changing the chirp parameter, which is similar to that of beam ellipticity except that ellipticity always increases the critical wave action.
Abstract: The spatiotemporal self-focusing of chirped optical pulses propagating in a nonlinear dispersive medium has been studied analytically and numerically. The analytic theory shows that the critical power for self-focusing occurring in a dispersive media changes quadratically with the chirp parameter in both two and three dimensions. It is found that the critical wave action depends on the sign of the total chirp parameter. Analytic results show that the effect of chirp is similar to that of beam ellipticity except that ellipticity always increases the critical wave action. Numerical simulations are used to study the effect of chirp and group-velocity dispersion on self-focusing. It is shown numerically and analytically that the self-focusing process can be controlled by changing the chirp parameter.

Patent
02 Nov 1994
TL;DR: In this paper, a method and apparatus for controlling the detailed strain field imposed on a guided-wave Bragg grating is presented, which allows the grating to be tuned over a broad spectral range or chirped in a manner that permits it to compensate for a wide range of dispersion with or without a shift in the Grating center wavelength.
Abstract: The present invention provides a method and apparatus for controlling the detailed strain field imposed on a guided-wave Bragg grating. The spectral and temporal form of radiant energy traveling down optical fibers is important in many fields, including communications and sensing. The spectral distribution and center wavelength of short optical pulses is of particular importance. In the communication field the information carrying capacity of an optical fiber can be limited by the dispersion experienced by short pulses as they propagate along an optical fiber. Chirped fiber optic intracore Bragg gratings can be used to compensate for this dispersion. However, a grating with a different degree of chirp would be required for each particular communication system. This control of the intra-grating strain distribution allows the grating to be tuned over a broad spectral range or chirped in a manner that permits it to compensate for a wide range of dispersion with or without a shift in the grating center wavelength. In addition, precise control of the strain distribution along a grating can be used to manufacture a variety of spectral filters.

Journal ArticleDOI
TL;DR: In this paper, a novel method for simultaneous 3D imaging using chirped ultrashort light pulses is proposed, which is applicable for imaging of the shape of moving objects, or surface testing or inspection.
Abstract: A novel method for simultaneous 3-D imaging using chirped ultrashort light pulses is proposed. Simultaneous imaging without scanning is possible by means of the conversion between the time and the color axes mediated by the chirped pulse. The theoretical limitation of the spatial resolution is discussed. We proved the efficacy of this method by measuring an area composing three flat steps made of three gauge blocks of different lengths with an accuracy of better than 0.3 mm. This technique is applicable for imaging of the shape of moving objects, or surface testing or inspection.

Journal ArticleDOI
TL;DR: In this article, a simple and accurate technique for evaluating the chirp factor of Mach-Zehnder modulators is presented, based on the dependence of the α parameter of the modulators on the applied bias voltage.
Abstract: A simple and accurate technique for evaluating of the chirp factor of Mach-Zehnder modulators is presented. The method is based on the dependence of the α parameter of the modulators on the applied bias voltage. It is shown that experimental results and theory are in very good agreement.

Patent
20 Oct 1994
TL;DR: In this paper, a passive surface acoustic wave identification tag (SAW-ID tag) was proposed for identifying articles at enhanced ranges using pulse compression techniques and a large number of coding possibilities.
Abstract: A Passive Surface Acoustic Wave Identification Tag ("SAW-ID tag") device utilizes pulse compression techniques and a large number of coding possibilities for identifying articles at enhanced ranges. The SAW-ID tag device provides a piezoelectric substrate having bus bars, spaced electrode taps between the bus bars and a built-in antenna, with an input chirped SAW transducer having a dispersive, complementary matched filter response to an input expanded chirp signal from an expanded linear FM chirp waveform actively generated by a nearby chirp transmitter. The input expanded chirp signal is fed into the input chirped SAW transducer through the built-in antenna, to compresses the input expanded chirp signal into a narrow, compressed pulse signal propagating toward the electrodes taps. The spacing of the electrode taps establishes the desired unique time-ordered coding. The electrode taps sample the narrow, compressed pulse signal and provide narrow, compressed pulse samples propagating a coded, pulse train output transmitted via the antenna to a nearby interrogation means to identify the tagged article. The preferred embodiment is an SAW-ID tag device with a piezoelectric quartz substrate about 1 inch long, a built-in dipole antenna and five (5) spaced electrode taps, establishing several million coding possibilities. Also disclosed are methods of identifying articles using an SAW-ID tag device, and methods of selectively disconnecting and connecting electrodes to facilitate mass production.

Journal ArticleDOI
TL;DR: Hamiltonian systems describing particle motion in a wave with time-dependent (chirped) frequency are studied and the wave is found to form a single-node separatrix moving in the phase space at a rate proportional to that of the frequency change.
Abstract: Hamiltonian systems describing particle motion in a wave with time-dependent (chirped) frequency are studied. The wave is found to form a single-node separatrix ([ital bucket]) moving in the phase space at a rate proportional to that of the frequency change. Particles trapped inside the bucket undergo convection, while untrapped particles colliding with the bucket get a resonant kick, in phase space. In toroidal systems, these effects can result in a large radial convective flux roughly proportional to the size of the bucket and the frequency chirping. Possible applications of this novel mechanism to tokamak plasmas are discussed.