Showing papers on "Chirp published in 2002"

Attosecond Streak Camera

Abstract: An electron generated by x-ray photoionization can be deflected by a strong laser field Its energy and angular distribution depends on the phase of the laser field at the time of ionization This phase dependence can be used to measure the duration and chirp of single sub100-attosecond x-ray pulses

Analysis and optimization of optical parametric chirped pulse amplification

Abstract: Optical parametric amplification can give particularly high values for gain, gain bandwidth, energy, efficiency, and wave-front quality. In combination with chirped pulse amplification, in a technique we call optical parametric chirped pulse amplification, it offers the prospect of generating peak powers up to 100 PW and intensities greater than 1024 W/cm2 with existing technology. Here we study the technique in detail using both analytical and computational techniques, and the limit of validity of the analytical approach is identified. The effects of spectral phase, energy extraction, signal pulse chirp, and pump pulse nonuniformity are analyzed, and optimization techniques are proposed and discussed.

Wide tunable double ring resonator coupled lasers

Abstract: A double ring resonator coupled laser (DR-RCL) is proposed and analyzed in this letter. Benefiting from the uniform peak transmission, narrow bandwidth and other superior characteristics of traveling wave supported high-Q resonators, DR-RCLs offers many promising advantages over the conventional tunable lasers, including ultra wide wavelength tuning range, high side mode suppression ratio, uniform threshold and efficiency, narrow linewidth, low frequency chirp, and simple fabrication. A DR-RCL with a moderate optical loss /spl sim/10 dB/cm in the ring resonators can achieve a wavelength tuning enhancement of /spl sim/50. With such a large tuning leverage, DR-RCLs could utilize the electrooptic effects to achieve ultrafast tuning speed with a wide tuning range covering the material gain bandwidth.

Application of the fractional Fourier transform to moving target detection in airborne SAR

Abstract: As a useful signal processing technique, the fractional Fourier transform (FrFT) is largely unknown to the radar signal processing community. In this correspondence, the FrFT is applied to airborne synthetic aperture radar (SAR) slow-moving target detection. For airborne SAR, the echo from a ground moving target can be regarded approximately as a chirp signal, and the FrFT is a way to concentrate the energy of a chirp signal. Therefore, the FrFT presents a potentially effective technique for ground moving target detection in airborne SAR. Compared with the common Wigner-Ville distribution (WVD) algorithm, the FrFT is a linear operator, and will not be influenced by cross-terms even if multiple moving targets exist. Moreover, to solve the problem whereby weak targets are shadowed by the sidelobes of strong ones, a new implementation of the CLEAN technique is proposed based on filtering in the fractional Fourier domain. In this way strong moving targets and weak ones can be detected iteratively. This combined method is demonstrated by using raw clutter data combined with simulated moving targets.

High-power passively mode-locked semiconductor lasers

Abstract: We have developed optically pumped passively mode-locked vertical-external-cavity surface-emitting lasers. We achieved as much as 950 mW of mode-locked average power in chirped 15-ps pulses, or 530 mW in 3.9-ps pulses with moderate chirp. Both lasers operate at a repetition rate of 6 GHz and have a diffraction-limited output beam near 950 nm. In continuous-wave operation, we demonstrate an average output power as high as 2.2 W. Device designs with a low thermal impedance and a smooth gain spectrum are the key to such performance. We discuss design and fabrication of the gain structures and, particularly, their thermal properties

Generation of single intense short optical pulses by ultrafast molecular phase modulation

Abstract: Pulses with durations below 4 fs have been generated using the method of ultrafast molecular phase modulation. A laser pulse shorter than the molecular vibrational or rotational period obtains spectral broadening during propagation along a hollow waveguide filled with previously impulsively excited Raman active gases. The induced time dependent phase, frequency, and frequency chirp are controllable by changing the delay between excitation and probe pulse within the molecular vibrational period.

Electron-yield enhancement in a laser-wakefield accelerator driven by asymmetric laser pulses.

Abstract: The effect of asymmetric laser pulses on electron yield from a laser wakefield accelerator has been experimentally studied using > 10{sup 19} cm{sup -3} plasmas and a 10 TW, > 45 fs, Ti:Al{sub 2}O{sub 3} laser. Laser pulse shape was controlled through non-linear chirp with a grating pair compressor. Pulses (76 fs FWHM) with a steep rise and positive chirp were found to significantly enhance the electron yield compared to pulses with a gentle rise and negative chirp. Theory and simulation show that fast rising pulses can generate larger amplitude wakes that seed the growth of the self-modulation instability and that frequency chirp is of minimal importance for the experimental parameters.

Stepped-frequency pulse-train radar signal

Abstract: The stepped-frequency radar signal is revisited. The delay-Doppler response of pulse-trains with different interpulse frequency codings is investigated, including linear, nonlinear, up-down and Costas codings. Different approaches to sidelobe reduction are considered. Both matched filter and stretch processing are discussed.

Responses of Neurons in Cat Primary Auditory Cortex to Bird Chirps: Effects of Temporal and Spectral Context

Abstract: The responses of neurons to natural sounds and simplified natural sounds were recorded in the primary auditory cortex (AI) of halothane-anesthetized cats. Bird chirps were used as the base natural stimuli. They were first presented within the original acoustic context (at least 250 msec of sounds before and after each chirp). The first simplification step consisted of extracting a short segment containing just the chirp from the longer segment. For the second step, the chirp was cleaned of its accompanying background noise. Finally, each chirp was replaced by an artificial version that had approximately the same frequency trajectory but with constant amplitude. Neurons had a wide range of different response patterns to these stimuli, and many neurons had late response components in addition, or instead of, their onset responses. In general, every simplification step had a substantial influence on the responses. Neither the extracted chirp nor the clean chirp evoked a similar response to the chirp presented within its acoustic context. The extracted chirp evoked different responses than its clean version. The artificial chirps evoked stronger responses with a shorter latency than the corresponding clean chirp because of envelope differences. These results illustrate the sensitivity of neurons in AI to small perturbations of their acoustic input. In particular, they pose a challenge to models based on linear summation of energy within a spectrotemporal receptive field.

Single-shot supercontinuum spectral interferometry

Abstract: We have developed a single-shot spectral interferometer using the supercontinuum pulse generated by self-focusing in atmospheric pressure air. The diagnostic can be used to measure ultrafast refractive index transients either in a direct frequency-to-time mapping mode or in a full Fourier transform mode. In the direct mapping mode, temporal resolution is shown to be strongly restricted by the pulse chirp. In the transform mode, the ultimate temporal resolution is limited by the supercontinuum pulse bandwidth and the maximum pump-induced phase shift.

A fast refinement for adaptive Gaussian chirplet decomposition

Abstract: The chirp function is one of the most fundamental functions in nature. Many natural events, for example, most signals encountered in seismology and the signals in radar systems, can be modeled as the superposition of short-lived chirp functions. Hence, the chirp-based signal representation, such as the Gaussian chirplet decomposition, has been an active research area in the field of signal processing. A main challenge of the Gaussian chirplet decomposition is that Gaussian chirplets do not form an orthogonal basis. A promising solution is to employ adaptive type signal decomposition schemes, such as the matching pursuit. The general underlying theory of the matching pursuit method has been well accepted, but the numerical implementation, in terms of computational speed and accuracy, of the adaptive Gaussian chirplet decomposition remains an open research topic. We present a fast refinement algorithm to search for optimal Gaussian chirplets. With a coarse dictionary, the resulting adaptive Gaussian chirplet decomposition is not only fast but is also more accurate than other known adaptive schemes. The effectiveness of the algorithm introduced is demonstrated by numerical simulations.

Klystron instability of a relativistic electron beam in a bunch compressor

Abstract: In this paper, we consider a klystron-like mechanism of amplification of parasitic density modulations in an electron bunch passing a magnetic bunch compressor. Analytical expressions are derived for the small-signal gain. The effect of wakefields in front of the bunch compressor is analyzed by using a model of linear compression which assumes linear correlated energy chirp and linear dependence of a path length on energy deviation. Analysis of the density modulation growth due to coherent synchrotron radiation inside bends of the magnetic bunch compressor is done for the simplified case of no correlated energy chirp (no compression). Analytical results of this paper can be used for benchmarking numerical simulation codes.

Chirp characteristics of dual-drive. Mach-Zehnder modulator with a finite DC extinction ratio

Abstract: In this letter, we present theoretical and experimental studies on the chirp of the dual-drive Mach-Zehnder (MZ) modulator with a finite DC extinction ratio. The chirp of the modulator was measured using the fiber response peak method and compared with the theoretically calculated values. The results show that the residual chirp of the MZ modulator could be substantially reduced by driving the modulator in a push-pull mode with unequal amplitude signals.

Digital false-target image synthesiser for countering ISAR

Abstract: The paper presents the design of a pipelined, all-digital image synthesiser capable of generating false-target images from a series of intercepted inverse synthetic aperture radar (ISAR) chirp pulses to provide a novel RF imaging decoy capability. The image synthesiser modulates the phase samples from a phase-sampling digital RF memory (DRFM) that stores intercepted ISAR pulses. The synthesiser contains a parallel array of (identical) complex digital modulators with one modulator for each false-target range bin. The binary phase samples from each intercepted ISAR pulse are applied one at a time to the modulator array. To synthesise the image, each modulator requires a set of phase and gain coefficients that are derived from the range-Doppler description of the false-target to be synthesised. An ISAR image compression algorithm is presented to reconstruct the image using the synthesiser output pulses. A ship with 32 range bins is presented as an example of a false-target input, and simulations are used to quantify the image synthesis capability of the architecture. The image quality as a function of the number of ISAR pulses integrated is also numerically evaluated. The programmable design permits real-time alteration of modulation coefficients, allowing rapid and adaptive shifting among different types of targets offering a low-cost decoy capability using readily available DRFM technology.

Maximum likelihood parameter estimation of superimposed chirps using Monte Carlo importance sampling

Abstract: We address the problem of parameter estimation of superimposed chirp signals in noise. The approach used here is a computationally modest implementation of a maximum likelihood (ML) technique. The ML technique for estimating the complex amplitudes, chirping rates, and frequencies reduces to a separable optimization problem where the chirping rates and frequencies are determined by maximizing a compressed likelihood function that is a function of only the chirping rates and frequencies. Since the compressed likelihood function is multidimensional, its maximization via a grid search is impractical. We propose a noniterative maximization of the compressed likelihood function using importance sampling. Simulation results are presented for a scenario involving closely spaced parameters for the individual signals.

Spectral shaping of supercontinuum in a cobweb photonic-crystal fiber with sub-20-fs pulses

Abstract: Multiple approaches to generate a smooth, powerful, and stable supercontinuum in cobweb photonic-crystal fibers were undertaken by use of 18-fs pulses. These approaches include utilization of incident pulses with various chirp, power, and polarization states, as well as fibers with different lengths and core sizes. For long fibers (tens of centimeters) the supercontinuum contains a finely modulated structure that can be smoothed when the oscillator is in a regime of relaxation oscillations. Short fibers provide a supercontinuum free of gaps. By optimization of these parameters supercontinua exceeding one octave with modulations of less than 10 dB have been generated.

Single-shot femtosecond coherent anti-Stokes Raman-scattering thermometry

Abstract: A new time-domain single-shot detection technique based on nonresonant femtosecond coherent anti-Stokes Raman scattering is introduced for the determination of temperatures in flames and combustion processes. Chirped probe pulses are used to map the time evolution of the molecular dynamics initiated by a pump and a Stokes pulse onto the spectrum of the coherent anti-Stokes Raman-scattering signal pulses. An analytical description of this mapping process is given, which specifies ranges of linear and nonlinear time-frequency mapping, and an experimental realization is presented for single-shot thermometry on H2.

Full-polarization matched-illumination for target detection and identification

Abstract: This paper investigates the optimization of the full-polarization radar transmission waveform and the receiver response to maximize either target detection or identification performance. Application of such full-polarization matched-illumination techniques to simulated VHF-band frequency response data of mobile surface targets yields a significant performance improvement over that corresponding to chirped full-polarization transmission waveforms.

Quantitative SNR analysis for ISAR imaging using joint time-frequency analysis-Short time Fourier transform

Abstract: V.C. Chen recently presented an inverse synthetic aperture radar (ISAR) imaging technique using the joint time-frequency analysis (JTFA), which has been shown having a better performance for maneuvering targets over the conventional Fourier transform method. The main reason is because the frequencies of the radar returns of the maneuvering targets are time varying and a JTFA is a technique that is suitable for such signals, in particular a JTFA may concentrate a wideband signal, such as a chirp, while it spreads noise. We quantitatively study the signal-to-noise ratio (SNR) in the ISAR imaging using one of the typical JTFA techniques, namely the short time Fourier transform (STFT). We show that the SNR increases in the joint time-frequency (TF) domain over the one in the time or the frequency domain alone both theoretically and numerically. This quantitatively shows the advantage of the JTFA technique for the ISAR imaging.

ECCM capabilities of an ultrawideband bandlimited random noise imaging radar

Abstract: Investigated here is high-resolution imaging of targets in noisy or unfriendly radar environments through a simulation analysis of the ultrawideband (UWB) continuous-wave (CW) bandlimited random noise waveform. The linear FM chirp signal was selected as a benchmark radar waveform for comparison purposes. Simulation of the recovery of various types of target reflectivity functions (TRFs) for these waveforms were performed and analyzed. In addition, electronic counter-countermeasure (ECCM) capabilities for both types of systems were investigated. The results are compared using the error between the interference (jamming)-free recovered TRF and the recovered TRF under noisy conditions as a function of the signal-to-interference/jamming ratio (SIR/SJR). Our analysis shows that noise waveforms possess better jamming immunity (of the order of 5-10 dB improvement over the linear FM chirp) due to the unique radar correlation processing in the receiver.

Carrier-envelope offset dynamics and stabilization of femtosecond lasers

Abstract: Summary form only given. Control of the carrier-envelope offset (CEO) phase enables optical waveform synthesis and is of considerable importance in optical precision frequency metrology. Here we report on stabilization of the CEO frequency to an external rf reference oscillator with a jitter of 20 mrad (rms). This unprecedented low value corresponds to a relative timing jitter between laser carrier and envelope of 10 attoseconds (rms). Our stabilization is based on a careful investigation of sources of CEO phase fluctuations. We use chirped mirrors together with a prism sequence as well as mirror-only dispersion compensation in a Kerr-lens mode-locked Ti:sapphire laser.

Electroabsorption modulated laser for long transmission spans

Abstract: Strain-compensated AlGaInAs quantum-well electroabsorption modulated lasers (EMLs) transmit at 10 Gbits/s on uncompensated transmission spans of >75 km of standard fiber and >225 km of MetroCor fiber. Details of the design, fabrication, and testing are presented. A complex-coupled distributed feedback (DFB) grating is used to enable high output power. The epitaxy and chip structure are described. The paper studies what is needed to accomplish long-span transmission in terms of minimum physical requirements for laser mode control, facet reflection, index grating strength, laser-modulator matching, laser-modulator electrical isolation, modulator extinction ratio, modulator capacitance, linewidth enhancement factor, etc. The interaction of the modulator with the laser is analyzed and a requirement for this structure is computed for the electrical isolation resistance between modulator and laser contacts. Stability of the laser source is discussed and a method is derived for determining the grating's gain-coupling coefficient at operating power. Chirp due to the modulator is analyzed. Nonzero chirp of the modulator is shown to have beneficial impact on the quality of the signal after transmission. The effect of a bias-dependent alpha parameter is analyzed. Because bit-error rate is a strong function of mean alpha parameter and not a strong function of the range of alpha during nonreturn-to-zero modulation, we determine that tuning the chirp of the EML modulator to suit different fiber types (MetroCor, SMF-28, etc.) is practical. Specific tradeoffs are also required. Experimental results verify the analysis.

Measurement of the intensity-dependent atomic dipole phase of a high harmonic by frequency-resolved optical gating.

Abstract: The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.

Parameter Estimation for Galactic Binaries by the Laser Interferometer Space Antenna

Abstract: We calculate how accurately parameters of the short-period binaries (10-4 Hz f 10-2 Hz) will be determined from the gravitational waves by the Laser Interferometer Space Antenna (LISA). In our analysis, the chirp signal, , is newly included as a fitting parameter, and dependence on the observation period or wave frequency is studied in detail. Implications for gravitational wave astronomy are also discussed quantitatively.

Asymptotic soliton train solutions of the defocusing nonlinear Schrodinger equation

Abstract: evolution governed by the defocusing nonlinear Schrodinger equation. At first, wave breaking phenomenon is studied in the limit of small dispersion. A solution of the Whitham modulational equations is found for the case of dissipationless shock wave arising after the wave breaking point. Then, asymptotic soliton trains arising eventually from a large and smooth initial pulse are studied by means of a semiclassical method. The parameter varying along the soliton train is calculated from the generalized Bohr-Sommerfeld quantization rule, so that the distribution of eigenvalues depends on two functions—intensity r 0(x) of the initial pulse and its initial chirp v 0(x). The influence of the initial chirp on the asymptotic state is investigated. Excellent agreement of the numerical solution of the defocusing NLS equation with predictions of the asymptotic theory is found.

Frequency specificity of chirp-evoked auditory brainstem responses

Abstract: This study examines the usefulness of the upward chirp stimulus developed by Dau et al. [J. Acoust. Soc. Am. 107, 1530–1540 (2000)] for retrieving frequency-specific information. The chirp was designed to produce simultaneous displacement maxima along the cochlear partition by compensating for frequency-dependent traveling-time differences. In the first experiment, auditory brainstem responses (ABR) elicited by the click and the broadband chirp were obtained in the presence of high-pass masking noise, with cutoff frequencies of 0.5, 1, 2, 4, and 8 kHz. Results revealed a larger wave-V amplitude for chirp than for click stimulation in all masking conditions. Wave-V amplitude for the chirp increased continuously with increasing high-pass cutoff frequency while it remains nearly constant for the click for cutoff frequencies greater than 1 kHz. The same two stimuli were tested in the presence of a notched-noise masker with one-octave wide spectral notches corresponding to the cutoff frequencies used in the first experiment. The recordings were compared with derived responses, calculated offline, from the high-pass masking conditions. No significant difference in response amplitude between click and chirp stimulation was found for the notched-noise responses as well as for the derived responses. In the second experiment, responses were obtained using narrow-band stimuli. A low-frequency chirp and a 250-Hz tone pulse with comparable duration and magnitude spectrum were used as stimuli. The narrow-band chirp elicited a larger response amplitude than the tone pulse at low and medium stimulation levels. Overall, the results of the present study further demonstrate the importance of considering peripheral processing for the formation of ABR. The chirp might be of particular interest for assessing low-frequency information.

Tunable chromatic dispersion compensation in 40-Gb/s systems using nonlinearly chirped fiber Bragg gratings

Abstract: Chromatic dispersion management and tunable compensation are essential features of 40-Gb/s wavelength-division-multiplexed (WDM) systems. In this paper, we demonstrate both single and multi channel 40-Gb/s tunable dispersion compensation using nonlinearly chirped FBGs (NC-FBGs). For single channel compensation, we show that the NC-FBG can be tuned over a moderate range (/spl sim/400 ps/nm) with tolerable third-order dispersion (<200 ps/nm/sup 2/) within the channel's data bandwidth (intrachannel third-order dispersion). For multichannel systems, we demonstrate 4/spl times/40-Gb/s tunable dispersion compensation using sampled NC-FBGs in two configurations. First, we show that a single, sampled NC-FBG with fairly low intrachannel third-order dispersion induces negligible penalty on all four channels. This solution has a limited dispersion tuning range because of the deleterious intrachannel third-order dispersion. We show a moderate tuning range from -300 ps/nm to -700 ps/nm. Second, we demonstrate that two inverse, concatenated, sampled NC-FBGs can cancel the high deleterious intrachannel third-order dispersion, thus extending the dispersion tuning range. This solution provides both positive and negative dispersion values by stretching the two NC-FBGs separately. A tuning range of -300 ps/nm to +300 ps/nm with zero intrachannel third-order dispersion is shown.

Spatiotemporal measurements based on spatial spectral interferometry for ultrashort optical pulses shaped by a Fourier pulse shaper

Abstract: We measured the space–time profile of ultrashort optical pulses shaped with a Fourier pulse shaper. Spatial chirp, which originated in the space–time coupling at the pulse shaper, was observed directly with two-dimensional spatial spectral interferometry. By analyzing the two-dimensional fringe pattern, we successfully obtained a spatial and a temporal distribution in both amplitude and phase for shaped pulses. Numerically predicted spatiotemporal patterns were compared with experimental results.

Enhanced X(3) interactions of unamplified femtosecond Cr:forsterite laser pulses in photonic-crystal fibers

Abstract: Enhancement of nonlinear optical interactions in the core of a photonic-crystal fiber allows several χ(3) processes to be simultaneously observed in the field of unamplified 30-fs pulses of a Cr:forsterite laser. Subnanojoule fundamental-radiation pulses of this laser experience spectral broadening arising from self-phase modulation and generate the third harmonic at 410–420 nm. Third-harmonic pulses also appear spectrally broadened at the output of the fiber as a result of the cross-phase-modulation effect. This catalog of enhanced χ(3) processes observed in photonic-crystal fibers opens the way for using such fibers for frequency conversion of low-energy femtosecond pulses with simultaneous chirp control and subsequent pulse compression.

A novel chirp modulation spread spectrum technique for multiple access

Abstract: This paper proposes a new approach in chirp modulation spread spectrum (CMSS). A good selection of the parameters of the linear chirps, which are used in chirp modulation, combined with antipodal signaling significantly reduces the multiple access interference (MAI) even for small time-bandwidth products. This makes such systems attractive for efficient spread spectrum multiple access. Equal energy and equal bandwidth characterize the set of chirps being used for multiple access. Analytical expressions for the time-bandwidth efficiency and the bit error rate (BER) in additive white Gaussian noise (AWGN) channels are derived. The theoretical results are then confirmed through simulations. The simulation results show that the proposed CMSS technique outperforms existing CMSS techniques in terms of bit error rate and time-bandwidth efficiency. Moreover, its performance approaches those attained by direct sequence spread spectrum (DSSS) systems.