Showing papers on "Chirp published in 1998"

Linear frequency-modulated signal detection using Radon-ambiguity transform

Abstract: A novel time-frequency technique for linear frequency modulated (LFM) signal detection is proposed. The design of the proposed detectors is based on the Radon transform of the modulus square or the envelope amplitude of the ambiguity function (AF) of the signal. A practical assumption is made that the chirp rate is the only parameter of interest. Since the AF of LFM signals will pass through the origin of the ambiguity plane, the line integral of the Radon transform is performed over all lines passing through the origin of the ambiguity plane. The proposed detectors yield maxima over chirp rates of the LFM signals. This reduces the two-dimensional (2-D) problem of the conventional Wigner-Ville distribution (WVD) based detection or the Radon-Wigner transform (RWT) based detector to a one-dimensional (1-D) problem and consequently reduces the computation load and keeps the feature of "built-in" filtering. Related issues such as the finite-length effect, the resolution, and the effect of noise are studied. The result is a tool for LFM detection, as well as the time-varying filtering and adaptive kernel design for multicomponent LFM signals.

System, method, and sensors for sensing physical properties

Abstract: Several sensors are provided for determining one of a number of physical roperties including pressure, temperature, chemical species, and other physical conditions. In general, the sensors feature a resonant circuit with an inductor coil which is electromagnetically coupled to a transmitting antenna. When an excitation signal is applied to the antenna, a current is induced in the sensor circuit. This current oscillates at the resonant frequency of the sensor circuit. The resonant frequency and bandwidth of the sensor circuit is determined using an impedance analyzer, a transmitting and receiving antenna system, or a chirp interrogation system. The resonant frequency may further be determined using a simple analog circuit with a transmitter. The sensors are constructed so that either the resonant frequency or bandwidth of the sensor circuit, or both, are made to depend upon the physical properties such as pressure, temperature, presence of a chemical species, or other condition of a specific environment. The physical properties are calculated from the resonant frequency and bandwidth determined.

Theory of double-chirped mirrors

Abstract: A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated /spl ap/30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses.

Noncollinearly phase-matched femtosecond optical parametric amplification with a 2000 cm -1 bandwidth

Abstract: An optical parametric amplifier generating as short as 14 fs pulses in a visible region has been constructed in a noncollinear phase-matching configuration. The group-velocity matching between the signal and idler enormously broadens the gain bandwidth up to 2000 cm−1, which is mainly limited by the chirp of the seed pulses. Pulses shorter than 20 fs are tunable from 550 to 690 nm by scanning the delay-line of the pump beam.

Tunable nonlinearly chirped grating

Abstract: A nonlinearly chirped fiber grating (100) for achieving tunable dispersion compensation, chirp reduction in directly modulated diode lasers (102), and optical pulse manipulation. A dynamical dispersion compensation mechanism can be implemented in a fiber communication system based on such a nonlinearly chirped fiber grating (100).

Polarization mode dispersion induced pulse broadening in optical fibers.

Abstract: A simple and exact analytical expression is derived for the amount of broadening that a pulse suffers when it is subjected to the combined actions of polarization mode dispersion, chromatic dispersion, and chirping. The theory is then applied to various manifestations of second-order polarization mode dispersions.

Chirp optical coherence tomography of layered scattering media

Abstract: A new noninvasive technique that reveals cross sectional images of scattering media is presented. It is based on a continuous wave frequency modulated radar, but uses a tunable laser in the near infrared. As the full width at half maximum resolution of 16 mm is demonstrated with an external cavity laser, the chirp optical coherence tomography becomes an alternative to conventional short coherence tomography with the advantage of a simplified optical setup. The analysis of two-layer solid phantoms shows that the backscattered light gets stronger with decreasing anisotropic factor and increasing scattering coefficient, as predicted by Monte Carlo simulations. By introducing a two-phase chirp sequence, the combination of lateral resolved perfusion and depth resolved structure is shown. © 1998 Society of Photo-Optical Instrumentation Engineers. [S1083-3668(98)00503-6]

Temporal phase control of soft-x-ray harmonic emission

Abstract: We report a strong dependence of the soft-x-ray spectra generated by high harmonic emission on the chirp of the excitation pulse, when an ultrashort laser drives the process. For identical pulse durations, distinct harmonic peaks can be observed for positively chirped excitation pulses, while for negatively chirped pulses, the harmonic peaks become irregular. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. This work resolves an outstanding discrepancy between theory and experiment by demonstrating that high-order harmonic generation driven by short-duration pulses can result in distinct harmonic peaks. This work conclusively demonstrates the role of the intrinsic phase in determining harmonic emission spectra, and control this phase during the emission process.

Quantum Control of Population Transfer in Green Fluorescent Protein by Using Chirped Femtosecond Pulses

Abstract: We demonstrate that the methods of quantum control can be applied successfully to very large molecules in room temperature liquid solution. Chirped femtosecond pulses are used to excite a green fluorescent protein mutant in both buffered aqueous solution and solid acrylamide gel. At high energy densities, the fluorescence shows a strong chirp dependence, with positively chirped pulses transferring almost 50% more population to the excited state than negatively chirped pulses. By measuring the photobleaching rate in the gel as a function of pulse chirp, we find that the data are consistent with the bleaching of the protein being due to a thermal mechanism rather than to an excited-state photoreaction.

System outage probability due to first- and second-order PMD

Abstract: A theoretical approach is proposed that allows one to quantify the impact of fiber polarization mode dispersion (PMD) on optical binary transmission taking into account not only first-order polarization mode dispersion, but also signal distortion induced by second-order PMD. Using this approach the impact of the spectral signal width on PMD-induced system outage probability could be studied for the first time. An analysis of 10-Gb/s transmission exhibits that, as long as the mean PMD remains below the commonly accepted limit (about 10 ps) for negligible outage, a linear chirp of up to 30 GHz does not lead to an additional increase of the system outage. This result confirms that low bandwidth modulation schemes (external modulator, low chirp laser) do not suffer from additional outage degradation due to second-order PMD.

Efficient generation of narrow-bandwidth picosecond pulses by frequency doubling of femtosecond chirped pulses.

Abstract: We demonstrate efficient generation of picosecond narrow-bandwidth pulses by frequency mixing of broadband opposite chirped pulses in a type I doubling crystal. This procedure allows us to produce picosecond pulses that are perfectly synchronized with femtosecond pulses. The experiment shows a decrease of the initial bandwidth by a factor of more than 30, while a high conversion efficiency is maintained.

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers

Abstract: A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of ?-conjugated oligomers and semiconductor nanocrystals.

Dispersion tuning of a linearly chirped fiber Bragg grating without a center wavelength shift by applying a strain gradient

Abstract: We propose a new method for controlling the chirp of a linearly chirped fiber Bragg grating (FBG) without a center wavelength shift by using beam bending. The beam consists of a plastic sleeve enclosing a 10-cm-long chirped FBG and a metal rod. The grating pitch could be varied to give positive or negative chirp as well as zero chirp (i.e. uniform pitch) as applying displacement to one end of the beam without rotation. The dispersion at the two extremes of mechanical displacement were -791 ps/nm and +932 ps/nm. The center wavelength shift of the FBG was as small as 0.09 nm over the dispersion tuning range.

Stepped-frequency processing by reconstruction of target reflectivity spectrum

Abstract: This paper describes a processing technique for combining stepped-frequency waveforms efficiently to obtain higher range resolution. Essentially this method involves the reconstruction of a wider portion of the target's reflectivity spectrum by combining the individual spectra of the transmitted narrow-bandwidth pulses in the frequency domain. This paper describes the signal processing steps involved, and shows simulation results which validate and illustrate the method.

Chirp z-transform based SPECAN approach for phase-preserving ScanSAR image generation

Abstract: The scan mode synthetic aperture radar (ScanSAR) image impulse response is derived in the time domain, and particular attention is given to the analysis of the phase, which is important for several applications, and especially in interferometric ScanSAR systems. A new algorithm for phase-preserving azimuth focusing of ScanSAR data, that extends the basic SPECAN procedure, is presented. The proposed algorithm avoids the interpolation step needed to achieve a constant azimuth pixel spacing by replacing the standard Fourier transform used in the SPECAN procedure with an appropriate chirp z-transform. The relationship between the modified SPECAN algorithm and the standard range-Doppler approach is also discussed. Experiments on real and simulated data are carried out to validate the theory.

Chirped pulse enhancement of multiphoton absorption in molecular iodine

Abstract: Chirp effects on three photon absorption yields in I2 vapor are investigated using femtosecond pulses with center wavelengths in the region 550–600 nm. Enhancements of as much as a factor of 3 are observed for chirped pulses with respect to transform-limited, zero chirp pulses. Theoretical considerations and model calculations suggest that wave packet dynamics play an important role.

Dispersion-managed breathers with average normal dispersion.

Abstract: Analytic and numerical evidence is presented that demonstrates that a dispersion-managed breather can be supported in an optical fiber even when the average dispersion is in the normal regime. This nonlinear behavior, which is contrary to guiding-center theory, is shown to originate from the reversible dynamics associated with the strong quadratic chirp that is generated in both the anomalous and the normal dispersion regimes.

Chirped soliton interaction in strongly dispersion-managed wavelength-division-multiplexing systems.

Abstract: We theoretically analyze nonlinear interactions between chirped solitons in dispersion-managed wavelength-division-multiplexing (WDM) systems. We employ the perturbation method to evaluate frequency and chirp shifts caused by collisions among different WDM channels. It is shown that a chirped soliton suffers less frequency shift and time displacement than an ideal soliton, indicating its potential applicability for WDM systems.

Simple method for the complete characterization of an optical pulse.

Abstract: A novel and simple method to measure the amplitude and the phase of optical pulses is presented. The technique basically involves modulating the optical pulse train in a particular manner and then directly examining the resultant optical spectrum. This experimental measurement technique, which is extremely accurate and sensitive and can be implemented with an all-fiber setup, permits direct measurement of the phase of the optical signal in the frequency domain. Experimental results demonstrate the use of this measurement technique for characterizing optical pulses at 10 GHz from a gain-switched laser diode.

Quantum control of the yield of a chemical reaction

Abstract: Order of magnitude enhancement in the concerted elimination pathway leading to I2 product formation in the photodissociation reaction of CH2I2 by the use of positively chirped 312 nm femtosecond laser pulses is demonstrated. The maximum yield is found for chirps of 2400 fs2 while the minimum is found near −500 fs2. Multiphoton excitation with 624 nm pulses results in the opposite effect, where the maximum yield is found near −500 fs2. The enhancement as a function of chirp is found to depend on the wavelength and intensity of the laser pulses. These results offer new experimental evidence for quantum control of chemical reactions.

Adaptive control of pulse phase in a chirped-pulse amplifier.

Abstract: Using experimental feedback, we demonstrate that a chirped-pulse amplifier can adaptively learn to compensate for the higher-order phase dispersion that is inherent in the amplification process. A genetic algorithm-based search routine is used to repetitively update the pulse phase in a programmable pulse stretcher during a plasma breakdown experiment to maximize the magnitude of spectral blueshift. Reductions in pulse duration from 37 to 30 fs and substantially better wing structure are typically obtained as a result of the optimization.

Second-harmonic generation from regeneratively amplified femtosecond laser pulses in BBO and LBO crystals

Abstract: The spectral and temporal characteristics and optical-conversion efficiency of ∼150-fs laser pulses at 400 nm generated by second-harmonic generation (SHG) of a regeneratively amplified mode-locked Ti:sapphire laser were investigated both theoretically and experimentally. The theoretical investigation was done by taking into account cubic nonlinearity, pulse walk-off, group-velocity dispersion, Kerr nonlinearity, quadratic broadening, frequency chirping of the fundamental pulse, and higher-order nonlinear mixing such as backconversion and optical parametric processing. The experimental studies of the effects of crystal length and pumping intensity on the pulse duration, the spectrum, and the optical-conversion efficiency of the SHG were carried out in BBO and LBO crystals of various thicknesses and compared with the theory. It was found that in a non-transform-limited pulse, the most significant contribution to the temporal and spectral distortion of the ∼150-fs SHG pulses is mainly due to the chirping of the fundamental beam and self-phase modulation at high pumping intensity and long crystal length. The optimum crystal length and pumping intensity for obtaining a high optical-conversion efficiency and a pure spectrum in SHG are also calculated and experimentally investigated. It was found that a transform-limited fundamental pulse is essential to obtain a high conversion efficiency and to preserve the temporal profile of the second-harmonic pulse. It is also found that for a non-transform-limited ∼150-fs pulse, a 0.5–0.6-mm BBO crystal and a modest pumping intensity of ∼40 GW/cm2 are the most suitable for SHG.

Wavelength shift correction technique for a laser

Abstract: A wavelength shift correction system for a laser system is provided for correcting wavelength chirps. This wavelength shift correction system includes a learning algorithm (44) that learns characteristics of a wavelength chirp from a laser (34) and a computer system (46) that executes the learning algorithm and provides wavelength correction control signals based on the learned characteristics to reduce the magnitude of the wavelength shift of the present wavelength chirp and subsequent wavelength chirps.

Generation of over 140-nm-wide super-continuum from a normal dispersion fiber by using a mode-locked semiconductor laser source

Abstract: The super-continuum (SC) spectrum flatly broadened over 140-nm 10-dB bandwidth is generated by using a mode-locked semiconductor laser as a pulse source at 1550 nm. In our SC generation system, nearly Fourier-transform limited 0.9-ps pulses obtained through linear and nonlinear pulse compression are incident on a dispersion-flattened fiber having a small normal group-velocity dispersion. While the pulses pass through the normal dispersion fiber, the pulse waveform is adapted to suit the accumulation of a linear chirp, which leads to the flat and wide-band SC spectrum.

A robust ultra-broad-band wireless communication system using SAW chirped delay lines

Abstract: Design and performance of a low-cost wireless communication system for indoor and industrial environments are presented. The system is based on chirp-signal transmission to achieve a robust communication link. For the chirp expansion and compression, surface acoustic wave chirped delay lines fabricated from LiTaO/sub 3/-X112rotY are used. Center frequency, bandwidth, and chirp rate are 348.8 MHz, 80 MHz, and /spl plusmn/40 MHz//spl mu/s, respectively. An optimized square-root weighting was chosen to reduce the sidelobes of the compressed pulse to -42 dB compared to the correlation peak. The chirp filters have been deployed in a hardware demonstrator for data rates of up to 5 Mb/s. Limiting factors for the data rate according to simulations and measurements are mainly intersymbol interferences due to the time overlapping of consecutive symbols and, to a lower extent, the multipath propagation.

Optimal processing of marine high-resolution seismic reflection (Chirp) data

Abstract: Chirp frequency-modulated (FM) systems offer deterministic, repeatable source-signatures for high-resolution, normal incidence marine seismic reflection data acquisition. An optimal processing sequence for uncorrelated Chirp data is presented to demonstrate the applicability of some conventional seismic reflection algorithms to high-resolution data sets, and to emphasise the importance of a known source-signature. An improvement of greater than 60dB in the signal- to-noise ratio is realised from correlating the FM reflection data with the transmitted pulse. Interpretability of ringy deconvolved data is enhanced by the calculation of instantaneous amplitudes. The signal-to-noise ratio and lateral reflector continuity are both improved by the application of predictive filters whose effectiveness are aided by the repeatability of the Chirp source.

Doppler-sensitive active sonar pulse designs for reverberation processing

Abstract: The performance of active sonars operating in shallow water is often limited by the reverberation level (Waite, 1998). If a target is moving relative to the reverberating scatterers, it may be possible to isolate its Doppler-scaled echo from the zero-Doppler reverberation provided that the Doppler shift is greater than the bandwidth of the transmitted pulse. In practice, this generally implies the use of narrowband long-duration continuous-wave (CW) pulses. The major disadvantages of this method are the poor range resolution of such pulses leading to poor reverberation processing with low Doppler targets. To detect slowly moving or stationary targets, linear period modulation (LPM) chirps are often used in preference. Several new classes of pulse design have been proposed which theoretically provide superior reverberation processing to CW pulses by virtue of their comb-like spectra. The paper reviews the theory behind Newhall trains, sinusoidal frequency modulated pulses and geometric comb waveforms, comparing their theoretical reverberation processing gains against CW and LPM pulses using the Q-function. Experimental results from low-frequency active sonar sea trials are also presented to verify theoretical predictions.

Reflection coefficient calculation from marine high resolution seismic reflection (Chirp) data and application to an archaeological case study

Abstract: Chirp sub-bottom profilers produce high-resolution images of the near-surface. An attribute of the sea-bed reflection in chirp data are fluctuations in polarity between adjacent traces. Two models are proposed and presented to explain this: the first incorporates changes in an acoustic impedance gradient at the sea bed; the second uses changes in the thickness of the uppermost sediment layer. Mixing of adjacent traces produces a consistent polarity for the sea-bed reflector. Reflection coefficients are calculated, using amplitude information derived from single-traces, and polarity information from trace mixing, with application to a marine archaeological case study. The reflection coefficient calculated for the top of a buried 18th century wooden wreck is -0.26.