Showing papers on "Pulse compression published in 1998"

Dynamic spatial replenishment of femtosecond pulses propagating in air

Abstract: We present numerical simulations of nonlinear pulse propagation in air whereby an initial pulse is formed, absorbed by plasma generation, and subsequently replenished by power from the trailing edge of the pulse. This process can occur more than once for high-power input pulses and produce the illusion of long-distance propagation of one self-guided pulse.

Pulse-front-matched optical parametric amplification for sub-10-fs pulse generation tunable in the visible and near infrared

Abstract: A noncollinear optical parametric amplifier is presented that generates transform-limited sub-10-fs pulses that are tunable in both the visible and the near infrared (NIR).?The pulse-front-matched pump geometry realizes tilt-free signal amplification, and pulses as short as 6.1 fs can be obtained from 550 to 700 nm. The large angular dispersion of the idler specific to the group-velocity-matching interaction is effectively eliminated by a grating–telescope compensator, and 9-fs NIR pulses are also successfully obtained from 900 to 1300 nm. This is believed to be the first tunable sub-10-fs light source.

Design, performance, and applications of a coherent ultra-wideband random noise radar

Abstract: Ram M. NarayananYi XuPaul D. HoffmeyerUniversity of Nebraska—LincolnCenter for Electro-OpticsDepartment of Electrical EngineeringLincoln, Nebraska 68588-0511E-mail: rnarayanan@unl.eduJohn O. CurtisU.S. Army Waterways Experiment StationEnvironmental LaboratoryVicksburg, Mississippi 39180-6199Abstract. A novel coherent ultra-wideband radar system operating inthe 1- to 2-GHz frequency range has been developed recently at theUniversity of Nebraska. The radar system transmits white Gaussiannoise. Detection and localization of buried objects is accomplished bycorrelating the reflected waveform with a time-delayed replica of thetransmitted waveform. Broadband dual-polarized log-periodic antennasare used for transmission and reception. A unique signal-processingscheme is used to inject coherence into the system by frequency trans-lation of the ultrawideband signal by a coherent 160-MHz phase-lockedsource prior to performing heterodyne correlation. The system coher-ence allows the extraction of a target’s polarimetric amplitude and phasecharacteristics. This paper describes the unique design features of theradar system, develops the theoretical foundations of noise polarimetry,provides experimental evidence of the polarimetric and resolution capa-bilities of the system, and demonstrates results obtained in subsurfaceprobing applications.

Pulse elongation and deconvolution filtering for medical ultrasonic imaging

Abstract: Range sidelobe artifacts which are associated with pulse compression methods can be reduced with a new method composed of pulse elongation and deconvolution (PED). While pulse compression and PED yield similar signal-to-noise ratio (SNR) improvements, PED inherently minimizes the range sidelobe artifacts. The deconvolution is implemented as a stabilized inverse filter. With proper selection of the excitation waveform an exact inverse filter can be implemented. The excitation waveform is optimized in a minimum mean square error (MMSE) sense. An analytical expression for the power spectrum of the optimal pulse is presented and several techniques to numerically optimize the excitation pulse are shown. The effects of PED are demonstrated in computer simulations as well as ultrasonic images.

Pulse compression for weather radars

Abstract: Wideband waveform techniques, such as pulse compression, allow for accurate weather radar measurements in a short data acquisition time. However, for extended targets such as precipitation systems, range sidelobes mask and corrupt observations of weak phenomena occurring near areas of strong echoes. Therefore, sidelobe suppression is extremely important in precisely determining the echo scattering region. A simulation procedure has been developed to accurately describe the signal returns from distributed weather targets, with pulse compression; waveform coding. This procedure is unique and improves on earlier work by taking into account the effect of target reshuffling during the pulse propagation time which is especially important for long duration pulses. The simulation procedure is capable of generating time series from various input range profiles of reflectivity, mean velocity, spectrum width, and SNR. Results from the simulation are used to evaluate the performance of phase coded pulse compression in conjunction with matched and inverse compression filters. The evaluation is based on comparative analysis of the integrated sidelobe level and Doppler sensitivity after the compression process. Pulse compression data from the CSU-CHILL radar is analyzed. The results from simulation and the data analysis show that pulse-compression techniques indeed provide a viable option for faster scanning rates while still retaining good accuracy in the estimates of various parameters that can be measured using a pulsed-Doppler radar. Also, it is established that with suitable sidelobe suppression filters, the range-time sidelobes can be suppressed to levels that are acceptable for operational and research applications.

High pulse rate pulse power system

Abstract: A high pulse rate pulse power source (20) for supplying controlled high energy electrical pulses at rates of 2000 Hz or greater. The source (20) includes a pulse generating circuit (30) including a charging capacitor (42), a solid state switch (46) and a current limiting inductor (48). Pulses generated in the pulse generating circuit (30) are compressed in at least two pulse compression circuits (61, 65) and a step-up pulse transformer (56) increases peak voltage to at least 12,000 volts. A very fast regulated power supply is provided for charging the charging capacitor (42) in less than 400 microseconds and a pulse control system including a programmed processor (102) controls the charging of the charging capacitor (42) to an accuracy of less than about one percent at a rate of at least 2000 charges per second.

325 nm bandwidth supercontinuum generation at 10 Gbit/s using dispersion-flattened and non-decreasing normal dispersion fibre with pulse compression technique

Abstract: 325 nm bandwidth (at 20 dB) supercontinuum generation at 10 Gbit/s using dispersion-flattened uniform normal dispersion fibre along with a pulse compression technique is experimentally demonstrated with a pumped 3.8 ps, 1552 nm hybrid modelocked semiconductor laser. Application to a 50 nm wavelength tunable source is also demonstrated.

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 novel pulse TOA analysis technique for radar identification

Abstract: A new processing technique is presented for identifying individual pulse trains in a buffer load of pulse time-of-arrival (TOA) data from radar warning receiver sets currently in use for electronic warfare (EW). The method is based on the numerical characteristics of the matrix of the differences of the TOA data points. In particular, for a uniform pulse repetition interval (PRI) emitter, this matrix is a symmetrical harmonic Toeplitz matrix the inverse of which has banded tridiagonal form. This form leads to an estimator for the PRI in terms of the trace of the inverted matrix.

Subfemtosecond pulse generation by molecular modulation

Abstract: This Letter suggests and analyzes a technique for producing subfemtosecond pulses of radiation. We will show that two laser beams whose frequency difference is slightly offset from a molecular transition will, for an appropriate choice of gas pressure and cell length, generate a spectrum of Raman sidebands whose Fourier transform is a periodic train of subfemtosecond pulses. The essence of the technique is the concurrent generation of a frequency modulated (FM) waveform and the use of group velocity dispersion to temporally compress this waveform. By numerically modeling this process in molecular deuterium sD2d, we calculate a generated train of pulses with a pulse spacing of 11.1 fs and a pulse length of 0.21 fs. The coherence rab (Fig. 1) of the driven molecular transition is central to this work. This coherence is established by driving the molecular transition slightly off resonance with driving lasers of sufficient intensity that the product of their Rabi frequencies exceeds the product of the detuning from the molecular electronic states and the detuning from the Raman transition. With the linewidth of the applied laser pulses small as compared to the Raman detuning, the magnitude of the molecular coherence approaches 0.5, and its sign is determined by the sign of the Raman detuning. For the conditions of the previous paragraph, the generation and phase-slip lengths are of the same order, and Raman sideband generation proceeds collinearly and very differently than in the conventional low coherence regime. In essence, the molecular motion now modulates the electronic refractive index much in the same way that a low frequency electric field modulates the refractive index of a polar crystal. Incident optical frequencies become frequency modulated with peak (sinusoidal) frequency deviations and spectral bandwidths that substantially exceed the width of the visible spectrum. This allows the extension of compression techniques, such as the use of group velocity dispersion as described here, to a new regime of short pulse compression. In pertinent prior work, (1) the authors have noted how an electromagnetically induced transparency (EIT)-like excitation may be used to generate a broad comb of coherent sidebands, but the Bessel-function nature of the spectrum and the possibility of pulse compression were not

Nonlinear source for the generation of high-energy few-cycle optical pulses

Abstract: We investigate the evolution of optical pulses in a hollow waveguide filled with noble gas at pulse intensities for which tunnel ionization dominates the nonlinear response of the gas. A numerical analysis reveals that the spectral chirp generated by the plasma nonlinearity is to a good approximation linear over the whole pulse spectrum and can be compensated in a dispersive delay line. Our calculations predict the generation of 3–4-fs optical pulses with energies of a few milijoules. To our knowledge, these energies are an order of magnitude greater than the pulse energies that have been realized to date in hollow-fiber compressors based exclusively on the nonlinear Kerr effect.

High-quality laser diode pulse compression in dispersion-imbalanced loop mirror

Abstract: High-quality femtosecond pulse generation from a 1.5 µm gain-switched DFB laser diode in a compact nonlinear compressor based on a dispersion-imbalanced fibre loop mirror is reported. An overall compression ratio of 80 is achieved, resulting in generation of near-transform-limited pulses with negligible tails and a duration of 270 fs.

High pulse rate pulse power system with resonant power supply

Abstract: A high pulse rate pulse power source for supplying controlled high energy electrical pulses at rates of 2000 Hz or greater. The source includes a pulse generating circuit including a charging capacitor, a solid state switch and a current limiting inductor. Pulses generated in the pulse generating circuit are compressed in at least two pulse compression circuits and a step-up pulse transformer increases peak voltage to at least 12,000 volts. A very fast regulated resonant power supply is provided for charging the charging capacitor in less than 400 microseconds and a pulse control system including a programmed processor controls the charging of the charging capacitor to an accuracy of less than about one percent at a rate of at least 2000 charges per second.

High-power 100-fs pulse generation by frequency doubling of an erbium–ytterbium-fiber master oscillator power amplifier

Abstract: Frequency doubling of an erbium-ytterbium-fiber master-oscillator-power-amplifier system is demonstrated. Simultaneous amplification and pulse compression in multimode erbium-ytterbium-doped fibers produces high-quality near-diffraction and near-bandwidth-limited 100-fs pulses at a wavelength of 1.62microm with an average power of 230 mW at a repetition rate of 52 MHz. Periodically poled LiNbO(3) allows for frequency doubling with a conversion efficiency of 51%, producing near-bandwidth-limited 105-fs pulses with an average power of 117 mW and a pulse energy of 2.3 nJ at a wavelength of 810 nm.

Practical spread spectrum pulse compression for ultrasonic tissue imaging

Abstract: Spread spectrum pulse compression is a signal processing algorithm that enhances critical system performance parameters such as signal-to-noise ratio, peak power requirements, minimum detectable signal, and total dynamic range. For this research, a digital, real-time, Barker coded, bi-phase modulator was designed and constructed, as well as a simple ultrasonic test tank containing both synthetic targets and excised goat's liver. Upon reception and demodulation of the spread spectrum ultrasonic echo, cross-correlation with a sidelobe suppression filter was performed. Due to limitations such as narrow bandwidth, and very short minimum ranges, a practical ultrasonic pulse compression system must be restricted to short code lengths. For 13 bit Barker code compression, the expected increase in signal-to-noise ratio of 11 dB was realized; at the same time greater than 30 dB of instantaneous dynamic range was maintained.

Pulse wave detecting device and pulse measurer

Abstract: The present invention relates to a pulse wave detecting device for detecting pulse waves, and to a pulse measurer employing this pulse wave detecting device. The present invention addresses the problem of obtaining a pulse wave signal in which the noise components have been suitably removed from a pulse waveform, and of determining the pulse rate with high accuracy based on this pulse wave signal. The method for deriving the pulse wave signal and pulse rate is as follows. The pulse wave signal from pulse wave detecting sensor unit (30) is temporarily stored in buffer (503). When impulse noise is detected in the pulse wave signal in buffer (503) by impulse noise detecting means (505), the band pass for first digital filter (506) becomes a hill-shaped curve centered on the frequency corresponding to the preceding pulse rate, and impulse noise in the pulse wave signal output from buffer (503) is decreased. Thereafter, overall noise and body movement components are decreased in the pulse wave signal by means of second digital filter (507) and third digital filter (508). The signal is then subjected to frequency analysis by frequency analyzer (509), and the pulse rate is calculated from the results of this analysis.

Method and apparatus for color flow imaging using coded excitation with single codes

Abstract: A color flow imaging technique uses coded excitation on transmit and pulse compression on receive. Coded excitation allows a long transmit pulse to be compressed on receive such that most energy is concentrated in a short interval. Multiple transmit firings of the same coded pulse sequence are focused at the same transmit focal position with the same transmit characteristics. The receive signals are compressed utilizing matched or mismatched filtering. These techniques can be used to maximize color flow sensitivity in deep-lying regions. Alternatively, for a given transmit acoustic burst length and dosage, the spatial resolution can be improved without compromising sensitivity.

Adiabatic Bragg soliton compression in nonuniform grating structures

Abstract: Adiabatic soliton compression is an attractive optical pulse compression scheme that relies on monotonic decreasing of the fiber dispersion along the soliton’s propagation path. This scheme requires kilometers of specialty fiber, and only a few dispersion profiles are practically feasible. We propose adiabatic soliton compression in a nonuniform fiber Bragg grating. The Bragg soliton propagates in the passband of a grating, where one or more of its parameters (e.g., the grating pitch) varies along the propagation direction. The capability of manufacturing almost arbitrary grating profiles and hence engineering practically any dispersion profile makes this all-fiber pulse compressor, which is to our knowledge novel, a very versatile component. Additionally, the large dispersion in the spectral vicinity of the grating stop band leads to very short devices (tens of centimeters as opposed to kilometers).

Pulse compression using coupled-waveguide structures as highly dispersive elements

Abstract: Pulse compression is experimentally demonstrated by using a coupled-waveguide structure consisting of an InGaAsP/InP semiconductor as a highly dispersive element. In this structure, waveguide dispersion associated with supermodes is significantly enhanced. The maximum temporal compression factor was 4.3 (compression from 2.2 to 0.514 ps). This result indicates that the coupled-waveguide structure has the potential for use in a simple, compact, transmissive, and sign-free dispersion compensator.

Long-span repeaterless transmission systems with optical amplifiers using pulse width management

Abstract: This paper proposes pulse width management in order to extend the repeater spacings of repeaterless transmission systems with optical amplifiers. First, the dependency of receiver sensitivity on duty ratio, receiver response, and fiber dispersion is clarified by numerical analysis. Next, the calculation results of sensitivity as a function of signal format and receiver basedband response are verified experimentally. Moreover, we show that pulse width management which uses return-to-zero (RZ) format with large duty ratio (/spl sim/0.7) at the transmitter and pulse compression at the receiver increases the repeater gain by /spl sim/4.5 dB compared to conventional systems employing nonreturn-to-zero (NRZ) format. Record repeater spacing of 300 km is realized at 10 Gb/s by utilizing pulse width management.

Generation of 10 GHz, 200 fs Fourier-transform-limited optical pulse train from modelocked semiconductor laser at 1.55 [micro sign]m by pulse compression using dispersion-flattened fibre with normal group-velocity dispersion

Abstract: The authors report the generation of 10 GHz, 200 fs Fourier-transform-limited optical pulses at 1.55 /spl mu/m. The 3 dB spectral width of 1.7 ps optical pulses generated from a modelocked semiconductor laser is broadened from 1.8 to 22 nm, while the pulses travel through a dispersion-flattened fibre with a small normal group-velocity dispersion. Linear up-chirp is induced in the spectral broadening process, and the up-chirped pulses are compressed down to 200 fs by using a proper amount of anomalous dispersion of optical fibres. The compressed pulses are almost Fourier-transform limited, and the compression factor is as high as 8.5.

Meteorological radar apparatus

Abstract: A meteorological radar apparatus which calculates a shift of the pulse synchronization of a transmission pulse signal output from a transmission unit and corrects the transmission timing of the transmission pulse signal based on the shift of the pulse synchronization so that the Doppler velocity of a reference target becomes zero, thereby preventing deterioration in the measurement accuracy of the Doppler velocity caused by the shift of the pulse synchronization of the transmission pulse signal

Cross phase modulation squeezing in optical fibers.

Abstract: A novel method of producing squeezed vacuum uses cross phase modulation between a linearly polarized pump signal and the orthogonal polarized vacuum. Here we report on such cross phase modulation using 1-nJ 150-fs pulses from a low noise stretched pulse laser. The nonlinear medium was a single mode fiber and the noise reduction was 3 dB.

Characterization of 1.55-μm pulses from a self-seeded gain-switched Fabry-Perot laser diode using frequency-resolved optical gating

Abstract: The intensity and frequency chirp of picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode have been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemode suppression ratio (SMSR) range of 15-35 dB show that higher SMSR's are associated with an increasingly linear frequency chirp across the output pulses. This complete pulse characterization allows the conditions for optimum pulse compression to be determined accurately, and indicates that transform-limited, pedestal free pulses can be obtained at an SMSR of 35 dB.

Compression of optical pulses spectrally broadened by self-phase modulation with a fiber bragg grating in transmission.

Abstract: We demonstrate experimentally the compression of optical pulses, spectrally broadened by self-phase modulation occurring in the rod of a mode-locked Q-switched YLF laser, with an unchirped, apodized fiber Bragg grating in transmission. The compression is due to the strong dispersion of the Bragg grating at frequencies close to the edge of the photonic bandgap, in the passband, where the transmission is high. With the systems investigated, an 80-ps pulse, which is spectrally broadened, owing to self-phase modulation, with a peak nonlinear phase shift of ΔΦ = 7, is compressed to approximately 15 ps, in good agreement with theory and numerical simulations. The results demonstrate that photonic bandgap structures are promising devices for efficient pulse compression.

Global optimization of pulse compression in chirped pulse amplification

Abstract: We show theoretically that a global minimization of the spectral phase modulation leads to short pulses over a broad range of compressor parameters in chirped pulse amplification. The best way to achieve the optimization is debated. In summary, we have shown that a given compressor can be optimized by several means. Depending on the criteria used to optimize the compressor, the final pulse shape may differ slightly. In any case, an exact cancellation of the lower dispersion terms always gives the poorest results and one must definitely think in terms of global optimization of the residual phase modulation.

Fourier-transform-limited laser pulses tunable in wavelength and in duration (400-2000 ps)

Abstract: A combined system of an injection-seeded pulsed dye amplifier and a pulse compressor based on stimulated Brillouin scattering (SBS) is investigated. It allows for the generation of powerful pulses both tunable in wavelength and in duration. Online tuning of the pulse duration is possible due to the dependence of SBS compression on input energy. A range of 400-2000 ps at up to 100 mJ output energy is demonstrated. The output pulses are temporally and spectrally resolved to investigate the properties of this system. Coherent nearly Fourier-transform-limited pulses of variable pulse duration in the extreme ultraviolet (UV) are produced employing harmonic conversion. As an application of such pulses a single rotational line of H/sub 2/ at 98-nm wavelength is excited, demonstrating that the system may be used for laser-spectroscopic studies to simultaneously gain spectral as well as dynamical information.

Improvement of ultrasonic testing of concrete by combining signal conditioning methods, scanning laser vibrometer and space averaging techniques

Abstract: This paper deals with the combination of several techniques to improve the ultrasonic pulse echo testing of concrete elements when a laser doppler interferometer is used as the ultrasonic receiver. This techniques involves specially designed ultrasonic probes, a pulse compression technique, random speckle modulation and space time signal processing methods. The pulse echo technique is carried out by sending frequency modulated chirp signals and performing a cross-correlation between the received and the transmitted signals. In combination with the application of recently available ultrasonic concrete probes as transmitter, this leads to an improvement in the signal-to-noise ratio. A laser doppler interferometer, equipped with a random speckle modulator, is used as detector of the ultrasound. Finally, the data sets are processed with various methods, involving time signals of several space points. Examples are the space averaging and the synthetic aperture focusing technique (SAFT). The advantage of the suggested technique is demonstrated by practical measurements on a test specimen. The improvement in results as compared to standard laser interferometric measurements will increase the feasibility of laser interferometric detection for non-destructive testing in civil engineering.