Extinction pulses synthesis for radar target discrimination using /spl beta/-splines, new E-pulse conditions

The extinction pulse method has been proven to be a suitable method for radar target discrimination using the natural resonance annihilation concept. The standard procedure for extinction pulse (E-pulse) construction is based on an expansion on subsectional polynomials. In this paper it is proposed a new formalism for E-pulse construction using /spl beta/-splines. This formalism allows the E-pulse polynomial basis expansion to be treated in a unified theoretical framework and leads to a simplification of the original problem due to the linear nature of all the involved parameters. This new formulation has been also used to impose new conditions over the spectral contents of the E-pulses. These E-pulses constructed using the new conditions annihilate better the natural modes in the late-time radar target response, and provide a better discrimination rates than the classical scheme.     

增益分布对皮秒脉冲自相似放大的影响

采用数值模拟的方法,研究了增益分布对皮秒脉冲自相似放大的影响。构建了掺镱光纤内超短激光脉冲自相似放大的理论模型,以不同的泵浦方式、光纤长度和总增益系数实现不同的增益分布,探究了不同增益分布对增益光纤中皮秒脉冲的自相似放大过程和结果的影响。结果表明,皮秒脉冲在不同的增益分布下存在最佳的自相似放大结果,可以得到近变换极限的百飞秒高质量脉冲输出。发现同一信号光脉冲在增益光纤中演化至自相似放大过程时,正向泵浦方式下的演化速度比反向泵浦快。对于不同的增益光纤长度和总增益系数,正向泵浦方式下的信号光自相似区域主要集中在低入射脉冲能量和长波长区域,反向泵浦方式下的信号光自相似区域主要集中在高入射脉冲能量和短波长区域。     

Efficient compression of high-energy laser pulses

Limitations caused by stimulated Raman scattering (SRS) for laser pulse compression in traditional fiber-grating compressors are discussed. It is shown that the scheme utilizing fibers of length L exceeding the length of the dispersion walk off L_{walk off} of pump pulses and SRS permits one to obtain high-contrast compressed pulses, their energy being no more than a few nJ, and the ultimate width being proportional to the square root of the initial pulse width. For the compression utilizing fibers of length L<L_{walk off}, the pulse energies are not limited, but the compression factor is limited to the value of 32, and the compressed pulses have a low-intensity wide pedestal. A theoretical model of high-energy pulse compression with simultaneous pedestal suppression by the polarization technique using nonlinear birefringence of the fiber is discussed. This technique is compared to the spectral windowing technique     

Tunable dual-wavelength picosecond optical pulses generated from aself-injection seeded gain-switched laser diode

A novel scheme to generate dual-wavelength picosecond optical pulses has been demonstrated. The pulses are obtained by self-injection seeding of a gain-switched laser diode simultaneously at two different wavelengths. Optical feedback is provided by a partially reflecting fiber mirror. Single-mode pulses at alternate wavelengths around 1.3 μm have been generated. The pulse width varies from about 40 to 50 ps. The output wavelengths can be tuned by changing the electrical modulation frequency. Spacing between the wavelengths can also be controlled by adjusting the length of the dispersive fiber cavity. Our experimental data are in good agreement with the calculated results     

Experimental study of single picosecond light pulses

Single picosecond light pulses from a mode-locked Nd:glass laser are investigated by several methods. Their temporal structure is studied by two-photon fluorescence. The frequency spectra are measured quantitatively. The energy distribution is simultaneously investigated by three-photon fluorescence, photoelectric measurements, and quantitative studies of the contrast ratio of the two-photon fluorescence. The pulse shape is measured using a method based on the stimulated Raman effect. It is observed that the pulses are weakly asymmetric-the pulse decay is slower (exponential) than the pulse rise (Gaussian). Bandwidth-limited pulses of 4-8 ps are present in the leading part of the pulse train. Substantial frequency broadening develops as the pulse train reaches its maximum and a subpicosecond structure is formed in the trailing part of the pulse train.     

Generation of single l-ns pulses at 10.6 µ

Single pulses of 1-ns duration have been generated at 10.6-μ wavelength using a pulse-selection scheme in conjunction with an actively mode-locked CO2TEA oscillator. The experimental setup and its operating characteristics are presented. A technique is described, which yields pulse extinction ratios of approximately 900:1.     

Transient interactions of an EM pulse with a dielectric sphericalshell

The authors study the scattering interaction of short electromagnetic pulses with a spherical target. The target is assumed penetrable and they model it as an air-filled dielectric shell. The radar cross-section (RCS) of such a target is obtained and its resonance features are analyzed. A dielectric composition makes the resonance features become very prominent compared with the case of a perfectly conducting sphere. When the interrogating waveform is a pulse of short duration, the resonance features of the RCS can be extracted within the frequency band of the spectrum of the incident pulse. To verify their theoretical predictions they illuminate spherical targets with short, broadband pulses using an impulse radar system. The actual shape of the pulse that is incident on the targets is theoretically modeled using a digital filter design technique together with pulse returns from a reference target. They verify that the shape of the predicted, backscattered pulse that results from their design method agrees well with the experimental findings using three additional targets of different sizes and materials. They investigate in the combined time-frequency domain the development in time of the various frequency features of the spectra of backscattered pulses using time-windowed Fourier transforms. The methodology developed can handle broadband pulses of any sufficiently smooth spectrum, interacting with (lossy or lossless) dielectric scatterers, and can extract resonance features within the frequency band of the spectrum of the transmitted pulse. Accordingly, this method could be also used for assessing the performance of high-power impulse radar systems     

Pulse retiming based on XPM using parabolic pulses formed in a fiber Bragg grating

We experimentally demonstrate a novel all-optical all-fiberized pulse retiming scheme incorporating parabolic pulses generated in a linear fashion through pulse shaping in a superstructured fiber Bragg grating. The scheme relies on chirping the signal to be retimed using cross-phase modulation with the broader parabolic clock pulses, and subsequently retiming it through linear propagation in a dispersive medium. We demonstrate the cancellation of up to 4-ps root-mean-square timing jitter for /spl sim/2-ps data.     

双波长皮秒脉冲与可调谐脉宽产生的实验研究

为了提高多波长光脉冲设备的实用性,采用双波长工作原理,设计了一种稳定的双波长半导体环形光纤激光器。讨论了环形腔色散、滤波器脉宽以及调制深度等对输出脉冲的影响。结果表明,双波长光脉冲的重复率为5.0GHz,典型脉宽为20ps,双波长光脉冲具有良好的边模抑制比,当输出波长为1554.75nm和1557.21nm时,边模抑制比分别为29.1dB和29.7dB,其典型的输出功率分别为-6.7dBm和-6.1dBm,输出光谱的峰值功率波动小于0.5dB。这项研究对太赫兹源和多波长光脉冲设备的研究具有借鉴意义。     

Simultaneous generation of wavelength tunable two-coloredfemtosecond soliton pulses using optical fibers

Wavelength tunable two-colored femtosecond (fs) soliton pulse generation is proposed and demonstrated for the first time, using passively mode-locked fs fiber laser and polarization maintaining fibers. The wavelengths of the two soliton pulses can be changed arbitrarily by varying the power and polarization direction of the fiber-input pulse. Ideal two colored soliton pulses in which the pulsewidths are about 200 fs are generated in the wavelength region of 1.56-1.70 μm for 110-m fiber. The generated pulses are almost transform-limited ones     

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

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 H₂ 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     

Linear and nonlinear filters suppressing UWB pulses

In order to protect electronic systems against natural or man-made electromagnetic interferences with high energies and amplitudes nonlinear protection elements like spark gaps, varistors or Z-diodes are state-of-the-art countermeasures. Most of these protection circuits are designed for well studied transient interferences like lightning electromagnetic pulse, nuclear electromagnetic pulse or electrostatic discharge pulses. It is of special interest to investigate the response of common nonlinear protection elements at ultrawideband (UWB) pulses with significant amplitudes, rise times in the picosecond range and pulse durations of a few nanoseconds. It is discussed whether traditional protection concepts provide a sufficient protection against such extremely fast pulses. Furthermore the possibility of linear filtering is presented with focus on the protection of high frequency datalines.     

A sequentially self-seeded Fabry-Perot laser for two-dimensional encoding/decoding of optical pulses

Multiwavelength optical pulse trains with variable two-dimensional (2-D) code patterns formed by a single encoder have been generated from a self-seeded gain-switched Fabry-Perot (F-P) laser for the first time. Mechanically tunable cascaded fiber Bragg gratings (FBG) are used to construct the reconfigurable encoder, which acts as a discrete nonlinear dispersive component in the subharmonically pulse-gated external cavity of the laser to generate multiwavelength pulse trains with a variable 2-D code pattern defined by the settings of the FBG string. Four distinct repetitive patterns (corresponding to four different 2-D codes) of optical pulse trains, each made up of up to four pulses generated sequentially with different wavelengths, have been produced at a repetition rate of 250 MHz. The output pulses obtained by this method are much more intense and stable than those obtained from a free-running F-P laser. Furthermore, the different pulse patterns obtained show that the scheme can allow easy switching among different 2-D codes by simply reconfiguring the FBG string. Hence this laser with the embedded reconfigurable encoder should be a viable optical source for incoherent 2-D fiber-optic code-division multiple access (FO-CDMA) applications. Near-error-free transmission of data at 250 Mbit/s employing this laser/encoder over 9.5 km standard single mode fiber has been successfully demonstrated, thus confirming its performance and viability for FO-CDMA.     

Subnanosecond pulses from a KrF laser pumped SF6Brillouin amplifier

Experimental measurements are presented of pulse compression in a backward Brillouin amplifier using SF6gas pumped by 0.9 GHz linewidth KrF laser radiation. Average pulse durations of 390 ps and energy extraction effieiencies of 40 percent from 24 ns pump pulses have been demonstrated at pressures of 15.3 and 12.6 atm, respectively.     

Design of time-limited pulses

An approach for designing time-limited transmit pulses is proposed. In the approach, a time-limited pulse is represented by a weighted sum of cosine functions whose frequencies are integer multiples of one over the time duration of the transmit pulse. The weighting coefficients are derived to minimize the mean square error of received symbols for a given symbol timing offset, while restricting the out-of-band energy to a predefined level. The minimization is numerically performed by using the Zoutendijk?s feasible direction method. Performance comparison results are presented showing that the pulses derived by this approach not only have smaller mean square errors, but also smaller symbol error probabilities for BPSK signaling compared to the time-truncated versions of conventional squareroot Nyquist pulses.     

Parametric construction of Nyquist-I pulses

A novel parametric approach for constructing families of intersymbol-interference (ISI)-free pulses is presented and examined. Some new pulses so constructed have smaller maximum distortion, a more open receiver eye, and a smaller probability of error in the presence of symbol-timing error than the Nyquist raised-cosine pulse for the same excess bandwidth. The parametric approach gives more degrees of freedom in the design of ISI-free pulses, and subsumes previous ISI-free pulses as special cases. A number of theorems that relate time-domain behaviors of a pulse to the pulse's frequency spectrum are proved. A previously known result relating pulse tail-time decay to discontinuity of the pulse-frequency spectrum is corrected and clarified.     

Chebyshev series for designing RF pulses employing an optimal control approach

Magnetic resonance imaging (MRI) provides bidimensional images with high definition and selectivity. Selective excitations are achieved applying a gradient and a radio frequency (RF) pulse simultaneously. They are modeled by the Bloch differential equation, which has no closed-form solution. Most methods for designing RF pulses are derived from approximation of this equation or are based on iterative optimization methods. The approximation methods are only valid for small tip angles and the optimization-based algorithms yield better results, but they are computationally intensive. To improve the solutions and to reduce processing time, a method for designing RF pulses using a pseudospectral approach is presented. The Bloch equation is expanded in Chebyshev series, which can be solved using a sparse linear algebraic system. The method permits three different formulations derived from the optimal control theory, minimum distance, minimum energy, or minimum time, which are solved as algebraic constrained minimization problems. The results were validated through simulated and real experiments of 90 degrees and 180 degrees RF pulses. They show improvements compared to the corresponding solutions obtained using the Shinnar-Le Roux method. The minimum time formulation produces the best performance for 180 degrees pulses, reducing the excitation length in 4% and the RF pulse energy in 3%.     

Generation of orthogonal UWB shaping pulses based on compressed chirp signal

This study investigates a novel method to numerically generate orthogonal ultrawide band (UWB) shaping pulses based on compressed chirp signal. First, a pulse template with less than 1 ns duration time, which is used to construct a Hermitian matrix, is produced with a compressed chirp pulse. Sub-nanosecond orthogonal pulses are then generated for UWB by using the Hermitian matrix eigenvectors. The simulation results show that the power spectral density distribution of the UWB shaping pulses met the constraint of Federal communications commissions (FCC) spectral mask. The shaping pulses not only have higher spectrum utilization ratio and very short time duration but also have excellent autocorrelation and cross-correlation properties, which is an advantage to reduce the interference between multiusers. Especially, a method to produce sub-nanosecond orthogonal UWB shaping pulses by using a relatively longer duration chirp signal is presented.     

Synchronous amplification of subpicosecond pulses

In high-gain dye amplifiers, the effective storage time of the gain medium is only a few hundred picoseconds. Therefore, efficient amplification of ultrashort pulses places a stringent requirement on the synchronization between the pump pulse and the pulse to be amplified. We present a technique of short pulse generation using a dye laser synchronously pumped by a frequency-doubled CW mode locked Nd:YAG laser. Pulses as short as 70 fs are produced. The short optical pulses are subsequently amplified with two different synchronous amplification schemes using 100 ps pulses to establish the gain in the dye amplifier stages. Subpicosecond pulses with energies from a few hundred nanojoules at 500 Hz to a few hundred microjoules at 7 Hz can be obtained.     

Measurement of intense ultraviolet subpicosecond pulses usingdegenerate four-wave mixing

Background-free measurement of the third-order intensity autocorrelation function of ultraviolet subpicosecond pulses is demonstrated. The method provides the pulse duration and additional information about asymmetry and other interesting structural properties of the pulses. It is possible to obtain the complete autocorrelation function from a single pulse. Measurements of subpicosecond pulses from XeCl excimer laser amplifiers reveal a distinct satellite structure, which can be attributed to a quantum beat effect of the XeCl lasing transitions