Optimal Pulse Compression Research Articles

Evaluation of contrast enhancement ultrasound images of Sonazoid microbubbles in tissue-mimicking phantom obtained by optimal Golay pulse compression

In contrast enhancement ultrasound (CEUS), the vasculature image can be formed from nonlinear echoes arising from microbubbles in a blood flow. The use of binary-coded pulse compression is promising for improving the contrast of CEUS images by suppressing background noise. However, the amplitudes of nonlinear echoes can be reduced, and sidelobes by nonlinear echoes can occur depending on the binary code. Optimal Golay codes with slight nonlinear-echo reduction and nonlinear sidelobe have been proposed. In this study, CEUS images obtained by optimal Golay pulse compression are evaluated through experiments using Sonazoid microbubbles flowing in a tissue-mimicking phantom.

Group-velocity dispersion emulator using a time lens.

We report a novel method to continuously track the temporal evolution of an arbitrary complex waveform as it propagates through a group-velocity dispersion medium by using a single-frequency-driven phase modulator. The proposed method exploits the fact that the frequency spectrum of a given (input) waveform, following a suitable sinusoidal temporal phase modulation, exhibits the same shape as that of a dispersed version of the same temporal waveform after propagation through a prescribed amount of dispersion. In experiments, we track the dispersion-induced temporal evolution of different optical picosecond pulsed waveforms by tuning the frequency and/or amplitude of the phase modulation signal and observing the resulting shapes in the optical frequency domain. A good agreement is obtained between the measured spectra and predicted temporal shapes of the propagating waveform for different amounts of dispersion. Moreover, the method is successfully applied on a chirped optical pulse to find the optimal pulse compression conditions.

Improved multiphoton imaging in biological samples by using variable pulse compression and wavefront assessment

A variable prism-pair-based pulse compressor with wavefront aberration sensing was used to enhance multiphoton imaging in biomedical samples. This was incorporated into a custom-made microscope to reduce pulse temporal length, improve the quality of images of different layers of thick tissues and increase penetration depth. The laser beam aberrations were found to hardly change with the different experimental configurations of the pulse compressor. The optimum pulse compression state was maintained with depth within the tissue, independently of its thickness. This suggests that for each sample, a single experimental configuration is able to provide the best possible image at any depth location; although this needs to be experimentally obtained. Furthermore, a simple method based on laser average power reduction is presented to minimize the risk of photo-damage in biological samples. The use of pulse compression in multiphoton microscopy might have a potential for accurate and improved biomedical imaging.

Application of P4 Polyphase codes pulse compression method to air-coupled ultrasonic testing systems

Air-coupled ultrasonic testing systems are usually restricted by low signal-to-noise ratios (SNR). The use of pulse compression techniques based on P4 Polyphase codes can improve the ultrasound SNR. This type of codes can generate higher Peak Side Lobe (PSL) ratio and lower noise of compressed signal. This paper proposes the use of P4 Polyphase sequences to code ultrasound with a NDT system based on air-coupled piezoelectric transducer. Furthermore, the principle of selecting parameters of P4 Polyphase sequence for obtaining optimal pulse compression effect is also studied. Successful results are presented in molded composite material. A hybrid signal processing method for improvement in SNR up to 12.11dB and in time domain resolution about 35% are achieved when compared with conventional pulse compression technique.

Ambiguity functions, processing gains, and Cramer-Rao bounds for matched illumination radar signals

Ambiguity functions (AFs) are obtained for a radar using matched illumination (MI) transmit signals for the detection of range-spread targets in the presence of clutter. The transmit signals are adapted to target and interference spectra and are filtered optimally in the receiver; they are designed to maximize signal-to-interference-and-noise ratio (SINR) power ratios at receiver output. In this paper, expressions for processing gains, spread AFs, and Cramer-Rao bounds on range and Doppler estimates are derived based on likelihood functions. Here, for extended targets, AFs resulting from using optimal MI constant envelope waveforms obtained via phase retrieval techniques demonstrate superior resolution characteristics compared with classic linear frequency-modulated (LFM) signals employing optimal pulse compression. For various target and clutter spectral models, simulation results show that optimally filtered MI signals provide significantly enhanced SINR behavior compared with LFM radar signals. Hopefully, these results set the stage for the induction of MI signaling and receive techniques into conventional radar signal processing and pave the way for realization of one methodology for achieving cognition in radar systems.

Few-cycle fiber pulse compression and evolution of negative resonant radiation

We present experimental observations of the spectral expansion of fs-pulses compressing in optical fibers. Using the input pulse frequency chirp we are able to scan through the pulse compression spectra and observe in detail the emergence of negative-frequency resonant radiation (NRR), a recently discovered pulse instability coupling to negative frequencies. We observe how the compressing pulse is exciting NRR as long as it overlaps spectrally with the resonant frequency. Furthermore, we observe that optimal pulse compression can be achieved at an optimal input chirp and for an optimal fiber length. Our measurements are supported by simulations of the compressing pulse propagation. The results are important for Kerr-effect pulse compressors, to generate novel light sources, as well as for the observation of quantum vacuum radiation.

Femtosecond laser pulse compression using angle of incidence optimization of chirped mirrors

Optimal pulse compression following spectral broadening in a noble gas-filled hollow core fiber is a critical step towards producing isolated attosecond pulses. Here we present a systematic pulse shaping method based on the optimal selection of chirped mirrors and on the optimization of their angle of incidence. Feedback from second harmonic frequency resolved optical gating measurements is used to compute the optimal chirped mirror configuration of the next iteration in a genetic algorithm. Sub-5 fs pulses were achieved after a few iterations.

Optimal compression in synchronised time‐lens source for CRS imaging

An optimal pulse compression scheme is proposed for the synchronised time-lens source for coherent Raman scattering (CRS) imaging, using CRS signals as the criterion. The simulation results show that instead of compressing the pulse width of the time-lens source to its minimum, peak power should be optimised to maximise CRS signals. In terms of experiment, this can be done by optimising the second-harmonic signal or two-photon current of the time-lens output.

Filamentation-assisted self-compression of subpetawatt laser pulses to relativistic-intensity subcycle field waveforms

Filamentation-assisted spatiotemporal dynamics of ultrashort laser pulses in the regime of extreme light powers is shown to enable self-compression of subpetawatt laser pulses to relativistic field intensities and subcycle pulse widths. Our supercomputer simulations demonstrate compression of 6-J, 30-fs laser pulses at a central wavelength of 800 nm to 1.3-fs sub-100-TW broadband field waveforms and reveal the generation of relativistic-intensity subfemtosecond field transients as a result of such a pulse evolution scenario, with multiple filamentation avoided due to low gas pressures and the balance between Kerr and ionization nonlinearities steered toward optimal pulse compression due to the depletion of outer-shell ionization in a high-intensity laser field.

VLSI-based Real-time Signal Processing Solution Employing Four-phase Codes for Spread-spectrum Applications

Spread-Spectrum applications require a set of sequences with individually peaky autocorrelation and pair-wise cross correlation. Obtaining such sequences is a combinatorial problem. If the autocorrelation and cross correlation are taken in the aperiodic sense, then there are hardly any theoretical aids available. Thus, the problem of signal design referred to above is a challenging problem for which many global optimization algorithms like genetic algorithm, simulated annealing, and tunneling algorithm were reported in the literature. The paper aims at implementation of an efficient VLSI System for the design of an optimal pulse compression codes useful for Spread-Spectrum applications. The proposed VLSI system implements the modified genetic algorithm for identifying and generating the optimal pulse compression codes. The VLSI System is implemented on the field programmable gate array as it provides the flexibility of reconfigurability and reprogramability and it is a real-time signal-processing solution which identifies and generates optimal sequences.

Optimal pulse compression in long hollow fibers

The spectral broadening performance of 1 m and 3 m long hollow fibers are compared. The 3 m capillary clearly outperforms the 1 m one in terms of both transmission and achievable spectral broadening. Starting from 1.1 mJ 71 fs pulses at 780 nm, a spectral broadening ratio of 26 was achieved using a single 3 m long argon-filled hollow fiber. After compression the measured pulse duration was 4.5 fs corresponding to a compression ratio of 16 at an energy of 0.42 mJ. Both the pulse duration and the pulse energy were limited by the applied chirped mirrors.

Self-similar propagation and compression of chirped self-similar waves in asymmetric twin-core fibers with nonlinear gain

Ultrashort-pulse propagation in asymmetric twin-core fiber amplifiers is studied with the aid of self-similarity analysis of the nonlinear Schrödinger-type equation interacting with a source, variable dispersion, variable Kerr nonlinearity, variable gain or loss, and nonlinear gain. Exact chirped pulses that can propagate self-similarly subject to simple scaling rules of this model have been found. It is reported that the pulse position of these chirped pulses can be precisely piloted by appropriately tailoring the dispersion profile. This fact is profitably exploited to achieve optimal pulse compression of these chirped self-similar solutions.

Microwave pulse compression using a helically corrugated waveguide

Summary form only given. There has been a drive in recent years to produce ultra-high power short pulses for a range of applications. These high power pulses can be produced by microwave pulse compression. Sweep-frequency based microwave pulse compression using smooth bore hollow waveguides is one technique of pulse compression, however at very high powers this method has some limitation due to its operation close to cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cutoff frequency. If a Cherenkov type TWT is used to drive the compressor then it is inevitable that the low-frequency part of the amplification band is below the cutoff which then reflected from the compressor back to the amplifier resulting in its possible parasitic self-oscillation. If a relativistic BWO is used as a source of frequency-modulated pulses for a smooth-waveguide compressor, then the necessary frequency sweep that would be required can only be produced by using a difficult-to-realize voltage modulation on the BWO. The two combined problems of wave reflection from a compressor and optimum frequency modulation can be solved using a waveguide with a special helical corrugation of its inner surface. A special helical corrugation of a circular waveguide ensures an eigenwave having a strongly frequency-dependent group velocity far from cut-off, which makes the helically corrugated waveguide attractive for using as a compressor of pulses from very high power amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a PIC code are presented. In the experiments a 70 ns 1 kW pulse from a conventional TWT was compressed in a 2 metre long helical waveguide. The compressed pulse had a peak power of 10.9 kW and duration of 3 ns. In order to find the optimum pulse compression ratio the waveguide's dispersion characteristics must be well known. The dispersion of the helix was calculated using the PIC code MAGIC and verified using an experimental technique. For a helical compressor the optimum negative frequency sweep can quite naturally be realized at the falling edge of a relativistic BWO pulse. For example, if a BWO generates hundreds of MW during 50 ns over a voltage drop from 700 kV to 400 kV, this frequency modulated radiation can be compressed up to 10-20 times in power with efficiency higher than 50%. Future work detailing plans to produce short, ultra-high power (multi-GW) pulses discussed.

PULSE COMPRESSION USING A PERIODICALLY DIELECTRIC LOADED DISPERSIVE WAVEGUIDE

The study of periodically dielectric-slab-loaded ���¸ 10 waveguide structures with conductive walls and finite length is carried out by using wave propagation techniques. Based on this analysis, the geometrical dimensions and permittivity values are determined, in order to provide very high dispersion with low losses. Non-linear frequency modulated waves, incident to a finite length periodic loaded waveguide, are studied, to achieve optimum pulse compression, by taking into account all wave phenomena involved. A staggered-tapered structure of dielectric slabs inside the waveguide is utilized, by matching the incident waves, in order to minimize the reflected (at the input) and maximize the transmitted (at the output) waves of the compressor structure. The slabs longitudinal discontinuity nature prevents the appearance of field singularity points that could hinder the operation of the compression mechanism. Exact Fourier analysis is carried out to compute the compressed wave field intensities. Optimization techniques are used to achieve the best compression and matching conditions for various realistic dielectric materials, having permittivities �0 r in the range of 9 to 36 and loss factors tan(�/ ) in the range of 0.01 to 0.0001. Experimental results obtained by carrying out measurements on prototype waveguide structures, built in our laboratory, show good agreement with theory. The structure consists of an orthogonal cross section metallic wall waveguide, loaded with low loss dielectric plates, perpendicular to the propagation z-axis. The selection of this rather simple structure was motivated by its ability to be easily constructed. The relative dielectric permittivities of the interfacing layers are denoted by �0 r1 and �0 ro=1 (air), while at the input and output of the structure matching transformers are assumed, consisting of a finite number of non-periodic layers, as shown in fig. 1. The structure on both sides is considered to be connected to a free space infinite length waveguide. The proposed structure could also be considered as an one dimensional photonic crystal structure.

Tuning of laser pulse shapes in grating-based compressors for optimal electron acceleration in plasmas.

The temporal shape (rise time, fall time, skewness) of 50-200-fs Ti:sapphire laser pulses has been controlled by appropriate adjustment of a grating-pair compressor. It was found that the skewness of the laser pulse envelope is particularly sensitive to the third-order component of the spectral phase. Introducing such a third-order phase offset by detuning the grating pair relative to the optimum pulse compression settings allowed the generation of skewed pulses. As an example of an application, these skewed pulses were used to optimize a laser-plasma electron accelerator.

Controlled supercontinuum generation for optimal pulse compression: a time-warp analysis of nonlinear propagation of ultra-broad-band pulses

We describe the virtues of the pump-probe approach for controlled supercontinuum generation in nonlinear media, using the example of pulse compression by cross-phase modulation in dielectrics. Optimization of a strong (pump) pulse and a weak (probe) pulse at the input into the medium opens the route to effective control of the supercontinuum phases at the output. We present an approximate semi-analytical approach which describes nonlinear transformation of the input pulse into the output pulse. It shows how the input and the output chirps are connected via a time-warp transformation which is almost independent of the shape of the probe pulse. We then show how this transformation can be used to optimize the supercontinuum generation to produce nearly single-cycle pulses tunable from mid-infrared to ultraviolet.

Enhanced pulse compression induced by the interaction between the third-order dispersion and the cross-phase modulation in birefringent fibres

In this paper, we report on the enhanced pulse compression due to the interaction between the positive third-order dispersion (TOD) and the nonlinear effect (cross-phase modulation effect) in birefringent fibres. Polarization soliton compression along the slow axis can be enhanced in a birefringent fibre with positive third-order dispersion, while the polarization soliton compression along the fast axis can be enhanced in the fibre with negative third-order dispersion. Moreover, there is an optimal third-order dispersion parameter for obtaining the optimal pulse compression. Redshifted initial chirp is helpful to the pulse compression, while blueshifted chirp is detrimental to the pulse compression. There is also an optimal chirp parameter to reach maximum pulse compression. The optimal pulse compression for TOD parameters under different N-order solitons is also found.

Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 μm

We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-µm wavelength range with pulse durations as short as 50 fs at 5 µm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.

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

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.

Microwave pulse compression using diffraction gratings

Electromagnetic pulse compressors employing gratings of high breakdown strength in the autocollimation regime at the center frequency of the pulse for p polarization are investigated theoretically. A theory is devised on the basis of a numerical solution of the electromagnetic field equations in an integral-equation formulation. The case of the aberrationless approximation of the compression of a bell-shaped pulse with a quadratically modulated phase is considered analytically. A numerical experiment demonstrates the presence of strong distortions in the pulse after compression, which are caused by the cubic and higher terms in the expansion of the frequency dependence of the phase acquired by the wave in the compressor. The conditions for optimal pulse compression with allowance for aberrations are investigated.