Showing papers on "Pulse compression published in 1997"

Adaptive femtosecond pulse compression.

Abstract: A practical adaptive method for femtosecond optical pulse compression is demonstrated experimentally for the first time to our knowledge. The method is robust and capable of handling the general case of pulse compression, in which the input pulses are completely uncharacterized or partially characterized.

Optical pulse compression to 5 fs at a 1-MHz repetition rate.

Abstract: We report on the characterization and compression of the white-light continuum produced by injection of a 13-fs pulse from a cavity-dumped self-mode-locked Ti:sapphire laser into a single-mode fiber. Pulses as short as 5 fs were generated at repetition rates up to 1 MHz.

Apparatus and method for the generation of high-power femtosecond pulses from a fiber amplifier

Abstract: An apparatus generates femtosecond pulses from laser amplifiers by nonlinear frequency conversion. The implementation of nonlinear frequency-conversion allows the design of highly nonlinear amplifiers at a signal wavelength (SW), while still preserving a high-quality pulse at an approximately frequency-doubled wavelength (FDW). Nonlinear frequency-conversion also allows for limited wavelength tuning of the FDW. As an example, the output from a nonlinear fiber amplifier is frequency-converted. By controlling the polarization state in the nonlinear fiber amplifier and by operating in the soliton-supporting dispersion regime of the host glass, an efficient nonlinear pulse compression for the SW is obtained. The generated pulse width is optimized by utilizing soliton compression in the presence of the Raman-self-frequency shift in the nonlinear fiber amplifier at the SW. High-power pulses are obtained by employing fiber amplifiers with large core-diameters. The efficiency of the nonlinear fiber amplifier is optimized by using a double clad fiber (i.e., a fiber with a double-step refractive index profile) and by pumping light directly into the inner core of this fiber. Periodically poled LiNbO3 (PPLN) is used for efficient conversion of the SW to a FDW. The quality of the pulses at the FDW can further be improved by nonlinear frequency conversion of the compressed and Raman-shifted signal pulses at the SW. The use of Raman-shifting further increases the tuning range at the FDW. For applications in confocal microscopy, a special linear fiber amplifier is used.

Pulse compression during second-harmonic generation in aperiodic quasi-phase-matching gratings

Abstract: We propose a simple means for compressing optical pulses with second-harmonic generation. Aperiodic quasi-phase-matching gratings impart a frequency-dependent phase shift on the second-harmonic pulse relative to the fundamental pulse and can be engineered to correct for arbitrary phase distortions. The mechanism is discussed, and a detailed analysis of the compression of quadratic phase (linear frequency) chirped pulses is presented.

A novel-high energy pulse compression system: generation of multigigawatt sub-5-fs pulses

Abstract: Powerful techniques for spectral broadening and ultrabroadband dispersion control, which allow the compression of high-energy femtosecond pulses to a duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow-core fused silica fiber filled with atomic and molecular gases is studied under two excitation regimes with high-energy input pulses of 140 fs and 20 fs duration respectively. Conditions for optimum pulse compression are outlined considering the role of self-phase modulation and gas dispersion in the two regimes. With 20 fs input pulses and under optimum compression conditions we demonstrate a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high-throughput prism-chirped-mirror delay line. These pulses are the shortest generated to date at multigigawatt peak power. PACS: 42.65.Re; 42.65.Vh Ultrashort-pulse lasers are the most important experimental tools for investigating fast-evolving atomic and molecular dynamics in physics, chemistry, and biology. In the last few years, great technological advances have been made in the field of ultrafast pulse generation. New mode-locking techniques such as additive-pulse mode-locking and Kerr-lens mode-locking have been successfully used for femtosecond pulse generation from a wide range of solid-state laser oscillators [1]. Using chirped mirrors [2] for intracavity dispersion control, pulses down to 7.5 fs have been directly generated by a Kerr-lens mode-locked Ti:sapphire oscillator [3] and, more recently, 6.5-fs pulses have been obtained using broadband semiconductor saturable absorbers for self-starting [4]. Ti:sapphire amplifiers seeded by femtosecond laser oscillators can now generate pulses of 20–30 fs with gigawatt [5, 6] or terawatt [7–9] peak power at repetition rates in the kHz and 10 Hz regimes, respectively. Ultrashort pulses can also be generated by extracavity compression techniques, in which the pulses are spectrally broadened upon propagation in a suitable nonlinear waveguide and subsequently compressed in a carefully designed optical dispersive delay line. Spectral broadening of laser pulses by self-phase modulation (SPM) in a single-mode optical fiber is a well-established technique: pulses down to 6 fs were obtained in 1987 from 50-fs pulses from a mode-locked dye laser [10]. More recently 13-fs pulses from a cavity-dumped Ti:sapphire laser were compressed to 5 fs with the same technique [11]. However, the use of single-mode fibers limits the pulse energy to a few nanojoules. A powerful pulse compression technique based on spectral broadening in an hollow fiber filled with noble gases has demonstrated the capability of handling highenergy pulses (sub-mJ range) [12]. This technique presents the advantages of a guiding element with a large diameter mode and of a fast nonlinear medium with high threshold for multiphoton ionization. New concepts in the construction of dispersive delay lines have been applied in the development of specially designed chirped mirrors for fine control of cubic and quartic phase dispersion terms over a large spectral bandwidth [3]. The implementation of the hollow-fiber technique using 20-fs seed pulses from a Ti:sapphire system [5] and a high-throughput broadband dispersive delay line consisting of prisms and chirped mirrors has recently permitted the generation of multigigawatt sub-5 fs pulses [13]. In this paper we present a comprehensive analysis of compression experiments with high-energy femtosecond pulses performed using gas-filled hollow fibers. Spectral broadenings obtained in different gases are compared for 140-fs and 20-fs input pulses generated by Ti:sapphire laser systems, and the optimum conditions for pulse compression are outlined considering the role of SPM and gas dispersion. A new ultrabroadband prism-chirped-mirror dispersive delay line, characterized by a high throughput and dispersion control up to the fourth order, is described in detail. The paper is organized as follows. In Sect. 1 we provide a description of hollow fiber modes and discuss the major advantages of this device compared to optical fibers. Sect. 2 reports on typical spectral broadenings achieved under different excitation conditions. In Sect. 3 we report on the characteristics of the prism-chirped-mirror compressor and discuss the experimental results obtained with 20-fs input pulses. Under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ. These pulses are the

Higher order soliton pulse compression in dispersion-decreasing optical fibers

Abstract: Compression of higher order optical solitons in fibers with anomalous dispersion decreasing along their length is investigated. We demonstrate high-quality compression of pulses with initial soliton order 1

Efficient temporal compression of coherent nanosecond pulses in a compact SBS generator-amplifier setup

Abstract: A pulse compressor based on stimulated Brillouin scattering (SBS) in liquids is experimentally and theoretically investigated. It allows for the compression of Fourier-transform limited nanosecond pulses of several hundreds of millijoules of energy with both high conversion efficiency and a high temporal compression factor. The two-cell generator-amplifier arrangement is of a compact design not requiring external attenuation of the generator cell input energy. Pulses from an injection-seeded, frequency-doubled Nd:YAG laser of 300-mJ energy were compressed by a factor variable between 6 and 21 at up to 75% reflectivity. Deviation from unity SBS reflectivity is predominantly determined by optical component losses. The generation of 270-ps pulses with high beam quality was achieved in liquid methanol. These powerful pulses of variable duration are difficult to produce with common laser systems and are highly suited for the generation of high-harmonics in gases.

Fourth-order-dispersion limitations of aberration-free chirped-pulse amplification systems

Abstract: To obtain shorter pulses in chirped-pulse-amplification lasers, researchers have recently proposed several designs for aberration-free pulse stretchers. We examine the limitations of two aberration-free chirped-pulse-amplification systems and show that comparable results can be obtained with simpler, conventional pulse stretchers. In addition, we present a simple, quintic-phase-limited, aberration-free chirped-pulse-amplification system that can support ultrashort, high-contrast pulses.

Optimum dispersion profile for compression of fundamental solitons in dispersion decreasing fibers

Abstract: In this paper, we have investigated the effect of dispersion profile on the performance of pulse compression in a dispersion decreasing fiber (DDF). Four closely shaped profiles, namely linear, Gaussian, exponential, and hyperbolic, have been considered. We have shown that appropriate shaping of the dispersion profile, particularly when higher order effects are accounted for, is quite effective for adiabatic compression of fundamental solitons, producing high-quality compressed pulses in the sense that the soliton pulse is compressed as a single unit and pedestals do not form. It is concluded that the linear and Gaussian dispersion profiles are nearly optimum in regards to the quality of the compressed pulse and the amount of compression achievable, in particular in the case of subpicosecond soliton pulse compression.

Optical pulse compression in fiber Bragg gratings

Abstract: We report the first experimental demonstration of the optical pushbroom - a novel type of all-optical pulse compression. In the optical pushbroom high intensity pump pulses, tuned well away from the resonance of a Bragg grating, modify the transmission of a weak probe tuned near to the grating's photonic bandgap. The clarity of the experimental results and their close agreement with numerical simulations highlight the tremendous potential of the fiber environment for the detailed study and practical application of nonlinear Bragg grating effects.

Active high-power RF pulse compression using optically switched resonant delay lines

Abstract: We present the design and a proof of principle experimental results of an optically controlled high-power RP pulse-compression system. In principle, the design should handle a few hundreds of megawatts of power at X-band. The system is based on the switched resonant delay-line theory [1]. It employs resonant delay lines as a means of storing RF energy. The coupling to the lines is optimized for maximum energy storage during the charging phase. To discharge the lines, a high-power microwave switch increases the coupling to the lines just before the start of the output pulse. The high-power microwave switch required for this system is realized using optical excitation of an electron-hole plasma layer on the surface of a pure silicon wafer. The switch is designed to operate in the TE/sub 01/ mode in a circular waveguide to avoid the edge effects present at the interface between the silicon wafer and the supporting waveguide; thus, enhancing its power handling capability.

Signal processing architecture which improves sonar and pulse doppler radar performance and tracking capability

Abstract: A Fast Fourier Transformation (FFT) processing is disclosed that measures FFT output phase standard deviation over a number of consecutive FFT runs. The system corrects FFT output phase in an organized fashion for all potential signal filter offset positions while measuring changes in phase standard deviation, and selects the filter offset where the minimum standard deviation occurs. The system utilizes pseudo coherent integration to enhance traditional integration, where the pseudo coherent integration locates the mean phase shift within the number of FFTs integrated, and corrects all FFT runs by this mean shift value. The integration multiplies the magnitude of each FFT filter output by the cosine and sums all FFTs in the integration period for the respective filter. Detections are then declared based on outputs from a combination of a single filter detector, a sum coherent and traditional detector, a filtered coherent detector, and a filtered traditional detector, thereby utilizing various blended techniques across the filter bandwidth. Pulse Doppler radar architecture is predicated on more efficient time utilization during target dwells and is achieved by using managed PRF scheduling, frequency domain Doppler ambiguity resolution, determining signal position and phase within a Doppler filter extremely accurately, performing pseudo coherent integration and use of multi-strategy detection concepts. The PRF scheduling used in this radar architecture coupled with a frequency domain approach to perform Doppler corrected pulse compression and resolve Doppler ambiguity on the same scan provides the architecture to maintain same PRF during dwell and capitalize on integration.

Chirped nonlinear pulse propagation in a dispersion-compensated system

Abstract: We study nonlinear pulse propagation in an optical transmission system with dispersion compensation. A chirped nonlinear pulse can propagate in such a system, but eventually it decays into dispersive waves in a way similar to the tunneling effect in quantum mechanics. The pulse consists of a quadratic potential that is due to chirp in addition to the usual self-trapping potential and is responsible for the power enhancement and the decay.

Nonlinear second-harmonic pulse compression with tilted pulses.

Abstract: We demonstrate that the group velocities of interacting pulses can be adjusted by pulse tilting to optimize second-harmonic pulse compression for a variety of nonlinear crystals and wavelengths. As an experimental proof we present results of ninefold compression of 1.3-ps Nd:glass laser pulses in β-barium borate crystal.

Experimental observation of nonlinear pulse compression in nonuniform Bragg gratings.

Abstract: We demonstrate a scheme for optical pulse compression by cross-phase modulation that utilizes a nonuniform Bragg grating to work in reflection. Our scheme is similar to the conventional optical pushbroom, which works in transmission. This reflection geometry has the advantage of allowing the compressed signal to be observed easily, as it is spatially separate from the pump. This is to our knowledge the first nonlinear effect to be observed that requires a nonuniform grating.

Ultra-wideband radar using Fourier synthesized waveforms

Abstract: Traditional methods of ultra-wideband (UWB) radar signal generation suffer from several disadvantages such as low antenna radiation efficiency and lack of accurate control of signal parameters like pulse shape, pulse repetition interval (PRI), and its spectrum. UWB signals can be generated by expanding the desired radar waveform in a Fourier series and then synthesizing the waveform by generating the individual terms in the expansion from harmonically related oscillators. Signals thus produced overcome the disadvantages of traditional methods of UWB signal generation. Fourier series based method for generation of complex amplitude coded waveforms is developed which can be used to generate time domain equivalent of Barker and other codes for application in radar and communication areas. In radar applications, these coded waveforms, with accurate and stable waveform parameters, shall allow pulse compression and coherent integration. The additional processing gain provided by these operations reduces the need for high peak power in radar transmitters which is one of the bottlenecks in the implementation of operational UWB radars. This paper also describes a UWB radar concept which incorporates Fourier synthesized waveforms. Related digital signal processing issues are also discussed.

High-resolution optical chirped pulse gating

Abstract: We describe a system for achieving high-resolution range gating using optically chirped pulses. The technique converts signals from the time domain into signals in the frequency domain through a nonlinear, sum-frequency generation process. The technique is based on similar methods used in microwave radar. We draw analogies between our method and conventional and time-lens imaging processes, and present experimental results demonstrating the method.

Multipath time-delay estimation

Abstract: A transmitted and known signal is observed at the receiver through more than one path in additive noise. The problem is to estimate the number of paths and for each of them the associated attenuation and delay. It is a frequent problem in sonar, radar and geophysics. We propose an algorithm that is easy to implement, that has a reasonable computational load and seems to be able to solve the problem under more severe conditions (lower SNR) than previous methods.

Fast high-voltage pulse generator with nonlinear transmission line for high repetition rate operation

Abstract: A new application of the nonlinear transmission line (NLTL) for high-voltage pulse generation is reported. In this NLTL, a rise time of an input pulse voltage of 20-kV amplitude can be reduced from 500 to less than 120 ns. Using this circuit, we demonstrated excitation of a pulsed CO/sub 2/ laser, and obtained output energy of 129 mJ at an efficiency of 4.3%. Moreover, we find that the head-on collision of two solitons is quite effective to generate a high-voltage and short-width pulse. The input pulse is doubled in amplitude and sharpened in width, from 3.6 kV-300 ns to 11 kV-76 ns. With this method, the utilization of semiconductor devices such as the SI thyristor is possible as a primary switching device. Finally, a xenon lamp has been flashed at a repetition rate of 1 kHz.

Active optical pulse compression with a gain of 29.0 dB by using four-wave mixing in an optical fiber

Abstract: The four-wave mixing gain in an optical fiber strongly depends on the nonlinear phase mismatch when the pump power itself causes a nonlinear index change. The peak of the pump pulse has a much higher gain than its wings, and so this process can be used to obtain efficient pulse compression. A 93-ps signal pulse at 1569.9 nm is compressed to as short as 20 ps with a gain of 29.0 dB. The wavelength-converted signal at 1559.9 nm is simultaneously compressed to 21 ps and amplified by 28.9 dB as compared with the original signal.

Target detection method and device for wideband unambiguous pulse Doppler radar

Abstract: Coherent bursts of N wideband, low repetition frequency width-modulated pulses are transmitted, and they are received with pulse compression and then sampling. For each range gate and each speed hypothesis, a selection is made of the corresponding samples of N repetitions of a burst after compensation for the migration in distance. On each set of N samples, for a given speed hypothesis, a Fourier transform and a threshold-setting operation are performed. The distance and the unambiguous speed of the detected targets are then extracted.

Algorithm development for an airborne real-time STAP demonstration

Abstract: A description of STAP algorithms used in the May 1996 Real Time Multichannel Airborne Radar Measurement (RT-MCARM) program is presented. The system collected and processed IF sampled data from 16 phased array radar receiver channels, performing IF to baseband conversion, Doppler processing, adaptive clutter cancellation, pulse compression, and formation of multiple receive beams within an intentionally broadened transmit beam. Selective PRI stagger was implemented in Doppler regions of strong clutter to provide increased cancellation. An innovative method of adaptation with variable beam constraints was developed, which allows preservation of target phase across boundaries between range intervals with differing clutter statistics. In addition to improved angle estimation compared to unconstrained adaptive processing, the constraint makes it possible to perform pulse compression after beamforming thereby reducing the computational requirements.

Gain-Switched DFB Lasers

Abstract: We present a brief review of the semiconductor gain-switching mechanism, its principles and aspects of some problems to be overcome in order to produce well behaved pulses, i.e. frequency chirp and turn-on jitter. We also describe three experiments on gain-switched DFB lasers - one operating in the second telecommunication window, at 1300 nm, and the other two in the third, at 1550 nm - to generate pulses of duration of around 5 -7 ps. In these demonstrations various techniques have been employed: the use of either a dispersion compensating fibre or a linearly step-chirped fibre grating for pulse compression, and also CW light injection for the reduction of the pulse turn-on delay. The pulses generated in all experiments were suitable for high-speed photonic applications.

Spread spectrum pulse position modulation system

Abstract: A spread spectrum pulse position modulation/demodulation system which receives an input signal including frames and which includes first and second pulse position modulators. The first pulse position modulator modulates each frame by inserting a first pseudo noise code into a selected slot of each frame according to the input signal to generate a first spread spectrum pulse position modulating signal. The second pulse position modulator modulates each frame by inserting a second pseudo noise code into a selected slot of each frame according to the input signal to generate a second spread spectrum pulse position modulating signal. An adder adds together the first and second spread spectrum pulse position modulating signals to generate a third spread spectrum pulse position modulating signal. This third spread spectrum pulse position modulating signal may be transmitted by a radio transmitter, and then received by a radio receiver. The received third spread spectrum pulse position modulating signal can then be input into a demodulator for demodulating this third spread spectrum pulse position modulating signal. The demodulator includes at least one matched filter to output a first pulse signal corresponding to the first spread spectrum pulse position modulating signal and to output a second pulse signal corresponding to the second spread spectrum pulse position modulating signal.

An ultra-low sidelobe pulse compression technique for high performance radar systems

Abstract: A pulse compression technique based on deconvolution and linear prediction in the frequency domain is presented which offers approximately 20 dB improvement in time sidelobe performance compared to conventional matched filter and windowing techniques. The detection performance converges to that of a matched filter for weak signals, while the sidelobe level decreases to very low levels as a function of the signal strength. The performance of the deconvolution method has been studied using measured radar data from the Rome Laboratory Surveillance Facility, and is shown to be a practical technique for future radar systems. Issues such as sensitivity to target Doppler frequency, range cell straddling, dispersion in analog transmit and receive channels, and A/D converter errors are analyzed and the results presented.

Observation of space-charge driven beam instabilities in a radio frequency photoinjector

Abstract: This article describes an experimental determination of the correlated, longitudinal phase-space electron distribution produced by a radio frequency photoinjector. Measurements of the electron beam energy spectra and pulse shapes are analyzed to deduce the longitudinal phase space at the exit of the photoinjector rf cavity. Data were obtained for micro-pulse charges of 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 nC, and show different phenomena in the low-charge and high-charge regimes. At low beam charge, the uncorrelated energy spread increases with increasing charge density, while rf bunching appears to cancel any pulse-length elongation due to the space-charge forces. At high beam charge, the data show that the micro-pulse separates into three distinct sub-pulses of nearly equal charge, and with a temporal separation proportional to the relativistic plasma frequency. These effects are compared with the space-charge generated instabilities and virtual-cathode phenomena observed in lower voltage devices. The implications that these results have upon the fundamental limits of beam brightness and magnetic pulse compression limitations are discussed.

Validation of pulse compression techniques for meteorological functions

Abstract: A major technology barrier to the application of pulse compression techniques in meteorological radar is the presence of range sidelobes which mask and corrupt observations of weak weather phenomena in areas of strong extended meteorological scatterers or point target returns. Techniques for suppressing range sidelobes are well known, but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of doppler. Recent investigations have presented a "doppler tolerant" range sidelobe suppression technique. The thrust of the work described in this article is the extension of previous simulations to the transmission of dispersed/coded waveform pulses using the ELDORA X-Band weather research radar located at the National Center for Atmospheric Research (NCAR) Foothills Laboratory. This study shows that the use of Barker coded pulses along with pulse compression and doppler tolerant range sidelobe suppression provides: (1) increased sensitivity over a simple pulse of the same peak power, and nominal receive bandwidth by a factor equal to the time-bandwidth product, and (2) accurate spectral moment estimates.

High-repetition-rate picosecond pulse generation at 1.5 µm by intracavity laser frequency modulation

Abstract: We propose and experimentally demonstrate generation of transform-limited pulse trains at a 2.5-GHz repetition rate from a frequency-modulated erbium-ytterbium bulk glass laser by using an intracavity lithium niobate phase modulator and an external fiber Bragg grating filter. Light pulses with durations tunable from 40 to 80 ps and 0.2-mW fiber-coupled average power at 1535 nm have been obtained.

Pulse edge detector with wide dynamic range

Abstract: A pulse detector achieves high accuracy over a wide dynamic range in providing accurately spaced pulse edge markers that allow measurement of pulse to pulse time intervals. Received input pulses are transformed to a Gaussian like waveform and a single cycle AC signal having a first polarity initial half cycle is derived therefrom, accomplished by subtracting the Gaussian like waveform from a delayed, but substantially time overlapping, copy of that waveform. Upon completion of that initial half cycle, a pulse edge marker is generated. By measuring the time between the pulse edge marker obtained from one inputted pulse and that of the next, a measurement of the pulse to pulse interval is achieved that is of one nanosecond in accuracy.