Chirped Signal Pulse Research Articles

Ultrashort-pulsed optical parametric oscillator employing Brewster angle prism retroreflectors.

We demonstrate a synchronously-pumped optical parametric oscillator (OPO) cavity in which traditional dielectric mirrors are replaced by all-planar Brewster angle prism retroreflectors, also known as Pellin-Broca prisms. Exploiting total internal reflection, these prisms form a cavity supporting >350-fs chirped signal pulses that were externally compressible to sub-150-fs durations. This simple architecture produces wavelengths tuneable across 1100 - 1350 nm, suitable for basic multi-photon applications.

Design of the front-end system for a subexawatt laser of the XCELS facility

A concept of the front-end system of the XCELS (eXawatt Center for Extreme Light Studies) facility is presented. Its design is aimed at achieving high stability of laser radiation parameters and possibility of their control in a wide range. Optically synchronised chirped signal (wavelength 910 nm, bandwidth more than 100 nm, and duration ∼3 ns) and pump (wavelength 1054 nm, bandwidth ∼1 nm, and duration ∼4 ns) pulses for XCELS parametric amplifiers will be implemented at the output of the front-end system. Chirped femtosecond pulses with energies above 100 mJ [no more than 15 fs long after compression, with carrier-envelope phase (CEP) stabilisation] will have a repetition rate up to 100 Hz, which will allow one to implement active energy stabilisation and to minimise the angular jitter of the emitted beam at the XCELS output. The application of picosecond pumping in the parametric amplifier of the front-end system should provide a high contrast of femtosecond pulses. The pump pulse will be linearly frequency-modulated; this approach will not affect the parametric amplification efficiency but make it possible to use spectral methods to control the pump pulse shape in order to form a pulse of specified shape at the output of power amplifiers, even under conditions of their strong saturation.

Phase matched scanning optical parametric chirped pulse amplification based on pump beam deflection* *Project supported by Science and Technology Innovation Seedling Project of Sichuan Province, China (Grant No. 2018100) and Major Project of CDNU (Grant No. CS18ZDZ0511).

Combined with the optical beam deflection, a novel approach of phase matched broadband scanning optical parametric chirped pulse amplification (OPCPA) was proposed. For this scheme, there was no superfluous operations to the chirped signal pulse which propagated in a changeless direction straightforward, but the pump beam were deflected in space with time by passing through a KTN crystal, which was applied with varied driving voltage. The theories of phase matching of each chirped signal frequency based on pump beam deflection was analyzed detailedly. And the type-I amplification of chirped signal with 800 nm central wavelength and 20 nm bandwidth pumped by 532 nm in BBO crystal was simulated as a case in point. The simulation results showed that the spectral distribution of chirped signal pulse was almost the same as the initial form, i.e., there was nearly no narrowing on the amplified spectrum by using of the scanning OPCPA based on pump beam deflection. In addition, the simulations demonstrated that it was worth minimizing the voltage deviation applied to KTN crystal as much as possible for the sake of better waveform, larger bandwidth and higher conversion efficiency of amplified signal pulse in the proposed scanning OPCPA.

Comparative study on coherent noise in optical parametric and quasi-parametric chirped-pulse amplification

The coherent noise inherent to optical parametric chirped-pulse amplification (OPCPA) poses a fundamental limitation on the temporal quality of current ultrashort ultraintense lasers as well as their applications in strong-field physics research. Here we present a peer comparison of coherent noise between OPCPA and one of its alternatives — quasi-parametric chirped-pulse amplification (QPCPA). In the initial amplification stage termed as small-signal regime, the spectrum of chirped signal pulse in either OPCPA or QPCPA is exposed to the imperfections in pump pulse and optics, resulting in coherent noise pedestals in the time domain. While in the final amplification stage featured by amplifier saturation, the nonlinear growths of coherent noise become sharply different. We demonstrate that the non-parametric saturation behavior of QPCPA can greatly smooth the spectral distortions of signal, and thus bring down the coherent noise. Consequently, QPCPA scheme allows an improvement of pulse contrast by 2–3 orders of magnitude. The results presented in this paper provide a new perspective on high-contrast amplification of ultrashort pulses.

Doppler Shifting Technique for Generating Multi-Frames of Video SAR via Sub-Aperture Signal Processing

The signal processing technologies of synthetic aperture radar (SAR) have been vastly studied as diverse measurement modes for various applications. One of the technologies is Video-SAR (ViSAR). It is a land-imaging mode where a sequence of images is continuously formed on the spotlight mode. We propose a new signal processing method to generate the multi-frames of ViSAR using Doppler shifting technique via sub-aperture processing. The method is based on our designed wide-angle antenna, which is a corrugated horn shape with low side lobe and Gaussian beam pattern. This wide-angle antenna receives a wide Doppler band along the azimuth direction. Based on the characteristics of the antenna, we separate the wide Doppler band into each chirp pulse signal. Each of these separated signals has a different center time. Through each chirp pulse signal in Doppler domain, the proposed method generates each frame of video at a high frame rate. The proposed ViSAR processing can show high efficiency with low computation and without a fixed coordinate system. Moreover, it can also visualize a wider area on stripmap mode than the conventional ViSAR mode, and can derive the video results based on Range Doppler Algorithm (RDA). These advantages can lead to reduce economic costs and simplify the operating ViSAR system. We have performed the practical experiments using X-band chirp pulse SAR system mounted on an airplane to verify the proposed method.

Laser pulse reshaping with spectral scanning filtering based on optical Kerr effect

A novel approach for laser pulse reshaping with spectral scanning filtering based on Optical Kerr effect is proposed. The physical reshaping schematic is revealed and analyzed theoretically for the F-P interference filter in combination with an optical Kerr medium. The effects of the relative time-delays between pump and signal, the pump deviations and chirp errors on the reshaping performance have been investigated and discussed. Numerical simulations show that precise controlling parameters is required to better reshaping properties end. A flat top pulse with the central wavelength of 800nm, pulse duration 0.1ns is reshaped from a first order Gaussian chirped signal pulse. Furthermore, a flat top pulse is reshaped to parabolic shape, which demonstrated the flexibility of this reshaping approach.

Broadband, efficient, and robust quasi-parametric chirped-pulse amplification.

Quasi-parametric chirped pulse amplification (QPCPA) is a new scheme that enables the amplification of chirped signal pulses without back conversion by depleting the idler pulses. In this paper, we present a numerical study on the bandwidth, efficiency, and robustness of QPCPA. Self-locked phase among the interacting waves is found to be the underlying mechanism for the suppression of back conversion, which allows signal efficiency approaching to the quantum limit even under the phase-mismatch condition, and thus greatly increases the phase-mismatch tolerance of QPCPA. We demonstrate that QPCPA can break through the trade-off between the efficiency and bandwidth encountered in conventional optical parametric amplification, hence supporting highly efficient amplification of few-cycle pulses.

Energy Scaling of Infrared Femtosecond Pulses by Dual-Chirped Optical Parametric Amplification

We present an energy-scaling experiment on a femtosecond infrared pulse using dual-chirped optical parametric amplification (DC-OPA). A total output energy exceeding 100 mJ with 36-fs pulse duration was achieved in the infrared region, which is the highest energy ever reported for an ultrafast optical parametric amplifier scheme. We also discuss the generation of high-energy 15–30–THz pulses through difference frequency generation between chirped signal and idler pulses based on DC-OPA. By manipulating the phase of a seed pulse in the DC-OPA system, the spectral phase of a THz pulse can be indirectly controlled precisely, which is very helpful for compressing THz pulses.

Dispersion-corrected frequency-resolved optical gating.

The phase retrieval algorithm of a frequency-resolved optical gating (FROG) is generalized to handle traces seriously distorted by group delay dispersion and non-uniform phase-matching spectra arising from the nonlinear crystal. In our proof-of-concept experiments, 15 mm thick aperiodically poled lithium niobate was employed in FROG, and successfully reconstructed chirped signal pulses were actually stretched by >5 times inside the crystal. This method is particularly promising in the measurement of weak few-cycle pulses produced by supercontinuum generation in fibers.

Numerical analysis of broadband fiber optical parametric amplifiers pumped by two chirped pulses

An optical parametric amplifier pumped by two chirped pump pulses has been analytically and numerically investigated to predict the spectral gain bandwidth and its temporal distribution. We highlight that the properties of the amplifier depend strongly on the relative chirp values and a uniform spectrotemporal gain can be obtained when the two chirps have opposite signs. A chirped signal pulse owing a very broad bandwidth has also been injected and we demonstrate the amplification of ultrashort pulse with this type of amplifier.

Decrease spectral phase modulation with spectral scanning filter for coherent pulse contrast improvement in femtosecond laser system

Abstract A spectral scanning filter method for decreasing the spectral phase modulation to improve the coherent intensity contrast in the chirped pulse amplification system is proposed and studied theoretically, which is made use of the frequency offset between the chirped signal pulse and the prepulse (or the postpulse). The spectral scanning filter based on the rapid Fabry–Perot (F–P) interference for reducing the pre-pulse and post-pulse originated from the spectral phase modulation is analyzed theoretically and simulated numerically. The numerical simulation results show that the coherent pulse contrast can be improved by 0.41 F 2 for the chirped pulse and the compressed pulse, and the transmission efficiency of the chirped signal pulse can exceed 90% when the synchronizing time jitter ranges from −2 ps to 2 ps and the chirp rate parametric from −0.1 C / T 2 to 0.1 C / T 2 .

Four-wave-mixing-based optical parametric oscillator delivering energetic, tunable, chirped femtosecond pulses for non-linear biomedical applications.

A novel concept for an optical parametric oscillator based on four-wave mixing (FOPO) in an optical fiber is presented. This setup has the ability of generating highly chirped signal and idler pulses with compressed pulse durations below 600 fs and pulse energies of up to 250 nJ. At a fixed pump wavelength of 1040 nm, the emerging signal and idler wavelengths can be easily tuned between 867 to 918 nm and 1200 to 1300 nm, respectively, only by altering the cavity length. With compressed peak powers >100 kW and a repetition rate of only 785 kHz, this source provides tunable intense ultra-short pulses at moderate average powers. This setup constitutes a stable, simple and in many ways superior alternative to bulk state-of-the-art OPO light converters for demanding biomedical applications and non-linear microspectroscopy.

Ultrabroadband fiber optical parametric amplifier pumped by chirped pulses Part 2: sub-30-fs pulse amplification at high gain

We report numerical investigations on ultrashort pulse amplification with high gain in a fiber-based optical parametric amplifier pumped by a broadband chirped pulse. The amplifier has been tailored for ultrabroadband amplification with an analytical model and has key features to overcome gain narrowing and pulse distortion, which are usually observed in ion-doped fiber amplifiers. By combining chirped pump and signal pulses in a fiber, we highlight the possibility to amplify sub-30-fs pulses with standard ultrafast technologies.

SNR improvement based on non-collinear OPCPA with angular spectral dispersion in BBO crystal

The characteristics of the signal-to-noise ratio (SNR) improvement based on non-collinear optical parametric chirped pulse amplification (OPCPA) with angular spectral dispersion (ASD) was revealed and investigated. The angular dispersion formula was derived theoretically and the effects of crystal length, pump peak intensity, noise characteristics and angular dispersion on the SNR improvement and conversion efficiency were discussed. Furthermore, by optimizing the critical parameters, 35% conversion efficiency and more than 3 orders of magnitude of SNR improvement were achieved for the chirped signal pulse with a central wavelength of 800nm, pumped by 532nm in presented amplification scheme with BBO crystal.

FORWARD-LOOKING IMAGING OF SCANNING PHASED ARRAY RADAR BASED ON THE COMPRESSED SENSING

In this paper, a novel forward-looking imaging method based on the compressed sensing is proposed for scanning phased array radar (PAR) in order to improve the azimuth resolution. Firstly, the echo of targets is modeled according to the principle of PAR. Then, it is analyzed why some of the former methods as multi-channel deconvolution are inefiective based on the signal model. Using a widely accepted assumption that dominant scatterers in an interesting area are sparse or compressible, an imaging algorithm based on the compressed sensing is proposed and investigated. This method obtains its high range resolution by transmitting and compressing chirp pulse signal, and improves its azimuth resolution by utilizing the compressed sensing technique. The efiectiveness of the proposed method is illustrated and analyzed with simulations data.

Femtosecond OPO based on LBO pumped by a frequency-doubled Yb-fiber laser-amplifier system for CARS spectroscopy

We present detailed investigations of a femtosecond green-pumped optical parametric oscillator (OPO) based on lithium triborate. As pump source, a frequency-doubled Yb-fiber laser-amplifier system is used. The OPO generates signal pulses tunable over a spectral range from 780 to 940 nm and idler pulses tunable from 1630 to 1190 nm. More than 250 mW are generated in the signal beam and more than 300 mW in the idler beam. Without dispersion compensation chirped signal pulses with a pulse duration between 100 and 250 fs are measured. Using this system for coherent anti-Stokes Raman scattering spectroscopy, vibrational resonances between 1110 and 6760 cm−1 can be excited. Due to the chirped pulses, a spectral resolution of 100 cm−1 is achieved, which is 2.5 times higher compared to an excitation with time-bandwidth limited pulses.

Generation of synchronized signal and pump pulses for an optical parametric chirped pulse amplification based multi-terawatt Nd:glass laser system

Synchronized signal (650 ps) and pump (1.3 ns) pulses were generated using 4-pass geometry in a grating pair based pulse stretcher unit. The pump pulse has been further amplified in a high gain regenerative amplifier. This amplified pulse was used as the pump in an optical parametric chirped pulse amplification based Nd:glass laser system. As the chirped signal pulse and the pump pulse originated from the same oscillator, the time jitter between the pump pulse and the signal pulse can be <50 ps.

Generation of 30-fs ultraviolet pulses by four-wave optical parametric chirped pulse amplification

We report on the generation of approximately 30-fs ultraviolet pulses with approximately 10 microJ energy by means of four-wave optical parametric chirped pulse amplification in fused silica. The four-wave optical parametric amplifier is pumped by the second-harmonic of the Ti:sapphire laser and is seeded by visible broadband chirped signal pulses. The idler pulses are produced in the ultraviolet by four-wave mixing and are compressed in a medium with normal group velocity dispersion.

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Near-degenerative near-collinear phase-match geometry for broadband optical parametric chirped-pulse amplification (OPCPA) at 780 nm is calculated in comparison with nondegenerate noncollinear phase-match geometry. In an experiment on LBO-I near-degenerate near-collinear OPCPA, high gain with broad gain bandwidth (71 nm, FWHM) at 780 nm is achieved by using an 390-nm pumping pulse. The stretched broadband chirped signal pulse near 780 nm is amplified to 412 µJ with a pumping energy of 15 mJ, and the total gain is >3.7×106, which agrees well with the calculation. For a broadband (covering 100 nm) chirped signal pulse, the theoretical gain bandwidth has been attained experimentally for the first time.

Image generation in chirp pulse microwave computed tomography (CP‐MCT) by numerical computation: Computation of a human head model

AbstractThis paper considers chirp pulse microwave CT (CP‐MCT), in which the chirp pulse and signal processing technology are used to estimate the internal structure and temperature variation of biological objects. A method is developed in which the CT image to be acquired by actual measurement can be derived by numerical computation. The aftereffect function between the transmitting and receiving antennas is calculated by a Gaussian pulse and the FDTD method. After convolution of the function with the input chirp pulse signal on the time axis, the measured signal at a point is constructed by the same signal processing as in real measurement. This computation is repeated for each point on the translation‐scanning axis. The projection data are acquired and the CT image is generated. The validity of this computation procedure, which is called the aftereffect function method, is demonstrated by comparing the resolution and the measured temperature variation in actual measurements with the results of a simulation computation. Using the aftereffect function method, the point‐spread function of an imaging system which is difficult to measure, can be derived. An analytical model for the human head is constructed. Simulations are performed for the attenuation distribution and the temperature variation distribution. Based on the results, the feasibility of the biological imaging by CP‐MCT is discussed. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 88(9): 53–63, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20190