Showing papers on "Chirp published in 2011"

Polynomial Chirplet Transform With Application to Instantaneous Frequency Estimation

Abstract: In this paper, a new time-frequency analysis method known as the polynomial chirplet transform (PCT) is developed by extending the conventional chirplet transform (CT). By using a polynomial function instead of the linear chirp kernel in the CT, the PCT can produce a time-frequency distribution with excellent concentration for a wide range of signals with a continuous instantaneous frequency (IF). In addition, an effective IF estimation algorithm is proposed based on the PCT, and the effectiveness of this algorithm is validated by applying it to estimate the IF of a signal with a nonlinear chirp component and seriously contaminated by a Gaussian noise and a vibration signal collected from a rotor test rig.

A comparison of the pulsed, lock-in and frequency modulated thermography nondestructive evaluation techniques

Abstract: Pulsed, lock-in and frequency modulated thermography are three alternative nondestructive evaluation techniques. The defect imaging performance of these techniques are compared using: matched excitation energy; the same carbon fiber composite test piece and infrared camera system. The lock-in technique suffers from “blind frequencies” at which phase images for some defects disappear. It is shown that this problem can be overcome by using frequency modulated (chirp) excitation and an image fusion algorithm is presented that enhance phase imaging of defects. The signal-to-noise ratios (SNRs) of defect images obtained by the three techniques are presented. For the shallowest defects (depths 0.25 and 0.5 mm, 6 mm diameter), the pulsed technique exhibits the highest SNRs. For deeper defects the SNRs of the three techniques are similar in magnitude under matched excitation energy condition.

Generation of an isolated sub-40-as pulse using two-color laser pulses: Combined chirp effects

Abstract: Generation of an isolated sub-40-as pulse using two-color laser pulses: Combined chirp effects

High-Fidelity Adiabatic Passage by Composite Sequences of Chirped Pulses

Abstract: We present a method for optimization of the technique of adiabatic passage between two quantum states by composite sequences of frequency-chirped pulses with specific relative phases: composite adiabatic passage (CAP). By choosing the composite phases appropriately the nonadiabatic losses can be canceled to any desired order with sufficiently long sequences, regardless of the nonadiabatic coupling. The values of the composite phases are universal for they do not depend on the pulse shapes and the chirp. The accuracy of the CAP technique and its robustness against parameter variations make CAP suitable for high-fidelity quantum information processing.

83 W, 3.1 MHz, square-shaped, 1 ns-pulsed all-fiber-integrated laser for micromachining.

Abstract: We demonstrate an all-fiber-integrated laser based on off-the-shelf components producing square-shaped, 1 ns-long pulses at 1.03 μm wavelength with 3.1 MHz repetition rate and 83 W of average power. The master-oscillator power-amplifier system is seeded by a fiber oscillator utilizing a nonlinear optical loop mirror and producing incompressible pulses. A simple technique is employed to demonstrate that the pulses indeed have a random chirp. We propose that the long pulse duration should result in more efficient material removal relative to picosecond pulses, while being short enough to minimize heat effects, relative to nanosecond pulses commonly used in micromachining. Micromachining of Ti surfaces using 0.1 ns, 1 ns and 100 ns pulses supports these expectations.

High quality sub-two cycle pulses from compression of supercontinuum generated in all-normal dispersion photonic crystal fiber

Abstract: We demonstrate nonlinear pulse compression based on recently introduced highly coherent broadband supercontinuum (SC) generation in all-normal dispersion photonic crystal fiber (ANDi PCF). The special temporal properties of the octave-spanning SC spectra generated with 15 fs, 1.7 nJ pulses from a Ti:Sapphire oscillator in a 1.7 mm fiber piece allow the compression to 5.0 fs high quality pulses by linear chirp compensation with a compact chirped mirror compressor. This is the shortest pulse duration achieved to date from the external recompression of SC pulses generated in PCF. Numerical simulations in excellent agreement with the experimental results are used to discuss the scalability of the concept to the single-cycle regime employing active phase shaping. We show that previously reported limits to few-cycle pulse generation from compression of SC spectra generated in conventional PCF possessing one or more zero dispersion wavelengths do not apply for ANDi PCF.

Octave spanning supercontinuum in an As 2 S 3 taper using ultra-low pump pulse energy

Abstract: An octave spanning spectrum is generated in an As 2 S 3 taper via 77 pJ pulses from an ultrafast fiber laser. Chirp compensation allows the octave to be generated directly from the un-amplified laser output.

Stochastic Gravitational Wave Background from Coalescing Binary Black Holes

Abstract: We estimate the stochastic gravitational wave (GW) background signal from the field population of coalescing binary stellar mass black holes (BHs) throughout the universe. This study is motivated by recent observations of BH-Wolf-Rayet (WR) star systems and by new estimates in the metallicity abundances of star-forming galaxies that imply BH-BH systems are more common than previously assumed. Using recent analytical results of the inspiral-merger-ringdown waveforms for coalescing binary BH systems, we estimate the resulting stochastic GW background signal. Assuming average quantities for the single source energy emissions, we explore the parameter space of chirp mass and local rate density required for detection by advanced and third-generation interferometric GW detectors. For an average chirp mass of 8.7 M ☉, we find that detection through 3 years of cross-correlation by two advanced detectors will require a rate density, r 0 ≥ 0.5 Mpc–3 Myr–1. Combining data from multiple pairs of detectors can reduce this limit by up to 40%. Investigating the full parameter space we find that detection could be achieved at rates r 0 ~ 0.1 Mpc–3 Myr–1 for populations of coalescing binary BH systems with average chirp masses of ~15 M ☉ which are predicted by recent studies of BH-WR star systems. While this scenario is at the high end of theoretical estimates, cross-correlation of data by two Einstein Telescopes could detect this signal under the condition r 0 ≥ 10–3Mpc–3 Myr–1. Such a signal could potentially mask a primordial GW background signal of dimensionless energy density, ΩGW ~ 10–10, around the (1-500) Hz frequency range.

Modelling multi-pulse population dynamics from ultrafast spectroscopy.

Abstract: Current advanced laser, optics and electronics technology allows sensitive recording of molecular dynamics, from single resonance to multi-colour and multi-pulse experiments. Extracting the occurring (bio-) physical relevant pathways via global analysis of experimental data requires a systematic investigation of connectivity schemes. Here we present a Matlab-based toolbox for this purpose. The toolbox has a graphical user interface which facilitates the application of different reaction models to the data to generate the coupled differential equations. Any time-dependent dataset can be analysed to extract time-independent correlations of the observables by using gradient or direct search methods. Specific capabilities (i.e. chirp and instrument response function) for the analysis of ultrafast pump-probe spectroscopic data are included. The inclusion of an extra pulse that interacts with a transient phase can help to disentangle complex interdependent pathways. The modelling of pathways is therefore extended by new theory (which is included in the toolbox) that describes the finite bleach (orientation) effect of single and multiple intense polarised femtosecond pulses on an ensemble of randomly oriented particles in the presence of population decay. For instance, the generally assumed flat-top multimode beam profile is adapted to a more realistic Gaussian shape, exposing the need for several corrections for accurate anisotropy measurements. In addition, the (selective) excitation (photoselection) and anisotropy of populations that interact with single or multiple intense polarised laser pulses is demonstrated as function of power density and beam profile. Using example values of real world experiments it is calculated to what extent this effectively orients the ensemble of particles. Finally, the implementation includes the interaction with multiple pulses in addition to depth averaging in optically dense samples. In summary, we show that mathematical modelling is essential to model and resolve the details of physical behaviour of populations in ultrafast spectroscopy such as pump-probe, pump-dump-probe and pump-repump-probe experiments.

Observations of four types of pulses in a fiber laser with large net-normal dispersion

Abstract: Four different types of pulses are experimentally obtained in one erbium-doped all-fiber laser with large net-normal dispersion. The proposed laser can deliver the rectangular-spectrum (RS), Gaussian-spectrum (GS), broadband-spectrum (BS), and noise-like pulses by appropriately adjusting the polarization states. These kinds of pulses have distinctly different characteristics. The RS pulses can easily be compressed to femtosecond level whereas the pulse energy is restricted by the trend of multi-pulse shaping with excessive pump. The GS and BS pulses always maintain the single-pulse operation with much higher pulse-energy and accumulate much more chirp. After launching the pulses into the photonic-crystal fiber, the supercontinuum can be generated with the bandwidth of >700 nm by the BS pulses and of ~400 nm by the GS pulses, whereas it can hardly be generated by the RS pulses. The physical mechanisms behind the continuum generation are qualitatively investigated relating to different operating regimes. This work could help to a deeper insight of the normal-dispersion pulses.

Limiting effects on laser compression by resonant backward Raman scattering in modern experimentsa)

Abstract: Through resonant backward Raman scattering, the plasma wave mediates the energy transfer between long pump and short seed laser pulses. These mediations can result in pulse compression at extraordinarily high powers. However, both the overall efficiency of the energy transfer and the duration of the amplified pulse depend upon the persistence of the plasma wave excitation. At least with respect to the recent state-of-the-art experiments, it is possible to deduce that at present the experimentally realized efficiency of the amplifier is likely constrained mainly by two effects, namely, the pump chirp and the plasma wave wavebreaking.

Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection

Abstract: This work presents a small-signal analysis for investigating the transmission performance of optical orthogonal frequency division multiplexing signals with a directly modulated DFB laser (DML). The analysis shows the positive chirp of DMLs can intensify power fading after transmission with positive dispersion and provide power gain instead with negative dispersion. The power of subcarrier-to-subcarrier intermixing interference after square-law direct detection, however, is independent on the sign of dispersion.

Measuring the Chirp and the Linewidth Enhancement Factor of Optoelectronic Devices with a Mach–Zehnder Interferometer

Abstract: In this paper, a technique based on the use of a Mach-Zehnder (MZ) interferometer is proposed to evaluate chirp properties, as well as the linewidth enhancement factor (αH-factor) of optoelectronic devices. When the device is modulated, this experimental setup allows the extraction of the component's response of amplitude modulation (AM) and frequency modulation (FM) that can be used to obtain the value of the αH-factor. As compared with other techniques, the proposed method gives also the sign of the αH-factor without requiring any fitting parameters and, thus, is a reliable tool, which can be used for the characterization of high-speed properties of semiconductor diode lasers and electroabsorption modulators. A comparison with the widely accepted fiber transfer function method is also performed with very good agreement.

Transform-limited spectral compression by self-phase modulation of amplitude-shaped pulses with negative chirp

Abstract: Spectral compression by self-phase modulation of amplitude- and phase-shaped pulses is demonstrated as superior compared to pulses that have only been phase shaped. We synthesize linearly negatively chirped parabolic pulses, which we send through a nonlinear photonic crystal fiber, in which self-phase modulation compresses the spectrum of the pulses to within 20% of the Fourier transform limit.

Dual-chirped optical parametric amplification for generating few hundred mJ infrared pulses.

Abstract: An ultrafast high-power infrared pulse source employing a dual-chirped optical parametric amplification (DC-OPA) scheme based on a Ti:sapphire pump laser system is theoretically investigated. By chirping both pump and seed pulses in an optimized way, high-energy pump pulses can be utilized for a DC-OPA process without exceeding the damage threshold of BBO crystals, and broadband signal and idler pulses at 1.4 μm and 1.87 μm can be generated with a total conversion efficiency approaching 40%. Furthermore, few-cycle idler pulses with a passively stabilized carrier-envelope phase (CEP) can be generated by the difference frequency generation process in a collinear configuration. DC-OPA, a BBO-OPA scheme pumped by a Ti:sapphire laser, is efficient and scalable in output energy of the infrared pulses, which provides us with the design parameters of an ultrafast infrared laser system with an energy up to a few hundred mJ.

Self-amplified spontaneous emission free-electron laser with an energy-chirped electron beam and undulator tapering.

Abstract: We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

Multi-mode and multi-frequency guided wave imaging via chirp excitations

Abstract: Guided wave imaging has shown great potential for structural health monitoring applications by providing a way to visualize and characterize structural damage. For successful implementation of delay-and-sum and other elliptical imaging algorithms employing guided ultrasonic waves, some degree of mode purity is required because echoes from undesired modes cause imaging artifacts that obscure damage. But it is also desirable to utilize multiple modes because different modes may exhibit increased sensitivity to different types and orientations of defects. The well-known modetuning effect can be employed to use the same PZT transducers for generating and receiving multiple modes by exciting the transducers with narrowband tone bursts at different frequencies. However, this process is inconvenient and timeconsuming, particularly if extensive signal averaging is required to achieve a satisfactory signal-to-noise ratio. In addition, both acquisition time and data storage requirements may be prohibitive if signals from many narrowband tone burst excitations are measured. In this paper, we utilize a chirp excitation to excite PZT transducers over a broad frequency range to acquire multi-modal data with a single transmission, which can significantly reduce both the measurement time and the quantity of data. Each received signal from a chirp excitation is post-processed to obtain multiple signals corresponding to different narrowband frequency ranges. Narrowband signals with the best mode purity and echo shape are selected and then used to generate multiple images of damage in a target structure. The efficacy of the proposed technique is demonstrated experimentally using an aluminum plate instrumented with a spatially distributed array of piezoelectric sensors and with simulated damage.

Forty Gb/s hybrid silicon Mach-Zehnder modulator with low chirp

Abstract: We demonstrate a hybrid silicon modulator operating up to 40 Gb/s with 11.4 dB extinction ratio. The modulator has voltage-length product of 2.4 V-mm and chirp of −0.75 over the entire bias range. As a switch, it has a switching time less than 20 ps.

Spectral Nulling on Transmit via Nonlinear FM Radar Waveforms

Abstract: Modern radars operate in electromagnetic environments crowded with other RF users. Adaptive spectral nulling on transmit can reduce interference from and to these other RF users. An analytical theory is developed for spectral nulling by a minimal adjustment, or offset, to the phase of the radar pulse that maintains constant pulse amplitude. Numerical examples show that a small time-varying phase offset designed by the proposed method can produce deep spectral nulls at one or more chosen frequencies in the radar's spectral sidelobes with little other effect on the pulse spectrum or ambiguity function.

Comparison between pulsed laser and frequency-domain photoacoustic modalities: signal-to-noise ratio, contrast, resolution, and maximum depth detectivity.

Abstract: In this work, a detailed theoretical and experimental comparison between various key parameters of the pulsed and frequency-domain (FD) photoacoustic (PA) imaging modalities is developed. The signal-to-noise ratios (SNRs) of these methods are theoretically calculated in terms of transducer bandwidth, PA signal generation physics, and laser pulse or chirp parameters. Large differences between maximum (peak) SNRs were predicted. However, it is shown that in practice the SNR differences are much smaller. Typical experimental SNRs were 23.2 dB and 26.1 dB for FD-PA and time-domain (TD)-PA peak responses, respectively, from a subsurface black absorber. The SNR of the pulsed PA can be significantly improved with proper high-pass filtering of the signal, which minimizes but does not eliminate baseline oscillations. On the other hand, the SNR of the FD method can be enhanced substantially by increasing laser power and decreasing chirp duration (exposure) correspondingly, so as to remain within the maximum permissible exposure guidelines. The SNR crossover chirp duration is calculated as a function of transducer bandwidth and the conditions yielding higher SNR for the FD mode are established. Furthermore, it was demonstrated that the FD axial resolution is affected by both signal amplitude and limited chirp bandwidth. The axial resolution of the pulse is, in principle, superior due to its larger bandwidth; however, the bipolar shape of the signal is a drawback in this regard. Along with the absence of baseline oscillation in cross-correlation FD-PA, the FD phase signal can be combined with the amplitude signal to yield better axial resolution than pulsed PA, and without artifacts. The contrast of both methods is compared both in depth-wise (delay-time) and fixed delay time images. It was shown that the FD method possesses higher contrast, even after contrast enhancement of the pulsed response through filtering.

Spectral and temporal changes of optical pulses propagating through time-varying linear media

Abstract: We present universal formulas for the spectral and temporal output optical fields from a linear traveling-wave medium whose refractive index changes during its propagation within the medium. These formulas agree with known changes in central wavelength and energy that are associated with adiabatic wavelength conversion (AWC). Moreover, they reveal new changes to the optical pulses that have not been noticed, such as pulse compression and spectral broadening. Most significantly, we find that AWC alters the pulse power, pulse chirp, and pulse delay. All of these effects depend on whether the central wavelength is blueshifted or redshifted, the first sign of asymmetry to be reported for AWC. These findings impact the applications of AWC to optical signal processing in microphotonic and nanophotonic structures as well as in lightwave systems.

Determination of Sweep Linearity Requirements in FMCW Radar Systems Based on Simple Voltage-Controlled Oscillator Sources

Abstract: Linear frequency modulated (FM), or chirp, pulse compression is a widely used technique for improving the range resolution of radar systems, although it often places quite stringent demands on FM sweep linearity. This paper examines the impact of sweep nonlinearities on the performance of frequency modulated continuous wave (FMCW) radar systems, particularly those employing simple voltage-controlled oscillator (VCO) sources, using a new and straightforward approach based on the fractional slope variation (FSV). Modeled results are presented, assuming a square-law source nonlinearity representation, showing the effect of such nonlinearities on point-target response and range resolution. These results are then related to the standard definition of linearity. Measurements from a commercial VCO are finally used to convincingly validate the work, resulting in a simple and practical method to predict the impact of source nonlinearity, as defined by the FSV parameter, on the performance of an FMCW radar system.

Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse

Abstract: We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2+1+1 Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly driven two-photon transition $4s\leftarrow\leftarrow3s$, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark-shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state "bow-tie" level crossing allows the preparation of coherent superposition states.

All-optical magnetization recording by tailoring optical excitation parameters

Abstract: We investigate the dependency of all-optical magnetization switching on the properties of the exciting laser pulse by specifically tailoring all accessible laser parameters---pulse duration, wavelength, chirp, and bandwidth---over a wide range. Our results show that all-optical switching can be achieved with picosecond instead of femtosecond laser sources of various wavelengths, which considerably relaxes technological feasibility of this technique. The most striking implication is that, in contrast to all current knowledge, a strong photoinduced nonequilibrium in the electronic system is not necessary to achieve magnetization switching with light.

Modulator Bias and Optical Power Control of Optical Complex E-Field Modulators

Abstract: We describe novel techniques for closed-loop control of inner and outer Mach-Zehnder (MZ) biases in a dual parallel MZ (DPMZ) optical modulator. We also present a new technique based on modulator bias dithers to monitor the optical power contribution of each RF data path. The latter is demonstrated in an I -Q power balance control loop. We show that the new control methods are extremely robust in presence of modulator non-idealities, such as finite extinction ratios and non-zero chirp. We also demonstrate these techniques to be largely independent of RF drive waveform characteristics. Mathematical derivations of control transfer curves are provided. These are supported by simulation and measurement results. Additionally, we present a simple technique for polarization power balance in a dual-polarization modulation format. Finally, we discuss some practical details related to the implementation of the control loops described in this work.

A 1.5GHz-modulation-range 10ms-modulation-period 180kHz rms -frequency-error 26MHz-reference mixed-mode FMCW synthesizer for mm-wave radar application

Abstract: A frequency modulated continuous-wave (FMCW) radar using triangular modulation is one of the promising candidates for realizing a CMOS radar IC [1–3]. Range and velocity resolutions of the FMCW radar are determined by the bandwidth and period of triangular modulation [4]. A short-range measurement requires wide (several GHz) bandwidth, while a long-range measurement with high-velocity resolution requires moderate (hundreds of MHz) bandwidth and long (several ms) period. Furthermore, since frequency error in the FMCW signal deteriorates range and velocity accuracy, a highly linear frequency chirp signal is required. However FMCW radars reported so far [2,3] exhibit a period up to 1.5ms because a long period degrades the chirp linearity in a conventional analog PLL. In this work, an analog/digital mixed-mode 82GHz FMCW synthesizer with 1.5GHz bandwidth, a period from 1ms to 10ms and less than 180kHz rms frequency error is described. The achieved performance corresponds to range and velocity resolutions of 10cm and 1.4km/h, respectively.

Spectrally Resolved Maker Fringes in High-Order Harmonic Generation

Abstract: We investigate macroscopic interference effects in high-order harmonic generation using a Ti:sapphire laser operating at a 100 kHz repetition rate. The structure and behavior of spectral and spatial interference fringes are explained and analytically described by transient phase matching of the long electron trajectory contribution. Time-frequency mapping due to the temporal chirp of the harmonic emission allows us to observe Maker fringes directly in the spectral domain.

Particle manipulation in a microfluidic channel using acoustic trap

Abstract: A high frequency sound beam was employed to explore an experimental method that could control particle motions in a microfluidic device. A 24 MHz single element lead zirconate titanate (PZT) transducer was built to transmit a focused ultrasound of variable duty factors (pulse duration/pulse repetition time), and its 1–3 piezocomposite structure established a tight focusing with f-number (focal depth/aperture size) of one. The transducer was excited by the Chebyshev windowed chirp signal sweeping from 18 MHz to 30 MHz with a 50% of duty factor, in order to ensure that enough sound beams were penetrated into the microfluidic device. The device was fabricated from a polydimethylsiloxane (PDMS) mold, and had a main channel composed of three subchannels among which particles flowed in the middle. A 60~70 μm diameter single droplet in the flow could be trapped near the channel bifurcation, and subsequently diverted into the sheath flow by releasing or shifting the acoustic trap. Hence, the results showed the potential use of a focused sound beam in microfluidic devices, and further suggested that this method could be exploited in the development of ultrasound-based flow cytometry and cell sorting devices.

Chirp-like transmit waveforms with multiple frequency-notches

Abstract: This paper describes the construction of chirp-like constant-modulus transmit waveforms designed so as to possess multiple notches in their frequency spectra at user-specified frequencies. We propose an iterative projection algorithm with low computational complexity. In wide-band radar systems, such frequency-notched transmit waveforms are needed so as to avoid transmitting into frequency bands that are used by other systems such as for communication, navigation, etc.

Real-Time Interrogation of a Linearly Chirped Fiber Bragg Grating Sensor Based on Chirped Pulse Compression With Improved Resolution and Signal-to-Noise Ratio

Abstract: A novel approach to interrogating in real time a linearly chirped fiber Bragg grating (LCFBG) sensor based on spectral-shaping and wavelength-to-time (SS-WTT) mapping with improved interrogation resolution and signal-to-noise (SNR) ratio is proposed and experimentally demonstrated. The proposed system consists of a mode-locked laser source, an optical interferometer incorporating an LCFBG, and a dispersive element. The optical interferometer has a spectral response with an increasing free spectral range (FSR). The incorporation of the LCFBG in the interferometer would encode the sensing information in the spectral response as a change in the FSR. After SS-WTT mapping, a linearly chirped microwave waveform is obtained. The correlation of the linearly chirped microwave waveform with a chirped reference waveform would provide a sharp correlation peak with its position indicating the wavelength shift of the LCFBG. A theoretical analysis is carried out, which is validated by a numerical simulation and an experiment. The experimental results show that the proposed system can provide an interrogation resolution as high as 0.25 μe at a speed of 48.6 MHz.