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Showing papers on "Chirp published in 2007"


Journal ArticleDOI
TL;DR: The progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses is reviewed and adaptive femTosecond quantum control is realized, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm.
Abstract: We review the progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses. Due to its high particle density the condensed phase has both high relevance and appeal for chemical synthesis. Thus, in recent years different methods have been developed to manipulate the dynamics of condensed-phase systems by changing one or multiple laser pulse parameters. Single-parameter control is often achieved by variation of the excitation pulse’s wavelength, its linear chirp or its temporal subpulse separation in case of pulse sequences. Multiparameter control schemes are more flexible and provide a much larger parameter space for an optimal solution. This is realized in adaptive femtosecond quantum control, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm. Several experiments are presented that illustrate the different control concepts and highlight their broad applicability. These fascinating achievements show the continuous progress on the way towards the control of complex quantum reactions in the condensed phase.

294 citations


Journal ArticleDOI
26 Mar 2007
TL;DR: In this paper, the authors demonstrate error-free wavelength conversion at 320 Gb/s by employing a semiconductor optical amplifier that fully recovers in 56 ps. Error-free operation is achieved without using forward error correction technology.
Abstract: We demonstrate error-free wavelength conversion at 320 Gb/s by employing a semiconductor optical amplifier that fully recovers in 56 ps. Error-free operation is achieved without using forward error correction technology. We employ optical filtering to select the blue sideband of the spectrum of the probe light, to utilize fast chirp dynamics introduced by the amplifier, and to overcome the slow gain recovery. This leads to an effective recovery time of less than 1.8 ps for the wavelength converter. The wavelength converter has a simple configuration and is implemented by using fiber-pigtailed components. The concept allows photonic integration

234 citations


Journal ArticleDOI
TL;DR: It is found that the ratio of nonlinearity and dispersion linearizes the pulse chirp, reducing the spectral oscillations caused by self-phase modulation alone and giving rise to a nonlinear transfer function suitable for all-optical regeneration of high data rate signals.
Abstract: We report on the fabrication and optical properties of etched highly nonlinear As2S3 chalcogenide planar rib waveguides with lengths up to 22.5 cm and optical losses as low as 0.05 dB/cm at 1550 nm - the lowest ever reported. We demonstrate strong spectral broadening of 1.2 ps pulses, in good agreement with simulations, and find that the ratio of nonlinearity and dispersion linearizes the pulse chirp, reducing the spectral oscillations caused by self-phase modulation alone. When combined with a spectrally offset band-pass filter, this gives rise to a nonlinear transfer function suitable for all-optical regeneration of high data rate signals.

218 citations


Proceedings ArticleDOI
26 Aug 2007
TL;DR: In this article, a simple encoder with a 2D FFT and a random sampler is used to compress the raw SAR data by sampling the signal below Nyquist rate using ideas from Compressed Sensing.
Abstract: Synthetic Aperture Radar (SAR) is active and coherent microwave high resolution imaging system, which has the capability to image in all weather and day-night conditions. SAR transmits chirp signals and the received echoes are sampled into In-phase (I) and Quadrature (Q) components, generally referred to as raw SAR data. The various modes of SAR coupled with the high resolution and wide swath requirements result in a huge amount of data, which will easily exceed the on-board storage and downlink bandwidth of a satellite. This paper addresses the compression of the raw SAR data by sampling the signal below Nyquist rate using ideas from Compressed Sensing (CS). Due to the low computational resources available onboard satellite, the idea is to use a simple encoder, with a 2D FFT and a random sampler. Decoding is then based on convex optimization or uses greedy algorithms such as Orthogonal Matching Pursuit (OMP).

138 citations


Journal ArticleDOI
TL;DR: A method is proposed for obtaining time-frequency distributions of chirp signals embedded in nonstationary noise, with the two-fold objective of a sharp localization for the chirP components and a reduced level of statistical fluctuations for the noise.
Abstract: A method is proposed for obtaining time-frequency distributions of chirp signals embedded in nonstationary noise, with the two-fold objective of a sharp localization for the chirp components and a reduced level of statistical fluctuations for the noise. The technique consists in combining time-frequency reassignment with multitapering, and two variations are proposed. The first one, primarily aimed at nonstationary spectrum estimation, is based on sums of estimates with different tapers, whereas the second one makes use of differences between the same estimates for the sake of chirp enhancement. The principle of the technique is outlined, its implementation based on Hermite functions is justified and discussed, and some examples are provided for supporting the efficiency of the approach, both qualitatively and quantitatively

135 citations


Journal ArticleDOI
TL;DR: The Letter describes a technique for the spontaneous generation of chirped time-energy entangled photons that results in a temporally compressed, transform limited biphoton.
Abstract: The Letter describes a technique for the spontaneous generation of chirped time-energy entangled photons. Transmitting either photon through an appropriate dispersive medium results in a temporally compressed, transform limited biphoton. At maximum bandwidth, the biphoton is a single cycle in length, with a waveform that has the same characteristic shape as a classical single-cycle pulse.

127 citations


Journal ArticleDOI
TL;DR: In this paper, a method to coherently control electron dynamics is proposed using a few-cycle laser pulse in combination with a controlling field, which not only broadens the attosecond pulse bandwidth, but also reduces the chirp; thus an isolated $80\text{\penalty1000-\hskip 0pt} as}$ pulse is straightforwardly obtained, and even shorter pulses are achievable by increasing the intensity of the controlling field.
Abstract: A method to coherently control electron dynamics is proposed using a few-cycle laser pulse in combination with a controlling field. It is shown that this method not only broadens the attosecond pulse bandwidth, but also reduces the chirp; thus an isolated $80\text{\penalty1000-\hskip0pt}\mathrm{as}$ pulse is straightforwardly obtained, and even shorter pulses are achievable by increasing the intensity of the controlling field. Such ultrashort pulses allow one to investigate ultrafast electronic processes. In addition, the few-cycle synthesized pulse is expected to be useful for manipulating a wide range of laser-atom interactions.

119 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate numerically and experimentally a chirped mirror with controlled reflectivity and dispersion of up to 1.5 octaves, with residual group delay dispersion ripples <100 fs2 in all of this range.
Abstract: We demonstrate numerically and experimentally a chirped mirror with controlled reflectivity and dispersion of up to 1.5 octaves. A complementary pair of such mirrors has a reflectivity of 95% in the wavelength range 400–1200 nm with residual group delay dispersion ripples <100 fs2 in all of this range. The mirror pair allows one to compensate a chirp of the corresponding spectrum (with a smooth phase), resulting in sub-3-fs pulses.

118 citations


Journal ArticleDOI
TL;DR: In this article, the first pulse compression of ultrabroadband white-light continuum generated using both induced and self-phase modulations in an Ar-gas-filled hollow fiber was demonstrated.
Abstract: We demonstrate the first pulse compression of ultrabroadband white-light continuum generated using both induced- and self-phase modulations in an Ar-gas-filled hollow fiber. By feedback chirp compensation with a liquid crystal spatial light modulator and a modified spectral interferometry for direct electric-field reconstruction, 2.6-fs, 1.4-GW, 1.3-cycle transform-limited pulses are generated in the visible to near-infrared region.

118 citations


Journal ArticleDOI
TL;DR: It is shown that chirp group delay representations are potentially useful for improving ASR performance and presented one application in feature extraction for automatic speech recognition (ASR), which can be guaranteed to be spike-free.

98 citations


Journal ArticleDOI
TL;DR: In this article, a numerical model for the analysis of the chirp dynamics of quantum-dot (QD) semiconductor laser under large signal current modulation is presented, and the model is based on the multipopulation rate equation formalism, and it includes all the peculiar characteristics of the active QD material such as the inhomogeneous broadening of the gain spectrum, the presence of an excited state confined in the QDs and the nonconfined states due to the wetting layer and the barrier.
Abstract: We present a numerical model for the analysis of the chirp dynamics of quantum-dot (QD) semiconductor laser under large signal current modulation. The model is based on the multipopulation rate equation formalism, and it includes all the peculiar characteristics of the active QD material such as the inhomogeneous broadening of the gain spectrum, the presence of an excited state confined in the QDs and the presence of nonconfined states due to the wetting layer and the barrier. In this paper the model is applied to the analysis of the chirp of two QD single-mode lasers emitting from the ground state and from the excited state, respectively. In order to make comparisons of the chirp in various operating conditions, we define some equivalent parameters for quantifying the adiabatic and transient contributions to the chirp. These parameters are then used to analyze the chirp as function of the bias current, of the modulation depth and of the modulation frequency. All the various simulation results show that the carrier accumulation in the QD states, poorly involved in the stimulated emission process and the carrier dynamics in these states, can cause a nonzero chirp under current modulation even for the ideal condition of zero linewidth enhancement factor (or -parameter) at the laser threshold.

Journal ArticleDOI
TL;DR: In this article, a theory for phase-only sampled Bragg gratings was presented, and it was shown that a sampled FBG of N channels would require radicN/etamiddot times higher maximum reflective index modulation than that of the single-channel FBG.
Abstract: In this paper, we review our recent developments in the design and fabrication techniques for high-channel-count fiber Bragg gratings (FBGs). We have presented a theory for the phase-only sampled FBG and demonstrated that a sampled FBG of N channels would require radicN/etamiddot times higher maximum reflective index modulation than that of the single-channel FBG. We experimentally demonstrate a 45- and 81-channel linearly chirped FBG for nearly whole C-band dispersion compensation, which is fabricated with a novel diffraction precompensated phase mask. The grating specifications obtained agree well with the theoretical design. We have presented a novel method for multichannel FBG design, which enables us to design any kind of multichannel FBGs, where the spectrum response of each channel could be either identical or nonidentical. Particularly, the nine-channel nonlinearly chirped FBG, which is used as a simultaneous dispersion and dispersion slope compensator, has been demonstrated.

Journal ArticleDOI
TL;DR: A high-capacity ultrafast all-optical time demultiplexer that can be employed to retrieve 40 gigabits/second base-rate channels from a 640 Gb/s single-polarized signal is presented.
Abstract: We present a high-capacity ultrafast all-optical time demultiplexer that can be employed to retrieve 40 gigabits/second (Gb/s) base-rate channels from a 640 Gb/s single-polarized signal. The demultiplexer utilizes ultrafast effects of filtered chirp of a semiconductor optical amplifier. Excellent demultiplexing performance is shown at very low switching powers: +8 dBm (640 Gb/s data) and -14 dBm (40 GHz clock). The demultiplexer has a simple structure and, in principle, allows monolithic integration.

Journal ArticleDOI
TL;DR: In this article, flat-top sharp optical filters of gigahertz bandwidth are realized using stimulated Brillouin scattering (SBS) to convert double sideband to single sideband (SSB) modulations for 1-GHz wide linear-frequency-modulated (LFM) signals of arbitrary radio-frequency carrier.
Abstract: Flat-top sharp optical filters of gigahertz bandwidth are realized using stimulated Brillouin scattering (SBS). Pump chirp control of the SBS process enables versatile programming of the filter shape and bandwidth. The operating wavelength of the filters is tunable, and their frequency response is inherently aperiodic. Full widths at half maximum of 1.3-2.5 GHz are demonstrated, with a filtering selectivity up to 30 dB and an rms ripple of 0.5-1 dB. The filters are used to convert double sideband to single sideband (SSB) modulations for 1-GHz-wide linear-frequency-modulated (LFM) signals of arbitrary radio-frequency carrier. Such SSB modulation is highly instrumental for photonic implementations of true time delay, for example, in antenna beam-forming. The peak-side-lobe ratio of the processed LFM signal was -32 dB; its main lobe was broadened by only 4%. The integrated side-lobe ratio, which is limited by noise from spontaneous Brillouin scattering, was better than 21 dB, which is a sufficient value for most systems. The technique results in a large modulation index and does not lead to harmonic distortions.

Journal ArticleDOI
TL;DR: In this article, the authors present the first experimental demonstration of a fully electronic system for the temporal magnification of signals in the ultra-wideband regime, which employs a broadband analog multiplier and uses chirped electromagnetic-bandgap structures in microstrip technology.
Abstract: We present the first experimental demonstration of a fully electronic system for the temporal magnification of signals in the ultra-wideband regime. The system employs a broadband analog multiplier and uses chirped electromagnetic-bandgap structures in microstrip technology to provide the required signal dispersion. The demonstrated system achieves a time-magnification factor of five in operation on a 0.6-ns time-windowed input signal with up to 8-GHz bandwidth. We discuss the advantages and limitations of this technique in comparison to recent demonstrations involving optical components

Journal ArticleDOI
TL;DR: In this paper, a distributed feedback (DFB) semiconductor laser with equivalent phase shifts and chirps is proposed for the first time to our knowledge and is investigated numerically.
Abstract: A distributed feedback (DFB) semiconductor laser with equivalent phase shifts and chirps is proposed for the first time to our knowledge and is investigated numerically. As an example, it is shown that the desired λ/4 phase shift in a phase-shifted laser can be obtained equivalently by a specially designed sampling structure instead of an actual phase shift, while the external characteristics are unchanged. This novel DFB structure is advantageous in that it can be fabricated by standard holographic technology. Hence, the proposed scheme is expected to provide a low-cost method for fabricating a high-performance DFB semiconductor laser with complex structures.

Journal ArticleDOI
TL;DR: It is demonstrated that, the character of similariton interactions crucially depends on the sign of the similariton phase chirp, and can under certain conditions lead to the formation of molecule-like bound states of two similaritons.
Abstract: We obtain exact self-similar solutions to an inhomogeneous nonlinear Schrodinger equation, describing propagation of optical pulses in fiber amplifiers with distributed dispersion and gain. We show that there exists a one-to-one correspondence between such self-similar waves and solitons of the standard, homogeneous, nonlinear Schrodinger equation if a certain compatibility condition is satisfied. As this correspondence guarantees the stability of the novel self-similar waves, we refer to them as similaritons. We demonstrate that, the character of similariton interactions crucially depends on the sign of the similariton phase chirp. In particular, we show that the similariton interactions can under certain conditions lead to the formation of molecule-like bound states of two similaritons.

Patent
Hiroshi Nakamoto1
30 Oct 2007
TL;DR: In this article, an optical transmitter of the invention phase modulates an output light from a light source according to data using a phase modulator, and then supplies it to an intensity modulator.
Abstract: An optical transmitter of the invention phase modulates an output light from a light source according to data using a phase modulator, and then supplies it to an intensity modulator The intensity modulator performs pulse carving of the output light from the phase modulator in accordance with a clock signal in which a duty cycle is changed to a value different from 50% by a waveform transformation circuit, with an operating range being a slope from a trough to a crest in the photoelectric response characteristics As a result, it is possible to realize, at the same time, duty cycle and chirp characteristics that enable excellent transmission characteristics to be obtained

Journal ArticleDOI
TL;DR: In this paper, an approach to generate chirped microwave pulses based on optical spectral shaping and nonlinear chromatic-dispersion-induced wavelength-to-time mapping using all-fiber components is proposed and demonstrated.
Abstract: An approach to generating chirped microwave pulse based on optical spectral shaping and nonlinear chromatic-dispersion-induced wavelength-to-time mapping using all-fiber components is proposed and demonstrated. In the proposed approach, the spectrum of a femtosecond pulse is shaped by a two-tap Sagnac-loop filter that has a sinusoidal spectrum response. The spectrum shaped pulse is then sent to a dispersive element that has first- and second-order chromatic dispersions. Thanks to the nonlinear wavelength-to-time mapping of the dispersive element, a temporal pulse that has a central frequency in the microwave band with a large chirp is generated, which provides the potential for applications in high-speed communications and radar systems. Numerical and proof-of-concept experimental results are presented.

Journal ArticleDOI
TL;DR: In this paper, a study of the variation of the spectral shape and the harmonic distribution of the high-order harmonics generated from silver plasma on the frequency chirp of the driving laser radiation (793-nm 48-fs) is reported.
Abstract: A study of the variation of the spectral shape and the harmonic distribution of the high-order harmonics generated from silver plasma on the frequency chirp of the driving laser radiation (793 nm 48 fs) is reported. The results of the systematic study of the harmonic generation from the 21st order up to the 61st order (λ=13 nm) are presented. A tuning of the harmonic wavelength up to 0.8 nm can be accomplished by variation of the laser chirp.

Journal ArticleDOI
TL;DR: An ultrawideband frequency-modulated continuous-wave radar is developed and an excellent agreement between radar estimates of snow thickness with physical measurements is observed, achieving a correlation coefficient of 0.95 and a vertical resolution of about 3 cm.
Abstract: An accurate knowledge of snow thickness and its variability over sea ice is crucial in determining the overall polar heat and freshwater budget, which influences the global climate. Recently, algorithms have been developed to extract snow thicknesses from satellite passive microwave data. However, validation of these data over the large footprint of the passive microwave sensor has been a challenge. The only method used thus far has been with meter sticks during ship cruises. To address this problem, we developed an ultrawideband frequency-modulated continuous-wave radar to measure the snow thickness over sea ice. We synthesized a very linear chirp signal by using a phase-locked loop with a digitally generated chirp signal as a reference to obtain a fine-range resolution. The radar operates over the frequency range from 2-8 GHz. We made snow-thickness measurements over the Antarctic sea ice by operating the radar from a sled in September and October 2003. We performed radar measurements over 11 stations with varying snow thicknesses between 4 and 85 cm. We observed an excellent agreement between radar estimates of snow thickness with physical measurements, achieving a correlation coefficient of 0.95 and a vertical resolution of about 3 cm. Comparison of simulated radar waveforms using a simple transmission line model with the measurements confirms our expectations that echoes from snow-covered sea ice are dominated by reflections from air-snow and snow-ice interfaces.

Journal ArticleDOI
TL;DR: The experimental results agree with the theoretical relation between the group index and bandwidth and the directional coupler was fabricated from a silicon-on-insulator PC slab with airhole diameter chirping.
Abstract: We report on the fabrication and evaluation of a directional coupler consisting of two different chirped photonic crystal (PC) waveguides that can generate wideband, low dispersion slow light. The directional coupler was fabricated from a silicon-on-insulator PC slab with airhole diameter chirping. For both waveguides, we observed a group index of 60–80 near the photonic band edge and opposite dispersion characteristics. In the directional coupler, we evaluated a group index of 30–40 in a wavelength bandwidth of 32 nm. The experimental results agree with the theoretical relation between the group index and bandwidth.

Journal ArticleDOI
TL;DR: In this article, a scheme for electron acceleration by two crossing chirped lasers has been proposed, where an important effect of a frequency chirp of the laser is investigated.
Abstract: A scheme for electron acceleration by two crossing chirped lasers has been proposed. An important effect of a frequency chirp of the laser is investigated. Two high intensity chirped lasers, with the same amplitude and frequency, crossing at an arbitrary angle in a vacuum, interfere, causing modulation of laser intensity. An electron experiences a ponderomotive force due to the resultant field of lasers and gains considerable energy. For a certain crossing angle, the electron gains maximum energy due to the constructive interference. A frequency chirp of the laser plays an important role during the electron acceleration in a vacuum. The electron momentum increases due to the frequency chirp. Hence, the electron energy is enhanced during acceleration.

Journal ArticleDOI
TL;DR: This paper focuses on a new implementation of the first stage, i.e., the range resampling, using the chirp z-transform (CZT) and can achieve a PFA totally free of interpolation.
Abstract: Besides an inverse two-dimensional (2-D) Fourier transform, the polar format algorithm (PFA) for the spotlight synthetic aperture radar (SAR) image formation can be normally divided into two cascaded processing stages, which are called the range and azimuth resampling, respectively. This paper focuses on a new implementation of the first stage, i.e., the range resampling, using the chirp z-transform (CZT). The presented algorithm requires no interpolation. It works for the SAR system directly digitizing the echo signal, as well as that employing the dechirp-on-receive approach. The parameters of the CZT, including the frequency spacing and the start frequency, are derived to accommodate the PFA in both cases. Related filtering and compensation procedures are developed for the chirp z-transformed range signal with and without dechirp, respectively, in order to achieve a signal format entirely suitable for the azimuth resampling. Furthermore, incorporating the new algorithm with the existing CZT-based azimuth resampling and focusing algorithm, we can achieve a PFA totally free of interpolation. The presented approach has been validated by point target simulation, and the test is carried out with a very critical relative bandwidth of 30%

Journal ArticleDOI
TL;DR: A method is presented that uses this unconventional, cost efficient, basically nonlinear synthesizer concept, but is capable of linearizing the frequency chirp directly in hardware by means of defined sweep predistortion.
Abstract: Recently , we introduced a software linearization technique for frequency-modulated continuous-wave (FMCW) radar applications using a nonlinear direct digital synthesizer based frequency source. In this letter, we present a method that uses this unconventional, cost efficient, basically nonlinear synthesizer concept, but is capable of linearizing the frequency chirp directly in hardware by means of defined sweep predistortion. Additionally, the concept is extended for the generation of defined nonlinear frequency courses and verified on measurements with a 2.45-GHz FMCW radar prototype

Journal ArticleDOI
TL;DR: The role of high-range resolution is investigated as a function of the illuminating wavelength and classification performance as function of diversity space is examined.
Abstract: The information content of radar target signatures is a key aspect for automatic target recognition. The role of high-range resolution is investigated as a function of the illuminating wavelength. The classification performance is evaluated using (i) full-scale 2D inverse synthetic aperture radar images obtained from a stepped-frequency chirp modulation radar system and (ii) the corresponding sub-spectra of the target reflectivity function forming lower resolution images at differing centre frequencies. The classification performance as given by different combinations of RF frequencies are also evaluated and compared with the coherent reconstruction from the full bandwidth. Finally, the classification results are also computed using multiple aspects to sense the target. In this way, classification performance as function of diversity space is examined.

Journal ArticleDOI
TL;DR: A new signal processing method, which uses a modified chirp signal for air-coupled ultrasonic imaging, which provides an improvement in both the resolution of images and signal-to-noise ratios.
Abstract: A new signal processing method, which uses a modified chirp signal for air-coupled ultrasonic imaging, is described. A combination of the elliptical and Tukey window functions has been shown to give a better performance than the Hanning windowing used in most pulse- compression algorithms for air-coupled applications. The elliptical-Tukey chirp signal provides an improvement in both the resolution of images and signal-to-noise ratios. In addition, this type of signal also reduces the level of signal voltages required to drive the source transducer while maintaining the performance of the system. This approach, thus, may have wide interest in all forms of wide bandwidth ultrasonic imaging.

Journal ArticleDOI
TL;DR: In this article, the authors present theoretical and experimental evidence that the output spectrum depends in a predictable way on the input chirp, and that an initial amount of CH that leads to the exact preservation of the spectral width of a given pulse can be predicted.
Abstract: Cross-polarized wave (XPW) generation is used for the contrast improvement of ultra-intense femtosecond laser pulses in a double CPA configuration. We present theoretical and experimental evidence that the XPW output spectrum depends in a predictable way on the input chirp. Therefore, a chirp controlled pulse can experience a pulse duration shortening up to a factor of \(\sqrt{3}\), and an initial amount of chirp that leads to the exact preservation of the spectral width of a given pulse can be predicted.

Journal ArticleDOI
TL;DR: In this article, gas-filled hollow-core fiber compression and the optical filamentation technique are compared experimentally in a parameter regime suitable for intense few-cycle pulse generation, in particular, pointing stability, spectral properties, and spatial chirp are investigated.
Abstract: The gas-filled hollow-core fiber compression and the optical filamentation technique are compared experimentally in a parameter regime suitable for intense few-cycle pulse generation. In particular, pointing stability, spectral properties, and spatial chirp are investigated. It is found that in the case of filamentation, the critical parameter for pointing stability is gas pressure inside the generation cell whereas for the hollow-core fiber it is alignment that plays this role. The hollow-core fiber technique yields spectra that are better suited for chirped-mirror pulse compression whereas filamentation offers higher throughput and prospects for easy-to-implement self-compression. We present spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements that directly show the transition in the spectral phase of the output continua into the self-compression regime as the gas pressure is increased.

Journal ArticleDOI
TL;DR: Two novel control methods based on adiabatic passage are proposed to be implemented in coherent anti-Stokes Raman scattering (CARS) microscopy for noninvasive imaging of biological structure and dynamics and allow one to achieve chemical sensitivity with high resolution.
Abstract: Two novel control methods based on adiabatic passage are proposed to be implemented in coherent anti-Stokes Raman scattering (CARS) microscopy for noninvasive imaging of biological structure and dynamics. The first method provides optimal pulse-area control of the resonant vibrational transitions by using a pair of equally linear-chirped pulses. The second method, named the 'roof' method, utilizes the chirp sign variation at the central time and gives robust adiabatic excitation of the resonant vibrational mode. Both methods are robust with respect to suppression of the off-resonant transitions. The methods allow one to achieve chemical sensitivity with high resolution and can be used to obtain CARS spectra of biological molecules with efficiently suppressed background.