Showing papers on "Chirp published in 2008"

Large delay-bandwidth product and tuning of slow light pulse in photonic crystal coupled waveguide.

Abstract: This paper reports two advances in a slow light device consisting of chirped photonic crystal slab coupled waveguide on SOI substrate. One is concerning the delay-bandwidth product, indicating the buffering capacity of the device. We experimentally evaluated a record high value of 57 (a 40 ps delay and a 1.4 THz bandwidth). We also observed ~1 ps wide optical pulse transmission in the cross-correlation measurement. Regarding the pulse as a signal and considering the broadening of the pulse width due to the imperfect dispersion compensation in the device, storage of more than 12 signal bits was confirmed. The other is a wide-range tuning of the pulse delay. We propose a technique for externally controlling the chirping to permit variable delay. We demonstrate tuning of the pulse delay up to 23 ps, corresponding to a ~7 mm extension of the free space length.

Coherent anti-Stokes Raman microspectroscopy using spectral focusing with glass dispersion

Abstract: We demonstrate experimentally that coherent anti-Stokes Raman microspectroscopy with high spectral resolution is achieved using femtosecond laser pulses chirped up to a few picoseconds by glass elements of known group-velocity dispersion without significant intensity losses. By simply choosing the length of the glass, the chirp of Stokes and pump pulses is tailored to obtain a spectral resolution given by the Fourier limit of the chirped pulse duration. We show that for chirped pulse durations shorter than or comparable to the Raman coherence time, maximum signal occurs for a pump arriving after the Stokes pulse, a time-ordering effect confirmed by numerical simulations.

Photonic Generation of Chirped Millimeter-Wave Pulses Based on Nonlinear Frequency-to-Time Mapping in a Nonlinearly Chirped

Abstract: A novel approach to optically generating chirped millimeter-wave pulses with tunable chirp rate based on spectral shaping and nonlinear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed approach, the optical power spectrum of an ultrashort pulse from a fem- tosecond pulsed laser is shaped by a two-tap Sagnac loop filter that has a sinusoidal frequency response. The spectrum-shaped optical pulse is then sent to a nonlinearly chirped fiber Bragg grating (NL-CFBG) with a tunable nonlinear group delay to serve as a high-order dispersive device to perform the nonlinear frequency-to-time mapping. A chirped electrical pulse with a high central frequency and large chirp rate is then generated at the output of a high-speed photodetector. The NL-CFBG used in the proposed system is produced from a regular linearly chirped fiber Bragg grating based on strain-gradient beam tuning. A detailed theoretical analysis on the chirped pulse generation is developed, which is verified by numerical simulations and experi- ments. Millimeter-wave pulses with a central frequency of around 35 GHz and instantaneous frequency chirp rates of 0.053 and 0.074 GHz/ps are experimentally generated. Index Terms—Chirped pulse generation, chromatic dispersion, frequency-to-time mapping, microwave photonics, nonlinearly chirped fiber Bragg grating (NL-CFBG), pulse compression, radar.

Giant-chirp oscillators for short-pulse fiber amplifiers.

Abstract: A new regime of pulse parameters in a normal-dispersion fiber laser is identified. Dissipative solitons exist with remarkably large pulse duration and chirp, along with large pulse energy. A low-repetition-rate oscillator that generates pulses with large and linear chirp can replace the standard oscillator, stretcher, pulse-picker, and preamplifier in a chirped-pulse fiber amplifier. The theoretical properties of such a giant-chirp oscillator are presented. A fiber laser designed to operate in the new regime generates ~150 ps pulses at a 3 MHz repetition rate. Amplification of these pulses to 1 μJ energy with pulse duration as short as 670 fs demonstrates the promise of this new approach.

Radar waveform for automotive radar systems and applications

Abstract: This paper describes three different continuous wave (CW) radar waveforms which are applied in automotive radar sensors. The radar sensor should be able to detect all objects inside the observation area and to estimate range, radial velocity and azimuth angle simultaneously even in multi target situations. A classical Linear Frequency Modulation (LFM) waveform provides a very high range and velocity resolution. But in multiple target situations so-called ghost targets will occur. Alternatively, a Frequency Shift Keying (FSK) waveform delivers a very high velocity resolution and avoids any ghost target situation but will not resolve targets in range direction. Finally, a combination between LFM and FSK provides simultaneously a high range and velocity resolution, extremely short measurement time and avoids any ghost target situation. It combines the advantages of both FSK and LFM waveforms. In all three cases the azimuth angle estimation is based on two receive antennas and the monopulse technique.

Photonic Generation of Chirped Millimeter-Wave Pulses Based on Nonlinear Frequency-to-Time Mapping in a Nonlinearly Chirped Fiber Bragg Grating

Abstract: A novel approach to optically generating chirped millimeter-wave pulses with tunable chirp rate based on spectral shaping and nonlinear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed approach, the optical power spectrum of an ultrashort pulse from a femtosecond pulsed laser is shaped by a two-tap Sagnac loop filter that has a sinusoidal frequency response. The spectrum-shaped optical pulse is then sent to a nonlinearly chirped fiber Bragg grating (NL-CFBG) with a tunable nonlinear group delay to serve as a high-order dispersive device to perform the nonlinear frequency-to-time mapping. A chirped electrical pulse with a high central frequency and large chirp rate is then generated at the output of a high-speed photodetector. The NL-CFBG used in the proposed system is produced from a regular linearly chirped fiber Bragg grating based on strain-gradient beam tuning. A detailed theoretical analysis on the chirped pulse generation is developed, which is verified by numerical simulations and experiments. Millimeter-wave pulses with a central frequency of around 35 GHz and instantaneous frequency chirp rates of 0.053 and 0.074 GHz/ps are experimentally generated.

Photonic Generation of Chirped Microwave Pulses Using Superimposed Chirped Fiber Bragg Gratings

Abstract: A novel approach to generating linearly chirped microwave pulses in the optical domain based on spectral shaping and linear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed system, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is spectrum-shaped by an optical filter that consists of two superimposed chirped fiber Bragg gratings (SI-CFBGs) with different chirp rates. The SI-CFBGs form a Fabry-Perot cavity with a cavity length linearly dependent on the resonance wavelength, thus a spectral response with an increased or decreased free spectral range is generated. A chirped microwave pulse with the pulse shape identical to the shaped spectrum is obtained at the output of a high-speed photodetector thanks to the frequency-to-time mapping in a dispersive device. The proposed technique is experimentally demonstrated, a linearly chirped microwave pulse with a central frequency of 15 GHz and a chirp rate of 0.0217 GHz/ps is experimentally generated.

Slow and fast dynamics of gain and phase in a quantum dot semiconductor optical amplifier

Abstract: Gain and phase dynamics in InAs/GaAs quantum dot semiconductor optical amplifiers are investigated. It is shown that gain recovery is dominated by fast processes, whereas phase recovery is dominated by slow processes. Relative strengths and time constants of the underlying processes are measured. We find that operation at high bias currents optimizes the performance for nonlinear cross-gain signal processing if a low chirp is required.

Lossless equalization of frequency combs.

Abstract: Frequency combs obtained by sinusoidal phase modulation of narrowband cw lasers are widely used in the field of optical communications. However, the resulting spectral envelope of the comb is not flat. We propose a general and efficient approach to achieve flat frequency combs with tunable bandwidth. The idea is based on a two-step process. First, efficient generation of a train with a temporal flat-top-pulse profile is required. Second, we use large parabolic phase modulation in every train period to map the temporal intensity shape into the spectral domain. In this way the resulting spectral envelope is flat, and the size is tunable with the chirping rate. Two different schemes are proposed and verified through numerical simulations.

Optical parametric amplifiers using chirped quasi- phase-matching gratings I: practical design formulas

Abstract: Optical parametric amplifiers using chirped quasi-phase-matching (QPM) gratings offer the possibility of engineering the gain and group delay spectra. We give practical formulas for the design of such amplifiers. We consider linearly chirped QPM gratings providing constant gain over a broad bandwidth, sinusoidally modulated profiles for selective frequency amplification and a pair of QPM gratings working in tandem to ensure constant gain and constant group delay at the same time across the spectrum. The analysis is carried out in the frequency domain using Wentzel-Kramers-Brillouin analysis.

Exact spatial soliton solutions of the two-dimensional generalized nonlinear Schrödinger equation with distributed coefficients

Abstract: An improved homogeneous balance principle and an $F$-expansion technique are used to construct exact periodic wave solutions to the generalized two-dimensional nonlinear Schr\"odinger equation with distributed dispersion, nonlinearity, and gain coefficients. For limiting parameters, these periodic wave solutions acquire the form of localized spatial solitons. Such solutions exist under certain conditions, and impose constraints on the functions describing dispersion, nonlinearity, and gain (or loss). We establish a simple procedure to select different classes of solutions, using the dispersion and the gain coefficient in one case, or the chirp function and the gain coefficient in the other case, as independent parameter functions. We present a few characteristic examples of periodic wave and soliton solutions with physical relevance.

Inverse synthetic aperture radar imaging of ship target with complex motion

Abstract: High-resolution inverse synthetic aperture radar (ISAR) imaging and recognition of ship target is very important for many applications. Although the principle of ISAR imaging of ship target on the sea is the same as that of flying target in the sky, the former usually has more complex motion (fluctuation with the oceanic waves) than the latter, which makes the motion compensation very difficult. However, the change in phase chirp rate caused by the complex motion of ships will deteriorate the azimuth focusing quality. In this paper, we first model the complex motion of ship target with cubic phase terms (parameterised on chirp rate and its change rate), then a new ISAR imaging method, referred to as TC-DechirpClean, is proposed, which estimates the chirp rate and the change rate of chirp rate of all scatters in the time-chirp distribution plane. Both numerical and experimental results are provided to demonstrate the performance of the proposed method.

Passive Nonlinear Pulse Shaping in Normally Dispersive Fiber Systems

Abstract: We propose a novel approach to characterize the parabolically-shaped pulses that can be generated from more conventional pulses via nonlinear propagation in cascaded sections of commercially available normally dispersive (ND) fibers. The impact of the initial pulse chirp on the passive pulse reshaping is examined. We furthermore demonstrate that the combination of pulse pre-chirping and propagation in a single ND fiber yields a simple, passive method for generating various temporal waveforms of practical interest.

A chirped photonic-crystal fibre

Abstract: Photonic crystals have widely increased the facility to guide and confine light at wavelengths close to the optical wavelength1,2,3. Because they can include extremely sharp bends, photonic-crystal waveguides are a key element in future integrated optical devices4. Moreover, they enable the manipulation of the spontaneous emission properties of luminescent devices5, the localization of light in microcavities6, and they may serve to generate negative refraction7,8. A special class of these devices are the hollow-core photonic-crystal fibres9,10,11, which confine the light by means of a periodic cladding, consisting of several layers of identical cells. This design resonantly decreases the transmission losses of such fibres to values of a few dB km−1 in a narrow wavelength range. However, the rather narrowband transmission bands and the detrimental third-order dispersion characteristics of this single-cell design generally render application of such hollow-core fibres difficult in the femtosecond range12. Therefore, no fibre-based concept can currently provide guiding of sub-100 fs pulses over extended distances. By introducing a radial chirp into the photonic crystal, we here demonstrate a novel concept for photonic-crystal fibres that breaks with the paradigm of lattice homogeneity and enables a new degree of freedom in photonic-crystal-fibre design, eliminating much of the pulse duration restriction of earlier approaches. Hollow-core photonic-crystal fibres enable confinement of light on a much tighter scale than is possible with conventional fibre. But dispersion makes it difficult to transmit very short, sub 100 fs, pulses over long distances. A chirped structure could offer a solution.

Coupled-ring-resonator-based silicon modulator for enhanced performance.

Abstract: A compact silicon coupled-ring modulator structure is proposed. Two rings are coupled to each other, and only one of these rings is actively driven and over-coupled to a waveguide, which enables high modulation speed. The resultant notch filter profile is steeper than that of the single ring and has exhibited a smaller resonance shift and lower driving electrical power. Simulation results include: (i) potentially 60-Gb/s non-return-to-zero (NRZ) data modulation and over 20-dB extinction ratio can be achieved by shifting the active ring by a 20 GHz resonance shift, (ii) the frequency chirp of the modulated signals can be adjusted by varying the coupling coefficient between the two rings, and (iii) dispersion tolerance at 0.5-dB power penalty is extended from 18 to 85 ps/nm, for a 40-Gb/s NRZ signal.

Polar format agorithm using chirp scaling for spotlight SAR image formation

Abstract: A novel implementation of the polar format algorithm (PFA) using the principle of chirp scaling (PCS) for spotlight synthetic aperture radar (SAR) image formation is addressed. A PFA completely free of interpolation has been achieved in moderate squinted imaging geometry. The presented approach consists of the range and azimuth scaling of the polar samples, in which only fast Fourier transforms (FFTs) and complex vector multiplications are involved. Following different processing chains, the dechirped and the chirped SAR signals are treated separately in this paper. Relative to the classic polyphase filter that uses the long sinc kernel, the new solution is able to attain a comparable result, but is much quicker by exploiting inherent properties of the linear frequency modulated (LFM) signal. Point target simulation has validated the presented methodology.

Broadband excitation for short-time impedance spectroscopy.

Abstract: Frequency domain impedance measurements are still the common approach in assessing passive electrical properties of cells and tissues. However, due to the time requirements for sweeping over a frequency range for performing spectroscopy, they are not suited for recovering fast impedance changes of biological objects. The use of broad bandwidth excitation and monitoring the response as a function of time will greatly reduce the measurement time. The widespread usage of a square wave excitation is simple but not always the best choice. Here we consider different waveforms for excitation and discuss not only the advantages but also their limitations. Measurements in a miniaturized chamber where frequency and time domain measurements are compared show the suitability of different waveforms as excitation signals for the measurements of bio-impedance. The chirp excitation has been found to be most promising in terms of frequency range, signal-to-noise ratio and crest factor.

Synthetic Aperture Radar (SAR) Signal Generation

Abstract: This paper outlines the trend of signal generation in synthetic aperture radar particularly chirp (linear FM signal) generation using digital approach. A study in fundamental of FM signal and typical analog FM signal generation is highlighted. Various signal generation in SAR using digital techniques is discussed and finally the some of the digital chirp generators are presented.

Population transfer between two quantum states by piecewise chirping of femtosecond pulses: theory and experiment.

Abstract: We propose and experimentally demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The piecewise nature of the process suggests a possibility for the selective population transfer in complex quantum systems.

Demonstration of detuning and wavebreaking effects on Raman amplification efficiency in plasma

Abstract: A plasma-based resonant backward Raman amplifier/compressor for high power amplification of short laser pulses might, under ideal conditions, convert as much as 90% of the pump energy to the seed pulse. While the theoretical highest possible efficiency of this scheme has not yet been achieved, larger efficiencies than ever before obtained experimentally (6.4%) are now being reported, and these efficiencies are accompanied by strong pulse compression. Based on these recent extensive experiments, it is now possible to deduce that the experimentally realized efficiency of the amplifier is likely constrained by two factors, namely the pump chirp and the plasma wavebreaking, and that these experimental observations may likely involve favorable compensation between the chirp of the laser and the density variation of the mediating plasma. Several methods for further improvement of the amplifier efficiency in current experiments are suggested.

Numerical Study of a DFB Semiconductor Laser and Laser Array With Chirped Structure Based on the Equivalent Chirp Technology

Abstract: We propose that a Bragg grating with a chirp profile in a semiconductor distributed feedback (DFB) laser can be designed and fabricated through nonuniform sampling of a uniform grating based on the equivalent chirp technology (ECT). The characteristics, including the relationship between the input current and output power, the light distribution within the laser cavity, the lasing spectrum, and the linewidth, are numerically analyzed and compared with a DFB laser with a true chirped grating. The study shows that the proposed DFB laser with an equivalent chirp provides an identical performance to that of a conventional DFB laser. Based on the ECT, different chirp gratings in a DFB laser array can also be fabricated by simply using different sampling functions, with the grating period in all gratings kept constant. The key advantage of the proposed technique is that a DFB laser or laser array can be fabricated based on the conventional holography technology, which would simplify the fabrication.

Design of Novel Unapodized and Apodized Step-Chirped Quasi-Phase Matched Gratings for Broadband Frequency Converters Based on Second-Harmonic Generation

Abstract: The unapodized and apodized step-chirped gratings (SCGs) for broadband frequency converters based on quasi-phase-matched second-harmonic generation with pump depletion in lithium niobate waveguides have been theoretically modeled and simulated as a function of the number of sections, and compared with the linearly chirped gratings (LCGs) for the first time to our knowledge. It is shown that for the same length, using fewer sections with more segments and larger amounts of chirp, the efficiency and bandwidth of an SCG approach over that of an LCG and can be extensively improved with apodization. Moreover, the increasing chirp period and duty cycle for the SCG structure may provide a more convenient method for fabrication and poling. In addition, we present useful relations for the band-widths that help to find the appropriate number of segments in the proposed SCGs of a given length.

Performance of chirp spread spectrum in wireless communication systems

Abstract: This paper analyzes the structure and modem scheme of chirp-BOK system. In additive white Gaussian noise channel, an approach of studying the performance of chirp-BOK is presented and the BER-SNR result is evidently improved compared with traditional BOK. Furthermore, the BER of multi-user chirp-BOK system is discussed.

All-optical TDM to WDM signal conversion and partial regeneration using XPM with triangular pulses

Abstract: We propose a new all-optical, all-fibre scheme for conversion of time-division multiplexed to wavelength-division multiplexed signals using cross-phase modulation with triangular pulses. Partial signal regeneration using this technique is also demonstrated.

Information-theoretic matched waveform in signal dependent interference

Abstract: The design of transmit waveforms is critical to the performance of a radar system. Traditionally, pulsed and wideband chirp transmit waveforms are used. Our focus here is on the design of waveforms matched to extended targets. In the literature, optimum transmit waveform design in additive receiver noise has been investigated both from SNR maximization and information-theoretic approaches. In this paper, we extend the information-based approach to the signal-dependent interference problem. In particular, we investigate the use of information theory to generate the optimum waveform matched to a Gaussian distributed target ensemble with known spectral variance in the presence of signal-dependent clutter. Several waveform design examples are shown. We also apply this waveform technique to a cognitive radar application.

Optical frequency conversion, pulse compression and signal copying using triangular pulses

Abstract: We propose techniques of optical frequency conversion, pulse compression and signal copying based on a combination of cross-phase modulation using triangular pump pulses and subsequent propagation in a dispersive medium.

Obtaining information from time variation of sensing results

Abstract: Sensing results from moving objects, e.g. from photosensing emanating light or from impedance-based sensing, can indicate sensed time-varying waveforms with information about objects. For example, a sensed time-varying waveform can be compared with another waveform, such as a reference waveform produced by objects of a certain type, to obtain comparison results indicating motion-independent information about the object; time-scaling can adjust for displacement rate such as speed. Also, a modulation periodicity value can be obtained from a sensed time-varying waveform and used in obtaining information about an object; for example, a periodic modulation frequency can be used with a given time's chirp frequency to obtain phase information about an object's position. Or, where periodic modulation frequency indicates displacement rate, time scaling during comparison can use a scaling factor based on the frequency. Objects can move fluidically as in flow cytometry or through scanning movement, as in document scanning.

Metrologies for the phase characterization of attosecond extreme ultraviolet optics

Abstract: Extreme ultraviolet (EUV) optics play a key role in attosecond science since only with higher photon energies is it possible to achieve the wide spectral bandwidth required for ultrashort pulses. Multilayer EUV mirrors have been proposed and are being developed to temporally shape (compress) attosecond pulses. To fully characterize a multilayer optic for pulse applications requires not only knowledge of the reflectivity, as a function of photon energy, but also the reflected phase of the mirror. We develop the metrologies to determine the reflected phase of an EUV multilayer mirror using the photoelectric effect. The proposed method allows one to determine the optic's impulse response and hence its pulse characteristics.

Decrease of pulse-contrast in nonlinear chirped-pulse amplification systems due to high-frequency spectral phase ripples.

Abstract: It is analytically shown that weak initial spectral phase modulations cause a pulse-contrast degradation at the output of nonlinear chirped-pulse amplification systems The Kerr-nonlinearity causes an energy-transfer from the main pulse to side-pulses during nonlinear amplification The relative intensities of these side-pulses can be described in terms of Bessel-functions It is shown that the intensities of the pulses are dependent on the magnitude of the accumulated nonlinear phase-shift (ie, the B-integral), the depth and period of the initial spectral phase-modulation and the slope of the linear stretching chirp The results are applicable to any type of laser amplifier that is based on the technique of chirped-pulse amplification The analytical results presented in this paper are of particular importance for high peak-power laser applications requiring high pulse-contrasts, eg high field physics

Deception jamming modeling in radar sensor networks

Abstract: In this paper we create a modeling scenario which includes an airborne imaging radar platform, receiving platform and an electronic countermeasure (ECM) platform whose goal is to introduce false target images. We also assume a high-resolution stripmap SAR model operating with several common types of UWB signals, as well as the newly proposed OFDM radar waveforms. OFDM-coded radar signals possess a viable quality of high pulse diversity potential, which can provide for robust performance in various jamming scenarios. In this paper, a method of jamming known as dasiadeceptionpsila is considered and a jammer model which aspires to generate false targets using intercepted radar signals is used. Two approaches to creating a radar signal by the jammer are considered: instantaneous frequency (IF) estimator and digital RF memory (DRFM)-based reproducer. In both cases, the jammer aims to create a copy of a valid target image - but located elsewhere in the observed target scene - via resending the radar signal at certain time intervals. Radar imaging simulation based on backprojection algorithm in presence of a deception jammer is performed for the cases of linear frequency modulated (LFM), short-pulse Gaussian, frequency-hopped (FH) and OFDM ultrawideband waveforms. The comparisons in ECCM performance of these signals are made based on the analysis of simulated imagery.