Showing papers on "Chirp published in 1999"

Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems

Abstract: Polybinary, optical amplitude modulated phase shift keying (AM-PSK) polybinary, M-ary amplitude shift keying (ASK), and polyquaternary signaling schemes operating at 10 Gb/s are investigated in 1550-nm lightwave systems operating over standard, single-mode fiber. The premise for exploring these signal types is that they concentrate power at frequencies closer to the optical carrier where phase distortion of the optical field from chromatic dispersion is less severe. Issues such as modulator chirp, optimal level spacing in a 4-ary ASK signal, and phase modulated to amplitude modulated (PM-AM) noise conversion from a nonzero laser linewidth are studied. It is found that higher order polybinary signals do not offer an improvement in dispersion tolerance over a duobinary signal. 4-ary ASK is demonstrated to increase the dispersion-limited distance to 225 km experimentally and 350 km through simulation, but at the expense of a /spl sim/8 dB degradation in receiver sensitivity due to the increased number of levels and the signal dependence of signal-spontaneous beat noise. Furthermore, the linewidth requirement for a 4-ary ASK signal is less than 1 MHz in order to minimize the impact of PM-AM relative intensity noise (RIN) when transmitting over 200-300 km.

Photonic time stretch and its application to analog-to-digital conversion

Abstract: We demonstrate a new concept for analog-to-digital (A/D) conversion based on photonic time stretch. The analog electrical signal is intensity modulated on a chirp optical waveform generated by dispersing an ultrashort pulse. The modulated chirped waveform is dispersed in an optical fiber, leading to the stretching of its envelope. We have derived analytical expressions for the stretch factor and the resolution of the system. An analog-to-digital converter (ADC) consisting of the photonic time-stretch preprocessor and a 1-Gsample/s electronic ADC is demonstrated. This technique is promising for A/D conversion of ultrafast signals and, hence, for realization of the digital receiver.

A four-parameter atomic decomposition of chirplets

Abstract: A new four-parameter atomic decomposition of chirplets is developed for compact and precise representation of signals with chirp components. The four-parameter chirplet atom is obtained from the unit Gaussian function by successive applications of scaling, fractional Fourier transform (FRFT), and time-shift and frequency-shift operators. The application of the FRFT operator results in a rotation of the Wigner distribution of the Gaussian in the time-frequency plane by a specified angle. The decomposition is realized by using the matching pursuit algorithm. For this purpose, the four-parameter space is discretized to obtain a small but complete subset in the Hilbert space. A time-frequency distribution (TFD) is developed for clear and readable visualization of the signal components. It is observed that the chirplet decomposition and the related TFD provide more compact and precise representation of signal inner structures compared with the commonly used time-frequency representations.

Electrically tunable power efficient dispersion compensating fiber Bragg grating

Abstract: Novel devices only offer reasonable telecommunication solutions when they can be packaged and manufactured efficiently and at low cost. We demonstrate such a compact and power efficient tunable dispersion compensating fiber Bragg grating (FBG) device. The device relies on a distributed on-fiber thin-film heater deposited onto the outer surface of an unchirped FBG. Current flowing though the thin film generates resistive heating that is governed by the thickness profile of the metal film. A chirp in the grating is obtained by using a coating whose thickness varies with position along the length of the grating in a prescribed manner; the chirp rate is adjusted by varying the applied current. Using an electrical power of less than 1 W in a packaged device, we demonstrate a linearly chirped Bragg grating in which the dispersion is continuously tuned from -300 to -1350 ps/nm, with an average deviation from linearity of approximately 10 ps.

Broadband phase-matched difference frequency mixing of femtosecond pulses in GaSe: Experiment and theory

Abstract: Phase-matched difference frequency mixing within the broad spectrum of single 13 fs pulses from a cavity-dumped, mode-locked Ti:sapphire laser in a GaSe crystal allows the generation of femtosecond mid-infrared pulses, continuously tunable in the wavelength range from 7 to 20 μm. Pulse durations down to 95 fs at 9.3 μm are directly measured. Model calculations including the full dispersion of GaSe show that the chirp of the pump pulses allows to change both the pulse duration and conversion efficiency obtained in the nonlinear process, in agreement with the experiment.

Analytical design of double-chirped mirrors with custom-tailored dispersion characteristics

Abstract: We present a theory for the analytical design of double-chirped mirrors with special dispersion characteristics. A simple analytical equation takes an arbitrarily desired group delay dispersion (that also includes possible higher order dispersion) as an input function and gives the chirp law as an output. The chirp law determines the local Bragg wavelengths in the mirror. It allows the calculation of the thicknesses of the high- and low-index layers if the double chirp of the layers in the front part of the mirror is taken into account. We use this method to design a highly dispersive double-chirped semiconductor Bragg mirror and a double-chirped TiO/sub 2/-SiO/sub 2/ mirror for higher order dispersion compensation in optical parametric oscillators operating in the visible spectral range. The design formulas are applicable to general chirped Bragg gratings and provide insight into the reasons why certain dispersion characteristics might be impossible to achieve.

10-GHz, over 20-channel multiwavelength pulse source by slicing super-continuum spectrum generated in normal-dispersion fiber

Abstract: We demonstrate a multiwavelength pulse source which utilizes the super-continuum generated in a normal group-velocity dispersion fiber and an arrayed-waveguide grating filter. Over 20-channels of pulse train with a repetition rate of 10 GHz are realized, and each channel has almost the same pulsewidth. This uniformity originates from the fact that the super-continuum pulse generated in a normal-dispersion fiber has a rectangular waveform and a linear chirp.

Nonlinear frequency modulation chirps for active sonar

Abstract: Range sidelobe suppression in pulse-compression radars and sonars is conventionally performed using amplitude windowing in either the time or frequency domains. Using peak power limited transmitters, this inevitably causes a degradation in the signal-to-noise ratio available at the receiver output owing to shading or mismatch losses. Nonlinear frequency modulation (NLFM) chirps have been suggested as a solution to this problem and have been successfully implemented in a number of radar systems. Using appropriate NLFM functions it is possible to shape the energy spectrum of a chirp, achieving low-range sidelobes, without the need for any amplitude windowing and hence maintaining maximum efficiency. In the field of active sonar, NLFM chirps have been largely ignored because of their poor Doppler tolerance compared with conventional linear period modulation (LPM) chirps. It is shown that a combination of amplitude windowing and NLFM can be used to achieve a compromise between sidelobe suppression, Doppler tolerance and shading loss. Using a novel family of NLFM functions a significant performance advantage over LPM chirps can be attained over a useful range of target velocities. Experimental results support this argument.

Floquet theory for short laser pulses

Abstract: We develop adiabatic perturbation theory for quantum systems responding to short laser pulses, with or without a frequency chirp. Our approach rests on lifting the time-dependent Schrodinger equation to an extended Hilbert space, then applying standard perturbational techniques to Floquet states in this extended space, and finally projecting back to the physical Hilbert space. The same strategy also allows us to construct superadiabatic bases for monitoring the quantum evolution in the course of a pulse. These bases provide a diagnostic tool for improving the efficiency of pulse-induced population transfer. The formalism is applied to the selective excitation of molecular vibrational states by chirped laser pulses, which exploit either successive single-photon resonances or a multiphoton resonance, and by a STIRAP-like process.

Theoretical study of frequency chirping and extinction ratio of wavelength-converted optical signals by XGM and XPM using SOA's

Abstract: We develop a complete large-signal dynamic model of semiconductor optical amplifier (SOA)-based wavelength converters using cross-gain modulation (XGM) and cross-phase modulation (XPM). The large-signal dynamic model has been implemented using the modified transfer matrix method (TMM). This model takes into account longitudinal variations of carrier-induced refractive index, n-parameter, internal reflection, photon density, and amplified spontaneous emission noise at each small section. Therefore, we can accurately estimate frequency chirping, optical pulse patterns, and the extinction ratio as well as the dynamic characteristics of SOA's. Frequency chirping and the extinction ratio have been calculated for various parameters such as signal power and wavelength, CW power and wavelength, and facet reflectivity. Modulation bandwidth, defined by the 3-dB drop as the eye opening ratio (EOR) is investigated with current density, confinement factor, and cavity length.

An effective coded excitation scheme based on a predistorted FM signal and an optimized digital filter

Abstract: Presents a coded excitation imaging system based on a predistorted FM excitation and a digital compression filter designed for medical ultrasonic applications, in order to preserve both axial resolution and contrast. In radars, optimal Chebyshev windows efficiently weight a nearly rectangular spectrum. For the small time-bandwidth (TB) products available in ultrasound, the rectangular spectrum approximation is not valid, which reduces the effectiveness of weighting. Additionally, the distant range sidelobes are associated with the ripples of the spectrum amplitude and, thus, cannot be removed by weighting. The authors show that by using a predistorted chirp with amplitude or phase shaping for amplitude ripple reduction and a correlation filter that accounts for the transducer's natural frequency weighting, output sidelobe levels of -35 to -40 dB are directly obtained. When an optimized filter is applied on receive, contrast or resolution can be traded in for range sidelobe levels down to -86 dB. The digital filter is designed to efficiently use the available bandwidth and at the same time to be insensitive to the transducer's impulse response. For evaluation of the method, simulations were performed with the program Field II. A commercial scanner (B-K Medical 3535) was modified and interfaced to an arbitrary function generator along with an RF power amplifier (Ritec). Hydrophone measurements in water were done to establish excitation voltage and corresponding intensity levels (I/sub sptp/ and I/sub spta/) well below the safety limits. In vivo images of the liver from two healthy volunteers show apparent increased depth of penetration of about 4 cm at 4 MHz, for codes of length 25 /spl mu/s. Images taken from a non-attenuating wire phantom show that the -20 dB axial resolution for the coded scheme is as good as with pulse excitation (about 1.5 /spl lambda/), depending on the filter design criteria. The axial sidelobes are below -40 dB, which is the noise level of the measuring imaging system. The proposed excitation/compression scheme shows good overall performance and stability to the frequency shift due to attenuation. It increases the penetration depth, and still yields a high resolution and low adjustable sidelobes.

Evaluation of an ultrabroadband high-gain amplification technique for chirped pulse amplification facilities

Abstract: Recently, an amplification technique for ultrashort pulses was explored in detail in a theoretical paper by Ross et al. [Opt. Commun. 144, 125 (1997)]. The technique, based on nonlinear optics, is called optical parametric chirped pulse amplification. It has a number of features that, in principle, make it highly attractive. It primarily offers extremely large gains simultaneously with extremely large bandwidths. Additional attractions are virtually no spatial and temporal phase distortion of the amplified pulse, high efficiencies and a low thermal loading, reduced amplified spontaneous emission levels, small optical material lengths, and an inherent simplicity of implementation. We present an evaluation of the technique as a front end amplifier for the ultrashort pulse amplification chain of the Vulcan laser system. Such a device could replace some of the existing amplification in Nd:glass and additionally have a wider effect as a direct replacement of Ti:sapphire regenerative amplifiers on large-scale chirped pulse amplification scale facilities.

Algorithm for complete and rapid retrieval of ultrashort pulse amplitude and phase from a sonogram

Abstract: We describe an algorithm based on principal-component generalized projections that is capable of unambiguous retrieval of the amplitude and phase of an ultrashort pulse from a measurement of its sonogram. Running on a personal computer, the algorithm can complete more than 50 iterations per second which enables real-time display of an ultrashort pulse intensity and phase profile. Retrieval is demonstrated for a range of pulses with commonly encountered phase errors including linear chirp, self-phase modulation, and cubic spectral phase, and experimental results illustrate good agreement between the retrieved pulse characteristics and independent spectral measurements.

What can short-pulse pump-probe spectroscopy tell us about Franck-Condon dynamics?

Abstract: We examine the signal from pump-probe spectroscopy of a model system—nonrotating I2—at short time delays and compare signals calculated without approximation (a full quantum calculation), with a semiclassical Franck-Condon approximation, and with a classical simulation of the nuclear wave packet. In order to assess the complications of simulation and interpretation when the probe window lies in the spectroscopically and dynamically important Franck-Condon region, we concentrate on a case where pump and probe resonances are at the same internuclear distance. We find that the common practice of ignoring the pump-truncation effects of pulse overlap leads to an overestimate of the signal at short times. Moreover, both classical simulations and semiclassical Franck-Condon treatments can deviate significantly in form from the actual signal even with proper treatment of pulse overlap. The sources of these deviations can be seen in the evolution of the excited-state nuclear distributions calculated classically and under the semiclassical Franck-Condon approximation. Specifically, the differences in evolution of the classical and full quantum excited-state nuclear distributions are due to differing initial momentum distributions. We introduce an efficient method for calculating the pump-probe signal that takes advantage of the brevity of ultrashort pulses and can include pulse characteristics such as chirp. This short-pulse expansion method aids in the proper treatment of pulse-overlap and nonzero pulse duration and promises to simplify the incorporation of relaxation processes.

Distributed on-fiber thin film heaters for Bragg gratings with adjustable chirp

Abstract: This letter describes a fiber Bragg grating device that has tunable chirp. It relies on a distributed on-fiber resistive heater that consists of a thin metal film deposited onto the outer surface of a bare fiber; the thickness of this film varies continuously with position along the fiber. The physics of heat flow and diffusion in these structures leads to temperature gradients that follow, to a remarkably good approximation, the local resistance of the tapered metal film. This temperature distribution produces a chirp with a geometry that is defined by the thickness profile of the film and at a rate that can be adjusted by changing the current. Finite element modeling illuminates aspects of the flow of heat in these structures, and optical measurements demonstrate important characteristics of the devices.

Super-resolution of pulsed multipath channels for delay spread characterization

Abstract: Frequency selective channels can be measured by either a continuous wave frequency sweep, which gives directly the transfer function, or by sounding the channel with pulses, which results in the impulse response. When the sounding pulse becomes a chirp-type waveform, the two approaches become similar. Simple rectangular pulses can be used for partial, but usually sufficient, channel characterization. In this paper, the process is described for resolving impulse response components into bins smaller than the duration of the sounding pulse and smaller than the reciprocal of the channel bandwidth. From such "super-resolution", the delay spread of the propagation channel can be established accurately even when it is much less than the sounding pulse duration, allowing longer, higher energy pulses to be used for channel characterization. The process is demonstrated using subtractive deconvolution where a loop gain of unity is shown to be stable; and a modified inverse filter technique, in which the modification caters for the spectral zeros of the sounding pulse.

Chapter 7 – Chirped Fiber Bragg Gratings

Abstract: Publisher Summary This chapter examines different aspects chirped fiber Bragg gratings. Gratings that have a non-uniform period along their length are known as chirped. Chirp in gratings take different forms. The period varies symmetrically, either increasing or decreasing in period around a pitch in the middle of a grating. The chirp is linear and the period varies linearly with length of the grating, may be quadratic, or may even have jumps in the period. The performance of chirped gratings as a dispersive filter specifically for the compensation of chromatic dispersion is analyzed. An important feature of a dispersion-compensating device is the figure of merit. There are several parameters that affect the performance of chirped fiber Bragg gratings for dispersion compensation. These are the insertion loss, dispersion, bandwidth, polarization mode-dispersion, and deviations from linearity of the group delay and group delay ripple. It is found that the transfer matrix method is ideally suited to chirped gratings, since the grating is broken up into smaller sections of uniform period and/or refractive index profile. It is observed that the role played by the rear end of the grating is apparent when the coupling constant is apodized asymmetrically.

Femtosecond measurements of the time of flight of photons in a three-dimensional photonic crystal.

Abstract: We report on experimental measurements of the propagation behavior of short optical pulses in a three-dimensional photonic crystal in the visible spectrum. The propagation delay of 70 fs light pulses transmitted through a sample of a fcc synthetic opal at frequencies lying in a photonic stop band was measured directly using a time-of-flight technique. Taking into account spectral reshaping of the transmitted pulses as well as the residual frequency chirp of the incoming pulses, it is found that the pulses significantly slow down at the photonic band edges.

Synchronization for systems with antenna diversity

Abstract: This paper deals with the synchronization of systems with frequency domain signal processing and antenna diversity. The results can be applied for both, single carrier transmission with frequency domain equalization (FDE) and orthogonal frequency division multiplexing (OFDM). For the considered systems, carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. A synchronization concept for systems with antenna diversity is presented which uses one pilot FFT symbol per transmission frame. The pilot FFT symbol consists of two periods of a chirp signal.

Chirp-compensated 40-GHz mode-locked lasers integrated with electroabsorption modulators and chirped gratings

Abstract: 40-GHz short-pulse generation of monolithic mode-locked lasers integrated with electroabsorption modulators and chirped gratings is reported. Theoretical analysis of active mode-locking based on a Gaussian model is described. Chirped gratings are shown to compensate the optical-frequency chirp and extend the repetition frequency range. Nearly transform-limited pulses of less than 4 ps are experimentally generated over a wide range of repetition frequencies from 38 to 41 GHz.

Hybrid FM-polynomial phase signal modeling: parameter estimation and Cramer-Rao bounds

Abstract: Parameter estimation for a class of nonstationary signal models is addressed. The class contains combination of a polynomial-phase signal (PPS) and a frequency-modulated (FM) component of the sinusoidal or hyperbolic type. Such signals appear in radar and sonar applications involving moving targets with vibrating or rotating components. A novel approach is proposed that allows us to decouple estimation of the FM parameters from those of the PPS, relying on properties of the multilag high-order ambiguity function (ml-HAF). The accuracy achievable by any unbiased estimator of the hybrid FM-PPS parameters is investigated by means of the Cramer-Rao lower bounds (CRLBs). Both exact and large sample approximate expressions of the bounds are derived and compared with the performance of the proposed methods based on Monte Carlo simulations.

Pulse Compression of a High-Order Harmonic by Compensating the Atomic Dipole Phase

Abstract: High-order harmonic pulses of a femtosecond Ti:sapphire laser were compressed to the pulse width of 13 fs, for the first time, by compensating the intrinsic negative chirp of a harmonic with positive dispersive medium.

Fabricating fibre edge filters with arbitrary spectral response based on tilted chirped grating structures

Abstract: We report a new technique for fabricating fibre edge filters with arbitrary spectral response based on tilted fibre chirped gratings. The dependence of the spectral response on the tilt angle, chirp rate, fibre type and grating strength has been studied in detail. The near-linear wavelength response with tailorable slope efficiency has been demonstrated over the filtering ranges from 2 nm to 20 nm.

Method and apparatus for mode locking of external cavity semiconductor lasers with saturable Bragg reflectors

Abstract: External cavity semiconductor lasers using a saturable Bragg reflector as an external reflector are mode locked and produce output pulses of 1.9 ps from a semiconductor lasers without dispersion compensation. By coupling the output to a standard single mode fiber with a length of 35 m to compensate the linear chirp, the mode-locked pulse duration as short as 880 fs is achieved.

Synchronously pumped femtosecond optical parametric oscillator based on AgGaSe2 tunable from 2 μm to 8 μm

Abstract: The operation of a continuous-wave mode-locked silver gallium selenide (AgGaSe2) optical parametric oscillator (OPO) is reported. The OPO was synchronously excited by 120-fs-long pulses of 1.55-μm radiation at a repetition rate of 82 MHz. The 1.55-μm radiation is generated by a noncritically phasematched cesium-titanyl-arsenate (CTA)-OPO pumped by a mode-locked Ti:sapphire laser. The AgGaSe2-OPO generates signal and idler radiation in the range from 1.93 μm to 2.49 μm and from 4.1 μm to 7.9 μm, respectively. Up to 67 mW of signal wave output power has been obtained. The experimentally determined pulse duration and chirp parameters are in reasonable agreement with results from a numerical model taking into account group velocity mismatch, group velocity dispersion, self phase modulation, and chirp enhancement.

System for transmitting optical data

Abstract: The transmission system includes a first electro-optical modulator adapted to supply in response to an input electrical signal a controlled phase optical signal having an optical power modulated between low levels and high levels and a phase shift within each time cell that contains a low power level. To make it more flexible to use, the system includes a second electro-optical modulator controlled by the input signal and optically coupled to the first electro-optical modulator to apply to said controlled phase optical signal complementary power and/or phase modulation so as respectively to modify its extinction ratio and/or to apply a transient “chirp” to it.

All-optical clock recovery from NRZ data with simple NRZ-to-PRZ converter based on self-phase modulation of semiconductor optical amplifier

Abstract: The feasibility of all-optical clock recovery (CR) from non-return-to-zero (NRZ) data with a simple NRZ-to-pseudo-RZ (PRZ) converter based on the self-phase modulation (SPM) of a semiconductor optical amplifier (SOA) is empirically presented The SPM induces a frequency chirp at every leading edge of the NRZ data passing through the SOA The SPM-induced chirp component is extracted by an optical grating bandpass filter and then transformed to PRZ data with a low polarisation dependency of ~07 dB By transmitting the PRZ data into a mode-locked Er-doped fibre laser, all-optical CR has been achieved at 25 Gbit/s

Dual on-fiber thin-film heaters for fiber gratings with independently adjustable chirp and wavelength.

Abstract: Dual, independently addressable thin-film resistive heaters fabricated in a multilayer geometry on the surface of an optical fiber provide a new, flexible means for thermally tuning the properties of intracore gratings. In particular, control of the current that is applied to each of these heaters permits the chirp and the central wavelength of the grating to be adjusted independently. The designs and simple fabrication procedures for these types of device, the important physics of heat flow in them, and a tunable add-drop filter that demonstrates essential aspects of their operation are described.