Showing papers on "Chirp published in 1997"

Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators

Abstract: We demonstrate two techniques to reduce the effects of fiber chromatic dispersion in fiber-wireless systems incorporating external modulators. We theoretically and experimentally show that the achievable link distance can be increased by varying the chirp parameter of the modulator to give large negative chirp using a dual-electrode Mach-Zehnder modulator (MZM) biased at quadrature. In addition, we show that dispersion can be almost totally overcome by implementing a simple method using the dual-electrode MZM to generate an optical carrier with single sideband (SSB) modulation. We demonstrate the transmission of a 51.8-Mb/s pseudorandom bit sequence (PRBS) at 12 GHz over 80 km of standard single-mode fiber using the SSB generator and measure a bit-error-rate (BER) power penalty due to fiber dispersion of less than 0.5 dB for a BER equal to 10/sup -9/.

Study of the spatial and temporal coherence of high order harmonics

Abstract: We apply the theory of high-order harmonic generation by low-frequency laser fields in the strong field approximation to the study of the spatial and temporal coherence properties of the harmonics. We discuss the role of dynamically induced phases of the atomic polarization in determining the optimal phase matching conditions and angular distributions of harmonics. We demonstrate that the phase matching and the spatial coherence can be controlled by changing the focusing parameters of the fundamental laser beam. Then we present a detailed study of the temporal and spectral properties of harmonics. We discuss how the focusing conditions influence the individual harmonic spectra and time profiles, and how the intensity dependence of the dynamically induced phase leads to a chirp of the harmonic frequency. This phase modulation can be used to control the temporal and spectral properties of the harmonic radiation. Temporally, the harmonic chirped pulse can be recompressed to very small durations. Spectrally, chirping of the fundamental beam may be employed to compensate for the dynamically induced chirp and to control the individual harmonic spectrum. Finally, we discuss the short pulse effects, in particular nonadiabatic phenomena and the possibility of generating attosecond pulses. Comment: Latex file with 37 pages, 25 postscript figures. to appear in Advances in Atomic, Molecular and Optical Physics

Quasi-soliton propagation in dispersion-managed optical fibers.

Abstract: Combination of programmed chirp and dispersion profiles produced a solitonlike nonlinear pulse with a stationary structure (quasi-soliton). The quasi-soliton has more attractive properties than those of a soliton because of its reduced interaction and smaller peak power than a soliton and allows ∼100-Gbit/s transmission over a 125-dispersion distance with reasonable power.

Extraction of DFB laser rate equation parameters for system simulation purposes

Abstract: Values for the parameters in a rate equation description of a distributed feedback (DFB) laser must be chosen appropriately in order to obtain agreement between calculated and measured results for system performance. A technique is described for readily extracting values of the rate equation parameters using measurements of the threshold current and the optical power, resonance frequency and damping factor for a bias current well above the threshold current. When used to estimate system performance, it is shown that different sets of physically reasonable parameter values which reproduce the measurements yield the same dependence of the receiver sensitivity on fiber length. Calculated and measured results for the receiver sensitivity and time resolved chirp exhibit good agreement when the extracted parameter values are used.

High resolution SAR processing using stepped-frequencies

Abstract: This paper demonstrates a method to produce high resolution SAR images using stepped-frequencies. An advantage of the stepped-frequency approach is the reduction of the instantaneous bandwidth and sampling rate requirements of the radar system, as well as the possibility of skipping frequencies that might be corrupted due to external interfering frequency sources. The technique described involves the construction of a wide-bandwidth chirp pulse produced from a burst of narrow-bandwidth chirp pulses transmitted at stepped-frequency intervals. A description of this technique is given and simulation results are shown.

Ultrashort pulse reflection from fiber gratings: a numerical investigation

Abstract: We consider the linear reflection of ultrashort broadband pulses by uniform and nonuniform narrowband fiber gratings. We examine the effects of grating characteristics, including peak reflectivity, bandwidth, phase response (dispersion and chirp), and apodization on the reflection of such pulses from various gratings. A symmetric transform-limited 1-ps Gaussian pulse is assumed as the ultrashort broadband input to the gratings; the reflected pulses take on significantly different shapes and vary in duration. The prominent features observed are qualitatively explained in order to gain physical insight into the nature of the ultrashort pulse response and corresponding interaction. The results of this study indicate that there is the potential for a new class of applications, including temporal pulse shaping and novel devices for optical communications systems, by combining ultrashort broadband pulses with narrowband fiber gratings.

Use of Chirped Quasi-phase-matched materials in chirped pulse amplification systems

Abstract: The limitations on maximum pulse energies from a fiber-grating pulse compressor are circumvented by placing a chirped-period quasi-phase-matched (QPM) crystal after the fiber-grating pulse compressor. The crystal accomplishes second-harmonic generation and stretched-pulse compression at the second-harmonic in a single device. This hybrid compressor configuration enables a substantial increase in ultrashort pulse energies obtainable with a compact all-fiber chirped pulse amplification system. Furthermore, with such a QPM crystal the adjustable compensation of both linear and nonlinear frequency chirp in second-harmonic pulses is possible. This property makes a variety of compact, robust and simple ultrashort-pulse fiber amplifier designs possible. It also allows for certain tolerances in the design and manufacturing of a pulse amplification system. Capability to compensate an arbitrary frequency chirp allows nonlinear spectral-broadening techniques for achieving shorter second-harmonic pulse durations. Also, by employing chirped QPM crystals maximum energy throughput and good second-harmonic pulse quality can be achieved.

Dispersion-managed soliton dynamics.

Abstract: We obtain and solve algebraic eigenvalue equations that predict the dependence of the pulse energy of a dispersion-managed soliton on pulse duration, chirp, and dispersion-map parameters. We demonstrate that a variational ansatz based on a Gaussian pulse shape remains useful even when the actual pulse shape is not Gaussian, and we show that the enhancement factor saturates as the pulse duration decreases.

Directly modulated semiconductor laser having reduced chirp

Abstract: In accordance with the present invention, an in-line fiber Bragg grating is coupled to the output of a directly modulated DFB laser. The grating preferably rejects chirp induced frequencies of light emitted by the DFB laser. Accordingly, light transmitted through the grating is spectrally narrowed and has a higher extinction ratio, thereby decreasing bit error rate probabilities.

Transfer-matrix analysis of optical bistability in DFB semiconductor laser amplifiers with nonuniform gratings

Abstract: We present a transfer-matrix method capable of simulating the effects of nonuniform gratings on the filtering, amplification, and bistability characteristics of distributed feedback (DFB) semiconductor laser amplifiers. The linewidth enhancement factor is incorporated in a way that allows direct gain-tuning of the bistability hysteresis. As an example, we compare a /spl lambda//4 phase-shifted DFB amplifier with and without spatial chirp. For amplifiers driven to yield the same unsaturated peak amplifier gain, positive linear chirp widens the spectral range of low-threshold switching and increases the switching contrast.

A four-parameter atomic decomposition of chirplets

Abstract: A new four-parameter atomic decomposition of chirplets is developed for compact representation of signals with chirp components. The four-parameter atom is obtained by scaling the Gaussian function, and then applying the fractional Fourier transform (FRFT), time-shift and frequency-shift operators to the scaled Gaussian. The decomposition is realized by extending the matching pursuit algorithm to four parameters. For this purpose, the four-parameter space is discretized to obtain a dense subset in the Hilbert space. Also, a related time-frequency distribution is developed for clear visualization of the signal components. The decomposition provides a more compact and precise representation of chirp components as compared to the three-parameter ones.

All-optical Mach-Zehnder wavelength converter with monolithically integrated DFB probe source

Abstract: The first interferometric wavelength converter with an integrated continuous-wave (CW) probe source is presented. The two-port device operates independent of polarization for a broad range of input wavelengths. The input dynamic range is enhanced by an integrated input preamplifier. Input powers as low as -5.4 dBm in the fiber are converted with penalties as low as 0.6 dB at a bit rate of 2.5 Gb/s. The device is shown to operate with the same low penalty for same-wavelength operation as well. The chirp parameter /spl alpha/ is measured to be 2 for wavelength conversion. No chirp is added to the input signal for same-wavelength operation.

System identification using chirp signals and time-variant filters in the joint time-frequency domain

Abstract: We propose a novel method to identify an unknown linear time invariant (LTI) system in low signal-to-noise ratio (SNR) environment. The method is based on transmitting chirp signals for the transmitter and using linear time-variant filters in the joint time-frequency (TF) domain for the receiver to reduce noise before identification. Due to the TF localization property of chirp signals, a large amount of additive white noise can be reduced, and therefore, the SNR before identification can be significantly increased. This, however, cannot be achieved in the conventional methods, where pseudo-random signals are used, and therefore, noise reduction techniques do not apply. Our simulation results indicate that the method proposed outperforms the conventional methods significantly in a low SNR environment. This paper provides a good application of time-frequency analysis and synthesis.

Chirped nonlinear pulse propagation in a dispersion-compensated system

Abstract: We study nonlinear pulse propagation in an optical transmission system with dispersion compensation. A chirped nonlinear pulse can propagate in such a system, but eventually it decays into dispersive waves in a way similar to the tunneling effect in quantum mechanics. The pulse consists of a quadratic potential that is due to chirp in addition to the usual self-trapping potential and is responsible for the power enhancement and the decay.

38.5 km error free transmission at 10 Gbit/s in standard fibre using a low chirp, spectrally filtered, directly modulated 1.55 /spl mu/m DFB laser

Abstract: 1.55 /spl mu/m DFB lasers with extremely high bandwidth and low wavelength chirp for local distribution at 10 Gbit/s are described. Time resolved chirp measurements direct from the laser, and after a transient chirp reducing filter, show very small wavelength variations, allowing error free transmission over 18.4 km (without filter) and 38.5 km (with filter) in standard singlemode fibre without the use of dispersion compensation.

Analysis of fiber Bragg gratings for dispersion compensation in reflective and transmissive geometries

Abstract: Numerical analysis of the dispersion-compensating properties of fiber Bragg gratings (FBGs) in both reflective and transmissive modes is presented. First, the sensitivity of chirped, reflective gratings to the grating chirp parameter, index modulation, and grating length is examined, showing that apodization provides lower sensitivity to variations in these parameters. Second, we introduce a new transmissive geometry for grating-based dispersion compensation that utilizes the dispersive properties of a uniform Bragg grating in transmission.

Effect of Mach-Zehnder modulator DC extinction ratio on residual chirp-induced dispersion in 10-Gb/s binary and AM-PSK duobinary lightwave systems

Abstract: It is shown through numerical simulation that residual frequency chirping in lithium niobate external modulators, which accompanies devices with a finite-optical extinction ratio, always degrades the performance of lightwave systems that use optical AM-PSK duobinary signaling. This is unlike binary systems which will exhibit either an improvement or degradation, depending on the polarity of residual chirp.

Theory of stretched-pulse transmission in dispersion-managed fibers.

Abstract: Approximate analytical expressions for quasi-steady-state pulse parameters of stretched pulses in dispersion-managed fibers are derived. The analysis uses the same approach as that developed for the analysis of stretched-pulse fiber ring lasers but with improved calculation of nonlinearity-induced pulse chirping by means of an approximate variational method. The energy-enhancement factor of the stretched pulse is derived, showing reasonable agreement with numerical results.

Monolithic colliding pulse mode-locked semiconductor lasers

Abstract: In this paper we investigate the temporal and spectral properties of monolithic colliding pulse mode-locked (CPM) quantum-well lasers. A large-signal model for the CPM laser is derived, which explains the mode-locking behaviour of both passively and hybridly mode-locked CPM lasers. The pulse-shaping and wavelength chirp of the CPM pulses are found to be the result of a complicated interplay between the gain and saturable absorber dynamics. The experimentally measured pulse width dependence upon variation of gain current and applied reverse bias on the saturable absorber is explained by fast gain dynamics. The experimentally observed broadening of the optical power spectrum with gain current is found mainly to be due to the self-phase modulation in the gain sections.

Scannerless ladar architecture employing focal plane detector arrays and FM-CW ranging theory

Abstract: A laser transmitter is amplitude modulated with a chirp signal to illumin an entire scene or field of view. A mixing process occurs in the received light path using an electro-optic light modulator positioned just in front of the focal plane detector array. The detector array detects and integrates the mixed light signal over some field of view. Numerous image frames are recorded periodically in the time over the FM period. The Fourier transform taken over time for a pixel establishes the range to the target in that pixel. Performing the Fourier transform for all pixels yields a three-dimensional image of objects in the field of view. Such an arrangement yields a scannerless ladar possessing high range resolution with no range ambiguities.

Configurable chirp mach-zehnder optical modulator

Abstract: A multi-quantum-well Mach-Zehnder optical phase modulator selectively generates positive and negative frequency chirp. The modulator has an asymmetric input y-junction coupler and an asymmetric output y-junction coupler joined by interferometric arms defining two optical paths of unequal length so as to generate a fixed phase difference of π or an integral odd multiple thereof. The modulator has electrodes on each arm for receiving modulating electric fields in a push-pull relationship. A control electrode is provided on one of the arms for receiving a control electric field. The modulator, when driven without any control electric field generates negative frequency chirp, and selectively generates positive frequency chirp in response to adjustment of the control electric field. As an alternative, the length differential between respective arms can be selected to generate a phase difference of π/2, in which case control electrodes are required on each of the interferometric arms. A method of selectively generating a modulated optical signal having selectively configurable frequency chirp is also described.

Frequency domain reflectometer and method of suppressing harmonics

Abstract: Frequency domain reflectometer and method of locating faults in a transmission line utilized by the reflectometer are disclosed. The reflectometer suppresses harmonics that may be interpreted as faults that in actuality do not exist. In general, the reflectometer applies a sweep signal to the transmission line in order to obtain a reflected sweep response signal. Then, the reflectometer obtains a sweep response spectrum from the reflected sweep response signal. The reflected sweep response signal includes a plurality of spectral peaks which represent the frequency components of the reflected sweep response signal. Then, the reflectometer mathematically determines which spectral peaks of the reflected sweep response spectrum were generated due to harmonics in the reflected sweep response signal. After determining that a spectral peak is a harmonic of a fundamental spectral peak, the reflectometer subtracts a percentage of the fundamental spectral peak from the harmonic spectral peak, thereby suppressing a harmonic of the reflected sweep response signal. The reflectometer then obtains from the adjusted sweep response spectrum a location of an impedance mismatch in the transmission line.

Compensation of self-phase modulation in chirped-pulse amplification laser systems.

Abstract: Small amounts of self-phase modulation can lead to significant pulse distortions in chirped-pulse amplifiers. However, we show the surprising result that the strong chirp of the pulse can be exploited to remove these distortions completely by linear pulse shaping before amplification.

Modulation characteristics of semiconductor Mach-Zehnder optical modulators

Abstract: The transmission and chirp characteristics are described for two types of semiconductor Mach-Zehnder modulators, distinguished by the differential phase shift between the two arms of the interferometer in the unbiased state. The conventional modulator has a differential phase shift of 0 radians, while the /spl pi/-shift modulator has a differential phase shift of /spl pi/ radians. The nonlinear dependence on the applied voltage of the attenuation and phase constants of the optical signal propagating in the p-i-n waveguide leads to different characteristics for the two modulators. The influence of the splitting ratio of the Y-junctions is considered for single-arm and dual-arm (push-pull) modulation formats. The /spl pi/-shift modulator is shown to yield better transmission performance for 10 Gb/s systems compared to the conventional modulator.

A short discussion on the exact compensation of the SAR range-dependent range cell migration effect

Abstract: Efficient and precise compensation of the range cell migration (RCM) effect is a key point for a fast and accurate synthetic aperture radar (SAR) data processor. In particular the range-dependent nature of the range cell migration effect complicates the compensation operation. It has been recently shown that an exact compensation of the range-dependent RCM (RDRCM) phenomenon can be carried out either by applying the chirp scaling algorithm or the chirp z-transform procedure. This paper investigates the relationship between the two methods. In particular, it is shown that the chirp z-transform based approach represents a particular implementation of the chirp scaling algorithm. A final discussion is dedicated to show how the chirp z-transform and the chirp scaling procedure can be applied within a SAR data processing algorithm.

Aperiodic quasi-phasematching gratings for chirp adjustments and frequency conversion of ultra-short pulses

Abstract: An apparatus and method for simultaneous chirp adjustment and frequency conversion of an ultra-short input optical pulse A1 characterized by a center angular frequency ω1,0 in a non-linear optical material with a quasi-phasematching (QPM) grating exhibiting an aperiodic pattern of regions Dj constituting a grating. Passing the ultra-short input optical pulse A1 through the grating gives rise to a chirp-adjusted and frequency-converted output optical pulse A2. In the preferred embodiment the non-linear optical material is a Second Harmonic Generator (SHG) such that the output optical pulse A2 generated from the input optical pulse A1 is a chirp-adjusted second harmonic of said ultra-short input optical pulse A1. In the general case the method and apparatus use a transfer function D(Ω) derived from the equation: A2 (Ω)=D(Ω)·A1 2 (Ω), where A1 2 (Ω) is the Fourier transform of the square of input optical pulse A1 and A2 (Ω) is the Fourier transform of output optical pulse A2, to engineer gratings for frequency-conversion and chirp-adjustment.

Linear optical modulator for providing chirp-free optical signals

Abstract: A dual stage optical modulator is disclosed that uses optical predistortion to achieve a high degree of linearity and also provides signals having substantially no residual phase modulation, or chirp. The modulator allows fiber optic transmission over fibers having a zero dispersion wavelength that is significantly different from the operating wavelength of the source laser without a chirp-induced dispersion penalty. In one embodiment of the modulator, an input Y-junction splits an optical input signal into a first interferometer, also referred to as a phase/intensity modulator, having first and second interferometer arms and an electrode structure for modulating the split signal. The electrode structure includes a pair of ground electrodes and a modulating electrode for receiving a first RF modulating voltage and associated DC bias. The modulating electrode is positioned between the interferometer arms but is offset from the center of the arms for providing a first asymmetric electro-optic effect through the introduction of intensity modulation and residual phase shift in the split signals. The split signals are then mixed in unequal proportions in a directional coupler and are separated into a second phase/intensity modulator that is substantially similar to the first, but which receives a second RF modulating voltage. The signals from the second phase/intensity modulator are again mixed in unequal proportions in a second directional coupler which couples the signals and provides first and second optical output signals. Linearization is achieved through optical predistortion by adjusting the DC biases and attenuating the second modulating voltage with respect to the first such that any distortion generated in the first interferometer is canceled in the second interferometer. Chirp introduced in the signals by the first interferometer is reduced in the second interferometer and directional couplers and is eliminated in at least one of the output signals by appropriately selecting the amount of asymmetric phase distortion applied to the signal in each stage.

2.5 Gbit/s directly-modulated fibre grating laser for WDM networks

Abstract: Densely spaced wavelength-routed optical transport and access networks will require low cost, stable, low chirp narrow linewidth lasers operating on precisely specified wavelengths. Directly modulated semiconductor lasers with external fibre grating (FGL) can potentially satisfy the above requirements. Although 10 Gbit/s externally modulated soliton transmission using a modelocked FGL over 27000 km was reported, wavelength chirp and transmission performance of directly modulated FGLs have not been investigated at bit-rates higher than 155 Mbit/s for NRZ transmission. In this paper we report on the very low chirp performance of a directly modulated FGL at 2.5 Gbit/s, for the first time. Transmission over 300 km in standard fibre and >10000 km in dispersion shifted fibre was demonstrated.