Effect Of Frequency Chirp Research Articles

Semiconductor Optical Amplifier-Enabled Intensity Modulation of Adaptively Modulated Optical OFDM Signals in SMF-Based IMDD Systems

Detailed numerical investigations of the transmission performance of adaptively modulated optical orthogonal frequency division multiplexed (AMOOFDM) signals are undertaken, for the first time, in optical amplification- and chromatic dispersion compensation-free SMF IMDD systems using semiconductor optical amplifiers (SOAs) as intensity modulators. A theoretical model describing the characteristics of the SOA-based intensity modulators is developed, based on which optimum SOA operating conditions are identified. It is shown that the optimized SOA-based intensity modulators support a 30 Gb/s AMOOFDM signal transmission over a 80 km SMF, which doubles the transmission performance offered by directly modulated DFB lasers. The aforementioned performance enhancement is mainly due to a considerable reduction in the frequency chirp effect, resulting from the strong SOA gain saturation-induced decrease in SOA effective carrier lifetime. Relatively low extinction ratio and clipping of the SOA modulated signals are identified to be the key factors limiting the maximum achievable AMOOFDM transmission performance. In addition, results also indicate that both the optimum SOA operating conditions and the AMOOFDM transmission performance are insusceptible to variations in SOA parameters.

Combined effect of tight-focusing and frequency-chirping on laser acceleration of an electron in vacuum

The investigation [Gupta et al., Appl. Phys. Lett. 91, 211101 (2007)] for electron acceleration by a tightly focused laser beam is revisited by including the effect of laser frequency chirping. The frequency chirping plays an important role to enhance the electron energy if the laser is tightly focused. Due to the combined effect of frequency chirping and tight focusing of a laser beam, an electron can be accelerated for a longer time in vacuum. As a result, from the proposed investigation, the electron energy gain during the laser acceleration is found to be considerably higher.

Effects of frequency chirp on magnetron injection locking

The injection locking of a magnetron is theoretically analyzed when either the free running oscillator or the drive signal has a frequency chirp. It is found that complete phase locking of the signal cannot be achieved in either case. However, as long as the locking condition of Adler is well-satisfied instantaneously, a high degree of locking occurs during a major duration of the frequency chirps. The expected output phase variation is computed in terms of the noise in the free-running magnetron oscillator for the case of constant drive frequency.

Electron acceleration by a chirped circularly polarized laser pulse in vacuum in the presence of a planar magnetic wiggler

Ultrahigh-gradient acceleration, by a chirped circularly polarized (CP) laser pulse in the presence of a planar magnetic wiggler, is investigated numerically. A simple linear chirped profile of laser frequency has been introduced in our model. The combined effect of frequency chirping and the tapered wiggler magnetic field on the electron acceleration is discussed in detail. It is found that the chirping parameter, the wiggler field tapering parameter, the initial phase of the laser, the laser spot size and laser intensity all have a strong influence on the electron acceleration. If an optimal tapered wiggler magnetic field in the same direction as the magnetic field of the laser pulse is externally applied during the trailing part of the pulse, the electron can gain and retain significant energy in the form of cyclotron oscillations even after the passing of the laser pulse.

Effect of frequency chirp on supercontinuum generation in photonic crystal fibers with two zero-dispersion wavelengths

The effect of initial frequency chirp is investigated numerically to obtain efficient supercontinuum radiation in photonic crystal fibers (PCFs) with two closely spaced zero-dispersion wavelengths. The positive chirps, instead of zero or negative chirps, are recommended because self phase modulation and four-wave mixing can be facilitated by employing positive chirps. In contrast with the complicated and irregular spectrum generated by negative-chirped pulse, the spectrums generated by positive-chirped pulses are wider and much more regular. Moreover, the saturated length of the PCF, corresponding to the maximal spectrum width, can be shortened greatly and the efficiency of frequency conversion is also improved because of initial positive chirps. Nearly all the energy between the zero-dispersion wavelengths can be transferred to the normal dispersion region from the region within the two zero-dispersion wavelengths provided that the initial positive chirp is large enough.

Effect of frequency Chirp on the black and gray solitions

the behavior of the first-order black and gray solitons propagtedin optical fiber in the presence of frequency chirp is studied analytically and numerically results show that phase profile of black solitons changes abruptly

Effect of frequency Chirp on the black and gray solitions

the behavior of the first-order black and gray solitons propagtedin optical fiber in the presence of frequency chirp is studied analytically and numerically results show that phase profile of black solitons changes abruptly

Effect of frequency chirping on supercontinuum generation in dispersion flatted and dispersion decreasing fiber

The effect of frequency chirping on supercontinuum (SC) generation in dispersion flatted and dispersion decreasing fiber has been studied by numerical simulation based on the total field nonlinear Schrödinger equation. Our results show that a positive initial frequency chirp can significantly broaden the supercontinuum spectrum by up to approximate 80 nm, and the SC intensity increase about 5 dB. A range of optimal positive frequency chirps is identified to obtain the maximized supercontinuum bandwidth. The mechanism of this enhancement is also discussed detailedly through the evolutions of temporal and spectral width related to different pre-chirped pulses.

Effect of frequency chirping on supercontinuum generation in photonic crystal fibers

Pre-chirp of the input pulse has a significant effect on pulse evolution in a photonic crystal fiber. We present numerical simulations which show that the supercontinuum bandwidth increases with the linear chirp, and that the coherence of supercontinuum improves as frequency chirping increases. An optimal positive chirp is identified that maximizes the supercontinuum bandwidth, corresponding to the formation of only one red-shifting Raman soliton.

Effects of chirping on the dissociation dynamics ofH2in a two-frequency laser field

We present the effects of frequency chirping of laser pulses on (1+1)-photon resonance-enhanced dissociation dynamics of ${\mathrm{H}}_{2}.$ The dissociation occurs via two closely spaced nonadiabatically coupled intermediate levels which are in one-photon resonance or near resonance with the initial level. Predissociating levels embedded into continua are considered. When the first laser field is sufficiently intense and suitably chirped, the dissociation probability is enhanced by adiabatic rapid passage through the avoided crossing arising from the frequency swept radiative interaction. The whole population of the ground level can be effectively transferred to the intermediate levels by this technique facilitating the dissociation process by the second field. We also report the effect of frequency detuning and chirp width on the dissociation probability. Widths of the two peaks of the dissociation line shape increase with an increase in chirp width, resulting in the possibility of control in the dissociation yield. When the first field is a laser pulse of low intensity and constant frequency and the second laser frequency is chirped, predissociating levels take important parts in the dissociation dynamics and we obtain a signature of the nonadiabatic effect of the first step on the second step of photodissociation dynamics. This feature is due to the presence of the predissociating levels and the nonadiabatic mixing of two intermediate levels. All these results can be explained in terms of the adiabatic dressed levels.

Ionization dynamics in intense pulsed laser radiation. Effects of frequency chirping

Via a non-perturbative method we study the population dynamics and photoelectron spectra of Cs atoms subject to intense chirped laser pulses, with Gaussian beams. We include above threshold ionization spectral peaks. The frequency of the laser is near resonance with the 6s-7p transition. Dominant couplings are included exactly, weaker ones accounted for perturbatively. We calculate the relevant transition matrix elements, including spin-orbit coupling. The pulse is taken to be a hyperbolic secant in time and the chirping a hyperbolic tangent. This choice allows the equations of motions for the probability amplitudes to be solved analytically as a series expansion in the variable , where is a measure of the pulse length. We find that the chirping changes the ionization dynamics and the photoelectron spectra noticeably, especially for longer pulses of the order of au. The peaks shift and change in height, and interference effects between the 7p levels are enhanced or diminished according to the amount of chirping and its sign. The integrated ionization probability is not strongly affected.

Analysis of frequency chirping and extinction ratio of optical phase conjugate signals by four-wave mixing in SOA's

We present a complete large-signal dynamic model of a wavelength converter or optical phase conjugator based on four-wave mixing in semiconductor optical amplifiers. The modified transfer matrix method (TMM) is used for implementing the large-signal dynamic model. It is possible for this model to involve not only forward traveling waves, but also backward traveling waves. Also, this model includes longitudinal variation of carrier-induced reflective index, /spl alpha/-parameter, gain, carrier density, photon density, reflection, and amplified spontaneous emission noise at each small section. Therefore, we can accurately evaluate frequency chirping, optical pulse pattern, and extinction ratio of the conjugate signal. Th eye-opening penalty has been calculated to investigate the effects of frequency chirping and extinction ratio on transmission performance.

Effect of frequency chirp on soliton spectral sidebands in fiber lasers

Fiber lasers emit soliton pulses that exhibit discrete spectral sidebands generated through dispersive-wave resonances. The position of these soliton sidebands is shown to be affected by the amount of chirp acquired by the pulse, and the degree of chirp is determined by total cavity losses and gain dispersion. Our results show that the soliton chirp shifts the sideband frequencies and that sidebands can be generated even in the case of normal dispersion. The long- and short-cavity cases are discussed separately so that our results are applicable to all laser configurations.

Numerical modeling of soliton-dragging logic gates

The soliton-dragging logic gate is based on the soliton-dragging effect, which occurs when pulses of opposite polarization states in a birefringent optical fiber collide. The central frequencies of the two pulses and hence their velocities are shifted. The soliton-dragging effect is studied here for two solitons of the same amplitude, two solitons of different amplitudes, and two pulses that are not solitons. The dependences of the effect on the initial pulse separation, the initial pulse amplitudes, and the initial pulse profiles are all determined. The pulse interaction is studied by means of several different methods: (1) a complete solution of the coupled nonlinear Schrodinger equation, (2) a solution of ordinary differential equations that includes the effect of frequency chirp, (3) a solution of ordinary differential equations that neglects the effect of frequency chirp, and (4) the Born approximation. We compare the last three approaches with the first. We find that the second approach accurately predicts the velocity and the time shifts for the larger amplitude pulse in all cases of practical interest that we studied. Thus the second approach is useful in the design of soliton logic-based networks. The third approach is significantly less accurate, while the fourth approach is the least accurate of all.

Effect of frequency chirping on the performance of optical communication systems

The effect of frequency chirping on the performance of optical communication systems is analyzed theoretically using a simple but realistic model for the chirped pulses emitted by a directly modulated semiconductor laser. The effect of pulse-edge sharpness is investigated by considering the propagation of super-Gaussian chirped pulses in a dispersive single-mode fiber. The results are presented using the parameter values appropriate for a 1.55-μm optical communication system employing conventional silica fibers and are in qualitative agreement with the reported experimental observations.

Power spectrum of directly modulated single-mode semiconductor lasers: Chirp-induced fine structure

The time-averaged power spectrum of a single-mode semiconductor laser under direct current modulation is calculated using the small-signal analysis of the stochastic rate equations. The general formalism includes the effect of frequency chirping owing to current modulation as well as the effect of phase diffusion related to spontaneous emission. Chirp-induced dynamic line broadening is shown to result from a superposition of the additional unresolved sidebands generated at multiples of the modulation frequency on both sides of the optical line. The effect of intensity modulation is to introduce an asymmetry such that the low-frequency sideband has a relatively higher amplitude. The theory explains reasonably well the reported experimental observations. At the same time it provides physical insight into the dynamic process of frequency chirping and its relation to the frequency modulation. In particular, the frequency and the decay rate of transient relaxation oscillations govern the interplay between simultaneously occurring intensity and frequency modulations.

Effect of frequency chirping of light from a Q-switched ruby laser on the speckle in Fabry-Perot rings

It has been observed that, when a Fabry-Perot interferometer was illuminated through a diffuser with light from a Q-switched ruby laser, the speckle smeared out in the Fabry-Perot rings. This is interpreted as a movement of the individual speckle resulting from the frequency chirping of light from the ruby laser and the high resolution of the Fabry-Perot interferometer.