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ULTRASHORT PULSE SHAPING IN LINEAR RESONANT ABSORBERS

Gazi Habiba Akter

Published Dec 14, 2011• 0 citations
OpenAlex

Abstract

Pulse shaping is the technique which controls the ultra-short pulse shape, and it became of great technological interest because of its potential applications in laser pulse compression, digital communications, microscopy etc. We demonstrate the idea of pulse-shaping technique and pulse propagation with low energy losses in a resonant linear absorbing medium. This thesis presents the results of a study of the propagation of Gaussian and hyperbolic secant ultrashort chirped and chirp-free pulses in homogeneously and inhomogeneously broadened resonant linear absorbers. Changes to the pulse shape and energy loss factor are presented as the pulse propagates in the absorber. The Fast Fourier method is used to numerically determine both the normalized intensity profile and the pulse spectrum. Our results show that, for pulse durations shorter than the relaxation time, chirped pulses in absorbing media obey the area theorem, with their shape changing with the propagation distance. Simulation results of the spectra of chirped pulses clearly show the burning of a spectral ’hole’ as the pulse propagates, with the pulse energy pushed away towards the wings. When compared to chirp-free pulses, chirped pulses reshape faster and develop wings in their tail due to initial phase modulation. Simulation results of the energy loss factor show that chirped pulses propagating in resonant linear absorbers sustain less energy losses than do chirp-free pulses. A comparison of chirped secant and Gaussian pulses shows that secant pulses propagate with lower energy losses. Analytic solutions are presented for long-distance asymptotic expressions of initial rms spectral bandwidth as well as for the attenuation factor of chirped Gaussian pulses. These analytical results are in agreement with numerical simulations. The comparison of energy losses of short chirped Gaussian pulses and long pulses of any profile in linear absorbers is also discussed in the thesis.

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Pulse shaping is the technique which controls the ultra-short pulse shape, and it became of great technological interest because of its potential applications in laser pulse compression, digital communications, microscopy etc. We demonstrate the idea of pulse-shaping technique and pulse propagation with low energy losses in a resonant linear absorbing medium. This thesis presents the results of a study of the propagation of Gaussian and hyperbolic secant ultrashort chirped and chirp-free pulses in homogeneously and inhomogeneously broadened resonant linear absorbers. Changes to the pulse shape and energy loss factor are presented as the pulse propagates in the absorber. The Fast Fourier method is used to numerically determine both the normalized intensity profile and the pulse spectrum. Our results show that, for pulse durations shorter than the relaxation time, chirped pulses in absorbing media obey the area theorem, with their shape changing with the propagation distance. Simulation results of the spectra of chirped pulses clearly show the burning of a spectral ’hole’ as the pulse propagates, with the pulse energy pushed away towards the wings. When compared to chirp-free pulses, chirped pulses reshape faster and develop wings in their tail due to initial phase modulation. Simulation results of the energy loss factor show that chirped pulses propagating in resonant linear absorbers sustain less energy losses than do chirp-free pulses. A comparison of chirped secant and Gaussian pulses shows that secant pulses propagate with lower energy losses. Analytic solutions are presented for long-distance asymptotic expressions of initial rms spectral bandwidth as well as for the attenuation factor of chirped Gaussian pulses. These analytical results are in agreement with numerical simulations. The comparison of energy losses of short chirped Gaussian pulses and long pulses of any profile in linear absorbers is also discussed in the thesis.
This article presents a study of the propagation of small-area pulses of coherent light through matter. It is found that the small-area pulses exactly obey an area theorem which, in the case of an attenuating medium, requires that the pulse area decay to zero exponentially with increasing distance of propagation. This, however, does not necessarily imply that the pulse energy decays exponentially. Instead, it is found that pulses of duration comparable to or shorter than the transverse relaxation time of the medium T2{T}_{2}T2​ (including both homogeneous and inhomogeneous broadening) propagate with low energy loss. The results are explained in terms of simple physical arguments which indicate that the pulse envelope should oscillate between positive and negative values, causing the area to decrease without a comparable decrease in the pulse energy. Analytic solutions are presented for the case of a pulse whose envelope varies as ${t}^{k}{e}^{\ensuremath{-}t}$ for t>0t>0t>0, a rectangular pulse, an ultrashort pulse (i.e., pulse width much less than T2{T}_{2}T2​), and the leading portion of a step-function pulse. The propagation of a small-area Gaussian pulse is studied numerically.
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A theoretical analysis of the propagation of near resonant and linearly chirped light pulses through a resonant medium is presented under the restriction that the pulses are far enough off resonance or weak enough so that they do not significantly alter the atomic populations. If the pulses are of duration comparable to or less than the phase memory time T(2) of the atoms, it is found that the transient response of the medium gives rise to amplitude-and phase-modulating effects. Analytic solutions are presented to describe the propagation of an off-resonant pulse which is very short compared with T(2) and an off-resonant rectangular pulse. The evolution of a Gaussian pulse having a positive linear chirp is studied numerically.
Optical experiments reveal the fact that real laser pulses have random fluctuations in their amplitude and phase that crucially affect pulse evolution. Statistical optics considers the random nature of light to give a more realistic description of laser pulses behavior. Generation of ultrashort pulses, operating at frequencies close to internal transition frequencies of medium atoms, motivated the growing interest in studying on-resonance light-matter interactions. Moreover, there exist optical phenomena that only occur at optical resonance. Thus, near resonance light-matter interactions are of great importance. In this dissertation, statistical properties of ultrashort pulses propagating in different linear and nonlinear media near optical resonance is studied. Various partially coherent pulses are simulated and effects of initial statistical properties of pulses on their propagation in resonant absorbing and amplifying media are explored. The research performed throughout this thesis leads to following results. 1- Various classes of ultrashort self-similar partially coherent pulses are explored along with closed form expressions for their correlation functions. Also, the evolution of coherence properties of pulses upon their short-distance and long-distance propagation is investigated. 2- Generic partially coherent pulses are simulated and their global and local correlation properties upon propagation in resonant linear absorber media are studied. The evolution of the coherence functions of pulses shows that partially coherent pulses are strongly affected by the medium regarding to their coherence levels. A correlation area theorem is also derived to describe global correlation properties of stochastic pulses propagating in such media. 3- Evolution of small-area pulse coherence properties, propagating in linear amplifiers near optical resonance, is studied. Our simulation results reveal that more coherent pulses are amplified more effectively by the medium than are their less coherent counterparts. 4- Propagation of partially coherent pulses in resonant, inhomogenously broadened nonlinear media is simulated. Stochastic pulses of different coherence levels are generated to investigate effects of initial properties of pulses on their long-term evolution. We also provide evidence of self-induced transparency phenomena and soliton formation for relatively coherent pulses. Also, evolution of the coherence functions reveal that low-coherence pulses lose their coherence level upon propagation faster than do highly coherent pulses.
The conservation laws for coherent resonant propagation are studied, and an interpretation of these laws as conservation of pulse spectral moments is proposed. The nonlinear spectral density is expressed in terms of the scattering data for an initially hyperbolic secant pulse. In this way spectral moments are exactly determined at any distance and are used for a partial description of the propagating pulse. The conservation laws for the three lowest moments are investigated in detail. The evolution of the zero-order moment leads to an explicit formula for the attenuation of the electromagnetic energy. The rate at which the energy is absorbed depends critically on the pulse area. There is an essential difference between propagation of pulse with area smaller than $\ensuremath{\pi}$ and those with area larger than $\ensuremath{\pi}$. The evolution of the first moment corresponds to change of the pulse average frequency. The calculated shift of the average frequency due to detuning is compared with the experimental result of Slusher and Gibbs. The second-order moment is a measure of the pulse spectral width. The evolution of the Fourier second-order moment is used to explain the observed spectral narrowing of a $\ensuremath{\pi}$ pulse and broadening of a $3\ensuremath{\pi}$ pulse.
We investigate the areas of extremely short, intense (area ⩾π) resonant optical pulses on propagation through an inhomogeneously broadened atomic medium. Experimentally, there is no apparent change in the shape of such pulses on propagation, in contradiction to the area theorem. Here we show that, although the main part of the pulse is unchanged for short propagation distances (αl∼1), it is followed by a long weak tail formed by free induction decay from the excited atoms. The tail lengthens and oscillates on propagation through the medium and permits the area theorem to be obeyed. These oscillations, which depend critically on the Doppler broadening, are reflected in the spectral analysis of the propagated pulse. We also introduce another mechanism for pulse reshaping, which is a generalization of the McCall–Hahn pulse breakup and operates at long propagation lengths (αl≫1), where the number of absorbing atoms encountered by the pulse is comparable with the number of photons in the pulse.