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Research on the technology of laser pulse smoothing based on multiphoton absorption effect

Author: ChengKun
Tutor: JingFeng
School: Chinese Academy of Engineering Physics
Course: Optical Engineering
Keywords: laser fusion amplitude modulation multiphoton absorption excited stateabsorption
CLC: TN249
Type: PhD thesis
Year: 2012
Downloads: 67
Quote: 0
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Abstract


Amplitude modulation might damage the optics, adversely affects the laser performance and eventually would make the inertial confinement fusion (ICF) failed, and thus needs to be eliminated or efficiently controlled. By spectrum resolved transfer functions of traditional elements, amplitude modulation could be alleviated effectively to a certain extent. However, these components just act on specific spectrum structure and the spectrum is usually complex. Meanwhile these components are costly for precision manufacture and accuracy control, and hence restrict these technologies for practical applications. Therefore, a flexible amplitude modulation suppressing method aiming at complex spectrum and practical applications is urgently demanding.Suppression of amplitude modulation in time domain based on nonlinear absorption effect is proposed. The smoothing characteristics of multiphoton absorption are investigated. The effects of several parameters of multiphoton absorber on smoothing are analysed, the feasibility of modulation suppressing effect of two-photon absorber on shaped pulse is also demonstrated.The theoretical model coupling the propagation equation with the rate equations is employed to study the dynamics of nonlinear absorbers interacting with spatiotemporal modulation pulse, and the main smoothing rules are abtained. Spatiotemporal modulation can be alleviated simultaneously by two-photon absorption because of its intensity dependent characteristic. The following excited stateabsorption can suppress spatial modulation but induce temporal distortion, and reduce the suppressing temporal modulation effect. The modulation smoothing effect of two-photon absorption is demonstrated experimently. The temporal shape distortion caused by the excited state absorption is also observed. In comparison with traditional methods in spectrum domain, this approach needs no detailed spectrum structure, leading to convenient design and manufacture in practice.Different from traditional optical limiters, high beam quality, efficient amplitude modulation suppressing capability and high energy transmission are the general requirments in beam smoothing applications. In order to assess the ultimate performance of TPA on amplitude modulation suppressing and to see trends, the effects of several parameters such as ESA cross section, relaxation time of first singlet excited state, TPA cross section, molecules consentration and the photon numbers on the temporal pulse shape, modulation suppressing efficiency and transmittance are quantitatively analysised in detail.The feasibility of modulation suppressing effect of two photon absorber on shaped pulse is studied. In order to eliminate the contour distortion of shaped pulse, we propose an analytic method to preshape the input pulse. By defining a characterization parameter for shaped pulse modulation, the modulation suppressing effect of two photon absorption on shaped pulse is evaluated. This thesis is an exploration of new laser element based on new material and new mechanism, also a try to expand the laser pulse control technology to time domain.

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CLC: > Industrial Technology > Radio electronics, telecommunications technology > Photonics technology,laser technology > Laser technology, the maser > Laser applications
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