Neural Networks-Based Real-Time Determination of the Laser Beam Spatial Profile and Vibrational-to-Translational Relaxation Time Within Pulsed Photoacoustics

This paper is concerned with the possibilities of computational intelligence application for simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases by pulsed photoacoustics. Results regarding the application of neural computing and genetic optimization are presented through the use of feed forward multilayer perception networks and real-coded genetic algorithms. Feed forward multilayer perception networks are trained in an offline batch training regime to estimate simultaneously, and in real-time, laser beam spatial profile R(r) (profile shape class) and vibrational-to-translational relaxation time ?V?T from a given (theoretical) photoacoustic signals ?p(r,t). The proposed method significantly shortens the time required for the simultaneous determination of the laser beam spatial profile and relaxation time and has the advantage of accurately calculating the aforementioned quantities. Real coded genetic algorithms are used to calculate ?V?T by fitting the ?p(r,t) with the theoretical one. The previously developed methods determine the laser beam profile and relaxation time with sufficient precision, but the methods based on the application of artificial intelligence are more suitable for practical applications, such as the real-time in-situ measurements of atmospheric pollutants.

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Iterative method for determination of the laser beam profile and τV-T

Facta universitatis - series Physics Chemistry and Technology ◽

10.2298/fupct0801071r ◽

2008 ◽

Vol 6 (1) ◽

pp. 71-76

Author(s):

Mihailo Rabasovic ◽

Dragan Markushev

Keyword(s):

Relaxation Time ◽

Laser Beam ◽

Photoacoustic Signal ◽

Linear Process ◽

Beam Profile ◽

Photoacoustic Tomography ◽

Spatial Profile ◽

Photoacoustic Effect ◽

Laser Beam Profile

Measuring the vibrational-to-translational relaxation time ?V-T in gases is one of the first applications of the photoacoustic effect. The spatial profile of the laser beam is crucial in these measurements because the multiphoton excitation is investigated. The multiphoton absorption is a non-linear process. Because of this, the top hat profile is preferable. It allows one to deal with nonlinearity in a simple manner. In order to reveal the real laser beam profile, we have slightly changed the theoretical profiles in such a manner that the best matching is obtained between theoretical and experimental photoacoustic signals. Still, there was a question: Is it possible to deduce the laser beam profile directly from the photoacoustic signal, thus avoiding manual changing of the laser beam profile? According to this paper, it is possible. The appropriate method has been found in another photoacoustics application: photoacoustic tomography. Thus, the method for the simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time is presented in this paper. It employs pulsed photoacoustics and an algorithm developed for photoacoustic tomography.

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