Vol.6, No 1, 2008 pp. 71 - 76
DOI 10.2298/FUPCT0801071R
UDC 544.532
ITERATIVE METHOD FOR DETERMINATION OF
THE LASER BEAM PROFILE AND τV-T
Mihailo D. Rabasović, Dragan D. Markushev
Institute of Physics, Pregrevica 118, 11080 Belgrade – Zemun, Serbia
Abstract. Measuring the vibrational-to-translational relaxation
time τV-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.
Key words: Photoacoustic spectroscopy, laser beam profile, vibrational-to-translational
relaxation time.
ITERATIVNI METOD ZA ODREĐIVANJE PROFILA
LASERSKOG ZRAKA I τV-T
Merenje vibraciono-translacionog relaksacionog vremena τV-T u gasovima
je jedna od prvih primena fotoakustičkog efekta. Prstorni profil laserskog
zraka je veoma značajan u ovim merenjima jer je ispitivana višefotonska
pobuda. Višefotonska apsorpcija je nelinearan proces. Zbog ovoga je poželjno
koristiti laserski zrak sa homogenim prostornim profilom. Ovakav profil
omogućava da se uticaj nelinearnosti eliminiše na jednostavan način.
Da bismo odredili stvarni prostorni profil zraka, neznatno smo menjali
teorijski profil zraka dok nismo dobili najbolje slaganje između teorijskog
i eksperimentalnog fotoakustičkog signala. Ipak, postavlja se pitanje
da li je moguće direktno iz fotoakustičkog signala odrediti prostorni
profil laserskog zraka i na taj način izbeći ručnu promenu teorijskog
profila laserskog zraka. U ovom radu je pokazano da je to moguće. Odgovarajući
metod za ovo je pronađen u drugoj primeni fotoakustike, fotoakustičkoj
tomografiji. u ovom radu je prezentovan metod za istovremeno određivanje
prostornog profila laserskog zraka i vibraciono-translacionog relaksacionog
vremena. Ovaj metod koristi impulsnu fotoakustiku i algoritam razvijen
za potrebe fotoakustičke tomografije.
