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SPECTROSCOPIC STUDY OF ds/dT IN COMMERCIAL FILTERS BY USING THE THERMAL LENS
TECHNIQUE
FRANCISCO DIAS LIMA1,2; LUIS PEDRO DE SOUZA ANDRADE1
1 Universidade Estadual de Mato Grosso do Sul, C.P. 351, Dourados, MS.
2 Instituto de Física de São Carlos, Universidade de São Paulo, C.P. 369, São Carlos, SP.
Introduction: The change in the optical path with the heat deposited per unit volume (ds/dQ) is a parameter
that measures the TL distortion induced within a given material by laser beam, and it is related with the
temperature coefficient of the optical path length change by ds/dT = (1/cp)ds/dQ, in which  is the density
and cp is the specific heat of the sample. This study is very important to classify the better wavelength region
to work with these filters. TL technique [1] has proved to be a convenient method to determine the thermooptical properties of transparent materials, like for instance the thermal diffusivity, D, ds/dT, and for
luminescent materials, the fluorescence quantum efficiency, . The TL effect is created when an excitation
laser beam passes through the sample and the absorbed energy is converted into heat, changing the
refractive index of the sample and therefore producing a lens-like optical element within the sample. By
measuring a probe laser beam intensity in the far field the thermo-optical properties can be measured. Aim:
The subject of this work is developing a spectroscopic study of the change in the optical path with the heat
deposited per unit volume (ds/dQ) in commercial filters by using the Thermal Lens (TL) technique. Material
and methods: In our measurements, the excitation wavelength was fixed at 514 nm (Ar+ laser)
corresponding to the absorption band of the filter, and the TL was probed from 655 to 688 nm with a tunable
cw dye laser. Figure 1 shows the wavelength dependence of ds/dQ and the absorption spectra in the same
wavelength range for the RG-610 filter. Results and discussion: We can note that ds/dQ increases when
the wavelength decreases, nearing to the absorption band. A similar behavior was also observed in a
chalcohalide glass and the interpretation suggest that ds/dQ is usually described by a Sellmeier equation [2].
In a TL measurement, the thermal diffusivity can also be determined, and for the RG-610 filter, the obtained
value was 3.510-3 cm2/s, which is approximately 2.7 times higher than the obtained value for chalcohalide
glass [1-2]. Conclusions: In conclusion, our results show that the studied filters exhibit low ds/dT (or ds/dQ)
value for energy (wavelength) far from their absorption bands.
9
-6
-1
12
6
3
ds/dQ ( 10 cm /J )
Absorption coef.
ds/dQ
8
3
4
0
650
660
670
680
Wavelength ( nm )
Absorption coef. ( cm )
16
0
690
Figure 1: The open circle indicates the optical path change with the heat deposited per unit
volume, ds/dQ, and the solid line is the absorption coefficient, vs the probe beam wavelength for
the RG-610 commercial filter.
References:
[1] S. M. Lima, J. A. Sampaio, T. Catunda, A. C. Bento, L. C. M. Miranda, and M. L. Baesso, J. Non-Cryst.
Solids 273 (2000) 215-227.
[2] S. M. Lima, A. A. Andrade, T. Catunda, R. Lebullenger, F. Smektala, Y. Jestin and M. L. Baesso, J. NonCryst. Solids 284 (2001) 203-209.
Key-words: Thermal lens; filter; thermo-optical parameters.
Acknowledgment: FUNDECT/MS.
Realização:
Apoio: