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UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-23 17:44 |
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Conference: Bucharest University Faculty of Physics 2009 Meeting
Section: Optics, Spectroscopy, Plasma and Lasers
Title:
A He-Ne laser for methane detection
Authors:
M. Ristici*, I. Gruia*, D.M. Ristici**, V.A. Gruia***
Affiliation:
* University of Bucharest
** 4R Optics SRL
*** Polytechnic University of Bucharest
E-mail
gruia_ion@yahoocom
Keywords:
Abstract:
Laser oscillation on 3s2→3p4 transition (λ=3,3922μm) of neon in He-Ne active medium was obtained by Bloom in 1963. It was considered that his main importance consisted in its competition with the well-known red laser line 632,8 nm. The gain coefficient for the infrared radiation is about 400% m-1, about 100 times higher than the red line so that without special care, they oscillate, in a 632,8 nm laser, together.
The infrared 3,3922 μm laser radiation was used in spectroscopy measurements for determining the structure of vibration-rotation band of methane. The emission frequency coincides closely to the one of the components of the P(F+) branch of the υ3 band of methane. The separation of the emission line and the nearest absorption line was measured to be 0,003±0,002 cm-1. Based on such research, many devices destined to detect the presence of methane in atmosphere were developed.
We realized a low power He-Ne laser oscillating on 3,3922 μm. The tube is made from Pyrex glass, with sided electrodes. The capillary is 1,6 mm inner diameter and 28 cm active length. The gas mixture is pHe:pNe=10 and total pressure 3 torr. The resonator consists of a 1 meter radius of curvature 96% reflexion as total reflecting mirror and, respectively, a fused silica plane-paralel plate placed at distance d=42 cm. The calculated reflexion coefficient on a surface of fused silica in the limit of normal incidence in air for 3,39 μm is about 3% so that the total reflexion of the plate is about 6%. The beam parameters were calculated: beam waist ws=0,73 mm and divergence θ=1,45 miliradians. For a discharge current of 6,5 mA we obtained 0,7 mW laser power.
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