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UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-24 9:11 |
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Conference: Bucharest University Faculty of Physics 2008 Meeting
Section: Optics, Spectroscopy, Plasma and Lasers
Title:
Competition in He-Ne laser. Didactic experiments.
Authors:
M. Ristici, I. Gruia, M. Rosu, B. Ionita
Affiliation:
University Bucharest- Faculty of Physics, Bucharest, Romania
E-mail
marin.ristici@4roptics.com
Keywords:
laser level competition, laser oscillations
Abstract:
In He-Ne cw plasma there are two upper laser levels (3s2, 2s2) and 18 accessible lower levels so that it is possible to generate many laser lines in visible and near infrared.
If the laser oscillates simultaneously on two or many laser transitions, all having the same upper level, the output of radiation is weaker than it would be the only oscillation. The situation is almost the same if the oscillating transitions had the same lower levels.
The transitions 3s2 → 3p4 (λ=3.39µm) and 3s2 → 2p4 (λ=633 nm) compete each other because their common upper level. The infrared line has a very high gain so that it is able to oscillate just when the reflection coefficient of mirrors is as low as 10 %.
The transitions 3s2 → 2p4 (λ=633nm) and 2s2 → 2p4 (λ=1.15µm) compete each other because their common lower level.
The experimental set-up consists on a laser tube, a special resonator, a high voltage variable current cw power supply, a laser power-meter, a methane cell and a set of permanent magnets.
The mirrors reflect 99% the 633 nm and 1.15µm radiations and about 20% the 3.39µm radiation. When the laser oscillates, the inside resonator beam consists of three radiations: one red-633nm and two infrared. The 3.39µm amplification is monitored by a transversal magnetic field (by Zeeman Effect) and, respectively, a methane cell placed inside resonator. The behavior of 633nm laser power for different discharge currents shows that it increases for large currents when infrared line has stopped but it decreases when laser oscillated simultaneously on both radiations.
The output beam is spectrally examined using a prism. It contains 633nm radiation, of course, but also 1084nm (2s2 → 2p4) and 1080nm (2s3 → 2p7) instead of the expected 1.15µm (2s2 → 2p4). That is because the strong oscillation on 633nm (3s2 → 2p4) determines the population of 2p4 level to increase so that the population inversion between 2s2 and 2p4 vanish.
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