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UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-24 9:03 |
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Conference: Bucharest University Faculty of Physics 2008 Meeting
Section: Optics, Spectroscopy, Plasma and Lasers
Title:
On the evolution of laser power and frequency
from the start till the stady regime for a He-Ne laser
Authors:
I. Gruia, M. Ristici, B. Ionita, M. Rosu
Affiliation:
Universitatea din Bucuresti, Facultatea de Fizica
E-mail
gruia_ion@yahoo.com>
Keywords:
He-Ne laser
Abstract:
To sustain laser action, usualy it is necessary to enclose a volume of suitably excited material in an optical cavity. Photons are multiply reflected through the lasing medium which greatly increases the probability of stimulated emission. The multi-pass nature of a typical laser, tagether with beam coherence, results in interference – induced longitudinal mode structure. Will be amplified only light whose wavelength satisfies the standing wave condition, m ë = 2 L, where L is the cavity length and m is a large integer refering to the number of nodes in the standing wave pattern. The spacing of adjacent, so-called cavity nodes, is given by , where c is the speed of light.
The multiple longitudinal mode structure described above gives rise to a power fluctuation phenomenon termed mode sweeping.
All unstabilized He-Ne lasers exhibit this effect which is due to thermal instability causing variation in the cavity length dL. As the cavity length changes, there is a small change in mode spacing given by:
d( )=(dL/L)
which is typically 10 kHz or less under normal conditions. However, the absolute wavelength of each cavity mode is also changed by the variation in tube length:
dë=(dL/L) ë.
Using an optical spectrum analyser we determined the time interval necessaire that the frequencies of the generated laser beam to be in a given interval. It is important if we need to have for our measurements a beam stable in power output and frequency.
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