Physics/7. Optics
Doctor
of Sc. Rena J. Kasumova
Physics Department,
Baku State University, Azerbaijan, rkasumova@azdata.net
Parametric generation in Zn0.75Mg0.25Se
crystal
Abstract
In the present work investigation is
carried out of parametric intracavity interaction with account for phase change
of interacting waves of pump, signal and idler waves in the materials for IR
range of spectrum on case of Zn1-xMgxSe crystal. In work
there has been made an analysis of threshold character of parametric generation
in crystal of IR range Zn0.75Mg0.25Se. Increase in
efficiency of intracavity conversion is possible at high entry values of
pumping intensity.
Keywords:
intracavity parametric interaction, middle IR,
constant - intensity approximation.
Recent years there are intensively
investigated the new lasers of IR range. Thus, we can note solid-state lasers
of IR range on the basis of Cr2+:ZnMgSe and Fe2+:ZnSe.
These tunable compact quantum generators work with pumping in the region of
generation of ions Cr2+ and Fe2+ respectively in the
chalcogenide matrix [1-4]. Two versions of conversion of energy of coherent
optical radiation into energy of parametric waves are possible: intracavity
conversion and conversion outside of laser resonator [5].
As is known, parametric generation of
light waves has threshold character, which is determined by losses in a medium
and resonator. At exceeding effort over losses parametric generation is
realized. Therefore, high intensities of pumping are required to start this
process. In the constant–intensity approximation [6] we determine threshold
value of pump wave amplitude. At greater powers of pumping and lengths of
interaction the threshold amplitude of pumping looks as () [7].
(1)
From the expression (1) it is seen that
with increasing losses, condition of parametric gain, i.e.
(2)
is fulfilled at
greater values of pumping amplitude.
Fig. 1.
Dependencies of threshold pump intensity on linear losses () in the crystal at phase matching condition D=0 for crystal length of =0.5 cm
[1].
For investigation of threshold
intensity of pumping in case of mixed crystal Zn0.75Mg0.25Se
well make numerical calculation using analytical expression for complex
amplitude of signal wave, obtained in the constant-intensity approximation [8].
With this, we choose the task parameters according to conditions of existing
experiment [1].
In Fig. 1 there are offered dependences
of parametric strengthening of signal wave on losses. As it was
expected with an increase of losses the minimum value of pump intensity,
determining the threshold character of the process, according to (1),
increases. In conditions of phase matching, from the numerical analysis (1) it
is followed that in Zn0.75Mg0.25Se crystal the threshold
intensity of pumping at cm-1 is
equal to mJ, at cm-1 is
equal to mJ, and at cm-1 makes
up value to mJ. Note that in [1]
in Zn0.75Mg0.25Se crystal there has been experimentally
obtained threshold energy of pumping equal to 3 mJ (the thickness of the sample
was 5 mm). From comparison of the results we may estimate that in the
experimental sample losses make up a value cm-1.
Fig. 2. Dependences of gain coefficient of signal wave in Zn0.52Mg0.48Se
crystal as a function of the reduced
length calculated in the constant-intensity
approximation for =1 cm-1,
=0.5, R2s=55%
[1] at pump intensity of Ipo=40 kW/cm2 (curve
4), 30 kW/cm2 (curve 3), 20 kW/cm2 (curve 2), 10 kW/cm2
[1] (curve 1).
Dynamics of parametric conversion after
two passages of crystal Zn0.75Mg0.25Se is cited in Fig. 2
for case of nonzero phase mismatch in dissipative medium of crystal. With the
increase in pumping intensity conversion efficiency nonlinearity increases. So,
increase in pumping two times leads to
more than double fold (2.12 times) increasing efficiency .
Thus, in work there has been made an
analysis of threshold character of parametric generation in crystal of IR range
Zn0.75Mg0.25Se. Increase in efficiency of intracavity
conversion is possible at high entry values of pumping intensity.
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