Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1067190
FIELD OF TME INVENTION
1 The present invention relates to laser excitation and
2 in particular to a system for providing a confined region of
3 excitation for a laser medium.
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, -BACKGROUND OF TH~ INVENTIO~
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4 '~ Excitation of a lasing medium, for example, a flowing `
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dye solution, to a lasing condition is typically achieved by
6 ,, the application of excitation radiation from a flashlamp or
7 i, laser source to the medium in order to create the "population
8 ,l inversion" that is employed in lasing. For use in many
9 '" applications, and of particular interest in the present
invention for use in isotope separation cr uranium enrichment,
11 ~ high power and high pulse rate lasing of a flowing dye
12 , solution is des`ired for lasers which are used to produce the
13 ~ radiant energy for isotopically selective photoexcitation.
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r~l14 An application for such a laser includes the uranium enrich- ;
ment technique shown in United States Patent 3,772,519.
16 As power and pulse rate requirements increase, it
: 17 becomes increasingly important to make efficient use of the
18 sources of excitation radiation used to create the population
-.~ 19 inversion and at the same time to prevent excitation, and
~; 20 resulting heating, of portions of the laser medium other
- ~ 21 than those directly employed in the generation of a laser
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beam. Efficiency~ in the generation o~ laser medium excita-
tion permits the realization of optimum power and pulse rate
output while confinement of the excited region, particularly
in side pumped laser mediums, prevents gradual deterioration
of laser output in the boundary of the laser medium with the
pump window~
BRIEF SUMMAR~ O~ THE INVENTION
In the present invention a pumping system for a
laser medium is described which provides both high efficiency
and limited heating by shifting the region of excitation of
the laser medium out from the boundary of the laser medium.
This contributes to high efficiency that in turn results in
high power output and at the same time reduces effects con-
tributing to the deterioration of lasing which would occur
where only the boundary between the laser medium and the
window for application of excitation is excited.
In the exemplary preferred embodiment, the excita-
tion is achieved by a focusing system for an elongate flash-
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lamp excitation source which provides a real approximately
same size image of the flashlamp plasma centrally within a
flowing laser medium. The generation of a real image concen-
~ trates the usable excitation radiation within a well defined
!.,~ region of generally cylindrical shape and permits it to be
-;~i located away from the medium boundary. This cylindrical shape
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defines the laser beam to be generated or amplified and assures that -
there will be a minimum of unused, but excited, laser medium.
In accordance with this invention there is provided a system ;~:
for imaging the discharge of a laser excitation source to a laser medium .
comprising: at least one source of laser excitation radiation having an
elongate generally linear region emitting excitation radiation in
substantially all directions; a flow channel having a window in at least
one of first and second facing generally parallel channel bordering .
surfaces; said window extending along the elongate direction of said :
linear region; a laser medium flowing through said channel and between ~
said first and second surfaces thereof in a direction transverse to the -
elongate direction of said linear region and capable of excitation to a
lasing condition; an optical system partially surrounding said at least
one source for providing a real image of said region emitting excitation
radiation through said window within a limited portion of said medium
between and apart from said first and second surfaces to provide an excited
region of generally elongate cylindrical dimension.
In accordance with another aspect of this invention there is
provided a system for forming a region of excited laser medium from a
20 real image of a line source of excitation radiation comprising: a flowing
dye solution; first and second cylindrical windows bordering said flowing
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dye solution; first and second specular reflectors curving about said first
: and second windows respectively on the side away from said flowing dye solution; first and second flashlamps between said first and second
:; reflectors and first and second windows respectively and having a region
of plasma in a generally elongate, linear shape transverse to the flow
direction of said dye solution and emitting excitation radiation; said
first and second windows, reflectors and flashlamps being associated to
. provide focusing of the excitation radiation to coincident, generally
30 ~ similar dimensioned, elongate linear regionswithin said dye channel apart.' ~
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from borders thereof with said first and second windows.
BRIEF DESCRIPTION OF THE DRAWING
These and other features of the present invention are more fully
set forth below in the detailed description of the preferred embodiment,
presented solely for purposes of illustration and not by way of limitation,
and in the accompanying drawing of which:
Figure 1 is a sectional view of a laser excitation system
according to the present invention; and
Figure 2 is an orthogonal sectional view of the illustration
.
of Figure 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
: The present invention contemplates a system for applying
:, excitation radiation to a laser medium in a confined cylindrical volume
of the laser medium to promote efficiency and limit the region of medium
' heating The invention is particularly useful in exciting a flowing
medium such as a dye solution used in a process for isotopically selective
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1 photoexcitation as~applied to uranium enrich~ent. Th~
2 system of the present invention may best be understood by
3 reference to the sectional view of an exemplary embodiment
4 sho~ in Fig. 1. In Fig. 1, a dye channel 12 is formed
between facing walls 14 and 16 having respective, typically
6 quartz windows 18 and 20 in the form of cylindrical segments
7 extending into and out of the page, set therein. The channel 12
8 contains a flowing dye solution such as is employed in laser
9 oscillators or amplifiers intended for use in isotope separation
10, as described in the above United States Patent 3,772,519,
incorporated herein by reference and assigned to the same
2 assignee as the present invention.
13On either side of the windows 18 and 20, away from the
14,,dye channel 12, are located respective flashlamps 22 and 24
lS which may be o conventional design, although they are
16 preferably smàll in cross-section. The ~lashlamps 22 and 24
17 consist of respective external jackets 26 and 28 within
18 which generally cylindrical, or elongate linear plasma ..
19 , regions 30 and 32 are created by electrîcal discharge through `
the gas within the quartz jacket 26 and 28. The plasmas 30
21 , and 32 are typically produced fron pulse discharges which
22 are of particular utility in the system o isotope separation
23 described in the above-referenced patent.
24 , The flashlamps 22 and 24 are partially surrounded by
cylindrical reflectors 34 and 36 typically elliptical in
26 cross-section having axes parallel to the axes of the cylindrical
27 windows 18 and 20. The reflectors 34 and 36 provide specular
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1 reflection of excitation radiation 38 from the flashlamps 22
2 and 24 onto paths 40 which are allowed to focus by cylindrical
3 windows 18 and 20 to a region 42 thereby creating a real
4 image within the region 42 of each plasma region 30 and 32.
The windows 18 and 20 have generally plane surfaces 44
6 ~ and 46 bounding the flowing dye solution within the dye
7 channel 12 and these surfaces are coplanar with the smooth
8 inner surfaces of the channel edge plates or walls 14 and 16.
9 ' The opposite surfaces 48 and 50 of the windows 18 and 20 ~`
respectively are curved to allow focusing, in conJunction
11 , with the reflectors 34 and 36, of plasma regions 30 and 32
12 ~, into the region 42. In the case where the reflectors 34
13 j and 36 are eliptical as described above, the curved sur-
14 , faces 50 and 52 of thé lenses 18 and 20 typically have a
circular cross-section or where cost is not a limitation, an
16 ' aspheric cross-section to optimize focusing. Other cross-
17 , sectional shapes for the reflectors 34 and 36 and lens
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; 18 ~ surfaces 50 and 52 are usable which permit the generation of
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19 " a real image of the plasma cylindrical discharge regions 30
l and 32 within the region 42.
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~ 21 The result of this imaging is the generation within the
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; 22 ~ dye channel 12 of a real image o the plasmas 30 and 32 with
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23 minimum magnification, i.e. generally same size ima~ing, and
24 with minimum aberrations. This results in the concentration
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: 25 of flux from the flashlamps within a narrow, high radiance
26 distribution within the dye volume, a distribution particularly
27 desired for efficient use of the flashlamn radiation.
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1 Design considerations for the shape of the reflectors 34
2 and 36 and the windows 18 and 20 at their curved surfaces 50
3 and 52 include maximu~ energy transfer efficiency; minimum
4 image 42 cross-sectional area; minimum radiation outside the
central image 42; and balancing of aberration from the
6 flashlamp jackets 22 and 24. In the case of elliptic reflec-
7 tors 34 and 36, these goals are achieved with windows 18 -;
8 ; and 20 which produce a minimum deviation in the flashlam~
9 'radiation. Other combinations of window and reflector
configurations, (e.g. parabolic) may be used although the
11 J~ exemplary one shown is preferred.
12 ,,3 To complete the description of a laser amPlifier or
13 oscillator using the flashlamp focusing system of the present
14 linvention, reference is made to Fig. 2 showing a sectional ;
!Iview along lines indicated in Fig. 1. As sho~m there, the
16 ends of the dye channel 12 which are above and below the
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17 plane of the page in Fig. 1 are bounded by windows 54 and 56
18 through which the laser beam to be amplified by the system
19 of the present invention, or the laser beam generated when
; 20 used as an oscillator, passes as the beam 58. The beam 58
21 ` will typically correspond in cross-sectional area to the
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22 cross-sectional area of the excited region 42. This region
23 is a limited area of the total area available within the dye
24 channel 12 between the lens surfaces 44 and 46 which limits
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~25 the generation of heat near the surfaces within the dye
; 26 medium, generally a problem in transverse pumped laser
- 27 ~mediums. In addition, by limiting the region of excitation
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to a confined area such as the region 42, corresnonding to
2 the cross-sectional area oE.the desired laser beam 58,
; 3 maximum efficiency in the use of the radiation from the
4 1ashlamps 22 and 24 may be achieved. High pulse rate is
also possible since there will be little excitation of the
6 laser medium outside of the region 42 particularly along the
7 lines of medium flow.
8 With the objectives of the present invention and the .-
9 manner of a~hieving them described above, it is intended not
i~to limit the specific apparatus for achieving them to the
11 exemplary embodiment described. Accordingly, it is intended
- 12 il to define the scope of the invention only as indicated in
13 ',the following claims.
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