United States Patent
`nu 3,619,588
`
`[72]
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`[21]
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`Inventor
`
`Appl. No.
`Filed
`Patented
`Assignee
`
`Keith W. Chambers
`Pinawa, Manitoba, Canada
`877,752
`Nov. 18, 1969
`Nov. 9, 1971
`Atomic Energy of Canada Limited
`Ottawa, Ontario, Canada
`
`HIGHLY COLLIMATED LIGHT BEAMS
`10 Claims, 1 Drawing Fig.
`240/1 R,
`U.S. Cl........................................................
`,
`331/94.5, 350/294
`Int. Cl......................................................... F21k 2/00
`Field of Search............................................
`240/1;
`331/94.5; 350/294
`
`[56]
`
`References Cited
`UNITED STATES PATENTS
`
`3,242,806
`3,433,555
`
`3/1966 Hine ........................... ..
`3/1969 Tomlinson .................. ..
`FORElGN PATENTS
`
`8/1921 Norway ...................... ..
`32,974
`Primary Examiner—Louis R. Prince
`Assistant Examiner—Frederick Shoon
`Attomey—Cushman, Darby & Cushman
`
`350/294
`331/94.5 X
`
`350/294
`
`ABSTRACT: This application discloses a light source consist-
`ing of a gas-filled pressure vessel to which light from a laser is
`admitted and focused to a point. Light emitted by the gas at
`the focal point is directed by a mirror system to an output
`pupil to provide a highly collimated light beam.
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`I
`HIGHLY COLLIMATED LIGHT BEAMS
`This invention relates to the production of highly collimated
`light beams.
`Such a highly collimated light beam finds many uses in
`physics, one such use being as a spectroscopic flash in certain
`photolysis experiments. An intense light flash can be produced
`by a laser beam focused in a gas.
`According to the present invention, a light source adapted
`to produce a highly collimated light beam comprises a pres-
`sure vessel; a gaseous filling in said pressure vessel of a gas
`able to produce by breakdown light of the required spectral
`content; an entrance pupil through which light from a laser
`can enter the said pressure vessel; an output pupil through
`which light can pass from the said pressure vessel; a first opti-
`cal system disposed within the pressure vessel and arranged to
`focus light entering the said entrance pupil inside the gas in
`the pressure vessel to a focal point; and a second optical
`system disposed within the pressure vessel and arranged to
`direct light passing from the said focal point out of the vessel
`through the said output pupil.
`The invention will now be described, by way of example,
`with reference to the accompanying drawing, which is a sec-
`tional side elevation of a high-pressure gas cell and shows a
`laser arranged to energize that cell.
`The high-pressure gas cell 1 comprises a cylindrical body 3
`formed at its two ends with flanges 5 and 7 respectively. The
`inside of this body has a matte black finish, such as is used in
`optical equipment. Clamped to flange 5, by bolts which are
`not shown,
`is a glass window 9, a sealing washer 11 lying
`between the window and the flange. Fastened to the outside of
`the window 9, by a suitable adhesive, is an opaque screen 13
`formed with a central aperture 15. Similarly, clamped to
`flange 7 is a glass window 17 provided with a sealing washer
`19 and an opaque screen 21 formed with a central aperture
`23. The body 3 is formed with a port 25 into which is screwed
`a nipple 27 connected to orthodox air-removal and gas-inser-
`tion apparatus, by which the gas cell can first be evacuated
`and then can be filled with any desired gas at a desired operat-
`ing pressure.
`Mounted inside the cell 1 are three mirror systems, namely
`an inwardly directed concave parabolic mirror 31 positioned
`adjacent flange 5, and inwardly directed concave parabolic
`mirror 33 positioned adjacent flange 7, both of these mirrors
`being of the same diameter and practically filling the cross
`section of body 3, and a much smaller double-sided parabolic-
`mirror system having a concave mirror 35A directed towards
`mirror 33, the mirror system being located on the central axis
`of the body 3 and positioned somewhat further from the mir-
`ror 31 than from the mirror 33. Mirrors 31 and 33 are held in
`place respectively against fixed rings 37 and 39 by circlips 41
`and 43, while mirror system 35A, 35B is located relative to the
`body and the other mirrors by a thin but rigid three anned
`spider 45. Thus its arms can have a thickness of 0.5 cm. and a
`width of 1.00 cm.
`lndicated in the drawing are: a focal point P; a central aper-
`ture P1 in mirror 33 forming an entrance pupil; a central aper-
`ture P2 in mirror 31 forming an output pupil; the diameters
`D1, D2, D3 and D4 respectively of mirrors 31, 33, 35B and
`35A; the focal lengths F1 of mirror 33, F2 of mirror 31, F3 of
`mirror 35B and F4 of mirror 35A.
`The mirrors used in the preferred embodiment are front alu-
`minized parabolic mirrors of quality similar to that found in
`astronomical image forming systems. They have the smallest
`attainable ratio of diameter to focal length. The diameters M1
`and M2 should not exceed 20 cm. The front surface coatings
`should give high reflectivity in the wavelength ratio 200 to 800
`nanometer (l nanometer equals 10” meter). The following
`relationships also exist:
`a. DI/Fl=D2/F2=D3/F3=D4/F4
`b. Fl=F2
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`c. (F3-F4) is as small as practicable
`d. F4=F2/l0=Fl/I0
`c. Pl=D3
`l". P2==D4/I0
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`3,619,588
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`Also shown in the drawing is a laser unit 51 the collimated
`beam of light from which has a diameter approximately equal
`to diameter D3. ln order to provide a laser beam of the same
`diameter as the entrance pupil P1, the laser unit 5 includes a
`laser producing a narrow beam together with a beam expand-
`ing telescope to produce a beam of the desired width. Such
`telescopes are provided as standard items by manufacturers of
`lasers :and simply attach to the output head of the laser. The
`part of the glass window 21 which underlies the aperture 23 is
`formed as a selective mirror 55 in known manner, which selec-
`tive mirror transmits incoming light from the laser at most
`frequencies but reflects light at the frequency which originates
`within the gas cell.
`The arrangement of mirrors shown constitutes two as-
`tronomical telescope systems, one Cassegrain and consisting
`of mirrors 31 and 35A. and aperture P2, and the other
`Gregorian and consisting of mirrors 33 and 35B and aperture
`P1. All the parabolic reflecting surfaces have a common focal
`point (P) and the samef-number.
`The high-pressure gas cell is prepared for use by evacuation
`and by filling with the desired gas under the appropriate pres-
`sure. Thus, for example, it can be filled with xenon gas to a
`pressure of 3 to 5 atmospheres to give a distribution of spec-
`tral radiance similar to that of a high-pressure xenon arc lamp.
`Xenon lamps emit strong medium- and long-wave ultraviolet
`radiation with a continuous spectrum and have several radia-
`tion maxima in the short-wave infrared range, i.e., between
`8,000 and 10,000 A. For any particular arrangement (gas),
`the cell could be energized by a ruby laser for pulsed output or
`a high-power continuous-wave laser (eg., 100 W C0, gas
`laser) for continuous output. The laser 51 is then energized,
`and the collimated beam of light from the laser enters the cell
`through the selective mirror 55 and the entrance pupil at P1.
`After reflection at the surfaces of mirrors 35B, 33 and 31, the
`laser beam is brought to a focus at point P; the “residual” laser
`beam passes out of the device through the exit pupil at P2,
`after an appropriate number of reflections at mirrors 3] and
`35A.
`
`Light rays emitted at the point P due to breakdown of the
`gaseous medium by the focused laser beam, and which un-
`dergo their first refiection at mirror 35A, pass out of the
`system via output pupil P2 as a narrow, collimated beam after
`an appropriate number of reflections at mirrors 31 and 35A.
`Light rays emitted at the point P and which are reflected
`first at mirror 31 will pass out of the system via the entrance
`pupil P1 but will be returned by the selective mirror 55, and
`ultimately follow the same path as those rays emitted at point
`P which are first reflected at mirror 35A.
`Emitted rays whose wavelength is equal to that of the laser
`beam will be lost through selective mirror 55; intensity losses
`at this wavelength will, however, be made up by the residual
`laser beam.
`The useful output from the cell 1 is thus emitted from the
`output pupil P2. This light output corresponds, essentially, to
`the emission spectrum of the gaseous medium used, and is in
`the form of an intense, narrow and highly collimated beam.
`lclaim:
`l. A light source adapted to produce a highly collimated
`light beam having a spectral content different from that of
`light from a laser light source, and comprising:
`a. a pressure vessel;
`b. a gaseous filling in said pressure vessel of a gas able to
`produce by breakdown light of the required spectral con-
`tent;
`'
`c. an entrance pupil through which light from said laser light
`source can enter the said pressure vessel;
`d. an output pupil through which light can pass from the
`said pressure vessel;
`'
`e. a first mirror system disposed within the pressure vessel
`and arranged to focus light entering the said entrance
`pupil inside the gas in the pressure vessel to a focal point‘.
`and
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`f. a second mirror system disposed within the pressure ves-
`sel and arranged to direct light produced by breakdown
`of the gas passing from the said focal point out of the ves-
`sel through the said output pupil as a highly collimated
`light beam.
`2. A light source according to claim 1, and in which the first
`mirror system comprises:
`a. a first circular concave paraboloidal mirror;
`b. a first central aperture in that mirror;
`c. a circular convex paraboloidal mirror facing the concave
`mirror and having a diameter much smaller than that of
`the convex mirror;
`d. a second circular concave paraboloidal mirror facing the
`said first circular concave mirror;
`e. a second central aperture, in the said second concave
`mirror; the focal points of the first and second concave
`mirrors being coincident with the focal point of the con-
`vex mirror, and the light entering the space between the
`first concave mirror and the convex mirror through the
`said first aperture being directed by the second concave
`mirror to the said focal point.
`3. A light source according to claim 2, and in which the
`diameter of the convex mirror is substantially equal to the
`diameter of the said central aperture.
`4. A light source according to claim 2, and in which each
`mirror is a front-surfaced mirror.
`5. A light source according to claim 1, and in which the
`second mirror system comprises:
`a. said second concave mirror;
`b. said second central aperture in that mirror;
`c. a third circular concave paraboloidal mirror facing said
`second concave mirror and having a diameter much
`smaller than that of said second concave mirror;
`the focal points of said second and said third concave mir-
`rors being coincident, and light originating at the said
`focal point passing out of the space between the mirrors
`by reflection first from the third concave mirror and
`second by multiple reflection between the second and
`third concave mirrors.
`6. A light source according to claim 5, and in which each
`mirror is a front-surfaced mirror.
`7. A light source according to claim 1, and in which:
`a. the first mirror system comprises:
`i. a first circular concave paraboloidal mirror;
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`ii. a first central aperture in that mirror;
`iii. a circular convex paraboloidal mirror facing the con-
`cave mirror and having a diameter substantially equal
`to the diameter of the said first central aperture;
`iv. a second circular concave paraboloidal mirror facing
`the said first circular concave mirror;
`v. a second central aperture, in the said second concave
`mirror;
`b. the second mirror system comprises:
`i. said second concave mirror;
`ii. said second central aperture in that mirror;
`iii. a third circular concave paraboloidal mirror facing
`said second concave mirror and having a diameter
`much smaller than that of said second concave mirror;
`c. the focal points of said second and third concave mirrors
`being coincident;
`light entering through the entrance pupil passing by
`reflection at the convex minor and the first and second
`concave mirrors to the said focal point;
`e. light originating at the said focal point passing altemative-
`ly:
`i. after a first reflection at the third concave mirror by
`multiple reflections between the third concave mirror
`and the second concave mirror out through the second
`central aperture to a point of use;
`ii. after a first reflection at the second concave mirror, by
`reflection at said first concave mirror and said convex
`mirror but through said first central aperture.
`8. A light source according to claim 7, and in which each
`mirror is a front-surfaced mirror.
`.
`_
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`9. A light source according to claim 7, and in which:
`-a. selective mirror is provided at the entrance pupil;
`b. this selective mirror permits entry into the pressure vessel
`of light having the frequency of the laser;
`c. this selective mirror returns into the pressure vessel light
`originating at the said focal point which has a frequency
`markedly different from that of the said laser light;
`d. such returned light is caused to return via said convex
`mirror, said first and second concave mirrors to the focal
`point and to pass thence to said third concave mirror for
`multiple reflection towards the output pupil.
`10. A light source according to claim 9, and in which each
`mirror is a front-surfaced mirror.
`It
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