Patent Application: US-3361908-A

Abstract:
through the use of a relatively inexpensive third mirror on a novel folded hybrid unstable resonator configuration , the optimum output coupling for a given laser design can be explored quickly and easily with a minimum of intracavity mirror alignment . no changes in either the radii of curvature of the three cavity optics or their spacing are required for this exploration . in addition to providing techniques for purposefully and systematically introducing mirror edge effects or avoiding edges effects altogether , the invention provides that output beams of different width can be advantageously explored in a relatively simple and straightforward manner . the invention provides that higher geometric magnification cavity designs may be made compatible with low diffraction output coupling in a configuration that uses only three totally reflecting optics .

Description:
fig5 shows a folded three mirror embodiment of a hybrid unstable resonator in accordance with the concepts of the present invention as viewed perpendicular to the longer transverse side of the gain medium cross section . the fig5 depiction emphasizes a high geometric magnification , negative branch confocal pair communicating with a comparatively large aperture planar cavity end mirror . this configuration addresses a combination of high geometric magnification and modest to low fractional output coupling . also depicted in fig5 are two features , designated by reference numerals 1 and 2 , positioned at the upper edge of mirror m 1 and the lower edge of mirror m 3 , respectively . feature 1 on mirror m 1 is a contoured edge nominally taught as part of prior art technology as the element responsible for radiation reflected from the forward or expanding wave of a hybrid unstable resonator that acts as the source of radiation initially feeding the reverse wave of such cavities . in accordance with the invention , feature 2 on mirror m 3 augments the scattering function of feature 1 by providing a source of scattered radiation that is symmetrically located with respect to feature 1 on the opposite side of the diffraction coupled laser output beam . such augmentation may , for example , include employing a curve or scattering edge different than that used on mirror m 1 . also , because mirror m 3 is envisioned as a relatively inexpensive totally reflecting optical element , a variety of edge shapes can be investigated to determine the optimum edge or surface treatment on mirror m 3 for a desired modal output . moreover , in the high magnification , low diffraction output coupling regime as shown in fig5 , the surface of mirror m 3 might advantageously employ a hole grating arrangement or employ fresnel reflecting features to enhance laser operation on a single co 2 transition or otherwise assist in restricting the laser output to a single co 2 band . fig6 shows a positive branch version of fig5 . in this arrangement , the reflecting surface of mirror m 1 is convex while the reflecting surface of mirror m 2 is concave so that the confocal condition for a cavity of length l is given by l = f 2 − f 1 and where the common focal plane for the confocal m 1 / m 2 mirror pair is located to the right of mirror m 2 . like the arrangement in fig5 , mirror m 3 has a lower scattering feature 2 that is symmetrically located opposite scattering feature 1 on mirror m 1 . fig7 shows a folded three mirror embodiment of the disclosed hybrid unstable resonator concept as viewed perpendicular to the longer transverse side of the gain medium cross section . the fig7 depiction emphasizes a moderate geometric magnification , negative branch confocal pair communicating with a comparatively small aperture planar cavity end mirror . this configuration addresses a resonator design whereby the aperture of the third cavity mirror is set to reduce the aperture of the magnified forward standing wave mode by about 50 %. fig8 shows a folded three mirror embodiment of the disclosed hybrid unstable resonator concept as viewed perpendicular to the longer transverse side of the gain medium cross section . the fig8 depiction emphasizes a moderate to low geometric magnification , positive branch confocal pair communicating with a comparatively small aperture cavity end mirror . this configuration addresses a resonator design whereby the aperture size and the front surface of the third cavity mirror could be specified to force oscillation on a single line or encourage lasing on a single band of the co 2 molecule . fig9 shows a folded five mirror embodiment of the disclosed hybrid unstable resonator concept as viewed perpendicular to the longer transverse side of the gain medium cross section . this depiction shows a moderate geometric magnification , negative branch confocal pair communicating with three additional planar cavity end mirrors . this configuration addresses a resonator design whereby the aperture of the third and fourth cavity mirrors are set to relay the full width of the intracavity radiation leaving the confocal pair but where the fifth cavity mirror set to allow a less than the full beamwidth of the intracavity flux to exit the five mirror cavity and where the remaining collimated piece is returned to repass the gain medium and eventually communicate with the confocal pair . this embodiment addresses the issue of achieving a cavity free spectral range corresponding to a cavity length as long as three times the confocal mirror spacing . referring back to fig5 , a high magnification negative branch hybrid unstable resonator is shown as viewed perpendicular to the longer side of an asymmetric transverse cross section gain medium . asymmetric laser gain media such as this are well known to practitioners of unstable resonator art and are taught and described in ref . 1 , 2 , 6 , 11 , 12 , 13 , 14 , 15 , 16 and 17 among others . the shorter side of such an asymmetric transverse cross section would be arranged to support a waveguide or freespace mode of operation . the gain media might be slab or slice co 2 rf or dc excited media , dye laser media , coil media or slab solid state laser media . for a slab co 2 gain region , the discharge electric field would be directed parallel to the shorter side of the transverse cross section , while for a slice co 2 gain region , the discharge electric field would be oriented perpendicular to the shorter side of the transverse cross section . in the view depicted in fig5 , the asymmetric transverse cross section is normal to the longitudinal cavity axis formed by connecting mirrors m 1 , m 2 and m 3 . in this embodiment , the confocal mirror pair m 1 / m 2 is shown to communicate with mirror m 3 to form a u shaped or vee shaped cavity . the illustrated m 1 / m 2 mirror combination is intended to represent a geometric magnification of magnitude about m = 5 , but where the size of mirror m 3 is set to allow only about 10 % diffractive output coupling from the m 1 , m 2 , m 3 cavity . the confocal requirement for the cavity in fig5 is l = f 1 + f 2 , where l is the spacing between mirror m 1 and mirror m 2 and f 1 is the focal length of mirror m 1 and f 2 is the focal length of mirror m 2 . for a geometric magnification m = f 2 / f 1 = 5 and l = 1 meter , the confocal requirement is satisfied when f 1 = ⅙ m concave and f 2 = ⅚ m concave . a conventional confocal hybrid unstable resonator with a geometric magnification of 5 would normally have an output coupling of c = 1 − 1 / m or a coupling of 80 %. thus , this embodiment illustrates that high geometric magnification and low diffractive output coupling are now compatible . to increase the output coupling , one only has to translate mirror m 3 parallel to the longer side of the asymmetric cross section gain medium so that the distance between mirrors m 1 and m 3 is increased . to the extent that this translation can be accomplished while maintaining the front surface of mirror m 3 normal to the cavity longitudinal axis , then it is obvious that the output coupling can be adjusted without changing the spacing between mirrors m 1 and m 2 , their focal lengths or altering the alignment of mirrors m 1 , m 2 or m 3 or any combination thereof . it is well known to those with skill in the art of hybrid unstable resonators that the essential optical feedback necessary for intracavity mode regeneration in a prior art unstable resonator is generated by the single edge feature , designated feature 1 ( ref . 18 ) in the lower right hand corner of fig5 . for low geometric magnification resonators taught by the prior art , this single edge would be located near the upper edge of the intracavity mode and the gain medium of fig5 . therefore , the embodiment of the invention illustrated in fig5 effectively doubles the source of this essential feedback by providing two scattering edges rather than one edge as is common in prior art hybrid unstable resonators . furthermore , to the extent that tailoring the edges of the feedback ( or output coupling ) mirror in unstable resonators is known to provide a means to tailor the intensity distribution of the circulating intracavity mode , it is clear that this level of incidental feedback is dwarfed by the level of intracavity flux returning to the m 1 / m 2 confocal mirror pair from the front surface of mirror m 3 . as will be appreciated by those skilled in the art , with regard to fig5 and similarly to all of the figures of the disclosed embodiments of the invention , when intra - cavity light reflected from mirror m 3 returns to the m 1 / m 2 mirror combination , the radiation is successively de - magnified ( ref . 19 , 20 ) on each cavity round trip . on the first round trip traversal in this reverse direction , the width of the intracavity mode is reduced by a factor of 1 / m . this reduction in aperture is just enough to not allow the intracavity flux to intercept mirror m 3 . on each successive round trip in the reverse cavity mode direction , the reflected flux from mirror m 3 continues to be demagnified until diffraction effects dominate this process . at this point , the intracavity flux from mirror m 3 can no longer be demagnified and starts to feed the forward or expanding mode of the standing wave cavity ( ref . 19 , 20 ). because the return radiation from mirror m 3 alternates to either side of the m 1 / m 2 optical centerline , the feedback of cavity flux into the reverse cavity standing wave mode by mirror m 3 can serve to homogenize any deleterious or undesirable amplitude or phase feature inadvertently generated by the upper edge of mirror m 1 , the lower edge of mirror m 2 or the front surface of mirror m 3 itself . in summary , mirror m 3 in fig5 serves to ( 1 ) provide one of two scattering edges , symmetrically located on each side of the diffraction coupled output beam , wherein the edges generate feedback for the reverse standing wave cavity mode , ( 2 ) provide a high level of cavity flux to feed the reverse standing wave cavity mode , ( 3 ) provide a systematic coupling between the commonly recognized forward or expanding mode in a standing wave unstable resonator to the less commonly recognized converging wave mode in such a resonator , ( 4 ) provide a means where the free spectral range of the cavity can be decreased , ( 5 ) provide a means to enjoy the benefits of high geometric magnification and low diffractive output coupling , ( 6 ) provide a means whereby the size of the output beam can be varied , and ( 7 ) provide a means whereby the magnitude of the output coupling can be varied from a very low value to the maximum value determined by the ratio of f 2 / f 1 without changing the value of f 1 , f 2 , the spacing between f 1 and f 2 or the alignment of mirrors m 1 , m 2 or m 3 . as stated above , fig6 shows the positive branch confocal hybrid unstable resonator counterpart to that illustrated in fig5 . the confocal condition for this configuration is f 2 − f 1 = l , where l is the spacing between mirror m 2 and mirror m 1 and f 2 and f 1 are the respective focal lengths of the confocal mirror pair m 2 / m 1 . mirror m 3 is depicted in this embodiment as a planar optic spaced a distance l from mirror m 2 . for a geometric magnification m = 5 and a m 2 m 1 spacing of l = 1 m , the confocal requirement is satisfied when f 2 = 1 . 25 m concave and f 1 = 0 . 25 m convex . like its negative branch counterpart , mirror m 3 in fig6 is aligned to return all of a predetermined portion of the cavity flux reflecting from mirror m 2 directly to the confocal mirror pair m 2 / m 1 . on this basis , mirror m 3 operates in completely different manner than element 22 of fig4 (&# 39 ; 687 patent ) since the teaching of the &# 39 ; 687 patent is for element 22 to return a predetermined amount of all of the radiation incident on its front surface . the predetermined portion of cavity flux returned by mirror m 2 to the confocal pair in the instant invention is set by the size of mirror m 3 as measured along the longer side of gain medium transverse cross section . in the embodiment depicted in fig6 , the predetermined portion of cavity flux reflected by the total reflecting front surface of mirror m 3 is such as to allow a diffractive output coupling of about 10 %. as with fig5 , feature 1 of cavity optic mirror m 1 serves to provide the essential feedback necessary to regenerate the forward or expanding wave mode of the mirror m 1 / m 2 portion of a prior art hybrid unstable resonator . this essential prior art feedback is augmented by feature 2 on mirror m 3 and by the reflected radiation from the front surface of mirror m 3 . clearly , for a high geometric magnification , the magnitude of direct reflection from mirror m 3 dwarfs the scattering generated by features 1 and 2 on mirrors m 1 and m 3 . based on the description of the embodiment of the invention shown in fig5 , it should be clear to those skilled in the art of hybrid unstable resonators that both the size of the diffracted output beam in fig6 and the level of diffraction coupling from the three mirror cavity can be varied without changing the alignment or curvatures of mirrors m 1 , m 2 or m 3 or the spacing between any of the three cavity optic as long as mirror m 3 is translated in one dimension such that the front surface of mirror m 3 is maintained so as to exactly reflect the flux coming from mirror m 2 toward mirror m 3 back towards mirror m 2 . the embodiment of the invention shown in fig7 depicts a confocal negative branch hybrid unstable resonator according to the teachings herein with a geometric magnification of about m = 1 . 25 and thus would have a geometric output coupling of c = 1 − 1 / m = 20 %. for a confocal pair separation of length l = 1 m , the confocal condition thus requires that f 1 = 0 . 444 m concave and f 2 = 0 . 555 m concave . in the embodiment of fig7 , the size of mirror m 3 is arranged to permit a diffractive output coupling of about 10 %, which is about half the geometric output coupling of a prior art m = 1 . 25 hybrid branch unstable resonator . it is notable that the m = 5 design of fig5 and the m = 1 . 25 design of fig7 utilize curvatures for mirrors m 1 and m 2 that are not all that different even though the diffraction output coupling for each embodiment can be adjusted according to the teachings herein to be about 10 % in both cases if so desired . if the longer transverse dimension of the gain space cross section in fig5 through 7 is denoted w , then it can be seen by comparing fig5 and 7 , that the location of the output beam can be made to fall at a position of w / 2 by judiciously choosing the size of mirror m 3 and the geometric magnification of the confocal mirror pair m 1 / m 2 according to the teachings of the present invention . furthermore , in view of fig6 , this degree of freedom is available to designs of either negative or positive branch . as stated above , fig8 shows another embodiment of a confocal positive branch hybrid unstable resonator made according to the teachings of the present invention with mirror m 3 sized to yield a diffractive output coupling in the range of 10 %. for a depicted geometric magnification of about m = 1 . 25 and a spacing between mirror m 1 and mirror m 2 of 1 m , a focal length of f 2 = 5 m concave and f 1 = 4 m convex is necessary to satisfy the confocal requirement . for a prior art hybrid unstable resonator , this would yield a diffractive output coupling of 20 %. fig9 shows a negative branch confocal mirror pair m 1 / m 2 employed in conjunction three additional cavity extending optics mirrors m 3 , m 4 and m 5 . in this depiction , f 2 / f 1 = 2 and l , which is the distance between cavity optics mirrors m 1 and m 2 is made such that l = f 2 + f 1 . if l is 1 m , then the confocal requirement is satisfied when f 1 = ⅓ m and f 2 = ⅔ m . in the fig9 embodiment , mirrors m 3 and m 4 serve to lengthen the cavity by essentially two additional cavity confocal lengths before the intracavity flux encounters the output mirror m 5 . in this way , the fig9 embodiment is configured to provide a means to achieve a lower overall cavity free spectral range than could be obtained with a conventional prior art hybrid unstable resonator whose cavity length was only l = f 1 + f 2 . to those who are skilled in the art of diffusion cooled , rf pumped slab or slice co 2 lasers or diode or flashlamp pumped solid state lasers it should be realized that the preferred embodiments herein described are by way of illustration and not limitation . therefore various modifications may be made without departing from the spirit and scope of disclosed invention . 1 ) ref . 1 , u . s . pat . no . 4 , 719 , 639 , “ carbon dioxide slab laser ”, by john tulip , filed 8 jan . 1987 . 2 ) ref . 2 , u . s . pat . no . 5 , 048 , 048 , “ gas laser device ”, by j . nishimae , k . yoshizawa , m . take , filed 9 aug . 1990 . 3 ) ref . 3 , u . s . pat . no . 5 , 123 , 028 , “ rf excited co 2 slab laser ”, by j . l . hobart , j . m . yarborough , j . dallarosa and p . gardner , filed 12 oct . 1990 . 4 ) ref . 4 , a . e . siegman and r . arrathoon , “ modes in unstable optical resonators and lens waveguides ”, ieee j . quantum electronics , vol . qe - 3 , pp . 156 - 163 , april 1967 . 5 ) ref . 5 , u . s . pat . no . 5 , 392 , 309 , “ laser apparatus includes an unstable resonator and a shading means ”, by j . nishimae , k . yoshizawa and k . kumamoto , filed 16 sep . 1993 . 6 ) ref . 6 , u . s . pat . no . 6 , 144 , 687 , “ laser ”, by p . e . jackson , filed 4 sep . 1998 . 7 ) ref . 7 , w . f . krupke and w . r . sooy , “ properties of an unstable confocal resonator co 2 laser system ”, ieee j . quantum electronics , vol . qe - 5 , pp . 575 - 586 december 1969 . 8 ) ref . 8 , r . j . freiberg , p . p . chenuasky and c . j . buczek , “ an experimental study of unstable confocal co 2 resonators ”, ieee j . quantum electronics , vol . qe - 8 , pp . 882 - 892 , december 1972 . 9 ) ref . 9 , u . s . pat . no . 3 , 969 , 685 , “ enhanced radiation coupling from unstable laser resonators ”, by p . p . chenausky and r . j . freiberg , filed 6 dec . 1974 . 10 ) ref . 10 , u . s . pat . no . 4 , 123 , 150 , “ stable resonators for radial flow lasers ”, by e . a . sziklas , filed 21 apr . 1977 . 11 ) ref . 11 , a . gabi , r . hertzberg and s . yatsiv , “ radio - frequency excited stripline co and co 2 lasers ”, paper tukky cleos conference , 1980 . 12 ) ref . 12 , p . e . jackson , h . j . baker and d . r . hall , “ co 2 large - area discharge laser using an unstable - waveguide hybrid resonator ”, appl . phy . lett . vol . 54 , no . 20 , may 1989 , pp . 1950 - 1952 . 13 ) ref . 13 , international patent application number pct / ru2003000220 , “ laser with hybrid - unstable ring resonator ”, v . e . sherstobitov and 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