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Title:
THE GENERATION OF MULTIPLY FOLDED OPTICAL PATHS
Document Type and Number:
WIPO Patent Application WO/1990/004865
Kind Code:
A1
Abstract:
This new technique for stacking multiply folded optical paths in an extended region of a laser resonator within a gaseous medium. The path segments are connected together by a set of mirrors located along the resonator perimeter. Moreover, this invention involves a system of stacking unit tiles, each minim containing a multiply folded path, into larger domains. A multiplicity of mirrors is located in the laser resonator around the perimeter of stacked minims. A longer optical path is achieved without a corresponding increase in the number of folding elements, by stacking the minims together and thereby extending the length of the folding elements. The resulting domains fully cover the plane with a grid of evenly spaced and intersecting optical paths. The laser resonator can be used in either a laser oscillator or amplifier, and in either pulsed or continuous wave mode. In one embodiment of this invention, a waveguide gas laser system has ceramic blocks (49) located within the laser resonator (47), and each block has a multiply folded waveguide channel (57) formed therein. The larger multiply folded optical path is formed by stacking the ceramic blocks (49) together and aligning the waveguide channels. The reflecting mirrors (59) of the laser resonator (47) may be integrally attached to the ceramic block (49) enhancing the durability of the laser resonator and reducing in size.

Inventors:
CANTONI, Armando WOKER, Gary, M.
Application Number:
PCT/US1989/004646
Publication Date:
May 03, 1990
Filing Date:
October 17, 1989
Export Citation:
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Assignee:
CALIFORNIA LABORATORIES INC.
International Classes:
H01S3/03; H01S3/07; H01S3/081; (IPC1-7): H01S3/081
Foreign References:
US3248671A
US3361987A
US4169251A
US4429398A
US4438514A
US4493087A
GB2033648A
US4815094A
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Claims:
CLAIMS
1. A laser resonator, which comprises: an active medium within the resonator capable of supporting laser action; a plurality, of solid blocks disposed within the lase resonator, each block having a smaller, multiply folded waveguide channel formed therein; and a multiplicity of mirrors disposed within the resonator, the mirrors being located around the perimeter of the solid blocks, the mirrors being arranged to produce a larger, multiply folded optical path along the waveguide channels.
2. The laser resonator of Claim 1, wherein the active medium is a gas, a solid, or a liquid.
3. The laser resonator of Claim 1, wherein the waveguide channels are evenly spaced across the blocks.
4. The laser resonator of Claim 1, wherein the waveguide channels intersect each other.
5. The laser resonator of Claim 1, wherein the waveguide channels are substantially located within the same plane.
6. The laser resonator of Claim 1, wherein the mirrors are coated to front reflect optical radiation.
7. A process for forming a laser resonator with a multiply folded optical path, which comprises: providing an active medium within the resonator capable of supporting laser action; providing a plurality of solid blocks, each of the blocks having more than one smaller, multiply folded waveguide channel formed therein; arranging a multiplicity of mirrors around the perimeter of the resonator; and stacking the blocks together such that the mirrors are located around the stacked blocks, and aligning the waveguide channels to form a larger, multiply folded optical path.
8. The process of Claim 7, wherein the waveguide channels are evenly spaced across the solid blocks.
9. The process of Claim 7, wherein the waveguide channel intersect each other.
10. The process of Claim 7, wherein the waveguide channels ar substantially located within the same plane.
11. The process of Claim 7, wherein the mirrors are coated t front reflect optical radiation.
12. A laser resonator, which comprises: an active medium within the laser resonator capable o supporting laser action; a plurality of minims disposed within the laser resonator the minims being stacked and aligned with respect to each other and a multiplicity of mirrors located around the perimeter o the minims within the laser resonator, each of the mirrors bein coated to front reflect optical radiation, the mirrors bein aligned with respect to each other and with respect to th minims to produce a multiply folded optical path within th minims.
13. The laser resonator of Claim 12, wherein the active mediu is a solid, a liquid, or a gas.
14. The laser resonator of Claim 12, wherein the minims eac have a substantially rectangular shape.
15. The laser resonator of Claim 14, wherein the length of th folded optical path (L) is essentially equal to: where the rectangular minim has an "a" dimension along an axis and a "b" dimension along another axis, "i" is the number o minims along the aaxis; and "j' is the number of minims alon the baxis.
16. The laser resonator of Claim 12, wherein the minims eac ha%*e a substantially triangular shape.
17. A process for forming a multiply folded optical path withi a laser resonator, which comprises: providing an active medium capable of supporting las action within the resonator; positioning a multiplicity of mirrors around the perimete of the minims, each mirror being coated to front reflect optica radiation; and stacking and aligning a plurality of minims together to for a multiply folded optical path within the minims.
18. The laser resonator of Claim 17, wherein the minims eac have a substantially rectangular shape.
19. The laser resonator of Claim 18, wherein the length of th folded optical path (L) is essentially equal to: where the rectangular minim has an "a" dimension along an axis and a "b" dimension along another axis, "i" is the number o minims along the aaxis; and "j" is the number of minims alon the baxis.
20. The laser resonator of Claim 17, wherein the minims eac have a substantially triangular shape.
Description:
THE GENERATION OF MULTIPLY FOLDED OPTICAL PATHS CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application to U. Patent Application S.N. 052,977, entitled "Multiply Folded Las Systems" by Armando Cantoni, filed on May 22, 1987.

TECHNICAL FIELD

This invention relates to laser resonators and g laser systems in which the multiply folded optical path lengthened by the stacking of smaller, multiply folded optic paths.

BACKGROUND ART

Gas lasers offer limited power output capabilities p unit length. The upper limit for a sealed, waveguide C laser is about 0.6 watts/cm: therefore, a 30 watt CO2 las has a resonator length of about 50 cm. The resulting syst architectures are mechanically complex, are not rugged o reliable, and have restricted portability. When wavegui lasers are used with a ceramic bore material, long resonato lengths imply considerable manufacturing difficulties because o tight machine tolerances, poor yields, and high productio costs.

Chenausky et al, in U.S. Patents 4,429,398 an 4,438,514, addresses the length problem by using mirrors to fol the optical path, achieving some reduction in overall resonato length at the expense of a modest increase in width. In th '398 patent two parallel waveguides are optically coupled by pair of folding mirrors within a common block. Thi configuration results in a finite amount of diffraction losse that can be reduced but not eliminated by minimizing th distance traveled in free space. The '514 patent shows progressive improvement which uses a third diagonal waveguide that connects, end-to-opposite-end, the two paralle waveguides. Since the waveguides are serially connected, th laser cavity will have a series of parallel waveguides, or Z-folded waveguide arrangement.

De Maria, in U.S. Patent 3,361,987, increases activ gain length in a polysided folded laser path in resonator employing solid or liquid amplifier media, with a continuou folded optical path, using total internal reflection. Nakagom et al in U.K. Application No. 2,033,648A employs a simila configuration for achieving long optical paths in semiconducto light amplifiers. The use of total internal reflection by D Maria and Nakagome requires that the refractive indices of th laser medium and the matching medium be selected to achieve th proper angle of reflection. The matching medium may surroun the laser medium, or may be deposited on the end faces of th solid medium. De Maria suggest the use of a ruby laser elemen immersed in liquid oxygen. The required index matchin signi icantly increases the difficulty of manufacturing an operating the laser. In gas lasers with the index of refractio differing from unity by a few parts per million, total interna reflection cannot be used as an optical path folding technique.

Patent Application Number 052,977, entitled 'MULTIPL FOLDED LASER SYSTEMS' by Armando Cantoni, was filed in the U.S Patent Office on May 22, 1987. This invention discloses a lase resonator having multiply folded optical paths in a region employing a series of front surface reflecting elements. I these novel and unique configurations, a dramatic reduction i the size of the laser resonators is achieved by multiply foldi the optical paths. The Cantoni application is incorporat herein by reference.

Successful implementation of laser-based systems c greatly benefit from a more compact design of the las resonator. In particular, the length of the optical las cavity appears to be the limiting parameter. Virtually a applications that require protability and ruggedness a enhanced by a reduction in overall resonator size.

SUMMARY OF THE INVENTION It is the primary object of this invention to increa the length of the optical path without a corresponding increa in the size of the laser resonator, by fully covering a plane o

optical path segments which are connected by front surfac reflecting elements. Moreover, longer optical paths can b generated simply by stacking smaller elements, which alread contain a number of path segments, into larger elements tha fully cover the enlarged plane with a grid of optical pat segments.

It is another object of this invention to stack smal multiply folded optical paths into a larger multiply folde optical path, thereby producing a more space-efficient desig and simplifying the alignment for the larger optical path. Still another object of this invention is to simplif the design of a waveguide laser resonator, by stacking plurality of solid blocks which contain the waveguide channels.

These objects are met by stacking multiply folde optical paths in an extended region of a laser resonator withi a gaseous medium, whereby path segments are connected an aligned together along the resonator perimeter.

Moreover, this invention involves a system of stackin minims (unit tiles), each minim containing a multiply folde path, into larger domains. A multiplicity of mirrors can b located in the laser resonator around the perimeter of th stacked minims. A long optical path is achieved without corresponding increase in the number of folding elements, b stacking the minims together and extending the length of th folding elements. The resulting domains fully cover the plan with a planar grid of evenly spaced and intersecting optica paths. In a similar manner, smaller modules can be stacked t form larger domains.

The principles of this invention are preferabl incorporated into a waveguide gas laser system having cerami blocks located within the laser resonator, each block havin multiply folded waveguide channels formed therein. The large multiply folded optical path is formed by stacking the cerami blocks together and aligning the waveguide channels. The reflecting mirrors of the laser resonator may be integrall attached to the ceramic block, enhancing .the durability of the

laser and reducing its size. The active resonator can be us for either a laser oscillator or amplifier, and in either puls or continuous wave mode.

The novel features which are believed to characteristic of this invention, which increase the length the optical path by stacking smaller, multiply folded optic paths together, with further objects and advantages thereo will be better understood from the following description connection with the accompanying drawings in which the present preferred embodiments of the invention are illustrated by way example. It is expressly understood, however, that the drawin are for purposes of illustration and description only, and a not intended as a definition of the limits of the inventio Throughout the following description and drawings, identic reference numbers refer to the same component through t multiple drawings of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is closed loop minim in the shape of triangle.

FIG. IB is closed loop minim in the shape of rectangle.

FIG. IC is closed loop minim in the shape of rhombus.

FIG. 2A is a stacked, closed loop triangul configuratio .

FIG. 2B is the stacked, closed loop triangul configuration of Fig. 2A showing three potential, distinc multiply folded optical paths.

FIG. 2C is the embodiment of a resonator obtained utilizing one of the multiply folded optical paths in Fig. 2B.

FIG. 3A is a stacked, rectangular, closed lo configuration.

FIG. 3B is another embodiment of a stacke rectangular, closed loop configuration.

FIG. 4 is a stacked, rectangular, open lo configuration.

FIG. 5A is another embodiment of a stacked rectangular, open loop configuration.

FIG. 5B is a stacked, rectangular, closed loo configuration which yields an "unacceptable" module.

FIG. 5C is a stacked, rectangular, closed loo configuration which yields another "unacceptable" module.

FIG. 5D' is a rectangular open loop module, where th optical path enters the module at one corner and exits th module at the opposite corner.

FIG. 5D is an "acceptable" stacked, rectangular, ope loop configuration obtained by stacking two modules identical t the module depicted in FIG. 5D.

FIG. 5E' is a rectangular open loop module, where th optical path enters the module at one corner and exits th module at an adjacent corner. FIG. 5E is another "acceptable" stacked, rectangular closed loop configuration obtained by stacking two module identical to the module depicted in FIG. 5E.

FIG. 5F' is a rectangular closed loop module.

FIG. 5F is another "acceptable" stacked, rectangular closed loop geometry obtained by stacking two modules identica to the module depicted in FIG. 5F.

FIG. 6 is a three-dimensional, rectangular, open loo pattern.

FIG. 7 is a segmented perspective view of rectangular, two-dimensional laser resonator.

FIG. 8 is a cutaway elevation of the laser resonato structure taken along line 8-8 in FIG. 7.

FIG. 9 is a cutaway elevation of the sealed laser head.

FIG. 10 is a perspective view of the sealed laser hea shown in FIG. 9. DETAILED DESCRIPTION OF THE INVENTION

Many portions of this description are depicted in term of planar geometries for purposes of illustration. However, th teachings of this invention are equally applicable to non-plana configurations .

A plane may be filed in a controlled and uniform mann by using a single polygon form, such as triangles, quadrangle or hexagons. The hexagons and more complex polygon configurations are a combination of triangles or quadrangle and therefore are not discussed herein. Instead, triangles a quadrangles are used as representative elements for purposes illustration, and the principles of this invention are equal applicable to more complex polygonal configurations.

FIG. 1A is a plane surface element of least surfa area (minim) 21 in the shape of an equilateral triangle. Min 0 21 carries optical path 19 through three mirrors 23 with nod 17 located at the juncture of optical path 19 and mirrors 23. the spacing between nodes 17 is "a", the overall path length "3a", minim 21 has a perimeter of "6a", and the length of ea side is "2a".

-, _- The stacking of triangular minims yields triangul modules. The number of minims in any successive row (N) is function of the number of rows (R) , and can be calculated follows:

N = 2R - 1 Hence, the number of minims in the second row is 3, a the number of minims in the third row is 5. The total number

20 minims (NM ) in a triangular configuration is calculated follows:

N M = R 2

Hence, a three-row configuration will have 9 mini

25 (1+3+5). Similarly, the total number of sides (N s ) in t triangular module is:

Ns = 3R

The overall perimeter (P) of a stacked equilater

30 triangular module is:

P = 6aR, where 2a is one side of the minim. The area (A) of a stack

35

equilateral triangular module is equal to: For all triangular configurations except for a singl minim, the number of node reflections (CN ) and the number o node crossings (C ) are fixed:

FIG. 2A shows an equilateral module 27 obtained b stacking nine equilateral minims in three rows. The dashe lines trace the optical path contained in each of the nin minims utilized in the stacking. FIG. 2B depicts the module 27 and three possible close loop, multiply folded optical paths contained therein. Optica path Li fully traces the optical paths contained in the corne minims utilized in the stacking, and only partially traces th optical paths contained in the remaining minims. Optical pat

L2 , while still a viable closed loop path, traces none of th original paths contained in the minims utilized in the stacking but rather follows the sides which are shared by neighborin minims in the stacking. The lengths of optical paths Li an 2 are identical and equal to the perimeter of the modul

(18a). Optical path L3 partially traces the optical paths o some of the minims used in the stacking, and traces none of th path of the corner minims used in the stacking. Its length i equal to half the perimeter of the module (9a). Functionally optical path L3 is equivalent to a path obtained, not b stacking the nine minims to form the module 27, but by enlargin the minim used in the stacking to a size equal to that of th module 27.

FIG. 2C depicts an optical resonator that can b obtained from module 27 by placing three fold mirrors 23 alon the perimeter of the module. Partial reflector 33 and tota reflector 31 are located at the same node, to define a close loop optical path Li , represented by a dashed line. Th dotted path L3 represents the total path length contained i the minims utilized in this stacking which is not traced by th optical path Li and therefore cannot contribute to th

resulting optical pathlength.

While it is possible to increase the optical pat length in a resonator by stacking a number of triangular minims the examples above clearly illustrate the intrinsic limitatio of this approach. For a given stacking (module size), the tota path length in a triangular configuration is fixed and canno exceed the perimeter of the module. As the module size (numbe of minims used in the stacking) increases, the ratio of th total path length to the sum of the path length contained in the stacked minims decreases, resulting in an inefficien utilization of the module area. Finally, the resulting optica path, such as Li in FIG. 2B, may result in non-unifor coverage of the module (since it fully traces the minim path i the three corner minims, and only partially in the remainin minims). This results in non-uniform thermal loading across th module, when the configuration is used as a resonator in activ devices such as lasers or amplifiers.

FIG. IB is a plane minim 25 in the shape of rectangle. Minim 25 carries optical path 19 through fou mirrors 23 again with nodes 17 located at the juncture o optical path 19 and mirrors 23. FIG. IC depicts a typica rhombic minim 16.

With reference to FIG. 3A, module 26 is obtained b stacking six square minims 29 in three columns of two row each. Internal reflections 18 are intersections of optical pat 19 with itself. The resulting grid of multiply folded optica paths can be uniquely defined by four operational parameters:

"i" is the number of nodes along one edge of th module (three are shown along the x-axis i FIG. 3A) ;

"j" is the number of nodes along the orthogonal edg of the module (two are shown along the y-axis i

FIG. 3A);

"a" is the spacing between nodes along the "i" edge and

b" is the node spacing along the "j" edge. The resulting module is determined by specifying "i" and "j" and its size can be scaled by a choice of grid parameters "a and "b." Hence, the configuration in FIG. 3A is designated a M(3,2), where index i = 3, and index j = 2. In a stacked, closed loop configuration such as show in FIG. 3A, total reflector 31 and partial reflector 33 ar located at the same node. In a stacked, closed loo configuration, index "i" and index "j" are each integers (1, 2 3, 4, 5, ). The most effective stacking is a close loop, "quasi-square" configuration. The "quasi-square" is module having indices which differ by plus or minus one, such a M(3,4) or M(5,4), and is preferred from a functional point o view because it maximizes the length of the optical path whil minimizing the surface area. When the optical path is folded i a "quasi-square", a given path length has a minimum number o nodes. When used as a laser resonator, module 26 may have an polygonal shape, but use of rectangular folding patterns wil permit the closest packing of optical path in a given area. I has been found that the best "quasi-square" arrangement for th fold mirrors is 45° reflections (square minims where a = b).

FIG. 3B depicts a preferred embodiment, as one corne has been removed to provide an entrance and an exit for optica path 19. Single optical substrate 35 is inserted in the remove corner, normal to incident segments of optical path 19 Substrate 35 may be "split-coated" to incorporate both tota reflector 31 and partial reflector 33, and installed in a singl gimbal mount for proper alignment tuning to the waveguide grid The use of single optical substrate 35 permits lower cos optics, since only one mount is needed. Alignment is easier since partial reflector 33 and total reflector 31 need not b independently aligned. Waveguide lasers with plano-plano tota and partial reflectors are suggested, since flat reflecto elements are preferred in this approach.

An open loop configuration, wherein partial reflecto

33 and total reflector 31 are located at different nodes, i

depicted in FIG. 4, which is designated as M(1.5,2.5). Here partial reflector 33 is located in one corner, and tota reflector 31 is located in the opposite corner. This preserve an optical axis between the entrance and exit channels, with a o fset defined by channel spacing. When the reflectors ar located at opposite corners, the "i" index is equal to hal integers (1/2, 3/2, 5/2, 7/2 ) and the "j" index i also equal to half integers. This is the preferre configuration for use as a single pass amplifier, since th output beam is parallel to the input beam. When it is necessar to introduce special optical elements for use in the resonato (such as prisms or gratings for wavelength control piezoelectric elements for cavity control, or intracavit electro- or acousto-optical elements, or total or partia reflectors with finite radii of curvature), open loo configurations that allow for independent mounting for suc elements are preferred.

Another type of open loop configuration is shown

FIG. 5A, which is designated as M(2.5,2). In the configurati where the reflectors are located at adjacent corners, one ind is an integer and the other is a half integer.

The set of acceptable closed loop configurations a generated by stacking minims to yield modules defined by choice of integer values for indices "i" and "j". FIG. 5B whi is designated as M(2,4), and FIG. 5C which is designated

M(3,3) are examples of unacceptable configurations. Bo examples illustrate closed loop, multiply folded optical pat in a rectangular and a square module, respectively. So possible optical paths tends to concentrate in certain regio of the module, resulting in non-uniform coverage of the modu area. As used herein, the term "acceptable" refers only those modules obtained by a stacking that results in an optic grid with uniform coverage of the area of the module.

It has been found that any number of minims can stacked to generate modules with "acceptable" multiply fold optical paths; provided that the resulting stack is a modu

having indices that do not share an integer or half integ greater than one as a common denominator. Since 2 and 4 a each divisible by an integer other than 1 (the number being 2 that configuration which is shown in FIG. 5A is unacceptabl Similarly, since 3 and 3 are each divisible by an integer oth than 1 (the number being 3) the resulting pattern which is sho in FIG. 5B is also unacceptable. No square pattern consisti of the same number of minims in the "i" and "j" directions wi yield an acceptable pattern. The fundamental Rectangular Generator Matrix, sho below as Table I, depicts acceptable (X) and unacceptable ( rectangular geometries. The matrix is symmetrical, in th i ,j = Mj ,i . Once a module has been chosen from t Generator Matrix, the indices "i" and "j" have been specifie The parameters of the resulting laser resonator are uniquel defined by the choice of the node spacing "a" and "b", whic specifies the distance between reflection points on the mirrors

TABLE I RECTANGULAR GENERATOR MATRIX i/j 1 1.5 2 2.5 3 3.S 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

X = Acceptable 0 = Not Acceptable

This Matrix can be readily expanded to determine acceptab stacking patterns for larger geometries, by applying the test whether or not the indices share an integer or half-integ common denominator greater than 1.

It is also possible to stack either closed or open lo modules to generate larger closed or open loop modules. T rules that govern this type of stacking are the same that gove the stacking of minims, i.e., the module obtained by stacki smaller modules may not have indices that share comm denominators larger than 1. FIG. 5D' is a rectangular open lo module where the optical path enters at one corner and exits t module at the opposite corner. FIG. 5D depicts the rectangula open loop module obtained by stacking the two smaller modul depicted in FIG. 5D' . In this example:

M( 2.5, 3.5) + M( 2.5, 3.5 ) = M((2)2.5, 3.5) = M(5,3.5 )

FIG. 5E' is a rectangular open loop module where t optical path enters the module at one corner and exits t module at an adjacent corner. FIG. 5F' is a rectangular clos loop module. FIG. 5E depicts a rectangular, closed loop modu obtained by stacking the two smaller modules, depicted in FI 5E', and FIG. 5F shows a rectangular, closed loop modu obtained by stacking two smaller modules depicted in FIG. 5F In the notation used herein:

M( 2.5 , 3 ) + M(2.5 ,3 ) = ((2)2.5, 3) = M(5,3) and

M(2,3) + M(2,3) = M( ( 2 )2, 3 ) = M(4,3)

A particularly useful embodiment of this property the creation of a larger, multiply folded waveguide device stacking ceramic blocks of a given size in a modular approac rather than manufacturing specialized larger ceramic blocks.

The module parameters "a", "b" , "i", and "j" can used to define the following laser parameters in two-dimension grids:

TABLE II OPEN LOOP RECTANGULAR GEOMETRIES

LASER PARAMETER

Laser Dimensions

Total Path Length

Number of Node Reflections

Number of Internal Crossings

TABLE III CLOSED LOOP RECTANGULAR GEOMETRIES

LASER PARAMETER

Laser Dimensions

Total Path Length

Number of Node Reflections

Number of Internal Crossin - l

In closed loop configurations, no provision has bee made in these expressions for a cutaway corner.

A resonator can be designed by selecting values fo these parameters and by solving the above expressions. Fo example, a path length (L) of 50 cm may be selected to minimiz the total number of nodes. This is important in applications to lasers with a medium or low gain, since each node introduces losses due to less than perfect mirror reflectivity. A close loop "quasi-square" is selected where i = 2, and j = 3. If square minims (with a = b) are used, than "a" is calculated to be 2.95 cm for the node spacing, A is calculated to be 52 square cm, X is calculated to be 5.89 cm, and Y is calculated to be 8.84 cm.

Conversely, if the path length in a given surface area is to be maximized for a laser with a medium of high gain, regardless of the number of nodes, a different approach is used. The parameters selected are A = 52 square cm (the same as the previous example), i = 4, j = 5 (again a closed loop, "quasi-square" geometry) , and again square minims are used (a = b). Then "a" is calculated to be 1.61 cm for the node spacing, X is calculated to be 6.45 cm, and Y is calculated to be 8.06 cm, producing a path length (L) of 91 cm.

FIG. 6 is a simplified depiction of how the principles of this invention can be applied to non-planar configurations. A simplified open loop geometry is shown, having four fold mirrors 37, although three mirror would work equally well. Total reflector 31 and output coupler 33 are arranged relative to the optical path 19 and mirrors 37, so as to progressively change the elevation of the path with each successive reflection within the laser cavity.

FIG. 7 and FIG. 8 depict the preferred embodiment of laser resonator 47 within a waveguide gas laser system, which includes polygonal ceramic block 49, intersecting waveguides 57 formed in block 49, reflecting mirrors 59, with total reflecting mirror 63, output coupler 61 "split coated" on single optical

substrate 62.

A gaseous laser gain medium is contained in th waveguides. Any gas mixture capable of laser action is suitabl for use as a gain medium in this multiply folded waveguide ga laser structure. Although a waveguide CO2 laser is preferre as the gain medium, liquid and solid gain mediums may b utilized. Various types of dye lasers and laser diodes ar examples of liquid and solid gain mediums, respectively. Th waveguide CO2 laser, using a mixture of Xe, CO2 , z , an He in typical proportions of 1-10-20-65% is preferred.

When a rectangular module is used, opposing vertica outer surfaces 55 of the ceramic block 49 are parallel Reflecting mirrors 59 are mounted adjacent to and parallel t vertical outer surfaces 55 of block 49. The incident angle o the second fold mirror preferably is complementary to th incident angle on the first fold mirrors, to produce a regula and evenly spaced optical path. In the case of modules obtaine by stacking square minims, waveguides 57 are located at 45 angles with respect to the normal to the surface of reflectiv mirrors 59, with light reflected at a 90° angle upon eac incidence with a reflecting mirror.

Higher power output can be generated by seriall connecting more than one laser resonator 47, while using onl one output coupler 61 and one mirror 63, or by using one lase resonator 47 serially connected to one or more amplifie stages. Such stages may be standard amplifiers, or may make us of the multiply folded design of this invention. Use of t present design for amplification requires the substitution anti-re lection coated windows in place of output coupler 61 a mirror 63.

Ceramic block 49 is composed of a solid ceram material, such as aluminum oxide or beryllium oxide. Block has first horizontal surface 51 substantially parallel to seco horizontal surface 53, and four vertical outer surfaces 5

Block 49 contains a series of intersecting waveguides 57. T

waveguides may be cut into first horizontal surface 51 of th block, or may be drilled in the block (not shown). Rather tha the flat waveguide grooves 57 depicted in FIG. 7 an FIG. 8 waveguides with circular cross-sections may be used which ar drilled into block 49. Waveguides having circula cross-sections make alignment easier and result in a more stabl laser resonator mode structure. However, circular grooves ar more difficult to machine, since the grooves are drilled a acute angles relative to outer surfaces 55. When waveguide with square or rectangular cross-sections are used, a fla ceramic cover plate 45 may be used to provide all-cerami boundaries to the waveguide channels.

Lasers with longer optical path lengths (and therefor more power) can be obtained not only by machining larger blocks but also by stacking smaller blocks, as indicated above. At th points where the waveguides intersect, negligible intracavit diffraction losses are introduced. Waveguide cross-sectio dimensions are preferably in the range of 50 to 250 times th wavelength or radiation to be propagated in the waveguide, an the distance between intersections is at least 2.5 times th waveguide cross-section.

Reflecting mirrors 59 and substrate 62 are adjacent t vertical outer surfaces 55. Reflecting mirrors 59 and substrat 62 may be integrally attached to block 49, as shown in FIG. 8 Alternatively, the reflecting mirrors may be permanently mounte a small distance from the block, leaving a small gap (0.2 to 5. mm) for gas flow (as shown in FIG. 9). A spacer may be inserte between block 49 and substrate 62 to ensure a uniform distanc between these elements.

In general, reflecting mirrors 59 are substantiall parallel to vertical outer surfaces 55. However, other means o reflection, such as diffraction gratings or prisms, may b used. Generally, the gratings or prisms may not be parallel t the vertical outer surfaces, thereby increasing the size of th laser resonator 47. However, the use of the gratings or prism allows the laser output radiation to be tuned.

The purpose of the folding mirrors is to connec optically adjacent waveguide channels with minimum losses Standard statement-of-the-art reflectors at 10.6 microns (CO∑ wavelength include silicon substrates with enhanced an protected silver or gold coatings, with measured reflectivit (with a 45 degree angle of incidence) of 99.8% fo s-polarization (parallel to the plane of incidence to th mirror) and of 99.5% for p-polarization (perpendicular to th plane of incidence to the mirror). In the absence of any othe polarization determining elements, this difference dictate laser operations in a fixed s-polarization with waveguid modules of ten nodes or more. Fixed polarization lasers ar generally preferred to lasers with a random or varying plane o polarization. A typical folding mirror is 5 to 10 cm long, 6 m high, and 6 mm thick.

The multiple reflections increase the difficulty o aligning the laser resonator. Although individual mirror mounted at each node in a series of planes may be used (no shown) , this multiplicity of mirrors causes alignmen difficulties and generally results in increased costs. Th alignment problems are minimized by the use of a minimum numbe of reflecting mirrors 59, by having mirrors 59 and substrate 6 permanently mounted adjacent, or integrally attached, to bloc 49, and by locating more than one node on each mirror 59. Thus the distance between waveguides 57 and these optics i minimized. This resonator design has several advantages First, the optical path outside the waveguide channels i negligible, reducing diffraction losses. Second, the tota non-gain path is minimized. Third, permanently mountin reflecting mirrors 59 and substrate 62 enhances the stabilit ruggedness, and portability of the resonator.

Output coupler 61 is typically obtained from a zi selenide substrate, although other materials can also be use The substrate is antireflection coated on the surface away fro the ceramic waveguide, and coated for the appropria reflectivity on the surface facing the ceramic waveguide. T

reflectivity value depends upon the resonator length, resonato losses, and fill pressure, but is typically in the range of 75 to 95%. Typically, output coupler 61 and mirror 63 are abou the same size: about 6 to 9 mm in diameter, and 3 mm thick. Mirror 63, output coupler 61 and folding mirror 59 ar commercially available from Laser Optics, Inc.

FIG. 9 and FIG. 10 depict a cutaway view and perspective view respectively of sealed laser head 73. Lase resonator 47, including reflecting mirrors 59 and ceramic bloc 49, is situated between first electrode 75 and second electrode 77. The electrodes shown in FIG. 9 are external electrodes, situated above and below first horizontal surface 51 and secon horizontal surface 53 of block 49. Alternatively, th electrodes may be hybrid, with one electrode providing a metal boundary to the waveguide channels and one electrode belo second horizontal surface 53 of block 49, or both electrodes ma be used to provide top and bottom metal boundaries to the waveguide channels. The external electrode configuration is preferred, since it removes metal electrodes from the active discharge region, thereby improving the life of the laser. A discharge between the electrodes excites the gas medium, creating population inversion. The electrodes preferably receive power from a radio frequency generator, although other discharge sources may be used. Laser resonator 47 and first electrode 75 are contained within sealed volume 79 defined by second electrode 77, walls 81, and flange 83. Only laser resonator 47 need be within the sealed volume. The gas mixture is contained in sealed volume 79 and can flow into and out of waveguides 57 through gaps between reflecting mirrors 59 and ceramic block 49 (as shown in FIG. 9). Also, the gas mixture can be fully sealed within laser resonator 47, without the use of a larger sealed volume.

Insulated RF power feedthrough 90 is connected to first electrode 75. Sealed volume 79 is closed by flange 83, gasket 85, and sealing cap 87 which may be tightened against flange 83

with sealing bolts 89. Sealing bolts 89 permit access to lase resonator 47 for replacement or adjustment.

Walls 81, electrodes 75 and 77, flange 83, and sealin cap 87 are preferably made of stainless steel. Gasket 85 i made of malleable material, such as copper. FIG. 10 shows cylindrical laser head 73, which uses stainless steel tubing fo walls 81. Second electrode 77 seals volume 79 at one end, an flange 83, gasket 85, and sealing cap 87 close volume 79 at th opposite end of the cylinder.

Optical output port 91 and gas fill port 93 ar attached to walls 81. Optical output port 91 contains a anti-reflection coated window and is located with respect t output coupler 63 so as to allow power extraction from the lase resonator. Alternatively, block 49 may be mounted within seale laser head 73 so that output coupler 63 is situated in optica output port 91. Gas fill port 93 may be of the pinch off type or may be of a stopcock type for repetitive use where frequen exchanges of the gaseous laser gain medium are necessary.

Power transferred from the RF discharge of th electrodes to ceramic block 49 causes heating of the block This heat can cause instability and lower power output of th laser. The use of ceramic material with a high heat transfe capability, such as BeO, enhances the ability to dissipate hea from block 49.

The ceramic block 49 fulfills the dual role o insulating spacer between top RF electrode 75 and bottom R groundplane 77, and of cooling interface between the hot lase plasma and the bottom cooling plate. Beryllia is the preferr ceramic because of its high thermal conductivity, even though i is toxic in powder form. Beryllia is readily available and c be easily fabricated into waveguide grids. High purity alumi (98.5% or higher) may be used in lower power applications reduced thermal loads.

A prototype laser resonator has been built using 98.5% beryllia ceramic block with intersecting waveguides, a machined into a M(3.5,4) open loop structure. The structure h

13 nodes (R) , 21 crossings (C), "a" and "b" are each equal t 0.75 inches (square minims), and the path length (L) is 29. inches. Three of the fold mirrors are 1.75 inches long, 0.2 inches high, and 0.25 inches thick. The fourth mirror is 2.7 inches long, with the same height and thickness. All fou mirrors are precisely located around the perimeter of the bloc at a spacing 0.010 inches form the vertical outer surfaces o the block. Mirror reflectivity is measured at 99.7% pe reflection for s-polarization. A beryllia plate is used as top cover. The ceramic block and the fold mirror assembly res on an aluminum plate serving as a ground electrode and coolin plate, and carrying a serpentine cooling water channel. Th laser cavity is filled with a mixture of CO2 , N2. and H (1:1:3.8) with 5% added Xe. At a fill pressure of 70 Torr, wit output coupler reflectivity of 80% and RF power input of 30 watts, a continuous power output of 22 watts is achieved.

Although only geometries involving three and fou reflecting mirrors have been discussed herein, it is clear tha the principles of this invention can be applied by one skille in the art to more sophisticated structures, such as non-plana configurations or geometries with more than four mirrors Similarly, although much of the disclosure has involve waveguide lasers, the principles of this invention ar applicable to non-waveguide laser systems, such as free spac mode lasers .

Accordingly, there has been provided stackin configurations and methods for generating multiply folde optical paths, in accordance with the invention that satisfie all of the objectives set forth above. It is understood tha all terms used herein are descriptive rather than limiting. While the invention has been described in conjunction wit specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skille in the art in light of the disclosure herein. Accordingly, i is intended to include all such alternatives, modifications,

variations that fall within the spirit and scope of the appende claims.