SLAB LASER RESONATORS

Abstract: A slab laser resonator which discriminates in favour of a single mode operation of a waveguide slab laser and which may be applied to slab waveguide lasers of the gas discharge, solid state material, semiconductor material or liquid dye types. The slab laser resonator includes a periodic, one-dimensional spatial loss modulator (4) positioned between the waveguide slab (1) and one or both of the cavity mirror reflectors (2, 3), or alternatively along the slab face, such that the spatial loss modulator is periodic across the width of the slab. The periodicity of the spatial loss modulator (4) is determined by the slab optical thickness, which also effects the choice of the optical length of the slab, which is determined by the requirement that the spatial loss modulation be coherently self-imaged one or more times per round trip of the laser cavity. The major distinguishing feature of this approach is that, because of the low loss propagation within the waveguide slab (1), overall cavity losses in both the lateral and transverse directions are also very low. Resonators of this design produce a double lobed, antisymmetric radiation pattern, which may, in principle, be converted to a single lobed pattern, for example by means of a binary optical phase shifter.

Description: DESCRIPTION SLAB LASER RESONATORS The present invention relates to lasers and particularly to lasers such as gas liquid or solid state lasers employing rectangular cross section, slab- shaped gain media, wherein the mechanical structure which confines or constitutes the gain media also acts as an optical waveguide. It is well known in. the laser field that certain types of laser may be configured to have a gain medium with one or other forms of large area geometry. In one embodiment of the large area laser concept, the axial dimension is some multiple of the width, which in turn is substantially larger than the thickness, so forming a rectangular, slab configuration. One example of such an arrangement is the carbon dioxide slab laser as exemplified by the devices described in U.S. Patent No. 4719639 by Tulip and U.S. Patent No. 4930138 by Opower, where the slab dimensions are such as to allow optical waveguiding between large planar metal or dieletric surfaces which define the boundary of the laser gain medium. Slab-shaped gain media are also possible with fluid dye lasers, as described in U.S. Patent No. 4423511 by Morton, and with solid state lasers, such as neodymium YAG described in references such as Koechner "Solid State Laser Engineering" Section 7.3, pages 388-397, Springer-Verlag (1988); Chun et al "Resonant- mode analysis of single-mode face pumped lasers," Applied Optics 16, 1067-1069 (April 1977); and Jones et al IEEE Journal of Quantum Electronics QE-7, 534 (1971). One major benefit of such large area gain media is that they facilitate the scaling of laser power in two dimensions, in proportion to the area rather than the length of the gain medium, which latter concept constitutes the conventional scaling law for many lasers. In the case of gas discharge lasers, the magnitude and proximity of the large area planar optical guiding surfaces, which may also constitute the metallic electrodes, are highly beneficial in allowing the effective cooling of the gain medium. For example, in the case of gas lasers, such as carbon dioxide, such cooling can be efficiently provided by diffusion of heat to the closely-adjacent walls, so that gas flow is unnecessary even for high density electrical power deposition. It is well known that the successful use of high power lasers for .applications such as material processing and laser radar is strongly dependent upon the achievement of high optical quality in the beam generated by the laser, where the term "high optical beam quality" is generally well-understood by those skilled in the laser art. Thus, the achievement of high quality output beams is a fundamental requirement of large area or slab lasers, if such lasers are to find widespread use in such applications. In the case of lasers with slab-shaped gain media, where the physical separation of the planar surfaces defining the optical waveguide is selected appropriately, the component of the optical field in the direction at right angles to the large planar face of the slab (the transverse direction) propagates in a low order mode of the waveguide. Thus, for example, surface separations in the approximate range of 0.5 - 3.0 mm are suitable for low order waveguide propagation of radiation at the 10 m wavelength produced by carbon dioxide lasers. However, it is also essential to achieve adequate control over the characteristics of the intra-cavity optical field in the plane parallel to the planar slab surfaces (the lateral direction) , if laser output beams exhibiting the required propagation and focusing characteristics are to be obtained from such slab lasers. In prior art, the use of a hybrid of waveguide and one-dimensional unstable resonators has been proposed. In U.S. Patent No. 4423511, a laser resonator is claimed, wherein the resonator optical elements of the design provide the necessary optical surfaces to permit optical waveguiding in one plane, while at the same time comprising an unstable resonator in the orthogonal plane, parallel to the waveguiding system. A subsequent disclosure, U.S. Patent No. 4719639 teaches an embodiment of this basic design with claimed particular applicability to the diffusion-cooled carbon dioxide slab laser. This latter resonator arrangement is modified in so far as, although it constitutes a hybrid waveguide-unstable resonator, it is of an asymmetric rather than symmetric design. Such a resonator may employ a pair of high reflectivity mirrors in the confocal configuration, comprising either both of concave curvature or alternatively, one of convex and the other of concave curvature. Since in this design, the laser radiation is extracted from the optical resonator around the edge of one of the mirrors, by appropriate mirror alignment, the design is so proposed is intrinsically asymmetric. Similar one- dimensional unstable resonator designs have been used with excited lasers (0 L Bourne and P E Dyer Opt. Commun. 31, 193 (1979)), and with solid state lasers (K Kuba, T Yamamoto, S Yagi Paper JWA3 (p298) CLEO/IQEC Conference, Anaheim (1990)). In a subsequent disclosure in U.S. Patent No. 4930138, it is claimed that the hybrid waveguide - unstable resonator of the edge-coupled design, as described above and in U.S. Patent No. 4719639, may only be applied to lasers of limited length and width because of its essential asymmetric character. U.S. Patent No. 4930138 describes a Waveguide Laser System whose objective is to allow the successful operation of as large a surface area of gain medium as possible without significant loss in the optical quality of the beam extracted from the laser system. In U.S. Patent No. 4930138 it is taught that this objective may be achieved by the use of an oscillator-amplifier configuration employing one-dimensional waveguiding in both the oscillator and the amplifier segments of the system. Optical configurations are proposed which allow the waveguide oscillator and the waveguide amplifier sections to be constructed in series within a monolithic slab gain medium structure. It teaches how the single sided (asymmetric) output of the prior art (U.S. Patent No. 4719639) may be replaced with a double-sided (symmetric) configuration which also employs an amplifying and beam combining section. Alternatively, it proposes the use of a single-sided output which couples into a width-extended segment of the waveguide planar medium, where multiple passes are obtained between a convex and concave mirror pair. In the quest to scale waveguide slab lasers to high powers, it is necessary to increase the area of the slab gain medium. For example, in the case of slab carbon dioxide lasers, it is required to increase the discharge area according to the scaling laws described in references such as Appl. Phys. Lett. 54, 1833, (1989) by K.M. Abra ski, A.D. Colley, H.J. Baker, D.R. Hall. In practice, it is very desirable that such large area slab gain medium devices should achieve a high degree of spatial uniformity over the entire plan of the gain medium. Such spatial uniformity is necessary in terms of the gain medium excitation, (electrical or optical pump energy input) as well as the gain medium cooling characteristics, whether this be by diffusion, conduction or convection. At the higher powers obtainable from large slab lasers, it is also highly desirable that spatial uniformity be achieved in terms of the power density of the optical radiation over the whole area of the planar waveguide gain medium system. Failure to achieve the required degree of spatial uniformity of optical flux may result in undesirable gradients in the saturation and refractive index of the medium. In the case of gas discharge gain media, spatial non-uniformities may also result in undesirable localised gas temperature variations with consequent localised electrical impedance changes, which in turn may have serious consequences for the spatial uniformity of the discharge itself, and therefore of the laser gain medium. This highly desirable feature of a high degree of spatial uniformity in the power transformation characteristics becomes more difficult to achieve as the slab area increases, even for resonator designs which are spatially symmetric (for example, about the centre-line optical axis of the laser) , but which nevertheless may depart significantly from the desired level of uniformity over the entire slab medium area. There are a number of references in the scientific literature to the use of the self-imaging condition for periodic fields, often referred to as the Talbot effect. It is well-known that a coherent optical field incident upon a periodic transverse (grating-like) structure will reproduce the identical transverse periodicity in the field at planes transverse to the axis of propagation which are located at integer multiples of the distance, Z, from the periodic structure, such that Z τ given by: Z χ = 2 nd 2 X where Z τ is referred to as the Talbot distance, d is the spacing of the individual apertures in the periodic transmission structure, A is the wavelength, and n is the refractive index of the medium. This so-called self-imaging condition has, in prior art, been used as the basis of a technique for producing phase-locked or coherent operation of an array of multiple individual adjacent parallel laser oscillators, spatially arranged in a one-dimensional or two-dimensional geometric configuration. In such cases, phase-locking or the array elements has been achieved by arranging that light from a given element of the array of laser gain media couple into adjacent elements by diffraction, within a common extended laser cavity. By selecting the round trip from the output plane of the array elements to the common laser mirror, which constitutes the diffractive coupling zone, to be a half-Talbot distance or multiple thereof, the periodic field in the output plane of the array is re-imaged back upon itself after the round trip to the laser mirror. Thus, it is possible to increase the effectiveness of diffractive coupling between the array elements because of the self-imaging properties associated with Talbot distances. In prior art, the application of diffractive coupling using the Talbot effect has been demonstrated as the basis of a technique for producing phase- locking or arrays of diode lasers (see, for example, the paper "Coherent Addition of AIGaAS lasers using microlenses and diffractive coupling" by J. Leger, M.L. Scott and W.B. Veldkamp published in Applied Physics Letters 52, 1771 (1988)). The same basic technique may also be applied to arrays of carbon dioxide gas lasers, for example the paper "Effective phase-locking of an array of lasers" by V.V. Antyukhov et al, published in JETP Letters 44, 78 (1986). It is a first object of the invention to improve the performance characteristics of lasers which incorporate large area waveguide slab gain media by providing an optical resonator system which exhibits a high degree of spatial uniformity, not only in terms of power deposition and heat removal, but also with regard to the optical power density over the whole slab gain medium area, while simultaneously providing an output beam of high optical quality. The invention provides this improve uniformity in the operating characteristic over large lengths and widths of the gain medium, while providing a high quality coherent output beam from the laser. The first object of the invention is achieved in a planar waveguide configuration of the generic type characteristic of a number of classes of laser, such as for example the carbon dioxide slab laser described above, or solid state lasers exemplified by the neodymium: YAG slab laser. As will be described later the present invention exploits the principles of coherent imaging, applying them particularly to wave¬ guide slab lasers in order to produce predominantly single mode output from the resonator. A first aspect of the invention depends upon a combination of two essential features, namely; first, the mode propagation characteristics of electromagnetic fields in a planar rectangular slab waveguide bounded on all faces by low loss, reflective surfaces; secondly, the self-imaging characteristics exhibited by periodic grating-like structures when illuminated by coherent light. According to a first aspect of the present invention there is provided a slab laser resonator comprising in combination: i) a rectangular slab-shaped waveguide laser gain medium comprising smooth side and top walls which have surfaces which are highly reflecting at glancing incidence for the laser wavelength, and capable of providing efficient, low loss optical waveguiding by means of Fresnel reflection or by total internal reflection; ii) two or more laser reonator mirrors which comprise, or are positioned parallel to and closely adjacent with, the end faces of the said slab wave¬ guide, and one or all of which mirrors is partially transmitting to form a low-loss laser resonator with efficient optical power extraction; iii) a spatial loss modulator which provides a one dimensional periodic optical loss, and/or phase modulation, for laser radiation propagating generally parallel to the main optical axis of the slab laser resonator, which spatial loss modulator comprises a periodic grating-like structure positioned adjacent to, or incorporated within, one or more of the resonator mirrors, and/or along the length of the slab waveguide surfaces parallel to the said main optical axis of the slab laser resonator, such that the loss modulation itself is caused to be distributed in the direction parallel to and extending across the full extent of the width of the slab waveguide; and wherein the dimensions of the said slab wave¬ guide, the slab laser resonator length and periodicity of the said spatial loss modulator are selected so as to match each other and the requirements of coherent imaging, within slab waveguide whose width is selected to be an appropriate multiple of the said period of the spatial loss modulator, and where the length of the slab laser resonator is chosen to conform to the required distance necessary to achieve efficient coherent imaging of the periodic loss modulation at the required wavelength. The slab laser resonator according to the first aspect of the invention has particular applicability to the case where waveguiding, as opposed to free space gaussian propagation, occurs both in the transverse and lateral directions of the slab wave-guide. The output beam from a simple waveguide slab laser consists of a superposition of many different waveguide modes, whose amplitudes and frequencies vary with mirror spacing and alignment, resulting in outputs which exhibit poor spatial and temporal coherence. In this application, two related methods for stabilisation of the internal mode content of the slab laser resonator are described, such that the output beam is predominantly a single, high brightness spatial mode, which can be converted if necessary, external to the laser cavity by passive optical means, to a desirable beam format in terms of its far-field intensity distribution. Such optical means are, for example, phase correction by binary phase shifters (of the generic type described by J.R. Leger, G.J. Swanson and V.B. Veldkamp in Applied Optics, 26, 4391 (1987) or beam re-formatting by segmented mirror) . The first method of mode stabilisation of the waveguide slab laser resonator uses a spatially modulated end reflector and the principle of coherent imaging. One or both end mirrors of the slab laser cavity is divided into q zones across the lateral direction of the slab waveguide, for example with each zone being of width d = w/qm, where w is the width of the slab. In some cases the width of particular zones may be adjusted to compensate for optical non-uniformities in the gain medium. This division may be achieved by a number of possible methods, as specified in the invention claims listed below. In the following explanation of the invention, reference will be made to a grid of wires placed immediately in front of one of the resonator mirrors, as a representation of the generic method, but where it is understood that a similar explanation applies to other particular embodiments of the invention. It is well known that a laser oscillator will self-adjust its intra-cavity field to produce maximum output in the presence of localised optical losses, such as those induced by the spatially modulated mirror. In the case of a slab waveguide, this will require the adoption of a predominant spatial mode whose periodicity matches that of the modulated mirror, i.e. the EH 1 waveguide mode. Other modes are intercepted by the wires of the grid assembly, and are suppressed because of the increased cavity losses. However, to discriminate strongly against unwanted modes, the necessary wire thickness also acts as a finite loss for the required mode, so reducing the laser output power and efficiency. In this invention, the loss of power caused by the spatially modulated end reflector is compensated by the appropriate selection of the dimensions of the waveguide slab resonator so as to produce the condition of coherent imaging. The coherent imaging condition for a waveguide slab requires that the cavity length, L should obey the relation: τ L = m n d 2 λ where m is a positive integer, d is the periodic zone width, defined earlier, and Λ is the laser wavelength, and n is the refractive index of the gain medium. This length corresponds to the round trip distance in the cavity being equal to a multiple of half of the characteristic Talbot distance, as defined in equation At the coherent imaging length, all waveguide modes which are odd harmonics of the fundamental mode are of equal frequency, and can operate as a phase- locked superposition. For the case of a grid of wires, the in-cavity field amplitude adjusts itself to avoid coupling power into the wires. The use of coherent imaging thus facilitates the selection of predominantly single mode operation without incurring excessively high cavity losses. The techniques of mode control presented so far can be extended to two dimensions, where the thickness of the slab is considerably larger than the size necessary to ensure a single waveguide mode in the transverse direction. The spatial loss modulation pattern now divides the end mirror into square or rectangular zones, for example by a grid of crossed wires. The cavity length condition, equation (2), must be simultaneously satisfied for the zone dimensions in both transverse and lateral directions, and the predominant mode now selected is now the form EH where p is an integer greater than unity. The second method, for stabilisation of a slab waveguide laser resonator to select a predominantly single high order waveguide mode, uses spatial modulation, of the widest of the radiation-confining walls of the waveguide. The periodicity of the modulation is given by d •= w/q, where w is the width of the waveguide slab and where the modulator takes the form of grooves or raised sections which are much narrower than d. The modulation may take the form of wider alternating grooves or raised sections. In both cases, the periodic modulation may be applied to either one of the side walls, or to both opposing faces, with accurate registration and alignment of the patterns on the respective faces. Modulation of the widest of the radiation-confining walls may beneficially be restricted to only short sections of the total length of the slab resonator, in zones spaced by a distance equal to half the Talbot length, Z τ or integer multiples thereof. Moreover, the cavity length should be some multiple of the Talbot half length, and the zones correctly positioned for coherent imaging. Such a registration allows spatial harmonics of the fundamental EH 1 mode to form a coherent superposition as for the case of the spatially-modulated end mirror. This coherent superposition, predominantly EH has reduced cavity loss and results in increased output power from the slab laser. It is a further object of the present invention to provide a laser system in which a waveguide slab structure is used to provide a phase locked operation of an array of laser oscillators. According to a second aspect of the present invention there is provided a laser system for providing phase-locked operation of an array of laser oscillators, comprising a planar array of waveguide gain elements each having lasers - such as for a waveguide gain media and one or more resonator mirrors. a slab waveguide, one end of which is positioned immediately adjacent to the output plane of the planar array of waveguide gain elements. the said slab waveguide having reflecting side walls to provide a region wherein diffractive coupling of the radiation emission from the array elements may occur under conditions of very low opticalloss in both the transverse and lateral directions, and which by virtue of choosing the dimensions of the said slab waveguide to match the periodicity of the array elements and the length requirements of the Talbot condition, produces very efficient low loss coherent imaging may be produced, as a result of which phase- locked operation of the laser array may be produced. In contrast to the prior art, which used coherent imaging in free-space to produce phase-locked operation of laser arrays, the laser system according to the second aspect of the present invention particularly concerns the use of techniques related to the Talbot effect to coherently couple the modes of the planar, rectangular geometry (slab) waveguide structure, or to achieve phase-locked operation of any array by virtue of coherent imaging in a waveguide slab structure. For a suitable selected geometry, the radiation pattern emanating from a slab waveguide gain medium with plane end reflectors is symmetric about two planes. In the transverse dimension, it is well known that the beam consists of a fundamental or low order waveguide mode. However, in the lateral direction, the beam may usually consist of a combination of high order waveguide modes. Some technique is therefore required to control the propagating waveguide mode in the lateral direction. Embodiments of the present invention will be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 (a) shows a slab laser resonator according to the first aspect of the present invention. Fig. 1 (b) shows the electric field amplitude near the end mirror where the wire grid is located. Figs. 2 (a) and (b) are schematic diagrams illustrating the two general methods of producing mode stabilisation, according to the first aspect of the invention. Figs. 3 (a) and (b) show two examples of the second general mode stabilisation method according to the first aspect of the invention. Figs. 4 (a) and (b) show the general geometry of the invention, with laser mirrors adjacent to or co-incident with the end faces of the slab gain medium. Figs, (a) to (c) show various embodiments of the first aspect of the invention as applied to electric discharge slab lasers. Fig. 6 shows an embodiment of the spatial loss modulation where the periodic loss extends over only part of the lateral width of the slab gain medium. Figs. 7 (a) and (b) depicts an embodiment of the first aspect of the invention where the spatial loss modulation consists of lines, grooves or raised portions etched or marked on one or both of the laser mirrors. Fig. 8 depicts an embodiment of the spatial loss modulator which consists or alternating coating zones of different reflectives. Figs. 9 (a) to (c) depict an embodiment of the spatial loss modulator which consists of a hole in one of the resonator mirrors. Figs. 19 (a) and (b) show an embodiment of the first aspect of the invention involving lines, grooves or raised portions suitably positioned on the large flat surfaces of the waveguide slab gain medium. Fig. 11 shows a waveguide slab medium consisting of a solid state material, to which the principle of the first aspect of the invention may be applied, as a monolithic structure by means of a suitable loss modulator. Fig. 12 shows a method of two-dimensional mode stabilisation according to the first aspect of the invention. Fig. 13 (a) and (b) shows two optical configurations involving multiple waveguide slab structures in series or parallel. Figs. 14 (a) and (b) shows two configurations where multiple waveguide slabs may be arranged and mode stabilised in parallel. Fig. 15 shows an embodiment of the second aspect of the invention in which the principle of coherent imaging in a waveguide slab structure is applied to the phase-locking of a linear array of laser oscillators. Fig. 16 shows an embodiment of the second aspect of the invention in which the principle of coherent imaging in a waveguide slab structure is applied to the phase-locking of a two-dimensional array of laser oscillators. The geometry of a slab laser resonator according to the first aspect of the present invention is illustrated in Fig. 4. A rectangular slab shaped waveguide laser gain medium 1, (hereinafter referred to as a slab waveguide) with dimensions and surfaces suitable for optical waveguiding is positioned between laser mirrors 2, 3. The mirrors are positioned very close to, as in Fig. 4 (a), or coincident with, as in Fig. 4 (b) , the ends of the slab waveguide. It is necessary that the side surfaces 9, 10 as well as the top surfaces 11, 12, of the slab waveguide be highly reflecting for glancing incidence beams at the laser wavelength. As shown in Fig. 4 (a) a grid of thin, parallel wires, 4, made from a metal (such as gold for example), or other suitable material, is positioned in the small space separating one end of the slab wave¬ guide from the adjacent mirror, 2. An alternative arrangement offering increased mechanical strength is an array of thin parallel plates or foils, oriented parallel to the cavity axis and positioned within the laser cavity in a manner similar to the grid of wires. The wires or foils are arranged to be equidistant from the mirror and orthogonal to the plane of the slab waveguide, as shown in Figs. 1 and 4. The laser mirrors have front surfaces 13, 14, which are coated to produce suitable reflectivity at the desired wavelength for efficient laser operation. The periodicity of the grid of wires must be chosen to be consistent with the dimensions of the slab waveguide, the laser wavelength, and the mirror separation, while the wire thickness must take account of resonator losses. The width of the slab waveguide may usually be chosen to be some convenient multiple of the slab waveguide thickness to facilitate the excitation of the EH 1 mode (q) is an integer) . The slab waveguide thickness itself is chosen to ensure fundamental waveguide mode operation in the transverse direction. These dimensions and the value of q also determine the periodicity and maximum required number of the wires in the parallel grid. Coherent imaging of this spatial pattern, which defines the allowed mode of the planar structure after one or more round trips of the cavity, can then be assured by selection of the round trip length of the laser resonator to be the corresponding half Talbot distance, or integer multiple thereof. At the coherent imaging length, all waveguide modes which are odd harmonics of the fundamental mode are of equal frequency, and can operate as a phase- locked superposition. For the case of a grid of wires 4, as shown in Fig. 1 (a), placed in front of a cavity mirror 2, in the arrangement illustrated in Fig. 2 (a), the in-cavity field amplitude adjusts itself to avoid coupling power into the wires. This is illustrated schematically in the diagram in Fig. 1 (b) , which shows zero field regions in the positions corresponding to the wire locations. The use of coherent imaging thus facilitates the selection of predominantly single mode operation without incurring excessively high cavity losses. In one embodiment of the first aspect of the present invention, shown schematically in Figs 5 (a) to (d) , the slab waveguide consists of an electric discharge excited gas mixture, such as that suitable for a carbon dioxide laser, for example one which is excited by a transverse radiofrequency discharge. The discharge may be maintained between a pair of fluid cooled metal electrodes, 15, 16. The upper surface 17 of the lower electrode 16 and the lower surface 18 of the supper electrode 15, are designed to be highly reflective at glancing incidence at the laser wavelength, and serve as optical waveguiding surfaces. The side boundaries 19, 20 of the slab waveguide 1, which are parallel with the optical axis BB', 21, at the sides of the slab waveguide (along direction AA' , 22), consist of ceramic spacers, which are ground or polished to produce high reflectivity at glancing incidence on their internal surfaces 23, 24. The separation of the electrodes 15, 16 in the case of the carbon dioxide slab laser may be in the approximate range of 0.5 - 3.0 mm. The volume 1 between the electrodes 15, 16 contains a suitable mixture of helium, nitrogen, carbon dioxide and other suitable gas additives for the efficient operation of a carbon dioxide laser, excited, for example, by a transverse radiofrequency electric field, dimension of the width and length of the slab waveguide and the resonator mirror separation are selected to obtain the desired power output from the laser and so as to match the waveguiding and Talbot imaging conditions. The output beam from the laser may be taken through either or both mirrors 26, 27. Another embodiment of the first aspect of the present invention, depicted in Fig. 5 (e) consists of a slab waveguide similar to that shown in Figs. 5 (a)- (d) , but in which the cooled metal RF electrodes 15, 16 are separated from the gas discharge volume 1, by dielectric strips 28, 29 instead of ceramic spacers. Yet another embodiment of the first aspect of the present invention, illustrated in Fig. 6 involves the use of a grid of wires, 30, with size and separation chosen as before, but which extends over only the central region of the slab waveguide. This reduced number of periodic apertures provides sufficient effective mode filtering because of the highly reflecting, kaleidoscope-like surfaces bounding the slab waveguide. Yet another embodiment of the first aspect of the present invention, illustrated n Fig. 7 (a) and (b) , requires a modification to one (or both) of the resonator mirrors as a replacement for the grid of fine wires, 4. In this embodiment a grid of fine lines 31, is deposited or etched on to the surface of the mirror. For a carbon dioxide laser, such etched lines might be approximately 50 m thick. The mirror substrate is then coated as usual with a multi-layer coating to produce the desired reflectivity. The periodicity of the line grating structure is chosen to match the dimensions of the particular slab waveguide. The effect of such a periodic grating on the mirror(s) is to control the lateral mode structure in a similar manner to the separate wire grid assembly described previously hereinabove. Yet another embodiment of the first aspect of the present invention uses an output coupler mirror 2, 3 (as shown in Fig. 2), whose front surface reflectivity varies periodically across the mirror, with different values in the alternating zones, 32, 33, as illustrated in Fig. 8, in a manner designed to simulate the periodicity of the relevant mode of the slab waveguide, as specified by observing the conditions necessary for coherent imaging of the periodic pattern. This embodiment of the invention differs from prior art, for example U.S. Patent No. 4884282 by W.B. Bridges, in that the periodicity of the varying reflectivity seg¬ ment is determined by the requirement to be matched to the waveguide slab and Talbot length dimensions, rather than being matched to the periodicity of an array of laser gain media. Yet another embodiment of the first aspect of the present invention, illustrated in Fig. 9 (a) , employs a hole 34, or alternatively multiple holes in one of the resonator mirrors, whose size, shape and location are designed to simulate one or more periods of the periodic spatial loss modulator. The output from the laser may be taken through this hole, or holes, which may be positioned centrally with respect to the slab waveguide (as shown in Fig. 9. ) , at either extreme edge of the mirror, or at positions which are some integral number of periods of the hole size from either end of the mirror. In a variation of this embodiment, instead of a physical hole, or holes, the output of the laser may be taken through one or more regions of the mirror of width equal to d (35 in Fig. 9 (b) ) where the reflectivity of the mirror is reduced as appropriate. For cases where the magnitude of the output coupling through the hole, or holes is in excess of the optimum value for the laser cavity, or for other reasons, the output coupling may be reduced by the use of an additional, partially reflecting mirror 36, shown in Fig. 9 (c) . The second method, according to the invention, for stabilisation of a waveguide slab laser to select a predominantly single high order waveguide mode, uses spatial modulation 5, of the widest of the radiation- confining walls of the waveguide, as illustrated schematically in Fig. 2 (b) . Two of the possible examples of this approach are illustrated schematically in Fig. 3 (a) and (b) . The periodicity of the modulation is given by d = w/q where w is the width of the waveguide slab and where, as in Fig. 3 (a) the modulation takes the form of grooves or raised sections 6, which are much narrower than d. In the implementation illustrated in Fig. 3 (b) , the modulation takes the form of wider alternating grooves or raised sections 7, 8. In both cases, the periodic modulation may be applied to either just one of the side walls, or to both opposing faces, with accurate registration and alignment of the patterns on the respective faces. Modulation 5, of the widest of the radiation-confining walls may beneficially be restricted to only short sections of the total length of the slab resonator, in zones spaced by a distance equal to half the Talbot length, Z„ or integer multiples thereof. Moreover, the cavity length should be some multiple of the Talbot half length, and the zones correctly positioned for coherent imaging. Such a registration allows spatial harmonics of the fundamental EH 1 mode to form a coherent superposition as for the case of the spatially-modulated end mirror. This coherent superposition, predominantly EH 1 has reduced cavity loss and results in increased output power from the slab laser. Yet another embodiment of the first aspect of the present invention illustrated in Fig. 10 (a) , involves the use of suitable lines, grooves or raised portions 6, positioned on the large flat surfaces 11, 12 of the slab waveguide. Mirrors 2, 3, are chosen as usual to optimise laser power efficiency. Modulation 6, of the widest of the radiation-confining walls may beneficially be restricted to only short sections of the total length of the slab resonator, as shown in Fig. 10 (b) in zones spaced by a distance equal to half the Talbot length, „ or integer multiples thereof. This embodiment may be realised, for example, in the case of a carbon dioxide slab laser, by suitably machining the slab waveguide surfaces, as illustrated in detail in Figs. 3 (a) and (b) . Alternatively, for example in the case of a solid state laser, the periodic markings may be etched or otherwise transferred to the planar surfaces. As shown in Figs. 3 (a) and (b) the slab wave¬ guide surfaces 7, 8 may exhibit either wide alternating grooves or wide alternating raised sections whose dimensions are chosen such that: d 1 + d„ = 2 /q where w is the slab width and q is an integer. In this modulation format, the value of d is set equal to i ( 1 + d 2 ). Yet another embodiment of the first aspect of the present invention, relates to one or both of the techniques exploiting the use of the principles of coherent imaging, described above, to produce mode stabilisation in slab waveguide solid state lasers, where the active medium is one of the well-known crystal or glass lasers, such as neodymium:YAG, neodymium:glass, alexandrite or other similar laser materials of this general type. All embodiments illustrated in Figs. 1 - 4 and Figs. 6 - 10 may be implemented with solid state slab waveguides. However, in addition, a monolithic embodiment, shown in Fig. 11, is particularly suitable. The slab waveguide 1, has all six faces optically polished, with side faces 37, 38 and 39, 40 parallel to each other and to the laser principal axis 41, and also orthogonal to the end faces 42, 43. The end faces are coated with multi¬ layer dieletric coatings of suitable reflectivity for efficient laser action. The reflectivity of the end fact 46, (or both end faces (46, 47) is also spatially modulated in a manner similar to previous embodiments, described above, so as to produce high order waveguide mode stabilisation by virtue of the principle of coherent imaging. Alternatively, a suitable lateral spatial modulation is included as a separate element in the resonator, in the manner illustrated in Fig. 2 (a) , where the ends of the slab gain medium may be anti- reflection coated to reduce resonator optical loss. In such embodiments of the principle, the sold state slab waveguide may be end, side or face pumped by diode laser arrays, flashlamps or other suitable pump sources using appropriate coupling optics to couple the pump light into the slab waveguide in a manner suitable to maximise the fraction of the incident pump radiation which is absorbed int he slab waveguide gain medium. Such embodiments may also include cases which the slab waveguide is provided with suitable cladding, which may also be dichroic, to enhance waveguiding action at both the pump and laser wavelengths. The slab laser resonator according to the first aspect of the present invention involving either periodic modulation in mirror reflectivity, or of the slab waveguide surface reflectivity, or of lines or grooves marked on the resonator mirror or mirrors, or of a separate intra-cavity spatial loss modulator, coupled with an appropriate choice of slab dimensions to facilitate coherent self-imaging, may also be applied to broad area semiconductor diode lasers, whether electrically or optically pumped. The slab laser resonator according to the first aspect of the present invention, involving either periodic modulation in mirror reflectivity, or of the slab waveguide surface reflectivity, or of suitable etched lines or grooves marked on the resonator mirror or mirrors, or of a separate intra-cavity spatial loss modulator, coupled with an appropriate choice of slab dimensions to facilitate coherent self-imaging, may also be applied to liquid slab dye lasers. Yet another embodiment of the first aspect of the present invention, the spatial loss modulation applied at or near the end mirrors, 2, 3 has a two dimensional grid pattern 44, as shown in Fig. 12, and is used when the thickness of the slab 1, is sufficiently large to produce higher order waveguide mode operation in this direction. The two dimensional spatial loss modulation pattern 44, can be produced by any one of the methods described previously hereinabove for producing one dimensional patterns. A laser system with multiple gain may be produced by using two or more slab waveguides arranged so as to be optically, if not physically, in series. In one such arrangement, shown in Fig. 13 (a), the first slab waveguide, 45, is configured as an oscillator in which the transverse and lateral mode structure is stabilised according to the method depending on coherent imaging, as described above. The second and succeeding slabs 46, are configured as optical amplifiers. In one version of the invention, the radiation emanating from the first slab waveguide module 45, is imaged using suitable optical elements 47, onto the front optical plane of the following slab amplifier 46, with similar optical coupling being effected between successive slab waveguide modules. The optical length of the amplifier sections may be selected to be one or more multiples of the characteristic Talbot length for the wavelength of interest. Two or more waveguide slabs 45, 46, may also be arranged geometrically in parallel, as illustrated in Fig. 13 (b) where the respective slab waveguide may be excited in series or parallel, and optically coupled in series, with appropriate folding and imaging optics, 48. In one version, radiation from a mode-stabilised oscillator, 45, is imaged on to multiple sequential amplifiers 46. In such cases, periodic spatial loss or phase modulation may also be incorporated in each section of the amplifier chain, with periodicity appropriate for efficient coherent imaging. By the inclusion of end mirrors 2, 3, the parallel slab structure may be configured as a multi-slab oscillator whose mode structure is controlled in much the same manner as the slab laser resonator according to the first aspect of the present invention. As shown in Fig. 14 (a) a number of slab wave¬ guides 49, may be arranged to be physically in parallel, with common resonator mirrors, 2, 3 and to share the same spatial loss modulator 4. The spatial loss modulator chosen to match the dimensions of the identical slab waveguides 49 and the conditions for efficient coherent imaging outlined above. In a further configuration, illustrated in Fig. 14 (b) , a single slab waveguide 50, configured as an oscillator which has transverse and lateral modes stabilised by coherent imaging techniques, as described above, is arranged to have its largest slab faces 51, orthogonal to the large slab faces 52, of a series of slab waveguides 53. The output from the mode stabilised slab oscillator 50 is optically coupled in such a manner as to simultaneously inject coherent light into each of the parallel slab waveguides 53, which may configured as amplifiers, regenerative amplifiers or injection-locked oscillators, so producing a coherent output from the whole assembly. Referring now to Fig. 15 there is shown a laser system in accordance with the second aspect of the present invention. As illustrated in Fig. 15, a slab waveguide structure 54, is positioned within a laser cavity, bounded by two resonator mirrors 2, 3, which may also incorporate a linear wire grating structure 4, (or one of the alternative spatial loss modulators described above) in series with a multi-element array of gain media 55. The length of the slab waveguide slab 54, within the laser resonator is chosen to match the conditions for coherent imaging, to produce a stable, phase-locked mode in the laser output. The slab waveguide region 54, is a region where diffractive coupling may occur under conditions of very low loss in both the transverse and lateral directions. Such low loss coupling is essential for power efficient operation of such coupled arrays, and represents a distinct advantage over the prior art, where such diffractive coupling, even with Talbot self-imaging occurs in the more lossy free space environment, rather than in the low loss waveguide structure. A variation on the laser system described with reference to Fig. 15, uses slab waveguide 54, not only to provide a region of very low loss diffractive coupling, but also as an intra-cavity region with optical gain. This may be accomplished by providing either electric discharge, optical or other form of excitation to the slab waveguide which simultaneously constitutes the low loss diffractive coupling region for the purpose of phase-locking an array of laser oscillators. Fig. 16 shows yet another variation on the laser system described with reference to Fig. 15. As shown in Fig. 16, the waveguide slab structure 54, may be applied to the phase-locking of a two dimensional array of lasers 56, which may be of the gas discharge, solid state, semiconductor or liquid dye types, by configuring the slab dimensions to produce coherent imaging in two dimensions, with the predominant mode EH 1 in the waveguide slab matching the periodicity of the array. A two-dimensional spatial loss modulator 44, may optionally be located at one end of the slab waveguide.

Claims: CLAIMS 1. A slab laser resonator comprising in combination: i) a rectangular slab-shaped waveguide laser gain medium comprising smooth side and top walls which have surfaces which are highly reflecting at glancing incidence for the laser wavelength, and capable of providing efficient, low loss optical waveguiding by means of Fresnel reflection or by total internal reflection; ii) two or more laser resonator mirrors which comprise, or are positioned parallel to and closely adjacent with, the end faces of the said slab wave¬ guide, and one or all of which mirrors is partially transmitting to form a low-loss laser resonator with efficient optical power extraction; iii) a spatial loss modulator which provides a one dimensional periodic optical loss, and/or phase modulation, for laser radiation propagating generally parallel to the main optical axis of the slab laser resonator, which spatial loss modulator comprises a periodic grating-like structure positioned adjacent to, or incorporated within, one or more of the resonator mirrors, and/or along the length of the slab waveguide surfaces parallel to the said main optical axis of the slab laser resonator, such that the loss modulation itself is caused to be distributed in the direction parallel to and extending across the full extent of the width of the slab waveguide; and wherein the dimensions of the said slab wave¬ guide, the slab laser resonator length and periodicity of the said spatial loss modulator are selected so as to match each other and the requirements of coherent imaging, within the slab waveguide whose width is selected to be an appropriate multiple of the said period of the spatial loss modulator, and where the length of the slab laser resonator is chosen to conform to the required distance necessary to achieve efficient coherent imaging of the periodic loss modulation at the required wavelength. 2. A slab laser resonator according to Claim 1, wherein the spatial loss modulator extends over the full width of the slab waveguide. 3. A slab laser resonator according to Claim 1, wherein the spatial loss modulator extends over a fraction of the width of the slab waveguide. 4. A slab laser resonator according to Claim 1, 2 or 3, wherein the spatial loss modulator comprises a grid of parallel fine wires, each wire being accurately separated from each of its neighbours by a distance chosen to match the dimensions of the wave- guide slab and the resonator, and to satisfy the Talbot length condition. 5. A slab resonator according to Claim 4, wherein the spatial loss modulator comprises a grid of parallel thin plates or foils of solid material, aligned parallel to the main optical axis of the slab laser resonator, each plate being accurately separated from each of its neighbours by a distance chosen to match the dimensions of the slab waveguides and the resonator, and to satisfy the Talbot length condition. 6. A slab resonator according to Claim 5, wherein the spatial loss modulator comprises a set of parallel lines, grooves or raised sections which are etched, deposited or otherwise marked on the surface of one or all of the laser mirrors, which are coated to provide the desired reflectivity for laser action, and where the periodicity of said lines, grooves, or raised sections is chosen to match the dimensions of the waveguide slab, and the resonator and to satisfy the Talbot length condition. 7. A slab resonator according to Claim 6, wherein the spatial loss modulator comprises a set of parallel lines, grooves or raised sections which are etched, deposited or otherwise marked on the surface of an otherwise optically transparent window positioned within the laser cavity, and where the periodicity of said lines, grooves or raised sections is chosen to match the dimensions of the slab waveguide, and the resonator and to satisfy the Talbot length condition. 8. A slab resonator according to Claim 7, wherein the slab waveguide is solid state and the spatial loss modulator comprises a set of parallel lines, grooves or raised sections which are etched, deposited or other¬ wise marked on the end surface of the solid state slab waveguide, and where the periodicity of said lines, grooves or raised sections is chosen to match the dimensions of the waveguide slab and the resonator, and to satisfy the Talbot length condition. 9. A slab resonator according to Claim 8, wherein the spatial loss modulator is provided by designing one or all of the laser mirrors such that across said mirror or mirrors there is a one-dimensional periodic variation in reflectivity of the mirror, or mirrors, at the desired laser wavelength, and where the periodicity of the modulation in reflectivity is chosen to match the dimension of the slab waveguide and the resonator length, and to satisfy the Talbot length condition. 10. A slab laser resonator according to Claim 1, wherein the spatial loss modulator comprises one or more holes in one of the laser mirrors, are chosen to match the slab waveguide dimensions and the Talbot imaging condition at the laser wavelength. 11. A slab laser resonator according to Claim 10 wherein an additional partially transmitting mirror is positioned behind the hole, or holes, in the laser mirror close to the slab waveguide so as to reduce the fractional output coupling from the slab laser resonator. 12. A slab laser resonator according to Claim 11, wherein the spatial loss modulator comprises one or more periods defined by differential reflectivity of one or both of the laser mirrors. 13. A slab laser resonator according to any preceding Claims, wherein the slab waveguide has a thickness which allows higher order waveguide modes, and the spatial loss modulator has periodicity in two orthogonal directions, resulting in predominantly a single EH waveguide mode, and where the periodicity of the two dimensional spatial loss modulator is chosen to match the dimensions of the slab waveguide, and the resonator and to meet the requirements for efficient Talbot imaging simultaneously in the two orthogonal directions. 14. A slab laser resonator according to Claims 1, 2 or 3, incorporating a periodic loss modulation technique which consists of suitable lines, grooves or raised portions, positioned on the large flat surfaces of the slab waveguide in the direction parallel to the main optical axis of the laser and with period and location determined by the slab dimensions. 15. A slab laser resonator according to Claim 13, incorporating a periodic loss modulation technique which consists of suitable lines, grooves or raised portions, positioned in localised regions along the length or the width of the large flat surfaces of the waveguide slab in the direction parallel to the main optical axis of the laser and with period and location determined by the slab dimensions, the resonator length and the requirement to achieve coherent imaging by virtue of the Talbot effect at the desired laser wavelength. 16. A slab laser resonator according to any of Claims 1 to 13, wherein the slab waveguide has dimensions such that the length and width are approximately or exactly equal, so forming a slab wave¬ guide which has exactly or approximately a square- shaped geometry, and wherein the spatial loss modulator may be disposed on two or more edges. 17. A slab laser resonator according to any of Claims 1 to 15, wherein the slab waveguide comprises a pair of large area planar metal electrode parallel plates, which are spaced by dielectric sidewalls. 18. A slab laser resonator according to Claim 1, comprising: a diffusion-cooled gas discharge apparatus, providing a slab-shaped waveguide optical gain medium, which consists a pair of parallel large area planar metal electrode surfaces, spaced by a distance, which for example with a carbon dioxide laser, might appropriately be in the range of about 1-3 millimeters, so as to confine an electrically excited gas discharge which provides suitable laser gain conditions, and which said slab dimensions are chosen so that, simultaneously, the electrode surfaces, along with the dielectric sidewalls of the said discharge chamber form a planar waveguide structure which exhibits low optical waveguiding losses at the electrode surfaces as a consequence of the high surface reflectivity at glancing incidence at the desired laser wavelength, and where the said electrode surfaces are cooled by fluid flow through the electrode structure, or by other means, so as to provide efficient diffusion cooling for the laser gas mixture. An optical resonator system comprising a pair of laser mirrors designed in size, shape, reflectivity, location and orientation to provide optical feedback to the laser cavity with low optical losses and such as to allow efficient power extraction through one or both laser mirrors, and where the surfaces of the said sidewalls of the discharge chamber and mirror surfaces are mutually orthogonal, and where the dimensions of the electrodes and their separation are chosen to both provide low loss optical waveguiding and to satisfy the requirements of efficient coherent imaging. a one-dimensional loss modulation structure as defined in Claims 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13 and 14 which is periodic across the width of the slab, or alternatively, a two-dimensional loss modulation structure, as defined in Claim 12, which is positioned at one or both ends of the said discharge module. 19. A slab laser resonator according to any of Claims 1 to 15, wherein the slab waveguide comprises a pair of parallel large area planar plates comprised of a dielectric material, which supports low loss optical waveguiding via Fresnel reflections, and which are in intimate contact with a pair of fluid-cooled metal electrodes, each of which may be either a single piece construction or segmented in a suitable fashion so as to facilitate the achievement of a uniform electrically excited gas discharge. 20. A slab laser resonator according to any one of Claims 17, 18 or 19, wherein the active laser gas is cooled predominantly by convective flow rather than by diffusion of heat to the cooled electrodes. 21. A slab laser resonator according to any one of Claims 1 to 16, wherein the slab waveguide is fabricated from a solid state material which exhibits laser gain when optically pumped by means of a light source coupled into the slab waveguide through one or more of the six optical surfaces thereof, and wherein the end faces of the slab waveguide are coated to provide reflective surfaces at the laser wavelength to produce efficient laser operation of the slab laser, and wherein some or all of the six surfaces of the slab waveguide are designed to incorporate optical coatings which support both high reflection at the laser wavelength as well as high transmission at the wavelength region of the optical pump source. 22. A slab laser resonator according to any one of Claims 1 to 16 wherein the slab waveguide is fabricated from a semiconductor material which exhibits laser gain when optically pumped or otherwise excited by the passage of electric current through a suitable junction region fabricated in the structure by well- known techniques of semiconductor laser diode manufacture. 23. A slab laser resonator according to any one of Claims 1 to 16, wherein the slab waveguide comprises a liquid dye laser gain medium, static or flowing, and confined in a suitable slab-shaped vessel, with mutually orthogonal confining walls which also serve to confine the intra-cavity laser field in a propagating waveguide mode. 24. A laser system with multiple gain comprising: two or more slab waveguide laser amplifiers, which are optically in series, optical means to enable the output face of one slab waveguide to be imaged onto the input face of the next succeeding slab waveguide, or otherwise for the optical radiation to be efficiently coupled between adjacent slab waveguides a pair of laser mirrors positioned one at each end of the said series of slab waveguide amplifiers, to provide optical cavity feedback and output coupling and a spatial loss modulator located in one or more locations in the laser cavity, the periodicity of each of which is designed to match the dimensions of the said slab waveguide and the resonator, as well as to accommodate the requirements of producing efficient coherent imaging. 25. A laser system according to Claim 24, wherein each slab waveguide consists of a diffusion-cooled or σonvectively-cooled gas discharge, an optically-pumped solid state gain medium, an optically or electrically pumped semiconductor gain medium or an optically pumped liquid dye medium. 26. A laser system for providing phase-locked operation of an array of laser oscillators, comprising a planar array of waveguide gain elements and one or more resonator mirrors, a slab waveguide, one end of which is positioned immediately adjacent to the output plane of the planar array of waveguide gain elements. the said slab waveguide having reflecting side walls to provide a region wherein diffractive coupling of the radiation emission from the array elements may occur under conditions of very low optical loss in both the transverse and lateral directions, and which by virtue of choosing the dimensions of the said slab waveguide to match the periodicity of the array elements and the length requirements of the Talbot condition, produces very efficient low loss coherent imaging may be produced, as a result of which phase- locked operation of the laser array may be produced. 27. A laser system according to Claim 26, wherein a spatial loss modulator matching the array periodicity is placed between the slab waveguide and the laser resonator mirror. 28. A laser system according to Claim 26 or 27, wherein the slab waveguide is either a hollow wave¬ guiding structure with reflecting sidewalls, or a slab of solid state material, which is coated or clad to have highly reflecting walls. 29. A laser system according to Claim 26, 27 or 28 in which the slab waveguide, wherein low loss diffractive coupling and coherent imaging occurs, also incorporates optical gain at the wavelength of the lasers in the array, so that the optical coupling region acts simultaneously as an optical amplifier. 30. A laser system according to Claims 26, 27, 28 or 29, wherein the slab waveguide is a slab of suitable excited gas discharge medium, solid state crystal, semiconductor or liquid dye material which exhibits gain at the wavelength of the laser array. 31. A laser system according to any one of Claims 26 to 30, wherein the array of laser gain media is a two dimensional array and that the slab waveguide has dimensions chosen to match the periodicity of the array in both directions and that efficient Talbot imaging is obtained simultaneously in both directions. 32. A rectangular waveguide slab shaped structure, capable of providing low loss optical propagation in the direction parallel to the optical axis, and involving low loss reflections from the parallel reflecting surfaces bounding the slab in either or both the transverse and lateral directions, which said slab structure, acting passively, and/or actively, may be used to control the oscillating cavity mode and frequency of a laser or array of lasers, by virtue of the particular choice of slab dimensions which, coupled with the use of resonator exploitation of intra-cavity coherent imaging.

Inventors: ABRAMSKI KRZYSZTOF MAREK, BAKER HOWARD JOHN, COLLEY ALAN DAVID, HALL DENIS RODERICK, ABRAMSKI, KRZYSZTOF, MAREK, BAKER, HOWARD, JOHN, COLLEY, ALAN, DAVID, HALL, DENIS, RODERICK
Applicants: UNIV HERIOT WATT, HERIOT-WATT UNIVERSITY
Application year: 1992
Main classification: H01S3
IPC codes: H01S3, H01S5