Abstract:
A discontinuous phase element ( 86, 204 ) is disposed between the reflector ( 20, 23 ) elements of an optical resonator in order to suppress unwanted modes propagating within the cavity, and to preferentially allow the existence of preferred modes within the cavity. The discontinuous phase element ( 204 ) operates by producing sharp changes in the phase distribution of the undesirable modes, so that their propagation losses are sufficiently high prevent their build-up. This is achieved by introducing a discontinuous phase change to these modes at locations where they have high intensity. At the same time, the desired modes suffer 0 or 2π phase change, or have low intensity at the discontinuity, and so are unaffected by the discontinuous phase element. Such elements can be used in a single element or a double element configuration, and can be used in passive cavities or active cavities, such as lasers. In addition to being able to improve the output beam quality of a laser by encouraging output of the lowest order mode beam, they can also be used to improve the maximum power output of solid state lasers by encouraging the output of specific higher order mode beams, without limiting dynamic range of the laser. They can also be used to compensate for birefringence distortion in the gain medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical resonators generally and to those used in lasers as specific examples of such resonators. 
     BACKGROUND OF THE INVENTION 
     In general, the intensity distribution of output light from optical resonators, especially those used in high power lasers with large apertures, is not uniform but has the form of a pattern of either spots or rings, or a combination of these. Such distributions of output light result from the fact that the light inside an optical resonator is distributed in similar patterns termed modes. The spots and rings of this intensity pattern are so distributed that adjacent spots and adjacent rings have opposite phases (π phase shift). 
     When only the fundamental mode (TEM  00 ) exists in a resonator, the output light distribution consists of only a single spot, and this mode is considered optimal. However, when higher order modes exist, the light distribution contains multiple spots or rings, so that the focusability and intensity of the output beam decreases. Such a decrease is undesirable for most applications involving laser light. 
     In order to improve the characteristics of the output light, external phase elements can be introduced into the optical path of the output beam in order to provide controlled phase or amplitude changes to any cross-sectional part of the beam. Examples of such phase elements are presented in the article “How phase plates transform and control laser beams” by Lee W. Casperson, published in Laser Focus World, May 1994, 223-228. In this article, the phase elements are designed to ensure that all portions of the output laser beam are in phase, thereby improving the far field intensity distribution. 
     With the introduction of a phase element outside the laser, the usual far field distribution of spots is replaced by a single, intense, on-axis spot surrounded by a pattern of much fainter spots. By this means, the intensity of light distribution resulting from a single high order mode can be increased by a factor of ten or more. Unfortunately, such an increase can only occur when the output light distribution results from a single stable mode. 
     Several methods have been proposed in order to obtain a specific, stable, high order mode, so the total laser output power is high, and the stability is such that the distribution of the beam can be improved by means of an external phase element. These methods have had limited success. For example, in the article “Single-mode selection using coherent imaging within a slab waveguide CO 2  laser” published in Applied Physical Letters, Vol. 60, 2469-2471 (1992), K. M. Abramski, H. J. Baker, A. D. Colly and D. R. Hall propose the insertion of a wire grid into the laser resonator for selecting a specific high order mode. Unfortunately, the grid and the adjacent resonator mirror are susceptible to damage through heating of the grid by the inter-cavity flux of energy, particularly during mirror alignment. 
     Another example involves the use of a conical resonator mirror to obtain a single higher order mode. This method was published by M. Rioux, P. A. Belanger and M. Cormier in the article “High-order circular-mode selection in a conical resonator” in Applied Optics, Vol. 16, 1791-1792 (1977). Unfortunately, it is difficult to predict and obtain a specific distribution of the higher order mode output laser beam in order to design the correct external phase element. In particular, it is difficult to match the orientation of the required external phase element to that of the output light distribution. 
     In order to obtain a single, stable, lowest order mode TEM  00 , an aperture must be inserted into the resonator. When the aperture is small, only the TEM  00  mode with the narrowest field distribution propagates, while higher order modes are eliminated because they are absorbed by the aperture. In general a laser operating with a pure TEM  00  mode has optimal beam quality, but relatively low output power. 
     In summary it is appreciated that the prior art shows how external phase elements can improve the quality of the output light distribution from an optical resonator, such as a laser. Unfortunately, this improvement cannot be practically implemented with most lasers, especially these operating in multi-mode, so the introduction of external phase elements are indeed limited to very special cases. Furthermore, wires and conical elements used for controlling the modes inside the optical resonator are not practical and tend to reduce the power of the output beam. Intra-cavity apertures, though widely used reduce the laser power output significantly. 
     The laser design limitations described above are applicable to both gas and solid state lasers. In addition, there are a number of mode design problems specific to solid state lasers, which limit their performance. Solid state lasers are particularly susceptible to a problem known as thermal lensing of the laser rod, which constitutes the gain material in such lasers. Thermal lensing occurs when the shape or the optical properties of the laser rod change because of the high thermal stresses set up within the rod at high input power levels. The optical elements in a solid state laser are designed to create an optimal mode shape while taking into account thermal lensing of the gain medium at the output power levels at which the laser is designed to operate. However, if the output power of the laser is changed, the thermal lensing of the gain medium changes, and the optical resonator no longer supports the optimal mode shape. Furthermore, in extreme cases the optical resonator becomes unstable and the output power of the laser drops to zero. 
     The input power levels to the gain medium over which the optical resonator remains stable define the dynamic range of the laser. It has been shown by Vittorio Magni in the article “Resonators for solid state lasers with large volume fundamental mode and high alignment stability” published in Applied Optics, Vol.25, 107-117 (1986) that the dynamic range of a solid state laser is inversely proportional to the width of the Gaussian mode ( 0 , 0 ) inside the gain medium. The maximum output power is proportional to the width of this mode. Therefore, the dynamic range is inversely proportional to the maximum output power from the laser. This means that when designing a laser, a compromise must be made between the maximum output power of the laser and its dynamic range. This significantly limits the laser&#39;s usefulness. 
     Another problem which limits the performance of solid state lasers arises from the optical distortions set up in the mode structure because of the birefringent nature of the lasing rod. The birefringence introduces areas of circular polarization into the mode because of the different phase shifts introduced along the different crystal axes, and thereby degrades the quality of the laser output beam. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved optical resonator with an improved output beam. 
     There is thus provided in accordance with a preferred embodiment of the present invention an optical resonator comprising reflector elements and with at least one discontinuous phase element disposed between the reflector elements. The reflector elements may be full reflectors or output couplers. 
     The optical resonator may be a passive or active optical resonator. If the optical resonator is an active optical resonator, it may be embodied in a laser, such as an axial laser or a ring laser. The resonator may be a stable or an unstable resonator. 
     The discontinuous phase element may be embodied in at least one reflector or output coupler or may be positioned adjacent to an optical element or may be placed inside the resonator at a defined point which images onto itself by reflection from one of the above optical components of the resonator. 
     The discontinuous phase element may provide discontinuous phase change as well as one or more of angular, linear or radial phase change. 
     The optical resonator may also comprise an external discontinuous phase element operative in addition to the internal element, in order to cancel distortions and eliminate phase discontinuities in the output beam from the optical resonator. 
     The invention operates by producing sharp changes in the phase distribution of the undesirable modes, so that their propagation losses are sufficiently high to prevent their existence. This is done by introducing a discontinuous phase change to these modes at locations where they have high intensity. 
     In general the intensity of resonator modes is distributed in well-defined patterns of spots or rings, the phases of neighboring spots or rings having opposite sign (π phase shift). It is possible to predict the field distribution (amplitude and phase) of any mode propagating. 
     This invention ensures that generally only one desired mode propagates in the optical resonator, with a defined intensity distribution. This is done by incorporating two discontinuous phase elements that introduce different phase changes to different parts (spots and rings) of the mode distribution. In addition, the elements are designed and aligned to ensure that the discontinuous phase change in each element falls at the interfaces between neighboring parts of the selected mode structure, where the intensity is very low. The phase changes introduced by each element must be of opposite sign, so that the sum of the combined phase change is either zero or a multiple of 2π. 
     If the distance between the two discontinuous phase elements inside the resonator is sufficiently short, then the total intensity distribution of the desired mode does not change as it passes through the two elements because the phase change introduced by one element is canceled by the other. The other, undesired modes have a different distribution of intensity, so that when they pass through the first discontinuous phase element, they suffer a sharp phase change at locations where their intensity is strong. This results in a strong divergence, so that the second discontinuous phase element can no longer cancel the phase changes introduced by the first element. These modes therefore suffer a loss of intensity and are suppressed. 
     Similar results can be achieved by using only a single discontinuous phase element. With the single element, the phase change introduced by the element must be either zero or π, and the element should be located near one of the resonator mirrors, preferably near the output coupler mirror. After the desired mode passes once through the element, all parts of its intensity distribution are in phase (zero or 2π phase difference). The mirror then reflects the modified mode so it passes once more through the element, and all parts of the intensity distribution return to their original phase. This is due to the fact that the total phase change introduced by the element is 2π, which is equivalent to no change at all. 
     It is possible for the second passage of the mode through the discontinuous phase element to take place at a point in the resonator far from that of the first passage, on condition that the two passages are made at planes which are conjugate image planes produced by some optical component in the resonator. 
     An additional phase element may be placed outside the resonator in order to further improve the output beam. This element can easily be characterized since the resonator output beam originates from a single well defined stable mode. 
     A suitably constructed discontinuous phase element may be inserted at the correct position into the cavity of a solid state laser designed with a conventional optical resonator to support a narrow Gaussian mode. The addition of the discontinuous phase element forces the resonator to oscillate in a high order mode, resulting in a laser which combines high maximum output power without the limitations of dynamic range of the conventional cavity. 
     Likewise, a suitably constructed discontinuous phase element inserted into the cavity of a solid state laser at the correct position, can also compensate for birefringence distortions introduced by the gain medium. 
     Likewise, a suitably constructed discontinuous phase element can be inserted into the cavity of a laser that has a flat output coupler and a curved full reflector. The placement of the discontinuous phase element adjacent to the flat output coupler leads to lower divergence of the output beam. 
     Likewise, a suitably constructed discontinuous phase element inserted into the cavity of an unstable resonator laser at the correct position, will force the resonator to oscillate in a high order mode, and will thereby prevent the high intensity center spot produced in unstable resonators operating in the lowest order mode. 
     Thus, in accordance with the above embodiments, single mode operation and high beam quality can be achieved even in a resonator with a large aperture and hence, high output power. The design of the discontinuous phase element can vary from one resonator to the other but there are several inherent features common to all such elements. The phases of these elements all have lines of discontinuity, and these lines are positioned where the intensity of the desired mode is expected to be very low. In addition, the discontinuous phase changes are introduced to the mode inside the resonator at planes which are located at defined distances from each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings, in which: 
     FIGS. 1A and 1B are schematic illustrations of axial lasers constructed and operative in accordance with a preferred embodiment of the present invention; FIG. 1A shows such a laser with a curved output coupler, while FIG. 1B shows a similar laser with a flat output coupler. 
     FIG. 2 is a schematic illustration of an output coupler arrangement useful in the laser of FIG. 1A, and including a combination of a conventional output coupler and a discontinuous phase element; 
     FIG. 3 is a schematic illustration of an output coupler arrangement useful in the laser of FIG.  1 A and including a hybrid element which consists of an output coupler and a discontinuous phase element. 
     FIG. 4 is a schematic illustration of a discontinuous phase element designed to select mode ( 6 , 1 ) in a rectangle resonator, 
     FIG. 5 is a schematic illustration of a discontinuous phase element designed to select mode ( 2 , 0 ) in a cylindrical resonator; 
     FIG. 6 is a schematic illustration of a discontinuous phase element combined with a spiral phase element. This element is designed to select mode ( 1 , 1 )* in a cylindrical resonator; 
     FIG. 7 presents a schematic illustration of a discontinuous phase element  212  designed to select mode ( 0 , 4 ) in a cylindrical resonator; 
     FIGS. 8A,  8 B,  8 C and  8 D are cross-sections of the intensity distribution of the first four modes having the lowest intensity loss, inside a rectangular conventional resonator; 
     FIG. 9 is an illustration of the amplitude distribution of mode ( 2 , 0 ) in a rectangular resonator; 
     FIG. 10 is an illustration of the amplitude distribution of mode ( 2 , 0 ) which was presented in FIG. 8C, after passing through the discontinuous phase element; 
     FIG. 11 is an illustration of the amplitude distribution of mode ( 1 , 0 ) in a rectangular resonator; 
     FIG. 12 is an illustration of the amplitude distribution of mode ( 1 ,O) which was presented in FIG. 11, after passing through the discontinuous phase element designed to select mode ( 2 , 0 ); 
     FIGS. 13A. 13B,  13 C and  13 D are cross-sections of the intensity distributions of the first four modes having the lowest intensity loss, inside a rectangular resonator with a discontinuous phase element designed to select mode ( 2 , 0 ); 
     FIG. 14 presents the intensity loss of the four lowest order modes in a rectangular resonator designed to select mode ( 2 , 0 ), as a function of the axial displacement of the discontinuous phase element from the output coupler; 
     FIG. 15 is a description of the far field of the output beam from a resonator with a discontinuous phase element designed to select mode ( 2 , 0 ). The beam converses to a single lobe with high intensity and good quality; 
     FIG. 16 is an illustration of an axial laser with one discontinuous optical element placed close to a plane which is imaged onto itself after reflection from the back mirror. This situation is optically equivalent to placing the element close to the output coupler; 
     FIG. 17 is an illustration of a ring laser with two discontinuous optical elements placed close to each other. A third element is placed outside the resonator in order to improve the output beam; 
     FIG. 18 is a representation of the linear polarization of the ( 0 , 0 ) mode in a cylindrical Nd:YAG crystal of a solid state laser; 
     FIG. 19 is a schematic illustration of the polarization distortion introduced into the ( 0 , 0 ) mode because of the birefringence of the crystal of FIG. 18; 
     FIG. 20 is a view of the polarization structure of the ( 0 , 2 ) cylindrical mode in the Nd:YAG crystal of a solid state laser, showing the π phase shift between adjacent parts of the mode structure; and 
     FIG. 21 is a schematic illustration of a single discontinuous phase element designed to select mode ( 0 , 2 ) and to compensate for birefringence in the Nd:YAG crystal of a solid state laser. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 which is a schematic illustration of an axial laser, as an example of an optical resonator, constructed and operative in accordance with a preferred embodiment of the present invention. The axial laser comprises reflectors, preferably a fill reflector  20  and a partial reflector or an output coupler  22 , disposed at opposite ends of an optical resonator  24 , which contains a gain medium  26 , and with a discontinuous phase element  86  located between the full reflector  20  and the output coupler  22 . 
     A mode, indicated by reference number  30 , is seen to propagate back and forth between the reflector  20  and the output coupler  22 , while passing through the gain medium  26 . A beam of light  32  is emitted through the output coupler  22 . The width of mode  30  inside the laser is influenced by an aperture  34 . 
     In accordance with a preferred embodiment of the invention, the laser of FIG. 1A is characterized in that it includes at least one discontinuous phase element operative to introduce discontinuous phase changes to the modes propagating inside the resonator, so that the divergence of the undesirable modes is greater than that of the desired mode. This results in a broadening of the undesirable mode intensity distribution, preventing their propagation in the laser. Thus the undesirable modes are suppressed, thereby improving the quality of the output beam. 
     Preferably, one discontinuous phase element is employed. This element should be placed close to the output coupler but not too close. Alternatively, the element can be placed near a plane that can be imaged onto itself by the optics of the laser. When two elements are used, they should be placed close to each other or close to two planes that can be imaged onto each other by the optics inside the laser. 
     There are a number of alternative optical configurations of such axial lasers, wherein the output coupler and full reflector can have different curvatures, concave or convex, or even be flat. FIG. 1B shows one such configuration, commonly used in industrial lasers, in which the output coupler  23  is flat, and the full reflector  20  is concave, like that of the laser shown in FIG.  1 A. Discontinuous phase elements can be used to improve the output beams of all these types of lasers. For example, a discontinuous phase element  86  inserted into the cavity configuration shown in FIG. 1B, next to the flat output coupler  23 , results in low divergence of the output beam  32 , while the curved full reflector may compensate for lensing effects of the gain medium. 
     Two alternative embodiments of an output coupler arrangement including a discontinuous phase element are now described with reference to FIGS. 2 and 3. 
     FIG. 2 is a schematic illustration of an output coupler arrangement useful in the laser of FIG.  1 A and including a combination of a conventional output coupler  80 , with a discontinuous phase element  86 . 
     The conventional output coupler is preferably coated with a partial reflecting coating on surface  82  and an antireflection coating on surface  84 . The discontinuous phase element is coated with an antireflection coating on both surfaces  88  and  90 . 
     The discontinuous phase element is constructed to be compatible with the desired mode intensity distribution. Specifically, it is desired to form phase discontinuities at locations where the desired mode has little or no intensity. The magnitude of each phase discontinuity is π, so that after the desired mode passes once through the discontinuous phase element, all parts of the cross-sectional intensity distribution of the desired mode have the same phase. After the second pass through the element the desired mode returns to its original distribution. 
     The embodiment of FIG. 3 combines elements  80  and  86  of the embodiment of FIG. 2 to achieve a single hybrid element  100  having less surfaces. The surface  102  of the hybrid element may have the same form as surface  90  in the embodiment of FIG. 2, while the partially reflective surface  104  may have the same form as surface  82  in embodiment of FIG.  2 . 
     FIGS. 4,  5  and  6  present examples of discontinuous phase elements constructed to select various types of modes. These elements may be realized by photolithograghic methods, by mechanical methods such as diamond turning, by mechanical methods such as graded index elements or any other method which produces an element that its changes the phase of a wave-front. 
     FIG. 4 presents a schematic illustration of a discontinuous phase element constructed to select mode ( 6 , 1 ) in a rectangle resonator. In mode ( 6 , 1 ) there are  14  sections in which the energy is concentrated. The element is designed to have constant phases at each location  90 . The discontinuous lines  95  are located at the boundaries of these sections. 
     FIG. 5 presents a schematic illustration of a discontinuous phase element  204  constructed to select mode ( 2 , 0 ) in a cylindrical resonator. The mode notation (r,θ) indicates the number of radial. r, and angular, θ, nodes in a cylindrical mode pattern The intensity distribution in the ( 2 , 0 ) mode is distributed in rings, where adjacent rings provide a π phase shift, and the discontinuities  200  are circles. 
     FIG. 6 presents a schematic illustration of a discontinuous phase element  208  combined with a spiral phase element. The element is constructed to select mode ( 1 , 1 )* in a cylindrical resonator. This cylindrical mode is unlike that previously described in FIG. 5, in that it has angular as well as radial phase distribution. It appears as two concentric rings. The circular discontinuities  210  select the desired radial distribution, and discriminate between other (n, 1 )* modes, which have a different number of rings, according to the value of n. The spiral element selects the desired angular phase distribution of the mode and ensures that modes with a different angular distribution suffer rapid broadening and decay after very few traverses of the element. 
     FIG. 7 presents a schematic illustration of a discontinuous phase element  212  constructed to select mode ( 0 , 4 ) in a cylindrical resonator. Such an element is insensitive to radial perturbations of the amplitude distribution of the mode, such as would accrue from the effects of thermal lensing in a solid state laser. 
     The mechanism for mode discrimination and consequent output beam quality improvement according to the present invention will now be described with the aid of FIGS. 8A-D,  9 ,  10 ,  11 ,  12 ,  13 A-D,  14  and  15 . 
     FIGS. 8A,  8 B,  8 C and  8 D present the intensity distributions of the first four modes having the lowest intensity loss, inside a rectangle conventional resonator. The intensity is distributed in a well defined sections  220  with significant intensities, separated by sections  222  with little, if any, intensity. 
     FIG. 9 shows the amplitude distribution of mode ( 2 , 0 ) in a rectangle resonator. The corresponding intensity distribution of this mode was presented in FIG.  8 C. The amplitude of this mode is positive in the central section  240  and negative in the two side sections  244 . 
     FIG. 10 shows the amplitude distribution of the mode ( 2 , 0 ) presented in FIG. 9, after passing once through the discontinuous phase element designed to select this mode. The sign of the amplitude in the central section  250  as well as in the side sections  254  are now all positive. This shape of the mode converges slowly as the mode propagates, and a converging output beam exits the output coupler located next to the discontinuous phase element. Some of the mode is reflected from the output coupler, as illustrated in FIGS. 2 and 3, and passes again through the discontinuous phase element. The optical path between the first passage and the second passage through the phase element is very short, so that after the second passage through the phase element, the amplitude distribution of the mode returns to its original distribution shown in FIG. 9 . 
     Other modes with a differing amplitude distribution are affected differently by the passage through the discontinuous phase element, so they rapidly decay and cease to exist as will be illustrated with the example shown in FIGS. 11 and 12. 
     FIG. 11 presents the amplitude distribution of mode ( 1 , 0 ). 
     FIG. 12 presents the amplitude distribution of mode ( 1 , 0 ) shown in FIG. 11, after if passing once through the discontinuous phase element designed to select mode ( 2 , 0 ). Two sharp discontinuous phase shifts  261  are introduced at locations where the amplitude of mode ( 1 , 0 ) is significant. These discontinuities ensure that mode ( 1 , 0 ) diverges rapidly so that it does not return to its original shape after the second passage through the phase element. As a result, a large intensity loss for mode ( 1 , 0 ) accrues and this mode ceases to exist. 
     FIGS. 13A,  13 B,  13 C and  13 D present the intensity distributions of the first four modes having the lowest intensity losses, inside a rectangle resonator with a discontinuous phase element designed to select mode ( 2 , 0 ). Only the intensity distribution of the desired mode ( 2 , 0 ), presented in FIG. 13A remains the same. The distributions of the other modes are distorted, and, being strongly attenuated, die out after a small number of resonator traverses. 
     FIG. 14 presents the losses for the four lowest order modes as a function of distance of the discontinuous phase element from the output coupler, for a rectangular axial resonator. The discontinuous phase element is designed to select mode ( 2 , 0 ). If the discontinuous phase element is placed very close or against the output coupler  280 , even the undesired modes are not distorted by the phase element, since there is no axial space for them to diverge before being reflected back for a second passage through the discontinuous phase element, where they are converted back to their original amplitude distribution. When the distance between the discontinuous phase element and the output coupler increases, then the undesired modes immediately start to diverge, resulting in sharply increased losses  282 , while the desired mode have little loss  283 . At a slightly increased distance, only the desired mode exist inside the laser. When the distance between the discontinuous phase element and the output coupler increases further, even the desired mode has a significant loss of intensity  284 . 
     FIG. 15 presents the far field intensity distribution of the output beam resulting from mode ( 2 , 0 ). As is evident, the beam converges to a single lobe with high intensity and good quality  285 . 
     Reference is now made to FIG. 16, which schematically presents a configuration of an axial laser with a single discontinuous phase element  302  placed at an arbitrary point inside the resonator and not close to the output coupler  304  or the reflector  306 . In this embodiment the phase element is placed close to a point that is being imaged to itself by the output coupler  304  and/or the reflector  306 , or by any other optical elements inside the resonator. This imaging configuration resembles the configuration with two phase elements placed close to each other. 
     In this embodiment the intensity distribution of the selected mode is different in each of two axial sections of the overall laser. In the first section  308 , on the output coupler side of the discontinuous phase element, the mode is converging and its crosssectional intensity is high because the phases of all parts of the intensity pattern are the same. In the second section  310 , on the reflector side of the discontinuous phase element, the shape of the mode is broad with low intensity because the phases of some parts of the intensity pattern are not the same. This embodiment is applicable, for example, in lasers which use the nonlinear properties of crystals for changing the radiation wavelength and/or for mode locking. The broader section  310 , with its high filling factor, is useful for efficiently obtaining increased gain from the gain medium  312 , while the narrow section  308 , with its high field intensity, is useful for wavelength conversion by the nonlinear crystal  314 . 
     Reference is now made to FIG. 17, which schematically presents a ring resonator having two discontinuous phase elements. The optical properties of these elements are the same as described earlier. The discontinuous phase elements  350  are designed to have opposite phases, so together do not influence the distribution of the desired mode. Other modes are distorted, however, as previously described in the axial laser case. In this configuration the phase discontinuities introduced by the elements may be arbitrary and need not be exactly π. The intensity distribution of the output beam is the same as that of the mode inside the laser. Thus, in order to improve the distribution of the output beam a third external phase element  352  must be introduced at the output from the laser. 
     Alternative configurations for this ring embodiment are similar to those presented for the axial resonator. The two phase elements can be combined into a single element with phase patterns on both sides. They can be placed far apart, provided that they are imaged onto each other by some optical element in the resonator. A single phase element can be used instead of two, provided that after one round-trip, the emerging wave-front from the element is imaged onto itself. 
     FIGS. 18,  19 ,  20  and  21  describe how a discontinuous phase element compensates for the problem of birefringence distortion in the gain medium of a solid state laser. 
     FIG. 18 is a schematic illustration of mode ( 0 , 0 ) marked as  401 . The arrow in the illustration represents the linear polarization of this mode. After this mode passes once through the distorting medium, for example a cylindrical Nd:YAG crystal, its polarization is distorted. 
     FIG. 19 is a schematic illustration of the polarization distortions introduced to the mode by the crystal due to birefringence. The primary axes of the polarization are marked as  403  and  405 . Along these axes, the polarization remains linear, but different phase shifts are introduced to the mode along them. In between these axes, the polarization of the mode becomes circular ( 407 ). The circular polarization and the phase shifts introduced to the different sections of the mode, distort the mode and consequently reduce the quality of the output beam of the laser. 
     FIG. 20 describes, schematically, mode ( 0 , 2 ) in cylindrical coordinates. The arrows represent a linear polarization where sections  412  and sections  414  of the mode have π phase shift between them, as previously described. After this mode passes through the distorting gain medium, small areas with circular polarization arise, although most of the energy of the mode is concentrated along the primary axes. However, the phase difference between the various sections  412  and  414  is no longer π. This phase distortion can be easily compensated by the use of a suitable discontinuous phase element. 
     FIG. 21 is a schematic illustration of a single discontinuous phase element designed to select mode ( 0 , 2 ) and to compensate for birefringence in the gain medium described in FIG.  19 . The phase difference between the various sections of the element ( 417  and  419 ) is π-Δ, where Δ is the phase difference introduced into the two sections of the mode ( 412  and  414 ) by the birefringence distortion of the gain medium. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.