IL131906A - Optical resonators with orthogonally polarized modes - Google Patents

Description

131906/2 OPTICAL RESONATORS WITH ORTHOGONALLY POLARIZED MODES YEDA RESEARCH & DEVELOPMENT CO. LTD. n»yi miun n nni? mm VP C: 35093 OPTICAL RESONATORS WITH ORTHOGONALLY POLARIZED MODES FIELD OF THE INVENTION The present invention relates in general to the field of optical resonators, and in particular to those used in lasers.

BACKGROUND OF THE INVENTION In general, the intensity distribution of light emerging from optical resonators, especially those used in high power lasers with large apertures,has a multimode pattern. In this pattern, the intensity is typically distributed in spots of different sizes, smaller in diameter than the aperture of the resonator. These spots have random phases, and in an unpolarized resonator, have different polarizations. This pattern of spots fills most of the gain medium volume, and thus efficiently extracts the power of the laser. However, the multimode pattern results in an output beam of relatively low brightness, compared to the diffraction limit of the resonator. This low brightness, in turn, limits the usage of the laser beam in many industrial, medical and military applications where a small, well-defined, focused spot or a well collimated beam is needed, such as in scribing, drilling, cutting, target designation and rangefinding.

Ever increasing attempts are being made to design laser resonators which emit a beam of high brightness and high power, i.e. lasing with a single low order mode that fills and utilizes most if not all of the gain medium.. This goal is difficult to achieve in a resonator with a high Fresnel number, i.e. with a wide aperture and short length, such as is desired for optimally compact lasers. In these resonators, there is hardly any loss discrimination between the different modes of oscillation and these resonators thus emit high divergence multimode beams. Particular examples of such resonators are those used in CW and pulsed solid state lasers.

A common approach used to control the modes of a resonator is to introduce an aperture inside the resonator. The aperture causes loss to higher order modes thus limiting their oscillation, so they virtually cease to exist. By use of the correct aperture, a laser can be made to emit in the fundamental mode TEMoo, which possesses the highest brightness of all possible modes. However, in a high Fresnel number resonator, this mode does not fill the entire gain medium diameter, resulting in poor efficiency of the laser.

A number of other methods have been proposed to obtain a specific, stable mode in a large lasing volume. For example, in the article "Single-mode selection using coherent imaging within a slab waveguide CO2 laser" published in Applied Physics Letters, Vol. 60, pp. 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. Other methods are reviewed and discussed in the co-pending patent applications "Optical Resonators with Discontinuous Phase Elements", Application No. PCT/IL98/00204, Publication No. WO98/50986 and "Optical Resonators with Spiral Phase Elements" Application No. PCT/EL97/00064, Publication No. WO97/34344. As discussed in the prior art, these methods have some performance limitations, or have some difficulties in practical implementation, such as an inability to sufficiently extract power from the gain medium.

One of the methods proposed in the above-mentioned patent applications has recently been demonstrated by Ram Oron et al. in the article "Discontinuous phase elements for transverse mode selection in laser resonators" in Applied Physics Letters, Vol. 74 (10), pp. 1373-1375, (1999). In this article, a method is described for causing a resonator to oscillate in a single mode, which need not necessarily be the fundamental TEM0o mode. This method involves introduction of phase elements, either discontinuous or continuous, into the resonator. The phase distribution of the phase elements impose different losses to different modes, thus discriminating between them, since only the mode with a phase distribution that matches that of the element suffers no loss. Since the phase element makes changes only to the phase of the radiation field within the cavity, it does not introduce extra loss to the resonator, unlike discriminators that modulate the amplitude such as apertures, wires, apodizers and the like.

The resulting single high order mode distribution, in contrast to a multimode distribution, has a controlled phase, and thus is more easily focusable. In addition, since the high order mode is larger in diameter than the fundamental mode, it more fully fills the gain medium diameter, resulting in higher resonator efficiency and an output beam of higher power.

Yet, the single high-order mode still does not utilize the entire gain medium volume, since modes of order higher than the fundamental mode have both zero intensity zones (nodes) and low intensity regions. In these regions, the electromagnetic field does not create stimulated emission which is the mechanism for extracting power from the lasing medium.

There therefore exists a serious need for a method of operating the resonator of a laser more efficiently, so as to effectively utilize a larger part of the lasing medium.

The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.

SUMMARY OF THE INVENTION The present invention seeks to provide an improved optical resonator of higher efficiency than previously available, together with an output beam of high brightness.

There is thus provided in accordance with a preferred embodiment of the present invention, an optical resonator with reflector elements at its extremities. Between the reflecting elements are disposed at least one polarizing element and at least two mode-controlling elements, each mode controlling element selectively presenting high attenuation to all but one mode, whether the fundamental mode or one of higher order. At least one of the reflecting elements may be a full reflector, and another a partially transmitting reflector operating as an output coupler.

The resonator may be an active optical resonator, as embodied in a laser, such as a linear laser or a ring laser, or a passive optical resonator. Also, the resonator may be a stable or unstable resonator. Each of the mode-controlling elements may be a separate element or may be embodied in at least one reflector or output coupler, or may be positioned adjacent to an optical element.

The invention, according to a preferred embodiment, operates in the following way. The polarizing element separates the two orthogonal polarizations of the beams propagating inside the resonator, such that a beam possessing one polarization propagates along one path and the beam with the orthogonal polarization propagates along another path. The two paths can be at an angle to each other, or parallel to each other but laterally displaced. Within the resonator there exists a common path for the two polarizations, and the gain medium is placed in this path. Each separate path together with the common path constitutes an independent resonator. In each of the separate paths the mode structure is controlled with any of the known methods for controlling modes, such as by means of apertures, phase elements, wires, apodizers, conical mirrors, diffractive elements and the like. The electromagnetic fields of the resonator beams do not directly interact in the common path because of the orthogonal polarizations of the beams. However, they do interact indirectly in the gain medium through the depletion of the gain by the electromagnetic field of the beams. Specifically, the gain of any region utilized by one of the beams is no longer available for utilization by the other beam.

Each of the modes has a region or regions or lobes of high intensity, and a region or regions or nodes of effectively zero or low intensity. Stable co-existence of two beams in the gain medium occurs when the mode control elements, one in each path, are designed and axially aligned in such a way that, within the gain medium, the region or regions of higher intensity of one mode fall on the lower intensity region or regions of the other mode. In this way the perturbation of one mode on the other is minimal.

On the other hand, because of the gain depletion mechanism, one mode acts effectively as a loss element to the other mode, thus limiting the onset of any modes other than the desired complementary polarized mode. In this way, undesired higher order uncontrolled modes are effectively eliminated. The two modes co-exist in equilibrium, complementing each other in the gain medium, and thus utilize a larger gain volume than a single mode would have utilized. The effective result is the ability to achieve utilization levels of the gain medium typical of multimode resonators, while at the same time maintaining the brightness typical of low order modes of the resonator.

Gain depletion can be utilized in a further preferred embodiment of the invention, whereby only a single mode control element is used in the resonator. This element defines the mode selectively excited in one polarization direction. The effect of the gain depletion is that other modes of the same polarization cannot freely exist in the resonator, because of the overlap and interference of the fields with the first mode selected by the mode control element. On the other hand, a second mode with orthogonal polarization can exist, on condition that the structure of this second mode is such that its regions of high intensity do not fall on the regions of the gain medium which have already been utilized by the regions of high intensity of the first mode. In this manner, the second orthogonal mode is naturally selected, without the necessity of a second mode control element, by the geometry of the resonator itself and by the geometry of the regions of the gain medium not utilized by the first mode.

According to further preferred embodiments of the invention, each of the mode control elements in the two paths can be such as to allow the existence of a set of modes rather than a single mode, each of the modes of the set having the same polarization. Thus, according to further preferred embodiments of the invention, there can respectively exist in the two paths of the resonator either two single modes of orthogonal polarization, or a single mode and a set of modes of orthogonal polarization, or two sets of modes, each set having polarization orthogonal to the other set. Throughout this specification, and in the claims, the use of the term "mode" and the term "set of modes", are used alternatively and equivalently, unless specifically indicated otherwise. If the mode control elements are such as to ensure that each beam essentially has only a single mode, the resultant output has a well-defined and controlled phase, low divergence and high brightness, as well as resulting from a high level of utilization of the gain medium.

The resultant output beam is generally extracted from the resonator by means of an output coupler, which may or may not be placed in the common path. The output beam has two orthogonal well-defined polarizations. In order to focus the output beam with conventional non-polarizing optics to a small focal spot for various applications, another phase element is preferably introduced outside the resonator to adjust the phase of the beam such that all the lobes of the beam have the same phase sign.

According to another preferred embodiment of the present invention, two output couplers can be provided, one in each of the resonator paths. By this means, two output beams with orthogonal polarizations are obtained from the resonator, each beam being associated with a different mode or set of modes.

According to yet another preferred embodiment of the present invention, both the polarizing element and the reflector elements are unified onto the same element. This unified element introduces a different phase shift for different polarizations thus controlling the modes independently in the two polarizations. In this preferred embodiment, the two paths of the resonator, one for each polarization, are degenerated into one.

Thus, in accordance with the above preferred embodiments, a stable combination of low order modes can be achieved in a large-aperture, short resonator possessing high brightness with high efficiency power extraction from the Iasing medium.

In accordance with yet another preferred embodiment of the present invention, there is provided an optical resonator consisting of reflectors, one or more polarizing elements, and one or more optical mode control elements, operative such that there exist in the resonator simultaneously, first and second sets of modes with orthogonal polarizations, each of the set of modes having one or more regions of higher intensity and one or more regions of lower intensity, and wherein there is essentially spatial coincidence of the one or more regions of higher intensity of the first set of modes and the one or more regions of lower intensity of the second set of modes.

There is further provided in accordance with yet another preferred embodiment of the present invention, an optical resonator as described above, and wherein one or more of the first and second sets of modes consists of a single mode, which could be the TEM0o mode of the resonator.

In accordance with still another preferred embodiment of the present invention, there is provided an optical resonator as described above, and wherein each of the one or more optical mode control elements is operative to attenuate all but one predetermined set of modes.

There is further provided in accordance with still another preferred embodiment of the present invention, an optical resonator as described above, and also consisting of a first and a second path, and wherein the polarizing element is operative such that radiation of the first set of modes with a first polarization passes along the first path of the resonator, and radiation of the second set of modes with a second polarization orthogonal to the first polarization, passes along the second path of the resonator, at least one of the paths consisting of one of the optical mode control elements, such that each of the paths supports a different one of the first and second sets of modes.

In accordance with further preferred embodiments of the present invention, there is also provided an optical resonator as described above, and wherein the resonator is an active or a passive resonator.

There is provided in accordance with yet a further preferred embodiment of the present invention, an optical resonator as described above, and also consisting of a gain medium located in a common path, through which passes the radiation of the first set of modes with a first polarization and the radiation of the second set of modes with a second polarization.

There is even further provided in accordance with a preferred embodiment of the present invention, an optical resonator as described above, and also consisting of two partial reflectors arranged such that two output beams are obtained from the cavity.

Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided an optical resonator as described above and wherein one of the polarizing elements and one of the mode control elements are unified.

There is also provided in accordance with further preferred embodiments of the present invention, an optical resonator as described above, and wherein the first and the second paths are at an angle to each other or parallel to each other.

In accordance with yet more preferred embodiments of the present invention, there is provided an optical resonator as described above, and wherein the polarizing element is a beam splitter, a birefringent crystal, or a thin film polarizer.

There is further provided in accordance with yet other preferred embodiments of the present invention, an optical resonator as described above, and wherein one or more of the mode control elements is a discontinuous or continuous phase element, a spatial amplitude modulation element, or is a reflective, diffractive or transmissive element.

In accordance with still another preferred embodiment of the present invention, there is provided an optical resonator as described above, and wherein the polarizing element has a subwavelength pattern.

There is further provided in accordance with still another preferred embodiment of the present invention, an optical resonator as described above and wherein the mode control elements are combined on the same physical optical element.

In accordance with further preferred embodiments of the present invention, there is also provided an optical resonator as described above, and being a ring optical resonator or an unstable optical resonator.

There is provided in accordance with yet a further preferred embodiment of the present invention, an optical resonator as described above, and wherein the reflectors have radii of curvature different from each other.

There is even further provided in accordance with preferred embodiments of the present invention, an optical resonator as described above, and wherein the polarizing element is made of calcite, YV04 or a-BBO.

Furthermore, in accordance with yet more preferred embodiments of the present invention, there is provided an optical resonator as described above and wherein the first set of modes with a first polarization is a TEM0o mode and the second set of modes with a second polarization is a TEM02 mode, or a TEM0i* mode, or a TEM04 mode, or a super-Gaussian mode.

There is also provided in accordance with a further preferred embodiment of the present invention, an optical resonator as described above and wherein both sets of modes are TEM0i modes.

In accordance with yet another preferred embodiment of the present invention, there is provided an optical resonator as described above and wherein the first set of modes with the first polarization consists of TEM0o and TEM04 modes, and the second set of modes with the second polarization is a TEM04 mode.

There is further provided in accordance with yet another preferred embodiment of the present invention a laser consisting of an optical resonator consisting of a gain medium, the resonator supporting a first and a second set of orthogonally polarized modes, each of the set of modes having one or more regions of higher intensity and one or more regions of lower intensity.

In accordance with still another preferred embodiment of the present invention, there is provided a laser as described above, and wherein one or more of the first and second sets of modes consists of a single mode.

There is further provided in accordance with still another preferred embodiment of the present invention a laser as described above, and wherein the first and the second set of modes are orthogonally polarized such that the gain medium is utilized by both of the sets of modes simultaneously.

In accordance with a further preferred embodiment of the present invention, there is also provided a laser as described above, and wherein the first and second sets of modes are arranged such that the one or more regions of higher intensity of the first set of modes and the one or more regions of lower intensity of the second set of modes are essentially spatially coincident, such that the gain medium is utilized by both of the sets of modes simultaneously.

There is provided in accordance with yet a further preferred embodiment of the present invention a method of simultaneously increasing the gain volume utilization of a laser consisting of a resonator with a gain medium, while at the same time maintaining the brightness typical of low order modes, consisting of the steps of providing one or more polarizing elements, and providing one or more optical mode control elements, such that the resonator supports simultaneously first and second sets of modes, each of the sets of modes having one or more regions of higher intensity and one or more regions of lower intensity.

There is even further provided in accordance with a preferred embodiment of the present invention, a method of simultaneously increasing the gain volume utilization of a laser while maintaining the brightness typical of low order modes, as described above, and also consisting of the step of arranging the one or more optical mode control elements such that there is essentially spatial coincidence of the one or more regions of higher intensity of the first set of modes and the one or more regions of lower intensity of the second set of modes, so that the volume of the gain medium is effectively utilized by both of the sets of modes simultaneously.

Furthermore, in accordance with yet another preferred embodiment of the present invention, there is provided a method of simultaneously increasing the gain volume utilization of a laser while maintaining the brightness typical of low order modes, as described above, and also consisting of the step of arranging the polarizing element such that the first set of modes and the second set of modes have orthogonal polarizations, so that the volume of the gain medium is effectively utilized by both of the sets of modes simultaneously.

There is also provided in accordance with a further preferred embodiment of the present invention, a method of simultaneously increasing the gain volume utilization of a laser while maintaining the brightness typical of low order modes, as described above, and wherein one or more of the first and second sets of modes consists of a single mode.

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: Fig. 1 is a schematic illustration of a linear resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention, with a thin film polarizing beam splitter; Fig. 2 shows a birefringent prism polarizing beam splitter for use in the resonator of Fig. 1, instead of the thin film polarizing beam splitter; Fig 3 is a schematic illustration of a resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention with a birefringent lateral beam displacer, separate mode control elements and separate reflectors; Fig 4 is a schematic illustration of another preferred embodiment of the beam displacer, mode control elements and reflectors used in the resonator of Fig. 3 and wherein the mode control elements and reflectors are each implemented on the same physical component; Fig 5 is a schematic illustration of a preferred embodiment of a combined beam-controlling element shown in Fig. 4; Figs 6A to 6E are schematic illustrations of mode controlling elements according to different preferred embodiments of the present invention; Fig. 6A is an aperture designed to select the TEM0o mode; Fig. 6B is a wire cross, to select the TEM02 mode; Fig. 6C is a discontinuous phase element designed to select the TEM02 mode; Fig. 6D is a continuous phase element designed to select the TEM01* mode; and Fig. 6E is an absorptive apodizer designed to select a super-Gaussian mode; Fig. 7 is a schematic illustration of a pair of mode selective phase elements and their relative orientation, designed and operative to select a combination of TEM01 modes; Fig 8 is a schematic view of the near-field intensity distribution and relative polarizations of the modes of the resonator of Fig. 1 or Fig. 3 using the mode selective elements of Fig.7; Fig 9 is a schematic illustration of an aperture and a mode selective phase element according to a preferred embodiment of the present invention, designed and operative to select the TEM0o and TEM02 modes respectively; Fig 10 is a schematic view of the near-field intensity distribution and relative polarizations of the modes of the resonator of Fig. 1 or Fig. 3 using the mode selective elements of Fig.9; Fig 1 1 is a schematic illustration of an aperture and a mode selective continuous phase element with a spiral phase distribution ranging from 0 to 2π, according to a preferred embodiment of the present invention, designed and operative to select the TEM0o and TEM0i* modes respectively; Fig 12 is a schematic view of the near-field intensity distribution and relative polarizations of the modes of the resonator of Fig. 1 or Fig. 3 using the mode selective elements of Fig. 1 1; Fig 13 is a schematic illustration of an aperture and a mode selective phase element according to a preferred embodiment of the present invention, designed and operative to select the TEM0o and TEM04 modes respectively; Fig 14 is a schematic view of the near-field intensity distribution and relative polarizations of the modes of the resonator of Fig. lor Fig. 3 using the mode selective elements of Fig. 13; Fig 15 is a schematic illustration of a resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention, with a unified polarizing and mode control element introduced close to the full reflector; Fig 16 is a schematic illustration of a unified polarizing and mode control phase element according to a preferred embodiment of the present invention, constructed and operative to select a TEM0o mode and a TEM04 mode with orthogonal polarizations; Fig. 17 is a schematic illustration of a unified polarizing and mode control phase element, for selecting a combination of two orthogonally polarized TEMoi modes; Fig 18 is a schematic illustration of a resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention, with a unified polarizing and mode control element introduced close to the output coupling reflector; Fig. 19 is a schematic illustration of a laser resonator constructed and operative in accordance with another preferred embodiment of the present invention with a polarizing beam splitter, and having two orthogonally polarized output beams, each emerging from an output coupling reflector; Fig. 20 is a schematic illustration of a laser resonator constructed and operative in accordance with another preferred embodiment of the present invention incorporating a polarizing birefringent beam displacer, and having two orthogonally polarized output beams, each emerging from an output coupling reflector; and Fig. 21 is a schematic illustration of a linear resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention, similar to that shown in Fig. 1, but using only a single mode control element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 1, which is schematic illustration of a linear resonator of a laser constructed and operative in accordance with a preferred embodiment of the present invention. The linear resonator preferably consists of reflectors, preferably full reflectors 20 and 21 and a partial reflector or an output coupler 22, a gain medium 24, a thin film polarizing beam splitter 26 and two mode control elements 28 and 29. The beam splitter 26 reflects one polarization, hereinafter called polarization "1", through control element 29 towards full reflector 21, and transmits the polarization orthogonal to polarization "1", hereafter called polarization "2". Two modes of oscillation, indicated by reference numbers 30 and 31 are thus established between the output coupler 22 and the reflectors 20 and 21 respectively. Radiation associated with both modes 30 and 31 propagates through the gain medium 24, as beam 32. However, the radiation associated with mode 30 propagates through the mode controlling element 28 while that of mode 31 propagates through the mode controlling element 29.

In accordance with a preferred embodiment of the invention, the laser resonator of Fig. 1 is characterized in that the elements 28 and 29 are operative to select modes 30 and 31 respectively such that they have different intensity and phase distributions, and element 26 is operative to provide that mode 30 has polarization "2" while mode 31 has polarization "1". Elements 28 and 29 are designed and oriented in such a way that the high intensity regions or lobes of mode 30 in the gain medium 24 fall on the nodes or low intensity regions of mode 31, and vice versa. Since modes 30 and 31 interact in the gain medium 24 only through the gain depletion which each one introduces, they can co-exist stably in the resonators. Other undesirable modes are suppressed by the mode controlling elements, thereby improving the quality of the output beam 33.

There are a number of alternative preferred optical configurations of such linear resonators. The output coupler 22 and full reflectors, 20, 21, may have surfaces of different radii of curvature, whether concave or convex, or may even be flat. The amount of curvature can be designed so as to compensate for the thermal lensing of the gain medium, particularly in solid state lasers. Furthermore, any of the end reflectors may be Porro prisms, phase conjugate mirrors, or any other type of appropriate reflector. In the preferred embodiment of Fig. 1, reflectors 20 and 21 need not be identical. Furthermore, provided the resonators formed by the elements 22, 26, 29, 21 and 22, 26, 28, 20 are designed in such a way that each correctly supports the respective modes 31 and 30 independently, with compensation for the thermal lensing of gain medium 24, the distances between the polarizing beam splitter 26 and the reflectors 20, 21, need not be identical. In fact, since each of the modes 30 and 31 generally occupies a separate portion of the gain medium 24, because of the different influence of the thermal distribution in the gain medium 24 on the distortion of the two modes, optimum performance is generally reached when the two resonators are different in design.

Fig. 2 is a schematic illustration of another preferred alternative to the thin film polarizing beam splitter 26 shown in Fig. 1, and is constructed, of a birefringent prism 27, such as a "Glan-Thompson" or "Glan-Laser" type of prism.

Reference is now made to Fig. 3, which is a schematic illustration of another preferred embodiment of the present invention. The beam splitter 26 of the embodiment of Fig. 1 is replaced by a polarizing beam displacer 34. The beam displacer 34 transmits one polarization, polarization "1", without any displacement, and transmits the polarization orthogonal to polarization "1", polarization "2", laterally displaced by a distance 35 from its line of propagation. The beam displacer is constructed of a birefringent material, preferably calcite, YVO4 or a-BBO.

Two modes of oscillation, indicated by reference numbers 30 and 31 are established between the output coupler 22 and the reflectors 20 and 21 respectively. The energy of both modes 30 and 31 propagate through the gain medium 24. In a similar manner to the configuration of Fig. 1, the energy of mode 30 propagates through the mode-controlling element 28, while that of mode 31 propagates through the mode-controlling element 29.

In accordance with this preferred embodiment of the invention, the laser resonator of Fig. 3 is characterized and operative in the same way as the embodiment of Fig. 1, except that the resonator comprised of the elements 22, 34, 29, 21 is not folded. The resonator section 34, 29, 21 is displaced laterally with respect to the section 34, 28, 20 instead of being at an angle to it. This alternative preferred embodiment is more compact than that of Fig. 1, and also has the advantages that the elements 28 and 29, and the reflectors 20 and 21 can be respectively combined, each pair on the same physical piece of optics.

Such an embodiment, with the two mode control elements on a single component, and the two end reflectors on another single component is shown in Fig. 4. The mode control elements, such as pattern 38, and aperture 39 are constructed on the same physical element 36, and also the reflectors 20, 21, on another single physical element 37. In the embodiment shown in Fig. 4., the reflectors 20 and 21 are preferably flat, but the element 37 could also be preferably constructed using a technique such as diamond turning, whereby the reflectors 20 and 21 could be given any suitable radius of curvature.

Fig. 5 is an example of a combined mode control element constructed on a single piece of optics 36. Pattern 38 is an etched or deposited phase pattern designed and operative to select the mode TEM02, while the aperture 39 is designed and operative to select the TEMoo mode. The centers of the pattern 38 and the aperture 39 are displaced by the distance 35, which is the exact displacement created by the polarizing beam displacer element 34.

Reference is now made to Figs. 6A to 6E, which present schematic illustrations of different preferred mode controlling elements.

Fig. 6A illustrates an element 43 having an aperture 41 designed to select the TEMoo mode. The aperture introduces loss to all modes higher than the TEMoo mode. It can be drilled or etched into a substrate. Generally, when such an aperture is introduced as the sole mode-limiting element into a high power laser resonator, the aperture tends to suffer damage at its edges. It is therefore preferably made of a high damage resistant materials such as Molybdenum or ceramic materials. However, when such an aperture is introduced in conjunction with other mode selectors, into the resonators of the present invention, such as those illustrated in Fig. 1 or Fig. 3, the other mode selectors too are operative in confining the mode TEM0o thus effectively preventing or significantly reducing damage to the aperture.

Fig. 6B illustrates an element 53 in the form of a cross made of thin wires designed and operative to select the TEM02 mode and higher modes. It introduces losses in the resonator to the TEM0o and the TEM0i modes, thus preventing their oscillation. In high power lasers, however, the wires are worn out by damage caused by the laser radiation.

Fig. 6C illustrates an element 47 in the form of a discontinuous phase element designed and operative to select the TEM02 mode. Since this element does not introduce amplitude loss, and its phase pattern matches that of the TEM02 mode, it prevents lasing of both higher order and lower order modes. The element can preferably be etched or deposited on any transparent optical material such as fused silica, glass, zinc selenide, or the like.

Fig. 6D illustrates an element 55 in the form of a spiral continuous phase element designed and operative to select the TEM01* mode.

Fig. 6E illustrates an element 51 in the form of an absorptive apodizer designed and operative to select a super-Gaussian mode Fig. 7 is a schematic illustration of an example of an element consisting of a combination of two discontinuous phase elements 40, 42, oriented in different directions. Each element is designed and operative to select a TEM0i mode. When introduced into a resonator according to the present invention, such as that shown in Fig. 1 or Fig. 3, mutually rotated at 90 degrees to each other, two TEM01 modes with orthogonal polarizations exist in the resonator.

Fig. 8 is a schematic illustration of the near-field intensity distribution of the combination of modes of the resonator of Fig. 1 or Fig. 3, resulting from the use of the discontinuous phase elements 40, 42, shown in Fig. 7. The arrows represent the polarizations of the high intensity regions of the modes. Regions 44 and 45 arise from the mode existing in the path containing mode control element 42, while regions 48 and 49 arise from the mode existing in the path with mode control element 40.

Fig. 9 is schematic illustration of a further preferred embodiment of a pair of mode control elements, consisting of an aperture 50 made in an element 50' that selects the TEM0o mode and a discontinuous phase element 52 that selects the TEM02 mode. When introduced into a resonator such as that shown in Fig. 1 or Fig. 3, a combination of modes with orthogonal polarizations exists in the resonator.

Fig. 10 is a schematic illustration of the near-field intensity distribution of such a combination of modes. The arrows represent the polarizations of the high intensity regions. The TEMoo mode fills the central zone 54 of the gain medium while the high intensity regions 56 of the TEM02 mode fill the outer zone, thus achieving good filling of the entire diameter of the gain medium 24.

According to yet another preferred embodiment of the present invention, modification of the mode control element, by predetermined changes in the phase shift of the sections of the phase element of Fig. 9 that selects the TEM02 mode, can be used for compensation of the birefringence introduced in high power solid state lasers, as described in the prior art.

Fig. 11 is schematic illustration of yet another preferred embodiment of the present invention showing a combination of an aperture 50 made in an element 50' that selects the TEM0o mode, and a continuous phase element 60 of spiral phase distribution that selects the TEM0i* mode. When introduced into a resonator such as that shown in Fig. 1 or Fig. 3, a combination of modes with orthogonal polarizations exist in the resonator.

Fig. 12 is a schematic illustration of the near-field intensity distribution pattern of such a combination of modes using the mode control elements shown in Fig. 11. The arrows represent the polarizations of the lobes. The central part 62 arises from the TEM0o mode, while the outer ring arises from the TEM0i* mode. Since both modes have rotational symmetry they complement each other efficiently and extract a high level of power from the gain medium volume.

Fig. 13 is schematic illustration of yet another preferred embodiment of the present invention, showing a combination of elements consisting of an aperture 50 that selects the TEMoo mode and a discontinuous phase element 66 that selects the TEMo mode. When introduced into a resonator such as those of Fig. 1 or Fig. 3, a combination of modes with orthogonal polarizations exist in the resonator.

Fig. 14 is a schematic illustration of the near-field intensity distribution lobes of such a combination of modes resulting from the use of the combination of mode control elements of Fig. 13. The arrows represent the polarizations of the high intensity regions. The central part 68 arises from the TEM0o mode, while the outer parts 70 arise from the TEM04 mode. Since the lobes 70 of the TEM04 modes are smaller than the high intensity regions 56 of the TEM02 mode shown in Fig. 10, the combination of the TEM04 mode with the TEM0o mode is more efficient in filling the entire cross-section of the gain medium.

Reference is now made to Fig. 15 which is a schematic illustration of a linear resonator of a laser constructed and operative in accordance with yet another preferred embodiment of the present invention. The resonator consists of a gain medium 24, a full reflective element 81, an output coupling reflective element 22 and a unified polarizing and mode control element 80. Inside the resonator a beam 32 travels back and forth between the reflectors 22 and 81. The beam consists of two sets of modes, each set at a different polarization, and the resonator is designed so that the high intensity regions of one set fall generally on the low intensity regions of the other set of modes.

Fig 16 is a schematic illustration of a preferred embodiment of a unified polarizing and mode control element 80, constructed and operative to select a combination of the mutual orthogonal polarized modes TEM0o and TEM04-Element 80 has an etched or deposited pattern on its face. It acts as a discontinuous phase element, in which the zones 84 create a phase shift of π with respect to the zones 82. The element with this specific phase shift pattern presents a low loss to the TEM04 mode thus preferentially selecting it to oscillate in the resonator. The central disk 83 has low loss in one polarization, and the zones 82 and 84 have low loss in the orthogonal polarization. Thus, when the element 80 is introduced into a laser resonator with the gain medium 24, the TEMoo mode which is selected by the zone 83 of the disk possesses one polarization, while the mode TEMo4 which is selected by the zones 82 and 84 has the orthogonal polarization. In one preferred embodiment the zones are constructed and operative to have a polarization dependent loss by etching or deposition of a diffractive grating having subwavelength period, as is known in the art. The element 80 can preferably be made of any material such as fused silica, glass, zinc selenide, or any other suitable material used for transmissive or reflective optical components.

Fig. 17 is a schematic illustration of yet another preferred embodiment of the present invention, showing a combined polarizing and mode control element 90 constructed and operative to select a combination of two orthogonally polarized TEMoi modes. Element 90 has an etched or deposited pattern on its surface. This pattern acts as a discontinuous phase element, in which zone 97 creates for both polarizations, a phase shift of π with respect to the zone 95. The patterns of zones 96 and 98 introduce different phase shifts to the two different orthogonal polarizations, zone 96 introducing a phase shift of π with respect to zone 95 for the Ύ polarization and no phase shift for the '2' polarization, and zone 98 introducing a phase shift of π with respect to the '2' polarization and no phase shift for the Ύ polarization. This is preferably achieved with the aid of a diffractive grating of subwavelength period, as described for the element shown in Fig. 16. Thus, the single element 90 acts in a similar manner to the two elements described in Fig. 5, allowing two TEMQI modes with orthogonal polarizations to co-exist when inserted into a laser resonator according to the present invention, preferably of the type shown in Fig. 15.

Fig 18 is a schematic illustration of yet another preferred embodiment of a laser resonator according to the present invention. The resonator consists of the same elements as the resonator of Fig 15, but the unified polarization and mode control element 80 is introduced close to the output coupling reflecting element 22. In this configuration, the lobes of the output beam 33 are generally in phase. As a result, no additional adjusting phase element is needed outside the resonator to properly focus the beam 33 to a small spot for various applications.

Fig. 19 is a schematic illustration of a laser resonator according to yet another preferred embodiment of the present invention. The resonator is similar to the embodiments of Fig. 1 and Fig. 2, but is constructed such that two laser output beams, each of a different polarization, emerge separately. The polarizing element 106 can preferably be either a thin film beam splitter 26 or a prism beam splitter 27. A total reflector 102 replaces the partial reflector (output coupler) 22 of Fig. 1, and two partial reflectors (output couplers) 100 and 101 replace the total reflectors 20 and 21 respectively of Fig. 1. In this embodiment, therefore, two output beams 103 and 104 are obtained. One output beam 103, possesses mode 30, and emerges through output coupler 100, and the other output beam 104 of mode 31, emerges through the output coupler 101. These two beams can then be combined into a single beam using an additional external optical system that includes a polarizing beam splitter (combiner) element such as 26 or 27.

Fig. 20 is a schematic illustration of a laser resonator according to yet another preferred embodiment of the present invention. The resonator is similar to the embodiment of Fig. 3, but two laser output beams, each of a different polarization, emerge separately. A total reflector 102 replaces the partial reflector (output coupler) 22 of Fig. 3, and two partial reflectors 100 and 101 replace the total reflectors 20 and 21 respectively of Fig. 3. Like the embodiment shown in Fig. 19, two output beams 103 and 104 exist. One output beam 103, has mode 30, and emerges through output coupler 100, and the other output beam 104 has mode 31, and emerges through the output coupler 101. These two beams can then be recombined into a single beam using an additional external beam displacer (combiner) element 34.

Reference is now made to Fig. 21, which is a schematic illustration of a laser resonator according to yet another preferred embodiment of the present invention. The resonator is similar to the embodiments of Fig. 1 , except that only one mode control element 29 is used in one arm of the resonator. This mode control element is operative to select one mode or set of modes of one polarization, while a second mode or set of modes of orthogonal polarization is preferentially selected by means of gain depletion of this first mode in the gain medium 24.

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 hereabove. 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. 23 131906/2

Claims (34)

CLAIMS We claim:

1. An optical resonator comprising: there exist in said resonator simultaneously, at least first and second sets of modes with orthogonal polarizations, each of said set of modes having at least one region of higher intensity and at least one region of lower intensity.

2. An optical resonator according to claim 1, and wherein said first set of modes and said second set of modes are aligned such that there is essentially spatial coincidence of said at least one region of higher intensity of said first set of modes and said at least one region of lower intensity of said second set of modes.

3. An optical resonator according to either of claims 1 and 2, and wherein at least one of said first and second sets of modes comprises a single mode.

4. An optical resonator according to claim 3, and wherein said single mode is essentially the TEM0o mode of said resonator.

5. An optical resonator according to any of claims 1 to 4, and wherein each of said at least two optical mode control elements is operative to attenuate essentially all but one predetermined set of modes.

6. An optical resonator according to any of claims 1 to 4, and also comprising a first and a second path, and wherein said polarizing element directs radiation of said first set of modes along said first path of said resonator, and radiation of said second set of modes along said second path of said resonator, 24 131906/2 each of said at least two optical mode control elements being disposed in one of said paths, such that each of said paths supports a different one of said first and second sets of modes.

7. An optical resonator according to any of the previous claims and wherein said resonator is a passive resonator.

8. An optical resonator according to any of the previous claims and wherein said resonator is an active resonator.

9. An optical resonator according to claim 8, and also comprising a common path, through which radiation of said first set of modes and of said second set of modes passes, and a gain medium located in said common path.

10. An optical resonator according to claim 9, and also comprising two partial reflectors arranged such that two output beams are obtained from said resonator .

11. An optical resonator according to either of claim 1 and claim 2, and wherein one of said at least one polarizing element and one of said at least two mode control elements are constructed on a single element.

12. An optical resonator according to claim 6, and wherein said first and said second paths are at an angle to each other.

13. An optical resonator according to claim 6, and wherein said first and said second paths are parallel to each other.

14. An optical resonator according to any of the previous claims, and wherein said polarizing element is selected from a group consisting of a beam splitter, a birefringent crystal, and a thin film polarizer. 25 131906/2

15. An optical resonator according to any of the previous claims, and wherein at least one of said at least two mode control elements is selected from a group consisting of a discontinuous phase element, a continuous phase element, a spatial amplitude modulation element, a reflective element, a transmissive element and a diffractive element.

16. An optical resonator according to any of the previous claims, and wherein said polarizing element has a subwavelength pattern

17. An optical resonator according to any of the previous claims, and wherein said mode control elements are disposed on the same physical optical element.

18. An optical resonator according to any of the previous claims, and being a ring optical resonator.

19. An optical resonator according to any of the previous claims, and being an unstable optical resonator.

20. An optical resonator according to any of the previous claims, and wherein said reflectors have radii of curvature different from each other.

21. An optical resonator according to any of the previous claims, and wherein said polarizing element is made of a material selected from a group consisting of calcite, YV04 and a -BBO.

22. An optical resonator according to any of the previous claims, and wherein said first set of modes comprises a TEMoo mode and said second set of modes comprises a mode selected from the group consisting of a TEM02 mode, a TEMQI* mode, a TEM04 mode and a super-Gaussian mode. 26 131906/2

23. An optical resonator according to any of the previous claims and wherein both sets of modes comprise TEMoi modes.

24. An optical resonator according to any of the previous claims, and wherein said first set of modes comprises TEMoo and TEMo4 modes, and said second set of modes comprises a TEM04 mode.

25. A laser comprising an optical resonator having a gain medium, said resonator comprising at least two optical mode control elements, such that said resonator simultaneously supports a first and a second set of modes, each of said set of modes having at least one region of higher intensity and at least one region of lower intensity, and wherein said modes are orthogonally polarized such that said gain medium is utilized by both of said sets of modes simultaneously.

26. A laser according to claim 25, and wherein at least one of said first and second sets of modes comprises a single mode.

27. A laser according to claim 25, and wherein said first and second sets of modes are arranged such that at least one region of higher intensity of said first set of modes and at least one region of lower intensity of said second set of modes are essentially spatially coincident.

28. A method of increasing the volume utilization of the gain medium of a laser, comprising the steps of: providing a laser comprising a resonator with said gain medium; providing at least one polarizing element; providing at least two optical mode control elements; and disposing said at least one polarizing element and said at least two mode control elements within said resonator, such that said resonator supports simultaneously first and second sets of modes with orthogonal polarization, each 27 131906/2 of said sets of modes having at least one region of higher intensity and at least one region of lower intensity.

29. A method of increasing the volume utilization of the gain medium of a laser according to claim 28, and wherein said step of providing at least two optical mode control elements comprises the step of providing two optical mode control elements.

30. A method of increasing the volume utilization of the gain medium of a laser according to claim 28, and also comprising the step of arranging said at least two optical mode control elements such that there is essentially spatial coincidence of at least one region of higher intensity of said first set of modes and at least one region of lower intensity of said second set of modes, so that the volume of said gain medium is effectively utilized by both of said sets of modes simultaneously.

31. A method of increasing the volume utilization of the gain medium of a laser according to any of claims 28 to 30, and wherein at least one of said first and second sets of modes are adapted to comprise a single mode.

32. An optical resonator comprising: reflectors; at least one polarizing element; a gain medium; and an optical mode control element selectively attenuating all but a first set of modes propagating in said resonator, said first set of modes having at least one region of higher intensity and at least one region of lower intensity; wherein gain depletion in said gain medium, from said at least one region of higher intensity of said first set of modes, is such that at least a second set of modes with polarization orthogonal to said first set of modes, and having at least 28 131906/2 one region of higher intensity and at least one region of lower intensity, also propagates in said resonator.

33. An optical resonator according to claim 32, and wherein said gain depletion is such that said at least one region of higher intensity of said second set of modes falls in a region of said gain medium having at least one region of lower intensity of said first set of modes.

34. An optical resonator according to claim 32, and wherein at least one of said first and second sets of modes comprises a single mode. For the applicant: Sanford T. Colb & Co. Advocates and Patent Attorneys C: 35093