UNSTABLE OPTICAL RESONATOR AND LASER

Abstract: An optical resonator comprising a substantially annular reflector (22) disposed about and generally facing a central axis and comprising at least one annular reflecting surface (22′) a central rotationally symmetric reflector (23) positioned along said central axis and comprising at least one central reflecting surface (23′), and a rotationally symmetric power extraction cavity (55) located between the annular reflector (22) and the central reflector (23).

Description: The present invention relates to unstable optical resonators and to lasers. All known lasers comprise the following three fundamental elements: a lasing medium which provides atoms, ions, or molecules that support light amplification, an energy source to excite the medium, and an optical resonator to provide feedback of the amplified light. One of the most common lasing media in current use in lasers is gas. Solid state lasers are also abundant and employed in industrial application. A common source for exciting the lasing medium is an electrical discharge, though many other means for excitation are also available. The optical resonators are of various shapes and constructions, as outlined in many publications such as those by M. W. Sasnett in "Comparing Industrial CO₂ Lasers" in Lasers & Applications, September 1984, pages 85-90 or by W. G. Burnell in "Review of CW High-Power Laser Technology," United Aircraft Research Laboratories - East Hartford, Connecticut, October 1973, UAR-M132. The optical resonators are constructed so as to provide for a high lasing volume and a high lasing mass. Constructions most common in gas lasers are of the "coaxial" type having a long and narrow shape such as a tube having two mirrors located at both ends, two electrodes located between the mirrors, and the gas being introduced into the tube so that it flows in the direction of the laser beam produced in the resonator. Such a construction has several disadvantages. The long distance between the mirrors makes it difficult to maintain an accurate permanent relative position between the mirrors, as is essential for the accurate operation of the laser. A further disadvantage derives from the large distance between the electrodes. Because the excitation voltage is proportional to distance and pressure, a laser operating with high lasing mass must use a very high excitation voltage, thereby causing many safety and technological problems. Furthermore, the high friction rate of the gas flowing along the walls of the tube increases its temperature and its staying time in the tube, thereby decreasing its lasing capability and requiring the application of a high power gas pump. In order to overcome these problems and to provide for more efficient lasers, lasers with other geometric constructions have been developed. One such construction is the so called "cross flow" construction wherein the resonator is defined by two mirrors of high surface area. The electrodes are coplanar with the mirrors, and the gas flows into the resonator perpendicularly to the direction of the laser beam. Such a construction allows for a significant shortening of the time of stay of the gas in the resonator. However, because the gas is not heated homogenously along the resonator the electrical discharge will be higher at those zones where the electrical resistance is lower and the gas temperature is higher i.e., the lower lasing zones. Another known construction is the "cross beam" type, wherein the electrical discharge and the gas flow are in the same direction and the mirrors are perpendicular thereto. In such constructions the non-homogenity in the temperature of the gas does not interfere with the electrical discharge. However, since the beam advances along non-homogenous thermal zones, excitation is predominant at the warmer zones, the least efficient zones in respect of lasing. An additional disadvantage of both the "cross beam" and the "cross flow" constructions is that only a very small part of the gas volume in the resonator is utilized for the production of the beam. In all known constructions, as described above, the excitation of the lasing medium is not symmetric and therefore a non-symmetric laser beam is produced. Furthermore, the high temperature along the resonator causes non-uniformity in the beam output, thereby imparing the symmetry of the beam's cross section and mode. The application of conical mirrors in optical resonators is known, as in U.S. Patent No. 4,164,366 which discloses a resonator formed with optically connected cavities comprising a power extraction cavity and a mode control cavity, means for coupling the two cavities, and a conical reflective surface. The known resonator disclosed therein is a non-symmetric complex system having many components, and thus the assembly and alignment of the resonator is complicated. European Published Patent Application No. 0100089 discloses a laser having a resonator comprising, interalia, substantially conical reflective surfaces for the emission of the laser beam. However, the known resonator is of a long and a non-symmetric structure, having a plurality of reflectors, thereby infering a complexity in structure and operation. U.S. Patent 4,025,172 describes a compound unstable resonator comprising a pair of axially disposed rotationally symmetric mirrors and a centrally disposed conical folding mirror. The power extraction cavity is defined to have a generally cylindrical configuration and to lie intermediate the pair of rotationally symmetric mirrors and the folding mirrors. German Offenlegungsschrift 24 45 597 also describes an unstable resonator. In this configuration, the power extraction cavity is not rotationally symmetric. Similarly, U.S. Patent 4,164,366 shows a resonator having a rotationally symmetric fold mirror and wherein the power extraction cavities are not rotationally symmetric. U.S. Patent No. 3,950,712 discloses an unstable optical resonator comprising: a substantially annular reflector disposed about and generally facing a central axis and comprising an annular reflecting surface; a central rotationally symmetric reflector positioned symmetrically along said central axis and comprising a conical central reflecting surface; and a rotationally symmetric power extraction cavity located between the annular reflector and the central reflector, the central reflector being arranged to reflect a portion of the light in the power extraction cavity out of the power extraction cavity as laser output. The unstable resonator cavity of which the power extraction cavity forms a part has an axial portion located between the central reflector and an end reflector spaced along the central axis. According to the present invention an unstable resonator as specified in the preamble to claim 1, is characterized in that the ends of the unstable resonator are defined by the annular reflector and the central reflector. There is defined a locus of optical axes extending between the annular reflector and the central reflector generally centrally of the power extraction cavity. The optical axes have a radial and, preferably, an axial component. For convenience, this locus of optical axes is hereinafter termed "optical axis". The optical axis defines an angle α with respect to the central axis. It should be understood that, because of the rotationally symmetrical structure of the resonator of the invention, the optical axis comprises an infinite number of optical axes extending between the annular reflector and the central reflector. The optical axis forms a planar ring surface when angle α equals 90 degrees or a curved surface of a truncated cone when angle α is smaller than 90 degrees. Thus the optical axis mentioned in the specification refers actually to such surfaces. The "optical axis" represents the main direction of the beams between the annular reflector and the central reflector and it is normal to the center of an imaginary line connecting the edges of the annular reflector. The resonator may include means for connecting the annular reflector to the central reflector without interfering with optical beams passing therebetween. The rotationally symmetric central reflector is one of the components of the resonator that contributes to the emission of a rotationally symmetrical laser beam having a high power density and a small diameter, with mainly gaussian distribution. The application of conical reflectors for the emission of a laser beam from a resonator is known to be problematic due to partial absorption of the incident light causing massive heating of the cone. Absorption of light is lower when the incident ray hits the reflective surface at a small angle with respect thereto. Therefore, the conical surface of the present invention preferably is a narrow cone, i.e. a cone with a large height to base diameter ratio for minimal absorption of light and minimal heating of the central reflector. A narrow cone having large surface area can also be more effectively cooled than a broad short cone with small surface area. The applicant has found that a significant increase in the reflective area of the central rotationally symmetric reflector can be obtained if it is configured so that the narrow cone lies on top of a much wider truncated cone. It is preferable that at least one segment of the central reflector has a curved surface area. The optical axis is planar when angle α is equal to 90 degrees. However, it is preferable that angle α is different from 90 degrees in order to prevent an amplification of those rays travelling between the reflective surface of the annular reflector without hitting the central reflector. Such rays, if formed, would impair the intensity of the laser beam emitted from the resonator. Preferably the angle α is in the range of 85 to 60 degrees. The position of the central refelctor relative to the substantially annular reflector is chosen so that the "optical axis" lies between the center of the annular reflector and the central reflector and is determined by angle α. The present invention also includes the use of the above described unstable resonator in lasers (Claims 7 - 9) and a gas laser apparatus (claim 10) employing the above described resonator in accordance with the present invention. The laser apparatus may comprise a laser or a laser amplifier. Preferably, the laser apparatus is a gas laser such as a carbon dioxide laser, wherein a mixture of carbon dioxide, nitrogen and helium comprises the lasing medium. However, the laser can also use: other gases such as a mixture of helium and neon or argon or others; media produced by a chemical reaction in the so-called "chemical laser"; solid media such as Nd-YAG, ruby, glass and others; or liquid media. The excitation of the lasing medium can be carried out by any of the known methods, such as by electrical discharge (AC, DC or radio frequency), optical pumping, chemical reaction or any combination of those techniques. When the lasing medium is a solid material, the excitation is preferably carried out by optical pumping such as by a flash lamp. When the lasing medium is gas, the exitation is preferably carried out by means of electrical discharge. Preferably, the electrodes to be used with the resonator are made of metal and shaped in a compact form, such as a dense net, a perforated metal sheet, or a honeycomb, in order to achieve the highest possible rate of excitation without blocking the gas flow through the electrodes. Alternatively, the electrodes can be made of carbon fibers in a "brush like" configuration, achieving high electrical concentration per unit area due to the very low thickness of the fibers. The electrodes may be positioned in the laser in such a way that their active surfaces lie parallel to the optical axis at both sides of the beam so that they do not interfere with the optical paths of the rays. Alternatively, both electrodes can be located on the same side of the beam, upstream of the gas flow. As a further alternative, more than two electrodes may be employed in a configuration where two or more electrodes are located on one side of the beam and one or more electrodes are located on the other side of the beam. The present invention will now be described by way of non-limiting embodiments with reference to the accompanying drawings, in which: Figure 1 is a cross sectional view of a resonator in accordance with a preferred embodiment of the present invention; Figure 2 is a top view of the resonator of Figure 1; Figure 3 is a schematic cross section view of a gas laser according to a preferred embodiment of the present invention; Figure 4 is a cross sectional view of electrodes according to a preferred embodiment of the present invention, and shows their relative position in the resonator of Figure 1; Figure 5 is a top view of the electrodes of Figure 4; and Figure 6 is a schematic cross sectional view of a solid state laser resonator in accordance with a preferred embodiment of the present invention. In Figure 1, the resonator 1 is defined by a substantially annular concave mirror 9 having reflective surface 9', and a central mirror 11 located along the central axis 17 of the resonator. A hollow cylinder 32 serves as a base for mirror 9 and is attached by several ribs 33 to cylindrical structure 31 which serves as a holder for mirror 11. The cylinder 32 and ribs 33 ensure that mirrors 9 and 11 form a concentric structure. Optical axis 18 is normal to annular mirror 9 and forms an acute angle α with the central axis 17. Opposite mirror 9 there is situated a segment 13 of mirror 11. The reflective surface area 13' of segment 13 has an annular convex shape having the same "optical axis" 18. Segment 13 is positioned on segment 12 of central mirror 11. The reflective surface 12' of segment 12 has a curved surface of a truncated cone perpendicular to the optical axis 18. On top of segment 13 are situated cone segments 29 and 28. Segment 29 has the shape of a truncated cone, with reflective surface 29', and segment 28 has the shape of a full narrow cone, with reflective surface 28'. Also shown in Figure 1 are electrodes 2 and 2' to be used in laser apparatus containing the resonator 1. For the sake of clarity, the entire cross sectional view of the electrodes is not shown, but only the location of their schematic cross section. A part 55 of the gain medium excited by the electrodes 2 and 2' is indicated by reference numeral 55. Mirrors 9 and 13 are confocal, and therefore resonator 1 is a confocal unstable resonator. The resonator has a large geometric magnification to provide for mode discrimination against higher order radial and azimuthal modes such that the lowest loss mode, typically the lowest order fundamental mode, is strongly favored. The resonator is a coverging-diverging optical system capable of expanding and compressing electromagnetic radiation circulating therein by repeated reflections between mirrors 9 and 13. In operation, the radiation within the resonator is compressed towards "optical axis" 18 by multiple reflections between the concave and convex surfaces of the mirrors into a diffraction dominated portion concentrated around optical axis 18 of the resonator. Eventually, diffractive spreading causes the compressed inwardly propagating radiation to expand, which transforms the compressed radiation into outwardly propagation radiation, having a lowest loss mode, which fills the entire surface area of mirror 13. The radiation further expands to mirror 9 where it is reflected to mirror 29 and then to mirror 28 and leaves mirror 28 as a collimated beam of radiation having a circular cross section. Part of the beam reflects from mirror 9 to mirror 12 and repeats the same track until it also emerges from mirror 28 and out of the resonator. Amplification also takes place between mirror 9 and 11 because electrodes 2 and 2' supply energy for excitation of the lasing medium in the volume between the mirror and the electrodes. Figure 2 shows a top view of the resonator 1 of Figure 1. Ribs 33 attach central mirror 11 to cylinder 32 on which the concave mirror 9 with reflective surface 9' is situated. Figure 3 shows a schematic cross section of a gas laser incorporating the resonator of Figure 1, and two concentric electrodes 2 and 2' parallel to optical axis 18, all placed in enclosure 3. Enclosure 3 is of substantially cylindrical shape, its central axis being the central axis 17 of the resonator. At one end of enclosure 3, a window 6 is provided to allow the laser beam to emerge from the upper segment of mirror 11. A blower 4 is provided for the circulation of the laser gas into and out of the resonator. Blower 4 is placed along central axis 17. Several heat exchange units fill most of the rest of the volume of the enclosure. In the figure, only one annular heat exchange unit 5 is shown. The exact location of the heat exchange units is not important. Preferably, the heat exchange units are comprised of thin metal tubings (not shown in Figure 3) having high heat conduction; cooling liquid is circulated in the metal tubing to allow for cooling of the laser gas. The lasing gas in Figure 3 is circulated by blower 4 which causes the gas to pass through electrode 2', resonator 1, electrode 2, heat exchange unit 5 and back to blower 4. The gas flow is indicated by arrows 100. Figure 4 shows a cross sectional view of electrodes 2 and 2' when positioned in resonator 1. Electrodes 2 and 2' each have the shape of a curved surface of a truncated cone and they are concentrically located in mutually spaced relationship about central axis 17. Electrode 2 is located above the optical path of the rays reflected between concave mirror 9 and central mirror 11, while electrode 2' is located below the optical path. Electrodes 2 and 2' are parallel to the optical path of the beam. Figure 5 is a top view of Figure 4, showing, for the sake of clarity, only the top electrode 2 having a net structure composed of radial metal wires 36 and concentric metal wires 37. There is a space 38 between the bottom edge 40 of the electrode and mirror 9 and a space 39 between the inner upper edge 41 of the electrode and mirror 11. Those spaces are necessary in order to prevent arcing between the electrodes and the metal surfaces of the mirrors. Figure 6 is a schematic cross sectional view of a solid state laser resonator 19 in accordance with the present invention. Resonator 19 has an annular mirror 20 having reflective surface 20' and a central mirror 11 positioned so that optical axis 18 forms angle α with the central axis 17. Mirror 11 has the same configuration as mirror 11 in Figure 1. The lasing medium 10 is of solid material such as Nd-Yag, and it fills the volume between mirror 20 and mirror 11. The optical path of rays in the resonator is identical to the optical path of rays in Figure 1 and the beam emerging from resonator 19, defined by extreme rays 14 and 27, has a circular cross section. Also shown schematically in Figure 6 are two annular flash lamps 7 and 8, used for the optical pumping of the lasing medium 10. The flash lamps are located out of the optical path of the beam.

Claims: An unstable optical resonator (1) comprising: a substantially annular reflector (9) disposed about and generally facing a central axis (17) and comprising at least one annular reflecting surface (9'); a central rotationally symmetric reflector (11) positioned symmetrically along said central axis (17) and comprising at least one conical central reflecting surface (12', 13', 28', 29'); and a rotationally symmetric power extraction cavity located between the annular reflector (9) and the central reflector (11), the central reflector (11) being arranged to reflect a portion of the light in the power extraction cavity out of the power extraction cavity as laser output; characterised in that the ends of the unstable resonator are defined by the annular reflector (9) and the central reflector (11). A resonator according to claim 1, wherein a locus of optical axes (18), having a radial component and extending between the annular reflector (9) and the central reflector (11) generally centrally of the power extraction cavity defines a curved surface of a truncated cone. A resonator according to claim 1 or claim 2, further comprising means (33) located outside the power extraction cavity and which connect the annular reflector (9) to the central reflector (11). A resonator according to any one of claims 1 to 3, wherein said central reflector (11) comprises a cone (28) mounted on two truncated cones (12,29). A resonator according to any one of claims 1 to 4, wherein the annular reflector (9) is concavely curved and one (13') of the at least one central reflecting surfaces is a convexly curved surface. A resonator according to claim 2, wherein the angle between the central axis (17) and the optical axes (18) lies in the range from 60 degrees to 85 degrees. Use of a resonator (1) according to any one of claims 1 to 6 in a gas laser. Use of a resonator (1) according to any one of claims 1 to 6 in a liquid laser. Use of a resonator (1) according to any one of claims 1 to 6 in a solid state laser. Gas laser apparatus comprising: an unstable resonator (1) according to claim 1; a pair of electrodes (2,2') disposed around and concentric with the central axis (17) of the resonator, the electodes being positioned radially outwards of the central reflector (11) but radially inwards of the annular reflector (9), outside the power extraction cavity and parallel to a locus of optical axes (18) having a radial component and extending between the annular reflector (9) and the central reflector (11) generally centrally of the power extraction cavity; an enclosure (3) having a window (6) opposite an apex of one (28') of the at least one conical central reflecting surfaces of the central reflector (11); means (4) for moving laser gas into the resonator (1) in a flow direction generally parallel to the central axis (17) of the resonator; and means (5) for cooling the laser gas in the enclosure (3). Laser apparatus according to claim 10, wherein said electrodes (2,2') are made of metal and are perforated in order to permit the laser gas to flow therethrough. Laser apparatus according to claim 10, wherein the electrodes (2,2') are made of a carbon fiber structure. Laser apparatus according to any one of claims 10 to 12, further comprising excitation means comprising D.C., A.C. or R.F. excitation means or any combination thereof. Laser apparatus according to any one of claims 10 to 13, wherein the means (5) for cooling the laser gas comprise heat exchange units positioned in the enclosure (3). Laser apparatus according to any one of claims 10 to 14, wherein the gas movement means (4) comprise a gas blower (4) positioned in the enclosure (3) substantially along the central axis (17) of the resonator (1).

Inventors: NIR, REUVEN
Applicants: THE WELDING INSTITUTE
Application year: 1986
Main classification: H01S3
IPC codes: H01S3