IMPROVED LASERS

Abstract: High power lasers for industrial use, mainly for materials processing, having an annular laser cavity and means for generating an axis-symmetric laser beam which is polarized. The conical mirrors can be axicons or waxicons with a selective coating. Method for producing a laser beam with preferred azimuthal and preferred radial polarization based on the use of conical intra-cavity mirrors on their equivalents.

Description: IMPROVEDLASERS Field of the Invention The general field of the invention is in the improvement of industrial high power lasers for materials processing applications and more specifically provides methods to control the mode of high power lasers using stable resonators to improve their metal cutting and welding performance. Background of the Invention In materials processing applications, and particularly in the cutting and welding of metals, the three primary laser characteristics that determine its performance are: the output power, beam quality, beam polarization. As is well known, laser beams with either random or linear polarization do not perform well in metal cutting and welding applications (see, e.g., Ref. 1). Beams with circular polarization typically perform much better (Ref. 1), and therefore the polarization of a laser is often circularized, typically outside of the laser cavity, to improve its metal processing performance. This is necessary since there is no technique known today to produce a circularly polarized beam inside the laser cavity. Furthermore, beams with random polarization cannot be circularly polarized without significant loss of power, and even linearly polarized beams, which are fairly easy to generate and therefore quite common in many lasers, require the addition of extra-cavity optical components (typically - a retardation plate) to produce the desired circular polarization. Thus the need to circularize the polarization of a laser beam has resulted, so far, in a loss of some optical power and/or required adding some optical components (and cost) to the laser system. We also refer to our European Patent No.0390 013 Bl, "Laser System 1 ' . Summary of the Invention The present invention is based on our discovery that axisymmetric polarized beams can be produced effectively, by following certain techniques described in the following. The novel lasers produce azimuthally or radially polarized beams, optionally with extra- cavity treatment, exhibit excellent materials processing performance. Description of the Preferred Embodiment Several techniques were developed to achieve the desired polarization control, both in cylindrical and in annular- lasers. These techniques can be divided into two categories, based on the use of: a.Polarization selective reflectors and coatings, which are optical elements within the resonator and which reflect different polarizaitions at different intensity and/or percentage. b.Brewster angle cones, which discriminate between different polarizations. According to a further feature of this invention it is possible to transform an azimuthally polarized beam to a radially polarized beam and vice versa, by means of: c.Extra-cavity auxiliary phase retarders The invention will be described with reference to the enclosed schematical Figures, which are not according to scale and in which: Fig. 1 is a sectional side view of a laser system of the invention; Fig. 2 is a sectional side view of another laser of the present invention; Figures 3a and 3b illustrate mirror arrangements resulting in polarized laser beams. (a)Two embodiments of this invention are described in Figures 1 and 2. Figure 1, shows an annular laser system, similar to that described in Ref. 2, corresponding to EP 0 390 013 Bl the laser output beam produced by this configuration in a TEM 01 . mode, which can be azimuthally polarized or radially polarized or have a polarization which is any combination of the two. By using an uncoated copper axicon, 20, and an uncoated waxicon, 18, a beam that is azimuthally polarized is produced. This is according to 4 the different reflective properties of the copper regarding the P and S linear polarizations. We verified this by using a polarizer/analyzer technique, placing a polarizer in the output beam paths of C0 2 lasers built essentially along the guidelines of Ref. 3, and recording the beam profile, while turning the polarizer around the beam axis. The output powers of these lasers were 1000W and 1600W. The results were clearly those of azimuthally polarized beams: the polarizer/analyzer extracted from the beam all energy that was polarized perpendicularly to the polarizer axis, leaving a beam profile that is typical of a TEM 01 mode, linearly polarized in the polarizer's axis direction. This is consistent with the classical explanation of the TEM 01 , mode in REF. 4. Different polarization selective high reflectivity coatings, which are commercially available and have different reflective properties regarding the P and S polarizations, on the waxicon and/or axicon mirrors, produce in the same configuration on Figure 1, a TEM 01 , mode with a radial or azimuthal polarization. This was verified using the above polarizer/analyzer technique. In any laser system which produces a beam with TEM 01 . mode, the polarization can be controlled to be azimuthal or radial, by implementing the above methods and selection of the resonator optics. (b)Figure 2 shows an annular laser, similar to the configuration in Figure 1, without polarization selective coating, but with an addition of a transparent conical part, 21, (for a C0 2 laser this part is made of ZnSe or other suitable material). The angle of the cone is the Brewster angle. As a result, the laser will operate with a preferred radial polarization that is also compatible with the TEM 01 . mode. The mirror arrangement shown in Figure 3 a (1) comprises a mirror 31 which reflects beam 32 by an angle of 90° and this is incident on mirror 33 which is not in the same plane as mirror 31. The same arrangement, viewed along arrow 34 is shown in Figure 3a(2) where the mirrors are again not in the same plane. A complimentary system that can be used to convert radial polarization into azimuthal polarization or vice versa is presented schematically in Figure 3a. This utilizes a pair of half-wave phase-retarding mirrors(or plates). The first mirror is mounted at 45° to the beam direction, reflecting it at 90°. The second mirror is mounted at 45° to the direction of the reflected beam, and its plane is not parallel to the plane of the first mirror, but is inclined to it at 45°. A radially polarized beam passing through such arrangement will become azimuthally polarized. Ac ording to Babinet's Principle, the same arrangement can turn an azimuthally polarized beam into a radially polarized beam. Another 4-mirror arrangement is illustrated with reference to Figure 3b. As shown in Figure 3b(l) the ray 35 is reflected by mirrors 36, 37, 38 and 39 as shown. The same mirrors are illustrated in Figure 3b(2), where the angle between mirrors 36 and 37 is equivalent to that of mirror 31 of Figure 3a(l), and mirrors 38 and 39 are the equivalent of mirror 33 of Figure 3a( 1) . Figure 3b depicts an alternative arrangement to achieve the same polarization conversion, that utilizes a pair of quarter-wave phase retarder instead of each single half-wave retarder. The advantage of this arrangement over the former one is that the beam exists in a direction parallel to the original beam direction. This was verified using a polarizer/analyzer technique, as mentioned above, without and with an optical setup, similar to the configuration in figure 3b. The results show clearly that there is complete conversion of the polarization. The system and methods described differ significantly from the well-known systems and methods whereby linear polarization that can be generated in other industrial lasers is converted into a circular polarization. In 7 the present invention, the polarization is axisymmetric, either azimuthal or radial. The necessity to create an axisymmetric polarized beam stems from the requirement of homogenous material processing in all directions. The capability to generate laser beams with time-constant axisymmetric polarizations enables their judicious usage to optimize different industrial applications, such as high- quality, high-speed cutting of highly-reflective materials, deep penetration welding etc. Specifically, excellent cutting speeds and quality have been achieved with bare aluminum and bare copper, as well as with mild steel and stainless steel. Also, butt welding in stainless steel has been achieved with deeper penetration and higher speeds than was reported for other lasers with similar power, which did not have either azimuthal or radial polarization. It is clear that the above description is by way of illustration only and is not to be construed in a limitative manner. References: l.C0 2 Laser Cutting, by John Powell, Springer-Verlag London Limited, 1993. 2.European Patent No. 0 390 013 Bl, granted June 1, 1994. 3.Annular (HSURIA) resonators: some experimental studies including polarization effects: R.A. Chodzko, S.B. Mason, E.B. Turner, W.W. Plummer, Jr. Applied Optics, Vol. 19, No.5, pp. 778-789, March 1980. 4. asers, by A.E. Sigman, Copyright© University Science Books, 1986.

Claims: CLAIMS : l.A process for producing an output laser beam with preferred azimuthal or with preferred radial polarization, utilizing essentially conical intra- cavity mirrors optionally with a polarization- selective coating, that is highly-reflective for the azimuthal polarization component only, and of lower reflectivity for the radial polarization component, to result in a preferred azimuthal polarization in the output beam, or for the second alternative, optionally with a polarization-selective coating, that is highly-reflective for the radial polarization component only, and of lower reflectivity for the azimuthal polarization component, to result in preferred radial polarization in the output beam. 2.A process according to claim 1 for obtaining an output laser beam with preferred azimuthal polarization, utilizing essentially conical intra- cavity mirrors optionally with a polarization- selective coating, that is highly-reflective for the azimuthal polarization component only, and of lower 9 00009 10 reflectivity for the radial polarization component, to result in a preferred azimuthal polarization in the output beam. 3.A process according to claim 1 for producing an output laser beam with preferred radial polarization, utilizing essentially conical intra-cavity mirrors optionally with a polarization-selective coating, that is highly-reflective for the radial polarization component only, and of lower reflectivity for the azimuthal polarization component, to result in a preferred radial polarization in the output beam. 4.A method according to any of claims 1 to 3, where the conical mirrors are axicons and waxicons, to accommodate an annular-cylindrical shape gain medium an axicon as the rear mirror, co-axially placed with respect to the waxicon, these mirrors facing each other, placed at the opposing ends of said gain medium and a partially reflecting output coupling mirror opposite to the central cone of the waxicon. 5.A method according to claim 1 in a resonator containing a waxicon to accommodate an annular- cylindrical shape gain medium, an axicon as the rear mirror, co-axially positioned with respect to the waxicon, facing each other, located at the opposing ends of said gain medium and a partially reflecting output coupling mirror opposite the central cone of the waxicon, with a hollow transparent cone having a Brewster angle between its axis and slope, said hollow cone positioned adjacent to the waxicon or the axicon so that its axis coincides with the main axis of the laser, and the beam passing through the axicon perpendicularly to the axis passes through the walls of said hollow cone, thus producing an output beam with a radial polarization. 6.An auxiliary extra-cavity polarization control method, utilizing two half-wave (180°) phase retarding elements, the first of them placed at 45° to the direction of the beam, the second element placed at 45° to the resultant beam, its plane at 45° to the plane of the first element, thus converting a radial polarization in the incident beam into an azimuthal polarization and vice versa. 7.Auxiliary polarization control method according to claim 6, with each of the half-wave (180°) phase retarders being a pair of two quarter-wave (90°) phase retarders, or by any number of phase retarders which give a half-wave phase retardation in the output. 8.A laser system, comprising essentially conical intra- cavity reflecting means, optionally provided with a selective coating, which is highly reflective for the azimuthal or radial polarization component, and weakly reflecting for the radial or azimuthal polarization component, respectively, generating an output beam with preponderant azimuthal or radial polarization, respectively. 9.A laser system according to claim 8, where the reflective elements are essentially conical mirrors coated with a selective coating. 10.A laser system according to claim 8, where the reflecting elements are coaxially arranged waxicons and axicons, facing each other and being positioned at both ends of an annular cylindrical gain medium configuration, and a partially reflecting output coupling mirror opposite the central cone of the waxicon, with a hollow transparent cone having a Brewster angle between its axis and slope, said hollow cone positioned adjacent to the waxicon or the axicon so that its axis coincides with the main axis of the laser, and the beam passing through the axicon perpendicularly to the axis passes through the walls of said hollow cone, thus producing an output beam with a radial polarization. 11.A laser according to claim 7, where there are provided two extra-cavity polarization control elements, which are two half-wave (180°) phase retarding elements, the first of them placed at 45° to the direction of the beam, the second element placed at 45° to the resultant beam, its plane at 45° to the plane of the first element, thus converting a 14 radial polarization in the incident beam into an azimuthal polarization and vice versa. 12.A laser according to claim 11, where each half wave (180°) phase retarding element comprises a pair of two quarter wave (90°) phase retarders. 13.A laser according to any of claims 7 to 17, where the reflecting elements are coated with a polarization selective copper coating. 14.A laser according to any of claims 7 to 12, where the transparent conical part, for C0 2 lasers, is made of ZnSe, and where the cone angle is a Brewster angle.

Inventors: KLUMEL GENADY, ZAJDMAN AVIGDOR, KLUMEL, GENADY, ZAJDMAN, AVIGDOR
Applicants: KLUMEL GENADY, OPTOMIC LASERS LTD, ZAJDMAN AVIGDOR, KLUMEL, GENADY, OPTOMIC LASERS LTD., ZAJDMAN, AVIGDOR
Application year: 1996
Main classification: H01S
IPC codes: H01S3