Abstract:
A novel dual-mirror mirror mount assembly for achieving polarization of a light beam in a gaseous laser is presented. The assembly includes a mirror mount structure open at one end and having a hollow cavity therein. A pair of mirrors are hard-sealed to the mirror mount structure. The first mirror is partially reflective and the second mirror is maximally reflective. The second mirror is arranged at a predetermined angle N with respect to the first mirror such that a light beam entering said mirror mount structure follows a beam path hitting the first mirror, reflecting off the first mirror and hitting the second mirror, and then retro-reflecting back on itself along the beam path of the entering light beam. The polarization function of a Brewster window is thus achieved without the use of an intra-cavity Brewster window.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to laser devices, and more particularly, to a novel dual-mirror mirror mount for polarizing light emitted from a laser without the use of an intra-cavity Brewster window. 
     BACKGROUND OF THE INVENTION 
     As increased applications of lasers are found due to the unique high-energy, high-precision properties of the output beam from such devices, the use of lasers throughout many areas of technology is becoming increasingly ubiquitous. As known by those skilled in the art, a laser is a very high frequency optical oscillator constructed from an amplifier and an appropriate amount of positive feedback. Lasers are used as critical components in a number of industries, including optical telecommunications, medical surgery, and manufacturing. 
     A typical gas laser comprises a plasma tubule discharge chamber enclosing a gaseous medium. An arc discharge is established through the gaseous medium, which serves to ionize the gas, thereby forming a plasma and elevating the electron energy states to the level required for lasing action. As the electrons recombine to lower energy states, light is emitted via spontaneous emission. Typically, a pair of optical resonator mirrors seal the two ends of the plasma tube so that light emitted by the plasma oscillates between the optical resonator mirrors and is amplified as it passes through the gaseous medium to achieve a lasing action in a manner known by those skilled in the art. 
     In a simple gaseous laser plasma discharge chamber with a cylindrical symmetry, the light output from the laser is randomly polarized. Each individual cavity mode has a linear polarization at any one time. However, the overall laser output is a time-varying mix of modes of different polarization. As a result, the output beam appears to be non-polarized when integrated over a fairly short period of time. Although the beam intensity is fairly constant, if the application involves polarization-dependent optics, then a polarizing intra-cavity Brewster window is employed which introduces sufficient loss in the plane of s-polarization (defined by the mode whose polarization vector for the electric field is perpendicular to the plane of incidence) so that only p-polarized output (defined by the mode whose polarization vector for the electric field is parallel to the plane of incidence) is produced. This occurs when the Brewster window is positioned at a Brewster&#39;s angle defined as: 
     
       
         2( b )=arctan( n ) 
       
     
     where n is the index of refraction of the window material and the index of refraction on either side of the window is assumed to be exactly 1. The Brewster window acts as a partial polarizer that ensures partial reflectivity for S-polarization and nominally zero reflectivity for p-polarization. Thus, the Brewster window provides maximum transmission efficiency at a preferred orientation for the polarization within the laser. The use of Brewster angle window assemblies is a standard technique that has been in use for many years, and, prior to the present invention, was the standard polarization method in commercial use for gas lasers. Polarization in gaseous lasers is described in greater detail in “Lasers and Electro-Optics: Fundamentals and Engineering” by Christopher C. Davis, Cambridge University Press, 1996 (ISBN 0-521-30831-3), which is incorporated herein by reference for all that it teaches. 
     To facilitate a better understanding of the advantages conferred by the present invention, a brief description of a conventional helium-neon laser  10  will be first described in conjunction with FIG.  1 . As illustrated, laser  10  includes a coaxial gas discharge chamber  12  defining a first end  2  and a second end  4  at opposite ends of the coaxial axis. Discharge chamber  12  comprises a concentric capillary bore  18  located coaxially therein. Typically, a support web  20  provides support to ensure centralization and better rotational stability of the capillary bore  18 . A cylindrical cathode  16  is positioned coaxially within the first end  2  of the discharge chamber  12 . 
     A first mirror mount assembly  40  is hard sealed to the first end  2 . First mirror mount assembly  40  includes a steel mirror mount  42  brazed to end plate  38 . A mirror substrate  44  is coated with a mirror coating  46  and hard-sealed to a mirror cup formed in the mirror mount  42  using a pre-formed glass frit  48 . End plate  38  is sealed to the first end  2  of discharge chamber  12  via a glass-to-metal seal  34 . 
     A second mirror mount assembly  50  is hard sealed to the second end  4  of discharge chamber  12 . Second mirror mount assembly  50  includes a steel mirror mount  52  brazed to end plate  68 . A mirror substrate  54  is coated with a mirror coating  56  and hard-sealed to a mirror cup formed in the mirror mount  52  using a pre-formed glass frit  58 . In the illustrative embodiment, second mirror mount assembly  50  includes an optional polarizing Brewster window  66 . Brewster window is positioned within the internal chamber of the mirror mount  42  and arranged at a Brewster angle with respect to coaxial axis of the capillary bore  18 . End plate  68  is sealed to the second end  4  of discharge chamber  12  formed by the glass capillary bore  18  via a glass-to-metal seal  64 . 
     The electrical anode  14  of the laser in this embodiment is formed by the steel mirror mount  58 . Electrical contacts to the cathode  16  are provided by support bonding straps  36  bonded to the cathode  16  and to the end plate  38 . In an illustrative 2 mW design, the resonator defined by the two mirrors  46  and  56  and the capillary bore  18  is typically of a hemispherical design with the bore diameter being 1.5 mm, mirror  54  being a flat mirror, and mirror  44  being a 30 cm concave mirror. The 30 cm concave mirror  44  is the output coupler which has a convex output radius to collimate the exiting radiation. Typical reflectivity for the high reflector is 99.9+%, while the output coupler  44  has a nominal 1% transmission. 
     An arc discharge is established by applying a voltage from a power supply (not shown) across the anode  14  and cathode  16 . The arc discharge causes the gasses within the discharge chamber  12  to be ionized, forming a plasma thereby. As the ions decay to lower energy states, light radiation is emitted in a manner well-known to those skilled in the art, and amplified by the optical resonator formed by mirrors  44 ,  54  and capillary bore  18  such that a lasing action occurs. 
     The current prior art configuration of a polarizing Brewster window mirror mount assembly as exemplified by mirror mount assembly  50  of the gas laser  10  shown in FIG. 1 is problematic. Because the Brewster window  66  is configured to reside within the mirror mount, manufacture of the mirror mount  50  is difficult because of the need to clean both sides of the window  66  during manufacture, the need to precisely position the window  66  at the Brewster&#39;s angle in order to prevent loss in efficiency (i.e., reduced power output) of the laser from deviation from the Brewster&#39;s angle, and the care required to mount the window in order to avoid stressing the window. 
     Accordingly, a need exists for a new and improved technique for polarizing a laser beam without the use of an internal Brewster angle window integrated into the mirror mount. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel method and apparatus for polarizing a laser beam without the use of a mirror mount with an internal integral Brewster window. In accordance with the method and apparatus of the invention, the present invention eliminates the Brewster window altogether and integrates two mirrors, one preferably at approximately 45° with respect to the other, along the exterior of the mirror mount structure. The mirror mount structure is open at one end and has a hollow cavity therein. A pair of mirrors are hard-sealed to the mirror mount structure. The first mirror is partially reflective and the second mirror is maximally reflective. The second mirror is arranged at a predetermined angle N with respect to the first mirror such that a light beam entering said mirror mount structure follows a beam path hitting the first mirror, reflecting off the first mirror and hitting the second mirror, and then retro-reflecting back on itself along the beam path of the entering light beam. 
     Because polarization is achieved using external mirrors rather than an integral internal mirror mounted within the mirror mount chamber, the mirror mount assembly of the invention is easier to manufacture, thereby resulting in higher manufacturing yields. Furthermore, since the polarization is achieved without employing an internal Brewster window, the cleaning issues associated with the internal window are eliminated. In addition, the angle of the mirrors is adjustable by bending the entire mirror mount as a unit. This simplifies angle adjustment and reduces the amount of accuracy required for setting the angle during manufacturer, thereby reducing complexity and cost of manufacture, and increasing the transmission efficiency due to the ability to achieve lower intracavity loss. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawing in which like reference designators are used to designate like elements, and in which: 
     FIG. 1 is a cross-section view of a conventional gaseous laser; 
     FIG. 2A is a cross-sectional view of a mirror mount assembly implemented in accordance with the invention; 
     FIG. 2B is a cross-sectional view of a mirror mount assembly implemented in accordance with the invention that has been adjusted for maximum transmission efficiency; and 
     FIG. 3 is a cross-section view of a gaseous laser implemented in accordance with the invention. 
    
    
     DETAILED DESCRIPTION 
     A novel technique and system that facilitates polarization of light emitted in a gaseous laser using a novel dual-mirror mirror mount assembly is described in detail hereinafter. The invention is described within the context of gaseous helium-neon lasers by way of example only and not limitation. The principles of the invention may be applied to any laser system requiring a polarizing mirror mount assembly. 
     Turning now to the novel features and results accompanying the present invention, there is shown in FIG. 2A a coaxial cross-sectional view of a mirror mount assembly  100  implemented in accordance with the invention. Assembly  100  comprises a mirror mount structure  154  forming a chamber  153  therein that is open at a first end  172  and hard sealed at in proximity to a second end  173  by a pair of mirrors  102  and  112 . Mirrors  102  and  112  are arranged at a predetermined acute angle φ with respect to one another. Angle φ is set such that the light beam emitted from the discharge chamber of the laser to which the mirror mount assembly  100  is attached retro-reflects back on itself along the beam path of the beam entering the mirror mount structure  154 . In the preferred embodiment, angle φ is 45°. In this embodiment, the beam  155  enters the mirror mount structure  154  and hits mirror  102  at an angle of incidence of approximately 45° with respect to the normal  165  of mirror  102 . Mirror  102  reflects the beam  155  symmetrically around the normal  165  of mirror  102  such that beam  155  hits mirror  112  at an angle substantially equal to the normal  175  of mirror  112 . Accordingly, mirror  112  reflects beam  155  back on itself such that the exit path of the beam  155  is the identical reverse of the entrance path of the beam. 
     In the preferred embodiment, the chamber of  153  mirror mount structure  154  is cylindrical along the coaxial axis  105  with respect to the open end  172  of the structure  154 . The sealed end  173  of the structure  154  comprises a pair of mirror cups  182  and  184  in which respective mirrors  102  and  112  are hard-sealed using glass frits  108  and  118  respectively. Mirror  102  preferably comprises a substrate  104  coated with a mirror coating  106  such that it provides the maximum reflectivity for s-polarization and partial reflectivity (e.g., 97% to 98%) for p-polarization. Likewise, mirror  112  preferably comprises a substrate  114  coated with a mirror coating  116 , and is substantially 99.9+% reflective such that it provides maximum reflectivity. Structure  154  is preferably made of stainless steel such as 4750 steel, which matches the coefficient of expansion of the mirror substrates  104 ,  114  and glass frits  108 ,  118  of the mirror seals. In a preferred embodiment, mirror mount structure  154  also forms a bendable thin-walled section  156  which allows the mirror mount assembly  100  to be adjusted as a unit to adjust the angle of incidence of the light beam  155 . This is an improvement over the prior art Brewster window adjustment techniques for maximizing transmission efficiency. Adjustment of the angle of the mirrors is simplified and achieved by adjusting the angle of the entire mirror mount  100 . FIG. 2B illustrates the mirror mount assembly  100  when adjusted by angle δ relative to the center axis  105  of the mirror mount structure  154 . As shown, light beam  155   a  enters mirror mount structure  154  and hits mirror  102  at angle δ with respect to the center axis  105  of the laser  200 . Mirror  102  performs partial polarization and substantially reflects beam  155  at an angle normal to axis  125 . Light beam  155  hits mirror  112 , which totally reflects the beam  155  back on itself to mirror  102  at the angle normal to axis  125 . Mirror  102  reflects the returned beam back along the path it entered at angle δ with respect to the center axis  105 . Accordingly, the partial polarization function typically performed by Brewster window or external Brewster window angle adjustment clamps as was done in the prior art. 
     Mirror mount assembly  100  also preferably includes a cylindrical steel end plate  152  having a hollow cylindrical cavity therein that is open at both ends of the coaxial axis of the cylinder. One open end of end plate  152  is brazed to the first end  172  of the mirror mount structure  154 , while the opposite open end of end plate  152  is hard-sealable to one end  204  of a laser  200 . 
     FIG. 3 is a cross-sectional view of a laser  200  employing the dual-mirror mirror mount  100  of the invention. As illustrated, laser  200  is identical to laser  10  of FIG. 1 except that mirror mount  50  in FIG. 1 is replaced with the dual-mirror mirror mount assembly of  100  of the invention in FIG.  3 . In particular, laser  200  includes a coaxial gas discharge chamber  212  defining a first end  202  and a second end  204  at opposite ends of the coaxial axis  125 . Discharge chamber  212  comprises a concentric capillary bore  218  located coaxially therein with a support web  220 . Cylindrical cathode  216  is positioned coaxially within the first end  202  of the discharge chamber  12 . 
     A first mirror mount assembly  240  is hard sealed to the first end  202 . Assembly  240  includes a steel mirror mount  242  brazed to end plate  238 . A mirror substrate  244  is coated with a mirror coating  246  and hard-sealed to a mirror cup formed in the mirror mount  242  using a pre-formed glass frit  248 . End plate  238  is sealed to the first end  202  of discharge chamber  212  via a glass-to-metal seal  234 . First mirror  244 ,  246  is the output coupler which has a reflectivity of 99.9+%, and a nominal 1% transmission. 
     In the preferred embodiment, the steel mirror mount  242  is made of 4750 steel, the end plate  238  is made of Kovar, and the mirror substrate  244  and glass frit  248  are made of BK-7 glass. The Kovar provides the proper expansion match for glass-to-metal sealing of the body parts, while the 4750 steel is matched to BK-7 glass used as a substrate material. Accordingly, all three materials have matched expansion coefficients. 
     Mirror mount assembly  100  is hard sealed to the second end  204  of discharge chamber  212 . Mirror mount assembly  100  is described in detail with respect to FIGS. 2A and 2B. 
     The electrical anode  214  of the laser is formed by the steel mirror mount structure  154 . Electrical contacts to the cathode  216  are provided by support bonding straps  236  bonded to the cathode  216  and to the end plate  238 . An arc discharge is established by applying a voltage from a power supply (not shown) across the anode  214  and cathode  216 . The arc discharge causes ionization of the gas, forming a plasma thereby. As the ions decay to lower energy states, light radiation is emitted and amplified by the optical resonator, resulting in lasing action. 
     In order to achieve the proper p-polarization in a laser implemented in accordance with the present invention, the amount of transmission that the angled mirror  112  has must have a transmission that is comparable to that of the output coupler, in this case the mirror  244 ,  246  in first mirror mount  240 . Accordingly, in the illustrative embodiment, the transmission of angled mirror  112  is approximately 1%, but can be as low as 0.1% or as high as 2- to 3% or higher depending on the output coupler design for the laser. 
     A novel technique and system that facilitates polarization of a light beam in a gaseous laser using a novel dual-mirror mirror mount has been described in detail above. It will be appreciated from a reading of the description that the present invention provides advantages over the prior art that were previously unattainable. In particular, because the mirror mount is constructed by mounting two mirrors on the walls of the mirror mount structure rather than by forming a Brewster window integral and internal to the structure chamber, manufacture of the mirror mount is significantly simplified. In addition, the elimination of the internal Brewster window within the cavity of the mirror mount allows for easier cleaning. Additionally, the elimination of the internal Brewster window reduces the number of parts, and simplifies the components necessary to adjust the angle of polarization. In particular, the dual-mirror mirror mount of the invention eliminates the need for any Brewster window adjustment clips, and allows for angle adjustment by simply adjusting the angle of the mirror mount as a single unit. 
     Although the invention has been described in terms of the illustrative embodiments, it will be appreciated by those skilled in the art that various changes and modifications may be made to the illustrative embodiments without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by the claims appended hereto.