Abstract:
High-damage-threshold output couplers with reflectivities suitable for use in high-power Q-switched lasers can be constructed from two pieces of high-damage-threshold bulk material. The output couplers are formed by a thin fluid-filled gap between parallel faces of bulk materials. This forms a reflective Fabry-Perot etalon with a large bandwidth. By avoiding the use of dielectric coatings to form the output coupler, a common source of damage—optical damage to the dielectric coating—can be avoided, making it possible to produce higher-performance lasers.

Description:
GOVERNMENT SUPPORT 
     This invention was made with government support under Contract Number F19628-95-C-0002 awarded by the Air Force. The government has certain rights in the invention 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the fields of lasers and optics. 
     A frequent problem in the performance of solid state lasers is optical damage to the dielectric coatings forming the output coupler. Such multilayer dielectric-film coatings are generally the weakest element in a laser system, and typically fail at intensities below 10 GW/cm 2  or fluences below 5 J/cm 2 . In high-gain pulsed lasers, the optical intensity at the output coupler is often larger than at other surfaces, making the output coupler a common source of problems. 
     In contrast to dielectric films, there are many bulk optical materials with a damage threshold in excess of 100 GW/cm 2 . As a result, polished etalons made from dielectric materials, such as quartz or sapphire, with highly parallel faces are often used as the output mirrors for pulsed high-power lasers. That is, the lasers are operated with a 100% mirror on one end and a polished etalon a few millimeters or a centimeter thick, generally with no additional coatings, as the output coupler on the other end. Since these lasers typically have large round-trip gains, they operate best with low-reflectivity output mirrors, and the uncoated dielectric etalon provides a simple way of achieving the necessary output mirror reflectivity. These uncoated etalons are simple to fabricate and can have very high optical-damage thresholds. 
     If a bulk etalon, as described above, is used in a solid-state laser, at least one end—the output end—of the solid-state gain medium must be treated to eliminate reflections at the solid-to-air interface. This could be done by depositing a dielectric antireflection coating on the gain medium, or by cutting the gain medium at Brewster&#39;s angle. The use of such a dielectric coating can result in a lower threshold for optical damage. Cutting the gain medium at Brewster&#39;s angle complicates the fabrication of the device and can lead to poorer performance. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an output coupler is formed of two bodies of bulk material separated by a fluid-filled gap between the highly parallel faces of the bodies. Preferably, the bodies are formed of a high-damage-threshold material such as rutile (TiO 2 ) and the spacing between the bodies is an odd multiple of one-quarter wavelength apart to achieve maximum reflectivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a first embodiment of the invention showing a partially reflecting output coupler formed by two transparent bulk dielectric materials (θ B , Brewster&#39;s angle; n 1 , refractive index of medium  16 ; n 2 , refractive index of medium  14 ). 
     FIG. 1A is a sectional view of one embodiment of spacer  18  along lines I—I. 
     FIG. 2 is a schematic of an alternate embodiment of the invention showing a partially reflecting output coupler comprising a compound etalon formed by a fluid-filled gap between two transparent bulk dielectric materials and a bulk dielectric etalon formed by the second dielectric material (n 1 , refractive index of medium  16 ; n 2 , refractive index of medium  14 ). 
     FIG. 2A is a sectional view along lines II—II of FIG.  2 . 
     FIG. 2B is a schematic as in FIG. 2 in which a compound etalon  10 ′ is formed. 
     FIG. 3 is a schematic of a further embodiment illustrating a stand-alone partially reflecting output coupler formed by a fluid-filled gap between two transparent bulk dielectric materials (θ B , Brewer&#39;s angle; n 1 , refractive index of medium  16 ; n 2 , refractive index of medium  14 ). 
     FIG. 4 is a schematic of another embodiment illustrating a stand-alone partially reflecting output coupler comprising a compound etalon formed by a fluid-filled gap between two transparent bulk dielectric materials and a bulk dielectric etalon formed by the second dielectric material (θ B , Brewster&#39;s angle; n 1 , refractive index of medium  116 ; n 2 , refractive index of medium  114 ). 
     FIG. 5 is a schematic of yet another embodiment showing a stand-alone partially reflecting output coupler comprising a compound etalon formed by a fluid-filled gap between two bulk dielectric etalons (n 1 , refractive index of medium  116 ; n 2 , refractive index of medium  114 ). 
     FIG. 6 is a schematic of a first laser embodiment of the invention showing a passively Q-switched laser with an air-gap etalon of the type shown in FIG. 1 as an output coupler. 
     FIG. 7 is a schematic of a second laser embodiment of the invention showing a passively Q-switched laser with a compound etalon of the type shown in FIG. 2 as an output coupler. 
     FIG. 8 is a graph showing the output-coupler reflectivities that can be achieved with output couplers of the types shown in FIGS. 1 and 2, with no dielectric coatings, with YAG as medium  16  (corresponding to n 1  in FIGS. 1 and 2) as a function of the refractive index of medium  14 . 
     FIG. 9 is a graph showing the reflectivities that can be achieved with stand-alone output couplers of the types shown in FIGS. 3,  4  and  5 , with no dielectric coatings, with a single medium, as a function of the medium&#39;s refractive index. 
    
    
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     First and second embodiments of the invention are shown in FIGS. 1 and 2, respectively, wherein the output face  16 A of a gain medium  16  is used as one side of an air-gap (or inert gas-filled) etalon  10  formed by the highly parallel faces  16 A and  14 A on the gain medium  16  and a second dielectric material  14 . The opposite side of the second dielectric material  14  can be provided with antireflection coating  20  as shown in FIG. 2 or cut at Brewster &#39;s angle θ B , to form the output facet  14 B as in FIG.  1 . Since this facet  14 B is external to the laser cavity, the dielectric coating  20  will see a lower intensity than if it were inside the cavity (on the gain medium), and a Brewster&#39;s angle cut becomes less critical. The maximum reflectivity is achieved when the length of the air-gap etalon is an odd multiple of one-quarter of the oscillating wavelength. In this case, the reflectivity of the etalon is given by          R   =     1   -     (         (     1   -       (         n   1     -   1         n   1     +   1       )     2       )                     (     1   -       (         n   2     -   1         n   2     +   1       )     2       )           (     1   +       (         n   1     -   1         n   1     +   1       )          (         n   2     -   1         n   2     +   1       )         )     2       )         ,                          
     where n 1  is the refractive index of the first dielectric material (the gain medium) and n 2  is the refractive index of the second dielectric material. The spacer  18  for forming the gap  12 , may be formed of any suitable material, such as quartz, sapphire or gold. The fluid in the gap  12  may comprise air or an inert gas, such as argon. An additional benefit of the air-gap etalon, compared to the bulk dielectric etalon, is that the air-gap etalon can be made extremely thin. Thinner etalons have a larger free spectral range than thicker etalons. By making the etalon only one (or a few) odd quarter wavelength(s) thick, the spectral profile of the output coupler can be extremely flat over the bandwidth of interest. Thin air gaps can be accurately fabricated by depositing the spacer  18  on one of the materials before bonding the two materials together. Alternatively, instead of a spacer, a shallow pocket to form an air gap  12 ′ can be accurately etched into one of the materials before they are joined along lines II—II of the embodiment of FIG.  2 . 
     In an alternate embodiment, shown in FIG. 2B, instead of forming a Brewster&#39;s angle, the opposite side of the second dielectric material  14  is polished to be parallel to the air-gap faces  16 A and  14 A, forming a compound etalon  10 ′. Such compound etalons will have bandwidths similar to the simple bulk etalons described in the background, except that the reflectivities can be much higher. The maximum reflectivity of the compound etalon  10 ′ is given by        R   =     1   -       (         (     1   -       (         n   1     -   1         n   1     +   1       )     2       )                       (     1   -       (         n   2     -   1         n   2     +   1       )     2       )     2           (     1   +     2                   (         n   1     -   1         n   1     +   1       )          (         n   2     -   1         n   2     +   1       )       +       (         n   2     -   1         n   2     =   1       )     2       )     2       )     .                              
     There are also benefits to the air-gap etalon that make it useful as a stand-alone high-damage-threshold output coupler for an optical cavity, independent of the gain media, as shown in FIGS. 3,  4  and  5 . It may be advantageous, in some applications, to put a dielectric coating on one or more of the flat surfaces to fine tune the reflectivity of the output coupler. 
     In FIG. 3, a stand-alone output coupler  100  is shown which can be used to efficiently couple input power  110  from an optical cavity or device to output power  120 . The input power  110  is coupled to an optical bulk medium  116  forming one side of the partial reflector or output coupler  100 . The input face  116 B is cut at a Brewster&#39;s angle θ B . The output face  116 A is formed substantially planar and spaced parallel to a similarly planar face  114 A on bulk medium  114  by spacer  118 . Again, the space  112  may be filled with air or an inert gas, and the gap is preferably an odd multiple of one-quarter the optical wavelength. 
     FIG. 4 is identical to FIG. 3, except that the output face  114 B of body  114  has an optional dielectric coating  122  as in FIG. 2 rather than being formed at a Brewster&#39;s angle. Likewise, FIG. 5 is identical to the embodiment of FIG. 4 except that both bodies  116  and  114  are formed without Brewster&#39;s angles θ B  on the respective input and output faces, and instead may use optional dielectric coatings  122 . As described in FIG. 2, compound etalons can be formed by eliminating the dielectric coatings  122  in FIGS. 4 and 5. 
     Referring now to FIGS. 6 and 7, preferred embodiments of high-power miniature lasers will now be described in which the output couplers of the invention are used to advantage to extract high power from the laser cavity  200 . As shown in FIG. 6, a typical passively Q-switched laser is comprised of a body of material  212  such as Nd 3+ :YAG forming a gain medium which is coupled, normally by bonding, to a saturable absorber crystal  214 , for example Cr 4+ :YAG. Both media are polished flat on opposing faces and mounted in parallel normal to the optic axis. The active media may be capped with transparent media  210  and  220 , for example undoped YAG, to help control thermal problems. Elements  212  and  214  and optional elements  210  and  220  form a laser cavity  200  bounded at the pump side facet  216 A by an input coupler  216  in the form of a dielectric coating which is highly reflective at the laser-cavity oscillating frequency and highly transmissive of the pump light  215  from a pump source (not shown). 
     The output face  200 A of the cavity  200  is bonded to an output coupler  218  of the invention in the form of the body  220  (or  214  if optional element  220  is not used) of polished flat transparent solid dielectric material, such as YAG, which interfaces with a second body  222  of optical material, such as rutile, with a high threshold for optical damage. In accordance with the invention, the two opposing faces of the bodies  220  and  222  are separated an odd number of ¼ wavelengths by spacer  224 , leaving a gap  226  in which air or an inert gas is disposed. The opposing faces are preferably flat and parallel to each other and normal to the optical axis of the laser cavity  200 . The output face  230  is either formed at the Brewster&#39;s angle θ B  as in FIG. 6 or as shown in FIG. 7 flat and provided with an optional dielectric coating  240 . FIG. 7 is otherwise identical to FIG.  6 . 
     In any of the above embodiments, the use of a birefringent medium as one of the two dielectric materials can result in a polarizing output coupler. 
     Further details of Q-switched lasers can be found in the following U.S. Pat. Nos. 4,982,405; 5,132,977 and 5,394,413 of Zayhowski, incorporated herein in their entirety by reference. 
     The output coupler reflectivities that can be achieved with YAG as the medium  16  in FIGS. 1 and 2B are shown in FIG. 8 as a function of the refractive index of the medium  14 . 
     The values of the reflectivity for the cases where the elements  14 ,  114  and  116  are composed of a single medium and are rutile, YAG, sapphire or quartz, and no dielectric coatings are used, is shown below in Table 1 versus each embodiment depicted in FIGS.  1 - 5 . The wavelength of interest is 1.064 μm. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Reflectivity 
               
             
          
           
               
                 Material 
                 Index 
                   
                   
                   
                   
                 FIG. 5 
               
               
                   
               
               
                 Rutile n e * 
                 2.740 
                 0.4431 
                 0.7456 
                 0.5851 
                 0.8232 
                 0.9315 
               
               
                 Rutile n o * 
                 2.480 
                 0.4057 
                 0.6986 
                 0.5188 
                 0.7690 
                 0.8996 
               
               
                 YAG 
                 1.818 
                 0.2867 
                 0.5107 
                 0.2867 
                 0.5107 
                 0.6927 
               
               
                 Sapphire 
                 1.750 
                 0.2722 
                 0.4837 
                 0.2578 
                 0.4699 
                 0.6517 
               
               
                 Quartz 
                 1.540 
                 0.2243 
                 0.3887 
                 0.1655 
                 0.3250 
                 0.4873 
               
               
                   
               
               
                 *Rutile is highly birefringent and has two entries, corresponding to the ordinary (n o ) and extraordinary (n e ) polarization.  
               
             
          
         
       
     
     FIG. 9 is a plot of reflectivities achievable with stand-alone output couplers of the types shown in FIGS. 3,  4  and  5  with a single medium and no dielectric coatings as a function of the medium&#39;s refractive index. 
     EQUIVALENTS 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims.