Abstract:
A method for thermally compensating lenses in an optical system for high power lasers includes the steps of providing a fused silica lens to collimate a high power laser beam and positioning that lens in collimating relation to the laser beam. A focusing lens assembly is provided to focus the collimated laser beam and is positioned in focusing relation to the collimated laser beam. At least one lens having a negative dn/dT to offset a heat-induced change in index of refraction of the fused silica lens is included as a part of the collimating lens assembly and as a part of the focusing lens assembly. The lens having a negative dn/dT is selected from a group of glasses having a negative dn/dT. The power of the lenses is balanced with an offsetting negative dn/dT so that the optical system maintains its focus over a wide temperature range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates, generally, to the art of lasers. More particularly, it relates to a lens for high power lasers. 
     2. Description of the Prior Art 
     One or more high quality fused silica lenses are used to focus high power fiber lasers in excess of one kilowatt (1 kW). More specifically, at least one high quality fused silica lens is used to collimate laser light emitted from a fiber laser where the fiber ranges in diameter from 50 microns to over 300 microns. After it has been collimated, the light is directed to a focusing lens assembly made of one or more high quality fused silica lenses that focus the light on to a surface to be cut, drilled, scribed, marked or welded. 
     Although the fused silica lens material is highly transmissive, some radiation is either absorbed or scattered within the lens, causing the lens to heat up. All optical glass materials have certain thermal properties that change the focus characteristics of a lens as they heat up. In particular, the thermal coefficient of expansion α and change in index of refraction n as a function of temperature (dn/dT) alter the power of a lens. The power of a lens is influenced by these two properties and is called the thermal power of a lens: 
     
       
         
           
             
               Ψ 
               p 
             
             = 
             
               [ 
               
                 
                   
                     
                       ⅆ 
                       n 
                     
                     / 
                     
                       ⅆ 
                       T 
                     
                   
                   
                     ( 
                     
                       n 
                       - 
                       1 
                     
                     ) 
                   
                 
                 - 
                 α 
               
               ] 
             
           
         
       
     
     The power of a lens is therefore altered as a function of temperature by the following equation: 
               ΔΦ   =     Φ   ⁡     [           ⅆ   n     /     ⅆ   T         (     n   -   1     )       -   α     ]         ,         
where Φ is the power of the lens. The change in power then is the original power of the lens times the thermal power of the lens Ψ p .
 
ΔΦ=ΦΨ p  
 
     Fused silica has a very low coefficient of thermal expansion, a very high transmission throughout the ultraviolet to near infrared wavelengths of the electromagnetic spectrum, and low scattering qualities. It is currently the most cost effective glass for the task. However, as indicated by the above equations, it is, along with all other optical glasses, susceptible to focal power changes as the temperature of the glass increases. The problem of focal power changes due to heat is problematic with conventional fiber lasers having average powers now in excess of 20 kW. 
     The coefficient of thermal expansion (CTE) for fused silica is about 0.5×10 −6 /° K and has a dn/dT of ˜10×10 −6 /° K. A fused silica lens having a nominal focal length of 200 mm will have a change in focus of more than 350 microns for a 100° C. temperature increase. Although this is not a huge amount for such a long focal length lens, when used as a collimator for a fiber it has a substantial impact on how the light is collimated from the fiber. 
     A paper written by Abt et al,  Focusing High - Power, Single Mode Laser Beams , Photonics Spectra Magazine, May 2008, discusses this problem and shows focus shifts of between 1 and 2 mm through a range of powers of 100 watts to 900 watts of laser power using a variety of fused silica lens and gradium index glasses. Steele et at describe a similar behavior with a CO2 laser in  Spot Size and Effective Focal Length Measurements for a Fast Axial Flow CO  2  Laser , in a paper released by the Department of Energy. Thermal lensing in window materials is further discussed by Klein in  Materials for High - Energy Laser Windows: How thermal Lensing and Thermal Stress Control Performance , SPIE Proceedings Vol. 6666, 66660Z1 (2007). 
     The conventional method for handling thermal lensing of an optical system is to let the system thermally stabilize for three or four minutes, followed by readjusting the focus of the collimation optics and the focusing optic. This is a very undesirable and expensive delay in a production environment. 
     U.S. Pat. No. 5,128,953 discloses a method for aiding the cooling of a lens by placing a small gap between the focus lens and a debris shield. This method has utility with low power lasers. It does not solve the collimation problems of high power fiber lasers without adding additional optics which should be avoided with high power lasers. The prior art requires an additional window, i.e. an extra optic, without improving the optical performance and without providing a means to provide a cooling gas when a multi element lens is required. 
     European patent application EP 1 791 229 A1 discloses a method for reducing thermal lensing with the use of radially polarized light and stress birefringence in ZnSe. This approach has very limited utility and is not practical for high power fiber lasers which are not polarized. 
     Athermalization is commonly applied to mid infrared optical systems but not specifically applied to high power lasers. These systems are adapted to compensate thermal changes over a broad spectrum within the infrared. Athermalization is discussed by Smith,  Modern Optical Engineering , McGraw Hill 2000 and in  Practical Optical System Layout and Use of Stock Lenses , McGraw Hill, 1997 and by Fishcer et al in  Optical System Design , McGraw Hill, 2008. These texts teach to achromatize and athermalize by solving three simultaneous equations: 
     
       
         
           
             Φ 
             = 
             
               
                 Φ 
                 a 
               
               + 
               
                 Φ 
                 b 
               
             
           
         
       
       
         
           
             ΔΦ 
             = 
             
               
                 
                   φ 
                   a 
                 
                 ⁢ 
                 
                   Φ 
                   a 
                 
               
               + 
               
                 
                   φ 
                   a 
                 
                 ⁢ 
                 
                   Φ 
                   b 
                 
               
             
           
         
       
       
         
           
             
               
                 ⅆ 
                 Φ 
               
               
                 ⅆ 
                 T 
               
             
             = 
             
               
                 
                   Ψ 
                   a 
                 
                 ⁢ 
                 
                   Φ 
                   a 
                 
               
               + 
               
                 
                   Ψ 
                   b 
                 
                 ⁢ 
                 
                   Φ 
                   b 
                 
               
             
           
         
       
     
     Where Φ is the power of the lens system, Φ a  and Φ b  are the powers of the individual lens elements; φ a  and φ b  are the chromatic powers of each element and Ψ a  and Ψ b  are the thermal powers of the lenses. The chromatic power of a lens is the inverse of its abbe value v. 
     In view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the prior art could be overcome. 
     SUMMARY OF THE INVENTION 
     The long-standing but heretofore unfulfilled need for a collimation means for high power lasers is now met by a new, useful, and non-obvious invention. 
     The inventive structure includes a thermal compensation lens assembly made of two or more optical elements where a first element compensates for the thermal power of a second element so as not to alter the overall power of the system. In the inventive system, the extra optics improves the optical performance of the lens system and provides a means to provide a cooling gas. 
     This invention improves conventional technology by harnessing the thermal advantages of fused silica and offsetting the change in index of refraction with a second material having a negative dn/dT. While most glasses exhibit a positive change in index of refraction as the material increases in temperature, some glasses, such as CaF2, BaF2, LiF2, NaCl, KCl, have negative dn/dTs. There are other glasses that have negative dn/dTs and the invention is not limited to the glasses listed here which are illustrative of the concept. By balancing the power of the lenses in the system with an offsetting dn/dT, the optical system maintains its focus over a wide temperature range. 
     As stated earlier, the power of a two lens system is given by:
 
Φ=Φ a +Φ b  
 
     Where Φ a  is the power of the first element and Φ b  is the power of the second element. If the lens system is subjected to a temperature change its power is changed by:
 
ΔΦ=Ψ a Φ a +Ψ b Φ b  
 
     Which is the sum of the power of each element times its thermal power Ψ. In order to obtain a zero shift in focus, the powers of the two lens must balance as follows:
 
Ψ a Φ a =−Ψ b Φ b  
 
     A laser is a monochromatic light source so the chromatic power of the lens is irrelevant and the above equations are all that is needed to athermalize a laser focusing lens. An ideal situation is where the coefficient of expansion α of each glass is identical and the dn/dT of a first material is the exact negative of a second material. That ideal is unattainable but the offsets in α and the dn/dT is compensated for by varying the thickness and curvatures of the elements to achieve more exact absolute values of each ΨΦ. 
     A primary object of the invention is to maintain a constant focal position, within the Rayleigh range (depth of focus) of the system as the temperature varies between ambient and a thermally stabilized temperature. 
     Another object is to provide thermal management to the optical elements to dissipate the absorbed heat as laser power is increased. 
     These and other important objects, advantages, and features of the invention will become clear as this description proceeds. 
     The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the description set forth hereinafter and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
         FIG. 1  depicts a prior art high power fiber laser light emitted from the output of a fiber and refocused onto a surface; 
         FIG. 2  depicts the  FIG. 1  lenses with a multi-thermal configuration; 
         FIG. 3  is an enlarged view of the focus of  FIG. 2 ; 
         FIG. 4  depicts a conventional fiber collimator for a high power fiber laser; 
         FIG. 5  is a spot diagram of the collimation lens doublet depicted in  FIG. 4 ; 
         FIG. 6  depicts the invention configured as a fiber collimator for a high power fiber laser; 
         FIG. 7  is the spot diagram for the collimator depicted in  FIG. 6 ; 
         FIG. 8  depicts the invention configured as a focusing objective for a high power fiber laser; 
         FIG. 9  is the spot diagram for the focusing objective depicted in  FIG. 8 ; 
         FIG. 10  is an enlarged view of the ray trace of the objective depicted in  FIG. 8 ; 
         FIG. 11  is a spot diagram of a fused silica and CaF2 doublet that have been thermally optimized for minimal focal shift variation; 
         FIG. 12  is an end view of a lens holding apparatus; 
         FIG. 13  is a sectional view taken along line  13 - 13  in  FIG. 12 ; and 
         FIG. 14  is a sectional view taken along line  14 - 14  in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a diagrammatic representation of a conventional arrangement of lenses, denoted as a whole by the reference numeral  10 . 
     More particularly,  FIG. 1  depicts a high power fiber laser light emitted from the output of a fiber  12 . Lens  14  collimates light  16  and lens  18  refocuses the light onto focal point  20  that is formed on the surface of a material to be cut, drilled, scribed, marked, welded or otherwise treated. Both lenses are made from high quality fused silica. 
       FIG. 2  depicts the  FIG. 1  lenses but a multi-thermal configuration has been set up where lenses  14  and  18  are at ambient temperature of 25° C., followed by increase to 50° C., 75° C. and 125° C. respectively. Focus  20  shifts towards said lens due to the temperature increase. Each configuration is overlaid with an offset of two millimeters (2 mm) to show the variation in focus as the temperature is increased. 
       FIG. 3  is an enlarged view of the focus of  FIG. 2 . The focus shift from the ambient condition at the top of  FIG. 3  to the highest temperature, at the bottom of  FIG. 3 , is 0.644 mm. 
       FIG. 4  depicts a conventional fiber collimator for a high power fiber laser  22 . Fused silica doublet  24  including plano-convex lens  24   a  and bi-convex lens  24   b  that produces a suitable level of collimation with minimal wavefront error. 
       FIG. 5  is a spot diagram of the collimation lens doublet  24  depicted in  FIG. 4 . The airy disk radius (diffraction limited divergence) for this lens system is 0.074 mradians. As the temperature values increase past 50° C. the rays begin to diverge beyond the diffraction limit and the outermost sets of rays extend to 0.161 mradians at 125° C. 
       FIG. 6  depicts the invention configured as a fiber collimator for a high power fiber laser. First lens  26  is a fused silica lens and second lens  28  is made from Schott N-PSK53A. 
       FIG. 7  is the spot diagram for the novel collimator depicted in  FIG. 6 . The divergence of the system at the higher temperature has been reduced to 0.083 mradians which is very close to the diffraction limit of 0.0701 mradians. 
       FIG. 8  depicts the invention as a focusing objective for a high power fiber laser. First lens  30  is a fused silica lens and second lens  32  is made from Schott N-PSK53A. 
       FIG. 9  is the spot diagram for the focusing objective depicted in  FIG. 8 . The rms spot radius is 2.129 microns and the geometric radius is 3.936 microns which is well below the diffraction limit of 8.737 microns. 
       FIG. 10  is an enlarged view of the ray trace of the objective depicted in  FIG. 8  showing no distinguishable difference in the focal position  20  as the temperature of the lens system increases over the range of 25° C., 50° C., 75° and 125° C., top to bottom, respectively. 
       FIG. 11  is a spot diagram of a fused silica and CaF2 doublet that has been thermally optimized for minimal focal shift variation. 
     An end view of a lens holding apparatus  40  is provided in  FIG. 12 . Apparatus  40  includes annular compression end cap  42  that cooperates with annular reference end cap  44  as depicted in  FIG. 13  to hold annular main housing  46  therebetween. Lenses  30 ,  32  are separated at their respective peripheral edges by annular invar separator  48  that is positioned radially inwardly of main housing  46  as depicted. Annular retaining ring  50  cooperates with a plurality of circumferentially spaced apart wavespring seats, collectively denoted  52  to hold annular wavespring  54  therebetween. Wavespring  54  is rated at about sixty five pounds (65 lbs). 
     The lenses are cooled by separate circuits in fluid communication with couplings  56  of which there are four (4) as indicated in said  FIG. 13 . Intralens gas fitting  58 , depicted in  FIGS. 12 and 14 , provides fluid communication from a remote gas source, preferably Helium having a thermal conductivity of 0.142 W/m° C. which is nearly 6 times greater than air (0.024 W/m° C.) and therefore can better conduct the heat away from the lenses, to the space between lenses  30 ,  32 . Port  60  provides a vent for the gas. If helium is used, it can be recycled and if air is used, it can be carried away from the lens assembly. In any event, the cooling gas must be free of residues or particulates that could contaminate the lens. 
     Couplings  56  and  58  are water cooling input and output ports. Each lens has its own cooling source and the water must be circulated to help regulate the temperature of the lens assembly. A laser with tens of kilowatts produces a tremendous amount of heat on the assembly from both absorbed light and back reflected light from the lens surfaces. For example, if there is a half percent (0.5%) loss per surface for a two (2) element optic, this represents a two percent (2%) loss and two percent (2%) of ten kilowatts (10 kW) is two hundred watts (200 W). This is a significant amount of power that has to be dissipated. 
     In all embodiments, at least one of the elements that make up the multi-element lens has a negative dn/dT value and at least one other lens has a positive dn/dT value. 
     A fused silica element and a N-PSK53A element are employed in the preferred embodiment. The N-PSK53A glass is a moldable glass. Moldability is advantageous in fabricating an aspheric surface that helps reduce spherical aberration. N-PSK53A may restrict the level of laser power used because it has more than twice the softening point compared to fused silica. 
     A second embodiment includes a CaF2 element and a fused silica element. CaF2 and fused silica are both good materials for high power lasers ranging from the UV through the near infrared. The invention is not limited to just these two materials. The following is a table that shows useable glasses with negative dn/dT values and the corresponding α values: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Glass 
                 dn/dT (10 −6 /° C.) 
                 α (10 −6 /° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 BaF 2   
                 −15.2 
                 18.1 
               
               
                   
                 CaF 2   
                 −10.6 
                 18.85 
               
               
                   
                 LiF 2   
                 −12.7 
                 37 
               
               
                   
                 KBr 
                 −40.83 
                 43 
               
               
                   
                 KCl 
                 −33.2 
                 36 
               
               
                   
                 AgCl 
                 −61 
                 30 
               
               
                   
                 NaCl 
                 −36 
                 44 
               
               
                   
                 NaF 
                 −36.2 
                 36 
               
               
                   
                 SrF 2   
                 −12 
                 19.2 
               
               
                   
                 N- 
                 −4.3 
                 9.6 
               
               
                   
                 PSK53A 
               
               
                   
                 N-PK51 
                 −8.4 
                 12.4 
               
               
                   
                 N-PK52A 
                 −8.0 
                 13 
               
               
                   
                 N-FK51A 
                 −7.3 
                 12.7 
               
               
                   
                 P-PK53 
                 −6.9 
                 13.3 
               
               
                   
                 N-FK5 
                 −2.6 
                 9.2 
               
               
                   
                   
               
             
          
         
       
     
     Glasses typically used for high power lasers and their corresponding dn/dT and α values include: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Glass 
                 dn/dT (10 −6 /° C.) 
                 α (10 −6 /° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Fused 
                 10 
                 0.52 
               
               
                   
                 Silica 
               
               
                   
                 Sapphire 
                 13 
                 8.4 
               
               
                   
                 BK7 
                 2.4 
                 7.1 
               
               
                   
                 MgF2 
                 2.3 and 1.7 
                 13.7 and 8.5 
               
               
                   
                 ZnS 
                 38.7 
                 6.5 
               
               
                   
                 ZnSe 
                 61 
                 7.8 
               
               
                   
                 Si 
                 160 
                 4.15 
               
               
                   
                 Ge 
                 396 
                 5.7 
               
               
                   
                   
               
             
          
         
       
     
     The tables show that it is unlikely to get a perfect match of dn/dT and comparable CTEs. The thickness of the lens elements and their corresponding curvatures are the variables that can be optimized for best athermalization of a high power laser optic. This is accomplished by building a suitable merit function in an optical design software program to optimize the lens thickness, curvatures and spacing to keep the focus shift under the nominal Rayleigh range value for the lens system. A study of the tables indicates that a combination of glasses can be configured to accommodate athermalization for high power lasers ranging from the UV to the far infrared that includes, but is not limited to, excimer, Nd:YAG, Nd:YLF, fiber and CO 2  lasers. 
     The following table shows the optical prescription and thermal setting for a fused silica lens with a nominal effective focal length of 100 mm focusing light at 1.075 microns which is the typical wavelength of a high power fiber laser. The back focal length is given for each temperature variation and the corresponding difference of the focus shift to the nominal temperature is given as the “Delta” values. A positive “Delta” represents a decrease of focus towards the lens and a negative “Delta” represents an increase in focus away from the lens. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Config 
                 Config 
                 Config 
                 Config 
               
               
                   
                 Oper 
                 Surf 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 TEMP 
                 0 
                 20.000 
                 50.000 
                 75.000 
                 125.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 CRVT 
                 1 
                 0.018 
                 0.018 
                 0.018 
                 0.018 
               
               
                   
                 THIC 
                 1 
                 7.000 
                 7.000 
                 7.000 
                 7.000 
               
               
                   
                 GLSS 
                 1 
                 Fused 
                 Fused 
                 Fused 
                 Fused 
               
               
                   
                   
                   
                 Silica 
                 Silica 
                 Silica 
                 Silica 
               
               
                   
                 SDIA 
                 1 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 2 
                 −0.004 
                 −0.004 
                 −0.004 
                 −0.004 
               
               
                   
                 SDIA 
                 2 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 TEMP 
                 0 
                 20.000 
                 20.000 
                 20.000 
                 20.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 BFL 
                 2 
                 95.695 
                 95.642 
                 95.596 
                 95.497 
               
               
                   
                   
                 Delta 
                 0.000 
                 0.052 
                 0.098 
                 0.197 
               
               
                   
                   
               
             
          
         
       
     
     The next table shows the optical prescription for a fused silica doublet at 1.075 microns. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Config 
                 Config 
                 Config 
                 Config 
               
               
                   
                 Oper 
                 Surf 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 TEMP 
                 0 
                 25.000 
                 50.000 
                 75.000 
                 125.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 CRVT 
                 1 
                 0.009 
                 0.009 
                 0.009 
                 0.009 
               
               
                   
                 THIC 
                 1 
                 7.000 
                 7.000 
                 7.000 
                 7.000 
               
               
                   
                 GLSS 
                 1 
                 Fused 
                 Fused 
                 Fused 
                 Fused 
               
               
                   
                   
                   
                 Silica 
                 Silica 
                 Silica 
                 Silica 
               
               
                   
                 SDIA 
                 1 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 2 
                 −0.002 
                 −0.002 
                 −0.002 
                 −0.002 
               
               
                   
                 THIC 
                 2 
                 0.250 
                 0.250 
                 0.250 
                 0.250 
               
               
                   
                 SDIA 
                 2 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 3 
                 0.016 
                 0.016 
                 0.016 
                 0.016 
               
               
                   
                 THIC 
                 3 
                 7.000 
                 7.000 
                 7.000 
                 7.000 
               
               
                   
                 GLSS 
                 3 
                 Fused 
                 Fused 
                 Fused 
                 Fused 
               
               
                   
                   
                   
                 Silica 
                 Silica 
                 Silica 
                 Silica 
               
               
                   
                 SDIA 
                 3 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 4 
                 0.005 
                 0.005 
                 0.005 
                 0.005 
               
               
                   
                 SDIA 
                 4 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 TEMP 
                 0 
                 25.000 
                 25.000 
                 25.000 
                 25.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 BFL 
                 4 
                 91.958 
                 91.915 
                 91.869 
                 91.771 
               
               
                   
                   
                 Delta 
                 0.000 
                 0.044 
                 0.090 
                 0.188 
               
               
                   
                   
               
             
          
         
       
     
     The next table provides the prescription of the preferred embodiment for a doublet focusing a fiber laser at 1.075 microns. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Config 
                 Config 
                 Config 
                 Config 
               
               
                 Type 
                 Surf 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 TEMP 
                 0 
                 25.000 
                 50.000 
                 75.000 
                 125.000 
               
               
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                 CRVT 
                 1 
                 0.008 
                 0.008 
                 0.008 
                 0.008 
               
               
                 THIC 
                 1 
                 6.999 
                 6.999 
                 6.999 
                 6.999 
               
               
                 GLSS 
                 1 
                 Fused 
                 Fused 
                 Fused 
                 Fused 
               
               
                   
                   
                 Silica 
                 Silica 
                 Silica 
                 Silica 
               
               
                 SDIA 
                 1 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                 CRVT 
                 2 
                 −0.002 
                 −0.002 
                 −0.002 
                 −0.002 
               
               
                 THIC 
                 2 
                 0.250 
                 0.250 
                 0.251 
                 0.251 
               
               
                 SDIA 
                 2 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                 CRVT 
                 3 
                 0.015 
                 0.015 
                 0.015 
                 0.015 
               
               
                 THIC 
                 3 
                 6.993 
                 6.995 
                 6.997 
                 7.000 
               
               
                 GLSS 
                 3 
                 N-PSK53A 
                 N-PSK53A 
                 N-PSK53A 
                 N-PSK53A 
               
               
                 SDIA 
                 3 
                 12.700 
                 12.703 
                 12.706 
                 12.712 
               
               
                 CRVT 
                 4 
                 0.006 
                 0.006 
                 0.006 
                 0.006 
               
               
                 SDIA 
                 4 
                 12.700 
                 12.703 
                 12.706 
                 12.712 
               
               
                 TEMP 
                 0 
                 25.000 
                 25.000 
                 25.000 
                 25.000 
               
               
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                 BFL 
                 4 
                 92.341 
                 92.344 
                 92.345 
                 92.344 
               
               
                   
                 Delta 
                 0.000 
                 −0.003 
                 −0.004 
                 −0.003 
               
               
                   
               
             
          
         
       
     
     The focus shift over the 25 to 125° C. temperature range is only about three (3) to four (4) microns which is an improvement of forty seven (47) times over the fused silica doublet in the above table. 
     N-PSK53A glass may not hold up to extremely high laser powers. The following table therefore shows a prescription where fused silica is used in conjunction with CaF 2 . 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Config 
                 Config 
                 Config 
                 Config 
               
               
                   
                 Type 
                 Surf 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 TEMP 
                 0 
                 25.000 
                 50.000 
                 75.000 
                 125.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 CRVT 
                 1 
                 0.011 
                 0.011 
                 0.011 
                 0.011 
               
               
                   
                 THIC 
                 1 
                 7.000 
                 7.000 
                 7.000 
                 7.000 
               
               
                   
                 GLSS 
                 1 
                 Fused 
                 Fused 
                 Fused 
                 Fused 
               
               
                   
                   
                   
                 Silica 
                 Silica 
                 Silica 
                 Silica 
               
               
                   
                 SDIA 
                 1 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 2 
                 −0.002 
                 −0.002 
                 −0.002 
                 −0.002 
               
               
                   
                 THIC 
                 2 
                 0.250 
                 0.251 
                 0.251 
                 0.253 
               
               
                   
                 SDIA 
                 2 
                 12.700 
                 12.700 
                 12.700 
                 12.701 
               
               
                   
                 CRVT 
                 3 
                 0.016 
                 0.016 
                 0.016 
                 0.016 
               
               
                   
                 THIC 
                 3 
                 6.987 
                 6.990 
                 6.994 
                 7.000 
               
               
                   
                 GLSS 
                 3 
                 CaF2 
                 CaF2 
                 CaF2 
                 CaF2 
               
               
                   
                 SDIA 
                 3 
                 12.700 
                 12.706 
                 12.712 
                 12.723 
               
               
                   
                 CRVT 
                 4 
                 0.007 
                 0.007 
                 0.007 
                 0.007 
               
               
                   
                 SDIA 
                 4 
                 12.700 
                 12.706 
                 12.712 
                 12.723 
               
               
                   
                 TEMP 
                 0 
                 25.000 
                 25.000 
                 25.000 
                 25.000 
               
               
                   
                 PRES 
                 0 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                   
                 BFL 
                 4 
                 91.111 
                 91.125 
                 91.139 
                 91.164 
               
               
                   
                   
                 Delta 
                 0.000 
                 −0.014 
                 −0.028 
                 −0.053 
               
               
                   
                   
               
             
          
         
       
     
     The presence of CaF 2  causes the focal length of the system to increase by fifty three (53) microns as indicated by the negative sign in contrast to the fused silica doublet. The focal shift is improved three and one-half (3.5) times over the prior art and the optics can be used at very high average power levels. Inspection of the spot diagram of  FIG. 11  indicates that the size of the spot decreases as the lens system heats up. Therefore, even though the focal length is slightly increasing, it is further reducing aberrations and improving performance of the lens system and thereby making it more diffraction limited. 
     To further enhance the performance of the optical design, a separation ring is fabricated of low thermal expansion material such as Invar which has a CTE of 1.5×10 −6 /° C. This ring is a split ring that fits over the outside diameter of each element; the split allows expansion of the glass. The Invar separating ring adds a means to conduct heat away from the lenses. Each side of the Invar separator is further machined to follow the contour of the lenses for greatest area contact on the perimeter and the first few millimeters radially across the lenses. The main housing of the objective assembly is made of a brass material having water cooling ports; no liquid actually contacts the lenses. The Invar separator has a gas bleed hole in diametrically opposed relation to the split so that clean, dry purge gas can be used to further cool the optical elements. The optics are referenced to the last surface of the assembly and secured by a wave spring that seats against the first element. The assembly is allowed to expand towards the laser and not the work piece to further reduce focus shift due to thermal expansion of the materials involved. 
     It will thus be seen that the objects set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.