Abstract:
A Mersenne-Cassegrain telescope provided in a single block of glass in which opposed parabolic elements are precision milled through diamond turning of a glass boule, with the magnification power of the telescope determined by the differences in focal length between the two parabolas. The result is a volumetrically small telescope with pre-aligned surfaces that are maintained by the structural rigidity of the glass itself and in which thermal coefficients of expansion, vibration and the like have no effect due to the single glass element structure.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a compact single element monolithic eccentric Mersenne-Cassegrain telescope and its use in laser systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typical beam expanding telescopes,are refractive involving a negative input lens in line with a positive objective lens; or are reflective involving a concave primary or input mirror reflecting to a concave secondary mirror. In these cases, the ratio of focal lengths defines the power or magnification of the telescope. Such telescopes are typically mounted in metal housings, precision aligned, and sealed against environment. The problem with these prior telescopes is that they include alignment features or mechanisms adding weight and volume, and are prone to misalignment from outside sources. Thus, the disadvantages of such telescopes are that they are larger, less inherently stable, and subject to contamination. This is of concern when the telescopes are used in harsh military environments. 
         [0003]    Confronted with optical performance requirements vs. size/space allocations on a recent gimbal mounted military laser target designator countermeasure system, a new telescope needed to be designed which would enhance optical performance and system stability. The configuration would have to be the smallest, most compact telescope of comparable performance one could build. It is noted that conventional afocal, no common focus, telescopes consist of two parabolic surfaces with a common axis of rotation, but with different focal lengths, spaced and supported by stable structures. Just as the power of a sighting telescope describes image magnification mentioned above, the power of the telescope when used to project laser energy describes reduction of laser beam divergence to provide a tighter, smaller beam on the intended target. Beam divergence is reduced by the power of the telescope, and energy on target increases by the square of that power. 
         [0004]    There is therefore a requirement to provide a stable miniaturized telescope that will fit into the gimbal. Ordinarily when one, for instance, seeks to have an 8 power telescope. As described above, one would use refracting optics with a negative lens and expand the beam to a positive lens, with the ratio of the apertures defining the power out of the telescope. Thus, if one requires an 8 power telescope in order to get the beam focused for long range applications, one needs to design an 8 power telescope to fit into the tightly packaged gimbal. Size, weight, and alignment are the important design considerations, just as temperature, shock, and vibration are the important environmental concerns. 
         [0005]    It will be appreciated that most telescopes have multiple elements which creates a variety of alignment problems including spacing, temperature coefficients of expansion, vibration mounting and the like. In order to design such a telescope one has to match coefficient of expansion of the housing to the elements so that one doesn&#39;t move one element relative to the other and therefore defocus or misalign the telescope. 
         [0006]    It will also be appreciated that mounting and aligning multiple elements such as negative and positive lenses takes up a fair amount of space, regardless of whether or not environmental problems can be solved. 
         [0007]    Therefore in laser target designators and other gimbal mounted laser systems, there is a requirement for a telescope that is extremely compact and immune to environmental factors which can cause optical aberrations and unwanted beam divergence, and which also can contribute to aiming errors. 
       SUMMARY OF INVENTION 
       [0008]    The above problems are solved by providing a new telescope that is unique in that two mirror surfaces, primary and secondary, are formed in a single piece of glass and this glass is the stable metering structure maintaining their spatial relationship. The surfaces cannot go out of alignment as precision is machined in rather than aligned in. As the primary and secondary reflective surfaces are on a common optical substrate with no separating air interface, they cannot be contaminated or scratched, with coatings protecting the rear of the surfaces. One of the purposes is to be able to illuminate a target many miles away so as to focus target defining energy onto unexploded ordinances, submarines and the like, or for instance to countermeasure missiles that are aimed at an aircraft. 
         [0009]    More specifically, the telescope is an afocal telescope, i.e. one having no internal focus, that consists of an entrance window, primary and secondary parabolic mirrors, and an exit window, all created using a single piece of glass. Diamond turning lathes form and locate critically positioned surfaces to a precision exceeding conventional alignment methods. By virtue of the telescope being a single piece of glass, the telescope is both miniaturized and cannot go out of alignment from external influences of shock, vibration, or temperature. 
         [0010]    As will be appreciated, the traditional way to make telescope lenses is to take a glass blank, put it in a spindle and polish it for a long period of time to form a lens, and then assemble these lenses into a structure. This type of process is too inexact for the subject invention. 
         [0011]    In order to provide for the subject compact telescope, a single block of glass is utilized which is diamond turned to form a precision Mersenne-Cassegrain telescope involving negative and positive parabolas milled onto external surfaces of the glass block in which the parabolas have their foci on a common offset axis. Thereafter the exterior of the milled glass having the parabolic surfaces is coated with reflective material. Note that diamond turning is a technique perfected by The Corning Glass Company replacing older grinding and polishing operations on glass optics. 
         [0012]    Diamond turning involves a very high precision lathe running on air bearings, and provides the ability to shape glass to a very fine surface finish in which the surfaces themselves also have an extremely good relationship one surface to the other. 
         [0013]    As mentioned above, the block of glass is configured such that it takes on a Mersenne-Cassegrain configuration in which an incoming light beam having for instance an 8 milliradian divergence characteristic is reflected back by a negative parabolic reflector to an opposed positive parabolic surface, with the ratio of the focal lengths being 8:1, to provide an 8 power telescope. The result is an exit beam confined to a well collimated beam with a I milliradian divergence angle. 
         [0014]    In one embodiment, the block of glass is configured such that at the entrance port is a flat surface which passes the incoming light beam through the glass to the internally carried negative parabolic surface made reflective by coating the exterior of the glass surface with a reflective material. This negative parabolic surface at the end of the glass opposite the input end redirects the beam towards a positive parabolic surface at the input end of the block, this positive parabolic surface being provided with an exterior reflective coating. In one embodiment, this positive parabolic surface has a focal length that is 8 times that of the parabolic surface that forms the first optical focusing element. Upon reflection at the second coated parabolic surface, the beam emerges in a co-linear fashion so as to provide a diffraction limited collimated beam. 
         [0015]    In essence, the exterior surface of the glass block is milled to provide two parabolas which are on the same eccentric offset axis but do not have the same focus, with the relationships of their curvatures setting up the power of the telescope. 
         [0016]    By use of the single piece of glass one can create a complete Cassegrain telescope in a single optical element in a small confined space. 
         [0017]    The result is a block of glass with parabolas milled into its opposed surfaces that counteracts the effects of vibration, heat extremes and saturation. Note that if one were to have multiple optical elements, the coefficient of the expansion difference between the glasses and the supporting structure can result in alignment problems resulting in defocusing or adding astigmatism to the telescope. Thus, environmental factors become problematic in multi-element telescopes. 
         [0018]    By making the telescope of a single piece of glass all with the same coefficient of expansion, all thermally induced problems are avoided because everything within the telescope moves together as a single universe. Thus, as the telescope heats up everything moves equally and there is automatic recompensation. 
         [0019]    In short, all of the precision is built into the telescope itself. Therefore if the telescope were to be shifted slightly relative to the laser package it does not change initial laser alignment. 
         [0020]    The savings in alignment is marked with the subject invention. Once the laser has been aligned with its housing, all that is necessary with the subject invention is to take the monolithic eccentric Mersenne-Cassegrain telescope and attach it to the laser housing. 
         [0021]    In short with the subject telescope, precision is “machined in” rather than aligned at assembly. Critical surfaces cannot “get dirty” because they are within the “glass” body of the telescope. Critical alignments cannot change in harsh environments as they are part of the same structure. The telescope is smaller than equivalent telescopes and reflective systems are more compact. Moreover, mirrors do not introduce chromatic aberrations as do refractive systems and parabolas form a “perfect image” on axis. 
         [0022]    In summary, what is provided in a single block of glass is a Mersenne-Cassegrain telescope in which opposed parabolic elements are precision milled through diamond turning of a glass boule, with the magnification power of the telescope determined by the differences in focal length between the two parabolas. The result is a volumetrically small telescope with pre-aligned surfaces that are maintained by the structural rigidity of the glass itself and in which thermal coefficients of expansion, vibration and the like have no effect due to the single glass element structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other features of the subject invention will be better understood in connection with the Detailed Description, in conjunction with the Drawings, of which: 
           [0024]      FIG. 1  is a diagrammatic illustration of a conventional telescope operating in the receive mode; 
           [0025]      FIG. 2  is a diagrammatic illustration of a conventional telescope operating in the transmit mode; 
           [0026]      FIG. 3  is a diagrammatic illustration of a typical afocal telescope having no internal focus; 
           [0027]      FIG. 4  is a diagrammatic illustration of a conventional Mersenne-Cassegrain telescope; 
           [0028]      FIG. 5  is a diagrammatic illustration of a monolithic telescope fabricated from a single block of optical material having two parabolic exterior surfaces, showing the telescope being made from one half of a boule of the optical material into which the parabolic surfaces are milled; 
           [0029]      FIG. 6  is a diagrammatic illustration of a monolithic telescope formed from the boule of  FIG. 5 ; 
           [0030]      FIG. 7  is an end view of the telescope of  FIG. 6  showing a flat input beam surface and a parabolic input beam reflector; 
           [0031]      FIG. 8  is a cross sectional view of the telescope of  FIG. 6  showing the concave parabolic surfaces making up the telescope; 
           [0032]      FIG. 9  is a front view of the telescope of  FIG. 6  illustrating the area subtended by the output beam from the telescope, as well as the area subtended by the input beam parabolic surface; 
           [0033]      FIG. 10  is a cross sectional view of the telescope of  FIG. 6  illustrating the mounting of the milled optical element in a housing which surrounds the barrel of the telescope and which is provided with an input beam aperture; 
           [0034]      FIG. 11  is an isometric view of the telescope housing of  FIG. 10  showing the output beam face of the telescope; 
           [0035]      FIG. 12  is a bottom view of the housing of  FIG. 10  illustrating the aperture for the input beam; 
           [0036]      FIG. 13  is a diagrammatic and isometric view of the subject telescope illustrating a single glass block into which are milled the two parabolas, also showing an input beam reflected by a first parabolic surface to a second parabolic surface, with the exit of the beam reflected by the second parabolic surface; and, 
           [0037]      FIG. 14  is a diagrammatic and isometric bottom view of the telescope of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Referring now to  FIG. 1 , the conventional definition of a telescope is an optical instrument for enlarging the image of a distant object. Here it can be seen that an object  10  is viewed by an objective lens  12  which focuses the object to a focal point  14  to provide an image  16  of the object. Image  16  is viewed by an eye lens  18  such that when the image is viewed, the magnification is the focal length of the objective lens divided by the focal length of the eye lens. As will be appreciated,  FIG. 1  shows a conventional telescope operated in a receive mode. 
         [0039]    However, the telescope can also be used to compress a projected beam  20  in a transmit mode. In this case a primary optic or lens  22  is used to expand the incoming beam as illustrated at  24  so as to impinge on a secondary optic or lens  26  which expands and collimates the beam. 
         [0040]    The diameter of the incoming beam is shown at d 2  having a included angle of θ 2 , whereas the output beam has a diameter d 1  with the beam subtending an angle θ 1 . Note in  FIGS. 1 and 2  these optics may be lenses or mirrors, but the ratio of their focal lengths defines the magnification or power of the telescope. 
         [0041]    As shown in  FIGS. 1 and 2 , these telescopes have real foci where collimated or parallel light beams come to an internal focus between the lenses. 
         [0042]    However as shown in  FIG. 3 , telescopes may take on the form of an afocal telescope where there is no internal focus between for instance a primary lens  28  and secondary lens  30 . 
         [0043]    Referring to  FIG. 4 , what is depicted is a conventional Mersenne-Cassegrain telescope having a primary optic in the form of a parabolic reflector  40  on to which input beam  42  impinges. This beam is reflected by parabolic reflector  40  to the secondary optic  42  which is itself a parabolic surface out of which the expanded beam exits. 
         [0044]    It is noted that the radius of parabolic reflector  40  is illustrated by PR 1 , whereas the radius of the parabolic reflector  42  is denoted by PR 2 . The magnification of the Mersenne-Cassegrain telescope is therefore PR 2 /PR 1  or the ratios of the radii. 
         [0045]    It is noted that for systems projecting high fluence laser beams, real focus is undesirable due to high energy levels in an optic, and afocal designs are preferred. It is also noted that relative positioning of the optics is critical for maintaining consistent beam pointing and wave front quality. Thus considerable effort is made developing stable assemblies. 
         [0046]    How this is accomplished in the subject invention is shown by the monolithic telescope  48  of  FIG. 5  made of a single block  50  of optical material. Here a single optical element such as glass is provided by milling the exterior of the glass to provide the necessary parabolas. Note, the telescope consists of a single piece of glass  50  in the form of a glass boule that is provided with parabolic surfaces for parabolas  52  and  54  through the Corning Glass milling technology mentioned above. Note that in the configuration shown there are two parabolic surfaces which are concentric or coaxial but with different radii. 
         [0047]    As shown in  FIG. 6 , the foci  55  and  57  of the parabolas of telescope  48  lie on the same axis  59  but at different locations on the axis. Also, the incoming beam axis  61  and the outgoing beam axis  63  are parallel to but offset from the common parabola axis  59  giving rise to the eccentric configuration of the subject telescope. 
         [0048]    It is the different parabolic radii or PR, which create the telescope&#39;s magnification. Precision stability is achieved by configuring the two parabolic surfaces in a single piece of glass as shown in  FIG. 5  using high precision diamond turning lathes so that nothing can change the relative locations of the parabolic surfaces that are milled into the glass. 
         [0049]    Note there are limited number of materials that can be diamond turned, namely crystalline and amorphous materials. The process creates surface-to-surface relationships on the order of 0.10 to 0.15 wave RMS to 0.6 micron wavelength which quantifies a total through-put from the four surfaces and the internal bulk material, with the single optical material preserving the relationship over all environments. 
         [0050]    This precision is far superior to conventional grinding/polishing techniques: Furthermore, as the reflective surfaces of the telescope are on the glass surface, the reflective coatings cannot be contaminated or “dirty” and thus subject to high energy laser damage. 
         [0051]    As can be seen in  FIG. 6 , parabolic surfaces  52  and  54  of telescope  48  are provided with exterior reflective coatings  56  and  58 , again with the magnification of d 1 /d 2  equal to PR 2 /PR 1  providing for an increase in beam diameter from d 1  to d 2 . 
         [0052]    Referring to  FIG. 7 , the rear face  60  of telescope  48  includes a flat input surface  62 , with the edge  64  of the parabolic  52  being visible as shown. 
         [0053]    As shown in  FIG. 8 , the two convex parabolic surfaces  52  and  54  of telescope  48  are shown having been milled into glass block  50  to form a monolithic structure. 
         [0054]    Referring to  FIG. 9  from the top view of telescope  48 , region  66  defines the output beam from the telescope, the axis  68  of which is offset from the input beam axis  69 . 
         [0055]    In one embodiment, the housing for the monolithic telescope is illustrated in  FIGS. 10 ,  11  and  12  in which housing  70  surrounds milled block  50  and has an input aperture  72  to admit the incoming beam  74  which is reflected by the parabola  52  towards the parabola  54 , with the ray traces indicating an expansion of beam  74  to the diameter of output beam  76 . 
         [0056]    Referring to  FIG. 11  looking at the top of the telescope, the exit surface  78  of telescope  48  is shown which also carries the milled parabolic surface of parabola  52 . 
         [0057]    Referring to  FIG. 12 , the telescope housing  70  for telescope  48  includes input beam orifice  74  as illustrated. 
         [0058]    Referring now to  FIG. 13 , what is shown in isometric view is telescope  48  made up of a single glass block  50  having the parabolic surface for parabola  52  milled into the top telescope surface  78 . Also shown is the flat input surface  72  which input beam  74  traverses on its way to parabola  52  as shown by dotted lines  80 . The reflected beam is shown by dotted lines  82  to impinge on the reflective parabolic surface of parabola  54 , with the reflected beam having a diameter  86 . 
         [0059]    The reflective surface of parabola  54  on the exterior of the glass block reflects the incident light in a collimated manner out of the telescope as shown by dotted lines  86  which projects the expanded incoming beam out in a collimated fashion through face  78 . Here the exit beam has the diameter illustrated in dotted outline at  90 . Thereafter as illustrated by arrows  92  the beam is projected out to subtend the area illustrated by dotted line  94 . 
         [0060]    It is noted that the both incoming beam axis and the outgoing beam axis are offset from the axis on which the foci of the parabolas lie, giving rise to the eccentric configuration. 
         [0061]    Referring to  FIG. 14 , what is shown is an isometric view of the monolithic telescope  48  of  FIG. 13  looking up from the bottom of the glass block. Here it can be seen that flat surface  72  is ground into the parabolic surface of parabola  54  so that the incoming beam  74  passes through flat surface  72  on its way to the parabolic surface of parabola  52 . The reflected incoming beam is redirected by the parabolic surface of parabola  52  to the parabolic surface of parabola  54  as illustrated by dotted lines  82  and impinges as shown by dotted circle  96  on the parabolic surface of parabola  54 . Thereafter, as illustrated by dotted lines  86 , the expanded and collimated beam as illustrated by dotted lines  86  exits telescope  48  as illustrated at  92  so as to have the beam diameter illustrated at  94 . 
         [0062]    What is therefore shown is a monolithic telescope having parabolic surfaces milled into opposed ends of a block of glass to provide an eccentric offset beam that is highly collimated. The monolithic telescope is unaffected by environmental factors which makes the telescope extremely rugged, miniaturized and usable in any manner of laser applications. 
         [0063]    While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.