Abstract:
A beam terminator for a high-power laser beam comprises a thermally conductive body including a beam-trapping chamber. A tapered spiral channel extends into the conductive body for guiding the beam. The beam is partially absorbed while propagating along the channel to the trapping chamber. What remains of the beam is absorbed in the trapping chamber.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to terminators or beam dumps for laser beams. The invention relates particular to a beam dump for a laser beam having a very high intensity, for example, 300 W or greater. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    High-power solid-state lasers that deliver very high intensity beams for laser machining operations, after being turned on can take up to 5 minutes to reach a stable operating condition. Accordingly during any relatively brief period between operations it is preferable not to turn the laser off. During this period the beam must be terminated, safely, at something other than a work-piece. The beam is terminated at what is usually termed a beam-dump, a beam-terminator or a beam-trap by practitioners of the art. A high-intensity, repetitively pulsed laser beam can reach an average power level from a few hundred (W) to several kilowatts (kW). This makes the design of a beam-terminator very challenging. 
         [0003]    Several forms of beam terminators are described in the prior-art. By way of example, U.S. Pat. No. 4,864,098 discloses a beam dump in the form of a cone structure. When such a structure is used with a very high intensity beam, however, the tip of the cone can melt due to a large heat concentration. A simple multiple reflection beam dump arrangement is disclosed in U.S. Pat. No. 5,237,454. This structure arrangement is also susceptible to damage by a high-intensity beam as there is necessarily some beam absorption at each reflection and the first reflection is at near-normal incidence. U.S. Pat. No. 6,792,017 discloses an arrangement wherein a laser beam is trapped by a plurality of absorbing plates in an enclosure through which a cooling fluid is circulated. The beam accesses the enclosure through an optical window. While this arrangement is apparently relatively compact and effective in beam termination, the window could quickly become contaminated in an industrial environment. The contamination would absorb the laser beam which could lead to a run-away absorption condition with a risk of the window breaking and leaking cooling fluid. 
         [0004]    It is believed that none of the prior art beam-terminating arrangements is practical for industrial or commercial use with very-high power beams. There is a need for a compact, efficient beam-terminator that has a minimum of operational components and is suitable for use in an industrial environment. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed to a terminator for a high-power laser beam. In one aspect, a beam-terminator in accordance with the present invention comprises a thermally conductive body having beam-trapping chamber therein. A curved channel extends into the conductive body. One end of the channel is arranged to receive the laser beam and the other end is in optical communication with the trapping-chamber. 
         [0006]    In a preferred embodiment of the present invention the channel has a spiral form and has a width that is tapered over at least a portion of the length thereof and has a narrowest width in communication thereof with the trapping chamber. The channel has first and second opposite walls, with the first wall in the form of a spiral. An end loop of the first wall provides a wall of the trapping chamber. The second wall is shorter than the first wall and a space between an end of the second wall and the end loop of the first wall provides the optical communication with the trapping chamber. 
         [0007]    In another aspect of the present invention, the walls of the channel are partially reflective and partially absorptive for the laser radiation and the channel is configured such that the laser beam received thereby propagates along the channel via a plurality reflections from the walls, with a portion of the beam being absorbed on each reflection and a residual portion entering the trapping chamber. The channel is further configured such that the first of the plurality of reflections is at grazing incidence and is also configured such that different portions of the beam undergo a different number of reflections as the beam propagates along the channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0009]      FIG. 1  schematically illustrates one preferred embodiment of a beam terminator in accordance with the present invention including a block of a thermally conductive material having a beam trapping chamber with a tapered spiral channel extending into the block from an edge thereof and making one and one-half loops before terminating in optical communication with the trapping chamber. 
           [0010]      FIG. 2  is a three-dimensional view from one side schematically illustrating another preferred embodiment of a beam terminator in accordance with the present invention similar to the beam terminator of  FIG. 1  but wherein the tapered channel makes only one-half of a spiral loop before terminating in optical communication with the trapping chamber and wherein there is a fluid cooling arrangement for walls of the channel and the trapping chamber. 
           [0011]      FIG. 3  is a three-dimensional view from an opposite side of the beam terminator of  FIG. 2  schematically illustrating further detail of the fluid cooling arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of a beam-terminator in accordance with the present invention. Terminator  10  includes a body  12  of a thermally conductive material. Suitable materials include but are not limited to copper, copper alloys, aluminum, and aluminum alloys. It is even possible to construct the body from a thermally conductive ceramic such as beryllium oxide or boron nitride. 
         [0013]    Included in body  12  is a beam trapping chamber  20 . A curved channel  14  leads the beam to the beam trap. Channel  14  has opposite wall  16  and  18  that are used to guide the beam, through multiple reflections therefrom, toward chamber  20 . Channel  14  has an entrance width W E  larger than the diameter (or maximum dimension) D of a beam  22  being terminated such that radiation in the beam is entirely captured. 
         [0014]    Beam channel  14  is configured to allow the beam to propagate in a forward direction only until what remains of the beam is trapped in chamber  20 . To this end, the channel is curved preferably in a near-elliptical shape based approximately on a Cornu spiral or an Euler spiral. A characteristic of such spirals is that the curvature varies linearly with arc length, leading to a self-terminating circle, which circle, in the example of  FIG. 1 , defines chamber  20 . The spiral shape of the walls  16  and  18  is preferably selected such that that channel  14  is tapered over a final portion thereof from a maximum width W MAX  (larger than dimension D and W E ) to a minimum width in space  26  where wall  18  ends and wall  16  continues to form a wall of chamber  20 . In this example, channel  14  makes one and one-half loops before terminating in optical communication with trapping-chamber  20 . 
         [0015]    Channel  14  is arranged such that wall  16  thereof receives beam  22  at about grazing incidence, or on average, at least relatively high angle of incidence, for example greater than about 45°. A beam guide-tube  28  is provided to facilitate beam alignment. It can be seen that different portions of the beam are incident on wall  16  at different angles of incidence, although all are high. Walls of the channel are made such that less than 100% of radiation is reflected. Preferably the reflected portion of radiation is between about 70% and 90% and such that correspondingly between about 30% and 10% is absorbed. The walls can be made reflective by polishing the walls, or by forming a reflective coating thereon. The average incidence angle of the beam on wall  16  is selected to maximize the area of the wall on which the beam is incident to minimize the power density on the wall and avoid optical damage thereto. 
         [0016]    An effect of the curved tapered channel can be traced by following the path of certain portions of beam  22  in the channel. In  FIG. 1  upper and lower extreme rays  22 A and  22 B are depicted by respectively dashed and dotted lines. A central or axial ray  22 C is depicted by alternately dotted and dashed line. It can be seen that ray  22 A reflects only from wall  16  and because of the spiral form of the wall is incident thereon each time at about the same, near-grazing angle of incidence. In chamber  20  the near grazing incidence also occurs until the end of wall  16  in chamber  20  is reached at which point subsequent reflections in chamber  20  occur at decreasing incidence angles essentially trapping the ray in the chamber. Extreme ray  22 B after a second incidence on wall  16  is reflected onto wall  18  and thereafter is reflected from one wall to the other progressing toward trapping chamber  20 . Central ray  16  makes three reflections from wall  16  before being incident on wall  18 . In summary different portions of the beam make different amounts of reflections in progressing into the trapping chamber. 
         [0017]    This effect causes the beam after the first reflection to progressively become spread over the channel walls even when some rays of the beam are incident at relatively low angles. This spreads the absorption and consequent heating over the length of the channel walls for better heat-distribution and for better removal of the distributed heat by conduction. 
         [0018]    The beam, progressively diminishing in power, is forced into narrow entrance  26  of trapping chamber  20 . The shape of the chamber allows the beam to bounce inside in one direction and eventually be absorbed in the chamber wall. The relative narrowness of the entrance minimizes the chance of radiation escaping from the light-trapping chamber. It should be noted that even with absorption at each reflection off the walls of only 10% significantly less than half of the beam power would enter trapping chamber. 
         [0019]    It should be noted here that walls of the chamber and the channel are preferably as specular as possible in characteristics as it is the specular reflection that provides the desired guidance and trapping. Diffusely reflected or scattered light could be directed out of the channel and back toward the source of the beam. 
         [0020]      FIG. 2  is three-dimensional view from one side, arbitrarily designated here as the “front” side schematically illustrating another preferred embodiment  40  of a beam terminator in accordance with the present invention. Terminator  40  is similar to the beam terminator of  FIG. 1  with an exception that the tapered beam-guidance channel makes only one-half of a spiral loop before terminating in optical communication with the trapping chamber. Accordingly the channel, channel walls and the trapping chamber are designated by the reference numeral used in the description as beam terminator  10 . In addition, there is provided a channel  42  configured for cooling the walls of channel  14  and chamber  20 . This is depicted in phantom in  FIG. 2 . Further detail of cooling-channel  42  and fluid connections thereto are depicted in  FIG. 3 , which is a three-dimensional view from the “rear” side of terminator  40 . 
         [0021]    Referring in particular to  FIG. 3 , thermally conductive body  12  has a front cover  44  (not shown in  FIG. 2 ) for covering channel  14  and trapping-chamber  20 . A rear cover plate  46 , depicted in phantom in  FIG. 3 , covers cooling channel  42 , which is machined into the thermally conductive body  12  on the side thereof opposite beam-guide channel  14 . Cover  46  includes fluid connections  48  and  49  to channel  42  and engages a sealing ring  50  for preventing leakage of cooling fluid from the beam terminator assembly. It should be noted here that fluid connections to channel  42  are depicted in simplified form for convenience of illustration. Those skilled in the art may provide any suitable connections without departing from the spirit and scope of the present invention. 
         [0022]    Thermal calculations were carried out for a specific hypothetical example of beam-terminator  40 . In these calculations it was assumed that body  12  was made from copper (Cu). The height and width and thickness of the body were 4.0 inches, 4.0 inches, and 1.0 inches respectively. Spiral channel  14  was assumed to have a length of 10.0 inches and a maximum width of 0.75 inches and a minimum width (space  26 ) of 0.2 inches. Reflectivity and absorption of the channel walls  16  and  18  were assumed to be 90% and 10% respectively, which is about what can be expected for gold plated walls. Chamber  20  was assumed to Nickel plated, providing absorption of about 50%. The input beam was assumed to have an average power of (500 W) with a symmetrical, Gaussian intensity distribution therein. Water-cooling was assumed at a water input temperature t of 20° C. and a flow rate of 0.5 gallons per minute GPM. 
         [0023]    The ambient temperature of the environment was assumed to be 21° C. The lowest temperature of the body was calculated to be about 305° K and occurred at the lower edge of the body. The highest temperature was calculated to be about 331° K and occurred near the entrance to chamber  20 . 
         [0024]    In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.