Abstract:
A device for directing a beam of radiation at a target. The device includes a fiber laser for producing the beam of radiation, an aiming mechanism, for aiming the beam of radiation at the target, that moves independently of the fiber laser, and an optical fiber for conveying the beam of radiation to the aiming mechanism.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to devices for directing a beam of coherent radiation at a target and, more particularly, to a fiber-laser-based device for neutralizing unexploded ordinance and for similar applications. 
     Laser-based systems for neutralizing unexploded ordinance are known. One such system is the ZEUS™ system produced by the International Systems Operation division of Sparta Inc., located in Huntsville, Ala., USA.  FIG. 1  is a partial high-level schematic diagram of such a system  10 . A continuous wave laser  12 , a telescope  16  and a video camera  18  are mounted on a gimbaled platform  14 . These four components are controlled by an operator using a fire control console  20 . The operator uses video camera  18  to locate a target to be neutralized. More specifically, the operator turns and tilts platform  14  until the target is centered in a video screen in fire control console  20  that displays the images acquired by video camera  18 . Video camera  18  is boresighted to laser  12  and telescope  16  so that when the target is centered in the video screen, laser  12  and telescope  16  are aimed at the target and a beam  22  of coherent radiation that is emitted by laser  12  when laser  12  is activated and that is focused by telescope  16  strikes the target. The operator focuses telescope  16  on the target and activates laser  12  to generate coherent beam  22 . The operator continues to monitor the target using video camera  18  to verify that coherent beam  22  has indeed neutralized the target. 
     It would be advantageous to position laser  12  remotely from platform  14 . The advantages of such a system include that platform  14  could be made lighter and/or mechanically more stable if platform  14  does not need to bear the weight of laser  12 , and that a kinetic weapon such as a light or heavy machine gun could be mounted on platform  14  to supplement coherent beam  22  without the vibration of the kinetic weapon interfering with the operation of laser  12 . An optical fiber would be used to convey coherent beam  22  from laser  12  to telescope  16 . 
     Heretofore, two problems have prevented the implementation of such a system. Both problems are related to the high power of coherent beam  22  that is needed to neutralize the intended targets. 
     The first problem relates to the coupling of a conventional laser  12  (for example the Nd:YAG laser used in the Zeus system) to the optical fiber. To produce an adequately bright coherent beam  22  with a small spot size on the target, the combination of laser  12  and the optical fiber should have a low “beam parameter product” (BPP). To have a sufficiently low BPP, the optical fiber should have a narrow-diameter core. It is difficult to optically couple the high-power coherent beam  22  generated by laser  12  to a sufficiently narrow core without damaging the optical fiber. 
     The second problem is that the high power of coherent beam  22  gives rise to nonlinear effects such as Stimulated Raman Scattering and Stimulated Brillouin Scattering in the optical fiber. These nonlinear effects reduce the power of the coherent beam that finally emerges from the optical fiber and so limit the length of the optical fiber that can be used to couple laser  12  to telescope  16 . 
     SUMMARY OF THE INVENTION 
     A fiber laser is a laser whose lasing medium is an optical fiber that is doped with a suitable dopant such as ytterbium, neodymium, erbium or thulium. Very recently, such lasers have become available with powers in excess of 300 watts, making these lasers suitable for use in systems for neutralizing unexploded ordinance. Such lasers are available, for example, from IPG Photonics Corporation of Oxford. Mass., USA, from SPI Optics, Southampton UK and from Xtreme Technologies GmbH, Jena, Germany. These lasers would not suffer from the two problems described above in connection with the lasers of prior art systems. First, the optical fiber that couples the laser to telescope  16  would be a direct extension of the lasing medium, so that there would be no obstacle to coupling the laser to the optical fiber while achieving a suitably low BPP. Second, the nonlinear effects in the optical fiber would be greatly reduced. This is because the magnitude of the optical effects is proportional to the inverse square of the bandwidth of the coherent beam, and the coherent beams produced by fiber lasers have significantly wider bandwidths than the coherent beams produced by the lasers used heretofore in systems for neutralizing unexploded ordinance. 
     Therefore, according to the present invention there is provided a device for directing a beam of radiation at a target, including: (a) a fiber laser for producing the beam of radiation; (b) an aiming mechanism, for aiming the beam of radiation at the target, that moves independently of the fiber laser; and (c) an optical fiber for conveying the beam of radiation to the aiming mechanism. 
     Furthermore, according to the present invention there is provided a device including: (a) a laser for producing a beam of radiation having a power of at least about 300 watts; and (b) an optical fiber that is optically coupled to the laser to receive the beam of radiation; wherein a product of a power of the beam of radiation and a length of the optical fiber is at least about 4000 watt-meters. 
     Furthermore, according to the present invention there is provided a method of irradiating a target, including the steps of: (a) producing a beam of radiation, using a fiber laser; (b) optically coupling a proximal end of an optical fiber to the fiber laser to receive the beam of radiation; and (c) aiming a distal end of the optical fiber at the target. 
     Furthermore, according to the present invention there is provided a method of irradiating a target that includes a casing, including the steps of: (a) piercing the casing using a kinetic weapon, thereby creating an aperture in the casing; and (b) directing a beam of radiation into the target via the aperture. 
     The basic embodiment of a first device of the present invention includes a fiber laser for producing a beam of radiation, an aiming mechanism for aiming the beam of radiation at a target and an optical fiber for conveying the beam of radiation to the aiming mechanism. Unlike prior art gimbaled platform  14  of system  10 , that moves along with laser  12  because laser  12  is mounted on platform  14 , the aiming mechanism of the present invention moves independently of the fiber laser, which means that, in normal operation, the aiming mechanism can be moved without moving the fiber laser and that the fiber laser can be moved without moving the aiming mechanism. 
     Preferably, the beam of radiation produced by the fiber laser has a power of at least about 300 watts. More preferably, the beam of radiation produced by the fiber laser has a power of at least about 3000 watts. Most preferably, the beam of radiation produced by the fiber laser has a power of at least about 30,000 watts. 
     Preferably, the lasing medium of the fiber laser includes an optical fiber doped with ytterbium, neodymium, erbium or thulium. 
     Preferably, the optical fiber extends substantially from the fiber laser all the way to the aiming mechanism. 
     Preferably, the optical fiber is passive, meaning that it merely transmits the beam of radiation and does not amplify the beam of radiation in the manner of, e.g., an erbium-doped fiber amplifier. 
     Preferably, the optical fiber is at least about 30 centimeters long. More preferably, the optical fiber is at least about one meter long. Most preferably, the optical fiber is at least about three meters long. 
     Preferably, the device also includes an optical system for focusing the beam of radiation on the target. Most preferably, this optical system includes a telescope. 
     Preferably, the device also includes a kinetic weapon. The aiming mechanism aims both the beam of radiation and the kinetic weapon at the target. 
     Preferably, the device also includes a mobile platform on which the aiming mechanism is mounted. The fiber laser also may be mounted on the same platform as the aiming mechanism, or alternatively may be “detached” from that platform, i.e., mounted elsewhere than the same platform as the aiming mechanism. 
     The basic embodiment of a second device of the present invention includes a laser (not necessarily a fiber laser) for producing a beam of radiation and an optical fiber that is optically coupled to the laser to produce a beam of radiation. What distinguishes this device from prior art devices that are used e.g. for optical communication is that the power of the beam of radiation is at least about 300 watts and that the product of the power of the beam of radiation, the length of the optical fiber and the brightness (“M 2 ”) of the optical fiber is at least about 40,000 watt-meters. Preferably, product of the power of the beam of radiation, the length of the optical fiber and the brightness of the optical fiber is at least about 10 kilowatt-meters. Most preferably, product of the power of the beam of radiation, the length of the optical fiber and the brightness of the optical fiber is at least about 1000 kilowatt-meters. 
     The scope of the present invention also includes two methods of irradiating a target. 
     According to the first method, a fiber laser is used to produce a beam of radiation. One (“proximal”) end of an optical fiber is optically coupled to the fiber laser to receive the beam of radiation. The other (“distal”) end of the optical fiber is aimed at the target and the radiation beam emerging from the distal end of the fiber is focused on the target, preferably using a telescope. Preferably, the fiber laser produces a beam of radiation that has a power of at least about 300 watts. 
     According to the second method, that is directed at irradiating a target that includes a casing, first a kinetic weapon is used to make an aperture in the casing. Then a beam of radiation is directed into the target via that aperture. Preferably, the beam of radiation is produced using a laser such as a fiber laser. 
     The latest model of the ZEUS™ system is said to use a 2 Kw fiber laser as laser  12 . However, this laser, like its predecessor, is mounted on platform  14 , so that even the latest model of the ZEUS™ system does not enjoy the advantages of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a high-level schematic diagram of a prior art system for neutralizing unexploded ordinance; 
         FIG. 2  is a high-level schematic diagram of a system of the present invention for neutralizing unexploded ordinance. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a device for directing a beam of radiation at a target. Specifically, the present invention can be used for applications such as neutralizing unexploded ordinance. 
     The principles and operation of target irradiation according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Returning now to the drawings,  FIG. 2  is a high-level schematic diagram of a system  30  of the present invention. System  30  shares many of its components with prior art system  10 ; these components are indicated by the same reference numerals in  FIG. 2  as in  FIG. 1 . The main difference between system  10  and system  30  is that instead of laser  12  system  30  includes a fiber laser  32  that is not mounted on platform  14 . Instead, the doped optical fiber  34  that forms the lasing medium of fiber laser  32  is optically coupled to the proximal end  36  of a passive, flexible optical fiber  38  whose distal end  40  is mounted on platform  14  and is optically coupled to telescope  16 . In different configurations of system  30 , optical fiber  38  is 30 centimeters long, one meter long, three meters long, or even, in a particular configuration discussed below, 200 meters long. 
     Normally, optical fiber  38  is fabricated separately from fiber laser  32  and is optically coupled to doped optical fiber  34  by butting proximal end  36  against one end of doped optical fiber  34 . Alternatively, optical fiber  38  is integral with doped optical fiber  34 : one way to make fiber laser  32  and optical fiber  38  is to dope only one end of an optical fiber with a dopant such as ytterbium, neodymium, erbium or thulium. The doped end of the optical fiber is used as the lasing medium of fiber laser  32 , and the rest of the optical fiber becomes passive optical fiber  38 . 
     System  30  is used substantially in the same way as system  10 . Video camera  18  is boresighted to distal end  40  of optical fiber  38  and to telescope  16 . The operator of system  30  uses video camera  18  to locate the target to be neutralized, by turning and tilting platform  14  until the target is centered in the video screen of fire control console  20 . Distal end  40  of optical fiber  38  and telescope  16  thus are aimed at the target. The operator of system  30  then focuses telescope  16  on the target and activates fiber laser  32  to create a beam  42  of coherent radiation. This beam  42  is conveyed by optical fiber  38  to telescope  16  and is focused by telescope  16  onto the target. 
     As noted above, fiber lasers  32  with output powers of 300 watts, 3000 watts or even 30,000 watts now are available. The direct coupling of optical fiber  38  to fiber laser  32  and the relatively large linewidth of the beam  42  of coherent radiation that is emitted by laser  32  allow optical fiber  38  to be much longer than would be possible using laser  12  of prior art system  10 . This is because the non-linear gain of optical fiber  38  is approximately proportional to the inverse square of the linewidth of beam  42 . The higher the gain, the more severe are nonlinear effects such as backward Stimulated Raman Scattering and backward Stimulated Brillouin Scattering. Conventional continuous wave lasers, e.g. Nd:YAG lasers, have linewidths of less than 5 Å. Fiber lasers have linewidths as large as about 30 Å. Therefore, an optical fiber coupled to a high power fiber laser can be 36 times as long as an optical fiber coupled to a conventional high power continuous wave laser with the same brightness. 
     For a laser having output powers of 300 watts or more, the appropriate figure of merit for determining the maximum length of the optical fiber that can be coupled to the laser with high brightness at the distal end of the optical fiber is the product of the laser power, the length of the fiber and the equivalent brightness (“M 2 ”) of the fiber. According to the prior art, this figure of merit was restricted to under 40,000 watt-meters. According to the present invention, this figure of merit may be as great as 1440 kilowatt-meters. So, for example, with a prior art laser  12  that emits a beam  22  of coherent radiation having a power of 1000 watts, optical fiber  38  with a brightness of 10 had to be less than four meters long. By contrast, with a fiber laser  32  that emits a beam  42  of coherent radiation having a power of 1000 watts, optical fiber  38  with a brightness of 10 can be 144 meters long. 
     Platform  14  of system  30  also has mounted thereon a kinetic weapon in the form of a machine gun  44  to which video camera  18  also is boresighted, so that when distal end  40  of optical fiber  32  is aimed at the target, machine gun  44  also is aimed at the target. Like fiber laser  32 , machine gun  44  is operated via fire control console  20 . The operator of system  30  has the option of supplementing coherent radiation beam  42  with ammunition rounds fired from machine gun  44 . Because fiber laser  32  is not mounted on platform  16  of system  30 , the firing of machine gun  44  does not interfere with the operation of fiber laser  32 . 
     One application of machine gun  44  is to the neutralization of unexploded artillery shells. The casing of such a shell protects the explosives contained therein, so that it takes an unreasonably long time to neutralize such a shell using fiber laser  32  alone. According to the present invention, machine gun  44  is fired at the shell to puncture the shell&#39;s casing. Radiation beam  42  then is aimed and focused at the hole thereby created in the casing. 
     The term “casing” as used in the appended claims should be interpreted as including any kind of shield that prevents radiation beam  42  from reaching an explosive charge. For example, an explosive charge may be camouflaged behind a concrete barrier. Such a concrete barrier, being thermally insulating, renders fiber laser  32  by itself totally useless for neutralizing the concealed explosive charge. According to the present invention, machine gun  44  is used to disrupt the concrete barrier and allow access by beam  42  to the explosive charge that is to be neutralized. 
       FIG. 2  shows gimbaled platform  14  mounted on a mobile platform  46 . Actually, in many embodiments of the present invention, all of system  30  is mounted on a mobile platform. Suitable mobile platforms include jeeps, HMMWVs, armored personnel carriers, tanks and helicopters. Unlike prior art system  10  that requires the exposure of laser  12  to enemy fire along with the other components that are mounted on platform  14 , system  30  permits the placement of fiber laser  32  in a protected location within the mobile platform. In the alternative embodiment illustrated in  FIG. 2 , in which only gimbaled platform  14  is mounted on mobile platform  46 , mobile platform  46  typically is a robotic platform. It is in this embodiment that optical fiber  38  preferably is 200 meters long or longer, to allow remote operation of the components that are mounted on gimbaled platform  14  without placing the operator of system  30  at risk. 
     System  30  has applications beyond just neutralizing unexploded ordinance. In an urban combat setting, system  30  can be used to cut through metal bars and to destroy door locks to facilitate entry to buildings and vehicles. System  30  can be used by law enforcement personnel and rescue personnel for similar purposes in a civilian context. System  30  also can be used by law enforcement personnel to disable a moving vehicle by puncturing the tires of the vehicle. Among other civilian uses of system  30  is the cleaning of soot, grime and other deposits from building facades. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.