Abstract:
The present invention relates generally to ladar and more particularly to a tilted primary clamshell lens laser scanner for transmitting a generally collimated beam of radiation at a first frequency such that the beam rotates about a central axis so as to form a conical scanning pattern suitable for lidar applications. In addition, the ladar includes a system for transmitting at least one additional collimated beam of radiation at a second frequency at a wider angle then the first collimated beam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to the field of laser scanners for aircraft such as helicopters and, in particular, to a laser scanner of the type having additional peripheral scanning capability such that a large spatial volume is scanned in front of the aircraft.  
           [0003]    2. Description of Related Art  
           [0004]    Laser infrared radar, commonly referred to as ladar, is well known. In a typical ladar device, a solid state laser generates intense infrared pulses having beam widths as small as 30 seconds of arc. Ladar is commonly utilized to measure the density of clouds, smog layers, and other atmospheric discontinuities via the scattering effects afforded thereby. Ladar is also commonly utilized to track airborne objects such as balloons, smoke puffs, rocket trails, etc., via the beam reflections therefrom.  
           [0005]    As those skilled in the art will appreciate, the use of lidar is facilitated by various mechanisms, which effect scanning of the transmitted light beam. According to contemporary methodology, it is desirable to cause the collimated laser output of the ladar device to scan in a generally circular pattern wherein the beam itself forms a cone about an axis defined along the scanned direction. Thus, as the beam is swept, the conical scanning pattern defines a substantial spatial volume. In U.S. Pat. No. 5,471,326 “Holographic Laser Scanner And Rangefinder” by J. T. Hall, et al., discloses a scanner which not only scans in a circular pattern, but moves plus or minus 12.5 degrees in azimuth, greatly increasing the area covered.  
           [0006]    U.S. Pat. No. 5,465,142 “Obstacle Avoidance System For Helicopters And Other Aircraft” by Krumes, et al. comprises a scanner for transmitting a generally collimated beam of radiation such that the beam rotates about an axis to form a conical scanning pattern. The scanner includes a concave parabolic reflector having a geometric axis and a focus, a rotation mechanism for effecting rotation of the concave parabolic reflector about a rotation axis thereof which is angularly offset with respect to the geometric axis of the concave parabolic reflector. An opening is formed in the concave parabolic reflector near the rotation axis thereof. A convex parabolic reflector is disposed along the rotation axis of the concave parabolic reflector and has a focus, which is approximately co-located with the focus of the concave parabolic reflector.  
           [0007]    Directing collimated radiation through the opening formed in the concave parabolic reflector and onto the convex parabolic reflector while rotating the concave parabolic reflector about the rotation axis thereof effects transmission of a generally collimated beam of radiation such that the beam rotates about the rotation axis so as to form a conical scanning pattern.  
           [0008]    In U.S. Pat. No. 5,903,386 “Tilted Primary Clamshell Lens Laser Scanner” by M. V. Mantravadi, et al., the scanner transmits a generally collimated beam of radiation such that the beam rotates about an axis to form a conical scanning pattern. The scanner includes a concave parabolic reflector having a geometric axis and a focus point. A motor is used to rotate the reflector about an axis of rotation that is angularly offset with respect to the geometric axis of the reflector. A convex reflector is located at the focus point of the concave reflector on the rotational axis thereof. The convex reflector has a focus point co-located with the focus point of concave reflector. Thus collimated radiation directed through a hole in the concave reflector onto the convex reflector is reflected back to the concave reflector. Therefore, as the concave reflector rotates, the collimated beam of radiation is transmitted in a conical scanning pattern. The patent to Mantrvadi, et al. is a significant improvement to the prior art in that it is simple to manufacture. However, it would be desirable to provide the device with additional peripheral scanning capability such that a large spatial volume is scanned in front of the aircraft.  
           [0009]    Thus, it is a primary object of the invention to provide a laser type scanner for aircraft and the like.  
           [0010]    It is another primary object of the invention to provide a laser type scanner for aircraft and the like capable of scanning a large spatial volume in front of the aircraft.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention is a scanner for transmitting at least two generally collimated beams of radiation of different frequencies such that the beams rotate about an axis to form a conical scanning pattern. In detail, the scanner includes a housing having an open end. A frame member is rotatably mounted within the housing having a first end and an open second end extending to the open end of the housing and having an axis of rotation. A concave parabolic reflector is mounted within the frame member at its first end having a geometric axis and with the geometric axis angularly offset from the axis of rotation of the frame member. Preferably, the rotation axis is offset with respect to the geometric axis by an angle of between approximately 5 degrees and approximately 7 degrees. The concave reflector includes an opening near the axis of rotation.  
           [0012]    A convex parabolic reflector is disposed along the rotation axis and having a focus which is approximately co-located with the focus of the first reflector. This convex reflector is adapted to transmit at least one of the collimated beam of radiation and to reflect at least one collimated beam of radiation. A collimating lens is disposed at the opening formed in the concave reflector directing the at least two collimated beams of radiation through the opening formed therein and onto the convex reflector. A prism is mounted to the frame member behind the convex reflector co-incident therewith, which receives the at least one collimated beam of radiation transmitted through the convex reflector. A mirror is mounted to the frame member behind the prism for reflecting the at least one collimated beam of radiation diffracted by the prism radically outward from the axis of rotation. A circular diffraction screen is mounted on the housing, which extends about the axis of rotation for diffracting the at least one collimated beam of radiation reflected from the mirror forward.  
           [0013]    A motor is incorporated into the housing for rotating the frame member at high speed, on the order of 110 revolutions per second. A second motor is included for rotating the housing in azimuth, preferably plus and minus 15 degrees. Thus it can be seen that the present invention can effectively increase the degree of conical scanning of the output of a ladar device. The invention is relatively simple in construction and which is comparatively inexpensive to fabricate and maintain.  
           [0014]    In a second embodiment of scanner, the prism and mirror are replaced by a diffraction grating and the peripheral diffraction grating mounted in the housing is eliminated. Therefore, beams that pass through the convex reflector strike the diffraction grating and are diffracted and pass directly out of the housing. In a third embodiment of the scanner the convex reflector is designed to reflect at least two beams to the concave reflector. A second diffraction grating is positioned about the convex reflector at the front end on the frame member. Thus the two beams reflected off the first reflector pass through this second diffraction grating causing the two beams to be diffracted at different angles.  
           [0015]    The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiments of the invention are illustrated by way of examples. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of the scanning device partially broken away to show the azimuth control system.  
         [0017]    [0017]FIG. 2 is a cross-sectional view of the scanning device shown in FIG. 1 taken across the line  2 - 2   
         [0018]    [0018]FIG. 3 is a simplified view of FIG. 2 expanded to show the lasers used to provide the collimated beams of radiation and the optical system used to process the outgoing and return collimated laser beam.  
         [0019]    [0019]FIG. 4 is a partial simplified view of FIG. 2 illustrating a second embodiment of the invention.  
         [0020]    [0020]FIG. 5 is a partial simplified view similar to FIG. 4 illustrating a third embodiment of the invention.  
         [0021]    [0021]FIG. 6 is a simplified representation of the field of scanning the scanner accomplished during one revolution of the concave parabolic first reflector, convex second reflector, prism and mirror.  
         [0022]    [0022]FIG. 7 is a simplified representation of the field of scanning of the scanner accomplished during continued revolution of the concave parabolic first reflector, convex second reflector, prism and mirror, and a full azimuth sweep. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The laser scanner assembly of the present invention is illustrated in FIGS.  1 - 3 , with scanner assembly generally indicated by numeral  10 . Referring now to FIG. 1, the scanner  10  design is based upon the design disclosed in U.S. Pat. No. 5,903,386 “Tilted Primary Clamshell Lens Laser Scanner” by M. V. Mantravadi, et al., herein incorporated by reference. Thus any details not discussed herein may be found in the referenced patent. The Scanner  10  includes a hollow housing  12  having an open first end  14  and closed of second end  16  and a longitudinal axis  17 . The housing  12  is suspended from a support structure  19 , which includes a motor  20  for rotating the housing  12  in azimuth about a vertical axis  21 . There are numerous motor systems  20  that can be used. For example U.S. Pat. No. 5,465,142 “Obstacle Avoidance System For Helicopters And Other Aircraft” by Krumes, et al. discloses a suitable design and this patent is herewith incorporated by reference. Azimuth rotation angles are indicated by numerals  22 A and  22 B, typically plus or minus 15 degrees, but it may be more or less. A protective transparent cover  23  is mounted in the front end  14 . Just behind the front end  14  is a diffraction grating  24  that extends completely around the housing  12  only disrupted by support beams  25 .  
         [0024]    Still referring now to FIGS.  1 - 3 , rotatably mounted within the housing  12  is a hollow cup shaped frame member  30  supported by bearing  32 . The frame assembly  30 , includes axis of rotation  33  aligned with the longitudinal axis  17  of the housing  12 , an open front end  34  and closed rear end  36 . The rear end  36  incorporates an opening  38  aligned with the axis of rotation  33 . Mounted within the frame member  30  is a parabolic concave reflector (clamshell lens)  40  having a geometric axis  41 . The geometric axis  41  has an angular offset angle  44  to the rotation axis  33 . According to the preferred embodiment of the present invention this offset angle  44  is between approximately 5 degree and approximately 7 degrees, preferably approximately 6.3 degrees.  
         [0025]    The focus point  42  of the first reflector  40  is located at the front end  34  of the frame member  30 . The concave reflector  40  also includes an opening  46  aligned with the axis of rotation  33  and opening  38  in the frame member  30 . A preferred method of manufacture of the reflector  40  is provided in U.S. Pat. No. 5,465,142 “Obstacle Avoidance System For Helicopters And Other Aircraft” by Krumes, et al. An axial motor  48  is attached to the rear end  36  of the frame member  30  to provide high-speed rotation in the 110 Hz range. The motor  48  also includes an opening  50  there through aligned with openings  38  and  46 .  
         [0026]    A convex parabolic reflector  52  having a focus identical to the focus  42  of the first mirror  40  and mounted to the front end  34  of the frame member  30  at the axis of rotation  33  by means of spider beams  53 . Mounted on the convex reflector  52  is a prism  58  coupled to a mirror  60 , both aligned with diffraction grating  24 . The convex reflector  52  is made of a transparent glass such as clear zinc sulfide. A coating  54  is made of multiple layers of a dielectric material forming a dichroic filter. Suitable dielectric materials are titanium dioxide and silicon dioxide. Thus the convex reflector  52  can be designed to reflect laser beam pulses in a specific frequency range and transmit laser beams in a different frequency range.  
         [0027]    Mounted within the housing  12  behind the motor  48  and frame assembly  30  is a laser transmitter Assembly  62  comprising a laser transmitter module  64  transmitting at least two diverging pulsating laser beams, and as illustrated three beams one  74 A at 0.98μ meters, one  74 B at 1.3 μmeters and one  74 C at 1.51 μmeters. The three diverging laser beams  74 A, B and C pass through a focusing lens assembly  76  through a small slit  77  in a mirror  78  to a collimating less assembly  79  wherein the three beams are collimated. The collimated beams  74 A, B and C then pass through holes  50 ,  46  and  36  on to reflector  52 .  
         [0028]    The reflector  52  with coating  54 , is designed to reflect laser beam  74 A at the 0.98 μmeter wave length and pass laser beams  74 B at 1.31 μmeters and  74 C at 1.5 μmeters on to a prism  58 . Such a Beam  74 B is passes through the prism  58  and is reflected by mirror  60  to diffraction grating  24  where it is diffracted by an angle  80  of 30 degrees to the rotational axis  33 . The beam  74 C also passes through the prim  50  and is reflected by the mirror  60  to the diffraction grating  24  where it is diffracted by angle  82  of 60 degrees to the rotational axis  33 . The diffraction grating  24  may be comprise two different grating periods, one to diffract beam  74 B by a desired angle and one to diffract beam  74 C by a desired second angle. Alternately, a single grating period may be used for both beams  74 B and  74 C, in which case, if the grating period is chosen so as to diffract beam  74 B 30 degrees, then beam  74 C will be diffracted by an angle according to the following equation:  
         (SIN (θ))=λ/ d    
         [0029]    Where: θ is the exit angle,  
         [0030]    λ is the wavelength, and  
         [0031]    d is the period  
         [0032]    The laser beam  74 A is reflected on to the reflector  40  wherein it is transmitted forward at an angle  44  the rotational axis  33 . Beam Return signals, indicated by  74 A′,  74 B′ and  74 C′ travel a reverse path. Because the frame member  30  is rotating during the time-of-flight of the return signals  74 A′,  74 B′ and  74 C′, they strike the mirror  78  and are directed to detector assembly  85 . In detail, the return signals  74 A′,  74 B′ and  74 C′, pass through relay lens assembly  86 , which focuses the return beams to avalanche photo-detectors  88 .  
         [0033]    Note that as disclosed in the U.S. Pat. No. 5,465,142 “Obstacle Avoidance System For Helicopters And Other Aircraft” Krumes, et al., the space  89  between the first reflector  40  and second reflector  52  can be filled with a glass spacer also made of zinc sulfide glass. Since beams passing there through will be refracted by the glass, the amount of off set angle  44  can be reduced. In addition, the reflector  52  can be bonded to the glass, eliminating the spider beams  53 .  
         [0034]    A second embodiment of scanner, generally designated by numeral  10 A, is illustrated in FIG. 4. The housing  12 A includes a frame assembly  30 , and concave and convex reflectors,  40  and  52  respectively, which are all identical to previous embodiment. The difference is that the prism  58  and mirror  60  are replaced with a diffraction grating  90  and the diffraction grating  24  is eliminated. In the example illustrated in FIG. 4, the beam  74 B and  74 C pass through convex reflector  52  and strike the diffraction grating  90 . Diffraction gratings have a very high chromatic dispersion, so the beam  74 B and  74 C are deviated by different amounts. The optimum exit angles for the 3 beams at three different wave-lengths are dependent upon the azimuth range of motion, angles  22 A and  22 B. If the azimuth moves through plus or minus 15 degrees, then the optimum exit angles (for zero gaps in azimuth coverage) are 15, 45 and 75° from the axis  33  of the concave reflector  40 . The exit angle of the beam  74 A is determined by the tilt of the primary mirror. The exit angles of the 1.3 and 1.5 μm beams are determined by the diffraction grating period determined by the previously mentioned formula. If the grating period is chosen so as to deviate the beam  74 B by 45 degrees, then the beam  74 C will be deviated by 55 degrees, as shown in FIG. 4. Different wavelengths and grating parameters may be selected to optimize exit angles and diffraction efficiencies.  
         [0035]    In a third embodiment, illustrated in FIG. 5, The scanner  10 B includes a housing  12 B having a frame member  30 A. The main difference is that the convex reflector  52 A with a coating  54 A is designed to reflect beams  74 A and  74 B and transmit beam  74 C. Beam  74 C is diffracted by diffraction grating  90  as in the second embodiment. However the spider frame is replaced by a second diffraction grating  92 , which supports the reflector  52 A and grating  90 . The diffraction grating  92  diffracts the beams  74 A and  74 B outward from the concave reflector different amounts due their difference in frequencies.  
         [0036]    In all three embodiments, as the motor  48  rotates the frame member  30 , the beam  74 A is directed outward from the scanner  10  in a generally conical pattern about the axis of rotation  33  and primarily used for collision avoidance. The beams  74 B and  74 C are also directed outward in a conical pattern, but are primarily used for situation awareness. This can be seen in FIG. 6. By rotating the housing in azimuth as shown in FIG. 1, a more complete scanning pattern shown in FIG. 7 is accomplished.  
         [0037]    It is understood that the exemplary tilted primary clamshell lens laser scanner described herein and shown in the drawings represents only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. For example, the concave parabolic first reflector  40  and the convex parabolic reflector  52  may be comprised of various different materials. Also, various different types of reflective surfaces may be formed thereupon so as to effect desired reflection therefrom. Further, as those skilled in the art will appreciate, various different types of radiated energy may be utilized according to the present invention. For example, microwaves, acoustic energy, visible light, infrared, ultraviolet, etc. may be utilized.  
         [0038]    While the invention has been described with reference to particular embodiments, it should be understood that the embodiments are merely illustrative, as there are numerous variations and modifications, which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims.  
       INDUSTRIAL APPLICABILITY  
       [0039]    The invention has applicability to the aircraft industry.