Abstract:
A multi-beam LADAR apparatus and a method for use in a multi-beam LADAR system are disclosed. The apparatus includes a plurality of mission specific optics; a gimbal in which the mission specific optics are mounted; an off-gimbal laser; and a multi-fiber relay optically linking the laser output to the mission specific optics. The method includes gimbaling a plurality of mission specific optics; generating a laser signal off the gimbal; and optically relaying the laser signal to the mission specific optics through a plurality of discreet channels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention pertains to LADAR systems, and, more particularly, to a LADAR transmitter for use in a scanned illumination implementation.  
         [0003]     2. Description of the Related Art  
         [0004]     Many military and civilian remote sensing applications rely on optical techniques such as laser detection and ranging (“LADAR”). At a very high level, LADAR works much like the more familiar RADAR (“radio wave detection and ranging”), in which radio waves are transmitted into the environment and reflected back, the reflections yielding range and position information for the objects that generate them. LADAR does roughly the same thing, but using light rather than radio waves. Although there are some significant differences in performance, they are similar in at least this one basic respect.  
         [0005]     One type of LADAR system employs what is known as a “scanned illumination” technique for acquiring data. More technically, a LADAR transceiver aboard a platform transmits the laser signal to scan a geographical area called a “scan pattern”. The laser signal is typically a pulsed, split-beam laser signal. That is, the laser signal is typically transmitted in short bursts rather than continuously. The LADAR transceiver produces a pulsed (i.e., non-continuous) single beam that is then split into several beamlets spaced apart from one another by a predetermined amount. Each pulse of the single beam is split, and so the laser signal transmitted in is actually, in the illustrated embodiment, a series of grouped beamlets. The LADAR transceiver aboard the platform transmits the laser signal. The laser signal is continuously reflected back to the platform, which receives the reflected laser signal. Note, however, that some implementations employ a continuous beam, an unsplit beam, or a continuous, unsplit beam.  
         [0006]     Each scan pattern is generated by scanning elevationally, or vertically, several times while scanning azimuthally, or horizontally, once within the field of view for the platform. Thus, each scan pattern is defined by a plurality of elevational and azimuthal scans. The principal difference between the successive scan patterns is the location of the platform at the start of the scanning process. An overlap between the scan patterns is determined by the velocity of the platform. The velocity, depression angle of the sensor with respect to the horizon, and total azimuth scan angle of the LADAR platform determine the scan pattern on the ground. Note that, if the platform is relatively stationary, the overlap may be complete, or nearly complete.  
         [0007]     The platform typically maintains a steady heading while the laser signal is transmitted at varying angles relative to the platform&#39;s heading to achieve the scans. The optics package of the LADAR transceiver that generates and receives the laser signal is typically “gimbaled”, or mounted in structure that rotates relative to the rest of the platform. Exemplary gimbaled LADAR transceivers are disclosed in: 
        U.S. Letters Pat. No. 5,200,606, entitled “Laser Radar Scanning System,” issued Apr. 6, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky, et al.; and     U.S. Letters Pat. No 5,224,109, entitled “Laser Radar Transceiver,” issued Jun. 29, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky, et al. 
 
 However, there are many alternatives known to the art. 
       
 
         [0010]     For technical reasons, the entire optics package is typically gimbaled. More particularly, in conventional systems, the components that comprise the optical train through which the laser signal is generated and transmitted must be optically aligned. This optical alignment cannot be achieved when a part of the optical train is moving relative to the rest of the optical train. Thus, the LADAR transceiver has “on-gimbal” laser cavities and bulk optics to expand, collimate, segment, and align the laser output. This adds size, weight, complexity, and cost to the LADAR transceiver. The on-gimbal laser cavity also requires a fiber coupled Laser diode pump which is a significant cost driver. Furthermore, current delivery and alignment techniques for the bulk optics are inefficient, sensitive to tolerances and temperature, and limit the output power per channel and therefore limits the signal-to-noise ratio in a multi-beam LADAR system.  
         [0011]     The art has not found a successful solution to these types of problems associated with conventional gimbaled LADAR transmitters/transceivers. One approach employs a fiber laser to mitigate some of these problems. However, current fiber lasers and mode coupled fiber delivery approaches are limited either in their power tolerance (i.e., laser induced  5  damage threshold, or “LIDT”) or laser beam quality (e.g., times diffraction limit, or M 2 ) because they tend to rely on a single fiber optic channel. For example, a conventional single mode optical fiber has a very small mode field diameter, and therefore, higher energy densities at its fiber/air interface and lower LIDT. Increasing the mode field diameter without limiting the number of guided modes may improve LIDT, but it increases output M 2  reducing delivered beam quality.  
         [0012]     The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.  
       SUMMARY OF THE INVENTION  
       [0013]     The invention includes a multi-beam LADAR apparatus and a method for use in a multi-beam LADAR system. The apparatus comprises a plurality of mission specific optics; a gimbal in which the mission specific optics are mounted; an off-gimbal laser; and a multi-fiber relay optically linking the laser output to the mission specific optics. The method comprises gimbaling a plurality of mission specific optics; generating a laser signal off the gimbal; and optically relaying the laser signal to the mission specific optics through a plurality of discreet channels. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:  
         [0015]      FIG. 1  is a block diagram of a LADAR transmitter constructed and operated in accordance with the present invention;  
         [0016]      FIG. 2A - FIG. 2B  illustrate the small beam collimators and the large beam collimator of the LADAR transmitter of  FIG. 1  in plan, end views;  
         [0017]      FIG. 3  is a block diagram of one particular embodiment of the LADAR transmitter of  FIG. 1 ;  
         [0018]      FIG. 4A - FIG. 4C  provide additional detail of selected portions of the LADAR transmitter of  FIG. 3 ;  
         [0019]      FIG. 5  illustrates the near-field beam spatial overlap of the LADAR transmitter of  FIG. 3 ; and  
         [0020]      FIG. 6  illustrates the far-field beam separation of the LADAR transmitter of  FIG. 3 .  
         [0021]     While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0023]      FIG. 1  illustrates one particular embodiment of an LADAR transmitter  100  for use in a multi-beam LADAR system built and operated in accordance with the present invention. The LADAR transmitter  100  comprises an off-gimbal subassembly  103 , an on-gimbal assembly  106 , and a multi-channel fiber relay  109  between them. The off-gimbal subassembly  103  includes a laser  112 , capable of producing a laser signal  11   5 , and a plurality of small beam collimators  118 . In the illustrated embodiment, the small beam collimators  118  are arrayed as is shown best in  FIG. 2A , which is a view in the direction of the arrow  119 . The on-gimbal subassembly  106  includes a large beam collimator  121  and a LADAR sensor  124  mounted on a gimbal  127 . The multi-channel fiber relay  109  is comprised of, in the illustrated embodiment, the small beam collimators  118 , the large beam collimator  121 , and a plurality of optical fibers  122 , each optical fiber  122  defining a channel. The number of optical fibers  122  in the multi-channel fiber relay  109  is not material to the practice of the invention.  
         [0024]     The small beam collimators  118  and the large beam collimator  121  are, in the illustrated embodiment, Silicon Dioxide (SiO 2 ) laser fused collimators. Suitable small beam collimators and large beam collimators are commercially available off the shelf and are photonics market commodities. The small beam collimators  118  provide a uniform energy distribution from the laser signal  115  across the optical fibers of the multi-channel fiber relay  109 . As is best shown in  FIG. 2A , the small beam collimators  118  area arrayed in a hex-close pack with a ˜75% fill factor. The multi-channel fiber relay  109  relays the laser signal  115  through the multiple discreet channels defined by the optical fibers to the large beam collimator  121 . The multi-channel fiber relay  109  terminates in the single, large beam collimator  121  with, in the illustrated embodiment, a telecentric input to the large beam collimator  121 . The output  133  of the large beam collimator  121  is a plurality of laser signals, e.g., beamlets, that comprise a split beam laser signal. The total relay insertion loss of the illustrated embodiment is &lt;1.5 dB.  
         [0025]     The laser  112  may be implemented using any suitable laser known to the art. Suitable lasers  112  may include, for instance, a side-pumped laser, a diode-pump solid state Q-switched laser, and a side-pumped diode-pump solid state laser cavity. Note that, because it removes the laser  112  from the gimbal  127 , the present invention affords an extra degree of flexibility in implementing the laser  112  relative to the state of the art. Thus, some types and/or models of lasers ordinarily unsuitable for conventional LADAR systems may be suitable for use with the present invention. Exemplary of such lasers are pulsed fiber lasers and fiber coupled solid state lasers with passive or external Q-switch, and/or fiber optic amplifiers.  
         [0026]     For instance, the current expensive end-pumped cavity laser used in conventainal LADAR systems may be replaced with a more cost effective side-pumped laser, where the crystal/gain medium may be pumped directly with laser diodes. Fiber lasers and/or fiber optic amplifiers also become a practical and cost effective replacement, wherein the fiber is pumped and itself is the gain medium and cavity). Side-pumped laser outputs may also be “fiber coupled”. That is, the laser&#39;s output may be launched into the input of a fiber optic cable via a large beam fused collimators and terminated with another fused collimator on the output (as a means of delivering the laser energy from off gimbal to on-gimbal as suggested in the multi-channel fiber relay concept). Fiber lasers would only need to be terminated on the output with a fused collimator since the laser energy originates in the fiber optic waveguide.  
         [0027]     However, “fiber coupled” side pumped lasers and fiber lasers still represent a single fiber channel with limitations in the power handling capability, non-linear effects, and spectral broadening of a single fiber—risks mitigated by a multi-channel fiber relay concept. Naturally, there will be variations on the current multi-channel fiber relay concept that may be designed to accommodate a variety of fiber or fiber coupled lasers with greater power tolerance than a single channel fiber—all while consolidating the segmentation and beam conditioning required for multi-beam Ladar as prescribed in the multi-channel baseline herein.  
         [0028]     The LADAR sensor  124  comprises a plurality of mission specific optics. These mission specific optics may include one or more of a folding mirror, a prism, a scanner, an optical switch, and a beam expansion optical component, none of which are shown. The type of considerations that will influence the selection of mission specific optics include the design constraints like near-field beam separation, beam divergence, and far-field beam separation. For instance, some embodiments may add scanners and gimbals for accomplishing specific field of view and field of regard requirements.  
         [0029]     To further an understanding of the present invention, one particular embodiment of the LADAR transmitter  100  of  FIG. 1  will now be presented. Turning to  FIG. 3 , a LADAR transmitter  300  is shown. The LADAR transmitter  300  has many parts in common with the LADAR transmitter  100 , with like parts bearing like numbers.  
         [0030]     In the off-gimbal subassembly  103 ′, the laser  112 ′ comprises a side-pumped, 1064 nm cavity laser  303  pumped by one or more, preferably at least two, pump diodes  306 . The laser signal  115 ′ produced by the laser  112 ′ has a 0.9 mm beam spread. The off-gimbal subassembly  103 ′ also includes an optional diffractive optical element (“DOE”)  309  or other beam conditioning optics between the laser  112 ′ and the small beam collimators  118 ′. Other optics that might be employed include, for instance, an optical attenuator that might be employed for gain control purposes.  FIG. 4A  conceptually illustrates how the small beam collimators  118 ′ focus portions of the laser signal  115 ′ onto the individual optical fibers  124 ′ of the multi-channel fiber relay  109 ′.  
         [0031]     The optical fibers  124 ′ of the multi-channel fiber relay  109 ′ comprises seven single mode optical fibers  124 ′, one for each of the small beam collimators  118 ′. Each single mode optical fiber  124 ′ has a numerical aperture (“NA”) of 0.14. The optical fibers  124 ′ are fused to the small beam collimators  118 ′ and the large beam collimator  121 ′ using well known fabrication techniques. More particularly, with respect to the large beam collimator  121 ′, the single mode optical fibers  124 ′ are fused to a SiO 2  seed  400 , shown in  FIG. 4B , of the large beam collimator  121 ′. Suitable optical fibers  124 ′, like the small beam collimators  118 ′ and the large beam collimator  121 ′, are commercially available off the shelf.  
         [0032]     In the illustrated embodiment, the small beam collimators  118 ′ and the large beam collimator  121 ′ are fabricated to create male connector elements, or plugs. Each of the off-gimbal subassembly  103 ′ and the on-gimbal subassembly  106 ′ include female connector elements, or sockets, (not shown) into which the small beam collimator  118 ′ and the large beam collimator  121 ′ are plugged. In the illustrated embodiment, the connector of the large beam collimator  121 ′ is keyed. The multi-channel optical fiber relay  109 ′ in this particular embodiment therefore includes a simple keyed connector interface that provides a degree of modularity not only to the multi-channel optical fiber relay  109 ′, but also the off-gimbal subassembly  103 ′ and the on-gimbal subassembly  106 ′.  
         [0033]     In addition to the large beam collimator  121 ′, the on-gimbal subassembly  106 ′ also includes a total internal reflectance (“TIR”) prism  312  in addition to the LADAR sensor  124 ′. The laser signal  133 ′ exiting the large beam collimator  121 ′ comprises seven beamlets, as was discussed above. The beamlets exit the large beam collimator  121 ′ to the prism  312 , which spreads them to a total beam spread of 3.7 mm. The operation of the prism  312  is conceptually illustrated in  FIG. 4C .  
         [0034]     The on-gimbal subassembly  106 ′, like the off-gimbal subassembly  103 ′, may also include other beam conditioning optics. The LADAR sensor  124 ′ will typically include such beam conditioning optics to manipulate the laser signal  133 ′ suitable for the particular application. In one particular application, the LADAR sensor  124 ′ includes a holed mirror  500 , shown in  FIG. 5 , through which the LADAR sensor  133 ′ transmits the laser signal  133 ′ with a near-field beam spatial overlap  503  that results in a far-field beam separation  600 , shown in  FIG. 6 , for use in a LADAR system used in remote sensing applications such as reconnaissance.  
         [0035]     Thus, returning to  FIG. 3 , the LADAR system  300  includes an off-gimbal laser  112 ′ output (i.e., the laser signal  115 ′) coupled to a fiber bundle  315  (i.e., the optical fibers  124 ′) via a fused collimator (i.e., the small beam collimators  121 ′). The fiber bundle  315  relays the laser signal  115 ′ to the gimbal  127 ′ in discreet channels. The fiber bundle  315  terminates in a linear array fused to the large beam collimator  118 ′. The large beam collimator  118 ′ is selected for the required output beam size and divergence. Fiber spacing and lens focal length are selected for the desired angular spacing. The fused, large beam collimator  121 ′ is attached via a keyed connector aligned to the holed mirror. Segmented beamlets  603 , shown in  FIG. 6  (only one indicated), are transmitted through the holed mirror  500  as spatially overlapping but angularly separated beams as in conventional architectures.  
         [0036]     Thus, in its various aspects and embodiments, the invention provides one or more of the following: 
        a common seeker interface for the LADAR sensor.     channel equalization and elimination of loss due to diffraction efficiency of binary diffraction gratings (segmenters) in conventional multi-beam LADAR systems.     an off-gimbal laser delivery solution that reduces cost and complexity to the transmit optical path while increasing output power per channel and improving reliability over current systems. A fiber coupled relay facilitates the use of end-pumped laser cavities or diode arrays off-gimbal as lower cost alternatives. Also, laser generated heat becomes easier to manage, and space becomes available on-gimbal for multi-mode seeker concepts.     improved system signal-to-noise ratio. Multi-channel fiber relays require fewer components and provide more efficient delivery. The invention increases laser power per channel without risking optical damage to the fiber or inducing other non-linear optical effects. The net result is a lower loss transmit path with higher power handling capability for much greater power per channel in a multi-beam LADAR.     reduced system size and cost. Complex on-gimbal laser and transmit path optics alignment may be replaced with a single line replaceable unit (“LRU”) with simple keyed connector attachments.     enhanced reliability. The simplified approach has smaller part count, shorter optical path, and fewer critical surfaces. The resulting assembly is less susceptible to contamination and therefore has fewer opportunities for defects in environments.     a reusable modularity/Sensor. A relay provides interface and reformatting necessary to integrate a common LADAR sensor on multiple platforms, multi-mode seekers, and Laser solutions.     upward compatibility: A multi-channel fiber relay concept also facilitates adding COTS fiber optics, signal conditioning, and multiplexing products into future LADAR architectures. 
 
 As implied above, not every embodiment or aspect of the invention will necessarily manifest all these advantages. Also, further advantages may become apparent to those skilled in the art having the benefit of this disclosure. 
       
 
         [0045]     This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.