Abstract:
In an integrated gimbal and High-Powered Multiband Laser (HPMBL) for use in an infrared countermeasure apparatus in a pod mounted on an aircraft, the improvement comprises an optical bench that connects the optical path between side-by-side mounted gimbal and high power laser; and a kinematic mounting system that prevents optical bench bending.

Description:
RELATED APPLICATIONS 
     This Application is a divisional of U.S. application Ser. No. 12/228,032 filed Aug. 8, 2008 and claims rights under 35 USC §119(e) from U.S. Application Ser. No. 61/010,257 filed Jan. 7, 2008, the contents of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with United States Government assistance under Contract No. HSSCHQ-04-C-00342 awarded by the Department of Homeland Security. The United States Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to pod optical benches and more particularly to an integrated pod optical bench for use in mounting a laser and a directed IR countermeasure head in a counter-MANPADS application. 
     BACKGROUND 
     MANPADS are shoulder-fired infrared (IR) guided missiles for use against low flying aircraft. Many experts in the counterterrorism field believe that MANPADS may pose a danger to commercial airliners. Consequently, extensive efforts have between made to develop countermeasures to these weapons which are known as counter-MANPADS. 
     Such efforts have included adapting existing infrared countermeasure (IRCM) technologies for use in counter-MANPADS. For example, both gimbals and High-Powered Multiband Lasers (HPMBLs) are known in the art. Both prior gimbals and HPMBLs have known optical paths. A need, therefore, exists for a way to integrate a gimbal and an HPMPL optical path in a limited space such as in a pod to be mounted on an aircraft. 
     More particularly, it is desirable to be able to mount counter-MANPAD apparatus on a commercial airliner without having to intrude into the interior space of the aircraft. In order to do this, it has been suggested that a pod be mounted to the belly of the aircraft carrying the infrared countermeasure equipment. However, a pod carrying the entire system would be canoe-shaped and at least as long as a canoe. This is not desirable, both because of its massive size and because of the air flow problems that it causes; but more importantly because in order to maintain the equipment, the entire pod must be removed from the aircraft which is a time-consuming project. 
     It has been proposed that many of the elements of the infrared countermeasure system be distributed throughout the aircraft. However, the two elements that are critical for the countermeasure system are the high-powered laser and the directed infrared countermeasure, (DIRCM) head; and these elements must be co-located. 
     It is exceedingly important to manage the optical path between the laser and the DIRCM so as to maintain strict optical alignment for avoiding microradian errors in the directivity of the laser beam from the DIRCM head towards the target. 
     It is therefore important to mount the laser and the directed IR countermeasure head so that the optical path is maintained. In order to do this, an optical bench is utilized to mount the laser adjacent to the head. 
     The problem in a lateral mounting, which is desirable to minimize intrusion into the belly of the aircraft as would be the case when a laser is mounted on top of the head, is that side-by-side mounting requires an optical bench. As has been discovered, the optical bench warps during thermal loading due to the rigid mounting schemes used. 
     In an effort to minimize warping, heavy optical benches have been proposed, but the weight alone is enough to make this approach undesirable. 
     Also, even with the largest or most robust of the optical benches, warping still occurs which disturbs the original alignment between the laser and the head. Since the head and the laser are separate and are connected using an optical bench which has an integral optical path therein, warping of the bench causes laser aiming problems. 
     Thus, there is a requirement for the mounting of the optical bench to the airframe that thermal effects be minimized so that warping is not a problem. 
     Another problem is the replacement of the laser or the head while still maintaining the original alignment. One would like to be able to achieve interchangeability of the units without having to go through a realignment process. It is thus desirable to be able to install alignment features into the setup so that one could drop a new unit onto the optical bench and maintain the original alignment. 
     Typically, one does not want to have to replace the entire laser/directed IR countermeasure head assembly, especially if the optical bench is bolted to the frame of the aircraft. Moreover, it is very important that the mounting of the two components to the optical bench be repeatable. 
     As will be appreciated, when trying to mount a laser and a head side by side, the optical bench may be 48″ long by 12″ wide. The length of the optical bench is determined by the desire to have a low profile so that when the optical bench is mounted in a pod and the components are mounted side by side, the pod is unobtrusive when bolted to the belly of the aircraft. 
     As mentioned hereinbefore, one of the key aspects of the optical bench is that one needs to have some means of preventing flexing of the optical bench and the resulting misdirection of the laser beam. It was found that a rigid mounting of the optical bench to the airframe engendered warpage of the optical bench during thermal cycling. 
     SUMMARY OF INVENTION 
     According to the present invention, a strong back optical bench is used to construct the optical path between the gimbaled directed infrared countermeasure (DIRCM) and the head high power multi-band laser. This bench is designed to be very stiff to meet a very precise optical alignment requirement (&lt;500 μrad). As part of the bench a light pipe formed in the optical bench is used with two 45-degree mirrors on either end to establish the optical path between laser and head. The pipe is sealed to protect the optical path from debris and damage. 
     How optical bench warpage and flexure is avoided is now described. Rather than rigidly mounting the optical bench to the frame of the aircraft, a kinematic-style mounting is utilized in which one has a stiff but flexible mount at one end of the bench, with two rigid mounts at the other end of the bench. This three-point mounting system prevents the bench from warping and bowing during thermal cycling due to the flexing of the third point. Thus, in one embodiment, the kinematics mount is a three-point mount in which one has two rigid bolts at one end of the optical bench and a third mount that is intentionally designed with more flexibility than if one had a rigid bolted connection. Note that the stiff but flexible mount is designed to be weaker than the bench itself so that during thermal cycling, the mount flexes rather than the bench, thus keeping the bench flat by not inducing flex to the bench. 
     In one embodiment, the bench is mounted by the above-mentioned three-point mount to the frame of the aircraft and is made of 6061 aluminum. 
     As a result of the three-point mounting with one stiff but flexible coupling, any thermal expansion of the bench via-a-vis the airframe due to different coefficients of thermal expansion will not be taken up by the bench, but rather by the flexure of the stiff but flexible coupling. The result is that the thermal stress will not induce either standard or bending loads into the platform. This means that standard loading on the fixed bolts is minimized, as well as flexural loading of the bench, which would impact the aiming accuracy of the laser beam emitted by the IR countermeasure system. 
     The subject mounting scheme and alignment features allow for repeatability unit to unit so that the design can incorporate two line-replaceable units, namely the laser and the DIRCM head. These are bolted to the bench and are interconnected through a fixed optical path, with the units being interchangeable such that if one or the other fails, one is able to remove and replace it. One can do this without realigning the entire system so that all one has to do is make the mechanical and electrical connections to the two units. 
     Once having mounted the optical bench in the above manner to the air frame, the individual components are aligned through alignment pins and alignment features which in one embodiment involve having a pin and a bushing combination. Note that the pins and bushings need to be precisely located to maintain alignment. 
     In one embodiment, the laser beam is emitted from the laser in a direction perpendicular to the flat plane of the optical bench where it enters the optical bench in a channel and is re-directed at a right angle towards the head. When the beam reaches a position underneath the head, it is re-directed up through the bench into the directed infrared countermeasure head. Thus, the optical channel is provided with optics at either end to re-direct the light at right angles. 
     The stiff but flexible mount is strong enough to work structurally under all applied loads, but flexible enough so that it does not induce optical bench bending. In one embodiment, the stiff-flexible mount is made of a metal oval which flexes with thermal loading so that the oval bends and distorts as opposed to the optical bench. Thus, the stiff-flexible mount is the weak link in the system such that, while flexing, the mount is still strong enough to carry all of the applied loads. 
     Because of the three-point kinematic suspension which is the subject of the present invention, the bench avoids warping during thermal loading, thereby to eliminate very small microradian deflections, such that optical bench flexing of tenths of thousands of inches is avoided. Note that the optical bench design is limited by the distortion of the optical path that is tolerable, and is not governed by the structural survival of applied environmental loads. 
     Thus, the purpose of the subject invention is to provide an optical bench that is stiff enough to maintain the optical path and uses a stiff-flexible mount which slightly deflects to accommodate differential expansion between the airframe and the optical bench. 
     As will be appreciated, a very very small deflection of the optical bench, if allowed, would be multiplied by the optics involved, meaning that very small deflections are to be scrupulously avoided. It will thus be appreciated that if the optical bench is at all deflected, the beam-bending results in a laser aiming error which is multiplicative due to the optics. 
     The result of the ability to side-by-side mount the laser and head means minimal penetration into the airframe and results in an overall outside height which does not significantly add to drag. The apparatus mounted on the optical bench is shrouded in an external pod that is conformal to the aircraft structure, thus to provide the subject integrated pod optical bench design. 
     The goal to provide the minimum penetration to the platform and minimum structural modification is achieved through the side-by-side mounting system made possible through the rigid optical bench and its kinematic mount. The subject system therefore provides a low-impact way to install an infrared countermeasure system to commercial and other aircraft so that they can be protected. 
     In one embodiment, the laser and the head are aligned to the optical bench using bushings and pins to accurately locate the devices on the optical bench. Note that for precision alignment, the location of the pins and the bushings are to tenths of thousands of an inch, thus to assure repeatability. 
     In summary, an integrated pod optical bench design is used to mount a laser and a directed infrared countermeasure head to an aircraft in which the optical bench is kinomatically mounted having two rigid points of connection to the aircraft&#39;s airframe and a stiff but flexible mount to isolate the optical bench from thermal stresses due to differing thermal co-efficient of expansion between the air frame and the optical bench. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a diagrammatic illustration of the subject optical bench/laser/directed infrared countermeasure head assembly illustrating the internal optical path from the laser to the head; 
         FIG. 2  is a diagrammatic illustration of the optical path from the laser to the directed infrared countermeasure head through a channel in the optical bench which re-directs light from the laser through the optical bench to the head; 
         FIG. 3  is a diagrammatic illustration of the kinematic three-point system for mounting the optical bench to the aircraft airframe involving two rigid bolts and a stiff but flexible mount at the opposite end of the optical bench to provide a three-point suspension system; 
         FIG. 4  is a diagrammatic illustration of the stiff but flexible mounting apparatus called a flexure for one of the three-point suspension points; 
         FIG. 5  is a diagrammatic illustration of the flexure showing an oval-shaped or elliptical ring which can flex as illustrated by the dotted lines; 
         FIG. 6  is a diagrammatic illustration of a pin and bushing assembly for the directed infrared countermeasure head showing locating pins relative to an optical input, and rigid bolts which bolt the head to the optical bench; 
         FIG. 7  is a cross-sectional view of a portion of the optical bench having a bushing into which is inserted one of the locator pins of  FIG. 6 ; and, 
         FIG. 8  is a diagrammatic illustration of the utilization of precision pins to locate the laser housing to the optical bench. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an integrated pod optical bench design is shown in which an optical bench  10  carries a high energy laser  12  positioned over an optical channel orifice  14  which introduces light from the laser through an internal channel  18  to an infrared countermeasure head  16 , with the light going from the laser through orifice  14  where it is re-directed laterally as illustrated at  18  and is then re-directed up into head  16 . 
     Referring to  FIG. 2 , in diagrammatic form, laser  12  is affixed to horizontal surface  22  of optical bench  10 , such that the beam from the laser enters orifice  14  where it is re-directed by mirror  24  down the length of channel  18 , where it is again re-directed orthogonally by mirror  24  into the base of head  16 . 
     It is this optical bench with its two components that is bolted to air frame  30  which constitutes a portion of the underside of the aircraft. A pod shown in dotted outline  32  shrouds the equipment to provide a conformal path at the underside of the aircraft. 
     As mentioned hereinbefore, it is a prime concern that the optical bench not bend, bow or otherwise become distorted during thermal cycling in which there is a difference in thermal co-efficient of expansion between the optical bench and the air frame. 
     In order to provide the subject kinematic mount for the optical bench so as to eliminate the possibility of any flexure or bending and referring to  FIG. 3 , optical bench  10  is secured to the air frame using two rigid bolts  40  and  42  at one end of the optical bench and flexure  50  at the opposite end of the optical bench. The flexure is actually a stiff but flexible pivot which is secured at one portion to the air frame and another portion to the optical bench. 
     Referring to  FIG. 4 , flexure  50  is an apertured rectilinear structure embedded into optical bench  10  and in one embodiment has an elliptical cross-section as illustrated. 
     Referring to  FIG. 5 , it can be seen that flexure  50  has an elliptical aperture or hole  52  in a rectilinear ring-like structure  54 , with the structure  54  being affixed to the optical bench  10  by a bolt  56 , whereas the diametric opposite side of flexure  50  is affixed to the air frame platform  30  with a bolt  58 . Flexure  50  is provided in a cavity  60  in optical bench  10  in a loose fit such that any relative motion between the air frame platform and the optical bench is accommodated by distortion or flexing of flexure  50  as illustrated by dotted outline  54 ′. Thus, it can be seen that any relative movement between the air frame and the optical bench is accommodated by flexure  50 , such that there is no flexing, bowing or movement of the optical bench during thermal cycling. 
     For repeatable mounting of the various components and referring to  FIG. 6 , a flange  70  is provided on head  16  which is located on the optical bench through the utilization of locator pins  72  so as to locate the optical input  74  precisely at the aperture at one end of optical path  18 . Here, bolts  74  are used to secure the flange of the bench with its optical centerline centered upon the optical input  75  due to the positioning of the pins in bushings within the optical bench. 
     Referring to  FIG. 7 , a bushing  76  is precisely located in the optical bench with the DIRCM head pin  72  press fit into bushing  76  to locate the head with respect to the optical bench, and therefore maintain original optical alignment. 
     With respect to the laser, laser  12  as illustrated in  FIG. 8  is positioned on the optical bench by two locator pins, here shown at  80  and  82  on the top side of the laser housing so as to position the laser output  86  directly at aperture  14 . 
     Once aligned with the pins, the laser itself is held to the optical bench through bolt  84  to secure the laser to the optical bench, with the alignment being assured by the locator pins. As before, the locator pins go into precision bushings on the optical bench, with the locator pins being precisely positioned with respect to the optical output of the laser and aperture  14  (not shown in this figure) to repeatably locate the high energy laser with respect to the optical bench. 
     In summary, what is provided is an integrated optical bench for mounting a laser and infrared countermeasure head on an optical bench, with the alignment between the two units being preserved due to the rigidity of the optical bench and the mounting of the optical bench to the aircraft air frame using a kinematic mount, in one embodiment including a three-point mount in which two of the three points use rigid bolts and in which the third point spaced from these two points is a stiff but flexible mount involving a flexure that has a stiffness less still than the stiffness of the optical bench. 
     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.