Abstract:
A laser beam steering module includes an optics assembly that directs a first portion of a laser beam through an output aperture and a second portion of through a sensing path. The optics assembly adjusts a position of the laser beam through the output aperture and sensing path responsive to position control signals. A sensor array in the sensing path receives the second portion of the laser beam and in response thereto generates electrical beam position signals indicating a position of laser beam through the output aperature. The electrical beam position signals have values that are a function of a temperature of the sensor array and are used in generating the position control signals to adjust the position of the laser beam as a function of the values of the electrical beam position signals. A thermal stabilization circuit stabilizes the temperature of the sensor array responsive to thermal control signals.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority from U.S. Provisional Patent Application No. 60/576,542 filed on 3 Jun. 2004, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to countermeasures systems and methods, and more specifically to the control or steering of laser beams in directional infrared countermeasures systems and methods.  
       BACKGROUND OF THE INVENTION  
       [0003]     A threat launch detection system is a system that detects a weapon being directed at a target, with the target typically containing the threat launch detection system. In response to detecting a weapon directed at the target, which will be referred to as a threat or event throughout the present description, the threat launch detection system typically takes countermeasures to prevent the weapon from impacting the target. For example, an airplane may include a threat launch detection system designed to detect missiles fired at the airplane. When the system detects a missile, the system typically takes appropriate countermeasures in an attempt to prevent the missile from impacting the airplane, such as transmitting a signal to “jam” electronic circuitry in the missile that is guiding the missile towards the target.  
         [0004]     A conventional threat launch detection system is illustrated in  FIG. 1 , which more specifically depicts a block diagram of a directional infrared countermeasures (DIRCM) system  100 . The DIRCM system  100  includes a missile warning system  102  that detects the presence of a weapon or threat  104  directed at an airplane or other vehicle (not shown) containing the system. In the example of  FIG. 1 , the threat  104  is a missile that has been fired at the airplane containing the DIRCM system  100 . The missile  104  includes a guidance system (not shown) for sensing infrared energy emitted by the airplane and for directing the missile towards the airplane.  
         [0005]     The missile warning system  102  is typically a passive system that includes a sensor array (not shown) in combination with suitable optics (not shown) to provide a relatively wide field of view WFOV for missiles  104 . The wide field of view WFOV is the region of space surrounding the system  100  in which missiles  104  can be detected. The sensor array in the missile warning system  102  may be an array of ultraviolet (UV) or infrared (IR) sensors that capture a series of images within the field of view WFOV. Processing circuitry (not shown) in the missile warning system  102  analyzes the captured images to detect a threat and generates a coarse directional determination indicating an arrival angle at which the missile or other threat  104  is approaching the airplane containing the system  100 .  
         [0006]     The missile warning system  102  provides this determined arrival angle to a system controller  106  which, in response to the determined angle, applies signals to a fine tracking system  108  to position a fine track sensor (not shown) toward the threat  104  at the determined angle. More specifically, this fine track sensor in the system  108  is typically mounted on a gimbal (not shown) that rotates in response to the signals from the system controller  106  to direct the fine track sensor towards the determined angle and thereby toward the approaching missile  104 . The fine track sensor has a narrow field of view NVOV that is much smaller than the wide field of view WFOV to allow the fine tracking system  108  to precisely track the missile  104  or other threat positioned within the narrow field of view.  
         [0007]     The fine tracking system  108  further includes a jamming laser (not shown) that is also directed towards the missile  104  by the rotating gimbal. Once the gimbal has positioned the fine track sensor and jamming laser towards the missile  104 , the jamming laser is turned on and an infrared jamming laser beam from the laser illuminates the approaching threat  104  missile. This infrared laser energy is modulated in such a way that the when the guidance system in the missile  104  senses this energy the guidance system directs the missile away from the airplane. The fine tracking sensor in the fine tracking system  108  senses the position of the missile  104  during this time to accurately illuminate the missile  104  with energy from the jamming laser. This overall operation of the fine tracking sensor and jamming laser in the fine tracking system  108  may be referred to as “tracking” and “jamming” the threat  104 .  
         [0008]     The alignment of the jamming laser relative to a mounting datum (not shown) to which the laser is mounted and to the fine tracking sensor must be accurate for proper operation of the fine tracking system  108  in tracking and jamming the threat  104 , as will be appreciated by those skilled in the art. As a result, part of the installation and configuration or set-up of the DIRCM system  100  includes properly aligning the jamming laser to the mounting datum. This is typically a manual adjust and set procedure. For example, in one approach an installation person makes manual adjustments to mirrors that contained in optics that direct the laser beam. The installation person manually adjusts these mirrors while looking at the output location of the laser beam using a cooled IR camera to thereby properly align the laser beam. This alignment of the jamming laser beam, however, is time consuming and must be periodically repeated because the alignment tends to drift over time. Also, the alignment is a function of environmental conditions that may cause errors during operation of the tracking and jamming operation of the fine tracking system  108 .  
         [0009]     In another approach, the jamming laser includes a reference laser and this laser is visible by an un-cooled sensor. Control circuitry then automatically moves a mirror to position the reference laser beam in the correct location to thereby properly position the jamming laser beam. There is no guarantee with this approach that the reference laser is properly aligned to the jamming laser beam. Moreover, with this approach lateral movement of the reference laser beam can not be separated from angular movement and thus alignment accuracy of the jamming laser beam is compromised.  
         [0010]     Regardless of which one of these prior approaches for proper alignment of the jamming laser beam is utilized, the jamming laser beam generated by the jamming laser tends to some degree or another to change direction over time. This change is an inherent characteristic of lasers and presents a design issue for the DIRCM system  100  in that the system must have a beam divergence adequate to accommodate changes in direction or angle of the laser beam. Alternatively, the laser must be designed and manufactured in such a way as to ensure that changes in the direction or angle of the jamming laser beam are negligible.  
         [0011]      FIG. 2  is a graph illustrating typical shifts in beam position of a 1 milliradian radius laser beam as the beam shifts during ten minutes of operation of the laser generating the beam. The position of the laser beam in a first direction designated the X direction and in a second direction designated the Y direction are shown over seven short intervals T 1 -T 7 . During each of these time intervals T 1 -T 7 , the top line indicates shifting of the laser beam in the X direction and the bottom line indicates shifting of the beam in the Y direction. Over the total ten minute (600 seconds) time period covered by the graph, the centroid of the laser beam shifts over 2 milliradians in the Y direction as indicated by a position of the beam at the start of interval T 1  of approximately 7.3 milliradians and a position of the beam at the end of interval T 7  of approximately 5.2 milliradians. If the laser beam was expanded to accommodate this shift, and still given a 1 milliradian error budget to the rest of the system, 90% of the beam energy would be wasted, as will be appreciated by those skilled in the art.  
         [0012]     Current approaches that mechanically monitor and adjust the angle of the laser beam have several potential issues. The first is that sensors sensitive laser energy at around 4 microns need to be cooled. Second, mirror actuators are susceptible to vibration and shock and have limited frequency responses for adjusting the position of the laser beam. A third potential issue is that most control loops for controlling such mirrors and actuators are digital control loop and thus have the accompanying issue of aliasing with the movement and firing frequencies of the laser.  
         [0013]     There is a need for improved methods and systems for aligning the position of a laser beam and maintaining this alignment over time in countermeasures systems.  
       SUMMARY OF THE INVENTION  
       [0014]     According to one aspect of the present invention, a laser beam steering module includes an optics assembly adapted to receive position control signals. The optics assembly is operable to direct a first portion of a laser beam through an output aperture and a second portion of the laser beam through a sensing path. The optics assembly is further operable to adjust a position of the laser beam through the output aperture and the sensing path responsive to the position control signals. An uncooled sensor array is positioned in the sensing path to receive the second portion of the laser beam and is operable responsive to the received portion of the laser beam to generate electrical beam position signals indicating a position of laser beam through the output aperture. The electrical beam position signals having values that are a function of a temperature of the sensor array and are adapted to be used in generating the position control signals to adjust the position of the laser beam as a function of the values of the electrical beam position signals. A thermal stabilization circuit is coupled to the sensor array and is adapted to receive thermal control signals. The thermal stabilization circuit is operable responsive to the thermal control signals to stabilize the temperature of the sensor array. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a functional block diagram illustrating the operation of a conventional directional infrared countermeasure system.  
         [0016]      FIG. 2  is a graph illustrating typical shifts in beam position of a 1 milliradian radius laser beam as the beam shifts during ten minutes of operation of the laser generating the beam.  
         [0017]      FIG. 3  is a functional block diagram of a directional infrared countermeasure system including a fine tracking system having a laser steering module according to one embodiment of the present invention.  
         [0018]      FIG. 4  is a functional cross-sectional view of the laser steering module of  FIG. 3  according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]      FIG. 3  is a functional block diagram of a directional infrared countermeasures system  300  including a fine tracking system  302  containing a laser jamming component  304  according to one embodiment of the present invention. The laser jamming component  304  includes a laser assembly  306  that generates a laser beam  308  that is applied to a laser steering module  310 . In operation, the laser steering module  310  includes thermal-stabilization circuitry and optics that provide accurate sensing of a position of the laser beam  308  and that adjust for variations in this position over time to ensure proper operation of the fine tracking system  302 , as will be explained in more detail below. The countermeasures system  300  further includes a missile warning system  312  and a system controller  314  that operate in the same way as previously described for the missile warning system  102  and system controller  106  of  FIG. 1 . Thus, for the sake of brevity, the operation of the missile warning system  312  and system controller  314  will not again be described in detail.  
         [0020]     In the following description, certain details are set forth in conjunction with the described embodiments of the present invention to provide a sufficient understanding of the invention. One skilled in the art will appreciate, however, that the invention may be practiced without these particular details. Furthermore, one skilled in the art will appreciate that the example embodiments described below do not limit the scope of the present invention, and will also understand that various modifications, equivalents, and combinations of the disclosed embodiments and components of such embodiments are within the scope of the present invention. Embodiments including fewer than all the components of any of the respective described embodiments may also be within the scope of the present invention although not expressly described in detail below. Finally, the operation of well known components and/or processes has not been shown or described in detail below to avoid unnecessarily obscuring the present invention.  
         [0021]     The laser steering module  310  includes an optics assembly  316  that receives the laser beam  308  and splits the laser beam into a first portion  318  and a second portion  320 . The optics assembly  316  directs the second portion  320  of the laser beam  308  outward into a narrow field of view NFOV to “jam” a threat  322  initially detected by the missile warning system  312 . The optics assembly  316  directs the first portion  318  of the laser beam  308  to provide a far-field view of this portion of the beam to an uncooled array  324  of infrared sensors. In response to the first portion  318  of the laser beam  308 , the array  324  generates beam position signals  326  indicating a position of the laser beam. A thermal stabilization circuit  328  is coupled to the array  324  and controls or stabilizes the temperature of the sensors to ensure the temperature of the sensors is relatively constant or stable and in this way the beam position signals  326  from the sensors do not vary as a function of temperature.  
         [0022]     In response to the beam position signals  326  from the sensor array  324 , a controller  330  applies position control signals  332  to the optics  310  to thereby adjust the position of the first and second portions  318  and  320  of the laser  308 . In this way, the sensor array  324  senses the position of the first portion  318  of the laser beam  308  and generates corresponding beam position signals  326 . The position of the second portion  320  of the laser beam  308  is a function of the position of the first portion  318 , and in this way the controller  330  utilizes the beam position signals  326  to control the optics assembly  316  to thereby adjust the position of the second portion of the laser beam to properly illuminate threats  322  within the narrow field of view NFOV. The alignment or position of the jamming laser beam  320  relative to a mounting datum (not shown) and to a fine tracking sensor (not shown) must be accurate for proper operation of the fine tracking system  302  in tracking and jamming the threat  322 . Note that the controller  330  may also generate additional control signals  334  to control other components in the laser jamming component  304 , such as the thermal stabilization circuit  328 .  
         [0023]     In operation of the countermeasures system  300 , the missile warning system  312  detects the presence of a threat  322  directed at an airplane or other vehicle (not shown) containing the system. As previously mentioned, the missile warning system  312  is typically a passive system that includes a sensor array (not shown) in combination with suitable optics (not shown) to provide a relatively wide field of view WFOV for threats  322 . The sensor array in the missile warning system  102  may be an array of ultraviolet (UV) or infrared (IR) sensors that capture a series of images within the field of view WFOV. Processing circuitry (not shown) in the missile warning system  312  analyzes the captured images to detect a threat  322  and generates a coarse directional determination indicating an arrival angle at which the threat is approaching the airplane containing the system  300 .  
         [0024]     The missile warning system  312  provides this determined arrival angle to the system controller  314  which, in response to the determined angle, applies signals to the fine tracking system  302  to position a fine track sensor (not shown) toward the threat  322  at the determined angle. More specifically, this fine track sensor in the fine tracking system  302  is typically mounted on a gimbal (not shown) that rotates in response to the signals from the system controller  314  to direct the fine track sensor towards the determined angle and thereby toward the approaching threat  322 . The fine track sensor has the narrow field of view NFOV that is much smaller than the wide field of view WFOV to allow the fine tracking system  302  to precisely track a threat  322  positioned within the narrow field of view.  
         [0025]     Once the fine tracking system  302  is properly positioned and begins tracking the threat  322 , the laser assembly  306  generates the laser beam  308  and the optics assembly  316  directs the second portion  320  of this laser beam into the narrow field of view NFOV to “jam” the threat  322 . During this operation, the sensor array  324  develops the beam position signals  326  indicating the position of the first portion  318  of the laser beam and thereby indicating the position of the second portion  320  of the laser beam that is jamming the threat  322 . In response to the beam position signals  326  from the sensor array  324 , the controller  330  generates the position control signals  332  as required to control the optics assembly  316  to adjust the position of the first and second portions  318  and  320  of the laser beam  308 .  
         [0026]     In the system  300 , the sensor array  324 , controller  330 , and optics assembly  316  operate in combination to maintain the alignment or proper position of the second portion  320  of the laser beam  308 . As the response of the sensors in the sensor array  324  would normally change as a function of temperature, the thermal stabilization circuit  328  maintains the temperature of the sensor array at a desired value and thus eliminates or greatly reduces any such changes. Also, in the laser jamming component  304  of  FIG. 3 a  control loop including the sensor array  324 , controller  330 , and optics assembly  316  may be implemented entirely through analog circuitry, and in this way aliasing effects that may result with digital implementations of such a control loop are eliminated. Finally, note that the second portion  318  of the laser beam  308  provided by the optics assembly  316  provides the sensor array  324  with a far-field view of this portion of the laser beam. Such a far-field view ensures that the optics assembly  316  only corrects for angular changes of the laser beam  308  (and corresponding angular changes of the first and second portions  318  and  320 ) while displacement effects are ignored since correction of such displacement effects is not typically required for proper operation of the system  300 . The controller  330  may include flash memory (not shown) for storing various operating parameters of the system  300 .  
         [0027]      FIG. 4  is a functional cross-sectional view of the laser steering module  310  of  FIG. 3  according to one embodiment of the present invention. The laser beam steering module  310  includes a housing  400  having an entrance aperture  402  through which the laser beam  308  enters the housing. The entrance aperture  402  may be merely an opening or may include a panel that is transparent to the laser beam  308 , with different suitable materials being selected depending on the wavelength of the laser beam. In one embodiment, the aperture  402  has a diameter of approximately 1 cm.  
         [0028]     The laser beam  308  propagates through the entrance aperture  402  and reflects off a front surface of a micro-electromechanical system (MEMS) mirror  404 . Although the mirror  404  is described as being a MEMS component in the embodiment of  FIG. 4 , the mirror need not be a MEMS component but can be formed from other equivalent low-mass agile mirrors. The beam reflected off the MEMS mirror  404  is designated as a reflected beam  406 , and the MEMS mirror  404  rotates about two axes in response to applied control signals (not shown) to thereby redirect or steer the reflected beam  406  in a desired direction. More specifically, the MEMS mirror  404  rotates about the two axes to reposition the reflected beam  406  in two dimensions relative to a surface of a beam splitter  408 . The beam splitter  408  splits the reflected beam  406  into the first portion  318  ( FIG. 3 ) and into the second portion  320  ( FIG. 3 ), with the second portion typically containing 95% or more of the power of the laser beam  308  and the first portion containing the remaining power. The second portion  320  propagates an exit aperture  410  that may be merely an opening or may include a panel that is transparent to the second portion. This panel functions as an alignment datum for the rest of the fine tracking system  302  ( FIG. 3 ).  
         [0029]     The first portion  318  of the reflected beam  406  reflects off a front surface of a flat mirror  412 , with this reflected beam being designated as a beam  414 . The reflected beam  414  thereafter reflects off a front surface of an off-axis parabolic mirror  416  that functions to collimate the reflected beam and thereby generate a collimated beam  418 . The collimated beam  418  is incident upon the sensor array  324  ( FIG. 3 ), which is an un-cooled infrared quad cell detector in one embodiment of the present invention. Such a quad cell detector includes four infrared sensors arranged adjacent one another in two rows and two columns. The off-axis parabolic mirror  416  has a focal point at a front surface of the quad cell detector  324 , which develops the beam position signals  326  (not shown in  FIG. 4 ) in response to the incident collimated beam  418 .  
         [0030]     The collimated beam  418  provides the quad cell detector  324  with a far-field view of this beam since the collimated beam has parallel rays as would be the case from a source very far from the detector. The thermal stabilization circuit  328  ( FIG. 3 ) is physically coupled to the quad cell detector  324  to cool the detector and maintain each of the cells at a desired temperature. A heat sink  420  may be coupled to the stabilization circuit  328  to dissipate heat received from the detector  324 . The thermal stabilization circuit  328  may include comparator and reference circuitry utilized to control the temperature of the quad cell detector  324 . An electrical connector  422  is formed in the housing  400  and includes connections to components in the steering module  310  to provide power to such components and also to provide signals from such components, like the beam position signals  326  from the sensor array  324 , to the controller  330  ( FIG. 3 ).  
         [0031]     In the embodiment of  FIG. 4 , the exact relative orientation of the components in the laser steering module  310  can be tailored to the overall laser/beam director integration in the system. In the laser steering module  310  the quad cell detector  324  is ideally positioned precisely at the focal point of the parabolic mirror  416 . This is true because even though the quad cell detector  324  is insensitive to uniform defocusing of the collimated beam  418  any deviation from having the focal point of the parabolic mirror  416  at the quad cell detector means that the steering module  310  is susceptible to centroid shifts if the laser beam  308  is displaced rather than tilted. As the overall module  310  is not sensitive to displacements of the laser beam  308  but is sensitive to angle changes of the laser beam, the quad cell detector  324  must only provide information on angle, which is why the first portion  318  of the laser beam  308  is collimated with the parabolic mirror  416  rather than using a plane mirror.  
         [0032]     Dimensions are ordinarily not critical in the module  310 , but what is typically maintained is the shape of the module meaning no deformations of the optical path. Also note that threshold levels may need to be added to the beam position signals  326  from the quad cell detector  324  to ensure that corrections for “noise centroid” shifts in between pulses of the laser beam  308  are not attempted to be corrected for by the laser jamming component  304 . Electrical interconnections inside the module  310  may be via flex circuit.  
         [0033]     One skilled in the art will understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, many of the components described above may be implemented using either digital or analog circuitry, or a combination of both, and also, where appropriate, may be realized through software executing on suitable processing circuitry. It should also be noted that the functions performed by the components  302 - 334  in the system  300  of  FIG. 3  can be combined to be performed by fewer elements and divided and performed by more elements, depending upon the application of the system and other factors as well. Therefore, the present invention is to be limited only by the appended claims.