Abstract:
A laser autocollimator assembly that provides an objective, unambiguous, and simple means to achieve precise (for example, arc second or sub arc-second) alignment between the laser autocollimator assembly and a reference surface. The laser autocollimator assembly relies on optical parasitic interference between a reflected beam from the reference surface and the laser beam in the laser cavity that, when alignment is achieved, results in a disruption of the action of the laser, resulting in a reduction in the output power level of the laser beam. By monitoring the power level of the laser beam, for example monitoring the power level of the reflected beam, it can be determined that alignment has been achieved when the power level of the laser beam has been reduced to a minimum level. The power level can be automatically monitored, thereby eliminating the need for user interpretation.

Description:
[0001]    This invention was made with Government support under Contract Number N00030-14-C-0002 awarded by The Department of The Navy, Strategic Systems Programs. The Government has certain rights in the invention. 
     
    
     FIELD 
       [0002]    This disclosure relates to an autocollimator, more specifically to a laser autocollimator that can determine alignment between the laser autocollimator and a reference surface using optical parasitic interference between the laser beam and a return reflected beam. 
       BACKGROUND 
       [0003]    An autocollimator works by projecting light from a light source onto a target surface which deflects the light, and measuring deflection of the returned light against a scale such as cross hairs. In nearly all cases, user interpretation is required to determine whether alignment, defined as the perpendicularity of the light source to the target surface, has been achieved. 
       SUMMARY 
       [0004]    A laser autocollimator assembly is described that provides an objective, unambiguous, and simple means to achieve precise (for example, arc second or sub arc-second) alignment between the laser autocollimator assembly and a reference surface. The laser autocollimator assembly described herein relies on optical parasitic interference between a return reflected beam from the reference surface and the laser beam in the laser cavity that, when alignment is achieved, results in a disruption of the action of the laser, resulting in a reduction in the output power level (i.e. reduction in the optical gain) of the laser beam. By monitoring the power level of the laser beam, it can be determined that alignment has been achieved when the power level of the laser beam has been minimized. The minimized power level is an indication of alignment to the reference surface. In some embodiments, the power level can be automatically monitored, thereby eliminating the need for user interpretation. 
         [0005]    The power level can be monitored in any suitable manner. For example, the power level of a return reflected beam that results from the laser beam impacting on the reference surface can be monitored by a power detector. In another embodiment, the power of the transmitted laser beam from the laser transmitter can be monitored by a power detector. Any power level detection, whether of the transmitted laser beam, the return reflected beam, or other power level detection, that reflects the reduction in power resulting from the optical parasitic interference that occurs upon alignment can be utilized. 
         [0006]    The laser autocollimator assembly includes a laser transmitter, a beam splitter and a power detector. A laser beam from the laser transmitter is directed through the beam splitter onto a reference surface. The laser autocollimator assembly and the reference surface are adjusted relative to one another so that a reflected beam resulting from the laser beam impacting on the reference surface interferes with the laser beam transmitted from the laser transmitter. In some embodiments, the position of the laser autocollimator assembly can be adjusted relative to the reference surface to achieve alignment. In other embodiments, the position of the reference surface can be adjusted relative to the laser autocollimator assembly to achieve alignment. In still other embodiments, the positions of both the laser autocollimator assembly and the reference surface can be adjusted to achieve alignment. In one embodiment, the power detector detects the power level of the reflected beam, and when the detected power level of the reflected beam is minimized, the laser autocollimator assembly and the reference surface are determined to be in alignment. 
         [0007]    In some embodiments, when it is desired to maintain alignment once it is achieved, the positions of one or both of the laser autocollimator assembly and the reference surface can be automatically or manually adjusted so that the detected power level of the reflected beam is maintained at the minimized power level. In other embodiments, once alignment has been achieved, deflections of the reference surface relative to the laser autocollimator assembly can be automatically monitored based on displacement of the optical axis of the transmitted laser beam from the optical axis of the reflected beam, as well as by variations in the detected power level of the reflected beam from the minimized power level. 
         [0008]    In one embodiment, a display device that can be part of or connected to the power detector can display a digital readout of real-time position coordinates of the laser beam and the reflected beam relative to one another, including at initial alignment and during dynamic changes (for example flexure, misalignment, deformation, deflection, and the like) of the reference surface, to arc-second or sub arc-second accuracies through the use of a reflected beam position sensor. The display device can also display a digital readout of the detected power level, either separately from or in addition to the position coordinates. 
         [0009]    The described laser autocollimator assembly can be used to achieve precise, for example arc second or sub arc-second, initial alignment between the laser autocollimator assembly and the reference surface. In some embodiments, the laser autocollimator assembly and the reference surface can be separated by a large linear distance such as, but not limited to, greater than about 10 feet or more, or greater than about 40 feet or more. 
         [0010]    Example applications of the laser autocollimator assembly and techniques described herein can include, but are not limited to: optical alignment; civil and commercial surveying and alignment; monitoring of sway, flexure and other deformations of bridges, buildings and other structures; designing earthquake resistant structures; docking of aircraft, surface ships, underwater vehicles, and space craft; alignment during in-flight refueling of aircraft; alignment of laser weaponry; surgical applications; gem cutting; and many others. In one specific embodiment, the laser autocollimator assembly and the techniques described herein can be used to align an Inertial Navigation Unit (INU) to an Optical Reference Assembly (ORA). In another specific embodiment, the laser autocollimator assembly and the techniques described herein can be used for measuring/registering an antenna housing relative to itself or to a permanent reference “monument”. 
     
    
     
       DRAWINGS 
         [0011]      FIG. 1  illustrates an example of a laser autocollimator assembly described herein that is projecting a laser beam onto a reference surface. 
           [0012]      FIG. 2  is a three dimensional chart plotting detected laser power versus displacement of the laser autocollimator assembly in an x-direction and a y-direction, illustrating the drop off in power of the return reflected beam from the reference surface upon achieving alignment. 
           [0013]      FIG. 3  is a chart of the power of the return reflected beam versus an x-coordinate of a two-dimensional coordinate system established by the power detector of the laser autocollimator assembly illustrating the drop off in power of the return reflected beam in the x-coordinate direction upon achieving x-coordinate direction alignment. 
           [0014]      FIG. 4  is a chart of power of the return reflected beam versus a y-coordinate of the two-dimensional coordinate system established by the power detector of the laser autocollimator assembly illustrating the drop off in power of the return reflected beam in the y-coordinate direction upon achieving y-coordinate direction alignment. 
           [0015]      FIG. 5  depicts a representation of the profiles of both the transmitted laser beam and the return reflected beam viewed from the direction A-A in  FIG. 1 , with the profiles slightly misaligned but approaching alignment. 
           [0016]      FIG. 6  depicts a display that provides a digital readout of the real-time position coordinates and detected power of the return reflected beam as determined by the power detector of the laser autocollimator assembly. 
           [0017]      FIG. 7  depicts an embodiment where the position of the laser autocollimator assembly can be adjusted using actuators. 
           [0018]      FIG. 8  depicts an embodiment where the position of the reference surface can be adjusted using actuators. 
           [0019]      FIG. 9  depicts an embodiment that uses feedback control to maintain alignment of the transmitted laser beam and the return reflected beam. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    With reference initially to  FIG. 1 , an embodiment of a laser autocollimator assembly  10  is illustrated. The assembly  10  includes a laser transmitter  12 , a beam splitter  14 , and a power detector  16 . The laser transmitter  12  generates and transmits a laser beam  18  through the beam splitter  14  and onto a reference surface  20 . The reference surface  20  reflects a return beam or wave  22  back toward the assembly  10 . The beam splitter  14  deflects the return reflected beam  22  toward the power detector  16  which detects the power of the reflected beam  22 . 
         [0021]    Depending upon the relative orientations of the assembly  10  and the reference surface  20 , the reflected beam  22  interferes with the optical gain of the laser cavity in the laser transmitter  12 . This interference is referred to herein as optical parasitic interference. This interference corrupts the output power of the laser transmitter  12  which is reflected in a drop in the power output of the laser transmitter  12  and therefore a drop in the power of the transmitted laser beam  18  and a drop in the power of the resulting reflected beam  22 . By adjusting the relative positions of the assembly  10  and the reference surface  20 , the parasitic interference, and the resulting drop in power of the transmitted laser beam  18  and the reflected beam  22 , can be varied. Maximum interference, and thus maximum reduction in power, occurs when the transmitted laser beam  18  and the reflected beam  22  are aligned with one another. Therefore, by monitoring the power of the reflected beam  22 , one can determine alignment when a minimum power level of the reflected beam  22  is reached. 
         [0022]    The charts in  FIGS. 2-4  show the drop in power that occurs when the transmitted laser beam  18  and the reflected beam  22  become aligned in both an x-coordinate direction and a y-coordinate direction. At a point where the laser power is at its minimum, the assembly  10  can be considered aligned with the reference surface  20 , and the x-coordinate and the y-coordinate at alignment can be considered an origin (x=0, y=0) in an x, y coordinate system for example if one wishes to quantify subsequent deflections of the reference surface. 
         [0023]      FIG. 5  depicts a profile of the transmitted laser beam  18  and the reflected beam  22  viewed from the direction A-A in  FIG. 1 . The transmitted beam  18  includes an optical axis  34  and the reflected beam  22  includes an optical axis  36 . The optical axes  34 ,  36  of the two beams  18 ,  22  are offset from one another in an x-coordinate direction  30  (or horizontal direction when viewing  FIG. 5 ) and in a y-coordinate direction  32  (or vertical direction when viewing  FIG. 5 ). The relative positions of the assembly  10  and the reference surface  20  can be adjusted to minimize the x, y displacement so that the optical axis  34  of the transmitted beam  18  is substantially aligned with the optical axis  36  of the reflected beam  22  at which point the power measured by the power detector  16  will be at its minimum. It is to be noted that the measured minimum power will not be zero. However, the minimum power will be measurably less than the measured power when the transmitted beam  18  and the reflected beam  22  are not achieving maximum parasitic interference. 
         [0024]      FIG. 6  depicts a display  40  that can be part of, or suitably connected to, the power detector  16  of the assembly  10 . The display  40  can provide a digital readout of the real-time x, y position coordinates  42  of the optical axis  36  of the reflected beam  22  relative to the optical axis  34  of the transmitted beam  18 . In this example, it is assumed that the optical axis  34  of the transmitted beam is the origin (x=0, y=0). In the representative example illustrated in  FIG. 5 , it is seen that the optical axis  36  of the reflected beam  22  is located vertically below the optical axis  34  (i.e. a negative y-direction) a certain distance and located horizontally to the right of the optical axis  34  (i.e. a positive x-direction) a certain distance. The x-direction displacement and the y-direction displacement can be measured in, for example, inches or other units of measure. 
         [0025]    The display  40  can also provide a digital readout of the detected laser power  44  either separately from the x, y position coordinates  42  or in addition to the coordinates  42  as illustrated in  FIG. 6 . When the detected laser power reaches its minimum level, the reflected beam  22  is considered aligned with the transmitted beam  18 , and the assembly  10  is considered aligned with the reference surface  20 . In this example, the detected laser power  44  is displayed as a percentage of an expected maximum laser power. However, the detected laser power  44  can be visually displayed in other units of measure and in visual formats other than numerical numbers. 
         [0026]    In one embodiment, the detected laser power  44  can be the primary variable that determines when the transmitted laser beam  18  and the reflected beam  22  are aligned with one another, which in turn indicates whether the assembly  10  and the reference surface  20  are aligned. When the detected power level  44  is at its minimum level, the assembly  10  and the reference surface  20  are considered to be aligned. The detected power level  44  could be at its minimum level even though the x, y position coordinates  42  are not precisely zero. However, once the minimum power level  44  is reached, the corresponding x, y position coordinates  42  can be considered the origin for measuring any subsequent x, y positional displacements, or the display  40  can be zeroed out at that time so that the x, y position coordinates  42  are displaying zero. 
         [0027]    As indicated above, the minimum detected power level can be achieved by adjusting the relative positions of the assembly  10  and the reference surface  20 . Such adjustments can occur automatically or manually. For example,  FIG. 7  depicts an embodiment where the position of the assembly  10  can be adjusted. In this example, position adjustment of the assembly  10  can be achieved using a plurality of actuators  50   a,    50   b,    50   c  that are connected to a support structure  52  on which the assembly  10  is mounted. The support structure  52  can include a lower support  54  and an upper support  56  on which the assembly  10  is mounted. The lower support  54  and the upper support  56  form a three-dimensional universal mount whereby the lower support  54  and the upper support  56  can be tilted together by the actuators  50   a,    50   b  relative to an x-y plane and can be moved linearly up and down together by the actuators  50   a,    50   b  in a z-axis direction. In addition, the upper support  56  can be rotated relative to the lower support  54  by the actuator  50   c  about the z-axis. The actuators  50   a - c  can be any type of actuators suitable for achieving precise, fine positional adjustments of the assembly  10 . In some embodiments, the actuators  50   a - c  can be hydraulic, pneumatic, or piezoelectric actuators, or manually actuated. 
         [0028]    In other embodiments, the position of the reference surface  20  can be adjusted in order to achieve alignment.  FIG. 8  depicts an embodiment where the position of the reference surface  20  can be adjusted using one or more actuators  60   a,    60   b  that can be similar in construction to the actuators  50   a - c.  The actuators  60   a,    60   b  can be directly connected to the reference surface  20 , or the reference surface  20  can be mounted on a support structure (not illustrated) that is connected to the actuators  60   a,    60   b.    
         [0029]    In still other embodiments, the position of the assembly  10  can be adjusted (for example as illustrated in  FIG. 7 ) and the position of the reference surface  20  can be adjusted (for example as illustrated in  FIG. 8 ) to achieve alignment. 
         [0030]    Once alignment is achieved, a control system that is connected to the actuators  50   a - c  and/or  60   a - b  can detect deviations from the initial alignment, and can adjust the position of the assembly  10  relative to the reference surface  20  so that the detected output power level  44  is maintained at the minimum power level. For example, with reference to  FIG. 9 , a control system  70  is illustrated. The control system  70  uses feedback control to maintain alignment of the transmitted laser beam  18  and the reflected beam  22 . For example, the power detector  16  detects the power level, and based on deviations from the minimum power level at alignment, control signals  72   a,    72   b  are sent to a pitch and roll controller  74   a  and to an azimuth controller  74   b.  The pitch and roll controller  74   a  suitably controls the actuators  50   a,    50   b  to achieve the desired positional adjustments in pitch and roll directions (i.e. tilting of the assembly  10 ), while the azimuth controller  74   b  suitably controls the actuator  50   c  to achieve the desired positional adjustments in an azimuthal direction (i.e. rotation of the assembly  10  about the z-axis). In another embodiment, the controllers  74   a,    74   b  could alternatively be controlled via suitable controls signals based on x, y positional deviations from the x, y origin. 
         [0031]    In another embodiment, once alignment is achieved, the techniques described herein can be used to determine a magnitude and direction of a deflection of the reference surface  20  relative to the assembly  10 . For example, once alignment is achieved and the origin of the x, y coordinate system is established, deflections of the reference surface  20  and the assembly  10  relative to one another will result in deviation of the detected power level from its minimum value, but also a deviation in the x, y position coordinates. In one example, the x, y positional coordinates deviation can be used, together with the distance between the assembly  10  and the reference surface  20  which is known, to calculate the magnitude, as well as the direction, of the deflection of the reference surface using simple geometry. In another example, a table of detected power levels and corresponding deflection magnitudes can be established and stored in suitable memory. Thereafter, by accessing the table with the actual detected output power level, the corresponding deflection magnitude can then be determined. 
         [0032]    The examples disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.