Method and apparatus for aligning collimated light beams

An optical bench configured to hold two or more sources of collimated light utilizes prisms that when rotated generate rings of possible angular directions of light that overlap at one or more locations at which the two or more beams are coaligned.

BACKGROUND OF INVENTION

Soldiers are required to rapidly acquire, identify, and accurately fire on enemy targets and may use weapon-mounted sights with visible and infrared light sources generated by one or more lasers to produce collimated beams of light for use in daytime and nighttime missions. These sights may be mounted on small arms such as the M4A1 carbine and other weapons and are used to provide better target observation, illumination, and marking. Coaligned visible and infrared lasers may be boresighted to assist in operation of the weapon. Unlike visible lasers, infrared lasers are only viewable with a night vision device, a phosphorescence material, thermal imager, or other device of similar function. Coaligning visible and infrared lasers allows a soldier to boresight the infrared laser of a weapon mounted sight using just the visible laser (i.e. without the need for a night vision device to see the infrared light beam).

An optical bench sub-assembly located within a weapon mounted housing may be used to hold the electrical and optical components of the coaligned and collimated lasers. The housing may provide protection from unintended contact or debris. The housing may be coupled to a weapon with a suitable attachment mechanism, for example a rail grabber, slide-lock mechanism, or other clamp.

Mechanical adjustors extending through the housing and in contact with the optical bench sub-assembly may be used to steer the optical axis of the coaligned light beams relative to the housing. This may enable a user to boresight the previously coaligned light beams to some reference, such as a point of impact of a projectile at a known distance or a barrel mounted boresight laser.

The degree of coalignment between two collimated lasers can be quantified by measurement tools and expressed as an angle, typically given in thousandths of a radian (mRad). The degree of coalignment accomplished by passive, snap-together mechanical designs, that is designs which do not employ active feedback, are limited by a tradeoff between size and the precision of the components used. For example, the coalignment of a weapon mountable snap-together design might be realistically limited to about 15 mRad. For effective use at ranges of 200-800 meters, substantially better coalignment, e.g. less than 2.0 mRad, is desired which typically requires an active alignment scheme that employs feedback from a measurement device and adjustable compensators during the coalignment process.

DETAILED DESCRIPTION

FIG. 1Ais an isometric view of a weapon mountable sight100. The sight100may generate a first collimated light beam108, for example a visible light beam, and a second collimated light beam110, for example an infrared light beam, to provide better observation, illumination, and marking of a target. The first light beam108may be coaligned with the second light beam110. Coaligned is intended to mean that the two light beams (e.g. visible and infrared) are parallel to within an acceptable tolerance and collimated is intended to mean that the light has little or no divergence in each wavelength, i.e. it has a focus at infinity or some other distant point. The sight100may have a housing102for providing protection to internal components from unintended contact or debris. The sight100may be removably coupleable to a weapon504(seeFIG. 5) such as the M4A1 carbine or other weapon with a suitable attachment method, for example a rail grabber, slide-lock mechanism, or other clamp. Alternatively, a sight incorporating this embodiment may be incorporated in and/or formed as part of a weapon. For use after the sight100is coupled to a weapon, the housing102may have a first adjustor104and a second adjustor106to allow an operator to boresight the previously coaligned pair of first and second collimated light beams108,110with a projectile point of impact on a target at a known distance or with a boresight alignment tool, for example a barrel mounted boresight laser.

FIG. 1Bis an isometric view of an optical bench structure120. The optical bench structure120may have a pivot adjustor section120A, a dual barrel section120B, and a flexure section120C. The optical bench sections120A,120B, and120C may be a unitary piece or made of two or more pieces. The optical bench structure120can be made of metal, plastic, or other suitable material, or a combination thereof. The flexure section120C may have a groove112or other feature that allows the optical bench to be coupled to the housing102. The adjustors104,106may contact the pivot adjustor section120A to steer the coaligned pair of first and second collimated light beams relative to the housing102to allow the first beam108and second beam110to be boresighted to a weapon. The adjustors104,106may be orthogonally offset 90 degrees from each other to provide elevation and windage adjustment of the coaligned and collimated light beams108,110relative to the housing102. Springs or other biasing mechanisms may be used to provide a counter force to the adjustors104,106. Alternatively, electrically controllable actuators, for example MEMS or piezoelectric actuators, may be used to steer the light beams108,110.

FIG. 2Ais a section view andFIG. 2Bis an exploded view of an optical bench assembly130. The optical bench structure120may provide a frame to support electrical and optical components that may be secured to the optical bench structure120during the assembly process. The optical bench assembly130can incorporate two or more light sources of the same or different wavelength. In an exemplary embodiment two light sources are provided, one in the visible spectrum (400-750 nm), for example the output of a 635 nm laser diode, and one in the near infrared spectrum (750-3000 nm), for example the output of a 830 nm laser diode. A first laser diode202may be placed in the first barrel114of the optical bench structure120with its electrical leads extending out of an opening in the rear of the optical bench structure120. A laser retainer204may be placed in the first barrel114and contact the laser diode202. The laser retainer204may have external threads, grooves or other features that cooperate with threads or features on the inside surface of the first barrel114to hold the laser diode202in place. The first laser diode202may alternatively be inserted from the rear end of an optical bench. Epoxy or other adhesive or other suitable bond may be used to provide a more permanent bond between the laser retainer204and the optical bench structure120. Alternatively, epoxy, other adhesives, other suitable bonds, an adhesive free clip, or a compliant flexure may be used in place of a threaded retainer to secure the first laser diode202inside the first barrel114. The epoxy may be time, wavelength, or thermally sensitive to allow proper positioning prior to forming a permanent bond. Alternatively, the laser diode202and the laser retainer204may be hot or cold staked in place.

A collimating lens assembly206may next be placed in the first barrel114of the optical bench structure120. The collimating lens assembly206may have a collimating lens206A coupled to a lens retainer and/or lens cell206B as shown inFIG. 2A. The lens retainer and/or lens cell206B may have external threads, grooves or other features that cooperate with threads or features on the inside surface of the first barrel114. A slip or light press fit of precision bores between barrel114and collimating lens assembly206may be used as a positioning aid and to minimize precession during alignment. An assembler, using a hollow tool, may rotate or slide the collimating lens assembly206into the first barrel114until a collimated light beam extends through the hollow portion of the tool and onto a target or measurement device. The hollow tool may have end features that cooperate with features on the lens retainer and/or cell206B to allow it to rotate or slide the collimating lens assembly206. Epoxy or other adhesive or other suitable bond or mechanical restraint may be used to provide a more permanent stable bond between the collimating lens assembly206and the optical bench structure120. The epoxy may be time or wavelength or temperature sensitive to allow proper positioning prior to forming a permanent bond. The collimating lens assembly206may include a lens encapsulated by an overmold made of a suitable material, e.g. polyamide. Such an overmold is sometimes referred to as an IMA, in mold assembly, or as an overmold. Alternatively, a collimated light beam may be generated with a laser diode or other light source separated from a lens by a pin hole aperture. Lens assembly206could be manufactured as a single piece. For example, the elements of lens206A and the cell206B could be molded as a single piece out of an appropriate optical plastic, e.g. Zeonex® 480.

A prism208may next be placed in the first barrel114of the optical bench structure120. As shown inFIG. 3AandFIG. 3B, the prism208may have a central portion302surrounded by a rim portion304useful for mounting and orienting the prism208. As shown inFIG. 3C, the central portion302may have an incoming surface306spaced from and at a first angle α1to an outgoing surface308. Light entering the prism208may be refracted by the incoming surface306and light exiting the prism208may be refracted by the second surface308. Since the first surface306may be disposed at angle α1to the second surface308, the exiting light beam108may be directionally offset by an angle α2to the incoming light beam. The second angle α2may be a function of the first angle α1, the prism material, and the wavelength of the incoming light beam. The incoming surface306may be at an angle α3, for example 4-10 degrees off of perpendicular to the incoming light beam, to reduce reflection of the incoming light beam back towards the laser diode202, which could disrupt normal operation of the light source, for example by interfering with a power monitoring photodiode.

The prism208may have external threads, grooves, a precision outer diameter or other features that cooperate with threads or features on the inside surface of the first barrel114or the collimating lens assembly206. The prism208may be a moldable plastic, for example Zeonex® 480, an optical glass, or other suitable material. A plurality of prisms, each designed to refract light by a different angle α2, may be available to the assembler. Identifying features on the rim304may assist the assembler in distinguishing between these different prisms. The number of different available prisms may depend on a variety of factors including cost, precision of laser and lens alignment, and desired degree of resulting coalignment.

An assembler, using a hollow tool, may rotate the prism208(as discussed below) relative to the optical bench structure120with the exiting light beam extending on to a measurement device400(seeFIG. 4B). The hollow tool may have end features that cooperate with features on the prism208to allow it to rotate the prism208into a desired rotational position. The hollow tool may have component placement features, such as vacuum suction or grippers, to aid in the positioning of the prism208. Installation of prism208alters the direction of a light beam408exiting the collimating lens assembly206from a position on measurement tool400to a spot on a circle e.g., (108A,108B or108C) on measurement tool400. The location of the spot on measurement device400and the radius of the circle are driven by the characteristics and orientation of the prism208. By selecting one of the plurality of different prisms available, the light could be altered to a position on different circles, for example108A,108B or108C on measurement tool400. Rotation of prism208alters which position on the circle the light beam108falls onto. The measurements and calculations may allow for selection of an appropriate prism. The measurements and calculations may be employed during the assembly and alignment of the optical bench assembly130in the weapon mountable sight100. The weapon sight user is not exposed to the measurements and calculations associated with prism selection and orientation.

Likewise, laser diode210, laser retainer212, collimating lens assembly214with collimating lens214A, and prism216may be assembled and coupled to the optical bench structure120in similar fashion. Prism216may be rotated about its axis to generate a second ring of possible angular directions with the second collimated light beam110. Installation of prism216alters the direction of a light beam410exiting the collimating lens assembly214from a position on measurement tool400to a position on different circles, for example110A,110B or110C on measurement tool400. If prisms208and216have sufficient optical power to overlap the circles, a pair of prism orientations will exist where the light beams108and110are coaligned. Using measurement tool400, the choice of and orientation of prisms208and216can be made to achieve the most desirable coaligned solution. Typically, this desired solution is that closest to the mechanical neutral axis402of the optical bench structure120. Coalignment close to the mechanical neutral axis402of the optical bench structure120may reduce the amount of user manipulation of the first adjustor104and the second adjustor106required to boresight the weapon sight and may provide a greater range of boresight adjustment available to the user. Once the desired rotational positions of the prisms208,216has been determined, epoxy or other adhesive or other suitable bond may be used to provide a more permanent bond between the prisms208,216and the optical bench structure120. The epoxy may be time or wavelength or thermally sensitive to allow proper positioning prior to forming a permanent bond.

FIG. 4Ais a 3-D plot useful in explaining the relationship between the light beams408,410from the collimated light source before the light beams are refracted by prisms208,216respectively and a mechanical neutral axis402for the optical bench structure120or weapon mountable sight100. The collimated light beam408,410may be angularly displaced from the mechanical neutral axis402for the optical bench structure120by an angle α4in a first plane and an angle β4in a second and orthogonal plane. Placing an appropriately oriented prism in series with each collimated light source can reduce the angular displacement to a direction108,110closer to the mechanical neutral axis402for the optical bench structure120in addition to coaligning directions108and110.

FIG. 4Bshows a measurement tool400useful in coaligning exiting collimated light beams108and110. During factory alignment, the first collimated light beam408and the second collimated light beam410may be reflected off of a parabolic mirror, passed through an infinity conjugate lens, or aimed down an optical range and hit a detection surface which may be an optical target or focal plane array. Information is extracted from this detection surface to generate a quantitative representation. The measurement tool400can directly measure the angle of the respective collimated light beams408,410or108,110from the mechanical neutral axis402which may be the mechanical neutral axis for the weapon mountable sight100or optical bench structure120. The first angular light direction408is formed by the first laser diode202and first lens assembly206. The second angular light direction410is formed by the second laser diode210and second lens assembly214. The measurement tool is designed such that spatial offsets between light beams408,410or108,110(due to the light sources being physically offset in the optical bench structure120) do not adversely affect the angular measurements of measurement tool400. Any set of two or more light sources which overlap on measurement tool400are coaligned.

FIG. 4Bshows a first ring108A, a second ring108B, and a third ring108C of possible angular directions of light the first light beam108would trace as three different choices of prism208are rotated through a complete 360-degree rotation about its optical axis when prism208is placed in the path of light beam408. A fourth ring110A, a fifth ring110B, and a sixth ring110C of possible angular directions of light the light beam110would trace as three different choices of prism216are rotated through a complete 360-degree rotation about its optical axis when prism216is placed in the path of light beam410. Each of the three prisms208,216provides different angular degrees of refraction. The prism with the smallest first prism angle α1produces the smallest ring of possible angular directions of light,108A or110A, and the prism with the largest first prism angle α1produces the largest ring of possible angular directions of light,108C or110C. In the embodiment shown, the available prisms may generate four (4) mRad, twelve (12) mRad, and twenty (20) mRad of refraction. More or less choices and differing first prism angles α1may be used without departing from the scope of the invention.

As shown in this example, the first, second, third, fourth, fifth, and sixth rings intersect at twelve positions (shown by eleven circles and one square). At these twelve positions the light beams108and110are coaligned. Note that not all prism choices may result in a valid coaligned solution, for example circles108A and110A inFIG. 4Bdo not overlap. Given three choices of prism, the number of valid coaligned solutions can vary from zero to eighteen, the arbitrarily chosen representation inFIG. 4Bshows a configuration with (12) valid coaligned solutions. A processor coupled to the measurement tool400or a manual look-up table may help chose which prism the operator should place in the first barrel114and which prism the operator should place in the second barrel116based on which coaligned solution108,110is closest to the mechanical neutral axis402. In this example, the operator would install a first prism208configured to retract the incoming light beam40812 mRad (circle108B), and a second prism216configured to refract the incoming light beam41012 mRad (circle110B), with the hollow tool or other device and rotate the prisms208,216until the angular directions of light beams108and110are coaligned and aimed as close as possible to the mechanical neutral axis402. Prisms208,216may then be secured to the optical bench structure120. Note that there are two valid, coaligned solutions for every overlapping prism pair. The solution denoted by the square box onFIG. 4Bis the desired solution in the case where it is desired to aim the coaligned lasers as close as possible to the mechanical neutral axis402of optical bench structure120.

Once the beams108,110are coaligned and components are secured in place, the optical bench assembly130could then be inserted in the housing102. A user/shooter could then couple the housing102of the weapon mountable sight100to a weapon504and use the adjustors104,106to boresight the coaligned collimated light beams108,110as a combined pair in the optical bench assembly130to the weapon504.

The prism208may be a lens that utilizes geometry of the lens and/or index of refraction of the lens to refract the light beam at a slight angle. However, embodiments of the invention are not limited to this configuration, and other lens types and optical components may be used to generate the change in angle of the light beam, for example, mirrors and lenses with designed and controlled molecular orientation to control the refraction angle. Using diffraction gratings; be they ruled, holographic, or otherwise; to bend light at a slight angle are also a feasible implementation.

FIG. 5is a side view of a weapon504with a weapon mountable sight100coupled thereto consistent with an exemplary embodiment. When the weapon mountable sight100is pointed at a target506, the projected dots508,510from the coaligned collimated light beams108,110respectively may be offset from each other because of their physical offset in the optical bench. The dot508from the visible light source may be offset from a point of impact502of a projectile by a distance, “X” in the horizontal direction and a distance “Y” in the vertical direction when the target is at a known distance. The adjustors104,106may be used to steer the coaligned pair of collimated light beams108,110to be, within a finite angular tolerance, at a desired location relative to the center502of the target506to boresight the weapon mountable sight100to the weapon504. This desired location may be impacted by considerations such as the projectile dropping over distance, the type of projectile used, or other factors that impact the accurate firing of the weapon504.

Referring toFIG. 6, a first collimated light beam and a second collimated light beam may be coaligned according to an exemplary embodiment method600. The optical bench structure120may be used for the assembling process (block602). A first light source202may be secured to the optical bench structure120for producing a first light beam (block604). A second light source210may be secured to the optical bench structure120for producing a second light beam (block606). A first collimating lens206A may be secured to the optical bench structure120for collimating light from the first light source202(block608). A second collimating lens214A may be secured to the optical bench for collimating light from the second light source (block610). An appropriate prism may be chosen for each optical path based on a measurement taken. A first prism208may be used to refract the collimated light from the first light source (block612). A second prism216may be used to refract the collimated light from the second light source (block614). The first collimated light beam408and the second collimated light beam410may be coaligned by rotating one or more of the prisms208,216around a mechanical axis of the respective prism (block616). The prisms208,216may then be secured to the optical bench structure120or collimating lens assembly206,214providing two collimated and coaligned beams of light (block618).

It will be understood that the foregoing is only illustrative of the principles of the invention and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. Accordingly, such embodiments, for example focusing more than two light beams, will be recognized as within the scope of the present invention. Various aspects disclosed in the exemplary embodiments may be incorporated with aspects disclosed in other exemplary embodiments without departing from the scope of the invention. Persons skilled in the art will also appreciate that the present invention can be practiced by other than the previously described exemplary method, which are presented for purposes of illustration rather than of limitation and that the present invention is limited only by the claims that follow.