Optical scanning system using micro-electro-mechanical system (mems) micro-mirror arrays (MMAs)

An optical scanning system includes one or more Micro-Electro-Mechanical System (MEMS) Micro-Mirror Arrays (MMAs) used to scan a field-of-view (FOV) over a field-of-regard (FOR). The MEMS MMA is configured such that optical radiation from each point in the FOV does not land on or originate from out-of-phase mirror segments and a diffraction limited resolution of the optical system is limited by the size of the entrance pupil and not by the size of individual mirrors.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to optical scanning systems for passive/active sensors and transmitters, and particularly to the use of a Micro-Electro-Mechanical System (MEMS) Micro-Mirror Array (MMA) to scan the optical system's field-of-view (FOV) over a field-of-regard (FOR). The optical scanning system may be used as part of guided munitions, autonomous vehicles, cameras or the like.

Description of the Related Art

Gimbaled optical sensors are commonly used as part of guided munitions and possibly autonomous vehicles. Passive systems use light emissions e.g. IR or visible from a target to detect and track the target. Active systems use an on-board source to emit light e.g. IR or visible, or RF that is reflected from the target to detect and track the target. The active return may be used for ranging, simple guidance commands to a target centroid or active imaging. The on-board source may also be used for other applications. The passive and active systems are often combined.

A typical gimbaled optical sensor includes inner (nod) and outer (roll) gimbals positioned behind a protective dome or window that rotate about orthogonal axes to point an optical axis in a three-dimensional space to scan a FOV over a FOR. An off-gimbal detector is responsive to a band of wavelengths e.g. Visible or IR (SWIR, MWIR, NIR, LWIR, etc.) A telescope mounted on the inner gimbal along the optical axis collects light from the target within the FOV to form an intermediate image. Gimbal optics propagate the light over the outer (roll) and inner (nod) gimbals along an optical path while preserving image quality. Off-gimbal focus optics relay the intermediate image to the detector. In some applications, an Aperture Sharing Element (ASE) is positioned in a receive aperture to separate the incident light into different wavelength bands e.g. Visible and IR and direct the light to different detectors. In a passive system, the pointer detects only emissions from the target within the FOV of the telescope. In a passive system, pointing control of a transmitter is performed “open loop”, based only on the detection of the passive emissions of the target.

To add active capabilities, an off-gimbal optical source e.g., a laser, emits light in a narrowband around a specified wavelength. This transmit signal is routed along an optical path (free-space or fiber) along the gimbal axes to a transmit telescope where it is transmitted toward the target. The transmit telescope may be mounted off-axis from the receive telescope or a common Tx/Rx telescope may be used for both transmit (Tx) and receive (Rx). In the later case, an ASE may be positioned in a common aperture to couple the transmit signal from the optical source to the common Tx/Rx telescope and to couple the returned transmit signal and the passive emissions from the target to the detector. An additional ASE may be positioned in the receive path to separate the incident light into different wavelength bands and direct the light to different detectors. Processing of the active signal return again may provide for ranging, centroid guidance or active imaging. This allows for pointing control of a transmitter to be performed “closed loop” based on the desired and actual location of the laser spot on the target.

The aperture stop of an optical system is the aperture in the optical system that limits the diameter of the axial bundle of light that passes through the optical system from each point in the field of view. An entrance pupil is the image of the aperture stop as seen from object space. An exit pupil is the image of the aperture stop as seen from image space. The diffraction limited spot diameter refers to the smallest possible diameter that a distant point of light can be focused onto an image plane and determines the physical limit on the resolution that can be obtained in an imaging system. If the diameter of a focused point is large, the system will be low resolution. The diameter of the aperture stop (and its conjugates the entrance pupil and exit pupil) determines the best possible resolution that can be obtained from an imaging system (the diffraction limit). For imaging, the bundle of light from each point in the field of view passes through the entrance pupil (the image of the aperture stop in object space). If parts of the bundle of light are delayed and made out of phase to other parts of the bundle reaching the entrance pupil, then the diffraction limited resolution will not be determined by the size of the entrance pupil, but by the size of the largest segment of the axial bundle of light that is in-phase with itself.

For projecting light, the diffraction limit determines the smallest possible divergence of a collimated light source such as a laser. If parts of the bundle of light projecting out of the optical system are delayed and made out of phase (a large optical path difference) relative other parts of the bundle, the smallest possible divergence of the light source will be determined by the largest segment of the axial bundle of light that is in-phase with itself.

In certain applications, the optical sensor is positioned behind an optical window or dome, which may change the angle of each ray bundle associated with each point in the FOV and cause distortion in the image. Distortion may be mitigated by adding a corrector optical element.

SUMMARY OF THE INVENTION

The present invention provides an optical scanning system in which one or more Micro-Electro-Mechanical System (MEMS) Micro-Mirror Arrays (MMAs) is used to scan a field-of-view (FOV) over a field-of-regard (FOR). The MEMS MMA is configured in different embodiments such that the diffraction limited resolution of the system is determined by the size of the entrance pupil.

The optical scanning system includes an optical system in which one or more optical elements are configured to propagate optical radiation focused at a near point at an active optical component (e.g. an optical detector and/or source) and collimated at a distant point within the FOV. Optical radiation at each point in the FOV passes through the entrance pupil. Each MEMS MMA is responsive to command signals to partition the MMA into one or more mirror segments, each mirror segment including one or more mirrors, and to at least tip and tilt the mirrors in at least 2 degrees-of-freedom (DOF) to approximate a continuous reflective surface in each mirror segment at a specified scan angle to scan the FOV over the FOR such that optical radiation from each point in the FOV does not land on or originate from out-of-phase mirror segments and the diffraction limited resolution of the optical system is limited by the size of the entrance pupil and not by the size of individual mirrors.

In an embodiment, the MEMS MMAs are arranged to address different portions of the FOR. Only one of MEMS MMA is active to scan the FOV over its portion of the FOR. A fold mirror selects the active MEMS MMA to propagate optical radiation between the active optical component and the active MEMS MMA. The fold mirror may be a conventional gimbaled mirror another secondary MEMS MMA in which the mirrors are responsive to command signals to at least tip and tilt to select the active MEMS MMA. The fold mirror is similarly configured such that the system's diffraction limited resolution is limited by the size of the entrance pupil.

In an embodiment, the mirrors tip, tilt and piston in 3 DOF to scan the FOV over the FOR. In one case, the MEMS MMA is partitioned into a single mirror segment including all of the mirrors. The mirrors are tipped, tilted and pistoned to approximate a single continuous reflective surface at the specified scan angle. This case will require either a limited scan angle or a large dynamic range to piston (translate) the mirrors. In another case, the MEMS MMA is partitioned into a minimum number of mirror segments each having a maximum size as limited by a maximum translation z to approximate the continuous reflective surface at the specified scan angle. This case optimizes the size of the mirror segments based on the available maximum translation z and the specified scan angle. For larger scan angles, the size of the mirror segments may limit the systems diffraction limited resolution. In a third case, if the optical radiation is coherent at a specified wavelength, the MEMS MMA is configured using piston such that the optical path differences between mirror segments are a multiple of 2pi times the wavelength to maintain phase matching across the wavefront. This may be combined with the previous case to maximize the size of the mirror segments in order to minimize the number of discontinuities between mirror segments.

In another embodiment, the optical scanning system is configured such that the size of the entrance pupil is less than the size of one mirror segment. In this case, optical radiation from the entrance pupil passes across a single mirror segment and thus the diffraction limited resolution is limited by the size of the entrance pupil and not the size of a given mirror segment. If piston is available, multiple mirrors can be configured to approximate the continuous reflective surface thereby increasing the size of the mirror segment. The size of the entrance pupil may be designed so that it is less than the size of the smallest mirror segment required for the maximum scan angle. Light collection may be limited in this configuration.

In another embodiment, the MEMS MMA is positioned at or near an intermediate image plane of the optical system e.g., where optical radiation comes to an intermediate focus or is within the depth of focus of the intermediate focus. As optical radiation from the entrance pupil does not pass across multiple small out-of-phase mirror segments, the optical system performance is diffraction limited by the size of the entrance pupil and not the mirror segments. Any optical elements upstream of the MEMS MMA will have to be sized to accommodate scanning the FOV over the FOR.

In another embodiment, if piston is available, the mirrors may be translated to adjust for path length differences across a wavefront of the optical radiation such as may be induced by a window or dome. This can be implemented within a given mirror segment (comprising multiple mirrors) or between mirror segments and can be used to augment any of the above cases.

In another embodiment, the MEMS MMAs may be configured to approximate a base curvature that provides optical power, alone or in combination with other optical elements, to focus or collimate the optical radiation. This base curvature may be provided by tipping, tilting and pistoning the mirrors. Alternately, a plurality of flat MEMS MMAs may be mounted on flat facets of a support structure in which in combination the facets, hence the MEMS MMA approximate in a piecewise linear fashion the base curvature. Piston may be used to smooth the piecewise linear approximate, requiring less piston to do this than to provide the entire base curvature. In another embodiment, one or more flexible or curved MEMS MMAs may be mounted on a support structure that provides the base curvature.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an optical scanning system in which one or more Micro-Electro-Mechanical System (MEMS) Micro-Mirror Arrays (MMAs) is used to scan a field-of-view (FOV) over a field-of-regard (FOR). The MEMS MMA can provide SWaP-C (size, weight, power and cost) and other advantages such as scanning speed over conventional optical scanning systems. The MEMS MMA is configured in different embodiments such that the diffraction limited resolution of the system is determined by the size of the entrance pupil and not by the size of individual mirrors.

The optical scanning system may be configured as a sensor, active and/or passive, and/or a transmitter. The scanning system includes an optical system of one or more optical elements (transmissive or reflective) configured to propagate optical radiation focused at a near point at an active optical component (detector or source) and collimated toward a distant conjugate point within the FOV. Optical radiation at each point in the FOV passes through an entrance pupil. The optical system may have any number of different configurations that take collimated light in the far field and focus it onto the detector or take diverging light from the source and project collimated light. In terms of optical design, light is reversible i.e., an optical system that works to focus light from a distant point within a FOV to a near point can also work in reverse. Diverging light at a near point can propagate through the optical system and collimated toward a distant conjugate point. The optical scanning system can be designed across the optical spectrum including the visible, infrared (IR) and ultraviolet (UV) bands. Some embodiments are applicable to either coherent or inherent optical radiation whereas others are limited to coherent optical radiation at a specific wavelength.

The one or more MEMS MMAs are configured to scan the FOV formed by the optical system over a FOR. The MEMS MMAs and optical system are configured in such a manner that the system's diffraction limited resolution is determined by the size of the entrance pupil and not the size of an individual mirror. The MEMS MMA may also be used to compensate for distortions to the wavefront induced by, for example, an optical window or dome. The one or more MEMS MMAs may also be configured to provide a portion of the optical power required to focus/collimate the optical radiation.

Referring now toFIGS.1,2A-2B and3, in an embodiment an optical scanning system100is positioned behind an optical dome102of a guided munition to scan a FOV104within a larger FOR106to receive or transmit optical radiation. Optical scanning system100includes both an optical detector108(e.g., a focal plane array (FPA) and a Read Out Integrated Circuit (ROIC)) and an optical source110(e.g. a coherent or incoherent laser). An optical system112includes one or more optical elements114and115e.g. lenses or mirrors configured to propagate optical radiation focused at a near point at the active optical component e.g., optical detector108and optical source110via a beam splitter116and collimated toward a distant conjugate point within the FOV104. In this simplified configuration, the optical system includes a folding mirror114and a powered lens115. Optical radiation117at each point in the FOV passes through an entrance pupil118.

One or more MEMS MMAs120, each comprising a plurality of independently controllable mirrors122, are positioned in the optical path. Each MEMS MMA120is responsive to command signals from a MEMS MMA controller124to partition the MMA into one or more mirror segments126, each mirror segment including one or more mirrors122, and to at least tip128and tilt130the mirrors in at least 2 degrees-of-freedom (DOF) to approximate a continuous reflective surface132in each mirror segment at a specified scan angle provided by a scan controller134to scan the FOV104over the FOR106. The MEMS MMA may be configured to tip128, tilt130and piston136(translate in the z-direction) in 3 DOF. Each mirror is capable of at least “Tip” (rotation about an X-axis), “Tilt” (rotation about a Y-axis and “Piston” (translation along a Z-axis, perpendicular to the XY plane) where the X, Y and Z are orthogonal axes in a three-dimensional space.

In an embodiment, the MEMS MMA is preferably capable of tipping and tilting over range of at least −15°×+15° to steer over a range of +/−30°×30° and pistoning (translating) over a range of at least +/−15 microns (at least one-half wavelength in either direction) piston at a rate of at least 1 Hz (<1 millisecond). Further, the MEMS MMA must have a sufficient number of mirrors, mirror size/resolution, fill factor, range of motion, response time, response accuracy and uniformity across the array.

One such MEMS MMA as illustrated inFIGS.2A-2Bis described in U.S. Pat. No. 10,444,492 entitled “Flexure-Based, Tip-Tilt-Piston Actuation Micro-Array”, which is hereby incorporated by reference. As shown inFIGS.1-3of the '492 patent this MEMS MMA uses flexures to support each mirror at three fulcrum points (or vertices) of an equilateral triangle. The three different pairs of fulcrum points define three axes at 60 degrees to one another in the XY plane. Each mirror pivots about each axis to produce tip, tilt and piston in the XYZ space. This MEMS MMA is currently being commercialized by Bright Silicon technologies for “digitally controlling light.”

If optical radiation117from a ray bundle is reflected off of the individual mirrors122of the MEMS MMA120, the optical path differences across the mirrors can make portions of the ray bundle out-of-phase. This can degrade the imagery provided to optical detector108. For transmission, if the optical radiation from optical source110is re-directed from the individual mirrors122, the beam may diverge according to the limit imposed by an individual mirror rather than the entire surface of the MMA. In other words, the system's diffraction limited resolution is no longer determined by the size of the entrance pupil but rather by the size of an individual mirror. This degradation of the sensed imagery or projected beam is the apparent trade-off to the SWaP-C and scan rate advantages of using a MEMS MMA to scan the FOV over the FOR.

The present invention configures the MEMS MMA and optical system to overcome this apparent limitation such that optical radiation from each point in the FOV does not land on or originate from out-of-phase mirror segments and a diffraction limited resolution of the optical system is limited by the size of the entrance pupil and not by the size of the mirror segments. This preserves either the detected image quality or projected beam quality.

As will be described below, there are multiple different configurations of the MEMS MMA that provide the SWaP-C and scan rate advantages while preserving the system's diffraction limited resolution. Each has advantages and constraints. In a first case, the MEMS MMA is responsive to command signals to partition itself into a single mirror segment including all of the mirrors, which are tipped, tilted and pistoned to approximate a single continuous reflective surface at the specified scan angle to emulate a conventional gimbaled mirror. The constraint is limited scan angle for a maximum translation z of each mirror. In a second case, the MEMS MMA is responsive to command signals to adaptively partition itself into a minimum number of mirror segments for a specified scan angle. The larger the scan angle, the smaller the mirror segment that can be supported and the larger the number of mirrors. At large scan angles, the mirror segment may be small enough that the system's diffraction limited resolution is somewhat degraded. In a third case, limited to coherent optical radiation, piston of each mirror segment is controlled such that the optical path differences between the mirror segments are a multiple of 2*pi times the specified wavelength of the coherent optical radiation to maintain phase matching across a wavefront of the coherent optical radiation. In a fourth case, the system's entrance pupil and MEMS MMA's mirrors are sized such that the size of the entrance pupil is less than the size of one mirror segment. This may be combined with the second case to maximize the size of the mirror segments for a specified scan angle. In this case the amount of light collected may be limited. In a fifth case, the MEMS MMA is positioned at or near an intermediate image plane (or focus) in the optical system. The tradeoff is that any optical components upstream of the MEMS MMA will have to be sized to account for the scanning of the FOV.

If piston is available, the mirrors may be translated to adjust for path length differences across a wavefront of the optical radiation such as may be induced by a window or dome. This can be implemented within a given mirror segment (comprising multiple mirrors) or between mirror segments and can be used to augment any of the above cases.

In another embodiment, the MEMS MMAs may be configured to provide a base curvature that provides optical power, alone or in combination with other optical elements, to focus or collimate the optical radiation. This base curvature may be provided by tipping, tilting and pistoning the mirrors. Alternately, a plurality of flat MEMS MMAs may be mounted on flat facets of a support structure in which in combination the facets, hence the MEMS MMA approximate in a piecewise linear fashion the base curvature. Piston may be used to smooth the piecewise linear approximate, requiring less piston to do this than to provide the entire base curvature. In another embodiment, one or more flexible or curved MEMS MMAs may be mounted on a support structure that provides the base curvature.

As shown inFIG.4, in Case 1 a MEMS MMA200is partitioned into a single mirror segment202including all of the mirrors204exhibiting tip, tilt and piston capabilities. To provide a specified steering angle, all of the mirrors within the single mirror segment202are tipped and tilted about the X and Y axes, respectively, at the same tip and tilt angles. Each mirror within the single mirror segment202is pistoned (translated along the Z axis) by a requisite amount such that the mirrors approximate a single continuous reflective surface206across the single mirror segment. The maximum angle, tip or tilt, that can be achieved is given by arcsin(z/w) where z is the maximum piston stroke (translation along z) and w is the width of the mirror segment (e.g. for tip, the number of mirrors and width of each mirror along the Y axis). The range of scan angles that can be serviced by this configuration is limited by the maximum piston stroke z. This approach is viable for either coherent or incoherent optical radiation.

As shown inFIGS.4and5A-5B, in case 2 a MEMS MMA200is adaptively partitioned (based on the current scan angle) into one or more mirror segments202with each section including a plurality of mirrors204exhibiting tip, tilt and piston capabilities. To provide a specified steering angle, all of the mirrors within a segment and all of the mirrors between different segments are tipped and tilted about the X and Y axes, respectively, at the same tip and tilt angles. Each mirror within a segment is pistoned (translated along the Z axis) by a requisite amount such that the mirrors approximate a continuous reflective surface206across the segment. The maximum angle, tip or tilt, that can be achieved is given by arcsin(z/w) where z is the maximum piston stroke (translation along z) and w is the width of the section (e.g. for tip, the number of mirrors and width of each mirror along the Y axis). For larger values of w (more mirrors grouped into a section), the maximum tilt angle, hence steering angle is smaller. Therefore, for small steering angles the number of mirror segments N is small and the size of the mirror segment is large. This adaptive approach will satisfy the system's diffraction limited resolution constraint for small scan angles. For larger scan angles some degradation may occur. An additional benefit of this optimization is that the number of edge discontinuities between mirror segments is minimized, which minimizes diffraction. This approach is viable for either coherent or incoherent optical radiation.

Table 1 is a plot of mirror tip angle for which a continuous reflective surface across a mirror segment can be formed versus the number of mirror segments (grouping of mirrors) along an axis of the MEMS MMA. In this example, the MEMS MMA includes 20 mirrors (1 mm in width) positioned along the Y axis to tip about the X axis. Each mirror can tip and tilt +/−15° about the X and Y axes, respectively, and piston +/−35 microns along the Z axes.

As expected, as the mirror segment width increases to preserve diffraction limited resolution, the maximum tip decreases. Note, the diffraction spot size also decreases indicating less diffraction due to fewer edge discontinuities. For this specific MEMS MMA configuration e.g. number of mirrors, size of the mirror, max piston etc. the serviceable angular range to gain benefits from approximating a continuous reflective surface across multiple mirrors that form a section is quite small, 2 degrees or less. Although shown in Table 1 the entry for a section including only a single mirror is a degenerate case. For a single mirror, the limiting factor is the maximum tip (or tilt) angle of the mirror itself. In this example, that is 15 degrees, not 4 degrees as suggested by the geometry. As such, in this example, which is exemplary of typical MMA configurations, considerable scan angle range may be sacrificed in order to preserve the system's diffraction limited resolution (and reduce diffraction).

In light of this, configuring the MEMS MMA to approximate continuous reflective surfaces may not be practical for many typical applications in which a large range of scan angles is required. This approach may be limited to applications such as the long range steering as found in space-based systems or to compensate for small angular adjustments such as found on imaging platforms to compensate for vibration. A hybrid approach in which the MEMS MMA is configured as shown inFIG.3i.e. 1 mirror per mirror segment to service large scan angles and is configured as shown inFIGS.4and5A-5Bi.e. multiple mirrors per segment to service small scan angles may be viable, particularly where large angle scanning is relatively infrequent.

As shown inFIG.6, in case 3, a MEMS MMA300is partitioned into a plurality of mirror segments302each including one or more mirrors304. A ray bundle of coherent optical radiation306at a specified wavelength passes across multiple MMA segments or “sub-apertures”. The optical path differences across the mirror segments can make portions of the ray bundle out-of-phase. Optical radiation from each distant point in the FOV forms a “footprint” on the MEMS MMA. Different “footprints” are overlapping each other on the MEMS MMA. This would appear as a plurality of out-of-phase sub-apertures that would degrade image (or beam) quality. To compensate for this, the MEMS MMA300responsive to command signals pistons each of a plurality of mirror segments302such that the optical path differences between the mirror segments are a multiple of 2*pi times the specified wavelength to maintain phase matching across the ray bundle (or wavefront) of the coherent optical radiation. The multiple can be as large as necessary such that the requisite piston (or translation) of each mirror to both approximate the continuous reflective surface within a mirror segment and to maintain phase matching between mirror segments is serviceable by the maximum translation z of a given mirror. Case 3 may be augmented with the case 3 optimized grouping to form the largest mirror segments that can provide a given scan angle to further reduce diffraction.

As shown inFIG.7, in case 4, an optical scanning system400includes a MEMS MMA402that forms a Primary mirror, a fold mirror404and a focusing lens406configured to propagate optical radiation408collimated at a distant point within a field-of-view (FOV) and focus the optical radiation at a near point at an optical detector410. Optical radiation at each point in the FOV passes through an entrance pupil412. The optical system and MEMS MMA are configured such that the size of entrance pupil412is less than the size of one mirror segment414including one or more mirrors. The same principle is applicable for scanning of an optical beam. This approach is viable for either coherent or incoherent optical radiation.

In this case, optical radiation from the entrance pupil412passes across a single mirror segment and thus the diffraction limited resolution is limited by the size of the entrance pupil and not the size of a given mirror segment. If piston is available, multiple mirrors can be configured to approximate the continuous reflective surface thereby increasing the size of the mirror segment. The size of the entrance pupil may be designed so that it is less than the size of the smallest mirror segment required for the maximum scan angle. Light collection may be limited in this configuration.

As shown inFIG.8, in case 5 a MEMS MMA500is positioned at or near an intermediate image plane (focus)502of an optical scanning system504where near is defined as being within the intermediate image depth of focus. As optical radiation506(coherent or incoherent) from an entrance pupil508does not pass across multiple small out-of-phase mirror segments510, the optical system performance is diffraction limited by the size of the entrance pupil and not the mirror segments510. Any optical elements such as lens512upstream of the MEMS MMA500will have to be sized to accommodate scanning the FOV over the FOR. Any downstream optical elements such as lens514and optical detector516are unaffected. The same principle is applicable for scanning of an optical beam.

As previously mentioned, in certain applications the optical radiation must pass through an optical window or dome, which may change the angle of each ray bundle associated with each point in the FOV and cause distortion in the image. As shown inFIGS.9A-9C, if available, piston may be used in any of the cases described to maintain phase across the wavefront of the optical radiation to eliminate or mitigate any distortion induced by the window or dome. As shown inFIG.9A, sans a window or dome, a wavefront600of collimated rays601of optical radiation is flat, exhibiting constant phase. The mirror segments602are tipped and tilted at a specified scan angle to re-direct a reflected ray604of optical radiation and maintain the constant phase across the re-directed wavefront606. As shown inFIG.9B, optical radiation that passes through a window or dome has a curved wavefront610. Simply tipping and tilting the mirror segments in the same manner controls the scan angle of the reflected ray612but the path length is now different across each reflected ray and a distorted wavefront614is now flat but out-of-phase. As shown inFIG.9C, given a distorted wavefront620, additional piston612can be applied to mirror segments602so that constant path length is maintained across the mirror segments and reflected rays622and a corrected wavefront644is in-phase.

Referring now toFIGS.10A and10B, in an embodiment an optical scanning system700is positioned behind an optical dome702of a guided munition to scan a FOV704within a larger FOR706to receive or transmit optical radiation. Optical scanning system700includes both an optical detector708(e.g., a focal plane array (FPA) and a Read Out Integrated Circuit (ROIC)) and an optical source710(e.g. a coherent or incoherent laser). An optical system712includes one or more optical elements714and715e.g. lenses or mirrors configured to propagate optical radiation focused at a near point at the active optical component e.g., optical detector708and optical source710via a beam splitter716and collimated toward a distant conjugate point within the FOV704. In this simplified configuration, the optical system includes a steerable folding mirror714and a powered lens715. Optical radiation717at each point in the FOV passes through an entrance pupil718.

A plurality (e.g., six) of MEMS MMAs720are arranged to address different portions (FOR706) of a combined field-of-regard (FOR)722, Steerable fold mirror714is responsive to command signals from a scan controller734to direct optical radiation between the active optical component, detector708or source710, and one active MEMS MMA720at a time. The active MEMS MMA is responsive to command signals from a MEMS MMA controller736to partition the MMA into one or more mirror segments, each mirror segment including one or more mirrors, and to at least tip and tilt the mirrors to approximate a continuous reflective surface in each mirror segment at a specified scan angle provided by scan controller734to scan the FOV704over its portion706of the combined FOR722. The MEMS MMAs720may be configured using any of the aforementioned cases such that optical radiation from each point in the FOV does not land on or originate from out-of-phase mirror segments and a diffraction limited resolution of the optical system is limited by the size of the entrance pupil and not by the size of individual mirrors.

The steerable fold mirror714may be a conventional gimbaled mirror or another secondary MEMS MMA in which the mirrors are responsive to command signals to at least tip and tilt to select the active MEMS MMA. The steerable fold mirror is similarly configured such that the system's diffraction limited resolution is limited by the size of the entrance pupil. The primary MEMS MMAs and the secondary MEMS MMA may implement the same or different cases to maintain the system's diffraction limited resolution. For example, the primary MEMS MMAs could implement Case 3 whereby the different mirror segments are pistoned to maintain a path length difference of a multiple of 2pi times the wavelength of coherent light to maintain phase coherence across the wavefront while the secondary MEMS MMA could be positioned at an intermediate image plane in accordance with Case 5.