Patent Publication Number: US-2022229160-A1

Title: Optical system for object detection and location using a micro-electro-mechanical system (mems) micro-mirror array (mma) beamsteering device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to active optical system that use laser radiation to detect and locate objects, and more particular to a mechanical beamsteering device for steering a pulsed laser beam to identify the presence of an object and the range to the object. 
     Description of the Related Art 
     Typical active optical systems use laser radiation to detect and locate objects. Such systems detect an object by sensing reflections of laser radiation from the object. After the object has been detected, these systems locate the object by determining an angular location of the object and a range to the object. Conventional active optical systems illuminate an entire field of view of the system instantaneously with sufficient laser energy to detect even the lowest reflective objects within that field of view, and at a maximum desired range. A detector within the optical system includes a given number of pixels, each having a sufficient sensitivity to detect the reflected laser radiation from the entire field of view. The reflected laser radiation is analyzed to determine a distance (i.e., range) to the object. Typically, range is determined by direct detection (e.g., the “time of flight” principle), or by frequency modulation. 
     The size, weight, and power of a typical active optical system increase with the range and field of view of the active optical system. Longer ranges and wider fields of view require high powered laser sources, which directly increase the required input power and the size of the active optical system. Increases in range and field of view may also require additional components to reject waste heat generated by the high-powered laser source. Such components also increase the size, weight, and power of the optical system. 
     U.S. Pat. No. 10,267,915 entitled “Optical System for Object Detection and Location” includes a laser radar optical source positioned to emit a pulsed laser beam, a non-mechanical beamsteering device such as a liquid crystal waveguide (LCWG) positioned to scan the beam in a linear scan over a first area of a scene, a laser radar detector positioned to receive and integrate a reflection of the beam, a read-out integrated circuit (ROIC) configured to provide a first read-out signal based on the integrated reflection, and a controller configured to receive the first read-out signal, determine a range to the first area based on a time of flight of the pulsed laser beam, and identify a presence of an object within the scene based on a signal level of the first read-out signal, the first signal level corresponding to a reflectivity of a portion of the object within the first area of the scene. 
     SUMMARY OF THE INVENTION 
     Aspects and embodiments are generally directed to imaging systems and methods, and in particular to imaging systems and methods for detecting objects in low-visibility and other restricted view conditions. Aspects and embodiments include thermo-optical, electro-optical, solid-state and/or non-mechanical optical elements for improved size reduction, improved weight reduction, and reduced power consumption during operation. One example of an imaging system includes a mechanical beamsteering device in the form of a Micro-Electro-Mechanical System (MEMS) Micro-Mirror Array (MMA) positioned to scan a pulsed laser beam in a scan over an area of a scene. 
     As discussed in further detail herein, reflections of the pulsed laser beam from the area of the scene, and reflections from multiple consecutive linear scans over different areas of the scene, may be used to identify the presence of an object within the scene, locate and determine a range to that object, detect a spatial profile of that object, and/or generate an image of that object. In particular examples, the imaging system may locate small objects at great ranges (or other difficult to detect objects) based on the reflective characteristics of the scene and small objects therein. In addition to those benefits discussed herein, various aspects and embodiments provide a reduced weight, size, and power consumption imaging system for ground, mobile, maritime, airborne, and space, imaging environments. 
     According to certain aspects, provided is an imaging system. In one example, the imaging system includes a laser radar optical source positioned to emit a pulsed laser beam, a MEMS MMA positioned to receive the pulsed laser beam from the laser radar optical source and to scan the pulsed laser beam in a first scan over a first area of a scene, a laser radar detector positioned to receive and integrate, at a first active portion of the laser radar detector, a reflection of the pulsed laser beam from the first area of the scene, a read-out integrated circuit (ROIC) coupled to the laser radar detector and configured to provide at least a first read-out signal based at least in part on the integrated reflection of the pulsed laser beam corresponding to the first area of the scene, and a controller coupled to the ROIC and configured to receive the first read-out signal, determine a range to the first area of the scene based at least in part on a time of flight of the pulsed laser beam of the first linear scan, and identify a presence of an object within the scene based at least in part on a first signal level of the first read-out signal, the first signal level corresponding to a reflectivity of at least a portion of the object within the first area of the scene. 
     According to an embodiment, the MEMS MMA is further configured to scan the pulsed laser beam in a second linear scan over a second area of the scene, within a frame time, and the laser radar detector is positioned to receive and integrate, at a second active portion of the laser radar detector, a reflection of the pulsed laser beam from the second area of the scene. In one embodiment, the ROIC is further configured to provide at least a second read-out signal based at least in part on the reflection of the pulsed laser beam corresponding to the second area of the scene, and the controller is further configured to receive the second read-out signal and determine a range to the second area of the scene based at least in part on a time of flight of the pulsed laser beam of the second linear scan. The controller is further configured to generate an average signal level for the scene, the average signal level corresponding to an average of at least the first signal level of the first read-out signal and a signal level of the second read-out signal. In an embodiment, in identifying the presence of the object, the controller is further configured to compare the first signal level and the average signal level for the scene and detect a disparity between the first signal level and the average signal level. 
     In one embodiment, the MEMS MMA is responsive to command signals to partition itself into a plurality of segments, each segment including a plurality of mirrors to form and independently steer a plurality of pulsed laser beams at specified steering angles. In an embodiment, the mirrors in different segments reflect light at different wavelengths such that the plurality of pulsed laser beams provide a diversity of wavelengths. This may be accomplished by using a plurality of sources at different wavelengths, optical coatings at different wavelengths or a combination thereof. The plurality of pulsed laser beams may scan the same or different areas of the scene. One or more pulsed laser beams may scan the area in a repetitive pattern to identify the presence of objects while one or more pulsed laser beams are steered to a location at which an object is identified to further interrogate the object while the repetitive scan(s) is ongoing. 
     In one embodiment, the MEMS MMA includes the ability to tip, tilt and piston the mirrors to both shape and steer the pulsed laser beam. Each mirror rotates about X and Y axes, and translates along a Z axis orthogonal to the XY plane to tip, tilt and piston. In an implementation of the MEMS MMA, each mirror is supported at three vertices of an equilateral triangle. Lines defined by three different pairs of the vertices provide three axes at 60 degrees to one another in the XY plane, wherein each said mirror pivots about each said axes to produce tilt, tip and piston in the XYZ space. The piston capability allows the MMA to shape the beam to adjust size, focus or intensity profile or to produce deviations in the wavefront of the beam to compensate for path length differences or atmospheric distortion. 
     In one embodiment, the MEMS MMA is further configured to synchronize at least the first scan to correspond to the first active portion of the laser radar detector. In an embodiment, the laser radar detector includes a single linear array of detector elements. According to an embodiment, the MEMS MMA, the laser radar detector, and the ROIC are configured to operate in accordance with a search and track mode of operation, and during the search and track mode of operation the MEMS MMA and laser radar detector are synchronized. 
     Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. Various aspects and embodiments described herein may include means for performing any of the described methods or functions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG. 1  is a block diagram of an example imaging system according to aspects of the invention; 
         FIG. 2  is an example illustration of operation of the imaging system shown in  FIG. 1 , according to aspects of the invention; 
         FIGS. 3A and 3B  are additional example illustrations of operation of the imaging system shown in  FIG. 1 , according to aspects of the invention; 
         FIGS. 4A and 4B  are further example illustrations of operation of the imaging system shown in  FIG. 1 , according to aspects of the invention; 
         FIG. 5  shows an example side profile of an operator wearing the example imaging system shown in  FIG. 1 , according to aspects of the invention; 
         FIGS. 6A and 6B  are illustrations of an embodiment of a Tip/Tilt/Piston (“TTP”) MEMS MMA and a single mirror actuated to tip, tilt and piston; 
         FIGS. 7A and 7B  is an illustration of partitioning the MEMS MMA into multiple segments to form and steer multiple optical beams; 
         FIG. 8  is an illustration of using the piston capability to add optical power to the continuous surface; 
         FIG. 9  is an illustration of using the piston capability to make small deviations from the continuous surface to compensate for atmospheric distortion; and 
         FIG. 10  is an example process flow according to aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects and embodiments relate to optical systems and methods of using the same. In particular, aspects and embodiments are directed to laser radar imaging systems and methods for detecting objects at long ranges, in low-visibility conditions, and in other restricted view conditions. Various embodiments include thermo-optical, electro-optical, solid-state and/or non-mechanical optical elements for improved weight reduction and reduced power consumption during such conditions. A MEMS MMA is used to scan the pulsed laser beam over a scene. As further discussed below, reflections of one or more scanned pulsed laser beams from an illuminated area of a scene may be used to identify the presence of an object in the scene, locate and determine an angular-range and distance (i.e., range) to the object, detect a spatial profile of the object, and generate an image of the object. In particular examples, the imaging system may locate small objects at great ranges (or other difficult to detect objects) based on a comparison of one or more signal levels corresponding to the reflections of the one or more scanned pulsed laser beams. Specifically, signal levels corresponding to highly reflective areas (e.g., the object) may be compared to an average signal level of the scene to identify the presence of the object. Further, components of the imaging system may synchronize the one or more linear scans performed by MEMS MMA of the imaging system to correspond to an active portion of a detector. Accordingly, in addition to reducing the weight and size of the imaging system, synchronized scans may significantly improve the power consumption of the imaging system, when compared to conventional imaging systems. 
     As discussed above, during various conventional approaches to range finding, a laser source is positioned to illuminate an entire scene within a field of view (FOV) of the optical system. Such an approach not only consumes a great deal of power when providing the intensity and pattern of illumination necessary for range finding operations at great distances, but also makes detection of objects, such as signs, difficult. Decreased visibility of the scene as a result of environmental conditions, movement of the scene or system, or other operating conditions, can also significantly increase the difficulty and complexity in determining an accurate range, or generating an accurate image of those objects. 
     Some advanced active optical systems incorporate laser beam steering in an attempt to reduce the power of the optical system, while improving the ability to detect and locate objects. Conventional approaches have suggested positioning beamforming optics on a gimbal system, which would allow the system to steer illumination to only those areas of the scene desired for imaging, and hence, reduce the power consumption and improve the accuracy of the system. Unfortunately, such complex mechanical structures increase the cost and weight of the imaging system significantly, making mobile, maritime, airborne, and space applications much less practical. 
     Another approach uses a non-mechanical beamsteering device configured to steer a pulsed laser beam generated by a laser radar optical source in one or more linear scans over an area of a scene. In particular, one or more linear scans may be performed rapidly to precisely detect and locate an object within that scene. Non-mechanical beamsteering allows a significant reduction in the size, weight and power of the system, as only a small fraction of the field of view is illuminated at any instant in time. This reduces the required input power to the laser radar optical source, the size of the system, and removes the need for equipment that dissipates waste heat. However, the non-mechanical beamsteering waveguide and particularly the LCWG has been found to have a number of limitations, which may include, but are not limited to, steering a very narrow band of wavelengths about a center wavelength. Furthermore each material system e.g., substrates, coatings and liquid crystals, and voltage settings to steer the laser beam are unique to each center wavelength. Therefore to accommodate different wavelengths requires different LCWG devices and significant investment in materials, manufacturing, set-up and calibration etc. to design and field each device. The LCWG cannot manipulate the wavefront of the beam to, for example, focus the beam into a spot, to provide wavefront correction e.g. atmospheric distortion, or to compensate for path length differences across the beam. The LCWG can steer one and only one beam at the single wavelength. The LCWG cannot steer multiple beams of the same or different wavelengths. The LCWG is limited to receive the optical energy from a single optical source, it cannot combine the optical energy from multiple sources and focus that energy into a single focused optical beam to provide the active illumination. 
     Accordingly, various aspects and embodiments discussed herein provide an optical system configured to scan a pulsed laser beam to locate and range an object with the capability to manipulate the wavefront of the beam, to segment the beam into a plurality of independently steerable beams of the same or different wavelengths, while maintaining a reduced weight, size, and power consumption when compared to typical imaging systems. Various other advantages and benefits of the active imaging system and methods described herein are discussed below with reference to  FIGS. 1-10 . 
     It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation. 
       FIG. 1  is a block diagram of an example imaging system  100  in accordance with various aspects and embodiments discussed herein. Among other components, the imaging system  100  may include a laser radar optical source  102 , a mechanical beamsteering device such as MEMS MMA  104 , a laser radar detector  106 , a read-out integrated circuit (ROIC)  108 , a controller  110  and a wavefront sense and correction subsystem  132 . In certain embodiments, the optical system  100  may further include additional optical or electrical components such as lenses, filters, and amplifiers, not shown in  FIG. 1 . As shown, the laser radar optical source  102  may be positioned and optically coupled to the MEMS MMA  104 , which is configured to nominally re-direct light along an optical axis (indicated generally by arrow  124 ) and steer the pulsed laser beam about the optical axis. In various embodiments, the control circuit includes a controller  110  coupled and in communication with the ROIC  108 , the MEMS MMA  104 , and the laser radar optical source  102 . The controller  110  may also include an output node configured to provide on or more outputs (shown generally as indicator  112 ). As shown, the ROIC  108  may also be coupled to the laser radar detector  106 . 
     Though the components of the example imaging system  100  shown in  FIG. 1  may be shown and described as discrete elements in a block diagram, and may be referred to as “circuitry” or “circuit,” unless otherwise indicated, the components may be implemented as one of, or a combination of, analog circuitry, digital circuitry, or one or more microprocessors executing software instructions. The software instructions may include digital signal processing (DSP) instructions. Unless otherwise indicated, signal lines between components of the system  100  may be implemented as discrete analog, digital, or optical signal lines. Some of the processing operations may be expressed in terms of calculations or determinations by the controller  110  or other components. The equivalent of calculating and determining values, images, or other elements, can be performed by any suitable analog or digital signal processing techniques and are included within the scope of this disclosure. Such techniques are further discussed with reference to the controller  110  below. Unless otherwise indicated, control signals may be encoded in either digital or analog form; conventional digital-to-analog or analog-to-digital converters may be included, but are omitted from  FIG. 1  to simplify explanation. 
     In various embodiments, the laser radar optical source  102  is in optical communication with the MEMS MMA  104 . Optical source  102  emits a pulsed laser beam  115  that is reflected off a fold mirror  125  onto MEMS MMA  104 . While in various embodiments, the laser radar optical source  102  may include any suitable source of optical radiation. In particular embodiments, the laser radar optical source  102  includes a source of near-infrared (NIR) light. For example, the laser radar optical source  102  may include a diode laser configured to emit a pulsed laser beam having at an eye-safe wavelength of 1,550 nm. However, in various other embodiments other wavelengths may be used, such as 1,064 nm pulsed laser beam. In one particular implementation, the laser radar optical source  102  may emit a pulsed laser beam at 1 μJ, at pulse width of 10 nanoseconds, and a repetition rate of 10 kHz. Such a beam may enable detection of objects, such as signs, with autonomous vehicles, such as motor vehicles, railed vehicles, or watercrafts. In other examples, the optical source  102  may include a broadband source of radiation within the visible, SWIR, NIR or MWIR bands or spanning portions of those bands. In other examples, the optical source  102  may include multiple sources at different predetermined optical wavelengths configured to address different segments of the MEMS MMA. The mirrors of the MEMS MMA may have optical coatings that reflect a narrowband about a predetermined optical wavelength or that reflect a broadband about such a wavelength. Various combinations of sources and optical coatings may be configured to produce one or more steerable beams that include different optical wavelengths. A key attribute of the MEMS MMA is that neither the materials system of the MEMS MMA or the steering parameters (voltages) is a function of wavelength. 
     As  FIG. 1  shows, the MEMS MMA  104  may be positioned to receive the pulsed laser beam  115  from the laser radar optical source  102  via fold mirror  125 . For example, the MEMS MMA  104  may receive the pulsed laser beam by an optical fiber or free space coupling. Responsive to receiving the pulsed laser beam from the laser radar optical source  102 , the MEMS MMA  104  is controlled to scan the pulsed laser beam in one or more linear scans over an area of a scene (e.g., scene  114 ). For instance, the MEMS MMA  104  may receive one or more control signals, such as an applied voltage, from the controller  110  to tip and/or tilt the mirrors to adjust a direction of the scan. In various embodiments, the area of the scene over which the pulsed laser beam is scanned is less than an entire span of the scene  114 . The MEMS MMA may be partitioned to simultaneously generate and steer multiple pulsed laser beams (of the same or different wavelengths) over the same or different areas of the scene. The MEMS MMA may, for example, steer one or more pulsed laser beams in a repetitive scan pattern over the area to detect objects and the adaptively generate and steer one or more pulsed laser beams (of the same or different wavelength composition) to interrogate the detected object. 
     In certain embodiments, scanning the pulsed laser beam  115  in a linear scan over an area of the scene  114  may include scanning the pulsed laser beam over at least a first portion of an object (e.g., a sign  116 ) within that area.  FIG. 1  shows a triangular shaped sign  116  within the example scene  114  for convenience of description. As referred to herein, signs may include any informational object or device, such as traffic signs. In particular embodiments, objects may be small in size relative to the entire span of the scene  114 . For example, in one application, a sign may cover less than 5% of an entire area of a field of view of the laser radar detector  106 . For at least this reason, signs are challenging to accurately locate and image, and can be especially challenging when visibility is reduced. 
     As discussed in further detail below with reference to at least  FIGS. 3A-3B  and  FIGS. 4A-4B , in certain embodiments, the MEMS MMA  104  may be controlled to perform one or more consecutive linear scans over the scene  114 , each linear scan corresponding to a different area of the scene  114 . For instance, the MEMS MMA  104  may be controlled to scan the pulsed laser beam over a first area of the scene  114 , and responsive to completing the scan, scan the pulsed laser beam over a second area of the scene  114 . Such a process may be repeated for any number of desired scans, or until an entire spatial profile of the object is detected. Furthermore, one or more linear scans may be retraced for a previously scanned area to obtain further image data regarding that area of the scene  114 . Accordingly, with each consecutive linear scan of the MEMS MMA  104  over a different area of the scene  114 , the pulsed laser beam may be directed at different portions of the object within that scene  114 . While discussed in certain examples as being configured to perform linear scans, in certain other implementations, the MEMS MMA  104  may be controlled to perform one or more spot scans over the scene  114  to achieve the same effect. 
     In  FIG. 1 , the beamwidth of the pulsed laser beam is illustrated by lines  118 , and a field of view of the laser radar detector  106  is illustrated as lines  120 . As shown, the MEMS MMA  104  is configured to direct the pulsed laser beam within the field of view of the laser radar detector  106 . In particular, the area scanned (i.e., illuminated) by the pulsed laser beam may be less than the field of view of the laser radar detector  106 . In certain embodiments, the MEMS MMA  104  is configured to synchronize at least one or more linear scans of the pulsed laser beam with an activated portion of the laser radar detector  106 . For instance, the MEMS MMA  104  may direct the first linear scan over only the area of the scene corresponding to the active portion of the laser radar detector  106 . That, is in various embodiments the field of view of the active section of the laser radar detector  106  may define the area of the scene illuminated by the first linear scan. 
     In certain examples, it is desirable to compensate for atmospheric distortion, which varies with time. The wavefront sense and correction subsystem  132  includes a source  134  positioned to emit electromagnetic radiation e.g. SWIR in an optical beam preferably having a “flat-top” intensity profile. Source  134  may be a pulsed laser at 1064 nm. A beamsteering device  136  such as a rotating mirror, LCWG or MEMS MMA steers the beam to illuminate scene  126 . A wavefront sensor  138  measures the wavefront of the reflected optical beam. Alternate embodiments may combine some or all functions of the imaging and wavefront correction sensor into a single system including the optical source, beam steering and sensor. Controller  116  generates command signals to configure the MEMS MMA to compensate for the atmospheric distortion via Piston actuation primarily. 
     As best shown in  FIGS. 6A-6B , Micro-Electro-Mechanical System (MEMS) Micro-Mirror Array (MMA)  104  comprises a plurality of independently and continuously controllable mirrors  140  to form and steer the optical beam(s). Each mirror is capable of at least “Tip” (rotation about an X-axis) and “Tilt” (rotation about a Y-axis). In preferred embodiments, each mirror is also capable of “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. The Piston capability can be used generically speaking to “shape” the beam(s) that are reflected off of the MEMS MMA. More specifically, the Piston capability can be used to focus the beam such as to form a “fan” or “spot” beam. The Piston capability can also be used to adjust the size, divergence or intensity profile of the beam, produce deviations in the wavefront of the beam to compensate for atmospheric distortions, adjust phase to maintain a zero phase difference across the beam, add optical power to focus or defocus the beam or to improve the formation and steering of the beam by approximating a continuous surface across the micro-mirrors, which reduces unwanted diffraction to increase power in the optical beam. The MEMS MMA is preferably capable of steering an output laser beam over a range of at least −15°×+15° in tip and tilt (30°×30° and steering range) and +/−15 microns (at least one-half wavelength in either direction) piston at a rate of at least 1 KHz (&lt;1 millisecond). The independently controllable mirrors can be adaptively segmented to form any number of optical beams, adjust the size/power of a given optical beam, generate multi-spectral optical beams and to combine multiple input sources. 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 is 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 in FIGS. 1-3 of the &#39;492 patent this MEMS MMA uses flexures to support each mirror at three fulcrum points (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.” 
     In various embodiments, the laser radar detector  106  is positioned to receive, at an active portion of the detector (shown as active portion  122 ), a reflection of the pulsed laser beam reflected from the scene. For example, the laser radar detector  106  may include a photodetector having a spectral band chosen to match the wavelength of the pulsed laser beam emitted by the laser radar optical source  102 . In many instances, the reflection of the pulsed laser beam may correspond to an area of the scene  114  that includes a portion of an object (e.g., sign  116 ). When activated, individual photo-sensitive detector elements (e.g., unit cells) within the laser radar detector  102  integrate photons of radiation impinging on the detector element. A circuit within each detector element of the laser radar detector  102  accumulates charge corresponding to the incident radiation. At the end of an integration period, the ROIC  108  may be configured to deactivate the activated detector elements, and provide a read-out signal to the controller  110  based on the accumulated charge. As discussed in further detail below, each read-out signal may have a corresponding signal level. In at least one example, each signal level refers to the strength (e.g., power) of the read-out signal, which may be based on the flux of reflections received. 
     In various embodiments, the laser radar detector  106  may include an array of detector elements, such as complementary metal-oxide semiconductor (CMOS) sensors or charge coupled devices (CCD). As discussed in further detail below with reference to  FIGS. 3A-3B  and  FIGS. 4A-4B , in various embodiments the laser radar detector  106  may include one or more rows or columns of detector elements, and in certain embodiments include a single linear array of detector elements, or a single detector element. 
     As  FIG. 1  shows, the ROIC  108  may be coupled and in communication with at least the laser radar detector  106  and the controller  110 . In particular, the ROIC  108  may be coupled and in communication with each detector element of the laser radar detector  106 . The ROIC  108  is configured to activate detector elements of the laser radar detector  106  and generate one or more read-out signal based at least in part on the integrated charge corresponding to reflections of the pulsed laser beam received at the laser radar detector  106 . For instance, the ROIC  108  may include a double-buffer and a multiplexer, configured to sequentially draw charges from the laser radar detector and generate a continuous data stream to the controller  110 . In various embodiments, the ROIC  108  is configured to activate a portion of the laser radar detector  106  that corresponds to the area of the scene illuminated by a current linear scan. For example, the ROIC  108  may be synchronized with the MEMS MMA  104  to operate in a rolling-shutter mode of operation. 
     Accordingly, the ROIC  108  may be configured to activate detector elements of the laser radar detector  106  in a row-by-row arrangement. In such an embodiment, the MEMS MMA  104  directs the pulsed laser beam over the areas of the scene  114  corresponding to the activated rows, and not over those areas of the scene  114  that correspond to inactive detector elements. For example, the ROIC  108  may generate a synchronization signal, based on which the MEMS MMA  104  may synchronize with the laser radar detector  106 . However, in other embodiments, a synchronization signal may be generated by the controller  110 , or the laser radar optical source  102 , and provided to each of the ROIC  108  and the MEMS MMA  104 . After the integration period has expired, the ROIC  108  is configured to read-out the current values of each detector element within the activated row, deactivate that row, and activate a new row of detector elements. Such embodiments are particularly advantageous for range detection operations because they permit the detection of objects within a scene (e.g., signs) that would otherwise go undetected if the entire scene was illuminated. 
     While discussed herein as in at least one embodiment including an array of detector elements, in various embodiments, the laser radar detector  106  may include a single linear array, or a single detector element. Such embodiments may further reduce the size, weight, and cost, of the imaging system. Detector elements may be arranged in any suitable pattern, such as rows and columns.  FIG. 1  shows the laser radar detector  106  including an array composed of five rows of five detector elements for convenience of illustration. In particular implementations, the laser radar detector  106  may include a focal plane array (FPA) including 2048×2048 array of detector elements, a 320×256 array of detector elements, or a 640×480 array of detector elements. In certain embodiments, the beamwidth of the pulsed laser beam steered by the MEMS MMA  104  may correspond to the dimensions of the laser radar detector  106 , or the dimensions of the activated detector elements within the MEMS MMA  104 . In certain embodiments the rows, columns, and/or certain subsections of the FPA may be configured and preferably optimized to receive optical energy of different wavelengths. “Row” and “column” as used herein, may be interchanged according to various embodiments. That is, although “row” may generally be used to refer to a horizontal positioning and “column” may generally be used to refer to a vertical positioning, as used herein, either may be used to refer to a horizontal positioning and a vertical positioning relative to the other. 
     In various embodiments, the MEMS MMA  104  may be controlled to perform one or more linear scans of an area of the scene within a frame time of the imaging system  100 . Specifically, the frame time may coincide with a single frame of operation of the laser radar detector  106 . The frame rate of the laser radar detector  106  refers to the snapshot operation of each individual detector element of the laser radar detector  106 , and the frequency at which the imaging system  100  generates consecutive images (i.e., frames). In various embodiments, the rapid response of the MEMS MMA  104  (in contrast to conventional mechanical devices) permits the one or more linear scans to be synchronized with the frame rate of the imaging system  100 . For instance, in various embodiments each consecutive linear scan may start at the beginning of a frame of operation of the imaging system  100  and end at the end of the frame of operation. Similarly, the integration period of the detector elements within the laser radar detector  106  may substantially match the frame time of a single frame of operation. 
     The one or more read-out signals provided by the ROIC  108  may be received at the controller  110 . The controller  110  may include a single controller; however, in various other embodiments the controller  110  may consist of a plurality of controllers and/or control circuitry. While the controller  110  is illustrated separate from one or more components of the imaging system, in various embodiments, the controller  110  may be combined with one or more other components, such as the ROIC  108 . For instance, the controller  110 , ROIC  108 , and other combined components of the imaging system  100 , may include a combination of software-configured elements, control circuitry, signal processing circuitry, application specific integrated circuitry, or any combination of various hardware and logic circuitry for performing the various processes discussed herein. 
     In various embodiments, the controller  110  includes a processor, data storage, a user interface, and one or more interfaces for system components, such as a communication interface. The processor may be coupled to the data storage, the communication interface, and the one or more other interfaces, and be configured to perform a series of instructions that result in manipulated data stored and retrieved from the data storage. The processor may include a commercially available processor, such as a processor manufactured by INTEL, AMD, MOTOROLA, or FREESCALE. 
     In additional examples, the processor may be configured to execute an operating system. The operating system may provide platform services to application software. These platform services may include inter-process and network communication, file system management, and standard database manipulation. One or more of many operating systems may be used, and examples are not limited to any particular operating system or operating system characteristic. In some examples, the processor may be configured to execute a real-time operating system (RTOS), such as RTLinux, or a non-real time operating system, such as BSD or GNU/Linux. 
     The instructions stored on the data storage may include executable programs or other code that can be executed by the processor. The instructions may be persistently stored as encoded signals, and the instructions may cause the processor to perform the functions and processes described herein, such as determining a range to a scene (including a portion of an object), identifying the presence of an object in a scene, detecting a spatial profile of an object, aggregating object data based at least in part on the detected spatial profile of the object, and generating an image of an object based on one or more read-out signals, from the ROIC  108 . The data storage may include information that is recorded, on or in, the medium, and this information may be processed by the processor during execution of instructions. The data storage includes a computer readable and writeable nonvolatile data storage medium configured to store non-transitory instructions and data. In addition, the data storage includes processor memory that stores data during operation of the processor. 
     Accordingly, in various embodiments, the controller  110  may determine a range to a first portion of an object within the scene  114  based on a time of flight of a reflection of the pulsed laser beam from that portion of the object. Subsequent determinations of a range to other portions of the object (i.e., a second portion, a third portion, etc.) may be based on further reflections from those portions of the object. In particular, each reflection of the pulsed laser beam may correspond to a linear scan performed by the MEMS MMA  104 . For example, the range to the first portion of the object may be determined based on the round trip time of the pulsed laser beam, the amount of the reflected pulsed laser beam, and the amount of the pulsed laser beam that is blocked. In particular, the controller  110  may start a time counter, which is stopped when radiation is received at the laser radar detector. The controller  110  may start the time counter when the pulsed laser beam is emitted by directly controlling the laser radar optical source  102 , or by redirecting a small percentage of the output of the laser radar optical source  102 . With the speed of the emitted pulsed laser beam being a known value, the controller  110  is configured to determine the range to a portion of the sign, and in subsequent scans additional portions of the object. 
     In particular embodiments, the controller  110  is configured to identify the presence of an object within the scene, and within the FOV of the imaging detector  106 , based on the reflective characteristics (i.e., reflectivity) of at least a portion of that object. Specifically, the controller  110  may be configured to identify the presence of an object based on a signal level of one or more read-out signals from the ROIC  108 . As discussed above, the signal level of a particular read-out signal may depend on the flux of the reflections of the pulsed beam collected, and accordingly will depend on the reflective nature of the reflecting area (e.g., the object and the scene). As such, the controller  110  may detect one or more disparities between signal levels to identify the presence of an object within the scene. 
     In at least one example, the controller  110  may compare a signal level of a first read-out signal and an average signal level of the scene to identify the presence of at least a portion of the object. For instance, in response to receiving the one or more read-out signals from the ROIC  108 , the controller  110  may determine an average signal level for the scene by summing and averaging each respective signal level. Specifically, the controller  110  may calculate the average based on the known power and determined range. The average signal level will hence correspond to an average reflective level of the scene. In particular, the controller  110  may calculate an average signal level for those read-out signals that cover the entire field of view of the laser radar detector  106 . In certain embodiments, the controller  110  may detect a disparity between the signal level of the first read-out signal and the average signal level to identify the presence of at least a portion of the object. While in one example, the controller  110  may detect signal levels that are higher than the average signal level, in various other examples any disparity may be used. 
     In other similar implementations, the average signal level may be computed for a portion of the scene (i.e., an area less than the entire span of the scene) to further improve the accuracy of the imaging system  100 . While in one implementation, objects to be detected may be provided with pre-determined highly reflective properties, in various other embodiments, objects (e.g., traffic signs) may naturally include a highly reflective surface, such as a reflective paint or tape. As further discussed below, in various embodiments the controller  110  is further configured to track and/or further interrogate a previously identified areas corresponding to high signal levels with one or more additional linear scans. 
     In certain embodiments, the controller  110  may also be configured to detect a spatial profile of the object within the scene based at least in part on the one or more read-out signals from the ROIC  108 . Specifically, the controller  110  may be configured to detect a spatial profile based on a range to the areas of the scene scanned. As used herein, “spatial profile” may refer to the size, shape, external boundaries, or other surface characteristics of a particular object. Spatial profiles may be detected based on amplitude or range variations between reflections of the pulsed laser beam from portions of the object, and reflections of the pulsed laser beam from a background. That is differences in range between the object and the background. As discussed above, the area of the scene scanned by a linear beam may often include much more than a portion of the object within the scene. For instance, the area scanned may include various background scenery objects such as trees, foliage, buildings, and other landscape features. Accordingly, reflections received from the portions of the object scanned will be received at different times relative to reflections from the portions of the object (if those reflections are received at all). Accordingly, a step transition in the range to the portion of the object and the background may be used to ascertain that an edge or boundary of the object exists. Multiple linear scans of the scene may be used to detect every edge of the object, and construct a spatial profile of the object. 
     Turning briefly to  FIG. 2 , shown is an example illustration of the generation of a spatial profile of an object (e.g., a traffic sign  208 ) based on one or more read-out signals from the ROIC  108  of the example imaging system  100  shown in  FIG. 1 .  FIG. 2  is described with continuing reference to the imaging system  100  shown in  FIG. 1 . In particular,  FIG. 2  shows read out values corresponding to at least two linear scans (e.g., a first linear scan  202  and a second linear scan  204 ) combined by the controller  110  to generate an image (shown as a spatial profile  206 ) of the detected sign  208  based on a range to each area of the scene. The first linear scan  202  of a first area of the scene illuminates a first portion of the sign  208  and a background of the sign within that area. A first scan measurement scale  210  is shown to represent the range of the first portion of the sign  208  from the imaging system  100 , and a range of the background from the imaging system  100 . As discussed herein, the range to the first portion of the sign  208  and the background may be determined based on reflections of the pulsed laser beam from the first area of the scene. While shown in the first scan measurement scale  210  as located at a discrete range, in various embodiments the background of the scene may include various objects (e.g., buildings, trees, foliage, etc.) positioned at different distances from the imaging system  100 . Accordingly, in various implementations, the sign may be at one range from the imaging system  100 , and the background may appear to be at various different ranges from the imaging system  100 . 
     The second linear scan  204  of a second area of the scene illuminates a second portion of the sign  208  and the corresponding background, the second portion of the sign  208  being different from the first portion of the sign  208 . For instance, the second portion may be directionally left or right of the first portion when viewing the sign  208  from the imaging system. A second scan measurement scale  212  is shown to represent the range of the second portion of the sign  208  from the imaging system  100 , and a range of the background of the sign  208  within the second area from the imaging system  100 . As shown, scanning different areas of the scene in multiple linear scans enables the imaging system  100  to detect different portions of the sign  208 , which may each be sequentially “pieced together” to generate an image (e.g., shown as the spatial profile  206 ) of the sign  208 . 
     As discussed above, in various embodiments the controller  100  is configured to identify the presence of an object within a scene, such as the sign  208 , based on the comparison a signal level corresponding to a reflectivity of at least a portion of that object relative to an average signal level corresponding to the reflectivity of the remainder of the scene. Responsive to identifying the presence of the object based on a disparity between the compared signal levels, the system  100  may further interrogate the object to determine additional information regarding the object. For example, as shown in  FIG. 2 , the system  100  may perform multiple linear scans to construct a spatial profile of the object. While in one embodiment, the object (e.g., sign  208 ) may include pre-determined highly reflective properties, in various other embodiments, the controller  110  is configured to identify the presence of the object based on one or more signal levels corresponding to a high reflectivity relative to an average reflectivity of the scene. 
     Returning to  FIG. 1 , in various embodiments the controller  110 , responsive to detecting a spatial profile of the object, may aggregate additional information based on the detected spatial profile of that object. For instance, many traffic signs include alphanumeric, directional, or other information, printed on a surface of the traffic sign (e.g., a sign may have the letters “YIELD” printed on it). The controller  110  may use a look-up table to determine this additional object data based on the detected spatial profile. The object data look-up table may include an array that replaces a runtime computation with an indexing operation. For example, the object data look-up table may include an array of object data stored in static program storage, such as in a data store of the controller  110 . The controller  110  may then use the additional object data to provide instructions or control signals to system coupled equipment to perform one or more additional operations. For example, in response to determining that the spatial profile of a detected object resembles an inverted triangle with rounded corners, the controller  110  may aggregate data corresponding to a YIELD traffic sign, and provide one or more control signals (or other communications) to a speed control system of an autonomous vehicle to adjust a speed of that vehicle. In certain embodiments, the controller  110  may perform one or more machine learning algorithms to recognize pre-defined shapes, objects, external boundaries, or other surface characteristics of an object. 
     Similarly, in various embodiments the controller  110  may be configured to generate one or more images based on reflections of the pulsed laser beam received from the area of the scene  114  scanned. Read-out signals of the ROIC  108  corresponding to one or more consecutive linear scans performed by the MEMS MMA  104  may be used by the controller  110  to determine a range to each respective area of the scene, and linearly construct an image of the scene  114 . Detected spatial profiles or generated images may be provided as an output of the controller  110 . For instance, images may be provided to a user display. In particular embodiments, the system  100  further includes a housing  126  shaped to attach to a user. Specifically, the system  100  may include a goggle housing, with the fixed optical axis  124  aligned with a line of sight of the user (i.e., along boresight). In such an embodiment, images may be provided within the line of sight of the user to assist the user in walking.  FIG. 5  shows an example side profile of an operator wearing the optical system  100  according to certain implementations. In  FIG. 5 , the line of sight of the operator is illustrated as ghost line  502 . In various other embodiments, the housing  126  may be shaped to attach the system  100  to a vehicle or machinery, such as vehicle-mount housing or machinery-mount housing. In such embodiments, images may be provided to dynamically assist the vehicle, machinery, or user thereof, in operating or moving that vehicle or machinery. 
     Turning now to  FIGS. 3A and 3B , shown are example illustrations of operation of the imaging system  100  shown in  FIG. 1 .  FIGS. 3A and 3B  are discussed within continuing reference to the example imaging system  100  shown in  FIG. 1 . In particular,  FIG. 3A  shows a first linear scan  302  of a first area of a scene  304  including a first portion of an object (e.g., sign  306 ), and  FIG. 3B  shows a second linear scan  316  of a second area of the scene  304  including a second portion of the sign  306 . 
     As discussed in more detail with reference to  FIG. 1 , in various embodiments the laser radar detector  106  includes an array of photosensitive detector elements.  FIG. 3A  shows the laser radar detector  106  as having a five by five array of detector elements for the convenience of illustration. In particular implementations, synchronized activation of a portion of the detector elements and scanning of an area of the scene may significantly improve the efficiency and power consumption of the imaging system when compared to conventional laser radar imaging systems. In  FIGS. 3A and 3B , activated detector elements are indicated by hashed lines (e.g., active detector elements  308 ), inactive detector elements are indicated by the absence of hashed lines (e.g., inactive detector elements  310 ), and the corresponding scanned area of the scene is indicated by the first scan  302  and the second scan  316 . In particular, individual detector elements of the laser radar detector may be activated and deactivated in a row-by-row fashion. In  FIG. 3A , the first row of activated detector elements is indicated generally by arrow  312 .  FIG. 3B  shows a subsequent scan (i.e., the second linear scan  316 ) of a second area of the scene  304  including a second portion of the sign  306 . In various embodiments, after activating a first row of detector elements and scanning the first area of the scene  304  (as shown in  FIG. 3A ), the ROIC  108  is configured to activate a second row of detector elements and the MEMS MMA  104  is configured to scan the pulsed laser beam over a second area of the scene including at least a second portion of the sign  306 , and corresponding to the second activated row of detector elements. In  FIG. 3B , the second row of activated detector elements is indicated generally by arrow  314 . Read-out signals from the scans shown in  FIGS. 3A and 3B  may then be received by the controller  110  and used to generate an image of at least the sign  306  based on range. 
     The ROIC  108  and the MEMS MMA  104  may perform one or more imaging or tracking operations based on a raster scan pattern. In such an embodiment, the MEMS MMA  104  is configured to direct the pulsed laser beam and illuminate an area of the scene corresponding to the activated portion of the laser radar detector  106  during the raster scan pattern. In particular, such embodiments of the ROIC  108  may be configured to activate a row of detector elements of MEMS MMA in a raster pattern, the row of detector elements being less than the entire array of the laser radar detector  106 . In certain embodiments the rows, columns, and/or certain subsections of the detector may be configured and preferably optimized to receive optical energy of different wavelengths. In the raster scanning pattern, the ROIC  108  is configured to activate rows of detector elements within one or more horizontal strips. For instance, with reference to  FIG. 3A , the ROIC  108  may be configured to sweep from the top of the laser radar detector  106  to the bottom, or from the bottom to the top of the laser radar detector  106  in a row by row fashion. Simultaneously (e.g., based on the same synchronization signal), the MEMS MMA  104  is configured to scan the pulsed laser beam emitted by the laser radar optical source  102  over each area of the scene corresponding to the currently activated row of detector elements during the raster scan selected pattern. 
     In certain other embodiments, the MEMS MMA  104 , the laser radar detector  106 , and the ROIC  108 , are configured to operate in accordance with a search and track mode of operation. In a search and track mode of operation the MEMS MMA  104  and ROIC  108  are synchronized such that MEMS MMA  104  scans the field of view of the laser radar detector  106  during a searching mode of operation to locate an object. Location of the object may be based on one or more generated images, one or more signal levels, or detection of a spatial profile of the object, as discussed above. Once located, the system  100  may perform one or more tracking operations during which areas of the scene including the object are retraced with one or more additional linear scan to further obtain image data regarding the object. In further embodiments, the searching mode of operation may be initiated by receipt of an input (e.g., a user input) identifying an intended object to be detected. For instance, a user may indicate that they wish for the system to locate traffic signs, and in particular speed limit signs. 
     Turning now to  FIGS. 4A and 4B , shown are additional example illustrations of operation of the imaging system  100  shown in  FIG. 1 .  FIGS. 4A and 4B  are discussed with continuing reference to the example imaging system  100  shown in  FIG. 1 . In particular,  FIG. 4A  shows a first linear scan  402  of a first area of a scene  404  including a first portion of an object (e.g., sign  406 ), and  FIG. 4B  shows a second linear scan  412  of a second area of the scene  404  including a second portion of the sign  406 . 
     As discussed in more detail with reference to  FIG. 1 , in certain embodiments the laser radar detector  106  includes a single linear array of detector elements (e.g., a single linear row of detector elements indicated by arrow  408 ).  FIGS. 4A-4B  show the laser radar detector  106  of one such an embodiment. In such an embodiment, the system  100  may further include focusing optics, such as a cylindrical lens, to focus the received reflection onto the single linear array. In one example, the cylindrical lens has a short focal length to accept light over the entire full field of view. A single linear array of detector elements may further improve the size, weight, and cost, of the imaging system, when compared to conventional imaging systems. In such an arrangement, reflections of the pulsed laser beam may be directed to the same row of detector elements, regardless of the area scanned by the MEMS MMA. Accordingly, the same row of detector elements may be activated for each consecutive scan. 
     Activated detector elements are indicated in  FIGS. 4A-4B  by hashed lines (e.g., active detector elements  410 ). In particular,  FIG. 4A  shows the single linear row of detector elements activated for the first linear scan  402 . Similarly,  FIG. 4B  shows the single linear row of detector elements activated for the second linear scan  412 . In such an arrangement, each pixel receives reflections of the pulsed laser beam for multiple consecutive scans, and the controller uses knowledge of the corresponding area scanned to construct an image of the scene. In particular, the controller may assign the received signal from the ROIC to the appropriate area of the scene based on knowledge of the control signals provided to the beamsteering device (e.g., one or more beamsteering commands). 
     An advantage of the MEMS MMA is its ability to be partitioned on the fly into multiple segments and possibly sub-segments to simultaneously create, shape and steer multiple beams of the same or different wavelengths. The segments and sub-segments do not need to be equally sized, can be any portion of the array and can be changed on the fly in number, size or location. Diverse wavelengths can be realized without changing either the MEMS MMA or the steering parameters (voltages) through some combination of the optical source(s) and optical coatings on the mirrors. For example, mirrors in different segments may have optical coatings that reflect at different wavelengths an illuminated with a broadband source thereby creating discrete optical beams with different wavelengths. Alternately, multiple sources at different wavelengths may be used to illuminate different segments of the MMA in which the mirrors either have broadband optical coatings or optical coatings tuned to the source. 
     Referring now to  FIGS. 7A and 7B , an embodiment of an optical scanning system  600  includes four optical sources  602 ,  604 ,  606  and  608  that generate optical radiation  610 ,  612 ,  614  and  616  at different wavelengths. For example optical source  602  generates broadband optical radiation  610  that spans the short-wave IR (SWIR) band of approximately 0.9 to 1.7 micrometers. Optical sources  604 ,  606  and  608  generate narrowband optical radiation  612 ,  614  and  618  at 1.0, 1.3 and 1.6 micrometers, respectively A MEMS MMA  618  is partitioned into four segments  620 ,  622 ,  624  and  626  each including a plurality of mirrors capable of at least tip and tilt and possibly tip, tilt and piston. The mirrors in segments  620 ,  622 ,  624  and  626  are provide with optical coatings  628 ,  630 ,  632  and  634  that are reflective in a broadband to cover the SWIR band and narrowbands about 1.0, 1.3 and 1.6 micrometers, respectively. Optical radiation  610 ,  612 ,  614  and  616  illuminates the mirrors in segments  620 ,  622 ,  624  and  626 , respectively. In response to command signals, the mirror in each segment tip, tilt and possibly piston to form and steer a broadband SWIR beam  640  and narrowband beams  642 ,  644  and  646  at 1.0, 1.3 and 1.6 micrometers, respectively. In this example, a single broadband SWIR beam  640  is generated to direct optical radiation over an area of the scene. The other segments are further petitioned into sub-segments to generate multiple narrowband beams at each wavelength. This may be done, for example, to revisit objects detected by the broadband SWIR beam  640  to gain additional and more precise information about the object. The number and wavelength diversity of the beams can adapt on the fly in response to system requirements. 
     Another advantage of MEMS MMAs that have piston capability in addition to tip and tilt is the ability to “shape” the beam to integrate one or more optical functions with the beam steering capability. More specifically, the Piston capability can be used to focus the beam such as to form a “fan” or “spot” beam. The Piston capability can also be used to adjust the size, divergence or intensity profile of the beam, produce deviations in the wavefront of the beam to compensate for atmospheric distortions, adjust phase to maintain a zero phase difference across the beam, add optical power to focus or defocus the beam or to improve the formation and steering of the beam by approximating a continuous surface across the micro-mirrors, which reduces unwanted diffraction to increase power in the f optical beam. 
     As illustrated in  FIG. 8 , responsive to command signals a MEMS MMA  700  tips and tilts its mirrors  701  to form a reflective lens to focus optical radiation  702  into an optical beam  704 . The angle of the normal  706  of each mirror to the boresight  708  in the plane of the axis is the same for all mirrors in each concentric circle of mirrors. The MEMS MMA  700  pistons (translates  710 ) mirrors  701  to approximate a continuous mirror surface  712  that approximates a single surface free-space reflective optical mirror. Each mirror can suitably translate at least % wavelength and typically several wavelengths in either direction to form the continuous mirror surface  712 . The edge discontinuities and loss of power are minimized. 
     As illustrated in  FIG. 9 , responsive to command signals a MEMS MMA  800  can adjust the piston  802  of mirrors  804  to induce deviations from a continuous mirror surface  806 . This can be done to compensate for path length variation of the optical beam (to maintain zero phase across the beam), to correct for atmospheric distortion or both. Adjustments for path length variation can be calibrated offline and stored in a lookup table (LUT) as a function of scan angle. Adjustments for atmospheric distortion are done in real-time during operation of the active imaging system. In  FIG. 1 , source  134  emits electromagnetic energy in a similar band to illumination e.g., SWIR and beamsteering device  136  scans the optical beam onto scene  126 . The optical beam preferably has a flat-top across the cross-section of the beam. Wavefront sensor  138  measures the wavefront of the reflected beam to determine the effects of atmospheric distortion. Controller  116  computes the requisite piston adjustments required to correct the wavefront and provides them as command signals to the MEMS MMA. In high quality, high performing active imaging systems, the ability to accurately remove the effects of path length variation and atmospheric distortion is critical to achieving useful imagery of the scene, and important features identified within the scene. 
     As described above with reference to  FIGS. 1, 2, 3A-3B, 4A-4B, 5, 6A-6B, 7A-7B, 8 and 9  several embodiments perform processes that improve known schemes for laser radar imaging. In some embodiments, these processes are executed by an imaging system, such as the imaging system  100  described above with reference to  FIG. 1 . One example of such a process is illustrated in  FIG. 10 . According to this example, the process  900  may include the acts of configuring, calibrating and then partitioning a MEMS MMA, emitting a pulsed laser beam, Tip/Tilt/Piston the MEMS MMA mirrors to shape and steer one or more pulsed laser beams, scanning the pulsed laser beam(s) in a scan, receiving and integrating a reflection of the pulsed laser beam(s), providing a read-out signal(s) based on the integrated reflection of the pulsed laser beam(s), determining a range to at least a portion of an object based on the read-out signal(s), and identifying the presence of the object. Process  900  is described with continuing reference to the example imaging system  100  shown in  FIG. 1 . 
     It is appreciated that in various embodiments the acts  902 - 920  may be performed in the order discussed below. However, in various other embodiments, acts  902 - 918  may be performed in any other suitable order, and each particular act may be performed for any suitable number of iterations. For instance, in certain embodiments, the process  900  may include scanning the pulsed laser beam (as discussed with reference to act  910 ) at substantially the same time as, or prior to, emitting a pulsed laser beam (as discussed with reference to act  906 ). In another example, the process  900  may include performing multiple linear scans (as discussed with reference to act  910 ) prior to determining a range to a first portion of an object (as discussed with reference to act  916 ). 
     In act  902  the process  900  may include, configuring the MEMS MMA to, for example, provide just tip/tilt actuation or tip/tilt/piston actuation, and provide different sections of the MMA with different reflective coatings for independent multi-spectral operation or the same broadband coating to produce each optical beam with multiple spectral components. 
     In act  904  the process  900  may include calibrating the MEMS MMA to determine the command signals to shape a particular optical beam to provide, for example, focus and to provide specific steering angles. For each of these the command signals to tip/tilt/piston each mirror can be stored in different LUTs. 
     In act  906 , the process  900  includes emitting from the laser radar optical source  102  a pulsed laser beam. As discussed above with reference to  FIG. 1 , the laser radar optical source  102  may include any suitable source of NIR light. Accordingly, in various embodiments emitting a pulsed laser beam may include emitting a pulsed laser beam having an eye-safe wavelength of 1,550 nm. In other embodiments, emitting the pulsed laser beam may include emitting a laser beam having a wavelength of 1,064 nm. In one particular example, the laser radar optical source  102  may emit a pulsed laser beam at 1 μJ, at pulse width of 10 nanoseconds, and a repetition rate of 10 kHz. A single narrowband or broadband optical source may illuminate the entire MEMS MMA. Multiple sources may illuminate different sections of the MEMS MMA and be combined into a single beam. Multiple narrowband sources at different wavelengths may illuminate different sections of the MEMS MMA for independent multi-spectral scanning. 
     In act  908 , the process  900  may partition the MEMS MMA for multi-segment and/or multi-spectral. In act  510 , the mirrors within each partitioned are actuated to shape and steer the one or more optical beams at the same or different wavelengths. Beam “shaping” may include for example, focusing the beam or providing deviations of the wavefront to compensate for path length variation and/or atmospheric distortion. 
     In act  910 , the mirrors within each partitioned are actuated to shape and steer the one or more optical beams at the same or different wavelengths. Beam “shaping” may include for example, focusing the beam or providing deviations of the wavefront to compensate for path length variation and/or atmospheric distortion. In act  910 , the process  900  scans an optical beam(s) over at least a first area of the scene. Act  910  may further include scanning additional optical beams to revisit previously scanned portions or scanning additional optical beams in parallel over different areas of the scene. 
     In act  910 , the process  900  may include receiving the pulsed laser beam from the laser radar optical source at the MEMS MMA  104  positioned to steer the pulsed laser beam about the optical axis. For example, the MEMS MMA  104  may receive the pulsed laser beam by an optical fiber or free space coupling. Responsive to receiving the pulsed laser beam from the laser radar optical source  102 , act  910  includes scanning the pulsed laser beam in one or more linear scans over one or more areas of the scene to be imaged. For instance, the MEMS MMA  104  may receive one or more control signals, such as an applied voltage, to actuate the mirrors to tip/tilt/piston and direct the pulsed laser beam. 
     In certain embodiments, scanning the pulsed laser beam in a linear scan over an area of the scene may include scanning the pulsed laser beam over at least a first portion of an object within that area. As discussed with reference to  FIG. 1 , objects may include signs or any informational object or device. In particular embodiments, the process  900  may include controlling the MEMS MMA  104  to perform one or more consecutive linear scans over the scene, each linear scan corresponding to a different area of the scene. For instance, the process  900  may include scanning the pulsed laser beam over a first area of the scene, and responsive to completing the scan, scanning the pulsed laser beam over a second area of the scene. Such a process may be repeated for any number of desired scans, or until an entire spatial profile of the object has been detected. Furthermore, the process  900  may include scanning the pulsed laser beam over a previously scanned area of the scene to retrace a previous scan. In various embodiments, the scanned area of the scene is substantially less than an entire span of the scene. 
     Still referring to act  910 , in certain embodiments scanning the pulsed laser beam over the area of the scene during a linear scan may include directing the pulsed laser beam within the field of view of the laser radar detector  106 . In particular, the area scanned (i.e., illuminated) by the pulsed laser beam may be less than the field of view of the laser radar detector  106 . The process  900  may also include the act of synchronizing the one or more linear scans to correspond to the active portions of the laser radar detector  106 . That is, the MEMS MMA  104  may direct a first linear scan over only an area of the scene corresponding to a first active portion of the laser radar detector  104 . 
     In act  912 , the process  900  may include receiving and integrating, at an active portion of the laser radar detector  106 , a reflection of the pulsed laser beam corresponding to the area of the scene scanned. Responsive to receiving the reflection of the pulsed laser beam, the process  900  may further include providing at least one read-out signal from the ROIC  108  coupled to the laser radar detector  106 , the read-out signal being based at least in part on the integrated reflection of the pulsed laser beam (act  914 ). For multiple linear scans, such acts may include providing a first read-out signal for reflections corresponding to a first area of the scene (illuminated during a first linear scan), and providing a subsequent read-out signal for reflections corresponding to another area of the scene (illuminated during a subsequent linear scan). For one or more subsequent linear scans, the ROIC  108  may activate detector elements of the laser radar detector in a row-by-row arrangement. As such, acts performed with reference to  FIG. 10  may operate according to a rolling-shutter mode of operation. 
     In act  916 , the process  900  may include receiving the one or more read-out signals from the ROIC  108  at the controller  110 , and determining a range to a portion of the object based at least in part on a time of flight of the pulsed laser beam. The process  900  may further include determining a range to other portions of the object (i.e., a second portion, a third portion, etc.) based on further reflections from those portions of the object. In particular, each reflection of the pulsed laser beam may correspond to a linear scan performed by the MEMS MMA  104 . 
     In act  918 , the process  900  may include identifying a presence of an object within the scene based at least in part on a signal level of at least one read-out signal. Specifically, the signal level may correspond to a reflectivity of at least a portion of the object within the scanned area of the scene. As discussed above, the signal level of a particular read-out signal may depend on the flux of the reflections of the pulsed beam collected, and accordingly will depend on the reflective nature of the reflecting area (e.g., the object and the scene). As such, the process  900  may include detecting one or more disparities between signal levels to identify the presence of an object within the scene. 
     In certain examples, the process  900  may include comparing a signal level for a first read-out signal to an average signal level of the scene to identify the presence of at least a portion of the object. For instance, in response to receiving the one or more read-out signals from the ROIC  108 , the process  900  may include determining an average signal level for the scene by summing and averaging each respective signal level. The average signal level will hence correspond to an average reflective level of the scene. In further embodiments, the process  900  may include detecting a disparity between the signal level of the first read-out signal and the average signal level to identify the presence of at least a portion of the object. While in one example, the controller  110  may detect signal levels which are higher than the average signal level, in various other examples any disparity may be used. 
     In other similar implementations, the average signal level may be computed for a portion of the scene (i.e., an area less than the entire span of the scene) to further improve the accuracy of the imaging system  100 . While in one implementation, objects to be detected may be provided with pre-determined highly reflective properties, in various other embodiments, objects (e.g., traffic signs) may naturally include a highly reflective surface, such as a reflective paint or tape. 
     In further embodiments, the process  900  may include the act of detecting a spatial profile of the object based at least in part on one or more detected ranges to the object. As discussed above, “spatial profile” may refer to the size, shape, external boundaries, or other surface characteristics of a particular object. Spatial profiles may be detected based on variations in amplitude of the pulsed laser beam or range between the portion(s) of the object and a background. Such embodiments are further discussed above with reference to at least  FIG. 2 . In particular, multiple linear scans of the scene may be used to detect the edges or boundaries of the object, and construct a spatial profile of the object. In various embodiments, responsive to detecting a spatial profile of the object, the process  900  may include aggregating additional information based on the detected spatial profile of that object. 
     Similarly, in various embodiments the process  900  may include the act  918  of generating one or more images, possibly multi-spectral, based on the reflections of the pulsed laser beam received from the area of the scene scanned. Read-out signals of the ROIC  108  corresponding to one or more consecutive linear scans performed by the MEMS MMA  104  may be used by the controller to linearly construct an image of the scene. Further acts of process  900  may also include providing detected spatial profiles or generated images to one or more additional system or downstream components such as a user display. 
     Accordingly, aspects and embodiments provide optical systems and methods for detecting objects in low-visibility and other restricted view conditions. In particular, aspects and embodiments include thermo-optical, electro-optical, solid-state and/or non-mechanical optical elements for improved weight reduction and reduced power consumption during such conditions. For instance, one example of an imaging system includes a MEMS MMA positioned to scan a pulsed laser beam in a linear scan over an area of a scene. Reflections of the pulsed laser beam from the area of the scene, and reflections from multiple consecutive linear scans over different areas of the scene, may be used to identify the presence of an object within the scene, locate and determine a range to that object, detect a spatial profile of that object, and/or generate an image of that object. Such aspects and embodiments provide improved practicality for mobile applications, such as laser range finding systems for autonomous vehicles, autonomous machinery, user-wearable devices, and other maritime, airborne, or space systems. 
     Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.