Abstract:
An optical scanning device includes a substrate, which is etched to define an array of two or more parallel micromirrors and a support surrounding the micromirrors. Respective spindles connect the micromirrors to the support, thereby defining respective parallel axes of rotation of the micromirrors relative to the support. One or more flexible coupling members are connected to the micromirrors so as to synchronize an oscillation of the micromirrors about the respective axes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application claims the benefit of U.S. Provisional Patent Application 61/614,018, filed Mar. 22, 2012, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical scanning. 
     BACKGROUND 
     Various methods are known in the art for optical 3D mapping, i.e., generating a 3D profile of the surface of an object by processing an optical image of the object. This sort of 3D profile is also referred to as a 3D map, depth map or depth image, and 3D mapping is also referred to as depth mapping. 
     PCT International Publication WO 2012/020380, whose disclosure is incorporated herein by reference, describes apparatus for mapping, which includes an illumination module. This module includes a radiation source, which is configured to emit a beam of radiation, and a scanner, which is configured to receive and scan the beam over a selected angular range. Illumination optics are configured to project the scanned beam so as to create a pattern of spots extending over a region of interest. An imaging module is configured to capture an image of the pattern that is projected onto an object in the region of interest. A processor is configured to process the image in order to construct a three-dimensional (3D) map of the object. 
     U.S. Patent Application Publication 2011/0279648, whose disclosure is incorporated herein by reference, describes a method for constructing a 3D representation of a subject, which comprises capturing, with a camera, a 2D image of the subject. The method further comprises scanning a modulated illumination beam over the subject to illuminate, one at a time, a plurality of target regions of the subject, and measuring a modulation aspect of light from the illumination beam reflected from each of the target regions. A moving-mirror beam scanner is used to scan the illumination beam, and a photodetector is used to measure the modulation aspect. The method further comprises computing a depth aspect based on the modulation aspect measured for each of the target regions, and associating the depth aspect with a corresponding pixel of the 2D image. 
     U.S. Pat. No. 8,018,579, whose disclosure is incorporated herein by reference, describes a three-dimensional imaging and display system in which user input is optically detected in an imaging volume by measuring the path length of an amplitude modulated scanning beam as a function of the phase shift thereof. Visual image user feedback concerning the detected user input is presented. 
     U.S. Pat. No. 7,952,781, whose disclosure is incorporated herein by reference, describes a method of scanning a light beam and a method of manufacturing a microelectromechanical system (MEMS), which can be incorporated in a scanning device. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved scanning devices, as well as apparatus and methods for 3D mapping using such devices. 
     There is therefore provided, in accordance with an embodiment of the invention, an optical scanning device, including a substrate, which is etched to define an array of two or more parallel micromirrors and a support surrounding the micromirrors. Respective spindles connect the micromirrors to the support, thereby defining respective parallel axes of rotation of the micromirrors relative to the support. One or more flexible coupling members are connected to the micromirrors so as to synchronize an oscillation of the micromirrors about the respective axes. 
     In a disclosed embodiment, the substrate is etched to separate the support from the substrate surrounding the support, and to define further spindles connecting the support to the substrate, thereby providing a further axis of rotation of the support, which is perpendicular to the axes of rotation of the micromirrors. 
     In one embodiment, the one or more flexible coupling members include a belt, which is etched from the substrate and has first and second ends attached respectively to first and second ones of the micromirrors and is anchored to the support at a point between the first and second ends. The belt may be thinned relative to the substrate. 
     The one or more flexible coupling members may be coupled so as to cause the micromirrors to oscillate in phase, so that the micromirrors have the same angular orientation during oscillation, or alternatively so as to cause the micromirrors to oscillate in anti-phase. 
     Typically the device includes a reflective coating applied to the substrate on the micromirrors, wherein the substrate is a part of a silicon wafer. In a disclosed embodiment, the device includes an electromagnetic drive, which is coupled to drive the micromirrors to oscillate about the respective parallel axes. 
     There is also provided, in accordance with an embodiment of the invention, scanning apparatus, including a substrate, which is etched to define an array of two or more parallel micromirrors and a support surrounding the micromirrors, wherein the micromirrors are coupled to rotate in mutual synchronization about respective parallel first axes of rotation relative to the support while the support rotates about a second axis relative to the substrate. An electromagnetic drive is coupled to cause the micromirrors and the support to rotate respectively about the first and second axes. 
     In some embodiments, the electromagnetic drive includes a stator assembly, including at least one magnetic core having an air gap and at least one coil wound on the magnetic core, and at least one rotor, on which the support is mounted and which is suspended in the air gap so as to move within the air gap in response to a current driven through the at least one coil. In a disclosed embodiment, the support has a pair of wings, each connected to the substrate by a respective spindle, and the at least one rotor includes a pair of permanent magnets, each connected to a respective one of the wings. Additionally or alternatively, the electromagnetic drive and the current may be configured to cause the micromirrors to rotate about the first axes at a first frequency, which is a resonant frequency of rotation, while causing the support to rotate about the second axis at a second frequency, which is lower than the first frequency. 
     In a disclosed embodiment, the substrate is etched to define respective first spindles connecting the micromirrors to the support, thereby defining the respective parallel first axes of rotation of the micromirrors relative to the support, one or more flexible coupling members, which are connected to the micromirrors so as to synchronize an oscillation of the micromirrors about the respective first axes, and second spindles connecting the support to the substrate along the second axis. 
     In one embodiment, the apparatus includes a transmitter, which is configured to emit a beam including pulses of light toward the micromirror array while the micromirrors and the support rotate, so as to cause the micromirrors to scan the beam over a scene, a receiver, which is configured to receive, by reflection from the micromirror array, the light reflected from the scene and to generate an output indicative of a time of flight of the pulses to and from points in the scene, and a controller, which is coupled to process the output of the receiver during a scan of the beam so as to generate a three-dimensional map of the scene. 
     There is additionally provided, in accordance with an embodiment of the invention, a method for producing an optical scanning device. The method includes etching a substrate so as to define an array of two or more parallel micromirrors, a support surrounding the micromirrors, respective spindles connecting the micromirrors to the support, thereby defining respective parallel axes of rotation of the micromirrors relative to the support, and one or more flexible coupling members, which are connected to the micromirrors so as to synchronize an oscillation of the micromirrors about the respective axes. 
     There is further provided, in accordance with an embodiment of the invention, a method for scanning, which includes providing a substrate, which is etched to define an array of two or more parallel micromirrors and a support surrounding the micromirrors. The micromirrors are driven to rotate in mutual synchronization about respective parallel first axes of rotation relative to the support while driving the support to rotate about a second axis relative to the substrate. A beam of light is directed toward the micromirror array while the micromirrors and the support rotate, so as to cause the micromirrors to scan the beam over a scene. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of an optical scanning head, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic, pictorial illustration of a MEMS scanner, in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic rear view of a gimbaled micromirror array, in accordance with an embodiment of the invention; 
         FIG. 4  is a schematic detail view showing elements of the micromirror array of  FIG. 3 ; 
         FIG. 5  is a schematic front view of the micromirror array of  FIG. 3  in operation; and 
         FIG. 6  is a schematic diagram illustrating principles of operation of a gimbaled micromirror array, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     U.S. patent application Ser. No. 13/766,801, filed Feb. 14, 2013, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes depth engines that generate 3D mapping data by measuring the time of flight of a scanning beam. A light transmitter, such as a laser, directs short pulses of light toward a scanning mirror, which scans the light beam over a scene of interest. A receiver, such as a sensitive, high-speed photodiode (for example, an avalanche photodiode) receives light returned from the scene via the same scanning mirror. Processing circuitry measures the time delay between the transmitted and received light pulses at each point in the scan. This delay is indicative of the distance traveled by the light beam, and hence of the depth of the object at the point. The processing circuitry uses the depth data thus extracted in producing a 3D map of the scene. 
     For compactness, low cost, and low power consumption, the scanning mirror in this sort of scanning system may be produced using MEMS technology (possibly by means of the sorts of techniques that are described in the above-mentioned U.S. Pat. No. 7,952,781). To enhance the sensitivity of the system, it is advantageous that the mirror be as large as possible (typically with an active area in the range of 5-25 mm 2 ). At the same time, for 3D mapping, as well as other scanning applications, it is desirable that the mirror scan mechanically about at least one axis over large angles (typically ±10-25°) at high frequency (typically 2-10 kHz). (The scan range about the second scan axis may be even larger, but the scan frequency is typically lower.) The need for high scan frequency and range conflicts with the desire to increase mirror size, and it may be infeasible to make a single scanning mirror of the desired size, range, and frequency capabilities given the limitations of the material (such as a silicon wafer) from which the scanner is made. 
     Embodiments of the present invention that are described herein seek to overcome these design constraints by using an array of multiple, adjacent mirrors. The mirrors scan in mutual synchronization, and thus behave optically as though they were a single mirror, of dimensions equal to the size of the entire array. A weak mechanical link between the mirrors in the array is used to couple the oscillations of the mirrors and thus maintain the synchronization between them. 
     In the embodiments that are illustrated in the figures, the synchronized mirror array comprises two micromirrors, which operate in phase and are mounted on a gimbaled base for two-axis scanning. (The term “micromirror” is used herein simply to refer to very small mirrors, which are typically no more than a few millimeters across, although it may be possible to apply the principles of the present invention to larger mirrors.) Alternatively, such mirror arrays may comprise a larger number of mirrors, and may be deployed with or without gimbaling. Further alternatively or additionally, other forms of synchronization, such as anti-phased rotation of the mirrors in the array, can be implemented by appropriate design of the mirrors and the mechanical link between them. 
       FIG. 1  schematically illustrates elements of an optical scanning head  40  comprising a gimbaled micromirror array  100 , in accordance with an embodiment of the present invention. With the exception of the micromirror array itself, optical scanning head  40  is similar to the optical scanning head that is described in the above-mentioned U.S. patent application Ser. No. 13/766,801. A transmitter  44  emits pulses of light toward a polarizing beamsplitter  60 . Typically, only a small area of the beamsplitter, directly in the light path of transmitter  44 , is coated for reflection, while the remainder of the beamsplitter is fully transparent in the transmitted wavelength range (or even anti-reflection coated for it) to permit returned light to pass through to a receiver  48 . The light from transmitter  44  reflects off beamsplitter  60  and then a folding mirror  62  toward micromirror array  100 . A MEMS scanner  64  scans the micromirror array in X- and Y-directions with the desired scan frequency and amplitude. Details of the micromirror array and scanner are shown in the figures that follow. 
     Light pulses returned from the scene strike micromirror array  100 , which reflects the light via folding mirror  62  through beamsplitter  60 . To limit the amount of unwanted ambient light that reaches receiver  48 , a bandpass filter (not shown) may be incorporated in the receiver path, possibly on the same substrate as beamsplitter  60 . Receiver  48  senses the returned light pulses and generates corresponding electrical pulses. A controller  30  drives transmitter  44  and scanner  64  and analyzes the time delay between the transmitted pulses and the corresponding pulses from receiver  48  in order to measure the time of flight of each pulse. Based on this time of flight, the controller computes the depth coordinate of each point in the scene that is scanned by scanning head  40  and thus generates a depth map of the scene. 
     To enhance sensitivity of detection, the overall area of beamsplitter  60  and the aperture of receiver  48  are considerably larger than the area of the transmitted beam. It is also desirable that the micromirrors in micromirror array  100  be as large as possible, within the inertial constraints imposed by the scanner. For example, the area of each micromirror may be about 12.5 mm 2 , and the overall area of the micromirror array may be about 25 mm 2 . 
     The specific mechanical and optical designs of the optical head shown in  FIG. 1  are described here by way of example, and alternative designs implementing similar principles are considered to be within the scope of the present invention. 
       FIG. 2  is a schematic, pictorial illustration of MEMS scanner  64 , in accordance with an embodiment of the present invention. This scanner is produced and operates on principles similar to those described in the above-mentioned U.S. Pat. No. 7,952,781, but enables two-dimensional scanning of micromirror array  100 . The micromirror array is produced by suitably etching a semiconductor substrate  68  to separate micromirrors  102  in the array from a support  72  (also referred to as a gimbal), and to separate the support from the remaining substrate  68 . After etching, micromirrors  102  (to which a suitable reflective coating is applied) are able to rotate in the Y-direction relative to support  72  on spindles  106 , while support  72  rotates in the X-direction relative to substrate  68  on spindles  74 , which are coupled to wings  104  of support  72 . 
     Micromirrors  102  and support  72  are mounted on a pair of rotors  76 , which typically comprise permanent magnets. (Only one of the rotors is visible in this figure.) Rotors  76  are suspended in respective air gaps of magnetic cores  78 . Cores  78  are wound with respective coils  80  of conductive wire, thus creating an electromagnetic stator assembly. Although a single coil per core is shown in  FIG. 2  for the sake of simplicity, two or more coils may alternatively be wound on each core; coils may be wound at different places on the cores; and different core shapes may also be used. Alternative core and coil designs are shown, for example, in U.S. Provisional Patent Application 61/675,828, filed Jul. 26, 2012, which is incorporated herein by reference. 
     Driving an electrical current through coils  80  generates a magnetic field in the air gaps, which interacts with the magnetization of rotors  76  so as to cause the rotors to rotate or otherwise move within the air gaps. Specifically, coils  80  are driven with high-frequency differential currents so as to cause micromirror  46  to rotate resonantly back and forth about spindles  70  at high frequency (typically in the range of 2-10 kHz, as noted above). This resonant rotation generates the high-speed Y-direction raster scan of the output beam from engine  22 . At the same time, coils  80  are driven together at lower frequency to drive the X-direction scan by rotation of support  72  about spindles  74  through the desired scan range. Alternatively, other stator configurations and drive schemes may be used for these purposes, as described in the above-mentioned U.S. Provisional Patent Application 61/675,828, for example. The X- and Y-rotations together generate the overall raster scan pattern of micromirror  46 . 
     Assembly of optical head  40  from discrete optical and mechanical components, as shown in  FIG. 1 , requires precise alignment and can be costly. In alternative embodiments, all parts requiring precise placement and alignment (such as the light transmitter, receiver, and associated optics) may be combined in a single integrated package on a silicon optical bench (SiOB). This approach can save costs and may make the depth engine easier to handle. Various alternative designs of these sorts are shown in the above-mentioned U.S. patent application Ser. No. 13/766,801, and may be adapted, as well, for use with a micromirror array. 
       FIG. 3  is a schematic rear view of gimbaled micromirror array  100 , in accordance with an embodiment of the invention. Array  100  as pictured in  FIG. 3  differs in some details of shape and orientation from the micromirror array that is show in  FIGS. 1 and 2 , but its elements and principles of operation are the same. As noted earlier, array  100  comprises two parallel micromirrors  102 , which are connected to support  72  by respective spindles  106 . Magnetic rotors  76  are attached to wings  104  of support  72 , which are coupled to substrate  68  by spindles  74 , perpendicular to spindles  106 . In operation, rotors  76  are suspended within the air gaps of cores  78 , as shown in  FIG. 2  and explained above. Mirrors  102  are linked mechanically to one another by flexible coupling members in the form of belts  108 , as explained below. 
       FIG. 4  is an enlarged, detail view of micromirrors  102 , showing details of one of belts  108 . This belt is produced in the same photolithographic process in which the mirrors and their spindles are etched apart from substrate  68 . Belt  108  thus comprises a thin strip of silicon, typically about 10-100 μm wide, which is separated by grooves etched through the substrate from support  72  on one side and from micromirrors  102  on the other. The thickness of the belt (i.e., the dimension perpendicular to the wafer surface) may be the full thickness of the wafer. Alternatively, belt  108  may be thinned to alter the belt connection stiffness and to enable bending and stretching modes of the belt in addition to the torsion mode that is illustrated in  FIG. 5 . Each end of the belt is connected to a respective one of the micromirrors, and the belt is anchored to support  72  at a central pivot point  110 . 
       FIG. 5  is a schematic pictorial view of array  100  in operation, powered by a MEMS scanner as shown above. The MEMS scanner drives both micromirrors  102  to rotate simultaneously about the X-axis (as defined in  FIG. 2 ). The elastic force exerted by belts  108  couples the motion of the two micromirrors together, so that they rotate in perfect phase synchronization and have the same angular orientation during oscillation. Even if the actual force exerted by the belts is small, it is sufficient to maintain mechanical phase locking and thus synchronize the two adjacent oscillators (i.e., the micromirrors), which have approximately the same resonant frequency. Thus, array  100  behaves optically as though it were a single oscillating mirror, with dimensions equal to the combined dimensions of both micromirrors  102  together. 
     Physically speaking, spindles  106  act as torsion springs, and belt  108  adds a third spring to the system, coupling together the masses of micromirrors  102 . When the masses are coupled via this third spring, two modes of motion are possible: one in which, the masses move in the same direction, and the other in which the masses move in opposite directions. (Each mode has its own frequency, which is shared by both mirrors, as opposed to the individual frequencies of the two mirrors in the absence of a coupling member.) The stiffness of the third spring can be adjusted, even to the point at which belt  108  is the primary spring, exerting greater force than pivots  106 . 
       FIG. 6  is a schematic diagram illustrating principles of operation of a gimbaled micromirror array  200 , in accordance with an embodiment of the present invention. This figure illustrates how the principles described above may be extended to arrays of three micromirrors  202 ,  204 ,  206  (labeled M 1 , M 2  and M 3 ), or more. Mirrors M 1 , M 2  and M 3  are mounted on pivots  208  (such as the sort of spindles described above), represented as springs K 3 , K 4 , K 5 , while the mirrors are linked by belts  210  represented as springs K 1  and K 2 . This arrangement can be used to synchronize the rotation of the three mirrors in the same manner as in the two-mirror embodiments described above. The three (or more) mirrors may likewise be mounted together on a gimbaled support. Regardless of whether the array includes two, three, or more mirrors, the springs may be implemented either as the sort of pivots and belts that are shown in the preceding figures or using other sorts of flexible, elastic elements, which may be fabricated by any suitable technique that is known in the art. 
     Although the operation of micromirror array  100  is described above primarily in the context of optical head and 3D mapping, the principles of array  100  may similarly be applied in optical scanners of other types, for substantially any application requiring a compact, high-frequency resonant scanner. Such scanners may be driven magnetically, as in the embodiments described above, or using any other suitable sort of drive mechanism that is known in the art, including various types of magnetic and electrostatic drives, for example. Furthermore, as noted earlier, the mirrors may be coupled and driven so that while rotating at the same frequency, the mirrors are oriented at different angles during their respective scans. This latter mode of operation can be useful in synchronized multi-beam scanning systems. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.