Patent Publication Number: US-11025898-B2

Title: Detecting loss of alignment of optical imaging modules

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application 62/730,023, filed Sep. 12, 2018; U.S. Provisional Patent Application 62/818,123, filed Mar. 14, 2019; and U.S. Provisional Patent Application 62/833,718, filed Apr. 14, 2019. All of these related applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to opto-electronic devices, and particularly to beam-forming optics for optical emitters and applications of such emitters. 
     BACKGROUND 
     Light emitters, such as vertical-cavity surface-emitting lasers (VCSELs), are commonly integrated with a small lens (referred to as a microlens) that directs and collimates the emitted beam. An array of such microlenses may be fabricated integrally over a semiconductor substrate on which an array of emitters is formed, with the microlenses in alignment with the emitters. 
     Compact optical imaging modules are ubiquitous in portable digital devices, such as mobile phones and tablet computers. A typical module comprises imaging optics, comprising one or more lenses, and an image sensor located in the image plane of the optics. Even when the imaging optics and image sensor have been carefully aligned at the time of manufacture, the alignment may shift during the lifetime of the module in the field, for example due to mechanical shocks. 
     Methods for detecting and correcting for alignment shift are known in the art. For example, U.S. Patent Application Publication 2018/0041755 describes a method in which a scene is imaged using an imaging system, which includes an array of radiation sensing elements. The array includes first sensing elements with symmetrical angular responses and second sensing elements with asymmetrical angular responses, interspersed among the first sensing elements, with optics configured to focus radiation from the scene onto the array. The method includes processing first signals output by the first sensing elements in order to identify one or more areas of uniform irradiance on the array, and processing second signals output by the second sensing elements that are located in the identified areas, in order to detect a misalignment of the optics with the array. 
     SUMMARY 
     Some embodiments of the present invention that are described hereinbelow provide integrated emitter devices and methods for their production and use. Other embodiments provide methods and apparatus for detecting lens misalignment in an optical imaging module, including apparatus using integrated emitter devices. 
     There is therefore provided, in accordance with an embodiment of the invention, imaging apparatus, including a housing and imaging optics mounted in the housing and configured to form an optical image, at a focal plane within the housing, of an object outside the housing. An image sensor, including a matrix of detector elements, is positioned at the focal plane in alignment with the imaging optics and is configured to output an electronic image signal in response to optical radiation that is incident on the detector elements. At least one emitter is fixed within the housing and is configured to emit a test beam toward one or more reflective surfaces within the housing, which reflect the test beam toward the image sensor. A processor is configured to process the electronic image signal output by the image sensor in response to the reflected test beam so as to detect a change in the alignment of the image sensor with the imaging optics. 
     In some embodiments, the processor is configured to initiate a corrective action upon detecting that a magnitude of the change is greater than a predefined limit. In one such embodiment, the detected change includes a shift of the optical image on the image sensor, and the corrective action includes processing the electronic image signal so as to compensate for the shift. 
     In a disclosed embodiment, the at least one emitter is fixed at a location adjacent to the image sensor. 
     In some embodiments, at least one of the one or more reflective surfaces is an interior surface of the housing. In a disclosed embodiment, the interior surface is configured as an elliptical mirror, which focuses the reflected test beam to form a predefined geometrical figure on the image sensor, wherein the processor is configured to detect the change in the alignment responsively to movement of geometrical figure on the image sensor. 
     Additionally or alternatively, the imaging optics include one or more lenses having refractive surfaces, and the one or more reflective surfaces include one or more of the refractive surfaces of at least one of the lenses. In a disclosed embodiment, the imaging optics are configured to form the optical image of the object within a predefined spectral range, and the test beam is emitted at a wavelength outside the predefined spectral range, and at least one of the refractive surfaces includes a coating configured to pass the optical radiation within the predefined spectral range while reflecting the test beam. Alternatively or additionally, a pattern is formed on the at least one of the lenses in an area on which the test beam is incident, wherein the pattern causes the reflected test beam to form a predefined geometrical figure on the image sensor, and the processor is configured to detect the change in the alignment responsively to changes in the geometrical figure on the image sensor. 
     Further additionally or alternatively, the at least one emitter includes a radiation source, which is configured to generate the test beam, and a lens, which is configured to collimate and direct the test beam toward the one or more reflective surfaces. In one embodiment, the lens includes a microlens, such as a micro-prism-lens, which is decentered relative to the radiation source in order to direct the test beam at a desired propagation angle toward the one or more reflective surfaces. 
     In a disclosed embodiment, the at least one emitter includes a plurality of emitters disposed at different, respective locations within the housing. 
     There is also provided, in accordance with an embodiment of the invention, a method for imaging, which includes mounting imaging optics in a housing so as to form an optical image of an object outside the housing on an image sensor, including a matrix of detector elements, in alignment with the imaging optics at a focal plane of the imaging optics within the housing. At least one emitter is fixed within the housing so as to emit a test beam toward one or more reflective surfaces within the housing, which reflect the test beam toward the image sensor. An electronic image signal that is output by the image sensor in response to the reflected test beam is processed so as to detect a change in the alignment of the image sensor with the imaging optics. 
     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 sectional view of an opto-electronic device, in accordance with an embodiment of the invention; 
         FIG. 2  is a schematic sectional view of an opto-electronic device, in accordance with another embodiment of the invention; 
         FIG. 3  is a schematic sectional view showing a detail of the opto-electronic device of  FIG. 2 , in accordance with an embodiment of the invention; 
         FIGS. 4-8  are schematic sectional views of opto-electronic devices, in accordance with further embodiments of the invention; 
         FIGS. 9A-C  are schematic sectional diagrams illustrating a process for fabricating asymmetrical structures on a substrate, in accordance with an embodiment of an invention; 
         FIGS. 10A , B, C, D, and E are schematic sectional diagrams illustrating a process for producing microlenses on a semiconductor substrate, in accordance with an embodiment of the invention; 
         FIG. 11  is a flowchart that schematically illustrates a fabrication process of a micro-prism-lens, in accordance with an embodiment of the invention 
         FIG. 12  is a schematic sectional view of an optical imaging module with an emitter for detection of changes in alignment, in accordance with an embodiment of the invention; 
         FIG. 13  is a schematic sectional view of the optical imaging module of  FIG. 12 , showing the effect of a change in alignment; and 
         FIGS. 14-16  are schematic sectional views of optical imaging modules with emitters for detection of changes in alignment, in accordance with other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Emitters with Micro-Prism-Lenses 
     Portable electronic devices, such as cellular phones or tablets, commonly employ one or more integral light sources. These light sources may, for example, provide illumination for a scene recorded by a camera integrated into the device. They commonly comprise a vertically emitting light source, such as a VCSEL, and are constrained to a very small surface area. Moreover, the light sources in these applications may be required to tilt the beam of light emitted by a VCSEL with respect to its substrate and to collimate it. 
     A VCSEL without any additional collimating optics emits a highly diverging beam in a direction perpendicular to the substrate. Adding a microlens for beam collimation and tilt is not an optimal solution: Inside the microlens, the VCSEL beam has a low divergence due to the high refractive index of the microlens, leading to a small beam diameter before the collimating surface, which, in turn, leads to an inherent divergence due to diffraction. Using the microlens to achieve beam tilt by offsetting the optical axis of the lens from the VCSEL leads to a further reduction of the beam diameter in the direction of the tilt as a function of the cosine of the tilt angle. 
     Some embodiments of the present invention that are described herein address the above limitations so as to provide a compact optical element, which focuses and deflects the beam emitted by a solid-state emitter, such as a VCSEL, so that the beam exits the optical element at a high tilt angle and with large diameter, relative to solutions that are known in the art. In the disclosed embodiments, an emitter is formed on a semiconductor substrate with a planar surface, and a reflective layer (either using a reflective coating or total internal reflection) is formed on the planar surface adjacent to the emitter. A micro-prism-lens is then formed over the emitter. 
     The emitter emits a beam of light in a direction away from the planar surface, for example in a vertical direction relative to the surface. The surface of the micro-prism-lens has a first segment positioned above the emitter so as to reflect the emitted beam towards the reflective layer, which further reflects the beam towards a second segment of the surface of the micro-prism-lens. The second segment is formed so as to collimate and transmit the beam out from the micro-prism-lens. The term “collimate” is used in the context of the present description and claims to mean that the divergence of the emitted beam is substantially reduced, typically by at least 50% in angular terms, even if the emitted beam is not fully parallelized by the second segment. 
     A large-diameter collimated and tilted beam is achieved by two properties of the disclosed micro-prism-lens:
         1. The two internal reflections within the micro-prism-lens provide an increased propagation length before the collimating surface provided by the second segment, thus yielding an increased diameter of the collimated beam; and   2. The reflection of the beam from the first segment of the surface of the micro-prism-lens imposes the desired tilt on the beam without reducing its diameter.       

     Although the embodiments described below relate, for the sake of simplicity, to a single VCSEL emitter and micro-prism-lens, the principles of the present invention can readily be applied to emitters of other types, as well as to provide integrated arrays of micro-prism-lenses over arrays of VCSELs and other emitters. 
     Embodiments with Two Internal Reflections 
       FIG. 1  is a schematic sectional view of an opto-electronic device  21 , in accordance with an embodiment of the invention. 
     Optoelectronic device  21  comprises a planar semiconductor substrate  10 , for example comprising GaAs, on which a VCSEL  12  is formed by processes of semiconductor device fabrication that are known in the art. VCSEL  12  emits a beam of light  14  in a vertical direction relative to substrate  10 , for example at a wavelength between 650 nm and 1300 nm. (The terms “optical radiation” and “light” as used in the present description and in the claims refer generally to any and all of visible, infrared, and ultraviolet radiation.) A reflective layer  16  is formed on substrate  10 . In the present embodiment, reflective layer  16  comprises a metal, such as gold, which also serves as an electrode for VCSEL  12 , but of extended size so as to function also as a reflector. In alternative embodiments, other metals, such as aluminum, may be used instead of gold. In further alternative embodiments, emitters such as edge-emitting lasers may be used, wherein either the emitter is oriented so that its emitting face is parallel to substrate  10 , or a micro-mirror is used to direct the emitted beam in a direction away from the substrate, in a vertical direction or possibly angled relative to the vertical. Alternatively, reflective layer  16  may be separate and independent of the electrodes used to drive the emitter. 
     A micro-prism-lens  18  is formed over VCSEL  12 . Micro-prism-lens  18  comprises a material that is transparent at the wavelength of beam  14 , such as GaAs, fused silica, SiO 2 , epoxy, polymers, or glass. The height of micro-prism-lens  18  may vary from tens of microns, such as 20 or 30 microns, to 2 mm, and width from 60 microns to 6 mm. Micro-prism-lens  18  can be fabricated utilizing methods described below with reference to  FIGS. 9A-C ,  10 A-E, and  11 . Reflective coatings and/or anti-reflective coatings can then be deposited on the outer surface of micro-prism-lens, using thin film deposition and patterning techniques that are known in the art. 
     A first segment  20  of an outer surface  22  of micro-prism-lens  18  is positioned to receive and reflect beam  14  internally into a beam  24 . First segment  20  is either flat or concave (as viewed from outside), and is tilted so as to impose a deviation angle φ 1  between beams  14  and  24 . Beam  24  is reflected by gold layer  16  into a beam  26 , which impinges on a second segment  28  of surface  20 . Second segment  28  has a radius of curvature selected so as to collimate and transmit beam  24  into a beam  30 . As noted earlier, the term “collimate” is used in the broader sense of reducing the divergence of a beam, so that beam  30  may be diverging (but less than it would be without micro-prism-lens  18 ), parallel, or even converging. 
     The two internal reflections within micro-prism-lens  18 , from first segment  20  and from gold layer  16 , increase the propagation length from VCSEL  12  to second segment  28 , thus ensuring a sufficient diameter for transmitted beam  30 . The deviation angle φ is responsible for imposing a desired tilt onto beam  24 , which tilt then propagates through the reflection from gold layer  16  to beam  26  and subsequently to beam  30 . By a suitable choice of the design parameters of micro-prism-lens  18 , the deviation angle φ 1  may exceed 55 degrees, and the diameter of beam  30  at second segment  28  may exceed 70 microns. Details of an optical design are shown in  FIG. 3 , and described below. 
     Due to the use of a high-index material for micro-prism-lens  18 , such as GaAs (n=2.7), the reflection from first segment  20  takes place as a total internal reflection (TIR) as long as the angle of incidence of beam  14  on the first segment is larger than approximately 20 degrees. Additionally or alternatively, surface  22  may be coated by an anti-reflective (AR) coating  32  in order to reduce reflection losses of transmitted beam  30 . 
     In an alternative embodiment, the part of gold layer  16  that reflects beam  24  into beam  26  may be replaced by a reflective dielectric layer, which reflects beam  24  either by TIR or by virtue of a multilayer construction designed for high reflectivity. 
     The shape of first segment  20  may be determined based on considerations of both manufacturability and functionality: A planar shape may be easier to manufacture and less sensitive to tolerances, whereas a concave shape increases the divergence of beam  24 , which then increases the diameter of beam  30 . 
       FIG. 2  is a schematic sectional view of an opto-electronic device  23 , in accordance with another embodiment of the invention. 
     Opto-electronic device  23  is similar to device  21 , and the same labels in  FIG. 2  are used for items similar to those in  FIG. 1 . Opto-electronic device  23  comprises a micro-prism-lens  40 , which is similar to micro-prism-lens  18  of device  21 , with a flat first segment  42 , and functions in a similar fashion to device  21 . When device  23  is incorporated in an illumination array, an array of micro-prism-lenses  40  of this sort can be formed in alignment over a corresponding array of VCSELs  12 . 
       FIG. 3  is a schematic sectional view of a detail  44  of opto-electronic device  23 , showing an optical design, in accordance with an embodiment of the invention. For the sake of clarity, anti-reflective coating  32  has been omitted from the figure, but may be included in this embodiment, and second segment  28  is illustrated as a flat surface (with “R” to indicate its radius of curvature).  FIG. 3  shows an optical design that yields collimated beam  30  with a diameter of 80 microns and a tilt angle φ 2  54 degrees with respect to a vertical direction. The thickness H of micro-prism-lens  40  is 95 microns, the horizontal center-to-center separation L between first segment  42  and second segment  28  is 700 microns, and the radius of curvature R of the second segment is 705 microns, with the second segment convex toward the outside of the micro-prism-lens. Alternatively, by scaling all the dimensions by a factor of X, the same tilt angle φ 2  of 54 degrees for collimated beam  30  is achieved, with beam diameter D scaled down to D/X=80/X microns. 
       FIG. 4  is a schematic sectional view of an opto-electronic device  25 , in accordance with yet another embodiment of the invention. 
     Opto-electronic device  25  is similar to device  21 , and the same labels are used in  FIG. 4  for items similar to those in  FIG. 1 . Opto-electronic device  25  comprises a micro-prism-lens  50 , which is similar to micro-prism-lens  18  of device  21 , except for a first segment  52 , which is formed as a concave indentation in the flat outer surface of micro-prism-lens  50 . This concave shape is advantageous in increasing the divergence of beam  24 , relative to the preceding embodiments. 
     Alternative Embodiments 
       FIG. 5  is a schematic sectional view of an opto-electronic device  55 , in accordance with another embodiment of the invention. In this embodiment and the embodiments that follow, the beam from VCSEL  12  is reflected internally four or more times within the micro-prism-lens, thus increasing the optical path length and hence increasing the diameter of the beam that is projected out of the device. 
     Opto-electronic device  55  comprises the following items that are similar to those of device  21 , and are labeled with the same labels: planar semiconductor substrate  10 , VCSEL  12 , and gold layer  16 . As in device  21 , VCSEL  12  emits beam of light  14  in a vertical direction relative to substrate  10 . A micro-prism-lens  60  is formed over VCSEL  12 . Micro-prism-lens  60  comprises, as in device  21 , a material that is transparent at the wavelength of beam  14 . 
     A first segment  62  of a surface  64  of micro-prism-lens  60  comprises a first and a second sub-segment  62   a  and  62   b , respectively, wherein both sub-segments are planar, but are not co-planar. A second segment  66  of surface  64  is similar to second segment  28  of device  21 . Second segment  66  is coated with an AR coating  68 , whereas first segment  62  is coated with a reflective coating  70 , comprising aluminum, for example. 
     First sub-segment  62   a  receives beam  14  and reflects it into a beam  72  with an angle of deviation of φ 4 , after which beam  72  is reflected by gold layer  16  into a beam  74 . Beam  74  impinges on second sub-segment  62   b , and is reflected into a beam  76  with an angle of deviation of φ 5 . Beam  76  is reflected by gold layer  16  into a beam  78 , which is subsequently collimated and transmitted by second segment  66  into a beam  80 . 
     The two angles of deviation, φ 4  and φ 5 , can be controlled by adjusting the tilt angles of sub-segments  62   a  and  62   b , thus enabling control of both the tilt angle and the diameter of collimated beam  80 . In alternative embodiments (not shown in the figures), first segment  62  may comprise more than two sub-segments, such as, for example, three, four, or even five subsegments. Additionally or alternatively, one or more of the surfaces of the sub-segments of first segment  62  may be non-planar (i.e., concave or convex) for further control of the beam size. For example, a concave shape may be used to increase the diameter of collimated beam  80 , whereas a convex shape may be used for either increasing or decreasing the beam diameter. 
       FIG. 6  is a schematic sectional view of an opto-electronic device  85 , in accordance with yet another embodiment of the invention. 
     Opto-electronic device  85  comprises the following items that are similar to those of device  21 , and are labeled with the same labels: planar semiconductor substrate  10 , VCSEL  12 , and gold layer  16 . A micro-prism-lens  90  is formed over VCSEL  12  and comprises, as in device  21 , a material that is transparent at the wavelength of beam  14 . 
     A first segment  92  of a surface  94  of micro-prism-lens  90  is planar, and dimensioned so as to intercept at least two reflections from gold layer  16 , as will be detailed below. A second segment  96  of surface  94  is similar to second segment  28  of device  21 . Similarly to opto-electronic device  55 , second segment  96  is coated with an AR coating  98 , whereas first segment  92  is coated with a reflective coating  100 . 
     First segment  92  receives beam  14  and reflects it into a beam  102  with an angle of deviation of (p 6 , after which beam  102  is reflected by gold layer  16  into a beam  104 . Beam  104  impinges back on first segment  92 , and is again reflected to gold layer  16 , now as a beam  106  with an angle of deviation of (ρ 7 . Beam  106  is reflected by gold layer  16  into a beam  108 , which is subsequently collimated and transmitted by second segment  96  into a collimated beam  110 . A central ray  112  of beam  108  and a central ray  114  of beam  110  will be used for comparison in  FIG. 7 . 
     With an appropriate choice of width of first segment  92  and its tilt angle with respect to gold layer  16 , the first segment may intercept more than two reflections from the gold layer, for example, three, four, or five reflections. Moreover, the beams originating from different reflections from gold layer  16  and impinging on first segment  92  are allowed to overlap on the first segment. 
       FIG. 7  is a schematic sectional view of an opto-electronic device  95 , in accordance with a further embodiment of the invention. 
     Opto-electronic device  95  is identical to opto-electronic device  85  of  FIG. 6 , except for a second segment  122  of a micro-prism-lens  120 . For all the other items of opto-electronic device  95  the same labels are used as for device  85 , and from VCSEL  12  until second segment  122  in device  95 , the beams of light follow paths identical to those in device  85 . 
     In device  95 , second segment  122  transmits and collimates beam  108  into a beam  124  in such a way that the axis of beam  124  (as represented by a central ray  126  of the beam) is deviated from the axis of beam  108  (as represented by central ray  112 ) by an angle α with respect to a normal to the second segment, whereas in device  85  central ray  108  continues in a straight line relative to central ray  114 . Thus, by choosing the tilt of second segment  122  with respect to central ray  112 , the direction of collimated beam  124 , and specifically its central ray  126 , may be chosen to be either collinear with central ray  112  or at an elevated or lowered angle with respect to central ray  112 . The design parameters of micro-prism-lens  120  may be chosen to yield a desired tilt angle of central ray  126  with respect to substrate  10 . For example, central ray  126  may be perpendicular to substrate  10 , as shown by a dotted line  128 . Due to the increased diameter of beam  108  at second segment  122 , as previously described, a reduction of the diameter of beam  124  due to the non-normal tilt angle (α≠0) may be more easily tolerated. 
       FIG. 8  is a schematic sectional view of an opto-electronic device  135 , in accordance with another embodiment of the invention. 
     Opto-electronic device  135  is similar to section  44  of device  23  ( FIGS. 2, 3 ), and the same labels are used in  FIG. 8  for items similar to those in  FIGS. 1 and 2 . Opto-electronic device  135  comprises a micro-prism-lens  140 , which is similar to micro-prism-lens  40  of device  23 , except for a second segment  142 , which is formed either as a Fresnel-lens or as another type of diffractive optical element (DOE). This embodiment is suitable for designs wherein beam  26  impinges on second segment  142  at a low angle of incidence, such as 10° or less. Additionally or alternatively, first segment  42  may be implemented using a Fresnel-lens or another type of DOE. 
       FIGS. 9A-C  are schematic sectional diagrams illustrating a process for fabricating asymmetrical structures  160  on a substrate  162 , in accordance with an embodiment of an invention. Asymmetrical structures  160  of this sort can be used for the purpose of fabricating micro-prism-lenses  18  ( FIG. 1 ),  40  ( FIG. 2 ),  50  ( FIG. 4 ), ( FIG. 5 ),  90  ( FIG. 6 ),  120  ( FIG. 7 ), and  140  ( FIG. 8 ), as will be further described with reference to  FIGS. 10A-D . 
     Asymmetric structures  160  can be fabricated, for example, using the fabrication process described by Gimkiewicz et al., in “Fabrication of microprisms for planar optical interconnections by use of analog gray-scale lithography with high-energy-beam-sensitive glass,”  Applied Optics , Vol. 38, pp. 2986-2990 (1990), which is incorporated herein by reference. The fabrication process utilizes a HEBS (high-energy-beam-sensitive) glass mask  164 , wherein a gray-level pattern  166  has been exposed by electron-beam lithography, with further details given in the above-referenced publication by Gimkiewicz et al. 
       FIG. 9A  shows an exposure step of the fabrication process, wherein substrate  162 , coated by a photoresist layer  168 , is exposed through HEBS mask  164  by ultraviolet (UV) light  170 . 
       FIG. 9B  shows the result of a development step, wherein exposed photoresist layer  168  has been developed into asymmetrical photoresist structures  172 . 
       FIG. 9C  shows the result of an etch step, wherein the shapes of asymmetrical photoresist structures  172  have been, with the use of reactive-ion etching (RIE), transferred into substrate  162  to form asymmetrical structures  160 . 
     Although asymmetrical structures  160  have a simple prismatic form, more complicated forms comprising, for example, curved surfaces, may be produced by generating suitable gray-level patterns in HEBS mask  164 . Such structures, comprising microlenses  174  as an example, are illustrated in  FIG. 10A , below. Substrate  162  may be a substrate that is etchable by RIE and has the required mechanical properties, such as fused silica. 
       FIGS. 10A-D  are schematic sectional diagrams illustrating a process for transferring microlenses  174  on substrate  162  into a GaAs substrate  176  by the Confined Etchant Layer Technique (CELT), in accordance with an embodiment of the invention. The transfer process utilizes a CELT, described by Zhan et al. in “Confined Etchant Layer Technique (CELT) for Micromanufacture,”  Proc.  6 th IEEE International Conference on Nano/Micro Engineered and Molecular Systems , pp. 863-867 (2011), which is incorporated herein by reference. 
       FIG. 10A  shows a replication of microlenses  174  on substrate  162 , which serves as a mold, into a layer of a polymer, such as PMMA  178 . Although the embodiments of  FIGS. 10A-D  illustrate the transfer of simple microlenses  174 , more complex structures may be fabricated, as detailed in  FIGS. 9A-C , above. The replication process comprises spinning liquid PMMA over microlenses  174 , drying the liquid PMMA into solid PMMA  178 , and separating the PMMA from the microlenses, so that the curved shapes of the microlenses are replicated in the PMMA. 
       FIG. 10B  shows a start of an etching process for transferring the features on the layer of PMMA  178  into GaAs substrate  176 . A surface  180  of PMMA  178  has been coated by thin layers of titanium (Ti) and platinum (Pt). It has been brought into close proximity to GaAs substrate  176  (for example, within a hundred microns or tens of microns), with microlenses  174  facing the GaAs substrate. A space  182  between PMMA  178  and GaAs  176  has been filled with a mixture of an etchant and a scavenger, for example with bromide and cystine respectively used as etchant and scavenger. The etchant has been chosen to etch GaAs, but not the layer of inert Pt. The scavenger is used to control the process, as detailed by Zhan et al., cited above. 
       FIGS. 10C, 10D and 10E  show the progress of the transfer process, with space  182  continuously diminishing, until in  FIG. 10E , PMMA  178  and GaAs  176  have come into contact, and the transfer process has been completed. 
     Although PMMA is used as a mold in the embodiments described above, other materials, such as silicon or a platinum-iridium (Pt—Ir), alloy may be used as molds when suitably patterned. 
       FIG. 11  is a flowchart that schematically illustrates the fabrication process of a micro-prism-lens, in accordance with an embodiment of the invention. This flowchart illustrates schematically the processes shown in  FIGS. 9A-C  and  10 A-E for fabricating micro-prism-lenses  18  ( FIG. 1 ),  40  ( FIG. 2 ),  50  ( FIG. 4 ),  60  ( FIG. 5 ),  90  ( FIG. 6 ),  120  ( FIG. 7 ), and  140  ( FIG. 8 ). The additional fabrication steps of opto-electronic devices  21 ,  23 ,  25 ,  55 ,  85 ,  95 , and  135 , such as fabrication of a VCSEL and depositing of anti-reflective and reflective coatings, have not been included in the flowchart for the sake of simplicity, and their implementation will be apparent to those skilled in the art. In the description below, reference is also made to  FIGS. 9A-C  and  10 A-E. 
     The fabrication starts in a start step  184 . In parallel (but not necessarily concurrently), HEBS mask  164  is fabricated in a mask fabrication step  186 , and substrate  162  is coated by photoresist  168  in a photoresist coating step  188 . In an exposure step  190 , photoresist  168  is exposed through HEBS mask  164 , as shown in  FIG. 9A . In a development step  192 , exposed photoresist  168  is developed to produce structures  172 , as shown in  FIG. 9B . In an RIE step  194 , structures  160  are etched into substrate  162 , as shown in  FIG. 9C . 
     In a transfer step  196 , structures  174  on substrate  162  are transferred into PMMA  178 , as shown in  FIG. 10A . In a PMMA coat step  198 , thin layers of Ti and Pt are deposited on PMMA  178 . In an etch cell assembly step  200 , PMMA  178  is brought close to GaAs substrate  176 , and space  182  is filled with etchant and scavenger, as shown in  FIG. 10B . In an etch step  202 , structures on PMMA  178  are transferred into GaAs  176 , as shown in  FIGS. 10C-E . In a separation step  204 , PMMA  178  and GaAs  176  are separated from each other. The process ends in an end step  206 . 
     Features of the embodiments shown in  FIGS. 1-11  may be combined in additional ways, as will be apparent to those skilled in the art after reading the present description. 
     Detecting Loss of Alignment in Optical Imaging Modules 
     The position and tilt of the imaging optics relative to the image sensor in an optical imaging module can play a critical role in the performance of the module. In particular, when the imaging module is used in measurement applications, changes in alignment can lead to inaccurate measurements. For example, many depth mapping systems project a pattern of structured light onto a scene, and then analyze an image of the pattern that is captured by an imaging module in order to compute depth coordinates of objects in the scene by triangulation. A change of the position or tilt of the imaging optics relative to the image sensor can result in a significant error in the estimation of the depth. 
     Some embodiments of the present invention that are described herein address this problem by periodically sensing the alignment of the image sensor with the imaging optics, and initiating corrective action when a significant shift in alignment is detected. The disclosed embodiments make use of one or more dedicated emitters, which can be built into the optical imaging module and direct beams of radiation toward one or more of the internal surfaces in the module. These surfaces can include, for example, refractive entrance and exit faces of lenses in the imaging optics, as well as interior surfaces of the lens housing. The emitter or emitters are arranged so that the radiation reflected from the internal surface or surfaces is incident on the image sensor. Changes in the pattern of this reflected radiation on the image sensor give an indication of changes in the alignment of the optics with the image sensor. 
     The disclosed embodiments provide imaging apparatus, such as an optical imaging module, comprising imaging optics, which are mounted in a housing and form an optical image of an object outside the housing. An image sensor, comprising a matrix of detector elements, is positioned and aligned at the focal plane of the imaging optics within the housing. In addition to these standard imaging module components, at least one emitter is fixed within the housing and emits a test beam toward one or more reflective surfaces within the housing. These reflective surfaces, which reflect the test beam toward the image sensor, may comprise, for example, one or more of the refractive surfaces of the lenses in the imaging optics and/or an interior surface of the housing. The test beam may be a directional beam, which is aimed in the desired direction by the sort of micro-prism-lens that is described above. 
     The image sensor outputs electronic image signals in response to the optical radiation that is incident on the detector elements, including signals in response to the reflected test beam. (The emitter may be actuated to emit the test beam only for short intervals, for example when the imaging module is not in use.) If the alignment of the image sensor with the imaging optics changes, for example due to a mechanical shock, the electronic image signal output by the image sensor due to the reflected test beam will change, as well. A processor receives and processes this electronic image signal in order to detect such changes, and will initiate corrective action when the magnitude of the change is greater than a predefined limit. 
     For example, a change in the electronic image signal due to the test beam may indicate that the optical image formed by the imaging optics on the image sensor has shifted as the result of a shift in the optical center of the imaging optics. In this case, the processor may compensate for the shift, possibly by applying a counter-shift of the appropriate direction and magnitude to the electronic images output by the image sensor. 
     Embodiments of the present invention are thus useful in enhancing the accuracy of measurement applications, such as depth sensing, that use imaging modules. Such embodiments can be used to correct certain errors that may result from changes in alignment of the imaging module, and/or to issue an alert when an error is not readily correctable, and measurements are likely to be incorrect. The disclosed embodiments require only minimal hardware additions to imaging module designs, while taking advantage of the existing image sensor and associated image processing capabilities. 
     For the sake of simplicity in the embodiments that are shown in the figures and described below, only a single emitter is used in generating a test beam. In alternative embodiments, however, multiple emitters are disposed at different, respective locations within the housing, so that each directs a respective beam toward the reflective surfaces of the optics at a different respective angle. The emitters may be operated in sequence, thus generating a sequence of different electronic image signals from the image sensor. The processor can analyze these signals in order to extract a comprehensive picture of any changes in alignment that may have occurred. For example, two emitters may be positioned 90° apart around the optical axis and thus be used in detecting optical shifts along two orthogonal axes. Alternatively or additionally, two emitters may be positioned in close proximity to one another in order to increase the measurement precision. 
     Reference is now made to  FIGS. 12 and 13 , which are schematic sectional views of an optical imaging module  220 , in accordance with an embodiment of the invention.  FIG. 12  shows the components of module  220  in an initial, baseline position, for example following factory calibration of the module, while  FIG. 13  shows the effect of a shift in alignment. Although this figure relates only to alignment shift due to element decentering, the principles of the present invention may similarly be applied in detecting other sorts of misalignment, due to element tilt and deformation, for example. 
     Module  220  comprises imaging optics in the form of lenses  222 , labeled A, B, C and D for convenience, which are mounted in a housing  224 . Housing  224  may comprise a lens barrel or any other suitable sort of lens mount. Lenses  22  form optical images, at a focal plane within the housing, of objects outside the housing. An image sensor  226  is positioned at the focal plane in alignment with lenses  222 . Image sensor  226  typically comprises a matrix of detector elements, as is known in the art, and outputs an electronic image signal in response to optical radiation that is incident on the detector elements. (The term “optical radiation,” as used in the present description and in the claims, refers to electromagnetic radiation in any of the visible, infrared and ultraviolet ranges and may be used interchangeably with the term “light.”) 
     An emitter  228  is fixed within housing  224  and emits a test beam  230  toward one or more reflective surfaces within the housing, which reflect the test beam toward image sensor  226 . Emitter  228  typically comprises a miniature radiation source, such as a semiconductor laser (including both edge-emitting and vertical-cavity surface-emitting laser (VCSEL) types), or light-emitted diode (LED), and may be conveniently fixed at a location adjacent to image sensor  226 , for example on the same circuit substrate. In the present embodiment, emitter  228  emits test beam  230  toward a reflective interior surface  232  of housing  224 . 
     A processor  234  receives and processes the electronic image signal output by image sensor  226  in response to the reflected test beam, and thus detects possible changes in the alignment of the image sensor with the imaging optics. As noted earlier, only short actuation times of emitter  228  are needed for this purpose, so that the effect on the normal operation of module  220  is minimal. Processor  234  typically comprises a programmable microprocessor or microcontroller, which is programmed in software or firmware to carry out the functions that are described herein, and which has suitable input and output interfaces for receiving the electronic image signals from image sensor  226  and outputting alerts and control signals as appropriate. Typically (although not necessarily), processor  234  also performs other processing and control functions in module  220 , such as processing images that are formed on image sensor  226  by lenses  222 , for example for purposes of depth mapping. Alternatively or additionally, processor  234  comprises hardware logic circuits, which may be hard-wired or programmable. 
     In the example shown in  FIG. 12 , reflective surface  232  is shaped as a concave curved mirror, such as an elliptical mirror, which reflects and focuses test beam  230  so as to form a predefined geometrical figure on image sensor  226 . The reflective surface is polished but does not generally require an actual mirror coating. Processor  234  is configured to detect changes in the alignment responsively to movement of geometrical figure on the image sensor. 
     A change of this sort is shown in  FIG. 13 , in which housing  224  and lenses  222  have shifted relative to image sensor  226 , as indicated by an arrow  236 , with the result that the optical center of the imaging optics has shifted relative to the image sensor. In this case, processor  234  will detect that the geometrical figure formed on image sensor  226  by reflection of test beam  230  from surface  232  has shifted, as well. Alternatively, movements along other axes can be detected using this technique, including movements along the direction of the optical axis of lenses  222 , rather than perpendicular to the optical axis as shown in  FIG. 22 . 
     Surface  232  may advantageously be shaped as a cylindrical-elliptical mirror, so that the geometrical figure formed on image sensor  226  is a curved line. Processor  234  can detect movement of this line in the direction of arrow  236  (following a mechanical shock to module  220 , for example) with high resolution, particularly using sub-pixel detection algorithms, as are known in the art. The configuration of the elliptical mirror that is shown in  FIGS. 12 and 13  creates an amplification in the movement of the line formed on image sensor  226 , so that if housing  224  moves by a distance x, the line on the image sensor can move by 25x or more. Combining this movement amplification with sub-pixel detection enables processor  234  to detect relative shifts between the image sensor and imaging optics that are as small as 0.1 pixels, or possibly less. 
       FIG. 14  is a schematic sectional view of an optical imaging module  240 , in accordance with another embodiment of the invention. Elements in this figure, as well as in the figures that follow, that are identical or closely similar to their counterparts in optical imaging module  220  are marked with the same indicator numbers as in  FIG. 12 . Processor  234  is omitted from these figures for the sake of simplicity. 
     In module  240 , an emitter  242  emits a test beam  244  with high angular divergence toward both surface  232  of housing  224  and toward the refractive surfaces of lenses  222 . This arrangement gives rise to multiple reflections of beam  244  from the various lens, which are incident on image sensor  226 . For the sake of simplicity, however,  FIG. 23  shows only reflected beams  248  and  250 , which are reflected from and focused by respective concave surfaces  249  and  252  of lenses D and B, along with a reflected beam  246  from surface  232 . 
     Processor  234  processes the electronic image signals due to reflected beams  246 ,  248  and  250  (and possibly beams reflected from other lens surfaces, whether concave, convex or any other shape) in order to detect changes in the reflected beams that may be indicative of changes in alignment. The changes in this case may be either in the positions of individual lenses  222  or of housing  224  as a whole relative to image sensor  226 . A numerical ray-trace simulation can be used to analyze the pattern of reflected beams that appears on image sensor  226  and the influence of movement of various elements of module  240  on the pattern, including the effect of lens refraction on the test beam. Processor  234  can use the simulation results in matching and analyzing the patterns that are formed on the image sensor by the reflected beams, and can thus translate particular pattern changes into the movements that caused the changes. Additionally or alternatively, machine learning algorithms can be used in associating changes in the patterns on the image sensor with movements of elements of module  240 . 
     In many imaging applications, lenses  222  are configured to form optical images of objects in a scene within a certain predefined spectral operating range. Emitter  242  may be chosen to emit test beam  244  at a wavelength outside this spectral range. Frequently, some or all of the refractive surfaces of lenses  222 , such as surfaces  249  and  252 , are coated to pass the optical radiation within the predefined spectral operating range. For example, in visible imaging applications, the surfaces of lenses  222  may have anti-reflection coatings for visible light. When test beam  244  is at a sufficiently long infrared wavelength, it may be strongly reflected by these coatings, thus enhancing the intensity of reflected beams  248  and  250  and making it easier for processor  234  to detect them. The anti-reflection coatings on the lenses may be optimized for high reflection at the emitter wavelength. As another example, in depth mapping applications using structured light of a given wavelength, some of the surfaces of lenses  222  may have a bandpass coating, which passes the given wavelength while reflecting radiation outside the passband, including test beam  244 . 
     Alternatively, even when the wavelength of test beam  244  is inside the spectral operating range of the module  240 , the high incidence angle of the test beam on the lens surfaces will cause the test beam to be strongly reflected. The anti-reflection coatings on lenses  222  can also be optimized to reflect high-angle rays. 
     Furthermore, if a particular surface is of more interest than others, its coating can be designed accordingly to reflect the wavelength of emitter  242  with larger efficiency than other surfaces. 
     In general, however, the methods described herein can be used even without any modification of the coating designs, since a certain amount of radiation is always reflected from the optical surfaces, and the optical power of emitter  242  and/or the integration time of image sensor  226  can be adjusted accordingly. 
       FIG. 15  is a schematic sectional view of an optical imaging module  260 , in accordance with a further embodiment of the invention. Module  260  is similar to module  240 , as described above, except that in the present embodiment, a pattern  262  is formed on lens D in the area on which test beam  244  is incident. Pattern  262  causes the reflected test beam to form a predefined geometrical figure on the image sensor, such as a corresponding pattern of focused, reflected beams  264 . Processor  234  detects movement or changes in the pattern of reflected beams  264 , based on the corresponding electronic image signals output by image sensor  226 , and is thus able to detect changes in the alignment of lens D relative to image sensor. Other refractive surfaces in module  260  may have similar sorts of patterns for this purpose. Each such surface may have a distinctive pattern to enable their respective reflected beams to be more easily distinguished from one another. 
     Various sorts of patterns  262  may be used for the purposes of the present embodiment. For example, pattern  262  may comprise one or more radiation-absorbing markings on the lens surface. The markings can be made thin enough to minimize their effect on the normal operation of module  260 . A pattern comprising a few lines of this sort with an appropriate equal spacing between them will give rise to an interference pattern on image sensor  226 . Fourier analysis can be used to extract the extent of relative movement between lens D and image sensor  226  from changes in the interference pattern. Additionally or alternatively, pattern  262  may be reflecting, which induces a very strong pattern intensity on image sensor  226  and thus makes it possible to decrease the exposure time of the image sensor (and hence reduce its sensitivity to radiation from the scene). 
       FIG. 16  is a schematic sectional view of an optical imaging module  270 , in accordance with yet another embodiment of the invention. In this embodiment, the emitter comprises a radiation source  272 , which generates a test beam  276 , and a lens  274 , which collimates and directs the test beam toward one or more reflective surfaces in module  270 . The pictured example shows beams  78  and  80  reflected from the lower, concave surfaces of lenses D and B, respectively (while omitting the beams that are reflected from other surfaces for the sake of simplicity). Lens  274  may advantageously comprise a microlens, which is decentered relative to radiation source  272  in order to create the desired propagation angle within module  270 . Although lens  274  is shown in  FIG. 16  as a separate component from radiation source  272 , the lens may advantageously be a micro-prism-lens, which is integrated with a suitable radiation source, for example as shown in any of  FIGS. 1-8  and described above. 
     Beams  278  and  280  will form respective spots (of different sizes) on image sensor  226 , rather than lines or other large-scale features as in the preceding embodiments. Collimation by lens  274  thus increases the signal/noise ratio and sharpness of the pattern, as well as separating the reflections from the different surfaces on the image sensor. 
     Although the figures above show a specific sort of module design, the principles of the present invention are similarly applicable to other sorts of imaging modules, with different geometries and arrangements of lenses. All such alternative embodiments are considered to be within the scope of the present invention. 
     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.