Abstract:
This disclosure describes optical systems for projecting an irregular or complex pattern onto a region of space (e.g., a two-dimensional or three-dimensional object or scene). A respective light beam is emitted from each of a plurality of light sources. The emitted light beams collectively are diffracted in accordance with a plurality of different first grating parameters to produce a plurality of first diffracted light beams. The first diffracted light beams then collectively are diffracted in accordance with one or more second grating parameters.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to optical pattern projection. 
       BACKGROUND 
       [0002]    Optical pattern projection can be used in a variety of applications such as three-dimensional (3D) or depth mapping, area illumination, and LCD backlighting. 3D (or depth) mapping, for example, refers to a set of 3D coordinates representing the surface of an object. As part of the process of depth mapping, light (i.e., visible, infra-red, or other radiation) can be projected onto a region with a pattern of high quality (e.g., good resolution) and well-controlled intensity, so that depth values can be found reliably over a substantial part of an object or objects in a scene. 
         [0003]    In some applications, diffraction gratings are used in creating a desired projection pattern. A diffraction grating can be implemented, for example, as an optical surface that is etched, molded or deposited on the surface of a substrate. In some systems, first and second diffraction gratings are arranged in series to diffract an input optical beam. 
       SUMMARY 
       [0004]    This disclosure describes optical systems for projecting an irregular or complex pattern onto a region of space (e.g., a two-dimensional or three-dimensional object or scene). Such patterns can be used, for example, to provide enhanced textural contrast so as to facilitate stereo matching and/or depth calculations via added/adding scene/object texture, pattern distortion, triangulation and/or other depth imaging or sensing applications. 
         [0005]    For example, in one aspect, a method of projecting an optical pattern onto an object or scene includes emitting a respective light beam from each of a plurality of light sources. The emitted light beams collectively are diffracted in accordance with different first grating parameters (e.g., profile functions and grating periods) to produce first diffracted light beams. The first diffracted light beams then collectively are diffracted in accordance with one or more second grating parameters (e.g., profile functions and grating periods). 
         [0006]    In accordance with another aspect, an optical projection sub-assembly includes optical channels, light sources each of which is operable to emit light into a respective one of the channels, one or more first gratings that collectively provide a plurality of different first grating parameters, and one or more second gratings that collectively provide at least one second grating parameter. The one or more first gratings are arranged so that light emitted by the light sources is diffracted according to different ones of the first grating parameters to produce a plurality of first diffracted optical signals. The one or more second gratings are arranged to diffract the first diffracted optical signals. 
         [0007]    In some implementations, various features of the gratings, such as the fan-out angles of the first and second gratings, can be selected to produce a highly irregular or complex light pattern that is projected onto a region of space. 
         [0008]    Other aspects, features and advantages will be apparent from the following detailed description, the accompanying drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of an optical 3D mapping system. 
           [0010]      FIG. 2  illustrates an example of an optical projection sub-assembly. 
           [0011]      FIG. 3  illustrates another example of an optical projection sub-assembly. 
           [0012]      FIG. 4  illustrates a further example of an optical projection sub-assembly. 
           [0013]      FIG. 5  illustrates yet another example of an optical projection sub-assembly. 
           [0014]      FIG. 6  illustrates an example of an optical projection sub-assembly. 
           [0015]      FIG. 7  illustrates another example of an optical projection sub-assembly. 
           [0016]      FIG. 8  is a plan view of an array of light emitters. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    As illustrated in  FIG. 1 , an optical 3D mapping system  20  includes an imaging device  24  that has an optical projection sub-assembly  30  operable to generate and project an optical pattern onto a region  22  (e.g., a scene or object). The device  24  includes an image capture sub-assembly  28  operable to capture an image of the pattern appearing on the region  22 . An image processor  26  is operable to process image data generated by the device  24  to obtain a 3D map of the region  22 . In some cases, the image processor  26  computes the 3D map using triangulation-based 3D mapping. The image processor  26  can be implemented, for example, as a computer processor programmed in software to carry out the 3D mapping, or may be implemented in dedicated hardware, such as an integrated circuit. The image processor  26  may be separate from the imaging device  24  or may be implemented by dedicated circuitry within a housing of the imaging device  24  or otherwise associated with the imaging device. 
         [0018]      FIG. 2  illustrates an example of the optical projection sub-assembly  30 . As shown in  FIG. 2 , the optical projection sub-assembly  30  includes a light source  32 , such as a laser diode or vertical cavity surface emitting laser (VCSEL), operable to generate and emit a light beam  34 . In some instances, the light beam  34  has a narrow spectral emission (centered about a wavelength λ e.g., 0.850, 0.905, and/or 0.940 μm). The light beam  34  passes through the optical assembly  46 , which can include one or more beam shaping elements such as collimating lenses. The optical projection sub-assembly  30  further includes first and second gratings  36 ,  42 . Each diffraction grating  36 ,  42  can be implemented, for example, as a diffraction grating and/or diffractive optical element defined by grating parameters such as a profile function and grating period. In the illustrated example, the first grating  36  has a period d 1 , whereas the second grating  42  has a period d 2 , which may differ from d 1 . 
         [0019]    Each diffraction grating  36 ,  42  diffracts the light beam(s) incident on the particular grating. The angle of the light diffracted from a particular grating is defined by the angle of the incident light with respect to an axis  44  that is normal to the surface of the grating, the wavelength of the incident light, the characteristics of the grating (e.g., the grating period), and the diffraction grating equation. In general, the diffraction equation can be represented as follows: 
         [0000]    
       
         
           
             
               d 
               = 
               
                 
                   m 
                    
                   
                       
                   
                    
                   λ 
                 
                 
                   ( 
                   
                     
                       sin 
                        
                       
                           
                       
                        
                       α 
                     
                     + 
                     
                       sin 
                        
                       
                           
                       
                        
                       β 
                     
                   
                   ) 
                 
               
             
             , 
           
         
       
     
         [0000]    where d is the grating period of the grating, m is the diffraction order, λ is the wavelength of the light, α is the angle of the incident light relative to the axis  44  normal to the surface of the grating, and β is the angle of the diffracted light relative to the axis  44  normal to the surface of the grating. In general, there may be multiple orders of diffracted light (e.g., m=−2, −1, 0, +1, +2). In some cases, however, depending on the grating parameters, (i.e. the profile function) some orders can be suppressed. For example, blazed gratings can be optimized for a particular wavelength of light and can be designed to suppress specified diffraction orders (e.g., m=0). 
         [0020]    In the illustrated example of  FIG. 2 , the first grating  36  diffracts the incident light beam  34  into a plurality of first diffracted optical signals  37 . In this example, the plurality of first diffracted optical signals  37  are diffracted as orders m=±1, which represent the highest magnitude diffraction orders diffracted from the first grating  36 , in this example. Further they are labeled in  FIG. 2  as m 1  to indicate they are the highest magnitude diffraction orders diffracted from the first grating  36 . First diffracted optical signals  37  are incident on the second grating  42  and each is diffracted into five diffraction orders, m=0, ±1, ±2, in this example, where each light beam diffracted from second grating  42  is labeled  47 . The highest diffraction orders diffracted from the second grating  42  are labeled as m 2  to indicate that they are the highest magnitude diffraction orders diffracted from the second grating  36 , and they correspond to m=±2 in this example. In other implementations, each of the diffraction gratings  36 ,  42  may diffract each incident beam into a set of diffraction orders different from those of  FIG. 2 . 
         [0021]    An angle between diffraction orders of the same magnitude |m| of a particular grating (e.g., the angle between beams m 1 , and/or the angles between beams m 2 ) is herein termed a fan-out angle. While the diffracted angle of a particular diffraction order of a particular grating is dependent on the angle of light incident on the grating (i.e., according to the diffraction grating equation), the fan-out angle is independent of the angle of light incident on the grating. Consequently, the fan-out angles associated with each grating are independent of the angle of light incident on the grating. Moreover, the sum of 1) the fan-out angle of the highest diffraction order of the first grating (herein termed the first fan-out angle) and 2) the fan-out angle of the highest diffraction order of the second grating (herein termed the second fan-out angle) is the full-fan-out angle of the module. As shown in the example depicted in  FIG. 2 , a first fan-out angle is labeled as θ m1 , and a second fan-out angle is labeled as θ m2 . The sum of θ m1  and θ m2  is the full fan-out angle θ full  for the module, again where both θ m1  and θ m2  are associated with the highest diffraction orders of their respective diffraction gratings  36 ,  42 . The full-fan-out angle may be any of a range of angles depending on the application of the module, for example, the full-fan-out angle θ full  may be from 10° to 100° or even more (e.g., may even be up to 160°). The first and second fan-out angles of the first and second gratings  36 ,  42  of the illustrative example of  FIG. 2  may be calculated according to the following: 
         [0000]    
       
         
           
             
               θ 
               
                 m 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               2 
               × 
               
                 sin 
                 
                   - 
                   1 
                 
               
                
               
                 
                   m 
                    
                   
                       
                   
                    
                   1 
                    
                   λ 
                 
                 
                   d 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
       
         
           
             
               
                 θ 
                 
                   m 
                    
                   
                       
                   
                    
                   2 
                 
               
               = 
               
                 2 
                 × 
                 
                   sin 
                   
                     - 
                     1 
                   
                 
                  
                 
                   
                     m 
                      
                     
                         
                     
                      
                     2 
                      
                     λ 
                   
                   
                     d 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where all variables have been defined above. Further the full fan out angle may be calculated according to the following: 
         [0000]      θ full =θ m1 +θ m2 ,
 
         [0000]    where all variables are defined as indicated above. 
         [0022]    As shown in  FIG. 2 , the light beams  47  exiting the second grating  42  can be projected onto an object  48  (e.g., a scene or object). By choosing the characteristics of the diffraction gratings  36 ,  42 , a specified, pre-determined pattern can be projected onto the object  48  with a predetermined full-fan-out angle θ full . For example, a projector module with a light source  32  that emits light of 0.940 μm wavelength, a first fan-out angle θ m1  of 30°, a second fan-out angle θ m2  of 25° (where a full-fan-out angle of 55° is desired, for example), highest magnitude of diffraction orders of the first grating  36  is |1|, highest diffraction orders of the second grating  42  is |2|, the grating period d 1  may be 3.63 μm and the grating period d 2  may be 8.69 μm. 
         [0023]    In some cases, instead of placing the optical assembly  46  between the light source  32  and the first grating  36 , it can be located between the second grating  42  and the object  48 . Alternatively, an optical assembly  46  may be positioned between the light source  32  and the first grating  36  and an additional optical assembly  46  may be positioned between the second grating  42  and the object  48 . 
         [0024]    A projection sub-assembly that includes multiple optical channels, each of which has a grating period that differs from the grating period of other channels, may be provided to increase or enhance the irregularity of the projected pattern. An example is illustrated in  FIG. 3 , which shows an optical projection sub-assembly  30 A that includes three optical channels. In this case, there are three light sources  32 A,  32 B,  32 C, each of which emits a respective light beam  34 A,  34 B, and  34 C onto respective first regions  35 A,  35 B, and  35 C of the first grating  36 A. In some implementations the light sources  32 A,  32 B, and  32 C may emit the same wavelength, however in other implementations a single, some, or all light sources may emit different wavelengths from each other. In particular, the first grating  36 A includes various first regions  35 A,  35 B,  35 C, each of which has a respective grating period (i.e., d 1 A, d 1 B, d 1 C, respectively). Each [0025] incident light beam  34 A,  34 B,  34 C is diffracted into multiple first diffracted optical signals  37 A,  37 B,  37 C of multiple diffraction orders (e.g., m=±1), where say m=|1| is the maximum magnitude of diffraction orders associated with first regions  35 A,  35 B, and  35 C (denoted as m 1 A, m 1 B, m 1 C, respectively, in  FIG. 3 ). Further, the respective first fan-out angles (θ m1A , θ m1B , θ m1C ) for the diffracted beams depend, in part, on the respective grating periods. Thus, in general, when the grating periods d 1 A, d 1 B, d 1 C are non-equal, then the first fan-out angles θ m1A , θ m1B , θ m1C  will differ from each other; accordingly, the optical output from the first grating  36 A is irregular. For example, in the illustrative example depicted in  FIG. 3 , light beam  34 A,  34 B, and  34 C may have a wavelength of 0.940 μm and may be collimated via optical assembly  46 . First regions  35 A,  35 B, and  35 C may comprise grating periods of d 1 A=3.56 μm, d 1 B=3.60 μm, and d 1 C=3.63 μm, respectively. Consequently, when collimated beams of light are incident on first regions  35 A,  35 B, and  35 C, the beams of light diffract, where the highest magnitude diffraction orders (e.g., m 1 A, m 1 B, and m 1 C) emanating from each first region  35 A,  35 B, and  35 C possess first fan-out angles θ m1A , θ m1B , and θ m1C  of 30.6°, 30.3° and 30.0°, respectively. As each of the first fan-out angles associated with each first region  35 A,  35 B, and  35 C are different by a predetermined fan-out differential and are herein termed a first fan-out differential optical output from the first grating  36 A is irregular. In this example the predetermined first fan-out differential is 0.3° for all first fan-out angles, however, in other implementations the first fan-out differential may be greater than or less than 0.3° (e.g., the first fan-out differential may be as large as 10°, or as small as 0.1°, or may be larger or smaller depending on intended application). Further the first fan-out differential may be non-equal with respect to different channels of the first grating  36 A in order to provide greater irregularity in the optical output projected onto object  48 . For example, in the illustrated example above, first fan-out angles θ m1A , θ m1B , and θ m1C  may be 30.9°, 30.3° and 30.0°, respectively. 
         [0025]    First diffracted optical signals  37 A,  37 B,  37 C from the first grating  36 A then are incident on the second grating  42 . Even if the second grating  42  has a single grating period (d 2 ), the first diffracted optical signals  37 A,  37 B,  37 C from the first grating  36 A are incident on the second grating  42  at non-equal angles relative to axis  44 . Thus, each of the first diffracted optical signals (e.g.,  37 A,  37 B, and  37 C) will be diffracted by the second grating at a different respective angle. In the illustrated example of  FIG. 3 , it is assumed that the second grating  42  diffracts the first diffracted optical signals  37 A,  37 B,  37 C into five orders (i.e., m=±2, ±1, 0) each. However, the fan-out angles, for example the second fan-out angles θ m2A , θ m2B , θ m2C  (the angles between the highest magnitude diffraction orders m 2 A, m 2 B, and m 2 C, respectively, emanating from grating  42  in this example where m=|2|) are not dependent on the incident angles of first diffracted optical signals  37 A,  37 B,  37 C. Accordingly, as second grating  42  has a single grating period (d 2 ) in this example, the second fan-out angle associated with each channel (e.g., θ m2A , θ m2B , θ m2C ) are the same. When d 2  is 8.69 μm, for example, the second fan-out angle for each of θ m2A , θ m2B , θ m2C  is 25°. The full-fan out angle for each channel (θ fullA , θ fullB , θ fullC ), however, are different in this example. For example, θ fullA =55.6°, θ) fullB =55.3°, θ fullC  55.0°, where θ fullA =θ m1A +θ m2A , etc. Accordingly, the light beams  47 A,  47 B, and  47 C exiting the second grating  42  are incident onto an object  48  wherein the optical output is irregular. As noted above, in some cases, the optical assembly  46  may be placed between the second grating  42  and object  48 . 
         [0026]    In the example of  FIG. 3 , the first and second gratings may have a first N×M grating multiplicity (i.e., each incident light beam produces M×N diffracted beams), and a second R×S grating multiplicity (i.e., each incident light beam produces R×S diffracted beams). For example, the first grating  36 A may have a first 2×2 grating multiplicity, meaning that a beam of light incident on first grating  36 A is diffracted into two diffracted beams by a 1D grating (as illustrated in  FIG. 3 ) and four diffracted beams by a 2D grating (e.g., ±1, ±1). Similarly, second grating  42  may have a second 5×5 grating multiplicity, where a beam of light incident on the second grating  42  is diffracted into five diffracted beams by a 1D grating (as illustrated in  FIG. 3 ) and twenty-five diffracted beams by a 2D grating (e.g., ±2, ±1, 0, ±2, ±1, etc.). Accordingly, in the example illustrated in  FIG. 3  (as illustrated in 1D), each of the light beams  34 A- 34 C from the three light sources  32 A- 32 C results in ten diffracted beams exiting the second grating  42 . The particular grating multiplicity, diffraction orders and/or the number of diffraction orders at each first and second grating  36 A,  42  may differ from those illustrated by  FIG. 3  depending on application and desired irregularity and/or complexity of the resulting optical output. For example, in cases where the fan-out angles (e.g., first fan-out angles) are non-equal, the overall complexity of the resulting irregular pattern incident on object  48  can be increased by increasing the first or second grating multiplicities (the number of beams diffracted from each grating). Moreover, in some instances, the number of optical channels may differ from that shown in  FIG. 3  further increasing the complexity and/or irregularity of the resulting optical output incident on object  48 . 
         [0027]    In the example of  FIG. 3 , the lateral spacing between adjacent light sources is substantially the same. In particular, the lateral spacing (x, y) between the light sources  32 A and  32 B is the same as the spacing between light sources  32 B and  32 C. In other implementations, however, the lateral spacing between adjacent pairs of sources may differ. For example, as shown in the optical projection sub-assembly  30 B of  FIG. 4 , the lateral spacing between light sources  32 A and  32 B is (x, y), whereas the lateral spacing between light sources  32 B and  32 C is (x′, y′). These differences in lateral spacing can help enhance the complexity and/or irregularity of the projected pattern. 
         [0028]    In the foregoing examples, the second grating  42  is provided with a single grating period (d 2 ). However, in some instances, as shown in  FIG. 5 , the second grating  42 A also can have multiple second regions  39 A,  39 B,  39 C, each of which has grating period (d 2 A, d 2 B, d 2 C) that differs from the grating periods of the other regions. Accordingly, each region may be associated with non-equal second fan-out angles (e.g., θ m2A , θ m2B , θ m2C ) differing by a predetermined second fan-out differential, wherein the second fan-out differential may be equal or non-equal with respect to different channels of the second grating  42 A. For example, in some implementations the second fan-out differential may be 1.0°, greater than or less than 1.0° (e.g., the second fan-out differential may be as large as 10°, or as small as 0.1°, or may be larger or smaller depending on intended application). Further the second fan-out differential may be non-equal with respect to different channels of the second grating  42 A in order to provide greater irregularity in the optical output projected onto object  48 . 
         [0029]    In some cases, each first diffracted optical signal  37 A,  37 B,  37 C diffracted from a particular first region  35 A,  35 B,  35 C of the first grating  36 A is incident on a second region  39 A,  39 B,  39 C of the second grating  42  within the same optical channel. For example, the diffracted optical signal  37 A may be incident on the first second-region  39 A, the diffracted optical signal  37 B may be incident on the second second-region  39 B, and the diffracted optical signal  37 C may be incident on the third second-region  39 C. The grating periods (d 2 A, d 2 B, d 2 C) may be the same as, or different from, the corresponding gating periods (d 1 A, d 1 B, d 1 C) of the first grating  36 A. Providing multiple regions having different respective grating periods in the second grating  42 A can further increase the complexity of the optical pattern to be projected on an object  48  (i.e., object or objects within a scene). 
         [0030]    As described above, in some implementations, a single first grating, second grating and optical assembly  46  can be provided for multiple optical channels (i.e., multiple emitters). In other implementations, each light emitter may have its own first grating, second grating and optical assembly. Thus, as shown in the example of  FIG. 6 , light from a first light source  32 A is incident on an optical assembly  46 A, and light from a second source  32 B is incident on an optical assembly  46 B. The light passing output from the optical assembly  46 A is incident on the first gratings  36 A, and the light passing output from the optical assembly  46 B is incident on the first gratings  36 B. The grating period (d 1 A, d 1 B) of the first gratings can differ from one another. Further, the light beams diffracted from a given one of the first gratings  36 A,  36 B, can be incident on a respective second grating. Thus, in the illustrated example, the light beams diffracted from the first grating  36 A are incident on a second grating  42 A having a grating period d 2 A. Likewise, the light beams diffracted from the first grating  36 B are incident on a different second grating  42 B having a grating period d 2 B. The grating periods d 2 A, d 2 B also may differ from one another and from the grating periods d 1 A, d 1 B of the first gratings  36 A,  36 B. As in other implementations, in some cases, the optical assemblies  46 A,  46 B may be disposed between the second gratings  42 A,  42 A and the object  48  (i.e., object or objects within a scene). 
         [0031]    In some instances, it is desirable for each optical channel to be associated with an array of light emitters. Accordingly, as each first grating may be characterized by a first grating multiplicity (M×N) (i.e., each incident light beam produces M×N diffracted beams), and each second grating may be characterized by R×S diffraction (i.e., each incident light beam produces R×S diffracted beams). In such a system, the system produces M×N×R×S diffracted beams from a single emitter. An example is illustrated in  FIG. 7 , which shows five channels (A through E), and is described in greater detail in the following paragraphs. 
         [0032]    In the projection sub-assembly of  FIG. 7 , each channel (A through E) includes a 2×2 array of light sources  32 . Further, each channel includes a first grating  36  and a second grating  42 . In the illustrated example, each first grating  36  is characterized by 2×2 diffraction such that each incident light beam  34  produces four first diffracted optical signals  37  (e.g., m=+1 in two dimensions). Also, each second grating  42  is characterized by 5×5 diffraction such that each first diffracted optical signal  37  produces twenty-five diffracted light beams  43  (e.g., m=0, ±1, ±2 in two dimensions). Each channel thus produces four hundred diffracted output beams for the projected pattern. 
         [0033]    In the illustrated example of  FIG. 7 , each first grating  36  has a grating period (d 1 A, d 1 B, d 1 C, d 1 D, d 1 E) that differs from the other grating periods. In some cases, the grating periods (d 2 A through d 2 E) for the second gratings  42  are the same as one another, whereas in other cases, they may differ from one another. In a particular implementation, the grating periods d 1 A-d 1 E range from about 2.70-2.90 μm, whereas the grating periods d 2 A-d 2 E range from about 6.80-7.20 μm. Different values may be used for other implementations. 
         [0034]    The projection sub-assembly of  FIG. 7  also includes a respective optical assembly  46  for each channel. Here too, the optical assemblies  46  can be disposed between the second gratings  42  and the object  48 , or between the light sources  32  and the first gratings  36 . The diffracted beams from each channel can be projected as an irregular pattern onto an object or object  48 . In other implementations, the number of channels and/or the diffraction characteristics of the first or second gratings may differ from the example of  FIG. 7 . 
         [0035]    Although the projection sub-assembly of  FIG. 7  is shown as 1×N array of channels (where N=5), the projection sub-assembly may, in some cases, include a two-dimensional M×N array of channels, where both M and N are equal to or greater than two. Further, the array of light sources  32  for each channel can be arranged in any of wide range of arrangements.  FIG. 8  is a plan view showing an example of a 2×5 array of channels, each of which includes a 2×2 light source array  50  of light sources  32 . Other variations are possible. 
         [0036]    In the foregoing examples, each diffraction grating  36 ,  42  has grating parameters such as a grating profile function and a grating period. As described above, the grating periods of the first and/or second gratings can be varied to achieve a projected irregular optical pattern. In some implementations, instead of, or in addition to, varying the grating periods, other characteristics of the grating profile functions for the diffraction gratings  36 ,  42  can be varied to achieve specified diffracted light characteristics (e.g., diffraction orders and diffraction angles). Thus, in some cases, the grating profile function of each first grating  36  may differ from the grating profile function of the other first gratings. Depending on the implementation, the grating profile function of each second grating  42  may be the same as the grating profile functions of the other second gratings or may differ from the grating profile functions of the other second gratings. The grating profile function(s) of the second gratings  42  may be the same as or differ from the grating profile functions of the first gratings  36 . Further, for a projection sub-assembly that includes a two-dimensional array of channels, the grating profile functions of the first or second gratings may differ from one another in only one lateral direction or may differ from one another in both lateral directions. 
         [0037]    In some implementations, various features described in different ones of the foregoing examples can be combined. Further, various other modifications can be made within the spirit of the foregoing description. Thus, other implementations are within the scope of the claims.