Patent Publication Number: US-2021181390-A1

Title: Diffractive optical structure and a structured light projection device having the same

Description:
The subject matter herein generally relates to optical devices, in particular relates to a diffractive optical structure and a structured light projection device having the same. 
     BACKGROUND 
     Depth camera realizes 3D scanning, scene modeling, and gesture recognition by calculating different depths. For example, the combination of depth camera, TV, computer, and so on can realize somatosensory game to achieve the effect of game and fitness. A core component of a depth camera is optical projection module. In order to acquire information as to depths, the depth camera includes light emission module which produces a specific type of structured light. The structured light projection module is generally composed of light source, collimation module, and diffractive optical module (DOE). However, when light is incident to the DOE, spots formed by the DOE are scattered, and this structured light used in face recognition results in a low resolution of structure pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology are described, by way of embodiments, with reference to the attached figures. 
         FIG. 1  is a cross-section view of a diffractive optical structure in accordance with one exemplary embodiment. 
         FIG. 2  is a diagrammatic view of a structured light projection device in accordance with one exemplary embodiment. 
         FIG. 3  is a diagrammatic view of a light path of a diffractive optical structure in accordance with one exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain portions may be exaggerated to better illustrate details and features of the present disclosure. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other feature that the term modifies, such that the component need not be exact. For example, “substantially cylindrical” means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. The references “a plurality of” and “a number of” mean “at least two.” 
       FIG. 1  illustrates a diffractive optical structure  100  according to a first embodiment.  FIG. 2  is an optical path diagram of the diffractive optical structure provided in  FIG. 1 . The diffractive optical structure  100  receives and splits the light beam and projects a patterned light beam with uniform energy distribution and high contrast in a way of image superposition. By using the diffractive optical structure  100  for beam shaping, a uniform light or structured light field can be efficiently produced. The diffractive optical structure  100  includes a first diffractive element  10 , a second diffractive element  20  facing and spaced apart from the first diffractive element  10 , and an optical adhesive  30  filled between the first diffractive element  10  and the second diffractive element  20 . 
     The optical adhesive  30  can be a fir glue, a methanol glue, an unsaturated polyvinylene, a styrene monomer glue, an epoxy resin optical adhesive, an organic silicon resin adhesive, and the like, the optical adhesive being higher in light transmittance and in refractive index. A refractive index of the optical adhesive  30  is substantially equal to a refractive index of the first diffractive element  10 . The optical adhesive  30  increases efficiency of light transmission. In the present embodiment, a refractive index of the optical adhesive  30  is between 1.45 and 1.55. 
     When light is emitted from the first diffractive element  10 , the light is refracted by the optical adhesive  30  and directed to the second diffractive element  20 . The light is relatively concentrated to increase the amount of light passing through the second diffractive element  20 . That is, the optical adhesive  30  adjusts an angle of light entering the second diffractive element  20 , and prevents light from being incident on both sides of the second diffractive element  20 . Emission rate of light from the second diffractive element  20  is thus improved. 
     The optical adhesive  30  also prevents the first diffractive element  10  and the second diffractive element being forcibly deformed, and can effectively prevent foreign matter such as dust from entering between the first diffractive element  10  and the second diffractive element  20 , thereby reducing loss-rate of light. 
     In particular, the first diffractive element  10  includes a first grating structure  12 , the second diffractive element  20  includes a second grating structure  22 , the first grating structure  12  is arranged relative to the second grating structure  22 , and the optical adhesive  30  is arranged between the first grating structure  12  and the second grating structure  22 . The first diffractive element  10  and the second diffractive element  20  may be of a glass material or a polymer (plastic) material, generally fabricated by etching a transparent substrate surface of a glass or plastic material to a certain depth and with regular or irregular grating microstructures, by means of electron beam direct-writing or other means. 
     In particular, the first grating structure  12  comprises at least one first microstructural portion  120 , the second grating structure  22  comprises at least one second microstructural portion  220 , and the at least one first microstructural portion  120  faces the at least one second microstructural portion  220 . In the present embodiment, the number of first and second microstructural portions  120  and  220  are both three. The first microstructural portions  120  are spaced from each other, and the second microstructural portions  220  are spaced from each other. 
     Both the first microstructural portion  120  and the second microstructural portion  220  comprise a plurality of microstructures  101 . The first microstructural portion  120  and the second microstructural portion  220  separate incident light into sub-beams. The period, groove depth, and duty cycle of the first grating structure  12  and of the second grating structure  22  can be set according to the demand. For example, the period of the first grating structure  12  is 0.4 um. A microstructure morphology can be rectangular as an example, and can also be trapezoidal or other shape. The groove depth h=150 nm, and duty cycle is 0.3. The period, groove depth, and duty cycle of the second grating structure  22  can be the same or different from those of the first grating structure  12 . 
     In this embodiment, the diffractive optical structure  100  further includes at least one first resistor  40  and at least one second resistor  50 . The at least one first resistor  40  is formed on a surface of the first diffractive element  10  deviating from the first grating structure  12 , and the at least one second resistor  50  is formed on a surface of the second diffractive element  20  deviating from the second grating structure  22 . The first resistor  40  corresponds to the position of the at least one first microstructural portion  120 , and the second resistor  50  corresponds to the position of the at least one second microstructural portion  220 . 
     In the present embodiment, the first resistor  40  and the second resistor  50  are both plural, the first resistors  40  are spaced from each other, and the second resistors  50  are spaced from each other. Each first resistor  40  and a facing second resistor  50  together form a resistance pair, and the resistance pair is used to detect a capacitance value between the first grating structure  12  and the second grating structure  22 . When the first grating structure  12  and/or the second grating structure  22  are deformed due to external force or when there is a foreign body entering between the first grating structure  12  and the second grating structure  22 , the capacitance value between the first grating structure  12  and the second grating structure  22  will change, which can be detected by the first resistor  40  and the second resistor  50 . 
     The first resistors  40  are formed on the surface of the first diffractive element  10  facing away from the first grating structure  12  in a form of a coated film, the second resistors  50  are formed on the surface of the second diffractive element  20  facing away from the second grating structure  22  in the form of a plated film. The first resistor  40  and the second resistor  50  are made of transparent conductive material. The transparent conductive material is, for example, a tin oxide (ITO), a zinc oxide (IZO), an aluminum zinc oxide (AZO), a zinc oxide (GZO), a zinc oxide (ZnO), a tin oxide, or any combination thereof. 
     In the present embodiment, the diffractive optical structure  100  further includes two refractive index matching layers  60  disposed on the surfaces of the first resistor  40  and the second resistor  50 . The refractive index matching layer  60  is made of a transparent dielectric material. 
     The refractive index matching layer  60  can be a single layer or a composite layer formed by materials with different refractive index. The materials of the refractive index matching layer  60  may include, but are not limited to, niobium oxide, titanium oxide, tantalum oxide, zirconia, silicon oxide, magnesium oxide, or any combination thereof. The refractive index matching layer  60  can be used as the refractive index buffer layer, which reduces the refractive difference between the diffractive element and the transparent base layer  70 , while reducing the reflectivity. In this way, penetration and contrast are enhanced, and the quality of display improved. 
     In the present embodiment, the diffractive optical structure  100  further comprises two base layers  70  which are respectively formed on surfaces of the two refractive index matching layers  60  away from the first resistor  40  and the second resistor  50 . The base layer  70  may be made of polyethylene (PE), polycarbonate (PC), polyethylene terephthalate (Polyethylene Terephthalate (PET), or fused silica. 
     In the present embodiment, the diffractive optical structure  100  further comprises two anti-reflective film layers  80  formed on the surfaces of the two base layers  70  away from the refractive index matching layer  60 . The anti-reflective film layer  80  increases transmittance of light. 
     In the present embodiment, the diffractive optical structure  100  further includes a hollow cylindrical supporting frame  90 , opposite ends of the supporting frame  90  support the first diffractive elements  10  and the second diffractive element  20  respectively. The optical adhesive  30 , the first grating structure  12 , and the second grating structure  22  are located in the supporting frame  90 . 
       FIG. 3  illustrates a structured light projection device  200  according to a second embodiment. The structured light projection device  200  in  FIG. 3  includes a light emitting assembly  201 , an optical element  203 , and the diffractive optical structure  100 . 
     The light emitting assembly  201  may be an array of light sources or a backlight source. Specifically, the backlight emitting assembly  201  may be a liquid crystal display (LCD), light source. The array of the light emitting assembly  201  may be a VCSEL light source. 
     The optical element  203  is arranged on a light path of the light emitting assembly  201 . The optical element  203  collimates light emitted from the light emitting assembly  201 . In the embodiment, the optical element  2  is a convex lens. The structured light projection device  200  may include more than one collimation element  2 . 
     The diffractive optical structure  100  is arranged on a light path of the optical element  203 . The diffractive optical structure  100  expands light beams from the optical element  203  to form a fixed beam pattern and emit the fixed beam pattern outward. The diffractive optical structure  100  acts as a beam splitter, thus, for example, when a number of the beams transmitted to the diffractive optical structure  100  is one hundred, the first diffractive element  10  expands the light beam at a certain rate (such as 50), and can emit 5000 beams into the second diffractive element  20 . The second diffractive element  20  can expand the light beam at a certain rate (such as 20), and eventually 100000 beams are projected into space. Ideally, there will be 100000 spots (in some cases, there will be some overlapping spots, resulting in a reduction in the number of spots). 
     The structured light projection device  200  is mainly used for 3D face recognition. The diffractive optical structure  100  has two diffractive elements which have function of dispersing a beam into N beams and shaping it to achieve a preset spot effect. After beam splicing and shaping by the diffractive optical structure  100 , many light and dark spots will be formed to irradiate a face. According to the deformation degree and optical path of the light spot, a 3D face will be simulated. The brighter the light spot can be, the higher will be the resolution of 3D face recognition. 
     The structured light projection device  100  ( 200 ) provided by the disclosure does not increase an overall size of the structured light projection device  100  ( 200 ), and increases the number of reflections of light to increase the optical path, so as to realize optimization of the spots. 
     The embodiments shown and described above are only examples. Therefore, many such details are neither shown nor described. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, including in matters of shape, size, and arrangement of the portions within the principles of the present disclosure, up to and including the full extent established by the broad general meaning of the terms used in the claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the claims.