Patent Publication Number: US-2022214596-A1

Title: Array antenna capable of shifting phase of light

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Application No. PCT/KR2020/015752 filed on Nov. 11, 2020, which claims priority to and the benefit of Korean Patent Application No. 10-2019-0143858 filed in the Korean Intellectual Property Office on Nov. 11, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The exemplary embodiment relates to an array antenna capable of precisely shifting a phase of light. 
     BACKGROUND ART 
     The content described in this section merely provides background information for the present exemplary embodiment and does not constitute the prior art. 
     A LIDAR sensor for autonomous driving vehicles acquires three-dimensional special information by measuring the time taken for an incident pulse layer to be reflected from an object and return. According to a laser radiation method, the LIDAR is generally divided into a flash and a scanning method. The flash-type LIDAR is a method of simultaneously scanning a laser beam over a large area, and includes a 2D array-type light receiving element so that a receiving unit is capable of recognizing a reflected image. On the contrary, the scanning-type LIDAR includes performs point mapping on a three-dimensional space through vertical and horizontal rotation of a laser beam. Therefore, the scanning type LIDAR has less laser light source output and a simple receiving unit structure compared to the flash type LIDAR. 
     The scanning type LIDAR in the related art measures a 360° viewing angle by a mechanical motor rotation. However, the basic mechanical LIDAR cannot be used in unmanned aerial vehicles that require limited power and weight because the motor for rotation is heavy and the LIDAR consumes a lot of power, and there is a problem in that the mechanical rotation speed does not correspond to the rotation speed required for highway driving of autonomous vehicles. 
     Therefore, the scanning type LIDAR in the related dart considered the use of an optical phased array antenna. The optical phased array antenna uses several directional couplers to disperse the incident laser to each antenna element, and modulates the phase of the dispersed laser to output the laser in a desired direction. 
     However, a waveguide in the scanning method in the related art increases the size of the evanescent wave of the waveguide mode due to the low refractive index, so the waveguide may easily interact with an adjacent waveguide having the same propagation constant. In order to widen the limited horizontal viewing angle of the optical phased array antenna, a gap between the antenna elements needs to be close to a distance equal to half the wavelength (λ/2), and as the antenna elements get closer, the desired output phase distribution cannot be obtained due to cross-talk between adjacent elements. In the case of LIDAR intended to be installed in an autonomous vehicle, since normal operation is important for the safety of occupants and pedestrians, the LIDAR in the related art has a problem in that it is difficult to install the LIDAR in devices, such as autonomous vehicles. 
     SUMMARY OF THE INVENTION 
     An exemplary of the present invention has an object to provide an optical phase shifting array antenna which is lighter and cheaper by replacing mechanical rotation and is capable of accurately outputting light as desired. 
     An aspect of the present exemplary embodiment provides an optical phase shifting array antenna including: a receiving unit configured to distribute input light to a plurality of antenna element waveguides; a phase shifting unit configured to shift a phase of light propagated to each antenna element waveguide by applying heat to each antenna element waveguide; an output unit including a plurality of antenna elements, and configured to output the light, which is propagated to each antenna element waveguide after a phase is varied in the phase shifting unit, to each antenna element; and a base part configured to seat the receiving unit, the phase shifting unit, and the output unit. 
     According to the aspect of the present exemplary embodiment, the receiving unit includes a multi-mode interference coupler, a Y-junction coupler, or a directional coupler in order to distribute the input light to the plurality of antenna element waveguides. 
     According to the aspect of the present exemplary embodiment, in the phase shifting unit, the amounts of heat heating the respective antenna elements waveguide is different from each other. 
     According to the aspect of the present exemplary embodiment, the base part is formed by stacking a silicon substrate and a silicon oxide cladding on the silicon substrate. 
     According to the aspect of the present exemplary embodiment, each of the plurality of antenna elements includes diffraction gratings having a predetermined interval. 
     According to the aspect of the present exemplary embodiment, the diffraction grating is aperiodically formed. 
     According to the aspect of the present exemplary embodiment, in the diffraction gratings, the interval between the diffraction gratings decreases from an outer edge of the antenna element toward a center, and the interval between the diffraction gratings is maintained constantly within a preset range from the center of the antenna element, and the interval between the diffraction gratings increases again from the center of the antenna element toward the outer edge out of the preset range. 
     According to the aspect of the present exemplary embodiment, in the diffraction gratings, the interval between the diffraction gratings decreases from an outer edge of the antenna element toward a center, and the interval between the diffraction gratings increases again from the center of the antenna element. 
     According to the aspect of the present exemplary embodiment, the output unit outputs the light propagated to the antenna element waveguide in a direction vertical to a propagation direction of light. 
     As described above, according to the aspect of the present exemplary embodiment, the optical phase shifting array antenna of the present invention is lighter and cheaper by replacing mechanical rotation and is capable of accurately outputting light as desired, thereby being mounted as a LIDAR system in various devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an optical phase shifting array antenna according to an exemplary embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a micro heater according to an exemplary embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a phase shifting unit according to the exemplary embodiment of the present invention. 
         FIGS. 4 and 5  are a cross-sectional view and a perspective view of the antenna element according to the exemplary embodiment of the present invention. 
         FIGS. 6A to 6C and 7A to 7C  are graphs illustrating an output of an array antenna in the related art and an output of the optical phase shifting array antenna according to the exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention may have various modifications and exemplary embodiments and thus specific exemplary embodiments will be illustrated in the drawings and described. However, it is not intended to limit the present invention to the specific exemplary embodiments, and it will be appreciated that the present invention includes all modifications, equivalences, or substitutions included in the spirit and the technical scope of the present invention. In describing each drawing, like reference numerals in the drawings refer to the same or similar functions. 
     Terms including an ordinary number, such as first, second, A, and B, are used for describing various constituent elements, but the constituent elements are not limited by the terms. The terms are used only to discriminate one constituent element from another constituent element. For example, without departing from the scope of the invention, a first constituent element may be named as a second constituent element, and similarly a second constituent element may be named as a first constituent element. A term “and/or” includes a combination of multiple relevant described items or any one of the multiple relevant described items. 
     It should be understood that when one constituent element referred to as being “coupled to” or “connected to” another constituent element, one constituent element can be directly coupled to or connected to the other constituent element, but intervening elements may also be present. In contrast, when one constituent element is “directly coupled to” or “directly connected to” another constituent element, it should be understood that there are no intervening element present. 
     Terms used in the present application are used only to describe specific exemplary embodiments, and are not intended to limit the present invention. Singular expressions used herein include plurals expressions unless they have definitely opposite meanings in the context. In the present application, it will be appreciated that terms “including” and “having” are intended to designate the existence of characteristics, numbers, steps, operations, constituent elements, and components described in the specification or a combination thereof, and do not exclude a possibility of the existence or addition of one or more other characteristics, numbers, steps, operations, constituent elements, and components, or a combination thereof in advance. 
     All terms used herein including technical or scientific terms have the same meanings as meanings which are generally understood by those skilled in the art unless they are differently defined. Terms defined in generally used dictionary shall be construed that they have meanings matching those in the context of a related art, and shall not be construed in ideal or excessively formal meanings unless they are clearly defined in the present application. 
     Terms defined in generally used dictionary shall be construed that they have meanings matching those in the context of a related art, and shall not be construed in ideal or excessively formal meanings unless they are clearly defined in the present application. 
     In addition, each configuration, process, or method included in each exemplary embodiment of the present invention may be shared within a range that does not technically contradict each other. 
       FIG. 1  is a diagram illustrating a configuration of an optical phase shifting array antenna according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , an optical phase shifting array antenna  100  according to an exemplary embodiment of the present invention includes a receiving unit  110 , a phase shifting unit  120 , an output unit  130 , and a base unit  140 . 
     The optical phase shifting array antenna  100  is configured to adjust a direction of light output by shifting a phase of (laser) light to be irradiated for sensing within a LIDAR system (not illustrated). The optical phase shifting array antenna  100  may configure the LIDAR system together with a light source (not illustrated) which generates light and outputs (transmits) the generated light to the optical phase shifting array antenna and a sensing device (not illustrated) which receives reflected light that is reflected from a target and the like and senses the received light. 
     The receiving unit  110  receives light output from the light source (not illustrated), and distributes the received light to a plurality of antenna element waveguides by using a plurality of couplers. 
     The receiving unit  110  includes a first optical coupler for receiving the light output from the light source (not illustrated). A mode diameter of the light output from the light source (not illustrated) is usually different from a mode diameter of a waveguide  122 , which is to be described below, within the optical phase shifting array antenna  100  . The receiving unit  110  completely receives the light output from the light source by adjusting the mode diameter of the light output from the light source by using the first optical coupler  112 . The first optical coupler  112  may have a reverse-tapered structure in the cantilever-shaped silicon oxide waveguide to adjust the mode diameter of light. 
     The receiving unit  110  distributes the received light to the plurality of antennal element waveguides by using a second optical coupler  114 . Herein, the second optical coupler  114  may be implemented with a multi-mode interference coupler, a Y-junction coupler, or a directional coupler, and one or more second couplers are included to distribute the received light to the plurality of antennal element waveguides. 
     The phase shifting unit  120  changes a refractive index of each antenna element waveguide  122  and changes a phase of a waveguide mode. 
     The phase shifting unit  120  includes a micro heater  126  and an electrode  124  which is capable of supplying power to the micro heater  126 . The phase shifting unit  120  changes a refractive index of each waveguide  122  by applying Joule&#39;s heat to the antenna element waveguide  122  by using the micro heater  126 , and the change in the refractive index of the waveguide causes the phase change of the waveguide mode. The micro heater  126  and the electrode  124  are illustrated in detail in  FIG. 2 . 
       FIG. 2  is a diagram illustrating the micro heater according to the exemplary embodiment of the present invention. 
     As illustrated in  FIG. 2 , an electrode  124   a  and an electrode  124   b  are disposed in a diagonal direction on a base part  140  on both sides of the plurality of antenna element waveguides  122 , and one micro heater  126  is connected to each of the electrodes  124   a  and  124   b  while crossing over each waveguide  122 . 
     In this case, the micro heater  126  does not cross all the waveguides  122  at once and contacts the base part on each waveguide only once and is connected to the electrode, but contacts the base part on each waveguide  122  once or multiple times and is connected to the electrode. The micro heater  126  has the form in which the micro heater  126  continues in the direction of each waveguide  122  and is bent in the direction in which the waveguide is located (the direction perpendicular to the direction of the waveguide), and is in contact with the base part on the specific waveguide, and then the micro heater  126  is away from the base part on the waveguide again and then continues in the direction of the waveguide again, so that the micro heater  126  may be in contact with the base part on one or a plurality of waveguides at one time. The micro heater  126  may continuously change the position of the base part (on the waveguide) with which the direction is to be in contact, thereby changing the position of the base part on each waveguide and the number of times of the contact. For example, the micro heater  126  illustrated in  FIG. 2  is formed to be connected with the electrode  124   a,  and at first, the micro heater  126  is in contact with only the base part on the waveguide closest to the electrode  124   a  and then becomes distant, and gradually comes into contact with the base part on the waveguide that is farther away from the electrode  124   a  and then becomes distant, and finally is connected with the electrode  124   b  by crossing over all of the waveguides. Since the micro heater  126  is connected with the electrode by changing the number of times of the contact with the base part on each waveguide, even though only one micro heater  126  and two electrodes  124   a  and  126   b  are provided, it is possible to heat all of the waveguides at the same time, and it is possible to shift the degree of heating each waveguide. However,  FIG. 2  illustrates only the example in which the micro heater  126  is formed in a form in which the numbers of times of the contact with the base part on each waveguide are different, but the present invention is not limited thereto, and the micro heater  126  may have the form in which the numbers of times of the contact with the base part on some waveguides are the same (for example, the form in which the micro heater is connected with another electrode in the form in which the micro heater is first in contact with the base part on the waveguide furthest away from one electrode and then becomes distant, is gradually in contact with the base part on the waveguide that is closer to the one electrode, and then becomes distant, and then is in contact with the base part on the waveguide furthest from the one electrode again and then becomes distant). 
     The micro heater  122  may be formed by a following process. The micro heater may be manufactured by applying a platinum paste (Pt paste) in a form to be formed by using a micro pen and then irradiating a laser to the applied area. 
     Referring back to  FIG. 1 , the micro heater  126  is in contact with the base part on each waveguide and indirectly applies heat to the waveguide  122  to change a refractive index of the waveguide. A cross-section of the waveguide  122  in which the heater  126  is in contact is illustrated in  FIG. 3 . 
       FIG. 3  is a cross-sectional view of the phase shifting unit according to the exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the base part  140  includes claddings  310  and  320  and a substrate  330 . The substrate  330  is implemented with silicon, and is implemented in the form in which the claddings  310  and  320  formed of a silicon oxide (SiO 2 ) are deposited on the substrate  330 . In this case, the waveguide  122  is implemented with silicon and is disposed inside the claddings  310  and  320 , and the claddings are divided into the upper cladding  310  and the lower cladding  320  based on the waveguide  122 . The waveguide  122  has the form of being surrounded by the silicon oxide. Since the waveguide  122  is surrounded by the cladding implemented with the silicon oxide, so that there occurs an effect in reducing optical loss. 
     In this case, the micro heater  126  is in contact with the upper cladding  310  on the waveguide  122  to transmit heat to the waveguide  122 . That is, the micro heater  126  is bent to the base part, that is, the upper cladding  310 , on the wave guide in the direction of the waveguide, and is in contact with the upper cladding  310  on the waveguide  122  and transmits the heat to the waveguide  122 . 
     Referring back to  FIG. 1 , the output unit  130  maintains the phase distribution varied by the phase shifting unit  120 , and outputs the light to the upper portion (+z-axis) of the antenna that is the direction vertical to the propagation direction of the light. The output unit  130  may output the light in the z-axis direction on an x-y plane, and the output direction of the light output from the output unit  130  is steered according to the wavelength of the light input to the receiving unit  110  and the phase varied by the phase shifting unit  120 . 
     It is necessary to use an optimized waveguide width in the output unit  130  in order to decrease a distance between the antenna element waveguides  122 , and in this case, in order to reduce the size of the evanescent wave, a wide width needs to be used, but the distance between the elements that are close due to the wide width needs to be compensated. 
     A cross-section of one antenna element  132  within the output unit  130  is illustrated in  FIG. 4 , and a perspective view of one antenna element  132  is illustrated in  FIG. 5 . 
       FIGS. 4 and 5  are a cross-sectional view and a perspective view of the antenna element according to the exemplary embodiment of the present invention. 
     The antenna element is also the element disposed on the base part  140 , so that the antenna element is implemented in the same configuration as that of the cross-section of the phase shifting unit  120 . In the antenna element, the claddings  310  and  320  implemented with a silicon oxide (SiO 2 ) are deposited on the silicon substrate  330 , and the waveguide  122  is disposed inside the cladding to divide the claddings between the upper cladding  310  and the lower cladding  320 . As the waveguide  122  is implemented with silicon and the cladding made of the silicon oxide is positioned on the waveguide  122 , the optical loss is significantly reduced. Further, a diffraction grating  410  is formed on the upper portion of the waveguide, and since the diffraction grating has symmetry and aperiodicity, the diffraction grating  410  may be more smoothly formed on a material implemented with a silicon oxide rather than silicon. Accordingly, in the antenna element according to the exemplary embodiment of the present invention, as the cladding and the waveguide are implemented with the above-described material, it is easy to manufacture the antenna element and the optical loss may be reduced. 
     Since the silicon of the upper cladding  310  has a high refractive index, the upper cladding  310  serves to increase an effective refractive index value of the hybrid waveguide and reduces the mode size, but to minimize nonlinear loss due to 2-photon absorption, the upper cladding  310  has a thickness of 120 nm or less. The maximum value of the silicon thickness that is changed periodically to eliminate the vertical refractive index symmetry may be larger than that of the receiving unit or a modulating unit, and as the material covering the upper portion of the waveguide, a silicon nitride oxide having a larger refractive index than the silicon oxide may be used instead of the silicon oxide depending on the use environment of the LIDAR. 
     The antenna element includes the diffraction grating  410  in which the thickness of the upper cladding  310  is periodically changed in order for the antenna element to directionally emit light in an upper direction (+z-axis direction). The diffraction grating  410  may be formed by etching, but is not limited thereto, and may be formed by various methods, such as laser etching. In this case, a phase change occurs due to a change in thickness in the thin and thick portions of the diffraction grating  410 , and constructive interference is satisfied at the upper portion of the diffraction grating, and destructive interference is satisfied at the lower portion of the diffraction grating. Accordingly, the light propagating to the waveguide  122  in the antenna element is directed upward, and the thickness of the upper and lower silicon oxide claddings  310  and  320  surrounding the waveguide is close to a multiple of half the wavelength to satisfy constructive interference. 
     In this case, intervals  420  between the diffraction gratings  410  are intentionally formed aperiodically rather than equally spaced. Regardless of the size of the antenna element, in the antenna element, the diffraction grating  410  is formed with symmetry with respect to a center  430  of the antenna element, and the interval  420  between the diffraction gratings is formed aperiodically. 
     For example, as illustrated in  FIG. 4 , the interval  420  between the diffraction gratings  410  of the antenna element may have the form of decreasing from the outer edge of the antenna element toward the center  430 , and then increasing again after passing through the center  430 . Otherwise, the interval  420  between the diffraction gratings  410  of the antenna element may have the form of decreasing from the outer edge of the antenna element toward the center  430 , and being constantly maintained from the center  430  within a preset range, and increasing again from the center  430  to the outer edge out of the preset range. As such, as the interval  420  between the diffraction gratings  410  has aperiodicity and symmetry, the following effects occur. By breaking the periodicity of uniformly arranging the interval  420  between the diffraction gratings  410  and having aperiodicity and symmetry, a large amount of light may be output, so that the vertical light output is increased and the signal strength is increased, and accordingly, the signal strength to noise (S/N ratio) increases at a long distance. 
     The effect is supported by the graphs illustrated in  FIGS. 6A to 6C and 7A to 7C . 
       FIGS. 6A to 6C and 7A to 7C  are graphs illustrating an output of an array antenna in the related art and an output of the optical phase shifting array antenna according to the exemplary embodiment of the present invention. 
     Referring to  FIG. 6A , the radiation intensity is formed at equal intervals, so that it can be seen that the positions of the diffraction gratings in the array antenna in the related art are arranged at equal intervals. In this case, referring to  FIG. 6B , although a main lobe of constant power is generated at the center (0°), it can be seen that a side lobe of considerable power is generated even at a position deviating from the center. Further, referring to  FIG. 6C , since the intervals of the diffraction gratings in the array antenna in the related art are all the same, it can be seen that only the position of 0 has a very low S/N ratio value. 
     On the other hand, referring to  FIG. 7A , it can be seen that the interval between the radiation intensity becomes narrower at the center and the interval becomes wider as the distance from the center increases, so that It can be seen that the position of the diffraction grating in the optical phase shifting array antenna according to the exemplary embodiment of the present invention has aperiodicity and symmetry. In this case, referring to  FIG. 7B , a main lobe of constant power is generated at the center) (0°), and only a side lobe of significantly reduced power is minutely generated even if slightly deviating from the center (0°), so that it can be seen that the S/N ratio is significantly improved. This is confirmed more clearly with reference to  FIG. 7C . It can be seen that the S/N ratio is increasing as the interval between the diffraction gratings is narrower (closer to 0), and the S/N ratio is decreasing as the interval between the diffraction gratings is wide (away from 0). As described above, since the diffraction grating in the optical phase shifting array antenna according to the exemplary embodiment of the present invention has a narrow interval at the center and has a wider interval as it goes away from the center, only the main lobe of constant power and the side lobe of significantly reduced power occur. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Therefore, the embodiments disclosed in the present invention are intended to illustrate the scope of the technical idea of the present invention, and the scope of the present invention is not limited by the embodiment. The scope of the present invention shall be construed on the basis of the accompanying claims in such a manner that all of the technical ideas included within the scope equivalent to the claims belong to the present invention.