Patent Publication Number: US-2021165101-A1

Title: Optical device, range sensor using optical device, and mobile object

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure relate to an optical device that scans a laser beam, a range sensor provided with the optical device, and a mobile object. 
     BACKGROUND ART 
     In the operations of a mobile object such as a vehicle, a ship, and an aircraft, technologies to detect the position of an object in a wide range of angle are used. As one example of such sensing technologies, light detection and ranging (LiDAR) is known in the art. The LiDAR is remote sensing based on light, and uses time-of-flight (TOF) method in which the distance to the object is measured based on the length of time it takes while the laser beams that are emitted from the laser beam source are reflected by an object and then return to the sensor. The laser beams are scanned in a wide angle by a scanner such as a micro-electromechanical systems (MEMS) mirror and a polygon mirror. As known in the art, edge-emitting semiconductor laser that emits laser beams parallel to the substrate is used as a LiDAR laser beam source. As it is difficult to integrate such edge-emitting semiconductor laser, the use of a vertical-cavity surface-emitting laser (VCSEL) device as a light source is under study as the multiple light emitting elements of the VCSEL device can easily be integrated into the same substrate. The VCSEL device performs lasing in a direction perpendicular to the substrate. A configuration in which the laser beams that are emitted from a plurality of light-emitting points are focused on the surface of a movable mirror using a VCSEL array and are optically scanned by the movable mirror is known in the art (see, for example, PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Publication No. 2010-151958 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the known optical scanners, laser beams are scanned as the light emitted from the VCSEL device is directly focused on a movable mirror by an optical element such as a focus lens. Since the output power of each light emitting element is small, the output power of the VCSEL device is increased by two-dimensional integration of the light emitting elements. 
     In the LiDAR devices, the laser beams that are emitted from the laser beam source need to be scanned at a high angular resolution in order to achieve a sufficient measuring resolution and measurable distance. The angular resolution dθ of the scanning laser beam is expressed in dθ=2×tan-1[(a/ 2 )/f], using the size a of the light-emitting area of the laser beam source and the focal length f of the focus lens that concentrates the laser beams onto a movable minor. 
     In order to increase the angular resolution (or decrease dθ), the size a of the light-emitting area of the VCSEL may be reduced, or the focal length f of the focus lens may be increased. In actuality, there is a limit to the extent to which the size of the light-emitting area a can be reduced, so that the focal length f of the focus lens needs to be made longer. 
     The beam waist diameter W of the laser beams that are concentrated by the lens is calculated by an equation given below using the divergence angle φ of the laser beam and the focal length f of the lens. 
     W=2f×tan(φ/2) 
     If the focal length f of the lens is lengthened in order to achieve a high angular resolution, the beam waist diameter W becomes wider, and a movable minor with a large mirror dimension is required. However, it is desired that the movable minor in LiDAR system be downsized. As the minor dimension of the MEMS minor is smaller, high-speed and wide-range scanning can be achieved. A smaller polygon minor can perform scanning at high speed, and can make the system more compact. 
     LiDAR devices that perform high-speed and wide-range scanning require laser beams with a high angular resolution. However, when the focal length is long, the beam diameter cannot be stopped down. If the laser beams whose focal length is made longer for the purpose of increasing the angular resolution are focused on a downsized movable minor, since the beam diameter is larger than dimension of the movable mirror, and some part of light cannot be incident on the movable minor. In such cases, the radiation intensity of light for optical scanning decreases, and the detectable distance becomes shortened. 
     Embodiments of the present disclosure aim at achieving high-speed and wide-range optical scanning with a high angular resolution, without affecting the detection distance. 
     Solution to Problem 
     In order to deal with the above technical problems, an optical element is arranged in the optical path of each light emitting element of a VCSEL device, and it controls the divergence angle reduces the divergence angle of the laser beam emitted from the VCSEL device. Due to such a configuration, the laser beams are focused on a movable mirror with a small beam waist diameter, and a sufficient radiation intensity of incident light can be achieved. 
     More specifically, an optical device includes a light source provided with a plurality of surface-emitting laser elements to emit a laser beam, a scanner to scan the laser beam emitted from the light source, and an optical system disposed in an optical path between the light source and the scanner and to guide the laser beam to the scanner. The optical system includes a first optical element to control a divergence angle of the laser beam emitted from the light source and a second optical element to focus the laser beam that has passed through the first optical element onto a to-be-scanned surface of the scanner. 
     Advantageous Effects of Invention 
     Due to the above configurations, high-speed and wide-range optical scanning can be achieved with a high angular resolution, without affecting the detection distance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings are intended to depict example embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
         FIG. 1  is schematic diagram illustrating a light detection and ranging (LiDAR) device according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating an example configuration of a laser beam source according to an embodiment of the present disclosure. 
         FIG. 3A  and  FIG. 3B  are diagrams each illustrating the relative positions of a first optical element and a vertical-cavity surface-emitting laser (VCSEL) array, according to an embodiment of the present disclosure. 
         FIG. 4  is a diagram illustrating an example case in which a diffraction element array is used as a first optical element, according to an embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating the relative positions of a second optical element and the VCSEL array, according to an embodiment of the present disclosure. 
         FIG. 6  is a diagram illustrating optical paths of a projection optical system, according to an embodiment of the present disclosure. 
         FIG. 7  a diagram illustrating a movable mirror according to an embodiment of the present disclosure. 
         FIG. 8  is a diagram illustrating a movable mirror according to an alternative embodiment of the present disclosure. 
         FIG. 9  is a diagram illustrating the optical scanning performed by a light emitter, according to a first example of the present disclosure. 
         FIG. 10  is a diagram illustrating the optical scanning performed by a light emitter, according to a second example of the present disclosure. 
         FIG. 11  is a diagram illustrating the optical scanning performed by a light emitter, according to a third example of the present disclosure. 
         FIG. 12  is a diagram illustrating the optical scanning performed by a light emitter, according to a fourth example of the present disclosure. 
         FIG. 13  is a diagram illustrating the optical scanning performed by a light emitter, according to a fifth example of the present disclosure. 
         FIG. 14  is a diagram illustrating the optical scanning performed by a light emitter, according to a sixth example of the present disclosure. 
         FIG. 15A  is a diagram illustrating an intensity distribution of the laser beams, at a beam-waist position, that have passed through a microlens array (MLA), when some of the laser beams is incident on neighboring microlenses, according to an embodiment of the present disclosure.  FIG. 15B  is a diagram illustrating an intensity distribution of the light that is cast to a remote point by a movable minor, according to an embodiment of the present disclosure. 
         FIG. 16A ,  FIG. 16B , and  FIG. 16C  are diagrams each illustrating the relative positions of a light emitting element and the lens elements of a MLA. 
         FIG. 17A ,  FIG. 17B , and  FIG. 17C  are diagrams each illustrating the relative positions of light emitting elements and lens elements when a MLA of a double-convex type is used, according to an embodiment of the present disclosure. 
         FIG. 18  is a diagram illustrating an example embodiment in which the convex surfaces of lens elements are arranged on the light exiting side of a MLA. 
         FIG. 19A  is a diagram illustrating an angular distribution of the laser beam emitted through a MLA when the focal length of the MLA matches the distance between a VCSEL and the MLA, according to an embodiment of the present disclosure.  FIG. 19B  is a diagram illustrating an angular distribution of the laser beam emitted through a MLA when the focal length of the MLA is longer than the distance between a VCSEL and the MLA, according to an embodiment of the present disclosure. 
         FIG. 20A  and  FIG. 20B  are diagrams each illustrating the light-receptive width of a movable mirror and the beam-waist shape of a laser beam, according to an embodiment of the present disclosure. 
         FIG. 21A  and  FIG. 21B  are diagrams each illustrating the relation between the divergence angle and the beam diameter of a laser beam, according to an embodiment of the present disclosure. 
         FIG. 22  is a diagram illustrating the utilization efficiency of light for different scanning angles when scanning is performed with varying divergence angles, according to an embodiment of the present disclosure. 
         FIG. 23  is a diagram illustrating the relation between the amount of defocusing of a projector lens and the angular resolution in the sub-scanning direction of each divergence angle, according to an embodiment of the present disclosure. 
         FIG. 24  is a schematic diagram illustrating a mobile object provided with a LiDAR device according to an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
       FIG. 1  is a schematic diagram illustrating a light detection and ranging (LiDAR) device  100  according to an embodiment of the present disclosure. The LiDAR device  100  is an example of a range sensor that optically measures the distance. The LiDAR device  100  includes a light emitter  1  that emits a laser beam, a light receiver  2  that receives the reflected light Lref reflected by an object  40 , and a controller and signal processor  3  that controls the light emitter  1  and computes the distance based on the received reflected light. The light emitter  1  is an example of an optical device. 
     The light emitter  1  includes a laser beam source  11  that is an example of a light source, a movable minor  14  that serves as a scanner, and a projection optical system  16  that is disposed on the optical path between the laser beam source  11  and the movable minor  14 . The projection optical system  16  includes a first optical element  12  that controls the divergence angle of the laser beam that is emitted from the laser beam source  11 , and a second optical element  13  that converts the laser beam whose divergence angle has been controlled into a laser beam with a desired divergence angle and angular resolution. The movable mirror  14  scans a desired scanning range  4  with the laser beam that comes from the second optical element  13  and is incident on the movable minor  14  with a prescribed angular resolution. 
     The light receiver  2  includes a light-receiving element  21 , an optical filter  22 , and a condensing optical system  23 . The condensing optical system  23  focuses the reflected light Lref reflected by an object  40  on the light-receiving element  21  as passing through the optical filter  22 . The optical filter  22  is a filter through which only the wavelengths near the oscillation wavelength of the laser beam source  11  can pass through. The signal-to-noise (S/N) ratio of the light that is incident on the light-receiving element  21  improves as wavelengths on both sides of the oscillation wavelength are cut. The light-receiving element  21  is one or more avalanche photodiode (APD), and converts the light that has passed through the optical filter  22  into an electrical signal. 
     The controller and signal processor  3  includes a laser beam source drive circuit  31  that drives the laser beam source  11 , a control circuit  32  that controls the movement (or deflection angle) of the movable minor  14 , and a signal processing circuit  33  that computes the distance to the object  40 . The laser beam source drive circuit  31  controls the timing of light emission and light-emission intensity of the laser beam source  11 . For example, the controller and signal processor  3  is implemented by a large scale integration (LSI) chip, an integrated circuit (IC) chip such as a microprocessor, a logic device such as a field-programmable gate array (FPGA), and combined use of the IC chip and the logic device. 
     The light that is emitted from the laser beam source  11  is guided to the movable minor  14  by the first optical element  12  and the second optical element  13 , and the movable minor  14  irradiates the object  40  existing in the scanning range  4  with the scanning light. The reflected light Lref that is reflected by the object  40  passes through the condensing optical system  23  and the optical filter  22 , and is received by the light-receiving element  21 . The light-receiving element  21  outputs photoelectric current as a detection signal, according to the radiation intensity of incident light. 
     The signal processing circuit  33  computes the distance to the object  40 , which is calculated from the time difference between a detection signal and a light-emission timing signal supplied from the laser beam source drive circuit  31 . 
       FIG. 2  is a diagram illustrating an example configuration of the laser beam source  11  according to the present embodiment. For example, the laser beam source  11  in which a plurality of laser element groups, each of which is referred to as a layer, are arranged on the same plane is composed of a vertical-cavity surface-emitting laser (VCSEL) array  11 A (surface-emitting laser array). In the following description of the present disclosure, a surface-emitting laser element that forms each layer of the laser beam source  11  may be referred to simply as a light emitting element. The VCSEL array  11 A includes layers  111 - 1  to  111 - m  (that may collectively be referred to as the layers  111  in the following description), and each of the layers  111  is provided with a plurality of light emitting elements  112 - 1  to  112 - n  (that may collectively be referred to as the light emitting elements  112  where appropriate in the following description). 
     The light emitting elements  112  are elements that can be integrated into the same substrate, and the optical axis of each of the light emitting elements  112  is orthogonal to a plane on which the VCSEL array  11 A is arranged. 
     The timing at which light is emitted from each of the layers  111  is independently controlled by the laser beam source drive circuit  31 . Moreover, each of the layers  111  is controlled such that the multiple light emitting elements  112  included in that layer  111  emit light at the same time. 
     In  FIG. 2 , the multiple layers  111  are one-dimensionally arranged. However, no limitation is indicated thereby, and the VCSEL array  11 A in which the multiple layers  111  are two-dimensionally arranged may be used. The light emitting elements  112  of each of the layers  111  are minutely arranged with a prescribed pitch with an arrangement of a honeycomb structure. However, no limitation is intended thereby. The shape of the opening of the light emitting elements  112  is also not limited to a hexagonal shape. For example, the number of the layers  111  of the VCSEL array  11 A, the number of the light emitting elements  112  in each of the layers  111 , and the dimension or size of the light-emitting area are designed as desired according to, for example, the angular resolution, the scanning range, and the detection distance that are required for the LiDAR device  100 . 
       FIG. 3A  and  FIG. 3B  are diagrams each illustrating the relative positions of the first optical element  12  and the VCSEL array  11 A, according to the present embodiment. The first optical element  12  is an optical element that controls the divergence angle of the laser beam emitted from the VCSEL array  11 A. Any desired optical element array that can control the divergence angle of the VCSEL array  11 A, such as a microlens array (MLA) and a diffraction element array, may be adopted as the first optical element  12 . In the example embodiment as illustrated in  FIG. 3A  and  FIG. 3B , a MLA  12 A is used as the first optical element  12 . 
       FIG. 3A  is a diagram illustrating the VCSEL array  11 A and the MLA  12 A in an overlapping manner, in the direction parallel to the optical axis, according to the present embodiment.  FIG. 3B  is a side view of the VCSEL array  11 A and the MLA  12 A, according to the present embodiment. The MLA  12 A includes a plurality of lens groups  122  that correspond to the layout of the layers  111  of the VCSEL array  11 A. Each of the lens groups  122  includes a plurality of lens elements  121  as many as the light emitting elements  112  of the corresponding layer  111 . 
     Each of the lens elements  121  is arranged such that the optical axis matches the optical axis of the corresponding light emitting element  112 , and uses refraction of light to control the divergence angle of the laser beam output from each of the light emitting elements  112 . The lens diameter of the multiple lens elements  121  is designed such that the neighboring lens elements  121  do not interfere with each other, and the spacing between the lens elements  121  is narrower than the spacing between the light emitting elements  112  of the VCSEL array  11 A. 
     As illustrated in  FIG. 3B , the MLA  12 A is arranged on the opposite side of the VCSEL array  11 A such that the convex surfaces of the lens elements  121  are placed on the incident-side of the laser beams output from the VCSEL array  11 A. All the lens elements  121  of the MLA  12 A have the same focal length. The distance d between the VCSEL array  11 A and the incident plane of the MLA  12 A is designed to be equal to or shorter than the focal length of the lens elements  121  of the MLA  12 A. 
       FIG. 4  is a diagram illustrating an example case in which a diffraction element array  12 B is used as the first optical element  12 , according to the present embodiment. The diffraction element array  12 B includes a plurality of lens groups  122  that correspond to the layout of the layers  111  of the VCSEL array  11 A. Each of the lens groups  122  includes a plurality of diffraction lens  123  as many as the light emitting elements  112  of the corresponding layer  111 . 
     The diffraction lens  123  has a concentric cyclic pattern around an optical axis OA, and controls the divergence angle of the incident light based on this cyclic pattern. Each of the diffraction lenses  123  is arranged such that the optical axis matches the optical axis of the corresponding light emitting element  112 , and the lens surface of the diffraction lens  123  is placed on the incident-side of the laser beams. 
       FIG. 5  is a diagram illustrating the relative positions of the second optical element  13  and the VCSEL array  11 A, according to the present embodiment. The second optical element  13  is a projector lens that converts the laser beam whose divergence angle has been controlled by the MLA  12 A into a laser beam with a desired angle of divergence and angular resolution. 
     “f” denotes the focal length of the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged at approximately the focal point of the projector lens. The angle of divergence of the laser beam θ and the angular resolution dθare determined by the focal length f of the projector lens, the size S of the light-emitting area of the entirety of the VCSEL array  11 A, and the size a of the light-emitting area of each of the multiple layers  111 , using the First Equation and Second Equation given below. 
       θ=2×tan-1[( S/ 2)/f]  First Equation
 
         dθ= 2×tan-1[( a/ 2)/ f]   Second Equation
 
       FIG. 6  is a diagram illustrating optical paths of the projection optical system  16 , according to the present embodiment. The beam diameters of the laser beams that have passed through the projector lens (i.e., the second optical element  13 ) are minimized at the focal point of the projector lens. The point at which the beam diameter is minimized is referred to as a beam waist. The beam waist diameter W is expressed in the Third Equation using the divergence angle φ of the laser beam oscillating from the laser beam source and the focal length f of the projector lens. 
         W= 2 f ×tan-1(φ/2)   Third Equation
 
     As understood from the Third Equation as above, the beam waist diameter W can be decreased by decreasing the divergence angle φ of the laser beam source, and the light can be focused on the movable minor  14  whose area of mirror is small. 
       FIG. 7  is a diagram illustrating the MEMS mirror  14 A that serves as the movable mirror  14 . The movable minor  14  scans the laser beam that has passed through the projector lens (i.e., the second optical element  13 ) in a uniaxial or biaxial manner and irradiates the object  40  existing in the scanning range  4  with the scanned light. The MEMS minor  14 A is arranged at the beam waist position where the beam diameter of the laser beam that has passed through the second optical element  13  is minimized, i.e., approximately the focal point of the second optical element  13 . 
     The MEMS mirror  14 A includes a movable part  144  provided with a reflection mirror  145 , and a pair of serpentine beams  146  that support the movable part  144  at both sides of the movable part  144 . One end of each one of the serpentine beams  146  is fixed to a supporting substrate  143 , and the other end of the serpentine beams  146  is coupled to the movable part  144 . 
     In each one of the serpentine beams  146 , a meandering pattern is formed as a plurality of first piezoelectric members  147   a  and a plurality of second piezoelectric members  147   b  are arranged in alternating sequence having a plurality of turning portions between each pair of the first piezoelectric member  147   a  and the second piezoelectric member  147   b . An antiphase voltage signal is applied to a neighboring pair of the first piezoelectric member  147   a  and the second piezoelectric member  147   b . In other words, the phase of the voltage signal that is applied to the first piezoelectric member  147   a  is opposite to the phase of the voltage signal applied to the second piezo-electric member  147   b . As a result, the serpentine beam  146  is curved in Z-direction. 
     The direction of bending is opposite to each other between a neighboring pair of the first piezoelectric member  147   a  and the second piezoelectric member  147   b . The bending in opposite directions is accumulated, and the movable part  144  provided with the reflection mirror  145  moves up and down in a rotary motion around a rotation axis A. In the example case as illustrated in  FIG. 7 , the reflection mirror  145  has a rectangular shape. However, no limitation is intended thereby. For example, the re-flection mirror  145  of an elliptic shape or circular shape may be used. 
     A sinusoidal wave that has a drive frequency in view of a minor resonant mode around the rotation axis A is applied to the first piezoelectric member  147   a  and the second piezoelectric member  147   b  in opposite phase. As a result, a wide rotation angle can be achieved with low voltage. 
       FIG. 8  is a diagram illustrating a polygon minor  14 B that serves as another example of the movable mirror  14 , according to the present embodiment. The polygon mirror  14 B rotates around a rotation axis  141  at a constant velocity. In this example, plane mirrors  142   a  to  142   f  are disposed on the six inclined planes of a rotor of a hexagonal shape. As the rotor rotates on the rotation axis  141 , the incident angle of a laser beam changes on the mirror surface, and the laser beam can be scanned on the XZ plane. 
     The hades (inclination angles) of the plane mirrors  142   a  to  142   f  with reference to the rotation axis  141  are different from each other. As a prescribed hade (inclination angle) is given to each of the plane mirrors  142   a  to  142   f , the exit angle of a projector beam in the Y-axis direction is controlled. Every time the minor surface where the laser beam is reflected is changed, the angle at which the laser beam is output in the Y-axis direction changes. The scanning area in the Y-axis direction can be broadened according to the number of the mirror surfaces that the polygon mirror  14 B has. 
     FIRST EXAMPLE 
       FIG. 9  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a first example. The VCSEL array  11 A with one layer and the movable mirror  14  of a uniaxial-scanning type are used in the present example configuration. A uniaxial-scanning MEMS mirror  14 A 1  is used as a movable minor. 
     The laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A, and is incident on the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged near the focal length f on the incident side of the second optical element  13 . The MEMS minor  14 A 1  is arranged near the focal length f on the light exiting side of the second optical element  13 . The laser beam that has passed through the second optical element  13  is focused on the uniaxial-scanning MEMS mirror  14 A 1 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS mirror  14 A 1 . 
     In regard to the main scanning direction, the laser beam is scanned as the rotation angle of the MEMS mirror  14 A 1  is changed (see  FIG. 7 ), and in the scanning range  4  the position of a projection image  10  sweeps in the main scanning direction. The scanning range in the main scanning direction depends on the scanning angle of the MEMS minor  14 A 1 . On the other hand, the scanning range in the sub-scanning direction is determined by the angle of divergence θ in the sub-scanning direction of the VCSEL array  11 A. The angle of divergence θ using Equation 1 as above is determined based on the size S of the light-emitting area of the VCSEL array  11 A in the sub-scanning direction, and the focal length f of the projector lens (i.e., the second optical element  13 ). 
     The angular resolution dθ of the scanning laser beam in both the main scanning direction and the sub-scanning direction is determined by the focal length f of the projector lens (i.e., the second optical element  13 ) and the size a of the light-emitting area of each of the multiple layers  111  of the VCSEL array  11 A, based on the Second Equation as above. 
     There is a limit to the extent to which the size of the light-emitting area of each of the multiple layers  111  can be reduced. However, the laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A. Accordingly, a laser beam whose beam waist has been stopped down is incident on the reflection area  148  of the MEMS mirror  14 A 1 . Due to this configuration, the radiation intensity of light can be prevented from decreasing, and the laser beams can be scanned in a wide range while maintaining a high definition. 
     In place of the uniaxial-scanning MEMS mirror  14 A 1 , any desired mirror such as a polygon mirror, where the angle of the movable mirror uniaxially changes with reference to the laser beam, may be used. 
     SECOND EXAMPLE 
       FIG. 10  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a second example of the present disclosure. More specifically,  FIG. 10  illustrates an example configuration in which the VCSEL array  11 A with one layer and the movable mirror  14  of a biaxial-scanning type are used. A biaxial-scanning MEMS mirror  14 A 2  is used as a movable minor. As the MEMS mirror  14 A 2  rotates on a principal axis Rmain, the laser beams are scanned in the main scanning direction. As the MEMS mirror  14 A 2  rotates on a conjugate axis Rsub, the laser beams are in the sub-scanning direction. 
     The laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A, and is incident on the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged near the focal length f on the incident side of the second optical element  13 . The MEMS minor  14 A 2  is arranged near the focal length f on the light exiting side of the second optical element  13 . The laser beam that has passed through the second optical element  13  is incident on the biaxial-scanning MEMS mirror  14 A 2 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS minor  14 A 2 . 
     The MEMS mirror  14 A 2  changes its angle in the main scanning direction and the sub-scanning direction to scan a laser beam. Within the predetermined scanning range  4 , the projection image  10  repeats a sweep in the main scanning direction and a shift in the sub-scanning direction in alternating sequence. The scanning ranges in the main scanning direction and the sub-scanning direction depends on the scanning angle of the MEMS minor  14 A 2 . The angular resolution dθof the scanning laser beam in both the main scanning direction and the sub-scanning direction is determined by the focal length f of the projector lens (i.e., the second optical element  13 ) and the size a of the light-emitting area of each of the multiple layers  111  of the VCSEL array  11 A, based on the Second Equation as above. 
     Also in such an example configuration as above, the laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A. Accordingly, a laser beam whose beam waist has been stopped down is incident on the reflection area  148  of the MEMS minor  14 A 2 . Due to this configuration, the radiation intensity of light can be prevented from decreasing, and the laser beams can be scanned in a wide range while maintaining a high definition. 
     In place of the biaxial-scanning MEMS mirror  14 A 2 , any desired mirror such as a polygon mirror, where the angle of the movable minor biaxially changes with reference to the laser beam, may be used. 
     THIRD EXAMPLE 
       FIG. 11  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a third example of the present disclosure. In the present example, the VCSEL array  11 A that include at least two layers  111  in the sub-scanning direction and a uniaxial-scanning MEMS mirror  14 A 1  are used. 
     The laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A, and is incident on the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged near the focal length f on the incident side of the second optical element  13 . The MEMS minor  14 A 1  is arranged near the focal length f on the light exiting side of the second optical element  13 . 
     The timing of light emission of the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A is controlled on an individual basis, and the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A are driven in sequence. The laser beams that are emitted from the layers  111 - 1  to  111 - 4  are prevented from diverging by the corresponding lens group  122 - 1  to  122 - 4  of the MLA  12 A, and are incident on the projector lens (i.e., the second optical element  13 ). The positions  131 - 1  to  131 - 4  at which the laser beams are incident on the second optical element  13  vary in the sub-scanning direction depending on the layer  111  that is being driven. The laser beam that has passed through the second optical element  13  is focused by the corresponding one of reflection areas  148 - 1  to  148 - 4  of the uniaxial-scanning MEMS minor  14 A 1 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS mirror  14 A 1 . 
     When the layer  111 - 1  is being driven, a projection image  10 - 1  sweeps in the main scanning direction at the first position in the sub-scanning direction. When the layer  111 - 2  is being driven, a projection image  10 - 2  sweeps in the main scanning direction at the second position in the sub-scanning direction. When the layer  111 - 3  is being driven, a projection image  10 - 3  sweeps in the main scanning direction at the third position in the sub-scanning direction. When the layer  111 - 4  is being driven, a projection image  10 - 4  sweeps in the main scanning direction at the fourth position in the sub-scanning direction. 
     In a similar manner to the first example as above, the scanning range in the main scanning direction depends on the scanning angle of the MEMS mirror  14 A 1 . On the other hand, the scanning range in the sub-scanning direction is determined by the angle of divergence θ in the sub-scanning direction of the VCSEL array  11 A, i.e., the size S of the light-emitting area of the entirety of the VCSEL array  11 A, and the focal length f of the projector lens (i.e., the second optical element  13 ), using the First Equation as above. 
     The angular resolution dθ of the scanning laser beam in both the main scanning direction and the sub-scanning direction is determined by the focal length f of the projector lens (i.e., the second optical element  13 ) and the size a of the light-emitting area of each of the multiple layers  111  of the VCSEL array  11 A, based on the Second Equation as above. 
     In  FIG. 11 , a plurality of layers  111 - 1  to  111 - 4  do not output laser beams at the same time. Instead, the timing of light emission is controlled for each one of the layers  111 . As a result, measurement is performed independently in the four scanning areas divided in the sub-scanning direction. 
     There is a limit to the extent to which the size of the light-emitting area of each of the multiple layers  111  can be reduced. However, the laser beams that are emitted from the corresponding one of the layers  111  are prevented from diverging by the multiple lens groups  122  of the MLA  12 A. Accordingly, a laser beam whose beam waist has been stopped down can be incident on the reflection area  148  of the MEMS mirror  14 A 1 . Due to this configuration, the radiation intensity of light can be prevented from decreasing, and the laser beams can be scanned in a wide range while maintaining a high definition. 
     In place of the uniaxial MEMS mirror  14 A 1 , any desired mirror such as a polygon mirror, where the angle of the movable mirror uniaxially changes with reference to the laser beam, may be used. 
     FOURTH EXAMPLE 
       FIG. 12  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a fourth example of the present disclosure. In an example configuration as illustrated in  FIG. 12 , the VCSEL array  11 A that include at least two layers  111  in the sub-scanning direction and the biaxial-scanning MEMS mirror  14 A 2  are used. The MEMS mirror  14 A 2  rotates on a principal axis Rmain to scan the laser beams in the main scanning direction, and rotates on a conjugate axis Rsub to shift the laser beams in the sub-scanning direction. 
     The laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A, and are incident on the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged near the focal length f on the incident side of the second optical element  13 . The MEMS minor  14 A 1  is arranged near the focal length f on the light exiting side of the second optical element  13 . 
     In a similar manner to the third example, the timing of light emission of the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A is controlled on an individual basis, and the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A are driven in sequence. The laser beams that are emitted from the layers  111 - 1  to  111 - 4  are prevented from diverging by the corresponding lens group  122 - 1  to  122 - 4  of the MLA  12 A, and are incident on the corresponding area of the projector lens (i.e., the second optical element  13 ). 
     The laser beam that has passed through the second optical element  13  is concentrated on the reflection area of the biaxial-scanning MEMS mirror  14 A 2 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS mirror  14 A 2 . The timing of light emission of the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A is controlled on an individual basis, and the layers  111 - 1  to  111 - 4  of the VCSEL array  11 A are driven in sequence. The laser beams that are emitted from the layers  111 - 1  to  111 - 4  are prevented from diverging by the corresponding lens group  122 - 1  to  122 - 4  of the MLA  12 A, and are incident on the projector lens (i.e., the second optical element  13 ). The positions  131 - 1  to  131 - 4  at which the laser beams are incident on the second optical element  13  drive vary in the sub-scanning direction depending on the layer  111  that is being driven. 
     The laser beam that has passed through the second optical element  13  is concentrated on one of the corresponding reflection areas  148 - 1  to  148 - 4  of the biaxial-scanning MEMS minor  14 A 2 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS minor  14 A 2 . 
     When the biaxial-scanning MEMS minor  14 A 2  is inclined with reference to the conjugate axis Rsub by Osub-n, the projection images that are formed by the layers  111 - 1  to  111 - 4  are referred to as projection images  10 -n−1 to  10 -n−4, respectively. In a similar manner to the third example, when the layer  111 - 1  is being driven, a projection image  10 -n−1 sweeps in the main scanning direction at the first position in the sub-scanning direction. When the layer  111 - 2  is being driven, a projection image  10 -n−2 sweeps in the main scanning direction at the second position in the sub-scanning direction. When the layer  111 - 3  is being driven, a projection image  10 -n−3 sweeps in the main scanning direction at the third position in the sub-scanning direction. When the layer  111 - 4  is being driven, a projection image  10 -n−4 sweeps in the main scanning direction at the fourth position in the sub-scanning direction. 
     When the projection images  10 -n−1 to  10 -n−4 complete a sweep in the main scanning direction, the inclination of the biaxial-scanning MEMS minor  14 A 2  with reference to the conjugate axis Rsub turns to θsub-(n+1), and a sweep of projection images  10 -(n+1)-1 to  10 -(n+1)-4 in the main scanning direction is performed in sequence. By repeating the above operation, scanning in the sub-scanning direction can be performed in a wider range compared with the third example. 
     In a similar manner to the first example, the scanning range in the main scanning direction depends on the scanning angle of the MEMS minor  14 A 2  in the main scanning direction. On the other hand, the scanning range in the sub-scanning direction is determined by the First Equation and the scanning angle of the MEMS minor  14 A 2  in the sub-scanning direction. 
     The angular resolution dθ of the scanning laser beam in both the main scanning direction and the sub-scanning direction is determined by the focal length f of the projector lens (i.e., the second optical element  13 ) and the size a of the light-emitting area of each of the multiple layers  111  of the VCSEL array  11 A, based on the Second Equation as above. 
     In  FIG. 12 , a plurality of layers  111 - 1  to  111 - 4  do not output laser beams at the same time. Instead, the timing of light emission is controlled for each one of the layers  111 . Accordingly, measurement is performed within the scanning range  4  independently in the four scanning areas divided in the sub-scanning direction. 
     There is a limit to the extent to which the size of the light-emitting area of each of the multiple layers  111  can be reduced. However, the laser beams that are emitted from the multiple layers  111  of the VCSEL array  11 A are prevented from diverging by the multiple lens groups  122  of the MLA  12 A. Accordingly, a laser beam whose beam waist has been stopped down is incident on the corresponding reflection area  148  of the MEMS mirror  14 A 2 . Due to this configuration, the radiation intensity of light can be prevented from decreasing, and the laser beams can be scanned in a wide range while maintaining a high definition. 
     In place of the biaxial-scanning MEMS mirror  14 A 2 , any desired mirror such as a polygon mirror, where the angle of the movable minor biaxially changes with reference to the laser beam, may be used. 
     FIFTH EXAMPLE 
       FIG. 13  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a fifth example of the present disclosure. In the fifth example, the VCSEL array  11 A that include at least two layers  111  in the main scanning direction and the movable minor  14  of a uniaxial-scanning type or a biaxial-scanning type are used. In the present example as illustrated in  FIG. 13 , the biaxial-scanning MEMS mirror  14 A 2  is used. 
     In the first to fourth examples as above, the multiple layers  111  are scanned in a wide angle by the movable mirror  14 . By contrast, in the fifth example, scanning area is divided for each of the layers  111 . 
     The laser beam that is emitted from the VCSEL array  11 A is prevented from diverging by the MLA  12 A, and are incident on the projector lens (i.e., the second optical element  13 ). The VCSEL array  11 A is arranged near the focal length f on the incident side of the second optical element  13 . The MEMS minor  14 A 2  is arranged near the focal length f on the light exiting side of the second optical element  13 . 
     The laser beam that has passed through the second optical element  13  is concentrated on the corresponding reflection area of the MEMS mirror  14 A 2 , and is cast in a direction dependent on the angle that the incident laser beam forms with the MEMS mirror  14 A 2 . 
     Among the multiple layers  111  of the VCSEL array  11 A arranged in the main scanning direction, the laser beams that are emitted from the layer  111 - 1  are incident on the corresponding area of the second optical element  13 , and a projection image  101  is formed on a scanning area  411 . In a similar manner, the laser beams that are emitted from a layer  111 - k  (in the example of  FIG. 13 , k denotes an integer of 1 to 4) are projected onto the corresponding scanning area  41   k  by the second optical element  13 , and an optical image  10   k  is formed. 
     As the MEMS minor  14 A 2  performs scanning in the main scanning direction and the sub-scanning direction, the optical image  10   k  sweeps the entirety of the scanning area  41   k . The angular resolution dθ in this configuration in both the main scanning direction and the sub-scanning direction is based on the Second Equation as above. The scanning range 4 in the main scanning direction is determined by the product of the optical scanning range of the multiple layers  111  due to the scanning performed by MEMS minor  14 A 2  and the number of layers of the VCSEL array  11 A. The scanning range in the sub-scanning direction is determined by the scanning range of the layers  111  due to the scanning performed by MEMS minor  14 A 2 . The scanning range of the movable minor  14  may be changed such that the size of the scanning area  41  varies for each of the multiple layers  111 . 
     SIXTH EXAMPLE 
       FIG. 14  is a diagram illustrating the optical scanning performed by the light emitter  1 , according to a sixth example of the present disclosure. In the sixth example, the multiple layers  111  of the VCSEL array  11 A are two-dimensionally arranged in the main scanning direction and the sub-scanning direction, in comparison to the scanning in the fifth example as above. For purposes of simplification, the image of the MLA  12 A is omitted in  FIG. 14 . Moreover, in accordance with the two-dimensional arrangement of the layers  111 , the multiple lens groups  122  of the MLA  12 A are two-dimensionally arranged in  FIG. 14 . 
     In the present example, each of the layers  111  of the VCSEL array  11 A is referred to as a layer  111 - ij  (where each of i and j denotes a natural number). The laser beams that are emitted from the layer  111 - ij  are prevented from diverging by the corresponding lens group  122 - ij  of the MLA  12 A, and are incident on the corresponding reflection area  148   ij  of the MEMS mirror  14 A 2  after passing through the second optical element  13 . The optical image  10   ij  of the laser beams reflected by the MEMS mirror  14 A 2  is projected onto a scanning area  41   ij.    
     As the MEMS mirror  14 A 2  is scanned in the main scanning direction and the sub-scanning direction, the optical image  10   ij  scans the entirety of the scanning area  41   ij . The angular resolution dθ in the present example is based on the Second Equation as above in both the main scanning direction and the sub-scanning direction. The scanning range in both the main scanning direction and the sub-scanning direction is determined by the product of the optical scanning range of the layers  111  due to the scanning performed by MEMS mirror  14 A 2  and the number of layers of the VCSEL array  11 A in each of the scanning directions. The scanning range of the movable mirror  14  may be changed such that the size of the scanning area  41  varies for each of the multiple layers  111 . 
     Relative Positions of Light Emitting Elements of VCSEL and Lens Elements  FIG. 15A  and  FIG. 15B  are diagrams each illustrating the intensity distribution of a laser beam when the laser beams that are emitted from a light emitting element  112   n  are incident not only on the corresponding lens element  121   n  but also on an adjacent microlenses  121 (n+ 1 ), according to the present embodiment. 
       FIG. 15A  is a diagram illustrating an intensity distribution at the beam waist position, i.e., an intensity distribution on the reflection plane of the movable mirror  14 , according to an embodiment of the present disclosure. The light that is emitted from one of the light emitting elements  112  and is incident on the corresponding lens element  121  is focused on the center point, and indicates a high beam intensity. By contrast, the light that is incident on a lens element existing in a peripheral area is focused on a position different from the image forming position of the target lens element  121 , and forms an image at a position deviating from the center point, even at the beam waist position after passing through the second optical element  13 . The light that is focused on a peripheral area does not only end up as an optical loss without striking the movable mirror  14 , but also emerges as stray light at a beam-waist position. 
       FIG. 15B  illustrates an intensity distribution of the light that is cast (reflected) to a remote point by the movable mirror  14 , according to the present embodiment. The laser beams that are reflected by the movable mirror  14  are cast to a remote point, and the shape of the spot is dispersed in the Y-direction. Even when the shape of the spot is dispersed, the laser beams that are incident on the movable mirror  14  after passing through the corresponding lens element maintain the intensity to a certain degree. The stray light that leaks to the area around after passing through a neighboring lens element emerges as stray light of the scanning laser beams. The stray light that is present in the periphery of the light to be used to measure the distance leads to erroneous detection. 
     In addition to the necessity for the optical axes of the light emitting elements  112  and the lens elements  121  to match, it is desired that the light emitting elements  112  and the lens elements  121  be arranged such that laser beams do not enter neighboring lens elements. 
     As an example arrangement, the convex surfaces of the lens elements  121  of the MLA  12 A face the side of the light emitting elements  112  such that the emergence of stray light is prevented in the present embodiment. 
       FIG. 16A ,  FIG. 16B , and  FIG. 16C  are diagrams each illustrating the principle of the lens arrangement according to the present embodiment. 
     When the lens elements  121  are planoconvex lenses, the MLA  12 A is a lens element array of a plano-convex type in which the multiple lens elements  121  of a convex shape are formed on one side. Typically, as illustrated in  FIG. 16B , the flat side of a planoconvex lens faces the side of the light emitting elements  112  in view of the aberration. However, the effective diameter of each of the lens elements  121  is small in the case of the MLA  12 A, and as known in the art, the thickness of a MLA substrate is on the order of several hundred micrometers (μm) to several millimeters (mm) (for example, 100 μm to 10 mm). As illustrated in  FIG. 16B , when the flat side of the planoconvex lens faces the side of the light emitting elements  112 , at the timing when the laser beams that are emitted from a light emitting element  112   n  reach the convex surface of the lens element  121   n  the laser beams are dispersed equal to or wider than the diameter of the lens element  121   n , and strike a neighboring lens element  121 m. 
     In order to handle such a situation, in one example embodiment, as illustrated in  FIG. 16A , the convex surfaces of the lens elements  121  face the side of the light emitting elements  112 . In the case of LiDAR devices, it is satisfactory as long as the laser beams can be controlled within a specific scanning range with a prescribed angular resolution. Accordingly, the spherical aberration on the imaging plane does not have to be controlled in a strict sense. In other words, the operation of the light emitter  1  is almost not at all affected even if the flat side of the MLA does not face the side of the light emitting elements  112 . 
     However, when the distance between the light-emitting surface of the VCSEL and the MLA  12 A is too long in such a case as above where the convex surfaces of the lens elements  121  face the side of the light emitting elements  112 , as illustrated in  FIG. 16C , laser beams enter neighboring lens elements, and stray light emerges. In order to avoid such a situation, the convex surfaces of the lens elements  121  are arranged at distance where the optical axes of the light emitting elements  112  match the optical axes of the lens elements  121  and the laser beams that are emitted from the light emitting element  112  do not strike the neighboring ones of the lens elements  121 . 
       FIG. 17A ,  FIG. 17B , and  FIG. 17C  are diagrams each illustrating the relative positions of the light emitting elements  112  and the lens elements  121  when the MLA  12 C of a double-convex type that has lens elements  121   n   1  and  121   n   2  on both sides is used as the first optical element  12 , according to the present embodiment. As the lens elements  121   n   1  and  121   n   2  of convex type are arranged on both sides of the MLA  12 C, the divergence angle can further be controlled compared with a configuration with convex lenses on one side. In this configuration, each of the lens element  121   n   1  on the incident side of the MLA  12 C and the lens element  121   n   2  on the light exiting side of the MLA  12 C is coaxially arranged with the corresponding light emitting element  112   n.    
       FIG. 17A  is a diagram illustrating the appropriate relative positions of the light emitting element  112   n  and the lens elements  121   n   1  and  121   n   2 , according to the present embodiment. The laser beams that are emitted from the light emitting element  112   n  are incident on the convex surface of the lens element  121   n   1  before the beam diameter of those laser beams becomes wider than the lens diameter. The divergence angle of the incident laser beams is effectively controlled by the lens element  121   n   1  and the lens element  121   n   2 . 
       FIG. 17B  is a diagram illustrating an arrangement in which the distance between the light emitting element  112   n  and the lens element  121   n   1  on the incident side is too short, according to the present embodiment. On the incident side, the laser beams are not incident on the neighboring lens element  121 . However, at the timing when the laser beams reach the convex surface of the lens element  121   n   2  on the on the light exiting side, the beam diameter of those laser beams becomes wider than the lens diameter. 
       FIG. 17C  is a diagram illustrating an arrangement in which the distance between the light emitting element  112   n  and the lens element  121   n   1  on the incident side is too long, according to the present embodiment. However, at the timing when the laser beams reach the convex surface of the lens element  121   n   1  on the on the incident side, the beam diameter of those laser beams becomes wider than the lens diameter. 
     When the MLA  12 C of a double-convex type is used, it is desired to be configured such that the laser beams enter only the corresponding one of the lens elements  121  on both the light entering side and the light exiting side. Due to such a configuration, the emergence of stray light can be prevented. 
       FIG. 18  is a diagram illustrating an example embodiment in which the convex surfaces of the lens elements  121  are arranged on the light exiting side of the MLA  12 A. When the MLA substrate is so thin that the laser beams that are emitted from the light emitting element  112   n  do not enter a neighboring lens elements  121 m, the laser beams enter only the corresponding lens element  121   n , and the laser beams exit through the convex surface of that lens element  121   n . In such cases, the convex surfaces of the MLA  12 A may be arranged on the light exiting side as illustrated in  FIG. 18 . In other words, in such cases as above, the flat side of the MLA  12 A may be arranged to face the VCSEL array  11 A. When the flat side of the MLA  12 A faces the side of the light emitting elements  112 , a VCSEL substrate can be bonded together with a MLA substrate as a package at a wafer level. Accordingly, the productivity may increase and the cost may be reduced, which is advantageous. The thickness of the MLA substrate as illustrated in  FIG. 18  is designed as appropriate in view of the laser beams emitted from light emitting elements. 
     Focal Length of MLA and Distance between VCSEL and MLA When the first optical element  12  such as the diffraction element array  12 B and the MLA  12 A or  12 C is used, the divergence angles of the laser beams output from the multiple light emitting elements  112  of the VCSEL array  11 A can efficiently be controlled. However, if the light-emitting areas of the light emitting elements are increased to achieve high output power, the reducing effect on the divergence angle, which is caused by the first optical element  12 , may be affected in an imaging system. 
     If the light-emitting areas are increased in the VCSEL array  11 A where the light emitting elements  112  are arranged at equal distances, the beam diameter of the laser beams increases. In order to prevent the laser beams from entering the neighboring lens elements  121  of the MLA  12 A, the distance between the VCSEL array  11 A and the MLA  12 A needs to be shortened. On the other hand, in the imaging system, the focal length of the MLA is shortened, and the divergence angle increases. 
     In order to handle such a situation, in the present embodiment, the first optical element  12  whose focal length is equal to or longer than the distance d between the laser beam source  11  (that is composed of, for example, the VCSEL array  11 A) and the first optical element  12  (for example, the MLA  12 A) is used to achieve the reducing effect on the divergence angle. 
     When the focal length fMLA of the MLA  12 A is made longer than the distance d to the VCSEL array  11 A, the laser beams are deflected by a surface with reduced curvature (large radius of curvature) compared with curvature with which the laser beams are oriented in a parallel direction under ordinary circumstances. Accordingly, the refractive power of light from each light-emitting point is weakened. In other words, the divergence angle becomes small compared with when the focal length fMLA of the MLA  12 A is controlled to match the distance d between the VCSEL and the MLA. 
       FIG. 19A  and  FIG. 19B  are diagrams each illustrating an angular distribution of the laser beam emitted through a MLA, according to an embodiment of the present disclosure.  FIG. 19A  is a diagram illustrating an angular distribution when the focal length fMLA of the MLA matches the distance d between the VCSEL and the MLA (fMLA=d), according to the present embodiment.  FIG. 19B  is a diagram illustrating an angular distribution when the focal length fMLA of the MLA is longer than the distance d between the VCSEL and the MLA (fMLA&gt;d), according to the present embodiment. 
     In both  FIG. 19A  and  FIG. 19B , the incident plane of the MLA  12 A is arranged away from the laser beam source whose divergence angle is 18.5 degrees by 80 micrometers (μm) (d=80 μm), the radiance is used as a function of the angle in the horizontal direction (H) and the vertical direction (V) to measure the angle. 
     The angular distribution in  FIG. 19B  (fMLA&gt;d) is closer to a normal distribution than the distribution in  FIG. 19A  (fMLA=d), and the divergence angle in  FIG. 19B  is smaller than the divergence angle in  FIG. 19A . When laser beams with broad angular distribution are focused on the same beam diameter, the intensity gets higher at the center and the intensity gets lower in the perimeter zone compared with laser beams whose angular distribution is narrow (see the distribution profile of  FIG. 19B ). 
     The loss in light quantity at the movable mirror  14  is equivalent to the loss in light quantity of the perimeter zone of the laser beams that were not concentrated on the movable mirror  14 . Accordingly, even if the divergence angles of laser beams are all the same, the image of laser beams with broader angular distribution can be formed on the movable mirror  14 , with a smaller amount of loss in light quantity. As a result, the power of the laser beams that are emitted from the LiDAR device increases, and the distance to an object at a long distance can be measured. 
     When a MLA whose focal length is longer than the distance d between the VCSEL and the MLA is used, the loss in light quantity can be reduced in the imaging optical system towards the movable mirror  14 , and the LiDAR device can measure the distance with an improved level of performance. 
       FIG. 20A  and  FIG. 20B  are diagrams each illustrating the light-receptive width of the movable mirror  14  and the beam-waist shape, according to the present embodiment. In the LiDAR devices, laser beams are scanned upon varying the angle of the movable mirror  14  with reference to the incident laser beam. Regarding the uniaxial movable mirror  14 , the light-receptive width Deff of the movable mirror  14  is expressed in an equation given below, using the width D of the movable mirror  14  and the inclination θMEMS of the movable mirror  14  with reference to the incident laser beam. Deff=D×cosθMEMS 
     As the inclination of the movable mirror  14  increases, the light-receptive width becomes shorter. When wide-angle scanning is performed, the radiation intensity of the laser beam that is not focused on the movable minor  14  increases as the rotation angle of the movable mirror  14  increases, and the utilization efficiency of light may be affected. In the present embodiment, the divergence angle of the laser beams is reduced in advance by the first optical element  12 , to minimize the loss in light quantity even in the wide-angle scanning. 
       FIG. 21A  and  FIG. 21B  are diagrams each illustrating the relation between the divergence angle and the beam diameter of the laser beam source, according to the present embodiment.  FIG. 21A  is a diagram illustrating the beam diameter in the sub-scanning direction when laser beams with varying divergence angles are concentrated on the projector lens (i.e., the second optical element  13 ), according to the present embodiment. The beam diameters obtained when laser beams whose divergence angle is 18.5 degrees are concentrated by a projector lens whose focal length is 30 millimeters (mm) are obtained as a function of the distance from the projector lens, and are plotted as the square marks as illustrated in  FIG. 21A . The beam diameters obtained when the laser beams whose divergence angle is controlled to be 10.6 degrees by the MLA  12 A are concentrated by a projector lens whose focal length is 30 millimeters (mm) are obtained and plotted as black dots as illustrated in  FIG. 21A . The beam diameters obtained when the laser beams whose divergence angle is controlled to be 7.8 degrees by the MLA  12 A are concentrated by a projector lens whose focal length is 30 millimeters (mm) are obtained and plotted as triangular marks as illustrated in  FIG. 21A . 
     Regardless of the presence or absence of the MLA  12 A, the laser beams form their beam waists at a position away from the projector lens by 30 mm, which matches the focal length of the projector lens. If the divergence angle is further controlled by the MLA  12 A, the laser beams may be concentrated with an even smaller beam diameter. 
       FIG. 21B  is a graph in which beam waist diameters are plotted as a function of the divergence angle of the laser beam, according to the present embodiment. The black dots indicate measured values, and the dotted lines indicate values calculated based on the Third Equation as given above. The focal length f of the projector lens in the Third Equation is 30 mm. 
     The result of calculation matches the actual beam waist diameter, and the presence or absence of an MLA does not matter as long as the light-emitting system is concerned. The beam waist diameter after passing through the projector lens (i.e., the second optical element  13 ) can be reduced (see the Third Equation as above), depending on the divergence angle of the laser beam after that laser beam is concentrated by the projector lens. 
       FIG. 22  is a diagram illustrating the utilization efficiency of light for different scanning angles when scanning is performed with varying divergence angles, according to the present embodiment. In a similar manner to  FIG. 21A  and  FIG. 21B , a laser beam whose divergence angle is 18.5 degrees where no MLA is involved and laser beams with the divergence angles of 10.6 degrees and 7.8 degrees where a MLA is used are concentrated on the uniaxial-scanning MEMS minor  14 A by a projector lens whose focal length is 30 mm, respectively, at the beam waist position. The size of the MEMS mirror  14 A is 10 mm×14 mm in the horizontal direction and the vertical direction. More specifically,  FIG. 22  is a diagram illustrating the utilization efficiency of light for different scanning angles when the MEMS mirror  14 A rotates in the main scanning direction within the range of 140 degrees, according to the present embodiment. 
     As described above, as the divergence angle is smaller, the beam waist diameter W of the laser beams that are concentrated by the projector lens becomes small. Accordingly, the utilization efficiency of light improves when the scanning angle gets wider, and distance can be measured in a wide range of angle. For example, assuming that the measurable range is an area with utilization efficiency of light of 95% or greater, the scannable range of angle is increased from 55° to 100° when the divergence angle is controlled by the MLA from 18.5° to 10.6°. Further, the scannable range of angle is increased from 100° to 120° when the divergence angle is controlled by the MLA from 10.6° to 7.8°. 
     If such reduction in divergence angle as above is to be applied to a LiDAR device, the divergence angle of the VCSEL array  11 A is controlled by the MLA  12 A. As a result, the light can be concentrated on the movable mirror  14  whose area of mirror is small (for example, the length of a rectangular reflection mirror in the shorter-side direction is about 1 mm to 20 mm) in an efficient manner, and long distances can be measured in a wide range of angle. 
       FIG. 23  is a diagram illustrating the relation between the amount of defocusing of the projector lens and the angular resolution in the sub-scanning direction of each divergence angle, according to the present embodiment. The amount of defocusing of the projector lens indicates the distance in which the projector lens is moved from the focal point along the optical axis. The negative side in the horizontal axis indicates cases in which the projector lens is made close to the VCSEL array, and the positive side in the horizontal axis indicates cases in which the projector lens is moved away from the VCSEL array. 
     As a secondary effect achieved by decreasing the divergence angle, easing of the precision of installation of the projector lens is achieved. As understood from  FIG. 23 , as the divergence angle is decreased, variations a in angular resolution are small when the projector lens is displaced from the focal length. As the divergence angle of the VCSEL array  11 A is controlled due to the MLA  12 A, variations in the angular resolution of the projector lens (i.e., the second optical element  13 ) decrease, and easing of the precision of installation of the projector lens is achieved. 
       FIG. 24  is a schematic diagram illustrating a mobile object installed with the LiDAR device  100 , according to the present embodiment. For example, the mobile object is a vehicle  501 , and the LiDAR device  100  is attached to the upper side of the front windshield or the ceiling of the front seat of the vehicle  501 . For example, the LiDAR device  100  performs optical scanning in the directions of travel of the vehicle  501  to receive the light reflected by the object  40  existing in the directions of travel. By so doing, the LiDAR device  100  can recognize the object  40 . The light emitter  1  of the LiDAR device  100  performs optical scanning upon controlling the divergence angle of the laser beam in advance by the first optical element  12  such as a microlens array (MLA). Accordingly, the optical loss at a scanner such as the movable mirror  14  can be reduced, and the laser beam can be cast to a remote point with a high angular resolution. 
     Note that the position to which the LiDAR device  100  is attached is not limited to the front and upper side of the vehicle  501 , but may be attached to a side or on the rear of the vehicle  501 . In addition to the application to a vehicle, the LiDAR device  100  is applicable to any mobile object such as a flight vehicle such as an aircraft and a drone, and an autonomous mobile object such as a robot. By adopting the configuration of the light emitter  1  according to the embodiments of the present disclosure, the existence and position of an object can be detected in a wide range. 
     The present disclosure can be implemented in any convenient form, for example using dedicated hardware, or a mixture of dedicated hardware and software. The present disclosure may be implemented as computer software implemented by one or more networked processing apparatuses. The processing apparatuses can compromise any suitably programmed apparatuses such as a general purpose computer, personal digital assistant, mobile telephone (such as a WAP or 3G-compliant phone) and so on. Since the present disclosure can be implemented as software, each and every aspect of the present disclosure thus encompasses computer software implementable on a programmable device. The computer software can be provided to the programmable device using any conventional carrier medium (carrier means). The carrier medium can compromise a transient carrier medium such as an electrical, optical, microwave, acoustic or radio frequency signal carrying the computer code. An example of such a transient medium is a TCP/IP signal carrying computer code over an IP network, such as the Internet. The carrier medium can also comprise a storage medium for storing processor readable code such as a floppy disk, hard disk, CD ROM, magnetic tape device or solid state memory device. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), digital signal processor (DSP), field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions. 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2018-044805, filed on Mar. 12, 2018, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     REFERENCE SINGS LIST 
       1  Light emitter (optical device) 
       2  Light receiver 
       3  Controller and signal processor 
       11  Laser beam source 
       11 A VCSEL array 
       111  Layer (laser element group) 
       112  Light emitting element (surface-emitting laser element) 
       12  First optical element 
       12 A MLA (optical element array) 
       12 B Diffraction element array (optical element array) 
       121  Lens element (optical element) 
       13  Second optical element 
       14  Movable mirror 
       100  LiDAR device (range sensor) 
       501  Vehicle (mobile object)