Patent Publication Number: US-2023145018-A1

Title: Light emitting device and detection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2021-157015 filed Sep. 27, 2021. 
     BACKGROUND 
     (I) Technical Field 
     The present invention relates to a light emitting device and a detection apparatus. 
     (II) Related Art 
     JP2020-188239A describes that in a light emitting device, a general-purpose (normal) capacitor is further connected to a series circuit of a light emitting element and a transistor. 
     SUMMARY 
     There is a technique for causing a light emitting element to emit light by supplying a current to the light emitting element by an electric accumulation element that accumulates electric charge. An electric accumulation element may be provided in the resonant circuit in which resonance occurs, and the current in the resonant circuit may be supplied to the light emitting element. In such a case, in a case where the electric accumulation element is provided in the resonant circuit without limitation, an angle of inclination of a rising edge of light may be gradual due to a capacitance and an inductance of the electric accumulation element in accordance with the electric accumulation element provided. 
     Aspects of non-limiting embodiments of the present disclosure relate to a light emitting device and a detection apparatus that accelerate the rising edge of light as compared with the configuration provided in the resonant circuit without limiting the electric accumulation element. 
     Aspects of certain non-limiting embodiments of the present disclosure overcome the above disadvantages and/or other disadvantages not described above. However, aspects of the non-limiting embodiments are not required to overcome the disadvantages described above, and aspects of the non-limiting embodiments of the present disclosure may not overcome any of the disadvantages described above. 
     According to an aspect of the present disclosure, there is provided a light emitting device including: a substrate that is provided with at least a part of a resonant circuit in which resonance occurs; and a light emitting element that emits light in a case where a current in the resonant circuit is supplied, in which the substrate has an electric accumulation layer provided in the resonant circuit and accumulating electric charge, and the resonant circuit is not provided with an electric accumulation element having a thickness larger than a thickness of the electric accumulation layer and accumulating electric charge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment(s) of the present invention will be described in detail based on the following figures, wherein: 
         FIG.  1    is a diagram showing a configuration example of a detection apparatus; 
         FIG.  2    is a cross-sectional view of a light emitting device; 
         FIG.  3 A  is a perspective view of the light emitting device, and  FIG.  3 B  is a view of the light emitting device viewed from the left side of the light emitting device; 
         FIG.  4    is a diagram showing an electronic circuit of the light emitting device; 
         FIG.  5    is a diagram showing a relationship between the time period elapsed from the start of light emission by the light emitting device and the intensity of emitted light; 
         FIG.  6    is a diagram showing an electronic circuit of a light emitting device as a modification example; and 
         FIG.  7 A  is a table showing parameters for a detection apparatus,  FIG.  7 B  is a table showing a relationship of a relative permittivity, a length in the front-rear direction, a length in the right-left direction, and a thickness for a dielectric layer to satisfy about 10 pF as a capacitance, and 
         FIG.  7 C  is a table showing a relationship of a relative permittivity, a length in the front-rear direction, a length in the right-left direction, and a thickness for the dielectric layer to satisfy about 1000 pF as a capacitance. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG.  1    is a diagram showing a configuration example of the detection apparatus  1 . The detection apparatus  1  is a device which detects a distance from the detection apparatus  1  to an object. The object as a target to which the detection apparatus  1  detects the distance is hereinafter referred to as a target object T. In the present exemplary embodiment, the light detection and ranging (LiDAR) is used as a method for the detection apparatus  1  to detect a distance to the target object T. The LiDAR is to measure the distance to the target object T by detecting light. The LiDAR includes a scanning type LiDAR which scans light and a collective irradiation type LiDAR which collectively irradiates light within a predetermined angle range. In the present exemplary embodiment, the scanning type LiDAR detects the distance to the target object T. 
     In the LiDAR, the time of flight (TOF) is used. The TOF is to measure the distance to the target object T on the basis of a time period in which light travels. The TOF includes the indirect time of flight (iTOF) and the direct time of flight (dTOF). The iTOF is a method of measuring the distance to the target object T on the basis of a difference between a phase of the emitted light and a phase of the received light. The dTOF is a method of measuring the distance to the target object T on the basis of a time period from emission of the light to light reception. 
     The detection apparatus  1  includes a light emitting device  10 , a light receiving unit  20 , and a detection unit  30 . 
     The light emitting device  10  is a device which emits light. Examples of the light emitting device  10  include a vertical cavity surface emitting laser (VCSEL). The VCSEL is a laser which emits light in a direction perpendicular to a surface of a substrate. The light emitting device  10  of the present exemplary embodiment generates a pulsed current by resonance, and emits light by the generated current. 
     A configuration of the light emitting device  10  will be described in detail later. 
     The light receiving unit  20  as an example of the light receiving unit receives light based on irradiation of the light emitted from the light emitting device  10  to the target object T. In a case where the light receiving unit  20  receives light, electric charge is generated. Examples of the light based on the irradiation of the target object T with the light emitted from the light emitting device  10  include light emitted from the light emitting device  10  and reflected by the target object T, and light scattered by the target object T to which the light is emitted from the light emitting device  10 . The light emitted from the light emitting device  10  may be referred to as emitted light below. The light emitted from the light emitting device  10  and reflected by the target object T is hereinafter referred to as reflected light. The light scattered by the target object T, to which the light is emitted from the light emitting device  10 , is hereinafter referred to as scattered light. 
     Examples of the light receiving unit  20  include an optical sensor which detects light. Examples of the optical sensor include semiconductors such as a single photon avalanche diode (SPAD). 
      The detection unit  30  as an example of the detection unit detects the distance from the detection apparatus  1  to the target object T, on the basis of the light received by the light receiving unit  20 . The detection unit  30  has a time measurement unit  31  and a measurement unit  32 . 
     The time measurement unit  31  measures a time period. 
     The measurement unit  32  acquires information indicating a time period from emission of the light from the light emitting device  10  to generation of electric charge by the light receiving unit  20  from the time measurement unit  31 . Then, from the acquired information, the distance from the detection apparatus  1  to the target object T is measured. More specifically, the measurement unit  32  measures the distance from the detection apparatus  1  to the target object T from Expression (1). 
     
       
         
           
             L 
               
             = 
               
             
               
                 c 
                 × 
                 t 
               
             
             / 
             2 
           
         
       
     
     In Expression (1) , L is the distance from the detection apparatus  1  to the target object T. c is a speed of light. t is a time period from emission of the light from the light emitting device  10  to generation of electric charge by the light receiving unit  20 . In addition, the measurement unit  32  measures the distance from the detection apparatus  1  to the target object T by setting a time period from emission of the light from the light emitting device  10  to generation of electric charge by the light receiving unit  20  to a time period from emission of the light from the light emitting device  10  to light reception by the light receiving unit  20 . 
     The detection apparatus  1  is provided in a movable body  2  which is traveling. In the illustrated example, an automobile is shown as the movable body  2 . The movable body  2  is not limited to the illustrated example. The movable body  2  may be, for example, a drone, a train, a ship, an airplane, or the like. The movable body  2  may be a computer carried by a user of the detection apparatus  1 . 
     In the illustrated example, a human being is shown as the target object T, but the target object T is not limited to the human being. The target object T may be any object as long as the object reflects the light emitted from the light emitting device  10  or scatters the light by being irradiated with the light emitted from the light emitting device  10 . 
     The detection apparatus  1  of the present exemplary embodiment includes a distance of 0.1 m or more and 500 m or less as a detection target as a distance to the target object T. 
     Next, a configuration of the light emitting device  10   will be described. 
       FIG.  2    is a cross-sectional view of the light emitting device  10 . In  FIG.  2   , an upper side of a page showing the light emitting device  10  may be referred to as “upper side”, a lower side of the page may be referred to as “lower side”, and directions toward the sides may be referred to as “up-down direction”. A left side of the page showing the light emitting device  10  may be referred to as “left side”, a right side of the page may be referred to as “right side”, and directions toward the sides may be referred to as “right-left direction”. A front side of the page showing the light emitting device  10  may be referred to as “front side”, a rear side of the page may be referred to as “rear side”, and directions toward the sides may be referred to as “front-rear direction”. 
       FIG.  3 A  is a perspective view of the light emitting device  10 , and  FIG.  3 B  is a view of the light emitting device  10  viewed from the left side of the light emitting device  10 . It should be noted that  FIG.  2    is a cross-sectional view taken along the line II-II of  FIG.  3 B .  FIG.  3 A  shows a state in which the light emitting device  10  is disassembled for convenience of explanation.  FIG.  3 A  does not show, for convenience of explanation, configurations of a middle electric path layer  133 , a lower electric path layer  134 , an upper insulation layer  135 , a reinforcement layer  136 , and a lower insulation layer  137  which will be described later.  FIG.  3 B  does not show, for convenience of explanation, the configurations of the middle electric path layer  133 , the lower electric path layer  134 , the reinforcement layer  136 , and the lower insulation layer  137  which will be described later. 
     As shown in  FIG.  2   , the light emitting device  10  includes a light emitting element  11 , an operation unit  12 , and a substrate  13 . 
     The light emitting element  11  is an element which emits light in a case where a current is supplied. The light emitting element  11  emits light toward the upper side, which is a direction perpendicular to the surface of the substrate  13 . The light emitting element  11  is provided with an anode surface  11 A and a cathode surface  11 B. The anode surface  11 A is an upper surface of the light emitting element  11  and is a surface which forms the anode. The cathode surface  11 B is a bottom surface of the light emitting element  11  and is a surface which forms a cathode. 
     The operation unit  12  is an integrated circuit (IC) which operates the light emitting element  11 . The operation unit  12  is provided with a cathode terminal  121 , a ground terminal  122 , and a solder portion  123 . 
     The cathode terminal  121  is a terminal which forms a cathode. 
     The ground terminal  122  is a terminal which forms a ground. The ground is an electric path which serves as a reference for electric potential in an electronic circuit. 
     The solder portion  123  is solder which adheres to the substrate  13 . 
     The substrate  13  is a substrate on which a part of the resonant circuit is provided. A resonant circuit is an electronic circuit in which resonance occurs. The substrate  13  has a plurality of layers. More specifically, the substrate  13  has an upper electric path layer  131 , a capacitor layer  132 , the middle electric path layer  133 , the lower electric path layer  134 , the upper insulation layer  135 , the reinforcement layer  136 , and the lower insulation layer  137 . 
     The upper electric path layer  131  is a layer which forms an electric path. The upper electric path layer  131  is provided on the uppermost side of the substrate  13 . The cathode terminal  121  of the operation unit  12  is attached to the upper electric path layer  131 . 
     The capacitor layer  132  as an example of the electric accumulation layer is a layer for accumulating electric charge. That is, the capacitor layer  132  is an electric accumulation element which accumulates electricity. In addition, the capacitor layer  132  supplies a current to the light emitting element  11  by discharging the accumulated electric charge. The capacitor layer  132  is provided on the lower side of the upper electric path layer  131  in the substrate  13 . 
     The middle electric path layer  133  is a layer which forms an electric path. The middle electric path layer  133  is provided on the lower side of the capacitor layer  132  in the substrate  13 . 
     The lower electric path layer  134  is a layer which forms an electric path. The lower electric path layer  134  is provided on the lowermost side of the substrate  13 . 
     Each of the middle electric path layer  133  and the lower electric path layer  134  is made of a metal material. Examples of the metal material include copper. 
     The upper insulation layer  135  is a layer which insulates the upper electric path layer  131  and the capacitor layer  132 . The upper insulation layer  135  is provided between the upper electric path layer  131  and the capacitor layer  132  in the up-down direction. The ground terminal  122  and the solder portion  123  of the operation unit  12  are attached to the upper insulation layer  135 . 
     The reinforcement layer  136  is a layer which supplements a strength of the substrate  13 . The reinforcement layer  136  insulates the capacitor layer  132  and the middle electric path layer  133 . The reinforcement layer  136  is provided between the capacitor layer  132  and the middle electric path layer  133  in the up-down direction. 
     The lower insulation layer  137  is a layer which insulates the middle electric path layer  133  and the lower electric path layer  134 . The lower insulation layer  137  is provided between the middle electric path layer  133  and the lower electric path layer  134  in the up-down direction. 
     The upper insulation layer  135 , the reinforcement layer  136 , and the lower insulation layer  137  each are made of, for example, a prepreg. The prepreg is a material in which carbon fibers are preliminarily impregnated with a resin. 
     In the present exemplary embodiment, the ground terminal  122  of the operation unit  12  is connected to the capacitor layer  132  via a via hole V. The via hole is an opening portion for conduction between one end and the other end. By coating an inner circumferential surface of the opening portion with a metal material, one end and the other end are conductive. In the present exemplary embodiment, the ground terminal  122  and the capacitor layer  132  are conductive by providing the via hole V on the upper insulation layer  135 . 
     As shown in  FIGS.  3 A and  3 B , a cathode layer  1311  and an anode layer  1312  are provided on the upper electric path layer  131 . 
     The cathode layer  1311  is a layer which forms a cathode. The cathode surface  11 B of the light emitting element  11  is attached to the cathode layer  1311 . More specifically, the entire surface of the cathode surface  11 B is attached to the cathode layer  1311 . The cathode layer  1311  is formed longer in the right-left direction than the light emitting element  11 , and on the right side of the light emitting element  11 , there is a portion of the cathode layer  1311  to which the cathode surface  11 B is not attached. Then, the cathode terminal  121  of the operation unit  12  is attached to the portion of the cathode layer  1311  to which the cathode surface  11 B is not attached (refer to  FIG.  2   ). In addition, four cathode terminals  121  are provided in the operation unit  12  at equal intervals in the front-rear direction, and the four cathode terminals  121  are attached to the cathode layer  1311  of the upper electric path layer  131 . 
     The anode layer  1312  is a layer which forms an anode. The anode layer  1312  is provided on each of the front side and the rear side of the cathode layer  1311 . The anode layer  1312  is formed shorter in the right-left direction than the cathode layer  1311 . More specifically, the anode layer  1312  is formed such that the length in the right-left direction is the same as the length of the light emitting element  11 . The position of the anode layer  1312  in the right-left direction is aligned with the position of the light emitting element  11 . The anode surface  11 A of the light emitting element  11  and the anode layer  1312  are connected through a wire W made of a metal material. In the illustrated example, five wires W are connected to each of the anode layer  1312  provided on the front side of the cathode layer  1311  and the anode layer  1312  provided on the rear side of the cathode layer  1311 . 
     The cathode layer  1311  and the anode layer  1312  each are made of a metal material. Examples of the metal material include copper. 
     A ground layer  1321 , a dielectric layer  1322 , and an anode layer  1323  are provided on the capacitor layer  132 . The ground layer  1321 , the dielectric layer  1322 , and the anode layer  1323  are all formed in a rectangular shape. 
     The ground layer  1321  is a layer which forms a ground. The ground terminal  122  of the operation unit  12  is connected to the ground layer  1321  of the capacitor layer  132  via the via hole V (refer to  FIG.  2   ) . Although not shown, four ground terminals  122  are provided in the operation unit  12  at equal intervals in the front-rear direction, and the four ground terminals  122  each are connected to the ground layer  1321  via the via hole V. The ground layer  1321  is made of a metal material. Examples of the metal material include copper. 
     The dielectric layer  1322  is a layer which forms a dielectric substance. In the present exemplary embodiment, Faradflex (registered trademark) manufactured by Oak-Mitsui Inc. is used as the dielectric layer  1322 . 
     In the present exemplary embodiment, a thickness of the dielectric layer  1322  in the up-down direction is the thickness d (refer to  FIG.  2   ). A length of the dielectric layer  1322  in the front-rear direction is a length a (refer to  FIG.  3 A ) . A length of the dielectric layer  1322  in the right-left direction is a length b. An area of the dielectric layer  1322  is a product of the length a and the length b. In the present exemplary embodiment, the area of the dielectric layer  1322  is equal to each of the area of the ground layer  1321  and the area of the anode layer  1323 . 
     The anode layer  1323  is a layer which forms the anode. As shown in  FIG.  3 A , the anode layer  1323  of the capacitor layer  132  is connected to the anode layer  1312  of the upper electric path layer  131  via the via hole V. In the present exemplary embodiment, by providing the via hole V on the upper insulation layer  135 , the ground layer  1321  of the capacitor layer  132 , and the dielectric layer  1322 , the anode layer  1312  of the upper electric path layer  131  and the anode layer  1323  of the capacitor layer  132  are conductive. In the illustrated example, four via holes V are provided for each anode layer  1312  of the upper electric path layer  131 . Each of the four via holes V is provided at an equal interval in the right-left direction. 
     The anode layer  1323  is made of a metal material. Examples of the metal material include copper. 
     As shown in  FIG.  3 A , a power supply  14  and a resistor  15  are provided in the light emitting device  10 . 
     The power supply  14  supplies electric charge to the capacitor layer  132  of the substrate  13 . One end of the power supply  14  is connected to the resistor  15 , and the other end is connected to the ground layer  1321  of the capacitor layer  132 . 
     The resistor  15  as an example of the suppression unit has a predetermined electric resistance. One end of the resistor  15  is connected to the power supply  14 , and the other end is connected to the anode layer  1312  of the upper electric path layer  131 . That is, the resistor  15  is connected to the electric path between the power supply  14  and the capacitor layer  132 . In the present exemplary embodiment, electric charge transfer between the power supply  14  and a resonant circuit RC is restricted in accordance with the electric resistance of the resistor  15 . More specifically, the higher the electric resistance of the resistor  15 , the more difficult it is for electric charge to move between the power supply  14  and the resonant circuit RC. 
       FIG.  4    is a diagram showing an electronic circuit of the light emitting device  10 . 
     As shown in  FIG.  4   , a resonant circuit RC is provided in the electronic circuit of the light emitting device  10 . The light emitting element  11 , the operation unit  12 , and the capacitor layer  132  are provided in the resonant circuit RC of the present exemplary embodiment. Although not shown, in the resonant circuit RC, the anode layer  1312  of the upper electric path layer  131  is connected to the electric path between the light emitting element  11  and the capacitor layer  132 . In the resonant circuit RC, the cathode layer  1311  of the upper electric path layer  131  is connected to the electric path between the light emitting element  11  and the operation unit  12 . 
     A transistor  124  is provided in the operation unit  12 . The transistor  124  is an electronic switch which switches between a state in which the resonant circuit RC is conductive and a state in which the resonant circuit RC is not conductive, depending on the applied voltage. In a case where a voltage equal to or greater than a predetermined value is not applied to the transistor  124 , the transistor  124  is in an OFF state. In such a case, since the circuit to which the transistor  124  is connected is cutoff and the resonant circuit RC is being not conductive, no current is supplied to the light emitting element  11 . In a case where a voltage equal to or greater than a predetermined value is applied to the transistor  124 , the transistor  124  is in an ON state. In such a case, the circuit to which the transistor  124  is connected is connected, and the resonant circuit RC is being conductive. Then, in the state, in the resonant circuit RC, the impedance drops at a specific frequency and resonance occurs, and the pulsed current generated by the resonance is supplied to the light emitting element  11 . It should be noted that the specific frequency, that is, a frequency at which resonance occurs may be referred to as a resonance frequency below. The pulsed current may be simply referred to as a pulse below. 
     The resistor  15  is connected to an electric path between the power supply  14  and the resonant circuit RC. 
     In the present exemplary embodiment, in a case where the transistor  124  is in an OFF state, electric charge is supplied from the power supply  14  to the capacitor layer  132  via the resistor  15 , and a voltage is applied to the capacitor layer  132 . That is, the capacitor layer  132  is charged. 
     In a case where the transistor  124  is in an ON state, the capacitor layer  132  discharges electric charge and a current is supplied from the capacitor layer  132  to the light emitting element  11 . Therefore, the light emitting element  11  emits light. In a case where the capacitor layer  132  discharges electric charge, the transistor  124  is put into an OFF state again and the capacitor layer  132  is charged. As described above, in the present exemplary embodiment, charging of the capacitor layer  132  and light emission of the light emitting element  11  performed by supplying a current from the capacitor layer  132  to the light emitting element  11  are repeatedly performed. 
     As described above, the light emitting device  10  of the present exemplary embodiment generates a pulse by resonance at a specific frequency, and supplies the generated pulse to the light emitting element  11  to cause the light emitting element  11  to emit light. Here, in the dTOF which is a kind of TOF described above, it is not necessary to have a range of frequencies for generating a current, and a current may be generated by lowering the impedance in the electronic circuit at a single frequency. On the other hand, in the iTOF, since it is necessary to generate a current in a frequency range wider than a frequency range of the dTOF, it is necessary to lower the impedance in the electronic circuit in the wider frequency region. Therefore, the light emitting device  10  of the present exemplary embodiment is used as a light source for the dTOF on the basis of a property of lowering the impedance in the resonant circuit RC at a resonance frequency which is a single frequency and generating a pulse by resonance. However, the light emitting device  10  may be used as a light source for the iTOF. 
     The light emitting device  10  is necessary to shorten the time period from the start of emission of the light to the increase in intensity of the emitted light, that is, to accelerate the rising edge of the intensity of the emitted light. 
       FIG.  5    is a diagram showing a relationship between the time period, which elapses after the light emitting device  10  starts to emit light, and the intensity of the emitted light. The intensity of the emitted light has a sharper waveform than the waveform shown in  FIG.  5    due to a time delay in rising edge due to the influence of the relaxation oscillation.  FIG.  5    shows the intensity of the emitted light in a case where the influence of the relaxation oscillation is not taken into consideration. 
     In  FIG.  5   , the horizontal axis indicates the time period which elapses after the light emitting device  10  starts to emit light, and the vertical axis indicates the intensity of the emitted light Li. In the emitted light Li shown in  FIG.  5   , an angle, at which the rising edge of the intensity of the emitted light Li is inclined with respect to the passage of time, is an angle 6. In the following description, the rising edge of light intensity may be simply referred to as the rising edge of light. The angle θ, at which the rising edge of the emitted light is inclined with respect to the passage of time, may be referred to as an inclination angle θ. 
     As described above, the detection apparatus  1  of the present exemplary embodiment detects a distance to the target object T on the basis of the time period from the start of light emission to reception of reflected light or scattered light. Here, the lower limit value of the intensity of the emitted light necessary to generate the reflected light depends on the reflectance of the light in the target object T. More specifically, the higher the reflectance of light in the target object T, the lower the lower limit value of the intensity of the emitted light necessary to generate the reflected light. A reflectance of the light is a ratio of the intensity of the emitted light to the intensity of the reflected light. The reflectance of light is determined for each target object T. 
     Here, an intensity C1 shown in  FIG.  5    is a lower limit value of the intensity of the emitted light Li necessary to generate the reflected light from a target object O1 having a specific light reflectance. An intensity C2 shown in  FIG.  5    is a lower limit value of the intensity of the emitted light Li necessary to generate the reflected light from a target object O2 of which the reflectance is lower than the reflectance of the target object O1. In the following description, the lower limit value of the intensity of the emitted light Li necessary to generate the reflected light may be referred to as an optical lower limit value. 
     In a case where the detection apparatus  1  irradiates the target object O1 with the emitted light Li, when the time period T1 elapses after the detection apparatus  1  starts to emit light, the light having the intensity of the lower limit value C1 is emitted. In a case where the detection apparatus  1  irradiates the target object O2 with the emitted light Li, when the time period T2 elapses after the detection apparatus  1  starts to emit light, the light having the intensity of the lower limit value C2 is emitted. Here, in the case where the target object to be irradiated with the emitted light Li is the target object O2 as compared with the case of the target object O1, the time period from the start of light emission of the detection apparatus  1  to emission of the light of which the intensity is the optical lower limit value increases. In a case where the time period to emission of the light of which the intensity is the optical lower limit value increases, the time period from the start of light emission by the detection apparatus  1  to reception of the reflected light also increases. In such a case, even in a case where the distance from the detection apparatus  1  to the target object O1 and the distance from the detection apparatus  1  to the target object O2 are the same, regarding the time period from the start of light emission to reception of the reflected light by the detection apparatus  1 , the time period of the target object O2 is longer than the time period of the target object O1 by a difference between the time period T1 and the time period T2. In other words, depending on the reflectance of the target object T, there is a variation in time period from the start of light emission of the detection apparatus  1  to emission of the light of which the intensity is the optical lower limit value, and there is also a variation in the result detected as the distance from the detection apparatus  1  to the target object T by the variation in time period. Therefore, for example, although there is no particular limitation, it is desired that the time period from the start of light emission of the detection apparatus  1  to emission of the light of which the intensity is the optical lower limit value is small for each target object T. 
     In a case where the inclination angle θ is gradual, that is, in a case where the rising edge of the emitted light Li occurs slower, for each the target object T, the variation in time period from the start of light emission of the detection apparatus  1  to emission of the light of which the intensity is the optical lower limit value increases such that the difference between the time period T1 and the time period T2 increases. In a case where the rising edge of the light occurs faster, for each the target object T, the variation in time period from the start of light emission of the detection apparatus  1  to emission of the light of which the intensity is the optical lower limit value decreases such that the difference between the time period T1 and the time period T2 decreases. 
     As the current supplied to the light emitting element  11  increases, the intensity of the emitted light increases. Thus, the faster the rising edge of the current in the light emitting device  10 , the faster the rising edge of the emitted light. Therefore, in a case where the rising edge of the current occurs faster in the light emitting device  10 , for each target object T, the variation in time period from the start of light emission by the detection apparatus  1  to emission of the light of which the intensity is the lower limit value decreases. 
     Then, in the present exemplary embodiment, by generating a pulse by resonance in the resonant circuit RC, a pulse having a shorter pulse width and a faster rising edge is generated as compared with the case where the pulse is generated without resonance. As described above, in a case where the rising edge of the current occurs faster, the variation in time period from the start of resonance in the light emitting device  10  to the start of emission of the light by the light emitting element  11  also decreases. 
     Here, in the resonant circuit RC, in a case where the impedance is low at a frequency different from the resonance frequency, resonance may not occur. In such a case, even in a case where a current is generated in the light emitting device  10 , the rising edge of the current occurs slowly, and the rising edge of the emitted light also occurs slowly accordingly. 
     Therefore, in the present exemplary embodiment, the electric accumulation element provided in the resonant circuit RC is limited. More specifically, the electric accumulation element other than the capacitor layer  132  is provided in the resonant circuit RC. In such a case, as compared with the configuration in which the electric accumulation element is provided in the resonant circuit RC without limitation, a decrease in impedance at a frequency different from the resonance frequency is suppressed by an amount that increases in capacitance and inductance in the resonant circuit RC are suppressed. 
     In the present exemplary embodiment, the light emitting device  10  further includes a resistor  15  which is connected to a circuit between the power supply  14  and the resonant circuit RC and suppresses electric charge transfer between the power supply  14  and the resonant circuit RC. 
     In a case where the resonant circuit RC is affected by a circuit outside the resonant circuit RC due to electric charge being supplied from the power supply  14  to the capacitor layer  132  and the like while resonance is being generated, the resonance in the resonant circuit RC may be attenuated. Therefore, in the present exemplary embodiment, by connecting the resistor  15  to the circuit between the power supply  14  and the resonant circuit RC, the resonant circuit RC is suppressed from being affected by the power supply  14  while resonance is being generated in the resonant circuit RC. 
     In the present exemplary embodiment, the substrate  13  has a plurality of layers including the capacitor layer  132 , and the light emitting element  11  is provided to overlap the plurality of layers. 
     In a case where the light emitting element  11  is provided so as not to overlap the capacitor layer  132 , such as in a case where the light emitting element  11  is disposed away from the substrate  13  in the front-rear direction (refer to  FIG.  3 A ) or in the right-left direction, as compared with the case where the light emitting element  11  overlaps the capacitor layer  132 , the electric path between the light emitting element  11  and the capacitor layer  132  becomes longer. In other words, in a case where the light emitting element  11  is provided so as not to overlap with the capacitor layer  132 , as compared with the case where the light emitting element  11  overlaps the capacitor layer  132 , the electric path in the resonant circuit RC becomes longer. Therefore, in the present exemplary embodiment, the light emitting element  11  is provided to overlap the capacitor layer  132 . 
     In particular, in the present exemplary embodiment, the plurality of layers, with which the light emitting element  11  overlaps, on the substrate  13  include a cathode layer  1311  which is provided in the resonant circuit RC and through which the current supplied to the light emitting elements  11  passes. 
     Modification Example 
     Next, a modification example will be described. 
     The present exemplary embodiment has described that the resistor  15  which suppresses electric charge transfer between the power supply  14  and the resonant circuit RC is provided in the light emitting device  10 . Here, the suppression unit suppressing electric charge transfer between the power supply  14  and the resonant circuit RC is not limited to the resistor  15 . 
       FIG.  6    is a diagram showing an electronic circuit of the light emitting device  10  as the modification example. 
     In the modification example, as shown in  FIG.  6   , the transistor  16  is connected to the circuit between the power supply  14  and the resonant circuit RC instead of the resistor  15 . 
     The transistor  16  as an example of the suppression unit is an electronic switch which switches between a state in which the circuit from the power supply  14  to the resonant circuit RC is conductive and a state in which the circuit is not conductive, depending on the applied voltage. 
     In a case where a voltage equal to or greater than a predetermined value is applied to the transistor  16 , the transistor  16  is in an ON state. In such a case, the circuit to which the transistor  16  is connected is connected and the circuit from the power supply  14  to the resonant circuit RC is being conductive. Therefore, electric charge is supplied from the power supply  14  to the capacitor layer  132  of the substrate  13 , and the capacitor layer  132  is charged. At that time, the electric charge is supplied earlier than the case where the electric charge is supplied via the resistor  15  or the like having the resistance necessary for making the resonant circuit RC independent. 
     In a case where a voltage equal to or greater than a predetermined value is not applied to the transistor  16 , the transistor  16  is in an OFF state. In such a case, the circuit to which the transistor  16  is connected is cutoff, and the circuit from the power supply  14  to the resonant circuit RC is being not conductive. At this time, the electric charge does not move between the power supply  14  and the resonant circuit RC. In a case where the charging of the capacitor layer  132  is completed at this time, the current is supplied from the capacitor layer  132  to the light emitting element  11 . Therefore, the light emitting element  11  emits light. 
     As described above, in the present exemplary embodiment, the transistor  16  cuts off conduction in the circuit between the power supply  14  and the resonant circuit RC, thereby facilitating the generation of resonance in the resonant circuit RC. 
     Next, parameters of the detection apparatus  1  necessary to accelerate the rising edge of the light will be described. In the following description, it is assumed that the light emitting device  10  is used as a light source for dTOF. 
       FIG.  7 A  is a table showing parameters for the detection apparatus  1 . 
     The term “type” shown in  FIG.  7 A  means a type of LiDAR used in the detection apparatus  1 . The term “collective irradiation type” shown in “type” means that a collective irradiation type LiDAR is used in the detection apparatus  1 . The term “scanning type” shown in “type” means that a scanning type LiDAR is used in the detection apparatus  1 . 
     The term “inductance” shown in  FIG.  7 A  means an inductance in the resonant circuit RC. 
     The term “detection distance” shown in  FIG.  7 A  means a distance to the target object T as a detection target of the detection apparatus  1 . 
     The term “peak current” shown in  FIG.  7 A  means a maximum value of the current generated in a case where the transistor  124  is turned on. 
     The term “half-value width of the pulse” shown in  FIG.  7 A  means a half-value width of the pulse generated by resonance in the resonant circuit RC. The term “half-value width of pulse” can also be regarded as the time period necessary from the generation of a pulse to the rising edge of current. 
     The term “accuracy” shown in  FIG.  7 A  means an accuracy with which the detection apparatus  1  detects the distance to the target object T. The term “accuracy” can also be regarded as an error in the distance to the target object T detected by the detection apparatus  1 , which is generated for the time period necessary from the start of light emission by the detection apparatus  1  to the rising edge of the light. 
     The term “capacitance” shown in  FIG.  7 A  means a capacitance of the dielectric layer  1322  in the capacitor layer  132 . 
     The term “power supply voltage” shown in  FIG.  7 A  means a voltage applied by the power supply  14 . 
     The values shown in  FIG.  7 A  will be specifically described. In the detection apparatus  1  in which the “detection distance” is “5” or more and “20” or less, the “inductance” is 0.4. In the detection apparatus  1  in which the “detection distance” is “50” or more and “200” or less, the “inductance” is 0.8. 
     In any of the detection apparatuses  1  shown in  FIG.  7 A , the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. In other words, in a case where the dielectric layer  1322  having a capacitance of 10 pF or more and 1000 pF or less is used, the rising edge of the light occurs faster. 
       FIG.  7 B  is a table showing a relationship between the relative permittivity, the length a, the length b, and the thickness d for the dielectric layer  1322  to satisfy about  10  pF as a capacitance.  FIG.  7 C  is a table showing a relationship between the relative permittivity, the length a, the length b, and the thickness d for the dielectric layer  1322  to satisfy about 1000 pF as a capacitance. 
     The term “capacitance” shown in  FIGS.  7 B and  7 C  means a capacitance of the dielectric layer  1322 . 
     The term “er” shown in  FIGS.  7 B and  7 C  means a relative permittivity of the dielectric layer  1322 . 
     The term “w1” shown in  FIGS.  7 B and  7 C  means a length a (refer to  FIG.  3 A ) of the dielectric layer  1322  in the front-rear direction. 
     The term “w2” shown in  FIGS.  7 B and  7 C  means a length b (refer to  FIG.  3 A ) of the dielectric layer  1322  in the right-left direction. 
     The term “d” shown in  FIGS.  7 B and  7 C  means a thickness d (refer to  FIG.  2   ) of the dielectric layer  1322  in the up-down direction. 
     The examples shown in  FIGS.  7 B and  7 C  have described that the “w1” is the length a of the dielectric layer  1322  in the front-rear direction, and the “w2” is the length b of the dielectric layer  1322  in the right-left direction, but the present invention is not limited to this. The “w1” may be the length b of the dielectric layer  1322  in the right-left direction, and the “w2” may be the length a of the dielectric layer  1322  in the front-rear direction. 
     The capacitances shown in  FIGS.  7 B and  7 C  are calculated from Expression (2). 
     
       
         
           
             C 
               
             = 
               
             ε 
               
             × 
               
             ε 
             r 
             
               
                 w 
                 1 
                   
                 × 
                   
                 w 
                 2 
               
             
             / 
             d 
           
         
       
     
     In Expression (2), C is a capacitance of the dielectric layer  1322 . ε is a permittivity of the vacuum.  FIGS.  7 B and  7 C  show capacitances each rounded to the first decimal place. 
     The values shown in  FIGS.  7 B and  7 C  will be specifically described. In any of the detection apparatuses  1  shown in  FIG.  7 B , the capacitance of the dielectric layer  1322  is 10 pF or more. In other words, in the dielectric layer  1322 , the relationship of the relative permittivity, the length a in the front-rear direction, the length b in the right-left direction, and the thickness d in the up-down direction is the relationship shown in the drawing. In such a case, the capacitance of the dielectric layer  1322  is 10 pF or more. 
     In any of the detection apparatuses  1  shown in  FIG.  7 C , the capacitance of the dielectric layer  1322  is about 1000 pF. In other words, in the dielectric layer  1322 , the relationship of the relative permittivity, the length a in the front-rear direction, the length b in the right-left direction, and the thickness d in the up-down direction is the relationship shown in the drawing. In such a case, the capacitance of the dielectric layer  1322  is about 1000 pF. 
       FIG.  7 B  shows that the area of the dielectric layer  1322  is “1.15x10 -6  m 2 ” which is the minimum value among the values shown in  FIGS.  7 B and  7 C  in a case where the “er” is “20” and the “d” is “0.02”. However, in a case where the relative permittivity of the dielectric layer  1322  is made higher or the thickness d of the dielectric layer  1322  is made shorter, even in a case where the area of the dielectric layer  1322  is “10 -7  m 2 ”, the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. That is, an area of the dielectric layer  1322  necessary for the dielectric layer  1322  to satisfy the capacitance of 10 pF or more and 1000 pF or less may be at least “10 -7  m 2 ” or more. 
       FIG.  7 C  shows that the area of the dielectric layer  1322  is “5.7×10 -5  m 2 ” which is the maximum value among the values shown in  FIGS.  7 B and  7 C  in a case where the “εr” is “100” and the “d” is “0.05”. However, in a case where the relative permittivity of the dielectric layer  1322  is made lower or the thickness d of the dielectric layer  1322  is made longer, even in a case where the area of the dielectric layer  1322  is “10 -3  m 2 ”, the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. That is, an area of the dielectric layer  1322  necessary for the dielectric layer  1322  to satisfy the capacitance of 10 pF or more and 1000 pF or less may be “10 -3  m 2 ” or less. 
       FIG.  7 C  shows that the “d” is “0.001 mm” which is the minimum value among the values shown in  FIGS.  7 B and  7 C  in a case where the “er” is “10”, the “w1” is “4”, and the “w2” is “3”. However, in a case where the relative permittivity of the dielectric layer  1322  is made lower or the area of the dielectric layer  1322  is made narrower, even in a case where the “d” is “0.0005 mm”, the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. That is, a thickness d of the dielectric layer  1322  necessary for the dielectric layer  1322  to satisfy the capacitance of 10 pF or more and 1000 pF or less may be at least “0.0005 mm” or more. 
       FIG.  7 C  shows that the maximum value of “d” among the values shown in  FIGS.  7 B and  7 C  is “0.05 mm”. However, in a case where the relative permittivity of the dielectric layer  1322  is made higher or the area of the dielectric layer  1322  is made wider, even in a case where “d” is “0.1 mm”, the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. That is, a thickness d of the dielectric layer  1322  necessary for the dielectric layer  1322  to satisfy the capacitance of 10 pF or more and 1000 pF or less may be “0.1 mm” or less. 
       FIG.  7 C  shows that the “εr” is “1000” which is the maximum value among the values shown in 7B and  FIG.  7 C  in a case where the “w1” is “3”, the “w2” is “2”, and the “d” is “0.05”. However, in a case where the area of the dielectric layer  1322  is made smaller or the thickness of the dielectric layer  1322  is made longer, even in a case where “εr” is “10000”, the capacitance of the dielectric layer  1322  is 10 pF or more and 1000 pF or less. That is, a relative permittivity of the dielectric layer  1322  necessary for the dielectric layer  1322  to satisfy the capacitance of 10 pF or more and 1000 pF or less may be “10000” or less. 
     As described above, in the present exemplary embodiment, the dielectric layer  1322  has an area of 10 -7  m 2  or more and 10 -3  m 2  or less, a thickness of 5×10 -7  m or more and 10 -4  m or less, and a relative permittivity of 3 or more and 10 4  or less. In the detection apparatus  1 , the detection target includes a distance of 0.1 m or more and 500 m or less as the distance to the target object T. 
     In particular, in the present exemplary embodiment, the dielectric layer  1322  has an area of 10 -6  m 2  or more and 10 -4  m 2  or less, a thickness of 10 -6  m or more and 5×10 -5  m or less, and a relative permittivity of 3 or more and 10 3  or less. 
      In the present exemplary embodiment, the substrate  13  of the light emitting device  10  has a dielectric layer  1322  having a capacitance of 10 -11  F or more and 10 -9  F or less and being a dielectric substance, and has a capacitor layer  132  provided in the resonant circuit RC. 
     The present exemplary embodiment has described that an electric accumulation element other than the capacitor layer  132  is provided in the light emitting device  10 , but the present invention is not limited to this. 
     For example, not only the capacitor layer  132  but also an electric accumulation element having a thickness smaller than a thickness of the capacitor layer  132  may be provided on the substrate  13 . That is, an electric accumulation element having a thickness larger than a thickness of the capacitor layer  132  and accumulating electric charge may be provided in the resonant circuit RC. In other words, an electric accumulation element, which has a dielectric substance having a thickness larger than a thickness of the dielectric layer  1322 , may not be provided in the resonant circuit RC. 
     For example, in the range where the capacitance in the resonant circuit RC is 10 pF or more and 1000 pF or less, another electric accumulation element may be provided in the resonant circuit RC in addition to the capacitor layer  132 . That is, such an electric accumulation element of which the capacitance of the resonant circuit RC is greater than 1000 pF may not be provided in the resonant circuit RC. 
     Among the electronic circuits of the light emitting device  10 , an electric accumulation element may be provided in a circuit different from the resonant circuit RC. For example, in the light emitting device  10 , an electric accumulation element having a thickness larger than a thickness of the capacitor layer  132  or an electric accumulation element having a capacitance greater than 1000 pF may be connected to the electric path between the power supply  14  and the resonant circuit RC. 
     All the resonant circuits RC may be provided on the substrate  13 . That is, at least a part of the resonant circuit RC may be provided on the substrate  13 . 
     In the present exemplary embodiment, the example in which the detection apparatus  1  is provided in the movable body  2  has been described, but the detection apparatus  1  may be provided in an object of which the position does not change. In a case where the detection apparatus  1  is provided in an object of which the position does not change, the detection apparatus  1  measures the distance to the target object T, and thereby it is possible to grasp the positional relationship between the detection apparatus  1  and the target object T in a space where the target object T is present. 
     In the present exemplary embodiment, an example in which the light emitting device  10  is applied to the detection apparatus  1  has been described, but the target to which the light emitting device  10  is applied is not limited to the detection apparatus  1 . 
     For example, the light emitting device  10  may be applied to an apparatus that transmits light by combining a light emitting device  10 , a light transmission path, and a light receiving unit  20 . The light emitting device  10  may be applied to an apparatus that detects the internal structure of the target object T by irradiating the inside of the target object T such as a living body with the light of the light emitting device  10 . 
     The present exemplary embodiment has described that the VCSEL is used as the light emitting device  10 , but the light emitting device  10  may be a light emitting diode (LED). 
     The present exemplary embodiment has described that the light emitting element  11  emits light toward the upper side (refer to  FIG.  2   ), but the present invention is not limited to this. The light emitting element  11  may emit light toward the lower side, that is, the rear surface of the substrate  13 . 
     A transfer element, which is specialized for controlling the timing at which the light emitting element  11  emits light in a case where the current in the resonant circuit RC is supplied, may be provided on the substrate  13 . As a specific example of the transfer element, the light emitting element  11  may be configured by a light emitting thyristor connected in series with the VCSEL, and the transfer thyristor that supplies a signal to be emitted to the light emitting thyristor may be configured monolithically. 
     Although the exemplary embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the scope described in the above exemplary embodiments. It is clear from the description of the claims that the above-mentioned exemplary embodiment with various modifications or improvements is also included in the technical scope of the present invention. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.