Patent Publication Number: US-2022224081-A1

Title: Surface-emitting laser

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. Provisional Application No. 63/136,206, filed on Jan. 12, 2021, and U.S. Provisional Application No. 63/143,927, filed on Jan. 31, 2021. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a light-emitting device and, in particular, to a surface-emitting laser. 
     2. Description of Related Art 
     A semiconductor laser can be classified into a surface-emitting laser and an edge-emitting laser. The surface-emitting laser has various types, including a vertical cavity surface-emitting laser (VCSEL), a photonic crystal surface-emitting laser, a distributed feedback surface-emitting laser, etc. 
     In a distributed laser, a grating and reflection are generally continuous along a resonance cavity, instead of just being at two opposite ends of the resonance cavity. This changes the modal behavior considerably and makes the laser more stable. 
     However, for a distributed feedback surface-emitting laser or a photonic crystal surface-emitting laser, a metal electrode at the light-emitting side of the laser has an opening for light output, so that current is not spread uniformly in an active region below the opening and current crowding effect is thus formed. Therefore, the beam quality and beam control of the laser are adversely affected. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention is directed to a surface-emitting laser, which has good current spreading, good beam quality, and good beam control. 
     An embodiment of the invention provides a surface-emitting laser including a cladding layer, an active region, a first grating, a plurality of second gratings, a first electrode, and a second electrode. The active region is disposed on the cladding layer. The first grating is disposed on the active region. The second gratings are disposed on the active region and separately distributed among the first grating. A diffraction order of the first grating is different from a diffraction order of the second gratings. The first electrode is electrically connected to the cladding layer. The second electrode covers at least the first grating. 
     In the surface-emitting laser according to the embodiment of the invention, since the second gratings are separately distributed among the first grating, the first grating is used for laser feedback for more efficient reflection of the light emitted by the active region, and the second gratings are used for output coupling by diffraction. Therefore, the laser characteristics of the surface-emitting laser are better controlled. As a result, the surface-emitting laser has good beam quality and good beam control. Moreover, in the surface-emitting laser according to the embodiment of the invention, the second electrode covers at least the first grating, so that the surface-emitting laser has good and uniform current spreading, and current crowding effect is effectively reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1A  is a schematic perspective view of a surface-emitting laser according to an embodiment of the invention. 
         FIG. 1B  is a schematic cross-sectional view of the surface-emitting laser in  FIG. 1A . 
         FIG. 2A ,  FIG. 2B , and  FIG. 2C  are schematic local top views of three embodiments of the first grating and the second gratings of the surface-emitting laser in  FIG. 1B . 
         FIG. 3A ,  FIG. 3B , and  FIG. 3C  are schematic local cross-sectional views of three embodiments of the first grating and the second gratings of the surface-emitting laser in  FIG. 1B . 
         FIG. 4  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. 
         FIG. 5  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. 
         FIG. 6  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. 
         FIG. 7  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. 
         FIG. 8  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. 
         FIG. 9  is a schematic view showing light spots of the sub-beams and the openings of the surface-emitting laser in  FIG. 1A  and  FIG. 1B  respectively in a far-field and a near-field. 
         FIG. 10  is a schematic top transparent view of the surface-emitting laser showing the second electrode, the first grating, and the second gratings according to another embodiment of the invention. 
         FIG. 11A  is a schematic top transparent view of the surface-emitting laser showing the second electrode and the second gratings according to another embodiment of the invention. 
         FIG. 11B  is a perspective view of the surface-emitting laser in  FIG. 11A  showing different sub-beams due to different sub-electrodes. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 1A  is a schematic perspective view of a surface-emitting laser according to an embodiment of the invention, and  FIG. 1B  is a schematic cross-sectional view of the surface-emitting laser in  FIG. 1A . Referring to  FIG. 1A  and  FIG. 1B , the surface-emitting laser  100  in this embodiment includes a cladding layer  110 , an active region  120 , a first grating  200 , a plurality of second gratings  300 , a first electrode  130 , and a second electrode  140 . The active region  120  is disposed on the cladding layer  110 . The first grating  200  is disposed on the active region  120 . The second gratings  300  are disposed on the active region  120  and separately distributed among the first grating  200 . A diffraction order of the first grating  200  is different from a diffraction order of the second gratings  300 . The first electrode  130  is electrically connected to the cladding layer  110 . The second electrode  140  covers at least the first grating  200 . 
     In this embodiment, the second electrode  140  covers the first grating  200  and has a plurality of openings  142  above the second gratings  200 . The second electrode  140  is, for example, a metal electrode layer. Moreover, the diffraction order of the first grating  200  is greater than or equal to 1, and the diffraction order of the second gratings  300  is greater than or equal to 2. In this embodiment, the first grating  200  is a first order grating, and the second gratings  300  are second order gratings. When the first electrode  130  and the second electrode  140  are applied with a forward voltage, the active region  120  emits light  122 . The first grating  200  transmits and reflects the light  122  in a horizontal direction parallel to the active region  120  to form distributed feedback, i.e. laser feedback. The second gratings  300  diffracts the light  122  in a vertical direction perpendicular to the active region to form output coupling. The light  122  diffracted by the second gratings  300  passes through the openings  142  to travel out of the surface-emitting laser  100 . In other embodiments, when the second gratings  300  are third order gratings, the second gratings  300  diffract the light  122  along inclined directions. For example, the inclined directions and a normal direction of the active region  120  have an included angle of 60 degrees. When the diffraction order of the second gratings  300  is greater than or equal to 4, the second gratings  300  have other different inclined directions. 
     In the surface-emitting laser  100 , the laser feedback and output coupling are separated for better control of laser characteristics. The first grating  200  (i.e. the first order grating) is used for distributed feedback for more efficient reflection, and the second gratings  300  (i.e. the second order grating) is used below the openings  142  (i.e. output windows) for output coupling. Therefore, the surface-emitting laser  100  has good beam quality and good beam control. Moreover, the second electrode  140  (i.e. a metal electrode layer) covers the first grating  200 , so that current can be spread horizontally and uniformly in the second electrode  140 . As a result, the surface-emitting laser  100  has good and uniform current spreading, and current crowding effect is effectively reduced. 
     In this embodiment, the surface-emitting laser  100  further includes a substrate  150  disposed between the cladding layer  110  and the first electrode  130 . The first electrode  130  is, for example, a metal electrode layer covering a bottom surface of the substrate  150 . In this embodiment, the surface-emitting laser  100  further includes a contact layer  160  disposed between the first grating  200  and the second electrode  140  and covers the second gratings  300 . The contact layer  160  form good electrical contact for the first grating  200  and the second electrode  140 . 
     In this embodiment, the cladding layer  110  is an n-type semiconductor layer, and the first grating  200  and the second gratings  300  form a p-type semiconductor layer. However, in other embodiments, the cladding layer  110  is a p-type semiconductor layer, and the first grating  200  and the second gratings  300  form an n-type semiconductor layer. 
     In this embodiment, the second gratings  300  diffract light  122  emitted by the active region  120  into a plurality of sub-beams (e.g. the sub-beams  1221  and  1222  shown in  FIG. 1B ) traveling out of the surface-emitting laser  100 , and adjacent sub-beams  1221  and  1222  interfere with each other. For example, the sub-beam  1221  and the sub-beam  1222  may form constructive interference, so as to form a light spot in a far-field having good beam quality. The first grating  200  transmits and strongly couples light  122  between the second gratings  300  so as to ensure the coherence between the sub-beams. 
       FIG. 2A ,  FIG. 2B , and  FIG. 2C  are schematic local top views of three embodiments of the first grating and the second gratings of the surface-emitting laser in  FIG. 1B . Referring to  FIG. 1B  and  FIG. 2A  first, in this embodiment, the first grating  200  and the second gratings  300  are one-dimensional gratings. In this embodiment, each of the first grating  200  and the second gratings  300  includes a plurality of straight strips  210 ,  310  parallel to each other. Specifically, the first grating  200  includes a plurality of straight strips  210  arranged in a first direction D 1 , and the second grating  300  includes a plurality of straight strips  310  arranged in the first direction D 1 . Each of the straight strips  210  extends along a second direction D 2 , and Each of the straight strips  310  extends along the second direction D 2 . The first direction D 1  may be perpendicular to the second direction D 2 . In this embodiment, a pitch P 2  of the straight strips  310  of the second gratings  300  is greater than a pitch P 1  of the straight strips  210  of the first grating  200 . 
     Referring to  FIG. 1B  and  FIG. 2B , in this embodiment, the first grating  200  and the second gratings  300  are two-dimensional gratings. Each of the first grating  200  and the second gratings  300  includes two-dimensional line array or two-dimensional pillar array. Specifically, the first grating  200  may include two-dimensional line array  210   a , and the second gratings  300  may include two-dimensional line array  310   a . Alternatively, in another embodiment, the first grating  200  may include two-dimensional pillar array  210   b , and the second gratings  300  may include two-dimensional pillar array  310   b . In other embodiments, Each of the first grating  200  and the second gratings  300  may include two-dimensional polygon array. 
     Referring to  FIG. 1B  and  FIG. 2C , in this embodiment, each of the first grating  200  and the second gratings  300  includes two-dimensional hole array. Specifically, the first grating  200  includes two-dimensional hole array  210   c , and the second grating  300  includes two-dimensional hole array  310   c.    
     In  FIG. 2A ,  FIG. 2B , and  FIG. 2C , a pitch P 2  of the two-dimensional hole, line, or pillar array  310   c ,  210   a ,  210   b  of the second gratings  300  is greater than a pitch P 1  of the two-dimensional hole, line, or pillar array  210   c ,  210   a ,  210   b  of the first grating  200 . 
       FIG. 3A ,  FIG. 3B , and  FIG. 3C  are schematic local cross-sectional views of three embodiments of the first grating and the second gratings of the surface-emitting laser in  FIG. 1B . Referring to  FIG. 1B ,  FIG. 3A ,  FIG. 3B , and  FIG. 3C , each of the first grating  200  and the second gratings  300  includes a plurality of periodical units periodically arranged. Specifically, the first grating  200  includes a plurality of periodical units  220  periodically arranged, and the second grating  300  includes a plurality of periodical units  320  periodically arranged. A profile of each of the periodical units  220  and  320  in a cross-section perpendicular to the active region  120  is a step shape (as shown in  FIG. 3A ), a wave shape (as shown in  FIG. 3B ), or a triangular shape (as shown in  FIG. 3C ). In these embodiments, a pitch P 2  of the periodical units  320  of the second gratings  300  is greater than a pitch P 1  of the periodical units  220  of the first grating  200 . 
     In this embodiment, the material of the substrate includes gallium nitride (GaN), gallium arsenide (GaAs), indium phosphide (InP), gallium antimonide (GaSb), indium arsenide (InAs), sapphire, any other suitable material, or a combination thereof. The material of the cladding layer  110  may include GaN, aluminum gallium arsenide (AlGaAs), aluminum gallium arsenic antimonide (AlGaAsSb), any other suitable material, or a combination thereof. The active region  120  may have quantum structure, for example, quantum wells, quantum dots, etc. The material of the active region  120  may include indium arsenide (InAs) and gallium arsenide (GaAs); indium gallium arsenide (InGaAs) and gallium aluminum arsenide (GaAlAs); indium gallium arsenic antimonide (InGaAsSb) and aluminum gallium arsenic antimonide (AlGaAsSb); indium gallium nitride (InGaN) and gallium nitride (GaN); indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN); indium arsenide (InAs) and indium phosphide (InP); indium arsenic antimonide (InAsSb) and indium phosphide (InP); other suitable materials or a combination thereof. The material of the first grating  200  and the second gratings  300  may include GaAs, InP, GaSb, AlGaAs, AlGaAsSb, any other suitable material, or a combination thereof. The material of the contact layer  160  may include GaAs, InP, GaN, GaSb, gallium phosphide (GaP), InAs, AlGaAs, AlGaAsSb, transparent conductive material, any other suitable material, or a combination thereof, wherein the transparent conductive material may include indium tin oxide (ITO), indium zinc oxide (IZO), aluminum doped zinc oxide (AZO), any other suitable material, or a combination thereof. 
       FIG. 4  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. Referring to  FIG. 4 , the surface-emitting laser  100   d  in this embodiment is similar to the surface-emitting laser  100  in  FIG. 1B , and the main difference therebetween is as follows. In the surface-emitting laser  100   d  according to this embodiment, the cladding layer  110  and the first electrode  130   d  are disposed on the substrate  150 , and the cladding layer  110  and the first electrode  130   d  are disposed on the same side of the substrate  150 . 
       FIG. 5  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. Referring to  FIG. 5 , the surface-emitting laser  100   e  in this embodiment is similar to the surface-emitting laser  100   d  in  FIG. 4 , and the main difference therebetween is as follows. In the surface-emitting laser  100   d  in  FIG. 4 , the second gratings  300  diffract the light  122  emitted by the active region  120  to pass through the openings  142 , so that the light  122  travels out of the surface-emitting laser  100   d  through the openings  142 . However, in the surface-emitting laser  100   e  in  FIG. 5 , the second electrode  140   e  covers the first grating  200  and the second gratings  300 ; that is, the second electrode  140   e  does not have openings  142  as shown in  FIG. 4 . Moreover, the substrate  150   e  is pervious to the light  122  emitted by the active region  120 . That is to say, the second gratings  300  diffract the light  122  upwards and downwards. The upward light  122  is then reflected by the second electrode  140   e  (e.g. a metal electrode layer) downwards. The downward light  122  passes through the substrate  150   e  and then travels out of the surface-emitting laser  100   e . Therefore, the surface-emitting laser  100   e  is a bottom emitting laser. 
       FIG. 6  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. Referring to  FIG. 6 , the surface-emitting laser  100   f  in this embodiment is similar to the surface-emitting laser  100  in  FIG. 1B , and the main difference therebetween is as follows. The surface-emitting laser  100   f  according to this embodiment further includes a distributed Bragg reflection (DBR) layer  170 , wherein the cladding layer  110  is disposed between the active region  120  and the DBR layer  170 . In this embodiment, the DBR layer  170  is disposed between the cladding layer  110  and the substrate  150 . The second gratings  300  diffract the light  122  upwards and downwards. The downward light  122  is reflected by the DBR layer  170  upwards. The upward light  122  passes through the openings  142  and travels out of the surface-emitting laser  100   f . The DBR layer  170  prevents the light  122  from leaking from the bottom of the surface-emitting laser  100   f , so that the light efficiency of the surface-emitting laser is improved. 
       FIG. 7  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. Referring to  FIG. 7 , the surface-emitting laser  100   g  in this embodiment is similar to the surface-emitting laser  100  in  FIG. 1B , and the main difference therebetween is as follows. The surface-emitting laser  100   g  according to this embodiment further includes a mirror layer  180  disposed on side surfaces of the cladding layer  110 , the active region  120 , and the first grating  200 . In this embodiment, the mirror layer  180  is also disposed on side surfaces of the first electrode  130 , the substrate  150 , the contact layer  160 , and the second electrode  140 . When the first grating  200  transmits and reflects the light  122  in a horizontal direction, the mirror layer  180  serves as a facet mirror to reflects the light  122  back to the first grating  200  and prevents the light  122  from leaking out from the side surface of the first grating  200 . Therefore, the light efficiency of the surface-emitting laser  100  is improved. 
       FIG. 8  is a schematic cross-sectional view of the surface-emitting laser according to another embodiment of the invention. Referring to  FIG. 8 , the surface-emitting laser  100   h  in this embodiment is similar to the surface-emitting laser  100   g  in  FIG. 7 , and the main difference therebetween is as follows. The surface-emitting laser  100   h  according to this embodiment further includes a DBR layer  170 , wherein the cladding layer  110  is disposed between the active region  120  and the DBR layer  170 . In this embodiment, the DBR layer  170  is disposed between the cladding layer  110  and the substrate  150 . Besides, in this embodiment, the mirror layer  180  is also disposed on a side surface of the DBR layer  170 . 
       FIG. 9  is a schematic view showing light spots of the sub-beams and the openings of the surface-emitting laser in  FIG. 1A  and  FIG. 1B  respectively in a far-field and a near-field. Referring to  FIG. 1B  and  FIG. 9 , in the surface-emitting laser  100 , the openings  142  may be in an order arrangement or in a random arrangement. The second gratings  300  diffract the light  122  emitted by the active region  120  into a plurality of sub-beams  1220 . The sub-beams  1220  pass through the openings  142  in the near-field, respectively, and travel out of the surface-emitting laser  100  to the far-field, so as to form a plurality of light spots  1223  in the far-field, respectively. According to the pattern design of the openings  142  in the near-field and the diffraction order of the second gratings  300 , the pattern of the light spots  1223  in the far-field may be designed. Sub-beams  1220  from adjacent openings  142  may interfere with each other to form beam shaping so as to form a different light spot pattern in the far-field. The pattern of the light spots  1223  may be in an order arrangement, a random arrangement. In another embodiment, due to beam shaping, the sub-beams  1220  may form order or random bright lines in the far-field. In another embodiment, multiple sub-beams  1220  interference with properly designed openings  142  arrangement can result very small beam divergence and excellent beam quality. 
       FIG. 10  is a schematic top transparent view of the surface-emitting laser showing the second electrode, the first grating, and the second gratings according to another embodiment of the invention. Referring to  FIG. 10 , the surface-emitting laser  100   i  in this embodiment is similar to the surface-emitting laser  100  in  FIG. 1A  and  FIG. 1B , and the main difference therebetween is as follows. In the surface-emitting laser  100   i , the second electrode  140  includes a plurality of separate sub-electrodes  141  respectively covering different sets of the second gratings  300 , wherein the sub-electrodes  141  are independently driven. In this embodiment, the sub-electrodes  141  may be independently driven by a driver. For example, when the surface-emitting laser  100   i  needs to emit a light beam to an object in a long distance, all sub-electrodes  141  may be driven, and all second gratings  300  diffract the light  122  into the sub-beams  1220  to form a light beam with high intensity. When the surface-emitting laser  100   i  needs to emit a light beam to an object in a short distance, one or some of the sub-electrodes may be driven, and the other sub-electrodes may not be driven. For example, the middle one or five second gratings may diffract the light  122  into one sub-beam  1220  or five sub-beams  1220  to form a light beam with lower intensity, which may save the power consumption of the surface-emitting laser  100   i . In another embodiment, the diffraction order of the second gratings  300  may be 2, 3 or more, or a combination thereof, so that the light patterns provided by the surface-emitting laser  100   i  may have more variations. In another embodiment, the first electrode  130  may include a plurality of separate sub-electrodes respectively under different sets of the second gratings  300 , wherein the sub-electrodes are independently driven. The surface-emitting laser  100   i  in this embodiment may achieve flexible application in three-dimensional sensing field, better spatial resolution. Moreover, the sub-beams  1220  may be separately designed for better output coupling and beam control. Additionally, the sub-beams  1220  may be separately turned on for power adjustment or beam steering. 
       FIG. 11A  is a schematic top transparent view of the surface-emitting laser showing the second electrode and the second gratings according to another embodiment of the invention.  FIG. 11B  is a perspective view of the surface-emitting laser in  FIG. 11A  showing different sub-beams due to different sub-electrodes. Referring to  FIG. 11A  and  FIG. 11B , the surface-emitting laser  100   j  in this embodiment is similar to the surface-emitting laser  100   i  in  FIG. 10 , and the main difference therebetween is as follows. In the surface-emitting laser  100   j , the sub-electrodes  141  are concentrically arranged. Different sub-electrodes  141  may cover different sets of second gratings  300  having different diffraction orders. For example, the set of second gratings  300  covered by the middle sub-electrode  141  may have the second diffraction order, so that the second gratings  300  covered by the middle sub-electrode  141  may emit the sub-beams  1220  in the front direction. The set of second gratings  300  covered by the first inner ring of the sub-electrodes  141  may have the third diffraction order, and emit sub-beams  1220  in a divergent direction. When the sets of second gratings  300  covered by different rings of the sub-electrodes have different diffraction orders, the sets of second gratings  300  may respectively emit sub-beams  1220  with different divergent angle. The sub-electrodes  141  are independently driven. Therefore, by driving one, some, or all of the sub-electrodes  141 , the divergent angle of a light beam of the surface-emitting laser  100   j  may be adjusted. Moreover, when the sub-electrodes  141  are separately turned on, the surface-emitting laser  100   i  may achieve power adjustment, beam steering, or large angle sweeping. 
     In conclusion, in the surface-emitting laser according to the embodiments of the invention, since the second gratings are separately distributed among the first grating, the first grating is used for laser feedback for more efficient reflection of the light emitted by the active region, and the second gratings are used for output coupling by diffraction. Therefore, the laser characteristics of the surface-emitting laser are better controlled. As a result, the surface-emitting laser has good beam quality and good beam control. Moreover, in the surface-emitting laser according to the embodiments of the invention, the second electrode covers at least the first grating, so that the surface-emitting laser has good and uniform current spreading, and current crowding effect is effectively reduced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.