PATENT DOCUMENT

Publication Number: US-10263391-B2
Application Number: US-201715853800-A
Country: US
Kind Code: B2

Title: Horizontal external-cavity laser geometry

Abstract:
An optoelectronic device includes a semiconductor substrate and a vertical-cavity surface-emitting laser (VCSEL) light source formed on the substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate. A block of a transparent material is mounted on the surface of the substrate and forms, with the VCSEL, a resonant cavity at the predefined wavelength having an entrance face that is aligned with the beam axis and an exit face that is laterally displaced with respect to the entrance face along a cavity axis running parallel to the surface of the substrate.

Claims:
The invention claimed is: 
     
       1. An optoelectronic device, comprising:
 a semiconductor substrate; 
 a vertical-cavity surface-emitting laser (VCSEL) light source formed on the substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate; and 
 a block of a transparent material comprising opposing first and second sides that are parallel to the surface of the substrate, such that the first side is adjacent to and mounted on the surface of the substrate, and the block of the transparent material forms, with the VCSEL, a resonant cavity at the predefined wavelength having an entrance face on the first side of the block of the transparent material that is aligned with the beam axis and an exit face on the second side of the block of the transparent material that is laterally displaced with respect to the entrance face along a cavity axis running between the first and second sides of the block of the transparent material in a direction parallel to the surface of the substrate. 
 
     
     
       2. The optoelectronic device of  claim 1 , wherein the block of the transparent material comprises third and fourth sides, not parallel to the first and second sides, wherein the third side is configured to receive the emitted light through the entrance face and to reflect the received light along the cavity axis, and the fourth side configured to reflect the light propagating within the resonant cavity toward the exit face. 
     
     
       3. The optoelectronic device of  claim 2 , wherein the exit face is configured to reflect more than 60% of the light that is incident on the exit face at the predefined wavelength. 
     
     
       4. The optoelectronic device of  claim 2 , wherein the third and fourth sides are planar. 
     
     
       5. The optoelectronic device of  claim 2 , wherein the third and fourth sides are curved. 
     
     
       6. The optoelectronic device of  claim 1 , wherein the exit face comprises an optical element having a curved surface. 
     
     
       7. The optoelectronic device of  claim 1 , wherein the entrance face comprises a grating configured to diffract the emitted light into the block toward the cavity axis. 
     
     
       8. The optoelectronic device of  claim 1 , wherein the exit face comprises a grating configured to retro-reflect a first portion of the light propagating in the cavity while diffracting a second portion of the propagating light out through the exit face. 
     
     
       9. The optoelectronic device of  claim 1 , wherein the first side comprises both the entrance face and a grating configured to retro-reflect a first portion of the light propagating in the cavity and to reflect a second portion of the propagating light towards the exit face. 
     
     
       10. The optoelectronic device of  claim 1 , wherein the light propagates through the resonant cavity from the entrance face to the exit face by total internal reflection between first and second sides of the block of the transparent material. 
     
     
       11. The optoelectronic device of  claim 10 , wherein the block of the transparent material comprises a reflector formed on a third side of the block, perpendicular to the first and second sides, wherein the reflector is configured to reflect the light propagating within the resonant cavity. 
     
     
       12. The optoelectronic device of  claim 10 , wherein the block of the transparent material comprises a grating that is formed in one of the sides of the block and is configured to retro-reflect the light propagating through the cavity. 
     
     
       13. The optoelectronic device of  claim 1 , wherein the block of the transparent material comprises a volume grating adjacent to the entrance face and configured to diffract the emitted light into the block toward the cavity axis. 
     
     
       14. The optoelectronic device of  claim 1 , wherein the block of the transparent material comprises a volume grating configured to diffract a portion of the propagating light out through the exit face. 
     
     
       15. A method for producing an optoelectronic device, the method comprising:
 providing a vertical-cavity surface-emitting laser (VCSEL) light source formed on a semiconductor substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate; and 
 mounting a first side of a block of a transparent material on the surface of the substrate so as to form, with the VCSEL, a resonant cavity at the predefined wavelength having an entrance face on the first side of the block of the transparent material that is aligned with the beam axis and an exit face on a second side of the block of the transparent material, opposite the first side, that is laterally displaced with respect to the entrance face along a cavity axis running between the first and second sides of the block of the transparent material, in a direction parallel to the surface of the substrate, wherein the first side and the second side are parallel to the surface of the substrate. 
 
     
     
       16. The method of  claim 15 , wherein the exit face comprises an optical element having a curved surface. 
     
     
       17. The method of  claim 15 , wherein at least one of the entrance face and the exit face comprises a grating. 
     
     
       18. The method of  claim 15 , wherein the light propagates through the resonant cavity from the entrance face to the exit face by total internal reflection between first and second sides of the block of the transparent material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/555,063, filed Sep. 7, 2017, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optoelectronic devices, and particularly to laser systems. 
     BACKGROUND 
     A laser is made up of an optical gain medium and a resonant cavity, with the gain medium contained inside the resonant cavity. The resonant cavity defines characteristics of the output laser beam. External-cavity lasers typically include additional, external optical elements, defining a resonant cavity. The external cavity is typically significantly longer than the resonant cavity of the laser. The use of an external cavity increases the overall length of the laser, simultaneously reducing the divergence of the output beam and allowing better control over spatial mode and frequency selection. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved designs for semiconductor lasers with external resonator cavities. 
     There is therefore provided, in accordance with an embodiment of the invention, an optoelectronic device, including a semiconductor substrate and a vertical-cavity surface-emitting laser (VCSEL) light source formed on the substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate. A block of a transparent material is mounted on the surface of the substrate and forms, with the VCSEL, a resonant cavity at the predefined wavelength having an entrance face that is aligned with the beam axis and an exit face that is laterally displaced with respect to the entrance face along a cavity axis running parallel to the surface of the substrate. 
     In some embodiments, the block of the transparent material includes a first side adjacent to the substrate and including the entrance face, and a second side opposite the first side and including the exit face, wherein the first side and the second side are parallel to the surface of the substrate. In some of these embodiments, the block of the transparent material includes third and fourth sides, not parallel to the first and second sides, wherein the third side is configured to receive the emitted light through the entrance face and to reflect the received light along the cavity axis, and the fourth side configured to reflect the light propagating within the resonant cavity toward the exit face. In a disclosed embodiment, the exit face is configured to reflect more than 60% of the light that is incident on the exit face at the predefined wavelength. The third and fourth sides may be planar or curved. 
     In a disclosed embodiment, the exit face includes an optical element having a curved surface. 
     In one embodiment, the entrance face includes a grating configured to diffract the emitted light into the block toward the cavity axis. Additionally or alternatively, the exit face includes a grating configured to retro-reflect a first portion of the light propagating in the cavity while diffracting a second portion of the propagating light out through the exit face. 
     In another embodiment, the first side includes both the entrance face and a grating configured to retro-reflect a first portion of the light propagating in the cavity and to reflect a second portion of the propagating light towards the exit face. 
     In some embodiments, the light propagates through the resonant cavity from the entrance face to the exit face by total internal reflection between first and second sides of the block of the transparent material. In one embodiment, the block of the transparent material includes a reflector formed on a third side of the block, perpendicular to the first and second sides, wherein the reflector is configured to reflect the light propagating within the resonant cavity. In another embodiment, the block of the transparent material includes a grating that is formed in one of the sides of the block and is configured to retro-reflect the light propagating through the cavity. 
     In a disclosed embodiment, the block of the transparent material includes a volume grating adjacent to the entrance face and configured to diffract the emitted light into the block toward the cavity axis. Additionally or alternatively, the block of the transparent material includes a volume grating configured to diffract a portion of the propagating light out through the exit face. 
     There is also provided, in accordance with an embodiment of the invention, a method for producing an optoelectronic device. The method includes providing a vertical-cavity surface-emitting laser (VCSEL) light source formed on a semiconductor substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate. A block of a transparent material is mounted on the surface of the substrate so as to form, with the VCSEL, a resonant cavity at the predefined wavelength having an entrance face that is aligned with the beam axis and an exit face that is laterally displaced with respect to the entrance face along a cavity axis running parallel to the surface of the substrate. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-9  are schematic sectional views of optoelectronic devices configured as external-cavity lasers, in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     VCSELs (vertical-cavity surface-emitting lasers) are useful in a wide range of applications, due to their low power consumption, high reliability, scalability to high production volumes, and a reasonable quality of the emitted beam. 
     The spatial mode profile of the emitted beam, as well as its spectral selectivity, can be further improved by coupling an external cavity to the VCSEL. Such a device is typically formed from additional structures within the VCSEL substrate, or from either a dielectric or hollow structure with one end coupled to the output of the VCSEL and receiving the emitted beam. The second end, referred to as an output coupler, passes a portion of the propagating light out from the external cavity, and reflects a portion of the propagating light back into the external cavity, thus producing the required feedback within the laser cavity for sustainable generation of the output laser beam. The quantity of light reflected by the output coupler depends upon optical design requirements, with typical values of 60-95%. The external cavity is typically either placed in close proximity to the VCSEL or monolithically integrated with the VCSEL and fabricated as a part of the VCSEL fabrication process (and an anti-reflective coating may be formed over the VCSEL medium to prevent parasitic reflections within the cavity). The external cavity forms, together with the VCSEL, a coupled resonator, which will affect both the spatial and spectral mode structure of the light emitted from the laser. 
     Orienting the external cavity to be parallel to the beam emitted by the VCSEL (i.e., perpendicular to the surface of the substrate on which the VCSEL is formed) provides a simple and straightforward solution, but the resulting structure adds considerably to the thickness of the light source. Moreover, adding an external cavity that stands perpendicular to the substrate and substantially exceeding the typical substrate thickness, may complicate the fabrication process and require an accurate alignment of the external cavity with respect to the VCSEL. 
     The embodiments of the present invention that are described herein address the above limitations of fabrication and alignment so as to enable the fabrication of compact light sources comprising a VCSEL and an external cavity. The disclosed embodiments use a VCSEL formed on a substrate and configured to emit coherent light at a predefined wavelength along a beam axis perpendicular to a surface of the substrate, and a block of a transparent material mounted on the surface of the substrate and forming with the VCSEL a resonant cavity at the predefined wavelength. The block is typically solid, but may alternatively be hollow. The cavity has an entrance face that is aligned with the beam axis and an exit face that is laterally displaced with respect to the entrance face along a cavity axis running parallel to the surface of the substrate. 
     System Description 
       FIG. 1  is a schematic sectional view of an optoelectronic device  20 , in accordance with an embodiment of the invention. A VCSEL  22  is formed on a semiconductor substrate  24 , using fabrication techniques that are known in the art. VCSEL  22  emits coherent light at a predefined wavelength along a beam axis  26 . A block  28  of a transparent material, such as a suitable glass or other dielectric or semiconductor material, is mounted on a surface  30  of substrate  24 . Block  28  comprises a first side  32 , adjacent to surface  30 , and a second side  34  opposite to the first side, wherein both sides  32  and  34  are parallel to surface  30 . First side  32  comprises an entrance face  36  aligned with beam axis  26 . For purposes of the present embodiment, block  28  is typically 10-50 μm thick and 100-1000 μm long, wherein thickness is the dimension perpendicular to first side  32  and second side  34 , and length is the dimension parallel to these sides. The thickness and length of block  28  represent the external cavity design parameters. Blocks of larger or smaller dimensions may be used, depending on the combination of the application of the optoelectronic device and the emission area of the VCSEL, as well as the fabrication technologies and optical components that are used, as further described below. 
     Block  28  further comprises a third side  38 , which is not parallel to the first and second sides  32  and  34 , respectively. In the present embodiment, third side  38  is planar, and configured to receive the emitted light through entrance face  36  and to reflect the received light along a cavity axis  40  (marked by a dash-dot-dot-line). In the context of the present description and in the claims, the cavity axis is defined as the projection of a central ray of a laser beam  41  propagating in the cavity onto a plane that is parallel to surface  30 . In the present embodiment, cavity axis  40  coincides with this central ray. 
     Second side  34  comprises an exit face  42  that is laterally displaced with respect to entrance face  36  and VCSEL along cavity axis  40 . In other words, in contrast to vertical external cavities, exit face  42  is not aligned with entrance face  36  along beam axis  36 . 
     Block  28  further comprises a fourth side  44 , which is not parallel to first and second sides  32  and  34 . Fourth side  44  in this embodiment is planar, and configured to receive the light directed (i.e., reflected) by third side  38 , and to further direct (reflect) it to exit face  42 . Exit face  42  comprises a mirror coating  46  for output coupling that is configured to reflect typically 60-95% of the light received from fourth side  44 , and to transmit the light, which is not reflected or absorbed by the mirror coating, out from block  28  as coherent light  48 . 
     Third side  38  and fourth side  44  reflect the light propagating within block  28  based on total internal reflection, providing that the entrance angles of the light impinging onto the two sides, as well as the refractive index of the transparent material of block  28 , support total internal reflection. Alternatively, third side  38  and fourth side  44  may be coated with a reflective coating, such as aluminum. Further alternatively, diffraction gratings may be formed on third side  38  and fourth side  44  for achieving deflection (reflection) of the light within the operative diffraction orders of the gratings from the two sides. 
     Block  28 , including third side  38 , fourth side  44 , and exit face  42  with mirror coating  46 , together with VCSEL  22 , thus forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. The circulation of photons within this cavity is illustrated in  FIG. 1  by a circulating arrow  47  within beam  41 . 
     Entrance face  36  may additionally comprise an optical element  50 , such as a homogeneous or gradient-index refractive lens, or a transmitting diffractive optical element, for modifying the mode structure of the light propagating within block  28 . Furthermore, this sort of element with optical power may be incorporated in other embodiments. 
     Exit face  42  may be configured either as a planar or a curved reflective surface, or it may comprise a refractive optical element, such as a lens. Furthermore, this sort of curved surface or refractive optical element may be incorporated in other embodiments. 
     Additional electronic, optoelectronic, and mechanical components (not shown in the figures) may be incorporated in other embodiments. One or more radiation detectors may be added, wherein the detectors are located below block  28  and configured to receive light through the block. Support structures may be added to maintain the mechanical integrity of the optoelectronic device. 
       FIG. 2  is a schematic sectional view of an optoelectronic device  60 , in accordance with another embodiment of the invention. Optoelectronic device  60  is substantially similar to optoelectronic device  20  of  FIG. 1 , and identical or substantially identical items are labelled with the same labels as in  FIG. 1 . The primary difference between optoelectronic device  20  and optoelectronic device  60  is that, whereas the former comprises planar third and fourth sides  38  and  44 , respectively, the latter comprises a curved third side  62  and a curved fourth side  64 . The respective radii of curvature of curved third and fourth sides  62  and  64  may be either positive or negative. In the pictured embodiment, the positive radii of curvature cause the light impinging on the surface to converge after reflection. Alternatively, a negative radius of curvature will cause the light impinging on the surface to diverge after reflection. 
     By choosing the signs and magnitudes of the radii of curvature of curved sides  62  and  64  appropriately, the resonant cavity formed by block  28 , including curved sides  62  and  64  and exit face  42  with mirror coating  46 , together with VCSEL  22 , may be configured either as a stable resonator or as an unstable resonator, as is known in the art. In other embodiments (not shown in the figures), the external cavity may be configured as an unstable resonator by using mirrors with apodized coatings or high diffraction losses. 
       FIG. 3  is a schematic sectional view of an optoelectronic device  80 , in accordance with yet another embodiment of the invention. 
     As in the preceding embodiments, a VCSEL  82  is formed on a substrate  84  and emits coherent light at a predefined wavelength along a beam axis  86 . A block  88  of transparent material is mounted on a surface  90  of substrate  84 . Block  88  comprises a first side  92 , adjacent to surface  90 , and a second side  94  opposite to the first side, wherein both sides  92  and  94  are parallel to surface  90 . First side  92  comprises an entrance face  96  aligned with beam axis  86 . 
     Entrance face  96  comprises a first diffraction grating  98 , which receives and transmits the light emitted by VCSEL  82  through entrance face  96  and redirects the transmitted light by diffraction at an angle toward a cavity axis  100  (marked by a dash-dot-dot-line). Diffraction angle θ b  within block  88  depends on the wavelength λ and the angle of incidence θ i  of the propagating light, the working diffraction order m, the grating period d, and the refractive indices n i  of the medium before the grating and n b  of the material of block  88 : 
                     sin   ⁡     (     θ   b     )       =       1     n   b       ⁢     (         n   i     ⁢           ⁢     sin   ⁡     (     θ   i     )         +       m   ⁢           ⁢   λ     d       )               (   1   )               
When the output beam from VCSEL  82  is incident on grating  98  at normal incidence (θ i =0), the diffracted angle θ b  can be found from equation (1) as:
 
     
       
         
           
             
               
                 
                   
                     θ 
                     b 
                   
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           m 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           λ 
                         
                         
                           
                             n 
                             b 
                           
                           ⁢ 
                           d 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Diffraction grating  98 , as well as the other diffraction gratings described hereinbelow, can be made as surface relief or volume gratings, and can be produced by any suitable method that is known in the art, such as etching or stamping the surface of entrance face  96 , or holographic recording. Here again, cavity axis  100  is defined as the projection of a central ray  102  (marked by a dash-dot-line) of a laser beam  103  propagating in block onto a plane that is parallel to surface  90 . The diffracted light propagates within block  88  by being totally internally reflected between first side  92  and second side  94 . The condition for total internal reflection within block  88  is satisfied when the diffraction angle θ b  exceeds the critical angle θ c  within the block  88 , defined as: 
     
       
         
           
             
               
                 
                   
                     θ 
                     c 
                   
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         1 
                         
                           n 
                           b 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Alternatively, first and second sides  92  and  94  of block  88  can be coated with reflective coatings. 
     Second side  94  comprises an exit face  104  that is laterally displaced with respect to entrance face  96  along cavity axis  100 . Exit face  104  comprises a second diffraction grating  106 , which retro-reflects (in a so-called Littrow configuration) typically 60-95% of the light it receives, and transmits the remaining light, which is not reflected or absorbed by the grating, out from block  88  as coherent light  108 . 
     Block  88 , together with second diffraction grating  106  and VCSEL  82 , thus forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
     First and second diffraction gratings  98  and  106  may have additional optical power for altering the curvature and angle of the wavefront of the propagating light, and thereby modifying the mode structure and the propagation direction of the light within block  88 . Furthermore, this sort of diffraction grating with optical power may be incorporated in other embodiments. 
     Second diffraction grating  106  may be configured to diffract a single diffraction order or multiple diffraction orders of coherent light  108 . Multiple diffraction orders can be useful when a wide output beam spread is desired. Furthermore, this sort of diffraction grating with one or multiple diffraction orders may be incorporated in other embodiments. Although the transmitted portion of light through second diffraction grating  106  is shown exiting block  88  through the exit face  104  at 90° with respect to side  94 , the period and spacing of grating  106  can be selected, in accordance with the diffraction equation (1), so that the output beam will exit the cavity at a different angle with respect to side  94 . 
       FIG. 4  is a schematic sectional view of an optoelectronic device  120 , in accordance with still another embodiment of the invention. Optoelectronic device  120  is substantially similar to optoelectronic device  80  of  FIG. 3 , and identical or substantially identical items are labelled with the same labels as in  FIG. 3 . The primary difference between optoelectronic device  120  and optoelectronic device  80  of  FIG. 3  is that, whereas the former comprises second diffraction grating  106  formed on exit face  104 , the latter comprises a second diffraction grating  122  formed on first side  92  opposite exit face  104 . Second diffraction grating  122  is configured to retro-reflect typically 60-95% of the light it receives. The remaining light, which is not retro-reflected or absorbed by second diffraction grating  122 , is diffracted toward exit face  104 , where it exits out from block  88  as coherent light  124 . 
     Block  88 , together with second diffraction grating  122  and VCSEL  82 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
       FIG. 5  is a schematic sectional view of an optoelectronic device  140 , in accordance with a further embodiment of the invention. 
     A VCSEL  142  is formed on a substrate  144  and emits coherent light at a predefined wavelength along a beam axis  146 . A block  148  of transparent material is mounted on a surface  150  of substrate  144 . Block  148  comprises a first side  152 , adjacent to surface  150 , and a second side  154  opposite to the first side, wherein both sides  152  and  154  are parallel to surface  150 . First side  152  comprises an entrance face  156  aligned with beam axis  146 . 
     Entrance face  156  comprises a first diffraction grating  158 , which receives the emitted light through entrance face  156  and diffracts the received light toward a cavity axis  160  (marked by a dash-dot-dot-line). Here again, the cavity axis is defined as the projection of a central ray  162  (marked by a dash-dot-line) of a laser beam  163  propagating in block  148  onto a plane that is parallel to surface  150 . The diffracted light propagates within block  148  by either being totally internally reflected by first side  152  and second side  154 , or reflected due to surface coatings. 
     Block  148  further comprises a third side  164 , which is perpendicular to first side  152  and second side  154 . Third side  164  comprises a reflector  166 , which may comprise either a mirror or a diffraction grating. 
     Second side  154  comprises an exit face  168  that is laterally displaced with respect to entrance face  156  along cavity axis  160 . Exit face  168  comprises a diffraction grating  170 , which is configured to reflect specularly a majority of the light it receives, and to transmit a small proportion of the remaining light, which is not reflected or absorbed by the second diffraction grating, out from block  148  as coherent light  172 . The diffractive element  168  is passed by the light both in the forward and return directions, and is configured to have a sufficiently high reflectivity in order to maintain a desired ratio between output and recirculated power for the external cavity. 
     Block  148 , together with reflector  166  and VCSEL  82 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
     Modes of light propagating within block  148  may be selected by appropriate choice of the sizes of first diffraction grating  158  and second diffraction grating  170 , as well as using the angular selectivity of reflector  166 . Furthermore, this sort of mode selection may be incorporated in other embodiments. 
       FIG. 6  is a schematic sectional view of an optoelectronic device  180 , in accordance with yet a further embodiment of the invention. Optoelectronic device  180  is substantially similar to optoelectronic device  140  of  FIG. 5 , and identical or substantially identical items are labelled with the same labels as in  FIG. 5 . The primary difference between optoelectronic device  180  and optoelectronic device  140  of  FIG. 5  is that, whereas the former comprises diffraction grating  170  formed on exit face  168 , the latter comprises a diffraction grating  182  formed on first side  152  opposite exit face  168 . Diffraction grating  182  is configured to reflect specularly typically 60-95% of the light it receives, and to diffract the remaining light, which is not reflected or absorbed by the second diffraction grating, towards exit face  168 , where it exits from block  148  as coherent light  184 . 
     Block  148 , together with reflector  166  and VCSEL  142 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
       FIG. 7  is a schematic sectional view of an optoelectronic device  200 , in accordance with an additional embodiment of the invention. 
     A VCSEL  202  is formed on a substrate  204  and configured to emit coherent light at a predefined wavelength along a beam axis  206 . A block of transparent material  208  is mounted on a surface  210  of substrate  204 . Block  208  comprises a first side  212 , adjacent to surface  210 , and a second side  214  opposite to the first side, wherein both sides  212  and  214  are parallel to surface  210 . First side  212  comprises an entrance face  216  aligned with beam axis  206 . 
     Entrance face  216  comprises a first diffraction grating  218 , which receives the emitted light through entrance face  206  and diffracts the received light toward a cavity axis  220  (marked by a dash-dot-dot-line). Here again, the cavity axis is defined as the projection of a central ray  222  (marked by a dash-dot-line) of a laser beam  223  propagating in block  208  onto a plane that is parallel to surface  210 . The diffracted light propagates within block  208  by being totally internally reflected by first side  212  and second side  214 . 
     First side  212  further comprises a second diffraction grating  224  and a third diffraction grating  226 . Second diffraction grating  224  retro-reflects the light propagating within block  208 . Second side  214  comprises an exit face  228  opposite third diffraction grating  226 . Third diffraction grating  226  is configured to reflect specularly typically 60-95% of the light it receives, and to diffract the remaining light, which is not reflected or absorbed by the third diffraction grating, towards exit face  228 , where it exits from block  208  as coherent light  230 . 
     Block  208 , together with second diffraction grating  224  and VCSEL  202 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
     An advantage of the embodiment of  FIG. 7  is that all three diffraction gratings (first diffraction grating  218 , second diffraction grating  224 , and third diffraction grating  226 ) are fabricated on the same side of block  208 . 
       FIG. 8  is a schematic sectional view of an optoelectronic device  240 , in accordance with an alternative embodiment of the invention. 
     A VCSEL  242  is formed on a substrate  244  and emits coherent light at a predefined wavelength along a beam axis  246 . A block of transparent material  248  is mounted on a surface  250  of substrate  244 . Block  248  comprises a first side  252 , adjacent to surface  250 , and a second side  254  opposite to the first side, wherein both sides  252  and  254  are parallel to surface  250 . First side  252  comprises an entrance face  256  aligned with beam axis  246 . 
     Block  248  comprises a first volume grating  258  adjacent to entrance face  256 , which diffracts the emitted light into the block toward a cavity axis  260  (marked by a dash-dot-dot-line). Volume gratings comprise volumetrically distributed modulations of refractive index, and are typically fabricated using a holographic recording process. Photo-thermo-refractive glass is commonly used to produce volume gratings in laser applications, as it has relatively high laser damage threshold. Here, too, the cavity axis is defined as the projection of a central ray of a laser beam  262  propagating in block  248  onto a plane that is parallel to surface  250 . In this embodiment, cavity axis  260  coincides with the central ray of beam  262 . 
     Block  248  comprises a third side  264 , which is perpendicular to first side  252  and second side  254 . Third side  264  comprises a reflector  266 , which may comprise either a mirror or a diffraction grating. 
     Block  248  further comprises a second volume grating  268 . Second side  254  comprises an exit face  270 , aligned with second volume grating  268 . Second volume grating  268  diffracts a portion of the light propagating within block  248  toward exit face  270  and out of block  248  as coherent light  272 . 
     Block  248 , together with reflector  266  and VCSEL  242 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. The circulation of photons within this cavity is illustrated in  FIG. 8  by a circulating arrow  274  within beam  262 . 
       FIG. 9  is a schematic sectional view of an optoelectronic device  280 , in accordance with one more embodiment of the invention. 
     A VCSEL  282  is formed on a substrate  284  and configured to emit coherent light at a predefined wavelength along a beam axis  286 . A block  288  of transparent material is mounted on a surface  290  of substrate  284 . Block  288  comprises a first side  292 , adjacent to surface  290 , and a second side  294  opposite to the first side, wherein both sides  292  and  294  are parallel to surface  290 . First side  292  comprises an entrance face  296  aligned with beam axis  296 . 
     Block  288  comprises a first volume grating  298  adjacent to entrance face  296 , which diffracts the emitted light into the block toward a cavity axis  300  (marked by a dash-dot-dot-line). The cavity axis is defined as the projection of a central ray  302  (marked by a dash-dot-line) of a laser beam  303  propagating in block  288  onto a plane that is parallel to surface  290 . The reflected light propagates within block  288  by being totally internally reflected by first side  292  and second side  294 . 
     Block  288  comprises a third side  304 , which is perpendicular to first side  292  and second side  294 . Third side  304  comprises a reflector  306 , which may comprise either a mirror or a diffraction grating. 
     Block  288  further comprises a second volume grating  308 . Second side  294  comprises an exit face  310 , aligned with second volume grating  308 . Second volume grating  308  diffracts a portion of the light propagating within block  288  towards exit face  310  and out of block  288  as coherent light  312 . 
     Block  288 , together with reflector  306  and VCSEL  282 , forms a resonant cavity at a wavelength within the emission spectrum of the VCSEL. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Metadata:
Filing Date: 20171224
Publication Date: 20190416
Grant Date: 20190416
Priority Date: 20170907
Inventors: SHPUNT, ALEXANDER
SUTTON, ANDREW J.
SOSKIND, YAKOV G.
Assignee: APPLE INC
CPC Classifications: [{"code": "H01S3/08059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01S3/08009", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S3/0071", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/02236", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/183", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/02315", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/0233", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/0235", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/11", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/1838", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/1032", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/0215", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/18341", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S3/08009", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/11", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/1838", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/1032", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01S5/0215", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S5/183", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/141", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01S5/18341", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01S3/0071", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S3/08059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/0233", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01S5/02315", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 65518791