Patent Publication Number: US-2022214554-A1

Title: Optical combiner

Description:
TECHNICAL FIELD 
     The present disclosure relates to an optical combiner. 
     BACKGROUND ART 
     An optical combiner that couples a plurality of different wavelength lights to one optical fiber has been proposed, and various application examples have been proposed. Examples thereof include medical and biological applications such as confocal microscope and flow cytometry, and display applications such as head-up display, virtual reality (VR) glasses, and retinal scanning glasses (see, for example, Patent Literature 1). In recent years, as a means for realizing virtual reality (VR), augmented reality (AR), mixed reality (MR), and the like, ultra-compact display devices have been actively developed, and it is essential to couple lights having greatly different wavelengths, such as red (R), green (G), and blue (B) to one optical fiber as a light source thereof. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2018-510379 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present disclosure is to provide an optical combiner that couples lights having greatly different wavelengths. 
     Solution to Problem 
     The optical combiner of the present disclosure is 
     an optical combiner using a graded index lens that emits light incident on an incident end surface from an emission end surface, the optical combiner including; 
     partial regions each having different distances from the emission end surface of the graded index lens, on the incident end surface of the graded index lens, in which 
     a wavelength of light focused on an optical axis of the emission end surface is determined in advance for each of the partial regions. 
     In the optical combiner of the present disclosure, collimated light generation units using graded index lenses that each convert light having a wavelength determined for each of the partial regions into collimated light may be provided on the partial regions of the incident end surface. 
     The optical combiner of the present disclosure may further include a capillary that covers the graded index lenses in a circumferential direction and is common to all of the graded index lenses provided on the partial regions. 
     Advantageous Effects of Invention 
     According to the present disclosure, there is provided an optical combiner that couples lights having different wavelengths. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of ray tracing in the case of collimated lights having different wavelengths incident on a GRIN lens. 
         FIG. 2  shows an example of a refractive index wavelength dispersion of a GRIN lens. 
         FIG. 3  shows an example of a g value, a length at ¼ pitch, and a difference therein at each wavelength. 
         FIG. 4  illustrates a first example of an optical combiner according to a first embodiment. 
         FIG. 5  illustrates a second example of the optical combiner according to the first embodiment. 
         FIG. 6  illustrates a third example of the optical combiner according to the first embodiment. 
         FIG. 7  illustrates a fourth example of the optical combiner according to the first embodiment. 
         FIG. 8  illustrates a fifth example of the optical combiner according to the first embodiment. 
         FIG. 9  illustrates a sixth example of the optical combiner according to the first embodiment. 
         FIG. 10  illustrates an example of an optical combiner according to a second embodiment. 
         FIG. 11  is an explanatory view of an incident position of the optical combiner according to the second embodiment. 
         FIG. 12  illustrates an example of an optical combiner according to a third embodiment. 
         FIG. 13  illustrates an example of an optical combiner according to a fourth embodiment. 
         FIG. 14  illustrates an example of an optical combiner according to a fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the following embodiments. These examples are merely examples, and the present disclosure can be implemented in a form with various modifications and improvements based on the knowledge of those skilled in the art. Note that components having the same reference numerals in the present specification and the drawings indicate the same components. 
     As a compact optical combiner, a system using a gradient index (GRIN) lens of a refractive index distribution type has been proposed. Since the GRIN lens has a flat lens end surface, the GRIN lens has good connectivity with other optical components, particularly, an optical fiber, and can focus incident lights from a plurality of optical fibers into one optical fiber with a lens diameter of 1 mm or less and a lens length of 10 mm or less. 
     As a technique for coupling laser beams having different wavelengths such as R, G, and B, there are a prism, a filter, a fiber coupler, and the like, but the components thereof are all large. 
     In a planar optical waveguide circuit, a combiner element can be downsized by using a doping material having a high refractive index. However, since a planar optical waveguide circuit using a doping material having a high refractive index has a small core portion for guiding light, it is necessary to devise connection with a normal optical fiber, and thus downsizing of the entire component is difficult. 
     The optical combiner using the GRIN lens can be downsized and has high connectivity with other optical components, in contrast with the above description. However, in a combiner of RGB or the like having greatly different wavelengths, focal positions on the lens optical axis are different each other due to difference in wavelength, and coupling efficiency to the output fiber is deteriorated. 
       FIG. 1  illustrates an example of ray tracing in the case of collimated lights having different wavelengths are incident on a GRIN lens. In  FIG. 1 , ray tracing after focusing the lights is omitted. As illustrated in the drawing, in a case where collimated lights  101 ,  102 , and  103  having different wavelengths from each other are incident on a GRIN lens  111  in parallel to an optical axis  12 , distances z 101 , z 102 , and z 103  in the optical axis direction from an incident end surface  111 A to focal positions F 101 , F 102 , and F 103  for focusing on the optical axis  12  are different due to difference in wavelength. 
     The ray tracing of the GRIN lens is expressed by Equation 1. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     r 
                     1 
                   
                   = 
                   
                     
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           gz 
                           ) 
                         
                       
                       × 
                       
                         r 
                         0 
                       
                     
                     + 
                     
                       
                         1 
                         
                           
                             n 
                             0 
                           
                           ⁢ 
                           g 
                         
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           gz 
                           ) 
                         
                       
                       × 
                       
                         
                           r 
                           · 
                         
                         0 
                       
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     Here, parameters are as follows. 
     r 1 : distance from optical axis  12  (height) 
     g: gradient constant of GRIN lens, also referred to as g value 
     z: position in optical axis direction, i.e., distance in optical axis direction from incident position in the case of incidence position on incident end surface  111 A set as reference 
     r 0 : distance (height) from optical axis  12  at the time of incident on incident end surface  111 A 
     n 0 : refractive index at center of GRIN lens 
     {dot over (r)} 0 : incident angle on incident end surface  111 A 
     When light is incident in parallel to the optical axis  12 , the incident angle on the incident end surface  111 A is 0, and thus the second term on the right side of Equation 1 is 0. Therefore, a condition in which the light beam is on the lens optical axis is as follows. 
       (Expression 2) 
       cos( gz )× r   0 =0  [Equation 2]
 
     The position (z) of intersection with the optical axis  12  except for the optical axis incidence (r 0 =0) is a position in the case of cos (gz)=0. 
     This is 
       (Expression 3) 
         gz=π/ 2  [Equation 3]
 
     That is, this is the case of the ¼ pitch of the ray tracing of the GRIN lens. 
     Further, Equation 3 may be modified into the following Equation 4. 
       (Expression 4) 
         z =(π/2)/ g   [Equation 4]
 
     Therefore, the position (z) of intersection with the optical axis  12  can be obtained from Equation 4. 
     Here, the gradient constant of the GRIN lens, that is, the g value is represented by Equation 5. 
     
       
         
           
             
               
                 
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                     Expression 
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                     ⁢ 
                     5 
                   
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                     Equation 
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                     ⁢ 
                     5 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     g 
                     ⁡ 
                     
                       ( 
                       λ 
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       r 
                     
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                         1 
                         - 
                         
                           
                             ( 
                             
                               
                                 
                                   n 
                                   r 
                                 
                                 ⁡ 
                                 
                                   ( 
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                                   n 
                                   0 
                                 
                                 ⁡ 
                                 
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                                   λ 
                                   ) 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
     Equation 5 indicates that the gradient constant of the GRIN lens, that is, the g value is defined by the refractive index. When there is wavelength dispersion of the refractive index of the GRIN lens material, Equation 3 indicates that the position of intersection with the optical axis  12  differs depending on the wavelengths. 
       FIG. 2  illustrates an example of a refractive index wavelength dispersion of a GRIN lens having an amount of Ge doped of 11.8 wt %.  FIG. 3  shows the result of obtaining the g value, the length at ¼ pitch (z), and the difference between the RGB wavelengths of the GRIN lens having a lens diameter of 0.5 mm from Equations 4 and 5 on the basis of this dispersion. 
       FIG. 3  shows that the difference in the length at ¼ pitch between red with a wavelength of 700 nm and blue with a wavelength of 425 nm is about 200 μm. This is a big problem in practical use when light is made incident on one single mode fiber installed on the emission side because the coupling efficiency varies depending on the wavelengths. 
     Therefore, in the present disclosure, in the optical combiner using the GRIN lens, the incident positions of the lights  101 ,  102 ,  103  are respectively shifted in the optical axis  12  direction in accordance with the shift of the focal positions F 101 , F 102 , and F 103 . As a result, in the present disclosure, even if lights each have greatly different wavelengths, the lights can be incident on one single mode optical fiber  41  installed on an emission end surface  111 B. 
     First Embodiment 
       FIG. 4  illustrates an example of an optical combiner according to the present embodiment. In an optical combiner  11  of the present embodiment, a staircase-like step is provided on an incident end surface  11 A of the optical combiner  11 . 
     The incident end surface  11 A according to the present embodiment includes flat surfaces  11 A_ 101 ,  11 A_ 102 , and  11 A_ 103  perpendicular to an optical axis  12 . A distance z 101  from the flat surface  11 A_ 101  to an emission end surface  11 B, a distance z 102  from the flat surface  11 A_ 102  to the emission end surface  11 B, and a distance z 103  from the flat surface  11 A_ 103  to the emission end surface  11 B are different from each other. In the present embodiment, the flat surfaces  11 A_ 101 ,  11 A_ 102 , and  11 A_ 103  function as partial regions. 
     The distances z 101 , z 102 , and z 103  are each ¼ pitch calculated by applying the lens center refractive index n 0 (λ) at each wavelength λ to Equations 5 and 4. The distances z 101 , z 102 , and z 103  may each be a value obtained by adding an integral multiple of ½ pitch to ¼ pitch. As a result, in the present embodiment, the collimated lights  101 ,  102 ,  103  having different wavelengths can be focused on the optical axis  12  of the emission end surface  11 B. 
     A production method includes, for example, processing the flat surface  11 A_ 102  and the flat surface  11 A_ 103  in a GRIN lens having a length of z 101  by cutting to form rings in a stepwise shape. The production method may also include polishing a base material having the same lens shape and characteristics as those of the flat surface  11 A_ 101  so as to have a thickness of Th 101 , and polishing a base material having the same lens shape and characteristics as those of the flat surface  11 A_ 102  so as to have a thickness of Th 102 , and attaching the base materials to a GRIN lens having a length of z 103 . 
     Specifically, in a case where lights having three kinds of wavelengths illustrated in  FIG. 3  are incident, light having a wavelength of 700 nm is incident on a position  301  with the distance z 101  set to 5.0846 mm, light having a wavelength of 535 nm is incident on a position  302  with the distance z 102  set to 5.0047 mm, and light having a wavelength of 425 nm is incident on a position  303  with the distance z 103  set to 4.8779 mm. In this case, Th 102  is 0.1268 mm, and Th 101  is 0.2067 mm. 
     In the optical combiner produced in this way, light can be emitted from a position  201  on the same optical axis  12  regardless of the wavelengths, and the difference in coupling efficiency to the fiber at the different wavelengths can be eliminated. In the present embodiment, light can be focused on the emission position  201  anywhere on the plane corresponding to RGB on the incident side. Therefore, when the wavelengths are fixed and the optical combiner  11  is mass-produced, the mounting time can be shortened by adopting the present embodiment. 
     In the present embodiment, an example in which the unevenness is formed on the entire incident end surface  11 A has been described, but it is only required to provide the unevenness on at least a part of the incident end surface  11 A. Furthermore, the shapes of the incident position of the collimated light  101  on the flat surface  11 A_ 101 , the incident position of the collimated light  102  on the flat surface  11 A_ 102 , and the incident position of the collimated light  103  on the flat surface  11 A_ 103  are optional. Therefore, as the shape of the end surface  11 A of the present embodiment, any shape in which the distances z 101 , z 102 , and z 103  are different can be adopted. 
     For example, the longitudinal cross-sectional shape of the end surface  11 A may be a concave shape as illustrated in  FIG. 4 , a convex shape as illustrated in  FIG. 5 , or a combination of a concave shape and a convex shape. In addition, the shape of the boundary between the flat surfaces may be a ring shape centered on the optical axis  12 , such as the flat surfaces  11 A_ 101  to  11 A_ 103  illustrated in  FIGS. 4 and 5 , or may be a linear shape such as the flat surfaces  11 A_ 101  to  11 A_ 105  illustrated in  FIGS. 6 and 7 . 
     Furthermore, the flat surfaces  11 A_ 101 ,  11 A_ 102 , and  11 A_ 103  are not limited to be perpendicular to the optical axis  12 , and may be inclined with respect to the optical axis  12 . For example, the flat surfaces  11 A_ 101 ,  11 A_ 102 , and  11 A_ 103  illustrated in  FIG. 4  are preferably inclined symmetrically with respect to the optical axis  12  as illustrated in  FIG. 8 . This facilitates processing of the flat surfaces  11 A_ 101 ,  11 A_ 102 , and  11 A_ 103  with a drill, and can prevent scattering inside the optical combiner  11 . Here, the direction of inclination may be set such that the flat surfaces are inclined in the same direction as illustrated in  FIG. 9 . As the angle of inclination, an optional angle at which antireflection can be obtained can be adopted, but an angle at which deviation from the emission position  201  does not occur is preferable. For example, the angle of inclination with respect to a plane perpendicular to the optical axis  12  can be 10 degrees or less. 
     In addition, the position  201  and the vicinity thereof on the emission end surface  11 B are only required to be flat, and the entire emission end surface  11 B does not need to be flat. For example, as illustrated in  FIG. 7 , the position  201  on the emission end surface  11 B may not be perpendicular to the optical axis  12 , but may be inclined with respect to the optical axis  12 . As a result, reflection of the lights  101 ,  102 , and  103  on the emission end surface  11 B can be prevented. In addition, as illustrated in  FIG. 6 , the emission end surface  11 B may have a shape common to or symmetric with the incident end surface  11 A. 
     Second Embodiment 
       FIG. 10  illustrates an example of an optical combiner according to the present embodiment. In the present embodiment, an incident end surface  11 A of the optical combiner  11  is inclined with respect to an optical axis  12 . As a result, partial regions each having different distances z 101 , z 102 , and z 103  from the emission end surface  11 B are formed on the incident end surface  11 A. 
     In the incident end surface  11 A according to the present embodiment, the distance to the emission end surface  11 B is z 101  at the position on a straight line H 101  of the incident end surface  11 A, the distance to the emission end surface  11 B is z 102  at the position on a straight line H 102  of the incident end surface  11 A, and the distance to the emission end surface  11 B is z 103  at the position on a straight line H 103  of the incident end surface  11 A. In the present embodiment, regions on the straight lines H 101 , H 102 , and H 103  in the incident end surface  11 A function as partial regions. 
     In the present embodiment, collimated lights  101 ,  102 , and  103  each having different wavelengths are incident on incident positions  301 ,  302 , and  303  in parallel to the lens optical axis  12 . The distances z 101 , z 102 , and z 103  in the optical axis direction of the ray tracing from each of the incident positions to the position at which the collimated lights intersect with the lens optical axis are each ¼ pitch calculated from each wavelength as in the first embodiment. As a result, in the present embodiment, the collimated lights  101 ,  102 , and  103  of respective wavelengths can be focused on the optical axis  12  of the emission end surface  11 B. 
     For example, as long as the light has a predetermined wavelength, the light can be focused on the optical axis  12  of the emission end surface  11 B even when the light is incident from any position on the straight line H 102 . Specifically, as illustrated in  FIG. 11 , the incident end surface  11 AC in the C-C′ cross section and the incident end surface  11 AE in the E-E′ cross section have different angles from each other with respect to the optical axis  12 . However, each of the distances from the emission end surface  11 B to the incident end surfaces is the same z 102 . Therefore, if light  102 C incident from an incident position  302 C and light  102 E incident from an incident position  302 E have the same wavelength, they are focused on the same emission position  201 . 
     In the present example, unlike the example of the first embodiment, it is only required to form the obliquely cut incident end surface  11 A without performing special processing on the incident surface, and to select an incident position at which each distance is ¼ pitch, which is practically advantageous. The present embodiment can address any wavelength and further prevent reflection. 
     Note that, in the present embodiment, an example has been described in which the incident positions  301  to  303  are arranged on the half surface of the incident end surface  11 A, but the present disclosure is not limited thereto. For example, the incident positions  301  to  303  may be arranged on the entire incident end surface  11 A. 
     In the incident end surface  11 A of the present embodiment, the angle of the incident end surface  11 A with respect to the optical axis  12  may not be constant. For example, the inclination of the incident end surface  11 A may be a ring shape centered on the optical axis  12  as illustrated in  FIGS. 4 and 5 . 
     In addition,  FIG. 10  illustrates an example in which the entire surface of the incident end surface  11 A is a plane having a constant angle with respect to the optical axis  12 , but the present disclosure is not limited thereto. For example, a plane having a constant angle with respect to the optical axis  12  may be a part of the incident end surface  11 A. 
     Third Embodiment 
       FIG. 12  illustrates an example of an optical combiner according to the present embodiment. In the optical combiner  11  of the embodiment, collimated light generation units  21 ,  22 , and  23  are connected to an incident end surface  11 A. 
     As lights  101  to  103  incident on the incident end surface  11 A are closer to parallel to an optical axis  12 , light focusing properties at an emission position  201  become better, and coupling efficiency to the optical fiber (not illustrated) at an emission end surface  11 B increases. In light of this, the present embodiment includes the collimated light generation units  21 ,  22 , and  23  that convert the lights  101  to  103  incident on the incident end surface  11 A into collimated lights. 
     The collimated light generation units  21 ,  22 , and  23  are GRIN lenses having the same lens characteristics, and have lens lengths z 21 , z 22 , and z 23  for converting light emitted from an optical fiber  31  into collimated light. In the present disclosure, each wavelength of light focused on the optical axis of the emission end surface  11 B is determined in advance for each partial region of the incident end surface  11 A. Thus, the lens lengths z 21 , z 22 , and z 23  of the collimated light generation units  21 ,  22 , and  23  are set to be ¼ pitch corresponding to the wavelength determined for each partial region of the incident end surface  11 A. However, the lens length may be a value obtained by adding an integral multiple of ½ pitch to ¼ pitch. 
     The optical fibers  31 ,  32 , and  33  are connected to the incident end surfaces of the collimated light generation units  21 ,  22 , and  23 . When lights from the optical fibers  31 ,  32 , and  33  are incident on the collimated light generation units  21 ,  22 , and  23 , collimated lights are emitted from the collimated light generation units  21 ,  22 , and  23 . 
     When the optical axes of the collimated light generation units  21 ,  22 , and  23  are arranged in parallel to the optical axis  12 , collimated lights from the collimated light generation units  21 ,  22 , and  23  are incident on the incident end surface  11 A in parallel to the optical axis  12 . As a result, in the present embodiment, light focusing properties at the emission position  201  can be enhanced, and coupling efficiency to the optical fiber (not illustrated) connected to the emission end surface  11 B can be enhanced. 
     In the present embodiment, the application example to the optical combiner  11  of the second embodiment is used; however, the present disclosure is not limited to the optical combiner  11  of the second embodiment, and may be the optical combiner  11  of the first embodiment. 
     Fourth Embodiment 
       FIG. 13  illustrates an example of an optical combiner according to the present embodiment. collimated light generation units  21 ,  22 , and  23  having the same lens characteristics are used in the third embodiment, but in the present embodiment, the collimated light generation units  21 ,  22 , and  23  having different lens characteristics from each other are used. 
     In the present embodiment, a distance from an emission end surface  11 B to the incident end surface of the collimated light generation unit  21 , a distance from the emission end surface  11 B to the incident end surface of the collimated light generation unit  22 , and a distance from the emission end surface  11 B to the incident end surface of the collimated light generation unit  23  are all z 200 . 
     That is, a lens length z 21  of the collimated light generation unit  21 , a lens length z 22  of the collimated light generation unit  22 , and a lens length z 23  of the collimated light generation unit  23  are set such that each lens length difference is canceled out by difference in pitch length corresponding to each wavelength in the optical combiner  11 . For example, a difference between the lens length z 23  and the lens length z 21  is equal to a difference between a distance z 101  and a distance z 103 . 
     Furthermore, the collimated light generation unit  21  has a gradient constant such that the lens length z 21  is ¼ pitch relative to a wavelength corresponding to an incident position  301 . The collimated light generation unit  22  has a gradient constant such that the lens length z 22  is ¼ pitch relative to a wavelength corresponding to an incident position  302 . The collimated light generation unit  23  has a gradient constant such that the lens length z 23  is ¼ pitch relative to a wavelength corresponding to an incident position  303 . 
     In the present embodiment, by adopting the above configuration, the incident positions of the collimated light generation units  21 ,  22 , and  23  can be arranged on the same plane. Therefore, in the present embodiment, an optical fiber array can be used for optical fibers  31 ,  32 , and  33 , and the collimated light generation units  21 ,  22 , and  23  are easily connected to the optical fiber array. 
     In the present embodiment, the application example to the optical combiner  11  of the second embodiment is used; however, the present disclosure is not limited to the optical combiner  11  of the second embodiment, and may be the optical combiner  11  of the first embodiment. 
     Fifth Embodiment 
       FIG. 14  illustrates an example of an optical combiner according to the present embodiment. In the optical combiner  11  according to the present embodiment, the collimated light generation units  21 ,  22 , and  23  of the fourth embodiment are housed in a common capillary  30 . 
     In the fourth embodiment, the incident end surfaces of the collimated light generation units  21 ,  22 , and  23  are arranged on the same plane in a state in which emission end surfaces of the collimated light generation units  21 ,  22 , and  23  are connected to the incident end surface  11 A. Therefore, the collimated light generation units  21 ,  22 , and  23  can be fixed by the common capillary  30 . 
     The outer shape of the capillary  30  is optional, and is, for example, a cylindrical shape having an outer diameter common to the optical combiner  11 . In the present embodiment, since the optical combiner  11  includes the capillary  30 , the positions of the collimated light generation units  21 ,  22 , and  23  on the incident end surface  11 A are fixed. Therefore, in the present embodiment, by connecting the capillary  30  to the incident end surface  11 A, the light generation units  21 ,  22 , and  23  having a lens lengths suitable for the position of the incident end surface  11 A can be connected. In the present embodiment, the collimated light generation units  21 ,  22 , and  23  can be connected to the optical combiner  11  without adjusting the individual lens lengths of the light generation units  21 ,  22 , and  23  and individually adjusting the connection positions to the optical combiner  11 , whereby mountability is enhanced, and improvement of the yield and cost reduction can be achieved. 
     Note that, although an output optical fiber is not drawn in the explanatory views used as a reference in the present embodiment, in the optical combiner according to the present disclosure, one optical fiber may be connected to the output end  11 B. The output optical fiber is not limited to a single mode fiber, and may be a multi-mode fiber. 
     In addition, with respect to light wavelengths, it is obvious that wavelengths other than the exemplified wavelengths can be applied, and is not limited to the exemplified wavelengths. In addition, a plurality of collimated light generation units or optical fibers may be connected to one partial region. 
     The present disclosure can be applied to the information communication industry. 
     REFERENCE SIGNS LIST 
     
         
           11  Optical combiner 
           11 A,  111 A Incident end surface 
           11 A_ 101 ,  11 A_ 102 ,  11 A_ 103  Flat surface 
           11 B,  111 B Emission end surface 
           12  Optical axis 
           21 ,  22 ,  23  collimated light generation unit 
           31 ,  32 ,  33 ,  41  Optical fiber