Abstract:
A light receiving module includes a substrate, a light receiving element mounted on the substrate, and a resin package for covering the light receiving element. The top portion of the resin package is formed with a lens for collecting external light to the light receiving element. The lens includes a light incident surface formed with irregularities for light dispersion. The light receiving module further includes a tubular body accommodated in the resin package. The tubular body is tapered as proceeding toward the light receiving element, and has an inner surface for light reflection. Light collected by the lens is reflected by the inner surface of the tubular body, to be detected by the light receiving element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light receiving module used for optical communication. 
         [0003]    2. Description of the Related Art 
         [0004]    In optical communication, light is used as a communication medium, for transmitting a large amount of information at high-speed. An optical communication system includes a light emitting module as a light source and a light receiving module for detecting light emitted by the light emitting module. Examples of conventional light emitting module and light receiving module are disclosed in JP-A-2005-17678.  FIG. 5  in this document illustrates a light emitting module provided with a convex lens for collecting light. 
         [0005]      FIG. 10  of the present application illustrates a light receiving module as a reference example for better understanding of the technique according to the present invention. The illustrated light receiving module X includes a substrate  91 , a photodiode  92  mounted on the substrate  91 , and a resin package  93  covering the photodiode  92 . The resin package  93  is formed with a lens  93   a  for collecting light to the photodiode  92 . The lens  93   a  is a convex lens and focuses light entering from the outside on the photodiode  92  (see solid lines shown in  FIG. 10 ). 
         [0006]    The above light receiving module leaves room for improvement in the following points. Recently, as a way of increasing communication speed, a photodiode of the light receiving module has been downsized and thus a light receiving surface of the photodiode has been reduced. However, when the light receiving surface of the photodiode is small, focused light may not arrive at the light receiving surface. For example, as shown by phantom lines in  FIG. 10 , if light enters into the resin package  93  from a portion deviating to the right, the focused light also deviates from a light receiving surface  92   a  of the photodiode  92 . Thus, with the structure shown in  FIG. 10 , depending on the position from which light enters into the resin package  93 , the photodiode  92  may not properly receive the light, resulting in communication trouble. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention has been proposed under the above-described circumstances. It is therefore an object of the present invention to provide a light receiving module capable of reliably detecting light which carries information. 
         [0008]    A light receiving module according to the present invention comprises a light receiving element and a resin package for covering the light receiving element, the resin package being formed with a lens for collecting light to the light receiving element. The resin package includes a surface serving as a boundary between substances of different refractive indexes, the surface being formed with irregularities. Here, “substances of different refractive indexes” are typically the lens and the air surrounding the lens. When the resin package includes a plurality of layers made of different materials, the “substances” are one of the layers and another held in contact with the first layer. 
         [0009]    Preferably, the irregularities are rotationally symmetric with respect to a light axis of the lens. The surface with such irregularities is obtained by rotating one of a sinusoidal waveform, a-triangular waveform and a trapezoidal waveform around the light axis. 
         [0010]    Preferably, the light receiving module according to the present invention further comprises a tubular body which is positioned between the lens and the light receiving element and is tapered as proceeding toward the light receiving element. The tubular body includes a tapered inner surface for guiding light collected by the lens to the light receiving element, and also includes an opening near the lens which is larger than a light receiving surface of the light receiving element. 
         [0011]    Other features and advantages of the present invention will be apparent from the following description with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a sectional view illustrating a light receiving module according to a first embodiment of the present invention. 
           [0013]      FIG. 2  is a graph illustrating the difference between the light receiving module of  FIG. 1  and a comparative example. 
           [0014]      FIG. 3  is a sectional view illustrating a light receiving module according to a second embodiment of the present invention. 
           [0015]      FIG. 4  is a sectional view illustrating a light receiving-module according to a third embodiment of the present invention. 
           [0016]      FIG. 5  is a sectional view illustrating a light receiving module according to a fourth embodiment of the present invention. 
           [0017]      FIGS. 6A-6D  illustrate examples of irregularities formed on a lens. 
           [0018]      FIG. 7  is a sectional view illustrating a light receiving module according to a fifth embodiment of the present invention. 
           [0019]      FIG. 8  is a sectional view illustrating a light receiving module according to a sixth embodiment of the present invention. 
           [0020]      FIG. 9  is a graph illustrating a difference between the light receiving module of  FIG. 8  and a comparative example. 
           [0021]      FIG. 10  is a sectional view illustrating a light receiving module as a reference example. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Preferred embodiments of the present invention will be described below with reference to the accompanied drawings. 
         [0023]      FIG. 1  illustrates a light receiving module according to a first embodiment of the present invention. The illustrated light receiving module A 1  includes a substrate  1 , a photodiode  2 , a resin package  3 , and a tubular body  4 . The light receiving module A 1  works together with a light emitting module (not shown) for transmitting information, with the photodiode  2  used for receiving the light emitted from the light emitting module. 
         [0024]    The photodiode  2  and the resin package  3  are provided on the substrate  1 . 
         [0025]    The photodiode  2  generates electrical current corresponding to the amount of the received light. The photodiode  2  includes a light receiving surface  2   a  having a diameter of about 100 μm, for example. The photodiode  2  is located on a light axis. L so as to properly receive light transmitted through a lens  3   a.    
         [0026]    The resin package  3  is made of a resin material that permits the passage of light emitted from the light emitting module, and covers the photodiode  2 . The upper surface of the resin package  3  (i.e. the boundary surface with the air) serves as the lens  3   a  which is convex as a whole and has an undulating surface. As viewed in plan, the lens  3   a  has a diameter of about 3.3 mm. The lens  3   a  is formed with irregularities obtained by rotating a sinusoidal curve around the light axis L. The undulating form of the lens  3   a  may be defined by the following Formula 1 in a cylindrical coordinate system. The sixth term of Formula 1 represents a sine wave, H represents the amplitude of the sine wave (H=0.01 mm, for example), and WL represents the wavelength of the sine wave (WL=0.5 mm, for example). 
         [0000]    
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       
                         cr 
                         2 
                       
                       
                         1 
                         + 
                         
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   k 
                                 
                                 ) 
                               
                                
                               
                                 c 
                                 2 
                               
                                
                               
                                 r 
                                 2 
                               
                             
                           
                         
                       
                     
                     + 
                     
                       Ar 
                       4 
                     
                     + 
                     
                       Br 
                       6 
                     
                     + 
                     
                       Cr 
                       8 
                     
                     + 
                     
                       Dr 
                       10 
                     
                     + 
                     
                       H 
                       × 
                       cos 
                        
                       
                           
                       
                        
                       
                         
                           2 
                            
                           r 
                         
                         WL 
                       
                     
                     - 
                     z 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0027]    where r 2 =x 2 +y 2 . 
         [0028]    The resin package  3  accommodates the tubular body  4 . The tubular body  4  is made of a metal, for example, and positioned between the photodiode  2  and the lens  3   a , near the photodiode  2 . The tubular body  4  surrounds the light axis L, and has an inner diameter becoming smaller as proceeding toward the photodiode  2 . In other words, the tubular body  4  is tapered as proceeding toward the photodiode  2 . The tubular body  4  includes an inner surface  4   a  which is a tapered surface for reflecting light to be received by the photodiode  2 . The tubular body  4  includes a lower opening (opening at the side of the photodiode  2 ) with a diameter the same as that of the light receiving surface  2   a  of the photodiode  2 , and an upper opening (opening at the side of the lens  3   a ) with a diameter larger than that of the light receiving surface  2   a . With such structure, light enters from the relatively wider upper opening and is repeatedly reflected by the tapered inner surface  4   a , and then emitted out from the opening as large as the light receiving surface  2   a , toward the light receiving surface  2   a  (As can be easily understood, part of light is not reflected by the tapered inner surface  4   a , and is directly emitted toward the light receiving surface  2   a ). In this way, with the tapered inner surface  4   a , light is collected from an area larger than the light receiving surface  2   a , so that light entering into the light receiving surface  2   a  is increased. 
         [0029]    Next, the functions of the light receiving module A 1  will be described. 
         [0030]    In the light receiving module A 1 , the lens  3   a  is formed with irregularities. Thus, incident light is collected by the lens  3   a  and is suitably dispersed by the irregularities. This dispersion enlarges the area of light irradiation to the photodiode  2 . Here, the tapered inner surface  4   a  of the tubular body  4  is capable of guiding light from a relatively wide area to the photodiode  2 . Therefore, the light receiving module A 1  has a high light sensitivity. Description will be made below with reference to specific embodiments. 
         [0031]      FIG. 2  is a graph showing the light sensitivity of each of the light receiving module A 1  and a comparative example having a structure similar to that of the light receiving module A 1 . In the comparative example of the light receiving module, the lens is formed with no irregularities, similarly to the light receiving module X shown in  FIG. 10 , and the other structures are the same as those of the light receiving module A 1 . In the graph shown in  FIG. 2 , intensity of incident light at the photodiode is indicated on the longitudinal axis, and distance from the light axis L, which is the light incident position into the lens, is indicated on the horizontal axis. Further in the graph shown in  FIG. 2 , the lateral direction in  FIG. 1  is indicated as direction y, and the origin of the y-coordinate corresponds to the position of the light axis L. Still further, in the graph shown in  FIG. 2 , three lines with respect to three kinds of incident angles α are shown for each of the light receiving module A 1  and the comparative example. Each of the incident angles α indicates the angle between the light axis L and incident light. 
         [0032]    As can be seen from  FIG. 2 , in the comparative examples, when the incident angle α is 1.5° or 3.0° and the position in the direction y is −1, the light intensity at the photodiode is largely reduced. On the other hand, in the light receiving module A 1 , the light intensity is not largely reduced at any point. This indicates that the light receiving module A 1  is capable of stably receiving light at any light incident position, due to the irregularities formed at the lens  3   a.    
         [0033]      FIG. 3  illustrates a light receiving module according to a second embodiment of the present invention. The light receiving module A 2  in the figure includes a lens  3   a  formed with irregularities obtained by rotating a triangular waveform, and the other structures are the same as those of the light receiving module A 1 . 
         [0034]    Even the irregularities of the lens  3   a  are in such form of triangular waves, incident light is dispersed so that irradiated area on the light receiving surface  2   a  is increased. Thus, the light receiving module A 2  has a high light receiving sensitivity. Further, since the irregularities on the surface of the light receiving module A 2  are an aggregate of plan surfaces, forming of the lens  3   a  is easier than that of the light receiving module A 1 . 
         [0035]      FIG. 4  illustrates a light receiving module according to a third embodiment of the present invention. The illustrated light receiving module A 3  includes a lens  3   a  formed with irregularities obtained by rotating a trapezoidal waveform, and the other structures are the same as those of the light receiving modules A 1 , A 2 . 
         [0036]    Even the irregularities of the lens  3   a  are in such form of trapezoidal waves, incident light is dispersed so that irradiated area on the light receiving surface  2   a  is increased. Thus, the light receiving module A 3  has a high light receiving sensitivity. Further, since the irregularities on the surface of the light receiving module A 3  are an aggregate of plan surfaces, forming of the lens  3   a  is easier than that of the light receiving module A 1 . 
         [0037]      FIG. 5  illustrates a light receiving module according to a fourth embodiment of the present invention. In the light receiving module A 4  shown in  FIG. 5 , a resin package  3 , which is a simple body in the light receiving module A 1 , includes a lens layer  5 , an external frame  6 , and a protection layer  7 . The lens layer  5  is formed of a transparent resin, and includes a lens  3   a  projecting upward in  FIG. 5 . The lens  3   a  is, similarly to the light receiving module A 1 , formed with irregularities obtained by rotating a sinusoidal waveform. The external frame  6  is a cylindrical resin package mounted on the protection layer  7 , and supports the lens layer  5 . The external frame  6  accommodates an air layer  6   a . The protection layer  7  is made of a transparent resin, and protects the photodiode  2  and the substrate  1 , while supporting the tubular body  4 . Other structures of the light receiving module A 4  is the same as those of the light receiving module A 1 . 
         [0038]    In the light receiving module A 4 , similarly to the light receiving module A 1 , incident light is dispersed by the irregularities formed on the lens  3   a , so that irradiated area on the photodiode  2  is increased. Thus, the light receiving module A 4  is capable of receiving light stably, similarly to the light receiving module A 1 . Further, by setting the refractive index of the lens layer  5  to be larger than that of the air layer  6   a , light entered from the lens  3   a  is refracted between the lens layer  5  and the air layer  6   a , in a manner such that the focal position of the lens  3   a  becomes closer to the lens  3   a . Thus, in the light receiving module A 4 , incident light is focused at a distance shorter than that of the light receiving module A 1 , which contributes to downsize the module. 
         [0039]    In the above-described embodiments, the lens  3   a  is formed with wavy irregularities, however, may be formed with polygonal or circular irregularities as shown in  FIGS. 6A-6D , as viewed in the direction of the light axis L. In other words, the surface of the lens  3   a  may be formed with irregularities of polygonal cones, polygonal frustums, circular cones, or circular frustums. Even with such irregularities, incident light is dispersed. 
         [0040]      FIG. 7  illustrates a light receiving module according to a fifth embodiment of the present invention. The illustrated light receiving module A 5  is formed with irregularities at the lower surface of the lens layer  5  (a reverse surface  3   b  of the lens  3   a ), and the other structures are the same as those of the light-receiving module A 4 . The lens layer Sand the air layer  6   a  have different refractive indexes, and the reverse surface  3   b  of the lens  3   a  serves as the boundary surface of the layers. 
         [0041]    When light entering from the lens  3   a  and traveling toward the photodiode  2  passes through the reverse surface  3   b , the light is dispersed by the irregularities formed on the reverse surface  3   b . Thus, irradiated area on the light receiving surface  2   a  is increased, whereby the light receiving module A 5  has a high light receiving sensitivity. Further, since it is easier to form irregularities on the reverse surface  3   b  which is a plan surface, than to form irregularities on the curved surface of the lens  3   a , manufacture of the light receiving module A 5  is easier than that of the light receiving module A 4 . The irregularities on the reverse surface  3   b  are not limited to have the sinusoidal waveform, but the forms described in the above embodiments may be applied. 
         [0042]      FIG. 8  illustrates a light receiving module according to a sixth embodiment of the present invention. The illustrated light receiving module A 6  has the same structure as the light receiving module A 1  except that the tubular body  4  is omitted. With such structure, light is not collected from a large area as in the light receiving module A 1 , however, steps for forming the tubular body  4  are saved. In such light receiving module A 6 , as shown by solid lines in the figure, even light entered from a position apart from the light axis L arrives to the light receiving surface  2   a  due to dispersion at the lens  3   a . Description is made below with reference to examples. 
         [0043]      FIG. 9  shows a difference between the light sensitivities of the light receiving module A 6  and a comparative example having similar structures. The comparative light receiving module includes a lens without irregularities as the light receiving module X, and the other structures are the same as those of the light receiving module A 6 . 
         [0044]    As shown in  FIG. 9 , in the comparative example, the light intensity is relatively low when the incident angle α is not 0°. In the light receiving module A 6 , the light intensity is stably high, regardless of the incident angle and the position in the direction y. In other words, even when the light enters at a certain angle with respect to the light axis L, or enters from a position apart from the position immediately above the photodiode  2   a , as seen in the figure, the light receiving module A 6  has a high light sensitivity.