Abstract:
A light-receiving device including: a lens; and a light-receiving element optically coupled to the lens, a plurality of optical path divided by the lens crossing each other in a position of between the lens and the light-receiving element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-001622, filed on Jan. 7, 2011, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to a light-receiving device. 
         [0004]    (ii) Related Art 
         [0005]    In an optical semiconductor device such as an optical receiver, a light-receiving element receives an optical signal emitted from an emission edge of an optical fiber. It is preferable that an active diameter is small, in order to operate a light-receiving element at high speed. On the other hand, when a light intensity peak on a light-receiving face of a light-receiving element gets higher, current density of the area gets higher. This results in space-charge effect (saturation in light-receiving element). Japanese Patent Application Publication No. 05-224101, Japanese Patent Application Publication No. 06-21485 and Japanese Patent Application Publication No. 08-18077 disclose a defocusing technology as a measure. 
         [0006]    When a beam diameter is enlarged through the defocusing, the peak light intensity on the light-receiving face gets lower relatively. Thus, local increasing of current density on the light-receiving face is restrained, and the occurrence of the space-charge effect is restrained. However, when the beam diameter is enlarged, light may leak out of the light-receiving face, and an optical coupling efficiency may be reduced. 
       SUMMARY 
       [0007]    It is an object of the present invention to provide a light-receiving device achieving both restraint of space-charge effect of a light-receiving element and high optical coupling efficiency of a light-receiving element. 
         [0008]    According to an aspect of the present invention, there is provided a light-receiving device including: a lens; and a light-receiving element optically coupled to the lens, a plurality of optical path divided by the lens crossing each other in a position of between the lens and the light-receiving element. 
         [0009]    According to another aspect of the present invention, there is provided a light-receiving device including: a lens; and a light-receiving element optically coupled to the lens, an incoming light through the lens having a plurality of peak intensities on a light-receiving face of the light-receiving element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device in accordance with a comparative example; 
           [0011]      FIG. 2A  and  FIG. 2B  illustrate a schematic view of a beam diameter of an optical signal passing through a lens; 
           [0012]      FIG. 3  illustrates light intensity distribution of an optical signal received by a light-receiving face of a light-receiving element; 
           [0013]      FIG. 4A  illustrates three dimensional light intensity distribution of “Peak” of  FIG. 3 ; 
           [0014]      FIG. 4B  illustrates contour lines of the light intensity distribution of “Peak”; 
           [0015]      FIG. 5A  illustrates three dimensional light intensity distribution of “Defocus  4 ” of  FIG. 3 ; 
           [0016]      FIG. 5B  illustrates contour lines of the light intensity distribution of “Defocus  4 ” of  FIG. 3 ; 
           [0017]      FIG. 6  illustrates a light intensity distribution during a defocusing; 
           [0018]      FIG. 7  illustrates a relationship between light intensity at a center of an optical signal and an optical coupling efficiency; 
           [0019]      FIG. 8  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device in accordance with an embodiment; 
           [0020]      FIG. 9A  and  FIG. 9B  illustrate a schematic view for describing a positional relationship between an emission edge of an optical fiber, a lens and a light-receiving element; 
           [0021]      FIG. 10  illustrates a case where a plurality of peaks appear; 
           [0022]      FIG. 11  illustrates an optical coupling efficiency; 
           [0023]      FIG. 12A  and  FIG. 12B  illustrate another example of a light receiving element; 
           [0024]      FIG. 13  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device in accordance with a second modified embodiment; 
           [0025]      FIG. 14A  illustrates three dimensional light intensity distribution of the embodiment; 
           [0026]      FIG. 14B  illustrates contour lines of light intensity distribution of  FIG. 14A ; 
           [0027]      FIG. 15  illustrates experimental results; and 
           [0028]      FIG. 16  illustrates an example of a structure of an optical system. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    A description will be given of a comparative example. 
       Comparative Example 
       [0030]      FIG. 1  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device  200  in accordance with the comparative example. As illustrated in  FIG. 1 , the optical semiconductor device  200  has a light input portion  10 , a light focus portion  20  and a light-receiving portion  30 . An optical signal input from the light input portion  10  is a single wavelength light signal. The light focus portion  20  focuses the optical signal. The light-receiving portion  30  receives the focused optical signal. 
         [0031]    In the light input portion  10 , a holder  11  fixes a ferrule clasp  12 . A ferrule  13  is inserted into the ferrule clasp  12 . An optical fiber  14  penetrates the ferrule  13 . Outside the ferrule  13 , the optical fiber  14  is covered with a cover member  15 . An emission edge of the ferrule  13  and the optical fiber  14  is vertically cut with respect to an optical axis of the optical fiber  14 . 
         [0032]    A cap  21  fixes a lens  22  in the light focus portion  20 . The lens  22  is arranged so that a center of the lens  22  overlaps with the optical axis of the optical fiber  14 . The lens  22  is not limited specifically. The lens  22  is, for example, a spherical lens. 
         [0033]    In the light-receiving portion  30 , a sub mount  32  is provided on a stem  31 , and a light-receiving element  33  is mounted on the sub mount  32 . The light-receiving element  33  has only to be a semiconductor light-receiving element (a photo diode). The light-receiving element  33  may be a front-face illuminated light-receiving element or may be a back-face illuminated light-receiving element. An outputting terminal of the light-receiving element  33  is coupled to a lead  35  via a pre-amplifier  34 . A lead  36  is coupled to a power supply terminal of the light-receiving element  33 . An insulating member  37  such as a glass is provided between the leads  35  and  36  and the stem  31 . 
         [0034]    An optical signal transmitting in the optical fiber  14  is emitted to the lens  22  from an emission edge of the optical fiber  14 . The lens  22  adjusts a beam diameter inputting to a light-receiving face of the light-receiving element  33 . The light-receiving element  33  converts an incoming light into an electrical signal through photoelectric conversion. The pre-amplifier  34  amplifies the electrical signal output from the light-receiving element  33 . 
         [0035]      FIG. 2A  illustrates a schematic view of the beam diameter of the optical signal passing through the lens  22 .  FIG. 2B  illustrates an enlarged view around the light-receiving element  33 . As illustrated in  FIG. 2A , a spherical lens is used as the lens  22 . As illustrated in  FIG. 2B , a back-face illuminated photo diode is used as the light-receiving element  33 . 
         [0036]    The beam diameter of the optical signal output from the emission edge of the optical fiber  14  gets larger in a transmitting direction of the optical signal with the optical axis being a center. Thus, the beam diameter forms a Gaussian distribution. In the comparative example, the lens  22  is provided so that the optical axis of the optical signal passes through the center of the lens  22 . That is, the optical axis of the optical signal is vertical with respect to a tangential plane of the lens  22 . In this case, comatic aberration is avoided. Therefore, the optical signal passing through the lens  22  is distributed with the optical axis of the optical signal being a symmetrical optical axis. The lens  22  collects a light from the optical fiber  14  and adjusts the beam diameter of the optical signal received by the light-receiving element  33  to a predetermined value. 
         [0037]      FIG. 3  illustrates light intensity distribution of an optical signal received by the light-receiving face of the light-receiving element  33 . In  FIG. 3 , a horizontal axis indicates a distance (μm) from the center of the optical signal. A vertical axis indicates the light intensity (relative light intensity with respect to total amount of light).  FIG. 3  illustrates light intensity distribution of an optical signal in which a beam diameter is changed through defocusing. “Peak” indicates an optical signal without defocusing. “Defocus  1 ” to “Defocus  4 ” indicate an optical signal with defocusing. As illustrated in  FIG. 3 , the light intensity of the optical signal is the highest at the center of the optical signal. 
         [0038]      FIG. 4A  illustrates three dimensional light intensity distribution of “Peak” of  FIG. 3 .  FIG. 4B  illustrates contour lines of the light intensity distribution of “Peak”.  FIG. 5A  illustrates three dimensional light intensity distribution of “Defocus  4 ” of  FIG. 3 .  FIG. 5B  illustrates contour lines of the light intensity distribution of “Defocus  4 ” of  FIG. 3 . In  FIG. 4A  and  FIG. 5A , an x-axis (dx) and a y-axis (dy) indicate two-dimensional directions of the light-receiving face. A z-axis (p) indicates the light intensity. The contour lines of  FIG. 4A  and  FIG. 5A  indicate five steps between a peak and a bottom. In  FIG. 4B  and  FIG. 5B , an x-axis (dx) and a y-axis (dy) indicate two-dimensional directions of the light-receiving face. 
         [0039]    As illustrated in  FIG. 3  through  FIG. 5B , when the beam diameter gets smaller, the light intensity distribution places a disproportionate emphasis on the center of the optical signal, and the light intensity at the center of the optical signal gets larger. On the other hand, when the beam diameter gets larger, the light intensity distribution diffuses outward from the center of the optical signal, and the light intensity at the center of the optical signal gets smaller. 
         [0040]    When the light intensity exceeds a predetermined limit value, space-charge effect occurs in the light-receiving element  33 . It is therefore preferable that the beam diameter is increased through defocusing so that a maximum value of the light intensity is the limit value or less. However, in this case, the light intensity far from the center of the optical signal increases as the light intensity at the center of the optical signal decreases. 
         [0041]      FIG. 6  illustrates the light intensity distribution during the defocusing. In  FIG. 6 , a horizontal axis indicates a distance (μm) from the center of an optical signal, and a vertical axis indicates the light intensity. In  FIG. 6 , the light intensity at a position where the distance from the center of an optical signal is larger than 7.5 μm is a predetermined value or more. An optical coupling efficiency of the light-receiving element  33  is reduced when the light-receiving diameter of the light-receiving element  33  is 15 μm, because the optical coupling efficiency of the light-receiving element  33  is proportional to an integral value of the light intensity of  FIG. 6 . In this way, when the beam diameter gets larger, the optical coupling efficiency gets lower. 
         [0042]      FIG. 7  illustrates a relationship between the light intensity at the center of an optical signal (hereinafter referred to as a peak light intensity) and the optical coupling efficiency. In  FIG. 7 , a horizontal axis indicates the peak light intensity, and a vertical axis indicates the optical coupling efficiency. As illustrated in  FIG. 7 , when the peak light intensity is large, the optical coupling efficiency indicates approximately “1”. This is because the beam diameter gets smaller. In contrast, when the peak light intensity is small, the optical coupling efficiency gets smaller. This is because the beam diameter gets larger, and light leaks from the light-receiving face. 
         [0043]    As mentioned above, in the optical semiconductor device  200  in accordance with the comparative example, the space-charge effect is not restrained when the beam diameter is small, and the optical coupling efficiency gets smaller when the beam diameter is large. Therefore, the optical semiconductor device  200  of the comparative example cannot achieve both the restraint of the space-charge effect and the high optical coupling efficiency of a light-receiving element. 
       Embodiment 
       [0044]      FIG. 8  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device  100  in accordance with an embodiment. As illustrated in  FIG. 8 , the optical semiconductor device  100  is different from the optical semiconductor device  200  of  FIG. 1  in positions of the lens  22  and the light-receiving element  33  with respect to the optical axis of the optical fiber  14 . The same components as those illustrated in  FIG. 8  have the same reference numerals as  FIG. 1 . 
         [0045]      FIG. 9A  illustrates a schematic view for describing a positional relationship between an emission edge of the optical fiber  14 , the lens  22  and the light-receiving element  33 .  FIG. 9B  illustrates an enlarged view around the light-receiving element  33 . As illustrated in  FIG. 9A , in the embodiment, the center position of the lens  22  has an offset with respect to optical path of an optical signal emitted from the emission edge of the optical fiber  14 . Therefore, in the embodiment, the optical axis of the optical signal emitted from the optical fiber  14  passes through off the center of the lens  22 . In other words, the optical axis of the optical signal is not vertical with respect to a tangential plane of the lens  22 . In this case, the optical signal passing through the lens  22  is distributed asymmetrically with respect to the optical axis of the optical signal because of comatic aberration and spherical aberration. 
         [0046]    One of optical paths of an optical signal emitted from the lens  22  is hereinafter referred to as a first optical path, and another optical path is referred to as a second optical path. When the first optical path and the second optical path cross with each other between the lens  22  and the light-receiving face of the light-receiving element  33 , an optical signal passing on the first optical path and an optical signal passing on the second optical path interfere with each other. In this case, the optical signal passing on the first optical path and the optical signal passing on the second optical path strengthen with each other or weaken with each other according to the phase difference, because the optical path of the optical signal emitted from the emission edge of the optical fiber  14  has an offset with respect to the center of the lens  22 , passes through the lens  22 , and emitted from the lens  22 . As a result, a plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33 . 
         [0047]      FIG. 10  illustrates the case where a plurality of peaks appear. In  FIG. 10 , a horizontal axis indicates a distance (μm) from a center of an optical signal, and a vertical axis indicates light intensity.  FIG. 10  also illustrates the light intensity distribution of the comparative example. As illustrated in  FIG. 10 , when a plurality of peaks appear in the light intensity distribution, the light intensity places disproportionate emphasis on a center of an optical signal. Thus, light intensity off the center of the optical signal is reduced. Therefore, even if the maximum value of the light intensity is adjusted to be a limitation value or less, light intensity out of the light-receiving face of the light-receiving element  33  is reduced. The light intensity of the central peak is 0.12 or less. Both side peaks with respect to the central peak is 0.08 or more. 
         [0048]      FIG. 11  illustrates the optical coupling efficiency in this case. In  FIG. 11 , a horizontal axis indicates the peak light intensity, and a vertical axis indicates the optical coupling efficiency. As illustrated in  FIG. 11 , the peak light intensity is reduced and the reduction of the optical coupling efficiency is restrained, when a plurality of peaks appear in the light intensity distribution. 
         [0049]    In the embodiment, the position of the lens  22  and the light-receiving element  33  is determined with respect to the optical axis of the optical fiber  14  so that there is a difference between the phase of the optical signal of the first optical path and the phase of the optical signal of the second optical path and a plurality of peaks light intensity appear in the light-receiving face of the light-receiving element  33 . Therefore, restraint of the space-charge effect of the light-receiving element  33  and high optical coupling efficiency of the light-receiving element  33  are achieved. 
       First Modified Embodiment 
       [0050]      FIG. 12A  illustrates another example of a light receiving element. As illustrated in  FIG. 12A , a light focus portion  38  having curvature may be monolithically provided on the side of the light-receiving element  33 . In this case, as illustrated in  FIG. 12B , the light focus portion  38  further collects optical signals received by the light-receiving element  33 . 
       Second Modified Embodiment 
       [0051]      FIG. 13  illustrates a cross sectional view for describing an overall structure of an optical semiconductor device  100   a  in accordance with a second modified embodiment. As illustrated in  FIG. 13 , an emission edge of the optical fiber  14  may be cut obliquely with respect to the optical axis of the optical fiber  14 . In this case, adjusting an angle between the emission edge of the optical fiber  14  and the optical axis of the optical fiber  14  enlarges the free degree of the position of the optical fiber  14 , the lens  22  and the light-receiving element  33 . Thus, limitation of component arrangement in the optical semiconductor device  100   a  is lightened. And, it is possible to restrain incoming of a light reflected by the light-receiving element  33  into the optical fiber  14 . 
       Experimental Examples 
       [0052]    A description will be given of an experimental result of the optical semiconductor device  200  of the comparative example and an experimental result of the optical semiconductor device  100   a  of the second modified embodiment. Table 1 shows experimental conditions. As shown in Table 1, a spherical lens of material BK-7 having a diameter of 1.5 mm was used as the lens  22 . And, an optical fiber, of which angle of a cut-plane of an emission edge is 10 degrees, was used as the optical fiber  14 . A distance between the lens  22  and the emission edge of the optical fiber  14  was 0.8 mm. A distance between the lens  22  and the light-receiving element  33  was 2.5 mm. In the comparative example, the optical axis of the optical fiber  14  passes through the center of the lens  22  and is positioned at the center of the light-receiving face of the light-receiving element  33 . In the embodiment, the center of the lens  22  has an offset of 0.34 mm with respect to the optical axis of the optical fiber  14 . The center of the light-receiving face of the light-receiving element  33  has an offset of 0.55 mm with respect to a position extended from the center of the lens  22  in the optical axis direction. 
         [0000]    
       
         
               
               
               
             
               
               
             
               
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 COMPARATIVE  
               
               
                   
                 EMBODIMENT 
                 EXAMPLE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 TYPE OF LENS 
                 SPHERICAL LENS, DIAMETER OF  
               
               
                   
                 1.5 mm, MATERIAL BK-7 
               
               
                 DISTANCE FROM LENS 
                 0.8 mm 
               
               
                 TO OPTICAL FIBER 
                   
               
               
                 DISTANCE FROM LENS 
                 2.5 mm 
               
               
                 TO LIGHT-RECEIVING  
                   
               
               
                 ELEMENT 
                   
               
             
          
           
               
                 OFFSET FROM LENS 
                 0.34 mm 
                 0 mm 
               
               
                 TO OPTICAL FIBER 
                   
                   
               
               
                 OFFSET FROM LENS 
                 0.55 mm 
                 0 mm 
               
               
                 TO LIGHT-RECEIVING  
                   
                   
               
               
                 ELEMENT 
                   
                   
               
             
          
           
               
                 TYPE OF 
                 BACK-FACE ILLUMINATED PIN-PD  
               
               
                 LIGHT-RECEIVING  
                 INTEGRATED WITH MONO- 
               
               
                 ELEMENT 
                 LITHIC LENS, ACCEPTANCE  
               
               
                   
                 DIAMETER OF 15 μm 
               
               
                 WAVELENGTH OF 
                 1310 nm. DFB LASER 
               
               
                 LIGHT SOURCE 
               
               
                   
               
             
          
         
       
     
         [0053]      FIG. 14A  illustrates three dimensional light intensity distribution of the embodiment.  FIG. 14B  illustrates contour lines of the light intensity distribution of  FIG. 14A . As illustrated in  FIG. 14A  and  FIG. 14B , the light intensity places a disproportionate emphasis on the center of the optical signal. Thus, light intensity off the center of the optical signal is reduced. This is because a plurality of light intensity peaks appear according to the phase difference of the optical signals on a plurality of optical paths in the light-receiving face of the light-receiving element  33 . 
         [0054]      FIG. 15  illustrates the experimental results. In  FIG. 15 , a horizontal axis indicates optical power (dBm) received by the light-receiving element  33 . A left vertical axis indicates photocurrent (μA) obtained through photoelectric conversion. And a right vertical axis indicates optical coupling efficiency (A/W). In the experimental examples of  FIG. 15 , a target value of the optical coupling efficiency was set to be 0.75 A/W. As illustrated in  FIG. 15 , in the comparative example, when inputting power exceeds 0 dBm, the photocurrent was saturated and the optical coupling efficiency was reduced. However, in the embodiment, even if the inputting power was +6 dBm, the photocurrent was not saturated and the optical coupling efficiency was not reduced. With the results, it has been demonstrated that the restraint of space-charge effect of a light-receiving element and high optical coupling efficiency of the light-receiving element are achieved when the optical semiconductor device of the embodiment is used. 
       Structure of Optical System 
       [0055]      FIG. 16  illustrates an example of a structure of an optical system.  FIG. 16  illustrates a central optical axis coupling the emission edge, the lens and the light-receiving face and illustrates a positional relationship of the emission edge and the light-receiving face with respect to the center of the lens. In  FIG. 16 , an L-direction indicates the optical axis of the optical fiber  14 , and an X-direction indicates a position in a face having vertical relationship with the optical axis of the optical fiber  14 . “0” indicates an angle between the optical axis of the optical signal emitted from the emission edge of the optical fiber  14  and the optical axis of the optical fiber  14 . “φ” indicates the diameter of the lens  22 . “n i ” indicates refraction index of the lens  22  (approximately 1.5 to 1.6). Here, “L 1 ” indicates a position of the emission edge of the optical fiber  14  in the L-direction. “X 1 ” indicates a position of the emission edge of the optical fiber  14  in the X-direction. “L 2 ” indicates a position of the light-receiving face of the light-receiving element  33  in the L-direction. “X 2 ” indicates a position of the light-receiving face of the light-receiving element  33  in the X-direction. 
         [0056]    A description will be given of an example of conditions in which a plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33 . The followings are conditions in a case where a wavelength of an optical signal emitted from the optical fiber  14  is 1.2 μm to 1.6 μm. The cut-plane angle means an angle of a cut-plane sloping toward the lens side with respect to the optical axis (the L-direction) of the optical fiber  14 . When the cut-plane angle is zero degree, the edge face of the optical fiber  14  is in parallel with the X-direction. These conditions can be obtained by adjusting each parameter and determining favorable conditions with optical analysis simulation. 
         [0057]    (Condition 1) A plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33  when the cut-plane angle of the emission edge of the optical fiber  14  is 6 degrees, the diameter of the lens  22  is 1.5 mm, and L 2 /X 2  is 5.0. (Condition 2) A plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33  when the cut-plane angle of the emission edge of the optical fiber  14  is 10 degrees, the diameter of the lens  22  is 1.0 mm, and L 2 /X 2  is 2.6. (Condition 3) A plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33  when the cut-plane angle of the emission edge of the optical fiber  14  is 10 degrees, the diameter of the lens  22  is 1.5 mm, and L 2 /X 2  is 4.5. (Condition 4) A plurality of peaks appear in the light intensity distribution on the light-receiving face of the light-receiving element  33  when the cut-plane angle of the emission edge of the optical fiber  14  is 10 degrees, the diameter of the lens  22  is 2.0 mm, and L 2 /X 2  is 5.2. 
         [0058]    The present invention is not limited to the specifically disclosed embodiments and variations but may include other embodiments and variations without departing from the scope of the present invention.