Patent Publication Number: US-2013248694-A1

Title: Light-receiving-element module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2012-67948, filed on Mar. 23, 2012, the entire disclosure of which is incorporated by reference herein. 
     FIELD 
     This application relates generally to a light-receiving-element module. 
     BACKGROUND 
     Unexamined Japanese Patent Application KOKAI Publication No. 2001-223369 discloses a light-receiving-element module that includes an optical fiber which transmits a signal light, a lens which combines the signal light on a waveguide light receiving element, and an amplifier which amplifies an electrical signal output by the light receiving element through photoelectric conversion. 
     In order to increase the high-frequency characteristic of the waveguide light receiving element disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2001-223369, it is necessary to reduce a time of the carrier that travels through the light absorbing layer of a photo diode. Hence, it is necessary to make the width of the waveguide narrow, and to increase the refractive index of the lens that combines the light emitted from the optical fiber to the waveguide. 
     When combining signals emitted from the optical fibers with different wavelengths by a lens having a large refractive index, the chromatic aberration of the lights produced by the lens becomes great, and the positional misalignment of combined points becomes great, resulting in the reduction of the light receiving sensitivity of the light-receiving-element module. This technical issue occurs in the case of typical light receiving elements of a non-waveguide type. 
     The present invention has been made in view of the above-explained circumstance, and it is an object of the present invention to provide a light-receiving-element module that has greater high-frequency characteristic and light receiving sensitivity than those of conventional technologies. 
     SUMMARY 
     To achieve the object, a light-receiving-element module according to an aspect of the present invention includes: a first refractor that refracts an optical signal with a first wavelength or an optical signal with a second wavelength; a second refractor that further refracts the optical signal with the first wavelength and the optical signal with the second wavelength refracted by the first refractor so as to reduce a chromatic aberration; and a light receiving element that converts the optical signal with the first wavelength or the optical signal with the second wavelength refracted by the second refractor into an electric signal by photoelectric conversion. 
     According to the present invention, it becomes possible to provide a light-receiving-element module that has a greater high-frequency characteristic and light receiving sensitivity than those of conventional technologies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of this application can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  is a perspective view showing a light-receiving-element module according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the light-receiving-element module taken along a line A-A′ in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view showing a refractive part of a light-receiving-element module according to a second modified example of the embodiment of the preset invention; 
         FIG. 4  is a cross-sectional view showing a refractive part of a light-receiving-element module according to a fourth modified example of the embodiment of the present invention; and 
         FIG. 5  is a cross-sectional view showing a light-receiving-element module according to a sixth modified example of the embodiment of the present invention taken along a line A-A′. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiment 
     An explanation will be given of a light-receiving-element module  100  according to an embodiment of the present invention with reference to the accompanying drawings. 
       FIG. 1  is a perspective view showing the light-receiving-element module  100 , and  FIG. 2  is a cross-sectional view of the light-receiving-element module  100  taken along a line A-A′. 
     The light-receiving-element module  100  has a refractive unit  120  that refracts an optical signal output by an optical fiber  210  of a ferrule  200 , and a light receiving element  130  of a waveguide type that performs photoelectric conversion on the optical signal refracted by the refractive unit  120 . The refractive unit  120  and the light receiving element  130  are built in a package  110  coupled with the optical fiber  210 . Moreover, an amplifier element  140  that amplifies an electric signal output by the light receiving element  130  and an RF (Radio Frequency) substrate  150  that processes the amplified electric signal are also built in the package  110 . 
     The optical fiber  210  transmits an optical signal with a wavelength of 1310 nm (hereinafter, referred to as a first wavelength) and an optical signal with a wavelength of 1550 nm (hereinafter, referred to as a second wavelength) which are defined by the standard specification ITU-T G.693 of the international telecommunication union. 
     As shown in  FIG. 2 , the refractive unit  120  includes a first refractive part  121  and a second refractive part  122 . The first refractive part  121  is a convex lens having a positive refractive power, and converges the optical signal output by the optical fiber  210 . The second refractive part  122  is a concave lens having a negative refractive power, disperses the optical signal converged by the first refractive part  121 , and inputs the dispersed optical signal into the light receiving element  130 . The first and second refractive parts  121  and  122  reduce the chromatic aberration between the refracted optical signal of the first wavelength and the refracted optical signal of the second wavelength. 
     The optical fiber  210 , the first refractive part  121 , the second refractive part  122 , and the light receiving element  130  are disposed in such a way that a center point Pe of an end face of the optical fiber  210 , a principal point P 1  of the first refractive part  121 , a principal point P 2  of the second refractive part  122 , and a center point Pa of a light receiving surface of the light receiving element  130  are aligned over a same substantially straight line. This is in order to increase the light receiving efficiency of the light receiving element  130 . 
     A synthesized refractive power k of the first refractive part  121  and the second refractive part  122  can be expressed as a formula “k=k1+k2−dk1·k2”. 
     The first refractive part  121  and the second refractive part  122  are disposed at locations distant from each other by a distance d that satisfies the following conditional equations (1) to (3) relating to the synthesized refractive power k, and are configured to have curvatures r1 to r4 that satisfy the conditional equations (1) to (3). Hence, the synthesized refractive power k of the first refractive part  121  and the second refractive part  122  remains the same or is not likely to change throughout when the first wavelength is refracted and when the second wavelength is refracted. 
       Δ k=Δk 1 +Δk 2 −d (Δ k 1 ·k 2 +k 1 ·Δk 2)≦ε  (1)
 
       Δ k 1 =Δn 1( r 1 −r 2)  (2)
 
       Δ k 2 =Δn 2( r 3 −r 4)  (3)
 
     where: 
     k1 is the refractive power of the first refractive part  121 ; 
     k2 is the refractive power of the second refractive part  122 ; 
     Δk1 is the refractive power of the first refractive part  121  for the optical signal with the first wavelength−the refractive power of the first refractive part  121  for the optical signal with the second wavelength different from the first wavelength; 
     Δk2 is the refractive power of the second refractive part  122  for the optical signal with the first wavelength−the refractive power of the second refractive part  122  for the optical signal with the second wavelength; 
     d is a distance between the first refractive part  121  and the second refractive part  122 ; 
     r1 is a curvature of the first refractive part  121  at the optical-fiber side; 
     r2 is a curvature of the first refractive part  121  at the second-refractive-part- 122  side; 
     r3 is a curvature of the second refractive part  122  at the first-refractive-part- 121  side; 
     r4 is a curvature of the second refractive part  122  at the light-receiving-element- 130  side; 
     Δn1 is the refractive index of the first refractive part  121  for the optical signal with the first wavelength−the refractive index of the first refractive part  121  for the optical signal with the second wavelength; 
     Δn2 is the refractive index of the second refractive part  122  for the optical signal with the first wavelength−the refractive index of the second refractive part  122  for the optical signal with the second wavelength; and 
     ε is a predetermined constant. 
     The appropriate range of ε can be defined by those skilled in the art through a laboratory test, and the most appropriate value of ε is “0”. 
     According to such a configuration, since the first and second refractive parts  121  and  122  satisfy the conditional equation (1), there is no difference between the synthesized refractive power for the optical signal with the first wavelength and the synthesized refractive power for the optical signal with the second wavelength, or such a difference is insignificant. Hence, there is no chromatic aberration between the optical signal with the first wavelength and the optical signal with the second wavelength, or such chromatic aberration hardly occurs. Accordingly, even if the synthesized refractive power k of the first refractive part  121  and the second refractive part  122  is increased in order to improve the high-frequency characteristic, the light receiving sensitivity of the light receiving element does not decrease or hardly decreases. 
     According to this embodiment, the explanation was given of an example case in which the optical signals transmitted by the optical fiber  210  are the optical signal with the wavelength of 1310 nm (i.e., the first wavelength) and the optical signal with the wavelength of 1550 nm (i.e., the second wavelength), but the wavelength of the optical signal transmitted by the optical fiber  210  is not limited to those wavelengths. Moreover, according to this embodiment, although the light receiving element  130  is a waveguide type, the present invention is not limited to such a type, and for example, a facial light receiving element can be used. 
     First Modified Example of Embodiment 
     According to the above-explained embodiment, the explanation was given of the case in which the first refractive part  121  is a convex lens with a positive refractive power and the second refractive part  122  is a concave lens with a negative refractive power. However, the present invention is not limited to this configuration, and the first refractive part  121  may be a concave lens with a negative refractive power and the second refractive part  122  may be a convex lens with a positive refractive power. 
     Second Modified Example of Embodiment 
     According to the above-explained embodiment, the refractive unit  120  includes the two lenses that are the first refractive part  121  and the second refractive part  122 . According to a second modified example, however, as shown in  FIG. 3 , the refractive unit  120  includes three lenses that are a first refractive part  126 , a second refractive part  127 , and a third refractive part  128 . 
     The first refractive part  126  is a concave lens with a negative refractive power, and disperses the optical signal emitted from the optical fiber  210 . The second refractive part  127  is a convex lens with a positive refractive power, and converges the optical signal dispersed by the first refractive part  126 . The third refractive part  128  is a concave lens with a negative refractive power, disperses the optical signal converged by the second refractive part  127 , and inputs the dispersed optical signal into the light receiving element  130 . 
     The optical fiber  210 , the first to third refractive parts  126  to  128 , and the light receiving element  130  are disposed in such a way that the center point Pe of an end face of the optical fiber  210 , principal points P 6  to P 8  of the first to third refractive parts  126  to  128 , and the center point Pa of the light receiving surface of the light receiving element  130  are aligned over a same substantially straight line. 
     The first refractive part  126  and the second refractive part  127  are disposed at locations distant from each other by a distance d that satisfies the above-explained conditional equations (1) to (3), and are configured to have curvatures r1 to r4 that satisfy the conditional equations (1) to (3). 
     Moreover, a synthesized refractive power k′ of the second refractive part  127  and the third refractive part  128  can be expressed as a formula “k′=k2+k3−d′k2·k3”. 
     The second refractive part  127  and the third refractive part  128  are disposed at locations distant from each other by a distance d′ that satisfies the following conditional equations (4) to (6), and are configured to have curvatures r3 to r6 that satisfy the conditional equations (4) to (6). Hence, the synthesized refractive power k′ of the second refractive part  127  and the third refractive part  128  remains same or is not likely to change throughout when the first wavelength is refracted and when the second wavelength is refracted. 
       Δ k′=Δk 2 +Δk 3 −d ′(Δ k 2 ·k 3 +k 2 ·Δk 3)≦ε′  (4)
 
       Δ k 2 =Δn 2( r 3 −r 4)  (5)
 
       Δ k 3 =Δn 3( r 5 −r 6)  (6)
 
     Where: 
     k2 is the refractive power of the second refractive part  127 ; 
     k3 is the refractive power of the third refractive part  128 ; 
     Δk2 is the refractive power of the second refractive part  127  for the optical signal with the first wavelength−the refractive power of the second refractive part  127  for the optical signal with the second wavelength; 
     Δk3 is the refractive power of the third refractive part  128  for the optical signal with the first wavelength−the refractive power of the third refractive part  128  for the optical signal with the second wavelength; 
     d′ is the distance between the second refractive part  127  and the third refractive part  128 ; 
     r3 is a curvature of the second refractive part  127  at the first-refractive-part- 126  side; 
     r4 is a curvature of the second refractive part  127  at the third-refractive-part- 128  side; 
     r5 is a curvature of the third refractive part  128  at the second-refractive-part- 127  side; 
     r6 is a curvature of the third refractive part  128  at the light-receiving-element- 130  side; 
     Δn2 is the refractive index of the second refractive part  127  for the optical signal with the first wavelength−the refractive index of the second refractive part  127  for the optical signal with the second wavelength; 
     Δn3 is the refractive index of the third refractive part  128  for the optical signal with the first wavelength−the refractive index of the third refractive part  128  for the optical signal with the second wavelength; and 
     ε′ is a predetermined constant. 
     The appropriate range of ε′ can be defined by those skilled in the art through a laboratory test, and the most appropriate value of ε′ is “0”. 
     According to such a configuration, the synthesized refractive power of the first refractive part  126  and the second refractive part  127  remains the same or is not likely to change between the first wavelength and the second wavelength. Moreover, the synthesized refractive power of the second refractive part  127  and the third refractive part  128  remains the same or is not likely to change between the first wavelength and the second wavelength. Hence, the total synthesized refractive power of the first refractive part  126 , the second refractive part  127 , and the third refractive part  128  remains the same or is not likely to change between the first wavelength and the second wavelength. 
     Third Modified Example of Embodiment 
     According to the second modified example of the embodiment, the explanation was given of the case in which the refractive unit  120  includes the first, second and third refractive parts  126 ,  127 , and  128 , and the synthesized refractive power of the first refractive part  126 , the second refractive part  127 , and the third refractive part  128  remains the same or is not likely to change between the first wavelength and the second wavelength. However, the refractive unit  120  may be n number of refractive parts, adjoining refractive parts may be disposed at locations distant from each other by a distance that satisfies the above-explained conditional equations (1) to (3), and such refractive parts may have curvatures that satisfy the conditional equations (1) to (3). According to such a configuration, the synthesized refractive power of the n number of refractive parts remains the same or is not likely to change between the first wavelength and the second wavelength. 
     Fourth Modified Example of Embodiment 
     According to the above-explained embodiment, the explanation was given of the case in which the refractive unit  120  includes the two lenses that are the first refractive part  121  and the second refractive part  122 . However, as shown in  FIG. 4 , a tablet having the two lenses that are the first refractive part  121  and the second refractive part  122  put together may be used as the refractive unit  120 . 
     According to this configuration, since the refractive unit  120  is a tablet having the two lenses put together, the light-receiving-element module can be downsized in comparison with the case in which the refractive unit  120  is configured by the two lenses. 
     Fifth Modified Example of Embodiment 
     According to the above-explained embodiment, the refractive unit  120  includes the two lenses that are the first refractive part  121  and the second refractive part  122 . However, as shown in  FIG. 5 , the refractive unit  120  may include the first refractive part  121  that is a convex lens with a positive refractive power, and a second refractive part  123  which is formed on a surface of the first refractive part  121  at the light-receiving-element- 130  side and which is a diffraction grating with a negative refractive power. 
     The first and second refractive parts  121  and  123  are configured so as to satisfy the above-explained conditional equation (1). Hence, the synthesized refractive power k of the first refractive part  121  and the second refractive part  123  remains the same or is not likely to change throughout when the first wavelength is refracted and when the second wavelength is refracted. 
     Hence, according to such a configuration, the refractive unit  120  is configured by a lens and a diffractive grating formed on the surface thereof, and thus the light-receiving-element module can be downsized in comparison with the cases in which the refractive unit  120  is configured by the two lenses and the refractive unit  120  is configured by a tablet having the two lenses put together. 
     Sixth Modified Example of Embodiment 
     According to the fifth modified example of the embodiment, the explanation was given of the case in which the refractive unit  120  includes the first refractive part  121  that is a convex lens with a positive refractive power, and the second refractive part  123  that is a diffraction grating formed on the surface of the first refractive part  121  at the light-receiving-element- 130  side and having a negative refractive power. 
     However, the present invention is not limited to such a configuration, and the refractive unit  120  may include the first refractive part  121  that is a convex lens with a positive refractive power and the second refractive part  123  that is a diffraction grating formed on a surface of the first refractive part  121  at the optical-fiber- 210  side and having a negative refractive power. 
     Seventh Modified Example of Embodiment 
     Moreover, the refractive unit  120  may include the first refractive part  121  that is a concave lens with a negative refractive power, and the second refractive part  123  that is a diffraction grating formed on a surface of the first refractive part  121  and having a positive refractive power. The second refractive part  123  may be formed on a surface of the first refractive part  121  at the light-receiving-element- 130  side or on a surface of the first refractive part  121  at the optical-fiber- 210  side. 
     Having described and illustrated the principles of this application by reference to one or more preferred embodiments, it should be apparent that the preferred embodiments may be modified in arrangement and detail without departing from the principles disclosed herein and that it is intended that the application be construed as including all such modifications and variations insofar as they come within the spirit and scope of the subject matter disclosed herein. 
     The present invention is appropriate for an optical module used for, for example, an optical communication.