Patent Publication Number: US-2022221664-A1

Title: Method of manufacturing light receiving module

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application No. 2021-003503 filed in the Japan Patent Office on Jan. 13, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a method of manufacturing a light receiving module. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 8-18077 describes an analog photodiode (PD) module and a method of manufacturing the analog PD module. The analog PD module includes a package, a photodiode mounted on the package with a submount interposed therebetween, a ball lens mounted on the submount with a cap interposed therebetween, and an optical fiber optically connected to the ball lens. The optical fiber is held by a ferrule, and the ferrule is held by a ferrule holder. 
     In the method of manufacturing the analog PD module, an alignment operation for determining the positions of the PD, which is fixed to the above-mentioned package, the ball lens, and the optical fiber is performed. First, in the alignment operation, the PD that is sealed with a metal cap, the ferrule holder, and the ferrule to which the optical fiber is attached, are attached to a YAG welder having a three-axis (X, Y, and Z axes) alignment mechanism. The three-axis alignment mechanism is capable of adjusting the positions of the PD, the ball lens, and the optical fiber. 
     A laser is disposed on the side of the optical fiber, the side being opposite to the side on which the ball lens is disposed. Light emitted by the laser is output to the PD through the optical fiber and the ball lens. The PD detects the light emitted by the laser and input thereto through the optical fiber and the ball lens. In this manufacturing method, in order to suppress distortion due to non-linearity of conversion of photocurrent in the PD, the position of the optical fiber is set in such a manner that an end surface of the optical fiber comes closer to the ball lens side from a position where light sensitivity becomes maximum. 
     For example, when a light condensing diameter is smaller than a light receiving area of the PD, an electric field is concentrated on a light receiving portion of the PD, and extraction of carriers in the PD may be suppressed. The suppression of carrier extraction refers to the case where carriers are not sufficiently removed from the PD even when the PD stops receiving light. When suppression of carrier extraction occurs, there is a concern about deterioration of frequency characteristics such as an increase in light loss with an increase in the frequency of light. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present disclosure to provide a method of manufacturing a light receiving module capable of reducing the probability that suppression of extraction of carriers will occur and improving frequency characteristics. 
     A method of manufacturing a light receiving module according to the present disclosure includes positioning a light receiving element, positioning a lens that causes light to converge on the light receiving element, and positioning an optical fiber such that an end surface of the optical fiber is located at a position spaced apart from the lens by a certain distance. The certain distance is a distance that is offset from a focal length of the lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating a light receiving module according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram schematically illustrating a light receiving element, a lens, and an optical system of an optical fiber that are included in the light receiving module according to the embodiment. 
         FIG. 3  is a diagram schematically illustrating the light receiving element of the light receiving module according to the embodiment. 
         FIG. 4  is a flowchart illustrating steps of a method of manufacturing the light receiving module according to the embodiment. 
         FIG. 5  is a graph illustrating an exemplary result obtained by inputting first light and an exemplary result obtained by inputting second light in a step of measuring the position of an end surface of the optical fiber. 
         FIG. 6  is a graph illustrating a relationship between the distance from the end surface of the optical fiber to the lens and the light receiving rate of the light receiving element. 
         FIG. 7  illustrates a simulation result showing the state of light received by the light receiving element when the distance from the end surface of the optical fiber to the lens is a focal length (280 μm). 
         FIG. 8  illustrates a simulation result showing the state of light received by the light receiving element when the distance from the end surface of the optical fiber to the lens is 300 μm. 
         FIG. 9  illustrates a simulation result showing the state of light received by the light receiving element when the distance from the end surface of the optical fiber to the lens is 500 μm. 
         FIG. 10  is a graph illustrating a relationship between the distance from the end surface of the optical fiber to the lens and the radius of a light beam received by the light receiving element. 
         FIG. 11  is a graph illustrating a relationship between the distance from the end surface of the optical fiber to the lens and the peak intensity of a light beam received by the light receiving element. 
         FIG. 12  is a graph illustrating a relationship between the frequency of light and light loss when the distance from the end surface of the optical fiber to the lens is 550 μm. 
         FIG. 13  is a graph illustrating a relationship between the frequency of light and light loss when the distance from the end surface of the optical fiber to the lens is 300 μm. 
         FIG. 14  is a graph illustrating a relationship between input light power and sensitivity when the distance from the end surface of the optical fiber to the lens is 550 μm. 
         FIG. 15  is a graph illustrating a relationship between input light power and sensitivity when the distance from the end surface of the optical fiber to the lens is 300 μm. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description of Embodiment of Present Invention 
     First, the contents of an embodiment of the present disclosure will be listed and described. A method of manufacturing a light receiving module according to an embodiment of the present disclosure includes a step of positioning a light receiving element, a step of positioning a lens that causes light to converge on the light receiving element, and a step of positioning an optical fiber such that an end surface of the optical fiber is located at a position spaced apart from the lens by a certain distance. The certain distance is a distance that is offset from a focal length of the lens. 
     In the method of manufacturing the light receiving module, the light receiving element is positioned first, and then, the lens, which causes light to converge on the light receiving element, is positioned. The optical fiber is positioned such that the end surface thereof is located at the position spaced apart from the lens by the certain distance. The certain distance between the end surface of the optical fiber and the lens is a distance between a position that is offset from the focal position of the lens and the lens. The optical fiber is disposed at a position that is offset from the focus of the lens, so that concentration of an electric field onto the light receiving element can be suppressed, and thus, suppression of extraction of carriers in the light receiving element can be prevented from occurring. Therefore, the probability that suppression of extraction of carriers will occur is reduced, so that frequency characteristics can be improved. 
     The above-mentioned certain distance may be longer than the focal length. In this case, the optical fiber can be disposed at a position where the distance from the end surface of the optical fiber to the lens is longer than the focal length of the lens. 
     The above-mentioned certain distance may be shorter than the focal length. In this case, the optical fiber can be disposed at a position where the distance from the end surface of the optical fiber to the lens is shorter than the focal length of the lens. 
     The above-mentioned lens may be an aspherical lens. Even in the case where the lens is an aspherical lens, by disposing the optical fiber at a position that is offset from the focal position of the lens, the probability that suppression of extraction of carriers will occur can be reduced, so that the frequency characteristics can be improved. 
     The method of manufacturing the light receiving module further includes a step of setting the certain distance that is performed before the step of positioning the optical fiber. In the step of setting the certain distance, a distance by which a sensitivity of the light receiving element to light received by the light receiving element is equal to or higher than a certain percentage of a maximum sensitivity in the case where the optical fiber is disposed at a position spaced apart from the lens by the focal length may be set as the certain distance. In this case, the distance by which the sensitivity of the light receiving element to light received by the light receiving element is equal to or higher than the certain percentage of the maximum sensitivity is set as the certain distance. Thus, the certain distance can be set in such a manner that the sensitivity becomes close to the maximum sensitivity. 
     The step of setting the certain distance includes a step of inputting first light to the light receiving element and a step of inputting second light that has an intensity lower than that of the first light to the light receiving element. In the step of inputting the first light, a dip distance that is the distance from the lens to the optical fiber and by which the sensitivity of the light receiving element to light received by the light receiving element decreases is measured. In the step of inputting the second light, a distance that is the distance spaced apart from the dip distance and by which the sensitivity of the light receiving element to light received by the light receiving element is equal to or higher than the certain percentage of the maximum sensitivity may be set as the certain distance. In this case, the dip distance by which the sensitivity decreases when the first light having the intensity higher than that of the second light is input is measured. In this manner, the dip distance from the optical fiber to the lens by which the sensitivity decreases when the first light is input can be measured. In addition, a distance that is spaced apart from the dip distance and by which the sensitivity of the light receiving element to light received by the light receiving element is equal to or higher than the certain percentage of the maximum sensitivity when the second light is input is set as the certain distance. Thus, the certain distance can be set as a distance that is different from the dip distance and by which the sensitivity becomes close to the maximum sensitivity when the first light and the second light are used. 
     The lens is held by a lens holder, and the optical fiber is held by a tubular sleeve. The method of manufacturing the light receiving module may further include a step of positioning a joint sleeve that is joined to the lens holder and the sleeve, the step being performed after the step of positioning the lens. In this case, the lens holder holding the lens and the tubular sleeve holding the optical fiber can be joined to each other by the joint sleeve. 
     The light receiving diameter of the light receiving element may be larger than the light condensing diameter of the lens. Even in the case where the light receiving diameter of the light receiving element is larger than the light condensing diameter of the lens as mentioned above, by disposing the optical fiber at a position that is offset from the focal position of the lens, the probability that suppression of extraction of carriers will occur can be reduced, so that the frequency characteristics can be improved. 
     Details of Embodiment of Present Disclosure 
     A specific example of the method of manufacturing the light receiving module according to the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to the exemplary embodiment, which will be described below, and is to be determined by the claims, and it is intended that all the modifications within the scope of the claims and the equivalents thereof are included in the scope of the present invention. In the descriptions of the drawings, elements that are the same as each other or that correspond to each other are denoted by the same reference signs, and repeated descriptions will be suitably omitted. For ease of understanding, some portions in the drawings may be illustrated in a simplified manner or in an exaggerated manner, and the dimensional ratios and so forth are not limited to those illustrated in the drawings. 
     A light receiving module  1  according to the embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view illustrating the light receiving module  1  according to the embodiment. For example, the light receiving module  1  includes an optical receptacle  10  that is connected to an optical fiber and a light receiving unit  20  that is fixed to the optical receptacle  10 . The optical receptacle  10  includes a fiber stub (stub ferrule)  12 , a sleeve  14 , and a connector sleeve  16 . The fiber stub  12  includes a ferrule  11  and an optical fiber  13 . 
     The ferrule  11  has a tubular shape (or a cylindrical shape). The central axis of the ferrule  11  extends in the Z-axis direction, and the cross section of the ferrule  11  that is perpendicular to the central axis of the ferrule  11  has a circular shape. The ferrule  11  has a first end surface  11   b  and a second end surface  11   c  that are arranged in the Z-axis direction. The second end surface  11   c  is a surface that is brought into physical contact with a ferrule of an optical connector that is connected to the optical receptacle  10 , and the second end surface  11   c  is, for example, polished. The optical connector that is connected to the optical receptacle  10  is, for example, an SC connector. The first end surface  11   b  is a surface opposite the second end surface  11   c . The first end surface  11   b  is slightly inclined (e.g., at about four degrees or about eight degrees) with respect to a plane perpendicular to the central axis of the ferrule  11 . The ferrule  11  has an outer peripheral surface  11   d , which is a cylindrical surface. 
     The ferrule  11  further has a fiber holding hole  11   f . The fiber holding hole  11   f  extends in the Z-axis direction and is formed on the central axis of the ferrule  11 . A cross section of the fiber holding hole  11   f  has a circular shape, and the inner diameter of the fiber holding hole  11   f  is slightly larger than the outer diameter of the optical fiber  13 . An opening of the fiber holding hole  11   f  is included in the second end surface  11   c , and the other opening of the fiber holding hole  11   f  is included in the first end surface  11   b . The fiber holding hole  11   f  extends between the first end surface  11   b  and the second end surface  11   c  of the ferrule  11  in the Z-axis direction. The ferrule  11  is made of, for example, zirconia (ZrO 2 ). By making the ferrule  11  out of zirconia having high toughness and high Young&#39;s modulus, a physical contact can be appropriately performed at the second end surface  11   c.    
     The optical fiber  13  is, for example, a single-mode fiber and is a bare fiber from which a resin coating has been removed. The optical fiber  13  is made of, for example, a quartz. The optical fiber  13  extends such that its longitudinal direction (optical axis direction) is the same as the Z-axis direction and has an end surface  13   b  and an end surface  13   c . The optical fiber  13  is inserted into the fiber holding hole  11   f . The end surface  13   b  is exposed through the opening of the fiber holding hole  11   f , the opening being located on the side on which the first end surface  11   b  is present, and the end surface  13   c  is exposed through the other opening of the fiber holding hole  11   f , the other opening being located on the side on which the second end surface  11   c  is present. The end surface  13   c  is brought into contact with an end surface of the optical fiber of the optical connector that is connected to the optical receptacle  10 . The end surface  13   b  is optically coupled to a light receiving element  21  of the light receiving unit  20 . The light receiving element  21  is a photodiode. The outer diameter of the optical fiber  13  is, for example, 125 μm. 
     The sleeve  14  has a through hole  14   b  extending in the Z-axis direction and holds the fiber stub  12  inside the through hole  14   b . The sleeve  14  is made of, for example, a metal material such as a stainless steel. The sleeve  14  has a tubular shape extending in the Z-axis direction. The sleeve  14  has a proximal end surface  14   c , a distal end surface  14   d , and an outer peripheral surface  14   f  The proximal end surface  14   c  and the distal end surface  14   d  are arranged in the Z-axis direction, and the through hole  14   b  extends between the proximal end surface  14   c  and the distal end surface  14   d . The cross section of the through hole  14   b  that is perpendicular to the Z-axis direction has a circular shape. The proximal end surface  14   c  faces a connection sleeve  19  of the light receiving unit  20 . The fiber stub  12  is press-fitted into the through hole  14   b  of the sleeve  14  along the Z-axis direction. The outer peripheral surface  11   d  of the ferrule  11  is in contact with the inner surface of the through hole  14   b , and as a result, the fiber stub  12  is fixed to the sleeve  14 . 
     The connector sleeve  16  is a tubular member that extends in the Z-axis direction and is made of, for example, a ceramic. For example, the connector sleeve  16  is made of the same material as the ferrule  11  (e.g., zirconia). The inner diameter of the connector sleeve  16  is approximately equal to the outer diameter of the fiber stub  12 . The connector sleeve  16  has a proximal end  16   b  and a distal end  16   c  that are arranged in the Z-axis direction. The connector sleeve  16  has an outer peripheral surface  16   d  and an inner peripheral surface  16   f . The fiber stub  12  is inserted in an opening of the connector sleeve  16 , the opening being located on the side on which the proximal end  16   b  is present. 
     A portion of the connector sleeve  16  on the side on which the proximal end  16   b  is present is inserted in a gap formed between the outer peripheral surface  11   d  of the ferrule  11  and the sleeve  14 . Thus, the inner peripheral surface  16   f  of the connector sleeve  16  is in contact with the outer peripheral surface  11   d  of the ferrule  11 . The ferrule of the optical connector is inserted into the other opening of the connector sleeve  16 , the other opening being located on the side on which the distal end  16   c  is present. The second end surface  11   c  of the ferrule  11  and the end surface of the ferrule of the optical connector come into contact with each other in the connector sleeve  16 . As a result, the optical fiber  13  held by the ferrule  11  and the optical fiber held by the ferrule of the optical connector are optically coupled to each other with high coupling efficiency. 
     The light receiving unit  20  includes the light receiving element  21 , a lens holder  22 , a lens  23 , a stem  24 , and a plurality of lead pins  27 . 
     The stem  24  is an insulating member having a flat circular plate-like shape and has a flat main surface  24   b . The main surface  24   b  intersects the optical axis of the optical fiber connected to the optical receptacle  10  (i.e., the optical axis of the optical fiber  13 ). For example, the main surface  24   b  is perpendicular to the optical axis of the optical fiber connected to the optical receptacle  10  (the optical axis of the optical fiber  13 ). The stem  24  is made of, for example, a ceramic. 
     The lens holder  22  is a metal member having a substantially tubular shape. The central axis of the lens holder  22  extends along the optical axis of the optical fiber  13 . A proximal end  22   b  of the lens holder  22  in the optical axis direction of the optical fiber  13  is fixed to the main surface  24   b  of the stem  24  with an annular metal fitting  29  interposed therebetween. The annular metal fitting  29  has a first end surface  29   b  and a second end surface  29   c  in the optical axis direction. The proximal end  22   b  of the lens holder  22  is fixed to the first end surface  29   b  of the metal fitting  29 , and the main surface  24   b  of the stem  24  is fixed to the second end surface  29   c  of the metal fitting  29 . The metal fitting  29  has a window hole  29   d  that allows the light from the lens  23  to pass therethrough. 
     A distal end  22   c  of the lens holder  22  in the optical axis direction of the optical fiber  13  faces the connection sleeve  19  in the Z-axis direction. The connection sleeve  19  has a tubular shape. For example, the connection sleeve  19  is fixed to the sleeve  14  and the lens holder  22  by YAG welding. The lens holder  22  is made of, for example, an iron-nickel alloy. 
     The light receiving element  21  is optically coupled to the end surface  13   b  of the optical fiber  13 . The light receiving element  21  receives light from the optical fiber of the optical connector connected to the optical receptacle  10  and outputs a current signal having a magnitude corresponding to the intensity of the light. For example, the light receiving element  21  is mounted on a carrier  25  that mainly includes a dielectric (as an example, the carrier  25  is simply formed of a dielectric), and the carrier  25  is placed on an integrated circuit chip  26 . In other words, the light receiving element  21  is mounted on the integrated circuit chip  26  with the carrier  25  interposed therebetween. As the light receiving element  21 , various photodiodes such as a PIN photodiode and an avalanche photodiode (APD) can be used. For example, the carrier  25  is a flat plate member made of a ceramic or a quartz. The integrated circuit chip  26  is a semiconductor integrated circuit (IC) that receives a current signal from the light receiving element  21  and converts the current signal into a voltage signal. 
     The plurality of lead pins  27  are rod-shaped metal members each of which extends in a direction crossing the main surface  24   b  of the stem  24 . The plurality of lead pins  27  are fixed to the stem  24  by extending through the stem  24 . The plurality of lead pins  27  transmit and receive electrical signals and electrical power to and from the light receiving element  21  and the integrated circuit chip  26  that are arranged in a space defined by the stem  24  and the metal fitting  29 . The lens  23  is held inside the lens holder  22  and fixed to the inner peripheral surface of the lens holder  22  with a resin  23   b  interposed therebetween. The lens  23  is, for example, an aspherical lens. 
       FIG. 2  is a diagram illustrating an optical system A of the light receiving module  1 . The lens  23  is a converging lens formed of a light transmitting member and is positioned on the optical axis of the optical fiber  13 . The mode field diameter (MFD) of the end surface  13   b  of the optical fiber  13  is, for example, 8.0 μm or more and 9.2 μm or less. The lens  23  causes light L that is emitted from the end surface  13   b  of the optical fiber  13  to converge on the light receiving element  21 . 
     The wavelength of the light L is, for example, 1,310 nm. In order to prevent the light reflected by the light receiving element  21  from being incident on the lens  23 , for example, the optical axis of the lens  23  is slightly offset with respect to the optical axis of the optical fiber  13 . In addition, in the X-axis direction, the center of a light receiving surface of the light receiving element  21  is positioned to be offset with respect to the center of the lens  23  by 100 μm. A light receiving diameter R 1  of the light receiving element  21  for the light L is larger than a light condensing diameter R 2  of the lens  23 . As an example, the light receiving diameter R 1  is 230 μm, and the light condensing diameter R 2  is 50 μm or more and 60 μm or less. For example, the light receiving diameter R 1  may be 10 times or more or 20 times or more the light condensing diameter R 2 . 
       FIG. 3  is a plan view schematically illustrating the light receiving element  21 . As illustrated in  FIG. 3 , the shape of the light receiving element  21  when viewed in plan view (the shape of the light receiving element  21  when viewed in the Z-axis direction) is a rectangular shape. In the present embodiment, the light receiving element  21  is a test element group (TEG) chip. In other words, the light receiving element  21  is an evaluation chip. For example, the light receiving element  21  includes a light receiving portion  21   b  and an electrode  21   c . As an example, the electrode  21   c  includes two pad electrodes  21   d , an arc-shaped portion  21   f  that is disposed so as to surround the light receiving portion  21   b , and a connecting portion  21   g  that connects one of the pad electrodes  21   d  and the arc-shaped portion  21   f  to each other. 
     A method of manufacturing the light receiving module  1  according to the embodiment will now be described with reference to  FIG. 4 .  FIG. 4  is a flowchart illustrating steps of the method of manufacturing the light receiving module  1  according to the embodiment. The lens  23  is spaced apart from the end surface  13   b  of the optical fiber  13  by a certain distance. Before performing assembly of components and so forth, a step of setting the certain distance that is the distance between the lens  23  and the end surface  13   b  of the optical fiber  13  is performed first (step S 1 ). 
     In step S 1 , for example, the certain distance is set in the optical system A including the optical fiber  13 , the lens  23 , and the light receiving element  21 . The certain distance is the distance between a position that is offset from the focus of the lens  23  and the lens  23  and is, for example, longer than the focal length of the lens  23 . In step S 1 , a step of inputting first light and a step of inputting second light having an intensity lower than that of the first light are performed. Each of these steps will be described later. As an example, the intensity of the first light is 10 mW, and the intensity of the second light is 1 mW. The wavelength of the first light and the wavelength of the second light are each, for example, 1,310 nm. In the step of inputting the first light and the step of inputting the second light, the first light and the second light are each input to the light receiving element  21  from the optical fiber  13  through the lens  23  in the optical system A. 
     An exemplary result obtained by performing the step of inputting the first light and an exemplary result obtained by performing the step of inputting the second light are illustrated in the graph in  FIG. 5 . In the graph illustrated in  FIG. 5 , the horizontal axis denotes a distance Z between the lens holder  22  and the end surface  13   b  of the optical fiber  13 , and the vertical axis denotes the light sensitivity of the light receiving element  21 . Note that the distance Z may be the distance from a surface of the lens  23  to the end surface  13   b  of the optical fiber  13  in the Z-axis direction. The sensitivity indicates an absolute sensitivity (in units of A/W) that is obtained by dividing the magnitude of a photocurrent (A) by the energy of input light (W). 
     As illustrated in  FIG. 5 , in the case of inputting the first light, a dip D where the light sensitivity decreases appears when the distance Z is a certain value. The dip D may appear as a result of strong light being concentrated and input to the light receiving element  21  to saturation and making it difficult for carriers and a current to flow. In the step of inputting the first light, a dip distance that is the distance Z from the lens holder  22  to the optical fiber  13  and by which the sensitivity decreases. For example, the dip distance is approximately the same as the focal length of the lens  23 . It is understood that the dip distance in the case illustrated in  FIG. 5  is about 300 μm. 
     In the case of inputting the second light having an intensity lower than that of the first light, although the dip D does not appear, the sensitivity gradually decreases when the distance Z becomes equal to or larger than a certain value. It is understood that, in the case illustrated in  FIG. 5 , the sensitivity gradually decreases when the distance Z becomes 600 μm or larger. In the step of inputting the second light, a distance that is a distance spaced apart from the dip distance and by which the sensitivity is equal to or higher than a certain percentage of a maximum sensitivity is set as the certain distance. In the case illustrated in  FIG. 5 , for example, the certain percentage is 99%, and 550 μm that is the distance Z corresponding to 99% or more of the maximum sensitivity when the second light is input is set as a certain distance Z 1 . Alternatively, the certain distance Z 1  may be set to 500 μm or more and 600 μm or less. 
     As illustrated in  FIG. 1  and  FIG. 4 , after the certain distance Z 1  has been set, the light receiving element  21  is positioned (a step of positioning a light receiving element, step S 2 ). More specifically, the integrated circuit chip  26  and the carrier  25  are mounted onto the main surface  24   b  of the stem  24 , and the light receiving element  21  is mounted onto the carrier  25 . After the light receiving element  21  has been positioned, a component of the light receiving module  1  is positioned (a step of positioning a lens, step S 3 ). In this case, the metal fitting  29  is placed onto the main surface  24   b  of the stem  24 , and the lens holder  22 , which holds the lens  23 , and the connection sleeve  19  are mounted onto the metal fitting  29  (a step of positioning a connection sleeve). 
     After the connection sleeve  19  has been mounted, the optical fiber  13  is positioned in such a manner that the end surface  13   b  is located at a position that is spaced apart from the lens holder  22  by the certain distance Z 1  (a step of positioning an optical fiber, step S 4 ). In the above-described case, the sleeve  14  is disposed at a position where the certain distance Z 1  between the lens holder  22  and the end surface  13   b  of the optical fiber  13  is 550 μm. Then, alignment (XY alignment) of the optical fiber  13  in the X-axis direction and the Y-axis direction, each of which is perpendicular to the Z-axis direction, is performed (step S 5 ). After the alignment of the optical fiber  13 , the connection sleeve  19  is joined to the lens holder  22  by YAG welding, and the sleeve  14  is joined to the connection sleeve  19  by YAG welding (step S 6 ). Then, the series of steps of the method of manufacturing the light receiving module  1  is completed. 
     Next, advantageous effects of the method of manufacturing the light receiving module  1  according to the present embodiment will be described. In the method of manufacturing the light receiving module  1 , the lens  23  that causes light to converge on the light receiving element  21  is positioned after the light receiving element  21  has been positioned. The optical fiber  13  is positioned in such a manner that the end surface  13   b  is located at a position that is spaced apart from the lens  23  by the certain distance. The certain distance between the end surface  13   b  of the optical fiber  13  and the lens  23  is different from the focal length of the lens  23  and is offset from the focal length. By disposing the optical fiber  13  at a position that is offset from the focal length of the lens  23 , concentration of an electric field onto the light receiving element  21  can be suppressed, and thus, suppression of extraction of carriers in the light receiving element  21  can be prevented from occurring. Therefore, the probability that suppression of extraction of carriers will occur is reduced, so that the frequency characteristics can be improved. In the present embodiment, in addition to improvement of the frequency characteristics by reducing the probability that suppression of extraction of carriers will occur, the sensitivity to input light in a wide wavelength band can be improved, and the linearity between input light power and the sensitivity can be improved. These advantageous effects will be described in detail later in the description of an example. 
     The above-mentioned certain distance is longer than the focal length. Thus, the optical fiber  13  can be disposed at a position where the distance from the end surface  13   b  of the optical fiber  13  to the lens  23  is longer than the focal length. Alternatively, the above-mentioned certain distance may be shorter than the focal length. In this case, the optical fiber  13  can be disposed at a position where the distance from the end surface  13   b  of the optical fiber  13  to the lens  23  is shorter than the focal length. However, the case of disposing the optical fiber  13  at a position where the distance from the end surface  13   b  of the optical fiber  13  to the lens  23  is longer than the focal length has an advantage in that YAG welding can more easily be performed. 
     The lens  23  is an aspherical lens. Even if the lens  23  is an aspherical lens, by disposing the optical fiber  13  at a position that is offset from the focal length of the lens  23 , the probability that suppression of extraction of carriers will occur can be reduced, so that the frequency characteristics can be improved. 
     The method of manufacturing the light receiving module  1  according to the present embodiment includes the step of setting the certain distance that is performed before the step of positioning the optical fiber. In the step of setting the certain distance, a distance by which the sensitivity of the light receiving element  21  to light received by the light receiving element  21  is equal to or higher than a certain percentage (e.g., 99%) of the maximum sensitivity in the case where the optical fiber  13  is disposed at a position spaced apart from the lens  23  by the focal length is set as the certain distance. Accordingly, the distance by which the sensitivity of the light receiving element  21  to light received by the light receiving element  21  is equal to or higher than the certain percentage of the maximum sensitivity is set as the certain distance. Thus, the certain distance can be set in such a manner that the sensitivity becomes close to the maximum sensitivity. 
     As illustrated in  FIG. 5 , the step of setting the certain distance includes the step of inputting the first light to the light receiving element  21  and the step of inputting the second light that has an intensity lower than that of the first light to the light receiving element  21 . In the step of inputting the first light, a dip distance that is the distance from the lens  23  to the optical fiber  13  and by which the sensitivity of the light receiving element  21  to light received by the light receiving element  21  decreases is measured. In the step of inputting the second light, a distance that is the distance spaced apart from the dip distance and by which the sensitivity of the light receiving element  21  to light received by the light receiving element  21  is equal to or higher than the certain percentage of the maximum sensitivity is set as the certain distance Z 1 . In other words, the dip distance by which the sensitivity decreases when the first light having the intensity higher than that of the second light is input is measured. In this manner, the dip distance from the optical fiber  13  to the lens  23  by which the sensitivity decreases when the first light is input can be measured. In addition, a distance that is spaced apart from the dip distance and by which the sensitivity of the light receiving element  21  to light received by the light receiving element  21  is equal to or higher than the certain percentage of the maximum sensitivity when the second light is input is set as the certain distance. Thus, the certain distance can be set as a distance that is different from the dip distance and by which the sensitivity becomes close to the maximum sensitivity when the first light and the second light are used. 
     The lens  23  is held by the lens holder  22 , and the optical fiber  13  is held by the tubular sleeve  14 . The method of manufacturing the light receiving module  1  according to the present embodiment further includes a step of positioning the connection sleeve  19 , which is joined to the lens holder  22  and the sleeve  14 , the step being performed after the step of positioning the lens. Therefore, the lens holder  22  holding the lens  23  and the tubular sleeve  14  holding the optical fiber  13  can be joined to each other by the connection sleeve  19 . 
     The light receiving diameter R 1  of the light receiving element  21  is larger than the light condensing diameter R 2  of the lens  23 . Even in the case where the light receiving diameter R 1  of the light receiving element  21  is larger than the light condensing diameter R 2  of the lens  23  as mentioned above, by disposing the optical fiber  13  at a position that is offset from the focal position of the lens  23 , the probability that suppression of extraction of carriers will occur can be reduced, so that the frequency characteristics can be improved. 
     Example 
     An example of the method of manufacturing a light receiving module according to the present disclosure will be described below. Note that the present invention is not limited to the following example. In the example, various simulations relating to a light beam received by the light receiving element  21  were performed by using the optical system A of the light receiving module  1 . First, as illustrated in  FIG. 2 , the position of the optical fiber  13  was determined in such a manner that the center of the light beam coincided with the center of the light receiving surface of the light receiving element  21  when the distance between the end surface  13   b  of the optical fiber  13  and the lens  23  was 300 μm. A distance E between the lens  23  and the light receiving element  21  was set to 2,160 μm. The wavelength of the light beam was set to 1,310 nm, and the MFD of the end surface  13   b  of the optical fiber  13  was set to 9.2 μm. The end surface  13   b  of the optical fiber  13  was inclined at four degrees with respect to a plane perpendicular to the central axis of the ferrule  11 . An aspherical lens was used as the lens  23 . The light receiving diameter R 1  of the light receiving element  21  is 230 μm. In addition, in the X-axis direction, the center of the light receiving surface of the light receiving element  21  was positioned to be offset with respect to the center of the lens  23  by 100 μm. 
     In the above-described optical system A, the relationship between the distance Z from the end surface  13   b  of the optical fiber  13  to the lens holder  22  and the light receiving rate of the light receiving element  21  was simulated. The simulation result is illustrated in  FIG. 6 . As illustrated in  FIG. 6 , it was found that, in the optical system A, the light receiving rate of the light receiving element  21  is about 1.0 when the distance Z is in the range of 0 μm or more to 800 μm or less, whereas the light receiving rate of the light receiving element  21  gradually decreases from 1.0 when the distance Z becomes 800 μm or more. 
       FIG. 7  illustrates the light beam received by the light receiving element  21  when the distance Z is 280 μm (the focal length).  FIG. 8  illustrates the light beam received by the light receiving element  21  when the distance Z is 300 μm.  FIG. 9  illustrates the light beam received by the light receiving element  21  when the distance Z is 500 μm. It is understood from  FIG. 7  to  FIG. 9  that the diameter of the light beam in the light receiving element  21  becomes larger as the distance Z increases from the focal length. 
       FIG. 10  is a graph illustrating the relationship between the distance Z and the radius of a light beam in the light receiving element  21 . It is understood from  FIG. 10  that the radius of the light beam becomes larger as the distance Z increases or decreases from 280 μm (the focal length). When the distance Z was 300 μm, the radius of the light beam in the light receiving element  21  was 6.5 μm, and when the distance Z was 500 μm, the radius of the light beam in the light receiving element  21  was 26.7 μm. 
       FIG. 11  is a graph illustrating the relationship between the distance Z and the peak intensity of a light beam in the light receiving element  21 . It is understood from  FIG. 11  that the peak intensity of the light beam becomes lower as the distance Z increases or decreases from 280 μm (the focal length). When the distance Z was 280 μm, the peak intensity of the light beam was 18,300 W/mm 2 , and when the distance Z was 300 μm, the peak intensity of the light beam was 15,300 W/mm 2 . When the distance Z was 500 μm, the peak intensity of the light beam was 900 W/mm 2 , and when the distance Z was 550 μm, the peak intensity of the light beam was 610 W/mm 2 . In this manner, when the distance Z is 500 μm or more and 550 μm or less, the peak intensity can be significantly reduced compared with the case where the distance Z is 280 μm or more and 300 μm or less. As a result, it is presumed that the probability that suppression of extraction of carriers in the light receiving element  21  will occur can be reduced. 
       FIG. 12  is a graph illustrating the relationship between the frequency of light and light loss when the distance Z is 550 μm (a distance offset from the focal length).  FIG. 13  is a graph illustrating the relationship between the frequency of light and light loss when the distance Z is 300 μm.  FIG. 12  and  FIG. 13  illustrate results of simulations of measurement of the frequency characteristics. In the simulations indicating the frequency characteristics, the input light power was set to 1.62 mW, and the wavelength of the input light was set to 1,310 nm. 
     As illustrated in  FIG. 12  and  FIG. 13 , it was found that, in the case where the distance Z is 300 μm, which is close to the focal length, the loss generated with an increase in the frequency is larger than that in the case where the distance Z is 550 μm. In the case where the distance Z is 300 μm, it was found that the loss increases due to the dependence of reverse bias especially when the voltage is 5 V or 6 V. In contrast, in the case where the distance Z is 550 μm, it was found that the loss does not increase at a frequency of 1.0×10 9  Hz or lower even when the voltage is 5 V or 6 V and that the frequency characteristics are favorable. 
       FIG. 14  is a graph illustrating the relationship between input light power and sensitivity when the distance Z is 550 μm.  FIG. 15  is a graph illustrating the relationship between input light power and sensitivity when the distance Z is 300 μm.  FIG. 14  and  FIG. 15  illustrate results of simulations of measurement of the linearity between the input light power and the sensitivity. As illustrated in  FIG. 14  and  FIG. 15 , it was found that, in the case where the distance Z is 300 μm, which is close to the focal length, the sensitivity to the input light power is lower than that in the case where the distance Z is 550 μm. In the case where the distance Z is 300 μm, it was found that the sensitivity markedly decreases especially when the input light power exceeds 1.0×10 −3  W. In contrast, in the case where the distance Z is 550 μm, it was found that the sensitivity does not markedly decrease even when the input light power is strong and that the linearity between the input light power and the sensitivity is maintained. 
     The embodiment and the example of the method of manufacturing a light receiving module according to the present disclosure have been described above. However, the present invention is not limited to the above-described embodiment or example. In other words, it can be easily understood by those skilled in the art that various modifications and changes can be made within the scope of the present invention as described in the claims. 
     For example, in the above embodiment, the light receiving module  1  that includes the optical receptacle  10  to which an SC connector is connected has been described. However, the light receiving module may include an optical receptacle to which a connector other than an SC connector (e.g., an LC connector or the like) is connected. In the above embodiment, the light receiving module  1  that includes the lens  23  which is an aspherical lens has been described. However, the type of lens included in the light receiving module is not particularly limited, and the lens may be a spherical lens. In addition, in the above embodiment, the light receiving module  1  includes the connection sleeve  19  has been described. However, the light receiving module does not need to include the connection sleeve  19 . In this manner, the configuration of each component of the light receiving module and the contents and the order of the steps of the method of manufacturing the light receiving module are not limited to the above-described embodiments and examples and can be suitably changed.