Abstract:
An optical receiving device includes: a lens portion configured to refract incident light with a first wavelength and refract incident light with a second wavelength; a first beam splitter configured to let refracted light with the first wavelength transmit and reflect refracted light with the second wavelength; a second beam splitter configured to reflect transmitted light with the first wavelength; and a light receiver configured to receive reflected light with the first wavelength and reflected light with the second wavelength, wherein the first beam splitter and the second beam splitter are disposed so as to be separated by a difference in optical path length between the light with the first wavelength and the light with the second wavelength.

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. 2015-239617, filed on Dec. 8, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical receiving device and an optical receiving system. 
       BACKGROUND 
       [0003]    With an increase in communication capacity, optical communication systems using optical signals have been widely used. 
         [0004]    Examples of related art are disclosed in Japanese Laid-open Patent Publication Nos. 2006-119464 and 2004-325813. 
       SUMMARY 
       [0005]    According to an aspect of the embodiments, an optical receiving device includes: a lens portion configured to refract incident light with a first wavelength and refract incident light with a second wavelength; a first beam splitter configured to let refracted light with the first wavelength transmit and reflect refracted light with the second wavelength; a second beam splitter configured to reflect transmitted light with the first wavelength; and a light receiver configured to receive reflected light with the first wavelength and reflected light with the second wavelength, wherein the first beam splitter and the second beam splitter are disposed so as to be separated by a difference in optical path length between the light with the first wavelength and the light with the second wavelength. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0006]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  depicts an example of an optical receiving device; 
           [0008]      FIG. 2  depicts an example of reflection and transmission characteristics of beam splitters; 
           [0009]      FIG. 3  depicts an example of laser wavelength dependence of a beam waist position of a laser beam gathered by a lens unit; 
           [0010]      FIG. 4  depicts an example of laser wavelength dependence when a beam waist position with respect to a second lens is fixed; 
           [0011]      FIG. 5  is a schematic view depicting a comparison in spot diameter when a light receiving unit is disposed at a position D; 
           [0012]      FIG. 6  depicts an example of the optical receiving device; 
           [0013]      FIG. 7  depicts an example of the optical receiving device; 
           [0014]      FIG. 8  depicts an example of the optical receiving device; 
           [0015]      FIG. 9  depicts an example of the optical receiving device; 
           [0016]      FIG. 10  depicts an example of a relation between optical input power and PD current; and 
           [0017]      FIG. 11  depicts an example of reflection and transmission characteristics of beam splitters. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    For example, communication distances in optical communication for use in a data center or the like may be classified into the following two: 
         [0019]    (1) a distance within several hundreds of meters inside a server rack or between adjacent server racks; and 
         [0020]    (2) a distance equal to or longer than several hundreds of meters between server racks on the same floor or between server racks across a building. 
         [0021]    In the optical communication (1), a short-wavelength optical module is used, including a short-wavelength laser (for example, with a wavelength of 850 nm) and a photodetector which receives an optical signal generated by the laser. In the optical communication (2), a long-wavelength optical module is used, including a long-wavelength laser (for example, with a wavelength of 1310 nm, 1550 nm, or the like) and a photodetector which receives an optical signal generated by the laser. Thus, when a connection structure between servers is changed due to, for example, an improvement in performance or addition of a data center, switching is made between the short-wavelength optical module and the long-wavelength optical module so as to support each connection destination, for example, each connection distance. 
         [0022]    For example, optical signals from the short-wavelength laser and the long-wavelength laser may be received by the same optical receiving module. For example, two photodetectors are disposed via a beam splitter which reflects light with a specific wavelength and lets light with another wavelength pass through. A beam splitter which reflects a laser beam with a long wavelength output from an optical fiber is placed. A photodetector capable of receiving light with a long wavelength is disposed via a lens to a destination of the laser beam which is reflected by the beam splitter. A photodetector capable of receiving light with a short wavelength is disposed via a lens to a destination of a laser beam with a wavelength other than the long wavelength which passes through the beam splitter. 
         [0023]    In the above, while a short-wavelength optical module and a long-wavelength optical module may not be provided both, the number of components such as lenses and photodetectors is increased to make a module system large as a whole. Thus, it may be difficult to increase density for increasing transmission bands desired in optical communications for supporting a data center. The size of an electrical circuit such as a transimpedance amplifier (TIA) placed at a subsequent stage of the photodetector may be doubled or more. 
         [0024]    For example, the optical receiving device is used for optical interconnection inside a server rack for use in a data center or the like (short-wavelength optical communication within several hundreds of meters or so) and between server racks (long-wavelength optical communication on the order of several hundreds of meters or longer). 
         [0025]      FIG. 1  depicts an example of the optical receiving device. The optical receiving device includes a lens unit, a plurality of reflecting units (here, of two types) which reflect each of laser beams of a plurality of light beams with different wavelengths (here, of two types), and a light receiving unit which receives each of the plurality of light beams. The lens unit has, for example, lenses  11   a  and  11   b  which each refract incident light. The reflecting units have, for example, beam splitters  12   a  and  12   b.  In place of the beam splitter  12   b,  a mirror which completely reflects incident light beams of all wavelengths may be used. The light receiving unit may be, for example, a photodetector (photodiode)  13 . 
         [0026]    The lens  11   a  is disposed at a subsequent stage of a light-emission surface  10   a  of an optical fiber  10 , from which a laser beam transmitted from an optical transmitting device is output as an optical signal. The lens  11   b  is disposed at a previous stage of the photodetector  13 . 
         [0027]    The beam splitters  12   a  and  12   b  each support a specific different wavelength.  FIG. 2  depicts an example of reflection and transmission characteristics of the beam splitters. For example, as depicted in  FIG. 2 , the beam splitter  12   a  may have a characteristic of reflecting substantially 100% of light with a long wavelength λ 1  such as, for example, 1310 nm or 1550 nm, and letting substantially 100% of light with another wavelength (including light with a short wavelength λ 2  such as, for example, 850 nm) pass through (with a reflectance of substantially 0%). The beam splitter  12   b  may have a characteristic of reflecting substantially 100% of light with the short wavelength λ 2  and letting substantially 100% of light with another wavelength (including light with the long wavelength λ 1 ) pass through (with a reflectance of substantially 0%). 
         [0028]    The beam splitters  12   a  and  12   b  are placed so as to be separated by a difference d in optical path length between the light with the long wavelength λ 1  and the light with the short wavelength λ 2 , and have an inclination of ea and an inclination of θb, respectively, from a vertical direction. The difference d in optical path length may be uniquely determined from the focal lengths of the lenses  11   a  and  11   b,  a distance from the light-emission surface  10   a  of the optical fiber  10  to the lens  11   a,  a distance between the lenses  11   a  and  11   b,  and a distance from the lens  11   b  to a light-receiving surface  13   a  of the photodetector  13 . 
         [0029]    For example, the difference d in optical path length as an arrangement distance between the beam splitters  12   a  and  12   b  may be determined.  FIG. 3  depicts an example of laser wavelength dependence of a beam waist position of a laser beam gathered at the lens. The beam waist position refers to a position on an optical axis where the beam spot size of a laser beam has a minimum value.  FIG. 3  depicts a case in which the distance between the lenses  11   a  and  11   b  is fixed with respect to laser beams with different wavelengths λ 1  and λ 2 . The light-emission surface  10   a  of the optical fiber  10  is at a position A, the lens  11   a  is at a position B, and the lens  11   b  is at a position C. A beam waist position of the laser beam with the wavelength λ 1  is at a position D separated from the position C by a distance d 21 . A beam waist position of the laser beam with the wavelength λ 2  is at a position E separated from the position C by a distance d 22 . The beam waist positions D and E are determined by the focal lengths of the lenses  11   a  and  11   b,  a distance d 0  from the position A to the position B, a distance d 1  from the position B to the position C, and the wavelengths λ 1  and λ 2 . Thus, as depicted in  FIG. 3 , a different plane depending on the wavelength is at a beam waist position. 
         [0030]    In  FIG. 3 , for example, the photodetector  13  is disposed at the position D, where the laser beams with the wavelengths λ 1  and λ 2  are each received. As depicted in  FIG. 5 , in the photodetector  13 , the laser beam with the wavelength λ 1  is at its beam waist position, and the laser beam with the wavelength λ 2  has a beam diameter larger than the light-receiving surface of the photodetector  13 . In this case, light-receiving efficiency of the laser beam with the wavelength λ 2  has a low value, for example, on the order of 16%. This value is on the order of ⅛ of the light-receiving efficiency of the laser beam with the wavelength λ 1 . 
         [0031]      FIG. 4  depicts an example of laser wavelength dependence when the beam waist position with respect to the lens is fixed. For the laser beam with the wavelength λ 1 , the light-emission surface  10   a  of the optical fiber  10  is at the position A, the lens  11   a  is at the position B, and the lens  11   b  is at a position C 1 . The position D, which is the beam waist position of the laser beam with the wavelength λ 1 , is determined by the focal lengths of the lenses  11   a  and  11   b,  the distance d 0  from the position A to the position B, a distance d 11  from the position B to the position C 1 , and the wavelength λ 1 . For the laser beam with the wavelength λ 2 , it is assumed that the light-emission surface  10   a  of the optical fiber  10  is at the position A, the lens  11   a  is at the position B, and the lens  11   b  is at a position C 2 . The position E, which is the beam waist position of the laser beam with the wavelength λ 2 , is determined by the focal lengths of the lenses  11   a  and  11   b,  the distance d 0  from the position A to the position B, a distance d 12  (&gt;d 11 ) from the position B to the position C 2 , and the wavelength λ 2 . For example, when a distance from the lens  11   b  to the positions D and E of the light-receiving surface is fixed (distance d 2 ), a difference occurs in the distance between the lenses  11   a  and  11   b,  and this distance d 12 -d 11  may correspond to the difference d in optical path length. 
         [0032]    In  FIG. 1 , when a laser beam with the wavelength λ 1  is emitted from the light-emission surface  10   a  of the optical fiber  10 , the laser beam is gathered at the lens  11   a.  The laser beam is then reflected at the beam splitter  12   a,  and is optically coupled to the photodetector  13  via the lens  11   b.  When a laser beam with the wavelength λ 2  is emitted from the light-emission surface  10   a  of the optical fiber  10 , the laser beam is gathered at the lens  11   a.  The laser beam then passes through the beam splitter  12   a  without being reflected, is reflected at the beam splitter  12   b,  and is then optically coupled to the photodetector  13  via the lens  11   b.    
         [0033]    For example, the beam splitters  12   a  and  12   b  are disposed so as to be separated by the difference d in optical path length. With this, the difference in optical path length disappears between the laser beam with the wavelength λ 1  reflected at the beam splitter  12   a  and the laser beam with the wavelength λ 2  reflected at the beam splitter  12   b.  Thus, the laser beams with the wavelengths λ 1  and λ 2  have a common beam waist position on the same plane. The laser beams with different wavelengths have a common beam waist on the same plane. Thus, by matching the same plane and the light-receiving surface  13   a  of the photodetector  13 , a structure may be generated in which all laser beams are received at the same beam waist position, and highly-efficient optical reception (substantially 100% for both of the laser beams with the wavelengths λ 1  and λ 2 ) may be performed. 
         [0034]      FIG. 6  depicts an example of the optical receiving device. In  FIG. 6 , the focal lengths of the lenses  11   a  and  11   b  and a positional relation among the light-emission surface  10   a  of the optical fiber  10 , the lens  11   a,  the lens  11   b , the beam splitter  12   a,  the beam splitter  12   b,  and the light-receiving surface  13   a  of the photodetector  13  are depicted. In this case, the wavelength λ 1  may be 1550 nm, and the wavelength λ 2  may be 850 nm. 
         [0035]    In the above, without increasing the number of components of the optical receiving device, a plurality of light beams with different wavelengths may be received with high efficiency, and optical reception with high reliability may be performed. 
         [0036]      FIG. 7  depicts an example of the optical receiving device. In  FIG. 7 , an optical receiving device and an optical receiving method similar to those of  FIG. 1  are disclosed, but an arrangement state of the components of the optical receiving device is different. In  FIG. 7 , a component similar to that depicted in  FIG. 1  is provided with the same reference character, and its description may be omitted. 
         [0037]    The optical receiving device includes a lens unit, a plurality of reflecting units (here, of two types) which reflect each laser beam of a plurality of light beams with different wavelengths (here, of two types), and a light receiving unit which receives each of the plurality of light beams. The lens unit has, for example, lenses  11   a  and  11   b.  The reflecting units have, for example, beam splitters  12   a  and  12   b.  In place of the beam splitter  12   b,  a mirror which completely reflects incident light beams of all wavelengths may be used. The light receiving unit may be, for example, a photodetector  13 . 
         [0038]    The lens  11   a  is disposed at a subsequent stage of a light-emission surface  10   a  of an optical fiber  10 , from which a laser beam transmitted from an optical transmitting device is output as an optical signal. The lens  11   b  is disposed at a subsequent stage of the lens  11   a.    
         [0039]    The beam splitters  12   a  and  12   b  each support a specific different wavelength. For example, the beam splitters  12   a  and  12   b  are disposed at a subsequent stage of the lens  11   b.  For example, the beam splitters  12   a  and  12   b  are disposed between the lens  11   b  and the photodetector  13 . The beam splitter  12   a  may have a characteristic of reflecting substantially 100% of light with a long wavelength λ 1  such as, for example, 1310 nm or 1550 nm, and letting substantially 100% of light with another wavelength (including light with a short wavelength λ 2  such as, for example, 850 nm) pass through (with a reflectance of substantially 0%). The beam splitter  12   b  may have a characteristic of reflecting substantially 100% of light with the short wavelength λ 2  and letting substantially 100% of light with another wavelength (including light with the long wavelength λ 1 ) pass through (with a reflectance of substantially 0%). 
         [0040]    The beam splitters  12   a  and  12   b  are placed so as to be separated by a difference d in optical path length between the light with the long wavelength λ 1  and the light with the short wavelength λ 2 , and have an inclination of θa and an inclination of θb, respectively, from a vertical direction. The difference d in optical path length may be uniquely determined from the focal lengths of the lenses  11   a  and  11   b,  a distance from the light-emission surface  10   a  of the optical fiber  10  to the lens  11   a,  a distance between the lenses  11   a  and  11   b,  and a distance from the lens  11   b  to a light-receiving surface  13   a  of the photodetector  13 . 
         [0041]    When a laser beam with the wavelength λ 1  is emitted from the light-emission surface  10   a  of the optical fiber  10 , the laser beam is gathered at the lenses  11   a  and  11   b.  The laser beam is then reflected at the beam splitter  12   a,  and is optically coupled to the photodetector  13 . When a laser beam with the wavelength λ 2  is emitted from the light-emission surface  10   a  of the optical fiber  10 , the laser beam is gathered at the lenses  11   a  and  11   b.  The laser beam then passes through the beam splitter  12   a  without being reflected, is reflected at the beam splitter  12   b,  and is then optically coupled to the photodetector  13 . 
         [0042]    For example, the beam splitters  12   a  and  12   b  are disposed so as to be separated by the difference d in optical path length. The difference in optical path length disappears between the laser beam with the wavelength λ 1  reflected at the beam splitter  12   a  and the laser beam with the wavelength λ 2  reflected at the beam splitter  12   b.  Thus, the laser beams with the wavelengths λ 1  and λ 2  have a common beam waist position on the same plane. The laser beams with different wavelengths have a common beam waist on the same plane. Thus, by matching the same plane and the light-receiving surface  13   a  of the photodetector  13 , a structure may be generated in which all laser beams are received at the same beam waist position, and highly-efficient optical reception (substantially 100% for both of the laser beams with the wavelengths λ 1  and λ 2 ) may be performed. 
         [0043]      FIG. 8  depicts an example of the optical receiving device. In  FIG. 8 , the focal lengths of the lenses  11   a  and  11   b  and a positional relation among the light-emission surface  10   a  of the optical fiber  10 , the lens  11   a,  the lens  11   b , the beam splitter  12   a,  the beam splitter  12   b,  and the light-receiving surface  13   a  of the photodetector  13  are depicted. In this case, the wavelength λ 1  may be 1550 nm, and the wavelength λ 2  may be 850 nm. 
         [0044]    For example, the beam splitters  12   a  and  12   b  may be disposed between the lens  11   b  and the photodetector  13 , and the beam splitters  12   a  and  12   b  may be disposed at a previous stage of the lens  11   a.  For example, the beam splitters  12   a  and  12   b  may be disposed between the light-emission surface  10   a  of the optical fiber  10  and the lens  11   a.    
         [0045]    Without increasing the number of components of the optical receiving device, a plurality of light beams with different wavelengths may be received with high efficiency, and optical reception with high reliability may be performed. 
         [0046]      FIG. 9  depicts an example of the optical receiving device. In  FIG. 9 , an optical receiving device and an optical receiving method similar to those of  FIG. 1  are disclosed, but  FIG. 9  is different from  FIG. 1  in that a beam splitter as a component of the optical receiving device is different. In  FIG. 9 , a component similar to that depicted in  FIG. 1  is provided with the same reference character, and its description may be omitted. 
         [0047]    The optical receiving device depicted in  FIG. 9  includes a lens unit, a plurality of reflecting units (here, of two types) which reflect each laser beam of a plurality of light beams with different wavelengths (here, of two types), and a light receiving unit which receives each of the plurality of light beams. The lens unit has, for example, lenses  11   a  and  11   b.  The reflecting units have, for example, beam splitters  21  and  12   b.  In place of the beam splitter  12   b,  a mirror which completely reflects incident light beams of all wavelengths may be used. The light receiving unit may be, for example, a photodetector  13 . 
         [0048]    The beam splitters  21  and  12   b  each support a specific different wavelength. The beam splitter  21  has a characteristic of reflecting light with a long wavelength λ 1  such as, for example, 1310 nm or 1550 nm, with a reflectance lower than 100% (for example, on the order of 80%) and letting substantially 100% of light with another wavelength (including light with a short wavelength λ 2  such as, for example, 850 nm) pass through (with a reflectance of substantially 0%). The beam splitter  12   b  has a characteristic of reflecting substantially 100% of light with the short wavelength λ 2  and letting substantially 100% of light with another wavelength (including light with the long wavelength λ 1 ) pass through (with a reflectance of substantially 0%). 
         [0049]      FIG. 10  depicts an example of a relation between optical input power and PD current. The photodetector has different sensitivities depending on the wavelength of light. Thus, as depicted in  FIG. 10 , even if optical signals with same power are received, a current output from the photodetector differs depending on the wavelength. For example, a TIA for amplifying an electrical signal is placed at a subsequent stage of the photodetector, and the TIA may have a region called a saturated region, where amplification is not performed on more than or equal to a certain value. For example, a high-sensitivity TIA is placed for supporting a laser beam with the short wavelength λ 2  and, when an optical signal with the wavelength λ 1  is received, as indicated by a solid line in  FIG. 10 , a current (PD current) from the photodetector may exceed an input range of the TIA to reach the saturated region. 
         [0050]    For example, in view of the presence of the above-described saturated region, for at least a reflecting unit which supports light with the longest wavelength, for example, the beam splitter  21  which supports light with the long wavelength λ 1 , from among the plurality of reflecting units, the reflectance of light with the long wavelength λ 1  is adjusted.  FIG. 11  depicts an example of reflection and transmission characteristics of the beam splitters. For example, as indicated by a broken line of  FIG. 11 , the transmission and reflection characteristic of the beam splitter  21  which supports light with long wavelength λ 1  is adjusted so that the PD current is in a normal operation range of the TIA. For example, light with the long wavelength λ 1  is reflected with a reflectance lower than 100% (for example, on the order of 80%), and substantially 100% of light with another wavelength is let pass through. Thus, the power of the incident laser beam entering the photodetector  13  is reduced so as not to reach the saturated region of the TIA as indicated by a broken line in  FIG. 10 , and substantially similar reception characteristics may be obtained for both of the laser beams with the long wavelength λ 1  and the short wavelength λ 2 . 
         [0051]    Also in  FIG. 9 , as with  FIG. 8 , the beam splitters  21  and  12   b  may be disposed between the lens  11   b  and the photodetector  13 . The beam splitters  21  and  12   b  may be disposed between the light-emission surface  10   a  of the optical fiber  10  and the lens  11   a.    
         [0052]    In the beam splitter which supports light with the long wavelength λ 1 , the reflectance of the light with the long wavelength λ 1  is adjusted. However, this is not meant to be restrictive. In the beam splitter which supports light with the short wavelength λ 2 , the reflectance of the light with the short wavelength λ 2  may be adjusted. In this case, in the beam splitter which supports light with the short wavelength λ 2 , the reflectance for the short wavelength λ 2  may be adjusted to be lower than 100%, for example, on the order of 80%. In both of the beam splitter which supports light with the long wavelength λ 1  and the beam splitter which supports light with the short wavelength λ 2 , the reflectance of the light with the long wavelength λ 1  and the reflectance of the light with the short wavelength λ 2  may be adjusted. In this case, in both of these two beam splitters, the reflectance of the light with the long wavelength λ 1  and the reflectance of the light with the short wavelength λ 2  each may be adjusted to be lower than 100%, for example, on the order of 80%. 
         [0053]    In the above-described optical receiving device, since a plurality of light beams with different wavelengths are received with high efficiency without increasing the number of components of the optical receiving device, optical reception with high reliability may be performed. 
         [0054]    The optical receiving device may support light beams of two types with different wavelengths (long wavelength λ 1  and short wavelength λ 2 ), and this is not meant to be restrictive. The optical receiving device may support light beams of three types or more with different wavelengths. In this case, a plurality of reflecting units (beam splitters) which reflect a plurality of light beams are disposed so as to be separated by a difference in optical path length among the plurality of light beams. One light receiving unit (photodetector) may be provided, which receives each of the plurality of light beams. For example, for an optical receiving device which supports light beams of three types with different wavelengths, three reflecting units (beam splitters) which reflect light beams of three types are disposed so as to be separated by a difference in optical path length among the light beams of three types. For example, a first beam splitter and a second beam splitter may be disposed so as to be separated by a difference in optical path length between a first light beam with the shortest wavelength and a second light beam with an intermediate wavelength. The second beam splitter and a third beam splitter may be disposed so as to be separated by a difference in optical path length between the second light beam with the intermediate wavelength and a third light beam with the longest wavelength. 
         [0055]    The above-described optical receiving device may support both of communication for a relatively short distance, for example, inside a server rack or between adjacent server racks, and communication for a relatively long distance, for example, between server racks on the same floor or between server racks across a building. In the optical receiving device, both of shortwave and longwave light beams are received. Therefore, for example, the optical receiving device may not be switched based on shortwave or longwave. 
         [0056]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.