Patent Publication Number: US-2021181015-A1

Title: Photodetection device and laser device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from International Patent Application No. PCT/JP2019/024740 filed. Jun. 21, 2019, which claims priority from Japanese Patent Application No. 2018-118956 filed Jun. 22, 2018. The contents of both applications are incorporated herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a photodetection device and a laser device equipped with the photodetection device. 
     BACKGROUND 
     Fiber laser devices are used in various fields such as the field of laser machining and the field of medical care because such fiber laser device has excellent light gathering performance and a high power density, and provides the light having a small beam spot. In order to achieve good processing quality with such highly efficient laser device, the intensity of light propagating through an optical fiber needs to be accurately detected. 
     For example, Patent Literature 1 listed below describes a fiber laser device that estimates the intensity of light propagating through an optical fiber by detecting the light leaking from a joint between optical fibers. Patent Literature 2 listed below describes a sensor unit that estimates the intensity of light propagating through an optical fiber by detecting Rayleigh scattering of light propagating through the optical fiber. 
     [Patent Literature 1] WO 2012/073952 A1 
     [Patent Literature 2] WO 2014/035505 A2 
     The fiber laser device described in Patent Literature 1 listed above uses the light leaking from a joint between optical fibers, and the leaking light generates heat when taken out of an optical fiber. Such heat generation is more noticeable as the light propagating through an optical fiber has higher energy. Therefore, in the fiber laser device described in Patent Literature 1 listed above, the detector and a path to the detector are more affected by heat as the light propagating through an optical fiber has higher energy, and the relationship between a detection result and an intensity of light propagating through an optical fiber, which is estimated from the detection result, tends to be less linear. As a result, it can be difficult to accurately detect the intensity of light propagating through an optical fiber. 
     The sensor unit described in Patent literature 2 listed above detects Rayleigh scattering of light propagating through an optical fiber. However, it is difficult to determine in which direction the light for Rayleigh scattering is propagating through an optical fiber since Rayleigh scattering occurs in all directions. Therefore, it is difficult for the sensor unit described in Patent Literature 2 listed above to accurately detect the intensity of the light propagating in a predetermined direction through an optical fiber. In particular, it is more difficult to accurately detect the intensity of the light propagating in a predetermined direction through an optical fiber in processing a highly reflective material such as metal working because a reflective light that propagates in a direction opposite to the direction of the output laser can produce. 
     SUMMARY 
     Accordingly, one or more embodiments of the present invention provide a photodetection device being capable of improving the accuracy of detection of the intensity of the light propagating in a predetermined direction through an optical fiber, and a laser device equipped with the photodetection device, the photodetection device. 
     A photodetection device according to one or more embodiments of the present invention comprises: a plurality of first optical fibers; an optical combiner having one end face to which one end face of each of she first optical fibers is connected; a second optical fiber having one end face to which the other end face of the optical combiner is connected; a first photodetector that detects an intensity of light propagating through at least one of the first optical fibers; a second photodetector that detects Rayleigh scattering of light propagating through the second optical fiber; and a calculation unit (calculator) that calculates the intensity of light propagating in a predetermined direction through the first optical fibers or the second optical fiber, from a result of detection by the first photodetector and a result of detection by the second photodetector. 
     In the photodetection device of the present invention, a plurality of first optical fibers are connected to one end face of the optical combiner, and the second optical fiber is connected to the other end face of the optical combiner. When the plurality of first optical fibers and the second optical fiber are optically coupled via the optical combiner in this way, a difference arises between the ratio of the light propagating from the plurality of the first optical fibers to the second optical fiber and the ratio of the light propagating from the second optical fiber to the plurality of the first optical fibers. In other words, it is easier to propagate light from an end face of the optical combiner, which is connected to the second optical fiber, to the core of the second optical fiber, whereas it is difficult to propagate light from an end face of the optical combiner, which is connected to the first optical fibers, to the cores of the first optical fibers. This is conceivably because the light coming from the second optical fiber side is incident on a gap between the adjacent first optical fibers on the end face of the optical combiner and on the claddings of the first optical fibers when the plurality of first optical fibers is connected to an end face of the optical combiner. Meanwhile, the first photodetector detects the intensity of the light propagating through at least one of the first optical fibers. In the case where light propagates through the plurality of first optical fibers, if the intensity of the light propagating through at least one of the first optical fibers is known, the intensity of the light propagating through the plurality of the first optical fibers can be estimated by totalizing the intensities. The second photodetector detects the intensity of the light propagating through the second optical fiber by detecting Rayleigh scattering of the light propagating through the second optical fibers. Because of the aforementioned difference between the ratios, a difference in detection result arises between the first photodetector and the second photodetector, which are placed to sandwich the optical combiner as described above. The difference in detection result is dependent on the intensity of the light propagating from the first optical fibers toward the second optical fiber and on the intensity of the light propagating from the second optical fiber toward the first optical fibers. Therefore, the calculation unit can calculate the intensity of the light propagating from the first optical fibers toward the second optical fiber or the intensity of the light propagating from the second optical fiber toward the first optical fibers, from the result of detection by the first photodetector and the result of detection by the second photodetector. That is, the calculation unit can calculate the intensity of the light propagating through the first optical fibers or through the second optical fiber in a predetermined direction. 
     In addition, in the photodetection device of the present invention, at least the second photodetector detects Rayleigh scattering. Therefore, as compared with the case where leaked light is detected as in the fiber laser device described in Patent Literature 1 above, the relationship between a detection result and an intensity of light propagating through the second optical fiber estimated from the detection result can be kept linear even when the light propagating through the second optical fiber has a high intensity. Note that the method for detecting the intensity of the light propagating through the first optical fiber by using the first photodetector is not particularly limited. The light propagating through the plurality of first optical fibers is incident on the second optical fibers, whereas the light propagating through each of the first optical fibers has a lower intensity than the light propagating through the second optical fiber. Accordingly, even in the case where, for example, the first photodetector detects leaked light, the relationship between a detection result and an intensity of light propagating through the first optical fiber estimated from the detection result is more likely to be kept linear. Therefore, the photodetection device of the present invention can improve the accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction. 
     In addition, in one or more embodiments, the first photodetector detects the intensity of the light propagating through all the first optical fibers. 
     As described above, the first photodetector detects the intensity of the light propagating through at least one of the first optical fibers, so that the intensity of the light propagating through the plurality of first optical fibers can be estimated. However, in the case where light propagates through the plurality of first optical fibers, the first photodetector detecting the intensity of the light propagating through all the first optical fibers eliminates the need for estimating the intensity of the light propagating through the plurality of first optical fibers as described above, and it is made easier to detect the intensity of the light propagating through the plurality of first optical fibers. In addition, if the first optical fiber is very thin relative to the size of the sensor used as the first photodetector, arranging the plurality of first optical fibers in parallel allows a single sensor to detect the intensity of the light propagating through all the first optical fibers, and the sensor can be disposed easily. 
     Furthermore, in one or more embodiments, a light source is optically coupled to the other end face of each of some first optical fibers among a plurality of the first optical fibers, the light source is non-connected to the other end face of each of some other first optical fibers among a plurality of the first optical fibers, and the first photodetector detects the intensity of the light propagating through the first optical fibers that are non-connected to the light source. 
     In the case where the light source is optically coupled to the other end face of each of some first optical fibers among the plurality of first optical fibers and the light source emits light, only the light coming from the second optical fiber side propagated through the first optical fiber that is non-connected to the light source. In this case, the second photodetector detects the intensity of the light propagating in both directions through the second optical fiber, whereas the first photodetector detects the intensity of the light propagating from the second optical fiber side through the first optical fiber. Therefore, it is made easier to calculate the intensity of the light propagating in a predetermined direction through the first optical fiber and the second optical fiber, from a difference between a result of detection by the first photodetector and a result of detection by the second photodetector. 
     In one or more embodiments, the first photodetector detects the intensity of the light emitted from the other end face of the first optical fiber that is non-connected to the light source. 
     Through detection of the intensity of the light emitted from the end face of the first optical fiber, it is made easier to accurately detect the intensity of the light propagating from the second optical fiber side through the first optical fiber. 
     Furthermore, in one or more embodiments, light sources are each optically coupled to the other end face of each of the first optical fibers, each of the light sources is switched between a light emission state and a light non-emission state, and the first photodetector detects, when at least one of the light sources is in the light emission state, the intensity of the light propagating through the first optical fibers optically coupled to the light sources that are in the light non-emission state. 
     In the case where light sources are each optically coupled to the other end face of each of the first optical fibers and each of the light sources is switched between the light emission state and the light non-emission state, it is assumed that at least one of the light sources emits light while the other light sources do not emit light. Then, among the plurality of first optical fibers, some first optical fibers are in the state in which light is propagated toward the optical combiner while some other first optical fibers are in the state in which light is not propagated toward the optical combiner. In this case, the first optical fiber that is optically coupled to the light source in the light non-emission state does not propagate light toward the optical combiner but only propagates the light coming from the second optical fiber side. The first photodetector detects the intensity of the light propagating through the first optical fiber that is not propagating any light toward the optical combiner, so that the first photodetector detects the intensity of the light propagating from the second optical fiber side through the first optical fiber. Therefore, it is made easier to calculate the intensity of the light propagating in a predetermined direction through the first optical fiber and the second optical fiber, from a difference between a result of detection by the first photodetector and a result of detection by the second photodetector. 
     Furthermore, in one or more embodiments, the cladding mode stripper is provided on the second optical fiber, and the second photodetector is disposed closer to the optical combiner than the cladding mode stripper. 
     The ratio of the cladding mode light to the light that is emitted from the second optical fiber and returns to the second optical fiber is indefinite, and thus the cladding mode light may constitute an uncertainty in a result of detection by the second photodetector. Since the second photodetector is provided closer to the optical combiner than the cladding mode stripper, the cladding mode light out of the light that is emitted from the second optical fiber and returns to the second optical fiber can be released to the outside of the second optical fiber by the cladding mode stripper. Therefore, it can be easier for the second photodetector to accurately detect the intensity of the light propagating through the second optical fiber. 
     Furthermore, a laser device according to one or more embodiments of the present invention comprises: any one of the above-described photodetection devices; and a light source that emits light to be incident on the other end face of at least one of the first optical fibers. 
     As described above, the photodetection device of the present invention can improve the accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction. Therefore, the laser device equipped with the photodetection device can improve the accuracy of the control based on the intensity of light propagating through an optical fiber. 
     As described above, the present invention provides a photodetection device and a laser device equipped with the photodetection device, the photodetection device being capable of achieving higher accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a laser device according to a first embodiment of the present invention. 
         FIG. 2  is an enlarged perspective view of the joint between the optical fibers and the optical combiner shown in  FIG. 1 . 
         FIG. 3  is a schematic cross-sectional view of the photodetection device shown in  FIG. 1 . 
         FIG. 4  is a diagram illustrating a photodetection device according to a second embodiment of the present invention in a similar manner to  FIG. 3   
         FIG. 5  is a diagram illustrating a laser device according to a third embodiment of the present invention in a similar manner to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The photodetection device and the laser device according to one or more embodiments of the present invention will now be described in detail with reference to the drawings 
     First Embodiment 
       FIG. 1  is a diagram schematically illustrating a laser device according to a first embodiment of the present invention. As illustrated in  FIG. 1 , a laser device  1  of the present embodiment includes, as main components, a plurality of light sources  2 , a plurality of first optical fibers  3 , a photodetection device  4 , a second optical fiber  5 , and a control unit CP. 
     Each of the light sources  2  is a laser device that emits signal light having a predetermined wavelength, such as, for example, a fiber laser device or a solid-state laser device. In the case where the light sources  2  are fiber laser devices, the fiber laser devices may be resonator type fiber laser devices or may be master oscillator power amplifier (MO-PA) type fiber laser devices. The light emitted from each of the light sources  2  may be, for example, the light having a wavelength of 1070 nm. Each of the light sources  2  is connected to the first optical fiber  3  that propagates the light emitted from the light source  2 . 
     One end face of each of the first optical fiber  3  is connected to one end face of an optical combiner  10 , and the other end face of the first optical fiber  3  is optically coupled to the light source  2 . Accordingly, the first optical fiber  3  is an input optical fiber for inputting the light emitted by the light source  2  to the optical combiner  10 . 
       FIG. 2  is an enlarged perspective view of the joint between the optical fibers and the optical combiner shown in  FIG. 1 . 
     As shown in  FIG. 2 , in the present embodiment, one of the first optical fibers  3  is connected to the optical combiner  10  at the center of one end face of the optical combiner  10 . The other six first optical fibers  3  are disposed around the one first optical fiber  3  to be connected to the one end face of the optical combiner  10 . 
     Each of the first optical fibers  3  includes a core  31 , a cladding  32  surrounding the core  31 , and a coating layer  33  covering the outer perimeter surface of the cladding  32 . Note that  FIG. 2  shows, for convenience, the coating layer  33  of one of the first optical fibers  3  only, while the other first optical fibers  3  are cut on the side opposite to the optical combiner  10 . In addition, the coating layer  33  of each first optical fiber  3  is peeled off at one end on the optical combiner  10  side. 
     In the first optical fiber  3 , the core  31  has a higher refractive index than the refractive index of the cladding  32 . For example, the core  31  is formed of the quartz to which a dopant such as germanium (Ge) for increasing the refractive index is added, while the cladding  32  is formed of pure quartz. Alternatively, the core  31  may be formed of pure quartz to which no dopant is added, while the cladding  32  may be formed of the quartz to which a dopant such as fluorine (F) for decreasing the refractive index is added. The coating layer  33  is made of a material having a refractive index lower than the refractive index of the cladding  32 . Examples of the material contained in the coating layer  33  include an ultraviolet curable resin. 
       FIG. 3  is a schematic cross-sectional view of the photodetection device shown in  FIG. 1 . As illustrated in  FIG. 3 , the photodetection device  1  according to the present embodiment includes, as main components, the plurality of first optical fibers  3 , the optical combiner  10 , the second optical fiber  5 , a first photodetector  21 , a second photodetector  22 , a first AD conversion unit  23 , a second AD conversion unit  24 , and a calculation unit  25 . 
     The optical combiner  10  of the present embodiment is a tapered fiber in which a part of the optical combiner  10  on the first optical fiber  3  side has an unvarying outer diameter while another part of the optical combiner  10  on the second optical fiber  5  side has an outer diameter being gradually reduced. That is, the optical combiner  10  includes a non-reduced diameter portion  13  that has an unvarying outer diameter and a tapered portion  14  that is formed integrally with the non-reduced diameter portion  13  and has an outer diameter gradually reduced further away from the non-reduced diameter portion  13 . In the optical combiner  10 , one end face on the non-reduced diameter portion  13  side and one end face of each first optical fiber  3  are fusion spliced, while the other end face on the tapered portion  14  side and one end face of the second. optical fiber  5  are fusion spliced. 
     The optical combiner  10  of the present embodiment has a core-cladding structure. That is, the optical combiner  10  of the present embodiment includes a core  11  in which the plurality of first optical fibers  3  and the second optical fiber  5  are optically image-formed, and a cladding  12  that has a refractive index lower than the refractive index of the core  11  and surrounds the core  11 . The refractive index of the optical combiner  10  is not particularly limited; however, from the viewpoint of suppressing the reflection of the light incident on the core  11  of the optical combiner  10  from the first optical fiber  3 , the core  11  has a refractive index approximately equal to the refractive index of the central axis and its surrounding of the first optical fiber  3 . For example, the core  11  of the optical combiner  10  is made of a material similar to the material of the core  31  of the first optical fiber  3 , and the cladding  12  of the optical combiner  10  is made of a material similar to the material of the cladding  32  of the first optical fiber  3 . 
     However, the optical combiner  10  may not necessarily have a core-cladding structure, and the entirety of the optical combiner  10  may be a portion that propagates light. In this case, the entirety of the optical combiner  10  can be regarded as the core  11 , and the air around the optical combiner  10  can be regarded as a cladding. Furthermore, in this case, the entirety of the optical combiner  10  is formed of, for example, a material similar to the material of the core  31  of the first optical fiber  3 . 
     Note that the first optical fiber  3  included in the photodetection device  4  may be part of the first optical fiber  3  optically coupled to the light source  2 , or may be another optical fiber that is optically coupled to the first optical fiber  3  and has a configuration similar to the configuration of the first optical fiber  3 . 
     The second optical fiber  5  is an output optical fiber for outputting the light emitted from the optical combiner  10  to the subsequent stage. The second optical fiber  5  includes a core  51 , a cladding  52  surrounding the core  51 , and a coating layer  53  covering the outer perimeter surface of the cladding  52 . For example, the core  51  of the second optical fiber  5  is made of a material similar to the material of the core  31  of the first optical fiber  3 , the cladding  52  of the second optical fiber  5  is made of a material similar to the material of the cladding  32  of the first optical fiber  3 , and the coating layer  53  of the second optical fiber  5  is made of a material similar to the material of the coating layer  33  of the first optical fiber  3 . In addition, the second optical fiber  5  of the present embodiment is a multimode fiber. 
     In the present embodiment, the core  51  of the second optical fiber  5  has a diameter equal to the diameter of an end face of the core  11  of the optical combiner  10  on the tapered portion  14  side, and the cladding  52  of the second optical fiber  5  has an outer diameter equal to the outer diameter of an end face of the cladding  12  of the optical combiner  10  on the tapered portion  14  side. In addition, the end face of the second optical fiber  5  and the corresponding end face of the optical combiner  10  are fusion spliced such that their central axes are aligned with each other. Note that the coating layer  53  of the second optical fiber  5  is peeled off in the vicinity of the end face to be fusion spliced to the optical combiner  10 . 
     Furthermore, the second optical fiber  5  of the present embodiment includes a cladding mode stripper  55 . The cladding mode stripper  55  is disposed on the outside of the cladding  52  of the second optical fiber  5 . The cladding mode stripper  55  is not particularly limited as long as it is configured to release the cladding mode light propagating through the cladding  52  out of the second optical fiber  5 . The cladding mode stripper  55  of the present embodiment is formed by separately providing a plurality of high refractive index portions  55   h  outside the cladding  52 . The high refractive index portions  55   h  is made of a resin having a refractive index higher than the refractive index of the cladding  52 . 
     Note that in  FIG. 1 , nothing is connected to the end face of the second optical fiber  5  on the side opposite to the optical combiner  10 ; however, another optical fiber, a glass rod having a diameter larger than the diameter of the core  51  of the second optical fiber  5 , or the like may be connected to the end face of the second optical fiber  5  on the side opposite to the optical combiner  10 . 
     The first photodetector  21  is disposed upstream of the optical combiner  10  with respect to the direction in which the light from the light source  2  propagates, while the second photodetector  22  is disposed downstream of the optical combiner  10  with respect to the direction in which the light from the light source  2  propagates. In the present embodiment, the first photodetector  21  is disposed on the outside of one of the first optical fibers  3  to detect Rayleigh scattering of the light propagating through the one first optical fiber  3 . The second photodetector  22  is disposed on the outside of the second optical fiber  5  to detect Rayleigh scattering of the light propagating through the second optical fiber  5 . Each of the first photodetector  21  and the second photodetector  22  is formed of, for example, a photodiode. 
     In addition, the second photodetector  22  of the present embodiment is disposed closes to the optical combiner  10  than the cladding mode stripper  55  and is thermally separated from the cladding mode stripper  55 . The second photodetector  22  is disposed to be thermally separated from the cladding mode stripper  55 , whereby the second photodetector  22  can be less affected by the heat generated by the cladding mode stripper  55 . 
     Note that since Rayleigh scattering occurs in all directions, it is difficult to determine in which direction the light for Rayleigh scattering is propagating through an optical fiber by simply detecting Rayleigh scattering. For example, in cases where the laser device  1  is used for processing a highly reflective material such as metal working, the reflected light that propagates in a direction opposite to the emission direction in which the light is emitted from the second optical fiber  5  may propagate through the second optical fiber  5 . For such cases, the photodetection device  4  of the present embodiment can improve the accuracy of detecting the intensities of the light propagating in both directions through the first optical fiber  3  and the light propagating in both directions through the second optical fiber  5 , as will be described later in detail. 
     The first AD conversion unit  23  performs AD conversion on a signal from the first photodetector  21  and sends the resulting signal to the calculation unit  25 . The second AD conversion unit  24  performs AD conversion on a signal from the second photodetector  22  and sends the resulting signal to the calculation unit  25 . 
     As described later, the calculation unit  25  estimates the intensity of the light propagating through the first optical fiber  3  and the intensity of the light propagating through the second optical fiber  5 , through a calculation based on the result of detection by the first photodetector  21  as sent via the first AD conversion unit  23  and the result of detection by the second photodetector  22  as sent via the second AD conversion unit  24 . 
     The control unit CP shown in  FIG. 1  controls the light source  2  on the basis of a signal from the calculation unit  25 , as described later. Note that the calculation unit  25 , the first AD conversion unit  23 , and the second AD conversion unit  24  may be part of the control unit CP. That is, at least one of the calculation unit  25 , the first AD conversion unit  23 , and the second AD conversion unit  24  may be integrated with the control unit CP through the use of a single CPU. 
     The following describes operations and actions of the laser device  1  and the photodetection device  4  of the present embodiment. 
     First, when light is emitted from the individual light sources  2 , the light is incident on the core  11  from one end face of the optical combiner  10  via the first optical fibers  3 . The light incident on the core  11  of the optical combiner from the first optical fibers  3  reaches the tapered portion  11  of the optical combiner  10 . In the tapered portion  14 , at least part of the light propagates while being reflected on the interface between the core  11  and the cladding  12  of the optical combiner  10 . Every time the reflection is repeated, the divergence angle of the light is increased by the outer perimeter surface of the core  11  of the tapered optical combiner  10 . That is, the light reflected on the outer perimeter surface of the core  11  of the optical combiner  10  forms an increasingly large angle with the axial direction of the optical combiner  10 . Then, the light propagating through the tapered portion  14  is emitted at a predetermined divergence angle from the emission face of the optical combiner  10 , which is the end face of the optical combiner  10  on the tapered portion  14  side, and enters the core  51  from one end face of the second optical fiber  5  to propagate through the second optical fiber  5 . In this way, the light emitted from the light sources  2  sequentially propagates through the first optical fibers  3 , the optical combiner  10 , and the second optical fiber  5 , and is emitted from the other end face of the second optical fiber  5 . 
     The light emitted from the other end face of the second optical fiber  5  as described above is applied to a workpiece or the like. Furthermore, part of the light applied to the workpiece or the like may be reflected on a surface of the workpiece or like, and part of the reflected light may further return to the second optical fiber  5 . In the following description, the direction from the first optical fibers  3  toward the second optical fiber  5  may be referred to as the forward direction, while the direction from the second optical fiber  5  toward the first optical fibers  3  may be referred to as the reverse direction. 
     The light that propagates through the individual first optical fibers  3  in the forward direction is caused to be efficiently incident on the second optical fiber  5  by the optical combiner  10 . Therefore, the following calculation ignores loss of the light propagating in the forward direction through the first optical fibers  3 . Letting Pf be the intensity of light propagating in the forward direction through the first optical fiber  3 , and letting Pr be the intensity of light propagating in the reverse direction through the second optical fiber  5 , the intensity M2 of light obtained from Rayleigh scattering detected by the second photodetector  22  can be expressed by the following equation (1): 
         M 2 =NPf+Pr    (1)
 
     where N is the number of the first optical fibers  3  propagating the light from the light sources  2 . In the present embodiment, N=7 on the assumption that the light sources  2  are connected to all of the first optical fibers  3  and the light from the light sources  2  propagates through all of the first optical fibers  3 . Note that it is assumed here that the light with substantially the same intensity propagates from the light source  2  through every first optical fiber  3 ; however, if the intensity of the light differs among the first optical fibers  3  through which the light propagates, a calculation may be done by multiplying Pf by an appropriate coefficient. 
     On the other hand, part of the light propagating in the reverse direction through the second optical fiber  5  may be incident on the first optical fibers  3  through the optical combiner  10 , but the other part of the light is not incident on the first optical fibers  3 . Accordingly, letting α be the ratio of the light incident on the first optical fiber  3  to the light propagating in the reverse direction through the second optical fiber  5 , the intensity M1 of light obtained from Rayleigh scattering detected by the first photodetector  21  can be expressed by the following equation (2). That is, the intensity of the light propagating in the reverse direction through the first optical fiber  3  can be denoted as αPr. 
         M 1= Pt+αPr    (2)
 
     The result of detection by the first photodetector  21  is input to the calculation unit  25  through the first AD conversion unit  23 , while the result of detection by the second photodetector  22  is input to the calculation unit  25  through the second AD conversion unit  24 . Then, the calculation unit  25  performs calculations in accordance with the equations (1) and (2) above. Furthermore, from the equations (1) and (2) above, the intensity Pf of the light propagating in the forward direction through the first optical fiber  3  and the intensity Pr of the light propagating in the reverse direction through the second optical fiber  5  are calculated as in the following equations (3) and (4). 
         Pr =( M 2− NM 1)/(1− N α)   (3)
 
         Pf =(α M 2− M 1)/( Nα− 1)   (4)
 
     The value of α above can be obtained by conducting a test in advance in which light is propagated in the reverse direction through the first optical fiber  3  and the second optical fiber  5  Specifically, first, a calorimeter is disposed for measuring the energy of light emitted from the end face of the first optical fiber  3  on the upstream side. Then, light is propagated in the reverse direction from the second optical fiber  5  on the downstream side, and α can be defined as the ratio between the energy of the light thus caused to enter the second optical fiber  5  and the energy measured by the calorimeter. 
     In addition, in order to take into account any loss of the light propagating in the forward direction through the first optical fiber  3 , caused between the point where the intensity is detected by the first photodetector  21  and the point where the intensity detected by the second photodetector  22 , the following consideration can be given. The light intensity M2 obtained from Rayleigh scattering detected by the second photodetector  22  can be expressed by the following equation (5). In the equation, β is the ratio of the light that propagates in the forward direction from the point where the intensity is detected by the first photodetector  21  to the point where the intensity is detected by the second photodetector  22 . That is, the intensity of the light propagating in the forward direction through the second optical fiber  5  can be denoted as NβPf. 
         M 2= NβPf+Pr    (5)
 
     Then, from the equations (2) and (5) above, the intensity Pf of the light propagating in the forward direction through the first optical fiber  3  and the intensity Pr of the light propagating in the reverse direction through the second optical fiber  5  are calculated as in the following equations (6) and (7), 
         Pr =(α M 2− NM 1)/( Nαβ− 1)   (6)
 
         Pf =( NβM 1− M 2)/( Nαβ− 1)   (7)
 
     The value of β above can be obtained by conducting a test in advance in which light is propagated in the forward direction through the first optical fiber  3  and the second optical fiber  5 . Specifically, first, a calorimeter is disposed for measuring the energy of light emitted from the end face of the second optical fiber  5  on she downstream side. Then, light is propagated in the forward direction from the first optical fiber  3  on the upstream side, and β can be defined as the ratio between the energy of the light thus caused to enter the first optical fiber  3  and the energy measured by the calorimeter. 
     After the calculation unit  25  calculates Pf and Pr, where Pf is the intensity of the light in the forward direction and Pr is the intensity of the light in the reverse direction as described above, the control unit CP can perform predetermined control over the laser device  1  on the basis of the calculation result. For example, the control unit CP can perform control to adjust the output from the light source  2  in accordance with Pf, which is the intensity of the light in the forward direction, or perform control to intercept the light emitted from the laser device  1  when Pf, which is the intensity of the light in the reverse direction, exceeds an allowable value. In addition, the control unit CP may cause a display device (not illustrated) to show a warning or may cause a speaker (not illustrated) to produce a warning sound. 
     As described above, the photodetection device  4  of the present embodiment includes the plurality of first optical fibers  3 , the optical combiner  10  having one end face to which one end face of each of the first optical fibers  3  is connected, and the second optical fiber  5  having one end face to which the other end face of the optical combiner  10  is connected. The photodetection device  4  of the present embodiment further includes the first photodetector  21  that detects the intensity of the light propagating through one of the first optical fibers  3 , the second photodetector  22  that detects Rayleigh scattering of the light propagating through the second optical fiber  5 , and the calculation unit  25 . 
     In the photodetection device  4  of the present embodiment, when the plurality of first optical fibers  3  and the second optical fiber  5  are optically coupled via the optical combiner  10 , a difference arises between the ratio of the light propagating from the plurality of the first optical fibers  3  to the second optical fiber  5  and the ratio of the light propagating from the second optical fiber  5  to the plurality of the first optical fibers  3 . In other words, it is easier to propagate light from an end face of the optical combiner  10 , the end face being connected to the second optical fiber  5 , to the core  51  of the second optical fiber  5 , whereas it is more difficult to propagate light from an end face of the optical combiner  10 , the end face being connected to the first optical fibers  3 , to the cores  31  of the first optical fibers  3 . This is conceivably because, in the case where the plurality of first optical fibers  3  is connected to an end face of the optical combiner  10 , the light coming from the second optical fiber  5  side is incident on a gap between the adjacent first optical fibers  3  on the end face of the optical combiner  10  and on the claddings  32  of the first optical fibers  3 . 
     Meanwhile, the first photodetector  21  of the present embodiment detects the intensity of the light propagating through one of the first optical fibers  3 . In the case where light propagates through the plurality of first optical fibers  3 , if the intensity of the light propagating through at least one of the first optical fibers  3  is known, the intensity of the light propagating through the plurality of the first optical fibers  3  can be estimated by totalizing the intensities. The second photodetector  22  detects the intensity of the light propagating through the second optical fiber  5  by detecting Rayleigh scattering of the light propagating through the second optical fiber  5 . Because of the aforementioned difference between the ratios, a difference in detection result arises between the first photodetector  21  and the second photodetector  22 , which are placed to sandwich the optical combiner  10  as described above. The difference in detection result is dependent on the intensity of the light propagating from the first optical fibers  3  toward the second optical fiber  5  and on the intensity of the light propagating from the second optical fiber  5  toward the first optical fibers  3 . Therefore, as described above, the calculation unit  25  can calculate the intensity of the light propagating from the first optical fibers  3  toward the second optical fiber  5  or the intensity of the light propagating from the second optical fiber  5  toward the first optical fibers  3 , from the result of detection by the first photodetector  21  and the result of detection by the second photodetector  22 . That is, calculation unit  25  can calculate the intensity of the light propagating through the first optical fibers  3  or through the second optical fiber  5  in a predetermined direction. 
     In addition, in the photodetection device  4  of the present embodiment, at least the second photodetector  22  detects Rayleigh scattering. Therefore, as compared with the case where leaked light is detected as in the fiber laser device described in Patent Literature 1 above, the relationship between a detection result and an intensity of light propagating through the second optical fiber  5  estimated from the detection result can be kept linear even when the light propagating through the second optical fiber  5  has a high intensity. Note that the method for detecting the intensity of the light propagating through the first optical fiber  3  by using the first photodetector  21  is not particularly limited. Thus, for example, the first photodetector  21  may detect Rayleigh scattering of the light propagating through the first optical fiber  3 , or may directly detect the light branched from the first optical fiber  3 . The light propagating through the plurality of first optical fibers  3  is incident on the second optical fiber  5 , whereas the light propagating through each of the first optical fibers  3  has a lower intensity than the light propagating through the second optical fiber  5 . Accordingly, even in the case where, for example, the first photodetector  21  detects leaked light, the relationship between a detection result and an intensity of light propagating through the first optical fiber  3  estimated from the detection result is more likely to be kept linear. Therefore, the photodetection device  4  of the present embodiment can improve the accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction. 
     Furthermore, in the photodetection device  4  of the present embodiment, the cladding mode stripper  55  is provided on the second optical fiber  5 , and the second photodetector  22  is disposed closer to the optical combiner  10  than the cladding mode stripper  55 . The ratio of the cladding mode light to the light that is emitted from the second optical fiber  5  and returns to the second optical fiber  5  is indefinite, and thus the cladding mode light may constitute an uncertainty in a result of detection by the second photodetector  22 . Since the second photodetector  22  is provided closer to the optical combiner  10  than the cladding mode stripper  55 , the cladding mode light out of the light that is emitted from the second optical fiber  5  and returns to the second optical fiber  5  can be released to the outside of the second optical fiber  5  by the cladding mode stripper  55 . Therefore, it can be easier for the second photodetector  22  to accurately detect the intensity of the light propagating through the second optical fiber  5 . 
     Furthermore, the laser device  1  of the present embodiment includes the above-described photodetection device  4  and the light source  2  that emits light to be incident on the other end face of each of the first optical fibers  3 . As described above, the photodetection device  4  of the present embodiment can improve the accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction. Therefore, the laser device  1  equipped with the photodetection device  4  of the present embodiment can improve the accuracy of the control based on the intensity of light propagating through an optical fiber. 
     Second Embodiment 
     The following describes a second embodiment of the present invention in detail with reference to  FIG. 4 . Note that unless otherwise specified, the same reference numerals are given to the components identical or equivalent to the components in the first embodiment, and duplicate description is omitted. 
       FIG. 4  is a diagram illustrating a photodetection device according to the second embodiment of the present invention in a similar manner to  FIG. 3 . As shown in  FIG. 4 , the photodetection device  4  of the present embodiment is different from the photodetection device  4  of the first embodiment in that the first photodetector  21  detects the intensity of the light propagating through all the first optical fibers  3 . 
     As in the first embodiment above, the intensity of the light propagating through the plurality of first optical fibers  3  can be estimated by detecting the intensity of the light propagating through at least one of the first optical fibers  3 , the detecting being performed by the first photodetector  21 . However, in the case where light propagates through the plurality of first optical fibers  3 , the first photodetector  21  detecting the intensity of the light propagating through all the first optical fibers  3  eliminates the need for estimating the intensity of the light propagating through the plurality of first optical fibers  3  as described above, and it is made easier to detect the intensity of the light propagating through the plurality of first optical fibers  3 . In addition, if the first optical fiber  3  is very thin. relative to the size of the sensor used as the first photodetector  21 , arranging the plurality of first optical fibers in parallel allows a single sensor to detect the intensity of the light propagating through all the first optical fibers  3 , and the sensor can be disposed easily. 
     Note that in the present embodiment, the first photodetector  21  detects the intensity of the light propagating through all the first optical fibers  3 , and therefore calculations can be done in accordance with the equations (1) to (7) with N=1 assuming that Pf represents the total intensity of the light propagating through all the first optical fibers  3 . 
     Third Embodiment 
     The following describes a third embodiment of the present invention in detail with reference to  FIG. 5 . Note that unless otherwise specified, the same reference numerals are given to the components identical or equivalent to the components in the first embodiment, and duplicate description is omitted. 
       FIG. 5  is a diagram illustrating a laser device according to the third embodiment of the present invention in a similar manner to  FIG. 1 . As shown in  FIG. 5 , in the photodetection device  4  included in the laser device  1  of the present embodiment, the light source  2  is optically coupled to the other end face of each of some first optical fibers  3  among the plurality of first optical fibers  3 , while the light source  2  is non-connected to the other end face of each of some other first optical fibers  3  among the plurality of the first optical fibers  3 . The first photodetector  21  detects the intensity of the light propagating through the first optical fiber  3  that is non-connected to the light source  2 . 
       FIG. 5  illustrates an example in which the light source  2  is non-connected to the other end face of the first optical fiber  3  that is connected to the center of the core  11  of the optical combiner  10 , while the light source  2  is connected to the other end face of each of the other first optical fibers  3 . However, the light source  2  may be non-connected to the other end face of each of the first optical fibers  3  other than the first optical fiber  3  that is connected to the center of the core  11  of the optical combiner  10 , or the light source  2  may be non-connected to the other end face of each of a plurality of first optical fibers  3 . 
     In the case where the light source  2  is optically coupled to the other end face of each of some first optical fibers  3  among the plurality of first optical fibers  3 , when the light source  2  emits light, only the light coming from the second optical fiber  5  side is propagated through the first optical fiber  3  that is non-connected to the light source  2 . In this case, the second photodetector  22  detects the intensity of the light propagating in both directions through the second optical fiber  5 , whereas the first photodetector  21  detects the intensity of the light propagating from the second optical fiber  5  side through the first optical fiber  3 . In other words, in the present embodiment, calculations can be done in accordance with the aforementioned equation (2) width Pf=0. Therefore, it is made easier to calculate the intensity of the light propagating in a predetermined direction through the first optical fiber  3  and the second optical fiber  5 , from a difference between a result of detection by the first photodetector  21  and a result of detection by the second photodetector  22 . 
     The first photodetector  21  of the present embodiment detects the intensity of the light emitted from the other end face of the first optical fiber  3  that is non-connected to the light source  2 . In this case, the first photodetector  21  is, for example, a photodiode that directly detects the light emitted from the other end face of the first optical fiber  3 . Through detection of the intensity of the light emitted from the end face of the first optical fiber  3 , it is made easier to accurately detect the intensity of the light propagating from the second optical fiber  5  side through the first optical fiber  3 . 
     The present invention has been described above by taking the embodiments as examples, but the present invention is not limited to these embodiments. 
     For example, in the embodiments described above, the light emitted from the light source  2  propagates through every first optical fiber  3  that is connected to the light source  2 . However, the light sources  2  each may be optically coupled to the other end face of each of the first optical fibers  3 , and each light source  2  may be switched between the light emission state and the light non-emission state. Then, the plurality of first optical fibers  3  is switched between the state in which light is propagated toward the optical combiner  10  and the state in which light is not propagated. That is, each of the plurality of light sources  2  may be individually controlled to emit light or stop emitting light and, among the first optical fibers  3  respectively connected to the light sources  2 , the light from the light sources  2  may propagate through some first optical fibers  3  and need not propagate through some other first optical fibers  3 . In this case, the first photodetector  21  detects, when at least one of the light sources  2  is emitting light, the intensity of the light propagating through the first optical fiber  3  optically coupled to the light source  2  that is in the light non-emission state. In other words, the first photodetector  21  detects the intensity of the light propagating through the first optical fiber  3  that is not propagating light toward the optical combiner  10 . In order to detect the intensity of the light propagating through the first optical fiber  3  in this way, the first photodetector  21  for detecting the intensity of the light propagating through the first optical fiber  3  is disposed, for example, for each of the first optical fibers  3 . Then, among the plurality of first photodetectors  21 , the first photodetectors  21  for detecting the intensity of the light propagating through the first optical fibers  3  optically coupled to the light sources  2  that are in the light non-emission state are only activated. In this case, the first photodetector  21  detects Rayleigh scattering of the light propagating through the first optical fiber  3 . 
     As described above, among the plurality of first optical fibers  3 , some first optical fibers  3  are switched to the state in which light is propagated toward the optical combiner  10  and some other first optical fibers  3  are switched to the state in which light is not propagated toward the optical combiner  10 . In this case, only the light from the second optical fiber  5  propagates through the first optical fiber  3  that is not propagating any light toward the optical combiner  10 . Through detection of the intensity of the light propagating through the first optical fiber  3  that is not propagating any light toward the optical combiner  10 , the detection being performed by the first photodetector  21 , the first photodetector  21  detects the intensity of the light propagating from the second optical fiber  5  side through the first optical fiber  3 . Therefore, as in the third embodiment, it is made easier to calculate the intensity of the light propagating in a predetermined direction through the first optical fiber  3  and the second optical fiber  5 , from a difference between a result of detection by the first photodetector  21  and a result of detection by the second photodetector  22 . 
     In addition, the number of the first optical fibers  3  connected to one end face of the optical combiner  10  is not particularly limited as long as the number is two or more. The first photodetector  21  is only needed to detect the intensity of the light propagating through at least one of the first optical fibers  3 . 
     Furthermore, in the examples described in the embodiments above, the cladding mode stripper  55  is provided on the second optical fiber  5 , and the second photodetector  22  is disposed closer to the optical combiner  10  than the cladding mode stripper  55 . However, the cladding mode stripper may be provided on the second optical fiber  5  closer to the optical combiner  10  than the second photodetector  22 , or may be provided on the first optical fiber  3 . 
     As described above, the present invention provides a photodetection device and a laser device that can improve the accuracy of detecting the intensity of light propagating through an optical fiber in a predetermined direction, and the photodetection device and the laser device are expected to be used in the fields of fiber laser devices, optical fiber communications, and so on. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
     
         
           1  . . . Laser device 
           2  . . . Light source 
           3  . . . First optical fiber 
           4  . . . Photodetection device 
           5  . . . Second optical fiber 
           10  . . . Optical combiner 
           21  . . . First photodetector 
           22  . . . Second photodetector 
           25  . . . Calculation unit 
         CP . . . Control unit