Patent Publication Number: US-2012032087-A1

Title: Light collecting optical fiber, photodetection system, optical coupling structure and radio ray detection system

Description:
FIELD OF THE INVENTION 
     The present invention relates to the light collecting optical fibers and the photodetection systems using the fibers, optical coupling structures and radioactive ray detecting units, particularly to photodetection technology by using the optical fibers. 
     BACKGROUND ART 
     A photodetection system is one of the important applications of the optical fiber. When a physical phenomenon generates a light, the generated light can be injected to a photodetector (for example, a photomultiplier,) and thus the physical phenomenon can be detected by detecting the light by the photodetector. The use of the optical fiber increases the degree of freedom in configuration of locations where the light is actually generated and where the detector is placed, thus makes it easier to configure the optical detection system. For example, by using the optical fiber, a photodetection system can be realized in which the place where the light being generated and the place where the light being detected are remote to each other. 
     In the photodetection system using the optical fibers, one of the requirements is to improve the injection efficiency of light into the optical fiber. The injection of the light into the optical fiber is generally made by inputting the light to the end surface of the optical fiber. And, improvement of the injection efficiency is made by adjusting the structure of the end portion where the end surface locates. For example, Japanese patent publication gazette S63-98610 discloses a technology to improve the efficiency of sending and receiving of optical signal by increasing the outer size of the end portion of the optical fiber. On the other hand, the Japanese patent publication gazette S63-303309 discloses an approach to improve the injection efficiency of light into the optical fiber by forming a reverse direction corn at the end portion and a lens at the end surface. 
     However, based on the study by the inventor, there is a limit in the approach injecting the light into the optical fiber from the end of the optical fiber. The approach of inputting light from the end surface of the optical fiber is not a preferable choice, particularly when the size of the light source is large, because the spatial area available is limited.
     [Patent reference 1] Japanese patent publication gazette S63-98610   [Patent reference 2] Japanese patent publication gazette S63-303309   

     DISCLOSURE OF THE INVENTION 
     Therefore, an objective of the present invention is to improve the injection efficiency of light into an optical fiber, particularly when the physical size of the light source is large. 
     SUMMARY OF THE INVENTION 
     According to one of the aspects of the present invention, a light collecting optical fiber comprises a plurality of optical waveguide portions and a light collecting portions inserted between two adjacent optical waveguide portions. Each of the plurality of optical waveguide portions comprises a core and a cladding layer surrounding the core. The light collecting portion is formed in a shape bulging out in radial direction, in order to collect an external light into the optical waveguide portion. The light collecting optical fiber constituted as above can collect light from intermediate portions of an optical fiber, and thus effectively increases the collection efficiency of light. For example, when the physical size of the light source is large, the light collecting optical fiber with high collection efficiency can be constituted by aligning desirable number of light collecting portions corresponding the spatial alignment of the light source and thus receiving light from the wide range of the light source. 
     The light collecting optical fiber may be formed so that light is also received from an end of a sensing portion of the optical fiber. For example, an end collecting portion having a shape bulging out in radial direction may be formed at the end of the sensing portion of the light collecting optical fiber. In another example, the light collecting optical fiber may be formed so that it reflects light at the end. The light collecting optical fiber may be formed so that light can be taken out from both ends of the optical fiber. 
     When the light collecting optical fiber is formed so that it reflects light at the end, a photodetection system detecting an incident location of the external light incident to the light collecting optical fiber can be configured. More specifically, the photodetection system is configured comprising, the light collecting optical fiber which is configured to reflect light at the end, a photodetector connected at a base end of the light collecting optical fiber, and a signal processor, receiving the output signal of the photodetector. From the output signal, the signal processor calculates, a first time when a first light component of a collected light collected from the external light by the light collecting optical fiber, arrives at the photodetector without being reflected at the end of the light collecting fiber, and a second time when a second light component of the collected light, arrives at the photodetector after being reflected at the end of the light collecting optical fiber. The signal processor calculates the incident location of the external light incident to the light collecting optical fiber from the first time and the second time. 
     The photodetection system detecting the incident location of the external light incident to the light collecting optical fiber can also be configured, when the light collecting optical fiber is configured so that the light can be taken out from both ends of the optical fiber. In one of the embodiments, the photodetection system comprises, a light collecting optical fiber which is configured so that the light can be taken out from both ends, a photodetector which is connected to one end of the light collecting optical fiber, a light reflecting means connected to the other end of the light collecting optical fiber, and a signal processor which receives the output signal of the photodetector. From the output signal, the signal processor calculates, the first time when the first light component of a collected light collected from the external light by the light collecting optical fiber, arrives at the photodetector without being reflected at the light reflecting means, and the second time when the second light component of the collected light, arrives at the photodetector after being reflected at the light reflecting means. The signal processor calculates the incident location of the external light incident to the light collecting optical fiber from the first time and the second time. 
     In another embodiment, a photodetection system is configured comprising, the light collecting optical fiber, a first photodetector connected to the first end of the light collecting optical fiber, a second photodetector connected to the second end of the light collecting optical fiber, and a signal processor receiving output signals from the first and the second photodetectors. The signal processor calculates the location of the light incident to the light collecting optical fiber, from the first time that the first light component arrives at the first photodetector and the second time that the second light component arrives at the second photodetector. 
     The above described light collecting optical fiber can be applied to an optical coupling structure, which realizes optical coupling with a light source using light guides. In one of the embodiments, when the light source is located on an extended line of the center axis of the light collecting optical fiber, the light guide is attached to the light source and is constituted to include a portion, which is so configured so that the further from the light source the portion is, the smaller the diameter of the portion. 
     In another embodiment, where a light emitting surface of the light source is aligned parallel to the center axis of the light collecting optical fiber, the light guide is attached to the light emitting surface, and has a body part, the outer surface of which plots a parabola in a cross sectional view perpendicular to the center axis of the light collecting optical fiber, with an axis of the parabola perpendicular to the light emitting surface. The light collecting optical fiber is aligned so that the center axis is at the focal point of the parabola. 
     In other embodiment, a light guide comprises a body part which is attached to the light emitting surface, and an end portion formed at the end of the body part and is attached to the light emitting surface. The body part has a surface shape which plots a first parabola in a cross section that is perpendicular to the center axis of the light collecting optical fiber, with an axis of the parabola perpendicular to the light emitting surface, where the center axis of the light collecting optical fiber is aligned at the focal point of the first parabola. The end portion has a surface shape which plots a second parabola in a cross section that includes the center axis and is perpendicular to the light emitting surface, with an axis of the parabola perpendicular to the light emitting surface, where the end collective portion of the light collective optical fiber is at the focal point of the second parabola. 
     A radioactive ray detector unit which detects radioactive ray is one of the embodiments of the light collecting optical fiber described above. A radioactive ray detector can be configured with the light collecting optical fiber and a scintillator, which is placed adjacent to the light collecting optical fiber. In one of the embodiments, a part of the light collecting optical fiber including at least the light collecting portion is inserted into the hole opened at the scintillator. Here, it is preferable to fill an optical gel having a refractive index between the refractive index of the scintillator and that of a core of the optical fiber, in the space between the surface of the hole and the light collecting optical fiber. 
     When the scintillator is a plastic scintillator, it is preferable that the light collecting optical fiber is embedded in the plastic scintillator so that the whole part of the surface of the light collecting optical fiber, which is inside the plastic scintillator, adheres to the plastic scintillator. 
     The scintillator may be a liquid scintillator. In this case, the radioactive ray detector unit will have a sealed housing which includes the liquid scintillator and at least the light collecting portions of the light collecting optical fiber. 
     Using the light collecting optical fiber, it is possible to configure the radioactive ray detector which detects the type of radioactive rays in addition to the fact that the radioactive rays were incident. In this case, a plurality of scintillators will be aligned adjacent to the light collecting optical fiber. The plurality of scintillators have sensitivity to different types of radioactive rays and also generates light with different wavelengths. 
     It is also possible to constitute a radioactive ray detection unit which detects radioactive ray images, using the light collecting optical fiber. In one of the embodiments, the radioactive ray detector comprises a number of the light collecting optical fibers and a scintillator structure having a number of scintillator blocks separated by slit and a base part which connects the plurality of scintillator blocks. The plurality of light collecting optical fibers are inserted into the holes formed in the scintillator blocks. Here, it is preferable that an optical gel having a refractive index between the refractive index of the scintillator and that of the core of the optical fiber, is filled into the space between the surface of the hole and the light collecting optical fiber. 
     By the present invention, the injection efficiency of light into the optical fiber can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional illustration of the structure of the light collecting optical fiber in one of the embodiments of the present invention. 
         FIG. 2  is an enlarged cross sectional illustration of the structure of the light collecting portion in the light collecting optical fiber of  FIG. 1 . 
         FIG. 3  is a table showing the results of experiments on external light collection by the light collecting optical fiber of the present invention. 
         FIG. 4A  is a cross sectional illustration of the structure of the light collecting optical fiber in another embodiment of the present invention. 
         FIG. 4B  is an enlarged cross sectional illustration of the structure of the end portion of the light collecting optical fiber of  FIG. 4A . 
         FIG. 4C  is a cross sectional illustration of the structure of the light collecting optical fiber of the present invention in further another embodiment. 
         FIG. 4D  is an enlarged cross sectional illustration of the structure of the end portion of the light collecting optical fiber of  FIG. 4C . 
         FIG. 5  is a cross sectional illustration of the structure of the light collecting optical fiber of the present invention, in further another embodiment. 
         FIG. 6  is a conceptual structure of the photodetection system under one embodiment of the present invention. 
         FIG. 7  is a conceptual structure of the photodetection system under another embodiment of the present invention. 
         FIG. 8  is a conceptual structure of the photodetectionsystem under further another embodiment of the present invention. 
         FIG. 9  is a conceptual structure of the photodetectionsystem under further another embodiment of the present invention. 
         FIG. 10  is a cross sectional illustration of the optical coupling structure under one embodiment of the present invention. 
         FIG. 11A  is a cross sectional illustration of the optical coupling structure under another embodiment of the present invention. 
         FIG. 11B  is a cross sectional illustration of the optical coupling structure under another embodiment of the present invention. 
         FIG. 12A  is a cross sectional illustration of the structure of the radioactive ray detector unit under one embodiment of the present invention. 
         FIG. 12B  is a cross sectional illustration of the structure of the radioactive ray detector unit under another embodiment of the present invention. 
         FIG. 12C  is a cross sectional illustration of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 13A  is a cross sectional illustration of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 13B  is a cross sectional illustration of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 13C  is a cross sectional illustration of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 14  is a bird&#39;s eye view of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 15  is a bird&#39;s eye view of the structure of the radioactive ray detector unit under further another embodiment of the present invention. 
         FIG. 16  is an illustration of a fabrication method of the scintillator body for the radioactive ray detector unit of  FIG. 15 . 
         FIG. 17  is an enlarged cross sectional view of the radioactive ray detector of  FIG. 15 . 
       EXPLANATION ON NOTATIONS 
       
           
             10 : light collecting optical fiber 
             10   a : center axis 
             10   b : end surface 
             1 : optical waveguide portions 
             2 : light collecting portions 
             3 : end collecting portion 
             4 : external light 
             5 : high reflection coating 
             6 : low refractive index coating 
             11 : core 
             11   a : surface 
             12 : cladding layer 
             12   a : surface 
             13 : cross section 
             21 ,  21   a ,  21   b : optical fiber 
             22 ,  22   a ,  22   b : photomultiplier 
             23 : signal processor 
             24 : external light 
             25 ,  25   a ,  26 ,  26   a ,  26   b : optical component 
             27 : optical fiber 
             28 : reflector 
             31 : light source 
             31   a : light emitting surface 
             32 : light guide 
             32   a : body part 
             33 : connecting sleeve 
             33   a : body part 
             33   b : receptacle tube 
             34 : optical fiber 
             35 : light shield tube 
             36 : light guide 
             36   a : body part 
             41 : scintillator 
             41   a : hole 
             42 : optical gel 
             43 : seal 
             44 : plastic scintillator 
             45 : enclosure container 
             46 : liquid scintillator 
             47 : seal 
             51 ,  52 ,  53 : scintillator 
             54 ,  55 ,  56 : radioactive ray 
             61 : scintillator body 
             62 : scintillator block 
             63 : base 
             64 : rotary teeth 
             65 : optical gel 
             66 : seal 
             67 : optical fiber 
         
      
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Configuration of a Light Collecting Optical Fiber 
       FIG. 1  shows a cross sectional view of the light collecting optical fiber  10  in one of the embodiments of the present invention. The light collecting optical fiber  10  has a plurality of optical waveguide portions  1 . The optical waveguide portions  1  comprises core  11  and cladding layer  12  surrounding the core  11  so that it functions as an optical fiber. That is, so that it guides light by total reflection of light. In one of the embodiments, the core  11  is made of quartz, the cladding layer  12  is made of fluorine resin. The optical waveguide portion has a circular cross sectional shape and its outer diameter is constant along the length direction. Each optical waveguide portions  1  is aligned so that its center axis fits in the center axis  10   a  of the light collecting optical fiber  10 . 
     The light collecting portion  2  is inserted between the two adjacent optical waveguide portions  1 . The light collecting portion  2  is formed by causing outward bulge in radial direction from the optical waveguide portions and is configured so that it can inject light from external into the optical waveguide portion  1 . In this embodiment, the light collecting portion  2  is formed so that it has a circular outer shape in a cross section perpendicular to the center axis  10   a  of the light collecting optical fiber  10 , where the outer diameter of the light collecting portion  2  is larger than that of the optical waveguide portions  1 . 
       FIG. 2  shows a cross sectional view of the structure of the light collecting portion in one of the embodiments of the present invention. The light collecting portion  2  also comprises of core  11  and cladding layer  12  as the optical waveguide portion  1  does. The light collecting portion  2  is formed in barrel shape, where the diameter r c  of the core  11  of which is larger than the diameter of the core  11  of the optical waveguide portion  1 . The light collecting portion  2  is configured so that the diameter r c  of the core increases gradually towards the cross section  13  (that is, it increases mathematically monotonically), having a maximum value at the cross section  13  which is perpendicular to the center axis  10   a  of the light collecting optical fiber  10 . Accordingly, the outer diameter r E  of the light collecting portion  2  also gradually increases toward the cross section  13 . At the cross section  13 , the rate of change of the diameter r c  of the core  11  (also the rate of change of the outer diameter r E  of the light collecting portion) is zero. The light collecting portion  2  is also configured so that the shape of the outer surface  11   a  of the core  11  and the shape of the outer surface  12   a  of the cladding layer  12  in a cross section including the center axis  10   a  of the light collecting optical fiber  10  form smooth curves. The structure of the light collecting portion  2  as described here contributes to collect and inject light efficiently to the optical waveguide portion  1 . 
     The light collecting optical fiber  10  shown in  FIG. 1  includes the light collecting portion  3  at the end of the optical waveguide portion. The light collecting portion  3  is also formed so that it bulges out in radial direction from the waveguide portion  1  and configured in order to collect and inject external light into the optical waveguide portion  1 . The light collecting portion  3  enables injecting external light into the optical waveguide portion not only from the end surface  10   b  but also from side surface. 
     The light collecting optical fiber  10  of  FIG. 1  can inject external light  4  into optical waveguide portion  1  not only from end surface  10   b  of the light collecting optical fiber  10  but also from side surface of the light collecting portion  2 ,  3  and thus can improve injection efficiency. The light collecting optical fiber  10  of such structure is particularly preferable when the size of the light source which generates the external light  4  is large. By designing locations and numbers of the light collecting portions  2 ,  3  in accordance with the size of the light source, the light collecting optical fiber  10  of  FIG. 1  can efficiently collect and inject external light into the optical waveguide portion  1 . 
     The applicant actually manufactured the light collecting optical fiber  10  experimentally, and measured the performance of collecting the external light.  FIG. 3  shows the results of those measurements. Here, the embodiments 1 and 2 represent the light collecting optical fibers fabricated. As for the reference 1, a conventional plastic optical fiber having no light collecting portions was used. In the embodiment 1, five light collecting portions were built in the mid portion of the light collecting optical fiber  10 , and further the light collecting portion  3  was also built in the end of the optical fiber. On the other hand, in the embodiment 2, two light collecting portions were built in the middle portion of the light collecting optical fiber  10 . In order to prove that the light collection from side surface of the light collecting portion  2  is possible, the end surfaces  10   b  of the light collecting optical fiber  10  of the embodiments 1 and 2 were shielded from external light. The end surface of the reference 1 was also shielded. Total length of the light collecting optical fiber  10  in the embodiments 1, 2 and the plastic optical fiber of the reference 1 was 67.5 mm. The light source was a U shaped fluorescent lamp of 20 W, the distance of which from the light collecting optical fiber  10  or the plastic optical fiber was about 20 cm. An optical power meter was connected at the base end of the light collecting optical fiber  10  or the plastic optical fiber, and measured light power collected by the light collecting optical fiber  10  or the plastic optical fiber. 
     As shown in  FIG. 3 , the reference 1 without the light collecting portion only collected 296 nW power from external light. While the embodiments 1 and 2 collected 5.69 μW and 3.20 μW, respectively. These results demonstrate that the light collecting portion  2  of the light collecting optical fiber  10  indeed has the function to collect the light from the external light. 
     The light collecting optical fiber  10  without the light collecting portion at the end of the optical fiber is one of the feasible options as shown in  FIGS. 4A to 4D . In these embodiments, the optical waveguide portion  1  at the end of the light collecting optical fiber  10  may be configured so that it reflects light at the end surface, for example, as shown in  FIG. 4A  and its enlarged view  FIG. 4B . For example, as shown in  FIG. 4B , a high reflection coating  5  may be formed at the end surface  10   b  of the optical waveguide portion  1  at the end of the light collecting optical fiber  10 . As for the high reflection coating  5 , for example, a metal coating may be used. In such a configuration, the light traveling toward the end of the light collecting optical fiber  10  is reflected towards the direction of the base end of the light collecting optical fiber  10 . 
     As shown in  FIG. 4C  and in its enlarged view  FIG. 4D , the light collecting optical fiber  10  may be configured so that it collects light from the end surface  10   b . In this case, the end surface  10   b  may be coated by a layer of a low refractive index. The layer of a low refractive index is formed of a material having a refractive index higher than that of air but lower than that of the core  11 , such as AMORPHOUS TEFLON (registered trademark), for example. The layer of a low refractive index is formed so that it is thicker at the center portion than the peripheral portion, by which the collecting efficiency of light is expected to be improved. 
     Further as shown in  FIG. 5 , the light collecting optical fiber  10  may be configured so that light outputs are taken out from both ends of the optical fiber. In this case, by connecting a first photodetector at one end of the light collecting optical fiber  10 , and a second photodetector at another end, light output from each end of the light collecting optical fiber  10  can be detected. 
       FIGS. 6 to 9  show examples for the photodetection systems using the light collecting optical fibers  10 . In the photodetection system shown in  FIG. 6 , the optical fiber  21  is connected to the base end of the light collecting optical fiber  10  as configured in  FIG. 1 , and the optical fiber  21  is further connected to the photomultiplier  22 . The output of the photomultiplier  22  is transmitted to the signal processor  23 . When the light collecting optical fiber  10  is exposed with external light  24 , the light collected by the light collecting optical fiber  10  is sent to the photomultiplier  22  via the optical fiber  21 . The photomultiplier  22  detects the light input from the optical fiber  21 . The signal processor  23  detects the input of the light to the light collecting optical fiber  10  from the output of the photomultiplier  22 . By the way, when the light collecting optical fiber  10  (or the optical waveguide  1  located at the end of it) is long enough, the light collecting optical fiber  10  may be directly connected to the photomultiplier  22 . 
     Referring to  FIG. 7 , it is possible to detect the incident location of external light, by using the light collecting optical fiber  10  configured to reflect light at the end, as shown in  FIGS. 4A and 4B . More specifically, when external light  24  was incident to the light collecting optical fiber  10 , a light component  25 , which is a component of light collected by the light collecting optical fiber  10 , and which travels toward base end of the light collecting optical fiber  10 , will input to the photomultiplier  22  without being reflected. On the other hand, a light component  26  which travels toward the end of the light collecting optical fiber  10 , will input to the photomultiplier after being reflected at the end. The signal processor  23  detects the location of light incident from the output of the photomultiplier  22 . More specifically, the signal processor  23  detects a time t 1  when the light component  25  that was not reflected at the end of the light collecting optical fiber  10  arrived at the photomultiplier  22 , and a time t 2  when the light component  26  that was reflected at the end of the light collecting optical fiber  10  arrived at the photodetector  22 . Here, time difference Δt=t 2 −t 1  depends on the distance from the photomultiplier to the incident location. In other words, an incident location close to the end of the light collecting optical fiber  10  causes a small time difference Δt, and a remote incident location causes a large time difference Δt. Therefore, the incident location of external light in the light collecting optical fiber  10  can be detected from the time difference Δt. The signal processor  23  calculates the time difference Δt from the time t 1  and the time t 2 , and estimates the location where the light was incident in the light collecting optical fiber  10  from the time difference Δt. 
     As shown in  FIGS. 8 and 9 , the incident location of external light can be detected when the light collecting optical fiber  10  which is configured to enable the light detection at both ends is used. In the configuration illustrated in the  FIG. 8 , one end of the light collecting optical fiber  10  is connected to the photomultiplier  22  via the optical fiber  21 , and the other end of the light connecting optical fiber is connected to the one end of the optical fiber  27 . The other end of the optical fiber  27  is connected to the reflector  28  which functions as a light reflecting means. A light component  25  of the light collected by the light collecting optical fiber  10 , which travels toward one end of the light collecting optical fiber  10 , inputs into the photomultiplier  22  via the optical fiber  21  without being reflected. On the other hand, a light component  26  that travels toward the other end of the light collecting optical fiber  10 , inputs into the photomultiplier  22  after being reflected by the reflector  28 . Based on the same principle as stated above, in this case the location of light incident in the light collecting optical fiber  10  can also be detected from the time difference Δt between the time t 1  when the light component  25  without being reflected at the end of the light collecting portion  10  inputs to the photomultiplier  22  and the time t 2  when the light component  26  reflected at the end of the optical fiber inputs to the photomultiplier  22 . 
     In the configuration of  FIG. 9 , one end of the light collecting optical fiber  10  is connected to the photomultiplier  22   a  via the optical fiber  21   a  and the other end of the light collecting optical fiber  10  is connected to the photomultiplier  22   b  via the optical fiber  21   b . The signal processor  23  detects the incident location of light in the light collecting optical fiber  10  from the outputs of the photomultipliers  22   a  and  22   b . More specifically, the signal processor  23  detects the time t 1  when a light component  25   a , which travels toward one end of the light collecting optical fiber  10 , inputs into the photomultiplier  22   a  and the time t 2  when a light component  26  which travels toward the other end of the light collecting optical fiber  10 , inputs into the photomultiplier  22   b . The time difference Δt=t 2 −t 1  depends on the location of light incident to the light collecting optical fiber  10 . For example, when the time difference is zero, it indicates that the light was incident to the location where the optical lengths from the photomultipliers  22   a  and  22   b  are equal. On the other hand, when the time difference is positive, it indicates that the incident location was closer to the photomultiplier  22   a  than the point of equal optical length from the photomultipliers  22   a  and  22   b . Conversely, if the time difference is negative, it means the incident location was closer to the photomultiplier  22   b . Thus, from the time difference the location of light incident can be detected. The signal processor  23  calculates the time difference Δt from the times t 1  and t 2 , and detects the incident location of light in the light collecting optical fiber  10 . 
     Further improvement in light injection efficiency can be achieved by embedding the light collecting optical fiber  10  of the present embodiment to the light guide, as shown in  FIGS. 10 ,  11 A and  11 B.  FIG. 10  is a cross sectional view showing the optical coupling structure to inject light to the light collecting optical fiber  10  from the light source located on the extended line of the center axis of the light collecting optical fiber  10 . The light guide  32  is attached to the light emitting surface  31   a  of the light source  31 . As for the light source  31 , a scintillator which emits lights by the input of radioactive ray may be used. The light guide  32  can be formed with a transparent resin such as acrylic, for example. 
     The light guide  32  comprises a body part  32   a  having the shape of a circular truncated cone and an insertion part having the shape of a column and formed at the smaller end of the body part  32   a . The outer diameter of the body part  32   a  decreases as the distance from the light emitting surface increases. The light collecting optical fiber  10  is embedded in the light guide  32  having the shape stated above. The light collecting optical fiber  10  is aligned so that its center axis fits in the center axis of the body part  32   a  of the light guide  32 , and the base end of the light collecting optical fiber  10  fits in the end surface of the insertion part  32   b . The insertion part  32   b  of the light guide  32  is inserted into the connecting sleeve  33 . The connecting sleeve  33  comprises a sleeve body part  33   a  and a receptacle tube  33   b . The receptacle tube  33   b  is bonded to the outer surface of the body part  33   a  and receives the insertion part  32   b  of the light guide  32 . A through hole is opened through the sleeve body part  33   a , through which the optical fiber  34  is inserted. The end of the optical fiber  34  is protected by the connecting sleeve  33  and is forced to contact with the base end of the light collecting optical fiber  10 , which enables the optical connection between the light collecting optical fiber  10  and the optical fiber  34 . The optical fiber  34  is inserted into the light shield tube  35 , and the light shield tube  35  is inserted into the hole of the sleeve body part  33   a  of the connecting sleeve  33 . 
     With the light coupling structure shown in  FIG. 10 , the light emitted from the light source  31  enters into the light collecting optical fiber  10  directly or after being reflected by the surface of the light guide  32 , whereby realizes efficient collection of the light emitted from the light source  31  by the light collecting optical fiber  10 . The light that was incident to the light collecting optical fiber  10  enters into the end of the optical fiber  34  and is further guided to the intended equipment. 
       FIGS. 11A and 11B  shows a cross sectional view of the optical coupling structure to inject light into the light collecting optical fiber  10  from the light source  31  aligned laterally to the light collecting optical fiber  10 . In explanation below, the XYZ Cartesian coordinate system defined as following is used: X axis is taken along the center axis of the light collecting optical fiber  10 , Y axis and Z axis are taken in directions vertical to the center axis of the light collecting optical fiber  10 , where Y axis is taken along the emitting surface  31   a , Z axis is taken perpendicular to the Y axis.  FIG. 11A  shows a cross sectional view in the XZ cross section, while  FIG. 11B  shows one in YZ cross section. 
     As shown in  FIG. 11A , the light source  31  is attached to the light guide  36 . As for the light source  31 , a scintillator which emits light by the irradiation of radioactive rays may be used. The light guide  36  is formed with a transparent resin such as acrylic. The light guide  36  comprises the body part  36   a  and the end part  36   b . The light emitting surface  31   a  of the light source  31  is attached to the body part  36   a  and the end part  36   b  of the light guide  36 . 
     The body part  36   a  of the light guide  36  is formed so that its surface plots a parabola in YZ cross section, having the axis of the parabola perpendicular to the light emitting surface  31   a . The light collecting optical fiber  10  is embedded in the body part  36   a  of the light guide  36 , so that the center axis of the light collecting optical fiber  10  is at the focal point  36   d  of the parabola. An advantage of this type of structure is that any light emitted vertically from the light emitting surface  31   a  and then enters into the body part  36   a  gathers on the light collecting optical fiber  10 , irrespective of the emitting point. This feature contributes to improve the light injection efficiency of the light collecting optical fiber  10 . 
     The end part  36   b  has a shape wherein the surface curves to form a parabola in the YZ cross section, where the axis of the parabola is perpendicular to the light emitting surface  31   a . Further, the end part  36   b  preferably has a shape wherein the surface curves to form a parabola in the XZ cross section also, where the axis of the parabola is perpendicular to the light emitting surface  31   a . Here, the light collecting portion  3  at the end of the light collecting optical fiber  10  preferably is at the focal point of the parabola plotted by the surface in the XZ cross section. The light collection efficiency of the light collecting optical fiber  10  can be effectively improved by this structure. 
     2. Detection of Radioactive Rays Using the Light Collecting Optical Fiber 
     Detecting radioactive rays is one of the preferable applications of the embodiments of the light collecting optical fiber  10  of the present invention. By aligning the light collecting optical fiber  10  close to (typically by embedding in) the scintillator which emits light corresponding to the incident radioactive ray (for example, X ray, β ray, gamma ray,) a radioactive ray detector system that detect radioactive ray can be constituted. The type of scintillator may be selected depending on the type of radioactive ray to be detected. By adopting the structure of the light collecting optical fiber  10  as described above, the injection efficiency of the light generated in the scintillator can be improved, thereby the sensitivity in detecting the radioactive ray can also be improved. 
       FIGS. 12  A to  12 C and  FIGS. 13  A to  13 C show cross sectional views of the structures of the radioactive ray detecting units wherein the light collecting optical fibers are embedded in the scintillators. 
     Referring  FIG. 12A , in one of the embodiments, a hole  41   a  is formed in the scintillator  41  and the light collecting optical fiber  10  is inserted into the hole  41   a . The portion of the light collecting fiber  10  including at least the light collecting portions  2  and  3  is installed in the hole  41   a . In  FIG. 12A , the light collecting optical fiber  10  that has been configured to take out light from the one end. As for the scintillator  41 , a plastic scintillator or inorganic crystal scintillators (for example, NaI, BGO, GSO, LSO, LaBr3) may be used. 
     An optical gel  42  is filled in the space between the light collecting optical fiber  10  and the inner face of the hole  41   a . The optical gel  42  has a refractive index between the refractive index of the light collecting optical fiber and that of the scintillator. The optical gel  42  is used to improve the optical coupling between the light collecting optical fiber  10  and the scintillator  41 , and thereby to improve the injection efficiency to the light collecting optical fiber  10 . In order to prevent the optical gel from leaking, the entrance of the hole  41   a  is sealed by the seal  43 , through which a through hole to insert the light collecting optical fiber  10  is formed. The base end of the light collecting optical fiber  10  is connected to the photodetector directly or via an optical fiber. 
     In the radioactive ray detecting unit with such a configuration, the scintillator  41  emits lights when the radio active ray to be detected enters into the scintillator  41 . The generated lights are collected by the light collecting optical fiber  10 . The collected lights are sent to the photodetector, where the incident event of the radioactive ray can be detected by detecting the light. The photodetection system, which detects the lights collected by the light collecting optical fiber  10  may be configured as shown in  FIG. 6 , for example. 
     When a plastic scintillator is used as the scintillator, the light collecting optical fiber  10  may be embedded in the plastic scintillator  44  so that a whole part of the surface of the light collecting optical fiber  10  that is within the plastic scintillator  44  adheres tightly to the plastic scintillator  44 , as shown in  FIG. 12  B. In such a configuration, a good optical coupling between the plastic scintillator  44  and the light collecting optical fiber  10  can be achieved. The structure shown in  FIG. 12  B can be easily achieved by embedding the light collecting optical fiber  10  when forming the plastic scintillator  44 . 
     On the other hand, the structure shown in  FIG. 12A  is preferable to the one shown in  FIG. 12B , when an inorganic material is used as the scintillator. In case of an inorganic scintillator, the structure shown in  FIG. 12B  would be hard to be achieved, since the inorganic scintillator is hard to treat by plastic forming. The structure shown in  FIG. 12A , which requires just forming a hole in the inorganic crystal scintillator, can be easily achieved. 
     A liquid scintillator may be used as a scintillator.  FIG. 12C  shows a cross sectional view for a radioactive ray detector unit, which utilizes a liquid scintillator. The light collecting optical fiber  10  is inserted into the enclosure container  45  and the liquid scintillator is encapsulated in it. The inlet port of the enclosure container  45  is sealed by the plug  47 . This structure can realize a detection of the radioactive ray detector. 
     In the radioactive ray detectors of  FIGS. 12A to 12C , the light collecting optical fiber  10  being configured to reflect light at the end may be used. In such a case, the radioactive ray detector which detects the incident location of the radioactive ray may be configured by using the configuration of the optical detection system as shown in  FIG. 7 . 
     As shown in  FIGS. 13A to 13C , the light collecting optical fiber  10  configured to take out lights from both ends may also be adopted. In such situations, the configuration of the light detection system of  FIG. 8  or  9  may be adopted in order to detect the incident location of the radioactive ray. 
       FIG. 14  shows a bird&#39;s eye view of the radioactive ray detector unit utilizing the light collecting optical fiber  10 . In the radioactive ray detector unit possessing the configuration shown in  FIG. 14 , three light collecting optical fibers  10  are arrayed in parallel and further they are embedded in three plate-like scintillators  51 ,  52  and  53  that are arrayed in the length direction of the light collecting optical fibers. The scintillators  51 ,  52  and  53  each have sensitivities to different types of radioactive rays, and further each emits light in a different wavelength. For example, the scintillator  51  has a sensitivity to gamma rays, the scintillator  52  has a sensitivity to beta rays, the scintillator  53  has a sensitivity to neutrons. When radioactive rays of a first type  54  (for example, gamma rays) are incident into the scintillator  51 , the scintillator  51  generates lights of a first wavelength which are collected by the light collecting optical fiber  10 . When radioactive rays of a second type  55  (for example beta rays) are incident into the scintillator  52 , the scintillator  52  generates lights in a second wavelength, which are collected by the light collecting optical fiber  10 . When radioactive rays of a third type  56  (for example, neutron rays) are incident into the scintillator  53 , the scintillator  53  generates lights in a third wavelength, which are collected by the collecting optical fiber  10 . The light collecting optical fiber  10  collects lights generated and output them. The radioactive ray detection unit of the above configuration is enabled to detect an incidence of a radioactive ray and a type of radioactive ray by connecting the light collecting optical fiber to a photodetector that can distinguish the wavelength of the incident light. 
     An image of light caused by the radioactive rays can be taken by an arrayed configuration of the scintillators and the light collecting optical fibers  10 .  FIG. 15  shows a bird&#39;s eye view of the radioactive ray detector, where the scintillators and the light collecting optical fibers  10  are configured as two dimensional arrays. Upon the scintillator body structure  61 , slits are formed in a matrix pattern, which forms the scintillator blocks  62 . The scintillator blocks  62  are used to actually detect the radioactive rays. The scintillator blocks are not separated completely but one side of the scintillator blocks are connected via the base part  63 . This structure enables a high density configuration of the scintillator blocks  62  to detect the radioactive rays. The scintillator body structure  61  shown in  FIG. 15  may be formed by cutting a plate-like scintillator by rotary teeth of a blade  64  down to the middle point of thickness direction. 
       FIG. 17  is a cross section showing a detail of the radioactive ray detector block  62 . In  FIG. 17 , the arrow  68  indicates a slit to separate the scintillator block  62 . Each of the scintillator blocks  62  has a hole, to which the light collecting optical fiber is inserted.  FIG. 15  indicates that the light collecting optical fibers are inserted only in a part of the scintillator blocks for simplicity. However, note that the light collecting optical fibers are inserted in every scintillator blocks. In  FIG. 17 , the optical gel  65  is filled in the space between the light collecting optical fiber  10  and the inner surface of the hole. The optical gel  65  has a refractive index between the refractive index of the light collecting optical fiber and that of the scintillator, thereby, the optical gel improves the optical coupling between the light collecting optical fiber  10  and the scintillator block  62 . The hole into which the light collecting optical fiber is inserted, is sealed with the plug  66 , which prevents the optical gel  65  from leaking. The plug  66  also functions as a connecting sleeve, which supports the light collecting optical fiber  10  and the optical fiber  67 . An end of the optical fiber  67  is pressed against the end of the light collecting optical fiber  10 , by which the optical fiber  67  is optically connected with the light collecting fiber  10 . The other end of the optical fiber  67  is connected to the photodetector. Thus, the light incident into each light collecting optical fiber is detected by the photodetector. 
     Using the configuration described above, the radioactive ray incident into each scintillator block  62  can be detected and the image caused by the radioactive rays can be taken. The radioactive ray detector unit of the configuration shown in  FIG. 15  may be applied to a PET (Positron Emission Tomography) system, for example. 
     Although various embodiments of the light collecting optical fiber under the present invention are discussed as above, the present invention can be embodied in various other ways. Therefore, the present invention should not be interpreted as limiting to the above embodiments. Particularly, it should be noted that the light collecting optical fiber of the present invention can be applied to various applications other than the radioactive ray detector system. For example, the light collecting optical fiber of the present invention may be used as a light collecting part of a sunlight introduction system which injects sunlight from the light collecting part aligned on the roof into a lighting panel in a house via bundle of optical fibers.