Patent Publication Number: US-2023160630-A1

Title: Refrigeration system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a refrigeration system. 
     BACKGROUND 
     Japanese Unexamined Patent Publication No. 2018-151148 discloses a technique related to a cryogenic refrigeration system. The cryogenic refrigeration system includes a container that accommodates an object to be cooled. The container interferes with heat transfer from the outside. For example, the cryogenic refrigeration system disclosed in Japanese Unexamined Patent Publication No. 2018-151148 includes several radiation shields for suppressing thermal radiation. 
     In a refrigeration system, a container (chamber) may be opened for a variety of reasons. Immediately after a refrigerating operation is stopped, temperature of components disposed inside the chamber is low. Therefore, when the chamber is opened immediately after the refrigerating operation is stopped, there is a possibility that water vapor in the atmosphere condenses on the cooled components. As a result, frost builds up around the cooled components. 
     When the chamber is opened, the chamber is put on standby until the temperature of the components disposed inside the chamber reaches near-room temperature. The inside of the chamber is cooled to cryogenic temperature during refrigerating operation. Therefore, it is difficult for heat to flow into the chamber from the outside of the chamber. The components disposed inside the chamber are cooled to cryogenic temperature. Therefore, it takes a considerable time for the temperature of the components to reach near-room temperature, the components being disposed inside the chamber. As a result, a considerable time is required until the chamber is opened after the refrigerating operation is stopped. 
     SUMMARY 
     An object of the present disclosure is to provide a refrigeration system capable of shortening the time to open a chamber. 
     A refrigeration system that is one aspect of the present disclosure includes: a chamber forming a cooling region which accommodates an object and in which cooling is performed; a cooling unit that cools the object accommodated in the cooling region; and a heating unit disposed in the cooling region to generate heat. The heating unit includes a heating block that is disposed in the cooling region and that receives light to generate heat, and a light irradiation unit that irradiates the heating block with the light. The heating block includes a closed region isolated from the cooling region. The light irradiation unit irradiates the closed region with the light. 
     The refrigeration system includes the heating block disposed inside the chamber. The heating block is irradiated with the light to generate heat. The heating block can actively raise temperature of the object disposed inside the chamber. Therefore, temperature of members disposed inside the chamber can be quickly raised. As a result, the time taken to open the chamber can be shortened. 
     The closed region of the refrigeration system may be isolated from the cooling region by a lid unit fixed to the heating block. The lid unit may include the light irradiation unit. According to this configuration, the closed region can be reliably isolated from the cooling region. As a result, energy of the light can be efficiently converted into thermal energy. Further, the size of the heating unit can be reduced. 
     The closed region of the refrigeration system may include a light absorption region to be irradiated with the light. The light absorption region may have a light absorption surface having a higher absorptance for the light than an absorptance of a base material of the heating block. According to this configuration, energy of the light can be efficiently converted into thermal energy. 
     The light absorption region of the refrigeration system may have a tubular shape. The light absorption surface may be an inner peripheral wall surface surrounding the light absorption region. According to this configuration, the light absorption surface is easily and efficiently irradiated with the light emitted from the light irradiation unit. 
     The light absorption surface of the refrigeration system may have an undulating shape. According to this configuration, a surface area of the light absorption surface is increased. As a result, energy of the light can be further efficiently converted into thermal energy. 
     A bottom of the light absorption region in the refrigeration system may be defined by a light absorption hole bottom surface. The light absorption hole bottom surface may have a tapered shape. According to this configuration, a traveling direction of the light can be changed. As a result, the opportunity of the light being absorbed by the light absorption surface can be increased. 
     The closed region of the refrigeration system may include a light-emitting end exposure region where a light-emitting end of the light irradiation unit is disposed. An area of a cross section of the light-emitting end exposure region intersecting an optical axis of the light irradiation unit may be larger than an area of a cross section intersecting an optical axis of the light absorption region. According to this configuration, the generation of return light to be incident on the light-emitting end again can be suppressed. 
     The light irradiation unit may further include an optical fiber that guides the light, and an optical fiber holder holding the optical fiber and attaching the optical fiber to the heating block. According to this configuration, a position of the optical fiber with respect to the heating block can be held. As a result, the heating block can be stably irradiated with the light. 
     A thermal conductivity of a base material of the heating block in the refrigeration system may be larger than a thermal conductivity of a base material of the optical fiber holder. According to this configuration, heat can be satisfactorily provided from the heating block to the object. 
     The heating block of the refrigeration system may have a first main surface and a second main surface intersecting the first main surface. The light irradiation unit may be disposed on the first main surface. The second main surface may thermally contact the object. According to this configuration, heat can be satisfactorily provided from the heating block to the object. 
     A heat conductive member may be sandwiched between the second main surface and the object in the refrigeration system. According to this configuration, thermal resistance between the second main surface and the object can be lowered. 
     The cooling unit of the refrigeration system may include a chiller and a support table that is connected to the chiller and that is disposed in the cooling region. The support table may include a stage on which the object is disposed, and a column that supports the stage. The heating unit may be attached to the column. According to this configuration, the column can be actively heated. 
     The cooling unit of the refrigeration system may include a chiller and a support table that is connected to the chiller and that is disposed in the cooling region. The support table may include a stage on which the object is disposed, and a column that supports the stage. The heating unit may be attached to the stage. According to this configuration, the stage can be actively heated. 
     The object of the refrigeration system may include an optical sensor that outputs a signal in response to incident light. According to this configuration, the refrigeration system including the optical sensor can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view showing a refrigeration system of an embodiment. 
         FIG.  2    is a perspective view showing a heating unit included in the refrigeration system of  FIG.  1   . 
         FIG.  3    is an exploded perspective view showing an internal structure of the heating unit. 
         FIG.  4    is a cross-sectional view showing a closed space of the heating unit. 
         FIG.  5 A  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 1. 
         FIG.  5 B  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 2. 
         FIG.  6 A  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 3. 
         FIG.  6 B  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 4. 
         FIG.  7 A  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 5. 
         FIG.  7 B  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 6. 
         FIG.  7 C  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 7. 
         FIG.  8 A  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 8. 
         FIG.  8 B  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 9. 
         FIG.  8 C  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 10. 
         FIG.  8 D  is a side view showing disposition of a heating unit included in a refrigeration system of Modification Example 11. 
         FIG.  9 A  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 12. 
         FIG.  9 B  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 13. 
         FIG.  9 C  is a cross-sectional view showing a heating unit included in a refrigeration system of Modification Example 14. 
         FIG.  10 A  is a view showing disposition of a heating unit included in a refrigeration system of Modification Example 15. 
         FIG.  10 B  is a view showing disposition of a heating unit included in a refrigeration system of Modification Example 16. 
         FIG.  10 C  is a view showing disposition of a heating unit included in a refrigeration system of Modification Example 17. 
         FIG.  11 A  is a view showing disposition of a heating unit included in a refrigeration system of Modification Example 18. 
         FIG.  11 B  is a view showing disposition of a heating unit included in a refrigeration system of Modification Example 19. 
         FIG.  12    is a view showing a configuration in which a holding member is attached to a heating unit with a heat conductive member sandwiched therebetween. 
         FIG.  13 A  is a graph showing a result of Experimental Example 1. 
         FIG.  13 B  is a graph showing results of Experimental Example 2 and Experimental Example 3. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a refrigeration system of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference signs. In the description of the drawings, duplicated descriptions will be omitted. 
     As shown in  FIG.  1   , a refrigeration system  1  includes a vacuum chamber  2 , a cooling unit  3 , and a heating unit  4 . The refrigeration system  1  is a so-called cryostat. The refrigeration system  1  maintains a temperature of an object  91  at a predetermined cryogenic temperature. The object  91  is a sample for measurement, an electric drive element that operates favorably at cryogenic temperature (for example, an optical sensor), or the like. 
     The vacuum chamber  2  forms a cooling region S 1 . The cooling region S 1  is a region where the object  91  is disposed. In order to maintain the temperature of the object  91  at cryogenic temperature, suppressing heat transfer to the object  91  is required. The cooling region S 1  is set to vacuum to suppress heat transfer by thermal conduction to the object  91  through the air. Heat transfer to the object  91  occurs due to not only thermal conduction but also radiation. Specifically, heat from an inner wall surface of the vacuum chamber  2  is emitted as electromagnetic waves. Then, the emitted electromagnetic waves are absorbed by the object  91 . When suppressing heat transfer by such radiation is also required, a heat shield that blocks heat may be provided between an inner wall of the vacuum chamber  2  and the object  91 . 
     The vacuum chamber  2  includes a top plate portion  21 , a cylindrical portion  22 , and a chamber flange  23 . An external shape of the vacuum chamber  2  is an approximately cylindrical shape. An upper end of the vacuum chamber  2  is closed by the top plate portion  21 . A lower end of the vacuum chamber  2  is opened by an opening  2   a.  The vacuum chamber  2  forms a space surrounded by the top plate portion  21  and the cylindrical portion  22 . The space surrounded by the top plate portion  21  and the cylindrical portion  22  is the cooling region S 1 . A shape of the cooling region S 1  is a columnar shape. A back surface  2   b  of the top plate portion  21  is exposed to the cooling region S 1 . An inner peripheral surface  2   c  of the cylindrical portion  22  is also exposed to the cooling region S 1 . The shape of the vacuum chamber  2  is not limited to a cylindrical shape. The shape of the vacuum chamber  2  may be a rectangular cylindrical shape. The shape of the vacuum chamber  2  may be a spherical shape. 
     The chamber flange  23  is provided on an outer peripheral surface at the lower end of the vacuum chamber  2 . The vacuum chamber  2  is fixed to the cooling unit  3  by bolts attached to the chamber flange  23 . The cylindrical portion  22  of the vacuum chamber  2  is provided with a plurality of lead-out/in portions  24 . The lead-out/in portion  24  is a member with a lead-out/in port that is provided from the inside to the outside of the vacuum chamber  2 . It is preferable that the lead-out/in portion  24  has airtightness and heat insulation. For example, an optical fiber  41  to be described later penetrates through the lead-out/in portion  24 . When the object  91  is, for example, an optical sensor, a cable for supplying electric power to the optical sensor, an optical fiber for guiding measurement light from the outside of the vacuum chamber  2  to the optical sensor, a cable for extracting a signal of the optical sensor to the outside of the vacuum chamber  2 , and the like may be introduced into the vacuum chamber  2  from the lead-out/in portion  24 . The function of the lead-out/in portion  24  is not limited to introducing members. The lead-out/in portion  24  may have a function of an optical connector that connects an optical fiber inside the vacuum chamber  2  and an optical fiber outside the vacuum chamber  2 . The lead-out/in portion  24  may have a function of an electrical connector that connects an optical fiber inside the vacuum chamber  2  and an optical fiber outside the vacuum chamber  2 . The lead-out/in portions  24  are provided in the vicinity of the lower end of the vacuum chamber  2 . The lead-out/in portions  24  are provided opposite the closed top plate portion  21 . In other words, the lead-out/in portions  24  are located opposite the object  91  in the space inside the vacuum chamber  2 . In further other words, the lead-out/in portions  24  are provided in the vicinity of a chiller  31  to be described later. Therefore, a distance from the top plate portion  21  to the lead-out/in portions  24  approximately corresponds to a height of the vacuum chamber  2 . 
     The cooling unit  3  includes the chiller  31  and a support table  32 . The chiller  31  may be appropriately selected based on cryogenic temperature required by the refrigeration system  1  or the like. For example, a stirling chiller, a Gifford-McMahon chiller, or the like may be used as the chiller  31 . 
     The support table  32  supports the object  91 . The support table  32  functions as a heat path that transfers (conducts) heat from the object  91 . An essential object to be cooled is the object  91 . In the present embodiment, the object  91  is cooled by the chiller  31  through a holding member  92  to be described later and through the support table  32 . Therefore, when the temperature of the object  91  is raised, the holding member  92  and the support table  32  need to be heated. Therefore, in the present embodiment, the object  91 , the holding member  92 , and the support table  32  are set as an object T to be heated. When the cooling unit  3  does not include the support table  32 , the object T to be heated may be formed of the object  91  and the holding member  92 . When the cooling unit  3  does not include the support table  32  and the holding member  92 , the object T to be heated may be formed of only the object  91 . Heat of the object  91  disposed on the support table  32  is transferred to the chiller  31  through the support table  32 . The support table  32  is inserted from the opening  2   a  of the vacuum chamber  2 . The support table  32  is disposed approximately coaxially with a central axis of the vacuum chamber  2 . A base end of the support table  32  is disposed on a lower end side of the vacuum chamber  2 . The base end of the support table  32  is thermally connected to the chiller  31 . A tip of the support table  32  is disposed on a top plate portion  21  side of the vacuum chamber  2 . 
     For example, the support table  32  includes a first column  321 , a first cooling stage  322 , a second column  323 , and a second cooling stage  324 . The configuration of the support table  32  is not limited to this configuration. The support table  32  may be formed of one column and one stage. The support table  32  may be formed of three or more columns and three or more stages. The first column  321 , the first cooling stage  322 , the second column  323 , and the second cooling stage  324  are thermally connected to each other. An external shape of the first column  321  and of the second column  323  is, for example, a columnar shape. An external shape of the first cooling stage  322  and of the second cooling stage  324  is, for example, a disk shape. The first column  321 , the first cooling stage  322 , the second column  323 , and the second cooling stage  324  are disposed coaxially with each other. 
     A base end of the first column  321  having a cylindrical shape or a columnar shape is attached to the chiller  31 . A lower surface of the first cooling stage  322  is attached to a tip of the first column  321 . A base end of the second column  323  is attached to an upper surface of the first cooling stage  322 . A lower surface of the second cooling stage  324  is attached to a tip of the second column  323 . 
     As shown in  FIG.  2   , the object  91  such as an optical sensor is disposed on an upper surface  324   a  of the second cooling stage  324 . One example of the optical sensor is a semiconductor optical element that operates favorably at cryogenic temperature. Examples of the semiconductor optical element that operates favorably at cryogenic temperature include a superconducting single-photon detector (SSPD) and a superconducting nanowire single-photon detector (SNSPD). One or a plurality of cables are connected to the optical sensor. The upper surface  324   a  of the second cooling stage  324  is an object disposition surface on which the object  91  is disposed. For example, the holding member  92  for holding the object  91  is attached to the upper surface  324   a  of the second cooling stage  324 . The upper surface  324   a  of the second cooling stage  324  contacts the holding member  92 . 
     A cross section of the holding member  92  shown in  FIG.  2    is an L shape. The holding member  92  includes a holding member base  921  and a holding member upright portion  922 . A thermal conductivity of a material forming the holding member  92  is the same as a thermal conductivity of a material forming a heating block  44 . The thermal conductivity of the material forming the holding member  92  is higher than the thermal conductivity of the material forming the heating block  44 . The material forming the holding member  92  is, for example, copper (oxygen-free copper) or aluminum (aluminum alloy). The holding member base  921  includes a lower surface  921   a.  The lower surface  921   a  contacts the upper surface  324   a  of the second cooling stage  324 . The object  91  is attached to one surface  922   a  of the holding member upright portion  922 . The object  91  may be attached to an upper surface of the holding member base  921 . Heat of the object  91  is transferred to the support table  32  including the second cooling stage  324 , through the holding member  92 . 
     The heating unit  4  is attached to the other surface  922   b  of the holding member upright portion  922 . The heating unit  4  supplies heat to the object  91  and to the support table  32  through the holding member  92 . The heating unit  4  supplies heat to the object T to be heated through the holding member  92 . 
     The holding member upright portion  922  is disposed between the object  91  and the heating unit  4 . The object  91  does not directly face the heating unit  4 . In this specification, such disposition refers to that “the object  91  is disposed in a place where the object  91  cannot be seen from the heating unit  4 ”. According to this disposition, even if light L leaks from the heating unit  4 , adverse effects due to the leaked light being incident on the object  91  that is, for example, an optical sensor can be suppressed. For example, the sensitivity of the optical sensor can be prevented from decreasing. The leaked light is unfavorable light. The unfavorable light is, for example, laser light that has leaked into the vacuum chamber  2  from the heating unit  4  during operation of the heating unit  4 . The unfavorable light is light of an indoor lighting that intrudes into the vacuum chamber  2  through the optical fiber. Further, the unfavorable light is light that has leaked from the heating unit  4 . 
     A shape of the holding member  92  is not particularly limited. The shape of the holding member  92  may be randomly set according to a size or shape of the object  91 . The object  91  may be attached to directly contact the upper surface  324   a  of the second cooling stage  324  without the holding member  92  sandwiched therebetween. The heating unit  4  may be attached to directly contact the upper surface  324   a  of the second cooling stage  324  without the holding member  92  sandwiched therebetween. A mode of attachment of the object  91  and the heating unit  4  to the second cooling stage  324  will be described as a modification example later. 
     As shown in  FIG.  3   , the heating unit  4  includes the heating block  44  and a light irradiation unit  90 . More specifically, the light irradiation unit  90  includes the optical fiber  41 , a ferrule  42 , and an optical fiber holder  43 . The light irradiation unit  90  is fixed to the heating block  44  by joining (screwing) using a bolt  45 A and a bolt  45 B. The heating unit  4  is also fixed to the holding member  92  by joining (screwing) using a bolt  45 C and a bolt  45 D (refer to  FIG.  2   ). In  FIG.  3   , an illustration of the bolt  45 C and the bolt  45 D is omitted. 
     A closed region S 2  formed by the heating unit  4  will be described. As shown in  FIG.  4   , the heating unit  4  includes the closed region S 2  separated from the cooling region S 1 . “Being closed” means that the light L with which the closed region S 2  is irradiated from the light irradiation unit  90  does not leak from the closed region S 2  to the cooling region S 1 . In other words, the closed region S 2  is a closed space. The closed region S 2  is formed by closing an opening of a closed hole  44 H that is a hole having a bottom surface provided in the heating block  44 , with a lid unit C. In the present embodiment, a lid unit C is the light irradiation unit  90 . As a result, the closed hole  44 H is closed in a state where the closed hole  44 H is isolated from the cooling region S 1 . The closed hole  44 H has different inner diameters. The closed hole  44 H is formed of two holes formed coaxially with each other. The two holes are a light absorption hole  443  and a fiber exposure hole  442 . The light absorption hole  443  forms a light absorption region S 21 . The fiber exposure hole  442  forms a fiber exposure region S 22  (light-emitting end exposure region). The lid unit C may be formed of a combination of the light irradiation unit  90  and another member. The lid unit C may be formed of only another member separate from the light irradiation unit  90 . The closed region S 2  is a region defined by the optical fiber  41 , the ferrule  42 , the optical fiber holder  43 , and the heating block  44 . 
     The closed region S 2  includes the light absorption region S 21  and the fiber exposure region S 22 . An inner diameter of the fiber exposure region S 22  is larger than an inner diameter of the light absorption region S 21 . A surface defining an upper end of the fiber exposure region S 22  includes an outer holder tube tip surface  431   b,  a ferrule tip surface  42   b,  and a light-emitting end  412 . A surface defining a lower end of the fiber exposure region S 22  includes an exposure hole bottom surface  442   b.  A surface defining an inner periphery of the fiber exposure region S 22  includes an inner holder tube inner peripheral surface  432   a  and an exposure hole inner peripheral surface  442   a.  A surface defining a lower end of the light absorption region S 21  includes a light absorption hole bottom surface  443   b.  A surface defining an inner periphery of the light absorption region S 21  includes a light absorption surface  443   a.  An upper end of the light absorption region S 21  is opened by an opening provided in the exposure hole bottom surface  442   b.    
     A laser light source LS (refer to  FIG.  1   ) is disposed outside the vacuum chamber  2 . The light absorption region S 21  of the heating unit  4  is irradiated with laser light output from the laser light source LS, through the light irradiation unit  90  as the light L. In the refrigeration system  1 , an object to be heated is not directly irradiated with the light L for heating. A wavelength band of the laser light source LS is not particularly limited. The heating unit  4  converts energy of the light L into thermal energy. The heating unit  4  generates heat using the thermal energy. The heat is transferred to the holding member  92  through the heating unit  4 . 
     The optical fiber  41  guides the light L from the outside of the vacuum chamber  2  to the heating unit  4 . The optical fiber  41  is introduced into the vacuum chamber  2  through the lead-out/in portion  24  provided in the vacuum chamber  2 . The optical fiber  41  is connected to an external device. The external device is the laser light source LS. 
     The optical fiber  41  is made of a glass material. When compared to a cable including a conductive member for making electrical connection, a thermal conductivity of the optical fiber  41  is lower than a thermal conductivity of the cable. For example, the cable includes a metal electrical lead, and the metal electrical lead is phosphor bronze. In this case, a thermal conductivity of the metal electrical lead is 50 W/m·K. A thermal conductivity of the optical fiber  41  made of quartz is 1.5 W/m·K. Therefore, the thermal conductivity of the optical fiber  41  made of quartz can be suppressed to approximately 1/30 of that of the cable including the metal electrical lead. As a result, thermal conduction from the outside of the vacuum chamber  2  to the inside of the vacuum chamber  2  through the optical fiber  41  made of quartz is more significantly suppressed than thermal conduction through the cable including the metal electrical lead. A significant reduction of thermal conduction greatly affects the vacuum chamber  2  that is maintained at cryogenic temperature. The optical fiber  41  includes a light-incident end  411  (refer to  FIG.  1   ) and the light-emitting end  412 . The light-incident end  411  is connected to the laser light source LS. The optical fiber  41  is formed of a plurality of optical fibers. For example, two optical fibers may be optically connected to the lead-out/in portion  24 . The light-emitting end  412  is exposed to the fiber exposure region S 22  to be described later. 
     The optical fiber  41  is physically in contact with the vacuum chamber  2  through the lead-out/in portion  24 . Therefore, the optical fiber  41  can be a heat transfer path in the vacuum chamber  2 . However, as described above, the thermal conductivity of the optical fiber  41  is low. Therefore, heat transmitted through the optical fiber  41  is substantially negligible. A heat quantity generated by the heating unit  4  depends on an output of the light L (laser power). Even in the case of guiding light having strong energy, outer dimensions and the like of the optical fiber  41  are not affected. A small cross-sectional area contributes to increasing thermal resistance from the viewpoint of the heat transfer path. In order to maintain the temperature of the object  91  at cryogenic temperature, it is important to reduce heat inflow from the outside of the vacuum chamber  2 . Therefore, a large thermal resistance in the heat transfer path is advantageous in maintaining a cryogenic temperature state. Therefore, as a path that introduces energy for heating, the optical fiber  41  is superior to, for example, a cable for electrical connection used in a heating method using resistance heating. The optical fiber  41  has a predetermined length inside the vacuum chamber  2 . The lead-out/in portion  24  into which the optical fiber  41  is introduced is disposed in the vicinity of the lower end of the vacuum chamber  2 . The light-emitting end  412  of the optical fiber  41  is disposed in the heating unit  4 . The heating unit  4  is disposed on the second cooling stage  324 . The second cooling stage  324  is installed in the vicinity of the top plate portion  21 . The optical fiber  41  has a length approximately from the lower end and the upper end of the vacuum chamber  2 . A length of the optical fiber  41  also contributes to increasing thermal resistance from the viewpoint of the heat transfer path. 
     As shown in  FIG.  3   , the ferrule  42  is attached to the optical fiber  41 . A shape of the ferrule  42  is a columnar shape. The ferrule  42  is made of a material having a low thermal conductivity such as zirconia. The ferrule  42  has a ferrule inner peripheral surface  42   a,  the ferrule tip surface  42   b,  a ferrule base end surface  42   c,  and a ferrule outer peripheral surface  42   d.  The ferrule inner peripheral surface  42   a  forms a through-hole. The ferrule inner peripheral surface  42   a  extends from the ferrule tip surface  42   b  to the ferrule base end surface  42   c.  The light-emitting end  412  of the optical fiber  41  is inserted into the through-hole formed by the ferrule inner peripheral surface  42   a.    
     The ferrule base end surface  42   c  is exposed to the cooling region S 1 . The ferrule tip surface  42   b  is exposed to the inside of the heating unit  4  (closed region S 2 ). In other words, the ferrule tip surface  42   b  is exposed to the closed region S 2 . For example, the ferrule tip surface  42   b  may be flush with the light-emitting end  412  of the optical fiber  41 . The ferrule outer peripheral surface  42   d  is held by the optical fiber holder  43 . Specifically, the ferrule  42  is inserted into the optical fiber holder  43 . At least one of the ferrule tip surface  42   b  and the ferrule base end surface  42   c  is located outside the optical fiber holder  43 . In the example shown in  FIG.  3   , both the ferrule tip surface  42   b  and the ferrule base end surface  42   c  protrude from the optical fiber holder  43 . 
     The optical fiber holder  43  fixes the optical fiber  41  to the heating block  44 . Specifically, the ferrule  42  into which the optical fiber  41  is inserted is attached to the optical fiber holder  43 . The optical fiber holder  43  is fixed to the heating block  44 . As a result, the optical fiber  41  is fixed to the heating block  44 . 
     For example, metal materials are used as materials (base materials) forming the optical fiber holder  43  and the heating block  44 . The metal material that is a base material of the optical fiber holder  43  has a lower thermal conductivity than that of the metal material that is a base material of the heating block  44 . The metal material that is a base material of the optical fiber holder  43  is, for example, stainless steel (SUS). The metal material that is a base material of the heating block  44  is, for example, an aluminum alloy. It is difficult for heat to be transmitted from the heating block  44  to the optical fiber holder  43 . 
     The optical fiber holder  43  includes an outer holder tube portion  431 , an inner holder tube portion  432 , and holder flanges  433 A and  433 B. 
     A shape of the outer holder tube portion  431  is a cylindrical shape. The outer holder tube portion  431  has an outer holder tube inner peripheral surface  431   a,  the outer holder tube tip surface  431   b,  an outer holder tube base end surface  431   c,  and an outer holder tube outer peripheral surface  431   d.  The outer holder tube inner peripheral surface  431   a  extends from the outer holder tube tip surface  431   b  to the outer holder tube base end surface  431   c.  The ferrule  42  is inserted into a through-hole formed by the outer holder tube inner peripheral surface  431   a.  The optical fiber  41 , the ferrule  42 , and the outer holder tube portion  431  are coaxial with each other. The outer holder tube tip surface  431   b  is exposed to the closed region S 2 . Specifically, the outer holder tube tip surface  431   b  is exposed to the fiber exposure region S 22 . An opening is formed in the outer holder tube tip surface  431   b. As shown in  FIG.  3   , the ferrule tip surface  42   b  of the ferrule  42  inserted into the outer holder tube inner peripheral surface  431   a  may protrude from the outer holder tube tip surface  431   b.  The ferrule tip surface  42   b  may be flush with the outer holder tube tip surface  431   b.  The ferrule tip surface  42   b  may not protrude from the outer holder tube tip surface  431   b.  The outer holder tube base end surface  431   c  is exposed to the cooling region S 1 . As shown in  FIG.  3   , the ferrule base end surface  42   c  of the ferrule  42  inserted into the outer holder tube inner peripheral surface  431   a  may protrude from the outer holder tube base end surface  431   c.    
     A shape of the inner holder tube portion  432  is a cylindrical shape. The inner holder tube portion  432  has the inner holder tube inner peripheral surface  432   a,  an inner holder tube outer peripheral surface  432   b,  and an inner holder tube tip surface  432   c.  The inner holder tube portion  432  protrudes from the outer holder tube tip surface  431   b  of the outer holder tube portion  431 . A base end of the inner holder tube portion  432  is integrated with the outer holder tube portion  431 . The inner holder tube portion  432  is coaxial with the outer holder tube portion  431 . An inner diameter of the inner holder tube portion  432  is, as one example, the same as an outer diameter of the outer holder tube portion  431 . As another example, the inner diameter of the inner holder tube portion  432  may be larger than the outer diameter of the outer holder tube portion  431 . The inner diameter of the inner holder tube portion  432  is larger than an outer diameter of the ferrule  42 . An outer diameter of the inner holder tube portion  432  is larger than the outer diameter of the outer holder tube portion  431 . A space surrounded by the ferrule tip surface  42   b,  the light-emitting end  412 , the inner holder tube inner peripheral surface  432   a,  and the outer holder tube tip surface  431   b  is the fiber exposure region S 22 . In other words, a region surrounded by the inner holder tube inner peripheral surface  432   a  is the fiber exposure region S 22 . The fiber exposure region S 22  faces the inner holder tube inner peripheral surface  432   a  and the outer holder tube tip surface  431   b.    
     A pair of the holder flanges  433 A and  433 B fix the optical fiber holder  43  to the heating block  44 . The holder flanges  433 A and  433 B extend from the outer holder tube outer peripheral surface  431   d  in a radial direction of the outer holder tube portion  431 . A direction in which the holder flange  433 B extends is opposite a direction in which the holder flange  433 A extends. 
     The holder flange  433 A has a flange hole  433   a,  a flange attachment surface  433   b,  and a flange main surface  433   c.  The flange hole  433   a  is a through-hole. The flange hole  433   a  extends from the flange attachment surface  433   b  to the flange main surface  433   c.  The bolt  45 B is inserted into the flange hole  433   a.  An inner diameter of the flange hole  433   a  is the same as an outer diameter of the bolt  45 B. The inner diameter of the flange hole  433   a  is slightly larger than the outer diameter of the bolt  45 B. The flange attachment surface  433   b  contact the heating block  44 . The flange main surface  433   c  faces the cooling region S  1 . The bolt  45 B is inserted from an opening of the flange hole  433   a  formed in the flange main surface  433   c.  A head of the bolt  45 B is pressed against the flange main surface  433   c.  A screw portion of the bolt  45 B protrudes from the flange attachment surface  433   b.    
     The holder flange  433 B has the flange hole  433   a,  the flange attachment surface  433   b,  and the flange main surface  433   c.  The holder flange  433 B extends in a direction opposite the holder flange  433 A. Therefore, the outer holder tube portion  431  is located between one flange hole  433   a  and the other flange hole  433   a.  Therefore, a pair of the bolts  45 A and  45 B interpose the outer holder tube portion  431  therebetween. The only difference is that the holder flange  433 B is provided at a position different from that of the holder flange  433 A. Therefore, a detailed description regarding the holder flange  433 B will be omitted. 
     The heating block  44  is irradiated with the light L to generate heat. The heating block  44  guides the generated heat to an object to be heated. The object to be heated is, as one example, the holding member  92 . In order to satisfactorily transfer heat, the heating block  44  is made of a material having a high thermal conductivity. For example, the heating block  44  may be made of aluminum. It is desirable that heat generated in the heating block  44  is guided to the holding member  92  without loss. The loss refers to that some of heat transferred to the heating block  44  is not transferred to the holding member  92 . Heat is transferred from the heating block  44  by thermal conduction and by thermal emission. Heat transfer from the heating block  44  to the holding member  92  by thermal conduction can be realized by bringing the heating block  44  into contact with the holding member  92 . 
     Heat is released to the cooling region S 1  from a portion of the heating block  44  that does not contact the holding member  92 , by thermal emission (thermal radiation) as light (electromagnetic waves). This phenomenon causes energy loss. A surface of the heating block  44  that does not contact the holding member  92  may have a low emissivity. In order to reduce the emissivity, for example, the surface that does not contact the holding member  92  may be mirror-finished. 
     A shape of the heating block  44  is a rectangular parallelepiped shape. The shape of the heating block  44  may be a cubic shape. The shape of the heating block  44  may be a columnar shape. In the following description, it is assumed that the shape of the heating block  44  is a rectangular parallelepiped shape. The heating block  44  has a block main surface  44   a  (first main surface) and a block heat-outputting surface  44   b  (second main surface). The optical fiber holder  43  is attached to the block main surface  44   a.  Openings of a pair of block screw holes  441 A and  441 B are formed in the block main surface  44   a.    
     Female screws are formed in the pair of respective block screw holes  441 A and  441 B. The screw portions of the bolts  45 A and  45 B are screwed to the respective female screws. The pair of block screw holes  441 A and  441 B may be through-holes. When the pair of block screw holes  441 A and  441 B are through-holes, one openings are formed in the block main surface  44   a,  and the other openings are formed in a block bottom surface  44   c.  The pair of block screw holes  441 A and  441 B may be blind holes having respective bottom surfaces. When the pair of block screw holes  441 A and  441 B are blind holes, openings are formed only in the block main surface  44   a,  and openings are not formed in the block bottom surface  44   c.  Axes of the pair of block screw holes  441 A and  441 B are parallel to an axis of the closed region S 2 . The pair of block screw holes  441 A and  441 B interpose the closed region S 2  therebetween. More specifically, the pair of block screw holes  441 A and  441 B interpose the fiber exposure region S 22  and the light absorption region S 21  therebetween. 
     The pair of block screw holes  441 A and  441 B are cavities. Therefore, the pair of block screw holes  441 A and  441 B do not substantially contribute to heat transfer. There may occur a difference in thermal resistance between when the block screw hole  441 A or the block screw hole  441 B exists on a heat path and when the block screw hole  441 A or the block screw hole  441 B does not exist on the heat path. The block heat-outputting surface  44   b  and the pair of block screw holes  441 A and  441 B can also be associated with each other in terms of positional relationship. Heat is desired to be actively transferred to the block heat-outputting surface  44   b.  Therefore, neither of the pair of block screw holes  441 A and  441 B is provided between the light absorption region S 21  and the block heat-outputting surface  44   b.  On the other hand, heat transfer to block side surfaces  44   d  and  44   e  that do not output heat is desired to be suppressed. Therefore, one block screw hole  441 A may be provided between the light absorption region S 21  and one block side surface  44   d.  The other block screw hole  441 B may be provided between the light absorption region S 21  and the other block side surface  44   e.    
     An opening of the fiber exposure hole  442  is also formed in the block main surface  44   a.  The inner holder tube portion  432  is disposed in the fiber exposure hole  442 . The fiber exposure hole  442  is for forming the fiber exposure region S 22 . The fiber exposure hole  442  is surrounded by the exposure hole inner peripheral surface  442   a  and the exposure hole bottom surface  442   b.  The exposure hole inner peripheral surface  442   a  faces the inner holder tube outer peripheral surface  432   b.  A slight gap may be formed between the exposure hole inner peripheral surface  442   a  and the inner holder tube outer peripheral surface  432   b.  The exposure hole bottom surface  442   b  faces the inner holder tube tip surface  432   c.  A gap is provided between the exposure hole bottom surface  442   b  and the inner holder tube tip surface  432   c.  The exposure hole bottom surface  442   b  does not contact the inner holder tube tip surface  432   c.  An opening of the light absorption region S 21  is formed in the exposure hole bottom surface  442   b.    
     The light absorption region S 21  is formed on an optical axis  41 S of the optical fiber  41 . The light absorption region S 21  is the light absorption hole  443  surrounded by the light absorption surface  443   a  having a circular shape in a plan view. The light absorption hole  443  is coaxial with the fiber exposure hole  442 . An axis  443  S of the light absorption hole  443  and an axis of the fiber exposure hole  442  overlap the optical axis  41 S of the optical fiber  41 . An inner diameter of the light absorption hole  443  is smaller than an inner diameter of the fiber exposure hole  442 . A difference between the inner diameter of the light absorption hole  443  and the inner diameter of the fiber exposure hole  442  appears as the exposure hole bottom surface  442   b  of the fiber exposure hole  442 . 
     The light absorption surface  443   a  has an undulating shape to increase absorptance (emissivity) for the light L. For example, an undulating shape such as a female screw shape is formed on the light absorption surface  443   a.  In other words, the light absorption surface  443   a  is obtained by forming a spiral projection on an inner peripheral surface of the light absorption hole  443 . The light absorption surface  443   a  is not limited to a portion on which an undulating shape is provided. For example, the light absorption surface  443   a  may be a portion to which surface treatment such as alumite treatment to be described later is applied. The light absorption surface  443   a  may be a portion having an undulating shape to which surface treatment is applied. 
     An undulating shape such as a female screw shape is formed from an opening of the light absorption hole  443  toward a bottom. An undulating shape such as a female screw shape may be provided on the entirety of the inner peripheral surface of the light absorption hole  443 . An undulating shape such as a female screw shape may be provided on a part of the inner peripheral surface of the light absorption hole  443 . In the illustrated example of  FIG.  3   , an undulating shape such as a female screw shape is not provided on a portion in the vicinity of the bottom surface of the light absorption hole  443 . 
     According to the undulating shape such as a female screw shape, an area that can be irradiated with the light L can be increased. According to the undulating shape such as a female screw shape, a microscopic direction of the light absorption surface  443   a  is inclined with respect to the optical axis  41 S of the optical fiber  41 . According to this inclination, of the light L incident on the light absorption surface  443   a  in a direction along the optical axis  41 S, the light L that is not absorbed by the light absorption surface  443   a  is reflected in a direction different from an incident direction. As a result, the light L is diffusely reflected, so that a so-called optical path length is lengthened. Therefore, the opportunity of the light L being incident on the light absorption surface  443   a  can be increased. As a result, the light L emitted from the optical fiber  41  can be satisfactorily absorbed. Since a traveling direction of the light L emitted from the optical fiber  41  is changed, the opportunity of the light being emitted from the optical fiber  41 , to be incident on the optical fiber  41  again can be reduced. Therefore, return light can be reduced. As a result, damage to the optical fiber  41  due to return light being incident on the optical fiber  41  (light-emitting end  412 ) again can be suppressed. The undulating shape is not limited to a female screw shape. The undulating shape may be a shape of a wall-shaped portion or of a protrusion portion formed on the light absorption surface  443   a.    
     The light absorption surface  443   a  is subjected to another processing of increasing an absorptance for the light L. The fact that the absorptance for the light L is high may be defined as, for example, that the absorptance of the light absorption surface  443   a  is higher than a reflectance. For example, the absorptance of the light absorption surface  443   a  may be defined as being higher than an absorptance of the base material forming the heating block  44 . The processing of increasing the absorptance for light is predetermined surface treatment processing. The processing of increasing the absorptance for light is, for example, processing of making the absorptance of the base material of the heating block  44  for the light L higher than the absorptance of the light absorption surface  443   a  for the light L. For example, when aluminum is used as the base material of the heating block  44 , the surface treatment processing is black alumite processing or plating process to increase the absorptance for light. For example, when black alumite is used, a black alumite coating that is a surface treatment layer is formed on a surface of the base material of aluminum. An absorptance of aluminum (polished surface) is 0.05. On the other hand, an absorptance of black alumite is 0.95. Therefore, the absorptance of black alumite is larger than the absorptance of aluminum. A color of the light absorption surface  443   a  is black. Therefore, thermal resistance between the base material of aluminum and the alumite coating is small. Heat is generated as a result of the alumite coating being irradiated with light. Heat can be satisfactorily transmitted from the alumite coating to the base material of aluminum. 
     The surface treatment processing is applied to at least a portion of the inner peripheral surface of the light absorption hole  443 , on which an undulating shape such as a female screw shape is provided. The surface treatment processing may be applied to the entirety of the inner peripheral surface of the light absorption hole  443 . The surface treatment processing may be applied to a portion on which an undulating shape such as female screw shape is not provided. The surface treatment processing may be applied to the bottom surface of the light absorption hole  443 . The surface treatment processing may be applied to the exposure hole bottom surface  442   b  described above. The surface treatment processing may be applied to the exposure hole inner peripheral surface  442   a  described above. The surface treatment processing may be applied to the entirety of an inner peripheral surface of the heating block  44 , which forms the closed region S 2 . 
     The bottom of the light absorption hole  443  is defined by the light absorption hole bottom surface  443   b.  The light absorption hole bottom surface  443   b  has a tapered shape that is reduced in diameter as the distance from the light-emitting end  412  of the optical fiber  41  increases. For example, the light absorption hole bottom surface  443   b  is an inclined surface having a conical shape. The light absorption hole bottom surface  443   b  is inclined with respect to the optical axis  41 S of the optical fiber  41 . According to such a shape, the light L incident on the light absorption hole bottom surface  443   b  is reflected in a direction different from an incident direction. As a result, the light L is diffusely reflected, so that the optical path length of the light L can be lengthened. As a result, the opportunity of the light L being incident on the light absorption surface  443   a  can be increased. Damage to the optical fiber  41  due to the light L being specularly reflected and being incident on the optical fiber  41  again can be suppressed. 
     A relationship between the light-emitting end  412  of the optical fiber  41 , a position of the light absorption hole  443 , and an inner diameter D 1  of the light absorption hole  443  will be described with reference to  FIG.  4   . As shown in  FIG.  4   , the light L emitted from the light-emitting end  412  travels while spreading based on a numerical aperture (NA) of the optical fiber  41 . From the point of view that heat is generated by the absorption of the light L, it is desirable that all the light L emitted from the light-emitting end  412  is guided to the light absorption hole  443 . For example, it is assumed that a diameter D 2  of the spread light L is larger than the inner diameter D 1  of the light absorption hole  443  at the opening of the light absorption hole  443 . In this case, the exposure hole bottom surface  442   b  is irradiated with some of the light L. On the other hand, as shown in  FIG.  4   , it is assumed that the diameter D 2  of the spread light L is smaller than the inner diameter D 1  of the light absorption hole  443  at the opening of the light absorption hole  443 . In this case, all the light L is guided to the light absorption hole  443 . A state of the latter can be realized by a predetermined relationship between the numerical aperture (NA) of the optical fiber  41 , a distance along the optical axis  41 S from the light-emitting end  412  to the opening of the light absorption hole  443 , and the inner diameter of the light absorption hole  443 . 
     As shown in  FIG.  2   , the block heat-outputting surface  44   b  is a surface that thermally contacts the holding member  92 . Any object T to be heated may thermally contact the block heat-outputting surface  44   b.  For example, the object  91  and the second cooling stage  324  that contact the holding member  92  may thermally contact the block heat-outputting surface  44   b.  The block heat-outputting surface  44   b  is a surface different from the block main surface  44   a.  In the example of  FIG.  2   , the block heat-outputting surface  44   b  is orthogonal to the block main surface  44   a.  The block heat-outputting surface  44   b  may be parallel to the block main surface  44   a.  Namely, the block heat-outputting surface  44   b  may be the block bottom surface  44   c  to be described later. 
     The heating block  44  further has the block bottom surface  44   c,  the block side surfaces  44   d  and  44   e,  and a block back surface  44   f  (refer to  FIG.  2   ). The block bottom surface  44   c  is parallel to the block main surface  44   a.  The block bottom surface  44   c  may be used as a heat-outputting surface. The block side surfaces  44   d  and  44   e  are orthogonal to the block heat-outputting surface  44   b.  The block back surface  44   f  faces and is parallel to the block heat-outputting surface  44   b.  The block side surfaces  44   d  and  44   e  and the block back surface  44   f  are not used as heat-outputting surfaces. Therefore, mirror finishing may be applied to the block side surfaces  44   d  and  44   e  and to the block back surface  44   f  to reduce emissivity. The block heat-outputting surface  44   b,  the block side surfaces  44   d  and  44   e,  and the block back surface  44   f  are four side surfaces forming the heating block  44 . The block heat-outputting surface  44   b,  the block side surfaces  44   d  and  44   e,  and the block back surface  44   f  can be distinguished by block fixing holes  444 A and  444 B. 
     A pair of the block fixing holes  444 A and  444 B are holes into which the bolts  45 C and  45 D are inserted to fix the heating block  44  to the holding member  92 . The pair of block fixing holes  444 A and  444 B are through-holes. The pair of block fixing holes  444 A and  444 B extend from the block back surface  44   f  to the block heat-outputting surface  44   b.  Namely, one surface in which openings of the pair of block fixing holes  444 A and  444 B are formed is the block heat-outputting surface  44   b.  The other surface in which openings of the pair of block fixing holes  444 A and  444 B are formed is the block back surface  44   f.  The openings of the pair of block fixing holes  444 A and  444 B are not formed in the block side surfaces  44   d  and  44   e.  The pair of block fixing holes  444 A and  444 B are provided to interpose the closed region S 2  therebetween. More specifically, the pair of block fixing holes  444 A and  444 B are provided to interpose the fiber exposure region S 22  therebetween. 
     Operation of Refrigeration System Including Heating Unit 
     The refrigeration system  1  can perform, for example, the following operation. First, the chiller  31  is driven to cool the second cooling stage  324  to a predetermined temperature. The predetermined temperature is, for example, two Kelvin. Next, the chiller  31  is stopped. Next, laser light is incident from the laser light source LS. The laser light is guided to the heating unit  4  by the light irradiation unit  90 , as the light L. The heating block  44  irradiated with the light L generates heat. The heat generated by the heating block  44  is transferred to the holding member  92  through the block heat-outputting surface  44   b.  In other words, the heat generated by the heating block  44  is transferred to the object T to be heated through the block heat-outputting surface  44   b.  After it is confirmed whether or not the temperature of the holding member  92  that is the object T to be heated has risen to a predetermined temperature, the emission of the light L that is laser light is stopped. The predetermined temperature is, for example, room temperature. 
     Actions and Effects 
     The refrigeration system  1  includes the vacuum chamber  2  forming the cooling region S 1  which accommodates the object  91  and in which cooling is performed; the cooling unit  3  that cools the object  91  accommodated in the cooling region S 1 ; and the heating unit  4  disposed in the cooling region S 1  to generate heat. The heating unit  4  includes the light irradiation unit  90  including the optical fiber  41  that guides light provided from the outside of the vacuum chamber  2 , and the heating block  44  that is disposed in the cooling region S 1  and that receives the light L emitted from the optical fiber  41 , to generate heat. The heating block  44  includes the closed region S 2  configured to be isolated from the cooling region S  1 . The light irradiation unit  90  irradiates the closed region S 2  with the light L. 
     The refrigeration system  1  includes the heating block  44  disposed inside the vacuum chamber  2 . The heating block  44  is irradiated with the light L to generate heat. The heating block  44  can actively raise temperature of the object T to be heated disposed inside the vacuum chamber  2 . Therefore, temperature of members disposed inside the vacuum chamber  2  can be quickly raised. As a result, the time taken to open the vacuum chamber  2  can be shortened. 
     The refrigeration system  1  can increase temperature rising rate inside the vacuum chamber  2  when maintenance of the refrigeration system  1  is performed. Further, an object disposed inside the vacuum chamber  2  may absorb gas. The absorbed gas can be quickly gasified by the heating of the heating unit  4 . A heat quantity flowing into the vacuum chamber  2  due to thermal conduction can be reduced by employing the optical fiber  41 . As a result, the number of cables for electrical connection provided inside the vacuum chamber  2  can also be increased. Moreover, the refrigeration system  1  can also perform cooling performance evaluation of the chiller  31 . In Experimental Example 1 to be described later, the cooling performance evaluation will be described. 
     The closed region S 2  of the refrigeration system  1  is isolated from the cooling region S 1  by the lid unit C fixed to the heating block  44 . The lid unit C includes the light irradiation unit  90 . According to this configuration, the closed region S 2  can be reliably isolated from the cooling region S 1  by the lid unit C. As a result, energy of the light can be efficiently converted into thermal energy. Further, the light irradiation unit  90  can serve as at least a part of the lid unit C. As a result, the size of the heating unit  4  can be reduced. 
     The closed region S 2  of the refrigeration system  1  includes the light absorption region S 21  with which the light L is irradiated. The light absorption region S 21  has the light absorption surface  443   a  having a higher absorptance for the light L than that of the base material of the heating block  44 . According to this configuration, energy of the light L can be efficiently converted into thermal energy. 
     The light absorption region S 21  of the refrigeration system  1  has a tubular shape. The light absorption surface  443   a  is an inner peripheral wall surface surrounding the light absorption region S 21 . According to this configuration, the light absorption surface  443   a  is easily and efficiently irradiated with the light L emitted from the optical fiber  41 . 
     The light absorption surface  443   a  of the refrigeration system  1  has an undulating shape. According to this configuration, a surface area of the light absorption surface  443   a  is increased. As a result, energy of the light L can be more efficiently converted into thermal energy. 
     A bottom of the light absorption region S 21  in the refrigeration system  1  is defined by the light absorption hole bottom surface  443   b.  The light absorption hole bottom surface  443   b  has a tapered shape. According to this configuration, the traveling direction of the light L is changed. As a result, the opportunity of light being absorbed by the light absorption surface  443   a  can be increased. 
     The absorptance of the light absorption surface  443   a  for the light L is larger than the reflectance of the light absorption surface  443   a  for the light L. According to this configuration, energy of the light L can be further efficiently converted into thermal energy. 
     The closed region S 2  of the refrigeration system  1  includes the fiber exposure region S 22  in which the light-emitting end  412  of the optical fiber  41  is disposed. An area of a cross of the fiber exposure region S 22  intersecting the optical axis  41 S of the optical fiber  41  is larger than an area of a cross intersecting an optical axis of the light absorption region S 21 . According to this configuration, the generation of return light to be incident on the optical fiber  41  again can be suppressed. 
     The light irradiation unit  90  further includes the optical fiber  41  that guides the light L, and the optical fiber holder  43  that holds the optical fiber  41  and that attaches the optical fiber  41  to the heating block  44 . According to this configuration, a position of the optical fiber  41  with respect to the heating block  44  can be held. As a result, the heating block  44  can be stably irradiated with light. 
     The thermal conductivity of the base material of the heating block in the refrigeration system  1  is larger than the thermal conductivity of the base material of the optical fiber holder  43 . According to this configuration, heat can be satisfactorily provided to the object  91 . 
     The heating block  44  of the refrigeration system  1  has the block main surface  44   a  and the block heat-outputting surface  44   b.  The light irradiation unit  90  is disposed on the block main surface  44   a.  The block heat-outputting surface  44   b  thermally contacts the holding member  92  to which the object  91  is attached. According to this configuration, heat can be satisfactorily provided from the heating block  44  to the object  91  through the holding member  92 . 
     The object  91  of the refrigeration system  1  includes an optical sensor that outputs a signal in response to incident light. According to this configuration, the refrigeration system  1  including the optical sensor can be obtained. 
     The refrigeration system  1  of the present disclosure is not limited to the above embodiment. 
     Modification Examples 1 to 4 illustrate several structures in which the optical fiber  41  is attached to the heating block  44 . 
       FIG.  5 A  shows a heating unit  4 A included in a refrigeration system  1 A of Modification Example 1. The heating unit  4 A includes the ferrule  42 , the optical fiber holder  43 , the heating block  44 , and an optical fiber connector  46 . The heating unit  4 A of Modification Example 1 is different from the heating unit  4  of the embodiment in that the heating unit  4 A includes the optical fiber connector  46 . The optical fiber connector  46  holds a light-emitting end  412  side of the optical fiber  41 . The optical fiber connector  46  is inserted into the outer holder tube portion  431  of the optical fiber holder  43 . The optical fiber holder  43  is fixed to the heating block  44  using fastening members such as the bolts  45 A and  45 B. 
       FIG.  5 B  shows a heating unit  4 B included in a refrigeration system  1 B of Modification Example 2. The heating unit  4 B of Modification Example 2 is the same as the heating unit  4 A of Modification Example 1 in terms of components. The heating unit  4 B includes the ferrule  42 , the optical fiber holder  43 , the heating block  44 , and the optical fiber connector  46 . The heating unit  4 B of Modification Example 2 is different from the heating unit  4 A of Modification Example 1 in the position of the optical fiber  41  with respect to the heating block  44 . Specifically, the light-emitting end  412  of the heating unit  4 A of Modification Example 1 protrudes from the outer holder tube tip surface  431   b.  The optical fiber  41  protrudes from the optical fiber holder  43 . Therefore, in Modification Example 1, the light-emitting end  412  of the optical fiber  41  is inserted into the heating block  44 . The light-emitting end  412  of the heating unit  4 B of Modification Example 2 does not protrude from the outer holder tube tip surface  431   b.  The optical fiber  41  does not protrude from the optical fiber holder  43 . Therefore, in Modification Example 2, the light-emitting end  412  of the optical fiber  41  is disposed inside a through-hole surrounded by the outer holder tube inner peripheral surface  431   a.    
       FIG.  6 A  shows a heating unit  4 C included in a refrigeration system  1 C of Modification Example 3. The heating unit  4 C includes the optical fiber connector  46  and the heating block  44 . The heating unit  4 C of Modification Example 3 is different from the heating unit  4  of the embodiment in that the heating unit  4 C includes the optical fiber connector  46 . Further, the heating unit  4 C of Modification Example 3 is different from the heating unit  4  of the embodiment in that the heating unit  4 C does not include the optical fiber holder  43 . The optical fiber connector  46  holding the optical fiber  41  in Modification Example 3 is directly fixed to the heating block  44 . A configuration for fixing the optical fiber connector  46  to the heating block  44  is not particularly limited. 
       FIG.  6 B  shows a heating unit  4 D included in a refrigeration system  1 D of Modification Example 4. The heating unit  4 D includes the heating block  44 . The heating unit  4 D of Modification Example 4 is different from the heating unit  4  of the embodiment in that the heating unit  4 D does not include the optical fiber holder  43 . Further, the heating unit  4 D of Modification Example 4 also does not include the optical fiber connector  46 . The optical fiber  41  of the heating unit  4 D of Modification Example 4 is directly fixed to the heating block  44 . For example, the optical fiber  41  may be held by a lid member  47  made of resin. 
     In the description of the embodiment, the object  91  has been described as being disposed in a place where the object  91  cannot be seen from the heating unit  4 . According to such disposition, even if the light L leaks from the heating unit  4 , adverse effects due to the leaked light being incident on the object  91  that is, for example, an optical sensor can be suppressed. For example, even when the light L leaks from the heating unit  4 , the sensitivity of the optical sensor can be prevented from decreasing. Modification Examples 5 to 10 illustrate several configurations in which the object  91  is disposed in a place where the object  91  cannot be seen from the heating unit  4 . 
       FIG.  7 A  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 E of Modification Example 5. As shown in  FIG.  7 A , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is disposed on a lower surface  324   b  (back surface) of the second cooling stage  324 . 
       FIG.  7 B  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 F of Modification Example 6. As shown in  FIG.  7 B , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is disposed on an outer peripheral surface  324   c  of the second cooling stage  324 . 
       FIG.  7 C  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 G of Modification Example 7. As shown in  FIG.  7 C , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is also disposed on the upper surface  324   a  of the second cooling stage  324 . Further, a partition  94  that partitions the object  91  off from the heating unit  4  is provided between the object  91  and the heating unit  4 . 
       FIG.  8 A  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 H of Modification Example 8. As shown in  FIG.  8 A , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is also disposed on the upper surface  324   a  of the second cooling stage  324 . An object casing  95  that accommodates the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . A casing that accommodates the heating unit  4  is not disposed. 
       FIG.  8 B  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 K of Modification Example 9. As shown in  FIG.  8 B , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is also disposed on the upper surface  324   a  of the second cooling stage  324 . A unit casing  96  that accommodates the heating unit  4  is disposed on the upper surface  324   a  of the second cooling stage  324 . A casing that accommodates the object  91  is not disposed. 
       FIG.  8 C  shows disposition of the object  91  and the heating unit  4  included in a refrigeration system  1 M of Modification Example 10. As shown in  FIG.  8 C , the object  91  is disposed on the upper surface  324   a  of the second cooling stage  324 . The heating unit  4  is also disposed on the upper surface  324   a  of the second cooling stage  324 . The unit casing  96  that accommodates the heating unit  4  is disposed on the upper surface  324   a  of the second cooling stage  324 . The object casing  95  that accommodates the object  91  is also disposed on the upper surface  324   a  of the second cooling stage  324 . 
     MODIFICATION EXAMPLE 11 
       FIG.  8 D  shows the object  91  and the heating unit  4  disposed in a refrigeration system  1 N of Modification Example 11. As shown in  FIG.  8 D , it is also possible not to employ the configuration in which the object  91  is disposed in a place where the object  91  cannot be seen from the heating unit  4 . The object  91  is disposed on the upper surface  324   a  of the second cooling stage  324  and the heating unit  4  is disposed thereon. Any member that partitions the object  91  and the heating unit  4  off from each other is not disposed between the object  91  and the heating unit  4 . Such disposition refers to that “the object  91  is disposed in a place where the object  91  cannot be seen from the heating unit  4 ”. 
     MODIFICATION EXAMPLE 12 
       FIG.  9 A  shows a heating block  44 P included in a refrigeration system  1 P of Modification Example 12. In the heating unit  4  of the embodiment, processing for enhancing the absorption of light is applied to the inner peripheral surface and the like surrounding the closed region S 2 . Namely, the block main surface  44   a,  the block heat-outputting surface  44   b,  the block bottom surface  44   c,  the block back surface  44   f,  and the block side surfaces  44   d  and  44   e  of the heating block  44  are not provided with a surface treatment layer such as black alumite. As shown in  FIG.  9 A , a surface treatment layer of the heating block  44 P of Modification Example 12 is not provided on the block main surface  44   a,  the block heat-outputting surface  44   b,  the block bottom surface  44   c,  the block back surface  44   f,  and the block side surfaces  44   d  and  44   e  of the heating block  44 . In  FIG.  9 A , an illustration of some components of the heating block  44 P is omitted. 
     The optical axis  41 S of the optical fiber  41  included in the heating unit  4  of the embodiment overlaps the axis  443  S of the light absorption hole  443  (light absorption region S 21 ). In other words, the optical axis  41 S of the optical fiber  41  overlaps the axis of the light absorption region S 21 . The relationship between these axes is not limited to overlapping.  FIG.  9 B  shows a heating unit  4 Q included in a refrigeration system  1 Q of Modification Example 13. As shown in  FIG.  9 B , the optical axis  41 S of the optical fiber  41  of Modification Example 13 is parallel to the axis  443 S of the light absorption hole  443  (light absorption region S 21 ). In other words, the optical axis  41 S of the optical fiber  41  is parallel to the axis of the light absorption region S 21 . However, the optical axis  41 S of the optical fiber  41  does not overlap the axis  443  S of the light absorption hole  443 . In other words, the optical axis  41 S of the optical fiber  41  does not overlap the axis of the light absorption region S 21 .  FIG.  9 C  shows a heating unit  4 R included in a refrigeration system  1 R of Modification Example 14. As shown in  FIG.  9 C , in Modification Example 14, the optical axis  41 S of the optical fiber  41  is inclined with respect to the axis  443  S of the light absorption hole  443  (light absorption region S 21 ). In other words, the optical axis  41 S of the optical fiber  41  is inclined with respect to the axis of the light absorption region S 21 . 
     OTHER MODIFICATION EXAMPLES 
     In the embodiment, the number of the optical fibers  41  that guides light to the heating unit  4  is 1. As shown in  FIG.  10 A , two optical fibers  41   f  and  41   s  may be connected to the heating unit  4 . 
     Further, the number of the optical fibers  41  that guides light to the heating unit  4  may be more than 2. 
     In the embodiment, only the heating unit  4  is attached to the second cooling stage  324 . The number of the heating units  4  included in the refrigeration system  1  may be a plural number. For example, as shown in  FIG.  10 B , the refrigeration system  1  may further include the heating unit  4 B attached to the first cooling stage  322 , in addition to the heating unit  4 A attached to the second cooling stage  324 . The heating unit  4 A includes an optical fiber  41 A. The heating unit  4 B includes an optical fiber  41 B. As shown in  FIG.  10 C , the refrigeration system  1  may further include the heating unit  4 C attached to the second column  323 , in addition to the heating unit  4 A attached to the second cooling stage  324 . The heating unit  4 C includes an optical fiber  41 C. As shown in  FIG.  11 A , the refrigeration system  1  may further include the heating unit  4 D attached to the first column  321 , in addition to the heating unit  4 A attached to the second cooling stage  324 . The heating unit  4 D includes an optical fiber  41 D. As shown in  FIG.  11 B , the refrigeration system  1  may include the heating unit  4 C attached to the second column  323  and the heating unit  4 D attached to the first column  321 , in addition to the heating unit  4 A attached to the second cooling stage  324 . 
     In the embodiment, the heating block  44  is brought into direct contact with the holding member  92 . As shown in  FIG.  12   , the heating block  44  may be fixed to the holding member  92  with a member sandwiched therebetween. For example, the heating block  44  and the holding member  92  may be connected to each other with a heat conductive member  97  made of relatively soft metal (indium or the like). As a result, a contact area of the heat conductive member  97  with respect to the heating block  44  is increased. Further, a contact area of the holding member  92  with respect to the heat conductive member  97  is increased. Namely, thermal resistance from the heating block  44  to the holding member  92  decreases due to an increase in contact area. As a result, heat can be satisfactorily transferred from the heating block  44  to the holding member  92 . 
     EXPERIMENTAL EXAMPLE 1 
     In Experimental Example 1, a heating capacity of the heating unit  4  was evaluated. Specifically, in Experimental Example 1, it was confirmed that the temperature of an object could be increased by operation of the heating unit  4 . The graph of  FIG.  13 A  shows a relationship between light output and internal temperature. The horizontal axis shows an output of light provided for heating. The horizontal axis shows light output of light with which the heating unit  4  is irradiated. The vertical axis shows internal temperature of the vacuum chamber  2  at the time temperature equilibrium is reached after the start of light irradiation at each light output. In Experimental Example 1, the internal temperature is temperature of the second cooling stage  324 . The heating unit  4  was attached to the second cooling stage  324 . The object of Experimental Example 1 was the second cooling stage  324 . Referring to a graph G 13   a,  it was confirmed that the internal temperature also increased with an increase in light output. When energy of input light increased, thermal energy generated by the heating unit  4  also increased. It was found that the temperature of the second cooling stage  324  that was an object also increased with an increase in thermal energy. It was found that the relationship between the light output and the internal temperature was an approximately proportional relationship. 
     In the refrigeration system  1  including the heating unit  4  having such a characteristic, a cooling capacity of the chiller  31  can also be evaluated. A heat quantity supplied to the inside of the vacuum chamber  2  can be controlled by controlling the output of laser light. In a state where a predetermined heat quantity per unit time is supplied to the inside of the vacuum chamber  2 , the setting is such that a heat quantity equal to the heat quantity supplied to the chiller  31  is removed. When the chiller  31  normally operates, a provided heat quantity and a removed heat quantity are equal. As a result, the temperature of the object disposed inside the vacuum chamber  2  does not change. Conversely, when the chiller  31  does not operate normally, namely, when the cooling function of the chiller  31  is degraded, a provided heat quantity and a removed heat quantity are not in balance. As a result, the temperature of the object disposed inside the vacuum chamber  2  rises. An inspection as to whether or not the chiller  31  operates normally can also be performed through such a test. 
     EXPERIMENTAL EXAMPLES 2 and 3 
     In Experimental Examples 2 and 3, the time from when the operation of the chiller  31  was stopped to when the temperature rose to a temperature at which the vacuum chamber  2  could be opened was confirmed. Room temperature was set as the temperature at which the vacuum chamber  2  could be opened. In Experimental Example 2, as a comparative example, the condition was that heating by the heating unit  4  was not performed. In Experimental Example 3, as a comparative example, the condition was that heating by the heating unit  4  was performed.  FIG.  13 B  shows a change in internal temperature over time. The horizontal axis shows elapsed time from the timing when the operation of the chiller  31  is stopped. The vertical axis shows internal temperature. In Experimental Examples 2 and 3, objects were the first cooling stage  322  and the second cooling stage  324 . In Experimental Example 3, the heating unit  4  was attached to the first cooling stage  322 , and the heating unit  4  was also attached to the second cooling stage  324 . A maximum output of light provided to the heating unit  4  was 9 W in all cases. 
     Graphs G 13   b  and G 13   c  of  FIG.  13 B  show results of Experimental Example 2. The graph G 13   b  shows temperature of the first cooling stage  322 . According to the graph G 13   b,  the time taken for the temperature of the first cooling stage  322  to rise to room temperature was approximately 12 hours. The graph G 13   c  shows temperature of the second cooling stage  324 . According to the graph G 13   c,  the time taken for the temperature of the second cooling stage  324  to rise to room temperature was approximately 15 hours. When the heating unit  4  was not used, it was found that the vacuum chamber  2  could not be opened for approximately 15 hours from when the operation of the chiller  31  was stopped. 
     Graphs G 13   d  and G 13   e  of  FIG.  13 B  show results of Experimental Example 3. The graph G 13   d  shows temperature of the first cooling stage  322 . According to the graph G 13   d,  the time taken for the temperature of the first cooling stage  322  to rise to room temperature was approximately six hours. When the heating unit  4  was not used, the time taken for the temperature of the first cooling stage  322  to rise to room temperature was approximately 12 hours. Therefore, it was found that the time taken for the temperature of the first cooling stage  322  to rise to room temperature could be shortened by approximately six hours by using the heating unit  4 . The graph G 13   e  shows temperature of the second cooling stage  324 . According to the graph G 13   e,  the time taken for the temperature of the second cooling stage  324  to rise to room temperature was also approximately six hours. When the heating unit  4  was not used, the time taken for the temperature of the second cooling stage  324  to rise to room temperature was approximately 15 hours. Therefore, it was found that the time taken for the temperature of the second cooling stage  324  to rise to room temperature could be shortened by approximately nine hours by using the heating unit  4 . When the heating unit  4  was used, it was found that the vacuum chamber  2  could be opened when approximately six hours elapsed from when the operation of the chiller  31  was stopped. It was found that compared to when the heating unit  4  was not used, the standby time from when the chiller  31  was stopped could be shortened by approximately nine hours.