DESCRIPTION FIBER FUSE TERMINATOR, FIBER LASER, AND OPTICAL TRANSMISSION LINE [Technical Field] [0001] The present invention relates to a fiber fuse terminator, fiber laser, and an optical transmission line, which can terminate a fiber fuse in an optical transmission line, a light filter laser, and the like through which high-power light is propagated, and can prevent damage to transmission equipment, a light source, and the like. Priority is claimed on Japanese Patent Application No. 2008-216485, filed August 26, 2008, the content of which is incorporated herein by reference. [Background Art] [0002] In recent years, in the field of optical communications, as transmission capacity increases, the intensity (power) of light which is propagated in optical fibers increases. In addition, in optical fiber lasers, as the laser output of the optical fiber lasers increases, high-power light in a range from several hundred W to several thousand W is propagated in the optical fibers. [0003] In optical fibers in which the high-power light is propagated, there is a possibility that a fiber fuse occurs due to overheating caused by dust and the like attached to an end surface thereof or overheating caused by local-bending of the optical fiber, resulting in damage not only the optical fibers but also devices or apparatuses connected to the optical fibers (for example, refer to Non-Patent Documents 1 and 2). [0004] FIGS. 1 and 2 respectively show a side view and a cross-sectional view illustrating a single mode optical fiber (SMF) through which the fiber fuse passes. In the drawings, the reference numeral 10 represents an optical fiber, the reference numeral 11 represents a core, and the reference numeral 12 represents a cladding. As shown in the drawings, in the optical fiber 10 through which the fiber fuse passes, voids 1 periodically occur in the center core 11. Since the voids prevent the propagation of light through the optical fiber, the passage of the fiber fuse is a fatal obstacle to the communication system, the optical fiber laser, and the like. Once the fiber fuse occurs, it will continue to pass through the optical fiber and the waveguide structure of the optical fiber will be damaged unless the intensity of the light propagating in the optical fiber drops below a threshold value. The threshold of the optical intensity varies depending on the structure of the optical fiber and the like. In the present specification, the threshold value of the optical intensity for terminating the fiber fuse is referred to as the "fiber fuse threshold value". [0005] As techniques for terminating the fiber fuse midway along the optical fiber in order to protect optical transmission lines or apparatuses, the following techniques are known. Patent Document 1 describes a technique of terminating the fiber fuse in which power density in the core is reduced by partially expanding a mode field diameter (MFD) of a part of a single mode optical fiber. Patent Document 2 describes an optical fiber transmission line in which a graded index (GI) optical fiber is inserted midway on the optical fiber transmission line to create an enlarged-core portion, thereby terminating the fiber fuse phenomenon. Patent Document 3 describes a technique of terminating the fiber fuse phenomenon by providing an optical attenuator of a photonic crystal fiber type midway on the transmission line. Non-Patent Document 3 describes that the fiber fuse can be terminated by etching a cladding of an optical fiber to thin the outer diameter of the optical fiber to approximately twice the MFD. For example, in the case where the MFD is 9.5 urn, when the outer diameter is 10.5 to 33 urn, the fiber fuse can be terminated. In addition, Non-Patent Document 3 describes that the outer diameter of the etched portion of the optical fiber required for terminating the fiber fuse has little effect on the emission strength of the laser. Non-Patent Document 4 examines the characteristics, with respect to fiber fuses, of a "microstructured fiber" which is provided with a center portion surrounded with 30 holes (having diameters of approximately 1 (am, and a center-to-center distance of approximately 2 jam) and allows single-mode propagation with MFD of approximately 2 µm at a wavelength of 1.06 µm. According to Non-Patent Document 4, the fiber fuse threshold value of the "microstructured fiber" is more than 10 times that of a conventional SMF having approximately the same MFD. [0006] As a fusion-splice technique of a hole-assisted optical fiber (HAF) which includes at its center a core with a refractive index higher than a cladding and holes in the cladding, the following technique has been known. Non-Patent Document 5 describes a technique that intermittent discharge or sweep discharge is performed on an optical fiber in which holes are disposed around a core of a general SMF to collapse the holes in a tapered shape, so that the optical fiber is fusion-spliced to the SMF with an average splicing loss of 0.05 dB. [Patent Documents] [0007] [Patent Document 1] Japanese Patent No. 4070111 [Patent Document 2] Japanese Patent No. 4098195 [Patent Document 3] Japanese Patent Application, First Publication No. 2005-345592 [Non-Patent Documents] [0008] [Non-Patent Document 1] R. Kashyap and K. J. Blow, "Observation of catastrophic self-propelled self-focusing in optical fibres", Electronic Letters, January 7, 1998, Vol. 24, No. 1, pp. 47-48. [Non-Patent Document 2] Shin-ichi Todoroki, "Origin of periodic void formation during fiber fuse", August 22, 2005, Vol. 13, No. 17, pp. 6381-6389. [Non-Patent Document 3] E. M. Dianov, I. A. Bufetov and A. A. Frolov, "Destruction of silica fiber cladding by fiber fuse effect", OFC2004, 2004, TuB4. [Non-Patent Document 4] E. Dianov, A. Frolov and I. Bufetov, "Fiber Fuse effect in microstructured fibers", OFC2003, 2003, FH2. [Non-Patent Document 5] Suzuki Ryuji et al., "A study of fusion splicing techniques for holey fiber", Suzuki Ryuji et al., Institute of Electronics, Information and Communication Engineers, 2004 Electronics Society Conference, C-3-119. [Disclosure of the present invention] [Problems to be Solved by the present invention] [0009] However, conventional technologies have the following problems. In the technique described in Patent Document 1 (method of terminating the fiber fuse by expanding the MFD of a part of the SMF), it is difficult to reduce the splicing loss between the optical fiber of which the MFD is expanded and a general SMF. In order to reduce the splicing loss between the optical fiber of which the MFD is expanded and the general SMF, there is a need to diffuse a dopant in the core of the SMF in a tapered shape, or to prepare various types of optical fibers having different MFDs and splice them in multiple stages; this is extremely expensive. In the technique described in Patent Document 2 (method of terminating the fiber fuse by inserting the GI fiber), there is a problem of considerable loss at a portion where the light is combined between the GI fiber and the SMF. In order to reduce the loss, it is necessary to enlarge the diameter of the light entering from the SMF by providing a GI fiber portion having a length of 1/4 of a pitch so as to reduce the power density of the light, and then, to reduce the diameter of the light by again providing a GI fiber portion having a length of 1/4 of a pitch, thereby allowing the light to enter the next SMF; this configuration is complex and expensive. In the technique described in Patent Document 3 (method of terminating the fiber fuse by inserting the optical attenuator of a photonic crystal fiber type), since the waveguide is structured only by the holes, there is a defect that the splicing loss in the fusion-splice portion is increased. Furthermore, since the optical attenuator itself has a large insertion loss, the loss in the transmission line is also increased. In the technique described in Non-Patent Document 3 (method of terminating the fiber fuse by etching the outer diameter of the optical fiber to approximately twice the MFD), it is difficult to achieve the intended outer diameter due to problems such as melting of the optical fiber caused by incorrect hydrogen fluoride (HF) processing time, resulting in poor manufacturability. Also, since post-processing is required, the cost increases. Furthermore, the localized thin outer diameter of the optical fiber results in weak mechanical strength. Moreover, to etch the cladding, after removing a part of a resin coating of the optical fiber, the cladding is immersed into a strong-acting chemical solution such as HF, which is a difficult operation. In Non-Patent Document 4, although one concrete example is given of the "microstructured fiber" where the fiber fuse threshold is higher than in a general SMF, there is no detailed explanation of the method for forming the holes. Also, no consideration is given to whether, when the microstructured fiber is spliced to an SMF, the microstructured fiber can terminate a fiber fuse arising in the SMF. Moreover, the problem of considerable splicing loss with the SMF due to the lack of a core having a high refractive index remains unsolved. [0010] The present invention has been made in the above circumstances, and an object is to provide a fiber fuse terminator which can be manufactured at a low cost and can be spliced to a single mode optical fiber at low loss and a method of terminating a fiber fuse. [Means for Solving the Problems] [0011] A fiber fuse terminator which is used to terminate a fiber fuse according to one aspect of the present invention includes an optical fiber which includes a core and a cladding having holes extending in a longitudinal direction thereof, in which: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; and when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W≤0.45xDfiber is satisfied. In the fiber fuse terminator according to one aspect of the present invention, when it is assumed that a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, furthest from the center of the core, of the hole that is furthest from the core is Rmax, and a sectional area of a region between a circle having a radius of Rmax around the center of the core and a circle having a radius of Rmin around the center of the core is S, a sectional area of a portion where the holes are provided in the region between the circle having the radius of Rmax and the circle having the radius of Rmin may be no less than 20% of the sectional area S. In the fiber fuse terminator according to one aspect of the present invention, each end of the optical fiber may be fusion-spliced to a single-mode optical fiber without holes, and the fusion-splicing loss per one point thereon is no greater than 0.50 dB. In the fiber fuse terminator according to one aspect of the present invention, the number of the holes of the optical fiber may be no less than 3. In the fiber fuse terminator according to one aspect of the present invention, a resin coating may cover a portion of a surface of the optical fiber, excepting a fusion-splice portion between the optical fiber and the single-mode optical fiber and a periphery thereof; and a flameproof protective layer may cover the fusion-splice portion and the periphery thereof of the surface of the optical fiber. In the fiber fuse terminator according to one aspect of the present invention, each end of the optical fiber may be fusion-spliced to the single-mode optical fiber by intermittent discharging or sweep discharging. In the fiber fuse terminator according to one aspect of the present invention, a length of the optical fiber may be no less than 1 mm. A fiber fuse terminator which is used to terminate a fiber fuse according to another aspect of the present invention includes an optical fiber which includes a core and a cladding having one layer of holes extending in a longitudinal direction thereof, in which: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W≤0.45xDfiber is satisfied; and when it is assumed that a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, furthest from the center of the core, of the hole that is furthest from the core is Rmax, and a sectional area of a region between a circle having a radius of Rmax around the center of the core and a circle having a radius of Rmin around the center of the core is S, a sectional area of a portion where the holes are provided in the region between the circle having the radius of Rmax and the circle having the radius of Rmin is no less than 20% of the sectional area S. A fiber laser according to one aspect of the present invention includes: a pumping light source; a rare-earth doped optical fiber; and a fiber fuse terminator having an optical fiber which includes a core and a cladding having holes extending in a longitudinal direction thereof, in which: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; and when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W≤0.45xDfiber is satisfied. In the fiber laser according to one aspect of the present invention, an isolator may be further provided, and the fiber fuse terminator may be disposed at an output side of the isolator. An optical transmission line according to one aspect of the present invention uses an optical fiber, in which the fiber fuse terminator of the present invention is inserted into the optical transmission line. [Advantageous Effects of the Invention] [0012] According to the fiber fuse terminator of the present invention, a fiber fuse that occurs in an optical fiber of an optical transmission line, an optical fiber laser, and the like can be terminated, thereby preventing damage to transmission equipment, a light source, and the like. The fiber fuse terminator of the present invention can be manufactured at low cost, and can be spliced to a single-mode fiber with low splicing loss, enabling it to contribute to increasing the transmission capacity and the laser output. [Brief Description of the Drawings] [0013] FIG. 1 is a side view schematically illustrating an example of a state in which a fiber fuse passes through a single mode optical fiber. FIG. 2 is a cross-sectional view schematically illustrating an example of a state in which a fiber fuse passes through a single mode optical fiber. FIG. 3 is a cross-sectional view illustrating a hole-assisted optical fiber which has 4 holes in a surrounding region of a core according to a first embodiment of the present invention. FIG. 4 is a side view schematically illustrating an example of a state in which a fiber fuse occurred in a single mode optical fiber passes through a conventional optical fiber. FIG. 5 is a side view schematically illustrating an example of a state in which a fiber fuse occurred in a single mode optical fiber is stopped at a splice place between the single mode optical fiber and a hole-assisted optical fiber of the present invention. FIG. 6 is a side view schematically illustrating an example of a state in which a fiber fuse occurred in a single mode optical fiber is stopped at the middle of a hole-assisted optical fiber of the present invention. FIG. 7 is a cross-sectional view illustrating a hole-assisted optical fiber which has 2 holes according to a modified example of the first embodiment of the present invention. FIG. 8 is a cross-sectional view illustrating a hole-assisted optical fiber which has 3 holes according to a modified example of the first embodiment of the present invention. FIG. 9 is a cross-sectional view illustrating a hole-assisted optical fiber which has 6 holes according to a modified example of the first embodiment of the present invention. FIG. 10 is a cross-sectional view illustrating a hole-assisted optical fiber which has 8 holes according to a modified example of the first embodiment of the present invention. FIG. 11 is a cross-sectional view illustrating a hole-assisted optical fiber which has 60 holes disposed in a plurality of layers in a surrounding region of a core according to a second embodiment of the present invention. FIG. 12 is a cross-sectional view illustrating a hole-assisted optical fiber which has 12 holes according to a modified example of the second embodiment of the present invention. FIG. 13 is a view illustrating an exemplary configuration of a measurement system for evaluating terminating performance of a fiber fuse. FIG. 14 is a graph illustrating the relationship between incident power and invasion distance of a fiber fuse in Experiment 3. FIG. 15 is a cross-sectional view illustrating the diameter of a melted portion of a single mode optical fiber. FIG. 16 is a graph illustrating the relationship between incident power and diameter of a melted portion in Experiment 3. FIG. 17 is a cross-sectional view schematically illustrating a structure of a fiber Q which is used in Experiment 10-1. FIG. 18 is a cross-sectional view schematically illustrating a structure of a fiber R which is used in Experiment 10-2. FIG. 19 is a view illustrating an exemplary configuration of a Yb-doped optical fiber laser using a fiber fuse terminator of the present invention. FIG. 20 is a view illustrating an exemplary configuration of an Er-doped optical fiber laser using a fiber fuse terminator of the present invention. [Mode for Carrying Out the present invention] [0014] In the following, the present invention will be described with reference to the accompanying drawings based on exemplary embodiments of the present invention. The HAF (Fiber A) having the cross section shown in FIG. 9 is used as the optical fiber 55 to be measured, and Experiments 1-1 and 1-2 were performed by changing the incident power. The experiment numbers 1 -1 and 1 -2 in Table 1 show the parameters of Fiber A and the experiment conditions. As for Fiber A, the cladding diameter Dfiber is 125 µm, the number of the holes is 6, Rmin is 8.5 µm, W is 7.3 µm, Rmax is 15.8 µm, and the MFD at the wavelength of 1.55 µm is 10.2 µm. In addition, 2xRmin/MFD is 1.67. As for Fiber A, when the incident wavelength is 1.55 µm, and when the incident power is 9.8 W (experiment number 1-1) and when the incident power is 3.0 W (experiment number 1-2), the terminating performance of the fiber fuse was investigated. In both Experiments 1-1 and 1-2, the value of 2xRrnin/MFD is 1.67, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 0.72, which is no less than 0.3. In addition, since the value of W is 7.3 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. As described above, when the values of 2xRmin/MFD, W/MFD, and 0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using the hole-assisted optical fiber. As a result, when the HAF of this experiment is used as a fiber fuse terminator and inserted in the middle of an optical transmission line or an optical fiber laser, the transmission loss can be suppressed, and the fiber fuse can be terminated. [0067] The HAF (Fiber C) having the cross section shown in FIG. 3 is used as the optical fiber 55 to be measured, and Experiments 3-1 to 3-5 were performed by changing the incident power. The experiment numbers 3-1 to 3-5 in Table 1 show the parameters of Fiber C and the experiment conditions. As for Fiber C, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 10.6 µm, W is 16.3 µm, Rmax is 26.9 µm, and the MFD at the wavelength of 1.55 urn is 10.4 µm. In addition, 2xRmin/MFD is 2.04. As for Fiber C, when the incident wavelength is 1.55 µm, and when the incident power is 8.1 W (experiment number 3-1), when the incident power is 4.7 W (experiment number 3-2), when the incident power is 2.1 W (experiment number 3-3), when the incident power is 1.7 W (experiment number 3-4), and when the incident power is 1.5 W (experiment number 3-5), the terminating performance of the fiber fuse was investigated. Further, the incident power of 1.5 W is a value close to the fiber fuse threshold value in a general SMF without holes, and the fiber fuse does not occur in a power lower than this incident power. In all the cases of experiments 3-1 to 3-5, the value of 2xRmin/MFD is 2.04, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.57, which is no less than 0.3. In addition, since the value of W is 16.3 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, in all the cases of the incident power, the fiber fuse slightly invaded the HAF from the SMF as shown in FIG. 6, but it stopped within 1 mm. As described above, when the values of 2xRmin/MFD, W/MFD, and 0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using the hole-assisted optical fiber. [0068] In Experiment 3, it was ascertained that the invasion distance is changed according to the incident power as shown in the graph of FIG. 14. As shown in FIG. 14, as the incident power is closer to the fiber fuse threshold value, the invasion distance is extended. [0069] To ascertain the cause of this phenomenon, a cross-section of a fiber where a fiber fuse occurred was observed. FIG. 15 is a schematic view of a cross-section of an SMF where a fiber fuse has occurred. In the drawing, the reference numeral 40 represents a core, the reference numeral 41 represents a void, the reference numeral 42 represents a melted portion, and the reference numeral 43 represents a cladding. In cross-section of the SMF, the black ring-shaped melted part 42 was observed around the void 41 generated in the core 40. This melted part 42 was melted by the passage of the fiber fuse. Results of measuring the diameter Dmelted of this melted part 42 are shown in FIG. 16. As shown in FIG. 16, as the incident power approaches the general SMF fiber fuse threshold Pth=1.5 W, the diameter Dmelted of the melted part 42 decreases sharply. [0070] Since the SMF 56 used in this experiment has an MFD of 10.4 µm at a wavelength of 1.55 urn, it can be considered that, also in the HAF of the measured optical fiber 55, as the incident power approaches the general SMF fiber fuse threshold, the diameter Dmeltedd decreases sharply. In FIG. 16, the 2xRmin value of fiber C used in Experiment 3 (i.e., 21.2 µm) is indicated by a horizontal broken line. [0071] As described above, the hole-assisted optical fiber of the present invention is provided with the holes such that the holes surround the core, so that the center portion (the core) of the hole-assisted optical fiber is adiabatically expanded outward (that is, toward the holes) in the radial direction so as to lower the temperature of the glass in the center portion. As a result, the fiber fuse is terminated. When the incident power becomes closer to the fiber fuse threshold value, the diameter Dmelted of the melted portion becomes smaller, so that the distance between the melted portion and the holes becomes large. For this reason, it is considered that, as the incident power approaches the fiber fuse threshold value, the effect of the holes on the fiber fuse is reduced, so that the phenomenon that the invasion distance is extended may occur. [0072] As shown in the graph of FIG. 16, it was ascertained that the diameter Dmelted of the melted portion depends on the incident power, and therefore, to determine whether a HAF structure will reliably terminate a fiber fuse, it is desirable to carefully consider the diameter Dmelted of the melted portion, particularly when the incident power is near the fiber fuse threshold. In view of the phenomenon whereby the fiber fuse stops after slightly invading into the HAF, it is clear that the length of the HAF is important. In Experiment 3, the longest invasion distance was 640 µm at an incident power of 1.5 W. Accordingly, the length of a HAF used as a fiber fuse terminator is preferably at least 1 mm. In addition, the fusion-splicing loss between the HAF and the SMF was a low value of 0.04 dB/point. [0073] The HAF (Fiber D) having the cross section shown in FIG. 10 is used as the optical fiber 55 to be measured, and Experiments 4-1 and 4-2 were performed by changing the incident power. The experiment numbers 4-1 and 4-2 in Table 1 show the parameters of Fiber D and the experiment conditions. As for Fiber D, the cladding diameter Dfiber is 125 µm, the number of the holes is 8, Rmin is 9.0 µm, W is 3.0 µm, Rmax is 12.0 µm, and the MFD at the wavelength of 1.55 µrn is 10.0 µm. In addition, 2xRmin/MFD is 1.80. [0074] As for Fiber D, when the incident wavelength is 1.55 urn, and when the incident power is 1.7 W (experiment number 4-1) and when the incident power is 8.0 W (experiment number 4-2), the terminating performance of the fiber fuse was investigated. In both Experiments 4-1 and 4-2, the value of 2xRmin/MFD is 1.80, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 0.30, which is no less than 0.3. In addition, since the value of W is 3.0 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. The fusion-splicing loss between the Fiber D and the SMF was a low value of no more than 0.03 dB/point. [0075] The HAF (Fiber E) having the cross section shown in FIG. 10 is used as the optical fiber 55 to be measured, and Experiments 4-3 and 4-4 were performed by changing the incident power. The experiment numbers 4-3 and 4-4 in Table 1 show the parameters of Fiber E and the experiment conditions. As for Fiber E, the cladding diameter Dfiber is 125 µm, the number of the holes is 8, Rmin is 10.2 µm, W is 3.2 µm, Rmax is 13.4 µm, and the MFD at the wavelength of 1.55 urn is 10.1 µm. In addition, 2xRmm/MFD is 2.02. [0076] As for Fiber E, when the incident wavelength is 1.55 µm, and when the incident power is 1.7 W (experiment number 4-3) and when the incident power is 8.0 W (experiment number 4-4), the terminating performance of the fiber fuse was investigated. In both Experiments 4-3 and 4-4, the value of 2xRmin/MFD is 2.02, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 0.32, which is no less than 0.3. In addition, since the value of W is 3.2 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. The fusion-splicing loss between the Fiber E and the SMF was a low value of no more than 0.03 dB/point. [0077] The HAF (Fiber H) having the cross section shown in FIG. 7 is used as the optical fiber 55 to be measured, and Experiments 5-1 and 5-2 were performed by changing the incident power. The experiment numbers 5-1 and 5-2 in Table 1 show the parameters of Fiber H and the experiment conditions. As for Fiber H, the cladding diameter Dfiber is 125 µm, the number of the holes is 2, Rmin is 8.5 µm, W is 14.5 µm, Rmax is 23.0 µm, and the MFD at the wavelength of 1.55 µm is 10.0 µm. In addition, 2xRmin/MFD is 1.70. [0078] As for Fiber H, when the incident wavelength is 1.55 µm, and when the incident power is 3.0 W (experiment number 5-1) and when the incident power is 10.0 W (experiment number 5-2), the terminating performance of the fiber fuse was investigated. In both Experiments 5-1 and 5-2, the value of 2xRmin/MFD is 1.70, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.45, which is no less than 0.3. In addition, since the value of W is 14.5 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. From this result, it can be seen that the fiber fuse can be terminated even though the number of the holes is small. The fusion-splicing loss between the Fiber H and the SMF was 0.50 dB/point. In Fiber H, of which the number of the holes is 2, since the number of the holes is small, the core is distorted when subjected to the fusion-splice, so that it is considered that the fusion-splicing loss becomes higher. On the other hand, in Fiber I in Example 6 (described later) of which the number of the holes is 3, the fusion-splicing loss for splicing with the SMF was a low value of 0.15 dB/point. It is clear from this that a large number of holes is desirable in the HAF, preferably three or more. [0079] The HAF (Fiber I) having the cross section shown in FIG. 8 is used as the optical fiber 55 to be measured, and Experiments 5-3 and 5-4 were performed by changing the incident power. The experiment numbers 5-3 and 5-4 in Table 1 show the parameters of Fiber I and the experiment conditions. As for Fiber I, the cladding diameter Dfiber is 125 µm, the number of the holes is 3, Rmin is 8.3 µm, W is 7.6 µm, Rmax is 15.9 µm, and the MFD at the wavelength of 1.55 µm is 9.8 µm. In addition, 2xRmin/MFD is 1.69. [0080] As for Fiber I, when the incident wavelength is 1.55 µm, and when the incident power is 3.0 W (experiment number 5-3) and when the incident power is 10.0 W (experiment number 5-4). the terminating performance of the fiber fuse was investigated. In both Experiments 5-3 and 5-4, the value of 2xRmin/MFD is 1.69, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 0.78, which is no less than 0.3. In addition, since the value of W is 7.6 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. From this result, it can be seen that the fiber fuse can be terminated even though the number of the holes is small. The fusion-splicing loss between the Fiber I and the SMF was 0.15 dB/point. It is clear from this that a large number of holes is desirable in the HAF, preferably three or more. [0081] The HAF (Fiber J) having the cross section shown in FIG. 12 is used as the optical fiber 55 to be measured, and Experiment 6-1 was performed. The experiment number 6-1 in Table 1 shows the parameters of Fiber J and the experiment conditions. Fiber J has a plurality of holes at different distances from the center of the core, and W is not equal to the hole diameter. As for Fiber J, the cladding diameter Dfiber is 125 µm, the number of the holes is 12, the hole diameter is 4.0 µm, Rmin is 8.6 µm, W is 15.0 µm. Rmax is 23.6 µm, and the MFD at the wavelength of 1.55 µm is 8.2 µm. In addition, 2xRmin/MFD is 2.10. [0082] As for Fiber J, when the incident wavelength is 1.55 µm and the incident power is 10.0 W, the terminating performance of the fiber fuse was investigated. In Experiment 6-1, the value of 2xRmin/MFD is 2.10. which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.83, which is no less than 0.3. In addition, since the value of W is 15.0 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiment, the fiber fuse was able to be terminated. The fusion-splicing loss between the Fiber J and the SMF was a low value of 0.10 dB/point. [0083] The HAF (Fiber K) having the cross section shown in FIG. 11 is used as the optical fiber 55 to be measured, and Experiment 6-2 was performed. The experiment number 6-2 in Table 1 shows the parameters of Fiber K and the experiment conditions. Fiber K has a plurality of holes at different distances from the center of the core, and W is not equal to the hole diameter. As for Fiber K, the cladding diameter Dfiber is 125 µm, the number of the holes is 60, the hole diameter is 3.9 µm, Rmin is 8.5 µm, W is 30.0 µm, Rmax is 38.5 µm, and the MFD at the wavelength of 1.55 µm is 8.1 µm. In addition, 2xRmin/MFD is 2.10. [0084] As for Fiber K, when the incident power is 10.0 W, the terminating performance of the fiber fuse was investigated. In Experiment 6-2, the value of 2xRmin/MFD is 2.10, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 3.70, which is no less than 0.3. In addition, since the value of W is 30.0 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiment, the fiber fuse was able to be terminated. The fusion-splicing loss between the Fiber K and the SMF was a low value of 0.12dB/point. [0085] The HAF (Fiber L) having the cross section shown in FIG. 9 is used as the optical fiber 55 to be measured, and Experiments 7-1 and 7-2 were performed by changing the incident power. The experiment numbers 7-1 and 7-2 in Table 1 show the parameters of Fiber L and the experiment conditions. Fiber L has holes that are equidistant from the center of the core, and W is equal to the hole diameter. As for Fiber L, the cladding diameter Dfiber is 125 µm, the number of the holes is 6, Rmin is 5.5 µm, W is 6.2 µm, Rmax is 11.7 urn, and the MFD at the wavelength of 1.06 µm is 5.8 µm. In addition, 2xRmin/MFD is 1.90. [0086] As for Fiber L, when the incident wavelength is 1.06 µm, and when the incident power is 8.0 W (experiment number 7-1) and when the incident power is 20.0 W (experiment number 7-2), the terminating performance of the fiber fuse was investigated. In both Experiments 7-1 and 7-2, the value of 2xRmin/MFD is 1.90, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.07, which is no less than 0.3. In addition, since the value of W is 6.2 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiments, the fiber fuse was able to be terminated in both the incident powers. The fusion-splicing loss between the Fiber L and the SMF was a low value of 0.20 dB/point. [0087] Also noted in the experiment number 7-2 (fiber L, incident power 20 W), there was a phenomenon in which a portion of UV-curable resin at the point where the fiber fuse was terminated was burned and carbonized. This UV-curable resin is a recoating over the fusion-splice portion between the HAF and the SMF. The reason is considered to be that, when the HAF terminated the fiber fuse occurring at high power of 20 W, high-power incident light leaked around the HAF, and this energy was absorbed into the UV-curable resin. Therefore, when using high-power incident light, the flameproof material as described above is preferably used as the HAF coating, or as the recoating for the fusion-splice portion between the HAF and the SMF. [0088] The HAF (Fiber N) having the cross section shown in FIG. 7 is used as the optical fiber 55 to be measured, and Experiment 7-5 was performed. The experiment number 7-5 in Table 1 shows the parameters of Fiber N and the experiment conditions. As for Fiber N, the cladding diameter Dfiber is 125 µm, the number of the holes is 2, Rmin is 5.5 urn, W is 4.5 µm, Rmax is 10.0 µm, and the MFD at the wavelength of 1.06 µm is 5.8 µm. In addition, 2xRmin/MFD is 1.90. [0089] As for Fiber N, when the incident wavelength is 1.06 µm and the incident power is 8.0 W, the terminating performance of the fiber fuse was investigated. In Experiment 7-5, the value of 2xRmin/MFD is 1.90, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 0.78, which is no less than 0.3. In addition, since the value of W is 4.5 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. As a result of the experiment, the fiber fuse was able to be terminated. The fusion-splicing loss between the Fiber N and the SMF was a low value of 0.22 dB/point. [0090] Experiment 9-1 was performed using the same HAF (Fiber C) as that of Example 2. The experiment number 9-1 in Table 1 shows the parameters of Fiber C and the experiment conditions. Similar to Example 2, as for Fiber C, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 10.6 µm, W is 16.3 urn, Rmax is 26.9 µm, and the MFD at the wavelength of 1.55 µm is 10.4 µm. In addition, 2xRmin/MFD is 2.04. As for Fiber C, when the incident wavelength is 1.55 µm and the incident power is 3.0 W, the terminating performance of the fiber fuse was investigated. In Experiment 9-1, the value of 2xRmin/MFD is 2.04, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.57, which is no less than 0.3. In addition, since the value of W is 16.3 µm and the value of 0.45xDfiber is 56.25 urn, W≤0.45xDt-,ber is satisfied. As a result of the experiment, the fiber fuse slightly invaded the HAF from the SMF as shown in FIG. 6, but stopped within 1 mm. As described above, when the values of 2xRmin/MFD, W/MFD, and 0.45xDfiber are in the above-mentioned ranges, the fiber fuse can be terminated using the hole-assisted optical fiber. [0091] The HAF (Fiber O) having the cross section shown in FIG. 3 is used as the optical fiber 55 to be measured, and Experiment 9-2 was performed. The experiment number 9-2 in Table 1 shows the parameters of Fiber O and the experiment conditions. As for Fiber O, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 7.5 µm, W is 14.3 urn. Rmax is 21.8 µm, and the MFD at the wavelength of 1.55 µm is 9.8 µm. In addition, the value of 2xRmin/MFD is 1.53, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.46, which is no less than 0.3. In addition, since the value of W is 14.3 µm and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDtiber is satisfied. [0092] As for Fiber O, when the incident wavelength is 1.55 µm and the incident power is 3.0 W, the terminating performance of the fiber fuse was investigated. As a result, the fiber fuse did not invade Fiber O, and the fiber fuse was able to be terminated. The fusion-splicing loss between Fiber O and the SMF was a low value of 0.15dB/point. [0093] The HAF (Fiber P) having the cross section shown in FIG. 3 is used as the optical fiber 55 to be measured, and Experiment 9-3 was performed. The experiment number 9-3 in Table 1 shows the parameters of Fiber P and the experiment conditions. As for Fiber P, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 5.5 µm, W is 16.7 µm, Rmax is 22.2 µm, and the MFD at the wavelength of 1.55 µm is 9.2 µm. In addition, the value of 2xRmin/MFD is 1.20, which is in a range no less than 1.2 and no more than 2.1. The value of W/MFD is 1.82, which is no less than 0.3. In addition, since the value of W is 16.7 urn and the value of 0.45xDfiber is 56.25 µm, W≤0.45xDfiber is satisfied. [0094] As for Fiber P, when the incident wavelength is 1.55 µm and the incident power is 3.0 W, the terminating performance of the fiber fuse was investigated. As a result, the fiber fuse did not invade Fiber P, and the fiber fuse was able to be terminated. The fusion-splicing loss between Fiber P and the SMF was 0.60 dB/point. According to the result, it can be considered that, when Rmin is too close to MFD/2, the fiber fuse can be terminated, but the core is distorted when subjected to the fusion-splice so that the fusion-splicing loss becomes higher. [0095] The HAF (Fiber B) having the cross section in which 4 holes are provided in one layer is used as the optical fiber 55 to be measured, and Experiments 2-1 and 2-2 were performed by changing the incident power. The experiment numbers 2-1 and 2-2 in Table 2 show the parameters of Fiber B used in this experiment and the experiment conditions. As for Fiber B, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 19.4 µm, W is 17.4 µm, Rmax is 36.8 µm, and the MFD at the wavelength of 1.55 µm is 10.8 µm. In addition, the value of 2xRmin/MFD is 3.59, which is larger than 2.1. As for Fiber B, when the incident wavelength is 1.55 µm, and when the incident power is 4.4 W (experiment number 2-1) and when the incident power is 2.0 W (experiment number 2-2). the terminating performance of the fiber fuse was investigated. As a result of the experiments, the fiber fuse passed through the HAF from the SMF, and the fiber fuse was not able to be terminated in either of the incident powers. The fusion-splicing loss between the HAF and the SMF was 0.03 dB/point. Even when the HAF of this comparative example is used as the fiber fuse terminator in the middle of an optical transmission line or an optical fiber laser, the fiber fuse was not able to be terminated. [0096] The HAF (Fiber F) having the cross section in which 8 holes are provided in one layer is used as the optical fiber 55 to be measured, and Experiments 4-5 and 4-6 were performed by changing the incident power. The experiment numbers 4-5 and 4-6 in Table 2 show the parameters of Fiber F and the experiment conditions. As for Fiber F, the cladding diameter Dfiber is 125 µm, the number of the holes is 8, Rmin is 12.0 µm, W is 3.5 µm, Rmax is 15.5 urn, and the MFD at the wavelength of 1.55 µm is 10.3 µm. In addition, the value of 2xRmin/MFD is 2.33, which is larger than 2.1. [0097] As for Fiber F, when the incident wavelength is 1.55 µm, and when the incident power is 1.7 W (experiment number 4-5) and when the incident power is 8.0 W (experiment number 4-6), the terminating performance of the fiber fuse was investigated. As a result of the experiments, the fiber fuse was not able to be terminated in either of the incident powers. The fusion-splicing loss between Fiber F and the SMF was a low value of no more than 0.03 dB/point. [0098] The HAF (Fiber G) having the cross section in which 8 holes are provided in one layer is used as the optical fiber 55 to be measured, and Experiments 4-7 and 4-8 were performed by changing the incident power. The experiment numbers 4-7 and 4-8 in Table 2 show the parameters of Fiber G and the experiment conditions. As for Fiber G, the cladding diameter Dfiber is 125 µm, the number of the holes is 8, Rmin is 14.8 µm, W is 4.2 µm, Rmax is 19.0 urn, and the MFD at the wavelength of 1.55 µm is 10.5 µm. In addition, the value of 2xRmin/MFD is 2.82, which is larger than 2.1. [0099] As for Fiber G, when the incident wavelength is 1.55 urn, and when the incident power is 1.7 W (experiment number 4-7) and when the incident power is 8.0 W (experiment number 4-8), the terminating performance of the fiber fuse was investigated. As a result of the experiments, the fiber fuse was not able to be terminated in either of the incident powers. The fusion-splicing loss between Fiber G and the SMF was a low value of no more than 0.03 dB/point. [0100] The HAF (Fiber M) having substantially the cross section shown in FIG. 9 is used as the optical fiber 55 to be measured, and Experiments 7-3 and 7-4 were performed by changing the incident power. The experiment numbers 7-3 and 7-4 in Table 2 show the parameters of Fiber M used in this experiment and the experiment conditions. As for Fiber M, the cladding diameter Dfiber is 125 µm. the number of the holes is 6, Rmin is 5.6 µm, W is 1.4 µm, Rmax is 7.0 µm, and the MFD at the wavelength of 1.06 urn is 5.9 µm. In addition, the value of 2xRmin/MFD is 1.9, which is in a range no less than 1.2 and no more than 2.1. In addition, since the value of W is 1.4 µm and the value of 0.45xDfiber is 56.25 jam, W≤0.45xDfiber is satisfied. However, since the value of W is small at 1.4 µm, the value of W/MFD becomes 0.24, which is smaller than 0.3. As for the HAF, when the incident power is 8.0 W (experiment number 7-3), and when the incident power is 20.0 W (experiment number 7-4), the terminating performance of the fiber fuse was investigated. As a result of the experiments, the fiber fuse was not able to be terminated in either of the incident powers. The fusion-splicing loss between the Fiber M and the SMF was a low value of 0.18dB/point. [0101] Experiments 8-2 and 8-3 were performed using the same HAF (Fiber C) as that of Example 2. The experiment numbers 8-2 and 8-3 in Table 2 show the parameters of Fiber C and the experiment conditions. When the incident wavelength is 1.31 µm (experiment number 8-2) and when the incident wavelength is 1.06 µm (experiment number 8-3), the terminating performance of the fiber fuse was investigated. Similar to Example 2. as for Fiber C, the cladding diameter Dfiber is 125 µm, the number of the holes is 4, Rmin is 10.6 µm, and W is 16.3 µm, and Rmax is 26.9 µm. However, since the incident wavelength is different from that of Example 2, the MFD is also different. The MFD at the wavelength of 1.31 µm is 9.3 µm, and the MFD at the wavelength of 1.06 µm is 8.3 µm. Therefore, the value of 2xRmin/MFD is 2.3 at the wavelength of 1.31 µm, and 2.6 at the wavelength of 1.06 urn, so that it is larger than 2.1 in both cases. As a result of experiments, in both the cases of Experiments 8-2 and 8-3, the fiber fuse was not able to be terminated. [0102] The HAF (Fibers Q and R) having the cross section shown in FIGS. 17 and 18 is used as the optical fiber 55 to be measured, and Experiments 10-1 and 10-2 were performed. The experiment numbers 10-1 and 10-2 in Table 2 show the parameters of Fibers Q and R and the experiment conditions. In Fiber K (the number of the holes is 60) used in Example 8 and Fiber O (the number of the holes is 4) used in Example 12, the refractive index of the core is higher than the refractive index of the material of the cladding excepting the portions of the holes. In this regard, as shown in FIGS. 17 and 18, fiber Q (the number of holes is 60, FIG. 17) and fiber R (the number of holes is 4, FIG. 18), which have a plurality of holes 32 in a medium 31 but do not have a high-refractive-index core at a center portion 33 of optical fibers 30 and 30A, were manufactured. As for the optical fibers, when the incident wavelength is 1.55 urn and the incident power is 10.0 W, the terminating performance of the fiber fuse was investigated. As a result, both of the optical fibers were able to terminate the fiber fuse. [0103] The splicing loss of Fiber Q was 0.80 dB/point, which is larger than the splicing loss of 0.12 dB/point of Fiber K which has the same number of holes as Fiber Q. In addition, the splicing loss of Fiber R was 0.75 dB/point, which is larger than the splicing loss of 0.15 dB/point of Fiber O which has the same number of holes as Fiber R. As clearly shown by this result, although a fiber with a structure without a core having a high refractive index with respect to a medium without holes can terminate a fiber fuse, it has a problem of considerable splicing loss. Since the splicing loss is significant, Fiber Q and Fiber R are not suitable for the fiber fuse terminator. [0104] The investigation on the examples and the comparative examples is described in the following. (1: Regarding "2xRmin/MFD") As shown in Tables 1 and 2, the above experiments revealed that various features of the configuration, such as the number of holes, the structure of holes, incident power (also termed "optical intensity"), and incident wavelength are related to fiber fuse terminating performance. One parameter used was "2xRmin/MFD". Using this parameter as an indicator, it is possible to unambiguously determine the terminating performance of the fiber fuse. The lowest value of 2xRmin/MFD was proved to be 1.2 by Experiment 9-3 (Example 13). Therefore, when 2xRmin/MFD is no less than 1.2, the fiber fuse can be terminated. While fibers with 2xRmin/MFD of less than 1.2 can be manufactured generally, they have a problem of relatively high splicing loss. The highest value of 2xRmin/MFD was proved to be 2.1 by the results of Experiments 6-1 and 6-2 (Examples 7 and 8). In addition, from the results of Experiments 3-1 to 3-5 (Example 2), when 2xRmin/MFD is 2.0, although there is slightly invasion of the fiber fuse into the HAF, since the invasion distance is small within 1 mm, there is no damage to the light source or the transmission equipment. Furthermore, in Experiments 6-1 and 6-2. the splicing loss is suppressed to low values of 0.10 dB/point and 0.12 dB/point. Disposing the holes near the core is thought to be effective in infallibly terminating fiber fuses. In this regard, as shown in Experiments 1-1 and 1-2 (Example 1), 2xRmin/MFD is preferably no more than 1.7. [0105] (2: Regarding "W/MFD*' and "W/Dfiber") According to the results of Fiber M of Experiments 7-3 and 7-4 (Comparative Example 4), it was proved that, when the ratio between the MFD at the used wavelength and the width W of the hole region is small (W/MFD = 0.22), even though 2xRmin/MFD is 1.9, in some cases the fiber fuse cannot be terminated. In addition, according to the results of Fiber D of Experiments 4-1 and 4-2 (Example 3), it was proved that, when 2xRmin/MFD is 1.8 and W/MFD is 0.3, the fiber fuse can be terminated. It follows that, by ensuring that W/MFD is no less than 0.3, the fiber fuse can be terminated more reliably. Moreover, when the cladding diameter of an optical fiber including the core and holes described above is assumed to be Dfiber, it is preferable that W≤0.45xDfiber be satisfied. If this is not satisfied, the ratio of the sectional area of the fiber occupied by the sectional area of the holes increases, and the fiber cannot maintain its strength. [0106] (3: Regarding Sectional Area of Holes) When the incident light has high power, not only the value of W/MFD but also the ratio of the area S defined by a region between a circle having radius Rmax around the core center and a circle having radius Rmin around the core center (hereinafter "hole region'") to the area of a region occupied by the holes is important. Fibers H and I used in Experiments 5-1 to 5-4 (Examples 5 and 6) have area ratios occupied by the holes of 23.0% and 23.6%, respectively, and each was able to terminate the fiber fuse when the incident power was 10W. Thus, when the holes occupy no less than 20% of the hole region, fiber fuses can be more reliably terminated, even at high power. Since there were cases such as fiber J of Example 7 where, even though the area ratio occupied by the holes is less than 10%, the fiber fuse was successfully terminated at incident power of 1 OW, this area ratio is not a necessary requirement of the present invention. [0107] (4: Regarding Splicing Loss) Generally, when splicing different types of optical fibers, acceptable splicing loss may be about 1 dB in consideration of a design margin for the transmission system. Thus, when splicing the both ends of a fiber fuse terminator, acceptable splicing loss for a single splice place is assumed to be approximately 0.5 dB. [0108] (5: Regarding Number of Holes) As is clear from the results of Examples 2, 5, and 6, splicing loss sharply decreases as the number of holes increases to 2, 3, and 4. As described above, to keep splicing loss per splice place below 0. 5 dB, at least three or more holes are preferable. [0109] (6: Regarding Length of HAF) The phenomenon seen in Experiments 3-1 to 3-5 (Example 2), in which the fiber fuse stops after a slight invasion, confirms that the length of the HAF (length of hole portion) is important when using it as a fiber fuse terminator. The longest invasion distance in Example 2 was 630 iim at incident power of 1.5 W. Therefore, the length of a HAF used as a fiber fuse terminator is preferably no less than 1 mm. More preferably, to cope with sharp elongations of the invasion distance such as the one shown in the graph of FIG. 14, it is preferable that the HAF length be approximately 10 mm. [0110] As shown in FIG. 19, a fiber fuse terminator 67 configured from an HAF having a length of 50 mm was incorporated in a part of an output part of an optical fiber laser apparatus 60 using a ytterbium (Yb)-doped double-cladding optical fiber (rare-earth doped optical fiber) 64. The fibers were fusion-spliced together. In FIG. 19, symbol x represents fusion-splice point. [0111] This Yb fiber laser has an oscillating wavelength of 1,060 nm, and an output power of 3 W. The optical fiber laser apparatus 60 also includes a multi-port coupler 62 with which a plurality of laser diodes (LD) 61 for excitation, which serve as excitation sources, are connected, fiber Bragg gratings (FBG) 63 and 65 inserted before and after the Yb-doped double-cladding optical fiber 64, and an isolator 66 for stopping a fiber fuse from proceeding any further when the fiber fuse terminator 67 has allowed the fiber fuse to pass. [0112] The optical fiber 68 at the output terminal is a single mode optical fiber in which the outer diameter is 125 µm, and the MFD at the wavelength of 1,060 nm is 7.1 urn. As for the HAF which is used as the fiber fuse terminator 67, the outer diameter is 125 urn, the MFD at the wavelength of 1,060 nm is 7.4 urn, the number of the holes is 6, Rmin is 6.3 µm, 2xRmin/MFD is 1.7, and W is 5.2 µm. [0113] In the optical fiber laser apparatus 60, when a fiber fuse was deliberately generated by increasing the temperature of the optical fiber 68 at the output terminal, it was possible to terminate the fiber fuse in the HAF 67. Also, since polyimide was used as the coating of the HAF 67 and as the recoating over the fusion-splice portions at both ends, the coating did not burn. The optical fiber was thus protected from the fiber fuse, enabling the apparatus to be repaired simply by replacing and reconnecting the output fibers (the HAF 67 and the SMF 68). [0114] As a comparative example, when the same test was performed without inserting the HAF 67, the fiber fuse that was deliberately generated in the optical fiber 68 at the output terminal stopped after damaging part of the isolator 66. To repair the apparatus, this expensive isolator had to be replaced. [0115] As shown in FIG. 20, a fiber fuse terminator 77 configured from an HAF having a length of 60 mm was incorporated in a part of an output part of an optical fiber laser apparatus 70 using an erbium (Er)-doped double-cladding optical fiber (rare-earth doped optical fiber) 75. The fibers were fusion-spliced together. In FIG. 20, symbol x represents fusion-splice point. [0116] This Er fiber laser has an oscillating wavelength of 1,550 nm, and an output power of 4 W. The optical fiber laser apparatus 70 also includes a DFB laser 71 with a wavelength of 1,550 nm, an isolator 72 for preventing light from returning to the DFB laser 71, a multi-port coupler 74 with which a plurality of laser diodes (LD) 73 for excitation, which serve as excitation sources, are connected, and an isolator 76 for stopping a fiber fuse from proceeding any further when the fiber fuse terminator 77 has allowed the fiber fuse to pass. [0117] The optical fiber 78 at the output terminal is a single mode optical fiber in which the outer diameter is 125 µm, and the MFD at the wavelength of 1,550 nm is 9.8 µm. As for the HAF which is used as the fiber fuse terminator 77, the outer diameter is 125 µm, the MFD at the wavelength of 1,550 nm is 10.0 µm, the number of the holes is 4, Rmin is 8.1 µm, 2xRmin/MFD is 1.6, and W is 7.0 µm. [0118] In the optical fiber laser apparatus 70, when a fiber fuse was deliberately generated by increasing the temperature of the optical fiber 78 at the output terminal, it was possible to terminate the fiber fuse in the HAF 77. Also, since polyimide was used as the coating of the HAF 77 and as the recoating over the fusion-splice portions at both ends, the coating did not burn. The optical fiber was thus protected from the fiber fuse, enabling the apparatus to be repaired simply by replacing and reconnecting the output fibers (the HAF 77 and the SMF 78). [0119] In addition, when a fiber fuse occurs in the Er-doped double-clad optical fiber 75, it propagates towards the LD 73 and the DFB 71. However, since a multi-mode optical fiber is used for the output of the LD, the fiber fuse does not propagate any further in the direction of the LD 73. Furthermore, since the DFB laser 71 has an output of approximately several mW, the fiber fuse does not propagate in the direction of the DFB laser 71. [Industrial Applicability] [0120] The fiber fuse terminator of the present invention terminates the fiber fuse in an optical transmission line or an optical fiber laser through which high-power light is propagated and prevents damage to the transmission equipment or the light source, so that it can be appropriately used. [Description of the Reference Symbols] [0121] 20, 120, 220, 320, 420, 20A, 120A: HOLE-ASSISTED OPTICAL FIBER (OPTICAL FIBER) 21: CORE 22: CLADDING 23: HOLE 67, 77: FIBER FUSE TERMINATOR We Claim: 1. A fiber fuse terminator which is used to terminate a fiber fuse, comprising: an optical fiber which includes a core and a cladding having holes extending in a longitudinal direction thereof, wherein: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width in a diameter direction of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; and when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W<0.45xDfiber is satisfied. 2. The fiber fuse terminator according to claim 1, wherein when it is assumed that a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, furthest from the center of the core. of the hole that is furthest from the core is Rmax, and a sectional area of a region between a circle having a radius of Rmax around the center of the core and a circle having a radius of Rmin around the center of the core is S, a sectional area of a portion where the holes are provided in the region between the circle having the radius of Rmax and the circle having the radius of Rmin is no less than 20% of the sectional area S. 3. The fiber fuse terminator according to claim 1, wherein each end of the optical fiber is fusion-spliced to a single-mode optical fiber having no holes, and the fusion-splicing loss per one point thereon is no greater than 0.50 dB. 4. The fiber fuse terminator according to claim 1, wherein the number of the holes of the optical fiber is no less than 3. 5. The fiber fuse terminator according to claim 3, wherein: a resin coating covers a portion of a surface of the optical fiber, excepting a fusion-splice portion between the optical fiber and the single-mode optical fiber and a periphery thereof; and a flameproof protective layer covers the fusion-splice portion and the periphery thereof of the surface of the optical fiber. 6. The fiber fuse terminator according to claim 3, wherein each end of the optical fiber is fusion-spliced to the single-mode optical fiber by intermittent discharging or sweep discharging. 7. The fiber fuse terminator according to claim 1, wherein a length of the optical fiber is no less than 1 mm. 8. A fiber fuse terminator which is used to terminate a fiber fuse, comprising: an optical fiber which includes a core without holes and a cladding having one layer of holes extending in a longitudinal direction thereof, wherein: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; when it is assumed that a diameter of the cladding of the optical fiber is Dfibcr, W<0.45xDfiber is satisfied; and when it is assumed that a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a distance, in the cross section perpendicular to the longitudinal direction of the optical fiber, between the center of the core and a position, furthest from the center of the core, of the hole that is furthest from the core is Rmax, and a sectional area of a region between a circle having a radius of Rmax around the center of the core and a circle having a radius of Rmin around the center of the core is S, a sectional area of a portion where the holes are provided in the region between the circle having the radius of Rmax and the circle having the radius of Rmin is no less than 20% of the sectional area S. 9. The fiber fuse terminator according to claim 8, wherein a length of the optical fiber is no less 1 mm. 10. A fiber laser comprising: a pumping light source; a rare-earth doped optical fiber; and a fiber fuse terminator having an optical fiber which includes a core and a cladding having holes extending in a longitudinal direction thereof, wherein: a refractive index of the core of the optical fiber is higher than a refractive index of a portion of the cladding excepting portions of the holes; when it is assumed that a mode field diameter at a used wavelength of the optical fiber is MFD, and a distance, in a cross section perpendicular to the longitudinal direction of the optical fiber, between a center of the core and a position, closest to the center of the core, of the hole that is closest to the core is Rmin, a value expressed by 2xRmin/MFD is no less than 1.2 and no more than 2.1; when it is assumed that a width, in a diameter direction, of a region where the holes present in the cladding is W, a value expressed by W/MFD is no less than 0.3; and when it is assumed that a diameter of the cladding of the optical fiber is Dfiber, W<0.45xDfiber is satisfied. 11. The fiber laser according to claim 10, further comprising an isolator, wherein the fiber fuse terminator is disposed at an output side of the isolator. 12. An optical transmission line using an optical fiber, wherein the fiber fuse terminator according to claim 1 is inserted into the optical transmission line. 13. An optical transmission line using an optical fiber, wherein the fiber fuse terminator according to claim 8 is inserted into the optical transmission line.