Patent Description:
In the field of material processing in which metal materials or the like are processed (e.g., cut, welded, shaved), laser processing using laser light has been increasingly used, replacing machining using a blade, a drill, or the like. Laser processing is superior both in processing accuracy and processing speed to machining. As a laser device for use in laser processing, a fiber laser has been attracting attention. The fiber laser is highly energy-efficient, and is capable of providing laser light with high beam qualities (i.e., with small beam diameter and small beam spreading angle).

A fiber laser is a laser device whose amplifying medium is a pump-gain fiber. Examples of the fiber laser include a resonator-type fiber laser and a MOPA (Master Oscillator Power Amplifire)-type fiber laser. As the pump-gain fiber, a double-clad fiber whose core is doped with a rare-earth element such as Yb is used. By introducing pump light into a cladding of the pump-gain fiber, it is possible to oscillate or amplify laser light.

The resonator-type fiber laser and the power amplifier (post amplifier) of a MOPA-type fiber laser use a plurality of laser diodes to generate pump light. The resonator-type fiber laser and the power amplifier (post amplifier) of the MOPA-type fiber laser use an optical combiner to combine pump lights generated by the respective laser diodes (see Patent Literatures <NUM> and <NUM>). The optical combiner is an optical component that includes a plurality of input lead fibers and a single output lead fiber, and serves to combine the lights introduced via the respective input lead fibers and to send out the combined light via the output lead fiber. In the fiber laser, each of the input lead fibers of the optical combiner is connected to a laser diode via a pump fiber, whereas the output lead fiber of the optical combiner is connected to the pump-gain fiber. The optical combiner may have, at its input end, a delivery fiber port for receiving and sending out signal light, in addition to the input lead fibers connected to the pump fibers. The pump lights generated by the respective laser diodes are combined by the optical combiner and then introduced into the cladding of the pump-gain fiber.

The input lead fibers of the optical combiner for pump light (hereinafter referred to as a pump combiner), and the pump fibers that are fusion spliced to the input lead fibers, are each an optical fiber that guides only pump light therethrough. It is common to use, as such an optical fiber, a single-clad fiber constituted by: a core made of quartz glass; a primary coating made of resin and lower in refractive index than the core; and a secondary coating that is higher in refractive index than the core, such that the pump light is guided through the core. Since the core is made of quartz glass, the core is capable of guiding high-power pump light. Furthermore, since the cladding is made of a resin that is significantly different in refractive index from quartz glass, the numerical aperture (hereinafter referred to as NA) of the core is large and thus the pump light from the laser diodes can be efficiently introduced into the core.

On the other hand, the output lead fiber of the pump combiner, and the pump-gain fiber that is fusion spliced to the output lead fiber, are each an optical fiber that guides both the pump light and laser light therethrough. It is common to use, as such an optical fiber, a double-clad fiber constituted by: a core made of quartz glass; a cladding made of quartz glass and lower in refractive index than the core; a primary coating made of resin and lower in refractive index than the cladding; and a secondary coating that is higher in refractive index than the core, such that the laser light is guided through the core and that the pump light is guided through the cladding.

Incidentally, fusion splicing between optical fibers has to be done after removing coatings from an end portion of each fiber. This means that the primary coating and the secondary coating are removed from areas containing the fusion splice points between the input lead fibers and the pump fibers of the pump combiner. Similarly, the primary coating and the secondary coating are removed also from an area containing the fusion splice point between the output lead fiber of the pump combiner and the pump-gain fiber. Furthermore, also when an optical fiber is subjected to some other processing other than fusion splicing, the coating(s) of the optical fiber has to be removed from an area containing the to-be-processed point. For example, in a case of a resonator-type fiber laser, the pump-gain fiber has its opposite ends connected to fiber Bragg gratings. The fiber Bragg gratings are each a double-clad fiber having therein a grating that causes Bragg reflection. From an area containing the grating, the primary coating and the secondary coating are to be removed. Such an area from which the primary coating and the secondary coating have been removed is hereinafter referred to as a "coating-removed section". The coating-removed section borders on two other sections in each of which the primary coating and the secondary coating are left unremoved. Of these two sections, one section that is closer to the light-entrance end than the coating-removed section is referred to as a "first coated section", whereas the other section that is closer to the light-exit end than the coating-removed section is referred to as a "second coated section".

In the coating-removed section, the exposed portion of the optical waveguide (core or cladding) has to be covered with a medium that is lower in refractive index than that optical waveguide. This is because, otherwise, the coating-removed section is not capable of confining light within the optical waveguide. Patent Literature <NUM> discloses an arrangement in which an exposed portion of an optical waveguide in a coating-removed section is covered with a low-refractive-index resin. Patent Literature <NUM> discloses an arrangement in which an exposed portion of an optical waveguide in a coating-removed section is covered with air (air cladding). Further examples of background art can be found in Patent Literatures <NUM>-<NUM> listed below.

Conventionally, a medium that is lower in refractive index than the primary coating has been selected as the medium which covers the exposed portion of the optical waveguide in the coating-removed section, for the following reason. In cases where the medium that covers the exposed portion of the optical waveguide in the coating-removed section is lower in refractive index than the primary coating, the result is that the NA of the optical waveguide in the first coated section, which is closer to the light-entrance end than the coating-removed section, is smaller than the NA of the optical waveguide in the coating-removed section. This makes it possible to prevent light with a large propagation angle (i.e., the angle between the direction of light propagation and the optical axis of the optical fiber), which cannot be confined within the optical waveguide in the coating-removed section, from entering the coating-removed section from the first coated section and leaking out of the optical waveguide at the coating-removed section.

However, if the medium that covers the exposed portion of the optical waveguide in the coating-removed section is lower in refractive index than the primary coating, the following issue arises. Specifically, in cases where the medium that covers the exposed portion of the optical waveguide in the coating-removed section is lower in refractive index than the primary coating, the result is that the NA of the optical waveguide in the coating-removed section is greater than the NA of the optical waveguide in the second coated section, which is closer to the light-exit end than the coating-removed section. Therefore, if light with a large propagation angle which cannot be confined within the optical waveguide in the second coated section is produced at the processed part of the coating-removed section, this light will enter the second coated section from the coating-removed section and leak out of the optical waveguide at the second coated section.

The material for the primary coating is selected from resin materials that have a small Young's modulus, that are highly transparent, and that are well adhesive to quartz, whereas the material for the secondary coating is selected from resin materials that are highly wear-resistant, that are highly resistant to external forces, and that are easily workable when wound on and unwound from a reel. As such, the secondary coating is made of a material with low transparency in many cases. Therefore, if the light leakage as described earlier occurs, the secondary coating in the second coated section may absorb the leaked light and generate heat. Such heat generation may cause a reduction in reliability of the optical fiber and thus should be avoided.

One aspect of the present invention was made in view of the above issue, and an object thereof is to provide an optical fiber which is arranged such that light leakage from an optical waveguide, which would occur in a coated section that is closer to the light-exit end than a coating-removed section, is prevented or reduced, and thus is more reliable than conventional optical fibers.

In order to attain the above object, an optical fiber in accordance with the independent claim <NUM> or alternatively in accodance with the independent claim <NUM> is provided. Further preferred optional embodiments are defined in the dependent claims.

According to one aspect of the present invention, it is possible to provide an optical fiber which is arranged such that light leakage from an optical waveguide, which would occur in a coated section that is closer to the light-exit end than a coating-removed section, is prevented or reduced, and thus is more reliable than conventional optical fibers.

The following description will discuss a configuration of an optical fiber <NUM> in accordance with Embodiment <NUM> of the present invention, with reference to <FIG>. The upper part of <FIG> schematically illustrates the optical fiber <NUM> having a fusion splice point P1 as a processed point, and the lower part of <FIG> is an enlarged partial longitudinal cross-sectional view of a portion containing the fusion splice point P1 of the optical fiber <NUM>. The right part of <FIG> shows (i) refractive index distribution of the optical fiber <NUM> in a coating-removed section I0 that contains the fusion splice point P1 (upper right part) and (ii) a transverse cross-sectional view of the optical fiber <NUM> in the coating-removed section I0 (lower right part). The left part of <FIG> shows (i) refractive index distribution in coated sections I1 and I2 bordering on the coating-removed section I0 (upper left part) and (ii) a transverse cross-sectional view of the optical fiber <NUM> in the coated sections I1 and I2 (lower left part).

As illustrated in the lower part of <FIG>, the optical fiber <NUM> includes: a core <NUM> in the form of a circular rod (the core <NUM> is one example of the "optical waveguide" recited in the claims); a primary coating <NUM> that is in the form of a circular tube and that covers the side surface of the core <NUM> (the primary coating <NUM> is one example of the "coating" recited in the claims); and a secondary coating <NUM> that is in the form of a circular tube and that covers the outer surface of the primary coating <NUM>. The core <NUM> is made of quartz glass, and the primary coating <NUM> and the secondary coating <NUM> are each made of resin. The core <NUM> has a refractive index n1, the primary coating <NUM> has a refractive index n2, and the secondary coating <NUM> has a refractive index n3 such that the refractive index n1, the refractive index n2, and the refractive index n3 satisfy the relationship: n2<n1<n3, as illustrated in the left part of <FIG>. Specifically, the optical fiber <NUM> is a single-clad fiber designed to confine light within the core <NUM> by utilizing the difference between the refractive index n1 of the core <NUM> made of quartz glass and the refractive index n2 of the primary coating <NUM> made of resin (such a difference is represented as "n1-n2"). Such an optical fiber <NUM> is used as, for example, some of the optical fibers constituting a fiber laser which only guide pump light therethrough, that is, pump fibers and input lead fibers of a pump combiner.

The optical fiber <NUM> is obtained by fusion-splicing end surfaces of two optical fibers each having the foregoing structure. As illustrated in the upper and lower parts of <FIG>, in the section I0 that contains the fusion splice point P1, the primary coating <NUM> and the secondary coating <NUM> have been removed. Such a section I0 is referred to as the "coating-removed section I0" in this specification. The coating-removed section I0 borders on two sections, one of which is referred to as the "first coated section I1" and the other of which is referred to as the "second coated section I2".

The optical fiber <NUM> is characterized in that, in at least part of (in Embodiment <NUM>, the entirety of) the coating-removed section I0, there is provided an intermediate-refractive-index resin part <NUM> that covers the side surface of the core <NUM>. The intermediate-refractive-index resin part <NUM> may be (<NUM>) a part that is formed by allowing a resin filled in a groove (in which the coating-removed section I0 of the optical fiber <NUM> is stored) in a metal block to cure. The intermediate-refractive-index resin part <NUM> may be (<NUM>) a part that is formed by inserting the coating-removed section I0 of the optical fiber <NUM> into a heat-shrinkable tube filled with a thermoplastic resin and allowing the heat-shrinkable tube to shrink. Alternatively, the intermediate-refractive-index resin part <NUM> may be (<NUM>) a part that is formed by recoating. The intermediate-refractive-index resin part <NUM> may be composed of a plurality of resin parts. In this case, the resin parts may be spaced from each other. The plurality of resin parts may be arranged so as to form a plurality of layers stacked together in the radial direction of the optical fiber <NUM>, or may be arranged so as to form a plurality of sections arranged in the axial direction of the optical fiber <NUM>. The plurality of resin parts may be composed of a single kind of resin material, or may be composed of two or more kinds of resin material.

The intermediate-refractive-index resin part <NUM> is composed of a resin that (<NUM>) is higher in transmittance than the secondary coating <NUM> (e.g., has a transmittance of not less than <NUM>%/mm) at the wavelength of light propagating through the core <NUM> and (<NUM>) has a refractive index n4 that is lower than the refractive index n1 of the core <NUM> and that is higher than the refractive index n2 of the primary coating <NUM> as illustrated in the right part of <FIG>. Since the refractive index n4 of the intermediate-refractive-index resin part <NUM> is lower than the refractive index n1 of the core <NUM>, the coating-removed section I0 is also capable of confining light within the core <NUM>. Furthermore, since the refractive index n4 of the intermediate-refractive-index resin part <NUM> is higher than the refractive index n2 of the primary coating <NUM>, the "light-confining ability" of the coating-removed section I0 is weaker than those of the first coated section I1 and the second coated section. More accurately, NA0, which is the NA of the core <NUM> in the coating-removed section I0, is smaller than NA1 and NA2, which are the NAs of the core <NUM> in the first coated section I1 and in the second coated section I2, respectively.

The light-confining ability of the coating-removed section I0 is weaker than the light-confining ability of the first coated section I1 and the second coated section. Therefore, the optical fiber <NUM> brings about the following effect when light propagates from the first coated section I1 to the second coated section I2 (from left to right in <FIG>). Specifically, NA0, which is the NA of the core <NUM> in the coating-removed section I0, is smaller than NA2, which is the NA of the core <NUM> in the second coated section I2. This makes it possible to eliminate or reduce the likelihood that light which cannot be confined within the core <NUM> in the second coated section I2 will enter the second coated section I2 from the coating-removed section I0 (because such light leaks out of the core <NUM> and escapes into the intermediate-refractive-index resin part <NUM> while propagating through the coating-removed section I0). That is, it is possible to eliminate or reduce the likelihood that the primary coating <NUM> and the secondary coating <NUM> in the second coated section I2 will absorb light leaked from the core <NUM> and generate heat and degrade.

Note that, NA1, which is the NA of the core <NUM> in the first coated section I1, is greater than NA0, which is the NA of the core <NUM> in the coating-removed section I0; therefore, light that cannot be confined within the core <NUM> in the coating-removed section I0 may enter the coating-removed section I0 from the first coated section I1. That is, the intermediate-refractive-index resin part <NUM> in the coating-removed section I0 may absorb light that has been leaked from the core <NUM> and that has escaped into the intermediate-refractive-index resin part <NUM>, and may generate heat and degrade. However, the intermediate-refractive-index resin part <NUM> has a larger cross section than the secondary coating <NUM> and is made of a resin that is higher in transmittance than the secondary coating <NUM>; therefore, even if the light leaked from the core <NUM> enters the intermediate-refractive-index resin part <NUM>, the power density of the entered light is sufficiently small and the conversion efficiency from the entered light to heat is also sufficiently low. As such, even if the intermediate-refractive-index resin part <NUM> generates heat, the quantity of heat is extremely small, and, even if the intermediate-refractive-index resin part <NUM> degrades, the extent of the degradation is extremely small. Furthermore, unlike the coated sections I1 and I2, the intermediate-refractive-index resin part <NUM> does not have any high-refractive-index region therein such as that of the secondary coating <NUM>, and therefore light is quickly scattered within the intermediate-refractive-index resin part <NUM>. As such, a heat generating region is widely dispersed and the temperature rise is prevented or reduced.

Furthermore, by employing an arrangement in which NA1min (which is the minimum value of the NA of the core <NUM> in the first coated section I1) is not greater than NA0 (which is the NA of the core <NUM> in the coating-removed section I0), it is possible to reduce the likelihood that the light that cannot be confined within the core <NUM> in the coating-removed section I0 will enter the coating-removed section I0 from the first coated section I1. Such an arrangement is achieved by, for example, bending the optical fiber <NUM> in the first coated section I1 as illustrated in <FIG>.

In so doing, a minimum bend radius R of the optical fiber <NUM> in the first coated section I1 may be set as below. Specifically, the NA1min (which is the minimum value of the NA of the core <NUM> in the first coated section I1) is given by the following Equation (<NUM>), where θbend is defined by the following Equation (<NUM>) and θcmax is defined by the following Equation (<NUM>), assuming that the diameter of the core <NUM> is D. Therefore, the minimum bend radius R of the optical fiber <NUM> in the first coated section I1 needs only be set such that the NA1min, which is given by Equation (<NUM>), is less than NA0 (which is the NA of the core <NUM> in the coating-removed section I0). <MAT> <MAT> <MAT>.

Note that, in Embodiment <NUM>, the medium that covers the side surface of the core <NUM> is the intermediate-refractive-index resin part <NUM>, which is uniform in refractive index; however, the present invention is not limited as such. Specifically, the medium that covers the side surface of the core <NUM> may be an intermediate-refractive-index resin part <NUM> that is non-uniform in refractive index, instead of the intermediate-refractive-index resin part <NUM> that is uniform in refractive index, provided that the refractive index of the intermediate-refractive-index resin part <NUM> at and near the interface between the intermediate-refractive-index resin part <NUM> and the core <NUM> is lower than the refractive index n1 of the core <NUM> and is higher than the refractive index n2 of the primary coating <NUM>. The intermediate-refractive-index resin part <NUM> may be partially absent. In this case, an exposed portion, of the side surface of the core <NUM>, which is not covered by the intermediate-refractive-index resin part <NUM> is preferably not in contact with any medium that has a refractive index equal to or higher than the refractive index n1 of the core <NUM>. This is because, if such a medium is in contact with the side surface of the core <NUM>, the core <NUM> may be unable to confine light therein and the light may leak out through the contact part. Thus, the exposed portion, of the side surface of the core <NUM>, which is not covered by the intermediate-refractive-index resin part <NUM> is preferably covered with a gas (air cladding) or some other resin part each of which has a refractive index lower than the refractive index n1 of the core <NUM>. Specific variations will be described later with reference to other drawings.

An optical fiber <NUM> as described below was prepared as an Example: an optical fiber <NUM> in which the diameter of the core <NUM> is <NUM>, the refractive index n1 of the core <NUM> is <NUM>, the refractive index n2 of the primary coating <NUM> is <NUM>, the refractive index n3 of the secondary coating <NUM> is <NUM>, and the refractive index n4 of the intermediate-refractive-index resin part <NUM> is <NUM>. The minimum bend radius R of the optical fiber <NUM> in the first coated section I1 was set to <NUM>. In this case, the minimum value of the NA of the core <NUM> in the first coated section I1 is calculated as <NUM>, the NA of the core <NUM> in the coating-removed section I0 is calculated as <NUM>, and the NA of the core <NUM> in the second coated section I2 is calculated as <NUM>.

The temperature of the optical fiber <NUM> in accordance with the Example was measured with the use of a thermal imager while introducing light of <NUM> kW. As a result, it was confirmed that (<NUM>) the temperature rose to the greatest extent at and near the fusion splice point P1 and (<NUM>) the amount of temperature rise at the fusion splice point P1 was <NUM>.

An optical fiber as described below was prepared as a Comparative Example: an optical fiber that is the same as the optical fiber <NUM> of the Example except that the refractive index n4 of the intermediate-refractive-index resin part <NUM> is <NUM>. In this case, the minimum value of the NA of the core <NUM> in the first coated section I1 and the NA of the core <NUM> in the second coated section I2 are calculated as the same values as those of the optical fiber <NUM> in accordance with the Example; however, the NA of the core <NUM> in the coating-removed section I0 is calculated as <NUM>.

The temperature of the optical fiber in accordance with the Comparative Example was measured with the use of a thermal imager while introducing light of <NUM> kW. As a result, it was confirmed that (<NUM>) the temperature rose to the greatest extent at the coating-removed-section-I0-side end of the second coated section I2 and (<NUM>) the amount of temperature rise at the coating-removed-section-I0-side end of the second coated section I2 was <NUM>.

The above results experimentally demonstrate that the light that cannot be confined within the core <NUM> in the second coated section I2 does not enter the second coated section I2 from the coating-removed section I0. That is, the above results experimentally demonstrate that the primary coating <NUM> and the secondary coating <NUM> in the second coated section I2 do not absorb light leaked from the core <NUM> and thus do not generate heat.

The following description will discuss a configuration of an optical fiber <NUM> in accordance with Embodiment <NUM> of the present invention, with reference to <FIG>. The upper part of <FIG> schematically illustrates the optical fiber <NUM> which has a fusion splice point P2, and the lower part of <FIG> is an enlarged partial longitudinal cross-sectional view of a portion containing the fusion splice point P2 of the optical fiber <NUM>. The right part of <FIG> shows (i) refractive index distribution of the optical fiber <NUM> in a coating-removed section I0 containing the fusion splice point (upper right part) and (ii) a transverse cross-sectional view of the optical fiber <NUM> in the coating-removed section I0 (lower right part). The left part of <FIG> shows (i) refractive index distribution in coated sections I1 to I2 bordering on the coating-removed section I0 (upper left part) and (ii) a transverse cross-sectional view of the optical fiber <NUM> in the coated sections I1 to I2 (lower left part).

As illustrated in the lower part of <FIG>, the optical fiber <NUM> includes: a core <NUM> in the form of a circular rod; an inner cladding <NUM> that is in the form of a circular tube and that covers the side surface of the core <NUM> (the inner cladding <NUM> is one example of the "optical waveguide" recited in the claims); an outer cladding <NUM> that is in the form of a circular tube and that covers the outer surface of the inner cladding <NUM> (the outer cladding <NUM> is one example of the "coating" recited in the claims); and an outer jacket <NUM> that is in the form of a circular tube and that covers the outer surface of the outer cladding <NUM>. The core <NUM> and the inner cladding <NUM> are each made of quartz glass, and the outer cladding <NUM> and the outer jacket <NUM> are each made of resin. The core <NUM> has a refractive index n0, the inner cladding <NUM> has a refractive index n1, the outer cladding <NUM> has a refractive index n2, and the outer jacket <NUM> has a refractive index n3 such that the refractive index n0, the refractive index n1, the refractive index n2, and the refractive index n3 satisfy the relationship: n2<n1<n0<n3, as illustrated in the left part of <FIG>. Specifically, the optical fiber <NUM> is a double-clad fiber designed to confine light within the core <NUM> by utilizing the difference between the refractive index n0 of the core <NUM> made of quartz glass and the refractive index n1 of the inner cladding <NUM> made of quartz glass (such a difference is represented as "n0-n1") and confine light within the inner cladding <NUM> by utilizing the difference between the refractive index n1 of the inner cladding <NUM> made of quartz glass and the refractive index n2 of the outer cladding <NUM> made of resin (such a difference is represented as "n1-n2"). Such an optical fiber <NUM> is used as, for example, some of the optical fibers constituting a fiber laser which guide laser light and pump light therethrough, that is, a pump-gain fiber and an output lead fiber of a pump combiner.

The optical fiber <NUM> is obtained by fusion-splicing end surfaces of two optical fibers each having the foregoing structure. As illustrated in the upper and lower parts of <FIG>, in the section I0 that contains the fusion splice point P2, the outer cladding <NUM> and the outer jacket <NUM> have been removed. Such a section I0 is referred to as the "coating-removed section I0" in this specification. The coating-removed section I0 borders on two sections, one of which is referred to as the "first coated section I1" and the other of which is referred to as the "second coated section I2".

The optical fiber <NUM> is characterized in that, in at least part of (in Embodiment <NUM>, the entirety of) the coating-removed section I0, there is provided an intermediate-refractive-index resin part <NUM> that covers the side surface of the inner cladding <NUM>. The intermediate-refractive-index resin part <NUM> is composed of a resin that (<NUM>) is higher in transmittance than the outer jacket <NUM> (e.g., has a transmittance of not less than <NUM>%/mm) at the wavelength of light propagating through the inner cladding <NUM>, and (<NUM>) has a refractive index n4 that is lower than the refractive index n1 of the inner cladding <NUM> and that is higher than the refractive index n2 of the outer cladding <NUM> as illustrated in the right part of <FIG>. Therefore, the optical fiber <NUM> is capable of confining light within the inner cladding <NUM> also in the coating-removed section I0; however, the light-confining ability of the coating-removed section I0 is weaker than those of the first coated section I1 and the second coated section I2. More accurately, NA0, which is the NA of the inner cladding <NUM> in the coating-removed section I0, is smaller than NA1 and NA2, which are the NAs of the inner cladding <NUM> in the first coated section I1 and the second coated section I2, respectively.

Because of the above characteristics, the optical fiber <NUM> brings about the following effect when light propagates from the first coated section I1 to the second coated section I2 (from left to right in <FIG>). Specifically, NA0, which is the NA of the inner cladding <NUM> in the coating-removed section I0, is smaller than NA2, which is the NA of the inner cladding <NUM> in the second coated section I2. This makes it possible to eliminate or reduce the likelihood that light which cannot be confined within the inner cladding <NUM> in the second coated section I2 will enter the second coated section I2 from the coating-removed section I0 (because such light leaks out of the inner cladding <NUM> and escapes into the intermediate-refractive-index resin part <NUM> while propagating through the coating-removed section I0). That is, it is possible to eliminate or reduce the likelihood that the outer cladding <NUM> and the outer jacket <NUM> in the second coated section I2 will absorb light leaked from the inner cladding <NUM> and generate heat and degrade.

Note that NA1, which is the NA of the inner cladding <NUM> in the first coated section I1, is greater than NA0, which is the NA of the inner cladding <NUM> in the coating-removed section I0; therefore, light that cannot be confined within the inner cladding <NUM> in the coating-removed section I0 (i.e., light whose NA is greater than NA0) may enter the coating-removed section I0 from the first coated section I1. That is, the intermediate-refractive-index resin part <NUM> in the coating-removed section I0 may absorb light coming from the inner cladding <NUM> and may generate heat and degrade. However, unlike the coated sections I1 and I2, the intermediate-refractive-index resin part <NUM> does not have any high-refractive-index region therein such as that of the outer jacket <NUM>, and therefore light is quickly scattered within the intermediate-refractive-index resin part <NUM>. As such, a heat generating region is widely dispersed and the temperature rise is prevented or reduced. Furthermore, since the intermediate-refractive-index resin part <NUM> is made of a resin that is transparent at the wavelength of light propagating through the inner cladding <NUM>, even if the intermediate-refractive-index resin part <NUM> generates heat, the quantity of heat is extremely small, and, even if the intermediate-refractive-index resin part <NUM> degrades, the extent of the degradation is extremely small.

Furthermore, by employing an arrangement in which NA1min (which is the minimum value of the NA of the inner cladding <NUM> in the first coated section I1) is less than NA0 (which is the NA of the inner cladding <NUM> in the coating-removed section I0), it is possible to eliminate the likelihood that the light that cannot be confined within the inner cladding <NUM> in the coating-removed section I0 will enter the coating-removed section I0 from the first coated section I1. Such an arrangement is achieved by, for example, bending the optical fiber <NUM> in the first coated section I1 as illustrated in <FIG>.

In so doing, a minimum bend radius R of the optical fiber <NUM> in the first coated section I1 may be set as below. Specifically, the NA1min (which is the minimum value of the NA of the inner cladding <NUM> in the first coated section I1) is given by the foregoing Equation (<NUM>), where θbend is defined by the foregoing Equation (<NUM>) and θcmax is defined by the foregoing Equation (<NUM>), assuming that the diameter of the inner cladding <NUM> is D. Therefore, the minimum bend radius R of the optical fiber <NUM> in the first coated section I1 needs only be set such that the NA1min, which is given by the foregoing Equation (<NUM>), is less than NA0 (which is the NA of the inner cladding <NUM> in the coating-removed section I0).

The optical fibers <NUM> and <NUM> in accordance with Embodiments <NUM> and <NUM> as described above are each an optical fiber in which its core <NUM> or <NUM> has the fusion splice point P1 or P2 formed therein; however, the present invention is not limited as such. That is, the present invention is applicable to any optical fiber in which some processing that necessitates removal of a coating(s) has been done. For example, the present invention is applicable to an optical fiber that has a grating formed therein, or the like.

<FIG> shows a partial longitudinal cross-sectional view illustrating one variation of the optical fiber <NUM> in accordance with Embodiment <NUM>. The optical fiber <NUM> in accordance with this variation is the same as the optical fiber <NUM> illustrated in <FIG>, except that a Bragg grating G1 is provided in place of the fusion splice point P1. The rest of the configuration of the optical fiber <NUM> in accordance with this variation is the same as that of the optical fiber <NUM> illustrated in <FIG>, and thus the descriptions therefor are omitted here.

Similarly to the optical fiber <NUM> illustrated in <FIG>, also according to the optical fiber <NUM> illustrated in <FIG>, NA0, which is the NA of the core <NUM> in the coating-removed section I0, is smaller than NA2, which is the NA of the core <NUM> in the second coated section I2. This makes it possible to eliminate or reduce the likelihood that light which cannot be confined within the core <NUM> in the second coated section I2 will enter the second coated section I2 from the coating-removed section I0. Thus, it is possible to eliminate or reduce the likelihood that the primary coating <NUM> and the secondary coating <NUM> in the second coated section I2 will absorb light leaked from the core <NUM> and generate heat and degrade.

<FIG> shows a partial longitudinal cross-sectional view illustrating one variation of the optical fiber <NUM> in accordance with Embodiment <NUM>. The optical fiber <NUM> in accordance with this variation is the same as the optical fiber <NUM> illustrated in <FIG>, except that a Bragg grating G2 is provided in place of the fusion splice point P2. The rest of the configuration of the optical fiber <NUM> in accordance with this variation is the same as that of the optical fiber <NUM> illustrated in <FIG>, and thus the descriptions therefor are omitted here.

Similarly to the optical fiber <NUM> illustrated in <FIG>, also according to the optical fiber <NUM> illustrated in <FIG>, NA0, which is the NA of the inner cladding <NUM> in the coating-removed section I0, is smaller than NA2, which is the NA of the inner cladding <NUM> in the second coated section I2. This makes it possible to eliminate or reduce the likelihood that light which cannot be confined within the inner cladding <NUM> in the second coated section I2 will enter the second coated section I2 from the coating-removed section I0. Thus, it is possible to eliminate or reduce the likelihood that the outer cladding <NUM> and the outer jacket <NUM> in the second coated section I2 will absorb light leaked from the inner cladding <NUM> and generate heat and degrade.

The foregoing optical fiber <NUM> in accordance with Embodiment <NUM> employs an arrangement in which the side surface of the core <NUM> is covered with the intermediate-refractive-index resin part <NUM> from all directions in every transverse cross section in the coating-removed section I0; however, the present invention is not limited as such. Specifically, the following arrangement may be employed: the side surface of the core <NUM> is covered with the intermediate-refractive-index resin part <NUM> only in some transverse cross sections in the coating-removed section I0. Alternatively, the following arrangement may be employed: the side surface of the core <NUM> is covered with the intermediate-refractive-index resin part <NUM> from only some directions in a transverse cross section.

(a) of <FIG> shows a partial longitudinal cross-sectional view (left) and a transverse cross-sectional view (right), each illustrating one variation of the optical fiber <NUM> in accordance with Embodiment <NUM>. The transverse cross-sectional view is taken along line αα in the partial longitudinal cross-sectional view. As illustrated in (a) of <FIG>, according to the optical fiber <NUM> in accordance with this variation, the intermediate-refractive-index resin part <NUM> is absent except in sections I01 and I02. That is, the side surface of the core <NUM>, only in some transverse cross sections in the coating-removed section I0 (that is, in each transverse cross section in the sections I01 and I02), is covered with the intermediate-refractive-index resin part <NUM> from all directions in that transverse cross section. In the coating-removed section I0, a portion of the side surface of the core <NUM> that is not covered with the intermediate-refractive-index resin part <NUM> needs only be covered by a gas or some other resin part each of which is lower in refractive index than the core <NUM>.

(b) of <FIG> shows a partial longitudinal cross-sectional view (left) and a transverse cross-sectional view (right), each illustrating another variation of the optical fiber <NUM> in accordance with Embodiment <NUM>. The transverse cross-sectional view is taken along line αα in the partial longitudinal cross-sectional view. As illustrated in (b) of <FIG>, according to the optical fiber <NUM> in accordance with this variation, the intermediate-refractive-index resin part <NUM> is absent in the range of ±<NUM>° from the top of the core <NUM>. That is, the side surface of the core <NUM>, in every transverse cross section in the coating-removed section I0, is covered with the intermediate-refractive-index resin part <NUM> only from some directions (±<NUM>° from the bottom of the core <NUM>) in that transverse cross section. In the coating-removed section I0, a portion of the side surface of the core <NUM> that is not covered with the intermediate-refractive-index resin part <NUM> needs only be covered by a gas or some other resin part each of which is lower in refractive index than the core <NUM>.

(c) of <FIG> shows a partial longitudinal cross-sectional view (top) and transverse cross-sectional views (bottom), each illustrating a further variation of the optical fiber <NUM> in accordance with Embodiment <NUM>. The transverse cross-sectional views are taken along line αα, line ββ, line γγ, line δδ, and line εε in the partial longitudinal cross-sectional view, respectively. As illustrated in (c) of <FIG>, the optical fiber <NUM> in accordance with this variation is such that the intermediate-refractive-index resin part <NUM> is (i) absent in a section I01 at the bottom side of the core <NUM> and (ii) absent in a section I02 at the top side of the core <NUM>. That is, the side surface of the core <NUM>, only in some transverse cross sections in the coating-removed section I0 (i.e., only in each transverse cross section in the sections I01 and I02), is covered with the intermediate-refractive-index resin part <NUM> only from some directions in that transverse cross section. In the coating-removed section I0, a portion of the side surface of the core <NUM> that is not covered with the intermediate-refractive-index resin part <NUM> needs only be covered by a gas or some other resin part each of which is lower in refractive index than the core <NUM>.

Note that the optical fiber <NUM> in accordance with Embodiment <NUM> can also be modified in a similar manner. Specifically, the following arrangement may be employed: the side surface of the inner cladding <NUM> is covered with the intermediate-refractive-index resin part <NUM> only in some transverse cross sections in the coating-removed section I0. Alternatively, the following arrangement may be employed: the side surface of the inner cladding <NUM> is covered with the intermediate-refractive-index resin part <NUM> from only some directions in a transverse cross section.

Lastly, the following description will discuss examples of application of the optical fibers <NUM> and <NUM> in accordance with Embodiments <NUM> and <NUM>, with reference to <FIG> schematically illustrates a configuration of a fiber laser FL.

The fiber laser FL is a laser device including, as a resonator, a pump-gain fiber PGF that has fiber Bragg gratings FBG1 and FBG2 connected to the opposite ends thereof. The fiber Bragg grating FBG1, which is on the forward side, is an optical fiber having a grating formed therein which serves as a mirror. The fiber Bragg grating FBG2, which is on the backward side, is an optical fiber having a grating formed therein which serves as a half mirror. The pump-gain fiber PGF used here is a double-clad fiber whose core is doped with a rare-earth element such as Yb. By introducing pump light into a cladding of the pump-gain fiber PGF, it is possible to allow this resonator to oscillate laser light.

In the fiber laser FL, laser diodes LD11 to LD16 and laser diodes LD21 to LD26 are used to generate the pump light. Furthermore, in the fiber laser FL, pump combiners PC1 and PC2 are used to introduce, into the cladding of the pump-gain fiber PGF, the pump light generated by the laser diodes LD11 to LD16 and the laser diodes LD21 to LD26. Each of the pump combiners PC1 and PC2 is an optical component that includes a plurality of input lead fibers and a single output lead fiber. Each of the pump combiners PC1 and PC2 serves to combine lights introduced thereto via the respective input optical fibers and to send out the combined light via the output lead fiber.

The input lead fibers of the pump combiner PC1 are connected to the laser diodes LD1i via pump fibers PF1i (i is <NUM> to <NUM>). The output lead fiber of the pump combiner PC1 is connected to one end of the pump-gain fiber PGF via the fiber Bragg grating FBG1. Pump lights generated by the laser diodes LD11 to LD16 are combined by the pump combiner PC1, and the combined light, as forward pump light, is introduced to the cladding of the pump-gain fiber PGF. Similarly, the input lead fibers of the pump combiner PC2 are connected to the laser diodes LD2j via pump fibers PF2j (j is <NUM> to <NUM>). The output lead fiber of the pump combiner PC2 is connected to the other end of the pump-gain fiber PGF via the fiber Bragg grating FBG2. Pump lights generated by the laser diodes LD21 to LD26 are combined by the pump combiner PC2, and the combined light, as backward pump light, is introduced to the cladding of the pump-gain fiber PGF. The laser light oscillated in the core of the pump-gain fiber PGF propagates through the core of the fiber Bragg grating FBG2, the core of the output lead fiber of the pump combiner PC2, the core of a delivery fiber port of the pump combiner PC2, and the core of a delivery fiber DF, and then is emitted outward.

In the fiber laser FL, the pump fibers PF11 to PF16, the pump fibers PF21 to PF26, and the input lead fibers of the pump combiners PC1 and PC2 are optical fibers in which the pump light propagates through the cores thereof. It is common to use, as each of these optical fibers, the optical fiber used in Embodiment <NUM>, that is, a single-clad fiber that includes a core made of quartz glass and a primary coating (cladding) made of resin. As such, optical fibers obtained by fusion-splicing the pump fibers PF11 to PF16 with the input lead fibers of the pump combiner PC1, respectively, and optical fibers obtained by fusion-splicing the pump fibers PF21 to PF26 with the input lead fibers of the pump combiner PC2, respectively, are suitable examples of application of the optical fiber <NUM> in accordance with Embodiment <NUM> (see <FIG>).

On the other hand, in the fiber laser FL, the output lead fibers of the pump combiners PC1 and PC2, the fiber Bragg gratings FBG1 and FBG2, and the pump-gain fiber PGF are optical fibers in which signal light propagates through the cores thereof and pump light propagates through the claddings thereof. It is common to use, as each of these optical fibers, the optical fiber used in Embodiment <NUM>, that is, a double-clad fiber that includes a core made of quartz glass, a cladding made of quartz glass, and a primary coating made of resin. As such, an optical fiber obtained by fusion-splicing these fibers is a suitable example of application of the optical fiber <NUM> in accordance with Embodiment <NUM> (see <FIG>). Furthermore, each of the pump combiners PC1 and PC2 itself is obtained by fusion-splicing the input lead fibers, which are single-clad fibers (or the delivery fiber DF, which is a double-clad fiber), with the output lead fiber, which is a double-clad fiber, and thus is a suitable Example of the present invention. Furthermore, each of the fiber Bragg gratings FBG1 and FBG2 is a double-clad fiber that has a grating formed therein, and thus is a suitable example of application of a variation of the optical fiber <NUM> in accordance with Embodiment <NUM> (see <FIG>).

Note that optical devices to which the optical fibers <NUM> and <NUM> in accordance with Embodiments <NUM> and <NUM> are applicable are not limited to fiber lasers. For example, an optical combiner is also one example of an optical device to which the optical fibers <NUM> and <NUM> in accordance with Embodiments <NUM> and <NUM> are applicable.

In order to attain the foregoing object, an optical fiber in accordance with the independent claim <NUM> or alternatively in accordance with the independent claim <NUM> is provided.

According to the above arrangements of the independent claims <NUM> or <NUM>, it is possible to prevent or reduce light leakage from the optical waveguide which would occur in a coated section that borders on the coating-removed section and that is closer to the light-exit end than the coating-removed section. This makes it possible to provide an optical fiber that is more reliable than conventional optical fibers.

The above optical fiber may be arranged such that (<NUM>) the side surface of the optical waveguide, in every transverse cross section in the coating-removed section, is covered with the medium from all directions in the every transverse cross section, (<NUM>) the side surface of the optical waveguide, only in some transverse cross sections in the coating-removed section, is covered with the medium from all or some directions in each of the some transverse cross sections, or (<NUM>) the side surface of the optical waveguide, in all or some transverse cross sections in the coating-removed section, is covered with the medium only from some directions in each of the all or some transverse cross sections. According to each of the above arrangements, it is possible to provide an optical fiber that is more reliable than conventional optical fibers.

The optical fiber in accordance with the invention is arranged such that: the coating-removed section borders on two sections, one of which is a first coated section in which the side surface of the optical waveguide is covered with the coating. In a preferred embodiment, the medium may have a refractive index set such that an NA of the optical waveguide in the coating-removed section is not less than the minimum value of an NA of the optical waveguide in the first coated section.

According to the above arrangement, it is possible to prevent or reduce light leakage from the optical waveguide that would occur in the coating-removed section.

The optical fiber in accordance with the present embodiment is preferably arranged such that the refractive index of the medium is set such that the NA of the optical waveguide in the coating-removed section is not less than NA1min, the NA1min being defined by the following equations (<NUM>) to (<NUM>): <MAT> <MAT> <MAT> where D represents a diameter of the optical waveguide, R represents a minimum bend radius in the first coated section, n1 represents a refractive index of the optical waveguide, and n2 represents a refractive index of the coating.

According to the above arrangement, it is possible to prevent or reduce light leakage from the optical waveguide that would occur in the coating-removed section, merely by bending the optical fiber in the first coated section such that the above conditions are satisfied.

The optical fiber in accordance with the present embodiment is preferably arranged such that the medium is transparent at a wavelength of light guided through the optical waveguide.

According to the above arrangement, it is possible to prevent or reduce the optical loss that would result from the absorption, by the medium, of light that is distributed in the periphery of the optical waveguide in the coating-removed section. It is also possible, even if light leakage from the optical waveguide occurs in the coating-removed section, to prevent the medium from absorbing the leaked light and generating heat.

The optical fiber in accordance with the present embodiment is preferably arranged such that the medium is a resin part that is lower in refractive index than the optical waveguide and that is higher in refractive index than the coating.

According to the above arrangement, it is not only possible to achieve the foregoing optical effects but also possible to obtain a mechanical effect that the coating-removed section of the optical fiber is reinforced.

The optical fiber in accordance with the present embodiment may be (i) a single-clad fiber that includes: a core made of quartz glass and serving as the optical waveguide; and a primary coating made of resin and serving as the coating or (ii) a double-clad fiber that includes: a core made of quartz glass; an inner cladding made of quartz glass and serving as the optical waveguide; and an outer cladding made of resin and serving as the coating.

According to any of the above arrangements, the foregoing effects are obtained.

The optical fiber in accordance with the present embodiment may be arranged such that the optical waveguide contains a fusion splice point or that the optical waveguide has a grating formed therein.

A fiber laser including any of the foregoing optical fibers also falls within the scope of the present embodiment.

Claim 1:
An optical fiber (<NUM>) comprising:
an optical waveguide (<NUM>), the optical waveguide (<NUM>) comprising a core (<NUM>);
a primary coating (<NUM>)serving as a coating (<NUM>) that is lower in refractive index than the optical waveguide (<NUM>) and that covers a side surface of the optical waveguide (<NUM>) such that the coating (<NUM>) is in direct contact with the side surface of the optical waveguide (<NUM>) except in a coating-removed section (I0); and
a secondary coating (<NUM>) covering an outer surface of the primary coating (<NUM>), wherein the secondary coating (<NUM>)has higher refractive index than the optical waveguide (<NUM>) and the primary coating (<NUM>),
wherein in the coating-removed section (I0), the coating (<NUM>) and the secondary coating (<NUM>) are removed from the whole circumference of the optical waveguide (<NUM>),
wherein the coating-removed section (I0) borders on a first coated section (I1) and a second coated section (I2),
wherein the first coated section (I1) is closer to a light-entrance end than the coating-removed section (I0),
wherein the second coated section (I2) is closer to a light-exit end than the coating-removed section (I0), and
in at least part of the coating-removed section (I0), the side surface of the optical waveguide (<NUM>) being covered with a medium (<NUM>) that is lower in refractive index than the optical waveguide (<NUM>) and that is higher in refractive index than the primary coating (<NUM>).