10 of the optical input portion 2 can leak out of the center core 10 into the outer core 30 of the optical process portion 3. The portion L of the laser beam that has leaked to the outer core 30 can be introduced into the outer core 30 of the optical output portion 4. A portion of the laser beam that has propagated through the center core 10 of the optical input portion 2 leaks to the outer core 30 of the optical process portion 3 and then returns to the center core 10 or propagates through the center core 10 of the optical process portion 3 to the center core 10 of the optical output portion 4 without leakage.

In the optical output portion 4, the relative refractive index difference Δ_C2between the center core 10 and the inner ring layer 20C is greater than the relative refractive index difference Δ_CFin the optical process portion 3. Therefore, the laser beams divided to the center core 10 and the outer core 30 of the optical output portion 4 then propagate through the center core 10 and the outer core 30, respectively, without leakage. Accordingly, the quantity of the laser beam propagating through the center core 10 and the quantity of the laser beam propagating through the outer core 30 can be adjusted to a desired ratio in the optical output portion 4 by properly adjusting a radius of curvature of the optical fiber 1 in the optical process portion 3.

Now a method of manufacturing an optical fiber 1 having such a configuration will be described. When an optical fiber 1 is to be manufactured, a base optical fiber 101 having the same structure as the aforementioned optical input portion 2 or optical output portion 4 is prepared as shown in FIG. 7A. Specifically, the base optical fiber 101 has a center core 110, an inner ring layer 120 that covers the center core 110, an outer core 130 that covers the inner ring layer 120, an outer ring layer 140 that covers the outer core 130, and a resin layer 150 that covers the outer ring layer 140. For example, the center core 110 and the outer core 130 are formed of SiO₂. A dopant having a property to lower the refractive index (fluorine in this example) has been doped to each of the inner ring layer 120 and the outer ring layer 140. Therefore, the refractive index profile of the base optical fiber 101 is the same as that shown in FIG. 3 or 5. For example, the center core 110 has an outside diameter of 100 μm, the inner ring layer 120 has an outside diameter of 110 μm, the outer core 130 has an outside diameter of 200 μm, and the outer ring layer 140 has an outside diameter of 360 μm. The base optical fiber 101 may have a refractive index profile in which the relative refractive index difference Δ between the outer core 130 and the outer ring layer 140 is greater than the relative refractive index difference between the center core 110 and the inner ring layer 120.

Then a portion 150A of the resin layer 150 is removed from the base optical fiber 101 so as to expose the outer ring layer 140. As shown in FIG. 7B, the exposed portion 103 of the outer ring layer 140 is heated to, for example, 1000° C. or higher from around the base optical fiber 101, for example, with use of a burner 190 or arc discharge. For example, the burner 190 is moved in a longitudinal direction by 50 mm to heat the exposed portion 103 of the outer ring layer 140. The heating causes diffusion of fluorine doped in the inner ring layer 120 and the outer ring layer 140. As a result, the refractive indices of the inner ring layer 120 and the outer ring layer 140 increase. The thickness of the outer ring layer 140 (80 μm) is much larger than the thickness of the inner ring layer 120 (5 μm). Additionally, fluorine in the outer ring layer 140 also diffuses into air. Therefore, the refractive index of the outer ring layer 140 does not greatly increase. Thus, the refractive index of the inner ring layer 120 increases more than that of the outer ring layer 140. As a result, the refractive index profile at the heated portion 103 changes into that shown in FIG. 4. The relative refractive index difference between the center core 110 and the inner ring layer 120 of the heated portion 103 becomes smaller than that of the unheated portions 102 and 104. Thus, an optical fiber 1 as shown in FIG. 1 is completed. The heated portion 103 of the base optical fiber 101 becomes the optical process portion 3 of the optical fiber 1. The unheated portions 102 and 104 become the optical input portion 2 and the optical output portion 4 of the optical fiber 1, respectively. A diffusion distance of a dopant such as fluorine varies according to a heating temperature or a heating duration. Therefore, the degree of an increase of the refractive index of the inner ring layer 120 can be controlled by properly adjusting the heating temperature or the heating duration in the aforementioned heating process. Thus, a desired refractive index profile can be obtained in the heated portion 103.

In the aforementioned example, the base optical fiber 101 is heated at one location along a longitudinal direction thereof. However, the base optical fiber 101 may be heated at a plurality of locations along the longitudinal direction thereof so as to form a plurality of portions where the inner ring layer 120 has an increased refractive index. Furthermore, in the above example, the portion 150A of the resin layer 150 is removed from the middle of the base optical fiber 101 before heating. For example, however, a portion of the resin layer 150 may be removed from an end of the base optical fiber 101. Then the end of the base optical fiber 101 may be abutted against another optical fiber. The portion where the optical fibers are abutted may be heated. In this case, fusion splicing of the base optical fiber 101 (optical fiber 1) and the other optical fiber can be achieved simultaneously with the aforementioned increase of the refractive index of the inner ring layer 120.

Furthermore, a dopant having a property to increase the refractive index (e.g., germanium (Ge) or phosphorus (P)) may be doped into the center core 110 of the base optical fiber 101, instead of doping a dopant having a property to lower the refractive index into the inner ring layer 120 of the base optical fiber 101. In this case, heating of the base optical fiber 101 causes diffusion of the dopant doped in the center core 110 to the inner ring layer 120. As a result, the refractive index of the inner ring layer 120 increases. Thus, an optical fiber having the same structure as the aforementioned optical fiber 1 can be obtained.

In the above examples, the optical fiber 1 is manufactured with use of diffusion of a dopant through heating. However, the refractive index of the inner ring layer 120 may be increased by releasing a stress between the center core 110 and the inner ring layer 120 of the base optical fiber 101 through heating. For example, when a material having a softening temperature lower than that of the inner ring layer 120 (for example, quartz glass to which germanium has been doped) is used for the center core 110, the inner ring layer 120 bears a spinning tension upon drawing (spinning) of the optical fiber. Therefore, a compressive stress remains in the center core 110 so that the refractive index of the center core 110 increases. When such a base optical fiber 101 is heated, the residual stress is released, so that the relative refractive index difference between the center core 110 and the inner ring layer 120 decreases. Accordingly, an optical fiber having the same structure as the aforementioned optical fiber 1 can be obtained.

Furthermore, there has been known that, when an ultraviolet ray having a wavelength around 250 nm is applied to quartz glass to which germanium has been doped, the refractive index of the quartz glass increases. Therefore, glass to which germanium has been doped may be used for the inner ring layer 120 of the base optical fiber 101, and an ultraviolet ray may be applied to the base optical fiber 101 to increase the refractive index of the inner ring layer 120. In this case, when germanium is doped into quartz, the doped quartz has an increased refractive index. Therefore, in order to lower the refractive index, fluorine or boron needs to be doped to the inner ring layer 120 in addition to germanium.

FIG. 8 is a cross-sectional view schematically showing an optical fiber 201 according to one or more embodiments of the present invention, and FIG. 9 is a cross-sectional view taken along line D-D of FIG. 8. As shown in FIG. 8, the optical fiber 201 includes an optical input portion 202 connected to a laser source such as a fiber laser, an optical process portion 203 having a structure capable of distributing light inputted from the optical input portion 202 into a desired one of a plurality of optical waveguide layers, and an optical output portion 204 that allows light distributed by the optical process portion 203 to propagate therethrough, as with the optical fiber 1 of the above-described embodiments. As shown in FIGS. 8 and 9, the optical fiber 201 of one or more embodiments has a center core 210, an inner ring layer 220 that covers the center core 210, a first outer core 230 that covers the inner ring layer 20, a first outer ring layer 240 that covers the first outer core 230, a second outer core 250 that covers the first outer ring layer 240, a second outer ring layer 260 that covers the second outer core 250, and a resin layer 270 that covers the second outer ring layer 260.

For example, the center core 210, the first outer core 230, and the second outer core 250 are formed of SiO₂. A dopant having a property to lower the refractive index (e.g., fluorine (F) or boron (B)) has been doped to each of the inner ring layer 220, the first outer ring layer 240, and the second outer ring layer 260. Thus, as shown in FIG. 10, the refractive indices of the inner ring layer 220, the first outer ring layer 240, and the second outer ring layer 260 are lower than the refractive indices of the center core 210, the first outer core 230, and the second outer core 250. In this manner, the refractive index of the inner ring layer 220, which is located adjacently outside of the center core 210 in a radial direction, is lower than the refractive index of the center core 210. Accordingly, the center core 210 serves as an optical waveguide layer that confines light therein and allows the light to propagate therethrough. Furthermore, the refractive index of the inner ring layer 220, which is located adjacently inside of the first outer core 230 in the radial direction, and the refractive index of the first outer ring layer 240, which is located adjacently outside of the first outer core 230 in the radial direction, are lower than the refractive index of the first outer core 230. Accordingly, the first outer core 230 serves as an optical waveguide layer that confines light therein and allows the light to propagate therethrough. Moreover, the refractive index of the first outer ring layer 240, which is located adjacently inside of the second outer core 250 in the radial direction, and the refractive index of the second outer ring layer 260, which is located adjacently outside of the second outer core 250 in the radial direction, are lower than the refractive index of the second outer core 250. Accordingly, the second outer core 250 serves as an optical waveguide layer that confines light therein and allows the light to propagate therethrough. In the example illustrated in FIG. 10, the relative refractive index difference Δ_C1between the center core 210 and the inner ring layer 220 is the same as the relative refractive index difference Δ_P11between the first outer core 230 and the first outer ring layer 240 and the relative refractive index difference Δ_P12between the second outer core 250 and the second outer ring layer 260.

As shown in FIG. 8, the resin layer 270 has been removed in the optical process portion 203 so that the second outer ring layer 260 is exposed to an outside. FIG. 11 is a graph showing a radial refractive index distribution of the optical process portion 203 on a cross-section taken along line E-E of FIG. 8. As shown in FIG. 11, the refractive index of the inner ring layer 220 is lower than the refractive index of the center core 210 in the optical process portion 203. However, the relative refractive index difference Δ_CFbetween the center core 210 and the inner ring layer 220 in the optical process portion 203 is smaller than the relative refractive index difference Δ_C1in the optical input portion 202. Furthermore, the refractive index of the first outer ring layer 240 is lower than the refractive index of the first outer core 230. However, the relative refractive index difference Δ_PF1between the first outer core 230 and the first outer ring layer 240 in the optical process portion 203 is smaller than the relative refractive index difference Δ_P11in the optical input portion 202. Moreover, the refractive index of the second outer ring layer 260 is lower than the refractive index of the second outer core 250. However, the relative refractive index difference Δ_PF2between the second outer core 250 and the second outer ring layer 260 in the optical process portion 203 is equal to or smaller than the relative refractive index difference Δ_P11in the optical input portion 202. The relative refractive index difference Δ_PF1is greater than the relative refractive index difference Δ_CF, and the relative refractive index difference Δ_PF2is greater than the relative refractive index difference Δ_PF1. Thus, the relative refractive index difference between the core and the ring layer gradually increases along a radially outward direction.

FIG. 12 is a graph showing a radial refractive index distribution of the optical output portion 204 on a cross-section taken along line F-F of FIG. 8. As shown in FIG. 12, the refractive index distribution in the optical output portion 204 is the same as the refractive index distribution in the optical input portion 202. In the example illustrated in FIG. 12, the relative refractive index difference Δ_C2between the center core 210 and the inner ring layer 220 is the same as the relative refractive index difference Δ_P21between the first outer core 230 and the first outer ring layer 240 and the relative refractive index difference Δ_P22between the second outer core 250 and the second outer ring layer 260.

In one or more embodiments, the relative refractive index difference between the center core 210 and the inner ring layer 220 varies along a longitudinal direction, and the relative refractive index difference between the first outer core 230 and the first outer ring layer 240 varies along the longitudinal direction. Specifically, the relative refractive index difference Δ_CFbetween the center core 210 and the inner ring layer 220A in the optical process portion 203 is smaller than the relative refractive index difference Δ_C1between the center core 210 and the inner ring layer 220B in the optical input portion 202 and the relative refractive index difference Δ_C2between the center core 210 and the inner ring layer 220C in the optical output portion 204. The relative refractive index difference Δ_PF1between the first outer core 230 and the first outer ring layer 240A in the optical process portion 203 is smaller than the relative refractive index difference Δ_P11between the first outer core 230 and the first outer ring layer 240B in the optical input portion 202 and the relative refractive index difference Δ_P21between the first outer core 230 and the first outer ring layer 240C in the optical output portion 204.

In this optical fiber 201, since the relative refractive index difference Δ_CFbetween the center core 210 and the inner ring layer 220A in the optical process portion 203 is smaller than the relative refractive index difference Δ_C1in the optical input portion 202, the optical confinement effect of the center core 210 lessens in the optical process portion 203 as compared to that in the optical input portion 202. Furthermore, since the relative refractive index difference Δ_PF1between the first outer core 230 and the first outer ring layer 240A in the optical process portion 203 is smaller than the relative refractive index difference Δ_P11in the optical input portion 202, the optical confinement effect of the first outer core 230 lessens in the optical process portion 203 as compared to that in the optical input portion 202.

Therefore, when the optical fiber 201 thus configured is bent at the optical process portion 203, a portion of the laser beam that has propagated through the center core 10 of the optical input portion 202 can leak out of the center core 210 into the first outer core 230 of the optical process portion 203. The portion of the laser beam that has leaked to the first outer core 230 can further leak into the second outer core 250. Thus, the quantity of the laser beam propagating through the center core 210, the quantity of the laser beam propagating through the first outer core 230, and the quantity of the laser beam propagating through the second outer core 250 can be adjusted to a desired ratio in the optical output portion 204 by properly adjusting a radius of curvature of the optical fiber 201 in the optical process portion 203.

With the optical process portion 203 of one or more embodiments, the relative refractive index difference Δ_PF1is greater than the relative refractive index difference Δ_CF. Therefore, when the radius of curvature of the optical fiber 201 in the optical process portion 203 is large, the laser beam only leaks to the first outer core 230. If the radius of curvature of the optical fiber 201 in the optical process portion 203 is made smaller, the laser beam can leak out of the first outer core 230 to the second outer core 250. Since the relative refractive index difference between the core and the ring layer gradually increases along a radially outward direction in this manner, cores to which light is to be introduced can be controlled from the center core to any of outer cores by properly adjusting the radius of curvature of the optical fiber 201 in the optical process portion 203.

The optical fiber 201 thus configured can be manufactured in the same manner as the optical fiber 1 of the above-described embodiments. Specifically, a portion of a resin layer 270 of a base optical fiber having a structure of the aforementioned optical input portion 202 or optical output portion 204 is removed so as to expose the second outer ring layer 260. The exposed portion of the second outer ring layer 260 is heated so as to diffuse fluorine doped in the inner ring layer 220, the first outer ring layer 240, and the second outer ring layer 260. The optical fiber 201 is thus manufactured. In order to ensure that the relative refractive index difference Δ_CFbetween the center core 210 and the inner ring layer 220 is smaller than the relative refractive index difference Δ_PF1between the first outer core 230 and the first outer ring layer 240 in the optical process portion 203, in one or more embodiments, the thickness of the inner ring layer 220 of the base optical fiber is adjusted to be smaller than the thickness of the first outer ring layer 240. Furthermore, in order to ensure that the relative refractive index difference Δ_PF1between the first outer core 230 and the first outer ring layer 240 is smaller than the relative refractive index difference Δ_PF2between the second outer core 250 and the second outer ring layer 260 in the optical process portion 203, in one or more embodiments, the thickness of the first outer ring layer 240 of the base optical fiber is adjusted to be smaller than the thickness of the second outer ring layer 260.

The optical fiber 201 of one or more embodiments includes two outer cores 230 and 250 and two outer ring layers 240 and 260. As a matter of course, however, one or more embodiments of the present invention are applicable to any optical fiber including three or more outer cores and three or more outer ring layers.

An optical fiber according to one or more embodiments of the present invention can be applied to a laser processing apparatus in the following manner. When an optical fiber according to one or more embodiments of the present invention is used for a laser processing apparatus, optical properties of a laser beam to be emitted to a workpiece can be changed readily and safely depending on a material of the workpiece.

FIG. 13 is a schematic view showing an arrangement of a laser processing apparatus 501 using an optical fiber according to one or more embodiments of the present invention. The laser processing apparatus 501 shown in FIG. 13 has a stage 510 configured to hold a workpiece W, a laser generator 520, a laser emission portion 530 (i.e., laser emitter) configured to direct a laser beam outputted from the laser generator 520 to the workpiece W on the stage 510, a process monitor 540 operable to detect a process condition of the workpiece W, and a controller 550 operable to control the laser generator 520 in response to a detection signal from the process monitor 540.

The laser generator 520 includes a plurality of laser sources 521, an optical combiner 522 operable to combine laser beams from those laser sources 521, a delivery fiber 523 extending from the optical combiner 522, an optical fiber 524 connected to the delivery fiber 523, and a fiber bend unit 525 configured to bend the optical fiber 524 with a predetermined radius of curvature. Each of the laser sources 521 generates an output laser beam having a predetermined wavelength (e.g., 1100 nm). For example, each of the laser sources 521 may be formed by a fiber laser including a master oscillator power amplifier (MOPA) or a fiber laser including an optical cavity.

Each of the laser sources 521 and the optical combiner 522 are connected to each other by an optical fiber 526. The delivery fiber 523 is connected to a downstream side of the optical combiner 522. The delivery fiber 523 and the optical fiber 524 are connected to each other at a fusion splice portion 527 by fusion splicing. The aforementioned optical fiber 1 or 201 can be used as the optical fiber 524. An example where the aforementioned optical fiber 1 is used as the optical fiber 524 will be described below.

The fiber bend unit 525 receives therein a portion of the optical fiber 524 (optical fiber 1) in a state in which it is bent. This fiber bend unit 525 is configured to adjust a radius of curvature of the optical fiber 1. For example, the fiber bend unit 525 may have a stationary portion 601 and a movable portion 603 that is movable on a rail 602. An optical process portion 3 of the optical fiber 1 being bent may be interposed between the stationary portion 601 and the movable portion 603. A radius of curvature of the optical process portion 3 can be adjusted by moving the movable portion 603 relative to the stationary portion 601 so as to change a distance between the stationary portion 601 and the movable portion 603. The mechanism for adjusting a radius of curvature of the optical fiber 1 is not limited to the illustrated example.

The controller 550 controls the laser sources 521 and the fiber bend unit 525 depending on the process condition of the workpiece W in response to the detection signal from the process monitor 540. For example, the controller 550 may control an electric current supplied to the laser sources 521 depending on the process condition of the workpiece W to adjust outputs of the laser beams emitted from the laser sources 521 or may move the movable portion 603 of the fiber bend unit 525 to adjust the optical properties of the laser beam outputted from the laser emission portion 530. The controller 550 may control the laser sources 521 and the fiber bend unit 525 in accordance with a preset program.

For example, in a case where a portion of a workpiece W to be processed is thin and is to be cut at a high speed, the controller 550 moves the movable portion 603 of the fiber bend unit 525 so as to increase a ratio of a laser beam outputted from a center core 10 of the optical fiber 1. In a case where a portion of a workpiece W to be processed is thick and is to be cut, the controller 550 moves the movable portion 603 of the fiber bend unit 525 in another direction so as to increase a ratio of a laser beam outputted from an outer core 30 of the optical fiber 1.

In the aforementioned example, the optical fiber 1 is bent at one location. Nevertheless, the optical fiber 1 may be bent at a plurality of locations. In a case where a high-power laser beam propagates through the center core 10, if the laser beam propagating through the center core 10 leaks to the outer core 30 at one location, then an optical loss may be caused by excessive leakage of the laser beam to the outer core 30. Such an optical loss can be reduced when the laser beam propagating through the center core 10 gradually leaks to the outer core 30 at a plurality of locations.

Although the above embodiments of the present invention have been described, the present invention is not limited to the aforementioned embodiments. It should be understood that various different forms may be applied to the present invention within the technical idea thereof.

As described above, according to one or more embodiments of the present invention, there is provided an optical fiber capable of readily outputting light having a desired optical property with a low loss. This optical fiber has a center core, an inner ring layer located outside of the center core in a radial direction, at least one outer core located outside of the inner ring layer in the radial direction, and at least one outer ring layer located outside of the at least one outer core in the radial direction. The inner ring layer has a refractive index lower than a refractive index of the center core. The at least one outer core has a refractive index higher than the refractive index of the inner ring layer. The at least one outer ring layer having a refractive index lower than the refractive index of the at least one outer core. A relative refractive index difference Δ_CFbetween the center core and the inner ring layer varies along a longitudinal direction so that the relative refractive index difference Δ_CFin at least one location along the longitudinal direction is smaller than a relative refractive index difference Δ_PFbetween the at least one outer core and the outer ring layer located adjacently outside of the at least one outer core in the radial direction.

According to this configuration, the optical confinement effect of the center core lessens in the at least one location along the longitudinal direction. When the optical fiber is bent at that location, a portion of light that has propagated through the center core can leak out of the center core into the outer core. Therefore, the quantity of light propagating through the center core and the quantity of light propagating through the outer core can be adjusted to a desired ratio by properly adjusting a radius of curvature of the optical fiber. Thus, light having a desired optical property can readily be outputted with a low loss.

In one or more embodiments, for more effectively reducing the relative refractive index difference Δ_CFbelow the relative refractive index difference Δ_PF, the thickness of the inner ring layer may be smaller than the thickness of the at least one outer ring layer.

The at least one outer core may include a plurality of outer cores, and the at least one outer ring layer may include the same number of outer ring layers as the at least one outer core. In this case, light propagating through the center core can be divided into the center core and the plurality of outer cores. Therefore, the optical property of the outputted light can be controlled more finely.