JPWO2018003574A1 - Amplifying optical fiber and laser device - Google Patents

Abstract

An inner cladding 12 including an active element-added core 11, an inner cladding 12 surrounding the core 11 and having a lower refractive index than the core 11, and an outer cladding 13 surrounding the inner cladding 12 and having a lower refractive index than the inner cladding 12. In the cross section perpendicular to the longitudinal direction to which the torsion about the central axis of the core 11 is added, the outer periphery of the inner cladding 12 has a rounded polygon corner, and the number of vertices of the polygon Where n is the diameter of the circumscribed circle C2 on the outer periphery of the inner cladding 12 and d2 is the diameter of the outer circumscribed circle C1 on the outer periphery of the inner cladding 12, and are defined by the following equations (1) and (2). The angularity c is 0.15 or more and 0.8 or less.
A = cos (π / n) (1)
c = {1- (d1 / d2)} / (1-A) (2)

Description

  The present invention relates to an amplification optical fiber and a laser device that can suppress the occurrence of a skew mode.

  The fiber laser device is used in various fields such as a laser processing field and a medical field because it has excellent light condensing performance, has a high power density, and can obtain light as a small beam spot. In such a fiber laser device, a rare earth doped fiber having a core doped with a rare earth element is used. Further, in rare earth doped fibers used in fiber laser devices, a double clad structure is generally applied in order to allow more excitation light to enter the core. The rare earth-doped fiber having a double clad structure has a core doped with a rare earth element, an inner clad surrounding the core, and an outer clad surrounding the inner clad, and the outer clad has a lower refractive index than the inner clad. At least a part of the excitation light incident on the inner cladding is reflected to the core side at the interface between the inner cladding and the outer cladding and is incident on the core to excite the rare earth element added to the core.

  However, in the rare earth doped fiber having the double clad structure as described above, when the cross-sectional shape of the inner cladding is circular, the excitation light continues to be reflected at a certain angle at the interface between the inner cladding and the outer cladding, and the excitation light is reflected on the core. It may propagate through the inner cladding without being incident. Light that propagates through the cladding without passing through the core is called skew light. When the skew light is generated, the excitation light incident on the core is reduced, so that the rare earth element added to the core is difficult to be excited.

  As a technique for suppressing the occurrence of a skew mode, for example, Patent Document 1 below discloses a technique in which an optical fiber having a clad having a polygonal cross section is twisted and fixed around a central axis. When the polygonal clad is fixed by being twisted in this way, it is considered that the excitation light propagating through the clad is repeatedly reflected on the outer peripheral surface of the clad while changing the reflection angle, and easily enters the core.

JP 2001-13346 A

  In general, an optical fiber having a clad having a polygonal cross-section as described above is manufactured by drawing an optical fiber preform having a polygonal cross-sectional shape at a portion to be a clad. However, there are cases where the corners are rounded due to heat during drawing and the cross-sectional shape of the cladding is close to a circle. When the cross-sectional shape of the clad becomes close to a circle in this manner, even if the clad is twisted as in the optical fiber described in Patent Document 1, the skew mode may not be suppressed as intended.

  Therefore, an object of the present invention is to provide an amplification optical fiber and a laser device that can further suppress the occurrence of a skew mode.

In order to solve the above problems, an optical fiber for amplification according to the present invention includes a core to which an active element is added, an inner clad surrounding the core and having a lower refractive index than the core, and surrounding the inner clad and being refracted from the inner clad. An outer cladding having a low rate, wherein the inner cladding is twisted about the central axis of the core, and the outer periphery of the inner cladding is rounded at polygonal corners in a cross section perpendicular to the longitudinal direction. When the number of vertexes of the polygon is n, the diameter of the circumscribed circle on the outer periphery of the inner cladding is d2, and the diameter of the inscribed circle on the outer periphery of the inner cladding is d1, the following formula (1 ) And the angularity c defined by the following formula (2) is 0.15 or more and 0.8 or less.
A = cos (π / n) (1)
c = {1- (d1 / d2)} / (1-A) (2)

  A laser apparatus according to the present invention includes the amplification optical fiber and at least one light source that emits light propagating through the amplification optical fiber.

  The circumscribed circle on the outer periphery of the inner cladding is a circle having the smallest area among the circles that can include the inner cladding in the cross section perpendicular to the longitudinal direction of the optical fiber, and this circumscribed circle is the outer periphery of the inner cladding as described above. Since the shape is rounded at the corners of the polygon, it touches each chamfered apex of the outer periphery of the inner cladding. The inscribed circle on the outer periphery of the inner cladding is a circle having the largest area among the circles formed inside the outer periphery of the inner cladding in a cross section perpendicular to the longitudinal direction of the optical fiber. It touches each side of the outer periphery of the cladding. Note that the diameter d1 of the inscribed circle and the diameter d2 of the circumscribed circle are obtained in an arbitrary cross section perpendicular to the longitudinal direction of the optical fiber.

  In the above optical fiber for amplification, the inner cladding is sandwiched between a core having a higher refractive index than the inner cladding and an outer cladding having a lower refractive index than the inner cladding, so that the excitation light incident on the inner cladding can enter the core. it can. In addition, the present inventors have found that the angularity c is 0.15 or more even when the outer periphery of the inner cladding in a cross section perpendicular to the longitudinal direction has a rounded polygonal corner. For example, it has been found that the occurrence of a skew mode can be suppressed by twisting the inner cladding as described above. When the occurrence of the skew mode is suppressed, the excitation light incident on the inner cladding is easily incident on the core to which the active element is added, and the light propagating through the core is easily amplified.

  A is equal to the ratio of the diameter of the inscribed circle to the diameter of the circumscribed circle (diameter of the inscribed circle / diameter of the circumscribed circle). Therefore, if the outer periphery of the inner cladding is an accurate regular polygon, d1 / d2 = A, and the angularity c is 1. Further, if the outer circumference of the inner cladding is circular, d1 / d2 = 1, and the angularity c is zero. Thus, the angularity c is a numerical value of 0 or more and 1 or less. When the angularity c is close to 1, the outer periphery of the inner cladding is close to an accurate regular polygon, and when the angularity c is close to 0. It means that the outer periphery of the inner cladding is nearly circular. Therefore, from the viewpoint of suppressing the occurrence of the skew mode, it is preferable that the angularity c is as close to 1 as possible. However, the present inventors have found that even when the angularity c is 0.8 or less, the occurrence of the skew mode can be suppressed by twisting the inner cladding as described above. In addition, when the squareness c is 0.8 or less, the heating temperature when drawing the optical fiber preform can be increased to some extent, so that the active element added to the core is prevented from crystallizing. Can be done. By suppressing the crystallization of the active element in this manner, it is possible to suppress an increase in transmission loss of the amplification optical fiber. In particular, in an optical fiber for amplification in which an active element is added to the core at a high concentration for high output, it is effective to suppress crystallization of the active element.

  The angularity c is more preferably 0.25 or more.

  When the angular degree c is 0.25 or more, the occurrence of the skew mode can be further suppressed by twisting the inner cladding as described above.

Further, when the number of rotations of the twist per 1 m in the direction parallel to the longitudinal direction is N, the following formula (3) is preferably satisfied, and the following formula (4) is more preferable.
c × N ≧ 0.75 (3)
c × N ≧ 2.5 (4)

  If the angularity c is 0.15 or more as described above, even if the angularity c is small to some extent, the condition of the above formula (3) is satisfied by increasing the rotational speed N of the torsion. As a result, the occurrence of the skew mode can be suppressed. Further, when the condition of the above formula (4) is satisfied, the occurrence of the skew mode can be further suppressed. In addition, when the degree of angularity c is large to some extent, even if the number of rotations N of the twist is reduced, the occurrence of the skew mode can be suppressed by satisfying the condition of the above expression (3). By reducing the number of rotations N of the twist, when the twist as described above is applied to the inner cladding during the manufacture of the amplification optical fiber, the manufacturing error of the amplification optical fiber can be suppressed. Hereinafter, the number of rotations of twist added to the inner cladding per 1 m length in the direction parallel to the longitudinal direction of the amplification optical fiber may be simply referred to as “twist amount N”.

  As described above, according to the present invention, an optical fiber for amplification and a laser device that can suppress the occurrence of a skew mode are provided.

It is the schematic which shows the laser apparatus concerning 1st Embodiment of this invention. It is a figure which shows the mode of a cross section perpendicular | vertical to the longitudinal direction of the optical fiber for amplification shown in FIG. It is the schematic which shows the laser apparatus concerning 2nd Embodiment of this invention. It is a graph which shows the relationship between the length of the optical fiber for amplification, and the quantity of the light absorbed with the optical fiber for amplification. It is a graph which shows the relationship of the skew suppression index | exponent with respect to the product of squareness degree c and the amount of twist N.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of an amplification optical fiber and a laser apparatus according to the present invention will be described in detail with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention, and are not intended to limit the present invention. The present invention can be changed and improved without departing from the spirit of the present invention. For ease of understanding, the scale of each figure may be different from the scale described in the following description.

(First embodiment)
FIG. 1 is a diagram showing a laser apparatus according to the first embodiment of the present invention. As shown in FIG. 1, the laser device 1 of this embodiment includes an amplification optical fiber 10, a pumping light source 20, an optical combiner 40, an optical fiber 35 connected to one side of the amplification optical fiber 10, and an optical fiber 35. A primary FBG (Fiber Bragg Grating) 31 provided, an optical fiber 36 connected to the other side of the amplification optical fiber 10, and a second FBG 32 provided on the optical fiber 36 are mainly provided. In the laser apparatus 1, a resonator is configured by the amplification optical fiber 10, the first FBG 31, and the second FBG 32.

  The excitation light source 20 is composed of a plurality of laser diodes 21, and in the present embodiment, the laser diode 21 is a Fabry-Perot type semiconductor laser made of, for example, a GaAs semiconductor and emits excitation light having a center wavelength of 915 nm. To do. Each laser diode 21 of the pumping light source 20 is connected to an optical fiber 25, and pumping light emitted from the laser diode 21 propagates through the optical fiber 25 as, for example, multimode light.

  FIG. 2 is a diagram showing a state of a cross section perpendicular to the longitudinal direction of the amplification optical fiber 10 shown in FIG. As shown in FIG. 2, the amplification optical fiber 10 covers a core 11, an inner cladding 12 that surrounds the outer peripheral surface of the core 11 without a gap, an outer cladding 13 that covers the outer peripheral surface of the inner cladding 12, and an outer cladding 13. And a covering layer 14 to be formed as a main structure, and a so-called double clad structure is provided. The refractive index of the inner cladding 12 is lower than the refractive index of the core 11, and the refractive index of the outer cladding 13 is lower than the refractive index of the inner cladding 12.

  As the material constituting the core 11, for example, an element such as germanium (Ge) for increasing the refractive index and an active element such as ytterbium (Yb) excited by excitation light emitted from the excitation light source 20 are added. Quartz. Examples of such active elements include rare earth elements, and examples of rare earth elements include thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), and erbium (Er) in addition to Yb. Can be mentioned. Furthermore, bismuth (Bi) etc. can be mentioned as an active element other than a rare earth element.

The inner cladding 12 is provided with a twist about the central axis of the core 11. Further, in the cross section perpendicular to the longitudinal direction, the outer periphery of the inner cladding 12 has a rounded polygonal corner. The outer periphery of the inner cladding 12 of the present embodiment has a regular heptagonal corner with a rounded shape. Specifically, the outer periphery of the inner cladding 12 in a cross section perpendicular to the longitudinal direction has an angularity c defined by the following formula (1) and the following formula (2) of 0.15 or more and 0.8 or less. . Here, n is the number of vertices of the polygon, and in this embodiment, n = 7 as described above. D2 is the diameter of the circumscribed circle C2 on the outer periphery of the inner cladding 12, and d1 is the diameter of the inscribed circle C1 on the outer periphery of the inner cladding 12.
A = cos (π / n) (1)
c = {1- (d1 / d2)} / (1-A) (2)

  Examples of the material constituting the inner cladding 12 include pure quartz to which no dopant is added. Note that an element such as fluorine (F) that lowers the refractive index may be added to the material of the inner cladding 12.

  The outer cladding 13 is made of resin or quartz. Examples of such a resin include an ultraviolet curable resin, and examples of quartz include quartz added with a dopant such as fluorine (F) that lowers the refractive index so that the refractive index is lower than that of the inner cladding 12. It is done.

  Examples of the material constituting the coating layer 14 include an ultraviolet curable resin. When the outer clad 13 is a resin, the material is an ultraviolet curable resin different from the resin constituting the outer clad.

  The optical fiber 35 connected to one side of the amplification optical fiber 10 includes a core to which no active element is added, an inner cladding that surrounds the outer peripheral surface of the core without a gap, and an outer surface that covers the outer peripheral surface of the inner cladding. A clad and a coating layer covering the outer clad are provided as main components. The core of the optical fiber 35 is configured substantially the same as the core 11 of the amplification optical fiber 10 except that no active element is added. The core of the optical fiber 35 is connected to the core 11 of the amplification optical fiber 10, and the inner cladding of the optical fiber 35 is connected to the inner cladding 12 of the amplification optical fiber 10. Further, a first FBG 31 as a first mirror is provided in the core of the optical fiber 35. Thus, the first FBG 31 is provided on one side of the amplification optical fiber 10. The first FBG 31 has a portion where the refractive index periodically increases along the longitudinal direction of the optical fiber 35, and the active element of the amplification optical fiber 10 in the excited state is adjusted by adjusting the cycle. Is configured to reflect light of at least a part of the light emitted. The reflectivity of the first FBG 31 is higher than the reflectivity of the second FBG 32, which will be described later, and it is preferable to reflect light having a desired wavelength of 90% or more among the light emitted by the active element, more preferably 99% or more. preferable. The wavelength of the light reflected by the first FBG 31 is, for example, 1090 nm when the active element is ytterbium as described above.

  An optical fiber 36 connected to the other side of the amplification optical fiber 10 includes a core to which no active element is added, a clad that surrounds the outer peripheral surface of the core without a gap, and a coating layer that covers the outer peripheral surface of the clad. Is provided as a main configuration. The core of the optical fiber 36 is connected to the core 11 of the amplification optical fiber 10, and the cladding of the optical fiber 36 is connected to the inner cladding 12 of the amplification optical fiber 10. The core of the optical fiber 36 is provided with a second FBG 32 as a second mirror. Thus, the second FBG 32 is provided on the other side of the amplification optical fiber 10. The second FBG 32 has a portion in which the refractive index is increased at a constant period along the longitudinal direction of the optical fiber 36, and at least some of the light reflected by the first FBG 31 is lower than the first FBG 31. It is configured to reflect with reflectivity. The second FBG 32 reflects light of at least a part of the light reflected by the first FBG 31 with a reflectance of 5% to 50%, more preferably with a reflectance of 5% to 10%. . In the present embodiment, nothing is connected to the other end of the optical fiber 36 opposite to the amplification optical fiber 10 side, but a glass rod or the like may be connected.

  In the optical combiner 40, the core of each optical fiber 25 and the inner cladding of the optical fiber 35 are connected. Therefore, the optical fiber 25 through which the pumping light emitted from each laser diode 21 propagates and the inner cladding 12 of the amplification optical fiber 10 are optically coupled via the inner cladding of the optical fiber 35.

  Next, the operation and action of the laser device 1 in this embodiment will be described.

  First, when pumping light is emitted from each laser diode 21 of the pumping light source 20, the pumping light enters the inner cladding 12 of the amplification optical fiber 10 through the inner cladding of the optical fiber 35. The inner cladding 12 is sandwiched between a core 11 having a refractive index higher than that of the inner cladding 12 and an outer cladding 13 having a refractive index lower than that of the inner cladding 12. The excitation light incident on the inner cladding 12 mainly propagates through the inner cladding 12. Is incident on the core 11. Thus, the excitation light incident on the core 11 excites the active element added to the core 11. The active element brought into an excited state emits spontaneous emission light having a specific wavelength. For example, when the active element is ytterbium, the spontaneous emission light at this time is light having a certain wavelength band including a wavelength of 1090 nm. The spontaneous emission light propagates through the core 11 of the amplification optical fiber 10, and a part of the wavelength light is reflected by the first FBG 31, and the light having the wavelength reflected by the second FBG 32 among the reflected light is reflected. It is reflected by the second FBG 32 and reciprocates in the resonator. Then, when the light reflected by the first FBG 31 and the second FBG 32 propagates through the core 11 of the optical fiber for amplification 10, stimulated emission occurs and this light is amplified, and the gain and loss in the resonator are equal. The laser oscillation state is entered. A part of the light resonating between the first FBG 31 and the second FBG 32 passes through the second FBG 32 and is emitted from the end of the optical fiber 36.

  By the way, in the amplification optical fiber 10, as described above, the outer periphery of the inner cladding 12 has a rounded polygonal corner in a cross section perpendicular to the longitudinal direction. That is, the outer peripheral surface of the inner cladding 12 has a plurality of surfaces with different angles. Further, the inner cladding 12 is provided with a twist about the central axis of the core 11. Such excitation light propagating through the inner cladding 12 can be easily reflected while changing the reflection angle at the interface between the inner cladding 12 and the outer cladding 13, and the occurrence of the skew mode can be suppressed. Therefore, in the amplification optical fiber 10, the excitation light is easily incident on the core 11, and the active element added to the core 11 is easily excited, so that the light propagating through the core 11 is easily amplified.

  Further, as described above, the inner cladding 12 has an angularity c of 0.15 or more and 0.8 or less. Even if the outer periphery of the inner cladding 12 in a cross section perpendicular to the longitudinal direction has a shape with rounded polygonal corners, the present inventors can provide the angularity c of 0.15 or more. It has been found that the occurrence of skew mode can be suppressed by adding the above twist to the inner cladding 12. From the viewpoint of making it easier to suppress the occurrence of the skew mode, it is preferable that the angularity c is 0.25 or more.

  The angularity c is 0 in the case of a circle and 1 in the case of an accurate regular polygon. Therefore, from the viewpoint of suppressing the occurrence of the skew mode, it is preferable that the angularity c is as close to 1 as possible. However, the present inventors have found that even when the angularity c is 0.8 or less, the occurrence of the skew mode can be suppressed by adding the above-described twist to the inner cladding 12. Further, when the degree of squareness c is 0.8 or less, the heating temperature at the time of drawing the optical fiber preform can be increased to some extent, so that the active element added to the core 11 may be crystallized. Can be suppressed. By suppressing the crystallization of the active element in this manner, an increase in transmission loss of the amplification optical fiber 10 can be suppressed. In particular, in the amplification optical fiber 10 in which the active element is added to the core at a high concentration for high output, it is effective to suppress the crystallization of the active element.

Further, it is preferable that the following formula (3) holds when the number of rotations of the twist added to the inner cladding 12 per 1 m in the direction parallel to the longitudinal direction is N, and the following formula (4) holds. It is more preferable.
c × N ≧ 0.75 (3)
c × N ≧ 2.5 (4)

  If the angularity c is 0.15 or more as described above, even if the angularity c is small to some extent, the condition of the above formula (3) is satisfied by increasing the twist amount N. The occurrence of the skew mode can be suppressed. Further, when the condition of the above formula (4) is satisfied, the occurrence of the skew mode can be further suppressed. Further, when the degree of angular tension c is large to some extent, even if the twist amount N is reduced, the occurrence of the skew mode can be suppressed by satisfying the condition of the above expression (3). When the twist amount N is reduced, when the optical fiber preform is rotated during the manufacture of the amplification optical fiber 10 and the above-described twist is added to the inner cladding 12, the manufacturing error of the amplification optical fiber 10 is increased. Etc. can be suppressed. Further, since the amount of twist N is reduced, it is possible to suppress a decrease in the spinning speed when the amplification optical fiber 10 is manufactured as described above. Therefore, the amplification optical fiber 10 in which the inner cladding 12 has a twist is provided. Easy to manufacture. From such a viewpoint, the upper limit of the twist amount N is preferably 30 or less, and more preferably 15 or less. Therefore, the upper limit of c × N is preferably 24 or less, and more preferably 12 or less, because the upper limit of the angularity c is 0.8 as described above.

  Further, the twist added to the inner cladding 12 is preferably a permanent twist. Here, the permanent twist means not a twist applied after manufacturing an untwisted optical fiber but a twist applied during the manufacture of the optical fiber. Since the twist added to the inner cladding 12 is a permanent twist, the refractive index of the core 11 is suppressed from fluctuating unevenly due to the elastic stress caused by the twist. For this reason, when light propagating through the core 11 propagates in multimode, mode coupling can be suppressed.

  When the amplification optical fiber 10 is manufactured by rotating the optical fiber preform as described above, the core 11 may be slightly spiraled and eccentric. In this case, the eccentric amount of the core 11 with respect to the center of the inscribed circle C1 is preferably 5 μm or less. In this way, the amount of eccentricity of the core 11 is suppressed, so that the amplification optical fiber 10 can be easily connected to another optical fiber.

(Second Embodiment)
Next, a second embodiment of the present invention will be described in detail with reference to FIG. In addition, about the component which is the same as that of 1st Embodiment, or equivalent, unless otherwise demonstrated, the same referential mark may be attached | subjected and the overlapping description may be abbreviate | omitted.

  FIG. 3 is a diagram showing a laser device according to the present embodiment. As shown in FIG. 3, the laser device 2 of the present embodiment is different from the laser device 1 of the first embodiment in that it is an MO-PA (Master Oscillator Power Amplifier) type fiber laser device. Therefore, the laser apparatus 2 of the present embodiment includes the seed light source 70 and the optical fiber 30 connected to the seed light source 70.

  The seed light source 70 is composed of, for example, a laser diode or a fiber laser. The optical fiber 30 mainly includes a core to which no active element is added, a clad that surrounds the outer peripheral surface of the core without a gap, and a coating layer that covers the outer peripheral surface of the clad. The seed light emitted from the seed light source 70 propagates through the core of the optical fiber 30.

  In the present embodiment, in the optical combiner 50, each optical fiber 25 is connected to one end of the amplification optical fiber 10 together with the optical fiber 30. Specifically, the core 11 of the amplification optical fiber 10 and the core of the optical fiber 30 are connected so that the core of the optical fiber 30 is optically coupled to the core 11 of the amplification optical fiber 10. Therefore, the seed light emitted from the seed light source 70 enters the core 11 of the amplification optical fiber 10 through the core of the optical fiber 30 and propagates through the core 11. Further, the core of each optical fiber 25 and the inner cladding 12 of the amplification optical fiber 10 are connected so that the core of each optical fiber 25 is optically coupled to the inner cladding 12 of the amplification optical fiber 10. Yes. Therefore, the pumping light emitted from each laser diode 21 of the pumping light source 20 is incident on the inner cladding 12 of the amplification optical fiber 10 via the optical fiber 25, propagates mainly through the inner cladding 12, and enters the core 11. Excites added active elements. For this reason, the seed light propagating through the core 11 is amplified by stimulated emission of the active element in an excited state, and the amplified seed light is emitted from the amplification optical fiber 10 as output light. The light emitted from the amplification optical fiber 10 is emitted through the optical fiber 36 in the same manner as in the first embodiment.

  Also in the present embodiment, the use of the amplification optical fiber 10 can suppress the occurrence of the skew mode.

  As mentioned above, although the said embodiment was demonstrated to the example about this invention, this invention is not limited to these. For example, in the above-described embodiment, an example in which the outer periphery of the inner cladding 12 has a regular heptagonal corner rounded in a cross section perpendicular to the longitudinal direction has been described. However, the outer periphery of the inner cladding 12 in the cross section perpendicular to the longitudinal direction is not particularly limited as long as it has a rounded polygonal corner. For example, the outer clad 12 has a rounded corner such as a regular hexagon or a regular octagon. There may be.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not limited to a following example.

<Example 1>
An optical fiber similar to the amplification optical fiber 10 was produced by the following method. First, an optical fiber preform composed of glass having the same refractive index distribution as the core 11 and the inner cladding 12 constituting the amplification optical fiber 10 was prepared. That is, an optical fiber preform having a regular heptagonal cross-sectional shape perpendicular to the longitudinal direction, in which the outer peripheral surface of a cylindrical material that becomes the core 11 is surrounded by the material that becomes the inner cladding 12 without gaps, was prepared. Next, this optical fiber preform was suspended so that the longitudinal direction was vertical. Then, the optical fiber preform was placed in a drawing furnace, and the lower end of the optical fiber preform was heated. Next, the glass melted from the lower end of the heated optical fiber preform was drawn out of the drawing furnace at a predetermined drawing speed and cooled. At this time, the drawing tension was adjusted so that the squareness c was 0.55. Further, the inner cladding 12 was twisted by drawing while rotating the optical fiber preform around the central axis. By changing the rotation speed of the optical fiber preform during drawing, the twist amount N was changed as shown in Table 1 below. Thereafter, the outer peripheral surface of the inner clad 12 was covered with the outer clad 13 and the coating layer 14 made of an ultraviolet curable resin or the like, so that an amplification optical fiber according to Example 1 was produced. The relative refractive index difference between the core and the inner cladding was 0.12%.

  The outer peripheral shape of the optical fiber preform (the outer peripheral shape of the inner cladding) in the cross section perpendicular to the longitudinal direction, the angularity c of the cross section perpendicular to the longitudinal direction of the inner cladding, the twist amount N of the inner cladding, and the value of c × N Are shown in Table 1 below together with other examples and comparative examples described below.

<Comparative Example 1>
An amplification optical fiber was fabricated in the same manner as in Example 1 except that the optical fiber preform was not rotated when drawing.

<Example 2>
An amplification optical fiber was fabricated in the same manner as in Example 1 except that the angular tension c was 0.25 and the twist amount N was changed as shown in Table 1 below.

<Example 3>
An amplification optical fiber was manufactured in the same manner as in Example 1 except that the angularity c was 0.15 and the twist amount N was changed as shown in Table 1 below.

<Example 4>
Example except that the outer peripheral shape of the optical fiber preform in the cross section perpendicular to the longitudinal direction is a regular hexagon, the angularity c is 0.34, and the twist amount N is changed as shown in Table 1 below. In the same manner as in Example 1, an amplification optical fiber was produced.

<Comparative example 2>
An amplification optical fiber was fabricated in the same manner as in Example 4 except that the optical fiber preform was not rotated when drawing.

<Example 5>
Example except that the outer peripheral shape of the optical fiber preform in the cross section perpendicular to the longitudinal direction is a regular octagon, the angularity c is 0.64, and the twist amount N is changed as shown in Table 1 below. In the same manner as in Example 1, an amplification optical fiber was produced.

<Comparative Example 3>
An amplification optical fiber was fabricated in the same manner as in Example 5 except that the optical fiber preform was not rotated when drawing.

<Comparative Examples 4-6>
An amplification optical fiber was manufactured in the same manner as in Example 1 except that the angularity c was 0.09 and the twist amount N was changed as shown in Table 1 below.

(Evaluation of skew mode suppression effect)
For the amplification optical fibers according to the above-described examples and comparative examples, the effect of suppressing the skew mode was evaluated by the method described below.

The skew mode suppression effect was performed by defining the skew suppression index γ as follows. The skew suppression index γ was defined by the following formula (5).
γ = α _L / α _S (5)

Here, α _L and α _S were determined as follows. First, the amplification optical fiber was wound in a spiral shape with an inner diameter of 130 mm. Hereinafter, the optical fiber wound in a spiral shape is referred to as a fiber coil. Thus, by winding the amplification optical fiber in a spiral shape, it is difficult for micro vents to occur, and the effect of suppressing the skew mode by micro vents is difficult to occur. Therefore, it is considered that the effect of suppressing the skew mode evaluated by the following method was hardly affected by the microvent.

  Next, light having a wavelength of 915 nm was incident on the inner peripheral end of the fiber coil, and the light emitted from the outer peripheral end of the fiber coil was measured with a power meter. By measuring the power of the light incident on the fiber coil in advance, the difference between the power of the light incident on the fiber coil and the power of the light emitted from the outer end of the fiber coil The loss of light, that is, the amount of light absorbed by the amplification optical fiber can be determined.

Next, the length of the optical fiber for amplification was changed by shortening the fiber coil by cutting it from the outer peripheral end, and the amount of light absorbed by the optical fiber for amplification was obtained by the same method as described above. . Thereby, the amount of light absorption according to the length of the amplification optical fiber can be obtained as shown in the graph of FIG. A graph as shown in FIG. 4 was created for each amplification optical fiber, and the following equation (6) was obtained as an approximate curve of a quadratic function passing through the plot.
y = ax ² + bx + c (6)

Here, since it is considered that the amount of light absorbed in the amplification optical fiber is preferably at least about 21 dB, the value of x when y = 21 is L, and α _L is the following formula (7). It was defined as
α _L = 21 / L (7)

On the other hand, α _s is the amount of light absorbed by the amplification optical fiber when the skew mode is not generated. That is, α _s can be the amount of light absorbed per unit length when the amplification optical fiber is short, and can be expressed by the following equation (8).

  The skew mode suppression effect was evaluated for each amplification optical fiber by γ defined as above. The results are shown in Table 1 and FIG. FIG. 5 is a graph showing the relationship of the skew suppression index with respect to the product of the angularity c and the number N of twists. In FIG. 5, the horizontal axis represents the product of the long-angle tension degree c and the twist amount N, and the vertical axis represents the skew suppression index γ.

  As can be seen from Table 1 and FIG. 5, in Comparative Examples 4 to 6 in which the angular degree c was 0.09, the value of the skew suppression index γ did not change even when the inner cladding was twisted. On the other hand, in Examples 1 to 5 in which the squareness degree c was 0.15 or more, the skew suppression index γ was increased by twisting the inner cladding. Therefore, it can be seen that by setting the angularity c of the inner cladding to a predetermined value or more, the occurrence of the skew mode is easily suppressed when twist is added to the inner cladding. Further, as can be seen from the comparison between Comparative Example 1 and Example 1, the comparison between Comparative Example 2 and Example 4, and the comparison with Example 5 of Comparative Example 3, the angularity c is not less than a predetermined value. , The skew suppression index γ increases as the twist amount N increases. Furthermore, as can be seen from FIG. 5, when the angularity c is equal to or greater than a predetermined value, the skew suppression index γ is substantially directly proportional to the product until the product of the angularity c and the number of twists N increases to some extent. .

  As described above, according to the present invention, an optical fiber for amplification that can suppress the occurrence of a skew mode is provided, and is expected to be used in the fields of processing machines, medical laser devices, and the like.

DESCRIPTION OF SYMBOLS 1, 2 ... Laser apparatus 10 ... Amplifying optical fiber 11 ... Core 12 ... Inner clad 13 ... Outer clad 14 ... Covering layer 20 ... Excitation light source 21 ... Laser Diode 31 ... 1st FBG
32 ... 2nd FBG
40, 50 ... optical combiner 70 ... seed light source

Claims (5)

A core to which an active element is added;
An inner cladding surrounding the core and having a lower refractive index than the core;
An outer cladding surrounding the inner cladding and having a lower refractive index than the inner cladding;
With
The inner cladding is added with a twist about the central axis of the core,
In the cross section perpendicular to the longitudinal direction, the outer periphery of the inner cladding is rounded at the corners of the polygon,
When the number of apexes of the polygon is n, the diameter of the circumscribed circle on the outer periphery of the inner cladding is d2, and the diameter of the inscribed circle on the outer periphery of the inner cladding is d1, the following formulas (1) and (2 An amplification optical fiber characterized in that the angularity c defined by (1) is 0.15 or more and 0.8 or less.
A = cos (π / n) (1)
c = {1- (d1 / d2)} / (1-A) (2) The amplification optical fiber according to claim 1, wherein the angularity c is 0.25 or more. The amplification optical fiber according to claim 1 or 2, wherein when the number of rotations of the twist per 1 m in a direction parallel to the longitudinal direction is N, the following expression (3) is satisfied.
c × N ≧ 0.75 (3) The amplification optical fiber according to claim 3, wherein the following expression (4) is satisfied.
c × N ≧ 2.5 (4) An amplification optical fiber according to any one of claims 1 to 4,
At least one light source that emits light propagating through the amplification optical fiber;
A laser device comprising:

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