US-12627114-B2 - Active element added-optical fiber, preform for active element added-optical fiber, resonator, and fiber laser device

US12627114B2US 12627114 B2US12627114 B2US 12627114B2US-12627114-B2

Abstract

An active element added-optical fiber includes a core, having a radius d and including a first region and a second region, and a cladding that surrounds an outer peripheral surface of the core without a gap and propagates light in a few mode. The first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element. The second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium. The active element is not added to a region within the second region from a radius rc to the radius d. The germanium is not added to a region within the first region from the central axis to a radius rb, and a concentration of the germanium is highest among the plurality of dopants.

Inventors

Tsukasa Hosokawa
Rintaro Kitahara
Ryoichi Nishimura

Assignees

FUJIKURA LTD.

Dates

Publication Date: 20260512
Application Date: 20210222
Priority Date: 20200327

Claims (19)

1 . An active element added-optical fiber comprising: a core, having a radius d, including a first region and a second region, and configured to propagate light in a few modes, and a cladding that surrounds an outer peripheral surface of the core without a gap, has a refractive index lower than a refractive index of the second region of the core, and propagates light in a few modes, wherein the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light, the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium, the active element is not added to a region within the second region from a radius rc to the radius d, the germanium is not added to a region within the first region from the central axis to a radius rb, a concentration of the germanium is highest among the plurality of dopants, the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc, 0.1d<ra<d, when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, and the local maximum value position exists in a region of 0.4d<r≤ra.
2 . The active element added-optical fiber according to claim 1 , wherein in the first diffusion region, a concentration of the active element decreases with increasing radius, and the concentration of the germanium increases with increasing radius, and in the first diffusion region, at a first matching point where a concentration of the active element equals the concentration of the germanium, the concentration of the active element is 70% or less of an average concentration of the active element in a region from the central axis to a radius 0.5ra or less.
3 . The active element added-optical fiber according to claim 1 , wherein at least one up-dopant other than the active element that increases refractive index is added to a region from the central axis of the core to a radius rd in the second region, the up-dopant and germanium are added to a second diffusion region from the radius rb to the radius rd, in the second diffusion region, a concentration of the up-dopant decreases with increasing radius, and the concentration of the germanium increases with increasing radius, and at a second matching point in the second diffusion region where a total concentration of the up-dopant equals the concentration of the germanium, the total concentration of the up-dopant is larger than 0 and is 50% or less of an average concentration of the up-dopant in a region from the central axis to a radius 0.5ra or less.
4 . The active element added-optical fiber according to claim 1 , wherein a slope of a refractive index profile at the radius d of the core is −0.035%/μm or less.
5 . The active element added-optical fiber according to claim 1 , wherein the core has a step index-shaped refractive index profile, the core has a V-value of 5 or more and 12 or less, and 0.39d≤ra≤0.78d.
6 . The active element added-optical fiber according to claim 1 , wherein a standard deviation of a relative refractive index difference in a region of 1.1ra<r≤0.9d is 0.004 or less.
7 . The active element added-optical fiber according to claim 1 , wherein the local maximum value position exists in a region of 0.338d≤r≤0.614d.
8 . The active element added-optical fiber according to claim 1 , wherein 0.595d≤ra≤0.716d.
9 . The active element added-optical fiber according to claim 1 , wherein an average concentration of the active element in a region of 0≤r≤0.1d is higher than an average concentration of the active element in a region of 0.1d<r≤ra.
10 . The active element added-optical fiber according to claim 1 , wherein an average refractive index in the region of 0≤r≤0.1d is higher than a refractive index at r=0.2d.
11 . The active element added-optical fiber according to claim 1 , wherein an average refractive index in a region of 0.1d≤r≤0.8ra is higher than an average refractive index in a region of 1.1ra≤r≤0.9d.
12 . The active element added-optical fiber according to claim 1 , wherein an average refractive index in a region of 0.1≤r≤0.8ra is lower than an average refractive index in a region of 1.1ra≤r≤0.9d.
13 . The active element added-optical fiber according to claim 1 , wherein an average refractive index in a region of 0.1d≤r≤0.8ra is equivalent to an average refractive index in a region of 1.1ra≤r≤0.9d.
14 . The active element added-optical fiber according to claim 1 , wherein an average relative refractive index difference with respect to the cladding in a region of 0.055d≤r≤0.1d is equal to or more than a relative refractive index difference at the local maximum value position, an average relative refractive index difference of the core with respect to the cladding is larger than 0% and 0.18% or less, and the local maximum value position is at 0.55d or less.
15 . A resonator comprising: the active element added-optical fiber according to claim 1 ; a first mirror configured to be optically coupled to the core of the active element added-optical fiber on one side of the active element added-optical fiber and reflect light having at least a part of wavelength of light emitted by the pumped active element; and a second mirror configured to be optically coupled to the core of the active element added-optical fiber on another side of the active element added-optical fiber and reflect light having at least a part of wavelength of the light reflected by the first mirror at a reflectance lower than a reflectance of the first mirror.
16 . A fiber laser device comprising: the active element added-optical fiber according to claim 1 ; and a light source configured to emit light that pumps the active element.
17 . An active element added-optical fiber comprising: a core, having a radius d, including a first region and a second region, and configured to propagate light in a few modes, and a cladding that surrounds an outer peripheral surface of the core without a gap, has a refractive index lower than a refractive index of the second region of the core, and propagates light in a few modes, wherein the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light, the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium, the active element is not added to a region within the second region from a radius rc to the radius d, the germanium is not added to a region within the first region from the central axis to a radius rb, a concentration of the germanium is highest among the plurality of dopants, the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc, 0.1d<ra<d, when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, an average concentration of the active element in a region of 0<r≤0.1d is higher than an average concentration of the active element in a region of 0.1d<r≤ra, and an average refractive index in the region of 0≤r≤0.1d is higher than a refractive index at r=0.2d.
18 . An active element added-optical fiber comprising: a core, having a radius d, including a first region and a second region, and configured to propagate light in a few modes, and a cladding that surrounds an outer peripheral surface of the core without a gap, has a refractive index lower than a refractive index of the second region of the core, and propagates light in a few modes, wherein the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light, the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium, the active element is not added to a region within the second region from a radius rc to the radius d, the germanium is not added to a region within the first region from the central axis to a radius rb, a concentration of the germanium is highest among the plurality of dopants, the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc, 0.1d<ra<d, when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, and an average refractive index in a region of 0.1d≤r≤0.8ra is higher than an average refractive index in a region of 1.1ra≤r≤0.9d.
19 . An active element added-optical fiber comprising: a core, having a radius d and including a first region and a second region, and a cladding that surrounds an outer peripheral surface of the core without a gap and propagates light in a few modes, wherein the first region is a region from a central axis of the core to a radius ra and contains ytterbium as an active element that can be pumped by light, the second region is a region to the radius d that surrounds the first region without a gap and contains a plurality of dopants, one of which is germanium, the active element is not added to a region within the second region from a radius rc to the radius d, the germanium is not added to a region within the first region from the central axis to a radius rb, a concentration of the germanium is highest among the plurality of dopants, the active element and the germanium are added to a first diffusion region from the radius rb to the radius rc, 0.1d<ra<d, when a distance from the central axis of the core in a radial direction is r, in a region of 0.2d<r≤0.9d, there is at least one local maximum value position, whose refractive index is locally maximum and is higher than an average refractive index in a region of 0≤r≤0.9d, and an average refractive index in a region of 0.1d≤r≤0.8ra is equivalent to an average refractive index in a region of 1.1ra≤r≤0.9d.

Description

TECHNICAL FIELD One or more embodiments of the present invention relate to an active element added-optical fiber, a preform for an active element added-optical fiber, a resonator, and a fiber laser device capable of suppressing deterioration of beam quality. BACKGROUND A fiber laser device, which is excellent in light condensing property, has high power density, and can obtain light that becomes a small beam spot, is used in various fields such as a laser processing field and a medical field. In such a fiber laser device, output of emitted light is increased. In general, a fiber laser device includes an active element added-optical fiber in which a rare earth element, which is an active element, is added to a core to amplify light. However, when the power density of light in an active element added-optical fiber increases, wavelength conversion of light due to stimulated Raman scattering is likely to occur, and light having an unintended wavelength can be emitted. In this case, the light reflected by a workpiece or the like returns to the fiber laser device and is amplified, and the amplification of the light having the wavelength to be amplified in design becomes unstable, and the output can become unstable. Examples of a means for suppressing such stimulated Raman scattering include increasing the effective cross-sectional area of the light propagating through the core, and as one of the methods for increasing the effective cross-sectional area, increasing the diameter of the core can be mentioned. Therefore, in order to suppress stimulated Raman scattering in the optical fiber, for example, an optical fiber having a core that enables propagation of light in a few mode is used. In the fiber laser device, the beam quality of emitted light is excellent from the viewpoint of light condensing property or the like. Therefore, there is a demand for suppressing excitation of light in modes other than the basic mode even when the effective cross-sectional area of light is increased by using an optical fiber having a core that enables propagation of light in the few mode as described above. Note that the beam quality is indicated, for example, by M2 (M square) or the like. Therefore, amplification of light in a higher-order mode is suppressed while using an active element added-optical fiber having a core that enables propagation of light in the few mode as in an active element added-optical fiber described in Patent Literature 1 below. By the way, in an active element added-optical fiber in which a rare earth element is added to the core, it is known that a phenomenon called photodarkening occurs in which the transmission loss of the core portion of the amplification optical fiber gradually increases as pumping light is incident and propagated and the intensity of the output laser light decreases with time. As one of methods for suppressing this photodarkening, for example, addition of aluminum to the core is performed. For example, Non Patent Literature 1 below discloses a configuration in which ytterbium is added only to a first region located in a center portion of a core and aluminum is added to the entire core including a second region surrounding the first region. In the optical fiber of Non Patent Literature 1 below, the concentration of aluminum is the highest among the elements added to the second region. [Patent Literature 1] JP 5124701 B1[Non Patent Literature 1] Proc. of SPIE Vol. 7580 758016-1 Fiber amplifier utilizing an Yb-doped large-mode-area fiber with confined doping and tailored refractive index profile. However, even when light can be propagated in the few mode as in Patent Literature 1, there is a demand for further suppressing deterioration of beam quality. For example, when the optical fiber of Non Patent Literature 1 is heated to be fusion-spliced with another optical fiber, the diffusion coefficient of aluminum in glass is large, so that the aluminum existing on an outer side of the core diffuses into a cladding, and the refractive index profile of the glass can change from the refractive index profile before heating. When the refractive index profile of the core and the cladding changes in this way, unnecessary higher-order modes are excited in the fusion-spliced portion, and the beam quality can deteriorate. Therefore, one of more embodiments of the present invention provide an active element added-optical fiber, a preform for an active element added-optical fiber, a resonator, and a fiber laser device capable of suppressing deterioration of beam quality. SUMMARY One of more embodiments of the present invention provide an active element added-optical fiber comprising a core and a cladding that surrounds an outer peripheral surface of the core without a gap and capable of propagating light in a few mode, wherein the core has radius d and includes a first region and a second region, the first region is a region from a central axis of the core to radius ra and to which ytterbium that is a