JP5178329B2 - Hole-type double clad fiber - Google Patents

Description

  The present invention relates to a hole-type double clad fiber having an end cap provided at an end and used for a resonator, a laser guide, or the like of a fiber laser apparatus, and more particularly to improvement of the end face shape of the end cap.

  Generally, at the end face of an optical fiber that guides high-power light, such as an optical fiber used in a high-power fiber laser device, pure quartz having a low thermal expansion coefficient that does not have a waveguide structure for transmitted light. By connecting the rod, the beam energy density at the emission end face can be reduced, and the laser resistance at the end face can be improved. In particular, in the case of a hole-type optical fiber having holes in the cladding, since the holes are exposed on the end face, impurities such as dust and moisture are likely to enter from the outside. For this reason, the end of the hole is heated and melted and collapsed to seal the hole and prevent contamination from entering. Thus, the part which does not have the waveguide structure intentionally added to the end face part of the optical fiber for the purpose of improving the laser resistance and preventing the intrusion of contamination is called an end cap.

  In particular, in a double clad fiber used for a high-power fiber laser, pump light is confined by a second clad provided outside the core and the first clad, and the portion where the pump light propagates is usually a pump guide. Called. By the way, in the double clad fiber, since the second clad is often formed from a polymer of a low refractive index resin, reliability becomes a problem due to heat generation in the polymer clad when the laser output becomes high. On the other hand, as schematically shown in FIG. 5, each of the hole-type doubles is configured such that the second cladding 130 is constituted by a plurality of holes 130a extending substantially in the fiber axis direction (substantially left-right direction in the figure). Since the clad fiber 100 can form a waveguide structure only with a quartz material, it has excellent heat resistance.

  In the case of such a hole-type double clad fiber 100, unlike the method of connecting the pure quartz rod described above, the end portion is heated and melted as described in Patent Document 1. The end cap 150 can be formed by eliminating the waveguide structure for the excitation light.

Further, when used as a resonator of a fiber laser device, as shown by an imaginary line in FIG. 5, a mirror 160 is directly formed on the end face of the end cap 150 (for example, a dielectric multilayer film is vapor-deposited). A resonator can be formed. Specifically, first, the end cap 150 is formed at the end of the hole-type double clad fiber 100, and then the end surface of the end cap 150 is vertically polished, and then the mirror 160 is formed.
JP 2005-321447 A

  However, in the above conventional hole-type double clad fiber, when coupled with other optical components or when a laser beam is irradiated to a predetermined laser processing point or the like, as shown by a solid line arrow in FIG. Therefore, there is a problem that the coupling efficiency is not good because the beam diameter of the laser light propagating through the end cap 150 is not reduced. For this, a technique for melting and connecting the front lens (see JP-A-8-114729) is known, but in that case, a new problem is caused in the strength at the connection point. become.

  In the conventional case, when the mirror 160 is formed on the end face of the end cap 150, a part of the excitation light that diverges during propagation of the end cap and reaches the surface of the mirror 160 is indicated by an imaginary line arrow in FIG. As shown, there is a problem that so-called “vignetting” of excitation light occurs in which the pump guide 120 does not return to the outer side but is reflected outwardly, resulting in a decrease in oscillation efficiency.

  The present invention has been made in view of such various points, and its main purpose is to add a device to the end face shape of the end cap when the end cap is formed at the end of the hole-type double clad fiber. As a result, for example, the coupling efficiency can be improved when used in combination with optical components, etc., and when used as a resonator, the vignetting of the excitation light is reduced to suppress the decrease in oscillation efficiency. There is to be able to do it.

  In order to achieve the above object, in the present invention, the end face of the end cap is aspherical.

  Specifically, in the present invention, a core for propagating predetermined light, a first clad provided so as to cover the outer periphery of the core, and provided so as to cover the outer periphery of the first clad, A second clad composed of a plurality of holes arranged so as to extend in the length direction of the first clad, and an open end of each hole of the second clad at the ends of the core, the first clad and the second clad It is premised on a hole-type double clad fiber provided with an end cap provided in a state of sealing.

  And at least the center part in the end surface of the said end cap shall be formed in the lens shape which bulges toward a fiber axial direction outward.

  In the above configuration, when coupled with other optical components, the aspherical surface can have a shape that changes in a direction in which the beam diameter of the predetermined light that has passed through the end cap decreases.

  On the other hand, when used as a resonator of a fiber laser device, that is, the first cladding is used as a pump guide for propagating pumping light, and is provided so as to cover the end face of the end cap. When a mirror that reflects at least a part of the excitation light that propagates while diffusing the cap is provided, the excitation light that has propagated through the end cap from the center of the fiber axis while diffusing the end cap to reach the end face Can be formed in a shape that reflects toward the first cladding by a mirror.

  Further, when used as a resonator of a fiber laser device, the more specific configuration is that the length of the end cap is L, the minimum value of the dimension from the fiber axis to the outer peripheral surface of the first cladding is p, and the pump is excited. When the propagation angle of light is θp, the radius of curvature R of the arrival region of the excitation light on the end face of the end cap is R ≦ (p / tan θp) + L. When the outer peripheral surface of the first cladding has a circular cross section centered on the fiber axis, the minimum value of the dimension from the fiber axis to the outer peripheral surface of the first cladding is the value of the outer peripheral surface of the first cladding. The radius.

  According to the present invention, when the end cap is formed at the end portion of the hole-type double clad fiber, the end face shape is a convex aspherical surface. When coupled with optical components, etc., it can contribute to improved coupling efficiency. When a mirror is formed on the end face and used as a resonator, vignetting of pumping light is suppressed to suppress a decrease in oscillation efficiency. Can contribute.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 schematically shows an end structure of a hole-type double-clad fiber according to an embodiment of the present invention, and this hole-type double-clad fiber 10 is provided on the output side of a fiber laser device. It is used as a laser guide for transmitting laser light as predetermined light generated by the fiber laser device to another optical component 20.

  The hole-type double clad fiber 10 is provided so as to cover a core 11 for propagating laser light, a first clad 12 provided so as to cover the outer periphery of the core 11, and an outer periphery of the first clad 12. A second cladding 13 composed of a plurality of holes 13a arranged so as to extend substantially in the fiber axis direction, a support layer 14 provided so as to cover the outer periphery of the second cladding, and this hole-type double An end cap 15 is provided at the end of the clad fiber 10 so as to seal the open end of each hole 13 a of the second clad 13.

  In the present embodiment, the end face of the end cap 15 is formed in an aspherical shape that bulges outward in the fiber axial direction (rightward in FIG. 1). Specifically, the aspherical shape is a shape in which the laser light emitted from the end face 16 focuses on a predetermined position on the optical component 20 as indicated by a thick solid line arrow in FIG.

  Here, the aspheric surface will be described in detail. Since the correction term can generally be ignored for the shape of the aspheric surface, in the hy coordinates where the position in the optical axis direction with the apex of the aspheric surface as the origin is h and the position in the radial direction is y, the following expression Given by [1].

h = c · y ² / {1+ [1- (1 + k) · c ² · y ² ] ^1/2 }. . . [1]
c: curvature of vertex of aspheric surface (reciprocal of radius of curvature R of same point)
k: Conical constant (coefficient of quadratic surface)
In the above formula [1], an aspherical surface is a hyperboloid when k <−1, a paraboloid when k = −1, an ellipsoid when −1 <k <0, and when k = 0. Is spherical, and when 0 <k <1, the surface is depolarized. In other words, “aspherical surface” in this specification is a concept including “spherical surface”.

  Further, as an example of a method for obtaining the end face shape as described above, a polishing sheet having a fine texture and flexibility may be used, and polishing may be performed by pressing the end face of the end cap 15 against the polishing sheet. That is, since the portion of the polishing sheet pressed against the end cap 15 is concavely curved, the peripheral portion of the end cap 15 can be polished more than the central portion of the fiber axis by the recess, and thus the end surface is aspherical. Will be formed.

  Therefore, according to the present embodiment, when the end cap 15 is formed at the end of the hole-type double clad fiber 10, the end surface of the end cap 15 is oriented in the direction in which the laser light emitted from the end surface focuses. Since the aspherical shape has a wavefront focusing effect that changes, when a front-end lens of another member is fused and connected to the end of the optical fiber (refer to Japanese Patent Laid-Open No. 8-114729) On the other hand, the coupling efficiency can be improved without causing a situation where the connection strength is insufficient.

  In the above embodiment, the case where the laser beam emitted from the end face of the end cap 15 is irradiated to the other optical component 20 has been described. For example, the end face of the end cap 15 is used as the end face of another optical fiber. It can also be applied in the case of performing physical contact with physical contact.

  In the above-described embodiment, the end face of the end cap 15 has a shape that changes so that the emitted laser light is focused. However, depending on the use conditions, at least the direction in which the beam diameter decreases. The shape can be changed to

  Furthermore, in the above embodiment, the case of a laser guide that guides laser light has been described. However, the hole-type double-clad fiber according to the present invention can be applied to various optical devices related to a fiber laser device. it can.

(Embodiment 2)
FIG. 2 schematically shows an end structure of a hole-type double clad fiber according to Embodiment 2 of the present invention, and this hole-type double clad fiber 10 is used as a resonator of a fiber laser device. .

  The hole-type double clad fiber 10 is provided so as to generate stimulated emission light by the excitation light and propagate the stimulated emission light, and to cover the outer periphery of the core 11, and to propagate the excitation light. A first clad 12; a second clad 13 comprising a plurality of holes 13a provided so as to cover the outer periphery of the first clad 12 and extending substantially in the fiber axis direction; and the second clad A support layer 14 provided so as to cover the outer periphery of 13 and a state in which the open end of each hole 13a of the second cladding 13 is sealed at the end of the hole-type double-clad fiber 10; And an end cap 15 having no waveguide structure for the excitation light.

  The outer peripheral cross-sectional shape of the first cladding 12 has a circular shape centered on the fiber axis. The second clad 13 has a function of confining the excitation light in the first clad 12, so that the first clad 12 functions as a pump guide. On the end face of the end cap 15, a mirror 16 made of a dielectric multilayer film that reflects at least a part of the excitation light that diffuses from the first cladding 12 side and propagates through the end cap 15 is formed.

  In this embodiment, the end face of the end cap 15 propagates while diffusing the end cap 15 from the center of the fiber axis and reflects the excitation light reaching the end face toward the first cladding 12 by the mirror 16. It is formed into a shape. That is, the mirror 16 is formed as a concave mirror by the shape of the end face.

  Specifically, the end surface of the end cap 15 is such that the excitation light that has propagated through the end cap 15 while diffusing from the start end of the end cap 15 on the fiber axis with a propagation angle θp with respect to the fiber axis is the first cladding. 12 is formed so as to reach the surface of the mirror 16 at a position separated by the radial dimension of the outer peripheral surface 12, pass through the end cap 15 as parallel light with respect to the fiber axis from that position, and enter the first cladding 12. In FIG. 2, L is the dimension of the end cap 15 in the fiber axial direction, and p is the radius of the outer peripheral surface of the first cladding 12.

  Therefore, according to the present embodiment, the end cap 15 is formed at the end of the hole-type double-clad fiber 10, and at least a part of the excitation light is reflected by the mirror 16 provided on the end face. When configuring, the excitation light propagating through the end cap 15 while being diffused from the fiber axis center with respect to the fiber axis at an angle of propagation θp is reflected in the inner direction of the first cladding 12. For most of the excitation light propagating through the cladding 12 and reflected by the mirror 16, vignetting in the outer direction of the first cladding 12 can be reduced, and therefore, the oscillation efficiency due to such vignetting of the excitation light can be reduced. It can contribute to suppression of the decrease.

  Further, since the excitation light is reflected as parallel light with respect to the fiber axis, the reflected light is not focused in the end cap 15, and therefore damage to the end cap 15 due to the increase in energy density can be avoided. it can.

  In the above embodiment, the case where the outer peripheral cross-sectional shape of the first cladding 12 is circular has been described. However, the present invention is also applied to other shapes such as a D-shaped cross section. be able to. However, in that case, it is preferable to substitute for p the minimum dimension between the fiber axis and the outer peripheral surface of the first cladding 12 instead of the radius of the outer peripheral surface of the first cladding 12.

  In the above embodiment, the case of the resonator of the fiber laser device has been described. However, the hole-type double clad fiber according to the present invention can be applied to other optical devices related to the fiber laser device. .

(Embodiment 3)
FIG. 3 schematically shows an end structure of a hole-type double clad fiber according to a third embodiment of the present invention. This hole-type double-clad fiber 10 is a fiber laser as in the second embodiment. Used as a device resonator.

  In the present embodiment, the end face of the end cap 15 has a shape in which all of the excitation light that has diffused from the first clad 12 and propagated through the end cap 15 and reached the end face is reflected by the mirror 16 toward the first clad 12. Is formed.

Specifically, when the length of the end cap 15 in the fiber axis direction is L, the radius of the outer peripheral surface of the first cladding 12 is p, and the propagation angle of the excitation light is θp, the propagation angle θp with respect to the fiber axis is In order for the excitation light that diverges at an angle and that passes through the end cap 15 at the radial position farthest from the fiber axis to return to the first cladding 12, the return path between the reflected path and the fiber axis The minimum value of the angle needs to be the same as the propagation angle θp. At this time, the position in the fiber axis direction where the excitation light starts to diffuse from the fiber axis is the front side (left side in FIG. 3) from the start end of the end cap 15 by the dimension of p / tan θp. Here, when the thickness dimension of the end cap 15 is L, the radius of curvature R at this time is
R = (p / tan θ) + L. . . [2]
This is the optimum radius of curvature for preventing vignetting of the excitation light.

Therefore, the radius of curvature R in the region where the excitation light reaches the end face of the end cap 15 is
R ≦ (p / tan θ) + L. . . [3]
As a result, all of the pumping light that propagates through the first cladding 12 and reaches the mirror 16 is reflected back to the first cladding 12, so that the vignetting of the pumping light by the mirror 16 is in principle. No longer exists. Since the other configuration of the present embodiment is the same as that of the second embodiment, description thereof is omitted.

  Therefore, according to the present embodiment, as in the case of the second embodiment, the end cap 15 is formed at the end of the hole-type double clad fiber 10, and at least a part of the excitation light is generated by the mirror 16 provided on the end face. When the resonator is configured to reflect, all the excitation light that propagates from the first cladding 12 to the end cap 15 and reaches the end face is reflected toward the first cladding 12. In principle, the vignetting of the excitation light toward the radially outward side is completely eliminated. Therefore, it is possible to further contribute to the suppression of the decrease in oscillation efficiency due to such vignetting of the excitation light.

  Also according to this embodiment, the reflected light is not focused in the end cap 15, so that damage to the end cap 15 due to the increase in energy density can be avoided.

  In the above-described embodiment, the core 11 is doped with a rare earth element, and the rare earth element absorbs excitation light to generate stimulated emission diffused light. It is possible to generate stimulated emission light by generating non-doped interaction and causing nonlinear interaction such as stimulated Raman scattering in the core by the excitation light.

  In the above embodiment, the case where the outer peripheral cross-sectional shape of the first cladding 12 is a circle centered on the fiber axis has been described. However, other shapes such as a rectangular shape, a polygonal shape, a D-shape, and the like are used. The present invention can also be applied to the case where there is nothing. In this case, however, it is preferable to substitute for p the minimum dimension between the center of the fiber axis and the outer peripheral surface of the first cladding, instead of the radius of the outer peripheral surface of the first cladding 12.

  Further, in the above embodiment, the curvature radius R is determined by the relational expression [3] only in the excitation light arrival area at the end face of the end cap 15, but the curvature by the relational expression [3] in the entire area of the end face. The radius R may be determined.

  Further, in the above embodiment, the case of the resonator of the fiber laser device has been described. However, the hole-type double clad fiber according to the present invention guides the laser beam emitted from the fiber laser device to a predetermined location. It can be applied to various optical devices such as a laser guide.

Here, an example of the first embodiment will be described. In this embodiment, the end cap length L is L = 200 μm, the pump guide radius p is p = 200 μm, and the propagation angle θp of excitation light in the pump guide is θp = 28.9 ° (the numerical aperture NA is NA = 0.7). The fiber radius r was set to r = 1000 μm. At this time, the optimum curvature radius R ′ of the end face is obtained by the above equation [3].
R ′ = (200 [μm] /0.552) +200 [μm] ≈562 [μm]
It becomes.

  In FIG. 4, the cross-sectional shape of the end surface of a present Example is shown. In addition, the comparative example shown together with the same figure is the cross-sectional shape of the conventional end surface grind | polished for physical contact (PC), The curvature radius R is R10 mm. The radius of curvature R during PC polishing is generally about R = 10 to 25 mm.

FIG. 1 is a longitudinal sectional view schematically showing an end structure of a hole-type double clad fiber according to Embodiment 1 of the present invention, together with optical components to be coupled. FIG. 2 is a longitudinal sectional view schematically showing an end structure of a hole-type double clad according to Embodiment 2 of the present invention. FIG. 3 is a view corresponding to FIG. 2 schematically showing an end structure of a hole-type double clad according to Embodiment 3 of the present invention. FIG. 4 is a characteristic diagram showing the cross-sectional shape of the end face of the example of Embodiment 3 together with the cross-sectional shape of the conventional end face subjected to PC polishing. FIG. 5 is a view corresponding to FIG. 2 schematically showing an end structure of a conventional hole-type double clad fiber.

Explanation of symbols

10 hole type double clad fiber 11 core 12 first clad 13 second clad 13a hole 15 end cap 16 mirror

Claims (3)

A core that propagates predetermined light; and
A first clad provided to cover the outer periphery of the core;
A second clad comprising a plurality of holes provided so as to cover the outer periphery of the first clad and extending in the length direction of the first clad;
A hole-type double-clad fiber provided with an end cap provided in a state in which the open end of each hole of the second cladding is sealed at the ends of the core, the first cladding, and the second cladding. And
At least the center part of the fiber axis on the end face of the end cap is formed as an aspherical surface that bulges outward in the fiber axis direction and changes in a direction in which the beam diameter of the predetermined light that has passed through the end cap decreases. Has been
The first cladding is a pump guide for propagating excitation light,
A mirror that is provided so as to cover the end face of the end cap and reflects at least a part of the excitation light that diffuses and propagates through the end cap;
The end face of the end cap is formed in a shape that reflects the excitation light that has propagated through the end cap from at least the center of the fiber axis and has reached the end face to the first cladding by the mirror. Characteristic hole-type double clad fiber. A core that propagates predetermined light; and
A first clad provided to cover the outer periphery of the core;
A second clad comprising a plurality of holes provided so as to cover the outer periphery of the first clad and extending in the length direction of the first clad;
A hole-type double-clad fiber provided with an end cap provided in a state in which the open end of each hole of the second cladding is sealed at the ends of the core, the first cladding, and the second cladding. And
At least the center portion of the fiber axis on the end face of the end cap is formed as an aspherical surface having a shape that bulges outward in the fiber axis direction and changes in a direction in which the beam diameter of the predetermined light passing through the end cap decreases. And
The first cladding is a pump guide for propagating the excitation light,
Provided so as to cover the end face of the end cap further comprises a mirror for reflecting at least a part of the excitation light diffusing substance propagates the end cap,
When the length of the end cap is L, the minimum value of the dimension from the center of the fiber axis to the outer peripheral surface of the first cladding is p, and the propagation angle of the excitation light is θp,
The radius of curvature R of the arrival area of the excitation light on the end face of the end cap is:
R ≦ (p / tan θp) + L
A hole-type double-clad fiber characterized by The hole-type double clad fiber according to claim 2 ,
The outer peripheral surface of the first clad has a circular cross section around the fiber axis,
Minimum value of the dimension from the fiber axis to the outer peripheral surface of the first cladding, vacancy type double clad fiber is the radius of the outer circumferential surface of the first cladding.

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