TECHNICAL FIELD

The present invention relates to a cladding mode stripper for removing cladding mode light from an optical fiber.

BACKGROUND

In the field of material processing in which metal materials or the like are processed (e.g., cut, welded, shaved), laser processing using laser light has been increasingly used, replacing machining using a blade, a drill, or the like. As a laser device for use in laser processing, a fiber laser has been attracting attention. For processing that requires laser light with higher power, a fiber laser system including a plurality of fiber lasers and a combiner for combining laser light waves generated by the respective plurality of fiber lasers is used.

The fiber laser is a laser device including, as a resonator, a pump-gain fiber that has fiber Bragg gratings connected to the opposite ends thereof, wherein one of the fiber Bragg gratings serves as a mirror and the other serves as a half mirror. The pump-gain fiber is typically a double cladding fiber whose core is doped with a rare-earth element such as Yb. Signal light generated by the resonator is guided to an object to be processed, with use of a delivery fiber connected to the fiber Bragg grating serving as the half mirror. The delivery fiber is typically a double cladding fiber as well.

In the fiber laser, an optical path of laser light generated by the resonator is constituted by the pump-gain fiber, the fiber Bragg grating, and the delivery fiber. The optical path includes at least a connection point between the pump-gain fiber and the fiber Bragg grating and a connection point between the fiber Bragg grating and the delivery fiber. At any of the connection points constituting the optical path, in a case where the optical fibers at the connection point have mismatch of cores (difference in core diameter, axial misalignment between the cores, or the like), cladding mode light is excited in a cladding of a downstream one of the optical fibers. For example, in a case where cores mismatch at the connection point between the fiber Bragg grating and the delivery fiber, cladding mode light is excited in a cladding of the delivery fiber. Further, also in a case where a disturbance such as lateral pressure or bending is applied to an optical fiber constituting the optical path, a higher order mode component of laser light guided through the core leaks into a cladding, so that cladding mode light is excited. In the fiber laser system, the combiner can be a source of cladding mode light, and a connection point between the combiner and an optical fiber constituting the optical path can also be a source of cladding mode light.

Cladding mode light which is guided through a cladding of an optical fiber constituting the optical path (i) causes a coating of the optical fiber or of an optical fiber downstream of the optical fiber to generate heat and (ii) causes a component of a processing head connected to an exit end of the delivery fiber to generate heat. That is, cladding mode light guided through a cladding of an optical fiber constituting the optical path can cause a decrease in reliability of the fiber laser or the fiber laser system. In view of this, an optical fiber constituting the optical path is provided with a cladding mode stripper for removing cladding mode light.

FIG. 13is a longitudinal sectional view illustrating a configuration of a conventional cladding mode stripper10. The cladding mode stripper10is constituted by a high-refractive-index resin part101which covers a portion of an inner cladding52of an optical fiber5which portion is exposed in a coating-removed section I. The high-refractive-index resin part101is made of a light-transmitting resin having a refractive index not less than that of the inner cladding52of the optical fiber5. This allows cladding mode light confined in the inner cladding52of the optical fiber5to leak into the high-refractive-index resin part101in the coating-removed section I.

Patent Literature 1 discloses a cladding mode stripper provided with a heat dissipation mechanism or a cooling mechanism. Patent Literature 2 discloses a cladding mode stripper including a high-refractive-index resin part having a refractive index that gradually increases along a light propagation direction.

Patent Literature 1

Japanese Patent Application Publication, Tokukaihei, No. 1-316705

Patent Literature 2

International Publication No. WO 2013/001734

In a conventional cladding mode stripper, part of cladding mode light which part has leaked out from an optical fiber in a coating-removed section may propagate inside a high-refractive-index resin part while being repeatedly reflected by a surface of the high-refractive-index resin part. The cladding mode light propagating inside the high-refractive-index resin part may cause the high-refractive-index resin part to generate heat and may degrade the high-refractive-index resin part, accordingly.

For example, in a case where 1% of laser light guided through a core of a fiber laser having an output of 1 kW becomes cladding mode light, the cladding mode power is as high as 10 W. In a case where a 10 W cladding mode is converted into heat in a high-refractive-index resin part having a length of several millimeters to several centimeters, a temperature rise enough to degrade the high-refractive-index resin part may occur.

SUMMARY

One or more embodiments of the present invention provide a cladding mode stripper that is arranged such that heat generation of a high-refractive-index resin part is less likely to occur as compared with a conventional cladding mode stripper.

A cladding mode stripper in accordance with one or more embodiments of the present invention is a cladding mode stripper, including a resin part which covers a coating-removed section of an optical fiber and has a refractive index not less than that of an outermost shell of the optical fiber in the coating-removed section, the resin part including an incident angle reducing structure formed on a surface of the resin part, the incident angle reducing structure being for reducing an incident angle or an average incident angle at which cladding mode light that has entered the resin part from the optical fiber enters the surface.

A cladding mode stripper in accordance with one or more embodiments of the present invention is a cladding mode stripper, including: a resin part which covers a coating-removed section of an optical fiber and has a refractive index not less than that of an outermost shell of the optical fiber in the coating-removed section; and a transparent member which supports the resin part and has a refractive index matching that of the resin part, the transparent member including a rough surface in a region of a surface of the transparent member which region is opposite to an interface between the transparent member and the resin part, cladding mode light that has entered the transparent member from the optical fiber through the resin part being scattered by the rough surface so that an average propagation angle of the cladding mode light is increased.

According to an aspect of the present invention, it is possible to provide a cladding mode stripper that is arranged such that heat generation of a high-refractive-index resin part is less likely to occur as compared with a conventional cladding mode stripper.

