WO2007015577A1

WO2007015577A1 - Combined light source

Info

Publication number: WO2007015577A1
Application number: PCT/JP2006/315697
Authority: WO
Inventors: Shinichi Shimotsu
Original assignee: Fujifilm Corporation
Priority date: 2005-08-04
Filing date: 2006-08-02
Publication date: 2007-02-08
Also published as: TW200714925A; JP2007041378A

Abstract

A combined light source (100) includes: a plurality of light sources (1) which emit light beams; a plurality of optical waveguides (2) which the light beams emitted from the plurality of light sources enter; an optical combiner (4) which is formed by joining portions of the cores of the plurality of optical waveguides into a single core, has a light-output end face, optically combines the light beams so as to produce an optically combined light beam, and outputs the optically combined light beam from the light-output end face; and a transparent medium (5) which has a light-entrance end face connected to the light-output end face of the light-combining means, where the cross-sectional area of the light-entrance end face of the transparent medium (5) is greater than the cross-sectional area of the light-output end face of the optical combiner (4).

Description

DESCRIPTION

COMBINED LIGHT SOURCE

Technical Field

The present invention relates to a combined light source which optically combines light beams emitted from light sources, by using optical fibers .

Background Art

In the conventional light-source modules and the like inwhich laser beams emitted from a plurality of light sources are optically combined, the laser beams are condensed by using an optical means such as a condensing lens, as disclosed in Japanese Unexamined Patent Publication No. 2005-032909.

In addition, the techniques for optically combining light beams by using multimode optical fibers are essential techniques for use with fiber lasers, and are currently under active development . As indicated in U.S. Patent Nos. 5,864,644 and 6,434,302, in the conventional systems where excitation light beams for a fiber laser are combined, a plurality of multimode optical fibers are arranged around a single-mode optical fiber which is located in the center, the plurality of multimode optical fibers and the single-mode optical fiber are bundled, and cores in near-end portions of the plurality of multimode optical fibers and the cladding and the core of the single-mode optical fiber are joined into a single core so as to combine the excitation laser beams which enter the plurality of multimode optical fibers.

In order to uniformize the cross-sectional intensity distribution of the combined light in the conventional systems disclosed in Japanese Unexamined Patent Publication No.2005-032909, it is necessary to make the combined light pass through a transparent medium such as a rod integrator. In this case, it is further necessary that the combined light be accurately incident on a light-entrance end face of the rod integrator after the combined light is condensed by the condensing lens. Therefore, the necessity for adjustment of the position of the rod integrator complicates assembly of such a system.

On the other hand, in the case where light beams are combined by using the techniques as disclosed in U. S. Patent Nos. 5,864,644 and 6,434,302, the combined laser light is conventionally emitted with a large numerical aperture in order to increase the efficiency in the excitation of the fiber laser. When the numerical aperture is great, it is possible to increase the output power of the outputted laser light and the degree of overlapping of the excitation light outputted from the plurality of multimode optical fibers with signal light outputted from the single-mode optical fiber, so that the amplification gain increases. However, in the case where the laser light is outputted with a large numerical aperture, the intensity of the laser light decreases. Therefore, it is not appropriate for a high-intensity light source to output laser light with a large numerical aperture .

Disclosure of the Invention The object of the present invention is to provide a combined light source which optically combines light beams emitted from a plurality of light sources, without use of the optical means such as a condensing lens so as to output combined light having a uniform cross-sectional intensity distribution. In order to accomplish the above object, the first aspect of the present invention is provided. According to the first aspect of the present invention, there is provided a combined light source comprising: a plurality of light sources which emit light beams; a plurality of optical waveguides which the light beams emitted from the plurality of light sources enter; a light combining means which is formed by integrating optical waveguide paths that extend from light emitting ends from the plurality of optical waveguide paths, has a light-output end face with a first cross-sectional area, optically combines the light beams so as to produce an optically combined light beam, and outputs the optically combined light beam from the light-output end face; and a transparent medium which has a light-entrance end face connected to the light-output end face of the light-combining means, where the light-entrance end face has a second cross-sectional area greater than the first cross-sectional area of the light-output end face.

Preferably, in the combined light source according to the first aspect of the present invention, the transparent medium has a core and a cladding, and is, for example, a light guide.

In order to accomplish the aforementioned object, the second aspect of the present invention is also provided. According to the second aspect of the present invention, there is provided a combined light source comprising: a plurality of light sources which emit light beams; a plurality of optical waveguides which the light beams emitted from the plurality of light sources enter; and a light-combining means which is formed by joining portions of the cores of the plurality of optical waveguides into a single core, has an output end, optically combines the light beams so as to produce a combined light beam, and outputs the combined light beam from the output end. In the above combined light source, the light-combining means has such a length as to uniformize the cross-sectional intensity distribution of the optically combined light during propagation of the optically, combined light through the light-combining means.

In the combined light sources according to the first and second aspects of the present invention, each of the plurality of optical waveguides may have a cladding as well as the core, and the light-combining means may be formed by joining substantially only the cores of the plurality of optical waveguides into a single core, or joining both of the cores and the claddings of the plurality of optical waveguides into a single optical waveguide.

In the combined light source according to the first aspect of the present invention, the light beams which enter the plurality of optical waveguides are optically combined in the light-combining means, and the combined light outputted from the light-combining means enters the transparent medium, so that the combined light outputted from the transparent medium has a uniform cross-sectional intensity distribution.

In addition, since the (second) cross-sectional area of the light-entrance end face is greater than the (first) cross-sectional area of the light-output end face, a portion of the combined light which leaks to the cladding, as well as the other portion of the combined light which propagates through the core of the light-combining means, can enter the transparent medium. Therefore, optical loss can be suppressed. Further, since the optical means such as a condensing lens is not used for the optical combining operation, and the light beams are optically combined in the light-combining means, the combined light outputted from the combined light sources according to the first and second aspects of the present invention can output stable combined light, and the cost of the optical means can be saved.

Furthermore, since the optical combining operation is performed in the light-combining means, the region in which the light beams are optically combined is not exposed to the atmosphere. Therefore, it is possible to prevent contamination which causes deterioration of the performance of the combined light source. On the other hand, in the conventional combined light sources where the optical means such as a condensing lens is used for the optical combining operation, the light-output end faces of optical waveguides and the light-entrance end face of the transparent medium are exposed to the atmosphere and contaminated, so that the performance of the conventional combined light sources deteriorate.

In the combined light source according to the second aspect of the present invention, the light-combining means has such a length as to uniformize the cross-sectional intensity distribution of the optically combined light during propagation of the optically combined light through the light-combining means. That is, the combined light source according to the second aspect of the present invention outputs the combined light having the uniform cross-sectional intensity distribution without use of a separate transparent medium. Therefore, it is possible to downsize the combined light source.

Brief Description of Drawings

FIG. lisa schematic view of a combined light source according to a first embodiment of the present invention.

FIGS . 2A to 2D are perspective views schematically illustrating representative stages in a process for producing an optical combiner which constitutes the combined light source according to the first embodiment. FIG. 3 is a cross-sectional side view schematically illustrating a cross section in the length direction of the combined light source according to the first embodiment.

FIGS .4A to 4D are cross-sectional views of the combined light source of FIG. 3 at representative positions. FIG .5 is a schematic view of a combined light source according to a second embodiment of the present invention.

Best Mode for Carrying Out the Invention

Preferred embodiments of the present invention are explained in detail below with reference to drawings .

First Embodiment

FIG.1 is a schematic view of a combined light source according to the first embodiment of the present invention. The combined light source 100 of FIG. 1 comprises light sources 1, lenses 2, multimode optical fibers (waveguides) 3, an optical combiner 4 (as the light-combining means) , and a rod integrator 5 (as the transparent medium) .

The light sources 1 are semiconductor lasers, light-emission diodes, or the like, and the multimode optical fibers 3 are made of quartz, glass, or plastic.

The lenses 2 are respectively arranged in the optical paths of light beams emitted from the light sources 1, and the light beams pass through the lenses 2, converge on the end faces of the multimode optical fibers 3, and are coupled to (enter) the multimode optical fibers 3.

Alternatively, the number of the light sources 1 may not be equal to the number of the multimode optical fibers 3. For example, the light sources 1 and the lenses 2 may be arranged so that a light beam emitted from one of the light sources 1 enters more than one of the multimode optical fibers 3 through more than one of the lenses 2.

The optical combiner 4 is formed by joining the cores in portions, near the output end, of the multimode optical fibers 3 into a single core, so that the light beams which enter the multimode optical fibers 3 are optically combined in the optical combiner 4 into an optically combined light beam, which enters the rod integrator 5. It is desirable that the cross-sectional area of the light-entrance end face of the rod integrator 5 be equal to or greater than the cross-sectional area of the light-output end face of the optical combiner 4. In this case, a portion of the combined light which leaks to the cladding, as well as the other portion of the combined light which propagates through the core of the light-combining means, can enter the rod integrator 5. Therefore, optical loss can be suppressed.

Although the rod integrator 5 is used as the transparent medium according to the first embodiment, the rod integrator 5 in the first embodiment may be replaced with any transparent medium which uniformizes the cross-sectional intensity distribution of the combined light entering the transparent medium and outputs the combined light with a uniform cross-sectional intensity distribution. Therefore, the transparent medium may be a large-diameter quartz fiber, a large-diameter plastic fiber, or the like, where the term "large-diameter" means a diameter which is equal to or greater than 100 micrometers. In addition, the transparent medium is not limited to the rod integrator having a rectangular parallelepiped shape, and any transparent medium having a round or quadrate cross section can be used.

Hereinbelow, a process for producing the optical combiner 4 is explained with reference to FIGS .2A to 2D, which are perspective views schematically illustrating representative stages in a process for producing an optical combiner which constitutes the combined light source according to the first embodiment.

In the first stage, the coating 31 in a predetermined portion A of each of a plurality of multimode optical fibers 3 is removed as illustrated in FIG. 2A. Then, the plurality of multimode optical fibers 3 are bundled, and the predetermined portions A of the multimode optical fibers 3 in which the coating 31 is removed are softened by heating the predetermined portions A so that the cores of the multimode optical fibers 3 in the heated portions are joined into a single core.

In the second stage, the bundle of the multimode optical fibers 3 containing the (single-core) portion joined as above are pulled from both ends so as to elongate the softened portion of the bundle as illustrated in FIG.2B. The diameter of the softenedportion of the bundle of the multimode optical fibers 3 is reduced by the elongation, so that a tapered structure is formed in the bundle of the multimode optical fibers 3 containing the joined portion. In the tapered structure, the diameter of the softened portion is smaller than the diameters of both ends of the bundle of the multimode optical fibers 3. Since the diameter of the softened portion is reduced, the confinement of light propagating through the softened portion is weakened. Therefore, it is possible to increase the mode field diameter. At this time, it is sufficient for the heated and softened portion of the bundle of the multimode optical fibers 3 to have a length of approximately 3 mm. In the case where the heated and softened portion of the bundle of the multimode optical fibers 3 has a length of approximately 3 to 20 mm, the heated and softened portion of the bundle of the multimode optical fibers 3 can form a slow taper structure by the elongation, so that the loss in the combined light can be reduced.

Next, as illustrated in FIG. 2C, the joined portion of the bundle of the multimode optical fibers 3 is cut at such a position that the numerical aperture NAi_nput and the cross-sectional area D_input at the cut surface 32 of the joined portion of the bundle of the multimode optical fibers 3 satisfy the relationship,

NAinput X Dinput ≤ NAoutput X D_Output/ ( 1 )

where NAoutput and D_output are respectively the numerical aperture and the cross-sectional area at the light-entrance end face of the rod integrator 5. Thereafter, the cut surface 32 (the light-output end face) of the joined portion of the bundle of the multimode optical fibers 3 is joined to the light-entrance end face of the rod integrator 5 by fusion or mechanical connection using a connector or the like, as illustrated in FIG. 2D. In the arrangement illustrated in FIG. 2D, the joined portion which is produced by the heating and elongation as above and is continuously connected to the respective multimode optical fibers 3 realizes the optical combiner 4 constituting the combined light source 100 according to the first embodiment.

FIG. 3 shows a cross section in the length direction of a portion of the combined light source 100 including the optical combiner 4 which is produced as explained above, and FIGS. 4A to 4D show cross sections of the portion of FIG. 3 at the positions which are respectively indicated in FIG. 3 by the dashed lines A, B, C, and D, where the cross sections are perpendicular to the length direction of the optical combiner 4.

At the position A, the multimode optical fibers 3 connected to the optical combiner 4 has a step-index structure in which a steplike change in the refractive index occurs at the boundary between each core and the cladding surrounding the core. The positions B, C, and D belong¹ to the aforementioned portion which is heated and elongated. Therefore, dopant atoms in the vicinity of the core-cladding boundary are diffused by heat so that the distribution of the refractive index becomes smooth. Further, when the outer diameter of the optical combiner 4 becomes small as illustrated in FIGS. 4C and 4D, light propagates through approximately the entire cross section of the optical combiner 4.

As explained above, the combined light source 100 according to the first embodiment is formed by making a bundle of the multimode optical fibers 3, joining the cores in the predetermined portion of the bundle of the multimode optical fibers 3 into a single core, cutting the joined portion so as to produce the cut surface 32 (the light-output end face) , and connecting the rod integrator 5 to the cut surface 32. Thus, the combined light source 100 can output combined light having a uniform cross-sectional intensity distribution. In addition, when the cross-sectional area of the light-entrance end face of the rod integrator 5 is equal to or greater than the cross-sectional area of the light-output end face 32 of the optical combiner 4, a portion of the combined light which leaks to the cladding, as well as the other portion of the combined light which propagates through the core of the optical combiner 4, can enter the rod integrator 5. Therefore, optical loss can be suppressed.

Further, since the optical means such as a condensing lens is not used for the optical combining operation, and the light beams are optically combined in the optical combiner 4, the combined light outputted from the combined light sources 100 according to the first embodiment can output stable combined light, and the cost of the optical means can be saved. Furthermore, since the optical combining operation is performed in the optical combiner 4, the region in which the light beams are optically combined is not exposed to the atmosphere. Therefore, it is possible to prevent contamination which causes deterioration of the performance of the combined light source. On the other hand, if the optical means such as a condensing lens is used for the optical combining operation, the light-output end faces of the multimode optical fibers 3 and the light-entrance end face of the rod integrator 5 will be exposed to the atmosphere and contaminated, so that the performance of the combined light sources will deteriorate.

Although the claddings, as well as the cores, in the near-end portions of the multimode optical fibers 3 are joined into the cladding in the combined light source 100 according to the first embodiment, alternatively, it is possible to join only the cores in the near-end portions of the multimode optical fibers 3 into a single core.

Second Embodiment

In the combined light source 100 according to the first embodiment, the rod integrator 5 is connected to the light-output end face 32 of the optical combiner 4. On the other hand, in the combined light source according to the second embodiment, the rod integrator 5 is not used, and an optical combiner 4a (the light-combining means) has the function of the transparent medium. FIG. 5 is a schematic view of the combined light source according to the second embodiment of the present invention. In FIG.5, elements and constituents which are equivalent to some elements or constituents in FIG. 1 are respectively indicated by the same reference numbers as FIG. 1, and descriptions of the equivalent elements or constituents are not repeated in the following explanations unless necessary.

The combined light source 200 of FIG. 5 comprises the light sources 1, the lenses 2, the multimode optical fibers 3, and the optical combiner 4a. The optical combiner 4a is produced in a similar manner to the optical combiner 4 in the combined light source 100 according to the first embodiment. The optical combiner 4a has a sufficient length to uniformize the cross-sectional intensity distribution of the combined light during propagation of the optically combined light through the optical combiner 4a. For example, the length of the optical combiner 4a is 5 millimeters or greater. That is, the combined light source 200 according to the second embodiment outputs combined light having a uniform cross-sectional intensity distribution without the connection of a separate transparent medium such as the rod integrator 5 to the light-output end face of the optical combiner 4a. Therefore, the combined light source 200 according to the second embodiment can be reduced in size.

Claims

CLAIMS 1. A combined light source comprising: a plurality of light sources which emit light beams; a plurality of optical waveguides having cores which said light beams emitted from said plurality of light sources enter; a light-combining means which is formed by joining portions of said cores of said plurality of optical waveguides into a single core, has a light-output end face with a first cross-sectional area, optically combines said light beams so as to produce an optically combined light beam, and outputs the optically combined light beam from the light-output end face; and a transparent medium which has a light-entrance end face connected to said light-output end face of said light-combining means, where the light-entrance end face has a second cross-sectional area greater than said first cross-sectional area of the light-output end face.

2. A combined light source according to claim 1, wherein said transparent medium is a light guide.

3. A combined light source comprising: a plurality of light sources which emit light beams; a plurality of optical waveguides which said light beams emitted from said plurality of light sources enter; and a light-combining means which is formed by joining portions of said cores of said plurality of optical waveguides into a single core, has an output end, optically combines said light beams so as to produce an optically combined light beam, and outputs the optically combined light beam from the output end; wherein said light-combining means has such a length as to uniformize a cross-sectional intensity distribution of said optically combined light during propagation of the optically combined light through the light-combining means.