WO2006129872A1

WO2006129872A1 - Combined light source

Info

Publication number: WO2006129872A1
Application number: PCT/JP2006/311376
Authority: WO
Inventors: Shinichi Shimotsu
Original assignee: Fujifilm Corporation
Priority date: 2005-05-31
Filing date: 2006-05-31
Publication date: 2006-12-07
Also published as: TW200704965A; TWI296721B; JP2006337399A

Abstract

In a combined light source (100): a plurality of light sources (1) emit light beams; the light beams propagate through a plurality of multimode optical fibers (3); and an optical combiner (4) is continuously joined to the plurality of multimode optical fibers (3), optically combines the light beams outputted from the plurality of multimode optical fibers (3), and outputs combined light.

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 multimode optical fibers .

Background Art

In the conventional systems, laser beams emitted from a plurality of light sources are optically combined by coupling the laser beams emitted from the plurality of light sources, at a light-entrance end face of an optical fiber arranged on the output side, by use of an optical means such as a condensing lens, as disclosed in Japanese Unexamined Patent PublicationNo.2004-273620.

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, conventionally, in the case 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 pluralityof 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 excitation laser beams which enter the plurality of multimode optical fibers . However, in the case where laser beams are combined by using an optical means such as a condensing lens, the light-entrance end face and the light-output end faces of optical fibers on the optical-means side are exposed to the atmosphere. Therefore, the light-entrance end face and the light-output end faces are likely to be contaminated. In addition, the cost of the optical means is unignorable.

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 (e.g., approximately 0.46) 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, when 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 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, byusingmultimode optical fibers without use of an optical means such as a condensing lens so as to output high-intensity combined light.

In order to accomplish the above object, the present invention is provided. According to the present invention, there is provided a combined light source comprising: a plurality of light sources which emit light beams; a plurality of multimode optical fibers through which the light beams propagate; and an optical combiner which is continuously joined to the plurality of multimode optical fibers, optically combines the light beams outputted from the plurality of multimode optical fibers, and outputs combined light.

Preferably, the combined light source according to the present invention may also have one or any possible combination of the following additional features (i) to (v) .

(i) The optical combiner may be realized by joining the cores of the plurality of multimode optical fibers into a single core in vicinities of the output ends of the plurality of multimode optical fibers. Alternatively, the optical combiner may be realized by an optical coupling device (i.e., an optical coupler).

(ii) The optical combiner may satisfy the condition,

(NAoutput/NAinput) ² > ( ∑A_input) /Aoutput ( D

where NAo_Utp_Ut is the numerical aperture at an output end of the optical combiner, Aoutput is the cross-sectional core area at the output end of the optical combiner, NAi_nput is the numerical aperture at each of input ends of the pluralityofmultimode optical fibers, and ΣA_input is the sum of cross-sectional core areas at the input ends of the plurality of multimode optical fibers.

(iii) The numerical aperture at the output end of the optical combiner is preferably smaller than 0.46, and is more preferably smaller than 0.22. In this case, the combined light source can output high-intensity combined light, i.e., it is possible to realize a high-intensity combined light source.

(iv) A lens may be formed integrally with each of the plurality of multimode optical fibers at the input end of the multimode optical fiber. Specifically, the lens may be a spherical

(convex) lens formed by machining of the fiber tip. Alternatively, the input end of each of the plurality of multimode optical fibers may be concavely shaped by machining an end face of the multimode optical fiber (which is originally perpendicular to the length direction of the multimode optical fiber) so that the central area of the end face is recessed. That is, it is possible to make the input end of each of the plurality of multimode optical fibers have a function of a concave lens. (v) The plurality of light sources may emit ultraviolet light beams. In this case, the present invention is remarkably effective in reducing contamination of the end faces of the optical fibers, compared with the conventional combined light sources using an optical means such as a condensing lens . The combined light source according to the present invention has the following advantages .

In the combined light source according to the present invention, the light beams emitted from the plurality of light sources are inputted into the plurality of multirαode optical fibers, and the light beams outputted from the plurality of multimode optical fibers are optically combined by the optical combiner, which is continuously joined to the plurality of multimode optical fibers. Therefore, no portion of the optical paths from the light-condensing points (at which the light beams emitted from the plurality of light sources are collected) to the output end of the optical combiner is exposed to the atmosphere, so that it is possible to solve the contamination problem which occurs in the conventional case where the optical means such as a condensing lens is used. That is, it is possible to minimize the portions of the optical paths exposed to the atmosphere in the combined light source, suppress deposition of contaminants on the optical fibers, and prevent performance deterioration of the combined light source. In particular, in the case where ultraviolet light beams are optically combined, gas in the optical paths in the atmosphere is intensely excited by the ultraviolet light beams, so that contaminants are seriously deposited on the optical fibers. However, the portions of the optical paths exposed to the atmosphere are minimized in the combined light source according to the present invention. Therefore, in the case where ultraviolet light beams are used, it is possible to remarkably suppress the deposition of contaminants, compared with the conventional combined light sources.

Further, in the case where light beams are optically combined by use of an optical means such as a condensing lens, it is difficult to stabilize the combined light since variations in the environment such as the temperature cause movement of light-condensing points at which light beams condense. However, the combined light source according to the present invention can output stable combined light, and therefore the reliability of the combined light source according to ^'the present invention is high. Further, since the optical means such as a condensing lens is not used, it is possible to simplify the construction of the combined light source, and save the cost of the optical means.

Brief Description of Drawings FIG.1 is a 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 a multimode optical combiner constituting 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 multimode optical combiner of FIG. 2D.

FIGS. 4A to 4D are cross-sectional views of the multimode optical combiner 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.

FIG. 6 is a magnified view of an input end of each of a plurality of multimode optical fibers constituting the combined light source according to the second embodiment.

FIG.7 is a schematic view of a combined light source according to a third embodiment of the present invention.

FIG. 8 is a magnified view of an input end of each of a plurality of multimode optical fibers constituting the combined light source according to the third embodiment.

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 3, an optical combiner 4, and an output-side optical fiber 5. 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 lenses 2. 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 output ends of the multimode optical fibers 3 are connected to the optical combiner 4, and a light beam obtained by optical combining in the optical combiner 4 is outputted from the output-side optical fiber 5. Alternatively, the output-side optical fiber 5 can be dispensed with. That is, the combined light beam may be directly outputted from the optical combiner 4.

The optical combiner 4 may be realized by an optical coupling device (i.e., an optical coupler). Alternatively, the optical combiner 4 may be realized by joining the cores of the multimode optical fibers 3 into a single core in vicinities of the output ends of the multimode optical fibers 3. That is, in the latter case, the joined portions of the multimode optical fibers 3 realize the optical combiner 4.

Hereinbelow, the above multimode optical combiner realized by joining the cores of the multimode optical fibers into a single core in vicinities of the output ends of the multimode optical fibers is explained. First, a process for producing the above multimode optical combiner is explained with reference to FIGS . 2A through 2D, which are perspective views schematically illustrating representative stages in the process for producing such a multimode optical combiner.

In the first stage, the coating 11 in a predetermined portion of each of a plurality of multimode optical fibers 10 is removed as illustrated in FIG.2A. Then, the plurality of multimode optical fibers 10 are bundled. Subsequently, the predetermined portions of the multimode optical fibers 10 in which the coating 11 is removed are softened by heating so that the cores of the multimode optical fibers 10 in the heated portions are joined into a single core.

In the second stage, the bundle of the multimode optical fibers 10 are pulled from both ends so as to elongate the softened portion of the bundle of the multimode optical fibers 10 as illustrated in FIG. 2B. The diameter of the softened portion of the bundle of the multimode optical fibers 10 is reducedby the elongation, so that a tapered structure is formed in the bundle of the multimode optical fibers 10. In the tapered structure, the diameter of the softened portion of the bundle of the multimode optical fibers 10 is smaller than the diameters of both ends of the bundle of the multimode optical fibers 10.

Next, the bundle of the multimode optical fibers 10 is cut so as to produce a multimode optical combiner 20 having the output end 22 as illustrated in FIG. 2C. That is, the cut surface of the bundle of the multimode optical fibers 10 becomes the output end 22 of the multimode optical combiner 20, which is to be joined to the input end of the output-side optical fiber 5. The output end 22 of the multimode optical combiner 20 is formed at such a position that the numerical aperture NAoutput and the cross-sectional core area Aoutput of themultimode optical combiner 20 at the output end 22 satisfy the relationship,

(NiWput/NAinput) ² > ( ∑A_inp_Ut) /Ao_Utput ( D

where NA_input is the numerical aperture at each of input ends of the plurality of multimode optical fibers 10, and ∑Ai_nput is the sum of cross-sectional core areas at the input ends of the plurality of multimode optical fibers 10.

In addition, in order to output high-intensity combined light, the numerical aperture NAo_Ut_put at the output end 22 of the multimode optical combiner 20 is low (preferably smaller than 0.46, and more preferably smaller than 0.22) .

Further, when the multimode optical combiner 20 is produced by bundling the plurality of multimode optical fibers 10 and joining the cores into a single core so as to satisfy the condition (1), it is possible to suppress the loss in the combined light.

Furthermore, when necessary, the output-side optical fiber 5 may be connected to the output end 22 of the multimode optical combiner 20 by fusion or the like as illustrated in FIG.2D. In this case, in the relationship (1), the value NZWput is the numerical aperture at the output end of the output-side optical fiber 5, and the value Aoutput is the cross-sectional core area at the output end of the output-side optical fiber 5. Alternatively, it is possible to prepare a multimode optical combiner which is produced in another way so as to satisfy the above relationship (1) , and simply connect such a multimode optical combiner to the output-side optical fiber 5.

FIG. 3 shows a cross section in the length direction of the multimode optical combiner 20 which is constructed as explained above, and FIGS . 4A to 4D show cross sections of the multimode optical combiner 20 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 multimode optical combiner 20.

At the position A, the multimode optical fibers 10 constituting the multimode optical combiner 20 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 in the multimode optical combiner 20. The positions B and C belong¹ to the aforementioned portion which is heated and elongated. Therefore, dopant atoms in the vicinityof 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 multimode optical combiner 20 becomes small as illustrated in FIG. 4C, light propagates through approximately the entire cross section of the multimode optical combiner 20. The present inventor has produced a sample of the above multimode optical combiner by bundling seven multimode optical fibers, joining the multimode optical fibers into a single core in a partial length of the bundle near the output end of the bundle, and connecting an output-side optical fiber to the output end, where the numerical aperture NAi_nput at the input end of each of the multimode optical fibers is 0.15, the core diameter Di_nput at the input end of each of the multimode optical fibers is 50 micrometers, the numerical aperture NZWput at the output end of the output-side optical fiber 5 is 0.22, and the core diameter D_out_put at the output end of the output-side optical fiber 5 is 185 micrometers. The present inventor has confirmed that seven light beams are optically combined into a single light beam by the above sample of the multimode optical combiner connected to the output-side optical fiber. As explained above, in the combined light source 100 according to the first embodiment, the light beams emitted from the light sources 1 are inputted into the multimode optical fibers 3 through the lenses 2, and are optically combined by the optical combiner 4. At this time, the entire optical paths from the light-condensing points at which the light beams emitted from the light sources 1 condense (i.e., the input ends of the multimode optical fibers 3) to the output end of the combined light are within the optical fibers . Therefore, it is possible to solve the contamination problem, which occurs in the portion of the conventional combined light sources in which the light beams are optically combined. That is, since the portions of the optical paths which are exposed to the atmosphere can be minimized, it is possible to suppress deposition of contaminants on the optical fibers, and therefore prevent deterioration of the performance of the combined light source 100. In particular, in the case where ultraviolet light beams are optically combined, gas in the optical paths in the atmosphere is intensely excited by the ultraviolet light beams, so that contaminants are seriously deposited on the optical fibers . However, the portions of the optical paths exposed to the atmosphere are minimized in the combined light source 100 according to the present embodiment. Therefore, in the case where ultraviolet light beams are used, it is possible to remarkably suppress the deposition of contaminants, comparedwith the conventional combined light sources .

Further, in the case where light beams are optically combined by use of an optical means such as a condensing lens, it is difficult to stabilize the combined light since variations in the environment such as the temperature cause movement of the light-condensingpoints at which light beams condense. However, the optical means such as a condensing lens is not used in the combined light source 100 according to the present embodiment. Therefore, the combined light source 100 can output stable combined light, and thus the reliability of the combined light source increases. Further, it is possible to simplify the construction of the combined light source, and save the cost of the optical means. Furthermore, since the numerical aperture at the output end of the optical combiner or the output-side optical fiber is smaller than 0.46 (more preferably smaller than 0.22), the combined light source 100 can output high-intensity combined light, i.e., it is possible to realize a high-intensity combined light source. Second Embodiment

In the combined light source 100 according to the first embodiment, the lenses 2 are arranged separately from the multimode optical fibers 3. On the other hand, in the combined light source according to the second embodiment, a lens is formed integrally with each of the plurality of multimode optical fibers at the input end of the multimode optical fiber by fiber-tip machining. 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.

The combined light source 100b of FIG. 5 comprises light sources 1, multimode optical fibers 3a, an optical combiner 4, and an output-side optical fiber 5. A spherical lens 2a is formed integrally with each of the multimode optical fibers 3a at the input end of the multimode optical fiber by fiber-tip machining.

Alternatively, the multimode optical fibers 3a can be produced by separately forming spherical lenses and the multimode optical fibers 3 (as in the first embodiment) , and joining the spherical lenses to the input ends of the multimode optical fibers 3, respectively.

FIG. 6 is a magnified view of the input end of each of the multimode optical fibers 3a constituting the combined light source

100a. Since each of the multimode optical fibers 3a has the spherical lens 2a at the input end, each of the multimode optical fibers 3a can receive the light beam from the corresponding one of the light sources 1 through a greater area. Therefore, the optical density in the vicinity of the area of the spherical lens 2a through which the light beam is received decreases, so that deposition of contaminants can be suppressed. In addition, in the above structure of the multimode optical fibers 3a, the light beams emitted from the light sources 1 are directly coupled to the multimode optical fibers 3 through the spherical lenses 2a, i.e. , the light-condensing points of the spherical lenses 2a are not exposed to the atmosphere. Therefore, the portions of the optical paths exposed to the atmosphere are minimized.

The output ends of the multimode optical fibers 3 are connected to the optical combiner 4, and a light beam obtained by optical combining in the optical combiner 4 is outputted from the output-side optical fiber 5. Alternatively, the output-side optical fiber 5 may be dispensed with. That is, the combined light may be directly outputted from the optical combiner 4.

The optical combiner 4 maybe realized by an optical coupling device (i.e., an optical coupler). Alternatively, the optical combiner 4 may be realized by joining the cores of the multimode optical fibers 3a into a single core in vicinities of the output ends of the multimode optical fibers 3a as in the first embodiment.

As explained above, since the portions of the optical paths which are exposed to the atmosphere can be minimized by forming the spherical lenses 2a integrally with the multimode optical fibers 3a (for example, by fiber-tip machining) , the portions of the optical paths which are exposed to the atmosphere can be minimized, and deposition of contaminants on the optical fibers can be suppressed. Thus, it is possible to prevent deterioration of the performance of the combined light source 100, so that the combined light source 100a according to the second embodiment can output stable combined light.

Third Embodiment

In the combined light source 100a according to the second embodiment, the lenses 2a are formed integrally with the plurality ofmultimode optical fibers at the input ends of themultimode optical fibers, for example, by fiber-tip machining. On the other hand, in the combined light source according to the third embodiment, the input end of each of the plurality of multimode optical fibers is concavely shaped by fiber-tip machining. FIG. 7 is a schematic view of the combined light source according to the third embodiment of the present invention. In FIG. 7, 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 .

The combined light source 100b of FIG. 7 comprises light sources 1, lenses 2, multimode optical fibers 3b, an optical combiner 4, and an output-side optical fiber 5. The input end face of each of the multimode optical fibers 3b (which is perpendicular to the length direction of the multimode optical fiber) is concavely shaped by fiber-tip machining as schematically illustrated in FIG.7. FIG. 8 is a magnified view of the input end of each of the multimode optical fibers 3b constituting the combined light source according to the third embodiment. As illustrated in FIG. 8, the input end face of each of the multimode optical fibers 3b is concavely machined so that the central area of the input end face is recessed, and the function of a concave lens 6 having a predetermined magnification power is realized at the input end of the multimode optical fiber. That is, the concave lens is formed integrally with each of the multimode optical fibers 3b at the input end of the multimode optical fiber. Since the input end of each of the multimode optical fibers 3a is formed as above, the pattern of the light beam condensed by each of the lenses 2 and inputted into the corresponding one of the multimode optical fibers 3b can be shaped, so that it is possible to increase the intensity of the combined light outputted from the combined light source 100b.

The output ends of the multimode optical fibers 3b are connected to the optical combiner 4, and a light beam obtained by optical combining in the optical combiner 4 is outputted from the output-side optical fiber 5. The optical combiner 4 maybe realized by an optical coupling device (i.e., an optical coupler). Alternatively, the optical combiner 4 may be realized by joining the cores of the multimode optical fibers 3 into a single core in vicinities of the output ends of the multimode optical fibers 3 as in the first embodiment.

The present inventor has produced a sample of the above multimode optical combiner. In the sample, the lens system constituted by the lenses 2 and the concave lenses 6 is arranged so that the spread of the light beam emitted from each of the light sources 1 is reduced by a factor of ten in the direction of the fast axis of each semiconductor laser and by a factor of three in the direction of the slow axis of each semiconductor laser. An output-side optical fiber 5 having a numerical aperture NZWpUt of 0.22 and a core diameter D_output of 50 micrometers at its output end is connected to the output end of the optical combiner 4 realized by sevenmultimode optical fibers 3b each having a numerical aperture NAi_nput of 0.10 and a core diameter D_input of 28 micrometers at its input end. The present inventor has confirmed that seven light beams are optically combined into a single light beam in the above sample.

Claims

CLAIMS 1. A combined light source comprising: a plurality of light sources which emit light beams; a plurality of multimode optical fibers through which said light beams propagate; and an optical combiner which is continuously joined to said plurality of multimode optical fibers, optically combines the lightbeams outputted fromthe plurality ofmultimode optical fibers, and outputs combined light.

2. A combined light source according to claim 1, wherein said plurality of multimode optical fibers respectively have cores and output ends, and said optical combiner is realized by joining the cores into a single core in vicinities of the output ends of the plurality of multimode optical fibers .

3. A combined light source according to either of claims 1 and 2, wherein said optical combiner satisfies a condition,

(NAoutput/NAinput) ² > ( ∑Ainpuϋ /Aoutput

where NAoutput is a numerical aperture at an output end of the optical combiner, Ao_Utp_Ut is a cross-sectional core area at the output end of the optical combiner, NAi_nput is a numerical aperture at each of input ends of the plurality of multimode optical fibers, and ∑Ai_nput is a sum of cross-sectional core areas at the input ends of the plurality of multimode optical fibers .

4. A combined light source according to either of claims 1 to 3, wherein said numerical aperture at an output end of the optical combiner is smaller than 0.46.

5. A combined light source according to either of claims 1 to 4, wherein a lens is formed integrally with each of the plurality of multimode optical fibers at an input end of said each of the plurality of multimode optical fibers •

6. A combined light source according to either of claims 1 to 5, wherein said plurality of light sources emit ultraviolet light beams .