Optical branching device and optical branching method using same

Abstract

To provide an optical branching device capable of making a plurality of emitted lights having been branched uniform in ratio of light quantities at respective wavelengths.SOLUTION: An optical branching device 10 comprises: a non-circular core optical fiber 11 which has a core 111 sectioned in a non-circular shape; and optical branching means 12 which branches a light from the non-circular core optical fiber 11 into a plurality.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical branching device and an optical branching method using the same. [Background technology]

[0002] Optical fibers having a core with a non-circular cross-sectional shape are known. For example, Patent Document 1 discloses an optical fiber for transmitting laser light from a laser light source, in which a first fiber having a core with a circular cross-sectional shape is connected to a second fiber having a core with a rectangular cross-sectional shape. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-48447 A Summary of the Invention [Problem to be solved by the invention]

[0004] By the way, as a means for constructing a multiple-emission light source device that emits multiple laser beams, first there is a method using multiple light sources. However, this method has a problem in that it is not possible to reduce the size, power consumption, and cost of the device.

[0005] There is also a method in which a single light source is used, and the laser light emitted from the light source is used as the incident light, and the laser light is optically branched by an optical branching device such as a waveguide structure, a fiber coupler, or a dielectric multilayer mirror. However, this method has a problem in that the branching ratio differs depending on the wavelength of the laser light, and the light intensity of the multiple laser lights after branching becomes non-uniform. For this reason, for example, if the incident light is a visible laser light in which three wavelengths of red (R), green (G), and blue (B) are mixed at a predetermined light amount ratio, the branching ratio of the wavelengths differs for each color, and therefore, in the multiple laser lights after branching, not only does the light intensity become non-uniform, but the color of the incident light is not maintained, and different colors are mixed together.

[0006] An object of the present invention is to provide an optical branching device in which the optical intensities of a plurality of output beams after branching are uniform. [Means for solving the problem]

[0007] The present invention is an optical branching device comprising a noncircular core optical fiber having a core with a noncircular cross-sectional shape, and optical branching means for branching light from the noncircular core optical fiber into a plurality of beams.

[0008] The present invention is an optical branching method for inputting light from an input end of the non-circular core optical fiber in the optical branching device of the present invention. Effect of the Invention

[0009] According to the present invention, by splitting light from a non-circular core optical fiber into a plurality of beams by the optical branching means, the optical intensities of the plurality of output beams after splitting can be made uniform. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a configuration of an optical branching device according to an embodiment. [Figure 2A] 1 is a cross-sectional view of an example of a non-circular core optical fiber. [Figure 2B]FIG. 2 is a cross-sectional view of another example of a non-circular core optical fiber. [Diagram 3] 1 is a diagram showing a configuration of a multi-emission light source apparatus using an optical branching device according to an embodiment; [Figure 4A] 1 is an explanatory diagram showing the operation when laser light is incident only on the core of a non-circular core optical fiber. FIG. [Figure 4B] 1 is an explanatory diagram showing the operation when laser light is incident on both the core and the cladding of a non-circular core optical fiber. FIG. [Diagram 5] 1 is a graph showing the relationship between the incident angle of light at the incident end of a rectangular core optical fiber and the transmission efficiency and in-plane distribution. [Figure 6A] 1 is a graph showing the relationship between the incident angle of light at the incident end of a rectangular core optical fiber and the exit angle and coupling efficiency of light from the exit end of an optical fiber bundle A. [Figure 6B] 13 is a graph showing the relationship between the incident angle of light at the incident end of a rectangular core optical fiber and the exit angle and coupling efficiency of light from the exit end of an optical fiber bundle B. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The embodiments will be described in detail below.

[0012] 1 shows an optical branching device 10 according to an embodiment. The optical branching device 10 according to the embodiment includes a non-circular core optical fiber 11 and an optical fiber bundle 12 (optical branching means) connected to an output end of the non-circular core optical fiber 11.

[0013] The noncircular core optical fiber 11 has a noncircular core 111 , a cladding 112 that covers the noncircular core 111 , and a jacket 113 that further covers the cladding 112 .

[0014] The noncircular core 111 has a noncircular cross-sectional shape. Here, the term "noncircular" in the present application refers to a shape in which the inscribed circle and the circumscribed circle do not coincide with each other. Specifically, the cross-sectional shape of the noncircular core 111 may be, for example, a square as shown in FIG. 2A or a rectangle as shown in FIG. 2B. The cross-sectional shape of the noncircular core 111 may be a polygonal shape such as a triangle or a pentagon, a D-shape, or the like. The maximum outer diameter of the noncircular core optical fiber 11 is preferably 50 μm or more and 3000 μm or less, more preferably 100 μm or more and 1000 μm or less. The length L of the noncircular core optical fiber 11 is preferably 10 mm or more and 300 mm or less, more preferably 50 mm or more and 200 mm or less. Examples of materials for forming the noncircular core 111 include undoped quartz, quartz doped with a dopant such as Ge that increases the refractive index, acrylic resin, and fluorine-based resin.

[0015] The cladding 112 preferably has a circular outer shape, but may have a non-circular outer shape such as a rectangle. The outer diameter of the cladding 112 is set in consideration of mechanical strength in relation to the size of the non-circular core 111, and is, for example, 125 μm to 2000 μm. The material from which the cladding 112 is formed has a lower refractive index than the material from which the non-circular core 111 is formed. Examples of the material from which the cladding 112 is formed include quartz, acrylic resin, and silicone-based resin doped with a dopant such as B or F that lowers the refractive index.

[0016] The outer diameter of the jacket 113 is, for example, 250 μm or more and 4000 μm or less. Examples of materials for forming the jacket 113 include silicone-based resin, acrylic resin, and fluororesin. When the clad 112 is made of quartz, the material for forming the jacket 113 is preferably silicone-based resin from the viewpoint of obtaining excellent adhesion with the quartz, and when the clad 112 is made of resin, the material for forming the jacket 113 is preferably acrylic resin or fluororesin. From the viewpoint of confining light within the clad 112, the material for forming the jacket 113 preferably has a lower refractive index than the material for forming the clad 112. The jacket 113 may have a two-layer structure with an outer layer of nylon-based resin or fluororesin.

[0017] The non-circular core optical fiber 11 may have a support layer made of, for example, quartz between the cladding 112 and the jacket 113. Alternatively, the non-circular core optical fiber 11 may not have the jacket 113, and may have a means for fixing and holding the outer circumferential surface of the cladding 112 so as to prevent the cladding 112 from being scratched.

[0018] The optical fiber bundle 12 is composed of a bundle of a plurality of optical fibers 121. At the input end of the optical fiber bundle 12, the plurality of optical fibers 121 are arranged so as to obtain high coupling efficiency with the noncircular core optical fiber 11, for example, in a close-packed manner. Each of the plurality of optical fibers 121 has a core 121a and a clad 121b covering the core 121a. The cross-sectional shape of the core 121a may be either circular or noncircular. The diameter of the core 121a and the outer diameter of the clad 121b are set so as to obtain high coupling efficiency with the noncircular core optical fiber 11. The core 121a and the clad 121b may be formed from the same material as that of the noncircular core optical fiber 11. The plurality of optical fibers 121 may all have the same configuration, or may include optical fibers having different configurations. Therefore, the plurality of optical fibers 121 may have different cross-sectional shapes and sizes of the cores 121a. At the output end side of the optical fiber bundle 12, a plurality of optical fibers 121 may branch out and be independent from each other.

[0019] The output end of the non-circular core optical fiber 11 and the input end of the optical fiber bundle 12 may be connected in close proximity to each other by a connector, or may be connected by an adhesive, or may be fusion spliced.

[0020] In the optical branching method using the optical branching device 10 according to the embodiment having the above configuration, when light is incident from the incident end of the noncircular core optical fiber 11, the incident light propagates through the noncircular core 111 of the noncircular core optical fiber 11. At this time, since the cross-sectional shape of the noncircular core 111 is noncircular, the multimode propagation mode of the light is promoted, and light is emitted from the exit end of the noncircular core optical fiber 11 in an exit pattern in which the amount of light is spatially spread uniformly.

[0021] Next, the light of this emission pattern with the spatially uniform light amount is incident on and propagates through the cores 121a of the multiple optical fibers 121 at the entrance end of the optical fiber bundle 12. At this time, the light is coupled with little difference in light amount to the multiple optical fibers 121 of the optical fiber bundle 12. Furthermore, by promoting the multimode propagation mode of light in the non-circular core optical fiber 11, the wavelength dependency of the emission pattern of the light amount of light emitted from the exit end is eliminated, and as a result, wavelength independence is realized even among the light branched and incident on the multiple optical fibers 121 of the optical fiber bundle 12.

[0022] Then, the light propagated through the multiple optical fibers 121 of the optical fiber bundle 12 is output as output light from each output end. At this time, the optical intensities of the multiple output lights after branching output from the optical fiber bundle 12 are uniform. Therefore, according to the optical branching device 10 according to the embodiment, by branching the light from the non-circular core optical fiber 11 into multiple lights by the optical fiber bundle 12, the optical intensities of the multiple output lights after branching can be made uniform.

[0023] Furthermore, for example, when laser light in which three wavelengths of red (R), green (G), and blue (B) are mixed at a predetermined light amount ratio is incident from the incident end of the noncircular core optical fiber 11, even if there is a wavelength difference between the three RGB colors of the incident laser light, the light of each color is converted into a multimode when propagating through the noncircular core 111 having a noncircular cross-sectional shape, and propagates while maintaining an intensity distribution that is strongly dependent on the cross-sectional shape of the noncircular core 111. At this time, the difference in the intensity distribution of the light of the three RGB colors becomes very small, and the influence of the wavelength difference is suppressed. As a result, laser light in which each of the light of the three RGB colors has been converted to have a spatially uniform light amount distribution is emitted from the exit end of the noncircular core optical fiber 11.

[0024] Then, the laser light, each of which has a spatially uniform light amount distribution of the three colors of RGB, branches and enters and propagates into each core 121a of the multiple optical fibers 121 at the entrance end of the optical fiber bundle 12, and is output from the multiple optical fibers 121 at the exit end of the optical fiber bundle 12 as output light of the same color with uniform light intensity and maintaining the color of the light incident on the noncircular core optical fiber 11. Even if the wavelength of the incident light fluctuates, there is no significant change in the uniformity of the output pattern of the light amount of the multimode light output from the exit end of the noncircular core optical fiber 11, and therefore the uniformity of the output light output from the multiple optical fibers 121 at the exit end of the optical fiber bundle 12 is hardly affected.

[0025] From the viewpoint of achieving a multi-mode light propagation mode, the noncircular core optical fiber 11 may be twisted along the length direction. The noncircular core optical fiber 11 may be divided into a plurality of pieces in the length direction, which are then recombined by rotating the pieces about their axes and fusing them. The noncircular core optical fiber 11 may have a tapered shape at the output end side thereof. The noncircular core optical fiber 11 may have a structure in which the sides forming the cross-sectional shape of the noncircular core 111 of the noncircular core optical fiber 11 are formed with unevenness or in a corrugated shape. The noncircular core optical fiber 11 may be bent. The noncircular core optical fiber 11 may be provided with a means for applying vibration to the noncircular core optical fiber 11.

[0026] 3 shows a multiple-emission light source device 20 using the optical branching device 10 according to the embodiment. The multiple-emission light source device 20 is used in fields such as displays, lighting, visible light communications, optical sensing, and medicine that utilize red (R), green (G), and blue (B) visible laser light.

[0027] The multiple-emission light source apparatus 20 includes the optical branching device 10 according to the embodiment, a laser light source 21 provided on the incident end side of the optical branching device 10, and a lens system 22 provided therebetween.

[0028] The laser light source 21 emits a laser light to be incident on the optical branching device 10 according to the embodiment. Examples of the laser light source 21 include a semiconductor laser, a solid-state laser, and a gas laser. The laser light source 21 may be composed of a single laser or a plurality of lasers. Therefore, the laser light source 21 may be composed of three lasers, a laser that emits red (R) laser light, a laser that emits green (G) laser light, and a laser that emits blue (B) laser light. The output of the laser light source 21 is, for example, several mW to several W, and a light source with a larger output can be applied by providing a cooling means. The laser light emitted by the laser light source 21 may be either continuous light or pulsed light.

[0029] The lens system 22 focuses the laser light from the laser light source 21 and makes it incident on the optical branching device 10 according to the embodiment. At this time, from the viewpoint of efficiently making the propagation mode of the laser light multimode, it is preferable that the lens system 22 focuses the laser light from the laser light source 21 and makes it incident on both the noncircular core 111 and the cladding 112 of the noncircular core optical fiber 11 at the same time.

[0030] 4A, when the laser light L from the laser light source 21 is incident only on the noncircular core 111 of the noncircular core optical fiber 11, if the length of the noncircular core optical fiber 11 is short, the electric field intensity distribution of the incident laser light L gathers near the center of the noncircular core 111, and the laser light L reaches the output end with a small number of repeated reflections at the interface between the noncircular core 111 and the clad 112, so that the multimode propagation mode of the laser light L does not progress sufficiently. As a result, there is a risk that the laser light L having an output pattern including a portion with an uneven light amount is emitted from the output end of the noncircular core optical fiber 11 to a portion corresponding to the vicinity of the interface between the noncircular core 111 and the clad 112.

[0031] On the other hand, when the laser light L from the laser light source 21 is simultaneously incident on both the noncircular core 111 and the clad 112 of the noncircular core optical fiber 11 as shown in Fig. 4B, the electric field intensity distribution of the incident laser light L spreads not only to the noncircular core 111 but also to the clad 112, and the component with a large incident angle therein is reflected many times at the interface between the noncircular core 111 and the clad 112, and even if the length of the noncircular core optical fiber 11 is short, the number of propagation modes increases and multimodes progress. As a result, the laser light L is emitted from the output end of the noncircular core optical fiber 11 in an output pattern with a uniform light amount, including a portion corresponding to the vicinity of the interface between the noncircular core 111 and the clad 112. In this case, the length of the noncircular core optical fiber 11 can be preferably 10 mm or more and 300 mm or less, more preferably 50 mm or more and 200 mm or less, so that the device can be made smaller and easier to manufacture.

[0032] In addition, if the material forming the jacket 113 has a lower refractive index than the material forming the cladding 112, not only the light incident on the cladding 112 but also the leaked light from the noncircular core 111 propagates while being reflected at the interface between the cladding 112 and the jacket 113. This leaked light is likely to undergo mode conversion because the noncircular core 111 is present in the cladding 112, and is likely to progress to multimode. As a result, the multimode laser light propagating through the noncircular core 111 and the multimode leaked laser light propagating through the cladding 112 are combined from the output end of the noncircular core optical fiber 11, and a laser light having an output pattern with very high uniformity in the amount of light is output.

[0033] Specific means for simultaneously making the laser light from the laser light source 21 incident on both the noncircular core 111 and the cladding 112 of the noncircular core optical fiber 11 include, for example, a method in which the laser light is made incident obliquely on the incident end of the noncircular core optical fiber 11 and the incident angle is set within a predetermined range, and a method in which the focused size of the laser light on the noncircular core optical fiber 11 is made larger than the noncircular core 111 of the noncircular core optical fiber 11. From the viewpoint of efficiently making the light propagation mode multimode, the former method among these is preferable.

[0034] More specific methods for setting the angle of incidence of the laser light into the noncircular core optical fiber 11 within a predetermined range include a method of adjusting the focusing distance by the lens system 22, and a method of simply arranging the lens system 22 so as to tilt the optical axis of the focused laser light with respect to the axis of the noncircular core optical fiber 11. Note that if the laser light from the laser light source 21 is directly incident on both the noncircular core 111 and the cladding 112 of the noncircular core optical fiber 11 simultaneously, the lens system 22 is not necessarily required.

[0035] The multiple emission light source device 20 configured as above can achieve smaller size, lower cost, and lower power consumption compared to a configuration using multiple laser light sources 21. In addition, since multiple emission light beams with uniform light intensity are emitted, there is no need to adjust the multiple laser light beams emitted from the multiple laser light sources to be uniform, as in a configuration using multiple laser light sources.

[0036] Here, a simulation performed using optical design and analysis software (Zemax OpticStudio, manufactured by Zemax, Inc.) will be described.

[0037] In the simulation, a model of an optical branching device in which a rectangular core optical fiber (non-circular core optical fiber) and an optical fiber bundle are arranged with a gap of 10 μm was used, and light having a divergence angle according to the incident angle and a unimodal Gaussian-shaped intensity distribution in both the horizontal and vertical directions was simulated by varying the incident angle at the input end of the rectangular core optical fiber.

[0038] First, we investigated the relationship between the incident angle of light at the input end of the rectangular core optical fiber and the transmission efficiency and in-plane distribution. Figure 5 shows this relationship. The transmission efficiency is expressed as a percentage of the ratio of the intensity of the light emitted from the rectangular core at the output end to the intensity of the light incident on the rectangular core (non-circular core) at the input end of the rectangular core optical fiber. The in-plane distribution is expressed as the maximum light intensity in the light intensity distribution on the output end face. MAX and the minimum light intensity is I min When In-plane distribution (%)={(I MAX -I min ) / (I MAX +I min )}×100 It is a value expressed as:

[0039] According to Fig. 5, the initial value of the transmission efficiency is 96% at an incident angle of 0°, and in region I where the incident angle is relatively small, there is almost no change even when the incident angle increases, but in region II where the incident angle is relatively large following region I, there is a tendency for the efficiency to decrease in proportion to the incident angle. The decrease in the transmission efficiency in region II is considered to be due to the promotion of leakage of light from the rectangular core to the cladding. The in-plane distribution also shows a similar decreasing trend to the transmission efficiency in region II. This means that the amount of light emitted from the rectangular core optical fiber becomes more uniform. The decrease in the in-plane distribution is considered to be due to the spread of the electric field distribution of light to the outside of the rectangular core due to leakage of light from the rectangular core to the cladding.

[0040] From these facts, in the relationship between the incident angle of light at the input end of a rectangular core optical fiber and the transmission efficiency, the incident angle at the bending point between region I and the following region II is defined as the critical incident angle θ T The incident angle of light into the rectangular core optical fiber is θ op θ op ≧θ T By setting the operation mode so as to satisfy the above condition, it is possible to obtain high uniformity of the light intensity of the light emitted from the rectangular core optical fiber. Such an operation mode is useful for applications such as optical sensors that require multiple light sources that emit light of the same wavelength with small differences in light intensity.

[0041] The critical angle of incidence θ for uniform light intensity at the turning point between region I and region II T is defined as the angle of incidence when the second-order differential equation of the relationship between the angle of incidence of light at the input end of a rectangular core optical fiber and the transmission efficiency is approximated by a fourth-order function and takes an extreme value.

[0042] According to FIG. 5, in the above simulation, the critical incident angle θ T is slightly smaller than 0.15. On the other hand, the numerical aperture (NA) of a rectangular core optical fiber calculated from the refractive index of the rectangular core and cladding is 0.2. Therefore, the critical incident angle θ Tis smaller than this numerical aperture (NA). This means that even at an incident angle smaller than the numerical aperture (NA), light leakage from the rectangular core to the cladding occurs. This is thought to be due to the fact that, structurally, a rectangular core optical fiber is more likely to generate higher-order modes than a circular core optical fiber.

[0043] Moreover, undulations are observed in the change in the in-plane distribution. This undulation is due to the combination of the incident angle and the structure of the rectangular core optical fiber. Therefore, by setting the incident angle taking this undulation into consideration, it is possible to obtain a higher uniformity in the amount of light emitted from the rectangular core optical fiber.

[0044] Next, for each of the optical fiber bundle A with a small core size and the optical fiber bundle B with a large core size, the relationship between the incident angle of light at the input end of the rectangular core optical fiber, the output angle of light from the output end of the optical fiber bundle, and the coupling efficiency was investigated. Figures 6A and 6B show the relationship. The coupling efficiency is the percentage of the amount of light output from the output end of the optical fiber bundle to the amount of light input to the input end of the rectangular core optical fiber.

[0045] 6A and 6B, the optical fiber bundles A and B, which have different core sizes, have quantitative differences in the output angle and coupling efficiency of light from the output end, but their behavior is generally the same. In both optical fiber bundles A and B, the coupling efficiency is very high in region X, where the incident angle is small. However, in region X, the light propagates through the rectangular core optical fiber while maintaining the incident beam shape. As a result, at the output end of the rectangular core optical fiber, the in-plane distribution of the output light is high, and therefore the light amount is not further uniformized, and the output light with the light amount not yet uniformized is branched in the optical fiber bundle.

[0046] As a result of various studies, it was found that in order to obtain output light with a more uniform light amount at the output ends of the optical fiber bundles A and B, it is preferable to set the angle of incidence of light into the rectangular core optical fiber to be equal to or larger than θ1 shown in the following formula (1), where L is the length of the rectangular core optical fiber and D is the maximum outer diameter. θ1=2.5×tan -1 (D / L) (1)

[0047] The length L of the rectangular core optical fiber is preferably 10 mm or more and 300 mm or less, more preferably 50 mm or more and 200 mm or less. The maximum outer diameter D is preferably 50 μm or more and 3000 μm or less, more preferably 100 μm or more and 1000 μm or less.

[0048] The coupling efficiency is high and stable in region Y of the incident angle subsequent to region X, and then attenuates in the subsequent region Z from a certain incident angle θ2.

[0049] As a result of various investigations, it was found that the incident angle θ2 at which the coupling efficiency starts to decay in the region Z is the critical incident angle θ T It was found that, using the maximum outer diameter D of the rectangular core optical fiber, and the diameter d of the core of the optical fiber constituting the optical fiber bundle, it can be expressed by the following formula (2). θ2=θ T ×(d / D) (2)

[0050] In an optical branching device intended to branch light in the visible light region, even an amount of light with a coupling efficiency that is half the maximum value is useful for display purposes. Considering this point, in order to obtain a high coupling efficiency between a rectangular core optical fiber and an optical fiber bundle, the incident angle θ of light at the input end of the rectangular core optical fiber is op is preferably set so as to satisfy the condition of the following formula (3), which includes a range in which the coupling efficiency is approximately 50% or more of the maximum value. θ1≦θ op ≦2.5×θ2=θ3(3)

[0051] However, from the viewpoint of reliably obtaining a high coupling efficiency between the rectangular core optical fiber and the optical fiber bundle, the incident angle θ of light at the incident end of the rectangular core optical fiber is op It is more preferable to set so as to satisfy the condition of the following formula (4). θ1≦θ op ≦×θ2(4)

[0052] It can be seen that the emission angle of light from the output end of the optical fiber bundle increases as the incident angle of light into the rectangular core optical fiber increases, but shows a tendency to saturate in region Z and eventually settles at a constant value.

[0053] In the above embodiment, the optical fiber bundle 12 is used as the light branching means, but the present invention is not limited to this, and a multi-core fiber having multiple cores, a lens, or the like may also be used. [Industrial Applicability]

[0054] The present invention is useful in the technical fields of an optical branching device and an optical branching method using the same. [Explanation of symbols]

[0055] 10 Optical branching device 11 Noncircular core optical fiber 111 Non-circular core 112 Clad 113 Jacket 12 Optical fiber bundle (optical branching means) 121 Optical Fiber 121a Core 121b Clad 20 Multiple output light source device 21 Laser light source 22 Lens system L Laser light

Claims

1. a noncircular core optical fiber having a core having a noncircular cross-sectional shape and a cladding having a circular cross-sectional outer shape covering the core; an optical branching means for branching light from the non-circular core optical fiber into a plurality of beams; An optical branching device comprising:

2. 2. The optical branching device according to claim 1, An optical branching device in which the cross-sectional shape of the core is rectangular.

3. 3. The optical branching device according to claim 1, An optical branching device in which the optical branching means is an optical fiber bundle.

4. The optical branching device according to claim 1, The non-circular core optical fiber has a twisted structure along its length.

5. The optical branching device according to claim 1, The non-circular core optical fiber is divided into a plurality of pieces in the length direction, which are rotated around their axes and fused to be recombined into an optical branching device.

6. The optical branching device according to claim 1, An optical branching device in which the non-circular core optical fiber is formed so that the portion on the output end side is tapered.

7. The optical branching device according to claim 1, The non-circular core optical fiber is an optical branching device in which unevenness or waves are formed on the sides that form the cross-sectional shape of the non-circular core.

8. The optical branching device according to claim 1, An optical branching device, wherein a bend is imparted to the non-circular core optical fiber.

9. The optical branching device according to claim 1, An optical branching device comprising a means for applying vibration to the non-circular core optical fiber.

10. An optical branching method for inputting light from an input end of a noncircular core optical fiber in an optical branching device comprising a noncircular core optical fiber having a core with a noncircular cross-sectional shape and optical branching means for branching light from the noncircular core optical fiber into a plurality of beams, comprising: The relationship between the incident angle of light at the incident end of the noncircular core optical fiber and the transmission efficiency is approximated by a fourth-order function, and the incident angle at which the second-order differential equation takes an extreme value is defined as the light amount uniforming critical incident angle θ Ｔ When the incident angle θ of light into the non-circular core optical fiber is ｏｐ θ ｏｐ ≧θ Ｔ An optical branching method that is set to satisfy the above condition.

11. An optical branching method for inputting light from an input end of a noncircular core optical fiber in an optical branching device comprising a noncircular core optical fiber having a core with a noncircular cross-sectional shape and optical branching means for branching light from the noncircular core optical fiber into a plurality of beams, comprising: The relationship between the incident angle of light at the incident end of the noncircular core optical fiber and the transmission efficiency is approximated by a fourth-order function, and the incident angle at which the second-order differential equation takes an extreme value is defined as the light amount uniforming critical incident angle θ Ｔ In addition, the length of the noncircular core optical fiber is L, the maximum outer diameter is D, and the diameter of the core of the optical fiber constituting the optical fiber bundle that is the optical branching means is d, and θ is calculated by the following formulas (1) and (2), respectively. １ and θ ２ When you define i １ =2.5×tan －１ (D / L) (1) i ２ =θ Ｔ ×(^ / D) (2) The incident angle θ of light into the non-circular core optical fiber ｏｐ so as to satisfy the condition of the following formula (3). i １ ≦θ ｏｐ ≦2.5×θ ２ (3)

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