JP7635936B2 - Optical branching device and optical branching method using same - Google Patents

Description

The present invention relates to an optical branching device and an optical branching method using the same.

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.

JP 2014-48447 A

The first method for constructing a multi-emission light source device that emits multiple laser beams is to use multiple light sources. However, this method has the problem that it is not possible to reduce the size, power consumption, or cost of the device.

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, which is optically branched using an optical branching device such as a waveguide structure, a fiber coupler, or a dielectric multilayer mirror. However, with this method, the branching ratio differs depending on the wavelength of the laser light, and therefore 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 the light intensity of the multiple laser lights after branching becomes non-uniform. In addition, the color of the incident light is not maintained, and different colors are mixed together.

The objective of the present invention is to provide an optical branching device in which the optical intensities of the multiple output beams after branching are uniform.

The present invention is an optical branching device that includes a noncircular core optical fiber having a core with a noncircular cross-sectional shape, and an optical branching means that branches light from the noncircular core optical fiber into multiple beams.

The present invention is an optical branching method for injecting light into the input end of the noncircular core optical fiber in the optical branching device of the present invention.

According to the present invention, by splitting the light from the noncircular core optical fiber into multiple beams using a beam splitter, the light intensities of the multiple output beams after splitting can be made uniform.

1 is a diagram illustrating a configuration of an optical branching device according to an embodiment. 1 is a cross-sectional view of an example of a non-circular core optical fiber. FIG. 2 is a cross-sectional view of another example of a non-circular core optical fiber. 1 is a diagram showing a configuration of a multi-emission light source apparatus using an optical branching device according to an embodiment; 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. 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. 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. 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. 11 is a graph showing the relationship between the incident angle of light at the input end of a rectangular core optical fiber and the output angle and coupling efficiency of light from the output end of an optical fiber bundle B.

The following describes the embodiments in detail.

Figure 1 shows an optical branching device 10 according to an embodiment. The optical branching device 10 according to the embodiment includes a noncircular core optical fiber 11 and an optical fiber bundle 12 (optical branching means) connected to the output end of the noncircular core optical fiber 11.

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.

The noncircular core 111 has a noncircular cross-sectional shape. Here, "noncircular" in this application refers to a shape in which the inscribed circle and the circumscribed circle do not coincide. 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 polygon 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 fluororesin.

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 materials 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.

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 a 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 an acrylic resin or a 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 a nylon-based resin or a fluororesin.

The noncircular core optical fiber 11 may have a support layer, for example made of quartz, between the cladding 112 and the jacket 113. Alternatively, the noncircular core optical fiber 11 may not have the jacket 113 and may have a means for fixing and holding the outer peripheral surface of the cladding 112 so as not to scratch it.

The optical fiber bundle 12 is composed of a bundle of multiple optical fibers 121. At the input end of the optical fiber bundle 12, the multiple 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 multiple optical fibers 121 has a core 121a and a clad 121b that covers 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 materials forming the core 121a and the clad 121b may be the same as those of the noncircular core optical fiber 11. The multiple optical fibers 121 may all have the same configuration, or may include ones with different configurations. Therefore, the multiple optical fibers 121 may have different cross-sectional shapes and sizes of the cores 121a. At the output end of the optical fiber bundle 12, multiple optical fibers 121 may branch out and be independent of each other.

The output end of the noncircular core optical fiber 11 and the input end of the optical fiber bundle 12 may be connected in close proximity by a connector, or may be connected by adhesive, or may be fusion spliced.

In the optical branching method using the optical branching device 10 according to the embodiment of the above configuration, when light is incident on the input 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, the noncircular cross-sectional shape of the noncircular core 111 promotes the multimode propagation of light, and light is emitted from the output end of the noncircular core optical fiber 11 in an output pattern in which the amount of light is uniformly spread spatially.

Then, the light of the 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 input end of the optical fiber bundle 12. At this time, the light is coupled to the multiple optical fibers 121 of the optical fiber bundle 12 with little difference in light amount. In addition, by promoting the multimode propagation mode of the light in the noncircular core optical fiber 11, the wavelength dependency of the emission pattern of the light amount of the light emitted from the output end is eliminated, and wavelength independence is realized even among the light branched and input to the multiple optical fibers 121 of the optical fiber bundle 12.

The light propagating through the multiple optical fibers 121 of the optical fiber bundle 12 is emitted as output light from each output end. At this time, the light intensity of the multiple output lights after branching that are output from the optical fiber bundle 12 is uniform. Therefore, according to the optical branching device 10 of the embodiment, by branching the light from the non-circular core optical fiber 11 into multiple lights by the optical fiber bundle 12, the light intensity of the multiple output lights after branching can be made uniform.

For example, when laser light having three wavelengths of red (R), green (G), and blue (B) mixed at a predetermined light amount ratio is input from the input end of the noncircular core optical fiber 11, even if there is a wavelength difference between the three RGB colors of the input laser light, each color of light is converted into a multi-mode when propagating through the noncircular core 111 whose cross-sectional shape is noncircular, 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 three RGB colors of light becomes very small, and the influence of the wavelength difference is suppressed. As a result, laser light in which each of the three RGB colors of light has been converted to have a spatially uniform light amount distribution is output from the output end of the noncircular core optical fiber 11.

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 input end of the optical fiber bundle 12, and is output from the multiple optical fibers 121 at the output end of the optical fiber bundle 12 as output light of the same color with uniform light intensity and maintaining the color of the input light to the noncircular core optical fiber 11. Even if the wavelength of the input 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 output 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 output end of the optical fiber bundle 12 is hardly affected.

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 around their axes and fusing them together. The noncircular core optical fiber 11 may have a tapered shape at the emission end side. The noncircular core optical fiber 11 may have a structure in which the edges 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 vibrating it.

Figure 3 shows a multi-emission light source device 20 using the optical branching device 10 according to the embodiment. This multi-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.

The multi-emission light source device 20 includes an optical branching device 10 according to an embodiment, a laser light source 21 provided on the input end side of the optical branching device 10, and a lens system 22 provided between them.

The laser light source 21 emits 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 even higher 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.

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 multi-modeizing the propagation mode of the laser light, 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.

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 as shown in FIG. 4A, 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 amount of light 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.

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 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 with an output pattern having 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.

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

Specific means for simultaneously irradiating the laser light from the laser light source 21 to 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 incident obliquely on the incident end of the noncircular core optical fiber 11 and the incident angle is 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 multi-mode, the former of these methods is preferred.

Specific methods for setting the angle of incidence of the laser light on the noncircular core optical fiber 11 within a predetermined range include a method of adjusting the focusing distance using a lens system 22, and a method of simply positioning the lens system 22 so that the optical axis of the focused laser light is tilted relative 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 at the same time, the lens system 22 is not necessarily required.

The multi-emission light source device 20 configured as described above can be made smaller, less expensive, and more energy efficient than a configuration using multiple laser light sources 21. In addition, since multiple emission lights with uniform light intensity are emitted, there is no need to adjust the multiple laser lights emitted from the multiple laser light sources to be uniform, as is the case with a configuration using multiple laser light sources.

Here, we explain the simulations performed using optical design and analysis software (Zemax OpticStudio, manufactured by Zemax).

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 between them was used. A simulation was performed in which 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 incident on the rectangular core optical fiber by varying the incident angle at the input end of the rectangular core optical fiber.

First, 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 was investigated. Figure 5 shows this relationship. The transmission efficiency is expressed as a percentage of the ratio of the intensity of light emitted from the rectangular core at the output end to the intensity of 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 follows, where _IMAX is the maximum light intensity and _Imin is the minimum light intensity in the light intensity distribution at the output end face:
In-plane distribution (%)={(I _MAX −I _min )/(I _MAX +I _min )}×100
It is a value expressed as:

According to Figure 5, the initial value of the transmission efficiency at an incident angle of 0° is 96%, 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 after region I, there is a tendency for the efficiency to decrease in proportion to the incident angle. The decrease in transmission efficiency in region II is thought 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 thought 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.

From these facts, in the relationship between the incidence angle of light at the input end of the rectangular core optical fiber and the transmission efficiency, a high uniformity in the amount of light can be obtained for the output light of the rectangular core optical fiber by _setting the incidence angle at the bending point between region I and the following region II as the critical incidence angle for _uniforming the amount of light θ _T and setting the incidence angle θ _op of light into the rectangular core optical fiber so as to satisfy the condition θ op ≧θ T. Such an operation mode is useful in applications such as optical sensors that require multiple light sources that emit light of the same wavelength with small differences in light amount.

The critical incident angle _θT for uniforming the amount of light at the bending point between Region I and Region II is defined as the incident angle at which the second-order differential equation of a quartic function that approximates the relationship between the incident angle of light at the incident end of the rectangular core optical fiber and the transmission efficiency takes an extreme value.

According to Fig. 5, in the above simulation, the critical incident angle _θT for uniforming the light amount is slightly smaller than 0.15. On the other hand, the numerical aperture (NA) calculated from the refractive indexes of the rectangular core and cladding of the rectangular core optical fiber is 0.2. Therefore, the critical incident angle _θT for uniforming the light amount is smaller than this numerical aperture (NA). This means that even at an incident angle smaller than the numerical aperture (NA), leakage of light from the rectangular core to the cladding occurs. This is thought to be due to the fact that, structurally, higher-order modes are more likely to occur in the rectangular core optical fiber than in the circular core optical fiber.

In addition, 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 account, it is possible to obtain a higher uniformity in the amount of light emitted from the rectangular core optical fiber.

Next, for optical fiber bundle A with a small core size and 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 this relationship. The coupling efficiency is the percentage of the amount of light output from the output end of the optical fiber bundle relative to the amount of light input to the input end of the rectangular core optical fiber.

According to Figures 6A and 6B, although there are quantitative differences in the exit angle and coupling efficiency of light from the exit end for optical fiber bundles A and B, which have different core sizes, it can be seen that the behavior is generally the same. In both cases of optical fiber bundles A and B, the coupling efficiency is very large 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 exit end of the rectangular core optical fiber, the in-plane distribution of the exit light is high, and therefore further uniformization of the light amount does not progress, and the exit light, whose light amount has not progressed to uniformization, is branched in the optical fiber bundle.

As a result of various studies, it has been found that in order to obtain output light with a further 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 as shown in the following formula (1), where L is the length of the rectangular core optical fiber and D is the maximum outer diameter.
θ ₁ =2.5×tan ⁻¹ (D/L) (1)

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.

The coupling efficiency is high and stable in an incident angle region Y following region X, and then attenuates in the following region Z from a certain incident angle _θ2 .

As a result of various studies, it was found that the incident angle _θ2 at which the coupling efficiency begins to attenuate in region Z is expressed by the following formula (2) using the critical incident angle _θT for uniforming the light amount, the maximum outer diameter D of the rectangular core optical fiber, and the core diameter d of the optical fiber constituting the optical fiber bundle.
θ ₂ = θ _T × (d/D) (2)

In an optical branching device intended to branch light in the visible light region, even an amount of light for which the coupling efficiency is half the maximum value is useful for display purposes. Taking this into consideration, in order to obtain a high coupling efficiency between a rectangular core optical fiber and an optical fiber bundle, it is preferable to set the incident angle θ _op of light at the incident end of the rectangular core optical fiber so as to satisfy the condition of the following formula (3), which includes a range in which the coupling efficiency is about 50% or more of the maximum value.
θ ₁ ≦θ _op ≦2.5×θ ₂ = θ ₃ (3)

However, from the viewpoint of reliably obtaining a high coupling efficiency between the rectangular core optical fiber and the optical fiber bundle, it is more preferable to set the incident angle θ _op of light at the incident end of the rectangular core optical fiber so as to satisfy the condition of the following formula (4).
θ ₁ ≦θ _op ≦×θ ₂ (4)

The emission angle of light from the output end of the optical fiber bundle increases as the incident angle of the light into the rectangular core optical fiber increases, but it shows a tendency to saturate in region Z and eventually settles at a constant value.

In the above embodiment, an optical fiber bundle 12 is used as the optical branching means, but this is not limited to this, and a multi-core fiber having multiple cores, a lens, etc. can also be used.

The present invention is useful in the technical field of optical branching devices and optical branching methods using the same.

10 Optical branching device 11 Non-circular core optical fiber 111 Non-circular core 112 Cladding 113 Jacket 12 Optical fiber bundle (optical branching means)
121 Optical fiber 121a Core 121b Clad 20 Multi-emission light source device 21 Laser light source 22 Lens system L Laser light

Claims (11)

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. The optical branching device according to claim 1,
An optical branching device in which the cross-sectional shape of the core is rectangular. 3. The optical branching device according to claim 1,
An optical branching device in which the optical branching means is an optical fiber bundle. 2. The optical branching device according to claim 1,
The non-circular core optical fiber has a twisted structure along its length. 2. 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. 2. The optical branching device according to claim 1,
The non-circular core optical fiber is an optical branching device in which a portion on the output end side is formed in a tapered shape. 2. 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. 2. The optical branching device according to claim 1,
An optical branching device, wherein a bend is imparted to the non-circular core optical fiber. 2. 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. 1. An optical branching method for an optical branching device including a noncircular core optical fiber having a core with a noncircular cross-sectional shape and an optical branching means for branching light from the noncircular core optical fiber into a plurality of beams, comprising the steps of:
a light branching method for setting an incident angle θ op of light into the noncircular core optical fiber so as to satisfy the condition θ _op ≧ θ _T , where 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 when the second-order differential equation takes an extreme value is defined as a critical incident _angle θ _T for equalizing the amount of light. 1. An optical branching method for an optical branching device including a noncircular core optical fiber having a core with a noncircular cross-sectional shape and an optical branching means for branching light from the noncircular core optical fiber into a plurality of beams, comprising the steps of:
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 a light amount uniforming critical incident angle _θT . In addition, the length of the noncircular core optical fiber is defined as L, the maximum outer diameter is defined as D, and the diameter of the core of the optical fiber constituting the optical fiber bundle serving as the light branching means is defined as d. When _θ1 and _θ2 are defined by the following equations (1) and (2), respectively:
θ ₁ =2.5×tan ⁻¹ (D/L) (1)
θ ₂ = θ _T × (d/D) (2)
An optical branching method, comprising: setting an incident angle θ _op of light into the non-circular core optical fiber so as to satisfy the condition of the following formula (3).
θ ₁ ≦θ _op ≦2.5×θ ₂ (3)

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