Mode converter and optical connector

Abstract

According to the present disclosure, it is possible to reduce the difference in mode conversion efficiency occurring between degenerate modes of propagation in an optical fiber.　A mode converter (10) according to the present disclosure is provided with: a cladding (11); and, in the cladding, a core (12) that has a refractive index greater than the refractive index of the cladding, and a plurality of refractive index modulation portions (13A, 13B) that are provided so as to obliquely intersect the core and have a refractive index different from that of the surroundings. The mutually different refractive index modulation portions (13A, 13B) intersect the core (12) in mutually different directions in a cross section orthogonal to the longitudinal direction of the core.

Description

Mode converter and optical connector

[0001] The present disclosure relates to a mode converter and an optical connector.

[0002] Patent Document 1 discloses a mode converter comprising a cladding having a constant refractive index, a core within the cladding having a refractive index greater than that of the cladding, and a refractive index modulation section arranged diagonally across the core and having a refractive index different from that of its surroundings.

[0003] International Publication No. 2023 / 073881

[0004] The technique described in Patent Document 1 has a problem in that a difference in mode conversion efficiency occurs between degenerate modes propagating within the optical fiber.

[0005] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide a mode converter and an optical connector that can reduce the difference in mode conversion efficiency that occurs between degenerate modes propagating in an optical fiber.

[0006] In order to solve the above-mentioned problems, a mode converter according to one aspect of the present disclosure includes a cladding, a core disposed within the cladding and having a refractive index greater than that of the cladding, and a plurality of refractive index modulation sections disposed so as to intersect the core obliquely, the refractive index modulation sections having a refractive index different from that of their surroundings, wherein the different refractive index modulation sections intersect with the core in different directions in a cross section perpendicular to the longitudinal direction of the core.

[0007] An optical connector according to one aspect of the present disclosure includes the above-described mode converter provided within a ferrule.

[0008] According to the present disclosure, it is possible to reduce the difference in mode conversion efficiency that occurs between degenerate modes propagating in an optical fiber.

[0009] 1 is a schematic diagram of a mode converter according to an embodiment of the present disclosure; FIG. 2 is a first cross-sectional view of a mode converter according to an embodiment of the present disclosure; FIG. 3 is a second cross-sectional view of a mode converter according to an embodiment of the present disclosure; FIG. 4 is an intensity distribution diagram of each light wave mode propagating in an optical fiber; FIG. 5 is a diagram showing coupling efficiency in a mode converter according to a comparative example; FIG. 6 is a diagram showing coupling efficiency in a mode converter according to an embodiment of the present disclosure; and FIG. 7 is a diagram showing the relationship between ellipticity and conversion efficiency in a mode converter according to an embodiment of the present disclosure.

[0010] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0011] [Configuration of Mode Converter] Fig. 1 is a schematic diagram of a mode converter according to an embodiment of the present disclosure. The mode converter 10 includes a cladding 11, a core 12, and a plurality of refractive index modulation sections. In Fig. 1, the mode converter 10 includes refractive index modulation sections 13A and 13B as the plurality of refractive index modulation sections. The number of refractive index modulation sections included in the mode converter 10 may be two or more.

[0012] The cladding 11 has a constant refractive index and extends with its longitudinal direction in the z-axis direction in Fig. 1. For example, the cladding 11 is formed in a cylindrical shape with the z-axis as its central axis.

[0013] The core 12 is provided within the cladding 11 and has a refractive index greater than that of the cladding 11. There is a relative refractive index difference Δ between the cladding 11 and the core 12. For example, the core 12 has a core radius a and is formed in a cylindrical shape with the z-axis as its central axis. The central axes of the cladding 11 and the core 12 are parallel. The central axes of the cladding 11 and the core 12 may also coincide. Two or more lightwave modes propagate through the core 12. The lightwave modes will be described later. The mode converter 10 is connected to a multimode optical fiber (not shown), and multiple lightwave modes propagated through the optical fiber are introduced into the core 12.

[0014] Each of the multiple refractive index modulation sections is provided so as to intersect the core 12 obliquely, and has a refractive index different from that of the surrounding area. For example, the refractive index modulation sections 13A and 13B each intersect the core 12 at a predetermined angle. The refractive index modulation sections 13A and 13B may also intersect the core 12 at different angles. The shape of the refractive index modulation section may be cylindrical. For example, the shape of the refractive index modulation section may be an elliptic cylinder, a circular cylinder, or a rectangular cylinder.

[0015] Furthermore, the different refractive index modulation sections intersect with the core 12 in different directions in a cross section perpendicular to the longitudinal direction of the core 12. For example, the central axes of the refractive index modulation sections are not parallel to the central axis of the core 12 but intersect with the central axis of the core 12. In the following, it is assumed that the refractive index modulation section 13A is arranged along the xz plane and intersects with the core 12 in the x-axis direction in a cross section (xy plane) perpendicular to the longitudinal direction of the core 12. It is also assumed that the refractive index modulation section 13B is arranged along the yz plane and intersects with the core 12 in the y-axis direction in the orthogonal cross section.

[0016] Although a single-core structure is shown in Figure 1, a multi-core structure having multiple cores may also be used. In the case of a multi-core structure, a refractive index modulation section is provided for each core under the same conditions as for the single-core structure. In the embodiment, calculations are performed using a step-index optical fiber, but similar effects can be obtained with other refractive index profiles, such as a graded-index optical fiber.

[0017] 2A and 2B are cross-sectional views of a mode converter according to an embodiment of the present disclosure. Cross sections CS1 and CS2 are cross sections perpendicular to the longitudinal direction of core 12, and FIGS. 1 and 2 are cross-sectional views of cross sections CS1 and CS2, respectively.

[0018] For example, the refractive index modulation portions 13A and 13B may be disposed in a region between the cross sections CS1 and CS2. The refractive index modulation portions 13A and 13B may have one end at the position of the cross section CS1 and the other end at the position of the cross section CS2.

[0019] The ratio of the cross-sectional area of ​​the refractive index modulation portions 13A and 13B to the cross-sectional area of ​​the core 12 is defined as "R ｓ ", and the ellipticity of the cross-sectional shape of the refractive index modulation portions 13A and 13B is "R ｅ The ellipticity is expressed by the length along the radial direction of the cladding 11 with respect to the cross-sectional shape of the refractive index modulation portions 13A and 13B as "r ｘ ", and the length along the direction perpendicular to the radial direction of the cladding 11 is "r ｙ " and the ellipticity is "R ｅ =r ｘ / r ｙ " is expressed as:

[0020] The refractive index modulation sections 13A and 13B are assumed to have a modulation amount δn. Here, the modulation amount is defined as the difference in the refractive index of the refractive index modulation section relative to the refractive index of the surrounding area of ​​the refractive index modulation section. The refractive index modulation sections 13A and 13B are assumed to intersect obliquely with the core 12 at an angle of 1.1 degrees. Additionally, the discussion will proceed assuming that the wavelength of light passing through the mode converter 10 is 1550 nm.

[0021] Figure 3 shows the intensity distribution of each light wave mode propagating through an optical fiber. Light rays propagate through an optical fiber by repeated total reflection in the core 12. Here, the inclination of the light rays is not allowed to be any arbitrary value; only light rays with a specific angle can propagate. Such a light ray configuration is called a light wave mode. Light wave modes that can propagate through the core 12 can be determined, for example, by electromagnetic field analysis.

[0022] For example, a linearly polarized mode exists in the light wave mode. ｍｌ Here, "m" is the mode order indicating how the intensity distribution of the transverse electric field of the light beam propagating through the core 12 changes in the angular direction, and "l" is the mode order indicating how the intensity distribution of the transverse electric field of the light beam propagating through the core 12 changes in the radial direction.

[0023] As shown in Fig. 3, differences in the angular and radial intensities of the optical wave modes occur depending on the mode order. When there are only a few refractive index modulation sections, for example, one refractive index modulation section, which are arranged diagonally across the core 12 and have a refractive index different from that of the surrounding area, the influence of the refractive index modulation section on the optical wave mode varies depending on the differences in the angular and radial intensities of the optical wave mode. This results in differences in the mode conversion efficiency between the optical wave modes.

[0024] [Mode conversion efficiency in comparative example] Next, the mode conversion efficiency in the comparative example will be described for comparison with the mode converter according to the embodiment of the present disclosure. Fig. 4 is a diagram showing the coupling efficiency in the mode converter according to the comparative example. In Fig. 4, the mode converter according to the comparative example is provided with an LP ０１ Mode, LP １１ａ Mode, LP １１ｂ The coupling efficiency to each optical mode when the mode is input is shown.

[0025] In the comparative example, the mode converter has only one refractive index modulation section 13B as a refractive index modulation section, and the other configuration is the same as that of the mode converter 10 shown in FIG.

[0026] In order to calculate the coupling efficiency in the comparative example, the core radius a was set to 6.5 μm and the relative refractive index difference Δ was set to 0.4%. ｓ is 0.057, and the ellipticity R of the cross-sectional shape of the refractive index modulation part is ｅ The modulation amount δn was set to 0.005.

[0027] LP １１ａ Since the mode has a symmetrical intensity distribution centered on the y-axis, it is not affected by the refractive index modulation part and is not affected by other light wave modes (LP ０１ Mode, LP １１ｂ Therefore, there is no coupling with the LP mode. １１ａ The conversion efficiency for the mode is 0. Here, the "conversion efficiency" is the sum of the coupling efficiencies from the lightwave mode of interest to other lightwave modes.

[0028] On the other hand, LP ０１ Mode and LP１１ｂ There is coupling between the modes. ０１ Mode and LP １１ｂ The conversion efficiency for the mode is not 0. According to Figure 4, it is 30 to 40%.

[0029] In this way, LP ０１ Mode, LP １１ｂ In the LP mode, mode conversion occurs, whereas in the １１ａ Therefore, in the comparative example, a difference in mode conversion efficiency occurs between the light wave modes.

[0030] [Mode Conversion Efficiency in the Present Disclosure] Next, the mode conversion efficiency in the present disclosure will be described. Fig. 5 is a diagram showing the coupling efficiency in the mode converter according to the embodiment of the present disclosure. In Fig. 5, the mode converter according to the comparative example is provided with an LP ０１ Mode, LP １１ａ Mode, LP １１ｂ The coupling efficiency to each optical mode when the mode is input is shown.

[0031] For the calculation of the coupling efficiency, the core radius a was set to 6.5 μm and the relative refractive index difference Δ was set to 0.4%. ｓ is 0.057, and the ellipticity R of the cross-sectional shape of the refractive index modulation part is ｅ The refractive index modulation portion 13A and the refractive index modulation portion 13B have the same structural parameters.

[0032] Unlike the comparative example shown in FIG. 4, by providing a plurality of refractive index modulation sections in the mode converter 10, ０１ Mode, LP １１ａ Mode, LP １１ｂ Mode conversion between modes occurs. Therefore, the difference in conversion efficiency between light wave modes can be reduced. ０１ Mode, LP １１ａ Mode, LP １１ｂ The average value of the mode conversion efficiency also increases. Therefore, it is clear that providing a plurality of refractive index modulation sections in the mode converter 10 is effective from the viewpoint of the absolute value of the conversion efficiency as well.

[0033] Furthermore, by orthogonally intersecting the core 12 in the cross section perpendicular to the longitudinal direction between the refractive index modulation section 13A and the refractive index modulation section 13B, the difference in conversion efficiency between the optical wave modes can be further reduced. Also, the average value of the conversion efficiency between the optical wave modes can be increased. By orthogonally intersecting the core 12 between the multiple refractive index modulation sections, the asymmetry between the x-axis and the y-axis is reduced, and coupling between the optical wave modes is promoted.

[0034] 6 is a diagram illustrating the relationship between the ellipticity and the conversion efficiency in a mode converter according to an embodiment of the present disclosure. As shown in FIG. 6, as the ellipticity increases and approaches 1, the conversion efficiency decreases. Therefore, in order to increase the conversion efficiency between optical wave modes, it is desirable that the refractive index modulation portion has a non-circular shape in the cross section perpendicular to the longitudinal direction.

[0035] By placing the mode converter 10 of the present disclosure within an optical component, a small optical component can be provided with the function of mixing lightwave modes. For example, the mode converter of the present disclosure may be placed within an optical connector. By placing the mode converter 10 of the present disclosure within a ferrule, the lightwave modes propagating through the optical connector plug are mixed.

[0036] As described above in detail, the mode converter according to the present disclosure includes a cladding, a core within the cladding having a refractive index greater than that of the cladding, and a plurality of refractive index modulation sections disposed obliquely across the core and having a refractive index different from that of its surroundings. The different refractive index modulation sections intersect with the core in different directions in a cross section perpendicular to the longitudinal direction of the core.

[0037] This reduces the difference in mode conversion efficiency between degenerate modes propagating in the optical fiber, and also increases the average value of the conversion efficiency of the optical wave modes, thereby increasing the absolute value of the conversion efficiency.

[0038] In the mode converter according to the present disclosure, the directions of intersection with the core in the different refractive index modulation sections may be orthogonal in a cross section orthogonal to the longitudinal direction. This can reduce the difference in conversion efficiency between optical wave modes. It can also increase the average value of conversion efficiency between optical wave modes. By orthogonally intersecting the core between the multiple refractive index modulation sections, asymmetry between the x-axis and the y-axis is reduced, and coupling between optical wave modes is promoted.

[0039] In the mode converter according to the present disclosure, the refractive index modulation portion may have a non-circular shape in a cross section perpendicular to the longitudinal direction, thereby increasing the conversion efficiency between light wave modes.

[0040] The optical connector according to the present disclosure has the above-mentioned mode converter disposed within the ferrule. This allows a small optical component to have the function of mixing lightwave modes. Placing the mode converter within the ferrule mixes the lightwave modes propagating through the optical connector plug.

[0041] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0042] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0043] 10 mode converter 11 cladding 12 core 13A, 13B refractive index modulation section CS1, CS2 cross section

Claims

1. A mode converter comprising: a cladding; a core within the cladding, the core having a refractive index greater than that of the cladding; and a plurality of refractive index modulation sections arranged to intersect obliquely with the core and having a refractive index different from that of its surroundings, wherein the different refractive index modulation sections intersect with the core in different directions in a cross section perpendicular to the longitudinal direction of the core.

2. The mode converter according to claim 1, wherein the different refractive index modulation sections are orthogonal in the direction of intersection with the core in a cross section perpendicular to the longitudinal direction.

3. The mode converter according to claim 2, wherein the refractive index modulation portion has a non-circular shape in a cross section perpendicular to the longitudinal direction.

4. An optical connector having a mode converter according to any one of claims 1 to 3 provided in a ferrule.

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