Mode converter and optical component including the mode converter

Abstract

The purpose of the present disclosure is to provide an optical fiber-type mode converter which is short and for which wavelength dependence is small.　The present disclosure is a mode converter characterized by comprising: cladding having a constant refractive index; a core which is disposed in the cladding and which has a refractive index greater than that of the cladding; and a modulated refractive index part which is disposed so as to diagonally cross the core and which has a refractive index that is different from the surrounding area.

Description

[Technical Field]

[0001] The present disclosure relates to fiber optic mode converters and optical components including mode converters. [Background technology]

[0002] Mode converters that mix modes within an optical path have been proposed to reduce differential mode loss and differential mode group delay. Mode converters are broadly classified into device-type mode converters that use planar lightwave circuits (see, for example, Non-Patent Document 1) and optical fiber-type mode converters that use optical fibers (see, for example, Non-Patent Document 2).

[0003] Device-type mode converters suffer from insertion loss and increased inter-mode loss due to mismatching of the mode field diameter at the connection point with the optical fiber. On the other hand, optical fiber-type mode converters that do not have these problems have traditionally used LPG (Long-Period Fiber Grating). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Y. Zhao et al., Broadband and low-loss mode scramblers using CO2-laser inscribed long-period gratings, optics letters, vol. 42, no. 12, pp. 2868-2871 [Non-patent document 2] T.Fujisawa et al, Wavefront-matching-method-designed six-mode-exchanger based on grating-like waveguide on silica-PLC platform, OFC 2020, Th1A.5. Summary of the Invention [Problem to be solved by the invention]

[0005] However, optical fiber mode converters using LPG have the drawback of being highly wavelength-dependent and long, at around 24 mm in length.

[0006] In order to solve the above-mentioned problems, the present disclosure aims to provide an optical fiber-type mode converter that has small wavelength dependency and is short in length, and also aims to provide various optical components that include the mode converter of the present disclosure. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the mode converter of the present disclosure is configured to have a refractive index modulation section that crosses the core of the optical fiber obliquely.

[0008] Specifically, the mode converter of the present disclosure is a cladding having a constant refractive index; a core within the cladding, the core having a refractive index greater than the refractive index of the cladding; a refractive index modulation portion provided so as to diagonally cross the core and having a refractive index different from that of its surroundings; A mode converter comprising: is.

[0009] By adopting such a structure, the mode converter of the present disclosure can be configured to have small wavelength dependency and a short length.

[0010] In addition, the mode converter of the present disclosure The core has a structure that propagates in 2LP mode, d / r c >=1.1 1deg<θ<2deg r y / r c <a(r x / r c ) b When 0<δn<0.005 a=-55δn+0.36 b=-95δn-0.5 When -0.006<δn<0, a=39δn+0.29 b=32δn-0.69 The present invention is characterized in that: where d is the distance between the center of the end of the refractive index modulation section and the major axis of the core, and r c is the radius of the core, θ is the angle between the major axis of the refractive index modulation portion and the major axis of the core, δn is the refractive index modulation amount, r x is the radius of the refractive index modulation section in the x direction, r y is the radius of the refractive index modulation section in the y direction, the refractive index modulation amount is the difference in the refractive index of the refractive index modulation section with respect to the refractive index around the refractive index modulation section, the x direction is the direction parallel to a plane including the major axis of the core and the major axis of the refractive index modulation section and perpendicular to the major axis of the core, and the y direction is the direction perpendicular to a plane including the major axis of the core and the major axis of the refractive index modulation section.

[0011] In addition, the mode converter of the present disclosure The core has a structure that propagates in 4LP mode, d / r c >=1.4 1deg<θ<2deg r y / r c <a(r x / r c ) b When 0<δn<0.007 a=-123.8δn+1.21 b=421.5δn-3.53 When -0.005<δn<0, a=42.6δn+0.3 b=-96.5δn-1.4 The present invention is characterized in that: where d is the distance between the center of the end of the refractive index modulation section and the major axis of the core, and r c is the radius of the core, θ is the angle between the major axis of the refractive index modulation portion and the major axis of the core, δn is the refractive index modulation amount, r xis the radius of the refractive index modulation section in the x direction, r y is the radius of the refractive index modulation section in the y direction, the refractive index modulation amount is the difference in the refractive index of the refractive index modulation section with respect to the refractive index around the refractive index modulation section, the x direction is the direction parallel to a plane including the major axis of the core and the major axis of the refractive index modulation section and perpendicular to the major axis of the core, and the y direction is the direction perpendicular to a plane including the major axis of the core and the major axis of the refractive index modulation section.

[0012] This disclosure also An optical fiber transmission line is characterized in that any one of the above-described mode converters is provided in multiple stages midway along an optical fiber.

[0013] This disclosure also An optical connector is characterized in that any of the above-described mode converters is provided in a ferrule.

[0014] This disclosure also An optical adapter is characterized in that any one of the above-described mode converters is provided in an optical propagation path.

[0015] This disclosure also An optical amplifier is characterized in that any one of the above-described mode converters is provided in an optical propagation path.

[0016] This disclosure also A mode multiplexer / demultiplexer is characterized in that any one of the above-described mode converters is provided at a multi-mode end.

[0017] The above-disclosed inventions can be combined as much as possible. [Effects of the Invention]

[0018] According to the present disclosure, it is possible to provide an optical fiber type mode converter that has small wavelength dependency and is short in length. [Brief explanation of the drawings]

[0019] [Figure 1]1A and 1B are diagrams illustrating the configuration of a mode converter according to the present disclosure. [Figure 2] 10A and 10B are diagrams illustrating the propagation distance dependence of the excitation efficiency of the mode converter of the present disclosure. [Figure 3] 10A and 10B are diagrams illustrating the electric field distribution of a mode converter according to the present disclosure. [Figure 4] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 5] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 6A] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 6B] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 7A] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 7B] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 8] 10A and 10B are diagrams illustrating the refractive index modulation amount dependency of a mode converter according to the present disclosure. [Figure 9] 10A and 10B are diagrams illustrating the excitation efficiency of modes of a mode converter according to the present disclosure. [Figure 10] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 11] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 12A] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 12B] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 13A] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 13B] 1A and 1B are diagrams illustrating the conversion efficiency and insertion loss of a mode converter according to the present disclosure. [Figure 14] 10A and 10B are diagrams illustrating the refractive index modulation amount dependency of a mode converter according to the present disclosure. [Figure 15]10A and 10B are diagrams illustrating wavelength characteristics of a mode converter according to the present disclosure. [Figure 16] 10A and 10B are diagrams illustrating wavelength characteristics of a mode converter according to the present disclosure. [Figure 17] 10 is an example in which mode converters according to the present disclosure are provided in multiple stages within an optical fiber transmission line. [Figure 18] 10 is an example in which a mode converter according to the present disclosure is disposed inside an optical connector. [Figure 19] 10 is an example in which a mode converter according to the present disclosure is disposed in an optical adapter. [Figure 20] 10 is an example in which a mode converter according to the present disclosure is disposed in an optical amplifier. [Figure 21] 10 is an example in which a mode converter according to the present disclosure is arranged in a mode multiplexer / demultiplexer. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0021] (Embodiment 1) The configuration of the mode converter of the present disclosure is shown in Figure 1. Figure 1(a) is an overall view, Figure 1(b) is an xz cross-sectional view at y=0, and Figures 1(c) and 1(d) are xy cross-sectional views seen from the right side of the drawing at the start and end points of the optical modulation section, respectively. In Figure 1, 10 is the mode converter, 11 is the cladding, 12 is the core, and 13 is the refractive index modulation section.

[0022] The mode converter of the present disclosure comprises a cladding 11 with a constant refractive index, a core 12 within the cladding 11, which has a refractive index greater than that of the cladding 11, and a refractive index modulation section 13 disposed obliquely across the core 12 and having a refractive index different from that of its surroundings. c , the refractive index of the core 12 is n core, the refractive index of the cladding 11 is n clad , the radius of the refractive index modulation portion 13 in the x direction is r x , the radius in the y direction is r y The x direction refers to a direction parallel to a plane including the major axis of the core 12 and the major axis of the refractive index modulation section 13, and perpendicular to the major axis of the core 12. The y direction refers to a direction perpendicular to a plane including the major axis of the core 12 and the major axis of the refractive index modulation section 13.

[0023] The refractive index modulation amount of the refractive index modulation portion 13 is denoted by δn. The refractive index modulation amount δn refers to the difference in the refractive index of the refractive index modulation portion relative to the refractive index of the surrounding area of ​​the refractive index modulation portion. The refractive index of the refractive index modulation portion may be greater or smaller than the refractive index of the surrounding area. The refractive index of the refractive index modulation portion 13 differs from that of the core 12 within the core 12 and differs from that of the clad 11 within the clad 11. The refractive index modulation portion 13 can be fabricated, for example, by femtosecond laser processing. Processing with a femtosecond laser increases the refractive index compared to before processing. Therefore, the refractive index of the refractive index modulation portion 13 is greater than that of the core 12 by the refractive index modulation amount δn within the core 12 and greater than that of the clad 11 by the refractive index modulation amount δn within the clad 11. The shape of the refractive index modulation portion 13 may be cylindrical. For example, it may be an elliptical cylinder, a circular cylinder, or a rectangular cylinder. In the embodiment, the shape of the refractive index modulation portion 13 was simulated as an elliptical cylinder.

[0024] The refractive index modulation section 13 is provided such that the center of its end starts from a position d μm away from the center of the core 12, passes through the center of the core 12, and extends to a position d μm away on the opposite side, and the major axis of the refractive index modulation section 13 and the major axis of the core 12 form an angle θ. The length L of the refractive index modulation section 13 in the major axis direction is given by L=2d / tanθ It is expressed as:

[0025] 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 type.

[0026] (Embodiment 2) LP 01 , LP 11 Consider an optical fiber in which two modes propagate. Consider two-mode transmission in the C band (1530 nm to 1565 nm), c = 6.5 μm, n core =1.45003, n clad =1.444(Δ core =0.4%), where n core , n clad , Δ core The relationship is Δ core =((n core 2 -n clad 2 ) / 2n core 2 ) 1 / 2 One scan of the femtosecond laser is approximately r x = 1.4 μm, r y It is possible to create a refractive index modulation section of approximately 2.1 μm. By processing the film multiple times, the amount of refractive index modulation can be controlled.

[0027] The LP was measured across the core with d=7μm, δn=0.004, and θ=1.3deg. 01 Mode, LP 11 The propagation distance dependence of the excitation efficiency when each mode is incident is shown in Fig. 2. Fig. 2(a) shows the LP 01 When the LP mode is input, Fig. 2(b) shows 11This shows the excitation efficiency of each mode when a mode is incident. z=200μm is the start point of the refractive index modulation section, and z=817μm is the end point of the refractive index modulation section. The excitation efficiency is the ratio of the power of the excited mode to the power of the incident mode.

[0028] The electric field distribution at this time is shown in Figure 3. Figure 3(a) shows the LP 01 When the LP mode is input, Fig. 3(b) shows 11 The electric field distribution when a mode is incident is shown. It can be seen that the mode is converted and the electric field distribution changes.

[0029] From Figure 2, LP 01 The conversion efficiency when the LP mode was input was 28.9%. 11 The conversion efficiency when two modes are input is 14.4%. Conversion efficiency refers to the ratio of the total power of the modes newly excited after passing through the refractive index modulation section to the power of the input mode. The insertion losses in this case are 0.24 dB and 0.42 dB, respectively. The insertion loss is expressed as the output power (total of two modes) normalized by the input power. LP 11 Mode is LP 01 Compared to the LP mode, the conversion efficiency is lower and the insertion loss is larger, so under the conditions of 2LP mode propagation described below, the LP mode has worse characteristics. 11 Each parameter is evaluated using the conversion efficiency and insertion loss when the mode is input.

[0030] It is desirable to have as high a conversion efficiency as possible, while it is also desirable to have as small an insertion loss as possible. Considering that multiple mode converters are arranged in a transmission line, it is desirable that the insertion loss be at least 0.5 dB or less.

[0031] We will check the effective range of each parameter. First, we will evaluate the dependency of the characteristics on the distance d from the center of the core to the start and end points of the refractive index modulation section. Here, we will use r x = 1.4 μm, r y = 2.1 μm, δn = 0.004. For the 2LP mode optical fiber, LP 11Figure 4(a) shows the conversion efficiency when the distance d is changed with the mode input, and Figure 4(b) shows the insertion loss. It can be seen that the conversion efficiency increases monotonically with increasing distance d in the region of d<7μm, and saturates at around 7μm. Similarly, no large fluctuations are seen in the insertion loss in the region of d>=7μm. This shows that it is desirable to convert the mode in the range of d>=7μm. Core radius r c When normalized at d / r = 6.5 μm, c >=1.1, high efficiency mode conversion is possible.

[0032] Next, we evaluate the dependence of the characteristics on the angle θ between the major axis of the core and the major axis of the refractive index modulation section. x = 1.4 μm, r y = 2.1 μm, d = 7 μm. For the 2LP mode optical fiber, LP 11 Figure 5(a) shows the conversion efficiency when the angle θ is changed when a mode is input, and Figure 5(b) shows the insertion loss. Figure 5(a) shows that the maximum value of the conversion efficiency exists in the range of 1°<θ<2°. It also shows that the angle θ dependency of the insertion loss is small. Therefore, it is necessary to keep the angle θ at 1°<θ<2°. In this case, the length of the mode converter in the major axis direction is L = 400 to 800 μm, which is about 1 / 30 the length of the conventional optical fiber mode converter that uses LPG.

[0033] The dependence of the characteristics on the refractive index modulation amount δn is evaluated. x = 1.4 μm, r y = 2.1 μm, d = 7 μm. For the 2LP mode optical fiber, LP 11Figure 6A(a) shows the conversion efficiency and Figure 6A(b) shows the insertion loss when the refractive index modulation δn is changed by a positive value with the input mode. The insertion loss is highly dependent on the refractive index modulation δn, increasing monotonically in proportion to δn. Since there is no parameter that reduces the loss below 0.5 dB in the range of δn > 0.005, it is necessary to set δn < 0.005 to reduce the insertion loss below 0.5 dB. Figure 6B(a) shows the conversion efficiency and Figure 6B(b) shows the insertion loss when the refractive index modulation δn is changed by a negative value. The insertion loss is highly dependent on the refractive index modulation δn, increasing monotonically in proportion to a decrease in δn. Since there is no parameter that reduces the loss below 0.5 dB in the range of δn < -0.006, it is necessary to set δn > -0.006 to reduce the insertion loss below 0.5 dB.

[0034] For 2LP mode optical fiber, LP 11 By injecting the mode, r x / r c and r y / r c The conversion efficiency when θ is set as a parameter is shown in Figures 7A(a), 7A(b), and 7A(c), and the insertion loss is shown in Figures 7A(d), 7A(e), and 7A(f). Here, θ = 1.3 deg, d = 7 μm, and δn is a positive value. In Figures 7A(a) and 7A(d), δn = 0.003, in Figures 7A(b) and 7A(e), δn = 0.004, and in Figures 7A(c) and 7A(f), δn = 0.005. The contour lines where the insertion loss is 0.5 dB and 1 dB are shown with dashed lines, and r x and r y It can be seen that the insertion loss increases as the value increases. This means that the insertion loss can be kept below 0.5 dB in the area below and to the left of the contour line. In the figure, the contour line where the insertion loss is 0.5 dB is shown by a solid white line that approximates an exponential function. (A) When δn=0.003, r y / r c =0.20(r x / r c ) -0.79 (B) When δn = 0.004, ry / r c =0.13(r x / r c ) -0.88 (C) When δn=0.005, r y / r c =0.09(r x / r c ) -0.95 is expressed as an approximate curve.

[0035] 2For LP mode optical fiber, LP 11 By injecting the mode, r x / r c and r y / r c The conversion efficiency when θ is set as a parameter is shown in Figures 7B(a), 7B(b), and 7B(c), and the insertion loss is shown in Figures 7B(d), 7B(e), and 7B(f). Here, θ = 1.3 deg, d = 7 μm, and δn is a negative value. In Figures 7B(a) and 7B(d), δn = -0.005, in Figures 7B(b) and 7B(e), δn = -0.004, and in Figures 7B(c) and 7B(f), δn = -0.003. The contour where the insertion loss is 0.5 dB is shown by a dashed line, and r x and r y It can be seen that the insertion loss increases as the value increases. This means that the insertion loss can be kept below 0.5 dB in the area below and to the left of the contour line. In the figure, the contour line where the insertion loss is 0.5 dB is shown by a solid white line that approximates an exponential function. (A) When δn=-0.005, r y / r c =0.1(r x / r c ) -0.84 (B) When δn = -0.004, r y / r c =0.12(r x / r c ) -0.83 (C) When δn = -0.003, r y / r c =0.18(r x / r c) -0.78 is expressed as an approximate curve.

[0036] r y / r c =a(r x / r c ) b The relationship between a, b and δn when δn is positive is shown in Fig. 8(a), and the relationship between a, b and δn when δn is negative is shown in Fig. 8(b). If δn>0, a=-55δn+0.36 b=-95δn-0.5 To satisfy the following: If δn<0, a=39δn+0.29 b=32δn-0.69 To satisfy the following: Set a and b, r y / r c =a(r x / r c ) b to satisfy r x and r y By setting

[0037] Furthermore, from Figures 7A(d), 7A(e) and 7A(f), it can be seen that there may be regions where the conversion efficiency is 10% or more in regions where the insertion loss is 1 dB or less. Similarly, from Figures 7B(d), 7B(e) and 7B(f), it can be seen that there may be regions where the conversion efficiency is 10% or more in regions where the insertion loss is 1 dB or less. From this, it can be seen that for a 2LP mode optical fiber, d / r c >=1.1 1deg<θ<2deg r y / r c <a(r x / r c ) b When 0<δn<0.005, a=-55δn+0.36 b=-95δn-0.5 When -0.006<δn<0, a=39δn+0.29 b=32δn-0.69 By providing a refractive index modulation section having a refractive index different from that of the surrounding area, which is arranged so as to obliquely cross the core in a structure that satisfies the above, it is possible to convert and mix modes with low loss, and to provide a short optical fiber type mode converter.

[0038] (Embodiment 3) LP 01 , LP 11 , LP 21 , LP 02 Consider a fiber in which four modes propagate. We consider four-mode transmission in the C band (1530 nm to 1565 nm). c = 7 μm, n core =1.454215(Δ core =0.7%). For example, r x = 2.8 μm, r y LP when = 7 μm, δn = 0.005, θ = 1.3 deg 01 , LP 11 , LP 21 , LP 02 Figure 9 shows the excitation efficiency of each mode after passing through the optical modulation section when each of the four modes is incident. It can be seen that each mode is converted into a different mode. 02 Since the mode conversion efficiency is the smallest and the insertion loss is the largest, the LP with the poorer characteristics is the one under the following 4LP mode propagation conditions. 02 Each parameter is evaluated using the conversion efficiency and insertion loss when the mode is input.

[0039] We will check the effective range of each parameter. First, we will evaluate the dependency of the characteristics on the distance d from the center of the core to the start and end points of the refractive index modulation section. Here, we will use r x = 2.6 μm, r y For a 4LP mode optical fiber with δ = 7.0 μm and δn = 0.005 and d changed, 02Figure 10(a) shows the conversion efficiency when the distance d is changed with the mode input, and Figure 10(b) shows the insertion loss. The conversion efficiency increases monotonically with increasing d in the region where d<10μm, and saturates at around 10μm. Similarly, the insertion loss does not show any significant fluctuations in the region where d>=10μm. This shows that it is desirable to convert the mode in the range of d>=10μm. Core radius r c When normalized at 7 μm, d / r c >=1.4, high efficiency mode conversion is possible.

[0040] Next, we evaluate the dependence of the characteristics on the angle θ between the major axis of the core and the major axis of the refractive index modulation section. x = 2.6 μm, r y = 7 μm, d = 10 μm. For a 4LP mode optical fiber, LP 02 Figure 11(a) shows the conversion efficiency when the angle θ is changed when a mode is input, and Figure 11(b) shows the insertion loss. Figure 11(a) shows that the maximum value of the conversion efficiency is in the range of 1°<θ<2°. Similarly, it can be seen that the minimum value of the insertion loss is in the range of 1°<θ<2°. Therefore, θ must be set to 1°<θ<2°. In this case, the length in the major axis direction of the mode converter is L = 400 to 800 μm, which is about 1 / 30 the length of the conventional optical fiber mode converter that uses LPG.

[0041] The dependence of the characteristics on the refractive index modulation amount δn is evaluated. x = 2.6 μm, r y = 7.0 μm, d = 10 μm. For a 4LP mode optical fiber, LP 02 Figure 12A(a) shows the conversion efficiency when a mode is input and the refractive index modulation amount δn is changed by a positive value, and Figure 12A(b) shows the insertion loss. The insertion loss is highly dependent on the refractive index modulation amount δn. Since there is no parameter that reduces the loss to 0.5 dB or less in the range of δn > 0.007, it can be seen that in order to reduce the insertion loss to 0.5 dB or less, δn must be at least < 0.007.

[0042] Figure 12B(a) shows the conversion efficiency when the refractive index modulation amount Δn is changed by a negative value, and Figure 12B(b) shows the insertion loss. The insertion loss is highly dependent on the refractive index modulation amount Δn, and increases monotonically in proportion to a decrease in Δn. Since there is no parameter in the range of Δn < -0.005 that reduces the loss to 0.5 dB or less, it can be seen that Δn must be at least > -0.005 to reduce the insertion loss to 0.5 dB or less.

[0043] 4For LP mode optical fiber, LP 02 By injecting the mode, r x / r c and r y / r c The conversion efficiency when θ is used as a parameter is shown in Figures 13A(a), 13A(b), 13A(c), and 13A(d), and the insertion loss is shown in Figures 13A(e), 13A(f), 13A(g), and 13A(h). Here, θ = 1.4 deg, d = 10 μm, and δn is a positive value. In Figures 13A(a) and 13A(e), δn = 0.004, in Figures 13A(b) and 13A(f), δn = 0.005, in Figures 13A(c) and 13A(g), and δn = 0.007 in Figures 13A(d) and 13A(h). The contour where the insertion loss is 0.5 dB is shown by a dashed line, and r x and r y It can be seen that the insertion loss increases as r increases. From this, the insertion loss can be kept below 0.5 dB in the area below and to the left of the contour line. Note that when δn=0.004, r x / r c <1 and r y / r c The insertion loss can be kept below 0.5 dB in the entire range of <1. In the figure, the contour where the insertion loss is 0.5 dB is shown by a solid white line that approximates an exponential function. (A) When δn=0.005, r y / r c =0.61(r x / r c ) -1.35 (B) When δn = 0.006, ry / r c =0.44(r x / r c ) -1.1 (C) When δn = 0.007, r y / r c =0.36(r x / r c ) -0.51 is expressed as an approximate curve.

[0044] 4For LP mode optical fiber, LP 02 By injecting the mode, r x / r c and r y / r c The conversion efficiency when θ is set as a parameter is shown in Figures 13B(a), 13B(b), and 13B(c), and the insertion loss is shown in Figures 13B(d), 13B(e), and 13B(f). Here, θ = 1.4 deg, d = 10 μm, and δn is a negative value. In Figures 13B(a) and 13B(d), δn = -0.005, in Figures 13B(b) and 13B(e), δn = -0.004, and in Figures 13B(c) and 13B(f), δn = -0.003. The contour where the insertion loss is 0.5 dB is shown by a dashed line, and r x and r y It can be seen that the insertion loss increases as the value increases. This means that the insertion loss can be kept below 0.5 dB in the area below and to the left of the contour line. In the figure, the contour line where the insertion loss is 0.5 dB is shown by a solid white line that approximates an exponential function. (A) When δn=-0.005, r y / r c =0.09(r x / r c ) -0.96 (B) When δn = -0.004, r y / r c =0.12(r x / r c ) -0.1 (C) When δn = -0.003, r y / r c =0.18(r x / r c ) -1.16 is expressed as an approximate curve.

[0045] r y / r c =a(r x / r c ) b The relationship between a, b and Δn when Δn is positive is shown in FIG. 14(a), and the relationship between a, b and Δn when Δn is negative is shown in FIG. 14(b). From FIG. 14(a), If δn>0, a=-123.8δn+1.21 b=421.5δn-3.53 Set a and b so that When δn<0 a=42.6δn+0.3 b=-96.5δn-1.4 Set a and b so that r y / r c =a(r x / r c ) b to satisfy r x and r y The loss can be reduced by setting

[0046] 13A and 13B, it can be seen that there may be regions where the conversion efficiency is 40% or more in the region where the insertion loss is 0.5 dB or less. d / r c >=1.4 1deg<θ<2deg r y / r c <a(r x / r c ) b When 0<δn<0.007, a=-123.8δn+1.21 b=421.5δn-3.53 When -0.005<δn<0, a=42.6δn+0.3 b=-96.5δn-1.4 By providing a refractive index modulation section having a refractive index different from that of the surrounding area, which is arranged so as to obliquely cross the core in a structure that satisfies the above, it is possible to convert and mix modes with low loss, and to provide a short optical fiber type mode converter.

[0047] In this embodiment, examples of a 2LP mode optical fiber and a 4LP mode optical fiber have been described, but a short optical fiber type mode converter can also be provided using other few mode optical fibers.

[0048] (Embodiment 4) Figure 15 shows the wavelength characteristics of the excitation efficiency when a refractive index modulation section is provided in the 2LP mode optical fiber described in embodiment 1. Figure 16 shows the wavelength characteristics of the excitation efficiency when a refractive index modulation section is provided in the 4LP mode optical fiber described in embodiment 2. Each shows the wavelength characteristics from 1450 nm to 1625 nm, assuming S+C+L band transmission.

[0049] Figure 15(a) shows the LP 01 When light of the LP mode is incident, Fig. 15(b) shows 11 It shows the excitation efficiency when light of the mode is incident, and r x = 1.4 μm, r y = 2.1 μm, d = 7 μm, θ = 1.3 deg, δn = 0.004. 01 When light of the LP mode is incident, Fig. 16(b) shows 11 When light of the LP mode is incident, Fig. 16(c) shows 21 When light of the LP mode is incident, Fig. 16(d) shows 02 It shows the excitation efficiency when light of the mode is incident, and r x = 2.6 μm, r y = 7.0 μm, d = 10 μm, θ = 1.4 deg, and δn = 0.005.

[0050] In both the 2LP mode optical fiber and the 4LP mode optical fiber, the conversion efficiency to another mode is higher on the shorter wavelength side depending on the mode, but it can be seen that mode conversion is possible across the entire 1460nm to 1625nm band. The reason for the higher conversion efficiency on the shorter wavelength side is thought to be that the difference in effective refractive index between modes is smaller on the shorter wavelength side, making conversion more likely to occur.

[0051] By providing a refractive index modulation portion that is disposed so as to diagonally cross the core and has a refractive index different from that of the surrounding area, it is possible to provide an optical fiber type mode converter that has little wavelength dependency.

[0052] In this embodiment, examples of a 2LP mode optical fiber and a 4LP mode optical fiber have been described, but an optical fiber type mode converter with small wavelength dependency can also be provided using other few mode optical fibers.

[0053] (Embodiment 5) Taking advantage of the fact that optical fiber has little wavelength dependency and is short in length, it is thought that if an optical fiber transmission line is constructed in which mode converters 10 are installed every few kilometers along a length of several tens of kilometers, as shown in Figure 17, the mode will be converted evenly. In Figure 17, 10 is the mode converter, 11 is the cladding, and 12 is the core. The amount of coupling that occurs using one refractive index modulation section is about 10 to 20%, but by installing mode converters in multiple stages within the optical fiber transmission line, it is possible to mix modes uniformly throughout the entire optical fiber transmission line, thereby reducing the modal group delay difference and the modal loss difference.

[0054] (Embodiment 6) Taking advantage of the fact that the mode converter of the present disclosure has little wavelength dependency and is a short optical fiber type, placing the mode converter of the present disclosure inside an optical component allows a small optical component to have the function of mixing modes. For example, Fig. 18 shows an example of placing the mode converter of the present disclosure inside an optical connector. In Fig. 18, 10 is the mode converter, 20 is the optical connector, 21 is the optical connector plug, 22 is the ferrule, and 30 is the optical adapter. When the mode converter 10 of the present disclosure is placed inside the ferrule 22, the mode propagating through the optical connector plug 21 is mixed.

[0055] (Embodiment 7) An example of arranging a mode converter of the present disclosure inside an optical adapter is shown in Fig. 19. In Fig. 19, reference numeral 10 denotes a mode converter, 20 denotes an optical connector, 21 denotes an optical connector plug, 22 denotes a ferrule, and 30 denotes an optical adapter. When the mode converter 10 of the present disclosure is arranged in the light propagation path inside an optical adapter 30, the modes propagating through the optical adapter 30 are mixed.

[0056] (Embodiment 8) FIG. 20 shows an example of a mode converter according to the present disclosure arranged within an optical amplifier. In FIG. 20, reference numeral 10 denotes a mode converter, 40 denotes an optical amplifier, 41 denotes a pumping light source, 42 denotes an optical coupler, and 43 denotes a rare-earth-doped optical fiber. The light to be amplified and pumping light from the pumping light source 41 are multiplexed by the optical coupler 42, and the amplified light is optically amplified by the rare-earth-doped optical fiber 43. When the mode converter 10 according to the present disclosure is arranged in the optical propagation path within the optical amplifier 40, the mode converter 10 mixes modes, averaging inter-modal gain differences and loss differences within the repeater. The mode converter 10 may be arranged either before or after the rare-earth-doped optical fiber 43, or in the middle of the rare-earth-doped optical fiber 43. While FIG. 20 shows an example of an optical fiber amplifier, the same applies to a semiconductor amplifier.

[0057] (Embodiment 9) An example in which a mode converter according to the present disclosure is arranged in a mode multiplexer / demultiplexer is shown in Fig. 21. In Fig. 21, 10 is a mode converter, 50 is a mode multiplexer / demultiplexer, 51 is an optical fiber, and 52 is a multimode optical fiber. A terminal into which single-mode light is input from multiple optical fibers 51 is called a single-mode end, and a terminal from which multimode light is output to multimode optical fiber 52 is called a multimode end. When single-mode light input from multiple optical fibers 51 to the single-mode end is multiplexed by mode multiplexer / demultiplexer 50 and multimode light is output from the multimode end to multimode optical fiber 52, mixing the modes with the mode converter according to the present disclosure makes the distribution between modes uniform. The same applies when demultiplexing multimode light into single-mode light.

[0058] As described above, the mode converter of the present disclosure can be configured to have small wavelength dependency and a short length. [Industrial Applicability]

[0059] The present disclosure is applicable to the communications industry. [Explanation of symbols]

[0060] 10 Mode Converter 11 Clad 12 cores 13 Refractive index modulation section 20 Optical Connector 21 Optical connector plug 22 Ferrule 30 Optical adapter 40 Optical Amplifier 41 Excitation light source 42 Optical Coupler 43 Rare-earth doped optical fiber 50-mode multiplexer / demultiplexer

Claims

1. a cladding having a constant refractive index; a core within the cladding, the core having a refractive index greater than the refractive index of the cladding; a columnar refractive index modulation portion provided so as to diagonally cross the center of the core; Equipped with the refractive index of the refractive index modulation portion differs from the refractive index of the core by a refractive index modulation amount δn within the core, and differs from the refractive index of the clad by the refractive index modulation amount δn within the clad, the distances from the center of the core to both ends of the refractive index modulation section are equal; A mode converter characterized by:

2. A cladding having a constant refractive index; a core within the cladding, the core having a refractive index greater than the refractive index of the cladding; a refractive index modulation portion provided so as to diagonally cross the core and having a refractive index different from that of its surroundings; Equipped with The core has a structure that propagates in 2LP modes, d / r ｃ >=1.1 1deg<θ<2deg r ｙ / r ｃ <a(r ｘ / r ｃ ) ｂ When 0<δn<0.005 a=-55δn+0.36 b=-95δn-0.5 When −0.006<δn<0, a=39δn+0.29 b=32δn-0.69 A mode converter characterized by satisfying the above. where d is the distance between the center of the end of the refractive index modulation section and the major axis of the core, and r ｃ is the radius of the core, θ is the angle between the major axis of the refractive index modulation portion and the major axis of the core, δn is the refractive index modulation amount, r ｘ is the radius of the refractive index modulation portion in the x direction, r ｙ is the radius of the refractive index modulation portion in the y direction, the refractive index modulation amount is the difference in the refractive index of the refractive index modulation portion with respect to the refractive index around the refractive index modulation portion, the x direction is the direction parallel to a plane including the major axis of the core and the major axis of the refractive index modulation portion and perpendicular to the major axis of the core, and the y direction is the direction perpendicular to a plane including the major axis of the core and the major axis of the refractive index modulation portion.

3. A cladding having a constant refractive index; a core within the cladding, the core having a refractive index greater than the refractive index of the cladding; a refractive index modulation portion provided so as to diagonally cross the core and having a refractive index different from that of its surroundings; Equipped with The core has a structure that propagates in a 4LP mode, d / r ｃ >=1.4 1deg<θ<2deg r ｙ / r ｃ <a(r ｘ / r ｃ ) ｂ When 0<δn<0.007 a=-123.8δn+1.21 b=421.5δn-3.53 When −0.005<δn<0, a=42.6δn+0.3 b=-96.5δn-1.4 A mode converter characterized by satisfying the above. where d is the distance between the center of the end of the refractive index modulation section and the major axis of the core, and r ｃ is the radius of the core, θ is the angle between the major axis of the refractive index modulation portion and the major axis of the core, δn is the refractive index modulation amount, r ｘ is the radius of the refractive index modulation portion in the x direction, r ｙ is the radius of the refractive index modulation portion in the y direction, the refractive index modulation amount is the difference in the refractive index of the refractive index modulation portion with respect to the refractive index around the refractive index modulation portion, the x direction is the direction parallel to a plane including the major axis of the core and the major axis of the refractive index modulation portion and perpendicular to the major axis of the core, and the y direction is the direction perpendicular to a plane including the major axis of the core and the major axis of the refractive index modulation portion.

4. 4. An optical fiber transmission line comprising the mode converter according to claim 1 provided in multiple stages midway along an optical fiber.

5. 4. An optical connector comprising a mode converter according to claim 1 provided in a ferrule.

6. 4. An optical adapter comprising the mode converter according to claim 1 provided in an optical propagation path.

7. 4. An optical amplifier comprising a mode converter according to claim 1 provided in an optical propagation path.

8. 4. A mode multiplexer / demultiplexer, comprising: a mode converter according to claim 1 provided at a multimode end.

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