Optical side input / output circuit

Abstract

An optical-side input / output circuit (10) comprises: a grating part (20) including a grating (21) formed in a core (11) of an optical fiber (13); and a tap part (30) including a tap waveguide (31) that branches from the core (11). When viewed from the extension direction of the optical fiber (13), the grating (21) is unevenly distributed in the core (11), and the distance (D) from the grating (21) to the tap part (30) is set to a value calculated from a beat length found from a propagation constant of light in two LP 11 modes that are mutually orthogonal and that can propagate through the core (11).

Description

Optical lateral input / output circuit

[0001] The present disclosure relates to an optical side input / output circuit.

[0002] Optical tapping is an optical multiplexing / demultiplexing technology that uses laser processing to form a tapped waveguide (optical waveguide) within an optical fiber. The tapped waveguide is formed within the cladding and branches from the core. The location where the tapped waveguide is formed can be set arbitrarily. Therefore, optical tapping is being considered as a technology that can flexibly accommodate the construction (e.g., expansion) of connection points in the ever-expanding optical communication network (see Patent Documents 1 and 2).

[0003] International Publication No. WO 2022 / 059205 International Publication No. WO 2023 / 275921

[0004] KS Lee and T. Erdogan, “Fiber mode conversion with tilted gratings in an optical fiber,” J. Opt. Soc. Am. A, vol. 18, no. 5, p. 1176, May 2001.

[0005] An optical lateral input / output circuit is an optical multiplexer / demultiplexer circuit that includes a grating formed in a core and a tap waveguide formed by optical tapping. In this circuit, the core and the tap waveguide can be optically coupled for light of a wavelength selected by the grating. In other words, the coupling efficiency between the core and the tap waveguide for light of the selected wavelength is an important parameter that determines the performance of the optical lateral input / output circuit.

[0006] The present disclosure has been made in consideration of the above circumstances, and aims to provide an optical lateral input / output circuit that is capable of controlling the coupling efficiency between a core and a tap waveguide for light of a specific wavelength.

[0007] One aspect of the present disclosure is an optical lateral input / output circuit configured with an optical fiber, comprising a grating section including a grating formed in a core, and a tap section including a tap waveguide branched from the core, wherein the grating is unevenly distributed within the core as viewed from the extending direction of the optical fiber, and the distance from the grating to the tap section is such that two LPs that are orthogonal to each other and can propagate through the core are arranged. １１ It is set to a value calculated from the beat length obtained from the propagation constant of the light in the mode.

[0008] According to the present disclosure, it is possible to provide an optical lateral input / output circuit capable of controlling the coupling efficiency between a core and a tap waveguide for light of a specific wavelength.

[0009] FIG. 1 is a diagram showing the configuration of an optical lateral input / output circuit according to this embodiment. １１ 3A is a cross-sectional view of a first example of a refractive index modulation section as viewed from the Z direction; FIG. 3B is a cross-sectional view of a second example of a refractive index modulation section as viewed from the Z direction; FIG. 4 is a cross-sectional view of an LP mode; ０１ Mode and LP １１ 5 is a diagram showing the analysis results of the excitation efficiency of the LP mode with respect to the area ratio of the refractive index modulation section. １１ａ 6 is a graph showing the excitation efficiency of the LP mode. １１ LP vs. angle of the electric field distribution of the mode １１ FIG. 7 shows the coupling efficiency of the LP mode. １１ 8 shows the relationship between the angle of the electric field distribution of the mode and the propagation distance. １１ 9 is a graph showing the beat length caused by the mode. FIG. 9 is a graph in which the beat length on the vertical axis in FIG. 8 is converted to a value that is 7% of the beat length. FIG. 10 is a diagram showing the coefficients k and m when the relative refractive index difference is changed. FIG. 11 is a graph showing the LP ０１ 12 is a diagram illustrating an example of a power supply system to which the optical lateral input / output circuit of the present embodiment is applied, showing an electric field distribution in the optical lateral input / output circuit when light of a 1000-nm mode is incident.

[0010] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the drawings, identical parts are designated by the same reference numerals and their description will be omitted. For convenience of description, mutually orthogonal X-axis, Y-axis, and Z-axis are defined. The Z-axis is the central axis of the optical fiber 13 (see FIG. 1). The extension direction of the Z-axis is referred to as the Z-direction, the extension direction of the X-axis as the X-direction, and the extension direction of the Y-axis as the Y-direction. As an example, a tap waveguide 31 (see FIG. 1), which will be described later, is disposed in a plane including the X-axis and the Z-axis (i.e., the XZ plane). Furthermore, light in the optical fiber 13 is assumed to propagate from left to right in FIG. 1.

[0011] 1 is a diagram showing the configuration of an optical lateral input / output circuit 10 according to this embodiment. As shown in FIG. 1, the optical lateral input / output circuit 10 is configured by an optical fiber 13 having a core 11 and a cladding 12. The optical fiber 13 according to this embodiment is a step-index optical fiber. The optical fiber 13 is also a multi-mode optical fiber that allows propagation of light up to the sixth mode, and is configured to transmit light in the fundamental mode and the LP mode. １１ It may also be a two-mode optical fiber capable of propagating light in two modes.

[0012] The optical lateral input / output circuit 10 includes a grating section 20 and a tap section 30. As described above, the optical lateral input / output circuit 10 is configured with an optical fiber 13. Therefore, both the grating section 20 and the tap section 30 include a core 11 and a cladding 12 as basic components.

[0013] First, the tap section 30 will be described. The tap section 30 is provided after the grating section 20 in the propagation direction of light in the optical fiber 13. The tap section 30 includes a tap waveguide 31 branched from the core 11. The tap waveguide 31 is formed in a cylindrical shape within the cladding 12. The tap waveguide 31 branches from the core 11 at an angle θt, extends to the outer peripheral surface 12a of the cladding 12, and is exposed to the outside of the optical fiber 13. The tap waveguide 31 can be fabricated by refractive index modulation using a femtosecond laser.

[0014] The coupling efficiency from the core 11 to the tap waveguide 31 strongly depends on the mode of light. The higher the mode, the larger the numerical aperture (NA) and therefore the easier it is to couple to the tap waveguide 31. By appropriately setting the parameters of the tap waveguide 31, it is possible to make only the higher mode transition to the tap waveguide 31. In this embodiment, the angle θt formed between the core 11 and the tap waveguide 31, the diameter of the tap waveguide 31, and the refractive index of the tap waveguide 31 are set to LP. １１ The value is set so that only the light in the selected mode propagates from the core 11 to the tap waveguide 31 (see Patent Document 1).

[0015] The tap waveguide 31 extends linearly from the core 11. A downstream portion 32 of the tap waveguide 31 may extend linearly as is to the outer peripheral surface 12a, or may be curved toward the outer peripheral surface 12a. In the latter case, the portion 32 extends, for example, in an arc shape with a curvature radius R, which can suppress internal reflection when light is emitted compared to the former case.

[0016] Next, the grating section 20 will be described. The grating section 20 includes a grating 21 formed in the core 11 of the optical fiber 13. The distance D from the grating 21 to the tap section 30 (tap waveguide 31) is a beat length L (described later). ｂ is equal to the value calculated from

[0017] The grating 21 is a so-called long-period fiber grating (LPG, LPFG) and has refractive index modulation portions 22 arranged at a pitch Λ along the Z direction. The refractive index of the refractive index modulation portions 22 is slightly larger or smaller than the refractive index of the portion of the core 11 where the refractive index modulation portions 22 are not provided.

[0018] The pitch Λ of the grating 21 is set to ０１ Mode to LP １１ The length of the grating 21 along the longitudinal direction of the optical fiber 13 is also set to a value that allows the light to pass through the grating 21 while being converted into the LP mode (see Patent Document 1). ０１ Mode to LP １１The grating 21 is set in consideration of the efficiency of conversion into the mode (see Patent Document 1). The grating 21 can be fabricated by ultraviolet irradiation, laser irradiation, or application of periodic stress.

[0019] It is desirable to form the grating 21 and the tap waveguide 31 on a single (i.e., single) optical fiber. This reduces the number of manufacturing steps compared to forming them on separate optical fibers and then fusing them. It also allows the distance D to be set accurately.

[0020] Here, the LP between the core 11 and the tap waveguide 31 １１ The coupling of modes is explained in FIG. １１ 10 is a diagram showing the electric field distribution of the mode. It is assumed that the core 11 and the tap waveguide 31 are arranged in the XZ plane, and the core 11 extends in the Z direction.

[0021] As shown in FIG. 2, the LP propagating through the core 11 (optical fiber 13) １１ There are two degenerate modes that are orthogonal to each other. For the sake of convenience, one of these two degenerate modes is referred to as the LP mode. １１ａ mode, the other to LP １１ｂ This is called a mode. １１ａ In the electric field distribution of the mode, two regions 15 of high electric field strength (so-called peaks of the electric field distribution) are arranged in the X direction on either side of the Y axis. That is, the arrangement direction of these two regions 15, 15 coincides with the arrangement direction of the core 11 and the tap waveguide 31. Therefore, the LP of the core 11 and the tap waveguide 31 １１ａ The modes couple with high coupling efficiency.

[0022] On the other hand, LP １１ｂ In the electric field distribution of the mode, two regions 15 with high electric field strength are arranged in the Y direction on either side of the X axis. That is, the arrangement direction of these two regions 15, 15 is perpendicular to the arrangement direction of the core 11 and the tap waveguide 31. Therefore, the LP of the core 11 and the tap waveguide 31 １１ｂ The modes are hardly coupled.

[0023] LP １１ When light of the mode propagates through the optical fiber 13, the electric field distribution in the core 11 is １１ａ Mode and LP１１ｂ The modes change while rotating so that they appear alternately. １１ａ Mode to LP １１ｂ The propagation length of light until the electric field distribution changes to the mode (or vice versa) is the beat length L calculated from the difference in propagation constants of both modes. ｂ Depends on.

[0024] LP of desired strength １１ａ In order to guide the mode from the core 11 to the tap waveguide 31, (1) LP １１ａ (2) Excited LP １１ａ Therefore, the optical lateral input / output circuit 10 according to this embodiment satisfies the following conditions: (a) When viewed from the Z direction, the grating 21 is unevenly distributed within the core 11; (b) The distance D from the grating 21 to the tap waveguide 31 is set to be equal to or smaller than the beat length L. ｂ Set based on.

[0025] First, the condition (a) will be explained. According to the description in Non-Patent Document 1, by forming a long-period fiber grating asymmetrically in the core, it is possible to excite only a specific degenerate mode. In addition, the grating 21 of this embodiment is unevenly distributed in the core 11, so that the LP ０１ From Mode no Hikari to LP １１ In other words, the LP ０１ LP mode light １１ Converts into light of mode.

[0026] 3A is a cross-sectional view of a first example of the refractive index modulation portion 22 as viewed from the Z direction. As shown in Fig. 3A, the refractive index modulation portion 22 has a circular cross section, and its radius an is smaller than the radius a of the core 11. In the core 11, the portions other than the portions where the refractive index modulation portion 22 is formed have the original (i.e., unmodulated) refractive index.

[0027] The refractive index modulation portions 22 are distributed asymmetrically in a cross section of the core 11 perpendicular to the Z axis (central axis). In other words, the refractive index modulation portions 22 are unevenly distributed in one direction (positive side of the X direction in the example of FIG. 3A ) perpendicular to the Z axis (central axis) within the core 11 when viewed from the Z direction. １１The two regions 15 (see FIG. 2 ) of the electric field distribution due to the mode are generated parallel to the direction in which the refractive index modulation portion 22 is unevenly distributed. For example, when two regions 15 aligned in the X direction are generated, the refractive index modulation portion 22 may be unevenly distributed on the positive side of the X direction or on the negative side of the X direction.

[0028] 3B is a cross-sectional view of a second example of the refractive index modulation section 22 as viewed from the Z direction. As long as the grating 21 is unevenly distributed within the core 11, the outer shape of the refractive index modulation section 22 as viewed from the Z direction is not limited to the circle shown in FIG. 3A or the semicircle shown in FIG. 3B. For example, as shown in FIG. 3B, the refractive index modulation section 22 may have a semicircular cross section having the same radius as the core 11. The semicircular cross section is located, for example, only on the positive side or the negative side in the X direction.

[0029] FIG. 4 shows the LP in the grating 21. ０１ When light of the mode is incident, ０１ Mode and LP １１ 10 is a diagram showing the analysis results of the excitation efficiency of the mode, where the horizontal axis represents the propagation distance of light from the front end of the grating 21, i.e., the grating length L ｇ The vertical axis indicates the excitation efficiency of each mode. In this analysis, the cross-sectional shape of the refractive index modulation portion 22 is assumed to be a circle as shown in FIG. 3A. The radius an of the refractive index modulation portion 22 is set to 4 μm, the radius a of the core 11 is set to 7 μm, the relative refractive index difference Δ of the core 11 is set to 0.4%, and the pitch Λ is set to 713 μm.

[0030] As shown in FIG. 4, the excitation efficiency of the LP10 mode gradually decreases as the light propagates. １１ Mode (e.g. LP １１ａ In other words, by distributing the grating 21 unevenly within the core 11 (i.e., forming it asymmetrically), the excitation efficiency of the LP mode increases as the light propagates. １１ The modes can be excited.

[0031] In a cross section perpendicular to the Z axis (center axis) of the optical fiber 13, the area of ​​the refractive index modulation portion 22 may be set to a value between 5% and 50% of the area of ​​the core 11. １１ａ5 is a graph showing the excitation efficiency of a mode. The area ratio is the ratio of the area of ​​the refractive index modulation portion 22 with a radius an to the area of ​​the core 11 with a radius a (see FIG. 3A). In this analysis, as shown in the graph, a circular refractive index modulation portion 22 is assumed whose center in the x direction is located at a position 1 / 2 of the radius a of the core 11 (x=a / 2) and whose center in the y direction is located at the center of the core 11. The area ratio changes depending on the value of the radius an. As shown in FIG. 5, by setting the area ratio within the range of 5% to 50%, an excitation efficiency of 90% or more can be obtained.

[0032] Next, the condition (b) will be explained. FIG. 6 shows the LP between the core 11 and the tap waveguide 31 with respect to the angle φ. １１ The angle φ is the angle between the X direction and the LP direction. １１ａ LP based on the electric field distribution of the mode １１ is the tilt angle (rotation angle around the Z axis) of the electric field distribution of the mode. １１ The electric field distribution of the mode is LP １１ａ When the electric field distribution of the LP mode coincides with that of the LP mode (see Figure 2), φ = 0°. １１ The electric field distribution of the mode is LP １１ｂ When it coincides with the electric field distribution of the mode (see FIG. 2), φ=90°. This analysis also assumes a tap waveguide 31 branching from the core 11 in the X direction.

[0033] When φ=0°, LP １１ The two regions 15 in the electric field distribution of the mode (i.e., the peaks of the electric field distribution) are distributed parallel to the extension direction of the tap waveguide 31 projected onto the XY plane. In this case, the coupling efficiency is 97%. On the other hand, when φ=90°, the LP １１ The two regions 15 in the electric field distribution of the mode are distributed perpendicular to the extension direction of the tap waveguide 31 projected onto the XY plane. In this case, the coupling efficiency is 0%, and it is clear that the light from the core 11 cannot be extracted to the tap waveguide 31.

[0034] FIG. 7 shows the LP １１ Angle φ of the electric field distribution of the mode and propagation distance L φ As described above, LP １１The light of the mode propagates while rotating the electric field distribution of the mode. However, as shown in FIG. 6, the smaller the angle φ, the higher the coupling efficiency. Therefore, the propagation distance L φ (i.e., distance D) １１ By setting the mode coupling efficiency to a desired value, the coupling efficiency between the core 11 and the tap waveguide 31 for light of a specific wavelength can be controlled.

[0035] For example, 90% or more LP １１ It is assumed that the coupling efficiency of the LP mode is to be obtained. In this case, the angle φ is set to 15° or less as shown in FIG. 6. １１ The propagation distance of light when the electric field distribution of the mode makes one rotation, i.e., the beat length L ｂ is a degenerate LP １１ It can be calculated by the following equation (1) using the difference in the propagation constants of the modes. Here, β １ is LP １１ａ Mode propagation constant, β ２ is LP １１ｂ is the propagation constant of the mode.

[0036] For example, when the radius a of the core 11 is 7 μm and the relative refractive index difference Δ of the optical fiber 13 is 0.4%, the beat length L ｂ The propagation distance L is 25 cm. φ As shown in FIG. 7, the propagation distance L φ By setting the distance D to 1.8 cm or less, the angle φ becomes 15° or less. φ (Distance D) is the beat length L ｂ By setting the value to 7% or less, a coupling efficiency of 90% or more can be obtained.

[0037] FIG. 8 shows the relationship between the radius a of the core 11 and the relative refractive index difference Δ of two LPs. １１ Beat length L generated by the mode ｂ 8 shows the cases where the relative refractive index difference Δ is 0.6%, 0.8%, 1.0%, 1.2%, and 1.4%. In each case, the beat length L ｂincreases monotonically with the radius a of the core 11.

[0038] FIG. 9 shows the beat length L ｂ , the beat length L ｂ 9 is a graph converted into a distance D at which a value of 7% of the refractive index difference Δ, i.e., a coupling efficiency of 90% or more, is obtained. From Fig. 9, it can be seen that when the relative refractive index difference Δ is constant, the distance D [cm] can be approximated by a linear function shown by the dotted line with respect to the radius a [μm] of the core 11. For example, when Δ = 0.6%, it can be approximated as D = 0.6 × a [μm] - 1.87, and if D = k × a [μm] + m, then k = 0.6 and m = -1.87.

[0039] 10 is a diagram showing the coefficients k and m when the relative refractive index difference Δ [%] is changed. Based on the values ​​shown in FIG. 10, the following equations (2) to (4) are used: D [cm] < k × a [μm] + m (2) k = 0.76 (Δ [%]) ２ -1.97Δ[%]+1.56...(3) m=-5(Δ[%]) ２ +11.65Δ[%]−7.58 (4) By setting the distance D so as to satisfy １１ It can be seen that mode rotation is suppressed and a coupling efficiency of 90% is obtained.

[0040] FIG. 11 shows the LP ０１ 6 shows the electric field distribution in the optical lateral input / output circuit 10 when light of mode 1 is incident. The grating 21 is formed from the position Z=5000 μm to the position Z=40800 μm. The tap waveguide 31 branches off from the position Z=40800 μm at an angle θt=0.1°. Therefore, the distance D in this case is 0 mm. The radius a of the core 11 is 7 μm, and the radius of the tap waveguide 31 is 3.78 μm. The angle φ (tilt) and pitch of the refractive index modulation section 22 are 0° and 711 μm, respectively. Therefore, as shown in FIG. 6, the coupling efficiency is 97%.

[0041] As shown in FIG. 11, the light propagates and the LP in the grating 21 ０１ The mode is attenuated, and conversely, the LP １１ａ The mode increases. The light is １１ａThe LP mode is dominant and reaches the entrance of the tap waveguide 31. １１ａ The coupling efficiency of the modes is 97%. １１ａ The light in the mode changes direction from the core 11 to the tap waveguide 31 and propagates through the tap waveguide 31 .

[0042] In this manner, in this embodiment, the grating 21 is arranged asymmetrically within the core 11, and the distance D from the grating 21 to the tap waveguide 31 is equal to the beat length L ｂ This allows the conventional LP １１ Compared to the mode excitation method, LP １１ａ The LP that selectively excites only the light of the mode １１ａ The light of the mode can be extracted to the tap waveguide 31 according to the rotation angle. １１ａ It is possible to provide an optical lateral input / output circuit 10 capable of controlling the mode coupling efficiency. １１ａ By setting the mode coupling efficiency to a desired value or higher, １１ａ It is possible to extract the light of the mode to the tap waveguide 31 with high efficiency.

[0043] 12 is a diagram showing an example of a power supply system to which the optical lateral input / output circuit 10 of the present embodiment is applied. In a power supply system 40A according to the first example, an optical fiber 41 for power supply and an optical fiber 42 for signals are laid. A node N of the optical fiber 41 is provided with an optical lateral input / output circuit 10A for power supply according to the present embodiment. Meanwhile, a node N of the optical fiber 42 is provided with an optical lateral input / output circuit 10B for signals according to the present embodiment. Each tap waveguide of the optical lateral input / output circuit 10A and the optical lateral input / output circuit 10B is connected to a corresponding device 51. The device 51 is, for example, a small antenna, an electromagnetic field sensor, or a thermal sensor.

[0044] The optical fiber 41 is connected to a power supply light (wavelength λ ｐ The feed light is branched at each node N (i.e., optical lateral input / output circuit 10A) and supplied to the device 51 via a tap waveguide.

[0045] When extracting the optical feed light from multiple points on the optical fiber 41, if optical couplers with the same branching ratio are used at each node N, the power of the branched optical feed light decreases as the optical coupler is positioned at the later stage. In order to avoid such a decrease in power, the grating length L ｇ may be set (adjusted) to a desired branching ratio (coupling efficiency). In this case, the power of the power supply light supplied from each node N to the device 51 is substantially optimized, so that excessive power consumption of the light source that generates the power supply light can be suppressed.

[0046] The signal optical fiber 42 propagates signal light that is input to and / or output from each device 51. For example, the wavelength of the signal light differs for each node N, and the wavelength λ ｓ１ ~λ ｓ４ The pitch Λ of the optical lateral input / output circuits 10 of each node N is also set (adjusted) accordingly. Therefore, in each node N, it is possible to selectively extract signal light of a wavelength that matches the corresponding device 51, and it is also possible to output light of different wavelengths output from each device 51 to the optical fiber 42.

[0047] In the power supply system 40B, one optical fiber 43 serves as both the optical fiber for power supply and the optical fiber for signals. An optical lateral input / output circuit 10A for power supply and an optical lateral input / output circuit 10B for signals are provided on each node N. The tap waveguides of the optical lateral input / output circuits 10A and 10B are connected to corresponding devices 51. The optical lateral input / output circuit 10 of this embodiment can selectively extract wavelengths. Therefore, the power supply system 40B using one optical fiber 43 can perform the same operation as the power supply system 40A using two optical fibers 41 and 42.

[0048] REFERENCE SIGNS LIST 10 (10A, 10B) Optical lateral input / output circuit 11 Core 12 Cladding 12a Outer circumferential surface 13 Optical fiber 15 Region 20 Grating section 21 Grating 22 Refractive index modulation section 30 Tap section 31 Tap waveguide 40A, 40B Power supply system 41 to 43 Optical fiber 51 Device

Claims

1. An optical lateral input / output circuit made of an optical fiber, comprising: a grating section including a grating formed in a core; and a tap section including a tap waveguide branched from the core, wherein the grating is unevenly distributed within the core when viewed from the extending direction of the optical fiber; and the distance from the grating to the tap section is such that two LPs that are mutually orthogonal and capable of propagating through the core are arranged. １１ The optical lateral input / output circuit is set to a value calculated from the beat length obtained from the propagation constant of the light in the mode.

2. The optical lateral input / output circuit according to claim 1, wherein the distance is set to 7% or less of the beat length.

3. The optical lateral input / output circuit according to claim 2, wherein the distance is set to satisfy the following formula: D<L×a+M L=0.76Δ ２ −1.97Δ+1.56 M=−5Δ2+11.7Δ−7.58 D: the distance [cm] a: the radius of the core [μm] Δ: the relative refractive index difference of the optical fiber [%] 4. An optical lateral input / output circuit as claimed in any one of claims 1 to 3, wherein in a cross section perpendicular to the central axis of the optical fiber, the area of ​​the refractive index modulation section forming the grating is set to a value between 5% and 50% of the area of ​​the core.