Lateral light input / output circuit

Abstract

A lateral light input / output circuit (10) comprises: a grating part (20) including a grating (21) formed on a core (11) of an optical fiber (13); and a tap part (30) including a tap waveguide (31) branched from the core (11). The relative refractive index difference of the optical fiber (13) and the radius of the core are set to values at which the full width at half maximum of light passing through the grating (21) is 20 nm or less.

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] CD Poole, CD Townsend, and KT Nelson, “Helical-Grating Two-Mode Fiber Spatial-Mode Coupler,” J. Lightwave Technol., vol. 9, no. 5, pp. 598-604, May 1991.

[0005] An optical lateral input / output circuit is an optical multiplexer / demultiplexer circuit that includes a grating formed in a core and a tapped waveguide formed by optical tapping. In this circuit, the core and tapped waveguide can be optically coupled for light of a wavelength selected by the grating. The bandwidth of light that passes through the grating is determined by the grating length and the structure of the optical fiber. For example, the longer the grating length, the narrower the bandwidth becomes. In other words, narrowing the bandwidth requires an increase in the size of the optical lateral input / output circuit.

[0006] The present disclosure has been made in view of the above circumstances, and aims to provide an optical lateral input / output circuit that can narrow the bandwidth while avoiding excessive size increase.

[0007] One aspect of the present disclosure is an optical lateral input / output circuit constructed using 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 relative refractive index difference of the optical fiber and the radius of the core are set to values ​​such that the full width at half maximum of light passing through the grating is 20 nm or less.

[0008] According to the present disclosure, it is possible to provide an optical lateral input / output circuit that can narrow the bandwidth while avoiding excessive size increase.

[0009] Fig. 1 is a diagram showing the configuration of an optical lateral input / output circuit according to this embodiment. Fig. 2 is a graph showing the relationship between the complete coupling length of a grating and the full width at half maximum. Fig. 3 is a diagram showing the full width at half maximum of the wavelength of light when the fiber structure is changed at a wavelength of 1550 nm. Fig. 4 is a diagram showing the full width at half maximum of the wavelength of light when the fiber structure is changed at a wavelength of 1550 nm. ４ Fig. 5 is a diagram showing the full width at half maximum of the wavelength of light when the fiber structure is changed at a wavelength of 1310 nm. Fig. 6 is a diagram showing an example of a communication system to which the optical lateral input / output circuit of this embodiment is applied.

[0010] Hereinafter, embodiments 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 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, and the extension direction of the X-axis is referred to as the X-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 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 as a host fiber having a core 11 and a cladding 12. The optical fiber 13 according to this embodiment is a step index type and has an LP ０１ Mode and LP １１However, the optical fiber 13 may be a multi-mode optical fiber that allows propagation of light up to the sixth mode, as long as it is capable of propagating light in the two modes described above.

[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 grating section 20 extracts the LP of light of a wavelength that is desired to be extracted to the tap waveguide 31 from the light propagating through the core 11 of the optical fiber 13. ０１ Only the mode is output to LP by the grating 21. １１ Convert to mode.

[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 １１ The 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 one (i.e., a single) optical fiber, since this reduces the number of manufacturing steps compared to forming them on separate optical fibers and then fusing them together.

[0020] According to Non-Patent Document 1, the full width at half maximum (FWHM) of the grating 21 is expressed by the following formula (1) using the frequency differential β' (expressed as Δβ' in Non-Patent Document 1) of the propagation constant difference between the mode before conversion and the mode after conversion and the complete coupling length Lc. Note that the mode before conversion in this embodiment is LP ０１ mode, and the converted mode is LP １１ The complete coupling length Lc is the length of one mode (for example, LP０１ mode) is the other mode (e.g. LP １１ This is the length of the grating 21 required for complete conversion into the original mode. In equation (1), c is the speed of light, λ ０ indicates the central wavelength of the grating 21. The complete coupling length Lc can be calculated from the coupling constant κ of the refractive index modulation portion 22 by the following formula (2) (see Non-Patent Document 1). From equation (2), it can be seen that the bandwidth decreases as the complete coupling length Lc increases.

[0021] However, when considering the optical lateral input / output circuit 10 as a single device, there is a practical upper limit to the settable complete coupling length Lc from a size perspective. Furthermore, to stably receive the light emitted from the optical lateral input / output circuit 10, it is necessary to fix the relative position between the exit of the optical lateral input / output circuit 10 (i.e., the core 11 or the tap waveguide 31) and the optical receiver. In other words, it is necessary to hold the entire device using a holding member. A device held entirely by a holding member reduces its storability. This further reduces the device size. For example, assuming that the device is to be housed in a current optical closure, the optical lateral input / output circuit 10, i.e., the device, is required to be approximately 5 cm or less in size. In this case, given that the total length of the tap waveguide 31 is approximately 1 cm, the upper limit of the length of the grating 21 is, for example, 4 cm.

[0022] Figure 2 is a graph showing the relationship between the perfect coupling length Lc of the grating 21 and the full width at half maximum. This analysis assumes a core radius of 7 μm and a relative refractive index difference of 0.4%. As shown in Figure 2, if the upper limit of the grating length is set to 4 cm, the full width at half maximum is found to be at most 60 nm.

[0023] As described above, the full width at half maximum also depends on the frequency derivative β' of the propagation constant difference. Therefore, the full width at half maximum can be changed by changing the structure of the optical fiber 13. The optical fiber structure here refers to the relative refractive index difference Δ of the optical fiber 13 and the radius a of the core 11 of the optical fiber 13 (hereinafter referred to as the core radius a), as will be described later.

[0024] As described above, the optical fiber 13 of the optical lateral input / output circuit 10 according to this embodiment is an LP １１ It is assumed that the optical lateral input / output circuit 10 is inserted into a single-mode fiber link. That is, a single-mode optical fiber (not shown) is connected to each of the optical inlet 13a (see FIG. 1) and outlet 13b (see FIG. 1) of the optical fiber 13 (core 11). The design conditions for the two-mode optical fiber in this case are as follows: (a) Two-mode operation in the wavelength range desired to be extracted to the tap waveguide 31; (b) Sufficiently small connection loss with the single-mode fiber.

[0025] For condition (a), by setting the longest wavelength of the multiple wavelength ranges to be extracted to 1625 nm, it is possible to cover a wide band, including the C band, the 1310 nm band used in access systems, and the 850 nm band frequently used for transmitting power supply light.For condition (b), the upper limit of the total connection loss at both ends of the single-mode fiber is set to 0.5 dB.

[0026] 3 shows the full width at half maximum (i.e., bandwidth) of the wavelength of light when the fiber structure is changed at a wavelength of 1550 nm. The full width at half maximum shown in FIG. 3 was calculated using finite element analysis to show the LP when the core radius and the relative refractive index difference were changed. ０１ Mode and LP １１ The propagation constant of the mode is derived, and then calculated based on the obtained propagation constant and equation (1). It is assumed that the wavelength of the light is 1550 nm, the refractive index of the cladding is 1.444, and the complete coupling length Lc is 4 cm. The white dotted line S １ is the LP at a wavelength of 1625 nm １１ The dotted line S in FIG. １ The upper right area R １ LP １１ This shows that light of different modes can exist.

[0027] The two white solid lines S in Figure 3 ２ａ , S ２ｂ indicates the combination of the core radius a and the relative refractive index difference Δ that results in a connection loss per connection point of 0.25 dB at a wavelength of 1550 nm.２ａ , S ２ｂ is the region R ２ and the boundary of the region R ２ In the region R, the total connection loss between the optical fiber 13 and the single-mode optical fiber at both ends is 0.5 dB or less at a wavelength of 1550 nm. ２ Within this range, the optical fiber 13 (core 11) and the single-mode optical fiber can be connected together with a total connection loss of 0.5 dB or less.

[0028] Two white dashed lines S ３ａ , S ３ｂ indicates the combination of the core radius a and the relative refractive index difference Δ at which the full width at half maximum is 20 nm. ３ｂ The region R surrounded by ３ indicates the region where the full width at half maximum is 20 nm. ３ Inside dashed line S ３ｂ and solid line S ２ｂ The region R surrounded (sandwiched) by ４ The region R is indicated by hatching. ４ The combination of the relative refractive index difference Δ and the core radius a in the above table can achieve a bandwidth of 20 nm or less while suppressing the total connection loss at both ends of the optical fiber 13 to 0.5 dB or less.

[0029] FIG. 4 shows the region R ４ The approximate line S of the boundary ａｐ１ , S ａｐ２ , S ａｐ３ The region R is shown by a solid line. ４ The core radius a and the relative refractive index difference Δ in the figure approximately satisfy the following condition: ａｐ１ , S ａｐ２ , S ａｐ３ indicate the boundaries of formula (3), formula (4), and formula (5), respectively. a [μm]>0.71Δ[%]+4.28 (3) a [μm]<3.33Δ[%]+1.45 (4) Δ[%]>1.15 (5) In other words, the core radius a and the relative refractive index difference Δ are set to values ​​that satisfy the condition of formula (6). 0.71Δ[%]+4.28<a [μm]<3.33Δ[%]+1.45 (6)

[0030] As shown in Figure 4, the lower limit of the relative refractive index difference Δ is 1.15%. On the other hand, there is no upper limit to the relative refractive index difference Δ. However, since it is generally difficult to manufacture optical fibers with a high relative refractive index difference Δ, the practical upper limit is considered to be 1.5%. In other words, the relative refractive index difference Δ may be set to a value that satisfies the condition of formula (7): 1.15<Δ[%]<1.5 (7)

[0031] 5 is a diagram showing the full width at half maximum of the wavelength of light when the fiber structure is changed at a wavelength of 1310 nm. The method for calculating the full width at half maximum shown in FIG. 5 is the same as the method for calculating the full width at half maximum shown in FIG. 3. As shown in FIG. 5, when the wavelength becomes shorter, the region R ３ In other words, when the values ​​of the core radius a and the relative refractive index difference Δ are fixed, the full width at half maximum tends to become smaller as the wavelength approaches the short wavelength side. This tendency is also seen in the connection loss. Although there is such a change, as shown in FIG. 5, in the region R ４ is the region R ３ In other words, as long as the relative refractive index difference Δ and the core radius a satisfy the relationships shown in formulas (6) and (7), a full width at half maximum of 20 nm or less can be obtained even at wavelengths shorter than 1550 nm.

[0032] From the above, by forming a grating 21 in which the relative refractive index difference Δ and the core radius a satisfy the relationships shown in equations (6) and (7) and which has a perfect coupling length Lc of 4 cm, it is possible to extract light from the tap waveguide 31 with a bandwidth whose full width at half maximum is 20 nm or less. In other words, it is possible to provide an optical lateral input / output circuit that can narrow the bandwidth while avoiding excessive size.

[0033] Although the complete coupling length Lc is assumed to be 4 cm, the complete coupling length Lc in this embodiment is not limited to this value. Even if the complete coupling length Lc is other than 4 cm, it is possible to obtain a full width at half maximum of 20 nm or less. As described above, from the perspective of size, it is desirable that the complete coupling length Lc be 4 cm or less. However, even if the complete coupling length Lc exceeds 4 cm, there is no problem with the optical characteristics of the optical lateral input / output circuit 10, and the complete coupling length Lc can be appropriately selected depending on the device to which the optical lateral input / output circuit 10 is connected and the installation location of the optical lateral input / output circuit 10.

[0034] (Application Example) FIG. 6 is a diagram showing an example of a communication system 40 to which the optical lateral input / output circuit 10 of this embodiment is applied. This communication system 40 includes an optical multiplexer / demultiplexer 42 and uses a WDM (wavelength division multiplexing) signal as a transmission signal. As shown in FIG. 6, in the communication system 40, an optical fiber 41, which is a single-mode optical fiber, is laid, and optical lateral input / output circuits 10A to 10D according to this embodiment are provided at each node N on the optical fiber 41. Both ends of the core 11 of each of the optical lateral input / output circuits 10A to 10D are connected to the optical fiber 41. Note that the number of optical lateral input / output circuits installed is not limited to four as shown in FIG. 6.

[0035] The optical multiplexer / demultiplexer 42 is a so-called arrayed waveguide grating, and as shown in FIG. １ , λ ２ , λ ３ , λ ４ The optical lateral input / output circuit 10A receives light of wavelength λ １ Only light of a desired wavelength is extracted to the tap waveguide and output to the destination facility (or device) 51A. The optical lateral input / output circuits 10B to 10D also extract only light of a desired wavelength and output it to the corresponding facilities 51B to 51D. In this way, by arranging the optical lateral input / output circuit 10 according to this embodiment in the optical fiber 41 while light of multiple wavelengths is propagating through the optical fiber 41, the destination of the light according to the wavelength can be controlled.

[0036] 10 (10A to 10D) Optical lateral input / output circuit 11 Core 12 Cladding 12a Outer circumferential surface 13 Optical fiber 13a Entrance 13b Exit 20 Grating section 21 Grating 22 Refractive index modulation section 30 Tap section 31 Tap waveguide 40 Communication system 41 Optical fiber 42 Optical multiplexer / demultiplexer 51A to 51D Facility or device N Node

Claims

1. An optical side input / output circuit composed 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 relative refractive index difference of the optical fiber and the radius of the core are set to values such that the full width at half maximum of the light passing through the grating is 20 nm or less.

2. The optical side input / output circuit according to claim 1, wherein the relative refractive index difference and the radius of the core are set to satisfy the following formula: 0.71Δ + 4.28 < a < 3.33Δ + 1.45, 1.15 < Δ < 1.5, where a is the radius of the core [μm], and Δ is the relative refractive index difference of the optical fiber [%].

3. The optical side input / output circuit according to claim 1 or 2, wherein the optical fiber is a two-mode optical fiber.

4. The optical side input / output circuit according to claim 3, wherein the total connection loss with a single-mode optical fiber at both ends of the optical fiber is 0.5 dB or less at a wavelength of 1550 nm.

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