Mode conversion device and method of design

Abstract

The present invention aims to provide: a mode conversion device that can design any bonding efficiency and full width at half maximum; and a design method therefor.　This mode conversion device is capable of propagating in at least two propagation modes, has a long period grating in the optical fiber core, and is characterized by the long period grating fulfilling several C1 relationships. Full width at half maximum FWHM is the wavelength band in which the bonding efficiency is half of the bonding efficiency during mode conversion at a central wavelength; C is bonding efficiency, Lc is full bond length, Lg is grating length, Λ is grating pitch, and Δβ is the propagation constant difference between two propagation modes at the central wavelength for the mode conversion target.

Description

[Technical field]

[0001] The present disclosure relates to a mode conversion device in which a long period grating (LPG) is formed in an optical fiber, and a design method thereof. [Background technology]

[0002] Non-Patent Document 1 discloses an LPG that selectively couples only light of a certain wavelength into the cladding mode of an optical fiber. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Craig D. Poole, etc., “Helical-Grating Two-Mode Fiber Spatial-Mode Coupler”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL 9, NO 5. MAY 1991 Summary of the Invention [Problem to be solved by the invention]

[0004] In an LPG, the relationship between the coupling efficiency and the full width at half maximum is unique, but if any of the following values ​​change: the complete coupling length, the fiber structure, or the central wavelength, the relationship between the coupling efficiency and the full width at half maximum also changes. For this reason, it is difficult to design the coupling efficiency and full width at half maximum of a mode conversion device equipped with an LPG. Note that "full width at half maximum" refers to the wavelength band where the coupling efficiency is half the coupling efficiency of the mode conversion at the central wavelength, and is the wavelength range in which the mode conversion of a mode conversion device is possible.

[0005] In order to solve the above problems, an object of the present invention is to provide a mode conversion device that allows designing of any coupling efficiency and full width at half maximum, and a method for designing the same. [Means for solving the problem]

[0006] In order to achieve the above object, the mode conversion device according to the present invention adjusts parameters based on the relationship between the wavelength derivative of the propagation constant difference Δβ between the two modes to be converted and the coupling efficiency and full width at half maximum.

[0007] Specifically, the mode conversion device according to the present invention is a mode conversion device having a long period grating in a core of an optical fiber with a step-index structure capable of propagating light in at least two propagation modes, and is characterized in that the long period grating satisfies the relationship of number C1.

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[0008] By forming an LPG with a grating pitch Λ and grating length Lg that satisfy the relationship of number C1 in the core of an optical fiber with a core radius a and a core relative refractive index difference Δ, it is possible to obtain a mode conversion device with the desired full width at half maximum FWHM and coupling efficiency C at the desired wavelength.

[0009] A specific design method is a design method for determining design parameters of a long-period grating to be installed in a core of an optical fiber having a step-index structure, which can propagate light in at least two propagation modes, comprising the steps of: The optical fiber has a core radius a (μm), a relative refractive index difference Δ (%), a central wavelength λ0 (nm) of light performing mode conversion, a coupling efficiency C, and a full width at half maximum FWHM (nm) given; From the core radius a (μm) and the relative refractive index difference Δ (%) of the optical fiber, the propagation constant difference Δβ (μm -1 ) and its wavelength derivative dΔβ / dλ are obtained by modal analysis. calculating the grating pitch Λ (μm) of said long period grating by the formula C2; Calculating the coefficient b in number C3; Complete bond length L with number C4 c Calculate (cm), and Number C5 and grating length L g Calculating (cm) It is characterized by:

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[0010] Therefore, the present invention can provide a mode conversion device and a design method thereof that can design any desired coupling amount and full width at half maximum.

[0011] The mode conversion device according to the present invention is characterized in that it further comprises a tap waveguide, which is located after the long period grating in the light propagation direction, and outputs light of a desired wavelength, which is converted by the long period grating from one mode to another mode among the light propagating through the core of the optical fiber, from a side of the optical fiber.

[0012] This mode conversion device can extract light of a desired wavelength with a desired power from light propagating through an optical fiber.

[0013] Here, in the mode conversion device according to the present invention, a set of the long period grating and the tap waveguide may be arranged in tandem in the optical fiber.

[0014] In this case, the long period gratings have different design parameters, as described in the formula C1, so that the wavelengths of the light converted from the one mode to the other mode are different from each other. Light of different wavelengths can be extracted from each tap waveguide.

[0015] In this case, the long period gratings may have different grating lengths Lg such that the wavelengths of the light converted from the one mode to the other mode are the same but the coupling efficiencies are different, and light of different desired powers can be extracted from each tap waveguide.

[0016] The above-mentioned inventions can be combined as much as possible. Effect of the Invention

[0017] The present invention can provide a mode conversion device and a design method thereof that can design any desired coupling efficiency and full width at half maximum. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating an overview of a mode conversion device having a long period grating (LPG). [Diagram 2] FIG. 13 is a diagram illustrating an example of a transmission spectrum when a long-period grating (LPG) is formed. [Diagram 3] FIG. 1 is a diagram illustrating an example of the relationship between the coupling amount and the full width at half maximum (FWHM) when a long period grating (LPG) is formed. [Figure 4]FIG. 13 is a diagram illustrating an example of the relationship between FWHM and the amount of coupling. [Diagram 5] 1 is a diagram illustrating the relationship between the product of FWHM and Lc and dΔβ / dλ. [Figure 6] 1 is a diagram illustrating the relationship between coupling efficiency C and coefficient b. [Figure 7] FIG. 2 is a diagram illustrating a design method according to the present invention. [Figure 8] 1A and 1B are diagrams illustrating a mode conversion device according to the present invention. [Figure 9] 1A and 1B are diagrams illustrating a mode conversion device according to the present invention. [Figure 10] 1A and 1B are diagrams illustrating a mode conversion device according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0020] (Embodiment 1) Figure 1 is a diagram explaining the outline of a mode conversion device with a long-period grating (LPG). By periodically applying perturbation, an incident mode (mode 1) is converted into another mode (mode 2). In this case, the period (Λ) at which the perturbation is applied and the propagation constant difference (Δβ) between mode 1 and mode 2 at the central wavelength (λ0) must satisfy the relationship in formula (1).

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[0021] Here, the length at which the incident mode 1 is completely coupled to mode 2 is called the perfect coupling length (Lc). Lc is determined by the amount of mode conversion per perturbation, and if the amount of mode conversion per location is large, the perfect coupling length becomes shorter. On the other hand, if the amount of mode conversion per location is large, the loss due to mode mismatch increases. In addition, the grating length is defined as Lg. When Lg = Lc, mode 1 is completely converted to mode 2 as shown in Figure 1 (a) as mentioned above. In addition, by adjusting Lg, the amount of coupling (C) from mode 1 to mode 2 can be controlled as shown in Figure 1 (b).

[0022] In this case, the coupling efficiency C at the wavelength λ0 is expressed by equation (2).

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[0023] FIG. 2 is a diagram for explaining an example of the transmittance of light with a central wavelength (λ0) of 1350 nm when a grating with Lc=3 cm is formed in an optical fiber with a core radius (a) of 4.5 μm and a relative refractive index difference (Δ) of the core to the cladding of 0.35%. Mode 1 and mode 2 explained in FIG. 1 are LP01 mode and LP11a mode, respectively. Here, "transmittance" refers to the transmittance of the LP01 mode when the LP01 mode is input. "Coupling efficiency" refers to the amount of attenuation at the central wavelength. Each line in FIG. 2 shows the transmittance when Lg / Lc is changed, and it can be seen that the coupling efficiency can be controlled by controlling Lg / Lc, and that the bandwidth tends to become narrower when the coupling efficiency increases (Lg is increased).

[0024] FIG. 3 is a diagram for explaining the relationship between the coupling efficiency and the full width at half maximum (FWHM). With λ0=1350 nm and Lc=3 cm, the core radius a and the core relative refractive index Δ, which are the optical fiber structure, are (a=5 μm, Δ=0.28%), (a=4.5 μm, Δ=0.35%), and (a=4 μm, Δ=0.44%).

[0025] From Figure 3, we can see that as the coupling efficiency increases (increasing Lg), the FWHM decreases, and that the relationship between FWHM and coupling efficiency depends on the optical fiber structure. The relationship between FWHM and coupling efficiency changes not only with the optical fiber structure, but also with changes in Lc or λ0. Here, we introduce the wavelength derivative (dΔβ / dλ) of the propagation constant difference Δβ between the two modes at wavelength λ0 as a parameter indicating the optical fiber structure. The propagation constant difference Δβ between the two modes can be found from the optical fiber structure by mode analysis.

[0026] Figure 4 is a diagram in which the relationship between FWHM and coupling efficiency explained in Figure 3 is re-plotted with dΔβ / dλ on the horizontal axis. In this diagram, data for different optical fiber structures (Δ, a) and central wavelengths (λ0) are overlaid. Figure 4 shows that the relationship between FWHM and dΔβ / dλ is uniquely determined for coupling efficiency, regardless of fiber structure or central wavelength.

[0027] Furthermore, by calculating the product of FWHM and Lc, the relationship between FWHM and dΔβ / dλ for the coupling efficiency is uniquely determined regardless of Lc. Figure 5 is a diagram explaining the relationship between the product of FWHM and Lc and dΔβ / dλ. Each plot shows the relationship between the product of FWHM and Lc and dΔβ / dλ obtained by calculation, and the dashed lines show the approximation function obtained for each coupling efficiency. The approximation function for each coupling efficiency is shown in equations (3) to (6).

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[0028] As shown in equations (3) to (6), it can be seen that the relationship between the product of FWHM and Lc and dΔβ / dλ can be approximated by an inverse proportional equation shown in equation (7).

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[0029] Thus, a mode conversion device having any desired coupling efficiency and bandwidth can be configured by providing an optical fiber with a grating structure that satisfies formulas (1), (2), (7), and (8). That is, the mode conversion device according to the present invention is characterized in that it has a long-period grating in the core of an optical fiber with a step-index structure that can propagate light in at least two propagation modes, and the long-period grating satisfies the relationship of number C1.

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[0030] Here, the calculation was performed using the conversion efficiency between the LP01 mode and the LP11 mode as an example, but the same can be considered for coupling between other modes, such as the LP01 mode and the cladding mode, the LP01 mode and the LP02 mode, the LP11 mode and the LP21 mode, etc. Furthermore, regardless of whether the optical fiber has a step index structure, it can also be considered in the same way for other structures such as a graded index structure.

[0031] (Embodiment 2) 7 is a flowchart for explaining a method for designing a long-period grating of the mode conversion device described in the first embodiment. The design method is a design method for determining design parameters of a long-period grating to be installed in a core of an optical fiber with a step-index structure, capable of propagating light in at least two propagation modes, the design method comprising the steps of: The core radius a (μm), the relative refractive index difference Δ (%), the central wavelength λ0 (nm) of the light performing mode conversion, the coupling efficiency C, and the full width at half maximum FWHM (nm) of the optical fiber are given (step S01); From the core radius a (μm) and the relative refractive index difference Δ (%) of the optical fiber, the propagation constant difference Δβ (μm -1 ) and its wavelength derivative dΔβ / dλ are obtained by mode analysis (step S02); Calculating the grating pitch Λ (μm) of the long period grating using a formula C2 (step S03); Calculating the coefficient b by the number C3 (step S04); Complete bond length L with number C4 c Calculating (cm) (step S05), and Number C5 and grating length L g Calculating (cm) (Step S06) It is characterized by:

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[0032] The design parameters are dΔβ / dλ, Lc, Lg, Λ, λ0, C, and FWHM, which indicate the optical fiber structure. First, in step S01, the core radius a (μm), relative refractive index difference Δ (%), central wavelength λ0 (nm) of the light performing mode conversion, coupling efficiency C, and full width at half maximum FWHM (nm) of the optical fiber are given as specification values.

[0033] In step S02, the propagation constant difference Δβ and its wavelength differential dΔβ / dλ at the central wavelength λ0 (nm) are obtained by mode analysis from the core radius a (μm) and the relative refractive index difference Δ (%) of the optical fiber structure. In step S03, the propagation constant difference Δβ is substituted into formula (C2) to calculate the grating pitch Λ (μm). In step S04, the coupling efficiency C, which is a specification value, is substituted into formula (C3) to calculate the coefficient b. In step S05, the coefficient b, the full width at half maximum FWHM of the specified value, and the wavelength differential dΔβ / dλ are substituted into formula (C4) to calculate the perfect coupling length Lc. In step S06, the grating length Lg is calculated by substituting the perfect coupling length Lc and the coupling efficiency C, which is a specification value, into formula C5.

[0034] Note that step S03 and steps (S04 to S06) may be performed simultaneously, or one of them may be performed first.

[0035] The wavelength range (FWHM) in which mode conversion is possible differs depending on the device using the LPG. For example, when using a wide band in mode multiplexing transmission or the like, a wider range is preferable. On the other hand, in the case of a tap device as described in embodiment 3, a narrower range is preferable. The key point of the design method of this embodiment is that it is possible to derive the grating pitch Λ and grating length Lg that match the application (specifications) of the device.

[0036] (Embodiment 3) 8 is a diagram illustrating a mode conversion device 301 of this embodiment. Mode conversion device 301 is characterized in that it further includes a tap waveguide 53 that is located after long period grating 21 in the light propagation direction and converts light of a desired wavelength propagating through core 51 of optical fiber 50 from one mode to another mode by long period grating 21, and outputs the converted light from a side surface of the optical fiber.

[0037] In this embodiment, one mode will be described as a fundamental mode, and the other modes as higher-order modes. The mode conversion device 301 includes a tap section 10 having a tap waveguide 53 for outputting light in a higher mode from a side surface of the optical fiber 50, among light propagating through a core 51 of the optical fiber 50; a grating section 20, which is located before the tap section 10 in the propagation direction of the light, and in which a grating 21 is formed in a core 51 of an optical fiber 50, for converting light of a desired wavelength from a fundamental mode to the higher mode; Equipped with.

[0038] The optical fiber 50 is a step-index fiber. The optical fiber 50 has a grating section 20 and a tap section 10 formed in this order in the longitudinal direction. The direction in which light can enter the tap waveguide 53 is defined as the optical waveguide direction. In FIG. 8, the optical waveguide direction is from left to right. The tap direction is defined as the direction in which the tap waveguide 53 faces from the core 51 to the side surface of the optical fiber 50. In FIG. 8, the tap direction is a direction inclined in the forward direction with respect to the optical waveguide direction.

[0039] The grating section 20 converts a desired amount of light having a wavelength to be extracted from the light propagating through the core 51 of the optical fiber 50 from LP01 mode to LP11 mode by the long period grating 21. The grating structure can be realized by, for example, femtosecond laser processing, CO2 laser processing, or pressing a grating.

[0040] The tap unit 10 has a tap waveguide 53 that extends from the center of the core 51 toward the side surface of the optical fiber 50 (the interface with the cladding 52) at an angle α. The tap unit 10 has an angle α between the tap waveguide 53 and the core 51, a diameter d t By controlling the refractive index of the tap waveguide 53, only the LP11 mode is selectively extracted from the core 51.

[0041] Here, the light coupled from the core 51 to the tap waveguide 53 is defined as tap light, and the light propagating directly through the core 51 is defined as transmitted light. For example, by connecting a photodetector 54 to the output end of the tap unit 10 (the side surface of the optical fiber 50), it is possible to extract and receive only the tap light from the optical fiber 50.

[0042] In the tap section 10, the coupling efficiency from the core 51 to the tap waveguide 53 strongly depends on the propagation mode of the light propagating through the core 51. This is because the higher the mode, the smaller the confinement and the easier it is to couple to the tap waveguide 53. For this reason, only the higher mode can be transitioned to the tap waveguide.

[0043] In order to couple only the higher-order mode to the tap waveguide 53, the refractive index and diameter d t The values ​​of are important. If these values ​​are too large, the NA of the tap waveguide 53 becomes large, and the LPO1 mode also becomes easily coupled, resulting in increased loss of transmitted light. On the other hand, if these values ​​are too small, the NA of the tap waveguide 53 becomes small, making it difficult for higher modes to couple, resulting in a decrease in the coupling efficiency of the tap light to the tap waveguide 53. In other words, the refractive index and diameter d t The value of needs to be determined appropriately.

[0044] Furthermore, in order to efficiently couple higher-order mode light to the tap waveguide 53 and to propagate fundamental mode light while keeping it confined in the core 51, it is necessary to make α sufficiently small and to allow the mode to transition adiabatically. If α is large, the LP01 mode will also be affected by the tap waveguide 51 and couple to the radiation mode, resulting in loss. For this reason, the upper limit of α is determined from the perspective of the loss of the LP01 mode. On the other hand, α can take any value greater than 0, but the total length L of the tap section 10 is limited by α. tap Therefore, the lower limit of α is determined from the viewpoint of the propagation loss of the tap waveguide 53 and the requirements for the overall length of the device.

[0045] A typical single mode fiber has a diameter of 50 d f For example, the tap part L tap In order to keep the distance to 5 cm or less, α needs to be set to 0.07° or more.

[0046] The grating section 20 has a grating 21 with a pitch Λ. For example, the grating 21 is a long-period fiber grating (LPG). In order to convert light of an arbitrary wavelength λ and a desired amount of light from the LP01 mode to the LP11 mode in the grating section 20, the grating 21 is molded according to the design parameters described in the first and second embodiments.

[0047] (Embodiment 4) 9 and 10 are diagrams for explaining a mode conversion device 302 of this embodiment. The mode conversion device 302 is characterized in that a set of the long period grating 21 and the tap waveguide 53 of the mode conversion device 301 described in FIG. 8 is vertically arranged in an optical fiber 50. In other words, the mode conversion device 302 is a system in which the mode conversion devices 301 are combined in multiple stages. The sets of the long period grating 21 and the tap waveguide 53 (mode conversion devices 301) are arranged at certain intervals (several meters to several kilometers), for example.

[0048] The long period gratings 21 of the mode conversion device 302 of FIG. 9 are characterized in that the design parameters described in number C1 are different such that the wavelengths of light converted from the one mode to the other mode are different from each other. 9, the mode conversion device 302 assigns a wavelength to each mode conversion device (301-1, 301-2, 301-3) and controls the extracted signal by the wavelength. By setting the design parameters of each mode conversion device according to the wavelength interval (center wavelength λ0 and bandwidth (FWHM)) of the wavelengths to be extracted, it is possible to extract signals at any wavelength interval.

[0049] The long period grating 21 of the mode conversion device 302 of FIG. 10 is characterized in that the grating lengths Lg are different so that the wavelengths of the lights converted from the one mode to the other mode are the same but the coupling efficiencies are different. The mode conversion device 302 in Fig. 10 assigns multiple mode conversion devices (301-1, 301-2, 301-3) to one wavelength. By sending a signal at one wavelength and controlling the coupling efficiency of each LPG, it is possible to extract the signal little by little at multiple stages. The mode conversion device 302 in Fig. 10 can output signals simultaneously from multiple locations at one wavelength. [Explanation of symbols]

[0050] 10: Tap section 20: Grating section 21: Long-period grating (LPG) 50: Optical fiber 51: Core 52: Clad 53: Tapped waveguide 54:Receiver 301, 302: Mode conversion device

Claims

1. A mode conversion device having a long period grating in a core of an optical fiber having a step index structure, capable of propagating light in at least two propagation modes, comprising: A mode conversion device, wherein the long period grating satisfies the relationship of number C1. [Number C1] however, λ is the wavelength (nm), FWHM is the wavelength band (nm) where the coupling efficiency is half the coupling efficiency of the mode conversion at the center wavelength, C is the coupling efficiency, L ｃ is the complete bond length (cm), L ｇ is the grating length (cm), Λ is the grating pitch (μm), and Δβ is the propagation constant difference (μm) between the two propagation modes at the central wavelength of the mode conversion target. －１ ), |dΔβ / dλ| has a unit of (10 １５ m-2).

2. a tap waveguide that is located after the long period grating in the light propagation direction and outputs light of a desired wavelength, which is converted from one mode to another mode by the long period grating, from a side surface of the optical fiber; 2. The mode conversion device of claim 1, wherein the tap waveguide extends from the center of the core toward a side of the optical fiber at an angle α.

3. 3. The mode conversion device according to claim 2, wherein the pair of the long period grating and the tap waveguide are tandemly arranged in the optical fiber.

4. The mode conversion device according to claim 3 , wherein the long period gratings have different design parameters, as defined by the formula C1, such that the wavelengths of light converted from the one mode to the other mode are different from each other.

5. The mode conversion device according to claim 3 , wherein the long period gratings have different grating lengths Lg such that the wavelengths of the light converted from the one mode to the other mode are the same and the coupling efficiencies are different.

6. A method for determining design parameters of a long period grating to be installed in a core of an optical fiber having a step index structure capable of propagating light in at least two propagation modes, comprising the steps of: The optical fiber has a core radius a (μm), a relative refractive index difference Δ (%), and a central wavelength λ of the light that performs mode conversion. ０ (nm), coupling efficiency C, and full width at half maximum FWHM (nm), The central wavelength λ of the mode conversion target is calculated from the core radius a (μm) and the relative refractive index difference Δ (%) of the optical fiber. ０ The difference in propagation constants Δβ (μm －１ ) and its wavelength derivative dΔβ / dλ are obtained by mode analysis; calculating the grating pitch Λ (μm) of said long period grating by equation C2; Calculating the coefficient b by the formula C3; Complete bond length L with number C4 ｃ (cm), and Number C5 and grating length L ｇ Calculate (cm) A design method characterized by: [Number C2] [Number C3] [Number C4] However, the full width at half maximum (FWHM) (nm) is the central wavelength λ ０ The wavelength band where the coupling efficiency is half that of the mode conversion coupling efficiency in (nm), |dΔβ / dλ|, is in units of (10 １５ m-2). [Number C5]

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