Long period fiber grating and optical transmission system

Abstract

The present application aims to provide an LPFG and an optical transmission system in which a mode conversion amount is independent of a polarization wave state and an electric field distribution. The LPFG of the present application is a cavity row (25-1, 25-2) of cavities 15 arranged in parallel with a center axis z and periodically at positions away from the center axis z in a core region 11 of a few-mode optical fiber 10. Each cavity row (25-1, 25-2) is located at a different position in a length direction of the few-mode optical fiber 10 (a position in the z direction is different), and the cavity rows (25-1, 25-2) are located at positions shifted by 90° on a cross section with the center (intersection of the x axis and the y axis) of the cross section of the core region 11 as a reference.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a long-period fiber grating capable of coupling multiple propagation modes and an optical transmission system including the same. BACKGROUND

[0002] Figure 1 is a diagram illustrating an optical transmission system based on mode multiplexing transmission using a few-mode fiber (FMF) using multiple propagation modes. The mode multiplexing transmission is attracting attention as a large-capacity transmission method because it can increase the transmission capacity by a factor of the number of modes. In transmission using an FMF, intermodal crosstalk occurs in the transmission path, and a MIMO (Multiple-Input Multiple-Output) equalizer is used to compensate for the intermodal crosstalk.

[0003] However, in the case where there is mode dependent loss (MDL), even if a MIMO equalizer is used, the performance of the transmission system is a problem. In addition, in the case where the differential mode delay (DMD) at the receiving end is large, the load of digital processing (DSP) involving MIMO becomes large, and thus reducing the load for long-distance transmission is a problem. Therefore, in order to reduce the influence of MDL or DMD, it has been proposed to use a mode scrambler to generate coupling between modes in a long-period fiber grating (LPFG) (for example, refer to Patent Literature 1).

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature: Japanese Patent Application Publication No. 2019-32440 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] As a method of implementing an LPFG, there are a method of applying stress and bending from the outside, a method of laser irradiation, and the like. These methods can only apply a refractive index change to one direction in the fiber cross section, and the LPFG has a problem that the amount of mode conversion depends on the polarization state and the electric field distribution of the propagation mode, and it is difficult to avoid this dependence of the amount of mode conversion.

[0009] Therefore, in order to solve the above problems, an object of the present application is to provide an LPFG and an optical transmission system in which the amount of mode conversion does not depend on the polarization state and the electric field distribution.

[0010] Means for solving the problem

[0011] To achieve the above object, the LPFG of the present application is formed with periodic cavities at positions apart from the central axis of an optical fiber.

[0012] Specifically, the LPFG of the present application is a long-period fiber grating formed in a few-mode optical fiber capable of propagating n (n is an integer of 2 or more) propagation modes, characterized in that,

[0013] a cavity row is arranged in parallel with the central axis of the core region of the few-mode optical fiber and periodically at positions apart from the central axis of the core region;

[0014] the cavity row is a plurality of rows;

[0015] each of the cavity rows is located at a different position in the length direction of the few-mode optical fiber;

[0016] the cavity rows are located at positions offset by 90° on a cross section of the core region with the center of the cross section as the origin.

[0017] Further, the optical transmission system of the present application includes:

[0018] a multi-mode optical fiber capable of propagating n (n is an integer of 2 or more) transmission modes; and,

[0019] the few-mode optical fiber is connected to the multi-mode optical fiber and formed with the long-period fiber grating.

[0020] By offsetting the cavity rows by 90°, mode conversion independent of the polarization state and the electric field distribution can be performed. Therefore, the present application can provide an LPFG and an optical transmission system in which the amount of mode conversion is independent of the polarization state and the electric field distribution.

[0021] Further, the cavity rows of the LPFG of the present application are preferably equal in cavity interval and in the number of cavities.

[0022] Further, in the LPFG of the present application,

[0023] the propagation modes of the few-mode optical fiber are 3;

[0024] the cavity rows are located at positions where the ratio to the radius of the core region on the cross section of the core region with the center of the cross section as the origin is 0.2 or more and 0.4 or less;

[0025] the ratio of the diameter of each cavity constituting the cavity row to the radius of the core region is preferably 0.3 or more and 0.43 or less.

[0026] In addition, the above-described inventions can be combined as much as possible.

[0027] Inventive Effects

[0028] The present application can provide an LPFG and an optical transmission system in which a mode conversion amount is independent of a polarization state and an electric field distribution. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a diagram illustrating an optical transmission system in which light is coupled between modes.

[0030] Figure 2 is a diagram illustrating an LPFG of the present application.

[0031] Figure 3 is a diagram illustrating an optical transmission system of the present application.

[0032] Figure 4 is a diagram illustrating an effect of an LPFG of the present application.

[0033] Figure 5 is a diagram illustrating an effect of an LPFG of the present application. DETAILED DESCRIPTION

[0034] Embodiments of the present application will be described with reference to the accompanying drawings. The embodiments described below are examples of the present application, and the present application is not limited to the following embodiments. In addition, in the present specification and the accompanying drawings, structural elements having the same reference numerals represent the same structural elements.

[0035] (Embodiment 1)

[0036] Figure 2 is a diagram illustrating an LPFG of the present embodiment. The present LPFG is a long-period fiber grating formed in a few-mode optical fiber 10 capable of transmitting n (n is an integer of 2 or more) transmission modes. Also, the present LPFG is a cavity row (25-1, 25-2) of cavities 15 arranged in parallel with a center axis z and periodically in the few-mode optical fiber 10 in a core region 11 at a position away from the center axis z of the core region 11. Each cavity row (25-1, 25-2) is located at a different position in the length direction of the few-mode optical fiber 10 (the position in the z direction is different),

[0037] The cavity rows (25-1, 25-2) are located at positions shifted by 90° on a cross section when the center (the intersection of the x axis and the y axis) of the cross section of the core region 11 is taken as the origin.

[0038] Figure 2 (A) is a cross-sectional view of the few-mode optical fiber 10. Figure 2 (B) is a diagram in which the few-mode optical fiber 10 is obliquely viewed and seen through. In addition, the symbol 12 is a cladding region.

[0039] The purpose of the present LPFG is to achieve a mode conversion independent of the polarization state and the electric field distribution by periodically arranging the cavities 15 in the transmission direction in the few-mode fiber 10. In order to achieve this purpose, the cavities 15 are periodically arranged inside the core region 11 of the few-mode fiber 10 by using external processing with a femtosecond laser. A plurality of cavities 15 periodically arranged is called a cavity row. The cavity row 25-1 is a row of cavities 15 whose center coordinates are on the x-axis (y-coordinate is 0), and the cavity row 25-2 is a row of cavities 15 whose center coordinates are on the y-axis (x-coordinate is 0). The cavity row 25-1 and the cavity row 25-2 are connected in series. That is, by arranging the cavity rows with a 90° shift from the z-axis, a mode conversion independent of the polarization state and the electric field distribution can be achieved.

[0040] Here, let the difference in the propagation constant between the coupled propagation modes be Δβ, and the interval Λ of the cavities 15 in the z-axis direction be given by mathematical expression 1.

[0041] [mathematical expression 1]

[0042]

[0043] Therefore, an LPFG for adjusting Λ to couple the desired two modes can be created (as shown in FIG. 2, for example, by arranging two cavity rows with a 90° shift from the z-axis). Also, if a plurality of LPFGs with different Λ are connected, coupling between a plurality of modes can also be achieved. Figure 2

[0044] Therefore, the LPFG of the present embodiment can achieve a mode conversion independent of the polarization state and the electric field distribution. However, the following conditions must be satisfied.

[0045] (A) The cavity interval Λ of the cavity row 25-1 is equal to the cavity interval Λ of the cavity row 25-2. This is to couple the desired modes.

[0046] (B) The number of cavities of the cavity row 25-1 is equal to the number of cavities of the cavity row 25-2. This is to eliminate the deviation in the amount of coupling between the polarization waves.

[0047] (C) The order of the cavity row 25-1 and the cavity row 25-2 arranged in the z-axis direction is irrelevant.

[0048] (D) The cavity row 25-1 and the cavity row 25-2 do not necessarily have to be arranged continuously.

[0049] (Embodiment Two)

[0050] Figure 3 ​is a diagram illustrating an optical transmission system 301 of the present embodiment. The optical transmission system 301 includes a multimode optical fiber 50 capable of propagating n (n is an integer of 2 or more) transmission modes, and a few-mode optical fiber 10 connected to the multimode optical fiber 50 and formed with the LPFG explained in Embodiment 1. Further, a symbol 30 is a mode combiner that combines a plurality of modes of light signals propagating in any of the transmission modes of the multimode optical fiber 50. A symbol 40 is a mode splitter that splits a plurality of modes propagating in the multimode optical fiber 50. Further, a symbol 60 is a connection portion.

[0051] The optical transmission system 301 is a multimode multiplex transmission system that uses the multimode optical fiber 50 that transmits n modes as a transmission path. The optical transmission system 301 connects a plurality of few-mode optical fibers 10 having appropriate LPFGs for coupling between desired two modes in the transmission path. As explained above, since the interval A of the cavities of the LPFG is given by the mathematical expression 1, by inserting a plurality of LPFGs having A so as to couple desired two modes in the transmission path, the optical transmission system 301 as a whole is able to couple between a plurality of modes. Further, it is also possible to further improve the effect of reducing MDL and DMD by inserting a plurality of LPFGs in the middle of the transmission path.

[0052] (Embodiment Three)

[0053] In the present embodiment, the effect of the cavity 15 formed in the core region 11 of the few-mode optical fiber 10 is explained. In the present embodiment, an example of a 3-mode optical fiber (2LP mode optical fiber) in which the multimode optical fiber 50 and the few-mode optical fiber 10 perform mode multiplex transmission in three transmission modes is explained. The LPFG of the few-mode optical fiber 10 couples the LP01 mode and the LP11 mode.

[0054] In the present embodiment, it is assumed that the cavity 15 is formed on the x-axis at a distance dl = 4 μm from the center of the core region 11. Further, it is assumed that the position of the cavity 15 is the center position of the cavity. The few-mode optical fiber 10 is of the step type, the core region 11 has a radius of 7 μm, and the relative refractive index difference of the core region 11 with respect to the cladding region 12 is 0.4%. Further, the cavity 15 has a diameter of 2 μm.

[0055] Figure 4 is a table explaining the mode conversion rate in the case where one cavity 15 is given on the x-axis as shown in Figure 2 (A). The x-polarized wave and the y-polarized wave both have about 4.5% of the LP01 mode coupled to the 11b mode (see Figure 4That is, it is possible to couple the LP01 mode to the LP1 lb mode by locating the cavity 15 on the x-axis at a position offset from the center of the core region 11. Likewise, it is possible to couple the LP01 mode to the LP1 la mode by locating the cavity 15 on the y-axis at a position offset from the center of the core region 11.

[0056] Therefore, by connecting in series the cavity column 25-1 in which the cavities 15 are periodically arranged on the x-axis and the cavity column 25-2 in which the cavities 15 are periodically arranged on the y-axis in the core region 11, it is possible to perform the mode conversion independent of the degenerate mode.

[0057] (Embodiment Four)

[0058] In the present embodiment, the distance dependency of the cavities 15 from the center of the core region 11 and the diameter dependency of the cavities 15 are described with respect to the mode conversion rate.

[0059] Figure 5 (A) is a graph showing the conversion rate from the LP01 mode to the LP1 la mode when the distance d1 of the cavity 15 from the center of the core region 11 is changed. Also, the position of the cavity 15 is assumed to be the center position of the cavity. Further, the diameter of the cavity 15 is 2 μm. The vertical axis is the mode conversion rate, and the horizontal axis is the value normalized by the radius of the core region 11 with respect to the distance d1.

[0060] When the cavity 15 is located at the center (the horizontal axis is zero), no mode conversion is performed. When the cavity 15 is offset from the center by about 2 μm, the largest mode conversion rate is shown. On the other hand, if the cavity 15 is further moved away from the center and closer to the cladding region 12, the mode conversion rate decreases. That is, in the case of coupling the LP01 mode and the LP1 l mode, the cavity 15 is located at a position at which the ratio of the distance d1 from the core center to the radius of the core region 11 is 0.2 or more and 0.4 or less, and preferably 0.29.

[0061] Figure 5 (B) is a graph showing the conversion rate from the LP01 mode to the LP1 la mode when the diameter of the cavity 15 is changed. Here, the distance d1 from the center of the cavity 15 is 2 μm. Also, the position of the cavity 15 is assumed to be the center position of the cavity. The first vertical axis is the mode conversion rate, the second vertical axis is the loss caused by the cavity 15, and the horizontal axis is the value normalized by the radius of the core region 11 with respect to the diameter of the cavity 15.

[0062] When the diameter of the cavity 15 is zero (in the case of no cavity), no mode conversion is performed. When the diameter of the cavity 15 is about 3 μm, the largest mode conversion rate is shown. On the other hand, if the diameter of the cavity 15 is further increased, the loss exceeds 1 dB, and the conversion rate decreases. That is, in the case where the LP01 mode and the LP11 mode are coupled, the ratio of the diameter of the cavity 15 to the radius of the core region 11 is 0.3 or more and 0.55 or less, and preferably 0.43 or less so that the loss is 1 dB or less.

[0063] [Supplementary note]

[0064] An object of the present application is to provide an LPFG capable of reducing the dependence of the mode conversion amount on the polarization state and the electric field distribution of the propagation mode.

[0065] Specifically, the present LPFG is a long-period fiber grating capable of coupling a plurality of propagation modes in a few-mode fiber (FMF) capable of propagating a plurality of n (n is an integer of 2 or more) propagation modes, characterized by comprising:

[0066] The LPFG includes:

[0067] a first cavity row in which a plurality of n cavities formed so as to be offset from the center of the cross section toward the x-axis direction are arranged at equal intervals (Λ) along the z-axis; and

[0068] a second cavity row in which a plurality of m cavities formed so as to be offset from the center of the cross section toward the y-axis direction are arranged at equal intervals (Λ) along the z-axis.

[0069] wherein the x / y / z axes are defined as follows.

[0070] x-axis: an axis passing through the center of the cross section and parallel to the cross section

[0071] y-axis: an axis passing through the center of the cross section, parallel to the cross section, and orthogonal to the first axis

[0072] z-axis: an axis passing through the center of the cross section and parallel to the waveguide direction of light in the LPFG

[0073] Explanation of reference numerals

[0074] 10: few-mode fiber

[0075] 11: core region

[0076] 12: cladding region

[0077] 15: cavity

[0078] 25-1, 25-2: cavity row

[0079] 30: mode combiner

[0080] 40: mode splitter

[0081] 50: multimode optical fiber

[0082] 60: connection portion

[0083] 301: optical transmission system

Claims

1. A long-period fiber grating formed in a few-mode fiber capable of propagating n modes, n being an integer of 2 or more, characterized by: a plurality of cavity rows arranged periodically in parallel with a center axis of a core region of the few-mode fiber at positions away from the center axis of the core region; each of the cavity rows being located at a different position in a length direction of the few-mode fiber; the cavity rows being located at positions shifted by 90° on a cross section of the core region with a center of the cross section as an origin; the few-mode fiber having 3 modes; the cavity rows being located at positions having a ratio of 0.2 or more and 0.4 or less to a radius of the core region on the cross section with the center of the cross section as the origin; and a ratio of a diameter of each cavity constituting the cavity rows to the radius of the core region being 0.3 or more and 0.43 or less. The cavity rows are equal in cavity spacing and number of cavities.

3. An optical transmission system comprising: a multi-mode fiber capable of propagating n transmission modes, n being an integer of 2 or more; and a few-mode fiber connected to the multi-mode fiber and formed with a long-period fiber grating according to claim 1 or 2. ​ ​ ​ ​ ​ 2. The long period fiber grating according to claim 1, wherein, ​ ​ ​ ​ ​

Images