US-12625031-B2 - Device, method and system for calculating power coupling coefficients between cores

Abstract

A device according to the present disclosure: acquires a group delay time difference between eigenmodes, at a specific wavelength, in a coupled two-core fiber; acquires spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber; and calculates an average power coupling coefficient between cores, at the specific wavelength, within an entire length of the coupled two-core fiber by using the group delay time difference, the spatial mode dispersion, and a length of the coupled two-core fiber.

Inventors

• Atsushi Nakamura
• Yusuke Koshikiya
• Nazuki HONDA

Assignees

• NTT, INC.

Dates

Publication Date

20260512

Application Date

20210802

Claims (4)

1. 1 . A method comprising: acquiring a group delay time difference between eigenmodes, at a specific wavelength, in a coupled two-core fiber; acquiring spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber; calculating an average power coupling coefficient between cores, at the specific wavelength, within an entire length of the coupled two-core fiber by using the group delay time difference, the spatial mode dispersion, and a length of the coupled two-core fiber, wherein the power coupling coefficient is calculated by using Expression C2, [ Math . C2 ]  Δ ⁢ τ = d ⁢ Δ ⁢ β d ⁢ ω ⁢ L h ( C2 ) where Δ τ represents the spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber, dΔβ/dω represents the group delay time difference between the eigenmodes, at the specific wavelength, in the coupled two-core fiber, L represents a fiber length of the coupled two-core fiber, and h represents a power coupling coefficient between cores; and optimizing design of the coupled two-core fiber using the power coupling coefficient.
2. 2 . A system that calculates a power coupling coefficient between cores of a coupled two-core fiber, the system comprising: a light source configured to inject light into the coupled two-core fiber; a light receiving unit configured to receive light propagating through the coupled two-core fiber; a processor interfaced with the light receiving unit; and a storage medium having computer program instructions stored thereon, when executed by the processor, perform to: acquire a group delay time difference between eigenmodes, at a specific wavelength, in a coupled two-core fiber using the light injected by the light source and received by the light receiving unit; acquire spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber using the light injected by the light source and received by the light receiving unit; and calculate an average power coupling coefficient between cores, at the specific wavelength, within an entire length of the coupled two-core fiber by using the group delay time difference, the spatial mode dispersion, and a length of the coupled two-core fiber, wherein the power coupling coefficient is calculated by using Expression C1, [ Math . C1 ]  Δ ⁢ τ = d ⁢ Δ ⁢ β d ⁢ ω ⁢ L h ( C1 ) where Δ τ represents the spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber, dΔβ/dω represents the group delay time difference between the eigenmodes, at the specific wavelength, in the coupled two-core fiber, L represents a fiber length of the coupled two-core fiber, and h represents a power coupling coefficient between cores.
3. 3 . The system according to claim 2 , wherein the coupled two-core fiber has a length in which an impulse response width is widened in proportion to a square root of a distance.
4. 4 . The system according to claim 2 , wherein the computer program instructions further perform to acquire the group delay time difference, at different wavelengths, individually acquire the spatial mode dispersion, at different wavelengths, individually, and calculate the power coupling coefficient, at a wavelength at which both the group delay time difference and the spatial mode dispersion are acquired.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a 371 U.S. National Phase of International Application No. PCT/JP2021/028654, filed on Aug. 2, 2021. The entire disclosure of the above application is incorporated herein by reference. TECHNICAL FIELD The present disclosure relates to a technique for calculating a power coupling coefficient between cores in a coupled two-core fiber. BACKGROUND ART A coupled multicore fiber is one of promising optical fibers as a medium for achieving future large-capacity optical communication. Important characteristics, such as an inter-mode loss difference, an inter-mode delay time difference, and a non-linear characteristic, in the coupled multicore fiber depend on a coupling degree of light intensity between cores. Thus, for optimizing design and grasping transmission characteristics of the coupled multicore fiber, it is important to grasp a power coupling coefficient representing the coupling degree of the light intensity between the cores. In particular, grasping the coupling degree in a coupled two-core fiber is important for designing the coupled multicore fiber having three or more cores and for grasping a transmission path. In Non Patent Literature 1 and Non Patent Literature 2, methods for measuring a power coupling coefficient of a non-coupled multicore fiber are disclosed. In these methods, backscattered light intensities, from a specific core and an adjacent core thereof, resulting from a test light pulse injected into the specific core are measured, and the power coupling coefficient is calculated from a ratio between the backscattered light intensities. The methods of Non Patent Literature 1 and Non Patent Literature 2 can also be applied to measurement of the power coupling coefficient of the coupled multicore fiber on the measurement principle thereof. On the other hand, by the power coupling coefficient of the coupled multicore fiber, which is extremely larger than the power coupling coefficient of the non-coupled multicore fiber, backscattered light intensities from one specific core that the test light pulse is injected into and an adjacent core thereof are caused to become equal to each other once the test light pulse propagates about several meters to several tens of meters. In such a case, in the methods of Non Patent Literature 1 and Non Patent Literature 2, a power coupling coefficient can be obtained only within a section of about several meters to several tens of meters, namely, until equality between the backscattered light intensities of the cores. That is, there is a problem that the power coupling coefficient within an entire length of a long optical fiber cannot be measured. CITATION LIST Non Patent Literature Non Patent Literature 1: M. Nakazawa, M. Yoshida, and T. Hirooka, “Nondestructive measurement of mode couplings along multi-core fiber using a synchronous multi-channel OTDR,” Optics Express, vol. 20, no. 11, pp. 12530-12540, 2012.Non Patent Literature 2: M. Ohashi, K. Kawazu, A. Nakamura, and Y. Miyoshi, “Simple backscattered power technique for measuring crosstalk of multi-core fibers,” in Proceedings of the 17th Opto-Electronics and Communications Conference (OECC), pp. 357-358, 2012.Non Patent Literature 3: Sakamoto et al., “Nondestructive measurement of mode couplings along multi-core fiber using a synchronous multi-channel OTDR,” Proceedings of the Institute of Electronics, Information and Communication Engineers (IEICE) General Conference p. 506, 2015.Non Patent Literature 4: T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto, and K. Nakajima, “Strongly-coupled multi-core fiber and its optical characteristics for MIMO transmission systems,” Optical Fiber Technology, vol. 35, pp. 8-18, 2017.Non Patent Literature 5: C. D. Poole, “Statistical treatment of polarization dispersion in single-mode fiber,” Optics Letters, vol. 13, no. 8, pp. 687-689, 1998. SUMMARY OF INVENTION Technical Problem To solve the above problem, an object of the present disclosure is to provide a device, a method, and a system for calculating an average power coupling coefficient between cores within an entire length of a coupled two-core fiber, which is even a long coupled two-core fiber. Solution to Problem Specifically, a device according to the present disclosure: acquires a group delay time difference between eigenmodes, at a specific wavelength, in a coupled two-core fiber;acquires spatial mode dispersion between the eigenmodes, at the specific wavelength, in the coupled two-core fiber; andcalculates an average power coupling coefficient between cores, at the specific wavelength, within an entire length of the coupled two-core fiber by using the group delay time difference, the spatial mode dispersion, and a length of the coupled two-core fiber. Specifically, a method according to the present disclosure includes: acquiring a group delay time difference between eigenmodes, at a specific wavelength, in a coupled two-core fiber;acquiring spatia