EP-3495859-B1 - INTER-MODE LOSS DIFFERENCE COMPENSATOR AND OPTICAL AMPLIFIER

EP3495859B1EP 3495859 B1EP3495859 B1EP 3495859B1EP-3495859-B1

Inventors

WADA, MASAKI
SAKAMOTO, TAIJI
MORI, TAKAYOSHI
AOZASA, SHINICHI
YAMAMOTO, TAKASHI
NAKAJIMA, KAZUHIDE

Dates

Publication Date: 20260513
Application Date: 20170801

Claims (3)

A differential mode attenuation compensator comprising: a first multi-mode optical fiber (11A) and a third multi-mode optical fiber (11B) in which a plurality of propagation modes propagate in a wavelength of a propagating optical signal; and a second multi-mode optical fiber (12) including a core and a clad, and arranged with a central axis aligned between the first multi-mode optical fiber and the third multi-mode optical fiber; the differential mode attenuation compensator (101) being configured by cascading two types of multimode fibers, that is, the first multi-mode optical fiber (11A), the second multi-mode optical fiber (12), and the third multi-mode optical fiber (11B) in this order; wherein each loss in the plurality of propagation modes is different in the first multi-mode optical fiber (11A) and the third multi-mode optical fiber (11B), which means that a loss in a propagation mode propagating through the first multi-mode optical fiber (11A) is different from a loss in a propagation mode that passes from the first multi-mode optical fiber (11A) through the second multi-mode optical fiber (12) and propagates through the third multi-mode optical fiber (11B); the number of propagation modes propagating through the second multi-mode optical fiber (12) is 5 or more; the core of the second multi-mode optical fiber (12) includes a ring-shaped core portion having a ring-shaped refractive index distribution; the core (17) has an inner ring portion having a first radius (a3) and an outer ring-shaped portion (19) extending radially in a region between the first radius (a3) and a second radius (a4); the core (17) has a ring-shaped refractive index distribution; the refractive index of the inner ring-shaped portion having the first radius (a3) is identical to that of the clad (18), and the outer ring-shaped portion (19) in said region between the first radius (a3) and the second radius (a4) has a relative refractive index difference (Δ4);in the second multi-mode optical fiber, a ratio (a3/a4) of the first radius (a3) to the second radius (a4) of the ring-shaped core portion is 0.1 or more; the first and third multi-mode optical fibers (11A,11B) have step-type refractive index profiles; and refractive index distributions of the first multi-mode optical fiber and the third multi-mode optical fiber are different from a refractive index distribution of the second multi-mode optical fiber.
The differential mode attenuation compensator according to claim 1, wherein the number of the plurality of propagation modes propagating through the first multi-mode optical fiber and the third multi-mode optical fiber is 5 or less.
An optical amplifier comprising: an optical amplification unit that amplifies an optical signal propagating in a plurality of propagation modes; and the differential mode attenuation compensator according to claim 1 or 2 arranged at a rear stage of the optical amplification unit in a propagation direction of the optical signal.

Description

Technical Field The present disclosure relates to a differential mode attenuation compensator that compensates a loss difference between modes in mode multiplexed transmission and an optical amplifier including the same. Priority is claimed on Japanese Patent Application No. 2016-153169 filed in Japan on August 03, 2016 and Japanese Patent Application No. 2016-153996 filed in Japan on August 04, 2016. Background Art In recent years, internet traffic is still increasing due to the diversity of services, and transmission capacity has been dramatically increasing due to an increase in the transmission speed and an increase in the number of multiplexed wavelengths by a wavelength division multiplexing (WDM) technique. Further, in recent years, it is expected that the transmission capacity will be further expanded by a digital coherent technique which has been extensively studied. In a digital coherent transmission system, frequency utilization efficiency has been improved by using multilevel phase modulation signals, but a higher signal-to-noise ratio is required. However, in a transmission system using a single mode fiber (SMF) in the related art, the transmission capacity is expected to saturate at 100 Tbit/sec as the boundary due to the input power limitation resulting from the nonlinear effect in addition to the theoretical limit, and it becomes difficult to further increase the capacity. In order to further increase the transmission capacity in the future, a medium realizing innovative transmission capacity expansion is needed. Therefore, mode multiplexed transmission using multi mode fiber (MMF) that uses a plurality of propagation modes in an optical fiber as channels and can be expected to improve the signal-to-noise ratio and space utilization efficiency has been attracting attention. A higher-order mode propagating through the fiber has been the cause of signal deterioration, but active utilization is considered for development of digital signal processes and multiplexing/demultiplexing techniques (for example, see NPL 2). In addition to the expansion of the transmission capacity, consideration is being made for long distance mode multiplexed transmission, and 527 km transmission using a non-coupling type 12-core fiber capable of three-mode propagation has been reported (for example, see NPL 3). In long-distance mode multiplexed transmission, in order to perform long distance transmission, a differential modal attenuation (DMA) generated in a transmission line and a differential modal gain (DMG) generated in an optical amplifier become important. Even in NPL 3, the mode dependent loss (MDL) including DMA and DMG is adjusted to 0.2 dB or less per span in order to realize long distance transmission. In NPL 3, by giving the LP01 mode a loss larger than the LP11 mode by about 3 dB by using a spatial filter type differential mode attenuation compensator it contributes to the reduction of MDL. Citation List Patent Literature [PL 1] US 5 633 974 A discloses an optical attenuator that uses a segment of attenuating fiber interposed in the optical path.[PL 2] EP 2 533 435 A1 relates to an optical transmission system with a multi-modefiber carrying a plurality of spatial modes, a multi-mode amplifier and an optical spatialmode gain equalizer.[PL 3] US 2016/142142 A1 discloses a spatial-mode multiplexing optical signalstreams onto a multimode optical fiber.[PL 4] US 6 434 311 B1 discloses an optical waveguide supporting a plurality of modes, including an annulus which attenuates a desired mode to a lesser degree than any other mode in the plurality of modes. Non-Patent Literature [NPL 1] N. Hanzawa et al., "Demonstration of Mode-Division multiplexing Transmission over 10 km Two-mode Fiber with Mode Coupler" OFC 2011, paper OWA 4[NPL 2] T. Sakamoto et al., "Modal Dispersion Technique for Long-haul Transmission over Few-mode Fiber with SIMO Configurations" ECOC 2011, We. 10. P1.82[NPL 3] K. Shibahara et al. "Dense SDM (12-Core × 3-Mode) Transmission Over 527 km With 33.2-ns Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization", J. Ligh tw. Technol., vol. 34, no. 1 (2016).[NPL 4] Nagase et al., "Characteristics of SC Type Optical Fixed Attenuator", Sociological Conference General Meeting C-5 99, 1989 Summary of the Invention Technical Problem However, a spatial gain equalizer in NPL 3 needs a complicated structure including a lens, a filter for imparting loss to a specific mode, or the like, in addition to a fiber, and there is a problem that a precise alignment work for preventing generation of crosstalk between propagation modes is required. Accordingly, an object of the present invention is to provide a differential mode attenuation compensator and an optical amplifier which are simple in construction and which do not require a precise alignment work. As an attenuator using a simple structure used in the 1.3 µm or 1.5 µm band, a method of generating a loss by sandwiching a thin met