(19)
(11)EP 3 754 391 A1

(12)EUROPEAN PATENT APPLICATION

(43)Date of publication:
23.12.2020 Bulletin 2020/52

(21)Application number: 20181162.7

(22)Date of filing:  19.06.2020
(51)International Patent Classification (IPC): 
G02B 6/02(2006.01)
G02B 6/036(2006.01)
G02B 6/024(2006.01)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 20.06.2019 CN 201910539222

(71)Applicant: Yangtze Optical Fibre and Cable Joint Stock Limited Company
East Lake High-tech Development Zone Wuhan Hubei 430073 (CN)

(72)Inventors:
  • Zhang, Xinben
    Wuhan, Hubei 430073 (CN)
  • Tang, Ming
    Wuhan, Hubei 430073 (CN)
  • Yang, Chen
    Wuhan, Hubei 430073 (CN)
  • Meng, Yue
    Wuhan, Hubei 430073 (CN)
  • Yang, Kun
    Wuhan, Hubei 430073 (CN)
  • Peng, Shen
    Wuhan, Hubei 430073 (CN)
  • Zhang, Chi
    Wuhan, Hubei 430073 (CN)
  • Tong, Weijun
    Wuhan, Hubei 430073 (CN)

(74)Representative: Laufhütte, Dieter 
Lorenz Seidler Gossel Rechtsanwälte Patentanwälte Partnerschaft mbB Widenmayerstraße 23
80538 München
80538 München (DE)

  


(54)POLARIZATION-MAINTAINING MULTI-CORE FIBER


(57) Disclosed is a polarization-maintaining multi-core fiber, including a plurality of fiber core areas and a main outer cladding. The plurality of fiber core areas includes one central fiber core area, and two or more than two outer fiber core areas equidistantly and uniformly arranged around the central fiber core area. The central fiber core area is a polarization-maintaining fiber core area. The outer fiber core areas each include a fiber core and an inner cladding surrounding a core layer. A portion outside the fiber core areas is the main outer cladding. The fiber can greatly enhance spectral efficiency of an optical transmission system, and improve fiber communication capacity. The arrangement of the polarization-maintaining fiber core area provides a waveguide structure with a function of maintaining polarized light, which can be used for transmission of local light. On the occasion of high capacity access networks or data center interconnection using self-coherent detection, the fiber can enhance certainty of polarization of local light, save automatic polarization controllers, reduce complexity of photoelectric devices and DSP on a receiving end, significantly decrease system latency, and thus reduce cost.




Description

Field of the Invention



[0001] The present disclosure relates to a polarization-maintaining multi-core fiber, and belongs to the technical field of optical communication transmission.

Background of the Invention



[0002] With the rise and popularization of large data volume services such as cloud computing, Internet of Things, high-definition televisions, virtual reality and the like, requirements for the transmission capability of fiber communication networks become increasingly higher. By using super-channel multiplexing technologies, the capacity of a single fiber can be increased to above 100 Tb/s, which is close to the Shannon theorem limit of a single fiber. There is thus an urgent need for in-depth researching and developing of ultra-large-capacity fiber communication technologies of high spectral efficiency so as to address the problem of further expanding fiber communication networks. Space division multiplexing (SDM) technologies provide a plurality of independent physical channels for parallel data transmission by using multi-core fibers (MCFs) or few-mode fibers (FMFs) or even multi-core/few-mode combined new type fibers, and can, in conjunction with self-coherent detection, greatly enhance the spectral efficiency of optical transmission systems, improve fiber communication capacity, and reduce cost. SDM technology is therefore considered to be an effective approach to address capacity crisis of single-mode fibers. Compared with larger inter-mode dispersion or inter-mode crosstalk present in few-mode fibers, inter-core crosstalk in multi-core fibers is easier to control. Multi-core fibers are therefore more advantageous in interconnection density, capacity expansibility, and transmission distance. A multi-core fiber based homologous homodyne self-coherent optical communication system can fully exert the advantage of high spectral efficiency of coherent optical communication, and meanwhile simplify DSP process to significantly reduce power consumption. The multi-core fiber based homologous homodyne self-coherent optical communication system is hence a competitive solution for realizing data center interconnection at a rate of 800 Gbps and above across a single wave in a short distance.

[0003] In multi-core fiber based self-coherent detection, a fiber typically does not have a polarization-maintaining function (for example, patent CN103399374A), and in practical use, evolution of a polarization state of the fiber is random, which requires additional use of an automatic polarization controller to track the polarization state of local light so as to control polarization of the local light transmitted to a receiving end. This increases power consumption and complexity of the system, and increases processing latency.

Summary of the Invention



[0004] An technical problem to be solved by the present disclosure is directed against the above described deficiencies in the existing technologies, and the present disclosure aims to provide a polarization-maintaining multi-core fiber which can effectively enhance certainty of a polarization state of local light, improve reliability of a system, decrease photoelectric devices on a receiving end, reduce complexity of DSP, to thereby decrease cost of the system.

[0005] In order to solve the above technical problem, the present disclosure adopts the following technical solution. A polarization-maintaining multi-core fiber includes a plurality of fiber core areas and a main outer cladding. The plurality of fiber core areas includes one central fiber core area, and two or more than two outer fiber core areas equidistantly and uniformly arranged around the central fiber core area. The central fiber core area is a polarization-maintaining fiber core area. The outer fiber core areas each include a fiber core and an inner cladding surrounding a core layer. A portion outside the fiber core areas is the main outer cladding.

[0006] According to the foregoing technical solution, provided are one to two layers of the outer fiber core areas, and each of the layers is provided with two or more than two equidistantly and uniformly arranged outer fiber core areas, the outer fiber core areas in each of the layers being equidistantly arranged from the central fiber core area.

[0007] According to the foregoing technical solution, provided is one layer of the outer fiber core areas, and each layer is provided with three to eight equidistantly and uniformly arranged outer fiber core areas.

[0008] According to the foregoing technical solution, the polarization-maintaining fiber core area includes a polarization-maintaining fiber core having a cross-section of a round shape and a polarization-maintaining inner cladding, and the polarization-maintaining fiber core area is provided symmetrically on two sides thereof with stress units (areas).

[0009] According to the foregoing technical solution, the stress units each have a cross-section of a round shape or an arc block shape.

[0010] According to the foregoing technical solution, the polarization-maintaining fiber core area includes a polarization-maintaining fiber core layer having a cross-section of a round shape and a polarization-maintaining inner cladding having a cross-section of an oval shape.

[0011] According to the foregoing technical solution, the polarization-maintaining fiber core area includes a polarization-maintaining fiber core layer having a cross-section of an oval shape and a polarization-maintaining inner cladding having a cross-section of an oval shape.

[0012] According to the foregoing technical solution, the fiber core of each of the outer fiber core areas has a step refractive index; the fiber core has a diameter of 5-15 µm; and a relative refractive index difference between the fiber core and the main outer cladding is 0.15%-0.45%.

[0013] According to the foregoing technical solution, the fiber core has a graded refractive index which is represented as n(r)=n0·[1-2Δ(r/a)m]0.5, 0≤r≤a, wherein a radius a of the fiber core is 2.5-25 µm; a relative refractive index difference Δ between a geometric center of the fiber core and the main outer cladding is 0.20%-1.50%; a characteristic parameter m is 1.5-2.5; r is a distance from any point on the fiber core to the geometric center; and n0 is a refractive index at the geometric center of the fiber core.

[0014] According to the foregoing technical solution, a relative refractive index difference between the inner cladding and the main outer cladding is -0.5%-0.05%; the inner cladding has a unilateral thickness of 1-10 µm; and the main outer cladding is a pure silica glass layer.

[0015] According to the foregoing technical solution, the fiber core and the inner cladding are provided therebetween with a transitional inner cladding. A ratio of a diameter of the transitional inner cladding to a diameter of the fiber core is 1.0-3.0; the transitional inner cladding is a pure silica glass layer.

[0016] According to the foregoing technical solution, polarization crosstalk at an operating wavelength of the polarization-maintaining fiber core area within 1000 m is not lager than -10 dB.

[0017] According to the foregoing technical solution, the main outer cladding contains a reference line extending along a length direction of the fiber. The reference line is located at an asymmetric portion of the fiber core areas of the multi-core fiber.

[0018] The present disclosure brings the following beneficial effects. 1. The multi-core fiber provides a plurality of independent physical channels for parallel data transmission, and can, in conjunction with self-coherent detection, greatly enhance the spectral efficiency of an optical transmission system, improve fiber communication capacity, and reduce cost. 2. The arrangement of the polarization-maintaining fiber core area provides a waveguide structure with a function of maintaining polarized light, which can be used for transmission of local light. On the occasion of high capacity access networks or data center interconnection using self-coherent detection, the present disclosure can enhance certainty of polarization of local light, save automatic polarization controllers, reduce complexity of photoelectric devices and DSP on a receiving end, significantly decrease system latency, and thus reduce cost. 3. Providing the polarization-maintaining fiber core at the center of the fiber can reduce external interference and facilitate connecting.

Brief Description of the Drawings



[0019] 

Fig. 1 is a cross-sectional structural view of a first embodiment of the present disclosure;

Fig. 2 is a cross-sectional structural view of a second embodiment of the present disclosure;

Fig. 3 is a cross-sectional structural view of a third embodiment of the present disclosure;

Fig. 4 is a cross-sectional structural view of a fourth embodiment of the present disclosure;

Fig. 5 is a cross-sectional structural view of a fifth embodiment of the present disclosure; and

Fig. 6 is a cross-sectional structural view of a sixth embodiment of the present disclosure.


Detailed Description of the Embodiments



[0020] The present disclosure is further described in detail below in conjunction with embodiments.

[0021] A first embodiment is as shown in Fig. 1. A polarization-maintaining multi-core fiber includes seven fiber core areas and a main outer cladding 101. The seven fiber core areas include one central fiber core area and six outer fiber core areas equidistantly and uniformly arranged around the central fiber core area. The six outer fiber core areas form a layer of outer fiber core areas. The central fiber core area is a polarization-maintaining fiber core area. The polarization-maintaining fiber core area includes a polarization-maintaining fiber core 301 having a cross-section of a round shape and a polarization-maintaining inner cladding 401. The polarization-maintaining fiber core area is provided symmetrically on two sides thereof with stress units 501. The stress units are located between the outer fiber core areas and the central fiber core area, and two outer fiber core areas adjacent to each of the stress units are symmetrical. The stress units each have a cross-section of a round shape, and the stress units are boron-doped stress units. A boron-doped stress area has a diameter of 20 µm. A core distance between the boron-doped stress area and the polarization-maintaining fiber core is 20 µm. The boron-doped stress area has a molar concentration of doped boron of 20%. Polarization crosstalk at the polarization-maintaining fiber core is -20 dB within 1000 m. The outer fiber core areas have core claddings of a same structure, including a fiber core 302 and an inner cladding 402 surrounding a core layer. A portion outside the fiber core area is the main outer cladding. The fiber core has a step refractive index. The fiber core has a diameter of 9 µm. A relative refractive index difference between the fiber core and the main outer cladding is 0.40%. A relative refractive index difference between the inner cladding and the main outer cladding is -0.05%. The inner cladding has a unilateral thickness of 2.5 µm. The main outer cladding is a pure silica glass layer, and has a diameter of 150 µm. The polarization-maintaining fiber core and the polarization-maintaining inner cladding have structures the same as the structure of the core cladding of the above described outer fiber core areas. The main outer cladding contains a reference line 601 extending along a length direction of the fiber. The reference line has a diameter of 1 µm. The reference line is located outside the outer fiber core areas and at an asymmetric portion of the outer fiber core areas. The main outer cladding is coated with a coating layer 201 of resin. The resin is acrylic resin, and the coating layer has a diameter of 245 µm.

[0022] A second embodiment is as shown in Fig. 2. The second embodiment is different from the first embodiment in that the polarization-maintaining multi-core fiber of the second embodiment includes eight outer fiber core areas equidistantly and uniformly arranged around the central fiber core area. Other structures are the same as those described in the first embodiment.

[0023] A third embodiment is as shown in Fig. 3. The third embodiment is different from the second embodiment in the following. The fiber core and the inner cladding are provided therebetween with a transitional inner cladding 702. The transitional inner cladding 702 has a diameter of 16 µm, and is a pure silica glass layer. The inner cladding layer has a diameter of 24 µm. Besides, the polarization-maintaining fiber core 301 and the polarization-maintaining inner cladding 401 are provided therebtween with a polarization-maintaining transitional inner cladding 701. The polarization-maintaining inner cladding and the polarization-maintaining transitional inner cladding have structures the same as those of the above described inner cladding and transitional inner cladding. Other structures in this embodiment are the same as those described in the first embodiment.

[0024] A fourth embodiment is as shown in Fig. 4. The fourth embodiment is different from the first embodiment in the following. The boron-doped stress area is of an arc block shape (bowtie shape). The arc block has an inner radius of 20 µm and an outer radius of 42 µm, and an angel between two side edges thereof of 90°, namely a radian of π/2. The boron-doped stress area is located on two sides of the polarization-maintaining fiber core. The boron-doped stress area has a molar concentration of doped boron of 20%. The stress units are located between the outer fiber core areas and the central fiber core area, aligning right to an outer fiber core area adjacent thereto. The polarization crosstalk at the polarization-maintaining fiber core is -24 dB within 1000 m. Other structures are the same as those described in the first embodiment.

[0025] A fifth embodiment is as shown in Fig. 5. The fifth embodiment is different from the first embodiment in the following. The polarization-maintaining fiber core 301 is of a round shape, and has a diameter of 9 µm. The relative refractive index difference between the polarization-maintaining fiber core and the main outer cladding is 1.0%. The polarization-maintaining inner cladding 401 is of an oval shape having a long axis of 42 µm and a short axis of 14 µm. The relative refractive index difference between the polarization-maintaining inner cladding and the main outer cladding is 0.5%. The polarization crosstalk at the polarization-maintaining fiber core is -18 dB within 1000 m. The coating layer is a polyimide coating layer. A long-time tolerable temperature of the fiber is 150°C. Other structures are the same as those described in the first embodiment.

[0026] A sixth embodiment is as shown in Fig. 6. The sixth embodiment is different from the third embodiment in the following. The polarization-maintaining fiber core 301 is of an oval shape having a short axis of 5 µm and a long axis of 22 µm. The polarization-maintaining inner cladding 401 is of an oval shape having a short axis of 10 µm and a long axis of 27 µm. The relative refractive index difference between the polarization-maintaining fiber core and the main outer cladding is 0.40%. The relative refractive index difference between the polarization-maintaining inner cladding and the main outer cladding is -0.05%. The polarization crosstalk at the polarization-maintaining fiber core is -18 dB within 1000 m. The coating layer is an acrylic resin coating layer. Other structures are the same as those described in the first embodiment.


Claims

1. A polarization-maintaining multi-core fiber, including a plurality of fiber core areas and a main outer cladding, wherein the plurality of fiber core areas includes one central fiber core area, and two or more than two outer fiber core areas equidistantly and uniformly arranged around the central fiber core area, wherein the central fiber core area is a polarization-maintaining fiber core area; the outer fiber core areas each include a fiber core and an inner cladding surrounding a core layer; and a portion outside the fiber core areas is the main outer cladding.
 
2. The polarization-maintaining multi-core fiber according to claim 1, wherein provided are one to three layers of the outer fiber core areas, and each of the layers is provided with two or more than two equidistantly and uniformly arranged outer fiber core areas, the outer fiber core areas in each of the layers being equidistantly arranged from the central fiber core area.
 
3. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein the polarization-maintaining fiber core area includes a polarization-maintaining fiber core having a cross-section of a round shape and a polarization-maintaining inner cladding, and the polarization-maintaining fiber core area is provided symmetrically on two sides thereof with stress units.
 
4. The polarization-maintaining multi-core fiber according to claim 3, wherein the stress units each have a cross-section of a round shape or an arc block shape.
 
5. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein the polarization-maintaining fiber core area includes a polarization-maintaining fiber core layer having a cross-section of a round shape and a polarization-maintaining inner cladding having a cross-section of an oval shape.
 
6. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein the polarization-maintaining fiber core area includes a polarization-maintaining fiber core layer having a cross-section of an oval shape and a polarization-maintaining inner cladding having a cross-section of an oval shape.
 
7. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein the fiber core of each of the outer fiber core areas has a step refractive index; the fiber core has a diameter of 5-15 µm; and a relative refractive index difference between the fiber core and the main outer cladding is 0.15%-0.45%.
 
8. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein the fiber core has a graded refractive index which is represented as n(r)=n0·[1-2Δ(r/a)m]0.5, 0≤r≤a, wherein a radius a of the fiber core is 2.5-25 µm; a relative refractive index difference Δ between a geometric center of the fiber core and the main outer cladding is 0.20%-1.50%; a characteristic parameter m is 1.5-2.5; r is a distance from any point on the fiber core to the geometric center; and n0 is a refractive index at the geometric center of the fiber core.
 
9. The polarization-maintaining multi-core fiber according to claim 1 or claim 2, wherein a relative refractive index difference between the inner cladding and the main outer cladding is -0.5%-0.05%; the inner cladding has a unilateral thickness of 1-10 µm; and the main outer cladding is a pure silica glass layer.
 
10. The polarization-maintaining multi-core fiber according to claim 9, wherein the fiber core and the inner cladding are provided therebetween with a transitional inner cladding, wherein a ratio of a diameter of the transitional inner cladding to a diameter of the fiber core is 1.0-3.0; the transitional inner cladding is a pure silica glass layer; and polarization crosstalk at an operating wavelength of the polarization-maintaining fiber core area within 1000 m is not lager than -10 dB.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description