空分复用光纤研究综述

涂佳静; 李朝晖

doi:doi:10.3788/AOS202141.0106003

光学学报, 2021, 41 (1): 0106003, 网络出版: 2021-02-23

空分复用光纤研究综述下载： 2728次特邀综述

Review of Space Division Multiplexing Fibers

涂佳静 ^1,*李朝晖 ^2,3,**

作者单位

¹ 暨南大学信息科学技术学院, 广东广州 510632

² 中山大学光电材料与技术国家重点实验室, 广东广州 510275

³ 南方海洋科学与工程广东省实验室, 广东珠海 519000

图 & 表

图 1. 标准单模光纤容量的物理限制。(a)非线性香农极限^[3];(b)光纤熔合损伤现象^[4];(c)熔融石英光纤的传输窗口图,黑色部分为标准光放大区间^[5]

Fig. 1. Reasons that make the capacity of single-mode fiber approach Shannon limit. (a) Nonlinear Shannon limit^[3]; (b) fiber fuse damage phenomenon^[4]; (c) transmission window of fused-silica fibers, where black region represents the standard optical amplification band^[5]

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图 2. 同质多芯光纤与异质多芯光纤的串扰对比。(a)同质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图;(b)异质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图

Fig. 2. Comparison of crosstalk between Homo-MCF and Hetero-MCF. (a) Crosstalk of Homo-MCF dependence on bending radius and change of effective index under bending condition; (b) crosstalk of Hetero-MCF dependence on bending radius and change of effective index under bending condition

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图 3. 单模多芯光纤按照外围辅助结构分类^[20]。(a)沟槽辅助式纤芯折射率分布图;(b)沟槽辅助式多芯光纤横截面图;(c)空气孔辅助式纤芯折射率分布图;(d)空气孔辅助式式多芯光纤横截面图;(e)光子晶体纤芯折射率分布图;(f)光子晶体多芯光纤横截面图

Fig. 3. Classification of single-mode multi-core fiber based on the assisted structure around core^[20]. (a) Refractive index profile of trench-assisted core; (b) cross-section of trench-assisted multi-core fiber; (c) refractive index profile of air-hole-assisted core; (d) cross-section of air-hole-assisted multi-core fiber; (e) refractive index profile of photonic crystal core; (f) cross-section of photonic crystal multi-core fiber

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图 4. 外侧纤芯1选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 4. Core selection of outer core 1^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as functions of r₁ and Δ₁ at λ =1550 nm and Λ=35 μm for the first outer cores in Hetero-TA-7-core fiber

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图 5. 外侧纤芯2选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 5. Core selection of outer core 2^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as function of r₁ and Δ₁ at λ=1550 nm and Λ=35 μm for the second outer cores in Hetero-TA-7-core fiber

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图 6. 纤芯1和目标纤芯间串扰对于n_eff,_q和K的函数关系^[42],其中(5)式中的n_eff,_p和n_eff,_q分别代表纤芯1和目标纤芯的有效折射率

Fig. 6. XT between core 1 and the undetermined core as a function of n_eff,_q and K^[42], where n_eff,_p and n_eff,_q in Eq. (5) are effective indexes of refraction of core 1 and the undetermined core, respectively

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图 7. 波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图^[42],其中CSR为中心纤芯2的纤芯选取区间,最大串扰l_XT,max是纤芯2和目标中心纤芯的串扰值

Fig. 7. n_effand A_effas functions of r₁ and Δ₁ at λ=1550 nm and λ=35 μm with CSR of center core 2 in Hetero-TA-7-core fiber^[42], where ${l_{XT}}_{, \max}$

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图 8. OAM模式、CV模式及LP模式之间的转关关系

Fig. 8. Conversion relationship between OAM, CV and LP modes

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图 9. 已报道的弱耦合中心下陷型阶跃光纤。(a)弱耦合中心下陷型阶跃6-LP模光纤^[49];(b)弱耦合中心下陷型阶跃7-LP模光纤^[50];(c)弱耦合中心下陷型阶跃4-LP模光纤^[51];(d)弱耦合中心下陷型阶跃6-LP模光纤^[52]

Fig. 9. Reported inner-depressed step-index few mode fibers with the weak coupling. (a) Inner-depressed step-index 6-LP-mode fiber with the weak coupling^[49]; (b) inner-depressed step-index 7-LP-mode fiber with the weak coupling^[50]; (c) inner-depressed step-index 4-LP-mode fiber with the weak coupling^[51]; (d) inner-depressed step-index 6-LP-mode fiber wi

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图 10. 减小模间DMGD的三种光纤解决方案。(a)低折射率沟槽辅助式双层阶跃光纤^[54];(b)低折射率沟槽辅助式渐变折射率光纤^[55];(c)按一定长度比例熔接正DMGD光纤与负DMGD光纤以达到0DMGD的补偿方法^[57]

Fig. 10. Fiber solutions for reducing DMGD between modes. (a) Trench-assisted dual-cladding step fiber with low refractive index^[54]; (b) trench-assisted graded-index fiber with low refractive index^[55]; (c) 0DMGD compensation method to connect positive DMGD fiber (p) and negative DMGD fiber(n) with low refractive index^[57]

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图 11. OAM模组通信的实验系统。(a)基于低损耗、低串扰的环芯光纤和4×4 MIMO处理技术,利用8个OAM模式以及10个波长,共传输80个通道16 Gbaud QPSK信号100 km^[65];(b)基于渐变折射率多模光纤实现了2.6 km无需MIMO的4个OAM模组传输^[66];(c)基于环芯光纤实现了13个OAM模组的弱耦合传输^[67]

Fig. 11. OAM mode groups multiplexing transmission system. (a) Low crosstalk and low attenuation ring-core fiber with 4×4 MIMO based OAM multiplexing transmission experiment involving ten wavelengths and eight OAM modes over a distance of 100 km, transmitting 16-Gbaud QPSK signals over all 80 channels^[65]; (b) graded-index multi-mode fiber without MIMO based OAM multiplexing transmission over 2.6 km transmitting 4 OAM mode groups^[

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图 12. 阶跃光纤的横截面、折射率分布及色散曲线。(a)阶跃光纤的横截面及折射率分布;(b)阶跃弱导光纤的色散曲线图^[68]

Fig. 12. Cross-section, refractive index distribution and dispersion curves of step-index fiber. (a) Cross-section and refractive index distribution of step-index fiber; (b) dispersion curves of weakly-guiding step-index fiber^[68]

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图 13. 环芯光纤的横截面、折射率分布及色散曲线^[68]。(a)环芯光纤的横截面及折射率分布;(b)d/(2a)=0.20时,环芯弱导光纤的色散曲线图;(c)d/(2a)=0.25时,环芯弱导光纤的色散曲线图;(d)d/(2a)=0.30时,环芯弱导光纤的色散曲线图

Fig. 13. Cross-section, refractive index distribution and dispersion curves of ring-core fiber^[68]. (a) Cross-section and refractive index distribution of ring-core fiber; (b) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.20; (c) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.25; (d) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.30

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表 1已报道的沟槽辅助式单模多芯光纤排布方式

Table1. Core arrangement of the reported trench-assisted single-mode multi-core fibers

Fiber	In Ref. [22]	In Ref. [23]	In Ref. [24]	In Ref. [25]	In Ref. [26]	In Ref. [27]
Cross section of thereported MCFs
Fiber structure	ORS	DRS	HCPS	TLS	New-typeHCPS	SLS
Core number	12	12	19	22	30	32
Core pitch /μm	37	44.6	35	41	30	29
Cladding diameter /μm	225	230	200	260	229	243
Average crosstalk@1550 nm	-32 dB/52 km	<-30 dB/1000 km	-42 dB/1 km	-45 dB/km	-50 dB/9.6 km	<-34.5dB/51.4 km
Loss /(dB·km-1)@1550 nm	0.199	0.186	0.227	0.21	—	0.24
Transmissioncapacity /(Tb·s-1)	1010	409	305	2150	—	—

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表 2已报道的OAM强导弱耦合环芯光纤

Table2. Reported OAM strongly-guiding and weakly-coupling ring-core fibers

Fiber	In Ref. [59]	In Ref. [60]	In Ref. [61]	In Ref. [62]	In Ref. [63]
Cross sectionof strongly-guidedOAM fiber			—
Refractiveindexdistribution				—	—
OAM modegroup	OAM5,1,OAM5,1,OAM7,1(12 OAMmodes in all)	OAM0,1,OAM1,1,OAM2,1 ,…,OAM7,1 (28 OAMmodes in all)	OAM1,1(2 OAMmodes in all)	OAM+1,1,OAM+2,1,OAM+3,1,OAM+4,1(4 OAMmodes in all)	OAM+1,1,OAM+2,1 (2 OAMmodes in all)
Min Δneff /10-4	~1	1.1	2.1	~2	~7

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表 3各阶CV模式的传播常数修正项

Table3. Correction of propagation constant for each CV mode

CV mode	dβ
H $E_{1,1}^{even}$ ,H $E_{1,1}^{odd}$	I₁
TE_0,1	0
TM_0,1	2(I₁+I₂)
H $E_{l + 1,1}^{even}$ ,H $E_{l + 1,1}^{odd}$ (l≥1)	I₁- I₂
E $H_{l - 1,1}^{even}$ ,E $H_{l - 1,1}^{odd}$ (l≥2)	I₁+I₂

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表 4三种SDM光纤方案的对比

Table4. Comparison among three SDM fiber schemes

Fiber	Multi-core fiber	Few-mode fiber	Few-mode multi-core fiber
Fiber fabrication	Mature	A bit complicated	Complicated
Fiber loss	Low	A bit high	High
Crosstalk	Low	Moderate/high	High
Alignment/splicer	Marker assisted/specialfusion splicer	Existing device	Marker assisted/specialfusion splicer
MUX/DEMUX	Space/waveguide/fiber	Space/waveguide/fiber	To be studied
Amplifier	Core/cladding pump	Core/cladding pump	Cladding pump
MIMO-DSP	N_core×(2×2)	2N_mode×2N_mode	N_core×(2N_mode×2N_mode)

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涂佳静, 李朝晖. 空分复用光纤研究综述[J]. 光学学报, 2021, 41(1): 0106003. Jiajing Tu, Zhaohui Li. Review of Space Division Multiplexing Fibers[J]. Acta Optica Sinica, 2021, 41(1): 0106003.

空分复用光纤研究综述下载： 2728次特邀综述

图 1. 标准单模光纤容量的物理限制。(a)非线性香农极限^[3];(b)光纤熔合损伤现象^[4];(c)熔融石英光纤的传输窗口图,黑色部分为标准光放大区间^[5]

Fig. 1. Reasons that make the capacity of single-mode fiber approach Shannon limit. (a) Nonlinear Shannon limit^[3]; (b) fiber fuse damage phenomenon^[4]; (c) transmission window of fused-silica fibers, where black region represents the standard optical amplification band^[5]

图 2. 同质多芯光纤与异质多芯光纤的串扰对比。(a)同质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图;(b)异质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图

Fig. 2. Comparison of crosstalk between Homo-MCF and Hetero-MCF. (a) Crosstalk of Homo-MCF dependence on bending radius and change of effective index under bending condition; (b) crosstalk of Hetero-MCF dependence on bending radius and change of effective index under bending condition

图 4. 外侧纤芯1选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 4. Core selection of outer core 1^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as functions of r₁ and Δ₁ at λ =1550 nm and Λ=35 μm for the first outer cores in Hetero-TA-7-core fiber

图 5. 外侧纤芯2选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 5. Core selection of outer core 2^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as function of r₁ and Δ₁ at λ=1550 nm and Λ=35 μm for the second outer cores in Hetero-TA-7-core fiber

图 6. 纤芯1和目标纤芯间串扰对于n_eff,_q和K的函数关系^[42],其中(5)式中的n_eff,_p和n_eff,_q分别代表纤芯1和目标纤芯的有效折射率

Fig. 6. XT between core 1 and the undetermined core as a function of n_eff,_q and K^[42], where n_eff,_p and n_eff,_q in Eq. (5) are effective indexes of refraction of core 1 and the undetermined core, respectively

图 7. 波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图^[42],其中CSR为中心纤芯2的纤芯选取区间,最大串扰l_XT,max是纤芯2和目标中心纤芯的串扰值

Fig. 7. n_effand A_effas functions of r₁ and Δ₁ at λ=1550 nm and λ=35 μm with CSR of center core 2 in Hetero-TA-7-core fiber^[42], where ${l_{XT}}_{, \max}$

图 8. OAM模式、CV模式及LP模式之间的转关关系

Fig. 8. Conversion relationship between OAM, CV and LP modes

图 9. 已报道的弱耦合中心下陷型阶跃光纤。(a)弱耦合中心下陷型阶跃6-LP模光纤^[49];(b)弱耦合中心下陷型阶跃7-LP模光纤^[50];(c)弱耦合中心下陷型阶跃4-LP模光纤^[51];(d)弱耦合中心下陷型阶跃6-LP模光纤^[52]

图 10. 减小模间DMGD的三种光纤解决方案。(a)低折射率沟槽辅助式双层阶跃光纤^[54];(b)低折射率沟槽辅助式渐变折射率光纤^[55];(c)按一定长度比例熔接正DMGD光纤与负DMGD光纤以达到0DMGD的补偿方法^[57]

图 12. 阶跃光纤的横截面、折射率分布及色散曲线。(a)阶跃光纤的横截面及折射率分布;(b)阶跃弱导光纤的色散曲线图^[68]

Fig. 12. Cross-section, refractive index distribution and dispersion curves of step-index fiber. (a) Cross-section and refractive index distribution of step-index fiber; (b) dispersion curves of weakly-guiding step-index fiber^[68]

图 13. 环芯光纤的横截面、折射率分布及色散曲线^[68]。(a)环芯光纤的横截面及折射率分布;(b)d/(2a)=0.20时,环芯弱导光纤的色散曲线图;(c)d/(2a)=0.25时,环芯弱导光纤的色散曲线图;(d)d/(2a)=0.30时,环芯弱导光纤的色散曲线图

表 1已报道的沟槽辅助式单模多芯光纤排布方式

Table1. Core arrangement of the reported trench-assisted single-mode multi-core fibers

表 2已报道的OAM强导弱耦合环芯光纤

Table2. Reported OAM strongly-guiding and weakly-coupling ring-core fibers

表 3各阶CV模式的传播常数修正项

Table3. Correction of propagation constant for each CV mode

表 4三种SDM光纤方案的对比

Table4. Comparison among three SDM fiber schemes

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空分复用光纤研究综述 下载： 2728次特邀综述

图 1. 标准单模光纤容量的物理限制。(a)非线性香农极限[3];(b)光纤熔合损伤现象[4];(c)熔融石英光纤的传输窗口图,黑色部分为标准光放大区间[5]

Fig. 1. Reasons that make the capacity of single-mode fiber approach Shannon limit. (a) Nonlinear Shannon limit[3]; (b) fiber fuse damage phenomenon[4]; (c) transmission window of fused-silica fibers, where black region represents the standard optical amplification band[5]

图 2. 同质多芯光纤与异质多芯光纤的串扰对比。(a)同质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图;(b)异质多芯光纤串扰对弯曲半径的依赖关系图以及弯曲状态下有效折射率变化示意图

Fig. 2. Comparison of crosstalk between Homo-MCF and Hetero-MCF. (a) Crosstalk of Homo-MCF dependence on bending radius and change of effective index under bending condition; (b) crosstalk of Hetero-MCF dependence on bending radius and change of effective index under bending condition

图 4. 外侧纤芯1选取方式[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下neff和Aeff对于r1和Δ1的函数关系图

Fig. 4. Core selection of outer core 1[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) neff and Aeff as functions of r1 and Δ1 at λ =1550 nm and Λ=35 μm for the first outer cores in Hetero-TA-7-core fiber

图 5. 外侧纤芯2选取方式[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下neff和Aeff对于r1和Δ1的函数关系图

Fig. 5. Core selection of outer core 2[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) neff and Aeff as function of r1 and Δ1 at λ=1550 nm and Λ=35 μm for the second outer cores in Hetero-TA-7-core fiber

图 6. 纤芯1和目标纤芯间串扰对于neff,q和K的函数关系[42],其中(5)式中的neff,p和neff,q分别代表纤芯1和目标纤芯的有效折射率

Fig. 6. XT between core 1 and the undetermined core as a function of neff,q and K[42], where neff,p and neff,q in Eq. (5) are effective indexes of refraction of core 1 and the undetermined core, respectively

图 7. 波长为1550 nm及Λ为35 μm条件下neff和Aeff对于r1和Δ1的函数关系图[42],其中CSR为中心纤芯2的纤芯选取区间,最大串扰lXT,max是纤芯2和目标中心纤芯的串扰值

Fig. 7. neff and Aeff as functions of r1 and Δ1 at λ=1550 nm and λ=35 μm with CSR of center core 2 in Hetero-TA-7-core fiber[42], where lXT,max

图 8. OAM模式、CV模式及LP模式之间的转关关系

Fig. 8. Conversion relationship between OAM, CV and LP modes

图 9. 已报道的弱耦合中心下陷型阶跃光纤。(a)弱耦合中心下陷型阶跃6-LP模光纤[49];(b)弱耦合中心下陷型阶跃7-LP模光纤[50];(c)弱耦合中心下陷型阶跃4-LP模光纤[51];(d)弱耦合中心下陷型阶跃6-LP模光纤[52]

图 10. 减小模间DMGD的三种光纤解决方案。(a)低折射率沟槽辅助式双层阶跃光纤[54];(b)低折射率沟槽辅助式渐变折射率光纤[55];(c)按一定长度比例熔接正DMGD光纤与负DMGD光纤以达到0DMGD的补偿方法[57]

图 12. 阶跃光纤的横截面、折射率分布及色散曲线。(a)阶跃光纤的横截面及折射率分布;(b)阶跃弱导光纤的色散曲线图[68]

Fig. 12. Cross-section, refractive index distribution and dispersion curves of step-index fiber. (a) Cross-section and refractive index distribution of step-index fiber; (b) dispersion curves of weakly-guiding step-index fiber[68]

图 13. 环芯光纤的横截面、折射率分布及色散曲线[68]。(a)环芯光纤的横截面及折射率分布;(b)d/(2a)=0.20时,环芯弱导光纤的色散曲线图;(c)d/(2a)=0.25时,环芯弱导光纤的色散曲线图;(d)d/(2a)=0.30时,环芯弱导光纤的色散曲线图

表 1已报道的沟槽辅助式单模多芯光纤排布方式

Table1. Core arrangement of the reported trench-assisted single-mode multi-core fibers

表 2已报道的OAM强导弱耦合环芯光纤

Table2. Reported OAM strongly-guiding and weakly-coupling ring-core fibers

表 3各阶CV模式的传播常数修正项

Table3. Correction of propagation constant for each CV mode

表 4三种SDM光纤方案的对比

Table4. Comparison among three SDM fiber schemes

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空分复用光纤研究综述下载： 2728次特邀综述

图 1. 标准单模光纤容量的物理限制。(a)非线性香农极限^[3];(b)光纤熔合损伤现象^[4];(c)熔融石英光纤的传输窗口图,黑色部分为标准光放大区间^[5]

Fig. 1. Reasons that make the capacity of single-mode fiber approach Shannon limit. (a) Nonlinear Shannon limit^[3]; (b) fiber fuse damage phenomenon^[4]; (c) transmission window of fused-silica fibers, where black region represents the standard optical amplification band^[5]

图 4. 外侧纤芯1选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 4. Core selection of outer core 1^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as functions of r₁ and Δ₁ at λ =1550 nm and Λ=35 μm for the first outer cores in Hetero-TA-7-core fiber

图 5. 外侧纤芯2选取方式^[42]。(a)Hetero-TA-7-core fiber的横截面;(b)波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图

Fig. 5. Core selection of outer core 2^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as function of r₁ and Δ₁ at λ=1550 nm and Λ=35 μm for the second outer cores in Hetero-TA-7-core fiber

图 6. 纤芯1和目标纤芯间串扰对于n_eff,_q和K的函数关系^[42],其中(5)式中的n_eff,_p和n_eff,_q分别代表纤芯1和目标纤芯的有效折射率

Fig. 6. XT between core 1 and the undetermined core as a function of n_eff,_q and K^[42], where n_eff,_p and n_eff,_q in Eq. (5) are effective indexes of refraction of core 1 and the undetermined core, respectively

图 7. 波长为1550 nm及Λ为35 μm条件下n_eff和A_eff对于r₁和Δ₁的函数关系图^[42],其中CSR为中心纤芯2的纤芯选取区间,最大串扰l_XT,max是纤芯2和目标中心纤芯的串扰值

Fig. 7. n_effand A_effas functions of r₁ and Δ₁ at λ=1550 nm and λ=35 μm with CSR of center core 2 in Hetero-TA-7-core fiber^[42], where ${l_{XT}}_{, \max}$

图 9. 已报道的弱耦合中心下陷型阶跃光纤。(a)弱耦合中心下陷型阶跃6-LP模光纤^[49];(b)弱耦合中心下陷型阶跃7-LP模光纤^[50];(c)弱耦合中心下陷型阶跃4-LP模光纤^[51];(d)弱耦合中心下陷型阶跃6-LP模光纤^[52]

图 10. 减小模间DMGD的三种光纤解决方案。(a)低折射率沟槽辅助式双层阶跃光纤^[54];(b)低折射率沟槽辅助式渐变折射率光纤^[55];(c)按一定长度比例熔接正DMGD光纤与负DMGD光纤以达到0DMGD的补偿方法^[57]

图 12. 阶跃光纤的横截面、折射率分布及色散曲线。(a)阶跃光纤的横截面及折射率分布;(b)阶跃弱导光纤的色散曲线图^[68]

Fig. 12. Cross-section, refractive index distribution and dispersion curves of step-index fiber. (a) Cross-section and refractive index distribution of step-index fiber; (b) dispersion curves of weakly-guiding step-index fiber^[68]

图 13. 环芯光纤的横截面、折射率分布及色散曲线^[68]。(a)环芯光纤的横截面及折射率分布;(b)d/(2a)=0.20时,环芯弱导光纤的色散曲线图;(c)d/(2a)=0.25时,环芯弱导光纤的色散曲线图;(d)d/(2a)=0.30时,环芯弱导光纤的色散曲线图