JP2015138115A - Multimode optic fiber and optic communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optic fiber capable of compensating a DMD in which a code of the DMD is different and which has a large absolute value.SOLUTION: A multimode optic fiber comprises: a center core disposed on a center of an optic fiber, and propagating in a basic mode and a higher mode; and a ring core disposed around the center core and propagating in the higher mode. In a communication wavelength, an execution refraction index of the center core and an execution refraction index of the ring core in the higher mode are almost equal, and difference between group delay of the basic mode propagating the center core and group delay of the higher mode propagating the ring core is predetermined group delay difference.

Description

  The present invention relates to a multimode optical fiber and an optical communication system using the multimode optical fiber.

  In an optical communication system, the transmission capacity is limited by nonlinear effects or fiber fuses generated in an optical fiber used for an optical transmission line. In order to relax this transmission capacity limitation, parallel transmission technology using a multi-core fiber having a plurality of cores in one optical fiber as an optical transmission line, or multi-mode light having a plurality of propagation modes in the core Spatial multiplexing technology such as mode multiplexing transmission technology using a fiber as an optical transmission line has been studied (for example, see Non-Patent Documents 1 and 2).

  In an optical communication system using a mode multiplex transmission technique, crosstalk occurs between modes in an optical transmission line. Therefore, a multi-input multi-output (MIMO) technique, which is a large capacity wireless technique, is applied. Attempts have been made to compensate for crosstalk.

  In an optical communication system using the MIMO technique, inter-mode crosstalk is compensated by digital processing when a signal is restored in an optical receiver. However, when the group delay difference (DMD: Differential Mode group Delay) between modes becomes large, digital processing necessary for signal restoration becomes complicated. For this reason, in the optical communication system using the MIMO technology, the transmission distance is limited to ten and several kilometers. Therefore, an optical transmission line that combines an optical fiber with positive DMD (pDMDF) and an optical fiber with negative DMD (nDMDF) as shown in FIG. 1 has been proposed (see, for example, Non-Patent Document 2). The optical transmission line of Non-Patent Document 2 compensates for DMD in the optical transmission line by compensating DMD generated in pDMDF with nDMDF and compensating DMD generated in nDMDF with pDMDF.

H. Takara et al. , “1.01-Pb / s (12 SDM / 222 WDM / 456 Gb / s) Cross-managed Transmission with 91.4-b / s / Hz Aggregate Spectral Efficiency”, in EcoC2012, paper Thr. 3. C. 1 (2012) T. T. et al. Sakamoto et al. , “Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber”, J. et al. Lightwave Technol. vol. 30, pp. 2783-2787 (2012). T. T. et al. Sakamoto et al. “Mode-Division Multiplexing Transmission System With DMD-Independent Low Complexity MIMO Processing”, J. Org. Lighttw. Technol. , Vol. 31, pp. 2192-2199. K. Sato et al. "Optimized graded index two-mode optical fiber with low DMD, large Aff and low bending loss", Optics Express, vol. 21, pp. 16231-16238. J. et al. D. Downie, "112 Gb / s PM-QPSK transmission systems with reach lengths enabled by optical fibers with ultra-low loss and verieffe effelect. SPIE, vol. 8284, 828403-1. A. H. Gnauck et al. , “2.5 Tb / s (64 × 42.7 Gb / s) Transmission Over 40 × 100 km NZDSF Using RZ-DPSK Format and All-Raman-Amplified Spans”, OFC2002, paper FC2-1. T. T. et al. Sakamoto, et al. , “Wide-Range Tunable Optical Delay Line Using Dual Concentric Core Fiber With Dispersion Coefficient of −2800 ps / nm / km”, J. et al. Lighttw. Technol. , Vol. 29, pp. 1920-1925 (2011) M.M. Taylor, "Coherent Detection for Fiber Optic Communications Using Digital Signal Processing", in Optical Amplifiers and Their Applications / Coherent Optical Technologies and Applications, Technical Digest (CD) (Optical Society of America, 2006), paper CThB1. M.M. Wada et al. "Modal Gain Controllable All-Type Fiber Amplifier Amplifier", OECC 2013, paper TuS4-5, 2013.

  However, in the optical transmission line of Non-Patent Document 2, the absolute values of the DMDs of the two types of optical fibers are not so different, and the two types of optical fibers have the same length in order to compensate for the DMD. Is required. Accordingly, the optical transmission line of Non-Patent Document 2 must be mixed with different types of optical fibers, which is not preferable from the viewpoints of cable manufacturing, DMD adjustment, and operation management.

  Therefore, a method of concentrating the DMD in the optical transmission path by installing a DMD compensation module in the repeater like the dispersion compensation module used in the chromatic dispersion compensation transmission path can be considered. However, no optical fiber capable of compensating for DMD having a different DMD sign and a large absolute value has been reported so far. For this reason, a DMD compensation module capable of compensating for a large DMD accumulated over the entire transmission span length cannot be configured, and there is a problem that an optical communication system capable of compensating concentrated DMD cannot be constructed.

  An object of the present invention is to provide an optical fiber that can compensate for DMDs having different DMD signs and large absolute values.

  The inventors have a multi-mode optical fiber having a center core and a ring core and having a propagation mode of 2 or more, and by combining the center core and the ring core only in a higher order mode, the DMD has a different sign and is absolutely It has been found that it is possible to compensate for large DMD values.

Specifically, the multimode optical fiber according to the present invention is:
A center core disposed in the center of the optical fiber and propagating the fundamental mode and higher order mode;
A ring core disposed around the center core and propagating higher order modes;
With
At a defined wavelength, the effective refractive index of the higher order mode of the center core and the effective refractive index of the higher order mode of the ring core are substantially equal,
The difference between the group delay of the basic mode propagating through the center core and the group delay of the higher-order mode propagating through the ring core is a determined group delay difference.

In the multimode optical fiber according to the present invention,
The normalized frequency calculated from the core radius and relative refractive index difference of the center core is 2.40 or more and 3.83 or less,
The predetermined group delay difference may be −7.85 ns / km or less.

  In the multimode optical fiber according to the present invention, the differential coefficient with respect to the difference wavelength at the predetermined wavelength may be negative.

Specifically, the optical communication system according to the present invention is:
An optical transmitter for transmitting a mode multiplexed optical signal of two modes;
An optical receiver for receiving the mode multiplexed optical signal;
An optical fiber connected between the optical transmitter and the optical receiver, wherein the differential coefficient with respect to the wavelength of the group delay difference between the two modes is positive;
A multimode optical fiber according to the present invention, inserted between the optical fiber and the optical receiver, to compensate for the group delay difference in the optical fiber;
Is provided.

Specifically, the optical communication system according to the present invention is:
An optical transmitter for transmitting a mode multiplexed optical signal of two modes;
An optical receiver for receiving the mode multiplexed optical signal;
An optical fiber that connects between the optical transmitter and the optical receiver and has a negative differential coefficient with respect to the wavelength of the group delay difference between the two modes;
A mode converter inserted between the optical fiber and the optical receiver and inverting the sign of the group delay difference generated in the optical fiber;
A multimode optical fiber according to the present invention for compensating for the group delay difference in the optical fiber inserted between the mode converter and the optical receiver and inverted by the mode converter;
Is provided.

Specifically, the design method of the multimode optical fiber according to the present invention is:
A multimode optical fiber design method comprising a center core disposed in the center of an optical fiber and a ring core disposed around the center core,
A center core adjustment procedure for obtaining a core radius and a relative refractive index difference of the center core so that the center core propagates two modes of a fundamental mode and a higher-order mode;
Calculating the group delay of the fundamental mode of the center core and the group delay of the higher order mode of the ring core when the center core and the ring core are present independently, respectively, and the group delay of the center core and the group of the ring cores A group delay difference adjustment procedure for obtaining a core radius and a relative refractive index difference of the center core and the ring core so that a delay difference satisfies a set group delay difference;
A ring core adjustment procedure for adjusting the refractive index of the ring core so that the effective refractive index of the higher order mode of the center core and the effective refractive index of the higher order mode of the ring core are substantially equal at a predetermined wavelength;
In order.

In the design method of the multimode optical fiber according to the present invention,
In the center core adjustment procedure, the core radius and relative refraction of the center core are set such that the normalized frequency calculated from the core radius and relative refractive index difference of the center core is 2.40 or more and 3.83 or less at the used wavelength. Find the rate difference,
In the group delay difference adjustment procedure, a core radius and a relative refractive index difference between the center core and the ring core may be obtained so that the determined group delay difference is −7.85 ns / km or less.

  The multimode optical fiber of the present invention can compensate for DMDs having different DMD signs and large absolute values. Therefore, the multimode optical fiber according to the present invention can construct a DMD compensation system capable of compensating concentrated DMD.

  In addition, by using the multimode optical fiber according to the present invention for the repeater arranged in the optical transmission line, even if the optical transmission line is configured only by an optical fiber having positive DMD (pDMDF), the optical transmission line DMD can be compensated. For this reason, the multimode optical fiber of the present invention can facilitate the improvement of the operation efficiency in the optical communication system and the adjustment of the DMD compensation.

2 shows an example of an optical transmission line for compensating for DMD related to the present invention. 2 shows an example of a refractive index distribution of a step type optical fiber. An example of the refractive index distribution of a graded optical fiber is shown. An example of a change in the mode field radius with respect to the core radius a when Δ = 0.35% is shown. 2 shows an example of a change in DMD with respect to a core radius a in a graded-type gradient index fiber. 2 shows an example of a change in DMD with respect to an α value in a graded-type gradient index fiber. 2 shows an example of a refractive index distribution of the DMD compensation fiber according to the first embodiment. 2 shows an example of DMD characteristics of the DMD compensation fiber according to the first embodiment. 2 shows an example of DMD characteristics of the DMD compensation fiber according to the first embodiment. 3 shows an example of DMD slope characteristics of the DMD compensation fiber of Embodiment 1. It shows an example of a refractive index distribution when the center core is present alone. An example of a refractive index distribution when a ring core is present alone is shown. An example of a change in group delay with respect to the core radius a and the relative refractive index difference Δ when only the center core exists is shown. An example of a change in group delay with respect to the core radius a and the relative refractive index difference Δ when only the ring core is present is shown. An example of the parameter in a ring core adjustment procedure is shown. In the structure center core and the ring core are both present, there is shown an example of a change in DMD characteristics for delta _2. An example of the optical communication system of Embodiment 2 is shown. 4 illustrates an example of a part of an optical transmission line according to a second embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

  The invention according to this embodiment is a group delay compensating optical fiber including a center core and a ring core, and the group delay of the LP01 mode in the center core and the group delay of the LP11 mode in the ring core when each core is assumed to exist alone. The core radius and the relative refractive index difference are set so that the difference between them and the desired DMD satisfies the desired DMD, and the effective refractive index of the LP11 mode of the center core and the effective of the LP11 mode of the ring core are set within the usable wavelength range. The difference in refractive index (level) between both cores is set while maintaining the set relative refractive index difference so that the refractive index matches. At the determined wavelength, the effective refractive index of the fundamental mode of the center core is different from the effective refractive index of the fundamental mode of the ring core. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 2 is an example of a refractive index profile of a step index fiber. There are a core region having a refractive index of n1 and a cladding region of n2, and n1> n2. In the structure of FIG. 2, the condition of n1> n2 is that the material of each region is pure quartz glass, germanium (Ge), aluminum (Al), phosphorus (P), or other impurities that increase the refractive index, or fluorine (F). It can be realized by using quartz glass to which impurities such as boron (B) are added to reduce the refractive index. For example, in the two-mode fiber described in Non-Patent Document 3 (core radius a = 7 μm, relative refractive index difference Δ = 0.4%), there are two propagation modes and the DMD is about 2 ns / km.

In order to reduce the DMD, the graded refractive index distribution shown in FIG. 3 is effective. In addition, about the shape of a core, it calculates | requires by the following formula | equation (1) using (alpha) parameter.

  For example, as described in Non-Patent Document 4, if the core radius a = 11.8 μm, the relative refractive index difference Δ = 0.36%, and α = 2.3, the two modes propagate and the DMD at the wavelength of 1550 nm is 0. It becomes. However, strictly speaking, DMD cannot be set to 0 due to an optical fiber manufacturing error.

  In order to grasp the worst value of DMD generated in the optical transmission line, it is necessary to quantitatively evaluate the manufacturing error. FIG. 4 shows the result of calculating the change in the mode field radius W of the fundamental mode with respect to the core radius when the relative refractive index difference Δ = 0.35%. Currently, ITU-T single mode fiber recommendation G.I. 652, the tolerance of MFD is defined as ± 0.6 μm, and when the radius a = 4.5 μm of a general single mode optical fiber is used as a reference, at least the core radius a is within a range of ± 0.8 μm or less. You can see that it changes. When the tolerance of MFD is ± 0.4 μm, the core radius changes within a range of ± 0.6 μm or less.

  As an optical fiber capable of transmitting two modes and having a low DMD, a two-mode graded-type refractive index distribution fiber having a refractive index distribution shown in FIG. 3 can be exemplified (for example, see Non-Patent Document 4). Targeting the two-mode graded graded index fiber described in Non-Patent Document 4, with respect to the core radius change when the relative refractive index difference Δ = 0.36%, the core radius 11.8 μm, and α = 2.3. The change in DMD is shown in FIG.

  In FIG. 5, in the optical transmission line, the lower limit of the core radius a is set to 11.8 μm in order to always obtain a positive DMD even in consideration of manufacturing errors. When the deviation of the core radius a is ± 0.6 μm or less, DMD <150 ps / km, and when the deviation of the core radius a is ± 0.8 μm or less, DMD <220 ps / km. From the graph shown in FIG. 5, it can be seen that the DMD can be up to 220 ps / km even when an optical fiber with DMD = 0 is manufactured as a target.

In order to reduce the DMD, a method for compensating for DMD by connecting an optical fiber having negative DMD (nDMDF) has been proposed. In general, if the DMD of an optical fiber having positive DMD (pDMDF) is τ _p (ps / km) and the length is L _p (km), the DMD of nDMDF is τ _n if the length of nDMDF to satisfy equation (2) and _{L n,} the cumulative DMD value at the receiving end may be set to zero.

However, as described in Non-Patent Document 2, at present because it does not differ significantly absolute value of DMD in pDMDF and nDMDF, _{L p} equivalent _{L n} is required, mix pDMDF and NDMDF the optical transmission path There must be. In this case, two types of cables, i.e., a cable incorporating pDMDF and a cable incorporating nDMDF, are mixed in the optical transmission line, and management operation becomes complicated. In addition, since adjustment of the length is difficult in the optical transmission line, there is a problem that adjustment of L _p and L _n is difficult.

  In order to solve the above problem, a DMD compensation module for compensating DMD is installed in the repeater like the dispersion compensation module used in the chromatic dispersion compensation transmission line, and the DMD in the optical transmission line is changed. A method of compensating can be considered. As a result, the optical transmission line can be configured with only a single fiber type, and it is easy to manufacture and manage the cable, and it is possible to adjust the cumulative DMD only by changing the fiber length of the DMD compensation module in the repeater. There is an advantage that DMD compensation can be easily adjusted.

  As described in Non-Patent Document 5 and Non-Patent Document 6, assuming that a typical span length is 50 to 100 km, the cumulative DMD generated in the optical transmission line is 220 ps / km × 50 to 100 km, that is, 11 ~ 22ns. In order to compensate the accumulated DMD value by the optical fiber of the DMD compensation module installed in the repeater existing for each span, it is only necessary to realize the accumulated DMD value of −11 to −22 ns. However, in order to compensate for a large cumulative DMD value, the length of nDMDF must be increased, and the length of the fiber that propagates between the relay apparatuses becomes longer, and transmission loss increases. In general, in order to ensure sufficient transmission quality in an optical communication system, it is desirable that the power penalty is 1 dB or less, and the power penalty received by the installation of nDMDF needs to be 1 dB or less.

In order to estimate the power penalty (OSNR degradation amount) generated due to excessive loss due to the installation of nDMDF, the following formula of performance index (FOM) described in Non-Patent Document 5 is used.

Ｌはファイバ長である。 _{Here, A eff} is the effective area of the optical fiber, alpha _dB loss of the optical fiber, _{L a} span _{length, L eff} is the effective length determined by the following equation (4).

L is the fiber length.

なお、下付き文字で「ｒｅｆ」がついている変数は、基準とするシステムのパラメータであり、基準システムに対してどの程度ＯＳＮＲが改善したかがＦＯＭの値で示される。 When the loss of the optical fiber is α _dB (dB / m), the relationship of Expression (5) is established.

The variable with “ref” in the subscript is a parameter of the reference system, and the FOM value indicates how much the OSNR is improved with respect to the reference system.

G. When the upper limit of loss of 0.4 dB / km described in 652 is set as the loss value of the optical fiber of the reference system, the span length is set to 100 km, and the excess loss allowed to suppress the OSNR degradation to 1 dB or less is calculated. 1.12 dB, that is, the excess loss due to the installation of nDMDF must be 1.12 dB or less. Similarly, if the loss of nDMDF is 0.4 dB / km, L _n must be 2.8 km or less. That is, in order to compensate for the maximum accumulated DMD value 22 ns generated in the optical transmission line, an nDMDF of −22 ns / 2.8 km = −7.85 ns / km is required.

On the other hand, when the optical communication system in the case of a span length of 50 km is used and the excess loss allowed to suppress the OSNR degradation to 1 dB or less is obtained, it is 1.25 dB, that is, the excess loss due to the installation of nDMDF is 1 .25 dB or less. When the loss of nDMDF similarly as 0.4dB / km, _{L n} would have to be less 3.125Km. That is, in order to compensate for the maximum accumulated DMD value 11 ns generated in the optical transmission line, nDMDF of −11 ns / 3.125 km = −3.5 ns / km is required.

  The DMD can be changed by adjusting the α value. FIG. 6 shows the change in DMD with respect to α when the core radius a = 11.8 μm and the relative refractive index difference Δ = 0.36%. By reducing α, DMD can be reduced, and when α = 1, it is about −3.2 ns / km. In an optical communication system with a span length of 50 km, a DMD of −3.5 ns / km or less is required for a repeater, but an optical fiber with α ≦ 1 has not been reported and is considered difficult to manufacture. In the optical fiber having the refractive index profile shown in FIG. 3, the DMD cannot be compensated with a power penalty of 1 dB or less in the repeater. The span length should be considered up to a maximum of 100 km. In that case, a DMD of −7.85 ns / km or less is required for the repeater, but the crazed type fiber related to the present invention cannot cope with it. That is, it can be seen that the related art cannot cope with the current optical communication system having a span length of 50 to 100 km.

  In addition, a configuration in which the optical transmission line is nDMDF and the optical fiber used for the DMD compensation module is pDMDF is conceivable, but even if α is increased and DMD is increased, it becomes about 1 ns / km, and the accumulated DMD is completely compensated. I can't. Further, as described in Non-Patent Document 3, even if a step type fiber is used, it is about 2 ns / km, so that the accumulated DMD cannot be similarly compensated.

Therefore, in this embodiment, a multimode optical fiber having a refractive index distribution shown in FIG. 7 is used as an optical fiber used for the DMD compensation module. The multimode optical fiber according to the present embodiment has a center core and a ring core. A center core is formed at a position with a radius of 0 or more and a ₁ or less, and a ring core is formed at a position with a radius of a ₂ or more and a ₃ or less.

For example, the calculation results of DMD when a ₁ = 3 μm, a ₂ = 4 μm, a ₃ = 7 μm, Δ ₁ = 1.2%, Δ ₂ = −0.4%, Δ ₃ = 0.2% As shown in FIG. It can be seen that DMD can realize −15 ns / km or less in a general wavelength range of 1530 nm to 1625 nm. The extremely large absolute value of DMD can be realized because the electric field transitions from the center core to the ring core and the mode characteristics change. This is due to the coupling between the modes propagating through the center core and the ring core. Can be explained (for example, see Non-Patent Document 7).

In Non-Patent Document 7, adjustment is made so that the fundamental mode is coupled to the mode of the ring core in order to control the dispersion value of the fundamental mode. On the other hand, the multimode optical fiber of the present embodiment is adjusted so that only the higher mode is coupled to the ring core in order to increase the DMD between the fundamental mode and the higher mode by setting the propagation mode to 2. The DMD between the fundamental mode and the higher order mode is greatly changed around the wavelength λ ₀ .

When the wavelength range of the wavelength characteristics of the DMD in FIG. 8 is expanded, as shown in FIG. 9, the DMD between the fundamental mode and the higher-order mode greatly decreases before and after the coupling wavelength λ ₀ . In the multimode optical fiber according to the present embodiment, as shown in FIG. 10, the wavelength at which the slope of the DMD with respect to the wavelength is the minimum is the coupling wavelength λ ₀ . At this coupling wavelength λ ₀ , the DMD changes most with respect to the wavelength, and the slope of the DMD with respect to the wavelength, that is, the differential coefficient is negative. That is, the multimode optical fiber of this embodiment has a negative DMD slope. For this reason, the multimode optical fiber of the present embodiment can compensate for the DMD slope in the optical transmission line by using pDMDF having a positive DMD slope in the optical fiber laid in the optical transmission line. In addition, since the multimode optical fiber according to the present embodiment can obtain the characteristics shown in FIG. 9 in the wavelength range of the wavelength characteristics of the DMD in FIG. 8, it is possible to realize DMD compensation in a wide wavelength range. Therefore, the multimode optical fiber of the present embodiment can collectively reduce the amount of digital processing required for signals of each wavelength in an optical communication system that performs WDM transmission.

The coupling wavelength λ ₀ can be controlled by changing the core radius and the refractive index difference as described in Non-Patent Document 7. Also, in order to control the DMD and DMD slope of the optical fiber used in the optical transmission line, respectively, as described in Non-Patent Document 2, it is realized by adjusting the refractive index distribution using a stepped refractive index distribution. it can.

  Also, by installing LP01 and LP11 mode converters at the incident end of the nDMDF, it is possible to reverse the sign of the DMD generated in the nDMDF in a pseudo manner, so that the nDMDF can be used for the transmission line. . Here, the mode converter can be realized by using a long-period grating as described in Non-Patent Document 9.

  By executing the multimode optical fiber design method according to the present embodiment, a multimode optical fiber having a desired negative DMD can be realized. The multimode optical fiber design method according to the present embodiment includes a center core adjustment procedure, a group delay difference adjustment procedure, and a ring core adjustment procedure in this order.

  In the multimode optical fiber having the center core and the ring core according to the present embodiment, as shown in FIGS. 11 and 12, the design can be performed from the optical characteristics when each core is present alone. That is, the difference between the group delay in the LP01 mode in the center core and the group delay in the LP11 mode in the ring core is the DMD obtained when both cores exist.

First, the center core adjustment procedure is executed. In the case where only the center core exists, the propagation mode needs to be the LP01 mode and the LP11 mode. Therefore, in this procedure, the normalized frequency V calculated from the core radius a ₁ of the center core and the relative refractive index difference Δ may be 2.40 or more and 3.83 or less at the used wavelength, and within the range a ₁ And Δ are adjusted.

なお、ｎ_１はセンターコアの屈折率である。 The normalized frequency V is expressed by the following equation.

N ₁ is the refractive index of the center core.

Next, a group delay difference adjustment procedure is executed. In this procedure, the group delay time between modes when the center core and the ring core exist independently is calculated, and the core radius and relative refractive index difference are determined so that the difference satisfies the desired DMD. To do. In the case of a ring core, the thickness of the ring core corresponds to the core radius. For example, FIGS. 13 and 14 show the group delay when the core radius a and the relative refractive index difference Δ are changed when the center core and the ring core exist alone. In FIG. 14, a ₃ −a ₂ in the case of a ₂ = 4 μm is defined as the core radius a of the ring core. The unit of group delay in FIGS. 13 and 14 is ns / km. It can be seen that the group delay is changed by controlling the core radius a and the relative refractive index difference Δ. Moreover, the structure which makes DMD -7.85ns / km or less can also be clarified by controlling them.

In a structure in which both a center core and a ring core exist, in order to realize the above DMD, only the higher order mode needs to be coupled from the center core to the ring core. Therefore, the ring core adjustment procedure is executed. In this procedure, mode coupling occurs at a wavelength where the effective refractive index of the LP11 mode of the center core and the effective refractive index of the LP11 mode of the ring core coincide with each other. Therefore, the effective refractive index is appropriately changed to control the coupling wavelength. Thereby, the effective refractive indexes of the center core and the ring core with respect to the higher order mode at the determined coupling wavelength are made substantially equal. At this time, as shown in FIG. 15, by applying Δ ₂ and changing the 0 level, the entire refractive index level of the ring core shown in FIG. It can be controlled.

Incidentally, a relationship of Equation (7) is a delta ₁ in FIG.
(Equation 7)
Δ ₀ = Δ ₁ + Δ ₂ (7)

For example, when a ₁ = 3 μm, a ₂ = 4 μm, a ₃ = 7 μm, Δ ₀ = 1.2%, Δ ₃ = 0.4%, a diagram showing changes in DMD with respect to changes in Δ ₂ is shown in FIG. 16 shows. It can be seen that the minimum value of DMD does not change greatly and only the coupling wavelength can be controlled.
By designing in such a procedure, a multimode optical fiber having a desired DMD can be designed.

(Embodiment 2)
FIG. 17 is a schematic diagram of an optical communication system according to the present embodiment. N types of optical signals emitted from the N optical transmitters 91 are combined in a multiplexer 92. The combined signal light enters the optical transmission line 93 and is demultiplexed to the M port by the demultiplexer 94 installed on the output side. The demultiplexed M kinds of signals are received by the M optical receivers 95, and the FIR filter 96 installed in the subsequent stage compensates for the signal degradation received by the optical transmission path 93.

  Here, this configuration is N-input M-output MIMO transmission, and N types of signals can be transmitted in parallel. For example, the optical transmission line 93 is a multimode optical fiber, and transmits a mode multiplexed optical signal in which at least two modes are mode multiplexed. The configuration for performing mode multiplexing may be the optical transmitter 91 or the multiplexer 92. The configuration for mode separation may be the duplexer 94 or the optical receiver 95. The FIR filter 96 can also compensate for mode dispersion, wavelength dispersion, and polarization dispersion.

  In order to acquire the electric field amplitude / phase information of the received signal, an optical receiver 95 including a local light source, a 90 ° hybrid, a balance receiver, an analog-digital converter, and a calculator may be used (for example, non (See Patent Document 8).

  The FIR filter 96 can compensate for the linear distortion generated in the optical transmission line 93, and by appropriately setting the tap delay amount / coefficient, another optical transmitter 91 generated in the optical transmission line 93 is obtained. Signal degradation due to interference, mode dispersion, chromatic dispersion, and polarization dispersion can be compensated. However, in the case of mode dispersion, if the DMD with the higher-order mode increases, the amount of calculation required for compensation becomes enormous, and therefore it is necessary to reduce the DMD.

Therefore, the related invention uses an optical transmission line as shown in FIG. In the optical transmission line shown in FIG. 1, pDMDF having a positive group delay difference and nDMDF having a negative group delay difference are alternately arranged to compensate for a group delay difference that changes according to propagation. The group delay difference can be reduced. However, two types of cables, that is, a pDMDF built-in cable and an nDMDF built-in cable are mixed in the optical transmission line 93, and the management operation becomes complicated. The adjustment of the length for difficult in the optical transmission path 93, there is a problem that it is difficult to adjust the length _{L n} of the length _{L p} and nDMDF of PDMDF.

  In the optical communication system according to the present embodiment, a pDMDF 931 is laid in the optical transmission line 93 as shown in FIG. 18, and a multimode optical fiber 932 is used for the DMD compensation module in the relay device 97. The multimode optical fiber 932 is the multimode optical fiber described in the first embodiment, and is an nDMDF having an absolute value of DMD of 7.85 ns / km or more.

  The DMD of the pDMDF 931 constituting the optical transmission line 93 is 220 ps / km at the maximum. For this reason, even in a system in which the span of the optical transmission line 93 is 100 km, the multi-mode light in which the power penalty in the optical transmission line 93 is 1 dB or less and the accumulated DMD value of the optical transmission line 93 is installed in the repeater 97 The fiber 932 can completely compensate.

  The low DMD optical fiber reported so far is about several tens of ps / km. For example, in Non-Patent Document 4, DMD is 36 ps / km or less. Based on this, when the DMD value of the nDMDF 932 required from the excess loss allowed to suppress the OSNR degradation to 1 dB or less in the 100 km span is obtained in the same manner as described above, the maximum accumulated DMD value 3 generated in the optical transmission line 93 is 3 In order to compensate for .6 ns, nDMDF of -3.6 ns / 2.8 km = -1.29 ns / km is required.

  As described above, when the manufacturing error based on the mass production of the optical fiber is not taken into consideration, the absolute value of the necessary DMD value of the multimode optical fiber 932 arranged in the relay device 97 may be small. Also in such an optical communication system, by using the multimode optical fiber of Embodiment 1, the fiber length for compensating for DMD can be shortened, so that the power penalty can be further reduced.

  It is possible to use nDMDF instead of pDMDF931 by installing a mode converter of LP01 and LP11 at the incident end of the multimode optical fiber 932. Here, the mode converter can be realized by using a long-period grating as described in Non-Patent Document 9.

  The present invention can realize an apparatus and a system for reducing the MIMO digital processing amount in the optical MIMO transmission system and reducing the operation efficiency.

91: Optical transmitter 92: Multiplexer 93: Optical transmission line 94: Demultiplexer 95: Optical receiver 96: FIR filter 97: Repeater 931: pDMDF
932: Multimode optical fiber

Claims (7)

A center core disposed in the center of the optical fiber and propagating the fundamental mode and higher order mode;
A ring core disposed around the center core and propagating higher order modes;
With
At a defined wavelength, the effective refractive index of the higher order mode of the center core and the effective refractive index of the higher order mode of the ring core are substantially equal,
A multimode optical fiber in which a difference between a group delay of a fundamental mode propagating through the center core and a group delay of a higher-order mode propagating through the ring core is a predetermined group delay difference. The normalized frequency calculated from the core radius and relative refractive index difference of the center core is 2.40 or more and 3.83 or less,
The multimode optical fiber according to claim 1, wherein the determined group delay difference is −7.85 ns / km or less.   The multimode optical fiber according to claim 1, wherein a differential coefficient with respect to a wavelength of the difference at the predetermined wavelength is negative. An optical transmitter for transmitting a mode multiplexed optical signal of two modes;
An optical receiver for receiving the mode multiplexed optical signal;
An optical fiber connected between the optical transmitter and the optical receiver, wherein the differential coefficient with respect to the wavelength of the group delay difference between the two modes is positive;
The multimode optical fiber according to any one of claims 1 to 3, wherein the multimode optical fiber is inserted between the optical fiber and the optical receiver and compensates for the group delay difference in the optical fiber.
An optical communication system comprising: An optical transmitter for transmitting a mode multiplexed optical signal of two modes;
An optical receiver for receiving the mode multiplexed optical signal;
An optical fiber that connects between the optical transmitter and the optical receiver and has a negative differential coefficient with respect to the wavelength of the group delay difference between the two modes;
A mode converter inserted between the optical fiber and the optical receiver and inverting the sign of the group delay difference generated in the optical fiber;
4. The multimode optical fiber according to claim 1, wherein the multimode optical fiber is inserted between the mode converter and the optical receiver and compensates for the group delay difference in the optical fiber inverted by the mode converter. 5. ,
An optical communication system comprising: A multimode optical fiber design method comprising a center core disposed in the center of an optical fiber and a ring core disposed around the center core,
A center core adjustment procedure for obtaining a core radius and a relative refractive index difference of the center core so that the center core propagates two modes of a fundamental mode and a higher-order mode;
Calculating the group delay of the fundamental mode of the center core and the group delay of the higher order mode of the ring core when the center core and the ring core are present independently, respectively, and the group delay of the center core and the group of the ring cores A group delay difference adjustment procedure for obtaining a core radius and a relative refractive index difference of the center core and the ring core so that a delay difference satisfies a set group delay difference;
A ring core adjustment procedure for adjusting the refractive index of the ring core so that the effective refractive index of the higher order mode of the center core and the effective refractive index of the higher order mode of the ring core are substantially equal at a predetermined wavelength;
A method of designing a multimode optical fiber having the following in order: In the center core adjustment procedure, the core radius and relative refraction of the center core are set such that the normalized frequency calculated from the core radius and relative refractive index difference of the center core is 2.40 or more and 3.83 or less at the used wavelength. Find the rate difference,
The core radius and relative refractive index difference of the center core and the ring core are determined so that the determined group delay difference is −7.85 ns / km or less in the group delay difference adjustment procedure. Multimode optical fiber design method.

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