JP5702707B2 - Optical fiber and optical fiber transmission system - Google Patents

Description

  The present invention relates to a multimode optical fiber capable of expanding communication capacity, and an optical transmission system technology using the same.

  In optical fiber communication systems, non-linear effects and fiber fuses that occur in optical fibers are problematic, and transmission capacity is limited. In order to alleviate these restrictions, it is necessary to reduce the density of light guided to the optical fiber, and a large core fiber has been studied (for example, see Non-Patent Documents 1 and 2).

  However, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited.

  Therefore, attempts have been made to apply multi-input multi-output (MIMO) technology, which is a technology for increasing the capacity in radio, to optical fiber transmission (see, for example, Non-Patent Documents 3 and 4).

  Optical MIMO technology can expand transmission capacity by using a multimode optical fiber as a transmission medium, and further eliminates the need for single-mode operating conditions, which are the limiting factors in large-core optical fiber, as described above. It is also a feature that is possible.

T. T. et al. Matsui, et al. , "Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConventionmm." Lightwave Technol. 27, 5410-5416, 2009. K. Mukasa, K .; Imamura, R.A. Sugizaki and T.A. Yagi, "Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160 μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600nm bandwidth", the Proceedings of OFC2008, OThR1, Feb. 2008. Akhil R.D. Shah, Rick C.I. J. et al. Hsu, Alireza Tarihat, Ali H. et al. Sayed, and Bahr Jalali, “Coherent Optical MIMO (COMIMO)”, J. Am. Lightwave Technol. 23, 2410-2419 (2005) B. C. Thomsen, "MIMO Enabled 40 Gb / s Transmission Using Mode Division Multiplexing in Multimode Fiber", in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper OThM6. 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. Y. Katsyuyama, M .; Tokuda, N.A. Uchida, and M.M. Nakahara, “New method for measuring V-value of a single-mode optical fiber”, Electron. Lett. , Vol. 12, pp. 669-670, Dec. 1976. X. Zhao, F.A. S. Choa, “Demonstration of 10-Gb / s transmissions over a 1.5-km-long multimode fiber usage techniques,” IEEE Phototechnologies Technologies. 14, pp. 1187-1189, 2002

  However, in optical fiber transmission using optical MIMO, when the group delay difference between modes increases, there is a problem that the amount of calculation of digital processing required for signal restoration increases.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a multimode optical fiber that can reduce the load of digital processing necessary for signal restoration, and an optical fiber transmission system using the same. To do.

  In the present invention, the optical fiber having a stepped profile in refractive index is used to reduce the group delay difference between the propagation modes, and the provision of this optical fiber reduces the digital processing load of the optical fiber transmission system. .

  Specifically, the multimode optical fiber according to the present invention includes a core whose refractive index decreases stepwise from the central axis toward the outside, and the outer side of the core, where the refractive index is any refractive index of the core. And a low refractive index layer that is between the core and the clad and has a refractive index equal to or lower than the refractive index of the clad.

  The multimode optical fiber according to the present invention has a core having a stepped profile and a low refractive index layer outside the core, and can reduce a group delay difference in light propagation modes. That is, the load of digital signal processing can be reduced by using the multimode optical fiber in an optical transmission system using optical MIMO. Therefore, the present invention can provide a multimode optical fiber that can reduce the load of digital processing necessary for signal restoration.

The core of the multimode optical fiber according to the present invention includes a first core on a central axis side and a second core outside the first core, and the first core has an outer radius of a ₁ and the low refraction. a ₁ relative refractive index with respect to the rate layer is delta, the second core, the outer peripheral radius a _2, the a ₂ specific refractive index for the low refractive index layer is delta, the low refractive index layer is an outer peripheral radius a _3, and the clad, the a specific refractive index for the low refractive index layer is _{Δ 3, a 1 = 4μm,} a 3 = 20μm, Δ 1 = 0.46%, when the delta ₃ = 0%, a ₂ = 7.5 to 10 μm and Δ ₂ = 0.3 to 0.36%. The multimode optical fiber can enhance the effect of reducing the load of digital processing necessary for signal restoration.

Specifically, the optical fiber transmission system according to the present invention includes the multimode optical fiber, an optical transmitter that outputs the signal light modulated by the input data signal to the multimode optical fiber,
An optical receiver that receives the signal light propagating through the multimode optical fiber and outputs a received signal; adjusts a delay amount and a tap coefficient of a tap to adjust the tap signal to the optical transmitter from the received signal of the optical receiver And an FIR (Finite Impulse Response) filter for restoring the input data signal.

  The optical fiber transmission system according to the present invention includes the multimode optical fiber, and can reduce the load of digital signal processing during reception. Therefore, the present invention can provide an optical fiber transmission system capable of reducing the load of digital processing necessary for signal restoration.

  The optical fiber transmission system according to the present invention includes a multiplexer that combines the signal light from the N optical transmitters and couples the signal light to the multimode optical fiber, and the signal that the multimode optical fiber propagates. A splitter that splits the light and couples it to each of the M optical receivers, wherein there are N FIR filters, and the optical transmission is performed using the received signals from the M optical receivers. An N-input M-output optical MIMO transmission for restoring each of the data signals input to the apparatus is realized.

  The optical fiber transmission system according to the present invention can reduce the load of digital signal processing at the time of reception in optical fiber transmission using optical MIMO.

  ADVANTAGE OF THE INVENTION According to this invention, the multimode optical fiber which can reduce the load of the digital processing required for the decompression | restoration of a signal, and an optical fiber transmission system using the same can be provided.

It is a figure explaining the optical fiber transmission system concerning the present invention. It is a figure explaining the structure of a FIR filter. It is a figure explaining the tap coefficient update procedure of a FIR filter. It is a figure explaining arrangement | positioning of a data part and a training symbol. It is a figure explaining the refractive index distribution of a step index type optical fiber. It is a figure explaining the group delay difference characteristic of a step index type optical fiber. It is a figure explaining the refractive index distribution of a graded index type optical fiber. It is a figure explaining the group delay difference characteristic of a graded index type optical fiber. It is a figure explaining the refractive index distribution of the multimode optical fiber which concerns on this invention. It is a figure explaining the group delay difference characteristic at the time of changing the relative refractive index (DELTA) ₁ of the multimode optical fiber which concerns on this invention. It is a figure explaining the group delay difference characteristic at the time of changing the relative refractive index (DELTA) ₂ of the multimode optical fiber which concerns on this invention. Is a diagram illustrating the group delay difference characteristics when the scaling ratio r _a multimode optical fiber according to the present invention was varied. It is a figure explaining the group delay difference characteristic in the 2 mode area | region of the multimode optical fiber which concerns on this invention. It is a figure explaining the group delay difference characteristic at the time of changing the relative refractive index (DELTA) ₃ of the multimode optical fiber which concerns on this invention. It is a figure which shows an example of the group delay difference characteristic of the 2 mode optical fiber which assumed utilization of MIMO in the optical fiber transmission system which concerns on this invention. It is a figure which shows an example of the group delay difference characteristic of the two-mode optical fiber when not using MIMO in the optical fiber transmission system which concerns on this invention. It is a figure explaining the group delay difference characteristic in 4 mode area | region of the multimode optical fiber which concerns on this invention. It is a figure which shows an example of the group delay difference characteristic of 4 mode optical fiber which assumed utilization of MIMO in the optical fiber transmission system which concerns on this invention. It is a figure which shows an example of the group delay difference characteristic of 4 mode optical fiber when not using MIMO in the optical fiber transmission system which concerns on this invention. It is sectional drawing explaining the multimode optical fiber which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments.

(Embodiment 1)
FIG. 1 is a schematic diagram illustrating a configuration of an optical fiber transmission system 301 according to the present embodiment. The optical fiber transmission system 301 includes a multimode optical fiber 10, an optical transmitter 11 that outputs signal light modulated by an input data signal to the multimode optical fiber 10, and signal light propagated by the multimode optical fiber 10. An optical receiver 14 that receives light and outputs a received signal; an FIR filter 15 that adjusts a tap delay amount and a tap coefficient to restore a data signal input to the optical transmitter 11 from the received signal of the optical receiver 14; .

  The optical fiber transmission system 301 multiplexes the signal light from the N optical transmitters 11 and couples it to the multimode optical fiber 10 and branches the signal light propagated by the multimode optical fiber 10. And branching devices 13 coupled to the M optical receivers 14 respectively, and there are N FIR filters 15, which are input to the optical transmitter 11 using received signals from the M optical receivers 14. Further, N-input M-output optical MIMO transmission for restoring the respective data signals is realized.

  N types of signals emitted from the N optical transmitters 11 are combined in the multiplexer 12. The combined signal light enters the multimode optical fiber 10 and is branched to the M port in the branching device 13 installed on the output side. The branched M types of signals are received by the M optical receivers 14, and the FIR filter 15 installed in the subsequent stage compensates for signal degradation received by the optical fiber. This configuration is N-input M-output MIMO transmission, and N types of signals can be transmitted in parallel. The FIR filter 15 can also compensate for mode dispersion, wavelength dispersion, and polarization mode dispersion.

  However, in order to compensate for various signal degradations by the FIR filter 15, the electric field amplitude and phase information of the received signal are necessary.

For example, a signal transmitted from the transmitter A as the nth symbol is x _A (n), a signal received as the nth symbol from the receiver B is y _B (n), and after passing through the FIR filter, x _A (n ) Is taken as u _A (n). That is, when x _A (n) = u _A (n), transmission can be performed without error. Here, various signals have electric field amplitude and phase information.

FIG. 2 is a diagram for explaining the configuration of the FIR filter 15. The FIR filter 15 is composed of L taps 151 for each received signal input, and if there are M optical receivers 14, there are M × L taps in total. Each tap 151 includes a delay element 152 and a control circuit 153 that controls the amplitude and phase of the received signal. For example, let τ _{i be} the delay amount of the delay element 152 _B- i of the i-th tap 151 _B -i of the FIR filter 15 to which the received signal y _B (n) is input, and w _B (i) be the tap coefficient. However, the _{τ 1 <τ 2 <··· <} τ L. If the signal after passing through the FIR filter with respect to the input of y _B (n) is z _B (n), the signal u _A (n) is obtained by adding z ₁ (n) to z _M (n). It is done.

In order to restore each data signal that is the basis of the signal light of x ₁ (n) to x _N (n), it is necessary to install N FIR filters shown in FIG.

  Further, in order to acquire the electric field amplitude and phase information of the received signal y (n), an optical receiver 14 composed of a local light source, a 90 ° hybrid, a balance receiver, and an analog-digital converter may be used. You may implement | achieve the function of the filter 15 using calculators, such as PC (for example, refer nonpatent literature 5).

  The FIR filter 15 shown in FIG. 2 can compensate for linear distortion generated in the optical fiber, and by appropriately setting the delay amount and tap coefficient of the tap 151, other transmitters generated in the optical fiber. Can be compensated for signal degradation caused by interference, mode dispersion, chromatic dispersion, and polarization mode dispersion.

  The tap coefficient for correctly restoring the received signal y (n) can be obtained using the adaptive equalization algorithm shown in FIG. It is assumed that a known training symbol 41 is added to the received signal y (n) in addition to the data part 42 as shown in FIG.

u _A (n) obtained when the received signals y ₁ (n) to y _M (n) pass through the FIR filter 15 must match x _A (n). When n <N _training , since the transmission symbol x _A (n) is known on the receiving side, the transmission symbol and the restored symbol can be compared, and an error e (n) = x _A ( n) The tap coefficient is controlled using an adaptive algorithm so that -u _A (n) becomes small. When y ₁ (n + 1) to y _M (n + 1) are received, e (n + 1) is calculated using u _A (n + 1) and x _A (n + 1) obtained using the previously determined tap coefficient, The tap coefficient is updated again using the adaptive algorithm, and the tap coefficient that minimizes the error is obtained by repeating the same procedure for the number of training symbols. After the coefficients are determined using all the training symbols 41, the subsequent data section 42 is restored by the FIR filter 15 using the determined tap coefficients.

  As the adaptive equalization algorithm, a Least Mean Square (LMS) algorithm or a Recursive Least Square (RLS) algorithm can be used.

  However, in the case of mode dispersion, if the group delay difference from the higher-order mode increases, the amount of calculation required for compensation becomes enormous, so the group delay difference needs to be reduced.

で表される。 FIG. 5 shows a refractive index profile of a conventional step index type optical fiber. In this structure, the core diameter is a, and the relative refractive index difference between the core and the clad is Δ. The relative refractive index difference is expressed as follows: n ₁ is the refractive index of the core and n ₂ is the refractive index of the cladding.

It is represented by

  For example, when the core radius a = 7 μm and Δ = 0.4%, when the propagation mode is calculated by general electromagnetic field analysis, the fundamental mode LP01 and the first higher-order mode LP11 are obtained at wavelengths of 1450 to 1625 nm. Propagate. FIG. 6 shows the result of calculation using the finite element method for the group delay difference between the LP01 and LP11 modes at this time. As is apparent from the figure, a group delay difference of 2.33 ns / km occurs at a wavelength of 1550 nm. In order to compensate for the mode dispersion received by the signal after transmission, an FIR filter having a delay corresponding to at least a group delay difference between the fundamental mode LP01 and the first higher-order mode LP11 is necessary. If this delay increases, the amount of calculation required for signal restoration using MIMO performed by the FIR filter increases, which is not preferable.

で表される。ここで、ａはコア外周の半径、ｒはコア内半径、αは屈折率の形状を決定する任意の正の実数である。 In order to reduce the group delay difference, a graded index optical fiber as shown in FIG. 7 is suitable. The refractive index distribution n in the radial direction of the fiber (r) is the refractive index of the core and n _1,

It is represented by Here, a is the radius of the outer periphery of the core, r is the radius of the core, and α is an arbitrary positive real number that determines the shape of the refractive index.

  FIG. 8 is a diagram showing the absolute value of the group delay difference when α = 2 in the above formula and a and Δ are changed. At this time, in order to limit the number of propagation modes to two at wavelengths of 1450 to 1625 nm, it is a condition that the LP21 mode, which is the second higher-order mode, does not propagate. The condition for not propagating was that the bending loss at a bending radius of 140 mm in the wavelength band used was 1 dB / m or more. This condition is based on the assumption that a bending radius of 140 mm is used to measure the cutoff wavelength as described in Non-Patent Document 6 and that the loss does not propagate at 1 dB / m or more as described in Non-Patent Document 1. Is based.

The structure satisfying this condition is a region where the core radius is smaller than the broken line BL _LP21 in the figure.

As for the bending loss of the number of propagation modes, the ITU-T G.G. As defined in 655, if the bending loss at a bending radius of 30 mm is 0.5 dB / 100 turn or less, this is a region where the core radius is larger than the dotted line BL _LP11 in the figure.

  Even when a graded index optical fiber is used, the group delay difference is 1 ns / km or more with a structure satisfying the above conditions.

  Therefore, the optical fiber transmission system 301 uses a multimode optical fiber 10 having a core 50 having a stepped profile and a low refractive index layer 53 outside the core 50 as shown in FIGS. 9 and 20. The multimode optical fiber 10 includes a core 50 having a refractive index that decreases stepwise from the center axis toward the outside, a cladding 54 that is outside the core 50 and has a refractive index smaller than any of the refractive indexes of the core 50, A low-refractive-index layer 53 that is between the core 50 and the clad 54 and has a refractive index equal to or lower than the refractive index of the clad 54.

In the following description, the core 50 having a stepped refractive index is referred to as a first core 51 and a second core 52 from the central axis toward the outside for each refractive index. In this structure, the radii of the first core 51 and the second core 52 are a ₁ and a ₂ , respectively, and the radius of the boundary between the low refractive index layer 53 and the clad 54 is a ₃ . Further, relative refractive index differences based on the refractive indexes of the low refractive index layers 53 of the first core 51, the second core 52, and the clad 54 are denoted by Δ ₁ , Δ ₂ , and Δ ₃ , respectively.

FIG. 10 shows that a ₁ = 4 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₂ = 0.34%, Δ ₃ = 0.08%, and the wavelength 1450 nm when Δ ₁ is changed. , 1550 nm and 1625 nm, the change in the group delay difference between the LP01 mode and the LP11 mode is shown. From FIG. 10, delta ₁ is increased when it can be seen that the group delay difference is small.

Similarly, in FIG. 11, the wavelength when a ₁ = 4 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, Δ ₃ = 0.08%, and Δ ₂ is changed. The change of the group delay difference of LP01 mode and LP11 mode in the case of 1450 nm, 1550 nm, and 1625 nm is shown. Than 11, it can be seen that the differential group delay is reduced by reducing the delta _2.

Next, in FIG. 12, a ₁ = 4 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, Δ ₂ = 0.34%, Δ ₃ = 0.08%, and a ₁ It shows the change of the group delay difference in the structure when multiplied by the respective scaling ratio r _a to ~a _3. With the change of r _a, it can be seen that the differential group delay at three wavelengths (1450,1550,1625nm) is changed. Here, when a 1 r _a, it can be seen that the absolute value of the group delay difference is less than 300 ps / miles in three wavelengths.

FIG. 13 shows, as an example, when a ₁ = 4 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, and Δ ₃ = 0%, the change in the absolute value of the group delay difference accompanying the change in a ₂ and Δ ₂ FIG. It can be seen that a group delay difference of 1 ns / km or less, which could not be realized with a graded index optical fiber, can be realized in a region where a ₂ = 7.5 to 10 μm and Δ ₂ = 0.3 to 0.36%.

To limit the two propagation modes in the above area may be increased propagation loss of 3 modes eyes, to realize a predetermined number of modes by adjusting the delta _3.

FIG. 14 shows the LP21 mode when a ₃ is changed with a ₁ = 4 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, Δ ₂ = 0.34%. The bending loss at a bending radius of 140 mm at a wavelength of 1450 nm is shown. By increasing the delta _3, it is possible to increase the bending losses _{BL LP21} of LP21 mode, it can be seen that a 1 dB / m or more. Further, although the bending loss BL _LP11 when the bending radius is 140 mm at the wavelength of 1625 nm in the LP11 mode is also shown, it is understood that the bending loss in the propagation mode is not affected.

  Note that the bending loss of the optical fiber is larger at the longer wavelength than the short wavelength, so the LP21 mode is calculated at 1450 nm, which is the shortest wavelength in use, and at 1625 nm, where the bending loss is maximized for the LP11 mode. The worst value of the characteristics at a wavelength of 1450 to 1625 nm can be grasped.

In this way, by using the multimode optical fiber 10, the group delay difference can be controlled by adjusting a ₂ and Δ ₂ in the core 50, and the number of propagation modes can be controlled by the relative refractive index Δ ₃ of the cladding 54. It becomes.

FIG. 15 shows the group delay when a ₁ = 4 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, Δ ₂ = 0.34%, and Δ ₃ = 0.08%. This is the wavelength characteristic of the difference. It can be seen that the group delay difference is suppressed to 1 ns / km or less over the use wavelength range of 1450 to 1625 nm.

When the bending loss at a bending radius of 30 mm in the LP01 and LP11 modes in this structure is calculated using the finite element method, they are 1 × 10 ⁻⁸ and 0.44 dB / 100 turn at a wavelength of 1625 nm, respectively, and at a use wavelength of 1450 to 1625 nm. ITU-TG. Meets 655 requirements. Similarly, the bending loss at a bending radius of 140 mm in the LP21 mode is calculated to be 5.5 dB / m at a wavelength of 1450 nm.

  The bending loss tends to be smaller at the short wavelength, and as described in Non-Patent Document 6, a bending radius of 140 mm is used for measuring the cutoff wavelength, and as described in Non-Patent Document 1, Based on the assumption that the loss does not propagate at 1 dB / m or higher, the loss is 1 dB / m or higher at 1450 nm. You will be limited to two.

  When a system that compensates for mode dispersion generated in a fiber by using an FIR filter and adaptive equalization technology without using MIMO is assumed (for example, see Non-Patent Document 7), propagation of higher-order modes is essential. Instead, it is sufficient that the bending loss of the LP01 mode, which is the basic mode, satisfies the regulation. By using this configuration, the effective area can be increased compared to the conventional single mode fiber, and the nonlinear effect generated in the optical fiber can be suppressed.

FIG. 16 shows the group delay when a ₁ = 7.5 μm, a ₂ = 15 μm, a ₃ = 22 μm, Δ ₁ = 0.348%, Δ ₂ = 0.252%, and Δ ₃ = 0.162%. This is the wavelength characteristic of the difference. It can be seen that the group delay difference is suppressed to 1 ns / km or less over the use wavelength range of 1450 to 1625 nm.

  The bending loss of the LP01 mode in this structure at a bending radius of 30 mm is 0.44 dB / 100 turn at a wavelength of 1625 nm, and ITU-TG. Meets 655 requirements. In addition, the bending loss of the LP21 mode at a bending radius of 140 mm is 30 dB / m at a wavelength of 1450 nm, and similarly, it is understood that the propagation mode is limited to two over the use wavelength range of 1450 to 1625 nm.

In the design shown in FIG. 15, the effective area was 127 μm ² , but by using the design of FIG. 16, the effective area is increased to 246 μm ² , and when the MIMO is not used, a more nonlinear effect is obtained. It becomes a structure that can suppress.

  When MIMO is used, the transmission capacity can be expanded if the number of modes is large. Therefore, a case is assumed where the number of propagation modes is limited to four. If the propagation modes are limited to four, the waveguide modes are LP01, LP11, LP21, and LP02, and LP31 does not propagate.

Therefore, the LP01 mode and each higher-order mode associated with changes in a ₂ and Δ ₂ when a ₁ = 4.5 μm, a ₃ = 20 μm, Δ ₁ = 0.78%, and Δ ₃ = 0.1%. FIG. 17 shows the maximum value among the absolute values of the group delay differences. When the mode increases, the group delay difference becomes large. Similarly, the group delay difference of 1 ns / km or less, which could not be realized by the graded index, is a ₂ = 8.2 to 9.5 μm, Δ ₂ = 0.54. It can be seen that it can be realized in the region of ˜0.56%.

FIG. 18 shows a case where a ₁ = 4.5 μm, a ₂ = 8.8 μm, a ₃ = 20 μm, Δ ₁ = 0.78%, Δ ₂ = 0.551%, and Δ ₃ = 0.1%. It is a wavelength characteristic of a group delay difference with LP01 mode. It can be seen that the group delay difference is suppressed to 1 ns / km or less over the use wavelength range of 1450 to 1625 nm. The bending loss of the LP02 mode in this structure at a bending radius of 30 mm is 0.078 dB / 100 turn at a wavelength of 1625 nm. In the optical fiber, the bending loss of the lower order mode is smaller than that of the higher order mode, and in the case of the 4-mode region, the bending loss of the other lower order modes is smaller than that of the LP02 mode, and the usable wavelength is 1450 to 1625 nm. In the ITU-TG. 655 will be satisfied. In addition, the bending loss of the LP31 mode at a bending radius of 140 mm is 9.2 dB / m at a wavelength of 1450 nm, and similarly, it is understood that the propagation mode is limited to four over the use wavelength range of 1450 to 1625 nm.

  As before, when assuming a system that does not use MIMO and compensates for mode dispersion that occurs in the fiber using an FIR filter and adaptive equalization technology, the bending loss of higher-order modes may not meet the specification. Good.

FIG. 19 shows a case where a ₁ = 8.5 μm, a ₂ = 16.8 μm, a ₃ = 25 μm, Δ ₁ = 0.4%, Δ ₂ = 0.305%, and Δ ₃ = 0.195%. It is a wavelength characteristic of a group delay difference. It can be seen that the group delay difference is suppressed to 1 ns / km or less over the use wavelength range of 1450 to 1625 nm. The bending loss of the LP01 mode in this structure at a bending radius of 30 mm is 8.2 × 10 ⁻³ dB / 100 turn at a wavelength of 1625 nm, and ITU-TG. Meets 655 requirements. In addition, the bending loss of the LP21 mode at a bending radius of 140 mm is 8.3 dB / m at a wavelength of 1450 nm, and similarly, it is understood that the propagation mode is limited to four over the use wavelength of 1450 to 1625 nm.

In the design shown in FIG. 18, the effective area is 94 μm ² , but in the case of the design not using the MIMO shown in FIG. 19, the effective area is increased to 270 μm ² . Therefore, it can be seen that when the MIMO is not used, the non-linear effect can be further suppressed.

The following describes the optical fiber transmission system of FIG.
(1)
N optical transmitters (N is an integer of 1 or more) for transmitting optical signals;
M optical receivers (M is an integer equal to or greater than N) for receiving the optical signal;
A multiplexer that multiplexes the light emitted from the N optical transmitters;
A multimode optical fiber having two or more propagation modes connecting between the optical transmitter and the optical receiver;
A branching device for branching the light emitted from the multimode optical fiber into M pieces;
An optical fiber transmission system comprising an FIR filter that compensates for waveform distortion of the received signal,
The optical receiver has a function of acquiring electric field amplitude / phase information of a received signal,
The multimode optical fiber comprises a first core and a second core having different refractive indexes,
The radius a ₁ and the refractive index Δ ₁ of the _first core and the radius a ₂ and the refractive index Δ _{2 of} the second core adjacent to the cladding side are a ₁ <a ₂ , 0 <Δ ₂ <Δ ₁ An optical fiber transmission system having the following relationship:

  The optical fiber transmission system of the present invention can realize large capacity and long-distance communication by suppressing nonlinear effects in the optical fiber or using multiple modes.

10: multimode optical fibers 11, 11-1, 11-2,..., 11-N: optical transmitter 12: multiplexer 13: branching devices 14, 14-1, 14-2,. 14-N: optical receiver 15: FIR filter 41: training symbol 42: data part 50: core 51: first core 52: second core 53: low refractive index layer 54: clad 151: tap 152: delay element 153: Control circuit 301: optical fiber transmission system

Claims (4)

A core whose refractive index decreases stepwise from the central axis toward the outside;
A cladding outside the core, the refractive index of which is smaller than any refractive index of the core;
A low refractive index layer that is between the core and the clad and has a refractive index less than or equal to the refractive index of the clad;
I have a,
The core consists of a first core on the central axis side and a second core outside the first core,
The outer peripheral radius of the first core is a ₁ , the relative refractive index difference with respect to the low refractive index layer is Δ ₁ ,
The outer peripheral radius of the second core is a ₂ , the relative refractive index difference with respect to the low refractive index layer is Δ ₂ ,
The inner radius of the low refractive index layer is a ₂ , the outer radius is a ₃ ,
The relative refractive index difference with respect to the low refractive index layer of the cladding is taken as delta _3,
a ₁ = 4 μm, a ₃ = 20 μm, Δ ₁ = 0.46%, Δ ₃ = 0%,
a ₂ = 7.5 to 10 μm, Δ ₂ = 0.3 to 0.36%,
The absolute value of the group delay difference between the LP01 mode and the LP11 mode is 1 ns / km or less.
A multimode optical fiber characterized by that .   A core whose refractive index decreases stepwise from the central axis toward the outside;
  A cladding outside the core, the refractive index of which is smaller than any refractive index of the core;
  A low refractive index layer that is between the core and the clad and has a refractive index less than or equal to the refractive index of the clad;
Have
  The core consists of a first core on the central axis side and a second core outside the first core,
  The outer radius of the first core is a ₁ , The relative refractive index difference with respect to the low refractive index layer is Δ ₁ ,
  The outer radius of the second core is a ₂ , The relative refractive index difference with respect to the low refractive index layer is Δ ₂ ,
  The inner peripheral radius of the low refractive index layer is a ₂ , The outer radius is a ₃ ,
  The relative refractive index difference of the cladding with respect to the low refractive index layer is Δ ₃ And when
  a ₁ = 4.5 μm, a ₃ = 20 μm, Δ ₁ = 0.78%, Δ ₃ = 0.1%
  a ₂ = 8.2 to 9.5 μm, Δ ₂ = 0.54 to 0.56%,
  The absolute value of the group delay difference between the LP01 mode and the LP11, LP21, and LP02 modes is 1 ns / km or less.
A multimode optical fiber characterized by that. The multimode optical fiber according to claim 1 or 2,
An optical transmitter that outputs the signal light modulated by the input data signal to the multimode optical fiber;
An optical receiver that receives the signal light propagated by the multimode optical fiber and outputs a received signal;
An FIR (Finite Impulse Response) filter that adjusts a tap delay amount and a tap coefficient to restore the data signal input to the optical transmitter from the received signal of the optical receiver;
An optical fiber transmission system comprising: A multiplexer that combines the signal light from N optical transmitters and couples the signal light to the multimode optical fiber;
A branching device for branching the signal light propagated by the multimode optical fiber and coupling the signal light to M optical receivers;
Further comprising
The FIR filter has N units, and realizes N-input M-output optical MIMO transmission that restores each data signal input to the optical transmitter using the received signals from the M optical receivers. The optical fiber transmission system according to claim 3.

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