Optical transmission system

Abstract

The present invention provides an optical transmission system that can satisfy the XT required by the modulation method even if there is a difference in inter-core loss in the MCF. The optical transmission system of the present invention includes: a multicore optical fiber having a plurality of core regions, in which the loss is different between at least two cores; a front excitation light source and a combining section for causing Raman amplification excitation light in the same direction as the signal light to be incident on each core of the multicore optical fiber; a rear excitation light source and a combining section for causing Raman amplification excitation light in the opposite direction to the signal light to be incident on each core of the multicore optical fiber; characterized in that the intensity ratio of the front excitation light and the rear excitation light (the ratio of the power of the excitation light is set to a prescribed value) is controlled to reduce the variation in inter-core crosstalk (XT).

Description

Technical Field

[0001] This disclosure relates to an optical transmission system using a multi-core optical fiber having multiple optical transmission cores. Background Technology

[0002] In optical transmission systems using multi-core fiber (MCF), crosstalk noise (XT) caused by light leakage from each fiber core to adjacent cores is a major cause of transmission quality degradation. The impact of XT on signal quality varies depending on the modulation scheme. For example, it is known that in QPSK modulation, a received XT (received XT) of -16 dB or higher, and in 16QAM modulation, a received XT of -24 dB or higher, results in an optical power loss of 1 dB or higher. Therefore, the inter-core spacing and core structure of the MCF are designed to meet the XT requirements of the modulation scheme (see, for example, Non-Patent Literature 1).

[0003] Existing technical documents

[0004] Non-patent literature

[0005] Non-patent literature 1: PJWinzer et al., “Penalties from In-Band Crosstalk for Advanced Optical Modulation Formats”, ECOC2011, Tu.5.B.7 (2011).

[0006] Non-patent literature 2: T. Takara et al., “1000-km 7-core fiber transmission of 10x 96-Gb / s PDM-16QAM using Raman amplification with 6.5W per fiber”, Opt. Exp. 20.9.10100 (2012).

[0007] Non-patent literature 3: T. Kitamura et al., “Cross-talk Characteristics of a Hybrid Multi-core Fiber Transmission System Using Distributed Raman Amplification”, OECC2013 TuS1-3 (2013). Summary of the Invention

[0008] The problem the invention aims to solve

[0009] However, in optical transmission systems using MCF, the received XT may deviate from its required design value due to differences in transmission loss between fiber cores. For example, the received XT may vary due to manufacturing quality deviations of individual fiber cores and fiber connection quality deviations, and may not meet the XT required by the modulation scheme.

[0010] Here, the received XT can be suppressed to the desired value by amplifying the intensity of the optical signal propagating in the fiber core. For example, non-patent documents 2 and 3 show examples of distributed Raman amplification in an optical transmission system using an MCF. However, non-patent documents 2 and 3 only show that amplification characteristics can be obtained in the same way as an SMF, and do not clarify the effect on the inter-fiber characteristic deviation of the aforementioned MCF.

[0011] In other words, the cited literature does not clearly explain how distributed Raman amplification is applicable to optical transmission systems using MCF.

[0012] Therefore, in order to solve the above problems, the purpose of this invention is to provide an optical transmission system that can meet the XT required by the modulation method even if there is a loss difference between fiber cores in the MCF.

[0013] Problem-solving methods

[0014] To achieve the above objectives, the optical transmission system of the present invention adjusts the intensity ratio of the excitation light for distributed Raman amplification incident from both directions.

[0015] Specifically, the optical transmission system of the present invention includes:

[0016] In multi-core optical fibers, the transmission loss differs between at least two of the multiple cores.

[0017] A front-excitation light source is used to incident Raman-amplified excitation light, which is incident on each core of the multi-core optical fiber in the same direction as the transmission direction of the optical signal; and,

[0018] A rear excitation light source is used to cause Raman amplified excitation light in the opposite direction to the transmission direction of the optical signal to be incident on each core of the multi-core optical fiber.

[0019] Its features are,

[0020] The intensity ratio of the Raman amplified excitation light output from the front excitation light source to the Raman amplified excitation light output from the rear excitation light source is adjusted so that the crosstalk noise on the receiving side is close to the design value.

[0021] Even if the received XT deviates from the design value due to fiber core loss differences, the variation of the received XT relative to the design value can be suppressed by adjusting the intensity ratio of the excitation light used for bidirectional Raman amplification to a specified range. Therefore, the present invention provides an optical transmission system that can meet the XT required by the modulation scheme even if there are inter-fiber loss differences in the MCF.

[0022] The specific adjustment range is shown below.

[0023] If the intensity ratio is set as the ratio R of the intensity of the Raman amplified excitation light output by the front excitation light source to the sum of the intensity of the Raman amplified excitation light output by the front excitation light source and the intensity of the Raman amplified excitation light output by the rear excitation light source, then the transmission loss difference Δα between the two fiber cores is adjusted to be within the following range:

[0024] When 0 dB / km < Δα ≤ 0.05 dB / km, 0.500 + 0.179Δα - 1.52 × 10 -2 / Δα≤R≤0.500+0.179Δα;

[0025] When -0.05dB / km ≤ Δα < 0dB / km, 0.500 + 0.179Δα ≤ R ≤ 0.500 + 0.179Δα - 1.52 × 10 -2 / Δα.

[0026] Invention Effects

[0027] This invention provides an optical transmission system that can meet the XT requirements of the modulation method even if there is a loss difference between fiber cores in the MCF. Attached Figure Description

[0028] Figure 1 This is a diagram illustrating the optical transmission system of the present invention.

[0029] Figure 2 This is a graph illustrating the relationship between the loss difference between the receiver XT and the fiber core.

[0030] Figure 3 This is a graph illustrating the relationship between the variation of the receiving XT relative to the design value and the transmission distance in the optical transmission system of the present invention.

[0031] Figure 4 This is a graph illustrating the relationship between the variation of the received XT relative to the design value and the crosstalk of the MCF in the optical transmission system of the present invention.

[0032] Figure 5 This is a graph illustrating the relationship between the intensity ratio of excitation light for bidirectional distributed Raman amplification and the loss difference between fiber cores in the optical transmission system of the present invention.

[0033] Figure 6 This is a graph illustrating the relationship between the intensity ratio of excitation light for bidirectional distributed Raman amplification and the loss difference between fiber cores in the optical transmission system of the present invention. Detailed Implementation

[0034] 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 these embodiments. Furthermore, in this specification and the accompanying drawings, the same reference numerals denote identical elements.

[0035] Figure 1 This is a diagram illustrating the optical transmission system of this embodiment. The optical transmission system of the present invention includes:

[0036] In multi-core optical fiber 50, the transmission loss differs between at least two of the multiple cores;

[0037] A front-excitation light source 11 causes Raman-amplified excitation light (front-excitation light) in the same direction as the transmission direction of the optical signal to be incident on each core of the multi-core optical fiber 50; and,

[0038] The rear excitation light source 12 causes Raman amplified excitation light (rear excitation light) in the opposite direction to the transmission direction of the optical signal to be incident on each core of the multi-core optical fiber 50.

[0039] Its features are,

[0040] The intensity ratio of the Raman amplified excitation light output from the front excitation light source 11 to the Raman amplified excitation light output from the rear excitation light source 12 is adjusted so that the crosstalk noise on the receiving side is close to the design value.

[0041] This optical transmission system is an optical transmission system using a bidirectional distributed Raman amplification multi-core fiber (MCF). Signal light generated by a number of transmitters 15 (15-1 to 15-N) equal to the number of cores in the MCF 50 is incident on the N cores of the MCF 50 via a fan-in device 13. At this time, excitation light from the front excitation light source 11 (11-1 to 11-N) used for front-excitation Raman amplification is combined in the signal light transmission path via a multiplexer 17, and each signal light is distributed Raman amplified. Signal light from each core of the MCF 50 extracted by the fan-out device 14 is received by receivers 16 (16-1 to 16-N) equal to the number of cores. Furthermore, excitation light from the rear excitation light source 12 (12-1 to 12-N) used for rear-excitation Raman amplification is combined in the signal light transmission path via a multiplexer 18, and each signal light is distributed Raman amplified.

[0042] Figure 2This is a graph illustrating the dependence of XT loss difference in an optical transmission system comprising a two-core MCF. The transmission distance is assumed to be 100 km, and the XT per unit length of the MCF is designed to be -50 dB / km. Furthermore, the loss per unit length of one core (core 1) is set to 0.19 dB / km. The loss per unit length of the other core (core 2) is varied relative to core 1 within a range of ±0.05 dB / km (i.e., 0.185–0.195 dB / km). Figure 2 In the diagram, the vertical axis represents the received XT (dB), and the horizontal axis represents the loss difference of fiber core 2 relative to fiber core 1 (dB / km).

[0043] The dashed line represents the dependence of the receiver XT (receiver XT) on the loss difference of fiber core 2 within the aforementioned range. Due to the variation in the loss difference of fiber core 2, the received XT starts to change from -30dB of the design value. On the other hand, the solid line represents the received XT when performing distributed Raman amplification based on bidirectional excitation. The gain is set such that the intensity of the incident light when the optical signal is incident on the MCF is at the same level as the intensity of the received light when the optical signal exits the MCF (netgain = 0dB). Let R be the ratio of the intensity of the forward excitation light to the sum of the intensities of the forward excitation light and the rear excitation light; here, R = 0.5. By performing bidirectional excitation with R = 0.5, the received XT is independent of the change in the loss difference of fiber core 2 and is equal to the design value. Figure 2 As shown in the diagram. Furthermore, the value of R is the same in all fiber cores.

[0044] Furthermore, the loss of fiber core 1, which serves as the reference, is set to 0.19 dB / km, but the same effect can be achieved even with other loss values. Additionally, the MCF is for two fiber cores, but the same applies to three or more fiber cores. Moreover, although netgain is set to 0 dB, if the incident and received light intensities are the same, the same effect can be achieved even if netgain is a value other than 0 dB.

[0045] Figure 3 This is a graph illustrating the transmission distance dependence of the received XT. The horizontal axis represents the transmission distance (km) in the MCF, and the vertical axis represents the deviation of the received XT from the design value (dB). With bidirectional excitation of distributed Raman amplification at R=0.5, the inter-fiber loss difference is 0.04dB / km. Over any transmission distance from 10 to 100km, the variation of the received XT from the design value is less than 0.02dB, effectively suppressed. That is, as in this optical transmission system, by performing bidirectional excitation of distributed Raman amplification, the deviation of the received XT from the design value can be suppressed independently of the transmission distance.

[0046] Figure 4This is a graph illustrating the fiber XT dependency of the receiving XT. Fiber XT is the XT amount per unit distance of MCF. Figure 4 In the diagram, the horizontal axis represents the fiber XT (dB / km), and the vertical axis represents the deviation of the received XT from the design value (dB). With bidirectional excitation of distributed Raman amplification at R=0.5, the inter-core loss difference is 0.04dB / km. Regardless of the MCF fiber XT's value (between -80 and -30dB / km), variations in the received XT relative to the design value are effectively suppressed. That is, as in this optical transmission system, by performing bidirectional excitation of distributed Raman amplification, deviations in the received XT from the design value can be suppressed independently of the MCF fiber XT.

[0047] Figure 5 This graph illustrates the deviation of the received XT from the design value, ΔXT, the value of R, and their relationship with the inter-core loss difference Δα. The horizontal axis represents the inter-core loss difference (dB / km), and the vertical axis represents the value of R. The solid line represents the relationship between R and Δα when ΔXT = 0 dB. Furthermore, the dashed lines represent the relationship between R and Δα when ΔXT = +0.1 dB and +1.0 dB.

[0048] The inter-core loss difference Δα is in the range of -0.05 to 0.05 dB / km, and ΔXT is R = +0.1 dB. +0.1 It can be expressed by the following formula.

[0049] [Mathematical Expression 1]

[0050] R +0.1 =0.500 + 0.179Δα - 1.54 × 10 -3 / Δα (1)

[0051] On the other hand, the inter-core loss difference Δα is in the range of -0.05 to 0.05 dB / km, and ΔXT is +1.0 dB R. +1.0 It can be expressed by the following formula.

[0052] [Mathematical Expression 2]

[0053] R +1.0 =0.500 + 0.179Δα - 1.52 × 10 -2 / Δα (2) Figure 6 It is also a graph illustrating the deviation of the received XT from the design value ΔXT, the value of R, and the relationship with the loss difference Δα between fiber cores. Figure 6 This graph specifically focuses on the relationship between R and Δα when ΔXT = 0 dB. For the inter-core loss difference Δα within the range of -0.05 to 0.05 dB / km, the optimal R for ΔXT = 0 is... opt It can be expressed by the following formula.

[0054] [Mathematical Expression 3]

[0055] R opt =0.500+0.179Δα (3)

[0056] As mentioned above, the inter-core loss difference Δα of the multi-core fiber 50 is in the range of 0 < Δα ≤ 0.05 dB / km. By setting R as the following formula, ΔXT can be suppressed to 0 to +1.0 dB.

[0057] [Mathematical Expression 4]

[0058] 0.500 + 0.179Δα - 1.52 × 10 -2 / Δα≤R≤0.500+0.179Δα (4) In addition, the inter-core loss difference Δα of the multi-core fiber 50 is in the range of -0.05≤Δα<0dB / km. By setting R as the following formula, ΔXT can be suppressed to 0~+1.0dB.

[0059] [Mathematical Expression 5]

[0060] 0.500+0.179Δα≤R≤0.500+0.179Δα-1.52×10 -2 / Δα (5)

[0061] (Effect)

[0062] Even if the transmission path is an MCF with inter-fiber loss differences, this optical transmission system can suppress the received XT from deviating from the design value.

[0063] Explanation of reference numerals in the attached figures

[0064] 11, 11-1, 11-2, ..., 11-N: Front excitation light source

[0065] 12, 12-1, 12-2, ..., 12-N: Rear excitation light source

[0066] 13: Fan-in

[0067] 14: Fan-out

[0068] 15, 15-1, 15-2, ..., 15-N: Transmitter

[0069] 16, 16-1, 16-2, ..., 16-N: Receiver

[0070] 17, 17-1, 17-2,..., 17-N: Combined Wave Department

[0071] 18, 18-1, 18-2,..., 18-N: Combined Wave Department

[0072] 50: Multi-core optical fiber.

Claims

1. An optical transmission system, comprising: In multi-core optical fibers, the transmission loss differs between at least two of the multiple cores. A front-excitation light source is used to cause Raman-amplified excitation light in the same direction as the transmission direction of the optical signal to be incident on each core of the multi-core optical fiber; and, A rear excitation light source is used to cause Raman amplified excitation light in the opposite direction to the transmission direction of the optical signal to be incident on each core of the multi-core optical fiber. Its features are, The intensity ratio of the Raman amplified excitation light output from the front excitation light source to the Raman amplified excitation light output from the rear excitation light source is adjusted so that the crosstalk noise on the receiving side is close to the design value. If the intensity ratio is set as R, which is the ratio of the intensity of the Raman amplified excitation light output from the front excitation light source to the sum of the intensity of the Raman amplified excitation light output from the front excitation light source and the intensity of the Raman amplified excitation light output from the rear excitation light source, then the transmission loss difference Δα between the two fiber cores is adjusted to be within the following range: When 0 dB / km < Δα ≤ 0.05 dB / km, 0.500 + 0.179Δα - 1.52 × 10 -2 / Δα≤R≤0.500+0.179Δα; When -0.05 dB / km ≤ Δα < 0 dB / km, 0.500 + 0.179Δα ≤ R ≤ 0.500 + 0.179Δα - 1.52×10 -2 / Δα.

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