Backward crosstalk measuring device and measuring method

Abstract

The objective of the present invention is to provide a measuring device and a measuring method capable of measuring a statistical characteristic of backward crosstalk in an uncoupled multi-core optical fiber.　A backward crosstalk measuring device 301 according to the present invention is a device for measuring backward crosstalk in bidirectional communication in an uncoupled multi-core optical fiber 50, and includes: a light source 31 that inputs test light of different wavelengths into any one core at one end of the multi-core optical fiber 50; a transmitted light receiver 34 that receives transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber 50; a scattered light receiver 35 that receives backscattered light output from another core other than said core, at said one end of the multi-core optical fiber 50; and an arithmetic circuit 36 that calculates, as backward crosstalk, the ratio of the light intensity of the backscattered light to the light intensity of the transmitted light for each wavelength of the test light.

Description

Backward crosstalk measuring device and measuring method

[0001] The present disclosure relates to a measurement device and a measurement method for measuring backward crosstalk in a multi-core optical fiber or an optical cable.

[0002] Space division multiplexing transmission technology has been attracting attention as a technology that can overcome the transmission capacity limit per single-mode optical fiber. Uncoupled multi-core optical fiber (MCF) has high compatibility with conventional single-mode optical fiber-based transmission systems, and expectations for its practical application are growing.

[0003] In a transmission system using uncoupled MCF, crosstalk limits the transmission distance, so reducing crosstalk is important. In recent years, bidirectional transmission has become popular, which reduces crosstalk in the entire system by transmitting signals in opposite directions between a pair of adjacent cores in an uncoupled MCF.

[0004] In this bidirectional transmission, the backscattered light of a signal propagating through one core leaks into an adjacent core, and becomes a crosstalk component in the adjacent core, which is called backward crosstalk. Crosstalk and backward crosstalk are explained using Figures 1 and 2.

[0005] 1 is a diagram illustrating crosstalk in unidirectional transmission in a multi-core optical fiber 50 having two cores. For communication, a transmitter Tx1, a receiver Rx1, a transmitter Tx2, and a receiver Rx2 are provided. Communication is established by placing a pair of a transmitter and a receiver at both ends of one core. For example, communication light is output from the transmitter Tx1, and the communication light that has passed through the core C1 is received by the receiver Rx1. When there are two cores, each core is treated as an independent transmission path, so two sets of transmitters and receivers are installed.

[0006] When communication is performed in the same direction, a portion of the communication light propagating through one core leaks from one core to another. This leaking light is called crosstalk. The magnitude of the crosstalk in the same direction is as follows: when communication light from transmitter Tx1 propagates through core C1, a portion of it couples with core C2 and becomes leaked light LL that travels through core 2. Then, communication light from transmitter Tx2 propagates through core C2 and is received by receiver Rx2. This is called transmitted light TL. The crosstalk in the same direction is the quotient obtained by dividing the optical intensity of the leaked light LL by the optical intensity of the transmitted light TL in the following equation. [Equation 1] Crosstalk in the same direction = Optical intensity of leaked light / Optical intensity of transmitted light (1)

[0007] Fig. 2 is a diagram illustrating backward crosstalk in bidirectional transmission in a multi-core optical fiber 50 having two cores. Bidirectional transmission is the same as unidirectional transmission in Fig. 1 in that it has a transmitter Tx1, a receiver Rx1, a transmitter Tx2, and a receiver Rx2 for communication, but transmits communication light in opposite directions to each other between a pair of adjacent cores. An advantage of bidirectional transmission is that it can reduce crosstalk compared to unidirectional transmission.

[0008] In bidirectional transmission, communication light propagating through a core leaks into an adjacent core, and the backscattered component becomes a crosstalk component in the adjacent core. This backscattered component is called backward crosstalk. In Figure 2, the backscattered light RL that travels from core C1 to core C2 and is scattered in core C2 returns to the receiver Rx2 side is the backward crosstalk component.

[0009] The backward crosstalk component is a component of the crosstalk component of the transmitter Tx1 that is scattered in the opposite direction to the output direction, and is therefore very weak light. Therefore, the crosstalk of the entire system in bidirectional transmission is smaller than the crosstalk in unidirectional transmission. Specifically, the crosstalk in bidirectional communication is improved by 20 dB compared to unidirectional communication. The backward crosstalk is the quotient obtained by dividing the light intensity of the backward scattered light RL by the light intensity of the transmitted light TL in the following equation. [Equation 2] Backward crosstalk = Light intensity of backward scattered light / Light intensity of transmitted light (2)

[0010] T. Hayashi et al. , “Crosstalk Variation of Multi-Core Fiber due to Fiber Bend”, inProc. ECOC' 10, We. 8. F. 6 (2010)

[0011] Understanding backward crosstalk is important in bidirectional transmission. Because backward crosstalk varies due to disturbances such as bending and twisting, it is necessary to measure statistical properties such as mean and variance to evaluate backward crosstalk. Non-Patent Document 1 discloses a method for measuring the statistical properties of crosstalk in uncoupled MCFs. However, this measurement method can measure the statistical properties of crosstalk for signals traveling in the forward direction (forward crosstalk in Figure 1), but cannot measure the statistical properties of backward crosstalk.

[0012] In other words, although it is important to grasp the backward crosstalk for bidirectional transmission, there is a problem that it is difficult to measure the statistical characteristics of the backward crosstalk. Therefore, in order to solve the above problem, an object of the present invention is to provide a measurement device and a measurement method capable of measuring the statistical characteristics of the backward crosstalk of an uncoupled multi-core optical fiber.

[0013] In order to achieve the above object, the measuring device according to the present invention inputs test light into one core of an uncoupled multi-core optical fiber while sweeping the wavelength, measures the transmitted light through the core and the backscattered light from another core, and calculates the backward crosstalk corresponding to each wavelength using the ratio between the transmitted light and the backscattered light.

[0014] Specifically, the measuring device according to the present invention includes: a light source that inputs test light of different wavelengths into any one core at one end of a multi-core optical fiber; a transmitted light receiver that receives transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber; a scattered light receiver that receives backscattered light output from cores other than the core at the one end of the multi-core optical fiber; and an arithmetic circuit that calculates, as backward crosstalk, a ratio of the optical intensity of the backscattered light to the optical intensity of the transmitted light for each wavelength of the test light.

[0015] Furthermore, a measurement method according to the present invention includes: injecting test light of different wavelengths into any one core at one end of a multi-core optical fiber; receiving transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber; receiving backscattered light output from a core other than the core at one end of the multi-core optical fiber; and calculating, as backward crosstalk, a ratio of the light intensity of the backscattered light to the light intensity of the transmitted light for each wavelength of the test light.

[0016] Here, the calculation circuit creates a histogram that counts the number of test lights with different wavelengths relative to the backward crosstalk value, and calculates the average and variance of the backward crosstalk, thereby obtaining the statistical characteristics of the backward crosstalk.

[0017] In order to shorten the measurement time, it is preferable that the scattered light receiver simultaneously receives the backscattered light output from all of the other cores.

[0018] The above inventions can be combined as much as possible.

[0019] The present invention provides a measurement device and a measurement method capable of measuring the statistical characteristics of backward crosstalk in an uncoupled multi-core optical fiber. The present invention can measure backward crosstalk in bidirectional transmission. Since this backward crosstalk corresponds to noise, evaluating the magnitude of the backward crosstalk makes it possible to check the quality of the optical fiber during installation and to inspect the optical fiber after installation, thereby providing a criterion for maintaining communication quality.

[0020] FIG. 1 is a diagram explaining crosstalk in unidirectional transmission in a multi-core optical fiber. FIG. 2 is a diagram explaining backward crosstalk in bidirectional transmission in a multi-core optical fiber. FIG. 3 is a diagram explaining the configuration of a measurement device according to the present invention. FIG. 4 is a diagram explaining a cross section of a two-core optical fiber. FIG. 5 is a diagram explaining the measurement principle of the measurement device according to the present invention. FIG. 6 is a diagram explaining a measurement method according to the present invention. FIG. 7 is a diagram explaining backward crosstalk measured by the measurement device according to the present invention. FIG. 8 is a diagram explaining backward crosstalk measured by the measurement device according to the present invention. FIG. 9 is a diagram explaining the structure of an optical cable. FIG. 10 is a diagram explaining the configuration of a measurement device according to the present invention.

[0021] 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. Note that components with the same reference numerals in this specification and drawings indicate the same components. Furthermore, although the present embodiment describes a case where the number of cores in a multi-core optical fiber is two, backward crosstalk can be measured in the same way even if the number of cores is three or more.

[0022] 3 is a diagram illustrating a backward crosstalk measurement device 301 according to this embodiment. The backward crosstalk measurement device 301 is a device for measuring backward crosstalk in bidirectional communication of an uncoupled multi-core optical fiber 50, and includes: a light source 31 that inputs test light of different wavelengths into any one core at one end of the multi-core optical fiber 50; a transmitted light receiver 34 that receives transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber 50; a scattered light receiver 35 that receives backscattered light output from cores other than the core at the one end of the multi-core optical fiber 50; and an arithmetic circuit 36 ​​that calculates, for each wavelength of the test light, a ratio of the optical intensity of the backscattered light to the optical intensity of the transmitted light as backward crosstalk.

[0023] The backward crosstalk measurement device 301 in Fig. 3 measures the two-core optical fiber in Fig. 4 as the multi-core optical fiber 50. The backward crosstalk measurement device 301 in Fig. 3 uses a wavelength swept light source as the light source 31, which outputs light by continuously changing the wavelength. Reference numerals 32 and 33 denote optical input / output devices that can input and output light to desired cores of the multi-core optical fiber 50. For example, the optical input / output devices (32, 33) are fan-in / fan-out devices. In this embodiment, since the multi-core optical fiber 50 to be measured is a two-core optical fiber, the optical input / output device 32 inputs test light from the light source 31 into a core C1 of the two-core optical fiber at one end of the two-core optical fiber and extracts backscattered light output from a core C2 of the two-core optical fiber. The optical input / output device 33 extracts transmitted light that has passed through the core C1 of the two-core optical fiber at the other end of the two-core optical fiber.

[0024] The scattered light receiver 35 receives the backscattered light from the core C2 extracted by the light input / output device 32. The transmitted light receiver 34 receives the transmitted light from the core C1 extracted by the light input / output device 33. The calculation unit 36 ​​calculates backward crosstalk from the acquired light intensities of the transmitted light and the backscattered light. The display unit 37 displays the result (backward crosstalk) calculated by the calculation unit 36.

[0025] FIG. 5 is a schematic diagram illustrating backward crosstalk measurement performed by the backward crosstalk measurement device 301. In contrast to the backward crosstalk description in FIG. 2 , in the backward crosstalk measurement device 301, the transmitter TX1 is the light source (wavelength swept light source) 31, the receiver RX1 is the photoreceiver 34, and the receiver RX2 is the photoreceiver 35. Comparing FIG. 2 with FIG. 5 , backward crosstalk, which is recognized as noise by the receiver RX2 in bidirectional transmission, is measured by the photoreceiver 35 in the backward crosstalk measurement device 301. Therefore, the backward crosstalk measurement device 301 can evaluate backward crosstalk using equation (2) using the optical intensity of the transmitted light TL measured by the photoreceiver 34 and the leakage light RL measured by the photoreceiver 35. Taking advantage of the fact that the optical intensity of the transmitted light is the same in both cores C1 and C2, the backward crosstalk measurement device 301 uses the light that passes directly through core C1 as the propagating light TL instead of the propagating light TL of core C2 in FIG. 2 .

[0026] 6 is a flowchart illustrating a backward crosstalk measurement method performed by the backward crosstalk measurement device 301. This method includes the steps of selecting two cores to be measured (a core to which test light is incident and a core to which leaked light is received) from among the cores of the multi-core optical fiber 50 (step S01), incident test light of different wavelengths (wavelength-swept test light in this embodiment) into any one core at one end of the multi-core optical fiber 50 (step S02), receiving transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber, and receiving backscattered light output from cores other than the core at one end of the multi-core optical fiber (step S03), and calculating, for each wavelength of the test light, a ratio of the optical intensity of the backscattered light to the optical intensity of the transmitted light as the backward crosstalk (step S05).

[0027] If the multi-core optical fiber 50 has three or more cores, the core that receives the leaked light is changed (the core that receives the test light is fixed) and the process is repeated from step S01 (step S04). That is, if the multi-core optical fiber 50 has n cores, one core is used for receiving the test light, and the leaked light from the remaining n-1 cores is measured. In Fig. 6, step S04 is between step S03 and step S05, but step S04 may be performed after step S05.

[0028] In addition, when the multi-core optical fiber 50 has three or more cores, it is also possible to connect photodetectors 35 to all cores other than the core onto which the test light is incident, and simultaneously receive the backscattered light output from all of the cores, as shown in Fig. 11. This eliminates the need to repeat step S04, thereby reducing the measurement time. Fig. 11 shows a backward crosstalk measurement device 302, which differs from the backward crosstalk measurement device 301 in that it includes photodetectors (35-1 to 35-n-1) that receive the backscattered light output from all cores other than the core onto which the test light is incident.

[0029] The calculation method of backward crosstalk and statistical characteristics performed by the calculation circuit 36 ​​in step S05 will now be described. While the explanation is given for a two-core optical fiber, the same applies to a three-core or more optical fiber. The calculation circuit 36 ​​creates a histogram that counts the number of test lights with different wavelengths relative to the backward crosstalk value, and calculates the average and variance of the backward crosstalk.

[0030] The wavelength dependency of backward crosstalk is calculated from the light intensity of the transmitted light TL and the light intensity of the leaked light (backscattered light) RL obtained in step S03, as shown in equation (3). Here, P １ (λ) is the light intensity of the transmitted light TL for each wavelength (wavelength dependence of the transmitted light), P ｂｓ２ (λ) is the light intensity of the leaked light (backscattered light) RL for each wavelength (wavelength dependency of the backscattered light).

[0031] The values ​​calculated by Equation (3) correspond to the backward crosstalk corresponding to each wavelength, and statistical characteristics such as a histogram of the backward crosstalk can be obtained using these values. Fig. 7 is a histogram illustrating the backward crosstalk before and after bending the multi-core optical fiber 50. The horizontal axis represents the magnitude of the backward crosstalk obtained by Equation (3) for each wavelength, and the vertical axis represents the frequency of the backward crosstalk for each magnitude. The dark gray bins represent data before bending the multi-core optical fiber 50, and the light gray bins represent data after bending the optical fiber 50.

[0032] Fig. 7 shows that bending the multi-core optical fiber 50 causes changes in the multi-core optical fiber 50, resulting in an increase in backward crosstalk. In this way, by acquiring a histogram of backward crosstalk, it is possible to measure and evaluate changes in backward crosstalk. Fig. 7 shows an example in which a bend is applied to the multi-core optical fiber 50, but the backward crosstalk also changes when a twist is applied.

[0033] Figure 8 shows the histogram of backward crosstalk after bending in Figure 7, with the fitting curve superimposed. The statistical distribution of crosstalk in an uncoupled multi-core optical fiber follows a chi-square distribution. Since the backward crosstalk is also considered to follow this statistical property, the mean and variance can be determined by fitting with the chi-square distribution shown in Equation (4). The parameters are as follows: XT: backward crosstalk, κ: mode coupling coefficient, β: propagation constant, R: bending radius, Λ: core spacing, L: fiber length.

[0034] Further statistical processing will be explained with reference to Fig. 9 . Fig. 8 has explained the calculation of the average and variance when measurements are made at a certain bending magnitude. Here, the magnitude of the backward crosstalk changes by changing the bending magnitude applied to the multi-core optical fiber 50. Fig. 9 is a graph plotting the average backward crosstalk when the bending magnitude applied to the multi-core optical fiber 50 is changed. There is a correlation between the bending magnitude applied to the multi-core optical fiber 50 and the average backward crosstalk, and the measurement data can be fitted with equation (5). In other words, the average backward crosstalk can be estimated even for bending magnitudes for which measurements have not been made. The parameters are the same as those in equation (4).

[0035] In this way, the mean and variance of the backward crosstalk and their bending dependencies can be obtained. Note that the calculation of the mean and variance of the backward crosstalk is possible even before the multi-core optical fiber is bent. The backward crosstalk measurement device 301 outputs the mean and variance of the backward crosstalk and their bending dependencies to the display unit 37. By checking the output values, the measurer can statistically grasp the backward crosstalk in the multi-core optical fiber 50.

[0036] (Embodiment 2) In the first embodiment, an evaluation method for a single multi-core optical fiber has been described. Usually, when an optical fiber is installed outdoors, it is laid as an optical cable in which a plurality of optical fibers are bundled. Therefore, it is also important to measure the backward crosstalk of a cabled multi-core optical fiber.

[0037] Fig. 10 is a diagram illustrating two main types of optical cable structures. Fig. 10(A) is a slotted type. In this structure, a concave groove is formed in a slot, and a tape fiber is housed in the groove. A tape fiber is a bundle of four or eight optical fibers in a tape shape. Fig. 10(B) is a non-slotted type, which differs from the previous slotted type in that it does not have a slot. In this structure, the tape fibers are bundled and covered with an outer jacket. In both structures, in this embodiment, the multiple optical fibers that make up the tape fiber are uncoupled multi-core optical fibers 50.

[0038] Optical cables are usually laid in a straight line. However, if an unintentional bend is applied to the cable during installation, stress is generated and applied to the fibers in the optical cable. As a result, the crosstalk characteristics of the multi-core optical fibers 50 in the ribbon fiber in the optical cable change. By performing the measurement described in the first embodiment on an installed optical cable, an increase in unintentional backward crosstalk can be detected.

[0039] Since optical cables are long, when measurements are performed using the configuration of the embodiment described in Fig. 3, the receiver 34 must be installed at a distance. In this case, for example, data from the receiver 34 can be transferred wirelessly or via a network, allowing calculation processing to be performed on the input side of the test light. In other words, the backward crosstalk measurement device 301 in Fig. 3 can measure everything from optical fibers in a laboratory environment to optical cables that are actually installed, and can be used in any environment.

[0040] (Effect) From the perspective of providing communication services, everything other than communication light is noise, and in order to provide stable communication services, it is required that the intensity of the signal light is always exchanged at a level equal to or greater than the intensity of the noise. In bidirectional transmission, backward crosstalk is noise, and it is desirable that its value be small.

[0041] For example, when providing bidirectional transmission as shown in Fig. 2, a crosstalk value (margin) that is tolerable for communication is designed into the communication system. At this time, by measuring the magnitude of the backward crosstalk using the backward crosstalk measurement devices (301, 302), it is possible to evaluate whether the optical fiber (or optical cable) has been laid within the tolerance range designed for the communication system (whether the measured value is within the margin). In other words, by using the backward crosstalk measurement devices (301, 302), it is possible to determine whether a stable communication service can be provided.

[0042] Furthermore, from the viewpoint of maintenance and operation of a communication system, the backward crosstalk measuring devices (301, 302) can also be used to inspect the backward crosstalk of laid optical fibers. As described above, measuring the backward crosstalk using the backward crosstalk measuring devices (301, 302) provides an important criterion for ensuring communication quality.

[0043] 31: Light source 32, 33: Optical input / output device 34: Photoreceiver 35, 35-1, . . . , 35-n-1: Photoreceivers 36: Arithmetic circuit 37: Display unit 50: Multi-core optical fiber 52: Cladding 301, 302: Backward crosstalk measuring device

Claims

1. A backward crosstalk measuring device comprising: a light source that inputs test light of different wavelengths into any one core at one end of a multi-core optical fiber; a transmitted light receiver that receives transmitted light, which is the test light that has passed through the core at the other end of the multi-core optical fiber; a scattered light receiver that receives backscattered light output from cores other than the core at one end of the multi-core optical fiber; and an arithmetic circuit that calculates, as backward crosstalk, the ratio of the optical intensity of the backscattered light to the optical intensity of the transmitted light for each wavelength of the test light.

2. The backward crosstalk measuring device according to claim 1, characterized in that the arithmetic circuit creates a histogram counting the number of test lights with different wavelengths relative to the backward crosstalk value, and calculates the average and variance of the backward crosstalk.

3. The backward crosstalk measuring device according to claim 1, wherein the scattered light receiver simultaneously receives the respective backward scattered lights output from all of the other cores.

4. A backward crosstalk measurement method comprising: injecting test light of different wavelengths into any one core at one end of a multi-core optical fiber; receiving transmitted light, which is the test light that has passed through the core, at the other end of the multi-core optical fiber; receiving backscattered light output from cores other than the core at one end of the multi-core optical fiber; and calculating, for each wavelength of the test light, the ratio of the optical intensity of the backscattered light to the optical intensity of the transmitted light as backward crosstalk.

Images