Optical characteristic measurement system and optical characteristic measurement method

Abstract

This optical characteristic measurement system is provided with: a light source capable of causing pulsed light to enter a first core of a multi-core optical fiber; an optical receiver capable of receiving second backscattered light from a second core of the multi-core optical fiber; and a calculation device that calculates a backscatter crosstalk index on the basis of at least the light intensity of the second backscatter light. The calculation device includes: a data processing unit that acquires the light intensity of the second backscattered light at each time received by the light receiver in correspondence with each position from one end to the measurement position; and an arithmetic processing unit that calculates, as a backscatter crosstalk index, a term including a ratio between the integrated value and a loss value indicating the transmission loss of the multi-core fiber when the light moves from one end of the first core to the measurement position. The integrated value is obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second core from the one end to the measurement position.

Description

Technical Field

[0001] The present disclosure relates to an optical characteristic measuring system and an optical characteristic measuring method.

[0002] This application claims the benefit of priority based on Japanese Application No. 2022-141933 filed on September 7, 2022, and all the contents described in the Japanese Application are incorporated herein by reference. Background Art

[0003] It is known that crosstalk occurs between multiple cores in a multi-core optical fiber having multiple cores. For example, Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2 disclose methods for measuring such crosstalk. In these methods, one or more OTDR (Optical Time Domain Reflectometer) devices are connected to a single end of a multi-core optical fiber, and a pulse test light is incident from the single end into one core of the multi-core optical fiber. Then, the backscattered light that returns to the single end due to backscattering in the multiple cores is input into the OTDR device, and the crosstalk is measured based on the intensity of the backscattered light.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-202827

[0007] Non-patent literature

[0008] Non-patent document 1: M. Nakazawa, at al., “Nondestructive measurement of mode couplings along a multi-core fiber using a synchronous multi-channel OTDR”, Opt. Express, Vol. 20, No. 11, pp. 12530-12540 (2012).

[0009] Non-patent document 2: A. Nakamura, at al., "Optical Time Domain Reflectometry for Simultaneously Characterizing Forward and Backward Crosstalk along Multi-coreFibers", IEEE, Journal of Lightwave Technology, pp. 1-7 (2022).

[0010] Non-patent document 3: Recommendation ITU-TG.650.1, "Definitions and test methods for linear, deterministic attributes of single-mode fiber and cable", (2020). Summary of the invention

[0011] An optical property measurement system according to one embodiment of the present disclosure is an optical property measurement system for measuring the optical property of a multi-core optical fiber having at least a first core and a second core. The optical property measurement system comprises: at least one light source optically coupled to the first core, capable of causing pulsed light to be incident on the first core from one end of the multi-core optical fiber; at least one optical receiver optically coupled to at least the second core, capable of receiving at least the second backscattered light of the first backscattered light and the second backscattered light emitted from the first core and the second core at one end respectively as the pulsed light is incident on the first core; and a computing device connected to the optical receiver in a communicable manner, calculating a backscatter crosstalk index based on at least the light intensity of the second backscattered light, the backscatter crosstalk index representing the backscatter crosstalk related to the light intensity of the second backscattered light that moves from the first core to the second core and returns to one end as a part of the pulsed light. The computing device includes: a data processing unit, which obtains the light intensity of the second backscattered light received by the optical receiver at each time corresponding to each position from one end to an arbitrary measurement position along the length direction of the multi-core optical fiber; and a computing processing unit, which calculates a term including a ratio of an integral value to a loss value as a backscatter crosstalk index, the loss value represents the transmission loss of the multi-core optical fiber when the light moves from one end to the measurement position in the first fiber core, and the integral value is obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second fiber core from one end to the measurement position. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a structural diagram showing an example of an optical property measurement system according to an embodiment.

[0013] Figure 2 It is used to illustrate Figure 1 A diagram showing the principle of crosstalk in a multi-core optical fiber.

[0014] Figure 3 It is shown Figure 1 A block diagram of an example of the functional structure of the computing device shown.

[0015] Figure 4 It is a graph showing the relationship between the position in the length direction of the multi-core optical fiber and the power coupling coefficient.

[0016] Figure 5 Graph showing the relationship between the position in the length direction of a multi-core optical fiber and the backscattering crosstalk.

[0017] Figure 6 is another graph showing the relationship between the position in the length direction of the multi-core optical fiber and the power coupling coefficient.

[0018] Figure 7 is another graph showing the relationship between the position in the length direction of the multi-core optical fiber and the backscattering crosstalk.

[0019] Figure 8 It is shown that the use Figure 1 Flowchart of an example of an optical property measurement method performed by the optical property measurement system shown.

[0020] Fig. 9 : is a flowchart showing another example of the optical property measurement method.

[0021] Fig.10 : is a flowchart showing another example of the optical property measurement method.

[0022] Fig.11 : is a flowchart showing another example of the optical property measurement method.

[0023] Fig.12 1 is a diagram showing a configuration of an optical property measurement system according to a modification example. DETAILED DESCRIPTION

[0024] [Technical problem to be solved by the invention]

[0025] As the crosstalk that may be generated between the cores of a multi-core optical fiber, there can be considered the optical signal that moves from the coupling source core to the coupling destination core and propagates in the same direction as the propagation direction of the optical signal in the coupling source core (hereinafter referred to as "parallel crosstalk"), and the optical signal that moves from the coupling source core to the coupling destination core and propagates in the opposite direction to the propagation direction of the optical signal in the coupling source core (hereinafter referred to as "opposite crosstalk"). Even in the opposite crosstalk, the crosstalk caused by backscattering (hereinafter referred to as "backscatter crosstalk") is often the main cause of the opposite crosstalk. When multi-core optical fibers are put into practical use, between adjacent cores, a signal is conducted in the direction of backscattering of the signal in another core relative to the signal conducted in one core. For this reason, measuring and evaluating the backscatter crosstalk is extremely important in the practical use of multi-core optical fibers.

[0026] However, Patent Document 1 and Non-Patent Document 1 do not disclose a method for deriving backscatter crosstalk. On the other hand, Non-Patent Document 2 discloses a method for deriving backscatter crosstalk. However, in this method, an assumption is made to simplify the derivation of backscatter crosstalk, and under this assumption, backscatter crosstalk may not be accurately evaluated.

[0027] The present disclosure provides an optical characteristic measurement system and an optical characteristic measurement method capable of more accurately evaluating backscattering crosstalk between cores of a multi-core optical fiber.

[0028] [Effects of the present disclosure]

[0029] According to the optical characteristic measurement system and the optical characteristic measurement method according to the present disclosure, it is possible to more accurately evaluate the backscattering crosstalk between the cores of a multi-core optical fiber.

[0030] [Description of Embodiments of the Present Disclosure]

[0031] First, the contents of the embodiments of the present disclosure are listed and described.

[0032] (1) An optical property measurement system according to one embodiment of the present disclosure is an optical property measurement system for measuring optical properties of a multi-core optical fiber having at least a first core and a second core. The optical property measurement system comprises: at least one light source optically coupled to the first core and capable of causing pulsed light to be incident on the first core from one end of the multi-core optical fiber; at least one optical receiver optically coupled to at least the second core and capable of receiving at least the second backscattered light of the first backscattered light and the second backscattered light emitted from the first core and the second core at one end respectively as the pulsed light is incident on the first core; and a computing device connected to the optical receiver in a communicable manner and calculating a backscatter crosstalk index based on at least the light intensity of the second backscattered light, the backscatter crosstalk index representing backscatter crosstalk related to the light intensity of the second backscattered light that moves from the first core to the second core as a part of the pulsed light and returns to the one end. The computing device includes: a data processing unit, which obtains the light intensity of the second backscattered light received by the optical receiver at each time corresponding to each position from one end to an arbitrary measurement position along the length direction of the multi-core optical fiber; and a computing processing unit, which calculates a term including a ratio of an integral value to a loss value as a backscatter crosstalk index, the loss value represents the transmission loss of the multi-core optical fiber when the light moves from one end to the measurement position in the first fiber core, and the integral value is obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second fiber core from one end to the measurement position.

[0033] In the method described in non-patent document 2, the backscatter crosstalk is derived under the assumption that the power coupling coefficient between the cores used to derive the backscatter crosstalk is constant and does not depend on the position of the multi-core optical fiber (that is, the power coupling coefficient has no position dependence). For this reason, when the power coupling coefficient has position dependence, there is a problem that the backscatter crosstalk cannot be accurately evaluated. Therefore, the inventors of the present disclosure have repeatedly studied the method for accurately evaluating the backscatter crosstalk by considering the position dependence of the power coupling coefficient. The backscatter crosstalk can be defined by the ratio of the intensity of the light caused by backscattering in the light that moves from the first core to the second core and returns to one end of the multi-core optical fiber to the intensity of the light that is incident from the one end to the first core and reaches the measurement position. In the process of repeated research, the inventors of the present disclosure found that the intensity of the light at the measurement position of the first core can be estimated by the loss value representing the transmission loss of the multi-core optical fiber when the light moves from one end to the measurement position in the first core. Furthermore, the inventors of the present disclosure have come up with the idea that the intensity of the light returning to the one end in the second core can be represented by the integral value of the light intensity of the second backscattered light corresponding to each position from the one end to an arbitrary measurement position when the pulse light is incident on the first core. Then, the inventors of the present disclosure have found that the term including the ratio of the integral value to the loss value can be regarded as a backscattered crosstalk index representing the backscattered crosstalk. If the integral value of the light intensity of the second backscattered light is used in this way, the backscattered crosstalk index can be calculated using the light intensity of all the second backscattered lights from the one end to the measurement position. Therefore, according to the above-mentioned optical characteristic measurement system, even in the case where the power coupling coefficient has position dependence, the backscattered crosstalk can be accurately evaluated.

[0034] (2) In the optical characteristic measurement system described in (1) above, the optical receiver may be optically coupled to the first fiber core, and may be capable of receiving the first backscattered light emitted from the first fiber core at one end. The data processing unit may also obtain the light intensity of the first backscattered light received by the optical receiver at each time corresponding to each position from the one end to the measurement position. The calculation processing unit may also use the light intensity of the first backscattered light corresponding to each position of the first fiber core from the one end to the measurement position to calculate the loss value representing the transmission loss of the multi-core optical fiber when the pulse light moves from one end to the measurement position in the first fiber core. In this case, the loss value representing the transmission loss at any position of the multi-core optical fiber can be more accurately calculated using the light intensity of the first backscattered light emitted from the first fiber core. As a result, the backscattered crosstalk index calculated as a term including the ratio of the integral value to the loss value can be used to more accurately evaluate the backscattered crosstalk at any position of the multi-core optical fiber.

[0035] (3) In the optical characteristic measuring system described in (1) or (2) above, when the loss value is set to D 11 , when the integral value is set to D2 [W·m], the calculation processing unit can also use the loss value represented by the following formula (1) and the integral value represented by the following formula (2) to calculate the ratio represented by the following formula (3). The inventors of the present disclosure have repeatedly studied the mathematical formula for calculating the backscatter crosstalk index and found that the ratio D2 / D represented by formula (3) can be 11 The term D2 / D is used as a backscatter crosstalk indicator that is proportional to the backscatter crosstalk. 11 If the term is used, the backscatter crosstalk can be accurately evaluated even when the power coupling coefficient between the first fiber core and the second fiber core has position dependence.

[0036] D 11 =exp(-αz L )…(1)

[0037]

[0038] Here, the position of the multi-core optical fiber along the length direction when one end is taken as the origin is set as z[m], and the light intensity of the second backscattered light corresponding to the position z[m] is set as P bs2 (z)[W], set the measurement position to z L [m], and the transmission loss coefficient of the multi-core optical fiber is set to α[ / m].

[0039] (4) In the optical characteristic measuring system described in (3) above, the processing unit may calculate the backscatter crosstalk index by multiplying the ratio represented by the formula (3) by the coefficient A1 [ / (W·m)] represented by the following formula (4) or the coefficient B1 [ / (W·m)] represented by the following formula (5). The inventors of the present disclosure have found in the process of repeatedly studying the mathematical formula for calculating the backscatter crosstalk index that the ratio D2 / D represented by the formula (3) can be multiplied by the coefficient A1 [ / (W·m)] represented by the following formula (4). 11 By multiplying the coefficient A[ / (W·m)]1 represented by equation (4) or the coefficient B1[ / (W·m)] represented by equation (5), the backscatter crosstalk can be directly calculated. 11 The value obtained by multiplying the coefficient A1 [ / (W·m)] or the coefficient B1 [ / (W·m)] can directly and accurately evaluate the backscatter crosstalk.

[0040]

[0041] The light intensity of the first backscattered light corresponding to one end is set to P bs1(0) [W], the light intensity of the pulse light is P0 [W], and the group velocity of the pulse light when moving in the multi-core optical fiber is V g [m / s], the pulse width of the pulse light is W[s], the scattering cross-sectional area of ​​the multi-core optical fiber is S, and the transmission loss coefficient of the multi-core optical fiber due to Rayleigh scattering is α R [ / m].

[0042] (5) In the optical property measurement system described in (2) above, when the transmission loss is set to D 12 [W 1 / 2 ], when the integral value is set to D2[W·m], the calculation processing unit may also use the loss value represented by the following formula (6) and the integral value represented by the following formula (7) to calculate the ratio represented by the following formula (8). The inventors of the present disclosure have found in the process of repeatedly studying the mathematical formula for calculating the backscatter crosstalk index that the ratio D2 / D represented by formula (8) can be 12 The term D2 / D is used as a backscatter crosstalk indicator that is proportional to the backscatter crosstalk. 12 If the term is used, the backscatter crosstalk can be accurately evaluated even when the power coupling coefficient between the first fiber core and the second fiber core has position dependence.

[0043]

[0044] Here, the position of the multi-core optical fiber along the length direction with one end as the origin is set as z[m], and the light intensity of the first backscattered light corresponding to the position z[m] is set as P bs1 (z)[W], the light intensity of the second backscattered light corresponding to the position z[m] is set to P bs2 (z)[W], set the measurement position to z L [m].

[0045] (6) In the optical characteristic measuring system described in (5), the arithmetic processing unit may multiply the ratio represented by the equation (8) by a coefficient A2[ / (W 1 / 2 ·m)] or the coefficient B2[ / (W 1 / 2 ·m)] to calculate the backscatter crosstalk index. The inventors of the present disclosure have repeatedly studied the mathematical formula for calculating the backscatter crosstalk index and found that by using the ratio D2 / D represented by formula (8) 12 Multiply by the coefficient A2[ / (W 1 / 2 ·m)] or the coefficient B2[ / (W 1 / 2 m)], the backscatter crosstalk can be directly calculated. Therefore, if the ratio D2 / D12 Multiply by the coefficient A2[ / (W 1 / 2 ·m)] or coefficient B2[ / (W 1 / 2 ·m)], the value obtained can directly and accurately evaluate the backscatter crosstalk.

[0046]

[0047] The light intensity of the first backscattered light corresponding to one end is set to P bs1 (0) [W], the light intensity of the pulse light is P0 [W], and the group velocity of the pulse light when moving in the multi-core optical fiber is V g [m / s], the pulse width of the pulse light is W[s], the scattering cross-sectional area of ​​the multi-core optical fiber is S, and the transmission loss coefficient of the multi-core optical fiber due to Rayleigh scattering is α R [ / m].

[0048] (7) The optical property measurement system described in any one of (2) to (6) above may further include an optical system that guides the pulse light from the light source to the first core and guides the first backscattered light from the first core and the second backscattered light from the second core to the optical receiver. In this case, it is easy to realize a structure in which the pulse light is incident on the first core and the first backscattered light and the second backscattered light emitted from the first core and the second core, respectively, are received by the optical receiver.

[0049] (8) The optical characteristic measurement system described in any one of (1) to (7) above may also include a display unit, which displays a double logarithmic graph indicating the relationship between the position of the multi-core optical fiber along the length direction and the backscatter crosstalk index. If a double logarithmic graph is used in this way, it is easy to understand the change in the backscatter crosstalk index relative to the position of the multi-core optical fiber. Thus, it is easy to evaluate the backscatter crosstalk.

[0050] (9) An optical characteristic measuring method according to one embodiment of the present disclosure is an optical characteristic measuring method using an optical characteristic measuring system, wherein the optical characteristic measuring system measures optical characteristics of a multi-core optical fiber having at least a first core and a second core. The optical characteristic measuring method comprises: a step of causing pulsed light to be incident on the first core from one end of the multi-core optical fiber; a step of receiving, by an optical receiver, at least the second backscattered light of the first backscattered light and the second backscattered light emitted from the first core and the second core at one end respectively as the pulsed light is incident on the first core; and a step of calculating a backscattered crosstalk index based on at least the intensity of the second backscattered light, wherein the backscattered crosstalk index represents backscattered crosstalk related to the light intensity of the second backscattered light that moves from the first core to the second core as a part of the pulsed light and returns to one end. The step of calculating the backscatter crosstalk index includes: the step of acquiring the light intensity of the second backscattered light received by the optical receiver at each moment corresponding to each position from one end to an arbitrary measurement position along the length direction of the multi-core optical fiber; and the step of calculating a term including the ratio of an integral value to a loss value as the backscatter crosstalk index, the loss value representing the transmission loss of the multi-core optical fiber when the light moves from one end to the measurement position in the first fiber core, and the integral value being obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second fiber core from one end to the measurement position.

[0051] In the method described in non-patent document 2, the backscatter crosstalk is derived under the assumption that the power coupling coefficient between the cores used to derive the backscatter crosstalk is constant and does not depend on the position of the multi-core optical fiber (that is, the power coupling coefficient has no position dependence). For this reason, when the power coupling coefficient has position dependence, there is a problem that the backscatter crosstalk cannot be accurately evaluated. Therefore, the inventors of the present disclosure have repeatedly studied the method for accurately evaluating the backscatter crosstalk by considering the position dependence of the power coupling coefficient. The backscatter crosstalk can be defined by the ratio of the intensity of the light caused by backscattering in the light that moves from the first core to the second core and returns to one end of the multi-core optical fiber to the intensity of the light that is incident from the one end to the first core and reaches the measurement position. In the process of repeated research, the inventors of the present disclosure found that the intensity of the light at the measurement position of the first core can be estimated by the loss value representing the transmission loss of the multi-core optical fiber when the light moves from one end to the measurement position in the first core. Furthermore, the inventors of the present disclosure have come up with the idea that the intensity of the light returning to the one end in the second core can be represented by the integral value of the light intensity of the second backscattered light corresponding to each position from the one end to an arbitrary measurement position when the pulse light is incident on the first core. Then, the inventors of the present disclosure have found that the term including the ratio of the integral value to the loss value can be regarded as a backscattered crosstalk index representing the backscattered crosstalk. If the integral value of the light intensity of the second backscattered light is used in this way, the backscattered crosstalk index can be calculated using the light intensity of all the second backscattered lights from the one end to the measurement position. Therefore, according to the above-mentioned optical characteristic measurement method, even in the case where the power coupling coefficient has position dependence, the backscattered crosstalk can be accurately evaluated.

[0052] [Details of the embodiments of the present disclosure]

[0053] Hereinafter, with reference to the accompanying drawings, specific examples of the measurement system and the measurement method involved in the embodiments of the present disclosure are described. The present disclosure is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the following description, the same reference numerals are marked on the same elements when describing the drawings, and repeated descriptions are appropriately omitted.

[0054] [Optical properties measurement system]

[0055] Figure 1: is a structural diagram showing an optical characteristic measurement system 1 involved in the present embodiment. The optical characteristic measurement system 1 is a system for measuring the optical characteristics of a multi-core optical fiber (MCF: Multi-core optical fiber, hereinafter referred to as "MCF") 10 as a measurement object. MCF10, for example, has a first core 11 and a second core 12. The first core 11 and the second core 12 extend from one end to the other end in the length direction of MCF10. The number of cores of MCF10 is not limited to two, and can be three or more. MCF10, for example, can be an uncoupled MCF capable of transmitting light using each core as an independent channel.

[0056] The optical characteristic measurement system 1 includes, for example, a measurement device 30 and a computing device 50. The measurement device 30 is, for example, an OTDR device that can measure the optical characteristics of the MCF 10 using the OTDR method. The OTDR method is widely used as a technique for evaluating the optical characteristics of the MCF 10 because it can easily measure the optical characteristics of the MCF 10 from a single end of the MCF 10. Specifically, the measurement device 30 causes the pulse light L p The pulse light L is incident on one end of the MCF 10 to be measured, and the distance distribution (light intensity distribution) of the light intensity of the backscattered light returning to the one end of the MCF 10 is measured. p One end of the MCF 10 to which light is incident is referred to as an “incident end 10 a ”, and the other end of the MCF 10 on the opposite side is referred to as an “exit end 10 b ”.

[0057] The operation device 50 uses the measurement result of the measurement device 30 to calculate the backscattering XT index representing the backscattering crosstalk (hereinafter referred to as "XT") between the first core 11 and the second core 12. The backscattering XT index is an example of the optical characteristics of the MCF 10. The "backscattering XT index" is an index value representing the change of the "backscattering XT". The index value representing the "backscattering XT" means a value that changes according to the change of the "backscattering XT", that is, a value that changes following the backscattering XT. The index value representing the "backscattering XT" can be, for example, a value proportional to the "backscattering XT" or a value representing the "backscattering XT" itself. In the present embodiment, the "backscattering XT index" is described as a value representing the "backscattering XT" itself.

[0058] Here, refer to Figure 2 , the principle of XT is explained. Figure 2 XT is a diagram for explaining the principle of XT. cw The case where the incident light is incident on the incident end 10a of the MCF 10. Figure 2 As shown, the continuous light L incident on the first fiber core 11 from the incident end 10a cw While backscattering and XT occur in the first core 11, it moves from the incident end 10a to the output end 10b. Figure 2 In the figure, backscattering is represented by a dashed arrow, and XT is represented by a double-dashed arrow. cw When backscattering occurs, in the first core 11, the continuous light L cw Part of the light A The light moves backward due to Rayleigh scattering, moves in the opposite direction in the first core 11, and returns to the incident end 10a.

[0059] In continuous light L cw In the event of XT, continuous light L cw A portion of the light L leaks from the first core 11 to the second core 12 due to mode coupling and propagates in the second core 12. Here, as XT, there is a continuous light L leaking from the first core 11 to the second core 12. cw Part of the light B The continuous light L propagating in the second core 12 and in the first core 11 cw The phenomenon of propagating in the same direction (hereinafter referred to as "parallel XT"), and the continuous light L leaking from the first core 11 to the second core 12 cw Part of the light C The continuous light L propagating in the second core 12 and in the first core 11 due to backscattering cw The phenomenon of propagation in the opposite direction (hereinafter referred to as "backscattering XT"). As the situation where backscattering XT occurs, the following two situations can be considered. One situation is that the continuous light L in the first core 11 cw Part of the light A Backscattering occurs, and then the light L A A part of the light L leaks from the first core 11 to the second core 12 due to the parallel XT, and a part of the leak moves toward the incident end 10a in the second core 12. cw A portion of the light L leaks from the first core 11 to the second core 12 due to the parallel XT, and a portion of the leaked light is backscattered in the second core 12. In these cases, light L propagating in the opposite direction in the second core 12 is generated. C .

[0060] If the continuous light L at the output end 10b of the first fiber core 11 cw The light intensity of the second fiber core 12 is set to P1, and the light L at the output end 10b of the second fiber core 12 is set to BThe light intensity of the second core 12 is set to P2, and the parallel XT is defined by the ratio P2 / P1 of the light intensity P2 at the output end 10b of the light caused by backscattering to the light intensity P1 at the output end 10b. On the other hand, if the light L at the incident end 10a of the second core 12 is C The light intensity is set to P b2 , then the backscattering XT is determined by the light intensity P at the exit end 10b b2 The ratio P of the light intensity P1 at the output end 10b b2 When the MCF 10 is put into practical use, when the first core 11 and the second core 12 are adjacent to each other, the optical signal is conducted in the second core 12 in the opposite direction to the optical signal conducted in the first core 11. Therefore, it is extremely important in the practical use of the MCF 10 to measure the backscattered XT which is the main cause of the XT that advances in the opposite direction in the XT.

[0061] Non-patent document 2 discloses a method for measuring parallel XT and backscattered XT using an OTDR device. In this method, a pulse test light output from the OTDR device is incident on a single end of the MCF, and the light intensities P1, P2, and P1 described above are estimated based on the backscattered light from each core of the single end. b2 The optical characteristics such as parallel XT and backscattering XT are derived from the corresponding light intensity. In the method of using the OTDR device in this way, even if it is difficult to connect the measurement devices to both ends of the MCF, the optical characteristics of the MCF can be easily derived based on the information obtained from the single end by connecting the OTDR device to only one end of the MCF. Furthermore, according to the method of using the OTDR device, the optical characteristics at any position of the MCF can also be measured non-destructively.

[0062] However, in the method disclosed in non-patent document 2, when deriving backscattering XT, it is assumed that the power coupling coefficient used to derive backscattering XT is constant regardless of the position in the length direction of the MCF (that is, the power coupling coefficient has no position dependence). For this reason, when the power coupling coefficient changes according to the position of the MCF, there is a problem that the backscattering XT cannot be accurately evaluated. Therefore, in this embodiment, a method for deriving backscattering XT using an OTDR device is focused on, and a method for accurately evaluating backscattering XT is proposed even when the power coupling coefficient has position dependence. The specific method will be described below.

[0063] Refer again Figure 1The measuring device 30 includes, for example, a light source 31, a light receiver 33, a circulator 35, and a switch 37. The light source 31, the light receiver 33, the circulator 35, and the switch 37 are connected, for example, by an optical waveguide such as a single mode optical fiber (SMF). The circulator 35 and the switch 37 are connected to the incident end 10a of the MCF 10 by an optical device such as a fan-in / fan-out (FIFO: Fan-In / Fan-Out).

[0064] The light source 31 is optically coupled to the first core 11 of the incident end 10a via the circulator 35. Specifically, the light source 31 is connected to the input port of the circulator 35 via an optical waveguide. The input and output ports of the circulator 35 are connected to the first core 11 of the incident end 10a via another optical waveguide. The light source 31 outputs pulsed light L p Pulse light source. Pulse light L p The pulse light L outputted from the light source 31 is a light whose peak value repeatedly appears at a predetermined time interval. p The pulse light L is incident on the first core 11 of the incident end 10a through the circulator 35. p The pulse light L is incident on the first core 11. p A portion of the light L returns to the incident end 10a due to backscattering. p The pulse light L is incident on the first core 11. p The other part moves from the first core 11 to the second core 12 due to backscattering XT and returns to the incident end 10a. Hereinafter, the light returning to the incident end 10a in the first core 11 is referred to as "first backscattered light L1", and the light returning to the incident end 10a in the second core 12 is referred to as "second backscattered light L2".

[0065] The optical receiver 33 is optically coupled to the second core 12 of the incident end 10a via the switch 37. Furthermore, the optical receiver 33 is optically coupled to the first core 11 of the incident end 10a via the switch 37 and the circulator 35. Specifically, the optical receiver 33 is connected to the output port of the switch 37 via an optical waveguide. The input port of the switch 37 is connected to the second core 12 of the incident end 10a via another optical waveguide. The other input port of the switch 37 is connected to the output port of the circulator 35 via another optical waveguide. The switch 37 switches the optical path connecting the two input ports and the output port.

[0066] The first backscattered light L1 emitted from the incident end 10a of the first core 11 is received by the optical receiver 33 via the circulator 35 and the switch 37. The second backscattered light L2 emitted from the incident end 10a of the second core 12 is received by the optical receiver 33 via the switch 37. Thus, the circulator 35 and the switch 37 constitute a circuit for transmitting the pulsed light L1 from the light source 31 to the optical receiver 33. p The optical system 39 guides the first backscattered light L1 from the first core 11 and the second backscattered light L2 from the second core 12 to the optical receiver 33 .

[0067] The light receiver 33 emits pulse light L from the light source 31 p The light intensity of the first backscattered light L1 is measured at a predetermined sampling period from the time of . Then, the light receiver 33 records the measured light intensity of the first backscattered light L1 as the first light intensity data D at each sampling period. L1 The light source 31 of the optical receiver 33 emits a pulse light L p The light intensity of the second backscattered light L2 is measured at a predetermined sampling period from the time of . Then, the light receiver 33 records the measured light intensity of the second backscattered light L2 as the second light intensity data D at each sampling period. L2 The sampling period is sent to the computing device 50. The predetermined sampling period can be set by a sampling period generating device synchronized with the light source 31.

[0068] The computing device 50 is connected to the measuring device 30 in a communicable manner. The computing device 50 uses the first light intensity data D from the measuring device 30. L1 and the second light intensity data D L2 Calculate backscatter XT. In the present embodiment, as described above, the "backscatter XT index" is used as a value representing the "backscatter XT" itself for explanation. For this reason, the "backscatter XT" can be appropriately replaced with the "backscatter XT index" for explanation below. The computing device 50 is configured as a computer that physically includes a CPU (Central Processing Unit), a RAM (Random Access Memory) and a ROM (Read Only Memory) as a main storage device, an auxiliary storage device such as a hard disk device, an input device such as an input key as an input device, an output device such as a display, and a communication module.

[0069] Each function of the computing device 50 is realized by reading the prescribed computer software into the hardware such as the CPU, RAM and ROM, and operating the input device, output device and communication module under the control of the CPU, and reading and writing the data in the main storage device and the auxiliary storage device. There is no particular limitation on the configuration of the computing device 50. The computing device 50 may not be configured outside the measuring device 30. The computing device 50 may also be configured inside the measuring device 30. That is, the computing device 50 may also be embedded in a part of the measuring device 30.

[0070] Figure 3 is a block diagram showing the functional structure of the computing device 50. Figure 3 As shown in FIG. 1 , the computing device 50 includes, for example, a data processing unit 51, a computing unit 53, and a result display unit 55 (an example of a “display unit”) as functional structures. The data processing unit 51 receives the first light intensity data D transmitted from the light receiver 33 at a predetermined sampling period. L1 , and store the first light intensity data D in time series order L1 Then, the data processing unit 51 emits the pulse light L from the light source 31. p The time from the moment when the light receiver 33 receives the first backscattered light L1 is converted into the length of the MCF10 from the incident end 10a. Thus, the data processing unit 51 stores the light intensity of the first backscattered light L1 received by the light receiver 33 at each moment in correspondence with each position of the MCF10 along the length direction from the incident end 10a to the exit end 10b.

[0071] The data processing unit 51 converts the first light intensity data D corresponding to each position along the longitudinal direction of the MCF 10 into L1 The time series data is the first time series data D T1 The data processing unit 51 may also obtain all the first light intensity data D from the incident end 10a to the emission end 10b of the MCF 10. L1 The timing of the first time series data D T1 The data processing unit 51 may also output the first light intensity data D L1 When the first light intensity data D is obtained L1 The first time series data D T1 The data is output to the calculation processing unit 53 in real time.

[0072] The data processing unit 51 receives the second light intensity data D transmitted from the light receiver 33 at a predetermined sampling period. L2 , and the second light intensity data D L2The light intensity of the second backscattered light L2 shown is stored in a time series order. Then, the data processing unit 51 emits the pulse light L from the light source 31. p The time from the moment of receiving the second backscattered light L2 to the moment when the optical receiver 33 receives the second backscattered light L2 is converted into the length of the MCF10 from the incident end 10a. Thus, the data processing unit 51 stores the light intensity of the second backscattered light L2 received by the optical receiver 33 at each moment in correspondence with each position along the length direction of the MCF10 from the incident end 10a to the exit end 10b. For example, after the measurement of the first fiber core 11 for one pulse is completed, the data processing unit 51 switches the switch 37 to perform the measurement of the second fiber core 12 for the next pulse, thereby being able to distinguish and obtain the first light intensity data D L1 and the second light intensity data D L2 More specifically, the data processing unit 51 can obtain the light intensity of the first backscattered light L1 obtained from the first core 11 N times when N pulses (N is an integer greater than 1) are incident on the first core 11, or can obtain the average of the light intensities as the first light intensity data D when the light receiver 33 is optically coupled to the first core 11. L1 In the first light intensity data D L1 After the measurement is completed and the switch 37 is switched to a state where the optical receiver 33 is optically coupled to the second core 12, the data processing unit 51 can obtain the light intensity of the second backscattered light L2 obtained from the second core 12 N times when the N pulses are incident on the first core 11, and can also obtain the average of the light intensity as the second light intensity data D L2 .

[0073] The data processing unit 51 converts the second light intensity data D corresponding to each position along the longitudinal direction of the MCF 10 into L2 The time series data is the second time series data D T2 The data processing unit 51 may also obtain all the second light intensity data D from the incident end 10a to the emission end 10b of the MCF 10. L2 The timing of the second time series data D T2 The data processing unit 51 may also output the second light intensity data D L2 When the second light intensity data D is obtained L2 The second time series data D T2 The data is output to the calculation processing unit 53 in real time.

[0074] The calculation processing unit 53 uses the first time series data D output from the data processing unit 51 T1 And the second time series data D T2, calculate the backscatter XT. First, refer again to Figure 2 The theoretical formula of backscattering XT is explained. Figure 2 As shown, suppose a continuous light L with light intensity P0 cw The case where the fiber is incident on the first core 11 from the incident end 10a.

[0075] If the light intensity at the output end 10b of the first core 11 is set to P1, and the light intensity at the output end 10b of the second core 12 is set to P2, the light intensity P1 and the light intensity P2 can be expressed by the following equations (11) and (12), respectively. Equations (11) and (12) can be derived based on the description of non-patent document 2, for example.

[0076] P1=exp(-αL)P0…(11)

[0077] P2=hLexp(-αL)P0…(12)

[0078] In equations (11) and (12), P0 represents the light intensity of the light incident on the first core 11, α represents the transmission loss coefficient of MCF10, h represents the power coupling coefficient between the first core 11 and the second core 12, and L represents the total length of MCF10 in the longitudinal direction. However, in equations (11) and (12), an approximation with hL being sufficiently less than 1 (hL<<1) is used. The approximation with hL<<1 means that XT is sufficiently small. Therefore, the approximation with hL<<1 is valid in the uncoupled MCF.

[0079] If the position of the MCF 10 in the longitudinal direction defined by the incident end 10a of the MCF 10 as a reference (i.e., the origin) is defined as z, and the light intensity of the backscattered light between the position z and the position z+dz is defined as dP b2 , then the light intensity dP b2 It can be expressed by the following formula (13).

[0080] dP b2 =2hzexp(-2αz)P0Sα R dz…(13)

[0081] In formula (13), S represents the scattering cross-sectional area of ​​MCF10, α R represents the transmission loss coefficient of MCF10 due to Rayleigh scattering. Then, if the light intensity dP is calculated from the incident end 10a to the output end 10b (i.e., from z=0 to z=L), b2 The light intensity dP obtained by integration b2 The integral value is set to P b2 , then the integral value P b2 It can be expressed by the following formula (14).

[0082]

[0083] Integral value P b2 = is equivalent to the light intensity at the incident end 10a of the second core 12. To this end, the backscattering XT can be obtained by the integrated value P of the second core 12: b2 The ratio P of the light intensity P1 at the output end 10b of the first core 11 b2 / P1. Therefore, if the backscatter XT is expressed as XT b , then XT b It can be expressed as the following formula (15).

[0084]

[0085] Next, a method for deriving the backscatter XT using an OTDR device is described. Here, it is assumed that a pulse light L having a pulse width W and a light intensity P0 p When the pulse light L is incident on the first core 11 at the incident end 10a, the pulse light L is guided in the first core 11 and the second core 12. p The group velocity is V g In this case, XT is calculated using the first backscattered light L1 and the second backscattered light L2 emitted from the first core 11 and the second core 12 of the incident end 10a, respectively. b .

[0086] If the light intensity of the first backscattered light L1 backscattered at the position z of the first core 11 is set to P bs1 (z), the light intensity of the second backscattered light L2 backscattered at the position z of the second core 12 is set to P bs2 (z), then the light intensity P bs1 (z) and light intensity P bs2 (z) can be represented by the following equations (16) and (17), respectively.

[0087]

[0088] However, in equations (16) and (17), an approximation is used in which hz is sufficiently less than 1 (hz<<1). Furthermore, in equations (16) and (17), boundary conditions are set such that the incident intensity of light incident on the first core 11 at the incident end 10a (z=0) is set to P0 and the incident intensity of light incident on the second core 12 is set to zero.

[0089] If we calculate the light intensity P in the same way as when we calculated the theoretical formula for backscattering XT bs2 (z) is integrated, and the light intensity P corresponding to each position from the incident end 10a to the emission end 10b is bs2The integral value of (z) (hereinafter referred to as "light intensity integral value D2") can be expressed by the following formula (16) using formula (14) and formula (15). The light intensity integral value D2 of formula (16) and the integral value P of the theoretical formula (14) described above are b2 correspond.

[0090]

[0091] On the other hand, in the method using the OTDR device, the light intensity P1 of the above-described theoretical formula (11) (i.e., the intensity of the light incident on the first core 11 from the incident end 10a and emitted from the emission end 10b) cannot be directly measured. Therefore, it is considered that exp(-αL) represents the light (in this embodiment, the "pulse light L p ”) is transmitted from the incident end 10a to the output end 10b, the transmission loss value of the MCF 10 (hereinafter referred to as “loss value D 11 ”), is a value that varies depending on the light intensity P1, considering the use of loss value D 11 To estimate the light intensity P1. Then, consider taking the light intensity integral value D2 represented by equation (18) and the loss value D represented by the following equation (19): 11 The ratio D2 / D 11 , then the backscatter XT can be calculated. The transmission loss coefficient α can be obtained, for example, by the method described in Non-Patent Document 3. However, in Non-Patent Document 3, the unit of the transmission loss coefficient α is expressed as (dB / unit length), but in the present disclosure, the unit of the transmission loss coefficient α is expressed as ( / unit length). The unit conversion can be performed by multiplying the transmission loss coefficient α in Non-Patent Document 3 by ln(10) / 10.

[0092] D 11 =exp(-αL)…(19)

[0093] Light intensity integral value D2 and loss value D 11 The ratio D2 / D 11 This is expressed as the following formula (20). Then, if the XT of formula (12) expressed by the theoretical formula described above is b Substituting into equation (20), equation (20) is rewritten as the following equation (21), and equation (21) is rewritten as the following equation (22).

[0094]

[0095] Then, if the coefficient A1 represented by the following equation (23) is used, equation (22) can be represented as the following equation (24).

[0096]

[0097] The coefficient A1 of equation (23) is calculated using the light velocity c and the effective refractive index n of MCF10. eff , as shown in the following equation (25): g . Effective refractive index n eff is a constant determined by the MCF 10. The pulse width W is a constant determined by the setting value for the light source 31.

[0098]

[0099] The light intensity P0 of the coefficient A1 can also be regarded as a constant determined by the setting value of the light source 31, but since the light intensity P0 is the pulse light L incident on the incident end 10a of the MCF 10, p Therefore, in reality, there may be a loss at the connection point from the light source 31 to the point where the light source 31 enters the MCF10, as well as an insertion loss of the medium connecting the light source 31 and the MCF10. However, the loss at the connection point is particularly affected by errors, etc., and therefore cannot be easily measured. Therefore, another constant can be used to replace the light intensity P0. For example, if z = 0 is substituted into equation (16), the light intensity P0 of the first backscattered light L1 at the incident end 10a is bs1 (0) is expressed as the following formula (26), and formula (26) is rewritten as the following formula (27).

[0100]

[0101] Therefore, the coefficient A1 represented by equation (23) can be rewritten as the coefficient B1 represented by the following equation (28). In this case, XT b (24) is rewritten as the following formula (29).

[0102]

[0103] Thus, by using the ratio D2 / D 11 By multiplying by the coefficient A1 of equation (23) or the coefficient B1 of equation (28), we can obtain XT of equation (24) or equation (29): b That is, the ratio D2 / D 11 The term multiplied by the coefficient A1 and the ratio D2 / D 11 The terms multiplied by the coefficient B1 can be used to represent XT bThe equation (24) represented by the coefficient A1 is assumed to be zero for the transmission path loss between the MCF10 and the light source 31 and the transmission path loss between the MCF10 and the light receiver 33. Therefore, taking these losses into account, if the transmission loss of the transmission path from the light source 31 to the light incident on the MCF10 is set to α in , the transmission loss of the transmission path from the MCF 10 to the light incident on the optical receiver 33 is α out , the coefficient A1 can also be rewritten as in the following formula (30).

[0104]

[0105] Indicates XT b The formula of is not limited to the above formula (24) or formula (29). For example, the loss value D of formula (24) or formula (29) 11 The transmission loss (exp(-αL)) can also be rewritten as follows. When the transmission loss coefficient α has position dependence, exp(-αL) can be defined as: p The ratio of the light intensity that decreases when advancing from the incident end 10a to the exit end 10b. To this end, exp(-αL) can be obtained by the light intensity P at the incident end 10a of the first core 11. bs1 (0) and the light intensity P at the output end 10b of the first core 11 bs1 However, when exp(-αL) is calculated using the first backscattered light L1, the light moving from the incident end 10a to the exit end 10b and then returning from the exit end 10b to the incident end 10a is measured, so the moving distance of the light is doubled. This is equivalent to the light being transmitted twice in the MCF 10, so by calculating the light intensity P bs1 (0) and light intensity P bs2 The square root of the ratio of (L) can be obtained to obtain exp(-αL). Therefore, exp(-αL) can be expressed as in equation (31).

[0106]

[0107] If equation (22) is substituted into equation (31), equation (22) is rewritten as equation (32) below. Here, the light intensity P in equation (32) is bs1 The square root of (L) can be used to represent the pulse light L p The value of the transmission loss of the MCF 10 when moving from the incident end 10a to the output end 10b (hereinafter referred to as “loss value D 12 ”). Therefore, if the loss value D represented by the following formula (33) is used 12And the coefficient A2 represented by the following equation (33), then equation (32) can be written as the following equation (35).

[0108]

[0109]

[0110] In the formula (34) represented by the coefficient A2, similarly to the formula (23) represented by the coefficient A1, it is assumed that the loss in the transmission path between the MCF10 and the light source 31 and the loss in the transmission path between the MCF10 and the light receiver 33 are both zero. Therefore, when these losses are taken into account, if the transmission loss in the transmission path from the light source 31 to the light incident on the MCF10 is α in , the transmission loss of the transmission path from the MCF 10 to the light incident on the optical receiver 33 is α out , then the coefficient A2 can also be rewritten as in the following formula (36).

[0111]

[0112] Furthermore, if the relationship of the above-mentioned equation (27) is used, equation (32) can also be rewritten as the following equation (37). In this case, if the coefficient B2 represented by the following equation (38) is used, equation (37) can be written as the following equation (39).

[0113]

[0114] Thus, by setting the ratio D2 / D 12 By multiplying by the coefficient A2 of equation (36) or the coefficient B2 of equation (38), we can also find XT of equation (35) or equation (39): b That is, the ratio D2 / D 12 The term multiplied by the coefficient A2 and the ratio D2 / D 12 The terms multiplied by the coefficient B2 can be used to represent XT b Item.

[0115] As described above, the backscattering XT can be obtained by any of the above equations (24), (29), (35) and (39). 11 and D 12 The light intensity P of the first backscattered light L1 can be used bs1 The light intensity integral value D2 can be obtained using the light intensity P of the second backscattered light L2. bs2Then, the coefficients A1, B1, A2, and B2 can be set as constants determined according to the setting of the light source 31 and the characteristics of the MCF 10. Therefore, if the light intensity P of the first backscattered light L1 is measured, bs2 (z) and the light intensity P of the second backscattered light L2 bs2 (z), the backscattering XT can be calculated. Since the light intensity integral value D2 included in any of the equations (24), (29), (35) and (39) is the light intensity P corresponding to each position from the incident end 10a to the exit end 10b of the MCF 10, bs2 (z), so no matter which formula is used, the backscattering XT is taken as the light intensity P at each position in the length direction of MCF10. bs2 As a result, even when the power coupling coefficient between the first core 11 and the second core 12 has position dependence, the backscattering XT can be accurately calculated and evaluated. However, when calculating XT of equation (24) or equation (29), b The formula (19) used in the above example is obtained under the assumption that the transmission loss coefficient α is constant and has nothing to do with the position in the longitudinal direction of MCF10. The loss value D in formula (19) is 11 = represents the transmission loss of MCF10 obtained under this condition. On the other hand, when calculating XT in equation (35) or equation (39), b The above assumption is not made in the formula (33) used when the transmission loss coefficient α varies according to the position in the longitudinal direction of MCF10, so the loss value D in formula (33) is 12 It can also accurately represent the transmission loss of MCF10. Therefore, if the position dependence of the transmission loss coefficient α is taken into account, the loss value D in equation (33) is 12 The loss value D can be compared with formula (19) 11 More accurately represents the transmission loss of MCF10.

[0116] In the above description, the backscatter XT corresponding to the total length L of the MCF 10 from the incident end 10a to the emission end 10b is calculated. However, the backscatter XT corresponding to the length of the MCF 10 at any measurement position from the incident end 10a can also be calculated. For example, if the measurement position is z L , the length of MCF10 from the incident end 10a to the measurement position can be expressed as z LTherefore, the above equations (24), (29), (35) and (39) can be rewritten as the following equations (40), (41), (42) and (43), respectively. By using these equations (40), (41), (42) and (43), it is possible to calculate the arbitrary measurement position z of MCF10 L Backscatter XT at .

[0117]

[0118]

[0119] The above specifically describes the method of deriving the backscattered XT using the OTDR device. However, the parallel XT can also be derived using the OTDR device. If the parallel XT is represented as XT f , then the light intensity P at the output end 10b (position z=L) of the first core 11 can be used bs1 (L) and the light intensity P at the emission end 10b (position z=L) of the second core 12 bs2 (L), as shown in the following formula (44): f About XT f The specific derivation method is recorded in non-patent document 2, for example.

[0120]

[0121] Based on the above method of deriving backscatter XT, refer to Figure 3 The flow of processing by the calculation processing unit 53 is described. Figure 3 As shown in FIG. 1 , the calculation processing unit 53 includes, for example, a loss value calculation unit 53a, an integral value calculation unit 53b, and an XT calculation unit 53c as functional structures. The loss value calculation unit 53a receives the first time series data D from the data processing unit 51. T1 , according to the first time series data D T1 , use the above formula (19) or formula (33) to calculate the loss value D 11 or D 12 The loss value calculation unit 53a will represent the loss value D 11 or D 12 Loss value data D 10 The integrated value calculation unit 53b receives the second time series data D from the data processing unit 51. T2 , according to the second time series data D T2 , the light intensity integral value D2 is calculated using the above-mentioned formula (18). The integral value calculation unit 53b converts the integral value data D representing the light intensity integral value D2 into 20 Output to the XT calculation unit 53c.

[0122] The XT calculation unit 53c uses the loss value data D outputted from the loss value calculation unit 53a and the integral value calculation unit 53b. 10 And integral value data D 20 , calculate the backscatter XT. For example, in the loss value data D 10 Indicates the loss value D 11 In the case of 11 The ratio D2 / D 11 (See equation (20)). Then, the XT calculation unit 53c calculates the ratio D2 / D 11 Multiply by coefficient A1 or coefficient B1 to calculate XT expressed by equation (24) or equation (29) b .

[0123] In the loss value data D 10 Indicates the loss value D 12 In the case of 12 The ratio D2 / D 12 Then, the XT calculation unit 53c calculates the ratio D2 / D 12 Multiply by coefficient A2 or coefficient B2 to calculate XT expressed by equation (35) or equation (39) b The XT calculation unit 53c includes the calculated XT b The calculation result D 30 Output to the result display unit 55. 30 In addition to XT b In addition, various parameters such as the power coupling coefficient and the position of the MCF 10 in the longitudinal direction are also included.

[0124] Next, use Figure 4 and Figure 5 , verifying the backscattering XT calculated by the XT calculation unit 53c. Figure 4 : is a graph showing the relationship between the position in the longitudinal direction of the MCF 10 and the power coupling coefficient between the first core 11 and the second core 12. Figure 5 In FIG. 1 , the horizontal axis represents the position (km) in the longitudinal direction of the MCF 10 , and the vertical axis represents the power coupling coefficient (dB / km). The graph G1 shows the change in the power coupling coefficient when there is a dependency on the position of the MCF 10 .

[0125] Figure 5 : is a graph showing the relationship between the position in the longitudinal direction of MCF10 and the backscattering XT. Figure 5In FIG. 1 , the horizontal axis represents the position (km) of the MCF 10 in the longitudinal direction, and the vertical axis represents the backscatter XT (dB). Graph G2 shows the theoretical formula of the backscatter XT when it is dependent on the position of the MCF 10 (specifically, “hL” in Formula (15) is replaced by “∫0 L The theoretical line is obtained by using the formula obtained by "hdz". The curve graph G3 shows the position dependence of the backscattering XT calculated using the method involved in this embodiment. The curve graph G4 shows the position dependence of the backscattering XT calculated using the method described in Non-Patent Document 2 as a comparative example.

[0126] In the method described in Non-Patent Document 2, as described above, the backscatter XT is calculated under the assumption that the power coupling coefficient has no position dependence. Figure 4 In the case where the power coupling coefficient has position dependence as in the curve G1 of Figure 5 As shown in FIG. 1 , the backscattering XT (graph G4) calculated using the method described in Non-Patent Document 2 deviates from the theoretical line (graph G2). In contrast, in the method according to the present embodiment, the light intensity P at each position in the longitudinal direction of the MCF 10 is calculated as described above. bs2 (z) are all used to calculate the backscatter XT, so the backscatter XT (graph G3) calculated using the method involved in this embodiment is a value that takes into account the position dependence of the power coupling coefficient and is consistent with the theoretical line (graph G2). In this way, it can be seen that according to this embodiment, even when the power coupling coefficient has position dependence, the backscatter XT can be accurately calculated.

[0127] use Figure 6 and Figure 7 , further verifying the calculated backscattering XT. Figure 6 and Figure 4 Similarly, it is a graph showing the relationship between the position in the longitudinal direction of the MCF 10 and the power coupling coefficient between the first core 11 and the second core 12. Figure 6 , a graph G11 showing the power coupling coefficient when it is dependent on the position of MCF10 and a graph G12 showing the power coupling coefficient when it is dependent on the position of MCF10 are shown. In the graph G11, the power coupling coefficient rises significantly when the position of MCF10 is near zero (i.e., near the incident end 10a) and then becomes a constant value. For example, when XT is large in the FIFO (Fan-in / Fan-out) device, which is an optical device for connecting MCF10 with the light source 31 and connecting MCF10 with the optical receiver 33, the power coupling coefficient shown in the graph G11 is obtained.

[0128] Figure 7 and Figure 5 Similarly, it is a graph showing the relationship between the position in the longitudinal direction of MCF10 and the backscattering XT. Figure 7 and Figure 5 The difference is shown as a double logarithmic plot. Therefore, Figure 7 The vertical and horizontal axes of the graph G21 are both logarithmic. The graph G21 shows the case where the power coupling coefficient has position dependence ( Figure 6 Graph G22 shows the case where the power coupling coefficient has no position dependence ( Figure 6 In the graph G22, in the portion where the position of MCF10 is close to zero (specifically, when the position of MCF10 is z and the transmission loss coefficient of MCF10 is α, the portion where αz<<1), the slope of the change in backscattering XT is 20 (dB / decade). The graph G23 shows a line where the slope of the change in backscattering XT is 20 (dB / decade).

[0129] like Figure 7 As shown in the curve G21, it can be seen that the slope of the change of backscatter XT when the power coupling coefficient has position dependence is inconsistent with the slope of the change of backscatter XT when the power coupling coefficient has no position dependence, that is, 20 (dB / decade). It can be seen that the backscatter XT is different between the case where the power coupling coefficient has position dependence and the case where the power coupling coefficient has no position dependence. Figure 7 If the backscatter XT is plotted in a double logarithmic plot, a straight line can be used to mark the Figure 5 Since the portion represented by the curve in FIG. 1 (i.e., the portion where the position of MCF10 is close to zero) is the portion, the backscatter XT in this portion can be easily evaluated. In the method described in non-patent document 2, even when the power coupling coefficient has position dependence, the backscatter XT having a slope of 20 (dB / decade) is obtained similarly to the graph G22.

[0130] The result display unit 55 uses the calculation result D output from the XT calculation unit 53c. 30 , displaying data indicating the relationship between the position in the longitudinal direction of MCF10 and the backscattering XT. For example, the result display unit 55 may display Figure 5 The curve graph and Figure 7 One side of the curve graph can also be displayed Figure 5 The curve graph and Figure 7 The result display unit 55 can Figure 4 The curve Figure 1 and displayed in Figure 5 The curve graph can also be Figure 6 The curve Figure 1 and displayed in Figure 7 Using the graph displayed on the result display unit 55 as described above, the position dependency of the backscattering XT and the like can be evaluated.

[0131] [Optical property measurement method]

[0132] Next, refer to Figure 8 An optical property measurement method using the optical property measurement system 1 will be described. Figure 8 This is a flowchart showing an example of an optical property measurement method.

[0133] First, the light source 31 generates pulse light L p The pulse light L is incident on the first core 11 from the incident end 10a of the MCF 10 (step S11). Next, the optical receiver 33 receives the pulse light L p The second backscattered light L2 is incident on the first core 11 and emitted from the second core 12 of the incident end 10a (step S12). If the optical receiver 33 receives the second backscattered light L2, the data processing unit 51 of the computing device 50 obtains the intensity of the second backscattered light L2 at each time received by the optical receiver 33 corresponding to each position along the length direction of the MCF10 from the incident end 10a to the exit end 10b (step S13). Then, the integral value calculation unit 53b of the computing device 50 calculates the light intensity integral value D2 obtained by integrating the light intensity of the second backscattered light L2 corresponding to each position of the MCF10 from the incident end 10a to the exit end 10b (step S14).

[0134] After step S11, the light receiver 33 receives the pulse light L p The first backscattered light L1 is incident on the first core 11 and emitted from the first core 11 at the incident end 10a (step S22). When the optical receiver 33 receives the first backscattered light L1, the data processing unit 51 of the computing device 50 obtains the intensity of the first backscattered light L1 at each time when the optical receiver 33 receives it, corresponding to each position along the length direction of the MCF 10 from the incident end 10a to the emission end 10b (step S23). Then, the loss value calculation unit 53a of the computing device 50 calculates the intensity representing the pulse light L1. p The transmission loss value D of MCF10 when moving from the incident end 10a to the output end 10b 11 or D 12 (Step S24).

[0135] Next, the XT calculation unit 53c of the computing device 50 calculates the light intensity integral value D2 and the loss value D 11 or D 12 Next, the XT calculation unit 53c calculates the backscatter XT (step S17) by multiplying the calculated ratio by a coefficient (step S16). Specifically, the XT calculation unit 53c calculates the ratio D2 / D 11 In this case, by changing the ratio D2 / D 11 Multiply by coefficient A1 or B1 to calculate XT expressed by equation (24) or equation (29) b On the other hand, the XT calculation unit 53c calculates the ratio D2 / D 12 In this case, by changing the ratio D2 / D 12 Multiply by coefficient A2 or B2 to calculate XT expressed by equation (35) or equation (39) b Then, the result display unit 55 displays the calculated XT b The step group for calculating the light intensity integral value D2 (specifically, steps S22, S23 and S24) and the step group for calculating the loss value D 11 or D 12 The step group (specifically, steps S12, S13 and S14) can be performed simultaneously or one after the other in time.

[0136] The optical property measurement method disclosed in the present invention is not limited to Figure 8 Example shown. Fig. 9 FIG. 1 is a flowchart showing another example of the optical property measurement method. Figure 8 In the example shown, the measurement of the first backscattered light L1 from the first core 11 and the measurement of the second backscattered light L2 from the second core 12 are performed in a single common measurement, but these measurements may be performed separately. Fig. 9 In the example shown, step S21 is provided separately from step S11. Step S21 is similar to step S11 in that the pulse light L p This is a step in which the light is incident on the first core 11 from the incident end 10a of the MCF 10, but is performed in a measurement different from step S11.

[0137] For example, step S21 may be performed before step S11, and the loss value D may be calculated in advance through steps S22, S23, and S24. 11 or D 12 After that, a series of processing from step S11 to step S17 is performed. That is, the loss value D is calculated. 11 or D 12The step group (specifically, steps S11, S12, S13 and S14) of calculating the light intensity integral value D2 may also be performed before the step group (specifically, steps S21, S22, S23 and S24) of calculating the loss value D2. 11 or D 12 The step group may also be performed after the step group of calculating the light intensity integrated value D2.

[0138] Fig.10 FIG. 1 is a flowchart showing another example of the optical property measurement method. Figure 8 In the example shown, the light intensity of the first backscattered light L1 emitted from the first core 11 of the incident end 10a is measured to calculate the loss value D 11 or D 12 However, for example, the loss value D may be measured by directly measuring the light emitted from the first core 11 of the emission end 10b. 11 In this case, for example, a device for measuring the light emitted from the first core 11 may be connected to the emission end 10b of the MCF 10, and the loss value D may be calculated by the device. 11 The light incident on the incident end 10a of the MCF 10 may be either "pulse light" or "continuous light". By measuring the light emitted from the output end 10b as the light is incident on the incident end 10a by a device connected to the output end 10b, it is possible to calculate the loss value D representing the transmission loss of the MCF 10 when the light (pulse light or continuous light) moves from the incident end 10a to the output end 10b. 11 Then, if Fig.10 As shown, the loss value D can also be obtained from the device 11 Then (step S31), the next steps S15 to S17 are performed. In step S31, if the loss value of the total length L of the MCF 10 is measured using a device connected to the emission end 10b, the loss value D can be expressed by exp(-αL) (refer to formula (19)). Therefore, by solving the formula D = exp(-αL), the transmission loss coefficient α can be estimated. Therefore, by substituting the estimated transmission loss coefficient α into exp(-αz L ), it is also possible to obtain the arbitrary measurement position z of MCF10 L The loss value D 11 .

[0139] Fig.11 FIG. 1 is a flowchart showing another example of the optical property measurement method. Figure 8 In the example shown, the backscatter XT is directly calculated. However, it is not necessary to directly calculate the backscatter XT. Fig.11 As shown, the light intensity integral value D2 and the loss value D 11 or D12 After the ratio is calculated (step S15), the calculation of multiplying the ratio by the coefficients A1, B1, A2 or B2 is not performed. Even if the calculation of multiplying by these coefficients is not performed, the calculated ratio can be used as a value proportional to the backscatter XT.

[0140] Backscatter XT is usually expressed in dB. In contrast, if a value proportional to backscatter XT is obtained, it is possible to understand how to express the position dependence of backscatter XT in dB units in arbitrary units. The coefficient A1, B1, A2, or B2 multiplied by the value proportional to backscatter XT is expressed in dB units. Figure 5 or Figure 7 Therefore, if the values ​​proportional to the backscatter XT are plotted in these graphs, the values ​​proportional to the backscatter XT show the same changes as the backscatter XT when they deviate from the backscatter XT. The values ​​proportional to the backscatter XT (i.e., the light intensity integral value D2 and the loss value D 11 or D 12 The ratio of (to) is a value that changes according to the change of backscatter XT, and therefore can be used as a backscatter XT index that represents backscatter XT. Fig.11 As shown, after step S15, the calculated ratio may be used as the backscatter XT index instead of being multiplied by the coefficient A1, B1, A2 or B (step S18). In this way, the "backscatter XT index" of the present disclosure may not be the backscatter XT itself, but may be a value proportional to the backscatter XT (i.e., the integral value of the light intensity D2 and the loss value D 11 or D 12 ratio).

[0141] The effects obtained by the optical characteristic measurement system 1 and the optical characteristic measurement method involved in the present embodiment described above are described. As described above, in the method described in non-patent document 2, the backscattering XT is derived under the assumption that the power coupling coefficient between the fiber cores used to derive the backscattering XT is constant and does not depend on the position of the MCF (that is, the power coupling coefficient has no position dependence). For this reason, when the power coupling coefficient has position dependence, there is a problem that the backscattering XT cannot be accurately evaluated. Therefore, the inventors of the present disclosure have repeatedly studied a method for accurately evaluating the backscattering XT by taking into account the position dependence of the power coupling coefficient.

[0142] The backscattering XT can be defined by the ratio of the intensity of the light due to backscattering among the light that moves from the first core 11 to the second core 12 and returns to the incident end 10a to the intensity of the light that enters the first core 11 from the incident end 10a and is emitted from the output end 10b. The inventors of the present disclosure have found in the process of repeated studies that the light intensity at the output end 10b can be expressed by the pulse light L p The transmission loss value D of the MCF 10 when moving from the incident end 10a to the output end 10b in the first core 11 is 11 or D 12 Furthermore, the inventors of the present disclosure have thought that the intensity of the light returning to the incident end 10a in the second core 12 can be estimated by making the pulse light L p The light intensity integral value D2 of the second backscattered light L2 corresponding to each position from the incident end 10a to the exit end 10b when incident on the first core 11 is represented. Then, the inventors of the present disclosure have found that the light intensity integral value D2 and the loss value D 11 or D 12 The term of the ratio of is regarded as backscatter XT. If the light intensity integral value D2 is used in this way, the backscatter XT can be calculated using the light intensity of all the second backscattered light L2 from the incident end 10a to the exit end 10b. Therefore, according to this embodiment, even when the power coupling coefficient has position dependence, the backscatter XT can be accurately evaluated.

[0143] As in the present embodiment, the calculation processing unit 53 may calculate the loss value D using the light intensity of the first backscattered light L1 corresponding to each position of the first core 11 from the incident end 10a to the emission end 10b. 11 or D 12 In this case, the loss value D indicating the transmission loss of the MCF 10 can be calculated more accurately using the light intensity of the first backscattered light L1 emitted from the first core 11. 11 or D 12 When the light intensity of the first backscattered light L1 corresponding to each position of the first core 11 is used in this way, for example, equations (40), (41), (42), and (43) can be used to more accurately calculate the value representing an arbitrary measurement position z of the MCF 10. L The transmission loss value D at 11 or D 12 As a result, it is possible to use the light intensity integral value D2 and the loss value D 11 or D 12 The ratio of the terms can be used to more accurately evaluate the arbitrary measurement position z of MCF10 LIn the present embodiment using the OTDR device, the loss value D in equation (19) is 11 And the loss value D of formula (33) 12 Both can be used as values ​​representing the transmission loss of MCF10. However, as described above, if the position dependence of the transmission loss coefficient α is taken into account, the loss value D of equation (33) is used. 12 The transmission loss of MCF10 can be expressed more accurately. That is, if equation (33) is used, a more accurate loss value D can be obtained that takes into account the position dependence of the transmission loss coefficient α. 12 If the loss value D is used 12 By calculating the backscatter XT, even when the transmission loss coefficient α has position dependence, the backscatter XT can be estimated more accurately.

[0144] As in the present embodiment, the calculation processing unit 53 may use the loss value D represented by equation (19): 11 The ratio D2 / D represented by equation (20) is calculated using the light intensity integral value D2 represented by equation (18). 11 The inventors of the present disclosure have found in the process of repeatedly studying the mathematical formula for calculating the backscatter XT index that the ratio D2 / D represented by formula (20) 11 The term represents a value proportional to the backscatter XT. If the ratio D2 / D 11 If the term is used, the backscattering XT can be accurately evaluated even when the power coupling coefficient between the first core 11 and the second core 12 has position dependence.

[0145] As in the present embodiment, the calculation processing unit 53 may also calculate the ratio D2 / D represented by equation (20) by 11 The backscatter XT is calculated by multiplying the coefficient A1 represented by the formula (23) or the coefficient B1 represented by the formula (28). The inventors of the present disclosure have found that by repeatedly studying the mathematical formula for calculating the backscatter XT, the ratio D2 / D represented by the formula (20) can be 11 By multiplying the coefficient A1 represented by equation (23) or the coefficient B1 represented by equation (28), the backscatter XT can be directly calculated. 11 The value obtained by multiplying the coefficient A1 or the coefficient B1 can directly and accurately estimate the backscatter XT.

[0146] As in the present embodiment, the loss value D expressed by equation (34) may also be used. 12 The ratio D2 / D represented by equation (35) or equation (39) is calculated by using the light intensity integral value D2 represented by equation (38). 12The inventors of the present disclosure have found in the process of repeatedly studying the mathematical formula for calculating the backscatter XT index that the ratio D2 / D represented by formula (35) or formula (39) 12 The term represents a value proportional to the backscatter XT. If the ratio D2 / D 12 If the term is used, the backscattering XT can be accurately evaluated even when the power coupling coefficient between the first core 11 and the second core 12 has position dependence.

[0147] As in the present embodiment, the calculation processing unit 53 may also convert the ratio D2 / D represented by equation (35) or equation (39) into 12 The backscatter XT is calculated by multiplying the coefficient A2 represented by the formula (36) or the coefficient B2 represented by the formula (38). The inventors of the present disclosure have found that by repeatedly studying the mathematical formula for calculating the backscatter XT, the ratio D2 / D represented by the formula (35) or the formula (39) can be 12 By multiplying the coefficient A2 represented by equation (36) or the coefficient B2 represented by equation (38), the backscatter XT can be directly calculated. 12 The value obtained by multiplying the coefficient A2 or the coefficient B2 can directly and accurately estimate the backscatter XT.

[0148] As in the present embodiment, the optical characteristic measurement system 1 may include an optical system 39 that converts the pulse light L from the light source 31 into p The first backscattered light L1 from the first core 11 and the second backscattered light L2 from the second core 12 are guided to the optical receiver 33. In this case, it is easy to make the pulse light L p The first backscattered light L1 and the second backscattered light L2 are incident on the first core 11 and emitted from the first core 11 and the second core 12 respectively, and are received by the optical receiver 33 .

[0149] As in the present embodiment, the optical characteristic measurement system 1 may also include a result display unit 55 that displays a double logarithmic graph showing the relationship between the position along the length direction of the MCF 10 and the backscattering XT. By using the double logarithmic graph in this way, it is possible to easily understand the change in the backscattering XT relative to the position of the MCF 10. Thus, it is possible to easily evaluate the position dependence of the backscattering XT.

[0150] The present disclosure is not limited to the above-described embodiments, and can be appropriately modified within the scope of the gist described in the claims. Fig.12 An optical property measurement system 1 according to a modified example is shown. Fig.12The optical characteristic measuring system 1A shown in the figure includes, for example, a plurality of light sources 31A, 31B and 31C, a plurality of light receivers 33A, 33B and 33C, and a plurality of circulators 35A, 35B and 35C. The MCF 10A has a third core 13 in addition to the first core 11 and the second core 12. Fig.12 In the example shown, three light sources 31A, 31B, and 31C, three light receivers 33A, 33B, and 33C, and three circulators 35A, 35B, and 35C are provided corresponding to the number of cores of the MCF 10A.

[0151] The light source 31A and the light receiver 33A are optically coupled to the first core 11 via the circulator 35A. Specifically, the light source 31A is connected to the input port of the circulator 35A, and the light receiver 33A is connected to the output port of the circulator 35A. The input and output ports of the circulator 35A are connected to the first core 11 at the incident end 10a. The pulse light L outputted from the light source 31A is p The first backscattered light L1 incident on the first core 11 of the incident end 10a and emitted from the first core 11 of the incident end 10a is received by the optical receiver 33A.

[0152] The light source 31B and the light receiver 33B are optically coupled to the second core 12 via the circulator 35B. Specifically, the light source 31B is connected to the input port of the circulator 35B, and the light receiver 33B is connected to the output port of the circulator 35B. The input and output ports of the circulator 35B are connected to the second core 12 of the incident end 10a. The pulse light output from the light source 31B can be incident on the second core 12 of the incident end 10a. The second backscattered light L2 emitted from the second core 12 of the incident end 10a is received by the light receiver 33B.

[0153] The light source 31C and the light receiver 33C are optically coupled to the third core 13 via the circulator 35C. Specifically, the light source 31C is connected to the input port of the circulator 35C, and the light receiver 33C is connected to the output port of the circulator 35C. The input and output ports of the circulator 35C are connected to the third core 13 of the incident end 10a. The pulse light output from the light source 31C can be incident on the third core 13 of the incident end 10a. The third backscattered light L3 emitted from the third core 13 of the incident end 10a is received by the light receiver 33C.

[0154] exist Fig.12 , showing the pulse light L p The pulse light L is incident on the first core 11 of the incident end 10a from the light source 31A and pIn this case, the optical receiver 33A measures the light intensity of the first backscattered light L1 at a predetermined sampling period, and uses the measured light intensity of the first backscattered light L1 as the first light intensity data D. L1 The optical receiver 33B measures the light intensity of the second backscattered light L2 at a predetermined sampling period, and uses the measured light intensity of the second backscattered light L2 as the second light intensity data D L2 The optical receiver 33C measures the light intensity of the third backscattered light L3 at a predetermined sampling period, and uses the measured light intensity of the third backscattered light L3 as the third light intensity data D L3 Sent to the computing device 50.

[0155] The computing device 50 uses the first light intensity data D L1 , the second light intensity data D L2 and the third light intensity data D L3 , the backscattering XT between the first core 11 and the second core 12 and the backscattering XT between the first core 11 and the third core 13 are calculated by the same method as in the above embodiment. p The pulse light L is incident on the second core 12 from the light source 31B, or p When the light source 31C is incident on the third core 13 , the backscattering XT between the second core 12 and the third core 13 can also be calculated.

[0156] In this way, the backscattering XT between any of the first core 11, the second core 12, and the third core 13 can be calculated by the same method as in the above-mentioned embodiment. Fig.12 The optical characteristic measurement system 1A shown in the figure can also obtain the same effect as the above-mentioned embodiment. Furthermore, according to the optical characteristic measurement system 1A including the plurality of light sources 31A, 31B and 31C and the plurality of light receivers 33A, 33B and 33C, even when measuring the backscattering XT for a combination other than the first core 11 and the second core 12, the backscattering XT between arbitrary cores of the MCF 10A can be easily calculated without changing the connection between the light source 31 and the MCF 10A and the connection between the light receiver 33 and the MCF 10A.

[0157] The number of light sources and the number of light receivers provided by the measuring device are not limited to the above-mentioned embodiments and variations, and can be appropriately changed. For example, the measuring device can include one light source and multiple light receivers, or multiple light sources and one light receiver. In the above-mentioned embodiments, the case where pulsed light is incident on the first core is exemplified. However, even in the case where pulsed light is incident on the second core, the backscattering XT can be calculated by the same derivation method as in the above-mentioned embodiments. In the above-mentioned embodiments and variations, the case where the MCF has the first core and the second core, and the case where the MCF has the first core, the second core, and the third core are described respectively. However, even in the case where the MCF has four or more cores, the same derivation method as in the above-mentioned embodiments can be applied by understanding one of the cores as the "first core" and the other as the "second core."

[0158] Description of Reference Numerals

[0159] 1.1A Optical property measurement system

[0160] 10, 10A Multi-core Fiber (MCF)

[0161] 10a Incident end

[0162] 10b Output end

[0163] 11 First Fiber Core

[0164] 12 Second core

[0165] 13 Third core

[0166] 30 Measurement device

[0167] 31, 31A, 31B, 31C Light Source

[0168] 33, 33A, 33B, 33C Optical receiver

[0169] 35, 35A, 35B, 35C Circulator

[0170] 37 Switch

[0171] 39 Optical System

[0172] 50 Computing device

[0173] 51 Data Processing Department

[0174] 53. Calculation Processing Unit

[0175] 53a Loss value calculation unit

[0176] 53b Integral value calculation unit

[0177] 53c XT Computing Department

[0178] 55 Result display unit (an example of a "display unit")

[0179] A1, A2, B1, B2 coefficients

[0180] D 11 , D 12 Loss value

[0181] D2 Light intensity integrated value (an example of “integrated value”)

[0182] D 10 Loss value data

[0183] D 20 Integral value data

[0184] D 30 Calculation results

[0185] D L1 First light intensity data

[0186] D L2 Second light intensity data

[0187] D L3 Third light intensity data

[0188] D T1 First time series data

[0189] D T2 Second time series data

[0190] L Total length

[0191] L1 First backscattered light

[0192] L2 Second backscattered light

[0193] L3 The third backscattered light

[0194] L cw Continuous light

[0195] L p Pulsed Light

[0196] The above equations (1) to (44) are valid in any unit system. The characters (variables) of physical quantities used in the above equations have the following dimensions in the MKS unit system. The scattering cross-sectional area S and the loss value D11 of the multi-core optical fiber are dimensionless quantities.

[0197] Determine the position z L [m]

[0198] Transmission loss coefficient of multi-core optical fiber α[ / m]

[0199] Transmission loss coefficient α R [ / m]

[0200] The group velocity V of the pulsed light g [m / s]

[0201] Pulse width of pulse light W[s]

[0202] Light intensity of pulse light P0[W]

[0203] The light intensity P of the first backscattered light corresponding to one end bs1 (0)[W]

[0204] The light intensity P of the first backscattered light bs1 [W]

[0205] The light intensity P of the second backscattered light bs2 [W]

[0206] Integrated value D2 [W·m]

[0207] Transmission loss D 12 [W 1 / 2 ]

[0208] Coefficient A1[ / (W·m)]

[0209] Coefficient B1[ / (W·m)]

[0210] Coefficient A2[ / (W 1 / 2 m)]

[0211] Coefficient B2[ / (W 1 / 2 ·m)].

Claims

1. An optical property measurement system for measuring optical properties of a multi-core optical fiber having at least a first core and a second core, the optical property measurement system comprising: at least one light source, optically coupled to the first fiber core, capable of causing pulsed light to be incident from one end of the multi-core optical fiber into the first fiber core; at least one optical receiver, optically coupled to at least the second fiber core, capable of receiving at least the second backscattered light of the first backscattered light and the second backscattered light emitted from the first fiber core and the second fiber core at the one end respectively as the pulse light is incident on the first fiber core; as well as a computing device connected to the optical receiver in a communicable manner, and calculating a backscatter crosstalk index based on at least the light intensity of the second backscattered light, the backscatter crosstalk index indicating backscatter crosstalk associated with the light intensity of the second backscattered light that moves from the first core to the second core and returns to the one end as a part of the pulsed light, The computing device comprises: a data processing unit that acquires the light intensity of the second backscattered light at each time received by the optical receiver corresponding to each position from the one end to an arbitrary measurement position along the length direction of the multi-core optical fiber; as well as The operation processing unit calculates an item including a ratio of an integral value and a loss value as the backscatter crosstalk index, wherein the loss value represents the transmission loss of the multi-core optical fiber when the light moves from the one end to the measurement position in the first fiber core, and the integral value is obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second fiber core from the one end to the measurement position.

2. The optical property measurement system according to claim 1, wherein: The optical receiver is optically coupled to the first fiber core and is capable of receiving the first backscattered light emitted from the first fiber core at the one end. The data processing unit acquires the light intensity of the first backscattered light at each time point received by the light receiver in correspondence with each position from the one end to the measurement position. The processing unit calculates the loss value indicating the transmission loss of the multi-core optical fiber when the pulse light moves from the one end to the measurement position in the first core using the light intensity of the first backscattered light corresponding to each position of the first core from the one end to the measurement position.

3. The optical property measurement system according to claim 1 or 2, wherein: When the loss value is set to D 11 , when the integral value is set to D2 [W·m], the arithmetic processing unit uses the loss value represented by the following equation (1) and the integral value represented by the following equation (2) to calculate the ratio represented by the following equation (3), D 11 =exp(-αz L )…(1) The position of the multi-core optical fiber along the length direction when the one end is taken as the origin is set as z[m], and the light intensity of the second backscattered light corresponding to the position z[m] is set as P bs2 (z)[W], the measurement position is set to z L [m], and the transmission loss coefficient of the multi-core optical fiber is set to α[ / m].

4. The optical property measurement system according to claim 3, wherein: The arithmetic processing unit calculates the backscatter crosstalk index by multiplying the ratio represented by the equation (3) by a coefficient A1 [ / (W·m)] represented by the following equation (4) or a coefficient B1 [ / (W·m)] represented by the following equation (5), The light intensity of the first backscattered light corresponding to the one end is set to P bs1 (0) [W], the light intensity of the pulse light is set to P0 [W], and the group velocity of the pulse light when moving in the multi-core optical fiber is set to V g [m / s], the pulse width of the pulse light is set to W [s], the scattering cross-sectional area of ​​the multi-core optical fiber is set to S, and the transmission loss coefficient of the multi-core optical fiber caused by Rayleigh scattering is set to α R [ / m].

5. The optical property measurement system according to claim 2, wherein: When the transmission loss is set to D 12 [W 1 / 2 ], when the integral value is set to D2 [W·m], the arithmetic processing unit uses the loss value represented by the following equation (6) and the integral value represented by the following equation (7) to calculate the ratio represented by the following equation (8), The position of the multi-core optical fiber along the length direction when the one end is taken as the origin is set to z[m], and the light intensity of the first backscattered light corresponding to the position z[m] is set to P bs1 (z)[W], the light intensity of the second backscattered light corresponding to the position z[m] is set to P bs2 (z)[W], let the measurement position be z L [m].

6. The optical property measurement system according to claim 5, wherein: The arithmetic processing unit multiplies the ratio represented by the equation (8) by a coefficient A2[ / (W 1 / 2 ·m)] or the coefficient B2[ / (W 1 / 2 m)] to calculate the backscatter crosstalk index, The light intensity of the first backscattered light corresponding to the one end is set to P bs1 (0) [W], the light intensity of the pulse light is set to P0 [W], and the group velocity of the pulse light when moving in the multi-core optical fiber is set to V g [m / s], the pulse width of the pulse light is set to W [s], the scattering cross-sectional area of ​​the multi-core optical fiber is set to S, and the transmission loss coefficient of the multi-core optical fiber caused by Rayleigh scattering is set to α R [ / m].

7. The optical property measurement system according to any one of claims 2 to 6, wherein: The optical property measurement system further includes an optical system that guides the pulse light from the light source to the first core and guides the first backscattered light from the first core and the second backscattered light from the second core to the optical receiver.

8. The optical property measurement system according to any one of claims 1 to 7, wherein: The optical characteristic measurement system further includes a display unit configured to display a double logarithmic graph indicating a relationship between a position of the multi-core optical fiber along the longitudinal direction and the backscatter crosstalk index.

9. An optical property measurement method, using an optical property measurement system, wherein the optical property measurement system measures optical properties of a multi-core optical fiber having at least a first core and a second core, the optical property measurement method comprising: A step of causing pulsed light to be incident on the first core from one end of the multi-core optical fiber; A step of receiving, by an optical receiver, at least the second backscattered light of the first backscattered light and the second backscattered light emitted from the first core and the second core at the one end respectively as the pulse light is incident on the first core; as well as a step of calculating a backscatter crosstalk index based at least on the light intensity of the second backscattered light, the backscatter crosstalk index representing backscatter crosstalk associated with the light intensity of the second backscattered light that moves from the first core to the second core as part of the pulsed light and returns to the one end, The step of calculating the backscatter crosstalk index comprises: A step of acquiring the light intensity of the second backscattered light at each time received by the light receiver corresponding to each position from the one end to an arbitrary measurement position along the length direction of the multi-core optical fiber; as well as A step of calculating a term including a ratio of an integral value and a loss value as the backscatter crosstalk index, wherein the loss value represents the transmission loss of the multi-core optical fiber when the light moves from the one end to the measurement position in the first fiber core, and the integral value is obtained by integrating the light intensity of the second backscattered light corresponding to each position of the second fiber core from the one end to the measurement position.

Images