Electric field distribution fluctuation period measuring method and electric field distribution fluctuation period measuring device

Abstract

The purpose of the present invention is to provide an electric field distribution fluctuation cycle measuring method and an electric field distribution fluctuation cycle measuring device capable of estimating the fluctuation cycle of an electric field distribution in an optical fiber in a short time.　The electric field distribution fluctuation cycle measuring device according to the present invention measures the fluctuation cycle of an electric field distribution in an arbitrary higher-order mode propagating in an optical fiber 51, and comprises: a test light injector 11 which converts pump light and probe light having a predetermined optical frequency difference from the pump light into the arbitrary higher-order mode, and injects the pump light into one end of the optical fiber 51 and injects the probe light into the other end of the optical fiber 51; and a gain analyzer 12 which acquires, in a distributed manner in the longitudinal direction of the optical fiber 51, the Brillouin gain of induced Brillouin scattering generated when the pump light and the probe light collide in the optical fiber 51, and sets the fluctuation cycle of the Brillouin gain in the longitudinal direction of the optical fiber 51 as the fluctuation cycle of the electric field distribution in the arbitrary higher-order mode.

Description

[Technical field]

[0001] The present disclosure relates to a method and an apparatus for measuring the longitudinal fluctuation period of the electric field distribution of a higher-order mode propagating through an optical fiber. [Background technology]

[0002] In recent years, with the rapid increase in transmission traffic, mode division multiplexing (MDM) communication using few-mode optical fiber (FMF) in which multiple spatial modes propagate has been attracting attention. In MDM communication, multiplexing is performed in multiple modes, but communication becomes difficult if the mode-dependent loss (MDL) is large. Therefore, it is important to evaluate the loss for each mode that occurs in the FMF transmission line. In addition, since loss occurs at the connection points between optical fibers in optical fiber transmission, it is necessary to evaluate the loss for each mode distributed along the optical fiber in order to evaluate the quality of each connection point after the transmission line is constructed.

[0003] The loss that occurs at the splice point depends on the electric field distribution of each propagation mode propagating through the optical fiber. Here, in the case of an FMF splice point, the electric field distribution of the fundamental mode is always constant during propagation, so the loss at the splice point is also constant. On the other hand, 11 In the case of higher modes such as the 10000 mode, the electric field distribution fluctuates during propagation, so the loss experienced changes even with the same connection. Therefore, when evaluating the quality of a connection point, it is desirable to be able to measure the loss after grasping the electric field distribution when passing through each connection point.

[0004] So far, a method for acquiring Brillouin gain in the cross-sectional direction of an optical fiber has been proposed as a method for acquiring information on the electric field distribution in an FMF (Non-Patent Document 1). In this method, an image sensor is used to measure the Brillouin gain in the cross-sectional direction of an optical fiber that occurs when a pump light incident from one side of the FMF and a probe light incident from the other side collide in the FMF. This method makes it possible to acquire a cross-sectional gain corresponding to the overlap of the electric field distributions of the pump light and the probe light in the FMF. Here, since the electric field distributions of the pump light and the probe light fluctuate when propagating through the FMF, what can be acquired from the cross-sectional gain is the relative relationship between the electric field distributions of the pump light and the probe light due to propagation through the optical fiber. By measuring the amount of this gain in a distributed manner in the longitudinal direction, the fluctuation period of the electric field distribution due to propagation can be acquired, and the electric field distribution in the optical fiber can be estimated from this fluctuation period and the electric field distribution of the incident higher-order mode. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] T. Oda et al., Proc. 7th APOS, Wed_8 (2018). Summary of the Invention [Problem to be solved by the invention]

[0006] However, since the measurement in Non-Patent Document 1 uses an image sensor, it takes a long time to measure the longitudinal direction of the optical fiber due to frame rate limitations, making it difficult to estimate the fluctuation period of the electric field distribution in the optical fiber in a short period of time.

[0007] Therefore, in order to solve the above problem, an object of the present invention is to provide an electric field distribution fluctuation period measuring method and an electric field distribution fluctuation period measuring device that can estimate the fluctuation period of the electric field distribution in an optical fiber in a short period of time. [Means for solving the problem]

[0008] In order to achieve the above object, the electric field distribution fluctuation period measurement method according to the present invention measures the fluctuation period of the electric field distribution of any higher-order mode propagating through an optical fiber by utilizing the fact that the difference (deviation) between the electric field distributions of the pump light and the probe light that generate stimulated Brillouin scattering light appears in the Brillouin gain.

[0009] Specifically, the electric field distribution fluctuation period measuring method according to the present invention is a method for measuring a fluctuation period of an electric field distribution of an arbitrary higher-order mode propagating through an optical fiber, comprising the steps of: converting pump light and probe light having a predetermined optical frequency difference from the pump light into the arbitrary higher-order mode; injecting the pump light into one end of the optical fiber and the probe light into the other end of the optical fiber; Obtaining a Brillouin gain of stimulated Brillouin scattering generated when the pump light and the probe light collide in the optical fiber in a distributed manner in the longitudinal direction of the optical fiber; and The variation period of the Brillouin gain in the longitudinal direction of the optical fiber is set to the variation period of the electric field distribution of the arbitrary higher order mode. Do the following.

[0010] Further, an electric field distribution fluctuation period measurement device according to the present invention is an electric field distribution fluctuation period measurement device for measuring a fluctuation period of an electric field distribution of an arbitrary higher order mode propagating through an optical fiber, comprising: a mode splitter that converts pump light and probe light having a predetermined optical frequency difference from the pump light into the arbitrary higher-order mode and inputs the pump light into one end of the optical fiber and the probe light into the other end of the optical fiber; a gain analyzer that acquires a Brillouin gain of stimulated Brillouin scattering generated when the pump light and the probe light collide in the optical fiber in a distributed manner in the longitudinal direction of the optical fiber, and determines a fluctuation period of the Brillouin gain in the longitudinal direction of the optical fiber as a fluctuation period of an electric field distribution of the arbitrary higher order mode; Equipped with.

[0011] This electric field distribution fluctuation period measuring method and device does not use an image sensor, but receives stimulated Brillouin scattered light with a photoelectric conversion element, so high-speed measurement is possible. Therefore, the present invention can provide an electric field distribution fluctuation period measuring method and electric field distribution fluctuation period measuring device that can estimate the fluctuation period of the electric field distribution in an optical fiber in a short time.

[0012] The electric field distribution fluctuation period measuring method and device according to the present invention are characterized in that the acquired Brillouin gain is the gain efficiency of the fundamental mode of the excited acoustic wave components. The polarization dependence of the Brillouin gain can be reduced, and fluctuations in the electric field distribution of the pump light and the probe light can be easily detected.

[0013] The above-mentioned inventions can be combined as much as possible. The electric field distribution fluctuation period measuring device according to the present invention can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. Effect of the Invention

[0014] The present invention can provide an electric field distribution fluctuation period measuring method and an electric field distribution fluctuation period measuring device that can estimate the fluctuation period of the electric field distribution in an optical fiber in a short time. [Brief description of the drawings]

[0015] [Figure 1] 1A and 1B are diagrams for explaining the Brillouin gain spectrum caused by differences in the electric field distribution of the propagation modes. [Diagram 2] 1A to 1C are diagrams illustrating the concept of an electric field distribution fluctuation period measuring method according to the present invention. [Diagram 3] 1 is a diagram illustrating an electric field distribution fluctuation period measuring device according to the present invention; [Figure 4] 1A to 1C are diagrams illustrating a method for measuring an electric field distribution fluctuation period according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0017] [Summary of the invention] In this invention, the Brillouin gain spectrum (BGS) of stimulated Brillouin scattering light generated in an optical fiber is measured. The observed BGS is determined by the overlap between the pump light, the probe light, and each acoustic mode excited by them. In particular, in an FMF where higher-order modes propagate, multiple peaks are observed due to multiple excitation of acoustic modes. In this invention, this characteristic is utilized to obtain the fluctuation period of the electric field distribution of the higher-order mode by distributively measuring the spectral shape corresponding to the overlap of the electric field distribution of the pump light and the probe light in the longitudinal direction.

[0018] [BGS of stimulated Brillouin scattering] The observed BGS is shown below.

[0019] The acoustic mode with mode order m (hereafter referred to as L m The gain efficiency G m (Δν) is a Lorentzian function that depends on the frequency difference Δν between the pump light and the probe light as shown below.

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[0020] Finally, the observed BGS is expressed as the sum of the gains due to the excited acoustic modes.

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[0021] [BGS in two-mode optical fiber] As an example, in a two-mode optical fiber, both the pump light and the probe light are LP 11 Consider the case of mode LP 11 Mode: LP 11a and L.P. 11b The electric field distribution changes with propagation through the optical fiber. Here, both the pump light and the probe light are LP 11a In the case where the pump light and the probe light are LP 11a , L.P. 11b In the case of , the overlap of the electric field distributions is different, and therefore the acoustic mode excited is different.

[0022] Figure 1 shows the LP 11 Figure 1(a) shows an example of BGS in the LP mode. 11a The BGS of the acoustic mode excited when the pump light and the probe light are LP 11a , L.P. 11b The BGS of the acoustic mode excited when 11 It can be seen that the acoustic mode excited differs depending on the electric field distribution of the mode. For example, when both the pump light and the probe light are LP 11a In the case of (the electric field distribution of the pump light and the probe light is the same), the L 01 There exists a mode, but the pump light and the probe light are LP 11a , L.P. 11b In the case of (where the electric field distributions of the pump light and the probe light rotate 90 degrees with the center of the optical fiber as the origin), L is included in the excited acoustic mode. 01 The mode does not exist.

[0023] Therefore, from the observed BGS, it is possible to obtain information on the overlap of the electric field distributions of the pump light and the probe light in the optical fiber. 11 Although the dependence of the BGS on the electric field distribution between modes is shown as an example, in reality, the overlap of the electric field distributions of the pump light and the probe light can be obtained by a similar method between other higher-order modes. Therefore, the present invention is applicable to all LP modes except the fundamental mode.

[0024] [Obtaining the fluctuation period of the electric field distribution of higher modes due to propagation] When obtaining information on the overlap of the electric field distributions of the pump light and the probe light, any acoustic mode of the BGS may be used. Here, we use the L 01 As mentioned above, from Fig. 1(a) and (b), the relative relationship between the electric field distribution of the pump light and the probe light gives the L 01 The peak state of the mode changes. The present invention utilizes this characteristic, 01The change in gain at the optical frequency difference ν1 (optical frequency difference between the pump light and the probe light) at the mode peak is obtained in the longitudinal direction. Figure 2 shows a conceptual diagram of obtaining the relative change in the electric field distribution of the pump light and the probe light due to propagation from BGS. As shown in Figure 2, the gain at the optical frequency difference ν1 increases at the point where the electric field distributions of the pump light and the probe light are identical, while the gain decreases when the electric field distributions are orthogonal. By obtaining the gain at this optical frequency difference ν1 distributed along the longitudinal direction of the optical fiber, it is possible to obtain the change in the electric field distribution due to the propagation of the pump light and the probe light in the optical fiber.

[0025] In addition, L 01 The use of modes has the following advantages: In actual measurements, it may be difficult to obtain the variation in the electric field distribution due to the polarization dependence of the Brillouin gain. For example, L 02 Consider the case where the gain due to the mode is measured by the optical frequency difference (optical frequency difference between the pump light and the probe light). 11a Mode and LP 11a The gain generated in the mode is L 02 Mode and L 21a It depends on the mode, but LP 11a Mode and LP 11b The gain generated in the mode is L 21b Thus, the gain generated is due to the LP 11 In addition, the Brillouin gain differs depending on the polarization of the pump light and the probe light, so even if the fluctuation in the gain of the acoustic wave component is observed, it can be difficult to distinguish whether it is due to a difference in polarization or a difference in the electric field distribution.

[0026] On the other hand, L 01 When measuring at a frequency where gain due to the LP mode occurs, 11a Mode and LP 11a Gain occurs in the LP mode. 11a Mode and LP 11b No gain is generated in this mode. 01By using the L mode (the optical frequency difference between the pump light and the probe light) 01 This has the advantage that, by setting the optical frequency difference such that a mode is generated, the effects of differences in polarization can be reduced and the fluctuations in the electric field distribution of the pump light and the probe light can be easily obtained.

[0027] [BGS measurement procedure] An example of the electric field distribution fluctuation period measurement method of the present invention is shown below. In this method, in order to obtain the overlap of the electric field distributions of the pump light and the probe light, it is sufficient to input the pump light from one side of the optical fiber and the probe light from the other side and obtain the BGS. As a measurement method for inputting light from both ends of the optical fiber and obtaining the BGS distributed in the longitudinal direction, Brillouin optical time domain analysis (BOTDA) and Brillouin optical correlation domain analysis (BOCDA) have been proposed, but in the present invention, any measurement method is acceptable as long as the BGS can be measured. An embodiment using BOTDA will be described below.

[0028] (Example) Measurement of stimulated Brillouin scattering light by counterpropagation of pump light and probe light In this embodiment, an example will be described in which pump light and probe light with different frequencies are input from both ends of an optical fiber, and the BGS of stimulated Brillouin scattering light generated by the collision of the lights is measured. 3 is a diagram illustrating an electric field distribution fluctuation period measuring device according to the present embodiment. This device is an electric field distribution fluctuation period measuring device that measures the fluctuation period of the electric field distribution of an arbitrary higher order mode propagating through an optical fiber 51, a test light injector 11 that converts pump light and probe light having a predetermined optical frequency difference from the pump light into the arbitrary higher mode and injects the pump light into one end of an optical fiber 51 and the probe light into the other end of the optical fiber 51; a gain analyzer (12) that acquires the Brillouin gain of stimulated Brillouin scattering generated when the pump light and the probe light collide in the optical fiber (51) in a distributed manner in the longitudinal direction of the optical fiber (51), and determines the fluctuation period of the Brillouin gain in the longitudinal direction of the optical fiber (51) as the fluctuation period of the electric field distribution of the arbitrary higher-order mode; Equipped with.

[0029] 4 is a flow chart for explaining an electric field distribution fluctuation period measuring method performed by the present device. This method is an electric field distribution fluctuation period measuring method for measuring the fluctuation period of the electric field distribution of any higher order mode propagating through the optical fiber 51, Converting pump light and probe light having a predetermined optical frequency difference from the pump light into the arbitrary higher-order mode (step S01); Injecting the pump light into one end of the optical fiber and the probe light into the other end of the optical fiber (step S02); A Brillouin gain of stimulated Brillouin scattering generated when the pump light and the probe light collide in the optical fiber is obtained in a distributed manner in a longitudinal direction of the optical fiber (step S03); and A fluctuation period of the Brillouin gain in the longitudinal direction of the optical fiber is set as a fluctuation period of the electric field distribution of the arbitrary higher-order mode (step S04). Do the following.

[0030] (Steps S01 and S02) In Fig. 3, the light output from the laser light generating means 21 that generates coherent light is converted to a desired wavelength by the wavelength tuning means, and then split into two by the splitting element 22. In this case, if the laser light generating means 21 is capable of controlling the wavelength, the wavelength tuning means is not necessary. One of the split lights is the pump light L pm After being pulsed by the pulse generator 23, the light is converted from the fundamental mode to the propagation mode of the object to be measured by the mode multiplexing / demultiplexing means 25 and is input to the optical fiber 51 to be measured.

[0031] The other branched light is the probe light L pbA frequency difference of about 10 to 11 GHz, which corresponds to the Brillouin frequency shift corresponding to the combination of the modes of the pump light and the probe light incident on the measured optical fiber 51, is given by the optical frequency controller 24. The optical frequency control means 24 is, for example, an external modulator such as an SSB modulator made of LiNb3. It is also possible to use two lasers with different frequencies (wavelengths) without using the optical frequency control means 24, and control the optical frequency difference between the two lasers by using separate light sources for the pump light and the probe light. The probe light L with the frequency difference given pb is converted to a propagation mode by the mode multiplexing / demultiplexing means 26 in the same manner as the pump light, and the pump light L pm The light is then incident on the optical fiber 51 to be measured from the opposite direction.

[0032] (Step S03) In the optical fiber 51 to be measured, the pump light L pm and the probe light L pb The collision of the probe light L pb Brillouin gain occurs at the pump light L pm The probe light L amplified by pb is sent to the photoelectric converter 31 by the optical circulator 30. The photoelectric converter 31 converts the probe light L pb The intensity of the reflected light is converted into an electrical signal by an A / D converter 32, which then converts the signal into digital data. A data extraction unit 33 and a gain analysis unit 34 analyze the Brillouin gain from this data.

[0033] The specific analysis method of the Brillouin gain is as follows. pm When no probe light L is incident pb Then, the reference intensity of the pump light L pm and the probe light L pb First, the signal intensity when the pump light and the probe light are incident is obtained. Next, the Brillouin gain is obtained from the difference between the signal intensity and the reference intensity. By repeating this process while changing the optical frequency difference between the pump light and the probe light, a BGS corresponding to the overlap of the electric field distributions of the pump light and the probe light, as shown in Figure 1, can be obtained.

[0034] (Step S04) In addition, the above-mentioned BGS can be measured in a distributed manner in the longitudinal direction of the optical fiber 51. Therefore, the data extraction unit 33 extracts the mode of an arbitrary acoustic wave component (for example, the fundamental mode L 01 ) (for example, the fundamental mode L that appears on the side of the lowest optical frequency difference of the BGS) 01 ), gain analyzer 34 analyzes the fluctuation of the Brillouin gain efficiency of the relevant mode in the longitudinal direction of optical fiber 51 (obtaining a waveform as shown in FIG. 2). Gain analyzer 34 regards the period of the fluctuation of the Brillouin gain efficiency as the fluctuation period of the electric field distribution of the propagation mode converted by the mode multiplexing / demultiplexing means (25, 26).

[0035] [Note] The measurement method and device of this embodiment are described below. (the purpose) An object of the present invention is to provide an electric field distribution fluctuation period measuring method capable of quickly measuring the period of fluctuation of the electric field distribution of a higher order mode propagating in an optical fiber through which multiple modes propagate.

[0036] (means) (1) This method is a method for measuring a period of fluctuation in an electric field distribution of a higher mode propagating in an optical fiber, comprising the steps of: The Brillouin gain spectrum of stimulated Brillouin scattering light generated when a probe light incident from one end of an optical fiber and a pump light incident from the other end collide in the optical fiber depends on the overlap of the electric field distributions of the pump light and the probe light. The present invention is characterized by comprising a fluctuation period evaluation means for measuring the fluctuation period of the electric field distribution that changes due to propagation from the longitudinal distribution of the Brillouin gain spectrum obtained in the optical fiber under test.

[0037] (2) In the method of (1) above, the difference between the components of the acoustic wave excited when the electric field distributions of the pump light and the probe light completely overlap and when they are perpendicular to each other is utilized, and the fluctuation period of the electric field distribution is measured from the longitudinal distribution of the gain efficiency of the acoustic wave components that depends on the overlap of the electric field distributions.

[0038] (3) The method according to (1) above is characterized in that the fluctuation period of the electric field distribution is measured by measuring the gain efficiency of the fundamental mode of the excited acoustic wave in a distributed manner in the longitudinal direction.

[0039] (4) This device is A pump light and a probe light generating means for inputting the pump light and the probe light into the optical fiber to be measured; an arbitrary mode input means for inputting the pump light and the probe light in an arbitrary propagation mode into the target optical fiber; a spectrum measuring means for measuring the Brillouin gain spectrum of stimulated Brillouin scattering light generated by the pump light and the probe light in the test optical fiber; a spectrum distribution measuring means for acquiring the spectrum obtained by the spectrum measuring means in a distributed manner in the longitudinal direction; an electric field distribution fluctuation period analysis means for acquiring a fluctuation period of the electric field distribution of a mode propagating through an optical fiber from the spectral distribution obtained by the spectral distribution measurement means; The present invention is characterized by comprising:

[0040] (effect) In this invention, the Brillouin gain spectrum of stimulated Brillouin scattering light generated by pump light and probe light in the FMF is measured by a photodetector. Since this spectrum depends on the overlap of the electric field distribution of the pump light and the probe light, the fluctuation period of the electric field distribution of the mode propagating in the optical fiber can be measured by measuring it in a distributed manner in the longitudinal direction. In addition, this invention enables faster measurement than the conventional measurement method using an image sensor. [Explanation of symbols]

[0041] 11: Test light injector 12: Gain analyzer 21: Laser light generating means 22: Branching element 23: Pulse generator 24: Optical frequency control means 25, 26: Mode multiplexing / demultiplexing means 30: Optical circulator 31: Photoelectric converter 32: A / D converter 33: Data extraction section 34: Gain analysis section

Claims

1. 1. A method for measuring a fluctuation period of an electric field distribution in a high-order mode propagating in an optical fiber, comprising the steps of: converting pump light into one of the higher-order modes and converting probe light having a predetermined optical frequency difference from the pump light into the other higher-order mode; Injecting the pump light into one end of the optical fiber and the probe light into the other end of the optical fiber; receiving, in the higher-order mode, the probe light, which passes through the optical fiber and is output from one end of the optical fiber and includes stimulated Brillouin scattering generated when the pump light and the probe light collide within the optical fiber; Obtaining a Brillouin gain of a desired acoustic wave mode from the stimulated Brillouin scattering in a distributed manner in the longitudinal direction of the optical fiber; and The variation period of the Brillouin gain in the longitudinal direction of the optical fiber is set to the variation period of the electric field distribution of the higher order mode. A method for measuring the period of electric field distribution fluctuations.

2. 2. The method for measuring the period of fluctuation of an electric field distribution according to claim 1, wherein the desired acoustic wave mode is a fundamental mode.

3. An electric field distribution fluctuation period measuring device for measuring a fluctuation period of overlap of electric field distributions between higher-order modes propagating in an optical fiber, comprising: a mode multiplexer / demultiplexer that converts pump light into one of the higher-order modes and converts a probe light having a predetermined optical frequency difference from the pump light into the other higher-order mode, and inputs the pump light into one end of the optical fiber and the probe light into the other end of the optical fiber; an opto-electrical converter that receives, in the higher-order mode, the probe light, which passes through the optical fiber and is output from one end of the optical fiber and includes stimulated Brillouin scattering that occurs when the pump light and the probe light collide within the optical fiber; a gain analyzer that acquires a Brillouin gain of a desired acoustic wave mode from the stimulated Brillouin scattering in a distributed manner in the longitudinal direction of the optical fiber, and determines a fluctuation period of the Brillouin gain in the longitudinal direction of the optical fiber as a fluctuation period of the electric field distribution of the higher-order mode; An electric field distribution fluctuation period measuring device comprising:

4. 4. The electric field distribution fluctuation period measuring device according to claim 3, wherein the gain analyzer determines a fundamental mode of the excited acoustic wave components as the desired acoustic wave mode.

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