Optical characteristic measurement device and method for measuring optical characteristics

Abstract

To provide an optical characteristic measurement device and a method for measuring optical characteristics capable of accurately acquiring a complex electric field intensity of signal light to be measured output from each core of a multicore optical fiber.SOLUTION: An optical characteristic measurement device 301 according to the present invention inputs repeatedly modulated signal light 21 to be measured, into a multicore optical fiber under test 31, extracts the signal light 21 to be measured output from each core of the multicore optical fiber under test 31 at individual positions (P1, P2) in a space 30, causes each extracted signal light to be measured to individually interfere, at the individual positions, with sampling pulse light having the same wavelength as that of the signal light to be measured and a different repetition period, and observes the spatial distribution of each interference light 37 with a two-dimensional PDA 37.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an optical characteristic measuring device and a measuring method for measuring the electric field distribution of a multi-core optical fiber. [Background technology]

[0002] A space division multiplexing transmission system using a multi-core optical fiber has been developed. In a multi-core optical fiber, signals are multiplexed into multiple cores for transmission. The signals propagating through the multi-core optical fiber are separated by a fan-in / out device and coupled to separate single-mode fibers, and then their complex electric field amplitudes are received.

[0003] When analyzing optical characteristics such as inter-core interference in a multi-core optical fiber itself, it is necessary to directly observe the change in spatial distribution of the complex electric field output from each core in the cross section of the multi-core optical fiber at the signal speed of optical communication (up to 100 Gbaud).For example, Non-Patent Document 1 discloses an optical signal measurement method for analyzing the complex electric field in the cross section of a multi-mode optical fiber that can keep up with the signal speed of optical communication (up to 100 Gbaud). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] S.Okamura, K.Osawa, C.Zhang, F.Ito, A.Nakamura, and Y.Koshikiya, “Ultrafast measurement of vector spatial modes by using two-dimensional linear optical sampling,” Opt. Lett., vol. 48, no. 10, pp. 2551-2554, May 15 2024, doi: 10.1364 / OL.490009. Summary of the Invention [Problem to be solved by the invention]

[0005] In Non-Patent Document 1, a repeatedly modulated signal light to be measured is input to a multimode optical fiber under test, and the signal light to be measured output from the multimode optical fiber under test is extracted into space and made to interfere with sampling pulse light having a different repetition period from the signal light to be measured, and the interference light is observed with a two-dimensional photodetector array.

[0006] However, unlike a multimode optical fiber, the core structure of a multicore optical fiber is not uniform in the circumferential direction, so the signal light to be measured output from each core may not necessarily be imaged on each element of the photodetector array.In addition, it is difficult to make the intensity of the sampling pulse light that interferes with the signal light to be measured output from each core uniform in the interference section.

[0007] That is, when observing an optical signal output from a multi-core optical fiber, there is a positional misalignment between each core of the multi-core optical fiber and each element of the photodetector array, and there is also a variation in the light intensity at the interference part of the sampling pulse light, and therefore, the configuration of Non-Patent Document 1 has a problem that it is difficult to correctly acquire the complex electric field intensity of the signal light to be measured output from each core.

[0008] In order to solve the above-mentioned problems, an object of the present invention is to provide an optical characteristic measuring device and a measuring method that can correctly acquire the complex electric field intensity of the signal light to be measured output from each core of a multi-core optical fiber. [Means for solving the problem]

[0009] In order to achieve the above object, the optical characteristic measuring device according to the present invention extracts the signal light to be measured output from each core individually into space, then causes each of the signal light to interfere with sampling pulse light, and then couples each of the interference lights to an element set for each core in a two-dimensional photodetector array.

[0010] Specifically, the optical property measuring device according to the present invention comprises: an optical branching unit that extracts, at individual spatial positions, repeatedly modulated signal light to be measured, which is input to any core at one end of the multi-core optical fiber under test and output from each of the cores at the other end of the multi-core optical fiber under test; a pulse generator that generates sampling pulse light having the same wavelength as the measured signal light but a different repetition period; an interference unit that individually causes interference between the signal light to be measured and the sampling pulse light at the individual positions; a two-dimensional photodetector array in which one or more photodetectors for each of the cores are arranged two-dimensionally and which receives interference light between the signal light to be measured and the sampling pulse light for each of the cores; a calculation unit that acquires, from the interference light, an electric field amplitude of the signal light to be measured that has passed through the multi-core optical fiber to be tested for each of the cores; an adjuster that adjusts the position of at least one of the other end of the multi-core optical fiber under test and the two-dimensional photodetector array so that each of the interference lights is received by one or more photodetectors for each of the cores; Equipped with.

[0011] Further, the optical property measuring method according to the present invention includes: inputting a repeatedly modulated signal light to be measured into an arbitrary core at one end side of the multi-core optical fiber to be tested; extracting the signal light to be measured output from each of the cores at the other end side of the multi-core optical fiber under test to individual positions in space; generating sampling pulse light having the same wavelength as the measured signal light but a different repetition period; causing the signal light to be measured and the sampling pulse light to interfere with each other at the individual positions; receiving, for each core, interference light between the signal light to be measured and the sampling pulse light using a two-dimensional photodetector array in which one or more photodetectors for each core are arranged two-dimensionally; acquiring an electric field amplitude of the signal light to be measured that has passed through the multi-core optical fiber under test for each core from the interference light; and adjusting the position of at least one of the other end of the multi-core optical fiber under test and the two-dimensional photodetector array so that each of the interference lights is received by one or more photodetectors for each of the cores; It is characterized by:

[0012] This optical property measuring device and method can adjust the relative position between each core of a multi-core optical fiber and a photodetector in a two-dimensional photodetector array using an adjuster, so that the signal light to be measured output from each core can be imaged on each photodetector in the two-dimensional photodetector array.

[0013] Furthermore, since the optical characteristic measuring device and measuring method prepare a sampling pulse light for each signal light to be measured output from the core, it is possible to cause sampling pulse light of the same intensity to interfere with all signal light to be measured output from the core.

[0014] In other words, the optical characteristic measuring device and the optical characteristic measuring method can eliminate the positional misalignment between each core of the multi-core optical fiber and each element of the photodetector array, and the variation in light intensity at the interference part of the sampling pulse light. Therefore, the present invention can provide an optical characteristic measuring device and an optical characteristic measuring method that can correctly acquire the complex electric field intensity of the signal light to be measured output from each core of the multi-core optical fiber.

[0015] The optical characteristic measuring device (measurement method) according to the present invention may further include a polarization separating unit that separates the interference light into polarizations and causes the two-dimensional photodetector array to receive the interference light for each polarization, thereby making it possible to obtain complex electric field intensity for each polarization component of the signal light to be measured.

[0016] The above inventions can be combined as much as possible. [Effects of the Invention]

[0017] The present invention can provide an optical characteristic measuring device and a measuring method that can correctly acquire the complex electric field intensity of the signal light to be measured output from each core of a multi-core optical fiber. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a diagram illustrating an optical property measuring device according to the present invention. [Figure 2] FIG. 1 is a diagram illustrating a linear optical sampling method. [Figure 3] 1 is a diagram illustrating an optical property measuring device according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components. In addition, although the present embodiment describes a case where the multi-core optical fiber under test has two cores, the multi-core optical fiber under test is not limited to having two cores. By changing the structure of the optical characteristic measuring device (the number of elements in the interference unit and the two-dimensional photodetector), it is possible to measure a multi-core optical fiber under test with three or more cores.

[0020] (Embodiment 1) FIG. 1 is a diagram illustrating an optical characteristic measuring apparatus 301 according to this embodiment. The optical characteristic measuring apparatus 301 includes: an optical branching device (lens 32) for extracting the repeatedly modulated signal light to be measured 21, which is input to any core at one end of the multi-core optical fiber 31 under test and output from each of the cores at the other end of the multi-core optical fiber 31 under test, to individual positions (P1, P2) in the space 30; a pulse generator (mode-locked laser 33) that generates sampling pulse light 22 having the same wavelength as the measured signal light 21 but a different repetition period; an interference unit 43 that individually causes interference between the signal light 21 to be measured and the sampling pulse light 22 at individual positions (P1, P2); a two-dimensional photodetector array 37 in which one or more photodetectors for each of the cores are arranged two-dimensionally and which receives interference light 27 between the signal light 21 to be measured and the sampling pulse light 22 for each of the cores; a calculation unit (44) for acquiring the electric field amplitude of the signal light (21) to be measured transmitted through the multi-core optical fiber (31) to be tested for each core from the interference light (27); an adjuster (41, 42) for adjusting the position of at least one of the other end of the multi-core optical fiber under test (31) and the two-dimensional photodetector array (37) so that each interference light (21) is received by one or more photodetectors for each of the cores; Equipped with.

[0021] The optical characteristic measuring device 301 measures the electric field amplitude of the signal light 21 to be measured that has passed through the cores of the multi-core optical fiber 31 to be tested as follows. First, the repeatedly modulated signal light under test 21 is input to any core at one end of the multi-core optical fiber under test 31. Here, the signal light under test 21 may be input to each core individually, or may be input to all the cores at once. The core to which the signal light under test 21 is input can be changed depending on the phenomenon to be observed.

[0022] Subsequently, the signal light to be measured 21 output from each core at the other end of the multi-core optical fiber 31 under test is extracted to individual positions (P1, P2) in the space 30. For example, an optical system such as a lens 32 is used to separate the light output from each core in the space 30 so that the light does not overlap.

[0023] Meanwhile, a pulse generator (mode-locked laser 33) or the like generates sampling pulse light 22 having the same wavelength as the measured signal light 21 but a different repetition period. Figure 2 shows an example of the relationship between the measured signal light 21 and the sampling pulse light 22. The measured signal light 21 has an arbitrary wavelength λ and a waveform that repeats with a period T. The sampling pulse light 22 has the same wavelength λ as the measured signal light 21, but has a pulse waveform that repeats with a period T+ΔT.

[0024] The interference unit 43 splits the sampling pulse light 22 into the same number of beams as the number of cores of the multi-core optical fiber 31 under test after the sampling pulse light 22 passes through an optical system such as a lens 32. Then, at individual positions (P1, P2), the sampling pulse light 22 and the signal light under measurement 21 are individually interfered with each other. By individually interfering in this way, the intensity of the sampling pulse light 22 that is made to interfere with each signal light under measurement 21 can be made uniform.

[0025] A two-dimensional photodetector array (two-dimensional PDA) 37, in which one or more photodetectors (PDs) for each of the cores are arranged two-dimensionally, receives the interference light 27 between the signal light 21 to be measured and the sampling pulse light 22 for each of the cores. In Fig. 1, the cores (interference light 27) and the PDs have a 1:1 relationship, but this relationship is not necessarily required. For example, a 2x2 PDA may be used, and two PDs may receive each of the interference light 27. Furthermore, if the multi-core optical fiber 31 to be tested has four cores, a 4x4 PDA may be used, and four PDs may receive each of the interference light 27.

[0026] Then, the calculation unit 44 obtains the electric field amplitude of the signal light 21 to be measured that has passed through the multi-core optical fiber 31 to be tested for each of the cores from the interference light 27. This optical signal measurement device (method) uses a linear sampling method, which measures the instantaneous complex amplitude of the signal light by coherent correlation detection between a short optical pulse (sampling pulse light 22) with the same wavelength as the signal light and the signal light 21 to be measured. As shown in Figure 2, this method slightly detunes (ΔT) the repetition period T of the signal light 21 to be measured and the period of the sampling pulse light 22, and detects the instantaneous amplitude 23 while shifting the interference point, thereby making it possible to observe the waveform 24 of the entire signal light.

[0027] The speed of the observable signal is determined only by the width (T+ΔT) of the sampling pulse light 22, and it can also accommodate ultra-high-speed signals. The electronic components used only need to be able to track this sampling pulse period, and components with a bandwidth of around several MHz can be used. Although the signal light that can be observed is limited to repetitive signals, it can also accommodate pseudo-random bit sequence (PRBS) signals used to verify communication systems. This method has the advantage of being able to measure the complex amplitude of ultra-high-speed multimode communications using low-speed, inexpensive electronic components.

[0028] When the interference unit 43 causes the measured signal light 21 and the sampling pulse light 22 to interfere with each other, it is preferable to adjust the position of the other end of the multi-core optical fiber 31 under test by the adjustment unit 41 and / or the position of the two-dimensional photodetector array 37 by the adjustment unit 42 so that each interference light 27 is received by one or more photodetectors for each of the cores.

[0029] The adjustment units (41, 42) are, for example, rotation mechanisms that adjust the core positions and the positions of each PD of the PDA 37. Deviations in the rotation direction can be corrected. The adjustment units (41, 42) may also be parallel movement mechanisms that adjust the core positions and the positions of each PD of the PDA 37 in the X and Y directions (plane directions perpendicular to the optical axis). The adjustment units (41, 42) enable the signal light (interference light) output from each core to be imaged on each element of the PDA 37.

[0030] (Embodiment 2) Fig. 3 is a diagram illustrating an optical characteristic measuring apparatus 302 according to this embodiment. The optical characteristic measuring apparatus 302 differs from the optical characteristic measuring apparatus 301 in Fig. 1 in that it further includes a polarization separation unit 36 ​​that separates the interference light 27 into polarizations and causes two-dimensional photodetector arrays (37-x, 37-y) to receive the interference light 27 for each polarization. Only the differences between this embodiment and the optical characteristic measuring apparatus 301 will be described.

[0031] The interference light 27 is separated into an x-polarized component and a y-polarized component by the polarizing beam splitter 36. The x- and y-components of the electric field amplitude of each of the separated lights are received by PDAs (37-x and 37-y). Using these signals (f four-channel signals per second in FIG. 3), the complex electric field amplitude of the signal light under test 21 that has propagated through the core can be determined (the electric field amplitude can be measured accurately by the optical characteristic measuring device 301). [Explanation of symbols]

[0032] 21: Signal light under test 22: Sampling pulse light 23: Instantaneous amplitude 24: Waveform 25: Sampling clock 27: Interference light 30: Space 31: Optical fiber under test (multi-core optical fiber) 32: Lens 33: Mode-locked laser 37, 37-x, 37-y: PDA 38: A / D conversion 39: Lens 41, 42, 42-x, 42-y: Adjustment section 43: Interference part 44: Arithmetic section 301, 302: Optical signal measuring device

Claims

1. an optical branching unit that extracts, at individual spatial positions, repeatedly modulated signal light to be measured, which is input to any core at one end of the multi-core optical fiber under test and output from each of the cores at the other end of the multi-core optical fiber under test; a pulse generator that generates sampling pulse light having the same wavelength as the measured signal light but a different repetition period; an interference unit that individually causes interference between the signal light to be measured and the sampling pulse light at the individual positions; a two-dimensional photodetector array in which one or more photodetectors for each of the cores are arranged two-dimensionally and which receives interference light between the signal light to be measured and the sampling pulse light for each of the cores; a calculation unit that acquires, from the interference light, an electric field amplitude of the signal light to be measured that has passed through the multi-core optical fiber to be tested for each of the cores; an adjuster that adjusts the position of at least one of the other end of the multi-core optical fiber under test and the two-dimensional photodetector array so that each of the interference lights is received by one or more photodetectors for each of the cores; An optical property measuring apparatus comprising:

2. 2. The optical characteristic measuring device according to claim 1, further comprising a polarization splitting unit that splits the interference light into polarizations and causes the two-dimensional photodetector array to receive the interference light for each polarization.

3. inputting a repeatedly modulated signal light to be measured into an arbitrary core at one end side of the multi-core optical fiber to be tested; extracting the signal light to be measured output from each of the cores at the other end side of the multi-core optical fiber under test to individual positions in space; generating sampling pulse light having the same wavelength as the measured signal light but a different repetition period; causing the signal light to be measured and the sampling pulse light to interfere with each other at the individual positions; receiving, for each core, interference light between the signal light to be measured and the sampling pulse light using a two-dimensional photodetector array in which one or more photodetectors for each core are arranged two-dimensionally; acquiring an electric field amplitude of the signal light to be measured that has passed through the multi-core optical fiber under test for each core from the interference light; and adjusting the position of at least one of the other end of the multi-core optical fiber under test and the two-dimensional photodetector array so that each of the interference lights is received by one or more photodetectors for each of the cores; An optical property measuring method comprising:

4. 4. The optical characteristic measuring method according to claim 3, wherein the interference light is separated into polarizations, and the interference light is received by the two-dimensional photodetector array for each polarization.

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