Spatial mode dispersion measuring device and spatial mode dispersion measuring method

Abstract

To provide a spatial mode dispersion measuring device and spatial mode dispersion measuring method capable of acquiring optical frequency dependency of spatial mode dispersion of a coupled multi-core fiber without using a reference path of reference light, and performing highly accurate measurement even when an optical fiber to be tested is long.SOLUTION: A spatial mode dispersion measuring device according to the present invention comprises a light beam shape sampling section 30 that measures a broadband impulse response waveform W1 by causing test pulse light propagated through a coupled multi-core optical fiber with reference pulse light, and an arithmetic section 40 that performs: Fourier transforming the waveform W1 to a broadband frequency response waveform W2; cutting out the waveform W2 by a desired optical frequency window as a narrowband frequency response waveform W4; inverse Fourier transforming the waveform W4 to a narrowband impulse response waveform W5 for an optical frequency band of the optical frequency window; and calculating spatial mode dispersion from the width of the waveform W5.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a spatial mode dispersion measurement device and a spatial mode dispersion measurement method for measuring spatial mode dispersion of a coupled multicore fiber. [Background technology]

[0002] Coupled multicore fibers are one of the promising optical fibers for realizing future high-capacity optical communications. Spatial mode dispersion in coupled multicore fibers is an important parameter that directly affects the signal processing load required to restore the transmitted signal received by a photodetector. Therefore, after constructing a transmission line using coupled multicore fibers, it is necessary to test the optical frequency dependence of spatial mode dispersion in the transmission band.

[0003] For example, Non-Patent Document 1 discloses a method for measuring an impulse response waveform using a wavelength sweep method. The spatial mode dispersion can be obtained from the spread of the impulse response waveform obtained by this method. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] M. Mazur et al., “Transfer matrix characterization of field-deployed MCFs,” Proc. ECOC 2020. Summary of the Invention [Problem to be solved by the invention]

[0005] The method of Non-Patent Document 1 uses wavelength-swept light as the test light, and therefore requires measuring the interference signal between the test light and the reference light output from the same light source. To achieve this, a reference path of approximately the same length as the optical fiber under test is required to transmit the reference light from the light source to the optical receiver. Therefore, the method of Non-Patent Document 1 has the problem that, when testing a transmission line installed in a real environment, the measurement accuracy deteriorates due to disturbances received by the reference path, making it difficult to accurately obtain the optical frequency dependence of spatial mode dispersion.

[0006] Therefore, in order to solve the above-mentioned problems, an object of the present invention is to provide a spatial mode dispersion measuring device and a spatial mode dispersion measuring method that can acquire the optical frequency dependence of spatial mode dispersion of a coupled multicore fiber without using a reference path of a reference light, and that can perform high-precision measurements even when the optical fiber under test is long. [Means for solving the problem]

[0007] In order to achieve the above object, the spatial mode dispersion measurement device according to the present invention acquires a wideband impulse response waveform by an optical linear sampling method that does not require a reference path, and acquires the optical frequency dependence of spatial mode dispersion by digital signal processing.

[0008] Specifically, the spatial mode dispersion measuring device according to the present invention comprises: a test light output unit that inputs test pulse light of an arbitrary wavelength at a repetition rate f into one core of the coupled multi-core optical fiber under test; a reference light output unit that generates a reference pulse light having the same wavelength as the test pulse light and a repetition frequency f-Δf different from the frequency f; an optical linear sampling unit that causes interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber to measure a wideband impulse response waveform; a calculation unit that performs a Fourier transform on the wideband impulse response waveform to a wideband frequency response waveform, cuts out the wideband frequency response waveform with a desired optical frequency window to obtain a narrowband frequency response waveform, performs an inverse Fourier transform on the narrowband frequency response waveform to obtain a narrowband impulse response waveform for an optical frequency band of the optical frequency window, and determines the width of the narrowband impulse response waveform as spatial mode dispersion; Equipped with.

[0009] Further, the spatial mode dispersion measuring method according to the present invention comprises: Injecting a test pulse light of an arbitrary wavelength into one core of the coupled multi-core optical fiber under test at a repetition rate of f; generating a reference pulse light having the same wavelength as the test pulse light and a repetition frequency f-Δf different from the frequency f; causing interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber and measuring a wideband impulse response waveform by an optical linear sampling method; Fourier transforming the wideband impulse response waveform into a wideband frequency response waveform; extracting the wideband frequency response waveform in a desired optical frequency window to obtain a narrowband frequency response waveform; inverse Fourier transforming the narrowband frequency response waveform into a narrowband impulse response waveform for an optical frequency band of the optical frequency window; and The width of the narrowband impulse response waveform is set to spatial mode dispersion. Do the following.

[0010] The optical linear sampling method uses broadband pulsed light as the test light, which does not require wavelength sweeping, and can therefore use light output from a light source different from that used for the test light as the reference light. This allows the light source for the reference light to be located on the optical receiver side, eliminating the need for a reference path for transmitting the reference light. This avoids degradation of measurement accuracy due to disturbances in the reference path.

[0011] Therefore, the present invention can provide a spatial mode dispersion measuring device and a spatial mode dispersion measuring method that can acquire the optical frequency dependence of spatial mode dispersion of a coupled multicore fiber without using a reference path of a reference light, and that can perform high-precision measurements even when the optical fiber under test is long.

[0012] It is preferable that the calculation unit of the spatial mode dispersion measurement device according to the present invention performs chromatic dispersion compensation on the wideband frequency response waveform or the narrowband frequency response waveform, thereby reducing the effect of chromatic dispersion and enabling the optical frequency dependence of spatial mode dispersion to be obtained with higher accuracy.

[0013] The spatial mode dispersion measuring device of 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.

[0014] The above inventions can be combined as much as possible. [Effects of the Invention]

[0015] The present invention can provide a spatial mode dispersion measuring device and a spatial mode dispersion measuring method that can acquire the optical frequency dependence of spatial mode dispersion of a coupled multicore fiber without using a reference path of a reference light, and that can perform high-precision measurements even when the optical fiber under test is long. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram illustrating an outline of a spatial mode dispersion measuring method according to the present invention. [Figure 2] 1 is a diagram illustrating a spatial mode dispersion measuring device according to the present invention. [Figure 3] 1 is a flowchart illustrating a spatial mode dispersion measuring method according to the present invention. [Figure 4] FIG. 10 is a diagram illustrating a process for determining spatial mode dispersion. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] [Summary of the Invention] FIG. 1 is a diagram for explaining an outline of a spatial mode dispersion measuring method according to the present invention. (Step 1) Measure the wideband impulse response waveform W1. (Step 2) The wideband impulse response waveform W1 is subjected to a Fourier transform (FFT) to obtain a wideband frequency response waveform W2. (Step 3) The effect of chromatic dispersion is removed from the wideband frequency response waveform W2 (chromatic dispersion compensation) to obtain the wideband frequency response waveform W3. In this step, the wideband frequency response waveform W3 is improved so that it approaches a rectangular shape. (Step 4) Extract the wideband frequency response waveform W3 using an arbitrary optical frequency window to obtain the narrowband frequency response waveform W4. For example, when evaluating a coupled multi-core optical fiber used in a transmission system that multiplexes wavelengths at 100 GHz intervals, extract the 100 GHz range accordingly. (Step 5) The narrowband frequency response waveform W4 is subjected to an inverse Fourier transform to obtain a narrowband impulse response waveform W5 for the extracted optical frequency band. Then, spatial mode dispersion is obtained from the width of the narrowband impulse response waveform W5. By removing the pulse broadening due to chromatic dispersion, the pulse width of the narrowband impulse response waveform W5 is narrower than that of the wideband impulse response waveform W1.

[0019] Note that chromatic dispersion compensation may be performed on the narrowband frequency response waveform W4 instead of the wideband frequency response waveform W2. That is, step 4 may be performed after step 2, and then step 3. Chromatic dispersion compensation will be described later.

[0020] [Embodiment] FIG. 2 is a diagram illustrating a spatial mode dispersion measuring apparatus 301 according to this embodiment. The spatial mode dispersion measuring apparatus 301 includes: a test light output unit 10 that inputs test pulse light of an arbitrary wavelength at a repetition frequency f into one core of the coupled multi-core optical fiber FUT to be tested; a reference light output unit 20 that generates a reference pulse light having the same wavelength as the test pulse light and a repetition frequency f-Δf different from the frequency f; an optical linear sampling unit 30 that causes interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber FUT and measures a wideband impulse response waveform W1; a calculation unit 40 that performs Fourier transform on the wideband impulse response waveform W1 to generate a wideband frequency response waveform W2, cuts out the wideband frequency response waveform W2 with a desired optical frequency window to generate a narrowband frequency response waveform W4, performs inverse Fourier transform on the narrowband frequency response waveform W4 to generate a narrowband impulse response waveform W5 for the optical frequency band of the optical frequency window, and sets the width of the narrowband impulse response waveform W5 to spatial mode dispersion; Equipped with.

[0021] The calculation section 40 preferably performs chromatic dispersion compensation on the wideband frequency response waveform W2 or the narrowband frequency response waveform W4.

[0022] 3 is a diagram illustrating a method for measuring the spatial mode dispersion of a coupled multi-core optical fiber FUT using a spatial mode dispersion measurement device 301. a step S01 of injecting a test pulse light of an arbitrary wavelength into one core of a coupled multi-core optical fiber FUT to be tested at a repetition rate f, generating a reference pulse light having the same wavelength as the test pulse light but a repetition rate f-Δf different from the frequency f, and measuring a wideband impulse response waveform W1 by an optical linear sampling method by causing interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber; Step S02: Fourier transforming the wideband impulse response waveform W1 into a wideband frequency response waveform W2; Step S04: extracting the wideband frequency response waveform W2 with a desired optical frequency window to generate a narrowband frequency response waveform W4; a step S05 of inverse Fourier transforming the narrowband frequency response waveform S04 into a narrowband impulse response waveform W5 for the optical frequency band of the optical frequency window; and Step S06: Set the width of the narrowband impulse response waveform W5 as the spatial mode dispersion Do the following.

[0023] Furthermore, this method preferably includes step S03, which performs chromatic dispersion compensation on the wideband frequency response waveform W2 or the narrowband frequency response waveform W4. Note that Fig. 3 illustrates a case where step S03 is performed after step S02 (i.e., chromatic dispersion compensation is performed on the wideband frequency response waveform W2). This method may also include step S03 between steps S04 and S05 (i.e., chromatic dispersion compensation is performed on the narrowband frequency response waveform W4).

[0024] Each step will be explained in detail. Step S01: At one end of the coupled multi-core optical fiber FUT, the test light output unit 10 inputs test pulse light of an arbitrary wavelength λ and a repetition rate f into one core. Note that the arbitrary wavelength λ is preferably the wavelength of signal light when the coupled multi-core optical fiber FUT is used as a transmission line. The test pulse light propagates through the coupled multi-core optical fiber FUT and is output from the other end of the coupled multi-core optical fiber FUT, and is input to the optical linear sampling unit 30. At the same time, the reference light output unit 20 inputs reference pulse light of an arbitrary wavelength λ and a repetition rate f-Δf to the optical linear sampling unit 30. The optical linear sampling unit 30 includes a polarizing beam splitter PBS, an optical splitter SP, an optical 90° hybrid 90H, a balanced photodiode BPD, and an A / D converter, and measures a wideband impulse response waveform W1 by optical linear sampling by causing interference between the test pulse light and the reference pulse light. Since there is no need to pass the reference pulse light through a coupled multi-core optical fiber FUT or another reference optical fiber, deterioration of measurement accuracy due to disturbances can be prevented.

[0025] Step S02: The impulse response acquisition unit 41 of the calculation unit 40 receives the wideband impulse response waveform W1 from the optical linear sampling unit 30. Then, the frequency analysis unit 43 of the calculation unit 40 performs a Fourier transform on the wideband impulse response waveform W1 in the time domain to obtain a wideband frequency response waveform W2 in the frequency domain.

[0026] Step S03: The chromatic dispersion compensator 42 receives the wideband frequency response waveform W2 from the frequency analyzer 43 and removes the effects of chromatic dispersion to obtain a wideband frequency response waveform W3. The chromatic dispersion compensator 42 passes the wideband frequency response waveform W3 to the frequency analyzer 43. Note that if step S03 is performed after step S04, the chromatic dispersion compensator 42 receives the narrowband frequency response waveform W4a from the frequency analyzer 43 and removes the effects of chromatic dispersion to obtain a narrowband frequency response waveform W4. The chromatic dispersion compensator 42 passes the narrowband frequency response waveform W4 to the frequency analyzer 43.

[0027] A specific method for compensating for chromatic dispersion is as follows. Chromatic dispersion can be compensated for by multiplying the wideband frequency response waveform W2 by the following equation:

number

[0028] It should be noted that if the influence of chromatic dispersion is negligibly small (if the coupled multi-core optical fiber FUT is short), there is no need to perform step S03.

[0029] Step S04: The frequency analysis unit 43 extracts the wideband frequency response waveform W3 using an arbitrary optical frequency window to obtain a narrowband frequency response waveform W4. If step S03 is performed after step S04, the frequency analysis unit 43 extracts the wideband frequency response waveform W2 using an arbitrary optical frequency window to obtain a narrowband frequency response waveform W4a.

[0030] Step S05: The frequency analysis unit 43 performs an inverse Fourier transform on the narrowband frequency response waveform W4 to obtain a narrowband impulse response waveform W5 in the time domain for the extracted optical frequency band.

[0031] Step S06: The spatial mode dispersion calculation unit 44 calculates the spatial mode dispersion SMD from the width of the narrowband impulse response waveform W5. The intensity waveform of the narrowband impulse response waveform W5 as shown in FIG. 4 is defined as I(t). First, the intensity waveform is calculated by the following equation at the center t of I(t). c Detect.

number

number

[0032] The spatial mode dispersion measurement method described in this embodiment is based on the assumption of an optical fiber in which multiple modes are strongly coupled during propagation, such as a coupled multicore fiber. This is because the impulse response waveform differs between a fiber with strong inter-mode coupling and a fiber with weak inter-mode coupling. Therefore, according to the present invention, it is possible to obtain the optical frequency dependence of spatial mode dispersion in an optical fiber with strong inter-mode coupling, including a coupled multicore fiber. [Explanation of symbols]

[0033] 10: Test light output section 20: Reference light output unit 30: Optical linear sampling unit 40: Arithmetic section 41: Impulse response acquisition unit 42: Chromatic dispersion compensation section 43: Frequency analysis section 44: Spatial mode dispersion calculation unit 301: Spatial mode dispersion measuring device

Claims

1. a test light output unit that inputs test pulse light of an arbitrary wavelength into one core of a coupled multi-core optical fiber under test at a repetition rate f; a reference light output unit that generates a reference pulse light having the same wavelength as the test pulse light and a repetition frequency f-Δf different from the frequency f; an optical linear sampling unit that causes interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber to measure a wideband impulse response waveform; a calculation unit that performs a Fourier transform on the wideband impulse response waveform to a wideband frequency response waveform, cuts out the wideband frequency response waveform with a desired optical frequency window to obtain a narrowband frequency response waveform, performs an inverse Fourier transform on the narrowband frequency response waveform to obtain a narrowband impulse response waveform for an optical frequency band of the optical frequency window, and determines the width of the narrowband impulse response waveform as spatial mode dispersion; A spatial mode dispersion measuring apparatus comprising:

2. 2. The spatial mode dispersion measuring apparatus according to claim 1, wherein the calculation unit performs chromatic dispersion compensation on the wideband frequency response waveform or the narrowband frequency response waveform.

3. Injecting test pulse light of an arbitrary wavelength into one core of the coupled multi-core optical fiber to be tested at a repetition rate f; generating a reference pulse light having the same wavelength as the test pulse light and a repetition frequency f-Δf different from the frequency f; causing interference between the test pulse light and the reference pulse light propagated through the coupled multi-core optical fiber and measuring a wideband impulse response waveform by an optical linear sampling method; Fourier transforming the wideband impulse response waveform into a wideband frequency response waveform; extracting the wideband frequency response waveform in a desired optical frequency window to obtain a narrowband frequency response waveform; inverse Fourier transforming the narrowband frequency response waveform into a narrowband impulse response waveform for an optical frequency band of the optical frequency window; and The width of the narrowband impulse response waveform is set to spatial mode dispersion. A spatial mode dispersion measurement method.

4. 4. The spatial mode dispersion measuring method according to claim 3, wherein chromatic dispersion compensation is performed on the wideband frequency response waveform or the narrowband frequency response waveform.

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