Apparatus and method for measuring intermode group delay difference.

Abstract

The purpose of the present invention to provide an inter-mode group delay difference measuring device and method that can measure a DMD with a simple configuration even if an FMF to be measured is at a long distance.　An inter-mode group delay difference measurement device 301 according to the present invention comprises: a light source 11 that outputs coherent light for which the optical frequency is changed at a predetermined modulation period; a branching element 12 that branches the light into two; a light input unit 13 that excites one branched light into a plurality of modes, guides the excited light into one end of an optical fiber 51 to be measured, the optical fiber 51 to be measured being a multi-mode optical fiber, and inputs the other branched light into one end of a reference optical fiber 52 in a single mode; an image sensor 15 that observes the interference light between the measurement light emitted from the other end of the optical fiber 51 to be measured and the reference light emitted from the other end of the reference optical fiber 52; and a computing unit 16 that detects the peaks of the interference light that appears when the modulation period is changed and measures the inter-mode group delay difference from the period of the peaks and the electric field distribution of the interference light at the peaks.

Description

Technical Field

[0001] The present disclosure relates to an apparatus and method for measuring the differential mode delay (DMD) between modes of a few-mode fiber.

Background Art

[0002] A few-mode fiber (FMF) is one of the promising optical fibers as a medium for realizing future high-capacity optical communication. In an FMF, multiple modes are transmitted as independent transmission paths, but crosstalk occurs where signals of each mode mix during propagation, so signal processing for compensating crosstalk is required on the receiving side. When compensating crosstalk using signal processing on the receiving side of an FMF, there is a problem that the group delay times of each mode are different, that is, the differential mode delay (DMD) is large and crosstalk cannot be compensated. Therefore, the DMD of an FMF is an important parameter in signal processing, and a method for measuring the DMD is required.

[0003] In DMD measurement, it is desirable to be able to determine the arrival time of light after the FMF emits and the mode of the arrived light. Therefore, in order to grasp the arrival time and the arrived mode, the method of Non-Patent Document 1 has been proposed. The method of Non-Patent Document 1 uses a wavelength-variable light source and an image sensor. Due to the delay time difference between each mode, the interference pattern between modes observed after the FMF emits changes. Specifically, due to the interference between modes, the intensity of the emitted light becomes periodic with respect to the wavelength, and the period changes depending on the delay time difference. The method of Non-Patent Document 1 obtains the delay time of each mode from the observed period and its electric field intensity distribution.

[0004] Furthermore, the method disclosed in Non-Patent Document 2 uses a low-coherence light source and an image sensor. The image sensor observes the interference light between the FMF (Fiber Microwave Output) light and the reference light that has passed through a single-mode fiber (SMF). The method in Non-Patent Document 2 utilizes the characteristic that the intensity of the interference light increases when the length of a specific mode of each mode in the FMF is the same as the length of the fundamental mode of the SMF. By measuring the interference light while changing the length of the reference path, the group delay time of each mode is measured. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] David R. Gray et al., “Real-Time Modal Analysis via Wavelength-Swept Spatial and Spectral (S2) Imaging”, IEEE Photon. Technol. Lett. 28(9), 1034-1037 (2016). [Non-Patent Document 2] Y. Abe et al., “Collective measurement of DMD in 6-mode 19-core fiber using low-coherence digital holography”, Proc. SPIE 11309, 1130904, (2020). [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the method described in Non-Patent Document 1 has a problem in that the period of the interference pattern increases as the delay time difference increases, making it difficult to measure the DMD when the FMF is several hundred meters or longer. Furthermore, the method described in Non-Patent Document 2 requires measurement while changing the length of the reference path, and especially when the DMD is large, the length of the reference path needs to be changed significantly, which presents the challenge of difficulty in measurement with a simple configuration.

[0007] Therefore, in order to solve the above-mentioned problems, the present invention aims to provide an intermode group delay difference measuring device and method that can measure DMD with a simple configuration even when the FMF to be measured is over a long distance. [Means for solving the problem]

[0008] To achieve the above objective, the intermode group delay difference measuring device according to the present invention measures the interference waveform of light with periodically changing optical frequency using an image sensor.

[0009] Specifically, the intermode group delay difference measuring device according to the present invention is A light source that outputs coherent light with a predetermined modulation period, A branching element that splits the aforementioned light into two, An optical incident unit that excites one of the aforementioned light beams into multiple modes and incidents it onto one end of the optical fiber under test, which is a multimode optical fiber, and inputs the other of the aforementioned light beams into one end of the reference optical fiber in a single mode, An image sensor that observes the interference light between the measurement light emitted from the other end of the optical fiber under test and the reference light emitted from the other end of the reference optical fiber, A calculator that detects the peak of the interference light that appears when the modulation period is changed, measures the group delay difference based on the difference in the modulation period in which the peak appears, and determines the type of mode from the electric field distribution of the interference light at the peak, It is equipped with.

[0010] Furthermore, the intermode group delay difference measurement method according to the present invention is Setting a modulation period that changes the optical frequency of coherent light. The aforementioned light is split into two, One of the aforementioned light beams is excited into multiple modes and incident onto one end of the optical fiber under test, which is a multimode optical fiber, while the other of the aforementioned light beams is input into one end of the reference optical fiber in a single mode. Observe the interference light between the measurement light emitted from the other end of the optical fiber under measurement and the reference light emitted from the other end of the reference optical fiber using an image sensor, and The process involves detecting the peak of the interference light that appears when the modulation period is changed, measuring the group delay difference based on the difference in the modulation period at which the peak appears, and determining the type of mode from the electric field distribution of the interference light at the peak. To do so.

[0011] This intermode group delay difference measurement apparatus and method has a simple configuration, which involves changing the optical frequency of coherent light, preparing a single reference optical fiber in parallel with the optical fiber under test, and observing the interference light obtained by interfering the light passing through the optical fiber under test and the reference optical fiber with an image sensor. In this configuration, by changing the modulation period that alters the optical frequency, the peaks of the interference light observed by the image sensor appear with time staggers for each mode. Furthermore, since the electric field distribution at the peak of the interference light can be obtained from the image sensor, it is possible to determine which mode each peak belongs to. Therefore, the time between the peaks can be used as the DMD (Dynamic Mode). Since this intermode group delay difference measurement apparatus and method measures between peaks, even when the FMF is several hundred meters or longer, the delay time difference is large, and the period of the interference pattern is large, the DMD can be measured without changing the reference optical fiber.

[0012] Therefore, the present invention provides an intermode group delay difference measuring device and method that can measure DMD with a simple configuration even when the FMF to be measured is over a long distance.

[0013] Furthermore, the aforementioned arithmetic unit can also be implemented using a computer and a program, and the program can be recorded on a recording medium or provided via a network.

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

[0015] The present invention can provide an inter-modal group delay difference measurement apparatus and a method thereof that can measure a deformed mirror device (DMD) with a simple configuration even when the fiber mode filter (FMF) of a measurement target is long-distance.

Brief Description of the Drawings

[0016] [Figure 1] It is a diagram for explaining an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 2] It is a diagram for explaining light output from a light source of an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 3] It is a diagram for explaining a measurement principle of an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 4] It is a diagram for explaining a measurement principle of an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 5] It is a diagram for explaining a measurement principle of an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 6] It is a diagram for explaining a measurement principle of an inter-modal group delay difference measurement apparatus according to the present invention. [Figure 7] It is a diagram for explaining an inter-modal group delay difference measurement method according to the present invention.

Embodiments for Carrying Out the Invention

[0017] <L 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. In the present specification and drawings, components with the same reference numerals indicate the same components.

[0018] FIG. 1 is a diagram for explaining an inter-modal group delay difference measurement apparatus 301 according to the present embodiment. The inter-modal group delay difference measurement apparatus 301 includes a light source unit 11 that outputs coherent light with a light frequency changed at a predetermined modulation period, a branching element 12 that branches the light into two, The optical incident unit 13 excites one of the aforementioned light beams into multiple modes and injects it into one end of the optical fiber 51 under test, which is a multimode optical fiber, and inputs the other of the aforementioned light beams into one end of the reference optical fiber 52 in a single mode. An image sensor 15 observes the interference light between the measurement light emitted from the other end of the optical fiber 51 under measurement and the reference light emitted from the other end of the reference optical fiber 52, A calculator 16 detects the peak of the interference light that appears when the modulation period is changed, measures the group delay difference based on the difference in the modulation period in which the peak appears, and determines the type of mode from the electric field distribution of the interference light at the peak. It is equipped with.

[0019] The optical fiber under test 51 is FMF. The reference optical fiber 52 is SMF. The light source 11 outputs coherent light whose optical frequency changes periodically, as shown in Figure 2. The modulator 11a can adjust the period (modulation frequency) during which the optical frequency changes. The branching element 12 splits the light output by the light source 11 into two. The light incidence unit 13 excites one of the two branches of light into multiple modes using the exciter 13a and then injects it into the optical fiber 51 under test. The light incidence unit 13 also injects the other branch of the light into the reference optical fiber 52 using the light incidence means 13b.

[0020] The light propagating through each optical fiber (51, 52) is output spatially by the collimator 17. The multiplexer 14 combines the two lights and causes interference to produce interference light. The image sensor 15 measures the electric field intensity distribution of the interference light. The arithmetic unit 16 analyzes the DMD from the measurement results of the interference light. In other words, the intermode group delay difference measuring device 301 is characterized by measuring the interference light of light with periodically modulated optical frequencies using an image sensor, and measuring the DMD from the change in the electric field intensity distribution.

[0021] The period (modulation frequency) over which the optical frequency of the light output by the light source 11 changes is variable. The intermode group delay difference measuring device 301 measures the DMD by changing this modulation frequency. Figures 3 to 6 illustrate the measurement principle for measuring DMD from changes in the electric field intensity distribution of interfering light. For simplicity, Figures 3 to 6 explain the case where the optical fiber 51 under test is a 2-mode optical fiber, but the same principle applies even if the optical fiber 51 under test is an optical fiber capable of propagating 3 or more modes.

[0022] Figure 3 illustrates the process of measuring interference light obtained by interfering a measurement light with a reference light. The measurement light emitted from the optical fiber 51 under test consists of mode 1 light and mode 2 light, which propagates more slowly. On the other hand, the light emitted from the reference optical fiber 52 is the reference light. The multiplexing element 14 combines the mode 1 and mode 2 measurement light with the reference light, and the image sensor 15 measures the interference light from these two lights.

[0023] Figure 4 illustrates the timing of the optical frequencies of the measurement light and the reference light at the time of their combined wave, when the modulation frequency is f1. When the modulation frequency of light source 11 is set to f1, assume that the phases of the light in mode 1 and the reference light coincide. In this case, the interference waveform of the light in mode 1 and the reference light will have a DC component. On the other hand, since the phases of the light in mode 2 and the reference light do not coincide, the interference waveform will fluctuate over time according to the difference in optical frequency (phase difference).

[0024] The image sensor 15 is characterized by operating at a sampling rate slower than the modulation frequency. The image sensor 15 generally has a low sampling rate (frame rate), making it difficult to observe components that fluctuate with phase difference (interference waveform of mode 2 light and reference light). On the other hand, the image sensor 15 can measure the DC component (interference waveform of mode 1 light and reference light). Therefore, by measuring with a sufficient exposure time (averaging time), the image sensor 15 can observe only the electric field distribution of mode 1 light, which is the DC component. In other words, when the modulator 11a is set to modulation frequency f1, the image sensor 15 measures the electric field distribution of mode 1 light.

[0025] Figure 5 illustrates the timing of the optical frequencies of the measurement light and the reference light at the time of their combined emission when the modulation frequency is f2. When the modulation frequency of light source 11 is set to f2, assume that the phases of the Mode 2 light and the reference light coincide. In this case, the interference waveform of the Mode 2 light and the reference light will have a DC component. On the other hand, since the phases of the Mode 1 light and the reference light do not coincide, the interference waveform will fluctuate over time according to the difference in optical frequency (phase difference).

[0026] As described above, because the image sensor 15 has a low sampling rate (frame rate), when the modulator 11a is set to modulation frequency f2, the image sensor 15 will measure the electric field distribution of light in mode 2.

[0027] The optical frequency f(t) of the light emitted from the light source 11 changes with time t as shown in the following equation (see Figure 2).

number

[0028] Here, f m Let's explain the relationship between and t. The effective refractive index of the optical fiber under test 51 is n, the speed of light in a vacuum is c, and the difference in length ΔL between the optical fiber under test 51 and the reference optical fiber 52 is f. m When the length is between N periods and N+1 periods, that is

number

number

[0029] According to this relationship, the modulation frequency f is set in modulator 11a. mBy changing the settings and measuring the interference waveform with the image sensor 15, the intensity of the interference waveform against time, as shown in Figure 6, can be obtained. In Figure 6, peak 61 is the intensity peak of the interference waveform of light in mode 1, and peak 62 is the intensity peak of the interference waveform of light in mode 2. The time difference between peak 61 and peak 62 is the difference in arrival time of light of each mode to the other end of the optical fiber 51 under test, and is the DMD (Dynamic Mode Degree).

[0030] Furthermore, since the interference waveform is measured by an image sensor, the electric field distribution at the time of the intensity peak can also be obtained. This makes it possible to determine which mode each peak represents.

[0031] In other words, the arithmetic unit 16 determines the modulation frequency f m While changing the settings, the peaks of the interference waveform observed by the image sensor 15 are captured, and the mode generating each peak is determined from the electric field distribution of the interference light when the peak occurs, and the time between peaks is defined as the respective DMD.

[0032] Figure 7 is a flowchart illustrating the method for measuring the intermode group delay difference performed by the intermode group delay difference measuring device 301. The intermode group delay difference measuring device 301 is Setting a modulation period for the light source 10 to change the optical frequency of the coherent light (step S01), The light is split into two, one of the light is excited into multiple modes and incident into one end of the optical fiber 51 to be measured, which is an FMF, and the other of the light is input to one end of the reference optical fiber 52 in a single mode (step S02). The image sensor 15 observes the interference light between the measurement light emitted from the other end of the optical fiber 51 under measurement and the reference light emitted from the other end of the reference optical fiber 52 (step S03), and Step S05: Detect the peak of the interference light that appears when the modulation period is changed, measure the group delay difference based on the difference in the modulation period in which the peak appears, and determine the type of mode from the electric field distribution of the interference light at the peak. To do so.

[0033] In step S01, the modulator 11a is used to periodically change the optical frequency of the light output by the light source 11. This period is the modulation frequency. In step S02, the light from the light source 11, which has been split into two by the branching element 12, is incident on the optical fiber 51 under test and the reference optical fiber 52, respectively. Here, the light incident on the optical fiber 51 under test is excited by the exciter 13a to each mode that can propagate in the optical fiber 51 under test. The other light is incident on the reference optical fiber 52 from the light incident means 13b while remaining in a single mode. In step S03, the interference light between the light propagated in the optical fiber 51 under test and the light propagated in the reference optical fiber 52 is observed by the image sensor 15. In step S04, it is determined whether or not to change the optical frequency. If the optical frequency has not been changed to within the predetermined range ("No" in step S04), the process returns to step S01, the optical frequency is changed, and steps up to S03 are repeated. If the optical frequency has been changed to within the predetermined range ("Yes" in step S04), step S05 is performed. In step S05, the calculation unit 16 detects multiple DC component peaks of the interference light from the results of observing the interference light while changing the optical frequency. The calculation unit 16 determines which mode each peak represents from the electric field distribution of the interference light at the time the peak occurs. The calculation unit 16 then measures the time between the peaks as the respective DMD.

[0034] Furthermore, the intermode group delay difference measuring device 301 can perform steps S01 to S05 by controlling each component (modulator 11a and calculation unit 16) with a control unit not shown in Figure 1.

[0035] As described above, the intermode group delay difference measuring device 301 measures the interference waveform of light with periodically modulated optical frequencies using an image sensor, so even for optical fibers under measurement that are several hundred meters or longer, the group delay time and DMD of each mode can be easily measured with a simple configuration. [Explanation of symbols]

[0036] 11:Light source 11a: Modulator 12: Branching method 13: Light incidence part 13a: Exciter 13b: Light incidence means 14: Multiplexing element 15: Image sensor 16: Arithmetic unit 17: Collimator 51: Optical fiber under measurement 52: Reference optical fiber 301: Intermode group delay difference measuring device

Claims

1. A light source that outputs coherent light with a predetermined modulation period, A branching element that splits the aforementioned light into two, An optical incident unit that excites one of the aforementioned light beams into multiple modes and incidents it onto one end of the optical fiber under test, which is a multimode optical fiber, and inputs the other of the aforementioned light beams into one end of the reference optical fiber in a single mode, An image sensor that observes the interference light between the measurement light emitted from the other end of the optical fiber under test and the reference light emitted from the other end of the reference optical fiber, A calculator that detects the peak of the interference light that appears when the modulation period is changed, measures the group delay difference based on the difference in the modulation period in which the peak appears, and determines the type of mode from the electric field distribution of the interference light at the peak, A device for measuring the delay difference between modes of a group.

2. The intermode group delay difference measuring device according to claim 1, characterized in that the image sensor operates at a sampling rate slower than the modulation period.

3. Setting a modulation period that changes the optical frequency of coherent light. The aforementioned light is split into two, One of the aforementioned light beams is excited into multiple modes and incident onto one end of the optical fiber under test, which is a multimode optical fiber, while the other of the aforementioned light beams is input into one end of the reference optical fiber in a single mode. Observe the interference light between the measurement light emitted from the other end of the optical fiber under measurement and the reference light emitted from the other end of the reference optical fiber using an image sensor, and The process involves detecting the peak of the interference light that appears when the modulation period is changed, measuring the group delay difference based on the difference in the modulation period at which the peak appears, and determining the type of mode from the electric field distribution of the interference light at the peak. A method for measuring the intermode group delay difference.

4. The method for measuring the intermode group delay difference according to claim 3, characterized in that the sampling rate of the image sensor is slower than the modulation period.

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