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    • 专利摘要:
    The invention relates to a method and apparatus for measuring a beam profile emitted at the output of a randomly coupled multi-core fiber. An apparatus comprises a light source, a unit of measurement and an analysis unit. The light emitted at the output of the light source is applied to one or more modes of a plurality of spatial modes of the fiber at an input end of the fiber. The measurement unit measures a sum of intensity profiles of individual light components outputted from respective modes of the plurality of spatial modes, by averaging an interference component between the plurality of spatial modes in a beam profile. combined light emitted at the output of the plurality of spatial modes at an output end of the fiber. The computing unit calculates a diameter MFD and / or a surface Aeff of the fiber based on the sum of the intensity profiles of the individual light components obtained by the measurement unit.
    公开号:FR3068792A1
    申请号:FR1856199
    申请日:2018-07-05
    公开日:2019-01-11
    发明作者:Tetsuya Hayashi；Takuji Nagashima
    申请人:Sumitomo Electric Industries Ltd；
    IPC主号:
    专利说明:

    BACKGROUND OF THE INVENTION
    Field of the Invention The present invention relates to a method and an apparatus for measuring a beam profile emitted at the output of a multicore fiber having a plurality of spatial modes which are randomly coupled.
    Description of the Related Art [0002] Spatial multiplexing optical fibers which are optical fibers having multiple spatial modes (multiple cores and / or multiple guided modes) have the advantage that the spatial density of the information transmitted can be increased, and are therefore attractive as a technology for ensuring efficient use of transmission paths with limited space, such as underground conduits and submarine cables.
    In particular, a coupled multi-core fiber (C-MCF, coupled multi-core fiber) in which guided modes are mutually coupled in a plurality of cores, comprises cores placed at a short distance from each other, and is therefore effective in increasing the spatial density of the information transmitted. As a result, C-MCF fiber enables high-density, high-capacity signal transmission when used in combination with multiple input multiple output (ΜΙΜΟ, multi-input multi-output) signal processing technology to distinguish between signals in a plurality of guided modes transmitted through the coupled cores.
    A randomly coupled multi-core fiber (RC-MCF, randomly-coupled multi-core fiber) is an example of a C-MCF fiber in which a coupling force between the cores is appropriately defined, so that 'there occurs a random coupling of modes due to the bending and twisting of the optical fiber. As a result, the rate of accumulation of differential mode delay (DMD) between modes can be reduced to the square root of the fiber length. Therefore, the amount and cost of computation in signal processing ΜΙΜΟ can be reduced through the use of RC-MCF fiber. Examples of RC-MCF fibers are described by Tetsuya Hayashi et al., In "Coupled-Core
    Multi-Core Fibers: High-Spatial-Density Optical Transmission Fibers with Low Differential Modal Properties »ECOC 2015, We. 1.4.1 (2015), by Taiji Sakamoto et al., In “Fiber Twisting- and Bending-Induced Adiabatic / Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber” J. OF LIGHTWAVE TECEtNOLOGY, vol. 34, n ° 4, pp. 1228-1237 (2016), and by Tetsuya Hayashi et al., In “Record-Low Spatial Mode Dispersion and Ultra-Low Loss Coupled Multi-Core Fiber for Ultra-Long-Haul Transmission” J. OF LIGHTWAVE TECHNOLOGY, vol. 35, n ° 3, pp. 450 ^ 157 (2017). A typical example of RC-MCF fiber has a mode coupling coefficient between cores greater than or equal to 1 [1 / m] or a power coupling coefficient between cores greater than or equal to 10 [1 / km].
    Compared with single core optical fibers which are currently in widespread use, RC-MCF fibers are more important, not only because the cores have a higher spatial density, but also because the non -optical linearity is reduced, because the light is dispersed among the cores due to the coupling of modes. As the non-linearity is reduced, the RC-MCF fibers produce less optical noise which is generated due to non-linear interference when high intensity light is propagated.
    SUMMARY OF THE INVENTION The purpose of the present invention is to provide a method and an apparatus for measuring a beam profile emitted at the output of an RC-MCF fiber.
    A method of measuring a beam profile emitted at the output of an optical fiber which is an RC-MCF fiber, according to the present invention comprises a measurement step and an analysis step. In the measurement step, the light emitted at the output of a light source is applied to one or more mode (s) of a plurality of spatial modes of the optical fiber, at an input end of the optical fiber, and a sum of intensity profiles of individual light components emitted from respective modes of the plurality of spatial modes is measured by averaging an interference component between the plurality of spatial modes in a combined light beam profile emitted at the output of the plurality of spatial modes, at an output end of the optical fiber. In the analysis step, an optical fiber output beam evaluation index is calculated based on the sum of the intensity profiles.
    In one embodiment, the measurement step can comprise the measurement of a near field radiation pattern (NFP, near field pattern) as the sum of the intensity profiles, and the step of analysis can include calculating an average beam evaluation index of all supermodes, based on the measured NFP diagram and the assumption that the measured NFP diagram is an average NFP diagram of beams emitted from the supermodes individual. In another embodiment, the measurement step may include measuring an NFP diagram as the sum of the intensity profiles, and the analysis step may include calculating an evaluation index of bundle of each of the cores included in regions separated from each other, so that each region includes one of the cores, based on the measured NFP diagram and an assumption that the measured NFP diagram is an NFP diagram of each of the hearts. In another embodiment, the measurement step can include measuring a far field pattern (FFP) as a sum of the intensity profiles, and the analysis step can understand the calculation of an average beam evaluation index of all cores, based on the measured FFP diagram and an assumption that the measured FFP diagram is an average FFP diagram of beams emitted from the individual cores.
    According to another aspect of the method according to the present invention, the measurement step can include determining an average wavelength of the beam profile to average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted from the plurality of spatial modes.
    Depending on the aspect, the measurement step may include determining an average wavelength of the beam profile to average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted from the plurality of spatial modes, while adjusting a line width Δί of the light source or a length or a radius of curvature of the optical fiber, so that the product τ · Δί of a dispersion τ modes of the optical fiber and a line width Δί is greater than or equal to 9.
    According to yet another aspect of the method according to the present invention, the measurement step can comprise the individual application of light components which are not mutually correlated from the light source, to all of the modes of the plurality of spatial modes. , at the input end of the optical fiber and the averaging of the interference component between the plurality of space modes in the beam profile of the combined light emitted at the output of the plurality of space modes.
    Depending on the aspect, the measurement step may include the individual emission of the uncorrelated light components, at the output of a plurality of light emitting elements which operate independently of each other.
    Alternatively, the measurement step may include dividing the light emitted at the output of a single light emitting element and having a line width Δί into light components to be applied to the respective spatial modes of the optical fiber and applying propagation times which differ from each other by at least 1.1 / Δί.
    Alternatively, the measurement step may comprise by dividing the light emitted at the output of a single light emitting element into light components to be applied to the respective spatial modes of the optical fiber and the temporal variation of polarizations of the light components.
    An apparatus for measuring a beam profile emitted from an optical fiber which is an RC-MCF fiber, according to the present invention comprises a light source, a measurement unit and an analysis unit. The light source emits light so that the light is applied to one or more mode (s) of a plurality of spatial modes of the optical fiber, at an input end of the optical fiber. The measurement unit measures a sum of intensity profiles of individual components of light emitted from the respective modes of the plurality of spatial modes, by averaging an interference component between the plurality of spatial modes in a beam profile. combined light emitted at the output of the plurality of spatial modes, at an output end of the optical fiber. The analysis step which calculates a fiber optic output beam evaluation index, based on the sum of the intensity profiles.
    In one embodiment, the measurement unit can measure an NFP diagram as the sum of the intensity profiles, and the analysis unit can calculate an average beam evaluation index of all the supermodes. , based on the NFP diagram and an assumption that the measured NFP diagram is an average NFP diagram of beams emitted at the output of the individual supermodes. In another embodiment, the measurement unit can measure an NFP diagram as the sum of the intensity profiles, and the analysis unit can calculate the beam evaluation index of each of the cores included in regions separated from each other, so that each region includes one of the cores, based on the NFP diagram and an assumption that the measured NFP diagram is an NFP diagram of each of the cores. In another embodiment, the measurement unit can measure an FFP diagram as the sum of the intensity profiles, and the analysis unit can calculate an average beam evaluation index of all cores, on the basis of the measured FFP diagram and an assumption that the measured FFP diagram is an average FFP diagram of beams emitted from the individual cores.
    According to another aspect of the apparatus according to the present invention, the measurement unit can determine an average wavelength of the beam profile to average the interference component between the plurality of spatial modes in the profile beam of the combined light emitted from the plurality of spatial modes.
    Depending on the aspect, the measurement unit can determine an average wavelength of the beam profile to average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted in exit from the plurality of spatial modes, while adjusting a line width Δί of the light source or a length or even a radius of curvature of the optical fiber, so that the product τ · Δί of a dispersion of modes τ of the optical fiber and of a line width Af is greater than or equal to 9.
    According to yet another aspect of the apparatus according to the present invention, the measurement unit can average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted from the plurality spatial modes that all individually receive uncorrelated light components from the light source at the input end of the optical fiber.
    Depending on the aspect, the light source may comprise a plurality of light emitting elements which operate independently of each other and emit the non-mutually correlated light components as output.
    Alternatively, the light source can be configured to divide the light emitted at the output of a single light emitting element and having a line width Af in light components to be applied to the respective spatial modes of the optical fiber measured and to apply propagation times that differ from each other by at least 1.1 / Δί.
    As a variant, the light source can be configured to divide the light emitted at the output of a single light emitting element into light components to be applied to the respective spatial modes of the measured optical fiber and to vary over time the polarizations of the light components in different diagrams.
    According to the present invention, a beam profile emitted at the output of an RCMCF fiber can be measured. In addition, a mode field diameter (MFD) and an effective area (A e ff) of each core of the RC-MCF fiber can be determined based on the result of the measurement.
    BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a conceptual diagram illustrating an FFP diagram measurement system.
    Figure 2 is a conceptual diagram illustrating an NFP diagram measurement system.
    Figure 3 shows examples of electric field distributions of NFP diagrams of an RC-MCF fiber.
    FIG. 4 shows examples of intensity distributions of NFP diagrams of the RC-MCF fiber.
    Figure 5 shows intensity distributions of NFP diagrams of the RC-MCF fiber measured at 0.5 second intervals, illustrating an example of variation over time.
    Figure 6 is a flow diagram of the fiber optic output beam profile measurement method according to one embodiment.
    Figure 7 is a conceptual diagram illustrating an example of the structure of a fiber optic output beam profile measurement apparatus according to the embodiment.
    FIG. 8 illustrates an example of the average NLP diagram of the light intensity of all the supermodes, in the case where an optical fiber object of the measurement is an RC-MCF fiber with four cores.
    Figure 9 is a graph illustrating the relationship between the ratio of the range of variation of the light intensity to the average light intensity etx-Af.
    Figure 10 is a graph illustrating the relationship between the ratio of the range of variation of the light intensity to the average light intensity and τ-ΔΤ
    DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is described below in detail, with reference to the accompanying drawings. In the description relating to the drawings, identical elements are designated by the same reference numerals, and a redundant description is therefore omitted. The present invention is not limited to the examples described below. The present invention is defined by the scope of the claims, and is deemed to include equivalents to the scope of the claims as well as all modifications within the scope.
    It is generally important to evaluate the performance of an optical fiber. The performance of an optical fiber is determined, for example, by a transmission capacity that can be achieved when the optical fiber is installed in an actual transmission system and by the expected splicing loss when the optical fiber is spliced with another fiber. optical. When the performance of an optical fiber is evaluated, it is generally also important to measure the profile (distribution of the electric field or distribution of the intensity) of a beam emitted at the output of a spatial mode of the optical fiber and of quantify the profile measured as a performance indicator.
    For single-mode fibers (SMF, single mode fibers), a mode field diameter (MFD) and an effective area (Aeff) are important performance indicators. The MFD diameter is an index relating to the loss by splicing caused when optical fibers are spliced together. The splice loss decreases as the MFD diameter increases and as the difference in MFD diameter between optical fibers decreases. Aeff is an index relating to non-linearity. The power of non-linear noise is inversely proportional to the square of A e ff. In other words, nonlinear noise decreases as Aeff increases.
    The non-linear noise which occurs during transmission through a single-mode fiber is also affected by the intensity of the light signal, by the bandwidth of the light signal, by the loss of transmission of the optical fiber and by the chromatic dispersion of the optical fiber. However, Aeff is a very important index in the quantification of the performances of optical fiber in terms of non-linearity. The definitions and methods for measuring the diameter MFD and A e ff of an SMF fiber are described by Rob Billington, in “Effective Area of Optical Fibers - Definition and Measurement Techniques”, National Physical Laboratory, and Recommendation G.650.1 from the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), "Definitions and test methods applicable to deterministic linear attributes of single-mode optical fibers and cables" (2010).
    A common technique for measuring the diameter MFD and Aeff of an SMF fiber is described below. Normally, a beam FFP diagram emitted at the output of the SMF fiber is measured and the diameter MFD is calculated on the basis of the FFP diagram. The FFP diagram is an electric field amplitude distribution or an intensity distribution over a hemisphere (far field (FF) centered on an output end of the optical fiber and having a sufficiently large radius (measurement distance The distance from the FFP diagram measurement should theoretically be infinite. However, according to ITU-T G.651.1, when the diameter MFD is 2w, the diameter of an optical receiver is b, and the length wave is λ, sufficient measurement accuracy can be ensured in the actual measurement of an optical fiber if the FFP diagram measurement distance is greater than or equal to 40 wb / λ.
    Figure 1 is a conceptual diagram illustrating an FFP diagram measurement system. A light source 10 is optically coupled to an input end of an optical fiber object of the measurement 2. A system of rectangular coordinates xy is defined with the origin of the center of an output end of the optical fiber 2. The coordinates (x, y) are local rectangular coordinates along an end surface, at the exit end of the optical fiber 2. The coordinates (r, 0) are polar coordinates which correspond to the coordinates (x , y). Here, φ χ is a divergence angle to the FFP diagram corresponding to the x axis, and φ γ is a divergence angle to the FFP diagram corresponding to the y axis.
    Assuming that the distribution of the electric field in the spatial mode of the SMF fiber is circularly symmetrical, the diameter MFD can be calculated from equation (1).
    * f / 2 | p.
    J o KpW | sin ^ cos (• Λ- / 2Ι, (1) £ P> wl sm
    Ρψ (φ) is the FFP diagram of the amplitude of the electric field at a radiation angle (polar angle) of φ. Ε φ (φ) can be considered as the measurement result of the FFP diagram for φ in a dimension at any 0.
    i— λ,
    MFD = V2π When E is the amplitude distribution of the electric field in an optical fiber mode, Ae "is defined as in equation (2).
    (2)
    In particular, when the amplitude distribution of the electric field in the mode is circularly symmetrical, A e ff can be expressed as in equation (3).
    4 _ (rO E i 2i> dr ) eff ~ ' P)
    Therefore, A e ff can be calculated from the intensity distribution | E | 2 in the fiber optic mode.
    The distribution of the electric field E in the mode can be considered to be equal to the distribution of the intensity of an NFP beam diagram emitted at the output of the optical fiber. Therefore, Aeff can be calculated from the intensity distribution of the NFP diagram. The NFP diagram is the distribution of the electric field or the distribution of the intensity on an output end surface (near field (NF), near field) of the optical fiber.
    Figure 2 is a conceptual diagram illustrating an NFP diagram measurement system. The NFP diagram of an optical fiber is very small, so it is difficult to measure the NFP diagram directly with high accuracy. Consequently, the NFP diagram is observed through a camera, for example, using an optical system with magnification comprising lenses 21 and 22. However, due to the diffraction limit of the optical system and problems with the dynamic range and linearity of the camera, the NFP diagram cannot be measured with sufficient accuracy.
    Consequently, during the actual measurement of the SMF fiber, the FFP diagram is firstly measured with precision, then the NFP diagram is determined from the FFP diagram. Then, Aeff is calculated using the NFP diagram. The variation in the amplitude of the electric field, from the NFP diagram to the FFP diagram, can be explained by Fraunhofer diffraction. Consequently, supposing that the distribution of the electric field in the mode is circularly symmetrical, a function E r (r) of r which represents the NFP diagram of the amplitude of the electric field and the function Ρψ (φ) of φ which represents the FFP diagram of the amplitude of the electric field can be converted into each other using the Hankel transform of order 0, as in equation (4).
    W x £ Er * ^ 4 ·)
    E r ( r ) * £ F V (^ sin ^) sin2 ^ The SMF fiber has a single spatial mode and the distribution of the electric field in the spatial mode is circularly symmetrical. Therefore, the diameter MFD and the area Aeff of the SMF fiber can be easily and accurately measured and evaluated using the method described above.
    In contrast, an RC-MCF fiber has a plurality of spatial modes. In addition, when light propagates through the RC-MCF fiber, a random optical coupling occurs between the spatial modes. Consequently, the diameter MFD and the surface area A ^ f of the RC-MCF fiber cannot be evaluated by means of a method similar to that implemented for the SMF fiber.
    The diameter MFD and the surface A ^ h are performance indices making it possible to quantify the amplitude distribution of the electric field in each mode of the optical fiber. Therefore, to evaluate the diameter MFD and the area Aeff of the RCMCF fiber, it is necessary to measure the NFP diagram and the FFP diagram, while the light is emitted at the output of only one of the modes which is to be evaluated. However, random optical coupling occurs between spatial modes, and the way in which coupling occurs varies over time.
    For example, the spatial modes of four cores coupled in an RCMCF fiber take the place of what are called "supermodes". Supermodes are distributed across all hearts. Figure 3 shows examples of electric field distributions of NFP diagrams of the RC-MCF fiber. Figure 4 shows examples of intensity distributions of NFP diagrams of the RC-MCF fiber. The RC-MCF fiber used here has four cores arranged in a square pattern, so that the center distance of the cores is 20 µm. The MFD diameter of each individual heart is about 10 µm. There is very little difference in NFP diagram intensity distribution between spatial modes.
    The profile of a beam emitted at the output of the RC-MCF fiber must be measured while the light is emitted at the output of only one of the spatial modes. However, the intensity of the NFP diagram of the light emitted from an actual RC-MCF fiber with a similar design varies over time. Figure 5 shows intensity distributions of NFP diagrams of the RC-MCF fiber measured at 0.5 second intervals, illustrating an example of variation over time. Cores do not emit light components having equivalent intensities, as shown in Figure 4. This shows that light components of a plurality of modes are emitted randomly.
    An optical fiber output beam profile measurement method and an optical fiber output beam profile measurement apparatus according to the present embodiment described below allow the measurement of an emitted beam profile at the output of the RC-MCF fiber. The diameter MFD and the area Aeff of each core of the RC-MCF 5 fiber can be determined on the basis of the measurement result.
    When all the modes of the RC-MCF fiber are excited, the intensity Infp of the NFP diagram can be expressed as in equation (5).
    Σ ^> ( λ '4
    Σ Ε Λ Χ ^
    Σ Ε η ( Χ ^ (5) n
    j ΑίΓΡ, / ι n
    = Σ l £ ”(* '^) | 2 + 2 Re Σ E n ( x 'y) E * m ( x ' y) _n * m (x, y) + 2Re '£ E n (x, y) E * t (x, y) _ηψιη
    E n is the profile of the complex amplitude of the near field electric field in mode n, and E n * is the complex conjugate number of E n . Re [x] is the real part of the complex number x. Infp, n represents the profile of the electric field intensity in the near field in the n mode.
    Similarly, when all the modes of the RC-MCF fiber are excited, the intensity Iffp of the FFP diagram can be expressed as in equation (6).
    (6) = Σ 7 ΡΡΡ, Π (^ - 5 ^) + 2Re Σ ^ _nïm
    F n is the profile of the complex amplitude of the far field electric field in mode n, and F n * is the complex conjugate number of F n . Iffp, n represents the profile of the electric field intensity in the far field in the n mode.
    The random variation, for example in the phase relationship between the modes of the RC-MCF fiber gives rise to a random variation of a component of variation of light intensity caused by interference between modes (interference component ) which is represented by the second term on the right side of the last line of each of equations (5) for the near field and (6) for the far field. Therefore, a reliable measurement cannot be performed.
    Consequently, by carrying out sufficient averaging of the interference component, so that the interference component tends towards 0, it is possible to rewrite equation (5) in the form of equation (7) . - ^ nfp (-L l) = X- ^ nfp.h (- ^ 5 y) Π) n
    Likewise, equation (6) can be rewritten in the form of equation (8).
    ΛτΡ (Py) = ΧΛγΡ.η) (θ) η
    Thus, Infp and Iffp can be measured reliably.
    To perform such a measurement, the optical fiber output beam profile measurement method and the optical fiber output beam profile measurement apparatus are designed as described below. Figure 6 is a flow diagram of the optical fiber output beam profile measurement method according to the present embodiment. FIG. 7 is a conceptual diagram illustrating an example of the structure of an optical fiber output beam profile measuring apparatus 1 according to the present embodiment.
    The fiber optic output beam profile measuring device 1 comprises the light source 10, a measuring unit 20 and an analysis unit 30. The optical fiber 2 is an RC-MCF fiber. An input optical fiber 3 is used to couple the light source 10 to the optical fiber 2 and can be an SMF fiber. A first end of the input optical fiber 3 is optically connected to the light source 10, and a second end of the input optical fiber 3 is optically connected to the input end of the optical fiber 2 at a point 4. The measurement unit 20 receives the light 5 emitted at the output of the output end of the optical fiber 2.
    In the SU measurement step, the light source 10 outputs the measurement light. The light emitted at the output of the light source 10 is guided through the input optical fiber 3 and applied to the optical fiber 2 which is an RC-MCF fiber, at the input end thereof, at a or more of the spatial modes of the optical fiber 2. The spatial mode or the spatial modes of the optical fiber 2 to which the light is applied is or are defined on the basis of a state of optical coupling between the input optical fiber 3 and the optical fiber 2 at the connection point 4. The light source 10 can comprise any type of light emitting element, such as a light emitting diode or a laser diode.
    In the measurement step SU, the measurement unit 20 measures the sum of the intensity profiles of individual components of light emitted at the output of the respective spatial modes, by averaging the interference component between the spatial modes in the profile of the beam of combined light emitted at the output of the plurality of spatial modes, at the output end of the optical fiber 2. The measurement unit 20 can comprise any type of light-receiving element, such as a photodiode and a certain lens system.
    In the analysis step S12, the analysis unit 30 calculates the output beam evaluation indices (MFD, Aeff) of the optical fiber 2, on the basis of the measurement result of the sum light intensity profiles obtained by the measurement unit 20. The analysis unit 30 may include a calculation element, such as a central processing unit (CPU), and a storage element, such as a memory.
    In the optical fiber output beam profile measuring apparatus 1 according to the present embodiment and the optical fiber output beam profile measuring method using this apparatus, the measuring unit 20 and the analysis unit 30 can be used in the three measurement modes described below. Two or more of the three measurement modes can be applied in combination.
    In a first measurement mode, the measurement unit 20 measures an NFP diagram as the sum of the intensity profiles of the individual components of light emitted at the output of the respective spatial modes of the optical fiber 2. Next, the the analysis unit 30 calculates the average beam evaluation indices of all the supermodes, assuming that the measured NFP diagram is the average NFP diagram of the beams emitted at the output of the individual supermodes.
    In a second measurement mode, the measurement unit 20 measures an NFP diagram as the sum of the intensity profiles of the individual components of light emitted at the output of the respective spatial modes of the optical fiber 2. Next, the the analysis unit 30 calculates the beam evaluation indices of each core assuming that the measured NFP diagram is the NFP diagram of each of the cores included in regions separated from each other, so that each region includes the one of the hearts.
    In a third measurement mode, the measurement unit 20 measures an FFP diagram as the sum of the intensity profiles of the individual components of light emitted at the output of the respective spatial modes of the optical fiber 2. Next, l the analysis unit 30 calculates the average beam evaluation indices of all the cores, assuming that the measured FFP diagram is the average FFP diagram of the beams emitted at the output of the individual cores.
    When the optical fiber 2 is an RC-MCF fiber with four cores, the average NFP light intensity diagram of all the supermodes is as shown in FIG. 8. When four quadrants are defined around the center of the output end of the optical fiber 2, each quadrant has a peak light intensity. Consequently, in each of the first to third measurement modes described above, the analysis unit 30 can execute the following process.
    In the first measurement mode, "Aeff" can be calculated from equation (2), using the intensity profile of the entire region of the four quadrants. This value is not the average Aeff area of the supermodes in the physical sense, but is "Aeff" calculated from the average NFP diagram of the supermodes. However, "Aeff" is not reliable enough as an evaluation proxy. A value corresponding to the area Aeff of each core mode can be calculated by dividing "Aeff" calculated by the number of core modes.
    In the second measurement mode, the area A e ff of each core can be calculated by calculating the area A e ff according to equation (2) for each of the regions of the four quadrants. In addition, the area Aeff of each core can also be calculated according to equation (3), using a polar coordinate system originating, for example, from the peak intensity point or the geometric center of each quadrant. Perpendicular bisectors of line segments connecting peak intensity points or geometric centers of hearts can serve as boundary lines that separate the regions of the hearts from each other.
    In the third measurement mode, as shown in equation (8), the measured FFP intensity diagram is the sum of the FFP intensity diagrams of the respective modes. In the case where each mode is considered to be the mode of each heart, if the measurement distance of the FFP diagram is large enough, the FFP intensity diagram measured can be considered simply as the sum of the FFP intensity diagrams of the respective cores . Unlike the NFP diagram in which the intensity peaks appear at different positions, the FFP diagrams of the respective cores have overlapping peaks at the position where the radiation angle is 0 degrees, when the surface of output end of the optical fiber 2 is perpendicular to the central axes of the cores. Therefore, it is possible to obtain the average FFP intensity diagram of the cores, simply by measuring the FFP intensity diagram. The measured FFP diagram is the average FFP diagram of the cores and is, of course, circularly symmetrical. Therefore, the diameter MFD of each core can be calculated from equation (1). In addition, the area Aes of each core can be calculated from equation (3) after conversion from the FFP diagram to the NFP diagram using equation (4).
    In any one of the first to third measurement modes, the measurement unit 20 measures the sum of intensity profiles of the individual components of light emitted at the output of the respective spatial modes, by averaging the interference component between the space modes in the beam profile of the combined light emitted at the output of the plurality of space modes, at the output end of the optical fiber 2. The interference component between the space modes can be averaged as well as it is described below.
    The interference component between the spatial modes in the beam profile of the combined light emitted from the plurality of spatial modes can be averaged by determining the average wavelength of the beam profile. For example, the light source 10 can vary the wavelength over time, using a light emitting element having a variable output light wavelength or by causing a plurality light emitting elements having different output light wavelengths successively emit light. In this case, the analysis unit 30 can determine the average wavelength on the basis of beam profiles measured by the measurement unit 20 for the respective wavelengths. Alternatively, the light source 10 may include a light emitting element which outputs broadband wavelength light (for example, a superluminescent diode (DSL)). In this case, the analysis unit 30 can determine the time average of the beam profile measured by the measurement unit 20.
    As a variant, the interference component between the spatial modes in the beam profile of the combined light emitted at the output of the plurality of spatial modes can also be averaged by individual application of mutually uncorrelated light components originating from the light source 10 in all space modes, at the input end of the optical fiber 2. In this case, the light source 10 can use light components emitted individually at the output of a plurality of light emitting elements which operate independently of each other, to individually apply light components which are not mutually correlated to all the space modes, at the input end of the optical fiber 2. As a variant, the light source 10 can divide the light emitted into output of a single light emitting element in light components to be applied to spatial modes respect ifs, and vary in time the polarizations of the light components to be applied to the respective spatial modes in different diagrams, so that uncorrelated light components are applied individually to all of the spatial modes, at the end of fiber optic input 2.
    The extent to which the interference component between the spatial modes in the beam profile of the combined light emitted at the output of the plurality of spatial modes at the output end of the optical fiber 2 is averaged can be evaluated based on a ratio (VAR) of the range of variation of light intensity to the average light intensity (range of variation / average). When the mode dispersion of the optical fiber 2 is τ and the line width of the light source 10 is Δί, VAR is a function of the product of τ and ΔΕ [0072] Figure 9 is a graph illustrating the relationship between VAR and τ · Δί, when the interference component between the spatial modes is averaged by determining the average wavelength of the beam profile. When τ · Δί is greater than or equal to 9, VAR is less than or equal to 0.05. When τ · Δί is greater than or equal to 22.5, VAR is less than or equal to 0.02. When τ · Δί is greater than or equal to 45, VAR is less than or equal to 0.01. When τ · Δί is greater than or equal to 90, VAR is less than or equal to 0.005. When τ · Δί is greater than or equal to 225, VAR is less than or equal to 0.002. When τ · Δί is greater than or equal to 450, VAR is less than or equal to 0.001.
    Consequently, the measurement is preferably carried out, while τ · Δί is greater than or equal to 9, by adjusting the line width of the light source 10 or the length or even the radius of curvature of the fiber. optical 2. Preferably also, the measurement is carried out while τ · Δ £ is greater than or equal to 22.5. More preferably, the measurement is carried out while τ · Δί is greater than or equal to 45. More preferably, the measurement is carried out while τ · Δί is greater than or equal to 90. Preferably still, the measurement is carried out while τ · Δί is greater than or equal to 225. Better still, the measurement is carried out while τ · Δί is greater than or equal to 450.
    When the interference component between the space modes in the beam profile of the combined light emitted at the output of the plurality of space modes is averaged by individual application of light components not mutually correlated from the light source 10 to all of the space modes, at the input end of the optical fiber 2, preferably, the light source 10 divides the light emitted at the output of a single light emitting element into light components to be applied to the space modes respective and applies propagation times which differ from each other by at least τ to the light components to be applied individually to all of the spatial modes.
    Figure 10 is a graph illustrating the relationship between VAR and τ · Δί in this case. When τ is greater than or equal to 1.10 / Δί, VAR is less than or equal to 0.05. When τ is greater than or equal to 1.26 / Af, VAR is less than or equal to 0.02. When τ is greater than or equal to 1.37 / Δί, VAR is less than or equal to 0.01. When τ is greater than or equal to 1.47 / Δί ', VAR is less than or equal to 0.005. When τ is greater than or equal to 1.59 / Δί,
    VAR is less than or equal to 0.002. When τ is greater than or equal to 1.67 / Δί, VAR is less than or equal to 0.001.
    Consequently, τ is preferably greater than or equal to 1.10 / Δί. Even more preferably, τ is greater than or equal to 1.26 / Δί. Even more preferably, τ is greater than or equal to 1.37 / Δί. Even more preferably, τ is greater than or equal to 1.47 / Δί. Even more preferably, τ is greater than or equal to 1.59 / Δί. Most preferably, τ is greater than or equal to 1.67 / Af.
    权利要求:
    Claims (11)
    [1" id="c-fr-0001]
    1. A method for measuring a beam profile emitted at the output of an optical fiber having a plurality of spatial modes which are randomly coupled, the method comprising:
    a measurement step in which the light emitted from a light source is applied to one or more modes of the plurality of spatial modes of the optical fiber at an input end of the optical fiber, and in which a sum of intensity profiles of individual light components emitted from respective modes of the plurality of spatial modes, by averaging an interference component between the plurality of spatial modes in a combined beam of light profile emitted from the a plurality of spatial modes at an output end of the optical fiber; and an analysis step in which a fiber optic output beam evaluation index is calculated based on the sum of the intensity profiles.
    [2" id="c-fr-0002]
    2. Method according to claim 1, in which:
    the measurement step includes measuring a near field radiation pattern, as the sum of the intensity profiles, and the analysis step includes calculating an average beam evaluation index of all supermodes, based on the measured near field radiation pattern and an assumption that the measured near field radiation pattern is a mean near field radiation pattern of beams emitted from the individual supermodes.
    [3" id="c-fr-0003]
    3. Method according to claim 1, in which:
    the measurement step comprises the measurement of a near-field radiation diagram, as the sum of the intensity profiles, and the analysis step comprises the calculation of a beam evaluation index of each of the cores included in regions separated from each other, so that each region includes one of the cores, based on the measured near-field radiation pattern and an assumption that the measured near-field radiation pattern is a radiation pattern in the near field of each of the cores.
    [4" id="c-fr-0004]
    4. Method according to claim 1, in which:
    the measuring step includes measuring a far field radiation pattern, as the sum of the intensity profiles, and the analyzing step includes calculating an average beam evaluation index of all cores, based on the measured far field radiation pattern and an assumption that the measured far field radiation pattern is a mean far field radiation pattern of beams emitted from the individual cores.
    [5" id="c-fr-0005]
    5. Method according to any one of claims 1 to 4, in which:
    the measuring step comprises determining a wavelength average of the beam profile to average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted from the plurality of spatial modes.
    [6" id="c-fr-0006]
    6. Method according to claim 5, in which:
    the measuring step comprises determining an average wavelength of the beam profile to average the interference component between the plurality of spatial modes in the beam profile of the combined light emitted from the plurality of spatial modes, while adjusting a line width Af of the light source or a length or a radius of curvature of the optical fiber, so that the product τ-Af of a dispersion of modes τ of the optical fiber and d 'a line width Af is greater than or equal to 9.
    [7" id="c-fr-0007]
    7. Method according to any one of claims 1 to 4, in which:
    the measurement step includes the individual application of uncorrelated mutually correlated light components from the light source, to all modes of the plurality of spatial modes, at the input end of the optical fiber and the averaging of the interference component between the plurality of space modes in the beam profile of the combined light emitted from the plurality of space modes.
    [8" id="c-fr-0008]
    8. Method according to claim 7, in which:
    the measurement step comprises the individual outputting of the uncorrelated mutually correlated light components from a plurality of light emitting elements which operate independently of each other.
    [9" id="c-fr-0009]
    9. Method according to claim 7, in which:
    the measurement step comprises the division of the light emitted at the output of a single light emitting element and having a line width Δί into light components to be applied to the respective spatial modes of the optical fiber and the application of time of propagation which differ from each other by at least 1.1 / Δί.
    [10" id="c-fr-0010]
    10. Method according to claim 7, in which:
    the measurement step comprises the division of the light emitted at the output of a single light emitting element into light components to be applied to the respective spatial modes of the optical fiber and the temporal variation of polarizations of the light components in different diagrams.
    [11" id="c-fr-0011]
    11. Apparatus for measuring a beam profile emitted from an optical fiber having a plurality of spatial modes which are randomly coupled, the apparatus comprising:
    a light source which emits light, so that the light is applied to one or more modes of the plurality of spatial modes of the optical fiber, at an input end of the optical fiber;
    a measurement unit which measures a sum of intensity profiles of individual components of light emitted from the respective modes of the plurality of spatial modes, by averaging an interference component between the plurality of spatial modes in a beam profile of combined light emitted at the output of the plurality of spatial modes, at an output end of the optical fiber; and an analysis unit which calculates a fiber optic output beam evaluation index, based on the sum of the intensity profiles.
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    JP2019015584A|2019-01-31|
    GB2565889A|2019-02-27|
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    优先权:
    申请号 | 申请日 | 专利标题
    JP2017132561A|JP6897373B2|2017-07-06|2017-07-06|Optical fiber emission beam profile measurement method and equipment|
    JP2017132561|2017-07-06|
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