A method for correcting one-way OTDR loss of a heterogeneous few-mode fiber fusion splice

Abstract

This invention belongs to the field of few-mode fiber loss correction technology, and specifically provides a method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices. The method includes the following steps: S1: forming a heterogeneous few-mode fiber fusion splice link; S2: generating a test curve using an OTDR; S3: obtaining the unidirectional apparent loss; S4: acquiring the structural parameters of the few-mode fiber; S5: determining the forward transmission mode and the reverse acquisition mode; S6: obtaining the lateral mode field distribution of the LP mode; S7: obtaining the normalized lateral mode field; S8: calculating the mode field overlap factor; S9: obtaining the backscattering correction factor; S10: correcting the unidirectional OTDR apparent loss; S11: outputting the corrected true insertion loss of the fusion splice. This invention can improve the accuracy of true insertion loss measurement at heterogeneous few-mode fiber fusion splices, avoid misjudgment of loss due to differences in Rayleigh scattering coefficients and mode field distributions between the two fibers, reduce the complexity of on-site testing, and improve testing efficiency.

Description

Technical Field

[0001] This invention relates to the field of few-mode fiber loss correction technology, specifically a method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices. Background Technology

[0002] As optical fiber communication systems evolve towards higher capacity, higher speed, and spatial division multiplexing, few-mode fiber, capable of supporting multiple spatial modes, has become an important transmission medium in new optical communication systems. During the construction, installation, and operation and maintenance of few-mode fiber links, fusion splice loss is a crucial indicator for evaluating link connection quality and transmission performance. For heterogeneous few-mode fiber links composed of different types of few-mode fibers, accurate measurement of fusion splice loss becomes even more complex due to differences in parameters such as fiber core structure, refractive index distribution, and mode field distribution between the two fibers.

[0003] Currently, optical time domain reflectometers (OTDRs) are commonly used in engineering to locate splices, connection points, and fault points in fiber optic links and to assess loss. An OTDR injects an optical pulse into the fiber under test and receives the backscattered and reflected light signals returning along the fiber to obtain a power change curve along the fiber. For fusion splices of ordinary single-mode fiber or the same type of fiber, the splice loss can usually be estimated based on the power abrupt change in the OTDR curve before and after the splice point. This method has advantages such as convenient testing and suitability for on-site inspection.

[0004] However, at the fusion splice of heterogeneous few-mode fibers, the loss measured by a unidirectional OTDR is not necessarily equal to the actual insertion loss of the splice. The fundamental reason is that the signal received by the OTDR is not the transmitted power after the splice, but rather the backscattered power returning along the fiber. Therefore, the power abrupt change in the OTDR curve before and after the splice is affected not only by the actual loss at the splice but also by differences in backscattering characteristics such as the Rayleigh scattering coefficient, backscattering acquisition capability, group velocity, and mode field distribution on both sides of the fiber, thus introducing additional bias into the unidirectional OTDR test results.

[0005] For few-mode fibers, different LP modes have different transverse field distributions and mode overlap characteristics. After Rayleigh scattering occurs in the forward transmission mode, the ability of the scattered light to be recaptured by the backscattering mode is closely related to the degree of mode field overlap. Therefore, in heterogeneous few-mode fiber connections, the two fibers not only have differences in material scattering characteristics but may also have differences in mode backscattering capabilities. Existing methods for directly reading abrupt changes in unidirectional OTDR curves struggle to distinguish between the actual splice loss and the apparent loss deviation caused by backscattering differences.

[0006] While bidirectional OTDR testing can mitigate the impact of differences in backscattering capabilities among different optical fibers to some extent, it requires testing from both ends of the link, making on-site operation complex and inefficient. This makes it difficult to implement in some long-distance links, already deployed links, or scenarios with limited maintenance conditions. Therefore, it is necessary to propose a unidirectional OTDR loss correction method suitable for heterogeneous few-mode fiber fusion splices. Under unidirectional testing conditions, this method combines fiber backscattering characteristics and mode acquisition characteristics to correct the apparent loss, thereby obtaining a more accurate true insertion loss. Summary of the Invention

[0007] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0008] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0009] A method for correcting loss in a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice includes the following steps:

[0010] S1: Connect the first few-mode fiber and the second few-mode fiber at the splice point to form a heterogeneous few-mode fiber fusion splice link.

[0011] S2: Connect the OTDR to one end of the first few-mode fiber, inject test light pulses into the heterogeneous few-mode fiber link under test by the OTDR, and receive the backscattered signal returned along the fiber. The OTDR forms a test curve distributed along the length of the fiber based on the returned signal.

[0012] S3: Determine the splice location of the first few-mode fiber and the second few-mode fiber based on the OTDR test curve, and read the power change of the curve before and after the splice point to obtain the unidirectional apparent loss obtained from the test from the first few-mode fiber to the second few-mode fiber.

[0013] S4: Obtain the structural parameters of the first few-mode fiber and the second few-mode fiber respectively;

[0014] S5: Based on the structural parameters, operating wavelength, and OTDR incident coupling conditions, determine the forward propagation mode and reverse capture mode that participate in backscatter correction during unidirectional testing.

[0015] S6: Based on the core refractive index, cladding refractive index, core diameter and operating wavelength of the first few-mode fiber and the second few-mode fiber, calculate their normalized frequency parameters respectively, and further determine the core transverse phase parameters and cladding transverse attenuation parameters of the corresponding LP modes. Then, obtain the transverse mode field distribution of the corresponding LP modes in the two fibers respectively.

[0016] S7: Normalize the LP mode field distribution obtained in S6 to obtain the normalized transverse mode fields corresponding to the first few-mode fiber and the second few-mode fiber.

[0017] S8: Based on the normalized mode fields of the first few-mode fiber and the second few-mode fiber, calculate the mode field overlap factor of the forward transmission mode after Rayleigh scattering and recaptured by the reverse capture mode.

[0018] S9: Based on the Rayleigh scattering coefficient, core refractive index and mode field overlap factor of the first and second few-mode fibers, determine the difference in backscattering capability of the two fibers returning to the OTDR under the same OTDR test conditions, and obtain the backscattering correction factor accordingly.

[0019] S10: Using the backscattering correction factor obtained from S9, the apparent loss of the unidirectional OTDR obtained from S3 is corrected, the deviation caused by the difference in backscattering capability between the two optical fibers is deducted or reduced, and the apparent power mutation in the unidirectional OTDR curve is converted into a result that is closer to the true transmission loss.

[0020] S11: Outputs the corrected true insertion loss of the fusion splice, and simultaneously outputs the unidirectional OTDR apparent loss, basic parameters of both optical fibers, normalized mode field, mode field overlap factor, backscattering correction factor, and comparison results before and after correction.

[0021] Preferably, in S1, the first few-mode fiber and the second few-mode fiber are few-mode fibers with different core diameters, refractive index distributions, Rayleigh scattering characteristics, or mode support conditions.

[0022] Preferably, the structural parameters in S4 include core refractive index, cladding refractive index, core radius or core diameter, operating wavelength, and Rayleigh scattering coefficient.

[0023] Preferably, in step S5, when the forward transmission mode and the reverse acquisition mode are mainly LP01 modes, the LP01-LP01 mode field overlap factor is used to characterize the main backward acquisition contribution; when other LP modes dominate, or multiple LP modes participate in transmission and acquisition together, the mode field overlap factor of the corresponding mode combination is used, or a weighted calculation is performed based on the mode excitation weight, transmission weight, and reception weight.

[0024] Preferably, in step S6, for weakly guided step-index few-mode fibers, analytical expressions for the core and cladding regions are used to solve the problem; for few-mode fibers with other refractive index distributions or non-ideal structures, the mode field distribution is obtained by numerical model solving, simulation calculation, or actual measurement, thereby obtaining the mode field distributions of the first and second few-mode fibers respectively.

[0025] Preferably, in step S3, the backscattered power obtained by extrapolating the OTDR fitting curve before the fusion splice to the fusion splice is denoted as P1, and the backscattered power obtained by extrapolating the OTDR fitting curve after the fusion splice to the fusion splice is denoted as P2. After converting this power abrupt change into a single-pass loss, the unidirectional OTDR apparent loss L measured from the first few-mode fiber to the second few-mode fiber is obtained. 12 The actual insertion loss at the fusion splice is L. true The backscattering correlation coefficients from the first few-mode fiber to the second few-mode fiber are K1 and K2, respectively. Since the abrupt change in the OTDR curve is represented by the backscattering power ratio in dB, and the splice insertion loss is defined according to the single-pass transmission loss, the difference in backscattering capability is reflected in the single-pass loss correction as follows: Then, the one-way apparent loss and the actual insertion loss satisfy the following relationship:

[0026] unidirectional OTDR apparent loss L 12 Includes the actual insertion loss L true In addition to the additional bias term caused by the difference in backscattering capabilities between the two sides, when the backscattering capability of the first few-mode fiber is stronger than that of the second few-mode fiber, K1 / K2 will cause the apparent loss to be larger. Therefore, it is necessary to adjust the K1 / K2 ratio from L. 12 The deviation item is deducted from the total.

[0027] Preferably, the specific calculation method for the normalized frequency parameter in S6 is as follows: for optical fiber q, where q=1,2, input its basic structure and optical parameters, including the core refractive index n. co,q Cladding refractive index n cl,q Core radius a q The operating wavelength λ, where the core radius a q From core diameter d q Based on the above parameters, calculate the normalized frequency parameter q of the optical fiber:

[0028] Among them, V q This is used to determine the types of LP modes that the few-mode fiber can support at the operating wavelength, and is used to subsequently solve the radial field distribution of the corresponding LP modes.

[0029] Preferably, for a given forward transmission mode LPlp Its transverse phase parameter of the fiber core is denoted as u. lp,q The lateral attenuation parameter of the cladding is denoted as w. lp,q Both conditions are met:

[0030] Among them, u lp,q and w lp,q The solution is obtained from the mode eigenvalue equation of few-mode fiber;

[0031] When the few-mode fiber is approximated as a weakly guided step-index fiber, the LP in fiber q lp The transverse field distribution of the mode is represented by analytical expressions for the core and cladding regions, when 0 ≤ r ≤ a q At that time, the mode field of the fiber core region is:

[0032] When r>a q At that time, the mode field of the cladding region is: .

[0033] Where r is the radial coordinate. Let a be the angular coordinate. q The core radius is... For a Bessel function of the first kind, For the second type of modified Bessel function, C q The cladding field amplitude coefficient, determined by the field continuity condition at r=a, is expressed as:

[0034] After obtaining the LP mode field distribution of fiber q, it is normalized. Let the normalization coefficient be A. lp,q Then the normalized transverse mode field is:

[0035] Wherein, the normalization constant A lp，q To ensure that the normalized mode field can be determined through cross-sectional integration, the energy normalization condition must be satisfied, i.e.:

[0036] In the formula, S represents the integration region of the fiber cross-section; through the above processing, the normalized mode fields of the first few-mode fiber and the second few-mode fiber are obtained respectively. and .

[0037] Preferably, after obtaining the normalized mode field, the mode field overlap factor is further calculated. For a typical forward transmission mode LP, this is done by... lp and reverse capture mode LPmn The mode field overlap factor in fiber q is expressed as:

[0038] In the formula, This represents the normalized transverse field distribution of the forward transmission mode in fiber q. This represents the normalized transverse field distribution of the reverse acquisition mode in fiber q. This indicates the conjugate operation.

[0039] Preferably, unidirectional OTDR testing mainly considers LP. 01 The incentive and reception of patterns, therefore previously given to LPs 01 After mode scattering, it is reversed by LP 01 The recaptured mode portion contributes primarily to the backscattering; in this case, the mode field overlap factor of fiber q is written as:

[0040] After obtaining the Rayleigh scattering coefficients, core refractive indices, and mode field overlap factors of both optical fibers, the ratio of the backscattering correlation coefficients of the two optical fibers is determined. For a given LP... 01 When the mode is the primary test mode, this ratio is expressed as:

[0041] In the formula, α R,1 and α R,2 n1 and n2 are the Rayleigh scattering coefficients of the first few-mode fiber and the second few-mode fiber, respectively; n1 and n2 are the core refractive indices of the first few-mode fiber and the second few-mode fiber, respectively. and LP of the first few-mode fiber and the second few-mode fiber, respectively 01 The mode field overlap factor is a formula that transforms the difference in backscattering capability between two optical fibers into a correction factor calculated from the fiber's fundamental parameters and mode field parameters.

[0042] Substituting this into the apparent loss correction relation for unidirectional OTDR, the backscattering bias term in the apparent loss is written in a computable mode parameter form, and the loss correction expression based on the mode backscattering correlation coefficient ratio is obtained:

[0043] In the formula, L true The corrected actual insertion loss at the weld joint; L 12 The apparent loss is obtained by unidirectional OTDR testing from the first few-mode fiber to the second few-mode fiber.

[0044] Compared with existing technologies, the advantages of this invention are: it can distinguish between the apparent loss measured by a unidirectional OTDR and the actual insertion loss at the splice, and compensates for the backscattering deviation in the apparent loss by introducing a correction factor corresponding to the difference in backscattering capabilities between the two optical fibers. Therefore, this invention can improve the accuracy of measuring the actual insertion loss at the splice of heterogeneous few-mode optical fibers and avoid misjudgment of loss caused by differences in Rayleigh scattering coefficients, core refractive indices, and mode field distributions between the two optical fibers.

[0045] This invention solves for the radial field distribution of the LP mode by inputting fundamental parameters such as the refractive index, core diameter, operating wavelength, and Rayleigh scattering coefficient of the few-mode fibers on both sides of the splice, and then normalizes the result. Finally, it calculates the mode field overlap factor, allowing the backscattering capability of the few-mode fiber to participate in loss correction as a calculable parameter. Therefore, this invention not only considers the differences in scattering characteristics of dissimilar fiber materials but also the impact of differences in the few-mode fiber mode field distribution on backscattering power, making the correction model more applicable to dissimilar few-mode fiber splicing scenarios.

[0046] This invention eliminates the need for separate testing at both ends of the fiber optic link; the corrected true insertion loss can be obtained using only the single-end OTDR test results and the fiber parameters on both sides. Therefore, this invention reduces the complexity of on-site testing and improves testing efficiency, making it particularly suitable for long-distance fiber optic links, existing links, single-end accessible links, and scenarios with limited on-site maintenance conditions.

[0047] This invention also provides a more reliable basis for loss determination in the evaluation of heterogeneous few-mode fiber fusion splice quality, project acceptance, link operation and maintenance, and fault diagnosis. By outputting a comparison of the loss before and after correction, misjudgments caused by apparent loss deviations in unidirectional OTDRs can be reduced, improving the consistency and reliability of few-mode fiber link detection results. Attached Figure Description

[0048] Figure 1 This is a flowchart illustrating an embodiment of the unidirectional OTDR loss correction method for heterogeneous few-mode fiber fusion splices according to the present invention.

[0049] Figure 2 This is a comparison diagram of the differences in LP01 mode field distribution between two heterogeneous optical fibers in an embodiment of a heterogeneous few-mode optical fiber fusion splice unidirectional OTDR loss correction method of the present invention.

[0050] Figure 3 This is a graph showing the variation of M1 / M2 with the core radius of fiber 2 in an embodiment of a heterogeneous few-mode fiber fusion splice unidirectional OTDR loss correction method of the present invention.

[0051] Figure 4This is a comparison chart of the mode overlap factor ratio and backscattering capability ratio in an embodiment of a heterogeneous few-mode fiber fusion splice unidirectional OTDR loss correction method of the present invention.

[0052] Figure 5 This is a diagram showing the relationship between the backscattering capability ratio and the unidirectional OTDR deviation in an embodiment of a heterogeneous few-mode fiber fusion splice loss correction method of the present invention.

[0053] Figure 6 This is a comparison diagram of errors before and after correction in an embodiment of a heterogeneous few-mode fiber fusion splice unidirectional OTDR loss correction method of the present invention.

[0054] Figure 7 This is a simulated OTDR step curve diagram under the same backscattering capability difference in an embodiment of the heterogeneous few-mode fiber fusion splice unidirectional OTDR loss correction method of the present invention. Detailed Implementation

[0055] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0056] This invention is applicable to scenarios where a first few-mode fiber and a second few-mode fiber are connected at a fusion splice, and the splice loss is measured and corrected using a unidirectional OTDR. The core idea of ​​this invention is that the fusion splice loss measured by a unidirectional OTDR is not directly equal to the actual insertion loss at the splice. The measurement result includes not only the actual fusion loss but also the apparent deviation introduced by the different backscattering capabilities of the few-mode fibers on both sides of the splice. Therefore, this invention establishes a relationship between the apparent loss of the unidirectional OTDR, the actual insertion loss, and the correlation coefficients of the backscattering of the two fibers, and further introduces a backscattering capture factor for the LP mode of the few-mode fiber to correct the apparent loss measured by the unidirectional OTDR, thereby obtaining a more accurate actual insertion loss at the fusion splice.

[0057] Specifically, a method for correcting loss in a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice includes the following steps:

[0058] S1: Connect the first few-mode fiber and the second few-mode fiber at the fusion splice point to form a heterogeneous few-mode fiber fusion splice link. The first few-mode fiber and the second few-mode fiber can be few-mode fibers with different core diameters, refractive index distributions, Rayleigh scattering characteristics, or mode support.

[0059] S2: Connect the OTDR to one end of the first few-mode fiber. Inject test light pulses into the heterogeneous few-mode fiber link under test through the OTDR and receive the backscattered signal returning along the fiber. The OTDR generates a test curve distributed along the fiber length based on the returned signal. Since the OTDR receives the backscattered power, rather than the transmitted light power after the fusion splice, the power change of the test curve at the fusion splice will be affected by both the actual fusion loss and the difference in backscattering capability between the two fibers.

[0060] S3: Determine the splice location of the first and second few-mode fibers based on the OTDR test curves, and read the power abrupt change in the curves before and after the splice point to obtain the unidirectional apparent loss measured from the first few-mode fiber to the second few-mode fiber. This apparent loss is not directly equivalent to the actual insertion loss, but rather includes the measurement result after considering the influence of the difference in backscattering characteristics between the two fibers.

[0061] S4: Obtain the core refractive index, cladding refractive index, core radius or diameter, operating wavelength, Rayleigh scattering coefficient, and other parameters for the first and second few-mode fibers, respectively. These parameters can be obtained from fiber design parameters, parameters provided by the manufacturer, experimental calibration results, simulation calculation results, or database queries. This step provides the basic data for subsequent calculations of the mode field distribution and backscattering correction factor of both fibers.

[0062] S5: Based on the structural parameters, operating wavelength, and OTDR incident coupling conditions of the few-mode fibers on both sides, determine the forward transmission mode and reverse acquisition mode involved in backscattering correction during unidirectional testing. When the forward transmission mode and reverse acquisition mode are mainly LP01 modes, the LP01-LP01 mode field overlap factor can be used to characterize the main backscattering acquisition contribution; when other LP modes dominate, or multiple LP modes participate in transmission and acquisition together, the mode field overlap factor of the corresponding mode combination can be used, or a weighted calculation can be performed based on the mode excitation weight, transmission weight, and reception weight.

[0063] S6: Based on the core refractive index, cladding refractive index, core diameter, and operating wavelength of the first and second few-mode fibers, calculate their normalized frequency parameters, and further determine the core transverse phase parameters and cladding transverse attenuation parameters of the corresponding LP modes. Then, obtain the transverse mode field distributions of the corresponding LP modes in both fibers. For weakly guided step-index few-mode fibers, analytical expressions for the core and cladding regions can be used for solving; for few-mode fibers with other refractive index distributions or non-ideal structures, numerical model solving, simulation calculations, or actual measurements can be used to obtain the mode field distributions of the first and second few-mode fibers.

[0064] S7: Normalize the LP mode field distribution obtained in S6 to obtain the normalized transverse mode fields corresponding to the first and second few-mode fibers. The normalization process is used to eliminate the difference in mode field amplitude scale, so that the mode field overlap factor calculated subsequently can reflect the influence of the mode field spatial distribution itself on the back-facing acquisition capability.

[0065] S8: Based on the normalized mode fields of the first and second few-mode fibers, calculate the mode field overlap factor for the forward transmission mode after Rayleigh scattering and recapture by the reverse acquisition mode. The more obvious the mode field overlap, the easier it is for the backward light formed by the forward mode scattering in the fiber to be recaptured by the reverse mode and returned to the OTDR, and the stronger the corresponding back-acquisition capability.

[0066] S9: Based on the Rayleigh scattering coefficient, core refractive index, and mode field overlap factor of the first and second few-mode fibers, the difference in backscattering capability returned to the OTDR under the same OTDR test conditions is determined, and a backscattering correction factor is obtained. This correction factor is used to characterize the additional deviation in the apparent loss of a unidirectional OTDR caused by the inconsistency in the backscattering capability of dissimilar few-mode fibers.

[0067] S10: Using the backscattering correction factor obtained in S9, the apparent loss of the unidirectional OTDR obtained in S3 is corrected, the deviation caused by the difference in backscattering capability between the two optical fibers is deducted or reduced, and the apparent power mutation in the unidirectional OTDR curve is converted into a result that is closer to the true transmission loss.

[0068] S11: Outputs the corrected true insertion loss of the fusion splice, and can simultaneously output the unidirectional OTDR apparent loss, fundamental parameters of both optical fibers, normalized mode field, mode field overlap factor, backscattering correction factor, and comparison results before and after correction. This result can be used for evaluating the quality of heterogeneous few-mode fiber fusion splices, project acceptance, link maintenance, and fault diagnosis.

[0069] In the above process, the apparent loss at the fusion splice of dissimilar few-mode fibers is first obtained through unidirectional OTDR testing. Assuming the first and second few-mode fibers are connected at the fusion splice, the OTDR injects a test light pulse from one end of the first few-mode fiber and receives the backscattered signal returning along the fiber. Since the OTDR receives the backscattered power, not the transmitted light power after the fusion splice, the power abrupt change in the OTDR curve before and after the fusion splice is affected not only by the actual insertion loss but also by the difference in backscattering capabilities between the two fibers.

[0070] The backscattered power obtained by extrapolating the OTDR fitting curve before the fusion splice to the fusion splice is denoted as P1, and the backscattered power obtained by extrapolating the OTDR fitting curve after the fusion splice to the fusion splice is denoted as P2. Since the power abrupt change in the OTDR curve includes the effect of the two-way transmission of the optical pulse after passing through the fusion splice, this power abrupt change is converted into a one-way loss to obtain the unidirectional OTDR apparent loss L measured from the first few-mode fiber to the second few-mode fiber. 12 The actual insertion loss at the fusion splice is L. true The backscattering correlation coefficients of fiber 1 and fiber 2 are K1 and K2, respectively. Since the abrupt change in the OTDR curve is represented by the backscattering power ratio in dB, and the splice insertion loss is defined according to the single-pass transmission loss, the difference in backscattering capability is reflected in the single-pass loss correction as follows: The apparent loss and the actual insertion loss satisfy the following relationship:

[0071] unidirectional OTDR apparent loss L 12 Includes the actual insertion loss L true And the additional bias term caused by the difference in backscattering capabilities between the two sides. When the backscattering capability of the first few-mode fiber is stronger than that of the second few-mode fiber, K1 / K2 will cause the apparent loss to be larger, so it is necessary to adjust the backscattering from L. 12 The deviation item is deducted from the total.

[0072] For an optical fiber q, where q = 1, 2, input its basic structure and optical parameters, including the core refractive index n. co,q Cladding refractive index n cl,q Core radius a q Operating wavelength λ. Wherein, the core radius a q It can be determined by the core diameter d q Based on the above parameters, the normalized frequency parameter q of the optical fiber can be calculated:

[0073] Among them, V q This is used to determine the types of LP modes that the few-mode fiber can support at the operating wavelength, and is used to subsequently solve the radial field distribution of the corresponding LP modes.

[0074] For a given LP lp The mode, whose core transverse phase parameter is denoted as u lp,q The lateral attenuation parameter of the cladding is denoted as w. lp,q Both conditions are met:

[0075] Among them, u lp,q and w lp,qThe mode eigenvalues ​​can be obtained by solving the mode eigenvalue equations of few-mode fibers. In this embodiment, for ease of explanation, it is assumed that the LP01 mode is mainly excited and received during the unidirectional OTDR test. Therefore, the focus is on solving the mode parameters corresponding to the LP01 mode in fiber 1 and fiber 2. This embodiment does not constitute a limitation on the types of modes to which this invention is applicable. When other LP modes or multiple LP modes participate in transmission and acquisition during the test, the back acquisition factor of the corresponding mode or mode combination can be solved according to the same calculation logic.

[0076] When a few-mode fiber can be approximated as a weakly guided step-index fiber, the transverse field distribution of the LPlp mode in fiber q can be represented by analytical expressions for the core and cladding regions, when 0 ≤ r ≤ a q At that time, the mode field of the fiber core region is:

[0077] When r>a q At that time, the mode field of the cladding region is: .

[0078] Where r is the radial coordinate. Let a be the angular coordinate. q The core radius is... For a Bessel function of the first kind, For the second type of modified Bessel function, C q The cladding field amplitude coefficient, determined by the field continuity condition at r=a, can be expressed as:

[0079] After obtaining the LP mode field distribution of fiber q, it is normalized. Let the normalization coefficient be A. lp,q Then the normalized transverse mode field is:

[0080] Wherein, the normalization constant A lp，q To ensure that the normalized mode field can be determined through cross-sectional integration, the energy normalization condition must be satisfied, i.e.:

[0081] In the formula, S represents the integration region of the fiber cross-section. Through the above processing, the normalized mode fields of the first few-mode fiber and the second few-mode fiber can be obtained respectively. and .

[0082] After obtaining the normalized mode field, the mode field overlap factor is further calculated. For a general forward LP... lp Mode and Reverse LP mnThe mode field overlap factor in fiber q can be expressed as:

[0083] In the formula, This represents the normalized transverse field distribution of the forward transmission mode in fiber q. This represents the normalized transverse field distribution of the reverse acquisition mode in fiber q. This indicates the conjugate operation. The mode field overlap factor characterizes the ability of a forward mode to be recaptured by a backward mode after Rayleigh scattering. The higher the degree of mode field overlap, the easier it is for the backward light scattered by the forward mode in the fiber to be captured by the backward mode and returned to the OTDR, and the stronger the corresponding backward capture capability.

[0084] In a preferred embodiment of the present invention, unidirectional OTDR testing mainly considers LP. 01 The incentive and reception of patterns, therefore previously given to LPs 01 After mode scattering, it is reversed by LP 01 The recaptured mode contributes primarily to the backscattering. Therefore, the mode field overlap factor of fiber q can be written as:

[0085] Because the core refractive index, cladding refractive index, core diameter, or refractive index distribution of heterogeneous few-mode fibers may differ, their LP 01 The mode field distribution may also be different, therefore, when connecting heterogeneous few-mode fibers, M 01-01 They are usually different. This difference affects the local backscattering capture capability of the two optical fibers and further affects the backscattering deviation in the apparent loss of a unidirectional OTDR.

[0086] After obtaining the Rayleigh scattering coefficients, core refractive indices, and mode field overlap factors of both optical fibers, the ratio of backscattering correlation coefficients between the two optical fibers can be determined. For LP... 01 When the mode is the primary test mode, this ratio can be expressed as:

[0087] In the formula, α R,1 and α R,2 n1 and n2 are the Rayleigh scattering coefficients of the first few-mode fiber and the second few-mode fiber, respectively; n1 and n2 are the core refractive indices of the first few-mode fiber and the second few-mode fiber, respectively. and LP of the first few-mode fiber and the second few-mode fiber, respectively 01 Mode field overlap factor. This formula transforms the difference in backscattering capabilities between two optical fibers into a correction factor that can be calculated from the fiber's fundamental parameters and mode field parameters.

[0088] Substituting this into the apparent loss correction relation for unidirectional OTDR, the backscattering bias term in the apparent loss can be written in a computable mode parameter form, and the loss correction expression based on the mode backscattering correlation coefficient ratio can be obtained:

[0089] In the formula, L true The corrected actual insertion loss at the weld joint; L 12 This is the apparent loss obtained from a unidirectional OTDR test from the first few-mode fiber to the second few-mode fiber. Using this formula, the apparent power abrupt change read from the unidirectional OTDR curve can be corrected to the true insertion loss, which is closer to the actual transmission loss.

[0090] In summary, the technical solution of the present invention is implemented according to the following calculation logic: first, the apparent loss L is obtained through unidirectional OTDR testing. 12 The process involves inputting basic parameters such as refractive index, core diameter, operating wavelength, and Rayleigh scattering coefficient of the two few-mode fibers. Then, the radial field distribution of the LP modes in both fibers is calculated and normalized. Next, the mode field overlap factor is calculated based on the normalized mode field. Finally, a backscattering correction factor is formed by combining the Rayleigh scattering coefficient, core refractive index, and mode field overlap factor to correct the apparent loss of the unidirectional OTDR, outputting the true insertion loss of the fusion splice. This scheme can reduce measurement errors caused by differences in backscattering capabilities between the two sides of heterogeneous few-mode fibers without relying on bidirectional OTDR measurements. It is suitable for evaluating the fusion splice quality of heterogeneous few-mode fibers, engineering acceptance, and link maintenance testing.

[0091] This invention is achieved through a combination of a higher-level mode description and a preferred implementation method: In the higher-level scheme, the forward transmission mode and the reverse capture mode can be any LP mode or a combination of multiple LP modes supported by the few-mode fiber; in the preferred implementation method, the LP01 mode is used as the main test mode, and specific calculations are performed in conjunction with the analytical mode field expression of the weakly guided step-index few-mode fiber. Therefore, this invention is applicable not only to test scenarios dominated by the LP01 mode, but also extends to scenarios involving other LP modes or multiple modes in heterogeneous few-mode fiber fusion splice loss correction.

[0092] Example 1:

[0093] like Figure 1As shown, this embodiment provides a method for correcting the loss of a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice. It is applicable to scenarios where a first few-mode fiber and a second few-mode fiber are connected at the splice, and the core diameter, refractive index distribution, Rayleigh scattering coefficient, or mode field distribution of the two fibers differ, causing the apparent loss measured by the unidirectional OTDR to not accurately reflect the true insertion loss of the splice. Specifically, this embodiment is implemented according to the following steps: "establishing a heterogeneous few-mode fiber fusion splice link—obtaining a unidirectional OTDR test curve—extracting apparent loss—inputting fiber basic parameters—determining the main LP modes—solving for the mode field distribution—normalizing the mode field—calculating the mode field overlap factor—determining the backscattering correction factor—correcting apparent loss—outputting the true insertion loss." This method can correct the backscattering deviation in the apparent loss of a unidirectional OTDR without relying on bidirectional OTDR testing, thereby obtaining a true insertion loss at the splice that is closer to the actual transmission loss.

[0094] In this embodiment, the method of the present invention mainly includes the following steps:

[0095] Step A: Establishing a heterogeneous few-mode fiber fusion splice link: First, fusion splice the first and second few-mode fibers at the test location to form a heterogeneous few-mode fiber fusion splice link. The first and second few-mode fibers can be few-mode fibers with different core diameters, core refractive indices, cladding refractive indices, Rayleigh scattering coefficients, refractive index distributions, or supported modes. The splice point serves as the test location and is used for subsequent unidirectional OTDR apparent loss measurement and true insertion loss correction.

[0096] In this embodiment, the first few-mode fiber is designated as fiber 1, and the second few-mode fiber is designated as fiber 2. The OTDR is connected from one end of fiber 1 and performs unidirectional testing in the direction of fiber 2. Since fiber 1 and fiber 2 are heterogeneous few-mode fibers, their backscattering capabilities may differ, therefore, it is necessary to correct the apparent loss measured by the unidirectional OTDR.

[0097] Step B: Acquire unidirectional OTDR backscattering curves and extract apparent loss: Connect the OTDR to one end of the first few-mode fiber, inject test light pulses into the heterogeneous few-mode fiber fusion splice link through the OTDR, and receive the backscattered signal returning along the fiber. The OTDR generates a test curve distributed along the fiber length based on the received backscattered signal.

[0098] After obtaining the OTDR test curve, the fusion splice location of the first and second few-mode fibers is determined based on the event positions in the curve. Stable backscattering sections are selected before and after the fusion splice for fitting, and the fitted curve is extrapolated to the fusion splice location to obtain the corresponding backscattering power change before and after the fusion splice. This power change is then converted into a one-way loss to obtain the unidirectional OTDR apparent loss tested from the first few-mode fiber to the second few-mode fiber. Since the OTDR receives the backscattered power returning along the fiber, rather than the transmitted light power after the fusion splice, this apparent loss includes not only the actual insertion loss at the fusion splice but also the additional deviation caused by the difference in backscattering capabilities of the two few-mode fibers.

[0099] Step C: Input the basic parameters of the few-mode fibers on both sides of the splice: After obtaining the apparent loss of the unidirectional OTDR, obtain the basic structural and optical parameters of the first and second few-mode fibers, respectively. The basic parameters include core refractive index, cladding refractive index, core radius or core diameter, operating wavelength, and Rayleigh scattering coefficient, etc.

[0100] The above parameters can be obtained from fiber design parameters, parameters provided by the manufacturer, experimental calibration results, or simulation calculation results. The purpose of this step is to provide basic data for subsequent LP mode parameter solving, mode field distribution calculation, mode field overlap factor calculation, and backscattering correction factor calculation.

[0101] Step D: Determine the main LP modes in the unidirectional test process: Based on the structural parameters, operating wavelengths, and OTDR incident coupling conditions of the first and second few-mode fibers, determine the LP modes that mainly participate in forward transmission and reverse capture in the unidirectional test process.

[0102] In a preferred embodiment of this example, the LP01 mode is selected as the primary test mode. This means that the OTDR-injected light primarily excites the forward LP01 mode, and after Rayleigh scattering, it is mainly recaptured by the reverse LP01 mode and returns to the OTDR. In this case, the portion scattered by the forward LP01 mode and recaptured by the reverse LP01 mode can be considered as the main backscattering contribution, used to characterize the local backscattering capability of the few-mode fibers on both sides of the splice. If other LP modes dominate in actual testing, the LP01 mode can be replaced with the corresponding forward transmission mode and reverse capture mode for calculation.

[0103] Step E: Solve for the radial field distribution of the LP mode in the two few-mode fibers: After determining the main LP mode, calculate the normalized frequency parameters of the two fibers based on the core refractive index, cladding refractive index, core diameter and operating wavelength of the first and second few-mode fibers respectively, and further solve for the core transverse phase parameters and cladding transverse attenuation parameters of the corresponding LP modes.

[0104] Subsequently, based on the obtained mode parameters, the radial field distributions of the LP modes in the core and cladding regions of the first and second few-mode fibers are calculated, respectively. For step-index few-mode fibers, the mode field in the core region can be described by a Bessel function of the first kind, and the mode field in the cladding region can be described by a modified Bessel function of the second kind. The field amplitude coefficient in the cladding region is determined by the field continuity condition at the core-cladding interface. Through the above processing, the transverse field distributions of the corresponding main LP modes in both fibers can be obtained.

[0105] Step F: Normalize the LP mode field: After obtaining the LP mode field distribution of the first few-mode fiber and the second few-mode fiber, normalize the mode fields of the two fibers respectively to obtain the corresponding normalized transverse mode fields.

[0106] This normalization process eliminates differences in mode field amplitude scales, ensuring that the subsequently calculated mode field overlap factor primarily reflects the influence of the mode field spatial distribution itself on the back-facing acquisition capability. After normalization, the mode fields of both optical fibers can be compared and integrated at a uniform scale, providing a foundation for subsequent solutions to the mode field overlap factor.

[0107] Step G: Calculate the mode field overlap factor: After obtaining the normalized transverse mode field, calculate the mode field overlap factor between the forward transmission mode and the reverse capture mode in the first few-mode fiber and the second few-mode fiber, respectively.

[0108] Specifically, the mode field overlap factor characterizes the ability of a forward LP mode, after Rayleigh scattering in an optical fiber, to be recaptured by a reverse LP mode and returned to the OTDR. If the lateral field distributions of the forward and reverse modes overlap significantly, it indicates that the backward light scattered by the forward mode is more easily captured by the reverse mode, and the fiber has a strong back-trapping capability; if the overlap is low, the corresponding back-trapping capability is weak.

[0109] In a preferred embodiment, when the primary test mode is LP01 mode, the LP01 mode field overlap factor of the first few-mode fiber and the second few-mode fiber are calculated respectively. Since the core diameter, refractive index distribution, or mode field distribution of dissimilar few-mode fibers may be different, the mode field overlap factors obtained by the two fibers are usually not exactly the same. This difference will further affect the backscattering power returned to the OTDR by the two fibers.

[0110] Step H: Determine the backscattering correction factor of the two optical fibers: After obtaining the Rayleigh scattering coefficient, core refractive index and mode field overlap factor of the two few-mode optical fibers, calculate the difference in backscattering capability between the first few-mode optical fiber and the second few-mode optical fiber under the same OTDR test conditions, and determine the backscattering correction factor accordingly.

[0111] This backscattering correction factor characterizes the additional bias in the apparent loss of a unidirectional OTDR caused by the inconsistency in the backscattering capabilities of dissimilar few-mode fibers. For cases where the LP01 mode is the primary test mode, the backscattering correction factor can be determined jointly by the Rayleigh scattering coefficients, core refractive indices, and LP01 mode field overlap factors of both fibers. This method transforms the backscattering bias, which is inherently difficult to separate directly from the OTDR curve, into a correction that can be calculated from fiber parameters and mode field parameters.

[0112] Step I: Correct the apparent loss of the unidirectional OTDR: Use the backscattering correction factor obtained in step H to correct the apparent loss of the unidirectional OTDR obtained in step B, deduct or reduce the additional deviation caused by the difference in backscattering capability of the two few-mode fibers, and thus obtain the true insertion loss of the fusion splice.

[0113] In this step, the change in backscattered power before and after the fusion point in the unidirectional OTDR curve, after single-pass conversion, is considered as the unidirectional apparent loss, rather than being directly used as the true insertion loss. By introducing a backscattering correction factor, the backscattering deviation term in the apparent loss can be compensated, making the corrected loss result closer to the actual transmission loss at the fusion point.

[0114] Step J: Output the actual insertion loss and related intermediate results: Finally, output the corrected actual insertion loss of the fusion splice. Depending on actual needs, the apparent loss of the unidirectional OTDR, the basic parameters of the first and second few-mode fibers, the LP mode field distribution, the normalized mode field, the mode field overlap factor, the backscattering correction factor, and the loss comparison results before and after correction can also be output simultaneously.

[0115] This embodiment demonstrates that the unidirectional OTDR loss correction method for heterogeneous few-mode fiber fusion splices proposed in this invention can achieve true insertion loss correction using unidirectional OTDR test results and fiber parameters on both sides under single-end access conditions. This method can reduce the apparent loss deviation caused by differences in Rayleigh scattering coefficient, core refractive index, and mode field distribution in heterogeneous few-mode fibers, improve the accuracy of loss measurement at heterogeneous few-mode fiber fusion splices, and is suitable for scenarios such as fusion splice quality evaluation, engineering acceptance, link maintenance, and fault diagnosis of heterogeneous few-mode fiber links.

[0116] Example 2:

[0117] This embodiment, based on Embodiment 1, further illustrates the proposed method for correcting unidirectional OTDR loss at fusion splices of heterogeneous few-mode fibers by incorporating a set of calculated parameters. This embodiment selects two segments of step-index few-mode fibers as the analysis object, with the first few-mode fiber located to the left of the fusion splice and the second few-mode fiber located to the right. Due to differences in core radius, core refractive index, and Rayleigh scattering coefficient between the two fibers, the backscattered power returned to the OTDR from both fibers differs under the same OTDR test conditions. If the power abrupt change before and after the fusion splice in the unidirectional OTDR curve is directly taken as the fusion splice loss, an apparent deviation caused by inconsistent backscattering capabilities will be introduced.

[0118] In this embodiment, the core radius of the first few-mode fiber is 8.0 μm, the core refractive index is 1.46520, the cladding refractive index is 1.45601, the Rayleigh scattering coefficient is 0.18 dB / km, and the operating wavelength is 1.55 μm; the core radius of the second few-mode fiber is 7.4 μm, the core refractive index is 1.46350, the cladding refractive index is 1.45601, the Rayleigh scattering coefficient is 0.20 dB / km, and the operating wavelength is also 1.55 μm. To facilitate verification of the effects before and after correction, in this simulation verification embodiment, the preset actual insertion loss at the fusion splice is 0.30 dB, and the apparent loss of the unidirectional OTDR is generated or calculated based on this actual insertion loss and the difference in backscattering capabilities of the two fibers; then, the apparent loss is corrected using the method of this invention, and the correction result is compared with the preset actual insertion loss.

[0119] Based on the above parameters, the normalized frequency parameter V of the first few-mode fiber is 5.3135, the LP01 mode parameters u are 2.0153 and w is 4.9164, and the mode field overlap factor M01-01 is 7.204560 × 10⁻⁶. 9 1 / m²; The normalized frequency parameter V of the second few-mode fiber is 4.4358, the LP01 mode parameters u are 1.9491, w is 3.9847, and the mode field overlap factor M01-01 is 7.791178 × 10⁻⁶. 9 1 / m². Therefore, the mode field overlap factor ratio M1 / M2 of the two optical fibers is 0.924707. Combining the Rayleigh scattering coefficients and core refractive index parameters of the two optical fibers, the backscattering capability ratio K1 / K2 is calculated to be 0.746409, corresponding to the unidirectional OTDR apparent loss deviation term 5log. 10 (K1 / K2) is -0.635116dB.

[0120] In this simulation verification embodiment, the preset actual insertion loss L at the fusion splice point is... trueThe value is 0.300000 dB. Since K1 / K2 is less than 1, it indicates that when testing from the first few-mode fiber to the second few-mode fiber, the backscattering capability of the fiber behind the splice is relatively stronger. Therefore, the apparent loss of the unidirectional OTDR will be less than the actual insertion loss, and may even manifest as an apparent gain. Based on the correction relationship, the apparent loss L of the forward unidirectional OTDR is calculated. 12 The reverse apparent loss is -0.335116 dB. L21 The average loss in both directions is 0.935116 dB, and the average loss in both directions is 0.300000 dB. After applying the unidirectional correction method based on K1 / K2 described in this invention, the corrected loss is 0.300000 dB, which is consistent with the preset true insertion loss, and the correction error is 0 dB.

[0121] Step A: Input heterogeneous few-mode fiber parameters and determine the analysis object: First, input the basic structural and optical parameters of the first and second few-mode fibers, including core radius, core refractive index, cladding refractive index, Rayleigh scattering coefficient, and operating wavelength. These parameters are used to characterize the structural differences, refractive index differences, and backscattering characteristics of the few-mode fibers on both sides of the splice.

[0122] In this embodiment, both the first and second few-mode fibers are treated as step-index few-mode fibers, and the weakly guided, weakly coupled LP-mode approximation is adopted. During testing, the excitation and reception of the LP01 mode are mainly considered; that is, the portion of the forward LP01 mode that is Rayleigh scattered and then recaptured by the reverse LP01 mode is taken as the main backscattering contribution.

[0123] Step B: Calculate the LP01 mode parameters and mode field distribution of the two few-mode fibers: After inputting the fiber parameters on both sides, calculate the normalized frequency parameters of the first and second few-mode fibers based on the core radius, core refractive index, cladding refractive index, and operating wavelength, respectively. Then, solve for the corresponding core transverse phase parameters and cladding transverse attenuation parameters according to the LP01 mode characteristic equation.

[0124] After obtaining the above mode parameters, the radial field distribution of the LP01 mode in the first and second few-mode fibers is calculated respectively. In the core region, the LP01 mode field is described by a first-type Bessel function; in the cladding region, the LP01 mode field is described by a second-type modified Bessel function, and the field amplitude coefficient in the cladding region is determined by the field continuity condition at the core-cladding interface.

[0125] like Figure 2As shown, the above calculations yield a comparison of the LP01 mode field distributions of the two dissimilar few-mode fibers. Due to the differences in core radius and refractive index parameters between the two fibers, the radial distribution range of their LP01 mode fields, the variation trend near the core, and the cladding attenuation characteristics differ. These differences further affect the back-side mode acquisition capability of the two fibers.

[0126] Step C: Normalize the LP01 mode field and calculate the mode field overlap factor: After obtaining the LP01 mode field distributions of the two few-mode fibers, the mode fields are normalized to obtain the normalized transverse mode fields corresponding to the first and second few-mode fibers. This normalization process is used to eliminate the difference in mode field amplitude scale, so that the subsequently calculated mode field overlap factor mainly reflects the influence of the spatial distribution difference of the mode field on the back-facing acquisition capability.

[0127] Subsequently, based on the normalized LP01 mode field, the LP01 mode field overlap factors of the first and second few-mode fibers were calculated respectively. The mode field overlap factor characterizes the ability of the forward LP01 mode to be recaptured by the reverse LP01 mode and returned to the OTDR after Rayleigh scattering. A higher degree of mode field overlap indicates that the backward light formed by scattering from the forward mode in the fiber is more easily captured by the reverse mode; a lower degree of mode field overlap indicates a weaker backward capture capability.

[0128] like Figure 3 As shown, by changing the core radius of the second few-mode fiber while keeping the parameters of the first few-mode fiber constant, the relationship between the mode field overlap factor ratio M1 / M2 and the core radius of the second few-mode fiber can be obtained. This figure illustrates that when the core diameter of dissimilar few-mode fibers changes, the radial distribution of the LP01 mode field also changes, resulting in a difference in the back-facing mode acquisition capability of the two fibers. In this embodiment, with a second few-mode fiber core radius of 7.4 μm, the calculated M1 / M2 is 0.924707, indicating that the LP01 mode field overlap factor of the second few-mode fiber is higher than that of the first few-mode fiber, and its back-facing acquisition capability is relatively stronger.

[0129] Step D: Calculate the backscattering capability ratio of the two optical fibers: After obtaining the mode field overlap factor of the two optical fibers, combine the Rayleigh scattering coefficient of the first few-mode fiber and the second few-mode fiber and the core refractive index to calculate the backscattering capability ratio K1 / K2 of the two optical fibers under the same OTDR test conditions.

[0130] In this embodiment, K1 / K2 comprehensively reflects the differences in Rayleigh scattering, core refractive index, and back-trapping capability of the LP01 mode between the two few-mode fibers. For cases where the LP01 mode is the primary test mode, this ratio is determined by the Rayleigh scattering coefficients, core refractive indices, and LP01 mode field overlap factors of both fibers.

[0131] like Figure 4 As shown, with the change in the core radius of the second few-mode fiber, both the mode field overlap factor ratio M1 / M2 and the backscattering capability ratio K1 / K2 change. M1 / M2 primarily reflects the difference in mode field distribution and backscattering capability between the two LP01 fibers, while K1 / K2 further integrates factors such as the Rayleigh scattering coefficient, core refractive index, and mode field overlap factor. Under the parameter conditions of this embodiment, M1 / M2 is 0.924707, and K1 / K2 is 0.746409. This result indicates that the difference in mode fields is further reflected in the difference in backscattering capability between the two fibers; therefore, introducing the mode field overlap factor is necessary in the unidirectional OTDR loss correction at heterogeneous few-mode fiber splices.

[0132] Step E: Determine the apparent loss deviation of the unidirectional OTDR: After obtaining the backscattering capability ratio K1 / K2, further calculate the apparent loss deviation term of the unidirectional OTDR caused by the difference in backscattering capabilities of the two optical fibers. This deviation term represents the additional loss or apparent gain introduced by the inconsistency of backscattering characteristics of dissimilar few-mode fibers in the unidirectional OTDR test results, in addition to the actual insertion loss at the fusion splice.

[0133] like Figure 5 As shown, the apparent loss deviation term of a unidirectional OTDR varies with the backscattering capability ratio K1 / K2. When K1 / K2 equals 1, it indicates that the backscattering capabilities of both optical fibers are consistent, and the unidirectional OTDR deviation term is close to 0. When K1 / K2 is greater than 1, the apparent loss of the forward unidirectional OTDR is larger than the actual insertion loss. When K1 / K2 is less than 1, the apparent loss of the forward unidirectional OTDR is smaller than the actual insertion loss, and there may even be an apparent gain.

[0134] In this embodiment, K1 / K2 is 0.746409, corresponding to the deviation term 5log. 10 (K1 / K2) is -0.635116 dB. Therefore, when the actual insertion loss is 0.300000 dB, the apparent loss of the forward unidirectional OTDR is -0.335116 dB. This result indicates that at the fusion splice of heterogeneous few-mode fibers, the apparent step in the unidirectional OTDR curve cannot be directly taken as the actual insertion loss and needs to be corrected based on the ratio of the backscattering capabilities of the two fibers.

[0135] Step F: Calculate the forward apparent loss, reverse apparent loss, and bidirectional average result: To further illustrate the impact of backscattering bias on the OTDR loss measurement results, this embodiment calculates the forward apparent loss obtained from the test direction from the first few-mode fiber to the second few-mode fiber, and the reverse apparent loss obtained from the test direction from the second few-mode fiber to the first few-mode fiber, under the condition that the actual insertion loss is preset to 0.30dB in the simulation verification.

[0136] When the backscattering capabilities of the two optical fibers differ, the apparent loss in the forward direction and the apparent loss in the reverse direction will deviate in opposite directions relative to the true insertion loss. If tests can be performed separately from both ends of the link, the impact of the backscattering difference on the loss result can be reduced by averaging the forward and reverse test results. However, in long-distance links, deployed links, or scenarios with only single-end access, testing from both ends is often inconvenient. Therefore, this embodiment further employs the unidirectional loss correction method proposed in this invention to obtain the corrected true insertion loss using only the unidirectional test results and the parameters of the two optical fibers.

[0137] Step G: Correct the apparent loss of the unidirectional OTDR: After obtaining the apparent loss of the unidirectional OTDR and the backscattering capability ratio K1 / K2, the unidirectional apparent loss is corrected using a backscattering correction factor. The deviation caused by the difference in backscattering capability of the two small-mode fibers is deducted or compensated to obtain the corrected true insertion loss of the fusion splice.

[0138] In this embodiment, since the preset true insertion loss in the simulation verification is 0.30 dB, the unidirectional apparent loss before correction and the corrected loss can be compared with this true insertion loss. The calculation results show that the unidirectional OTDR apparent loss before correction will deviate from the true insertion loss due to the difference in backscattering capabilities of the two optical fibers; after correction by the method of the present invention, the output loss can return to or approach the true insertion loss, thus demonstrating that the correction method of the present invention can effectively reduce the unidirectional OTDR apparent loss deviation.

[0139] like Figure 6 As shown, under the parameter conditions of this embodiment, without backscattering capability correction, the apparent loss of the forward unidirectional OTDR is -0.335116 dB, with an error of -0.635116 dB compared to the preset true insertion loss of 0.300000 dB; the apparent loss in the reverse direction is 0.935116 dB, with an error of 0.635116 dB compared to the true insertion loss. After correction using the method of this invention, the loss obtained based on K1 / K2 correction is 0.300000 dB, and the error after correction is 0 dB.

[0140] The results indicate that the difference in backscattering capability between the two sides of heterogeneous few-mode fibers leads to a significant deviation between the apparent loss of unidirectional OTDR and the true insertion loss. This invention, by introducing the backscattering capability ratio K1 / K2 and the mode field overlap factor, can effectively compensate for this deviation, restoring the unidirectional test results to or near the true insertion loss. Therefore, this invention can improve the accuracy of splice loss measurement in heterogeneous few-mode fibers without relying on bidirectional OTDR measurements.

[0141] Step H: Output correction results and related intermediate results: Finally, output the corrected actual insertion loss of the fusion splice, and simultaneously output the unidirectional OTDR apparent loss, basic parameters of the few-mode fibers on both sides, LP01 mode parameters, normalized mode field, mode field overlap factor, backscattering capability ratio, unidirectional OTDR deviation term, and error comparison results before and after correction.

[0142] In some implementations, OTDR step curves can also be output for different backscattering capabilities. For example... Figure 7 As shown, when the backscattering capabilities of the two optical fibers are the same, the OTDR step at the splice mainly reflects the actual insertion loss; when the backscattering capabilities of the two optical fibers are different, the OTDR step at the splice will increase or decrease, and may even manifest as an apparent gain. This result further illustrates that at the splice of heterogeneous few-mode optical fibers, the method described in this invention is needed to correct the apparent loss of the unidirectional OTDR.

[0143] In summary, under the heterogeneous few-mode fiber parameters used in this embodiment, the method of this invention first solves for the LP01 mode field distribution based on the structural and optical parameters of the few-mode fibers on both sides, then calculates the mode field overlap factor, and combines the Rayleigh scattering coefficient and the core refractive index to obtain the backscattering correction factor. Finally, this correction factor is used to correct the apparent loss of the unidirectional OTDR. The results show that this invention can effectively reduce the unidirectional OTDR loss measurement deviation caused by the difference in backscattering capabilities on both sides of the heterogeneous few-mode fiber, making the corrected loss result closer to the true insertion loss. This method is applicable to heterogeneous few-mode fiber fusion splice quality evaluation, engineering acceptance, link operation and maintenance testing, and OTDR loss measurement scenarios under single-end access conditions.

[0144] Although the present invention has been described above with reference to embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, as long as there is no structural conflict, the features in the disclosed embodiments can be combined with each other in any manner. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for correcting loss in a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice, characterized in that, Includes the following steps: S1: Connect the first few-mode fiber and the second few-mode fiber at the splice point to form a heterogeneous few-mode fiber fusion splice link. S2: Connect the OTDR to one end of the first few-mode fiber, inject test light pulses into the heterogeneous few-mode fiber link under test by the OTDR, and receive the backscattered signal returned along the fiber. The OTDR forms a test curve distributed along the length of the fiber based on the returned signal. S3: Determine the splice location of the first few-mode fiber and the second few-mode fiber based on the OTDR test curve, and read the power change of the curve before and after the splice point to obtain the unidirectional apparent loss obtained from the test from the first few-mode fiber to the second few-mode fiber. S4: Obtain the structural parameters of the first few-mode fiber and the second few-mode fiber respectively; S5: Based on the structural parameters, operating wavelength, and OTDR incident coupling conditions, determine the forward propagation mode and reverse capture mode that participate in backscatter correction during unidirectional testing. S6: Based on the core refractive index, cladding refractive index, core diameter and operating wavelength of the first few-mode fiber and the second few-mode fiber, calculate their normalized frequency parameters respectively, and further determine the core transverse phase parameters and cladding transverse attenuation parameters of the corresponding LP modes. Then, obtain the transverse mode field distribution of the corresponding LP modes in the two fibers respectively. S7: Normalize the LP mode field distribution obtained in S6 to obtain the normalized transverse mode fields corresponding to the first few-mode fiber and the second few-mode fiber. S8: Based on the normalized mode fields of the first few-mode fiber and the second few-mode fiber, calculate the mode field overlap factor of the forward transmission mode after Rayleigh scattering and recaptured by the reverse capture mode. S9: Based on the Rayleigh scattering coefficient, core refractive index and mode field overlap factor of the first and second few-mode fibers, determine the difference in backscattering capability of the two fibers returning to the OTDR under the same OTDR test conditions, and obtain the backscattering correction factor accordingly. S10: Using the backscattering correction factor obtained from S9, the apparent loss of the unidirectional OTDR obtained from S3 is corrected, the deviation caused by the difference in backscattering capability between the two optical fibers is deducted or reduced, and the apparent power mutation in the unidirectional OTDR curve is converted into a result that is closer to the true transmission loss. S11: Outputs the corrected true insertion loss of the fusion splice, and simultaneously outputs the unidirectional OTDR apparent loss, basic parameters of both optical fibers, normalized mode field, mode field overlap factor, backscattering correction factor, and comparison results before and after correction.

2. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, In S1, the first few-mode fiber and the second few-mode fiber are few-mode fibers with different core diameters, refractive index distributions, Rayleigh scattering characteristics, or mode support conditions.

3. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, The structural parameters in S4 include core refractive index, cladding refractive index, core radius or core diameter, operating wavelength, and Rayleigh scattering coefficient.

4. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, In S5, when the forward transmission mode and the reverse acquisition mode are mainly LP01 mode, the LP01-LP01 mode field overlap factor is used to characterize the main backward acquisition contribution; when other LP modes dominate, or multiple LP modes participate in transmission and acquisition together, the mode field overlap factor of the corresponding mode combination is used, or a weighted calculation is performed according to the mode excitation weight, transmission weight, and reception weight.

5. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, In S6, for weakly guided step-index few-mode fibers, analytical expressions for the core region and cladding region are used to solve the problem. For few-mode fibers with other refractive index distributions or non-ideal structures, the mode field distribution is obtained by numerical model solving, simulation calculation or actual measurement, thereby obtaining the mode field distribution of the first few-mode fiber and the second few-mode fiber respectively.

6. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, In step S3, the backscattered power obtained by extrapolating the OTDR fitting curve before the fusion splice to the fusion splice is denoted as P1, and the backscattered power obtained by extrapolating the OTDR fitting curve after the fusion splice to the fusion splice is denoted as P2. After converting this power abrupt change into a single-pass loss, the unidirectional OTDR apparent loss L measured from the first few-mode fiber to the second few-mode fiber is obtained. 12 The actual insertion loss at the fusion splice is L. true The backscattering correlation coefficients from the first few-mode fiber to the second few-mode fiber are K1 and K2, respectively. Since the abrupt change in the OTDR curve is represented by the backscattering power ratio in dB, and the splice insertion loss is defined according to the single-pass transmission loss, the difference in backscattering capability is reflected in the single-pass loss correction as follows: Then, the one-way apparent loss and the actual insertion loss satisfy the following relationship: unidirectional OTDR apparent loss L 12 Includes the actual insertion loss L true In addition to the additional bias term caused by the difference in backscattering capabilities between the two sides, when the backscattering capability of the first few-mode fiber is stronger than that of the second few-mode fiber, K1 / K2 will cause the apparent loss to be larger. Therefore, it is necessary to adjust the K1 / K2 ratio from L. 12 The deviation item is deducted from the total.

7. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 1, characterized in that, The specific calculation method for the normalized frequency parameter in S6 is as follows: For optical fiber q, where q=1,2, input its basic structure and optical parameters, including the core refractive index n. co,q Cladding refractive index n cl,q Core radius a q The operating wavelength λ, where the core radius a q From core diameter d q Based on the above parameters, calculate the normalized frequency parameter q of the optical fiber: Among them, V q This is used to determine the types of LP modes that the few-mode fiber can support at the operating wavelength, and is used to subsequently solve the radial field distribution of the corresponding LP modes.

8. The method for unidirectional OTDR loss correction at heterogeneous few-mode fiber fusion splices according to claim 4, characterized in that, For a given forward transmission mode LP lp Its transverse phase parameter of the fiber core is denoted as u. lp,q The lateral attenuation parameter of the cladding is denoted as w. lp,q Both conditions are met: Among them, u lp,q and w lp,q The solution is obtained from the mode eigenvalue equation of few-mode fiber; When the few-mode fiber is approximated as a weakly guided step-index fiber, the LP in fiber q lp The transverse field distribution of the mode is represented by analytical expressions for the core and cladding regions, when 0 ≤ r ≤ a q At that time, the mode field of the fiber core region is: When r>a q At that time, the mode field of the cladding region is: . Where r is the radial coordinate. Let a be the angular coordinate. q The core radius is... For a Bessel function of the first kind, For the second type of modified Bessel function, C q The cladding field amplitude coefficient, determined by the field continuity condition at r=a, is expressed as: After obtaining the LP mode field distribution of fiber q, it is normalized. Let the normalization coefficient be A. lp,q Then the normalized transverse mode field is: Wherein, the normalization constant A lp，q To ensure that the normalized mode field can be determined through cross-sectional integration, the energy normalization condition must be satisfied, i.e.: In the formula, S represents the integration region of the fiber cross-section; through the above processing, the normalized mode fields of the first few-mode fiber and the second few-mode fiber are obtained respectively. and .

9. A method for correcting loss in a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice according to claim 8, characterized in that, After obtaining the normalized mode field, the mode field overlap factor is further calculated for a typical forward transmission mode LP. lp and reverse capture mode LP mn The mode field overlap factor in fiber q is expressed as: In the formula, This represents the normalized transverse field distribution of the forward transmission mode in fiber q. This represents the normalized transverse field distribution of the reverse acquisition mode in fiber q. This indicates the conjugate operation.

10. A method for correcting loss in a unidirectional OTDR at a heterogeneous few-mode fiber fusion splice according to claim 9, characterized in that, One-way OTDR testing mainly considers LP 01 The incentive and reception of patterns, therefore previously given to LPs 01 After mode scattering, it is reversed by LP 01 The recaptured mode portion contributes primarily to the backscattering; in this case, the mode field overlap factor of fiber q is written as: After obtaining the Rayleigh scattering coefficients, core refractive indices, and mode field overlap factors of both optical fibers, the ratio of the backscattering correlation coefficients of the two optical fibers is determined. For a given LP... 01 When the mode is the primary test mode, this ratio is expressed as: In the formula, α R,1 and α R,2 n1 and n2 are the Rayleigh scattering coefficients of the first few-mode fiber and the second few-mode fiber, respectively; n1 and n2 are the core refractive indices of the first few-mode fiber and the second few-mode fiber, respectively. and LP of the first few-mode fiber and the second few-mode fiber, respectively 01 The mode field overlap factor is a formula that transforms the difference in backscattering capability between two optical fibers into a correction factor calculated from the fiber's fundamental parameters and mode field parameters. Substituting this into the apparent loss correction relation for unidirectional OTDR, the backscattering bias term in the apparent loss is written in a computable mode parameter form, and the loss correction expression based on the mode backscattering correlation coefficient ratio is obtained: In the formula, L true The corrected actual insertion loss at the weld joint; L 12 The apparent loss is obtained by unidirectional OTDR testing from the first few-mode fiber to the second few-mode fiber.

Images