Optical fiber three-dimensional refractive index measuring method based on differential interference optical tomography

Abstract

The invention belongs to the technical field of optical precision measurement, and particularly relates to an optical fiber three-dimensional refractive index measurement method based on differential interference optical tomography. The three-dimensional refractive index distribution of the optical fiber is reconstructed by extracting a differential phase field at a plurality of rotation angles. Compared with a traditional method, the method provided by the invention has the advantages that the natural weighted characteristic of the Hilbert transform of the differential phase field in the frequency domain is proposed, and the problem of over-dense sampling in a low-frequency region in the center of a frequency spectrum space is avoided; the method has the advantages that low-frequency oversampling is naturally suppressed, high-quality tomography can be performed on the optical fiber without manual frequency domain windowing filtering operation, three-dimensional refractive index distribution measurement is realized, Gibbs ringing artifacts are eliminated, and high-frequency details are reserved. According to the invention, isotropic micron / submicron resolution can be realized, and the method is suitable for accurate three-dimensional refractive index field measurement of optical fibers with complex structures.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical precision measurement, and particularly relates to a kind of optical fiber three-dimensional refractive index measurement method based on differential interference optical tomography. BACKGROUND

[0002] The refractive index distribution of optical fiber, as an inherent physical property of optical fiber, directly affects its mode characteristics, transmission bandwidth, dispersion performance and loss mechanism, and is also a core parameter determining the optical performance of optical fiber devices (such as couplers, fiber lasers, etc.). For special optical fibers such as microstructure optical fibers and photonic crystal fibers, the refractive index distribution in the core region has complex three-dimensional characteristics. Therefore, non-destructive and high-resolution three-dimensional measurement of the refractive index field inside such optical fibers is a key technical basis for optimizing fiber design processes and evaluating device performance.

[0003] At present, the main method for measuring the three-dimensional refractive index distribution of optical fiber is the optical projection tomography based on digital holographic microscopy. This method uses digital holographic technology to obtain the phase field distribution at multiple projection angles to reconstruct the three-dimensional refractive index distribution of the optical fiber. As a typical tomographic reconstruction inversion method, the filtered back-projection algorithm gives the tomographic reconstruction result in an analytical form, has a clear physical meaning and mathematical expression, and is commonly used in optical projection tomography reconstruction based on digital holography. Based on the Fourier center slice theorem, the inverse projection results at different projection angles are superimposed to obtain the complete expression of the three-dimensional data as the sample rotates. However, as the sample rotation angle increases, the projection data in the Fourier frequency domain presents a phenomenon of over-sampling in the central low-frequency region, which leads to a large proportion of low-frequency components in the reconstructed result, severely inhibiting the effective expression of high-frequency details (such as refractive index abrupt interfaces and micro / nano structure edges). The current method generally uses windowed frequency domain filtering operations such as the ramp filter (such as the Ram-Lak filter) to adjust the weight of the frequency domain data, which can partially alleviate the problem of low-frequency over-sampling, but cannot fundamentally eliminate the problem of over-sampling in the central low-frequency region, and will introduce Gibbs ringing artifacts, causing false oscillation stripes in the refractive index distribution image. The coupling effect of the above defects leads to a decrease in the spatial resolution of the reconstructed result, especially in the isotropic maintenance of axial resolution and lateral resolution, which significantly deteriorates and cannot meet the accurate characterization requirements of micron / submicron structure characteristics of special optical fibers.

[0004] The present application proposes a kind of optical fiber three-dimensional refractive index measurement method based on differential interference optical tomography. The light field acquisition process uses differential interference method, which has the advantage of stable optical path structure, and can use low coherence light as light source to avoid laser speckle noise in holographic interference method;The data reconstruction process is carried out by the method of Hilbert transform of differential phase distribution, which naturally suppresses the low-frequency over-sampling and can improve the measurement accuracy.

[0005] A digital holographic-based optical fiber refractive index three-dimensional distribution measuring device and method disclosed in patent No. 201510195927.X has the characteristics of using a digital holographic optical path to collect phase projection and using a filtered back-projection algorithm for reconstruction. The light field recording method thereof is essentially different from that of the present application, and the reconstruction method needs manual windowing and filtering operation, and cannot fundamentally eliminate the problem of over-dense sampling in the low-frequency region.

[0006] A quasi-cage type Mach-Zehnder interferometer for optical fiber refractive index measurement disclosed in patent No. 202010760843.7 has the characteristics of using a Mach-Zehnder optical path as a digital holographic recording optical path, and realizing the stability of the optical path system through the optimization of the mechanical structure. The light field recording method thereof is still holographic, which is essentially different from the differential interference type in the present application.

[0007] A micro-structured optical fiber high-resolution three-dimensional refractive index test method disclosed in patent No. 201911089764.1 has the characteristics of using an F-P cavity interference structure to record a digital hologram, and improving the measurement sensitivity by multiple reflections of the light beam in the F-P cavity through the micro-structured optical fiber. The light field recording method thereof is holographic, which is essentially different from the differential interference type in the present application.

[0008] An optical fiber three-dimensional refractive index measurement system and method based on cylindrical objective lens interference disclosed in patent No. 202411739338.9 has the characteristics of using a cylindrical objective lens to compensate for spherical aberration and match the optical characteristics of the optical fiber in the optical path system. However, the optical path is still holographic, which is essentially different from the differential interference type in the present application, and the optical path is more complex.

[0009] An optical material refractive index curve measurement device and method based on frequency domain interference disclosed in patent No. 202510025855.8 has the characteristics of using a broadband light source for interference, extracting information from the interference spectrum, and obtaining the information of the measured object. The optical path setting thereof has a reference arm and a measurement arm, which is essentially different from the interference of two beams of object light in the present application. The reconstruction method thereof needs manual windowing and manual filtering steps, which is essentially different from the Hilbert transform of the differential phase field in the present application. The present application is aimed at the measurement requirement of spectral dispersion characteristics, and the present application is aimed at the measurement requirement of spatial refractive index distribution, which form a complementary relationship in the field of optical detection without technical overlap. SUMMARY

[0010] In view of the reconstruction quality degradation problem caused by spectral sampling mismatch in the prior art, the present application proposes a micro-differential interference optical tomography-based optical fiber three-dimensional refractive index distribution measurement method, which adaptively suppresses low-frequency oversampling through Hilbert transform of the differential phase field, without manual frequency domain filtering, and significantly improves the measurement accuracy and spatial resolution.

[0011] The specific technical solutions of the present application are as follows:

[0012] In some embodiments of the present application, a fiber three-dimensional refractive index measurement method based on differential interference optical tomography is provided, which includes differential phase field recording based on differential interference, Hilbert transform of the differential phase to obtain natural weight containing spectral information, reconstruction of the phase delay distribution of each fault, and calculation of the three-dimensional refractive index distribution. In the differential interference imaging system, the optical fiber is placed, the optical fiber is rotated around the optical fiber axis, and the differential phase field at each angle θ m of the optical fiber is recorded. Along the pixel row (the number of rows is N) parallel to the differential direction, the differential phase data of the same pixel row at m angles is combined, that is, the optical fiber sample is divided into N slices, and N differential phase sinograms are constructed; the Hilbert transform of the differential phase sinogram has a natural weight containing characteristic in the frequency domain, that is, the frequency domain weight containing spectral information is obtained. The inverse Radon transform of the Hilbert transformed differential phase sinogram can reconstruct the phase delay distribution Φ(x, y, z) of each fault slice. Combined with the refractive index of the environment medium and the optical path length, the three-dimensional refractive index distribution n fiber (x, y, z) of the optical fiber is calculated by the derived formula.

[0013] In some embodiments of the present application, the quantitative differential device divides the transmitted light into two beams of object light, the two beams of object light are self-interfered, and interference fringes are formed on the photosensitive surface of the digital camera.

[0014] In some embodiments of the present application, the monochromatic light source is a laser or a monochromatic LED light source, and the central wavelength ranges from 400 nm to 800 nm.

[0015] In some embodiments of the present application, the rotation angle of the optical fiber covers 0° to 360°, and the number of rotations is 36 to 180 times.

[0016] In some embodiments of the present application, the Hilbert transform is used to adaptively suppress low-frequency components and enhance high-frequency components in the frequency domain.

[0017] In some embodiments of the present application, the inverse Radon transform is realized by the Fourier center slice theorem, which maps the frequency domain weight spectrum to the spatial domain and reconstructs the fault phase distribution.

[0018] In some embodiments of the present application, the refractive index of the environment medium is air or matching liquid, and the refractive index value is 1.0-1.5.

[0019] In some embodiments of the present application, the calculation formula of the three-dimensional refractive index distribution of the optical fiber is:

[0020]

[0021] n(x, y, z) = n0+ Δn(x, y, z)fiber (x, y, z) is the refractive index of the optical fiber at position (x, y, z), λ is the wavelength of the incident light, Φ is the three-dimensional phase retardation distribution of the optical fiber, l is the geometric path length of the incident light through the optical fiber, and n0 is the refractive index of the environment surrounding the optical fiber.

[0022] In some embodiments of the present application, the differential direction is parallel or perpendicular to the pixel row direction of the digital camera.

[0023] Compared with the prior art, the present application has the beneficial effects that,

[0024] By extracting the differential phase field under multiple rotation angles, the three-dimensional refractive index distribution of the optical fiber is reconstructed. Compared with the traditional method, the present application proposes to use the natural weighted characteristics of Hilbert transform of the differential phase field in the frequency domain to avoid the problem of oversampling in the low-frequency region of the center of the frequency spectrum space; the advantages are that the low-frequency oversampling is naturally suppressed, and no artificial frequency domain windowing filtering operation is needed, so that the optical fiber can be tomographically imaged with high quality, the measurement of the three-dimensional refractive index distribution is realized, the Gibbs ringing artifact is eliminated, and the high-frequency details are preserved. The present application can realize isotropic micron / submicron resolution, and is suitable for accurate three-dimensional refractive index field measurement of complex structure optical fibers. BRIEF DESCRIPTION OF DRAWINGS

[0025] Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The detailed description is merely meant to teach a person skilled in the art how to implement and practice the present application. The general principles defined herein can be applied to other embodiments, applications, and devices without departing from the scope of the present application. The following detailed description is not to be understood as limiting the present application. Rather, the appropriate scope of the application is indicated by the appended claims. Moreover, the same reference numerals are used throughout the several views to refer to same or like parts.

[0026] Figure 1 A schematic diagram of the principle of the differential interference optical projection tomography for measuring the refractive index distribution of the optical fiber is provided for the embodiments of the present application;

[0027] Figure 2 A schematic diagram of the structure of the differential interference optical projection tomography system is provided for the embodiments of the present application;

[0028] Figure 3 A schematic diagram of a tomographic cross-sectional data of a sample to be measured is provided for the embodiments of the present application;

[0029] Figure 4 A schematic diagram of the differential phase sinusoid of a tomographic cross-section of a sample to be measured is provided for the embodiments of the present application;

[0030] Figure 5 A schematic diagram of the result obtained by Hilbert transform of the differential phase sinusoid of a tomographic cross-section of a sample to be measured is provided for the embodiments of the present application;

[0031] Figure 6A phase delay distribution reconstruction result schematic diagram of a certain fault section provided by the embodiment of the present application. DETAILED DESCRIPTION

[0032] The specific embodiments of the present application are described in further detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present application, but are not used to limit the scope of the present application.

[0033] In order to better understand the purpose, structure and function of the present application, the present application is described in further detail below in conjunction with the accompanying drawings.

[0034] The present application proposes a fiber three-dimensional refractive index distribution measurement method based on differential interference optical projection tomography, which extracts a differential phase field at multiple rotation angles to reconstruct the three-dimensional refractive index distribution of the optical fiber, avoids the problem of oversampling in the low-frequency region of the spectrum space center, and does not need a frequency domain windowing filtering operation, so as to tomographically image the optical fiber with high quality and realize the measurement of the three-dimensional refractive index distribution thereof.

[0035] The purpose of the present application is to solve the reconstruction quality degradation problem caused by frequency spectrum sampling mismatch in the existing digital holographic microscopy-based tomography technology, and to propose a fiber three-dimensional refractive index distribution measurement method based on differential interference optical projection tomography. Through Hilbert transform of the differential phase field, natural weight-containing spectrum information is obtained, the low-frequency oversampling phenomenon is adaptively suppressed, the artificial windowing filtering operation is avoided, and the measurement accuracy and spatial resolution of the three-dimensional refractive index distribution of the optical fiber are significantly improved.

[0036] The present application adopts the following technical solutions:

[0037] The proposed fiber three-dimensional refractive index distribution measurement method based on differential interference optical projection tomography has the following characteristics:

[0038] 1. The three-dimensional refractive index distribution of the optical fiber is reconstructed by using the differential phase field at multiple projection angles;

[0039] 2. Hilbert transform is performed on the differential phase sinogram, and the phase field data after the transform has a natural weight-containing characteristic in the frequency domain;

[0040] 3. Radon inverse transform is performed on the Hilbert transform of the differential phase sinogram to obtain the phase delay distribution of a certain fault;

[0041] 4. A laser or a monochromatic LED is used to illuminate the side of the optical fiber with parallel light;

[0042] 5. A microscopic objective lens is used to collect the transmitted light;

[0043] 6. The transmitted light (i.e. object light carrying information of the object to be measured) collected by the microscope objective is split into two beams of object light by a quantitative differential interference device, and the two beams of object light are imaged in the form of differential interference on the light-sensitive surface of the digital camera;

[0044] 7. The differential direction is perpendicular or parallel to the pixel rows of the digital camera;

[0045] 8. The quantitative differential phase field is extracted on the light-sensitive surface of the digital camera;

[0046] 9. The optical fiber is axially rotated by a certain angle, and a series of quantitative differential phase fields at different angles are extracted through multiple rotations;

[0047] 10. Along the pixel rows parallel to the differential direction, the differential phase information at different angles for the current pixel row is extracted in order of the rotation angles to form a differential phase sinogram;

[0048] 11. Hilbert transform is performed on the differential phase sinogram;

[0049] 12. Radon inverse transform is performed on the Hilbert transform of the differential phase sinogram to obtain the tomographic result corresponding to the current pixel row;

[0050] 13. The tomographic reconstruction is performed row by row to obtain the three-dimensional phase delay of the optical fiber, and the environmental refractive index information is brought in to obtain the complete three-dimensional refractive index distribution of the optical fiber;

[0051] 14. Through the differential interference optical projection tomography method proposed, high-quality tomographic imaging of the optical fiber can be achieved without windowing and filtering, and the measurement of the three-dimensional refractive index distribution of the optical fiber is realized.

[0052] As shown in the accompanying drawings, Figure 1 In the present application, differential interference imaging of the optical fiber is performed at multiple angles to obtain the differential phase field at each projection angle, and the three-dimensional refractive index distribution of the optical fiber is reconstructed using Radon inverse transform, which can avoid the problem of spectral overcrowding in the low-frequency region of the spectral space and does not require manual frequency domain windowing and filtering operations.

[0053] Suppose the central axis of the optical fiber is along the y direction, and the incident plane wave illuminates the side of the optical fiber along the z direction. The transmitted light is collected on the opposite side of the illumination light source by the microscope objective, and the differential phase field information at the current projection angle θ is extracted on the light-sensitive surface of the digital camera through the quantitative differential interference device, with the differential direction being x direction. Then the three-dimensional refractive index distribution of the optical fiber is reconstructed by x sections one by one. Suppose the pixel rows or columns of the digital camera are parallel to the differential direction, and the differential phase field information extracted at different angles by the nth row of pixels parallel to the differential direction forms a differential phase sinogram, denoted as According to the Fourier derivative theorem, and using differential instead of derivative, the Fourier transform relationship between the phase field and the differential phase field is as follows:

[0054]

[0055] where F(·) represents Fourier transform, f D is the frequency coordinate component. The Hilbert transform of the differential phase field in the frequency domain is expressed as

[0056]

[0057] where H(·) represents Hilbert transform, sgn(·) is the sign function. Substituting equation (2) into equation (1), we can get

[0058]

[0059] Equation (3) shows that the spatial Hilbert transform of the differential phase field obtains the frequency domain spectrum with weight (the weight is |f D |), that is, the spatial Hilbert transform of the differential phase field in the frequency domain is equivalent to attenuating the low frequency component (|f D |→0) and enhancing the high frequency component (|f D |→±∞), which is consistent with the function of the slope filter, but without artificial intervention, thereby avoiding the problem of over-dense sampling in the low frequency region. The Radon inverse transform of equation (3) can be obtained as

[0060]

[0061] Equation (4) calculates the (y, z) distribution of the phase delay amount of the optical fiber at x=x n , that is, Φ(x n , y, z). Repeating the above process for the pixel rows x n =x1...x N , combining all the pixel row information, the three-dimensional phase delay distribution of the optical fiber

[0062]

[0063] Let the refractive index of the environment around the optical fiber be n0, then the three-dimensional refractive index distribution of the optical fiber can be calculated by the following formula

[0064]

[0065] where n fiber (x, y, z) is the refractive index of the optical fiber at (x, y, z), λ is the wavelength of the incident light, Φ is the three-dimensional phase delay distribution of the optical fiber, l is the geometric path length of the incident light through the optical fiber, and n0 is the refractive index of the environment around the optical fiber.

[0066] The specific steps of using the present invention to achieve the measurement of the three-dimensional refractive index distribution of optical fiber are as follows:

[0067] Step 1: Use laser or monochromatic LED to illuminate the side of the optical fiber placed along the y direction with parallel light along the z direction, and the microscope objective collects the current rotation angle θ m The transmitted light passes through the sample;

[0068] Step 2: Guide the transmitted light collected by the microscope objective to the quantitative differential interference device to form a differential interference image on the photosensitive surface of the digital camera. Assume that the differential direction is along the x-direction, and the digital camera pixel rows are perpendicular to the x-direction, with N rows in total.

[0069] Step 3: Quantitatively extract the differential phase distribution of the light field at the current rotation angle on the photosensitive surface of the digital camera, which is recorded as

[0070] Step 4: Axially rotate the fiber at a certain angle and repeat steps 1-3 to obtain the differential phase distribution at a series of rotation angles. Where m = 1 to M, i.e., rotate M times to cover a 360° rotation angle;

[0071] Step 5: Along the pixel rows parallel to the differential direction, extract the phase information at M rotation angles in order of rotation angles to form a differential phase sinusoidal graph A total of N differential phase sinusoidal graphs are obtained;

[0072] Step 6: Perform Hilbert transform on the N differential phase sinusoidal graphs one by one, as shown in equation (2), to obtain the Hilbert transform of the N differential phase sinusoidal graphs;

[0073] Step 7: Perform Radon inverse transform on the Hilbert transform of the N differential phase sinusoidal graphs one by one, as shown in equation (4), to obtain the phase distribution of N faults;

[0074] Step 8: Combine the phase distributions of the N slices in pixel order, as shown in equation (5), to obtain the three-dimensional phase delay reconstruction result Φ(x, y, z) of the optical fiber.

[0075] Step 9: Considering the refractive index n0 of the medium surrounding the optical fiber and the geometric path length l(x, y) of the incident light through the optical fiber, the three-dimensional phase delay reconstruction result Φ(x, y, z) of the optical fiber is substituted into equation (6) to obtain the three-dimensional refractive index distribution result n of the optical fiber. fiber (x,y,z).

[0076] by Figure 2 Taking the microscopic imaging system shown in the figure as an example, the differential interference optical projection tomography method of the present invention is further described.

[0077] according to Figure 2The illustrated building micro-imaging system, complete step 1, select monochromatic LED as monochromatic light source 1, central wavelength λ = 528 nm. After the beam collimation module 2 to obtain the plane wave, on the sample placed on 3 objective table illumination. By 4 micro-objective lens collected through the sample transmission light, the sample rotation angle is recorded as θ m For ease of illustration, Figure 3 Show a tomographic section of the sample to be measured in the simulation, recorded as n sample (x,z).

[0078] Step 2 is performed, using 5 beam splitter prism to guide the transmission light collected by the objective lens to 7 quantitative differential interference device, realizing the differential light splitting of the object light, two beams of object light are transmitted after 5 beam splitter prism, differential interference occurs on the photosensitive surface of 8 digital camera, the differential direction is along the x direction.

[0079] Step 3 is performed, quantitatively extracting the differential phase distribution of the light field under the current rotation angle on the photosensitive surface of the digital camera, recorded as

[0080] Step 4 is performed, rotating the sample around the y axis through 6 rotation module, and repeating steps 1-3 to obtain a series of differential phase distributions under a series of rotation angles Where m = 1 ~ M, i.e. rotating M times to cover 360° rotation angle;

[0081] Step 5 is performed, along the pixel row parallel to the differential direction, the phase information under M rotation angles is extracted in sequence, to form the differential phase sinusogram N differential phase sinusograms are obtained; Figure 4 One of the differential phase sinusograms is shown, which is generated by the tomographic section shown in Figure 3 The horizontal axis is the x axis and the vertical axis is the rotation angle 0°-360°.

[0082] Step 6 is performed, Hilbert transform is performed on each of the N differential phase sinusograms, as shown in equation (2), to obtain the Hilbert transform of the N differential phase sinusograms; Figure 5 The Hilbert transform of one of the differential phase sinusograms is shown, which is generated by the Hilbert transform of the differential phase sinusogram shown in Figure 4 The horizontal axis is the x axis and the vertical axis is the rotation angle 0°-360°.

[0083] Step 7 is performed, inverse Radon transform is performed on each of the N differential phase sinusogram Hilbert transforms, as shown in equation (4), to obtain N tomographic phase distributions; Figure 6 The phase distribution of one of the tomographic sections is shown, which is generated by the inverse Radon transform of the Hilbert transform of the differential phase sinusogram shown in Figure 5 Step 7 is performed, inverse Radon transform is performed on each of the N differential phase sinusogram Hilbert transforms, as shown in equation (4), to obtain N tomographic phase distributions;

[0084] Step 8: Combine the phase distribution of N tomograms in pixel order, as equation (5), to get the 3D phase retardation reconstruction result Φ(x, y, z) of the sample.

[0085] Step 9: Considering the medium refractive index n0of the surrounding environment where the sample is located and the geometric path length l(x, y) of the incident light through the sample, the 3D phase retardation reconstruction result Φ(x, y, z) of the sample is brought into equation (6), and the 3D refractive index distribution result n(x, y, z) of the sample can be obtained. sample (x, y, z).

[0086] In the description of the present application, it should be understood that the terms "center", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0087] The terms "first", "second" are only for descriptive purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0088] In the description of the present application, it should be noted that unless otherwise specified and limited, the terms "mounting", "connecting", "connecting" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be direct connection, or indirect connection through intermediate medium, or internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0089] The embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed by the embodiments, since it corresponds to the method disclosed by the embodiments, the description is relatively simple, and the related parts can be referred to the method part.

[0090] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for three-dimensional refractive index measurement of an optical fiber based on differential-interference optical tomography, characterized in that, The differential phase field recording based on differential interference is included, the Hilbert transform of the differential phase is performed to obtain natural weight containing spectral information, the reconstruction of the phase delay distribution of each fault is performed, and the calculation of the three-dimensional refractive index distribution is performed. wherein a fiber is put in a differential interference imaging system, and the fiber is rotated around the fiber axis, and the differential phase field at each angle θ m is recorded. The differential phase data of the same pixel row under m angles are combined along the pixel row parallel to the differential direction, the fiber sample is divided into N slices, and N differential phase sinograms are constructed, where N is the number of pixel rows of the digital camera. The Hilbert transform is performed on the differential phase sinogram to obtain frequency domain weight containing spectral information. The inverse Radon transform is performed on the Hilbert transformed differential phase sinogram to reconstruct the phase delay distribution Φ(x, y, z) of each fault slice. Combining the environmental medium refractive index and the optical path length, the three-dimensional refractive index distribution n fiber (x, y, z).

2. The method of claim 1, wherein the differential-interference-contrast-based optical tomography is a fiber-optic three-dimensional refractive-index measurement method. The object light carrying sample information is divided into two beams of object light using a quantitative differential device, the two beams of object light are self-interfered, and interference fringes are formed on the photosensitive surface of the digital camera.

3. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber three-dimensional refractive index measurement method is characterized by, Laser or monochromatic LED can be used as light source, with central wavelength ranging from 400 nm to 800 nm.

4. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber-optic three-dimensional refractive-index measurement method is characterized by, The rotation angle of the optical fiber covers 0° to 360°, and the number of rotations is 36 to 180.

5. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber-optic three-dimensional refractive-index measurement method is characterized by, The Hilbert transform is used to adaptively suppress low-frequency components and enhance high-frequency components in the frequency domain.

6. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber-optic three-dimensional refractive-index measurement method is characterized by, The inverse Radon transform is realized by the Fourier center slice theorem, maps the frequency domain weight spectrum to the spatial domain, and reconstructs the fault phase distribution.

7. The method according to claim 1, wherein the differential-interference-contrast optical tomography-based three-dimensional refractive-index measurement method is characterized by, The refractive index of the environmental medium is air or matching liquid, and the refractive index value is 1.0-1.

5.

8. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber-optic three-dimensional refractive-index measurement method is characterized by, The calculation formula of the three-dimensional refractive index distribution of the optical fiber is: where n fiber (x, y, z) is the refractive index of the fiber at position (x, y, z), λ is the wavelength of the incident light, Φ is the three-dimensional phase retardation profile of the fiber, / is the geometric path length of the incident light through the fiber, and n0is the refractive index of the environment surrounding the fiber.

9. The method of claim 1, wherein the differential-interference-contrast-based optical tomography fiber-optic three-dimensional refractive-index measurement method is characterized by, The differential direction is parallel or perpendicular to the pixel row direction of the digital camera.

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