System and method for measuring three-dimensional refractive index of optical fiber based on interference of cylindrical objective lens

Abstract

The invention discloses an optical fiber three-dimensional refractive index measuring system and method based on cylindrical objective lens interference. The system comprises a laser light source, a collimating and beam expanding lens group, a beam splitter prism, a rectangular prism, a reflector, a cylindrical objective lens, a tube lens, a CCD, a computer, a PZT, an electric rotating table, a cuvette and an optical fiber to be measured. According to the refractive index reconstruction method based on the system, an interference signal formed by a laser beam through a cylindrical objective lens is received by a CCD, an optical fiber projection phase diagram under full sampling is obtained by rotating the optical fiber at equal angles within the range of 1800, and three-dimensional reconstruction of the refractive index of the optical fiber is achieved by combining an optical calculation tomography back projection algorithm. The system uses a cylindrical objective lens different from a spherical objective lens, can effectively compensate spherical aberration and match optical characteristics of the optical fiber, improves imaging quality, realizes high-precision three-dimensional reconstruction of the refractive index of the optical fiber, and improves measurement precision of the fiber core diameter of the optical fiber. The method is suitable for high-precision nondestructive measurement of various types of optical fiber structures and internal defects, and is easy to apply.

Description

Technical Field

[0001] The invention belongs to the field of optical fiber refractive index detection, and in particular relates to an optical fiber three-dimensional refractive index measurement system and method based on cylindrical objective lens interference. Background Art

[0002] As a carrier for high-speed data transmission, optical fiber is widely used in communications, medical endoscopes, sensors, etc. The refractive index distribution of optical fiber is the key to affecting the transmission characteristics such as fiber mode, bandwidth, loss, and optical parameters such as cutoff wavelength and numerical aperture.

[0003] After decades of development and application, the detection methods of optical fiber refractive index have formed a variety of detection technologies with their own characteristics. There are various methods for measuring optical fiber refractive index based on different principles. The existing methods for measuring optical fiber refractive index mainly include refraction near-field method, near-field scanning method, end-face reflection method, atomic force etching method, focusing method and interference method. Among them, the near-field scanning method requires leakage correction and has low measurement accuracy, so it is only an alternative detection method; the end-face reflection method can only measure the relative refractive index; the refraction near-field method has accurate measurement and high resolution, but the flatness of the fiber end face directly affects the measurement accuracy, the operation is complicated, and the measurement is slow; the atomic force etching method can directly measure the refractive index distribution at each point on the end face of the measured fiber, but the measurement speed is slow and the corrosion range is not easy to control; although the focusing method does not need to cut the fiber, it has high requirements for the uniformity and stability of the light source, which restricts the application of this method; the interference method is the most sensitive method for measuring the refractive index of the optical fiber, and it is also the most popular measurement method in recent years. It is divided into thin-sheet interference method and transverse interference method. The thin-sheet interference method observes in the direction perpendicular to the fiber section, and has high measurement accuracy, but it needs to slice the fiber, and the sample preparation process is complicated. The transverse interference method that can be combined with microscopy does not need to cut the fiber. The fiber to be tested can be directly immersed in the matching liquid and placed in the system for measurement, which is more conducive to non-destructive and rapid detection of the optical fiber. Although the lateral interference method takes into account the cylindrical effect produced by the optical fiber cladding and selects a refractive index liquid that matches the cladding to eliminate the cylindrical effect, it still does not consider the problem of the cylindrical interface between the optical fiber core and the cladding, and does not better compensate for the aberrations produced by the optical fiber from the aspects of the optical fiber geometry and optical properties, thereby improving the imaging quality and measurement accuracy. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a system and method for measuring the three-dimensional refractive index of optical fiber based on cylindrical objective lens interference, which can realize non-destructive measurement of the refractive index of optical fiber and effectively compensate for spherical aberration and match the optical fiber geometry and optical properties, thereby improving the imaging quality and realizing high-precision three-dimensional reconstruction of the refractive index of optical fiber, and ultimately improving the measurement accuracy of the optical fiber core diameter.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a three-dimensional refractive index measurement system of optical fiber based on cylindrical objective lens interference, including a laser light source, a first lens, a second lens, a first beam splitter prism, a second beam splitter prism, a right angle prism, a first reflector, a first reflector, a first cylindrical objective lens, a second cylindrical objective lens, a tube lens, a CCD, a computer, a PZT, an electric rotating stage, a cuvette and an optical fiber to be measured; the PZT, the electric rotating stage and the CCD are respectively connected to the computer. The collimating beam expanding lens group includes a first lens and a second lens.

[0006] The light emitted by the laser light source is formed into a parallel light beam by a collimating beam expanding lens. The parallel light beam is incident on a first beam splitter prism and is divided into a coherent measuring light beam and a reference light beam. The reference light beam is reflected by a first reflector at 90° and enters a first cylindrical objective lens. It is magnified by the first cylindrical objective lens and incident on a second beam splitter prism. The measuring light beam enters a right-angle prism and is reflected into a second reflector. It is then reflected by the second reflector at 90° and incident on a fiber to be measured. The transmitted light carrying information of the fiber to be measured is magnified by a second cylindrical objective lens and then incident on a second beam splitter prism. The reference light beam and the measuring light beam entering the second beam splitter prism continue to merge into one light beam and interfere after passing through the second beam splitter prism. They are transmitted to a CCD through a tube lens to obtain an interference image.

[0007] The present invention also provides a method for reconstructing three-dimensional refractive index tomography of optical fiber based on the above cylindrical objective lens interferometric measurement system, using the above measurement system, comprising the following steps:

[0008] Step 1: Turn on the laser light source and wait for 30 minutes to ensure that the laser light source is stable.

[0009] Step 2: Remove the first cylindrical objective lens and the second cylindrical objective lens in the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, adjust the inclination angle of the second beam splitter prism to make the interference fringes on the CCD in a zero-fringe state, and then adjust the distance between the tube lens and the CCD to the focal length of the tube lens, and move the CCD back and forth along the optical axis to find the minimum position of the light spot, i.e., the imaging position.

[0010] Step 3: Install the first cylindrical objective lens and the second cylindrical objective lens back to their original positions in the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, and rotate the first cylindrical objective lens and the second cylindrical objective lens to ensure that the focal lines of the two cylindrical objective lenses overlap to obtain circular fringes.

[0011] Step 4: Strip off the coating of the optical fiber to be tested, install it on the optical fiber holder, adjust the position of the optical fiber to ensure that the optical fiber is located at the center of the electric rotating table and the center of the incident light spot, and ensure that the optical fiber is placed vertically on the horizontal plane.

[0012] Step 5. Block the reference light path, move the cylindrical objective lens of the test light path back and forth along the optical axis, adjust the working distance, obtain a clear image on the CCD, and ensure that the optical fiber is imaged at the center of the CCD target surface. Remove the blockage from the reference light path, adjust the cylindrical objective lens of the reference light path back and forth along the optical axis, and ensure that the distances between the two cylindrical objective lenses and the second beam splitter prism in the test and reference light paths are consistent. At this time, an interference image with optical fiber information and a background of zero fringes should be obtained on the CCD.

[0013] Step 6: Computer controls the phase shift module and sets the PZT moving step length to , CCD continuously collects interference images at different positions and stores them in the computer. The CCD pixel is nxm, n and m are both positive integers, and λ is the wavelength of the incident light.

[0014] Step 7: The computer controls the electric rotating stage to rotate, and the step length of the electric rotating stage is set to 1°. Step 6 is repeated each time the electric rotating stage rotates until it rotates to 180°.

[0015] Step 8. The computer uses a phase shifting algorithm to obtain the phase distribution of the optical fiber to be tested at 180 angles and obtain the background additional phase. Repeat step 6 to collect the interference image without the optical fiber to be tested to solve the system background phase, and subtract the corresponding background additional phase from the obtained phase at 180 angles to obtain the phase difference between the optical fiber and the matching liquid, thereby eliminating the influence of the system background.

[0016] Step 9: Establish a back-projection relationship between the phase difference and refractive index difference between the optical fiber to be tested and the matching liquid, and reconstruct the two-dimensional refractive index difference distribution of the optical fiber of P layers of pixels respectively by combining the filtered back-projection algorithm. Then, the reconstruction result of each layer of pixels is superimposed on the refractive index of the matching liquid to obtain the refractive index distribution of the optical fiber.

[0017] Step 10: The P-layer optical fiber refractive index reconstruction data are superimposed to obtain three-dimensional reconstruction data.

[0018] Compared with the prior art, the present invention has the following significant advantages:

[0019] (1) Compared with the traditional fiber refractive index detection system, the present invention can effectively compensate for spherical aberration while detecting the fiber refractive index distribution. At the same time, the cylindrical objective lens can better match the fiber geometry and optical properties, thereby improving the imaging quality and achieving high-precision three-dimensional reconstruction of the fiber structure, ultimately improving the measurement accuracy of the fiber core diameter.

[0020] (2) It can not only measure the refractive index distribution and internal defects of the optical fiber cross section, but also realize the three-dimensional refractive index tomography reconstruction of the optical fiber and observe the changes in the length direction of the optical fiber.

[0021] (3) The present invention is suitable for high-precision non-destructive measurement of various optical fibers, is simple and easy to operate, has low cost and is easy to apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference described in the present invention.

[0023] Figure 2 It is a schematic diagram of the structure of the spherical objective lens and the cylindrical objective lens described in the present invention.

[0024] Figure 3 1 is the Zemax simulation analysis result of the spherical objective lens and the cylindrical objective lens described in the present invention, (a) is the standard point diagram of the spherical objective lens, (b) is the standard point diagram of the cylindrical objective lens, (c) is the light fan diagram of the spherical objective lens, and (d) is the light fan diagram of the cylindrical objective lens.

[0025] Figure 4 The four-step phase shift continuous interference pattern required for the acquisition described in the present invention, (a) is the interference pattern with a phase change of 0, (b) is the interference pattern with a phase change of The interference pattern of (c) is the phase change of The interference pattern (d) is the phase change of The interference pattern of .

[0026] Figure 5 This is a distribution diagram of the optical path difference of a multimode optical fiber after the cylindrical objective aberration described in the present invention is compensated.

[0027] Figure 6 It is a two-dimensional refractive index distribution diagram of the cross section of a multimode optical fiber after the cylindrical objective aberration is compensated as described in the present invention.

[0028] Figure 7 It is the result of the three-dimensional refractive index tomography reconstruction of the multimode optical fiber after the cylindrical objective aberration is compensated as described in the present invention.

[0029] Figure 8 This is a diagram of the internal defect distribution of a single-mode optical fiber cross section after the cylindrical objective aberration is compensated as described in the present invention.

[0030] Fig. 9 1 is a comparison diagram of the error in the fiber core diameter when the spherical objective lens and the cylindrical objective lens described in the present invention are used to measure the refractive index of the fiber cross section. (a) is the measurement result of the spherical objective lens, and (b) is the measurement result of the cylindrical objective lens. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0032] The optical fiber three-dimensional refractive index measurement system and method based on cylindrical objective lens interference described in the present invention can effectively compensate for spherical aberration and match the optical fiber geometry and optical properties while realizing non-destructive measurement of the optical fiber refractive index, thereby improving imaging quality, realizing high-precision three-dimensional reconstruction of the optical fiber refractive index, and ultimately improving the measurement accuracy of the optical fiber core diameter.

[0033] Combination Figure 1 The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference of the present invention comprises a laser light source 1, a first lens 2, a second lens 3, a first beam splitter prism 4, a right angle prism 7, a first reflector 5, a second reflector 6, a first cylindrical objective lens 10, a second cylindrical objective lens 9, a second beam splitter prism 14, a tube lens 15, a CCD 16, a computer 17, a PZT 8, an electric rotating stage 11, a cuvette 12 and an optical fiber to be measured 13; the PZT 8, the electric rotating stage 11 and the CCD 16 are respectively connected to the computer 17. The collimating and beam expanding lens group comprises a first lens 2 and a second lens 3.

[0034] The laser light source 1, the collimating beam expansion lens group, the first beam splitter prism 4 and the reflector 5 are sequentially arranged on the first optical axis, the second reflector 6, the optical fiber to be measured 13, the second cylindrical objective lens 9, the second beam splitter prism 14, the tube lens 15 and the CCD 16 are sequentially arranged on the second optical axis, the first cylindrical objective lens 10 is located between the first reflector 5 and the second beam splitter prism 14, and the right angle prism 7 is connected to the optical path of the first beam splitter prism 4 and the second reflector 6. The right angle prism 7 is arranged on the PZT 8, and the PZT 8 can set the corresponding step length to drive the right angle prism 7 to move, the upper end of the optical fiber to be measured 13 is connected to the electric rotating stage 11, and the electric rotating stage 11 drives the optical fiber to rotate, and the lower end of the optical fiber is inserted into the cuvette 12 containing the refractive index matching liquid.

[0035] The light emitted by the laser light source 1 passes through the first lens 2 and the second lens 3 of the collimating beam expanding lens group to become a parallel light beam. The parallel light beam is incident on the first beam splitter prism 4 and is divided into a coherent measuring light beam and a reference light beam. The reference light beam is reflected 90° by the first reflector 5 and enters the first cylindrical objective lens 10. It is magnified by the first cylindrical objective lens 10 and incident on the second beam splitter prism 14. The measuring light beam enters the right-angle prism 7 and is reflected into the second reflector 6. Then, it is reflected 90° by the second reflector 6. The measuring light beam is incident on the optical fiber to be measured 13. The transmitted light carrying the information of the optical fiber to be measured is magnified by the second cylindrical objective lens 9 and then incident on the second beam splitter prism 14. The reference light beam and the measuring light beam entering the second beam splitter prism 14 continue to merge into one light beam through the second beam splitter prism 14 and interfere with each other. They are transmitted to the CCD 16 through the tube lens 15 to obtain an interference image, which is collected by the computer 17.

[0036] Furthermore, the first lens 2 and the second lens 3 are both plano-convex lenses, and the wavelength range is 400 nm to 700 nm. The focal length of the first lens 2 is smaller than that of the second lens 3, and the laser is expanded in the form of a Kepler beam expander; the first beam splitter prism 4 is the same as the second beam splitter prism 14, the splitting ratio is 5:5, and the wavelength range is 400 nm to 700 nm; the first reflector 5 is the same as the second reflector 6, and the wavelength range is 450 nm to 20 μm; the first cylindrical objective lens 10 is exactly the same as the second cylindrical objective lens 9, and both are aberration compensation cylindrical objective lenses; the tube lens 15 matches the first cylindrical objective lens 10 and the second cylindrical objective lens 9, and the tube lens 15 is a double-cemented lens, and the focal length is flexibly selected according to the magnification of the optical fiber 13 to be tested, and is located between the second beam splitter prism 14 and the CCD 15; the right-angle prism 7 and the PZT 8 form a phase shift module, the right-angle prism 7 is installed on the PZT 8, and the PZT 8 is connected to the computer 17, and the computer 17 controls the phase shift module to realize the phase shift of the system; the optical fiber to be tested The upper end of the optical fiber 13 is clamped by a fiber clamp and then installed on a one-dimensional translation stage. The one-dimensional translation stage is connected to the electric rotating stage 11. The lower end of the optical fiber 13 to be tested is vertically immersed in the matching liquid in the cuvette 12, wherein the refractive index of the matching liquid is close to the refractive index of the cladding of the optical fiber 13 to be tested, which can eliminate the cylindrical effect of the optical fiber 13 to be tested and avoid unnecessary light diffraction. The electric rotating stage 11 is connected to the computer 17, and the computer 17 controls the electric rotating stage 11 and then controls the rotation angle of the optical fiber 13 to be tested, so as to facilitate the collection of interference information of the optical fiber 13 to be tested at different angles.

[0037] The use of cylindrical objective lens is the innovation of the present invention. For the selection of objective lens in the system, firstly, it is considered that the first cylindrical objective lens 10 and the second cylindrical objective lens 9 can more effectively compensate for spherical aberration. Figure 2 The spherical objective lens structure shown in the figure, if the center of the objective lens is placed in the xy coordinate system for analysis, the cylindrical objective lens is actually obtained by replacing the spherical objective lens 9 with a cylindrical lens having the same curvature in the y-axis direction and infinite curvature in the x-axis direction. The Zemax simulated spherical and cylindrical objective lens point array diagram and light fan diagram are shown in Figure 3 As shown, Figure 3 (a) Figure 3 (b) The difference between the cylindrical objective lens spot and the spherical objective lens spot is that the cylindrical lens spot extends in the x-axis direction and remains unchanged in the y-axis direction, forming a spot close to a line. Therefore, according to the imaging characteristics of the cylindrical objective lens, when the system detects the optical fiber, the x-axis of the cylindrical objective lens must be parallel to the optical fiber to ensure clear imaging of the optical fiber, and the optical fiber is imaged and measured and analyzed in the y-axis direction; Figure 3 (c) Figure 3As shown in (d), in the y-axis direction, the spherical aberration of the cylindrical objective lens is significantly smaller than that of the spherical objective lens. The cylindrical objective lens can image the optical fiber more accurately and improve the subsequent reconstruction accuracy. On the other hand, the selection of the magnification M and the numerical aperture NA is the key to not losing the resolution of the first cylindrical objective lens 10, the second cylindrical objective lens 9 and the CCD 16 to the greatest extent. The idea is as follows:

[0038] In order to satisfy the Nyquist sampling criterion, the following must be ensured:

[0039] (1)

[0040] in, is the projected characteristic size, is the pixel pitch of the camera.

[0041] According to the Rayleigh criterion, the minimum resolution of a microscope objective is:

[0042] (2)

[0043] in, is the minimum resolution of the microscope objective, λ is the wavelength of the incident light, and NA is the numerical aperture of the microscope objective. Therefore, for a point object projected onto CCD16 by a microscope objective with a magnification of M, the condition is:

[0044] (3)

[0045] Since typical NA values ​​are between 0 and 1, only microscope objectives with high magnification can use the spatial frequency bandwidth of CCD 16 without losing resolution. However, in the system described in the present invention, it is desirable to image the relatively large optical fiber 13 to be tested onto CCD 16 using low magnification while having as high a NA as possible to maximize resolution.

[0046] A tomographic reconstruction method of an optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference comprises the following steps:

[0047] Step 1: Turn on the laser light source 1 and wait for 30 minutes to ensure that the laser light source is stable.

[0048] Step 2: Remove the first cylindrical objective lens 10 and the second cylindrical objective lens 9 of the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, adjust the inclination angle of the second beam splitter prism 14 to make the interference fringes on CCD16 in a zero-fringe state, then adjust the distance between the tube lens 15 and CCD16 to the focal length of the tube lens, and move CCD16 back and forth along the optical axis to find the minimum position of the light spot, i.e., the imaging position.

[0049] Step 3: Install the first cylindrical objective lens 10 and the second cylindrical objective lens 9 back to the original position of the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, rotate the first cylindrical objective lens 10 and the second cylindrical objective lens 9 to ensure that the focal lines of the two objective lenses coincide with each other to obtain circular fringes. If they do not coincide, the interference fringes will appear elliptical, which will introduce unnecessary aberrations into the system and affect the phase detection accuracy of the subsequent optical fiber 13 to be measured.

[0050] Step 4: strip the coating layer of the optical fiber 13 to be tested, install it on the optical fiber holder, adjust the position of the optical fiber 13 to be tested so that it is located at the center of the electric rotating table 11 and the center of the incident light spot, and ensure that the optical fiber 13 to be tested is placed vertically on the horizontal plane.

[0051] Step 5, block the reference light path, move the second cylindrical objective lens 9 back and forth along the optical axis, adjust the working distance, obtain a clear image on CCD16, and ensure that the optical fiber 13 to be tested is imaged at the center of the CCD16 target surface, remove the blockage from the reference light path, adjust the cylindrical objective lens 10 of the reference light path back and forth along the optical axis, ensure that the distances between the first cylindrical objective lens 10, the second cylindrical objective lens 9 and the second beam splitter prism 14 are consistent, and an interference image with optical fiber information and a background of zero fringes should be obtained on CCD16.

[0052] Step 6: The computer controls the PZT (8) to move in steps of , CCD16 continuously collects four interference images and stores them in computer 17, the pixels of CCD16 are nxm, n and m are positive integers, and λ is the wavelength of the incident light.

[0053] Step 7: The computer 17 controls the electric rotating table 11 to rotate, and sets the step length of the electric rotating table 11 to 1°. Step 6 is repeated each time the electric rotating table 11 rotates until it rotates to 180°.

[0054] Step 8, the computer 17 uses the phase shifting algorithm to obtain the phase distribution of the optical fiber 13 to be tested at 180 angles, and obtains the background additional phase; repeat step 6 again to collect the interference image without the optical fiber 13 to be tested to solve the system background phase, and subtract the corresponding background additional phase from the obtained phase at 180 angles to obtain the phase difference between the optical fiber 13 to be tested and the matching liquid, and eliminate the influence of the system background. The specific ideas are as follows:

[0055] (4)

[0056] in, represents the total light intensity of the interference pattern, is the background intensity of the interference pattern, It is the modulation system. is the initial phase information of the wavefront of the object to be measured, is the phase shifter, It is the coordinate information of the pixel point in the interference pattern.

[0057] First, obtain the interference image without phase shift, and then perform three phase shifts with a phase shift interval of π / 2. Each phase shift completes the acquisition of one frame of image, and a total of 4 frames of interference images with a phase difference of π / 2 are obtained. The image intensity information is:

[0058] (5)

[0059] (6)

[0060] (7)

[0061] (8)

[0062] in, represents the interference image without phase shift, represents the interference image corresponding to a phase shift of π / 2, represents the interference image corresponding to the shift, Represents the interference image corresponding to a phase shift of 3π / 2.

[0063] Then we get the relationship between the pixel information of the four frames:

[0064] (9)

[0065] Combining the above formula, we can get the information range of the optical fiber 13 to be tested: The wrapped phase is solved to obtain the phase distribution of the optical fiber 13 to be tested. Similarly, the optical fiber 13 to be tested is removed to obtain the system background phase distribution. The phase distribution of the optical fiber 13 to be tested minus the system background phase is used to obtain the phase difference distribution between the optical fiber 13 to be tested and the matching liquid.

[0066] Step 9: Establish a back-projection relationship between the phase difference and refractive index difference between the optical fiber 13 to be tested and the matching liquid, and reconstruct the two-dimensional refractive index difference distribution of the optical fiber 13 to be tested of the P layer pixels by using a filtered back-projection algorithm. The steps are as follows:

[0067] Step 9-1, convert the phase difference between the optical fiber 13 to be tested and the refractive index matching liquid at 180 degrees into the optical path difference, and the relationship is as follows:

[0068] (10)

[0069] in, is the phase difference between the optical fiber and the matching liquid, is the wavelength of incident light, Indicates the optical path difference.

[0070] Step 9-2, identifying the cladding edge of the optical fiber 13 to be tested in the optical path difference distribution based on the Sobel edge detection method, determining the center of the optical fiber 13 to be tested, intercepting the required range data, and calibrating the rotation center of the optical fiber 13 to be tested;

[0071] Step 9-3: Optical path difference at different angles The matrix representation is as follows:

[0072] (11)

[0073] in, represents the optical path difference of the i-th row and j-th column, i and j represent the number of rows and columns of the obtained optical path difference data matrix respectively, which are positive integers, and when the number of CCD pixels is nxm, i=n, ​​0 <j<m。

[0074] Will Split into several one-dimensional matrices by row :

[0075] (12)

[0076] (13)

[0077]

[0078] (14)

[0079] Step 9-4: The relationship between the optical path difference and the refractive index difference is as follows:

[0080] (15)

[0081] in, is the refractive index of the optical fiber, is the geometric path through the sample, n0 is the refractive index of the matching liquid, and the optical path difference can be regarded as the projection of the refractive index difference between the optical fiber 13 to be tested and the matching liquid. The sine graph is combined and iradon transformed to reconstruct the fiber refractive index difference of the first layer of pixels. The fiber refractive index difference is superimposed on the matching liquid refractive index to obtain the two-dimensional refractive index distribution of the fiber cross section. The same iradon transformation is repeated for the pixels from the second layer to the Pth layer, and finally the P-layer reconstruction data is obtained, where P is a positive integer and 0 <P≤i。

[0082] Step 10: Superimpose the P-layer optical fiber refractive index reconstruction data to obtain optical fiber three-dimensional reconstruction data.

[0083] Example 1

[0084] Combination Figure 1In this embodiment, the laser light source 1 adopts a helium-neon laser light source, model 05-LHR-121, with a wavelength of 632.8nm; the focal length of the first lens 2 is 12mm, and the focal length of the second lens 3 is 125mm; the focal length of the tube lens 15 is 200mm, and it matches the first cylindrical objective lens 10 and the second cylindrical objective lens 9 for imaging; the magnification of the first cylindrical objective lens 10 and the second cylindrical objective lens 9 is 10×, the numerical aperture is 0.28, and the working distance is 3.2mm; the electric rotating stage 11 and PZT8 are PRMTZ8 / M and NFL5DP20 / M of Thorlabs respectively; the model of CCD16 is GS3-U3-50S5M-C of Flir Company; the refractive index of the matching liquid is 1.451; but the device described in the present invention is not limited to this.

[0085] Regarding the selection of the objective lens in the system, first of all, it is considered that the first cylindrical objective lens 10 and the second cylindrical objective lens 9 can compensate for spherical aberration more effectively. In a specific imaging direction, the spherical aberration of the cylindrical objective lens is significantly smaller than that of the spherical objective lens. The cylindrical objective lens can image the optical fiber more accurately, thereby improving the subsequent reconstruction accuracy.

[0086] On the other hand, the selection of the magnification M and the numerical aperture NA is the key to not losing the resolution of the first cylindrical objective lens 10, the second cylindrical objective lens 9 and the CCD 16 to the greatest extent. The selection ideas of the magnification M and the numerical aperture NA are as follows:

[0087] First, in a conventional microscope, in order to meet the Nyquist sampling criterion, it is necessary to ensure that equation (1) holds. In this embodiment, the pixel size of CCD16 is 6.9 μm. Second, the minimum resolvable range of the microscope objective imaging is represented by equation (2):

[0088] Therefore, for a point object projected onto CCD16 by a microscope objective with a magnification of M, the condition is In this embodiment, λ=632.8nm, ξ=6.9μm, so In the case of , the resolution can be minimized. Since the typical NA value is between 0 and 1, only a microscope objective with a high magnification can use the spatial frequency bandwidth of CCD16 without losing resolution. However, in the system described in the present invention, it is desired to use a low magnification to image the relatively large optical fiber 13 to be tested onto CCD16, while having the highest possible NA to maximize the resolution, so a magnification of 10× is selected in this embodiment.

[0089] The reconstruction method of the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, the optical fiber 13 to be measured selects a multimode optical fiber with a cladding diameter of 125 μm and a core diameter of 51.9 μm, and includes the following steps:

[0090] Step 1: Turn on the laser light source 1 and wait for 30 minutes to ensure that the laser light source is stable.

[0091] Step 2: The system first removes the first cylindrical objective lens 10 and the second cylindrical objective lens 9, adjusts the inclination angle of the second beam splitter prism 14 to make the interference fringes in the zero-fringe state, and then adjusts the distance between the tube lens 15 and the CCD 16 to the tube lens focal length of 200 mm, and moves the CCD 16 back and forth along the optical axis to find the minimum spot position, i.e., the imaging position.

[0092] Step 3: Install the first cylindrical objective lens 10 and the second cylindrical objective lens 9 back to their original positions, and rotate the first cylindrical objective lens 10 and the second cylindrical objective lens 9 to ensure that the focal lines of the two objective lenses coincide to obtain circular stripes.

[0093] Step 4: Strip off the coating layer of the optical fiber 13 to be tested (cladding diameter 125 μm, core diameter 51.9 μm), install it on the optical fiber holder, adjust the position of the optical fiber 13 to be tested so that it is located at the center of the electric rotating table 11 and the center of the incident light spot, and ensure that the optical fiber 13 to be tested is placed vertically on the horizontal plane.

[0094] Step 5, block the reference light path, move the second cylindrical objective lens 9 back and forth along the optical axis, adjust the working distance to about 3.2 mm, obtain a clear image on CCD16, and ensure that the optical fiber 13 to be tested is imaged at the center of the CCD16 target surface, remove the blockage from the reference light path, adjust the first cylindrical objective lens 10 of the reference light path back and forth along the optical axis, ensure that the distances between the first cylindrical objective lens 10, the second cylindrical objective lens 9 and the second beam splitter prism 14 are consistent, and an interference image with optical fiber information and a background of zero fringes should be obtained on CCD16.

[0095] Step 6: The computer controls the moving step length of PZT8 to In this embodiment, the step length is 79.1 nm. CCD16 continuously collects four interference images and stores them in computer 17. The pixels of CCD16 used are 1024x1024, and the pixel size is 6.9 μm. The continuous interference images collected are as follows: Figure 4 (a)~ Figure 4 (d) as shown.

[0096] Step 7: The computer 17 controls the electric rotating table 11 to rotate, and sets the step length of the electric rotating table 11 to 1°. Step 6 is repeated each time the electric rotating table 11 rotates until it rotates to 180°.

[0097] Step 8, combine the four-step phase shifting algorithm and process it with the computer 17 to obtain the phase distribution of the optical fiber 13 to be tested at 180 angles, repeat step 6 again to collect the interference image without the optical fiber 13 to be tested to solve the system background phase, subtract the background additional phase from the phase at 180 angles, and obtain the phase difference between the optical fiber 13 to be tested and the matching liquid, thereby eliminating the influence of the system background.

[0098] Step 9: Establish a back-projection relationship between the phase difference and refractive index difference between the optical fiber 13 to be tested and the matching liquid, and reconstruct the two-dimensional refractive index difference distribution of the optical fiber 13 to be tested of 1024 layers of pixels by combining the filtered back-projection algorithm. The steps are as follows:

[0099] Step 9-1: According to formula (4), the phase difference between the optical fiber 13 to be tested and the refractive index matching liquid at 180 angles is converted into an optical path difference. The distribution of the optical path difference at a certain angle is as follows: Figure 5 shown.

[0100] Step 9-2: Based on the Sobel edge detection method, identify the cladding edge of the optical fiber 13 to be tested in the optical path difference distribution, determine the center of the optical fiber 13 to be tested, intercept the required range data, and calibrate the rotation center of the optical fiber 13 to be tested. In this embodiment, the data range is 801 pixels.

[0101] Step 9-3: Optical path difference at different angles The matrix representation is as shown in formula (5), The matrix size is 1024×801. Split into 1024 one-dimensional matrices by row.

[0102] Step 9-4: The relationship between the optical path difference and the refractive index difference is as shown in Formula 9. The optical path difference can be regarded as the projection of the refractive index difference between the optical fiber 13 to be tested and the matching liquid. The sine graph is combined and iradon transformed to reconstruct the fiber refractive index difference of the first layer of pixels. The fiber refractive index difference is superimposed on the matching liquid refractive index to obtain the two-dimensional refractive index distribution of the fiber cross section, such as Figure 6 As shown, the same iradon transformation is repeated for pixels from the 2nd layer to the 1024th layer, and finally 1024 layers of reconstructed data are obtained.

[0103] Step 10: 1024 layers of fiber refractive index reconstruction data are superimposed to obtain fiber 3D reconstruction data. The reconstruction result of the 3D refractive index of the fiber 13 to be tested is as follows: Figure 7 shown.

[0104] In this embodiment, a single-mode optical fiber with defects (cladding diameter 250 μm, core diameter 14 μm) is used as the optical fiber to be tested 13 for measurement. The cross-sectional refractive index distribution of the optical fiber to be tested 13 is as follows: Figure 8 As shown, the shape and location of the internal defects of the optical fiber are clearly shown.

[0105] Example 2

[0106] Example 2 is a comparative experiment. The method for measuring the three-dimensional refractive index of an optical fiber is the same as that described in Example 1. The difference is that the first cylindrical objective lens 10 and the second cylindrical objective lens 9 are replaced with spherical objective lenses with the same parameters, that is, the magnification is 10× and the numerical aperture is 0.28. The measurement results are shown in FIG. Fig. 9 (a) Fig. 9 (b) shows the refractive index distribution of the optical fiber cross section. The measurement results of the core and cladding diameters of the optical fiber 13 to be tested are calculated based on the four marked positions ①②③④ in the figure.

[0107] The actual diameter parameters of the optical fiber 13 to be tested are: core diameter 51.90 μm, cladding diameter 125.00 μm.

[0108] The measurement results of the spherical objective lens are: core diameter 54.20 μm, cladding diameter 125.55 μm; the measurement errors of core and cladding diameters are 0.0443 and 0.0044 respectively.

[0109] The measurement results of the cylindrical objective lens are: core diameter 52.14 μm, cladding diameter 125.56 μm; the measurement errors of core and cladding diameters are 0.0046 and 0.0045 respectively.

[0110] The measurement error of the core diameter of the optical fiber 13 to be measured using the cylindrical objective lens is significantly reduced.

[0111] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the principles of the present invention, and these should also be regarded as belonging to the scope of protection of the present invention.

Claims

1. A three-dimensional refractive index measurement system for optical fiber based on cylindrical objective lens interference, characterized in that: The device comprises a laser light source (1), a collimating beam expanding lens group, a first beam splitter prism (4), a right-angle prism (7), a first reflector (5), a second reflector (6), a first cylindrical objective lens (10), a second cylindrical objective lens (9), a second beam splitter prism (14), a tube lens (15), a CCD (16), a computer (17), a PZT (8), an electric rotating stage (11), a cuvette (12) and an optical fiber to be tested (13); the PZT (8), the electric rotating stage (11) and the CCD (16) are respectively connected to the computer (17); Light emitted by a laser light source (1) is formed into a parallel light beam by a collimating beam expanding lens. The parallel light beam is incident on a first beam splitter prism (4) and is split into a coherent measuring light beam and a reference light beam. The reference light beam is reflected by a first reflector (5) at 90 degrees and enters a first cylindrical objective lens (10). It is amplified by the first cylindrical objective lens (10) and incident on a second beam splitter prism (14). The measuring light beam enters a right-angle prism (7) and is reflected by a second reflector (6). It is then reflected by the second reflector (6) at 90 degrees and incident on a fiber to be measured (13). Transmitted light carrying information of the fiber to be measured is amplified by a second cylindrical objective lens (9) and then incident on a second beam splitter prism (14). The reference light beam and the measuring light beam entering the second beam splitter prism (14) are further combined into one light beam by the second beam splitter prism (14) and interfere with each other. They are transmitted to a CCD (16) through a tube lens (15) to obtain an interference image, which is collected by a computer (17).

2. The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference according to claim 1, characterized in that: The collimating beam expanding lens group comprises a first lens (2) and a second lens (3), the first lens (2) and the second lens (3) are both plano-convex lenses, and the wavelength range is 400 nm to 700 nm; the focal length of the first lens (2) is smaller than the focal length of the second lens (3); The first beam splitter prism (4) is the same as the second beam splitter prism (14), both have a beam splitting ratio of 5:5, and both have a wavelength range of 400 nm to 700 nm; The first reflector (5) and the second reflector (6) are the same, and both have a wavelength range of 450 nm to 20 μm.

3. The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference according to claim 1, characterized in that: The first cylindrical objective lens (10) is completely identical to the second cylindrical objective lens (9), and both are aberration-compensating cylindrical objective lenses; In addition to considering that the first cylindrical objective lens (10) and the second cylindrical objective lens (9) can more effectively compensate for spherical aberration, the selection of the magnification M and the numerical aperture NA is the key to not losing the resolution of the first cylindrical objective lens (10), the second cylindrical objective lens (9) and the CCD (16) to the greatest extent. The design method is as follows: In order to satisfy the Nyquist sampling criterion, the following must be ensured: (1) in, is the projected characteristic size, is the pixel pitch of the camera; According to the Rayleigh criterion, the minimum resolution of a microscope objective is: (2) in, is the minimum resolution of the microscope objective, λ is the wavelength of the incident light, and NA is the numerical aperture of the microscope objective. Therefore, for a point object projected onto the CCD (16) by a microscope objective with a magnification of M, the condition is: (3)。 4. The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference according to claim 3, characterized in that: The tube lens (15) matches the first cylindrical objective lens (10) and the second cylindrical objective lens (9); the tube lens (15) is a double-cemented lens, the focal length of which is flexibly selected according to the magnification of the optical fiber (13) to be tested, and is located between the second beam splitter prism (14) and the CCD (15).

5. The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference according to claim 4, characterized in that: The right-angle prism (7) and the PZT (8) form a phase shift module. The right-angle prism (7) is mounted on the PZT (8). The PZT (8) is connected to a computer (17), and the computer (17) controls the phase shift module.

6. The optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference according to claim 5, characterized in that: One end of the optical fiber (13) to be tested is clamped by an optical fiber clamp and then installed on a one-dimensional translation stage. The one-dimensional translation stage is connected to an electric rotating stage (11). The optical fiber (13) to be tested is vertically immersed in a matching liquid in a cuvette (12). The refractive index of the matching liquid is close to the refractive index of the cladding of the optical fiber (13) to be tested. The electric rotating stage (11) is connected to a computer (17). The computer (17) controls the electric rotating stage (11) and further controls the rotation angle of the optical fiber (13) to be tested. A CCD (16) collects interference images at different angles. The optical fiber phase information is obtained through processing by the computer (17). Then, a three-dimensional tomographic reconstruction of the optical fiber refractive index is realized in combination with a filtered back-projection algorithm to obtain the structural parameters and refractive index of the optical fiber (13) to be tested.

7. A method for reconstructing three-dimensional refractive index tomography of optical fiber using an optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, characterized in that: The following steps are involved: Step 1: construct the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference as described in any one of claims 1 to 6, turn on the laser light source (1), and wait for 30 minutes to ensure that the laser light source (1) is stable; Step 2, removing the first cylindrical objective lens (10) and the second cylindrical objective lens (9) in the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, adjusting the inclination angle of the second beam splitter prism (14) so ​​that the interference fringes on the CCD (16) are in a zero-fringe state, then adjusting the distance between the tube lens (15) and the CCD (16) to the focal length of the tube lens, and moving the CCD (16) forward and backward along the optical axis to find the minimum light spot position, i.e., the imaging position; Step 3, installing the first cylindrical objective lens (10) and the second cylindrical objective lens (9) back to the original positions of the optical fiber three-dimensional refractive index measurement system based on cylindrical objective lens interference, rotating the first cylindrical objective lens (10) and the second cylindrical objective lens (9) to ensure that the focal lines of the two cylindrical objective lenses overlap to obtain circular fringes; Step 4, stripping off the coating layer of the optical fiber (13) to be tested, mounting it on the optical fiber holder, adjusting the position of the optical fiber (13) to be tested so that it is located at the center of the electric rotating table (11) and the center of the incident light spot, and ensuring that the optical fiber (13) to be tested is placed vertically on a horizontal plane; Step 5, block the reference light path, move the second cylindrical objective lens (9) forward and backward along the optical axis, adjust the working distance, obtain a clear image on the CCD (16), and ensure that the optical fiber (13) to be tested is imaged at the center of the target surface of the CCD (16), remove the blockage from the reference light path, adjust the first cylindrical objective lens (10) of the reference light path forward and backward along the optical axis, ensure that the distances between the first cylindrical objective lens (10), the second cylindrical objective lens (9) and the second beam splitter prism (14) are consistent, and at this time, an interference image with optical fiber information and a background of zero fringes should be obtained on the CCD (16); Step 6: The computer controls the PZT (8) to move in steps of , CCD (16) continuously collects interference images at different positions and stores them in computer (17), the pixels of CCD (16) are nxm, n and m are both positive integers, and λ is the wavelength of incident light; Step 7, the computer (17) controls the electric rotating table (11) to rotate, and the step length of the electric rotating table (11) is set to 1°, and step 6 is repeated once each time the electric rotating table rotates until it rotates to 180°; Step 8, the computer (17) uses a phase shift algorithm to obtain the phase distribution of the optical fiber (13) under test at 180 angles; repeat step 6 again to collect the interference image when there is no optical fiber (13) under test to solve the system background phase, and subtract the corresponding background additional phase from the obtained phase at 180 angles to obtain the phase difference between the optical fiber (13) under test and the matching liquid, thereby eliminating the influence of the system background; Step 9, reconstructing the two-dimensional refractive index distribution of the optical fiber P layer pixels using a filtered back projection algorithm; Step 10: Superimpose the P-layer optical fiber refractive index reconstruction data to obtain three-dimensional reconstruction data.

8. The method for optical fiber three-dimensional refractive index tomography reconstruction according to claim 7, characterized in that: In step 3, the focal lines of the first cylindrical objective lens (10) and the second cylindrical objective lens (9) should overlap to ensure that the interference fringes of the light beam after passing through the second beam splitting prism (14) are circular.

9. The method for optical fiber three-dimensional refractive index tomography reconstruction according to claim 7, characterized in that: In step 8, the computer (17) uses a phase shifting algorithm to obtain the phase distribution of the optical fiber (13) under test at 180 angles; step 6 is repeated again to collect the interference image without the optical fiber (13) under test to solve the system background phase, and the phase under 180 angles obtained is subtracted from the corresponding background additional phase to obtain the phase difference between the optical fiber (13) under test and the matching liquid. The specific idea is as follows: (4) in, represents the total light intensity of the interference pattern, is the background intensity of the interference pattern, It is the modulation system. is the initial phase information of the wavefront of the object to be measured, is the phase shifter, It is the coordinate information of the pixel points in the interference pattern; First, obtain the interference image without phase shift, and then perform three phase shifts with a phase shift interval of π / 2. Each phase shift completes the acquisition of one frame of image, and a total of 4 frames of interference images with a phase difference of π / 2 are obtained. The image intensity information is: (5) (6) (7) (8) in, represents the interference image without phase shift, represents the interference image corresponding to a phase shift of π / 2, represents the interference image corresponding to the phase shift of π, represents the interference image corresponding to a phase shift of 3π / 2; Then we get the relationship between the pixel information of the four frames: (9) Combining the above formula, we can get the information range of the optical fiber (13) to be tested: The wrapped phase is solved to obtain the phase distribution of the optical fiber (13) to be tested. Similarly, the optical fiber (13) to be tested is removed to obtain the system background phase distribution. The phase distribution of the optical fiber (13) to be tested minus the system background phase is obtained to obtain the phase difference distribution between the optical fiber (13) to be tested and the matching liquid.

10. The method for optical fiber three-dimensional refractive index tomography reconstruction according to claim 7, characterized in that: In step 9, the two-dimensional refractive index distribution of the optical fiber P layer pixels is reconstructed using a filtered back projection algorithm, as follows: Step 9-1, convert the phase difference between the optical fiber (13) to be tested and the matching liquid at 180 angles into the optical path difference, and the relationship is as follows: (10) in, is the phase difference between the optical fiber (13) to be tested and the matching liquid, is the wavelength of incident light, It represents the optical path difference; Step 9-2, identifying the cladding edge of the optical fiber (13) to be tested in the optical path difference distribution based on the Sobel edge detection method, determining the center of the optical fiber (13) to be tested, intercepting the required range data, and calibrating the rotation center of the optical fiber (13) to be tested; Step 9-3: Optical path difference at different angles The matrix representation is as follows: (11) in, represents the optical path difference of the i-th row and j-th column, i and j represent the number of rows and columns of the obtained optical path difference data matrix respectively, which are positive integers, and when the number of CCD pixels is nxm, i=n, ​​0 <j<m; Will Split into several one-dimensional matrices by row : (12) (13) (14) Step 9-4: The relationship between the optical path difference and the refractive index difference is as follows: (15) in, is the refractive index of the optical fiber, is the geometric path through the sample, The optical path difference is the projection of the refractive index difference between the optical fiber and the matching liquid. The iradon transform is performed on the sine graph to reconstruct the fiber refractive index difference of the first layer of pixels. The fiber refractive index difference is superimposed on the matching liquid refractive index to obtain the fiber refractive index. The same iradon transform is repeated for the pixels from the second layer to the Pth layer, and finally the P-layer reconstruction data is obtained, where P is a positive integer and 0 <P≤i。

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