CN111198169A - Microstructure optical fiber high resolution three-dimensional refractive index testing method - Google Patents
Microstructure optical fiber high resolution three-dimensional refractive index testing method Download PDFInfo
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Abstract
The invention provides a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber. The method is characterized in that: the method comprises the steps of digital hologram recording, numerical reconstruction, unwrapping, error processing, three-dimensional phase distribution reconstruction and three-dimensional refractive index conversion based on an F-P cavity. The invention mainly provides a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber, which has higher sensitivity compared with the traditional microscopic imaging method. The invention has the advantages of simple structure, high sensitivity and accurate measurement. The invention can be used for high-resolution three-dimensional microscopic imaging of the microstructure optical fiber, and can be widely applied to nondestructive, unmarked and non-contact three-dimensional tomography of optical transparent objects and the like.
Description
(I) technical field
The invention relates to a method for testing the high-resolution three-dimensional refractive index of a micro-structural optical fiber, which can be used for high-resolution three-dimensional microscopic imaging of the micro-structural optical fiber, can be widely applied to nondestructive, unmarked and non-contact three-dimensional tomography of an optical transparent object and the like, and belongs to the technical field of microscopic imaging.
(II) background of the invention
The three-dimensional refractive index distribution of the microstructure fiber is an important inherent attribute, and for the optically transparent microstructure fiber, the three-dimensional refractive index distribution can reflect the information of the microstructure, the density and the like of a sample, so that the nondestructive, label-free and non-contact three-dimensional tomography is required to be realized.
In modern life science research, fluorescence imaging is commonly used to label samples to be tested. However, in the process of labeling, the sample to be tested will be affected to some extent, and the final research result will be affected. The digital holographic tomography technology is a novel imaging technology which is lossless, unmarked and non-contact, can reconstruct and obtain three-dimensional refractive index distribution information of the microstructure optical fiber, and is a research hotspot in recent years.
The digital holographic tomography technology combines the digital holographic microscopic imaging technology and the computed tomography technology, and is a new technology proposed in recent years. In recent years, various imaging methods using digital holography tomography have been proposed, but most of the ideas are to perform digital holography recording by combining with a mach-zehnder interference optical path.
More devices are used for Mach-Zehnder interference optical path imaging, the requirement on system stability is higher, and the operation becomes complicated. The method based on the Mach-Zehnder interference optical path has higher requirements on devices, and the optical path is more complex and difficult to debug, so that a new imaging method is urgently needed, the number of used devices is less, the optical path is simpler, the system stability is higher, the operation is simpler and more convenient, the sensitivity of the measurement on the microstructure optical fiber is higher, and the imaging resolution is higher.
The invention provides a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber. In principle, the light beam is reflected for multiple times in the F-P cavity and passes through the microstructure optical fiber for multiple times, and the optical path difference is accumulated in sequence, so that the measurement sensitivity is higher. The hologram formed by the F-P cavity interference has higher fineness than Mach-Zehnder interference, and the recording of the hologram is more accurate.
Patent CN201310082100.9 discloses an online reconstruction display system and method for digital holographic imaging, which is characterized in that a mach-zehnder interference digital holographic recording optical path is adopted, compared with the microstructure optical fiber high resolution three-dimensional refractive index testing method provided by the present invention, the new method provided by the present invention has higher measurement sensitivity.
Patent CN201610911993.7 discloses a dual-wavelength phase microscopic imaging system and method, and a corresponding phase recovery method, which is characterized in that a mach-zehnder interference optical path is adopted to realize a dual-wavelength coaxial phase-shift interference microscopic system.
Patent CN201710518263.5 discloses a three-refractive index tomography microscopic imaging system and method thereof, which can restore the three-dimensional refractive index information of the sample, but in the imaging optical path, it is essentially different from the present invention, and the required devices are more complex.
Patent CN201710904860.1 discloses an optical coherence tomography imaging system, which adopts mach-zehnder interference optical path, and is characterized in that optical fiber is adopted to simplify the system and reduce the cost, but the optical path structure is still more complex compared with the optical path structure of F-P cavity.
Patent CN201810145657.5 discloses a high resolution digital holographic diffraction tomography, which is characterized in that a mach-zehnder interference optical path structure is adopted, and a synthetic aperture method is used to obtain N synthetic high resolution holograms, thereby obtaining a high resolution three-dimensional refractive index representation of a measured sample. The structure is relatively more complex, and the invention is essentially different from the invention.
Patent cn201910136421.x discloses a super-resolution digital holographic imaging system and an imaging method, and the imaging system is characterized in that a transmission-type spatial light modulator is added in front of a traditional mach-zehnder interference optical path to modulate a light source. Compared with the invention adopting the optical path structure of the F-P cavity, the invention has essential difference.
The invention discloses a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber, which can be used for high-resolution microscopic imaging of the microstructure optical fiber and can be widely applied to the fields of nondestructive, unmarked and non-contact tomography of optical transparent objects and the like. The microstructure optical fiber high-resolution three-dimensional refractive index testing method adopts a digital holographic recording optical path based on an F-P cavity, and adopts a digital holographic tomography technology to perform chromatographic reconstruction on a digital hologram recorded by the F-P cavity optical path to restore the three-dimensional refractive index distribution of the microstructure optical fiber. Compared with the prior art, the digital holographic recording optical path based on the F-P cavity is adopted, so that the light beam is reflected in the F-P cavity for multiple times and passes through the microstructure optical fiber, and the measurement sensitivity is higher. The microstructure optical fiber high-resolution three-dimensional refractive index testing method has the advantages of simple structure, high sensitivity and higher resolution.
Disclosure of the invention
The invention aims to provide a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber, which has the advantages of simple structure, high sensitivity and higher resolution.
The purpose of the invention is realized as follows:
the microstructure optical fiber high-resolution three-dimensional refractive index testing method comprises digital hologram recording, numerical reconstruction, unwrapping, error processing, three-dimensional phase distribution reconstruction and three-dimensional refractive index conversion based on an F-P cavity. Recording the complex amplitude A of the incident F-P cavity light wave0And a digital hologram when the microstructure optical fiber sample is not placed, then placing the microstructure optical fiber in the F-P cavity, recording the optimal digital hologram containing the information of the microstructure optical fiber sample when theta is 0, and performing numerical reconstruction on the recorded digital hologram to obtain the complex amplitude distribution U of the transmitted lightTBy substituting a known value, the phase distribution δ at the angle can be obtainediUnwrapping the phase profile, the phase profile delta containing information of the microstructured fiber sampleiSubtracting the phase distribution delta when no microstructured fiber sample is placed0And obtaining the phase distribution delta only containing the microstructure optical fiber sample information. And rotating the microstructure optical fiber sample, recording the optimal digital holograms at different angles, and sequentially obtaining the phase distribution only containing the information of the microstructure optical fiber sample at the angle. And performing iRadon transformation on the phase distribution of all angles to reconstruct the three-dimensional phase distribution delta (x, y, z) of the microstructure optical fiber, and converting by a derivation formula to obtain the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructure optical fiber.
The microstructure optical fiber high-resolution three-dimensional refractive index testing method provided by the invention is suitable for a measuring system comprising a digital holographic recording light path 11 based on an F-P cavity, a microstructure optical fiber 12, a control module 13 and a calculation display module 14, and is shown in figure 2 a. The digital holographic recording light path 11 based on the F-P cavity comprises a light source, a beam expander, the F-P cavity, a microscope objective, an image collector and the like. The microstructured optical fiber 12 is an optical fiber having various microstructures. The control module 13 is composed of a computer, an instrument control unit and an instrument control interface, and controls and operates the image collector, the rotation control platform and the like to complete the recording of the digital hologram containing the microstructure optical fiber information. The calculation and display module 14 performs program processing on the recorded digital hologram, and displays the three-dimensional refractive index distribution information of the microstructured optical fiber 12 on line.
The digital holographic recording light path 11 based on the F-P cavity comprises a light source, a beam expander, the F-P cavity, a microscope objective, an image collector and the like. Preferably, the light source is a laser light source with a wavelength of 532nm, a beam expander with a corresponding wavelength is selected, an F-P etalon with a fixed cavity length is selected, a microscope objective with a magnification of 20 times is selected, and devices such as a Charged Coupled Device (CCD) with a pixel size of 3.45 μm × 3.45 μm are selected.
The microstructure optical fiber 12 is positioned in an F-P cavity in the digital holographic recording optical path 11 based on the F-P cavity, light beams are reflected for multiple times in the F-P cavity and pass through the microstructure optical fiber for multiple times, and optical path differences are accumulated in sequence, so that the measurement sensitivity is higher.
In the microstructure fiber high-resolution three-dimensional refractive index testing method, the cavity length of the F-P cavity is larger than the diameter of the microstructure fiber 12, so that the microstructure fiber 12 can be placed in the F-P cavity. The microstructured optical fiber 12 is coated and filled with an index matching fluid.
In the microstructure fiber high-resolution three-dimensional refractive index test method, the interface reflectivity of the F-P cavity is 0-1.
In the microstructure fiber high-resolution three-dimensional refractive index test method, the F-P cavity has cavity lengths with different lengths and can have different shapes and sizes. The cavity length of the F-P cavity may be adjustable, i.e., an F-P etalon, when the cavity length of the F-P cavity is fixed.
The microstructure optical fiber 12 is an optical fiber having various microstructures, and is a common single mode optical fiber, a special optical fiber, or the like having any three-dimensional refractive index distribution. The micro-structured optical fiber 12 is a micro object, and when a light beam passes through the micro-structured optical fiber 12, a cylindrical lens effect is generated, so that a refractive index matching fluid having a refractive index equivalent to that of the outermost layer of the micro-structured optical fiber 12 needs to be selected and filled. The microstructured optical fiber 12 may be a conventional single mode fiber, a specialty fiber, etc. having different sizes and structural distributions.
The control module 13 is composed of a computer, an instrument control unit and an instrument control interface, and controls and operates the image collector, the rotation control platform and the like to complete the recording of the digital hologram containing the microstructure optical fiber information. After the digital holographic recording light path 11 based on the F-P cavity is built, the control module 13 is utilized to control the rotary control platform to drive the micro-structural optical fiber 12 to rotate for a circle, and the CCD is controlled to collect and store the digital hologram containing the information of the micro-structural optical fiber 12.
The calculation and display module 14 performs program processing on the stored digital hologram, and displays the three-dimensional refractive index distribution information of the microstructured optical fiber 12 on line. According to the novel microscopic imaging method provided by the invention, the digital hologram acquired by the CCD is processed, and the high-resolution three-dimensional refractive index distribution of the microstructure optical fiber 12 can be obtained through reconstruction.
In the digital holographic recording optical path 11 based on the F-P cavity, the light beam is reflected and transmitted in the F-P cavity for multiple times, as shown in fig. 3b, the light beam passes through the microstructure fiber for multiple times, the three-dimensional information of the microstructure fiber is recorded, the optical path difference is accumulated in sequence, and the complex amplitude of the light beam finally penetrating through the F-P cavity is:
wherein, UTFor complex amplitude of transmitted light, A0Is the complex amplitude of the light wave 33 incident to the F-P cavity, R is the surface reflectivity inside the two parallel planar glass plates of the F-P cavity, as shown at 31 and 32 in fig. 3a, and δ is the phase distribution of the microstructured fiber.
Then when F-P cavity multiple beams interfere, the phase distribution obtained by digital holography is:
where n is the index of refraction of the medium within the cavity, d is the thickness of the F-P cavity, λ is the wavelength of the light source, and φ is the angle of incidence 34 of the light wave 33 incident on the F-P cavity.
The accumulation of the refractive index of the light beam passing through each point inside the microstructured optical fiber 12 along the propagation direction is the phase distribution obtained by the digital hologram, and when the refractive index difference between the inside of the microstructured optical fiber 12 and the ambient medium around the microstructured optical fiber 12 is small, the optical path difference is the accumulation of the refractive index along the beam path direction, and the relationship between the phase distribution and the refractive index distribution of the microstructured optical fiber 12 is:
where n (x, y, z) is the refractive index profile inside the microstructured fiber 12, the z-axis is the direction of beam propagation, n0Is the refractive index of the ambient medium surrounding the microstructured optical fiber 12.
For a common digital holographic recording light path based on Mach-Zehnder interference, a light source is divided into two beams by a beam splitter prism, one beam of light is used as object light wave through a microstructure optical fiber, and the other beam of light is used as reference light. The two beams of light are combined by the beam splitter prism and meet and interfere with each other on the CCD recording plane to form a hologram. The light beam only passes through the micro-structure optical fiber once, the three-dimensional information of the micro-structure optical fiber is recorded, and the optical path difference is accumulated once. The digital hologram recorded by CCD is reconstructed numerically and the complex amplitude distribution of the reconstructed object light wave is UMZ. Meanwhile, the phase distribution of the object light wave is obtained as follows:
wherein, UMZThe complex amplitude distribution of the reproduced object light wave is shown, Re represents the real part of the complex amplitude, and Im represents the imaginary part.
In the digital holographic recording optical path based on Mach-Zehnder interference, a light beam passes through the microstructure optical fiber along the propagation direction, phase difference is accumulated in the microstructure optical fiber along the propagation direction of the light beam, and the light beam only passes through the microstructure optical fiber once, so that the relationship between the phase distribution and the refractive index distribution of the microstructure optical fiber is as follows:
wherein n (x, y, z) is the refractive index distribution inside the microstructured fiber, the z-axis is the direction of beam propagation, n0Is the refractive index of the surrounding medium surrounding the microstructured optical fiber.
Comparing the formula (3) and the formula (5), it can be found that the microstructure optical fiber high resolution three-dimensional refractive index testing method based on the F-P cavity provided by the invention has the advantages that the light beam is reflected and transmitted for many times in the F-P cavity, passes through the microstructure optical fiber for many times, the three-dimensional information of the microstructure optical fiber is recorded, the optical path difference is accumulated in sequence, the measurement sensitivity is higher, the resolution is higher, and the method is superior to the common refractive index testing method based on Mach-Zehnder interference in principle.
Therefore, in the digital holographic recording optical path 11 based on the F-P cavity, the complex amplitude of incident light in the F-P cavity is firstly recorded, a digital hologram is recorded when the microstructure optical fiber is not put in, and the microstructure optical fiber is put in the F-P etalon. And recording the optimal digital hologram of the transmitted light with the angle containing the information of the microstructure fiber 12, performing numerical reconstruction on the recorded digital hologram to obtain the complex amplitude distribution of the digital hologram, substituting the complex amplitude distribution into the reflectivity R of the F-P cavity, and obtaining the phase distribution without the information of the microstructure fiber 12 and the phase distribution containing the information of the microstructure fiber 12 at the angle according to a formula (1). The phase distribution containing information of the microstructured optical fiber 12 is subtracted from the phase distribution not containing information of the microstructured optical fiber 12 to obtain the phase distribution of which the angle only contains information of the microstructured optical fiber 12. And the control module 13 controls the rotary control platform to drive the micro-structural optical fiber 12 to rotate for a circle, and controls the CCD to collect the digital hologram containing the information of the micro-structural optical fiber 12. By using the calculation display module 14, the phase distribution of each angle, which only contains information of the microstructured optical fiber 12, can be sequentially obtained by using the formula (1), and the three-dimensional phase distribution δ (x, y, z) of the microstructured optical fiber can be reconstructed by sequentially performing iRadon transformation on the phase distribution of the microstructured optical fibers 12 of all angles on each cross section. According to the formula (3), the high-resolution three-dimensional refractive index distribution of the microstructured optical fiber 12 can be obtained through conversion, and the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructured optical fiber 12 can be displayed on line through the calculation display module 14.
The invention relates to a method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber, which mainly comprises the following steps:
the first step is as follows: the complex amplitude distribution of the incident light in the F-P cavity is recorded, and the digital hologram without the microstructured optical fiber 12 in the F-P cavity is recorded. A microstructured optical fiber 12 is placed in the F-P cavity.
The second step is that: the control module 13 controls the rotation control platform to rotate the micro-structured optical fiber 12, records the optimal digital hologram after the micro-structured optical fiber 12 is placed at the angle, and intercepts a certain size.
The third step: and (3) carrying out numerical reconstruction on the recorded digital hologram, obtaining phase distribution according to the formula (1), and carrying out unwrapping treatment on the obtained phase distribution.
The fourth step: and subtracting the phase distribution obtained by unwrapping the optical fiber with the microstructure from the phase distribution obtained by unwrapping the optical fiber with the microstructure to obtain the phase distribution of which the angle only contains the phase information of the optical fiber sample with the microstructure.
The fifth step: the control module 13 controls the rotation control platform to rotate the microstructured optical fiber 12 for a circle, and the second step to the fourth step are repeated in sequence to obtain the phase distribution of each section of the microstructured optical fiber 12 at different angles.
And a sixth step: by calculating the display module 14, the phase distribution of the microstructured optical fiber 12 at all angles on each cross section is sequentially subjected to iRadon transformation, and the three-dimensional phase distribution δ (x, y, z) of the microstructured optical fiber 12 can be reconstructed.
The seventh step: the high-resolution three-dimensional refractive index distribution of the microstructured optical fiber 12 can be obtained by converting through the calculation display module 14 according to the formula (3), and the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructured optical fiber 12 can be displayed on line through the calculation display module 14.
The invention provides a microstructure optical fiber high-resolution three-dimensional refractive index testing method which comprises the steps of digital hologram recording, numerical reconstruction, unwrapping, error processing, three-dimensional phase distribution reconstruction and three-dimensional refractive index conversion based on an F-P cavity. Compared with the prior art, the digital holographic recording optical path based on the F-P cavity is adopted, so that the light beam is reflected in the F-P cavity for multiple times and passes through the microstructure optical fiber, and the measurement sensitivity is higher. The microstructure optical fiber high-resolution three-dimensional refractive index testing method has the advantages of simple structure, high sensitivity and higher resolution.
(IV) description of the drawings
FIG. 1 is a schematic diagram of a method for measuring a high-resolution three-dimensional refractive index of a microstructured optical fiber. The method comprises the steps of digital hologram recording based on an F-P cavity, numerical reconstruction, unwrapping, error processing, three-dimensional phase distribution reconstruction and three-dimensional refractive index conversion.
Fig. 2a is a structural diagram of a measurement system suitable for the microstructure optical fiber high resolution three-dimensional refractive index test method provided by the invention. The optical fiber comprises an F-P cavity-based digital holographic recording optical path 11, a microstructure optical fiber 12, a control module 13 and a calculation display module 14. FIG. 2b is a schematic diagram of an optical path for digital holographic recording based on F-P cavity in an embodiment of the present invention. In this embodiment, an F-P cavity based digital holographic recording optical path includes a light source 21, an attenuator 22, a beam expander 23, an F-P etalon 24, a micro-structured optical fiber 25, a micro-objective lens 26, a CCD27, and a computer 28.
Figure 3a is a schematic view of an F-P chamber of the present invention. The F-P cavity consists of two parallel planar glass plates with dielectric films of reflectivity R plated on their inward facing sides 31 and 32. Fig. 3b is a schematic diagram of the light beam incident on the F-P cavity, reflected multiple times in the F-P cavity, and transmitted. 33 is an incident light wave, 34 is an incident angle, 35, 37, 39 are schematic diagrams of transmitted light, and 36, 38 are schematic diagrams of reflected light.
FIG. 4 is a schematic diagram of an F-P cavity and a microstructured optical fiber in an embodiment of the invention. The rotation control platform 41 controls the rotation of the microstructured fiber 25, and the microstructured fiber 25 is inserted into a cuvette 42 filled with a refractive index matching fluid having a refractive index equivalent to that of the outermost layer of the microstructured fiber 25, and the propagation direction of the light beam is shown as 43.
FIG. 5 is a flow chart of the steps of the method for measuring the high resolution three-dimensional refractive index of the microstructured optical fiber according to the present invention.
(V) detailed description of the preferred embodiments
The invention is further illustrated below with reference to specific examples.
Example 1:
fig. 2b shows an embodiment of a digital holographic recording optical path based on an F-P cavity. The optical path includes a light source 21, an attenuator 22, a beam expander 23, an F-P etalon 24, a micro-structured optical fiber 25, a microscope objective lens 26, a CCD27, and a computer 28. The light beam is emitted from a light source 21, passes through an attenuator 22, the energy of the light beam is weakened, the light beam passes through a beam expander 23, the diameter of the light beam is enlarged, when the light beam passes through an F-P etalon 24 provided with a microstructure optical fiber 25, the light beam is reflected for multiple times in the F-P etalon 24, passes through the microstructure optical fiber 25 for multiple times, each transmitted light beam passing through the F-P etalon 24 passes through a microscope objective lens 26, is subjected to interference superposition on a CCD27, and the interference digital hologram is recorded by a CCD27 and is stored in a computer 28.
In the present embodiment, preferably, the light source 21 is a laser light source with a wavelength of 532nm, and a beam expander 23 with a corresponding wavelength is selected, an adjustable attenuator 22 is selected, an F-P etalon 24 with a cavity length of 60mm is selected, a microscope objective lens 26 with a magnification of 20 times is selected, and a CCD27 with a pixel size of 3.45 μm × 3.45 μm is selected.
In this embodiment, preferably, as shown in fig. 3a, the F-P etalon 24 is formed by two parallel flat glass plates coated with a dielectric film with a reflectivity of 0.8 on the inner surface to form the F-P etalon 24 with a cavity length of 60 mm. Preferably, the micro-structured fiber 25 is a dual-core fiber, the dual-core fiber with the coating layer removed is placed in a cuvette 42 filled with a refractive index matching fluid, the cuvette 42 is placed in the cavity of the F-P etalon 24, and as shown in fig. 4, the digital holographic recording optical path is adjusted so that the optical axis of the light beam 43 passes through the micro-structured fiber 25.
In this embodiment, preferably, the rotation control platform 41 is a precise rotating motor, and the control module precisely controls the microstructure optical fiber 25 to rotate around the axial direction of the optical fiber for one circle. The cuvette 42 is provided with a ceramic ferrule which can fix the microstructure fiber 25 to rotate around a shaft, and the refractive index of the filled refractive index matching fluid is the same as that of the fiber cladding.
In the present embodiment, the digital holographic recording optical path based on the F-P cavity comprises a light source 21, an attenuator 22, a beam expander 23, an F-P etalon 24, a microscope objective lens 26, a CCD27 and the like. The microstructured optical fiber 25 is a dual core fiber. The control module is composed of a computer 28, an instrument control unit and an instrument control interface, and controls and operates the CCD27, the rotary control platform 41 for controlling the rotation of the microstructure optical fiber 25 and the like, and completes the digital hologram recording containing the information of the microstructure optical fiber 25. The calculation display module processes the recorded digital hologram according to the microscopic imaging method provided by the invention and displays the three-dimensional refractive index information of the microstructure optical fiber on line.
In this embodiment, a method for testing a high-resolution three-dimensional refractive index of a microstructured optical fiber is used to process a digital hologram of the microstructured optical fiber 25 and display three-dimensional refractive index information of the microstructured optical fiber 25 on line, as shown in fig. 5, which mainly includes the following steps:
first step 51: the complex amplitude distribution of the light incident on the F-P etalon 24 is recorded and a digital hologram is recorded when the microstructured optical fiber 25 is not placed in the F-P etalon 24. A microstructured optical fiber 25 is placed in the F-P etalon 24.
Second step 52: the control module controls a precise rotating motor to rotate, so that the micro-structural optical fiber 25 rotates, the optimal digital hologram after the micro-structural optical fiber 25 is placed at the angle is recorded, and the optimal digital hologram with a certain size is intercepted.
Third step 53: the recorded digital hologram is reconstructed numerically.
The fourth step 54: the phase distribution of the microstructured fiber 25 is obtained according to the formula (1), and the unwrapping process is performed on the obtained phase distribution.
Fifth step 55: and subtracting the phase distribution obtained by unwrapping the optical fiber 25 which is not placed into the microstructure from the phase distribution obtained by unwrapping the optical fiber 25 which is placed into the microstructure to obtain the phase distribution only containing the phase information of the optical fiber 25.
Sixth step 56: the control module controls the rotary control platform to rotate the micro-structural optical fiber 25 for a circle, and the second step to the fifth step are repeated in sequence, so that the phase distribution of each section of the micro-structural optical fiber 25 at different angles can be obtained.
Seventh step 57: through the calculation and display module, the phase distribution of the microstructure optical fiber 25 with all angles on each cross section is subjected to iRadon transformation in sequence, and the three-dimensional phase distribution delta (x, y, z) of the microstructure optical fiber 25 can be obtained through reconstruction.
Eighth step 58: the high-resolution three-dimensional refractive index distribution of the microstructured optical fiber 25 can be obtained by converting through the calculation display module according to the formula (3), and the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructured optical fiber 25 can be displayed on line through the calculation display module.
The above description is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto. Various modifications and alterations of this invention will occur to those skilled in the art in view of the spirit and scope of this invention and are intended to be encompassed by the following claims.
Claims (7)
1. A method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber. The method is characterized in that: the method comprises the steps of digital hologram recording, numerical reconstruction, unwrapping, error processing, three-dimensional phase distribution reconstruction and three-dimensional refractive index conversion based on an F-P cavity. Recording the complex amplitude A of the incident F-P cavity light wave0And a digital hologram when the microstructure optical fiber sample is not placed, then placing the microstructure optical fiber in the F-P cavity, recording the optimal digital hologram containing the information of the microstructure optical fiber sample when theta is 0, and performing numerical reconstruction on the recorded digital hologram to obtain the complex amplitude distribution U of the transmitted lightTBy substituting a known value, the phase distribution δ at the angle can be obtainediUnwrapping the phase profile, the phase profile delta containing information of the microstructured fiber sampleiSubtracting the phase distribution delta when no microstructured fiber sample is placed0And obtaining the phase distribution delta only containing the microstructure optical fiber sample information. And rotating the microstructure optical fiber sample, recording the optimal digital holograms at different angles, and sequentially obtaining the phase distribution only containing the information of the microstructure optical fiber sample at the angle. Making iRadon transform on the phase distribution for all angles, i.e.The three-dimensional phase distribution delta (x, y, z) of the microstructure fiber can be reconstructed, and the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructure fiber can be obtained through conversion by a derivation formula.
2. A method for testing the high-resolution three-dimensional refractive index of a microstructure optical fiber. The method is characterized in that: the device is suitable for a measuring system comprising an F-P cavity-based digital holographic recording light path, a microstructure optical fiber, a control module and a calculation display module. The digital holographic recording light path based on the F-P cavity comprises a light source, a beam expander, the F-P cavity, a microscope objective, an image collector and the like. The microstructure optical fiber is an optical fiber with various microstructures. The control module consists of a computer, an instrument control unit and an instrument control interface, and is used for controlling and operating the image collector, the rotary control platform and the like to complete the recording of the digital hologram containing the microstructure optical fiber information. And the calculation display module is used for carrying out program processing on the recorded digital hologram and displaying the three-dimensional refractive index distribution information of the microstructure optical fiber on line.
3. The method of claim 2, wherein the microstructured optical fiber is located in an F-P cavity in a digital holographic recording optical path.
4. The method for testing the high-resolution three-dimensional refractive index of the micro-structured optical fiber according to claim 2, wherein the cavity length of the F-P cavity is larger than the diameter of the micro-structured optical fiber, and the interface reflectivity of the F-P cavity is 0-1.
5. The method of claim 2, wherein the F-P cavity has a cavity length with different lengths.
6. The method for testing the high-resolution three-dimensional refractive index of the microstructured optical fiber according to claim 1, wherein the microstructured optical fiber can be a common single-mode optical fiber, a special optical fiber and the like with any three-dimensional refractive index distribution.
7. The method for testing the high resolution three-dimensional refractive index of the microstructured optical fiber according to claim 1, comprising the following steps:
the first step is as follows: and recording the complex amplitude distribution of incident light in the F-P cavity and recording a digital hologram without the microstructure fiber in the F-P cavity. A microstructured optical fiber is placed in the F-P cavity.
The second step is that: the control module controls the rotary control platform to enable the micro-structural optical fiber to rotate, records the optimal digital hologram after the micro-structural optical fiber is placed at the angle, and intercepts a certain size.
The third step: and (3) carrying out numerical reconstruction on the recorded digital hologram, obtaining the phase distribution of the digital hologram according to the formula (1), and carrying out unwrapping treatment on the obtained phase distribution.
Wherein, UTFor complex amplitude of transmitted light, A0Is the complex amplitude of the light wave incident into the F-P cavity, R is the surface reflectivity of the inside of the F-P cavity, and δ is the phase distribution.
The fourth step: and subtracting the phase distribution obtained by unwrapping the optical fiber with the microstructure from the phase distribution obtained by unwrapping the optical fiber with the microstructure to obtain the phase distribution of which the angle only contains the phase information of the optical fiber sample with the microstructure.
The fifth step: and the control module controls the rotary control platform to rotate the microstructure optical fiber for a circle, and the second step to the fourth step are repeated in sequence to obtain the phase distribution of each section of the microstructure optical fiber at different angles.
And a sixth step: and through the calculation display module, the phase distribution of the microstructure optical fiber with all angles on each cross section is subjected to iRadon transformation in sequence, and the three-dimensional phase distribution delta (x, y, z) of the microstructure optical fiber can be reconstructed.
The seventh step: according to the formula (2), the high-resolution three-dimensional refractive index distribution of the microstructure optical fiber can be obtained through conversion, and the high-resolution three-dimensional refractive index distribution n (x, y, z) of the microstructure optical fiber can be displayed on line through the calculation display module.
Wherein n (x, y, z) is refractive index distribution inside the micro-structured fiber, z-axis is the direction of light beam propagation, λ is the wavelength of the light source, n0Is the refractive index of the surrounding ambient medium.
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