CN113324741B - System and method for evaluating large mode field optical fiber core refractive index fluctuation - Google Patents
System and method for evaluating large mode field optical fiber core refractive index fluctuation Download PDFInfo
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Abstract
The invention belongs to the technical field of optical fiber testing, and particularly discloses a system and a method for evaluating large mode field optical fiber core refractive index fluctuation. The system comprises a super-continuum spectrum light source, an optical isolator, a single-mode fiber beam splitter and a spectrometer, wherein the optical isolator is arranged at the output end of the super-continuum spectrum light source; the first input end of single mode fiber beam splitter welds with optical isolator's output, and its second output holds eccentric butt fusion with the second of the big mode field optic fibre that awaits measuring for the same pulse light is transmitted at the radial different positions of the optic fibre core that awaits measuring simultaneously, and the spectrum appearance is connected with single mode fiber beam splitter's third output. The method comprises the steps that supercontinuum light is injected into different fiber core positions at two ends of the large mode field optical fiber to be detected, so that the same pulse light is transmitted at different radial positions of the fiber core of the optical fiber to be detected at the same time, and finally the same pulse light is converged into a single-mode optical fiber beam splitter at the same time. The invention can rapidly evaluate the fluctuation of the refractive index of the large-mode-field optical fiber core, thereby rapidly screening the optical fiber.
Description
Technical Field
The invention belongs to the technical field of optical fiber testing, and particularly relates to a system and a method for evaluating large mode field optical fiber core refractive index fluctuation.
Background
For a high-power optical fiber laser, the large-mode-field optical fiber increases the effective area of the fiber core of the optical fiber and reduces the power density of laser in the fiber core, so that the temperature inside the fiber core of the optical fiber is effectively controlled, and the output power of the laser is greatly increased. However, large mode field fibers may allow transmission of several higher order modes, which may significantly degrade the beam quality of the output laser.
The refractive index profile of the large mode field fiber core is designed to be step-type, and the step-type refractive index profile can effectively reduce mode coupling between a fundamental mode and a high-order mode, so that the beam quality of output laser is improved. However, many optical fiber manufacturers adopt MCVD to fabricate large mode field optical fibers, and it is difficult to precisely control the refractive index profile of the core of the large mode field optical fiber, so that the refractive index of the core of many large mode field optical fibers fluctuates greatly along the radial direction. In the process of mass production of fiber lasers, even if external interference such as bending of optical fibers is adopted, the stability of the output power of each laser and the stability of the beam quality are difficult to guarantee. The output power and the beam quality of the lasers in different batches fluctuate, and the production progress is greatly influenced.
Based on the defects, the field needs to provide a method for simply evaluating the fluctuation of the refractive index of the large mode field optical fiber core urgently, and a simple and easy-to-operate optical fiber screening system is constructed, so that the fluctuation condition of the large mode field optical fiber core can be rapidly and qualitatively judged by simply constructing an experiment system, the repeatability is good, and the rapid screening of the optical fiber is realized in the production process.
Disclosure of Invention
The invention provides a system and a method for evaluating the fluctuation of the refractive index of a large-mode-field optical fiber core, aiming at the defects or improvement requirements of the prior art, wherein the system and the method for evaluating the fluctuation of the refractive index of the large-mode-field optical fiber core are correspondingly designed by combining the characteristics of the large-mode-field optical fiber and the characteristics of an optical fiber screening process, the structures and the specific arrangement modes of key components of the system and the method, such as a single-mode optical fiber beam splitter, a large-mode-field optical fiber to be tested, an isolator and a spectrometer, are researched and designed, and supercontinuum light is correspondingly injected into different fiber core positions at two ends of the large-mode-field optical fiber to be tested, so that the same pulse light is transmitted at different radial positions of the fiber core of the optical fiber to be tested at the same time, and finally, the same pulse light is converged into the single-mode optical fiber beam splitter. If the refractive index fluctuation of the large mode field optical fiber core is small, no obvious interference fringes can be observed on the spectrometer, and if the refractive index fluctuation of the large mode field optical fiber core is large, the interference fringes can be clearly observed on the spectrometer. The method is simple and convenient to operate and good in repeatability, and can be used for rapidly evaluating the fluctuation of the refractive index of the large-mode-field optical fiber core in actual production, so that the optical fiber can be rapidly screened.
To achieve the above objects, according to one aspect of the present invention, there is provided a system for evaluating refractive index fluctuation of a large mode field fiber core, comprising a supercontinuum light source, an optical isolator, a single mode fiber beam splitter, and a spectrometer, wherein,
the super-continuum spectrum light source is used for outputting a super-continuum spectrum;
the optical isolator is arranged at the output end of the supercontinuum light source;
the first input end of the single-mode fiber beam splitter is welded with the output end of the optical isolator, the first output end of the single-mode fiber beam splitter is welded with the first end of the large-mode-field fiber to be detected in a cladding alignment mode, the second output end of the single-mode fiber beam splitter is welded with the second end of the large-mode-field fiber to be detected, the fiber core position of the second output end of the single-mode fiber beam splitter deviates from the center of the fiber core of the second end of the large-mode-field fiber to be detected, and the fiber core position of the second output end of the single-mode fiber beam splitter is within the coverage range of the fiber core of the second end of the large-mode-field fiber to be detected;
the input end of the spectrometer is connected with the third output end of the single-mode fiber beam splitter, and the spectrometer is used for displaying the refractive index of the large-mode-field fiber core to be measured.
Further preferably, the single-mode fiber splitter has an output power ratio of 50: 50.
Preferably, the diameter of the fiber core of the large mode field optical fiber to be detected is 8-300 μm; the range of the cladding diameter of the large mode field optical fiber to be detected is 10-2000 mu m.
More preferably, the diameter of the fiber core of the single-mode fiber splitter ranges from 1 μm to 10 μm, and the diameter of the fiber cladding of the single-mode fiber splitter ranges from 10 μm to 500 μm.
Preferably, the spectral range of the supercontinuum light source at least covers the transmission band of the large mode field optical fiber to be measured.
Preferably, the spectrum measurement range of the spectrometer covers the emission spectrum of the supercontinuum light source, the minimum resolution of the spectrometer is not more than 0.05nm, and the dynamic range of the spectrometer is 50-80 dB.
Preferably, the second end of the large mode field fiber to be tested and the second output end of the single-mode fiber beam splitter are welded by an optical fiber welding machine, and an optical fiber clamp of the optical fiber welding machine is respectively matched with the second end of the large mode field fiber to be tested and the optical fiber size of the second output end of the single-mode fiber beam splitter;
the optical fiber diameter that this optical fiber splicer can the butt fusion covers at least the optical fiber diameter of the large mode field optic fibre that awaits measuring and the optical fiber diameter of the second output of single mode fiber beam splitter.
According to another aspect of the present invention, there is also provided a method for evaluating the refractive index fluctuation of a core of a large mode field optical fiber, which is implemented by using the system described above, and comprises the following steps:
s1, outputting a super-continuum spectrum by using a super-continuum spectrum light source;
s2, inputting the supercontinuum into a first input end of a single-mode fiber beam splitter for beam splitting after passing through an optical isolator, and simultaneously injecting the supercontinuum after beam splitting into different positions of a large-mode field fiber core to be detected, so that the same supercontinuum is transmitted at different radial positions of the large-mode field fiber core to be detected, and finally, the supercontinuum is simultaneously converged into the single-mode fiber beam splitter;
s3, displaying the spectrum converged into the single-mode fiber beam splitter in the step S2 by a spectrometer, and judging the fluctuation of the refractive index of the large-mode-field fiber core to be detected through the interference fringes of the spectrum.
Further preferably, the single-mode fiber splitter has an output power ratio of 50: 50;
preferably, the diameter of the fiber core of the large mode field optical fiber to be detected is 8-300 μm, and the diameter of the cladding of the large mode field optical fiber to be detected is 10-2000 μm;
more preferably, the diameter of the fiber core of the single-mode fiber splitter ranges from 1 μm to 10 μm, and the diameter of the fiber cladding of the single-mode fiber splitter ranges from 10 μm to 500 μm.
Preferably, in step S2, the first output end of the single-mode fiber splitter is fusion-spliced with the first end of the large-mode-field fiber to be measured in the cladding alignment mode, the second output end of the single-mode fiber splitter is fusion-spliced with the second end of the large-mode-field fiber to be measured in the fiber fusion splicer, the fiber core position of the second output end of the single-mode fiber splitter deviates from the center of the fiber core of the second end of the large-mode-field fiber to be measured, but the fiber core position of the second output end of the single-mode fiber splitter is within the coverage range of the fiber core of the second end of the large-mode-field fiber to be measured.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
1. according to the invention, supercontinuum light is injected into different fiber core positions at two ends of the large mode field optical fiber to be detected, so that the same pulse light is transmitted at different radial positions of the fiber core of the optical fiber to be detected at the same time, and finally, the same pulse light is converged into the single-mode optical fiber beam splitter at the same time. If the refractive index fluctuation of the large mode field optical fiber core is small, no obvious interference fringes can be observed on the spectrometer, and if the refractive index fluctuation of the large mode field optical fiber core is large, the interference fringes can be clearly observed on the spectrometer. The method is simple and convenient to operate and good in repeatability, and can be used for rapidly evaluating the fluctuation of the refractive index of the large-mode-field optical fiber core in actual production, so that the optical fiber can be rapidly screened.
2. The second end of the large mode field optical fiber to be detected and the second output end of the single-mode optical fiber beam splitter are respectively placed into an optical fiber fusion splicer, a fusion splicing program is set to be in a manual alignment mode, the position of the optical fiber is manually adjusted, the optical fiber core of the second output end of the single-mode optical fiber beam splitter deviates from the center of the optical fiber core of the large mode field optical fiber to be detected, the position of the optical fiber core of the second output end of the single-mode optical fiber beam splitter is still ensured to be within the range covered by the optical fiber core of the large mode field optical fiber to be detected, core deviation fusion splicing is carried out after position adjustment is completed, and in this way, supercontinuum light is accurately controlled to be injected into different optical fiber core positions at two ends of the large mode field optical fiber to be detected.
3. The invention carries out specific corresponding design on the fiber core diameter and the cladding diameter of the large mode field optical fiber to be measured and the fiber core diameter and the cladding diameter of the single mode optical fiber beam splitter, so that the power stability and the light beam quality are stable in the pulse light transmission process, and the spectrum finally displayed by the spectrometer is more accurate.
Drawings
FIG. 1 is a schematic structural diagram of a system for large mode field fiber core refractive index fluctuation according to an embodiment of the present invention;
fig. 2 is an effect diagram of cladding alignment welding of a first end of a large mode field fiber to be measured and a first output end of a single-mode fiber beam splitter having an output power ratio of 50:50 in a system for refractive index fluctuation of a fiber core of the large mode field fiber according to an embodiment of the present invention;
fig. 3 is a diagram illustrating an eccentric alignment welding effect of a second end of a large mode field optical fiber to be measured and a second output end of a single-mode optical fiber beam splitter, where an output power ratio of the second end of the large mode field optical fiber to be measured to the second output end of the single-mode optical fiber beam splitter is 50:50, in a system for refractive index fluctuation of a fiber core of the large mode field optical fiber according to an embodiment of the present invention;
FIG. 4 is a diagram illustrating an original emission spectrum of a supercontinuum light source in a method for large mode field fiber core refractive index fluctuation according to an embodiment of the present invention;
FIG. 5 shows a supercontinuum with interference fringes measured by a spectrometer due to the fluctuation of the refractive index of the large mode field optical fiber core in the method for the fluctuation of the refractive index of the large mode field optical fiber core according to the embodiment of the present invention.
In all the figures, the same reference numerals denote the same features, in particular: 1-large mode field fiber to be tested, 2-a first output end fusion point of the large mode field fiber to be tested and a single mode fiber beam splitter, 3-a first output end, 4-a single mode fiber beam splitter, 5-a first input end, 6-a fusion point of a first output end of the single mode fiber beam splitter and an input end of an optical isolator, 7-the optical isolator, 8-a supercontinuum light source, 9-the optical fiber fusion splicer, 10-a second output end, 11-a third output end, 12-the spectrometer, 13-a first end and 14-a second end.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in FIG. 1, the present invention provides a system and method for easily evaluating refractive index fluctuation of a large mode field fiber core, wherein the system utilizes a 50:50 single mode fiber splitter, an optical isolator, a supercontinuum light source, a spectrometer and a fiber fusion splicer. Through a single-mode fiber beam splitter of 50:50, supercontinuum light is injected into different fiber core positions at two ends of a large-mode-field fiber to be detected, so that the same pulse light is transmitted at different radial positions of the fiber core of the fiber to be detected at the same time, and finally, the pulse light is converged into the single-mode fiber beam splitter at the same time. If the refractive index fluctuation of the large mode field optical fiber core is small, no obvious interference fringes can be observed on the spectrometer, and if the refractive index fluctuation of the large mode field optical fiber core is large, the interference fringes can be clearly observed on the spectrometer. The method is simple and convenient to operate and good in repeatability, and can be used for rapidly evaluating the fluctuation of the refractive index of the large-mode-field optical fiber core in actual production, so that the optical fiber can be rapidly screened.
Specifically, as shown in fig. 1, 2 and 3, the system for evaluating the refractive index fluctuation of the large mode field optical fiber core comprises a supercontinuum light source 8, an optical isolator 7, a single-mode optical fiber beam splitter 4 and a spectrumAnd a spectrometer 12, wherein the supercontinuum light source 8 is configured to output a supercontinuum. In a preferred embodiment of the present invention, the supercontinuum light source 8 is coupled into a highly nonlinear fiber using an ultrashort pulsed laser. The supercontinuum light source 8 comprises a seed laser, a pulse frequency multiplier and a non-linear element, the seed laser being configured to provide seed pulses at a respective pulse frequency; a pulse frequency multiplier is configured to multiply the seed pulses and to convert the respective pulse frequency to pump pulses of a specified pulse frequency, wherein the specified pulse frequency is greater than the respective pulse frequency. The nonlinear element is configured to receive the pump pulse and convert the pump pulse into supercontinuum light that is output by the nonlinear element and has an output from about λ1Spanning to about lambda2Of the supercontinuum, lambda2-λ1>About 500 nm. An output of the nonlinear element is coupled to a single mode coupling unit to provide an output from the single mode coupling unit, wherein the light source output includes the output from the single mode coupling unit, the single mode coupling unit configured to suppress and shape the supercontinuum from the nonlinear element. The optical isolator 7 is arranged at the output end of the supercontinuum light source 8; the first input end 5 of the single-mode fiber beam splitter 4 is welded with the output end of the optical isolator 7, the first output end 3 of the single-mode fiber beam splitter 4 is welded with the first end 13 of the large-mode-field fiber 1 to be detected in a cladding alignment mode, the second output end 10 of the single-mode fiber beam splitter 4 is welded with the second end 14 of the large-mode-field fiber 1 to be detected, the fiber core position of the second output end 10 of the single-mode fiber beam splitter 4 deviates from the center of the fiber core of the second end 14 of the large-mode-field fiber 1 to be detected, but the fiber core position of the second output end 10 of the single-mode fiber beam splitter 4 is within the coverage range of the fiber core of the second end 14 of the large-mode-field fiber 1 to be detected; the input end of the spectrometer 12 is connected with the third output end 11 of the single-mode fiber beam splitter 4, and the spectrometer 12 is used for displaying the refractive index of the fiber core of the large-mode-field fiber 1 to be measured.
In the present invention, the single-mode fiber splitter 4 has an output power ratio of 50: 50. In the invention, the single-mode fiber beam splitter 4 with the output power ratio of 50:50 is adopted to split the pulse laser output by the continuous spectrum light source 8, then the split pulse laser is injected to different positions of the fiber core of the large-mode field fiber to be detected, then the same pulse light is transmitted at different positions of the fiber core of the large-mode field fiber to be detected in the radial direction at the same time, and finally the pulse light is converged into the single-mode fiber beam splitter, and in this way, whether the output power of the large-mode field fiber to be detected is stable or not is judged by finally obtaining the refractive index of the laser.
In one embodiment of the invention, a first end 13 of a large mode field fiber to be tested is welded with a first output end 3 of a 50:50 single mode fiber splitter, a welding program is set to be a cladding alignment mode, a second end 15 of the large mode field fiber to be tested and a second output end 10 of the single mode fiber splitter with an output power ratio of 50:50 are respectively put into a fiber welding machine 9, the welding program is set to be a manual alignment mode, the position of the fiber is manually adjusted, the fiber core of the second output end 10 of the single mode fiber splitter with the output power ratio of 50:50 deviates from the center of the fiber core of the large mode field fiber to be tested, the position of the fiber core of the second output end 10 of the single mode fiber splitter with the output power ratio of 50:50 is still ensured to be within the range covered by the fiber core of the large mode field fiber to be tested, core-off-core welding is carried out after the position adjustment is finished, a first input end 5 of the single mode fiber splitter with the output power ratio of 50:50 is welded with an output end of an optical isolator 7, the input end of the optical isolator 7 is connected with the output end of the supercontinuum light source 8, and the third output end 11 of the single-mode fiber beam splitter with the output power ratio of 50:50 is connected with the spectrometer 12. In this way, the supercontinuum light is injected into different fiber core positions at two ends of the large mode field optical fiber to be detected, so that the same pulse light is transmitted at different radial positions of the fiber core of the optical fiber to be detected simultaneously, and finally, the same pulse light is converged into the single-mode optical fiber beam splitter simultaneously. If the refractive index fluctuation of the large mode field optical fiber core is small, no obvious interference fringes can be observed on the spectrometer, and if the refractive index fluctuation of the large mode field optical fiber core is large, the interference fringes can be clearly observed on the spectrometer. The method is simple and convenient to operate, good in repeatability and capable of rapidly evaluating the fluctuation of the refractive index of the large-mode-field optical fiber core in actual production.
In the invention, the optical fiber fusion splicer melts the sections of the two optical fibers by using a high-voltage electric arc and simultaneously uses a high-precision motion mechanism to smoothly push the two optical fibers to fuse the two optical fibers into one, thereby realizing the coupling of an optical fiber mode field. Certainly, according to the rapid detection means of the present invention, the positions of injecting the pulsed light into the two ends of the large mode field fiber 1 to be detected are different, in an embodiment of the present invention, the first end 13 of the large mode field fiber is fusion-spliced with the first output end 3 of the 50:50 single mode fiber splitter, and the fusion-splicing procedure is set to be the cladding alignment mode, so that the pulsed light is injected into the center position of the fiber core of the large mode field fiber 1 to be detected, and meanwhile, the position of the fiber is manually adjusted, so that the fiber core of the second output end 10 of the 50:50 single mode fiber splitter deviates from the center of the fiber core of the large mode field fiber to be detected, and in this way, the pulsed light injected into the second section of the large mode field fiber 1 to be detected deviates from the center position of the fiber core. If the refractive index of the fiber core of the large mode field fiber 1 to be detected is small in amplitude along the radial direction, the first end and the second end are transmitted to the single mode fiber beam splitter 4, at the moment, the spectrum gathered into the single mode fiber beam splitter 4 is displayed by the spectrometer 12, and the stability of the refractive index of the large mode field fiber to be detected along the radial direction is judged by judging the spectral interference fringes, so that the stability of the output power and the stability of the light beam quality are judged.
In a preferred embodiment of the invention, the core diameter of the large mode field optical fiber 1 to be measured is 8-300 μm; the range of the cladding diameter of the large mode field optical fiber 1 to be detected is 10-2000 mu m.
In a preferred embodiment of the present invention, the diameter of the fiber core of the single-mode fiber splitter 4 is in a range of 1 to 10 μm, and the diameter of the fiber cladding of the single-mode fiber splitter 4 is in a range of 10 to 500 μm.
In a preferred embodiment of the present invention, the spectral range of the supercontinuum light source 8 is at least the transmission band of the large mode field fiber 1 to be measured.
In a preferred embodiment of the present invention, the spectrum measurement range of the spectrometer 12 covers the emission spectrum of the supercontinuum light source 8, the minimum resolution of the spectrometer 12 is not more than 0.05nm, and the dynamic range of the spectrometer 12 is 50-80 dB.
In a preferred embodiment of the present invention, the second end 14 of the large mode field optical fiber 1 to be tested and the second output end 10 of the single-mode optical fiber splitter 4 are fusion-spliced by using an optical fiber fusion splicer 9, and optical fiber clamps of the optical fiber fusion splicer 9 are respectively matched with the optical fiber sizes of the second end 14 of the large mode field optical fiber 1 to be tested and the second output end 10 of the single-mode optical fiber splitter 4. The optical fiber fusion splicer 9 can splice optical fibers with diameters at least covering the optical fiber diameter of the large mode field optical fiber 1 to be tested and the optical fiber diameter of the second output end 10 of the single-mode optical fiber beam splitter 4.
According to another aspect of the present invention, there is provided a method of evaluating large mode field fiber core refractive index fluctuations, comprising the steps of:
step one, a supercontinuum light source 8 is adopted to output a supercontinuum. As shown in fig. 4, the supercontinuum light source 8 uses an ultrashort pulsed laser coupled into a highly nonlinear fiber. More specifically, an optical isolator 7 is further arranged between the supercontinuum light source 8 and the single-mode fiber beam splitter 4, and the optical isolator 7 enables pulse laser output by the supercontinuum light source 8 to be transmitted to the single-mode fiber beam splitter 4 only in a single direction, so that the stability and the reliability of system measurement are further improved.
And step two, the first end of the large mode field fiber to be tested is welded with the first output end of the single-mode fiber beam splitter with the output power ratio of 50:50, and the welding procedure is set to be a cladding alignment mode.
And step three, respectively placing the second end of the large mode field optical fiber to be tested and the second output end of the single mode optical fiber beam splitter with the output power ratio of 50:50 into an optical fiber fusion splicer, setting a fusion splicing program to be a manual alignment mode, manually adjusting the position of the optical fiber to enable the optical fiber core of the second output end of the single mode optical fiber beam splitter with the output power ratio of 50:50 to deviate from the central position of the optical fiber core of the large mode field optical fiber to be tested, still ensuring that the optical fiber core of the second output end of the single mode optical fiber beam splitter with the output power ratio of 50:50 is within the range covered by the optical fiber core of the large mode field optical fiber to be tested, and performing core deviation fusion splicing after the position adjustment is completed. Meanwhile, the range of the diameter of the fiber core of the large mode field optical fiber to be detected is 8-300 mu m, and the range of the diameter of the cladding of the large mode field optical fiber to be detected is 10-2000 mu m. The single-mode fiber beam splitter 4 has an output power ratio of 50:50, a fiber core diameter of 1-10 μm, and a fiber cladding diameter of 10-00 μm.
And fourthly, welding a first input end of the single-mode fiber beam splitter with the output power ratio of 50:50 with an output end of the optical isolator, connecting the input end of the optical isolator with the output end of the supercontinuum light source, and connecting a third output end of the single-mode fiber beam splitter with the output power ratio of 50:50 with the spectrometer.
And fifthly, inputting the supercontinuum into a first input end 5 of a single-mode fiber beam splitter 4 for beam splitting after passing through an optical isolator 7, and simultaneously injecting the supercontinuum after beam splitting into different positions of the fiber core of the large-mode field fiber 1 to be detected, so that the same supercontinuum is transmitted at different radial positions of the fiber core of the large-mode field fiber 1 to be detected, and finally, the supercontinuum is simultaneously converged into the single-mode fiber beam splitter 4. If the refractive index fluctuation of the large mode field fiber core is small, no significant interference fringes can be observed on the spectrometer. If the refractive index fluctuation of the large mode field fiber core is large, interference fringes can be clearly observed on the spectrometer, as shown in fig. 5.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A system for evaluating the refractive index fluctuation of a large-mode-field optical fiber core is characterized by comprising a supercontinuum light source (8), an optical isolator (7), a single-mode optical fiber beam splitter (4) and a spectrometer (12),
the supercontinuum light source (8) is used for outputting a supercontinuum;
the optical isolator (7) is arranged at the output end of the supercontinuum light source (8);
the first input end (5) of the single-mode fiber beam splitter (4) is welded with the output end of the optical isolator (7), the first output end (3) of the single-mode fiber beam splitter (4) is welded with the first end (13) of the large-mode-field fiber (1) to be tested in a cladding alignment mode, the second output end (10) of the single-mode fiber beam splitter (4) is welded with the second end (14) of the large-mode-field fiber (1) to be tested, the fiber core position of the second output end (10) of the single-mode fiber beam splitter (4) deviates from the center of the fiber core of the second end (14) of the large-mode-field fiber (1) to be tested, and the fiber core position of the second output end (10) of the single-mode fiber beam splitter (4) is within the fiber core coverage range of the second end (14) of the large-mode-field fiber (1) to be tested;
the input end of the spectrometer (12) is connected with the third output end (11) of the single-mode optical fiber beam splitter (4), and the spectrometer (12) is used for displaying the refractive index of the fiber core of the large-mode-field optical fiber (1) to be measured.
2. The system for evaluating large mode field fiber core refractive index fluctuations of claim 1, wherein the single mode fiber splitter (4) has an output power ratio of 50: 50.
3. The system for evaluating the refractive index fluctuation of the large mode field optical fiber core according to claim 1, wherein the core diameter of the large mode field optical fiber (1) to be tested is in a range of 8-300 μm; the range of the cladding diameter of the large mode field optical fiber (1) to be detected is 10-2000 mu m.
4. The system for evaluating refractive index fluctuation of a large-mode-field optical fiber core according to claim 3, wherein the diameter of the optical fiber core of the single-mode optical fiber beam splitter (4) is in the range of 1-10 μm, and the diameter of the optical fiber cladding of the single-mode optical fiber beam splitter (4) is in the range of 10-500 μm.
5. The system for evaluating refractive index fluctuation of a large mode field optical fiber core according to claim 1, wherein the spectral range of the supercontinuum light source (8) covers the transmission band of the large mode field optical fiber (1) to be measured.
6. The system for evaluating refractive index fluctuations of a large mode field optical fiber core according to claim 1, wherein the spectral measurement range of the spectrometer (12) covers the emission spectrum of the supercontinuum light source (8), the minimum resolution of the spectrometer (12) is not greater than 0.05nm, and the dynamic range of the spectrometer (12) is 50-80 dB.
7. The system for evaluating the refractive index fluctuation of the large mode field optical fiber core according to claim 1, wherein the second end (14) of the large mode field optical fiber (1) to be tested is fusion-spliced with the second output end (10) of the single mode optical fiber splitter (4) by using an optical fiber fusion splicer (9), and fiber clamps of the optical fiber fusion splicer (9) are respectively matched with the fiber sizes of the second end (14) of the large mode field optical fiber (1) to be tested and the second output end (10) of the single mode optical fiber splitter (4);
the optical fiber fusion splicer (9) can splice optical fibers with diameters at least covering the optical fiber diameter of the large mode field optical fiber (1) to be tested and the optical fiber diameter of the second output end (10) of the single-mode optical fiber beam splitter (4).
8. A method of evaluating large mode field fiber core index fluctuations, implemented using the system of any of claims 1-7, comprising the steps of:
s1, outputting a supercontinuum by using a supercontinuum light source (8);
s2, the supercontinuum passes through an optical isolator (7) and then is input into a first input end (5) of a single-mode fiber beam splitter (4) for beam splitting, and the supercontinuum after beam splitting is simultaneously injected into different positions of a fiber core of the large-mode-field fiber (1) to be detected, so that the same supercontinuum is transmitted at different radial positions of the fiber core of the large-mode-field fiber (1) to be detected, and finally the supercontinuum is simultaneously converged into the single-mode fiber beam splitter (4);
s3, displaying the spectrum converged into the single-mode fiber beam splitter (4) in the step S2 by using a spectrometer (12), and judging the fluctuation of the core refractive index of the large-mode-field fiber (1) to be detected according to the interference fringes of the spectrum.
9. The method for evaluating the refractive index fluctuation of the large-mode-field optical fiber core according to claim 8, wherein the single-mode optical fiber beam splitter (4) has an output power ratio of 50: 50;
the range of the core diameter of the large mode field optical fiber (1) to be detected is 8-300 mu m, and the range of the cladding diameter of the large mode field optical fiber (1) to be detected is 10-2000 mu m;
the diameter range of the fiber core of the single-mode fiber beam splitter (4) is 1-10 mu m, and the diameter range of the fiber cladding of the single-mode fiber beam splitter (4) is 10-500 mu m.
10. The method of claim 8 for evaluating large mode field fiber core refractive index fluctuations, it is characterized in that in step S2, the first output end (3) of the single-mode fiber beam splitter (4) and the first end (13) of the large-mode-field fiber (1) to be tested are welded in a cladding alignment mode, the second output end (10) of the single-mode fiber beam splitter (4) and the second end (14) of the large-mode-field fiber (1) to be tested are welded by an optical fiber welding machine (9), and the fiber core position of the second output end (10) of the single-mode fiber beam splitter (4) deviates from the center of the fiber core of the second end (14) of the large-mode-field fiber (1) to be measured, but the fiber core position of the second output end (10) of the single-mode fiber beam splitter (4) is within the coverage range of the fiber core of the second end (14) of the large-mode-field fiber (1) to be measured.
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