Phase noise rejection anti-resonant hollow core fiber distributed backscattering measurement device

Abstract

The application belongs to the technical field of optical fiber measurement, and particularly relates to a phase noise suppression anti-resonant hollow core fiber distributed backscattering measurement device. When a traditional single-mode fiber circulator is used to test an anti-resonant hollow core fiber, single-mode-hollow core fusion and anti-reflection treatment need to be repeatedly performed, and the testing procedure is complex and time-consuming. The application proposes a special hollow core fiber circulator with an anti-resonant hollow core fiber pigtail built-in, and the hollow core fiber circulator is used in an anti-resonant hollow core fiber distributed backscattering measurement device. The special hollow core fiber circulator comprises a double-fiber collimator assembly, an optical path adjustment module, a circulator core assembly, a hollow core fiber collimator assembly and a circulator packaging shell, the second port of the special hollow core fiber circulator is integrated with an anti-resonant hollow core fiber pigtail matched with the mode field size of the anti-resonant hollow core fiber to be tested, space light collimation, beam size adjustment and pre-coupling calibration are performed inside the circulator, low-loss coupling between the single-mode fiber port and the hollow core fiber port is realized, and an anti-reflection coating is coated on all optical surfaces to reduce Fresnel reflection inside the device. The circulator is used in an optical frequency domain reflectometer, and when testing, the anti-resonant hollow core fiber to be tested only needs to be coupled with the anti-resonant hollow core fiber pigtail built-in the second port of the circulator, and single-mode-hollow core fusion, angle polishing or anti-reflection treatment do not need to be performed every time. The application greatly simplifies the preparation procedure before testing, effectively reduces the strong reflection peak generated by the direct butt joint of the single-mode fiber and the hollow core fiber, and realizes high-sensitivity detection and defect positioning of the anti-resonant hollow core fiber distributed backscattering signal.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber measurement technology, specifically relating to a phase noise suppression anti-resonant hollow optical fiber distributed backscattering measurement device. Background Technology

[0002] Antiresonant hollow fiber is one of the most revolutionary technological breakthroughs in the field of optical fiber in the past decade. It confines the light field within the air core through the antiresonant effect of the microstructure cladding, completely overcoming the intrinsic material limitations of traditional solid-core silica fiber. Compared to solid-core fiber, antiresonant hollow fiber possesses unique advantages such as an ultra-high laser damage threshold, extremely low nonlinear coefficient, ultra-wide low-loss transmission window, and low dispersion. It has become a core optical device in cutting-edge fields such as high-power laser transmission, optical fiber communication, and quantum key distribution, and is currently a hot topic in global optical fiber technology research and industrialization.

[0003] Backscattering characteristics directly reflect the structural uniformity, internal defect distribution, splice quality, and transmission loss of antiresonant hollow-core fibers. Especially in high-power laser transmission applications, minute structural defects within the fiber can lead to localized optical field enhancement, causing thermal damage and mode instability, severely limiting laser output power and long-term system stability. In ultrafast laser transmission, backscattering generates pulse broadening and noise, disrupting the pulse's time and frequency domain characteristics. Therefore, achieving high-sensitivity, distributed measurement of the backscattering characteristics of antiresonant hollow-core fibers is crucial for optimizing fiber fabrication processes, performance evaluation, and engineering applications. However, when testing antiresonant hollow-core fibers using OFDR, a strong Fresnel reflection peak as high as -14 dB is generated at the air-glass medium transition point due to the significant refractive index difference. According to the phase noise characteristics of the OFDR system, this strong reflection peak generates a large range of phase noise sidelobes, forming a test blind zone that completely masks the extremely low intrinsic backscattering signal of the antiresonant hollow-core fiber.

[0004] Optical frequency domain reflectometry (OFDR) is currently the preferred technique for characterizing the backscattering properties of antiresonant hollow fiber due to its millimeter-level spatial resolution, high sensitivity, and distributed measurement capabilities. However, existing OFDR testing systems all use optical circulators on traditional single-mode fiber pigtails. Because an extremely strong Fresnel reflection peak is generated at the junction of the antiresonant hollow fiber and the solid single-mode fiber, this strong reflection peak and its accompanying large-scale phase noise sidelobes completely mask the intrinsic weak backscattering signal of the antiresonant hollow fiber, preventing the system from detecting effective information.

[0005] To reduce reflection at the single-mode-hollow core fusion splice coupling point, existing technologies typically employ methods such as angle polishing or applying anti-reflection coatings to the splice end faces. In 2021, Michaud-Belleau et al. used an SMF-28 single-mode fiber pigtail to fusion splice an anti-resonant hollow core fiber, and added a graded-index multimode fiber as a mode field adapter at the splice point, achieving OFDR measurement of the extremely low backscattering coefficient of the anti-resonant hollow core fiber. However, this method still suffers from air-glass splice interface reflection and requires polarization control to suppress phase noise sidelobes caused by the far-end reflection peak. In 2023, Wei et al. used a short-segment angle-cut OM2 graded-index multimode fiber as a mode field adapter, fusion splicing one end to a single-mode fiber and abutting the other end to a flat-cut NANF end face via a five-dimensional displacement stage, achieving single-mode fiber-large-core hollow core fiber fusion coupling with low backscattering below −50 dB. In 2024, Wei et al. used an angle-cut graded-index multimode fiber mode field adapter to connect single-mode fiber and hollow fiber, and achieved hermetically sealed packaging through resin curing, reducing the echo reflection at the single-mode fiber-large-diameter hollow fiber connection to below −70 dB. However, these methods all require re-processing the end face and coupling calibration for each test, making the preparation process complex and time-consuming, and unable to meet the batch testing needs in the industrialization process of anti-resonant hollow fiber.

[0006] Therefore, there is an urgent need for a test scheme for anti-resonant hollow fiber OFDR that does not require repeated end-face processing and anti-reflection operations, can be quickly connected, and has stable and controllable reflectivity, in order to support the rapid development and industrial application of anti-resonant hollow fiber technology. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing anti-resonant hollow fiber OFDR testing, which suffers from Fresnel reflection at the coupling point between solid and hollow fibers, leading to a test blind zone and the suppression of weak scattering signals. This invention provides a simple, high dynamic range backscattering measurement device and method that requires no complex algorithms.

[0008] The objective of this invention is achieved through the following measures:

[0009] An optical frequency domain reflectometer with a built-in hollow fiber circulator includes a tunable laser source module (1), an interferometer module (2), and a signal detection, acquisition, and processing module (3).

[0010] The tunable laser source module (1) includes a tunable laser source (101) for querying the distributed characteristics of the anti-resonant hollow fiber under test.

[0011] The interferometer module (2) includes a first coupler (201), a special hollow fiber circulator (202), an anti-resonant hollow fiber to be tested (203), and a second coupler (204); the first output end (201a) of the first coupler (201) is connected to the first input end (204a) of the second coupler (204); the second output end (201b) of the first coupler (201) is connected to the first port (202a) of the hollow fiber circulator (202), the second port (202c) of the hollow fiber circulator (202) is used for docking and coupling with the anti-resonant hollow fiber to be tested (203), and the third port (202b) of the hollow fiber circulator (202) is connected to the second input end (204b) of the second coupler (204); the two output ends of the second coupler (204) are connected to the photoelectric balance detector (301);

[0012] The hollow fiber circulator (202) includes a dual fiber collimator assembly (4), an optical path adjustment module (5), a circulator core assembly (6), a hollow fiber collimator assembly (7), and a circulator housing (8). The dual fiber collimator assembly (4) integrates a first single fiber collimator (401) and a second single fiber collimator (402), which serve as the first port (202a) and the third port (202b) of the circulator, respectively. The optical path adjustment module (5) is located between the dual fiber collimator assembly (4) and the circulator core assembly (6) and is used to correct the beam transmission direction, adjust the beam incident angle, and reduce the difficulty of assembly alignment. The circulator core assembly (6) is the core for realizing non-reciprocal directional transmission of light. It contains birefringent crystals, waveplates, Faraday rotators, and other components, and has two transmission loops, which can make the forward / reciprocal direction of light transmission possible. The reverse-transmitted beam produces a fixed spatial displacement; the hollow fiber collimator assembly (7) integrates an anti-resonant hollow fiber pigtail (701) as the second port (202c) of the circulator. The core diameter, mode field diameter and numerical aperture of the anti-resonant hollow fiber pigtail (701) are matched with the anti-resonant hollow fiber (203) to be tested, so as to realize the mode field matching connection between hollow fibers at the test port; the circulator encapsulation shell (8) is used to fix the relative positions of the dual fiber collimator assembly (4), the optical path adjustment module (5), the circulator core assembly (6) and the hollow fiber collimator assembly (7), and to seal the components, preventing water vapor and other gases from entering the circulator and contaminating the surface of the components. All optical surfaces are coated with an anti-reflective coating.

[0013] The signal detection, acquisition and processing module (3) includes a photoelectric balance detector (301), a data acquisition card (302), and a host computer (303); the two input terminals of the photoelectric balance detector (301) are respectively connected to the two output terminals of the second coupler (204); the output terminal of the photoelectric balance detector is connected to the input terminal of the data acquisition card (302), and the output terminal of the data acquisition card (302) is connected to the host computer (303);

[0014] The pre-coupling loss of the built-in hollow fiber pigtail and the internal spatial optical coupling component is less than 3dB, and the return loss is below -60dB, avoiding the introduction of phase noise sidelobes of the swept light by strong reflection peaks.

[0015] Preferably, the splitting ratio of the first coupler (201) is 99:1, and the splitting ratio of the second coupler (204) is 50:50;

[0016] Preferably, the common-mode rejection ratio of the photoelectric balanced detector (301) is greater than 35dB, the detection bandwidth is not less than 15MHz, and the saturation optical power is greater than 5mW.

[0017] Preferably, the tunable laser source (101) is a narrow linewidth tunable laser with a linewidth of less than 10 kHz;

[0018] The working principle of this invention is as follows:

[0019] The test port of a traditional single-mode fiber circulator is usually a solid single-mode fiber pigtail. When it is directly connected to the anti-resonant hollow fiber under test, the two differ in core medium, mode field diameter, and numerical aperture, resulting in the single-mode fiber fundamental mode not fully overlapping with the anti-resonant hollow fiber fundamental mode, and generating a strong Fresnel reflection peak at the air-glass transition position. This invention sets the test port of a special hollow fiber circulator (202) to have an internal anti-resonant hollow fiber pigtail (202c), and matches the core diameter, mode field diameter, and numerical aperture of this pigtail with the anti-resonant hollow fiber under test (203). The special hollow fiber circulator (202) adopts a spatial optical collimation coupling structure. The beam from the first port (2202a) passes through the dual fiber collimator assembly (4), the optical path adjustment module (5), and the circulator core assembly (6), and is then coupled into the built-in anti-resonant hollow fiber pigtail (701) by the hollow fiber collimator assembly (7). The backscattered light generated by the anti-resonant hollow fiber under test (203) returns along the built-in anti-resonant hollow fiber pigtail (701) and is output to the second coupler (204) through the third port (202b) of the hollow fiber circulator (202). Through the above structure, the test port is changed from the traditional "single-mode fiber - hollow fiber" docking to "hollow fiber - hollow fiber" docking coupling, thereby improving the mode field matching degree and reducing the strong reflection peak.

[0020] Before leaving the factory, the hollow fiber circulator (202) completes the pre-coupling calibration between the internal single-mode fiber port, the spatial optical path, and the built-in anti-resonant hollow fiber pigtail (701). The beam direction, beam position, and coupling angle are adjusted by the optical path adjustment module (5) to match the optical field entering the built-in anti-resonant hollow fiber pigtail (701) with its fundamental mode field. During testing, it is only necessary to connect the end face of the anti-resonant hollow fiber (203) under test with the built-in anti-resonant hollow fiber pigtail (701) at the second port (202c). Since both are hollow fiber structures, the core medium and mode field size are similar, which can significantly reduce the strong Fresnel reflection peak generated by the direct connection between the traditional single-mode fiber pigtail and the anti-resonant hollow fiber, thereby reducing the masking of the weak backscattered signal by the phase noise side lobes carried by the strong reflection peak.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] No angle polishing or refractive index matching treatment is required on the mating end face, and it will not excite higher-order modes of anti-resonant hollow fiber, thus accurately characterizing the intrinsic backscattering characteristics of the fiber.

[0023] Polarization diversity technology can be used to simultaneously detect orthogonal polarization components, eliminating the influence of polarization fading on the measurement results and improving the stability and accuracy of the measurement.

[0024] It eliminates the need for single-mode hollow-core fusion splicing, end-face polishing, or refractive index matching for each test, significantly reducing test preparation time and supporting rapid batch testing of anti-resonant hollow-core fibers. Attached Figure Description

[0025] Figure 1 is a schematic diagram of the overall structure of a phase noise suppression anti-resonant hollow fiber distributed backscattering measurement device of the present invention.

[0026] Figure 2 is a schematic diagram of the internal structure of the hollow fiber circulator (202) of the present invention;

[0027] Figure 3 is a schematic diagram of an optical frequency domain reflectometer device for distributed backscattering testing of anti-resonant hollow optical fiber. Specific implementation methods

[0028] To more clearly illustrate the anti-resonant hollow fiber distributed backscattering measurement device with built-in hollow fiber circulator proposed in this invention, the following description of the invention in conjunction with embodiments and accompanying drawings is provided in more detail, but should not be construed as limiting the scope of protection of this invention.

[0029] This embodiment presents a schematic diagram of an optical frequency domain reflectometer device with a built-in hollow fiber circulator, as shown in Figure 3. It consists of a tunable laser source module (1), an auxiliary interferometer module (2), a main interferometer module (3), and a signal detection, acquisition, and processing module (4), wherein:

[0030] The tunable laser source module (1) outputs linear frequency modulated continuous light, which is injected along the input end of the first coupler (102). After being split by the first coupler, one path is injected into the auxiliary interferometer module (2) through the first output end (102a), and the other path is injected into the main interferometer module (3) through the second output end (102b).

[0031] The optical signal entering the auxiliary interferometer module (2) is first input to the first port of the first circulator (201), and output to the second coupler (202) through the second port; the second coupler splits the optical signal into two paths, one path is directly transmitted to the first Faraday rotator (203), and the other path is transmitted to the second Faraday rotator (204) through the first delay fiber (205); the two optical signals are reflected by the Faraday rotator and return along the original path, and are transmitted together to the first photoelectric balance detector (401) through the third port of the circulator to generate beat frequency interference;

[0032] The optical signal entering the main interferometer module (3) is split into two paths, a reference arm and a test arm, by the third coupler (301): the reference arm light passes through the polarization controller (302) and is input to the first input terminal (305a) of the fourth coupler (305); the test arm light is transmitted to the anti-resonant hollow fiber (304) under test through the special hollow fiber circulator (303), and the backscattered light generated returns along the original path and is input to the second input terminal (305b) of the fourth coupler (305) through the special hollow fiber circulator (303); the fourth coupler inputs the two paths of light to the first polarization beamsplitter (306) and the second polarization beamsplitter (307) respectively, and after being separated into orthogonal polarization components, they are sent to the second photoelectric balance detector (402) and the third photoelectric balance detector (403) respectively.

[0033] The acquisition card (404) in the signal detection, acquisition and processing module (4) synchronously acquires the analog electrical signals output by the three photoelectric balance detectors, performs analog-to-digital conversion and transmits them to the host computer (405); the host computer performs discrete Fourier transform, frequency sweep nonlinear correction and polarization power synthesis on the acquired digital signals, and finally obtains the distributed backscattering coefficient curve and internal defect location information of the anti-resonant hollow fiber under test along the length direction.

[0034] The selection of the main optoelectronic devices and their performance parameters in this invention are as follows:

[0035] The tunable laser source module (1) adopts a narrow linewidth tunable laser with a sweep frequency range covering the 1550nm communication band and stable output power; the first coupler (102) has a splitting ratio of 90:10 and a working wavelength covering the 1550nm communication band.

[0036] The first circulator (201) in the auxiliary interferometer module (2) is a three-port fiber optic circulator with a working wavelength of 1550nm; the second coupler (202) has a splitting ratio of 50:50; the first Faraday rotator (203) and the second Faraday rotator (204) are 45° Faraday rotators with a working wavelength of 1550nm; the first delay fiber (205) is a standard single-mode fiber used to construct the arm length difference of the auxiliary interferometer and realize frequency sweep nonlinear correction.

[0037] The third coupler (301) in the trunk interferometer module (3) has a splitting ratio of 99:1, the fourth coupler (303) has a splitting ratio of 50:50, the pigtails are all 1 meter long, and the working wavelength is 1550nm; the polarization controller (302) adopts a three-ring mechanical polarization controller with an overall length of 10m and a working wavelength of 1550nm; the special hollow fiber circulator (303) has a built-in large-core diameter nested anti-resonant nodeless fiber pigtail with an insertion loss of less than 3dB and a return loss of -70dB; the first polarization beam splitter (306) and the second polarization beam splitter (307) adopt fiber-type polarization beam splitters with a working wavelength of 1550nm; the anti-resonant hollow fiber to be tested (304) adopts a nested anti-resonant nodeless fiber with a length of 60m and a working wavelength of 1550nm;

[0038] The first photoelectric balance detector (401), the second photoelectric balance detector (402), and the third photoelectric balance detector (403) in the signal detection, acquisition and processing module (4) are high-speed photoelectric balance detectors with a common-mode rejection ratio of 35dB, a detection bandwidth of 80MHz, and a saturation optical power of 5mW; the acquisition card (404) is a high-speed data acquisition card with multi-channel synchronous acquisition and a sampling rate of 250MHz.

[0039] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A phase noise-suppressing anti-resonant hollow fiber distributed backscattering measurement device, comprising a tunable laser source module (1), an interferometer module (2), and a signal detection, acquisition, and processing module (3), characterized in that: The tunable laser source module (1) is used to output linear frequency modulated continuous light, which is input to the interferometer module (2) via the first coupler (201). The detector (301) in the signal detection, acquisition and processing module (3) converts the optical signal into an electrical signal, and the acquisition card (302) converts the continuous signal into a digital signal and transmits it to the host computer (303) for data demodulation. In the interferometer module (2), the first output end (201a) of the first coupler (201) is connected to the first input end pigtail (204a) of the second coupler (204), the second output end (201b) of the first coupler (201) is connected to the first port (202a) of the special hollow fiber circulator (202), the second port (202c) of the special hollow fiber circulator (202) is used to dock and couple with the anti-resonant hollow fiber (203) to be tested, forming a coupling point (203a), the third port (202b) of the special hollow fiber circulator (202) is connected to the second input end (204b) of the second coupler (204); the two output ends of the second coupler (204) are connected to the photoelectric balance detector (301). In the signal detection, acquisition and processing module (3), the photoelectric balance detector (301) receives two optical signals output by the second coupler (205) and performs beat frequency interference. The beat frequency interference signal is injected into the acquisition card (302) of the signal detection, acquisition and processing module (3) for signal analog-to-digital conversion. The converted signal is input to the host computer (303) and digital signal processing is performed to obtain the distributed backscattering intensity information of the anti-resonant hollow fiber (204) under test.

2. A hollow fiber circulator, comprising a dual fiber collimator assembly (4), an optical path adjustment module (5), a circulator core assembly (6), a hollow fiber collimator assembly (7), and a circulator enclosure (8), characterized in that: The hollow fiber circulator (302) includes a dual fiber collimator assembly (4), an optical path adjustment module (5), a circulator core assembly (6), a hollow fiber collimator assembly (7), and a circulator housing (8). The dual fiber collimator assembly (4) integrates a first single fiber collimator (401) and a second single fiber collimator (402), which serve as the first port (202a) and the third port (202b) of the circulator, respectively. The optical path adjustment module (5) is located between the dual fiber collimator assembly (4) and the circulator core assembly (6) to correct the beam transmission direction and reduce the difficulty of assembly alignment. The circulator core assembly (6) is the core for realizing non-reciprocal directional transmission of light. It contains birefringent crystals, waveplates, Faraday rotators, and other components, and has two transmission loops, which can make the forward / reciprocal direction of light transmission possible. The reverse-propagating beam produces a fixed spatial displacement; the hollow fiber collimator assembly (7) integrates an anti-resonant hollow fiber pigtail (701) as the second port (202c) of the circulator. The core diameter, mode field diameter and numerical aperture of the anti-resonant hollow fiber pigtail (701) are matched with the anti-resonant hollow fiber under test (203) to form a mode field matching connection between hollow fibers at the test port; all optical surfaces are coated with an anti-reflection coating.

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