Comb filter based on gradient refractive index two-mode optical fiber

Abstract

The invention belongs to the technical field of optical fiber filters, and particularly relates to a comb filter based on graded-index two-mode optical fibers, which comprises graded-index two-mode optical fibers and tapered graded-index two-mode optical fibers which are alternately connected in sequence, and the optical fibers at the two ends are gradient refractive index two-mode optical fibers. According to the invention, the gradient refractive index two-mode optical fiber is used to construct a bipyramid, and the optical fiber comb filter is realized; a double-cone gradient refractive index two-mode optical fiber is utilized to realize an optical fiber comb-shaped filtering effect. The filter can be directly manufactured by using an optical fiber tapering machine or high-voltage arc discharge, and has the advantages of easiness in production, convenience in use, easiness in fusion with a subsequent system and the like. The comb filter formed by the biconical gradient refractive index two-mode optical fiber can be directly applied to a 1550nm communication wave band, a 2mu m wave band and the like, an optical fiber laser or an optical fiber sensor is further constructed, and the comb filter has the advantages of being convenient to use, easy to fuse with a subsequent system and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber filters, particularly a comb-shaped optical fiber filter, and more specifically, relates to a comb filter based on a graded-index two-mode optical fiber. Background Art

[0002] Optical fiber comb filters are widely used in fields such as dense wavelength division multiplexing optical fiber communication systems, optical fiber sensors, multi-wavelength fiber lasers, and optical signal processing, and have received significant attention from researchers at home and abroad in recent years. In order to produce a comb filtering effect, optical fiber comb filters with various methods and structures have been proposed, mainly including comb filters based on interference effects such as Sagnac and Mach-Zehnder, and comb filters based on tapered optical fibers.

[0003] In the aspect of comb filters based on interference effects such as Sagnac and Mach-Zehnder, in the prior literature

Yaoyao Qi, et al.. Tunable all fiber multi-wavelength mode-locked laser with a large dynamic range using polarization controller coiled SMF-GIMF-SMF structure as both saturable absorber and comb filter[J]. Optical Fiber Technology, 2022, 74:103055.

Qixing Yu, et al.. Tunable Single and Multi-Wavelength Er-Doped Mode-Locked Fiber Laser Based on GIMF-PCF-GIMF[J]. IEEE Photonics Journal, 2023, 35(19):1043-1046.

[0004] In the aspect of comb filters based on tapered optical fibers, in 2006, K. Kieu [K. Kieu, M. Mansuripur. Tuning of fiber lasers by use of a single-mode biconic fiber taper[J]. Optical Letters, 2006, 31:2435-2437] first proposed the realization of comb filtering based on the taper of single-mode optical fibers for communication. Subsequently, comb filters based on tapered optical fibers have been widely studied, mainly including thick tapers with an enlarged waist diameter and thin tapers with a reduced waist diameter. In the case of thick tapers, previous literature [WANG Feng, et al.. A grapefruit microstructure fiber temperature sensor coated with liquid crystal based on waist-enlarged taper[J]. OPTOELECTRONICS LETTERS, 2024, 20(3):142-146.], [Gaolin Qin, et al.. Multi-wavelength tunable ring cavity fiber laser incorporated with a Mach–Zehnder interferometer filter based on waist-enlarged fiber bitapers[J]. OptiK, 2021, 248:168088.], [Saif A. Mohammed, et al.. Fiber laser with dual-channel and a selectable comb filter based on Vernier effect of parallel-connected two Mach–Zehnder interferometers[J]. Optical Engineering, 2023, 62(6):066102], [Jiandong Liu, et al.. Switchable multi-wavelength fiber laser based on a taper-coupled microbottle resonator[J]. Optics and Laser Technology, 2023, 158:108819.] etc. adopted the method of high-voltage arc to generate thick tapers, thereby producing the comb filtering effect.In terms of thin tapers, in the prior literature

Wanjing Peng,et al..Multiwavelength erbium-doped fiber laser based ona polarization-dependent in-line Mach-Zehnder interferometer[J].Optical andQuantum Electronics,2019,51:300.

J.A.Montenegro-Orenday,et al.Multi-wavelength fiber laser via evanescent field confinement with Al2O3nanolaminate-coated thinned fiber[J].Optical Fiber Technology,2023,81:103533.

Xiang Geng,et al..Switchable multi-wavelength fiber lasers based onasymmetric biconical fiber tapers[J].Optics Communications,2023,548:129837.

Hussein Alaa Al-Rubaiyee,et al..Vernier effect based on cascading two Mach–Zehnder interferometers for selectable comb filter and saturable absorberapplications in erbium-doped fiberlaser[J].OpticalFiber Technology,2024,84:103757.

Qi Zhao,et al..Tunable and wavelength interval precisely controllederbium-doped fiber laser by employing the fused taper technology[J].ChineseOptics Letters,2022,20(1):011402.

[0005] In addition, in the aspect of combining tapered fiber filtering and interferometer to generate composite comb filtering, in the prior literature [Boya Shi, et al.. Tunable and Switchable Multi-Wavelength Erbium-Doped Fiber Laser Based on Composite Structure Filter [J]. Photonics, 2022, 9: 768] and [Sagnac. Zefeng Li, et al.. Broadly Tunable Erbium-Doped Fiber Ring Laser Based on Tapered Polarization Maintaining Fiber [J]. IEEE Photonics Journal, 2022, 14(5): 1551405.], the tapered fiber filtering is combined with the Sagnac interference effect to generate a comb filtering effect.

[0006] In addition, the prior patent [Optical Fiber Comb Filter Based on Fiber Connector Connection Converter CN202120347454.1] reports a filter using a fiber connector connection converter, which requires the cooperation setting among a circuit board, a placement board and a limit slot to realize optical fiber comb filtering.

[0007] However, most of the above comb filters use single-mode fibers for communication to achieve comb filtering. For other special optical fibers, only the tapering of polarization-maintaining fiber and erbium-doped fiber can be used to achieve filtering. Summary of the Invention

[0008] The present invention is proposed to expand the generation method of a comb filter, and its purpose is to provide a method for forming a comb filter by drawing a double-tapered fiber using a graded-index two-mode fiber.

[0009] The present invention adopts the following technical solutions:

[0010] A comb filter based on a graded-index two-mode fiber includes a graded-index two-mode fiber and a tapered graded-index two-mode fiber. The graded-index two-mode fiber and the tapered graded-index two-mode fiber are alternately connected in sequence, and the fibers at both ends are both graded-index two-mode fibers.

[0011] In the above technical solution, the comb filter is a double-tapered graded-index two-mode fiber comb filter, which includes a first graded-index two-mode fiber, a first tapered graded-index two-mode fiber, a second graded-index two-mode fiber, a second tapered graded-index two-mode fiber, and a third graded-index two-mode fiber connected in sequence.

[0012] In the above technical solution, the output end of the first graded-index two-mode fiber is connected to the first tapered graded-index two-mode fiber, the output end of the first tapered graded-index two-mode fiber is connected to the second graded-index two-mode fiber, the output end of the second graded-index two-mode fiber is connected to the second tapered graded-index two-mode fiber, and the output end of the second tapered graded-index two-mode fiber is connected to the third graded-index two-mode fiber.

[0013] In the above technical solution, the length of the first graded-index optical fiber is not limited.

[0014] In the above technical solution, the length of the first graded-index optical fiber is not less than 0.5 m.

[0015] In the above technical solution, the first tapered graded-index two-mode fiber sequentially includes a lower taper region, a waist region, and an upper taper region. In the lower taper region, the diameters of the fiber cladding and the core gradually decrease, and in the upper taper region, the diameters of the fiber cladding and the core gradually increase.

[0016] In the above technical solution, the waist region diameter of the first tapered optical fiber is 24.3 μm, the length of the taper region is 1.2 cm, the corresponding taper angle is 0.5°, the extinction ratio is 17 dB, and the lower taper region and the upper taper region are symmetric structures.

[0017] In the above technical solution, the second tapered graded-index two-mode fiber sequentially includes a lower taper region, a waist region, and an upper taper region.

[0018] In the above technical solution, the second tapered graded-index two-mode fiber and the first tapered graded-index fiber have the same process and parameter settings, and the cone structure and each parameter are the same as those of the first tapered graded-index fiber.

[0019] In the above technical solution, the length of the second graded-index fiber affects the free spectral range of the comb fiber filter.

[0020] In the above technical solution, the length L of the second tapered graded-index two-mode fiber is related to the free spectral range Δλ of the comb filter and satisfies

[0021] where λ is the operating wavelength of the laser, and Δn eff is the refractive index difference between the cladding of the tapered fiber and the surrounding environment.

[0022] In the above technical solution, the length of the third graded-index fiber is not limited.

[0023] In the above technical solution, the length of the third graded-index fiber is not less than 0.5 m.

[0024] In the above technical solution, the comb filter has a linear structure.

[0025] Combining all the above technical solutions, the advantages and positive effects of the present invention are as follows:

[0026] 1. The present invention uses a tapered graded-index two-mode fiber to construct a dual-taper to realize a fiber comb filter.

[0027] 2. The present invention uses a dual-tapered graded-index two-mode fiber to achieve a fiber comb filtering effect. This filter can be directly fabricated using a fiber drawing taper machine or high-voltage arc discharge, and has the advantages of easy production, convenient use, and easy integration with subsequent systems.

[0028] 3. The comb filter formed by the dual-tapered graded-index two-mode fiber in the present invention can be directly applied to communication bands such as 1550 nm and 2 μm bands, and further construct a fiber laser or a fiber sensor, and has the advantages of convenient use and easy integration with subsequent systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present disclosure, and are used together with the specification to explain the principles of the present disclosure;

[0030] Figure 1 is a schematic diagram of the dual-tapered graded-index two-mode fiber comb filter of the present invention;

[0031] Figure 2It is a physical diagram of the tapered region of each single taper in the double-tapered graded-index two-mode fiber comb filter of the present invention; compared with the conventional graded-index two-mode fiber, the diameter gradually becomes smaller significantly;

[0032] Figure 3 is the waist region diameter of each single taper in the double-tapered graded-index two-mode fiber comb filter of the present invention;

[0033] Figure 4 is the filtering transmission spectrum of the double-tapered graded-index two-mode fiber comb filter of the present invention.

[0034] Wherein:

[0035] 1 First-segment graded-index two-mode fiber;

[0036] 2 First-tapered graded-index two-mode fiber;

[0037] 3 Second-segment graded-index two-mode fiber;

[0038] 4 Second-tapered graded-index two-mode fiber;

[0039] 5 Third-segment graded-index two-mode fiber;

[0040] 11 First port of the first-segment graded-index two-mode fiber;

[0041] 12 Second port of the first-segment graded-index two-mode fiber;

[0042] 21 First port of the first-tapered graded-index two-mode fiber;

[0043] 22 Second port of the first-tapered graded-index two-mode fiber;

[0044] 23 Third port of the first-tapered graded-index two-mode fiber;

[0045] 24 Fourth port of the first-tapered graded-index two-mode fiber;

[0046] 31 First port of the second-segment graded-index two-mode fiber;

[0047] 32 Second port of the second-segment graded-index two-mode fiber;

[0048] 41 First port of the second-tapered graded-index two-mode fiber;

[0049] 42 Second port of the second-tapered graded-index two-mode fiber;

[0050] 43 Third port of the second-tapered graded-index two-mode fiber;

[0051] 44 Fourth port of the second-tapered graded-index two-mode fiber;

[0052] 51 The first port of the third segment of graded-index two-mode fiber;

[0053] 52 The second port of the third segment of graded-index two-mode fiber. Detailed implementation manners

[0054] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following describes the detailed implementation manners of the present invention with reference to the accompanying drawings. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific implementations disclosed below.

[0055] The following further elaborates on the present invention with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0056] As Figures 1-3 shown, a dual-tapered graded-index two-mode fiber comb filter includes a first segment of graded-index two-mode fiber 1, with a length generally of 0.5 m or longer; one end 21 of the first segment of graded-index two-mode fiber 1 is connected to one end of the first tapered graded-index two-mode fiber 2; the other end 22 of the first tapered graded-index two-mode fiber 2 is connected to the second segment of graded-index two-mode fiber 3. Among them, between the lower tapered region 21 and 22, the waist region 22 and 23, and the upper tapered region 23 and 24 of the first tapered graded-index fiber 2, the corresponding parameters such as length and taper angle of these three parts can be controlled by adjusting the stretching speed and stretching time of the fiber taper machine. At the same time, these parameters are also related to parameters such as the extinction ratio of the filter spectrum. In the present invention, the waist diameter of the first tapered fiber drawn is 24.3 μm, the length of the tapered region is 1.2 cm, the corresponding taper angle is 0.5°, the extinction ratio is 17 dB, and the lower and upper tapered regions are symmetric structures; the second segment of graded-index two-mode fiber 3 is connected to the second tapered graded-index two-mode fiber 4; the length L of the second segment of graded-index two-mode fiber 3 is related to the free spectral range Δλ of the comb filter where λ is the working wavelength of the laser, and Δn effΔ is the refractive index difference between the cladding of the tapered optical fiber and the surrounding environment. A longer spacing L between the two tapered optical fibers corresponds to a smaller free spectral range. Similarly, a shorter spacing L corresponds to a larger free spectral range. In the present invention, the length of the second graded-index optical fiber is 13 cm, corresponding to a free spectral range of 2.84 nm. The tapered graded-index two-mode optical fiber 4 and the first tapered graded-index optical fiber 2 have the same process and parameter settings, and the cone structure and each parameter are the same as those of the first tapered graded-index optical fiber 2. The output end 44 of the tapered graded-index two-mode optical fiber 4 is connected to the third graded-index two-mode optical fiber 5. Generally, the third graded-index two-mode optical fiber 5, as a pigtail, has a length of 0.5 m or longer.

[0057] Furthermore, it can also be considered in this embodiment that one end 11 of the first graded-index two-mode optical fiber 1 serves as the input end of the entire fiber comb filter.

[0058] Furthermore, it can also be considered in this embodiment that the other end 12 of the first graded-index two-mode optical fiber 1 is connected to one end 21 of the first tapered graded-index two-mode optical fiber 2.

[0059] Furthermore, it can also be considered in this embodiment that the first tapered graded-index two-mode optical fiber 2 includes a lower taper region, a waist region, and an upper taper region. In the lower taper region, the diameters of the fiber cladding and the core gradually decrease, as Figure 2 shown; in the waist region, the diameter of the fiber cladding is about 20 μm or smaller, and the diameter of the fiber core is thin enough to be negligible; in the upper taper region, the diameters of the fiber cladding and the core gradually increase. When light is transmitted in the lower taper region, the light in the core gradually leaks into the cladding as the core diameter decreases; when it is transmitted to the waist region, a new refractive index difference is formed between the cladding and the environment outside the cladding, and more high-order mode light is excited in the cladding; when light is transmitted in the upper taper region, the light in the waist region is gradually transmitted in the core as the core diameter increases and finally completely enters the core.

[0060] Furthermore, it can also be considered in this embodiment that the input end 31 of the second graded-index two-mode optical fiber 3 is connected to the output end 24 of the first tapered graded-index two-mode optical fiber 2.

[0061] Furthermore, it can also be considered in this embodiment that the second tapered graded-index two-mode fiber 4 includes a lower taper region, a waist region, and an upper taper region. In the lower taper region, the diameters of the fiber cladding and the core gradually decrease; in the waist region, the diameter of the fiber cladding is about 20 μm or less, and the diameter of the fiber core is thin enough to be negligible; in the upper taper region, the diameters of the fiber cladding and the core gradually increase. When light is transmitted in the lower taper region, the light in the core gradually leaks into the cladding as the core diameter decreases; when it is transmitted to the waist region, a new refractive index difference is formed between the cladding and the external environment of the cladding, and more high-order mode light is excited in the cladding; when light is transmitted in the upper taper region, the light in the waist region is gradually transmitted into the core as the core diameter increases, and finally completely enters the core.

[0062] Furthermore, it can also be considered in this embodiment that the input end 41 of the second tapered graded-index two-mode fiber 4 is connected to the output end 32 of the second graded-index two-mode fiber 3.

[0063] Furthermore, it can also be considered in this embodiment that the output end 42 of the second tapered graded-index two-mode fiber 4 is connected to the input end 51 of the third graded-index two-mode fiber 5.

[0064] Furthermore, it can also be considered in this embodiment that the output end 52 of the third graded-index two-mode fiber 5 serves as the output end of the entire fiber filter.

[0065] Taking this embodiment as an example, the comb filter in the present invention has a linear structure, and its cavity contains a section of graded-index two-mode fiber, a tapered graded-index two-mode fiber, a section of graded-index two-mode fiber, a tapered graded-index two-mode fiber, and a section of graded-index two-mode fiber. As Figures 1-4 shown, the present invention uses the graded-index two-mode fiber to form a double-taper structure to achieve comb filtering. The present invention uses the double-tapered graded-index two-mode fiber to achieve the fiber comb filtering effect. This filter can be directly fabricated using a fiber tapering machine or high-voltage arc discharge, and has the advantages of easy production, convenient use, and easy integration with subsequent systems.

[0066] The above description is only a preferred embodiment of the present invention. It should be understood that the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The present invention is not limited to the forms disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications, and environments. Changes and alterations made by those skilled in the art without departing from the spirit and scope of the present invention shall fall within the scope of protection of the appended claims of the present invention.

Claims

1. A comb filter based on a graded-index two-mode fiber, characterized in that: It includes a graded-index two-mode fiber and a tapered graded-index two-mode fiber. The graded-index two-mode fiber and the tapered graded-index two-mode fiber are alternately connected in sequence, and the fibers at both ends are graded-index two-mode fibers.

2. The comb filter based on a graded-index two-mode fiber according to claim 1, characterized in that: The comb filter is a double-tapered graded-index two-mode fiber comb filter, which includes a first section of graded-index two-mode fiber, a first tapered graded-index two-mode fiber, a second section of graded-index two-mode fiber, a second tapered graded-index two-mode fiber, and a third section of graded-index two-mode fiber connected in sequence.

3. A comb filter based on a graded-index two-mode fiber according to claim 2, wherein: The length of the first section of graded-index fiber is not limited.

4. A comb filter based on a graded-index two-mode fiber according to claim 2, wherein: The first tapered graded-index two-mode fiber successively includes a lower taper region, a waist region, and an upper taper region. In the lower taper region, the diameters of the fiber cladding and the core gradually decrease, and in the upper taper region, the diameters of the fiber cladding and the core gradually increase.

5. A comb filter based on a graded-index two-mode fiber according to claim 4, characterized in that: The waist region diameter of the first tapered fiber is 24.3 μm, the taper region length is 1.2 cm, the corresponding taper angle is 0.5°, the extinction ratio is 17 dB, and the lower taper region and the upper taper region are symmetric structures.

6. The comb filter based on a graded-index two-mode fiber according to claim 2, wherein: The second tapered graded-index two-mode fiber and the first tapered graded-index fiber have the same structural composition.

7. A comb filter based on a graded-index two-mode fiber according to claim 6, characterized in that: The second tapered graded-index two-mode fiber and the first tapered graded-index fiber have the same technological process and parameter settings.

8. A comb filter based on a graded-index two-mode fiber according to claim 2, characterized in that: The length L of the second section of graded-index two-mode fiber is related to the free spectral range Δλ of the comb filter and satisfies where λ is the operating wavelength of the laser, and Δn eff is the refractive index difference between the cladding of the tapered fiber and the surrounding environment.

9. A comb filter based on a graded-index two-mode fiber according to claim 2, characterized in that: The length of the third section of graded-index fiber is not limited.

10. A comb filter based on a graded-index two-mode fiber according to any one of claims 1-9, characterized in that: The comb filter is of a linear structure.

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