A long period fiber grating based coupled level acoustic resonator

Abstract

The application discloses a kind of based on long period fiber grating coupling's level echo acoustic resonator, it includes first optical fiber, second optical fiber, first echo acoustic wall, second echo acoustic wall and the temperature control device of each echo acoustic wall;The first optical fiber has a taper region, and long period fiber grating is engraved on taper region, and the second optical fiber has first taper region and second taper region, and long period fiber grating is engraved on two taper regions respectively;First echo acoustic wall is coupled with the taper region of first optical fiber, and the first taper region of second optical fiber respectively;Second echo acoustic wall is coupled with the second taper region of second optical fiber;First echo acoustic wall and second echo acoustic wall are installed on temperature control device respectively, and temperature control device controls and adjusts echo acoustic wall temperature.The application can solve the technical problems that the physical strength of echo acoustic resonator coupled by taper fiber is low, is easily interfered by outside world, and echo acoustic wall is difficult to reach ideal single mode state.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic coupling for whispering gallery resonators, and specifically relates to a cascaded whispering gallery resonator based on long-period fiber grating coupling. Background Technology

[0002] The resonant light wave in the whispering-gallery microcavity undergoes total internal reflection at the boundary. The light energy propagates back and forth in the tiny annular region at the boundary of the whispering-gallery with minimal loss. Its mode has an extremely high quality factor and a very small mode volume, and therefore has important applications in fields such as optoelectronic oscillators, optical frequency combs, and narrow linewidth lasers.

[0003] Effective whispering-gallery coupling methods are mainly divided into prism coupling and tapered fiber coupling. Prism coupling has high stability but poor coupling efficiency. Tapered fiber coupling has higher coupling efficiency, but requires the core diameter of the tapered region to be less than 3 μm, resulting in low physical strength and instability due to environmental interference. Moreover, due to the influence of processing accuracy and coupling accuracy, it is difficult for actual whispering-gallery modes to achieve the ideal single-mode state, and several modes in the free spectral range often cannot meet the application requirements of single-mode. Summary of the Invention

[0004] The purpose of this invention is to provide a cascaded whispering-gallery resonator based on long-period fiber grating coupling, which can solve the technical problems of low physical strength, susceptibility to external interference, and difficulty in achieving ideal single-mode state in whispering-gallery resonators coupled with tapered fiber.

[0005] The technical solution adopted in this invention is:

[0006] A cascaded whispering resonator based on long-period fiber grating coupling includes a first fiber, a second fiber, a first whispering wall, a second whispering wall, and a temperature control device for each whispering wall.

[0007] The first optical fiber has a conical section with a long-period fiber grating etched on it. The second optical fiber has a first conical section and a second conical section, with long-period fiber gratings etched on each section. The first soundwall is coupled to the conical section of the first optical fiber and the first conical section of the second optical fiber, respectively. The second soundwall is coupled to the second conical section of the second optical fiber. The first and second soundwalls are mounted on a temperature control device, which controls and adjusts the temperature of the soundwalls. The first and second soundwalls together form a cascaded system.

[0008] The fundamental mode propagating in the fiber core is converted into a higher-order mode propagating in the cladding by a long-period fiber grating, which further leaks out of the fiber from the taper region. The long-period grating ensures a large core diameter in the taper region while maintaining a strong evanescent wave. The light energy is transmitted from the first fiber to the first whispering wall in the form of an evanescent wave, and then from the first whispering wall to the second fiber. At this time, the light wavelength in the second fiber is the mode of the first whispering wall. The light energy input from one end of the second fiber is transmitted from the second fiber to the second whispering wall in the form of an evanescent wave, and then from the second whispering wall back to the second fiber, and output from the other end of the second fiber. At this time, the second whispering wall mode filters the first whispering wall mode, achieving less mode or single-mode output.

[0009] A further proposed solution is to mount the first and second soundbars on a temperature control device, which includes a base, a TEC (thermal control unit), a heat sink base, a thermistor, and a temperature controller.

[0010] The TEC is placed on the base; the heat sink base is placed on the TEC; the thermistor is placed inside the heat sink base;

[0011] In use, the soundbar is placed on a heat sink base. The soundbar's temperature is effectively conducted to the heat sink base. After a thermistor detects the temperature of the heat sink base, it transmits the temperature data to a temperature controller. The temperature controller controls the TEC (thermal energy dissipation device) to cool or heat the soundbar, thereby regulating the soundbar's temperature and adjusting its resonant wavelength. As the soundbar's temperature increases, its resonant wavelength shifts towards longer wavelengths; as the soundbar's temperature decreases, its resonant wavelength shifts towards shorter wavelengths. This causes the second soundbar's mode to drift, thus allowing for precise filtering of the first soundbar's mode. Furthermore, it achieves maximum coupling efficiency with the long-period fiber optic grating.

[0012] A further proposed approach is that the resonant wavelength of the first whispering wall is located within the resonant bandwidth of the long-period fiber grating on the first fiber taper region and the resonant bandwidth of the long-period fiber grating on the first taper region of the second fiber (the first segment of the long-period fiber grating in the second fiber); the resonant wavelength of the second whispering wall is located within the resonant bandwidth of the long-period fiber grating in the second taper region of the second fiber (the second segment of the long-period fiber grating in the second fiber).

[0013] A further approach is to use the same type of fiber for the first and second fibers, with the taper region of the first fiber being the same as the first taper region of the second fiber, and the long-period fiber gratings on the taper region of the first fiber being the same as those on the first taper region of the second fiber, in order to ensure high coupling efficiency.

[0014] A further proposed solution is that the long-period fiber gratings on the first fiber cone region and the long-period fiber gratings on the first fiber cone region both achieve full coupling at the coupling point with the first whispering gallery.

[0015] The long-period fiber grating in the second cone region of the second optical fiber reaches full coupling at the coupling point with the second whispering gallery.

[0016] That is, it satisfies |k|L=aπ / 2, where k represents the coupling constant, L represents the grating length at the coupling point, and a is an odd number;

[0017] In the fully coupled state, all the energy of the core mode with a wavelength of the resonant wavelength is coupled to the cladding film.

[0018] A further proposed approach is to have the resonant wavelengths of the first and second whispering walls satisfy the phase-matching condition: λ = 2πn(T)r(T) / m, where n(T) represents the refractive index of the whispering wall, which varies with temperature T; r(T) is the radius of the whispering wall, which varies with temperature T; and m is the number of circumferential modes of the whispering wall.

[0019] A further proposed approach is that the resonant wavelengths of the long-period fiber gratings on the first and second fiber taper regions satisfy the phase-matching condition: λ = (nco - nc1)Λ, where nco represents the effective refractive index of the fiber core, nc1 is the effective refractive index of the cladding, and Λ represents the grating period.

[0020] A further proposed approach is to place the first and second echo walls within the cone region of the optical fiber, at the location where the coupling efficiency with the optical fiber is highest.

[0021] A further approach is to design the grating period and adjust the resonant wavelength of the long-period fiber optic grating to match the first and second whispering galleries to achieve the highest coupling efficiency.

[0022] The beneficial effects of this invention are as follows:

[0023] Long-period fiber gratings can convert the core mode into the cladding mode, ensuring strong evanescent wave leakage even with a large core diameter in tapered fiber. Therefore, it is not necessary to make the core diameter in the tapered region less than 3µm. The large core diameter of the tapered fiber ensures the physical strength of the fiber, thereby improving the stability and resistance to external interference of the entire resonator.

[0024] This invention cascades two soundbars and uses a temperature control device to adjust the second soundbar mode to precisely filter the first soundbar mode. This effectively solves the problem of multimode output of soundbars caused by processing and coupling precision in practical applications, and achieves multimode or single-mode output of soundbars.

[0025] By controlling the grating length L at the coupling point between the long-period grating and the whispering wall, the core mode and the cladding mode can be fully coupled. By controlling the grating period Λ of the long-period grating and the temperature T of the whispering wall, the resonant wavelength of the long-period fiber grating can be matched with the resonant wavelength of the whispering wall, thereby improving the coupling efficiency. Attached Figure Description

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:

[0027] Figure 1 This is a schematic diagram of a cascaded whispering-gallery resonator based on long-period fiber grating coupling;

[0028] Figure 2 This is a schematic diagram of a fiber taper region engraved with a long-period fiber grating;

[0029] Figure 3 This is a schematic diagram of the temperature control device;

[0030] Figure 4 This is a schematic diagram of optical spectrum transformation;

[0031] 1. First optical fiber; 2. Long-period fiber grating on the taper region of the first optical fiber; 3. First whispering wall; 4. Second optical fiber; 5. Long-period fiber grating on the first taper region of the second optical fiber; 6. Long-period fiber grating on the second taper region of the second optical fiber; 7. Second whispering wall; 8. Temperature controller of the temperature control device for the first whispering wall; 9. Temperature controller of the temperature control device for the second whispering wall; 2-1. Fiber core of the long-period fiber grating on the taper region of the first optical fiber; 5-1. Fiber core of the long-period fiber grating on the first taper region of the second optical fiber; 6-1. Fiber core of the long-period fiber grating on the second taper region of the second optical fiber; 2-2. 5-2. Long-period fiber grating core cladding on the first fiber taper region; 6-2. Long-period fiber grating cladding on the first taper region of the second fiber; 10. Heat sink base; 11. Thermistor; 12. TEC; 13. Base; 14. Evanescent wave spectrum of the long-period fiber grating of the first fiber; 15. Eigenmode of the first whispering wall; 16. Mode after coupling the first whispering wall to the first fiber; 17. Evanescent wave spectrum of the first whispering wall input to the second fiber; 18. Eigenmode of the second whispering wall; 19. Output light spectrum of the second fiber. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] See Figures 1-4A cascaded whispering-gallery resonator based on long-period fiber grating coupling is disclosed, comprising a first optical fiber 1, a second optical fiber 4, a first whispering-gallery 3, a second whispering-gallery 7, and a temperature control device. The first optical fiber 1 has a conical section with a long-period fiber grating (long-period fiber grating 2 on the conical section of the first optical fiber) etched on it. The second optical fiber 4 has a first conical section and a second conical section, each with a long-period fiber grating etched on it: a long-period fiber grating 5 on the first conical section and a long-period fiber grating 6 on the second conical section. The first whispering-gallery 3 is coupled to the conical section of the first optical fiber 1 and the first conical section of the second optical fiber 4, respectively; the second whispering-gallery 7 is coupled to the second conical section of the second optical fiber 4.

[0034] The fundamental mode propagating in the fiber core is converted into a higher-order mode propagating in the cladding by a long-period fiber grating, which further leaks out of the fiber from the cone region. The long-period grating ensures a large core diameter in the cone region while maintaining a strong evanescent wave. The light energy is transmitted from the first fiber 1 to the first whispering wall 3 in the form of an evanescent wave, and then from the first whispering wall 3 to the second fiber 4. At this time, the light wavelength in the second fiber 4 is the mode of the first whispering wall 3. The light energy input from one end of the second fiber 4 is transmitted from the second fiber 4 to the second whispering wall 7 in the form of an evanescent wave, and then from the second whispering wall 7 back to the second fiber 4. It is output from the other end of the second fiber 4. At this time, the second whispering wall mode filters the first whispering wall mode, achieving less mode or single-mode output.

[0035] The first soundbar 3 and the second soundbar 7 are respectively installed on the temperature control device. The temperature control device controls the temperature of the soundbar. That is, the temperature controller 8 of the temperature control device of the first soundbar controls and adjusts the temperature of the first soundbar 3, and the temperature controller 9 of the temperature control device of the second soundbar controls and adjusts the temperature of the second soundbar 7.

[0036] In this embodiment, the temperature control device includes a base 13, a TEC 12, a heat sink base 10 (copper heat sink base), a thermistor 11, and a temperature controller. The TEC 12 is placed on the base 13; the heat sink base 10 is placed on the TEC 12; and the thermistor 11 is placed inside the heat sink base 10. Taking the annual control of the first soundbar 3 as an example: during the first use, the first soundbar 3 is placed on the heat sink base 10, and the temperature of the first soundbar 3 can be effectively conducted to the heat sink base 10. After the thermistor 11 detects the temperature of the heat sink base 10, it transmits the temperature data to the temperature controller 8 of the temperature control device of the first soundbar. The temperature controller 8 of the temperature control device of the first soundbar controls the TEC 12 to cool or heat, thereby controlling and adjusting the temperature of the first soundbar 3, and thus adjusting the resonant wavelength of the first soundbar 3. When the temperature of the first soundbar 3 increases, its resonant wavelength will drift towards the longer wavelength direction; when the temperature of the first soundbar 3 decreases, its resonant wavelength will drift towards the shorter wavelength direction. The two temperature control devices in this embodiment, on the one hand, cause the mode of the second soundbar 7 to drift, thereby accurately filtering the mode of the first soundbar, and on the other hand, achieve the highest coupling efficiency with the long-period fiber grating.

[0037] In this embodiment, the resonant wavelength of the first whispering wall 3 is located within the resonant bandwidth of the long-period fiber grating 2 on the first fiber taper and the resonant bandwidth of the long-period fiber grating 5 on the first fiber taper; the resonant wavelength of the second whispering wall 7 is located within the resonant bandwidth of the long-period fiber grating 6 on the second fiber taper. The resonant wavelengths of the first whispering wall 3 and the second whispering wall 7 satisfy the phase matching condition: λ=2πn(T)r(T) / m, where n(T) represents the refractive index of the whispering wall, which varies with temperature T; r(T) is the radius of the whispering wall, which varies with temperature T; and m is the number of circumferential modes of the whispering wall.

[0038] In this embodiment, the first optical fiber 1 and the second optical fiber 4 are the same type of optical fiber, and the taper region of the first optical fiber 1 is the same as the first taper region of the second optical fiber 4. The long-period fiber grating 2 on the taper region of the first optical fiber is the same as the long-period fiber grating 5 on the first taper region of the second optical fiber, so as to ensure high coupling efficiency.

[0039] The long-period fiber grating 2 on the first fiber cone region and the long-period fiber grating 5 on the first fiber cone region of the second fiber both achieve full coupling at their coupling points with the first whispering wall 3; the long-period fiber grating 6 on the second fiber cone region achieves full coupling at its coupling point with the second whispering wall 7; that is, it satisfies |k|L=aπ / 2, where k represents the coupling constant, L represents the grating length at the coupling point, and a is an odd number. In the fully coupled state, all the energy of the core mode with the resonant wavelength is coupled to the cladding film.

[0040] The resonant wavelengths of the long-period fiber grating 2 on the first fiber cone region, the long-period fiber grating 5 on the first fiber cone region, and the long-period fiber grating 6 on the second fiber cone region satisfy the phase matching condition: λ=(nco-nc1)Λ, where nco represents the effective refractive index of the fiber core, nc1 is the effective refractive index of the cladding, and Λ represents the grating period.

[0041] The first whispering wall 3 and the second whispering wall 7 are located within the tapered region of the optical fiber, at the position where the coupling efficiency with the fiber is highest. The highest coupling efficiency can be achieved by designing the grating period and adjusting the resonant wavelength of the long-period fiber grating to match the first whispering wall 3 and the second whispering wall 4.

[0042] like Figure 1 As shown, the light wave is input through the first optical fiber 1. Figure 2 As shown, the long-period fiber grating 2 on the first fiber taper region has a periodic grating length of L2 and a grating period of Λ2 at the coupling point with the first whispering wall 3. When the following conditions are met...

[0043] When |k2|L2=aπ / 2, full coupling is achieved; where k2 is the coupling constant and a is an odd number. The optical modes propagating in the core 2-1 of the long-period fiber grating in the first fiber taper region are completely converted to the optical modes propagating in the cladding 2-2 of the long-period fiber grating in the first fiber taper region. Because the light wave propagates in the cladding 2-2 of the long-period fiber grating in the first fiber taper region, even with a large core diameter in the fiber taper region, strong evanescent wave leakage can still be guaranteed, and its resonant wavelength satisfies λ2=(nco-nc1)Λ2, where nco is the core refractive index and nci is the cladding refractive index. Figure 4 As shown, at this time, the evanescent wave spectrum 14 of the first fiber long-period fiber grating is a structure with a resonant wavelength of λ2 and a certain resonant bandwidth.

[0044] like Figure 1 As shown, the evanescent wave leaked from the long-period fiber grating 2 on the first fiber taper region couples with the first whispering wall 3. The relative positions of the first whispering wall 3 and the long-period fiber grating 2 on the first fiber taper region are adjusted to place them at the position with the maximum coupling efficiency. The resonant wavelength of the first whispering wall 3 satisfies λ3=2πn(T)r(T) / m, where n(T) represents the refractive index of the whispering wall, which varies with temperature T; r(T) is the radius of the whispering wall, which varies with temperature T; and m is the number of circumferential modes of the whispering wall.

[0045] This invention achieves maximum coupling efficiency by designing the grating period Λ2 of the long-period fiber grating 2 on the first fiber taper region and controlling the temperature of the first whispering wall 3, so that λ2 and λ3 are matched, and the first whispering wall mode is located within the resonant bandwidth of the long-period fiber grating 2 on the first fiber taper region. Figure 4As shown, at this time, within the first whispering wall 3, the mode 16 after the first whispering wall is coupled with the first optical fiber is the mode selection of the evanescent wave spectrum 14 of the long-period fiber grating of the first optical fiber by the eigenmode 15 of the first whispering wall.

[0046] like Figure 1 As shown, the evanescent wave leaked from the first whispering wall 3 couples with the long-period fiber grating 5 on the first conical region of the second fiber. The second fiber 4 and the first fiber 1 are of the same type. Similarly, the relative positions of the first whispering wall 3 and the long-period fiber grating 5 on the first conical region of the second fiber are adjusted to achieve the highest coupling efficiency. The grating period of the long-period fiber grating 5 on the first conical region of the second fiber is Λ5, and the grating length at the coupling point with the first whispering wall is L5. To achieve the highest coupling efficiency, the parameters of the long-period fiber grating 5 on the first conical region of the second fiber are the same as those of the long-period fiber grating 2 on the first conical region, i.e., Λ5 = Λ2 and L5 = L2. At this time, the long-period fiber grating 5 on the first conical region of the second fiber also reaches full coupling, meaning that all propagation modes in the fiber core 5-1 of the long-period fiber grating on the first conical region of the second fiber are converted to propagation modes in the cladding 5-2 of the long-period fiber grating on the first conical region of the second fiber. Figure 4 As shown, at this time, the evanescent wave spectrum 17 input from the first whispering wall to the second optical fiber is consistent with the mode 17 after the first whispering wall is coupled to the first optical fiber.

[0047] like Figure 1 As shown, the light wave continuing to propagate in the second optical fiber 4 couples with the second whispering wall 7 at the long-period fiber grating 6 on the second cone region of the second optical fiber. Similarly, the relative positions of the second whispering wall 7 and the long-period fiber grating 6 on the second cone region of the second optical fiber are adjusted to place them at the position with the maximum coupling efficiency. The grating period of the long-period fiber grating 6 on the second cone region of the second optical fiber is Λ6, the grating length at the coupling point with the second whispering wall 7 is L6, the coupling constant is k6, satisfying |k6|L6=aπ / 2, and the resonant wavelength is λ6=(nco-nc1)Λ6. The resonant wavelength of the second whispering wall 7 is...

[0048] λ7 = 2πn(T)r(T) / m. By designing the grating period Λ6 of the long-period fiber grating 6 on the second cone region of the second fiber, and controlling the temperature of the second whispering wall 7, λ6 and λ7 are matched, and the second whispering wall mode should be located within the resonant bandwidth of the long-period fiber grating 6 on the second cone region of the second fiber to achieve the highest coupling efficiency. Figure 2As shown, at this time, all the modes propagating in the long-period fiber grating core 6-1 of the second cone region of the second fiber are converted to the modes propagating in the long-period fiber grating cladding 6-2 of the second cone region of the second fiber, and further leak from the cone region into the second whispering wall 7 for notch filtering. The filtered light wave is then converted back from the long-period fiber grating cladding 6-2 of the second cone region of the second fiber to the long-period fiber grating core 6-1 of the second cone region of the second fiber for output. Figure 4 As shown, at this time, the output optical spectrum 19 of the second optical fiber is the result of filtering the evanescent wave spectrum 17 of the first whispering wall input to the second optical fiber by the second whispering wall eigenmode 18, thus realizing single-fiber mode output.

[0049] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A cascaded whispering-gallery resonator based on long-period fiber grating coupling, characterized in that: It includes a first optical fiber, a second optical fiber, a first soundbar, a second soundbar, and a temperature control device for each soundbar; The first optical fiber has a conical section with a long-period fiber grating etched on it. The second optical fiber has a first conical section and a second conical section, with long-period fiber gratings etched on each of the two conical sections. The first soundbar is coupled to the taper region of the first optical fiber and the first taper region of the second optical fiber, respectively; the second soundbar is coupled to the second taper region of the second optical fiber; the first and second soundbars are respectively mounted on a temperature control device, which controls and adjusts the temperature of the soundbars; the first and second soundbars together constitute a cascaded system. The fundamental mode propagating in the fiber core is converted into a higher-order mode propagating in the cladding by a long-period fiber grating, and further leaks out of the fiber through the taper region; the light energy is transmitted from the first fiber to the first whispering wall in the form of an evanescent wave, and then from the first whispering wall to the second fiber, at which time the light wavelength in the second fiber is the mode of the first whispering wall; the light energy input from one end of the second fiber is transmitted from the second fiber to the second whispering wall in the form of an evanescent wave, and then from the second whispering wall back to the second fiber, and output from the other end of the second fiber, at which time the second whispering wall mode filters the first whispering wall mode, realizing less mode or single-mode output.

2. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 1, characterized in that: The temperature control device includes a base, a TEC (thermal rectifier), a heat sink base, a thermistor, and a temperature controller. The TEC is placed on the base; the heat sink base is placed on the TEC; the thermistor is placed inside the heat sink base; In use, the soundbar is placed on the heat sink base. The temperature of the soundbar can be effectively conducted to the heat sink base. After the thermistor detects the temperature of the heat sink base, it transmits the temperature data to the temperature controller. The temperature controller controls the TEC to cool or heat, thereby controlling and adjusting the temperature of the soundbar, and thus adjusting the resonant wavelength of the soundbar.

3. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 1, characterized in that: The resonant wavelength of the first whispering wall is located within the resonant bandwidth of the long-period fiber grating on the first fiber taper and the resonant bandwidth of the long-period fiber grating on the first taper of the second fiber; the resonant wavelength of the second whispering wall is located within the resonant bandwidth of the long-period fiber grating on the second taper of the second fiber.

4. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to any one of claims 1-3, characterized in that: The resonant wavelengths of the first and second whispering walls satisfy the phase matching condition: λ = 2πn(T)r(T) / m, where n(T) represents the refractive index of the whispering wall, which varies with temperature T; r(T) is the radius of the whispering wall, which varies with temperature T; and m is the number of circumferential modes of the whispering wall.

5. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 4, characterized in that: The first and second soundwalls are located within the taper region of the optical fiber, at the position where the coupling efficiency with the optical fiber is highest.

6. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 1, characterized in that: The long-period fiber gratings on the first fiber cone region and the long-period fiber gratings on the second fiber cone region both achieve full coupling at their coupling points with the first whispering wall. The long-period fiber grating in the second cone region of the second optical fiber reaches full coupling at the coupling point with the second whispering gallery. In the fully coupled state, all the energy of the core mode with the resonant wavelength is coupled to the cladding film.

7. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 1 or 6, characterized in that: The resonant wavelengths of the long-period fiber gratings on the first and second fiber taper regions satisfy the phase matching condition: λ = (nco - nc1)Λ, where nco represents the effective refractive index of the fiber core, nc1 is the effective refractive index of the cladding, and Λ represents the grating period.

8. The cascaded whispering-gallery resonator based on long-period fiber grating coupling according to claim 1 or 6, characterized in that: The first optical fiber and the second optical fiber are the same type of optical fiber, and the taper region of the first optical fiber is the same as the first taper region of the second optical fiber. The long-period fiber grating on the taper region of the first optical fiber is the same as the long-period fiber grating on the first taper region of the second optical fiber.

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