A high-efficiency all-fiber single vector mode coupler

Abstract

The present invention relates to the technical field of couplers, and specifically discloses a high-efficiency all-fiber single vector mode coupler, comprising a polarization-maintaining polarizer, a polarization-maintaining single-mode optical fiber, an annular few-mode optical fiber and a long-period annular optical fiber grating connected in sequence, wherein the long-period annular optical fiber grating is inscribed by a CO2 laser, and its exposure direction is perpendicular or parallel to the slow axis of the polarization-maintaining single-mode optical fiber, the polarization-maintaining polarizer is used to fix the polarization state of the input light to linear polarization along the slow axis or fast axis of the polarization-maintaining single-mode optical fiber, and the annular few-mode optical fiber supports both LP01 and LP11 modes in the 1.0 micron or 1.5 micron band, and the effective refractive index difference between the single vector modes in the LP11 mode is greater than 10 ‑4 It can support low-crosstalk transmission of a single vector mode in optical fiber, realize efficient coupling of the fundamental mode to a single vector mode, and generate a high-purity single vector mode, solving the problem of the current optical fiber coupler stably generating a high-purity single high-order mode.

Description

Technical Field

[0001] The invention relates to the technical field of couplers, in particular to a high-efficiency all-fiber single vector mode coupler. Background Art

[0002] The single vector mode in the optical fiber has special polarization characteristics and mode field distribution. For example, the polarization of the TE01 mode is distributed along the angular direction, and its mode field distribution is annular. After focusing, it has a hollow intensity distribution, which has important application value in the field of optical tweezers. The TM01 mode has a radial polarization distribution, which has a unique advantage for surface plasmon excitation, and its spot size under tight focus can break through the limitation of the diffraction limit, which has important applications in microscopic imaging and optical storage. The two odd and even modes of the HE21 mode can generate an orbital angular momentum beam after superposition with a phase of π / 2, which can be applied to optical communication fields such as mode division multiplexing.

[0003] However, the generation of single vector modes in fiber lasers is currently limited by mode crosstalk. When propagating in the fiber, single vector modes will randomly couple to adjacent modes, greatly reducing mode purity and polarization characteristics. In addition, most long-period fiber gratings currently written use CO2 lasers or melt-tapered methods, and their refractive index modulation has strong directionality, and the coupling effect is affected by the polarization state of the incident light. The single vector mode fiber couplers currently proposed all rely on polarization controllers to adjust the polarization state and mode of the light beam, and are easily affected by environmental perturbations, making them less practical. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of poor controllability and low anti-interference ability of existing single vector mode fiber couplers. Based on polarization-maintaining single-mode fiber and long-period ring fiber grating, a high-efficiency all-fiber single vector mode coupler is designed.

[0005] To achieve the above object, the present invention provides the following technical solutions: a high-efficiency all-fiber single vector mode coupler, comprising a polarization-maintaining polarizer, a polarization-maintaining single-mode fiber, a long-period annular fiber grating, and annular few-mode fiber, wherein the polarization-maintaining polarizer is used to fix the polarization direction of the incident light to one of the slow axis or the fast axis of the polarization-maintaining fiber;

[0006] Wherein, the polarization-maintaining single-mode optical fiber is used to transmit a fundamental mode in a specific polarization direction;

[0007] Wherein, the long-period ring fiber grating is used to couple the incident fundamental mode to a specific single vector mode;

[0008] Wherein, the annular few-mode optical fiber is used to simultaneously transmit the LP01 and LP11 modes and reduce the coupling between single vector modes in the LP11 mode;

[0009] The pigtails at both ends of the polarization-maintaining polarizer are polarization-maintaining single-mode optical fibers. The polarization-maintaining single-mode optical fiber at the input end of the polarization-maintaining polarizer is connected to a laser source. The polarization-maintaining single-mode optical fiber at the output end of the polarization-maintaining polarizer is fused with the annular few-mode optical fiber. The long-period annular fiber grating adopts CO 2 The laser is inscribed on the annular few-mode optical fiber.

[0010] Preferably, a polarization-maintaining single-mode fiber stress axis and a polarization-maintaining single-mode fiber core are arranged inside the polarization-maintaining single-mode fiber, the polarization-maintaining single-mode fiber stress axis is located on both sides of the polarization-maintaining single-mode fiber core, an annular few-mode fiber core is arranged inside the annular few-mode fiber, and the polarization-maintaining single-mode fiber core is butt-jointed with the annular few-mode fiber core.

[0011] Preferably, the long-period annular fiber grating includes a long-period annular fiber grating groove, and the long-period annular fiber grating groove corresponds to a length of 0-5 cm of the annular few-mode fiber between one end of the polarization-maintaining single-mode fiber and the polarization-maintaining single-mode fiber, the period number of the long-period annular fiber grating is 20-60, and the period length is 200-1000 microns.

[0012] Preferably, the annular few-mode optical fiber adopts CO 2 When the laser writes the long-period annular fiber grating, a single-side exposure mode or a double-side exposure mode is adopted.

[0013] Preferably, the annular few-mode optical fiber adopts CO 2 When the laser writes the long-period annular fiber grating, the irradiation direction is perpendicular to the slow axis of the polarization-maintaining single-mode optical fiber.

[0014] Preferably, the annular few-mode optical fiber adopts CO 2 When the laser writes the long-period annular fiber grating, the irradiation direction is parallel to the slow axis of the polarization-maintaining single-mode optical fiber.

[0015] Preferably, the operating wavelength band of the polarization-maintaining single-mode optical fiber includes but is not limited to a 1.0 micron wavelength band and a 1.5 micron wavelength band.

[0016] Preferably, the operating band of the annular few-mode optical fiber includes but is not limited to a 1.0 micron band and a 1.5 micron band.

[0017] Preferably, the annular few-mode fiber supports the LP11 mode in both the 1.0 μm and 1.5 μm bands, and the corresponding effective refractive index differences between the TE01, TM01 and HE21 modes are greater than 10 -4 .

[0018] Preferably, the annular few-mode optical fiber is composed of a low refractive index layer, a high refractive layer and a cladding, and the low refractive index layer is one of pure silica or air, and the high refractive index layer of the annular few-mode optical fiber is annularly distributed germanium-doped silica.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The mode coupler of the present invention adopts polarization-maintaining single-mode optical fiber and annular few-mode optical fiber, has stable polarization state input and output, and has strong ability to resist environmental interference.

[0021] 2. The present invention uses CO 2 The laser writes a long-period grating, and by utilizing its asymmetry and the regulation of the polarization state of the incident light, efficient coupling of the fundamental mode to a single vector mode is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the device connection structure of the present invention as a whole;

[0023] Figure 2 It is a schematic diagram of the fusion of the polarization-maintaining single-mode optical fiber and the long-period annular fiber grating of the present invention;

[0024] Figure 3 A schematic diagram of a connection structure of an embodiment of the present invention;

[0025] Figure 4 Schematic diagram of the refractive index profile of the annular few-mode optical fiber of the present invention;

[0026] Figure 5 It is a schematic diagram of the transmission spectrum of the long-period annular fiber grating of the present invention;

[0027] Figure 6 Schematic diagram of the mode field distribution of the transformed light beam in an embodiment of the present invention (the white arrow represents the polarization direction of the analyzer).

[0028] In the figure: 1-polarization-maintaining polarizer; 2-polarization-maintaining single-mode optical fiber; 3-long-period annular fiber grating; 4-annular few-mode optical fiber; 5-polarization-maintaining single-mode optical fiber stress axis; 6-polarization-maintaining single-mode optical fiber core; 7-annular few-mode optical fiber core; 8-long-period annular fiber grating groove; 9-laser source. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] See also Figure 1-6 The present invention provides a technical solution: comprising a polarization-maintaining polarizer 1, a polarization-maintaining single-mode optical fiber 2, a long-period annular fiber grating 3 and annular few-mode optical fiber 4, wherein the polarization-maintaining polarizer 1 is used to fix the polarization direction of the incident light to one of the slow axis or the fast axis of the polarization-maintaining optical fiber; the polarization-maintaining single-mode optical fiber 2 is used to transmit a fundamental mode with a specific polarization direction; the long-period annular fiber grating 3 is used to couple the incident fundamental mode to a specific single vector mode; the annular few-mode optical fiber 4 is used to simultaneously transmit LP01 and LP11 modes and reduce the coupling between single vector modes in the LP11 mode; the pigtails at both ends of the polarization-maintaining polarizer 1 are both polarization-maintaining single-mode optical fibers 2, the polarization-maintaining single-mode optical fiber 2 at the input end of the polarization-maintaining polarizer 1 is connected to a laser source 9, the polarization-maintaining single-mode optical fiber 2 at the output end of the polarization-maintaining polarizer 1 is fused with the annular few-mode optical fiber 4, and the long-period annular fiber grating 3 adopts CO 2 The laser is written on the ring-shaped few-mode optical fiber 4.

[0031] When in use, the laser source 9 adopts a fully polarization-maintaining continuous fiber laser with an output laser center wavelength of 1068.2 nanometers, and its output end tail fiber model is PM980; the input end of the polarization-maintaining polarizer 1 is fused to the output end of the laser source 9 through the polarization-maintaining single-mode fiber 2; the polarization direction of the polarization-maintaining polarizer 1 is the slow axis, that is, only the light beam polarized along the slow axis direction can pass; the refractive index profile of the annular few-mode fiber 4 is as follows Figure 4 As shown, it supports both LP01 and LP11 modes in the 1064-nanometer band. The effective refractive indices of HE11, TE01, HE21 and TM01 modes in the annular few-mode fiber 4 are calculated by the finite element method to be 1.45529508, 1.4538347, 1.45367486 and 1.45347404, respectively. The long-period grating coupling formula calculates that the resonance period of the HE11 mode coupling to the TM01 mode at 1068 nanometers is 586 microns. The period length will be offset during actual writing. The writing direction of the long-period ring fiber grating 3 is parallel to the slow axis direction of the polarization-maintaining single-mode fiber 2. Single-side exposure writing is used. Its period length is 580 microns and the number of periods is 30. The transmission spectrum of the long-period ring fiber grating 3 is shown in FIG. Figure 5 As shown, its coupling rate at 1068.2 nanometers is 17dB, and the coupling rate from the fundamental mode to the TM01 mode exceeds 98%; the beam mode field distribution of the output of the laser source 1 after being converted by the single vector mode coupler is shown in Figure 6 As described above, from the polarization result of the analyzer, it can be known that its output is TM01 mode (the white arrow represents the polarization direction of the analyzer).

[0032] In this embodiment, a polarization-maintaining single-mode fiber stress axis 5 and a polarization-maintaining single-mode fiber core 6 are arranged inside the polarization-maintaining single-mode fiber 2, and the polarization-maintaining single-mode fiber stress axis 5 is located on both sides of the polarization-maintaining single-mode fiber core 6. An annular few-mode fiber core 7 is arranged inside the annular few-mode fiber 4, and the polarization-maintaining single-mode fiber core 6 is connected to the annular few-mode fiber core 7.

[0033] In this embodiment, the long-period ring fiber grating 3 includes a long-period ring fiber grating groove 8, and the long-period ring fiber grating groove 8 corresponds to the length of the ring few-mode fiber 4 between one end of the polarization-maintaining single-mode fiber 2 and the polarization-maintaining single-mode fiber 2, which is 0-5 cm. The number of periods of the long-period ring fiber grating 3 is 20-60, and the period length is 200-1000 microns. Generally speaking, the birefringence △n of the fundamental mode in the non-polarization-maintaining fiber is in the order of 10-6, so its beat length in the 1.0 micron band is about 1 meter. Therefore, the polarization direction of the fundamental mode of the polarization-maintaining single-mode fiber 2 coupled to the ring few-mode fiber 4 remains basically unchanged in the long-period ring fiber grating 3; for the use of CO 2 The refractive index modulation generated by the laser-written long-period annular fiber grating 3 is distributed along the laser irradiation direction, so it will undergo mode coupling in the laser irradiation direction, and the polarization state of the coupled light is consistent with the polarization state of the original incident light. When the polarization state of the incident light is parallel to the laser irradiation direction and the period of the long-period grating meets the coupling condition, the incident fundamental mode will be coupled to the radially polarized high-order mode (such as TM01 mode); when the polarization state of the incident light is perpendicular to the laser irradiation direction and the period of the long-period grating meets the coupling condition, the incident fundamental mode will be coupled to the angularly polarized high-order mode (such as TE01 mode).

[0034] When writing the long-period annular fiber grating 3, firstly, the polarization-maintaining single-mode fiber 2 at the output end of the polarization-maintaining polarizer 1 is fused with the annular few-mode fiber 4, and the polarization-maintaining single-mode fiber 2 is fixed on a rotatable fixture, and the stress axis direction of the polarization-maintaining single-mode fiber 2 is observed by an optical imaging system, and the fiber fixture is rotated so that the slow axis of the polarization-maintaining single-mode fiber 2 is parallel or perpendicular to the CO 2 The irradiation direction of the laser; finally, CO 2 The laser writes the long-period annular fiber grating 3 on the annular few-mode grating 4 at a position within 5 cm from the welding point between the polarization-maintaining single-mode fiber 2 and the annular few-mode fiber 4 .

[0035] In this embodiment, the operating wavelength band of the polarization-maintaining single-mode optical fiber 2 includes but is not limited to a 1.0 micron wavelength band and a 1.5 micron wavelength band.

[0036] In this embodiment, the operating wavelength band of the annular few-mode optical fiber 4 includes but is not limited to a 1.0 micron wavelength band and a 1.5 micron wavelength band.

[0037] In this embodiment, the annular few-mode fiber 4 supports the LP11 mode in both the 1.0 μm and 1.5 μm bands, and the corresponding effective refractive index differences among the TE01, TM01 and HE21 modes are all greater than 10 -4 .

[0038] In this embodiment, the annular few-mode optical fiber 4 is composed of a low refractive index layer, a high refractive layer and a cladding, and the low refractive index layer is one of pure silica or air, and the high refractive index layer of the annular few-mode optical fiber 4 is annularly distributed germanium-doped silica; the annular structure can reduce the degeneracy of the LP11 mode, so that the TE01, TM01 and HE21 modes are almost not coupled with each other when transmitted in the optical fiber.

[0039] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0040] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-efficiency all-fiber single vector mode coupler, characterized in that: include: Polarization-maintaining polarizer (1); Polarization-maintaining single-mode optical fiber (2); Long period ring fiber grating (3) and An annular few-mode optical fiber (4), wherein the polarization-maintaining polarizer (1) is used to fix the polarization direction of the incident light to be one of the slow axis and the fast axis of the polarization-maintaining optical fiber; Wherein, the polarization-maintaining single-mode optical fiber (2) is used to transmit a fundamental mode in a specific polarization direction; Wherein, the long-period ring fiber grating (3) is used to couple the incident fundamental mode to a specific single vector mode; The annular few-mode optical fiber (4) is used to simultaneously transmit the LP01 and LP11 modes and reduce the coupling between single vector modes in the LP11 mode; The pigtails at both ends of the polarization-maintaining polarizer (1) are polarization-maintaining single-mode optical fibers (2); the polarization-maintaining single-mode optical fiber (2) at the input end of the polarization-maintaining polarizer (1) is connected to a laser source (9); the polarization-maintaining single-mode optical fiber (2) at the output end of the polarization-maintaining polarizer (1) is fused with the annular few-mode optical fiber (4); and the long-period annular fiber grating (3) is inscribed on the annular few-mode optical fiber (4) using a CO2 laser; The polarization-maintaining single-mode optical fiber (2) is provided with a polarization-maintaining single-mode optical fiber stress axis (5) and a polarization-maintaining single-mode optical fiber core (6), the polarization-maintaining single-mode optical fiber stress axis (5) is located on both sides of the polarization-maintaining single-mode optical fiber core (6), the annular few-mode optical fiber (4) is provided with an annular few-mode optical fiber core (7), and the polarization-maintaining single-mode optical fiber core (6) is butt-jointed with the annular few-mode optical fiber core (7); The long-period annular fiber grating (3) comprises a long-period annular fiber grating groove (8), and the length of the long-period annular fiber grating groove (8) corresponding to the annular few-mode fiber (4) between one end of the polarization-maintaining single-mode fiber (2) and the polarization-maintaining single-mode fiber (2) is 0-5 cm, the period number of the long-period annular fiber grating (3) is 20-60, and the period length is 200-1000 microns; When the long-period annular fiber grating (3) is inscribed on the annular few-mode optical fiber (4) using a CO2 laser, the irradiation direction is perpendicular to the slow axis of the polarization-maintaining single-mode optical fiber (2).

2. A high-efficiency all-fiber single vector mode coupler according to claim 1, characterized in that: When the long-period annular fiber grating (3) is inscribed on the annular few-mode optical fiber (4) using a CO2 laser, the irradiation direction is parallel to the slow axis of the polarization-maintaining single-mode optical fiber (2).

3. A high-efficiency all-fiber single vector mode coupler according to claim 2, characterized in that: The operating wavelength band of the polarization-maintaining single-mode optical fiber (2) includes a 1.0 micron wavelength band and a 1.5 micron wavelength band.

4. A high-efficiency all-fiber single vector mode coupler according to claim 3, characterized in that: The working wavelength band of the annular few-mode optical fiber (4) includes a 1.0 micron wavelength band and a 1.5 micron wavelength band.

5. A high-efficiency all-fiber single vector mode coupler according to claim 4, characterized in that: The annular few-mode optical fiber (4) supports the LP11 mode in both the 1.0 micron and 1.5 micron wavebands, and the effective refractive index differences among the corresponding TE01, TM01 and HE21 modes are all greater than 10-4.

6. A high-efficiency all-fiber single vector mode coupler according to claim 5, characterized in that: The annular few-mode optical fiber (4) is composed of a low refractive index layer, a high refractive index layer and a cladding, wherein the low refractive index layer is one of pure silica or air, and the high refractive index layer of the annular few-mode optical fiber (4) is annularly distributed germanium-doped silica.

Images