A partially doped polarization maintaining active optical fiber

Abstract

The utility model discloses a kind of polarization maintaining active optical fibers based on partial doping, related to the polarization maintaining active optical fiber with big mode field or big fiber core diameter and simultaneously to the high-order optical fiber mode with radial and angular vector direction component has inhibitory effect, and smaller fundamental mode loss can be realized.

Description

Technical Field

[0001] This utility model relates to a polarization-maintaining active fiber based on partial doping, and more particularly, to a large-mode-field polarization-maintaining active fiber based on partial doping and polarization-maintaining technology, which can obtain fiber lasers with high power, high beam quality, and high polarization degree. Background Technology

[0002] Polarization-maintaining fiber lasers have been widely used in scientific research, industry, and medical applications. A 1-micron polarization-maintaining fiber laser is typically obtained through a polarization-maintaining resonant cavity or power amplifier structure using ytterbium-doped polarization-maintaining active fiber as the gain medium. Since polarization-maintaining lasers have a single-direction linear polarization vector, the ytterbium-doped fiber must possess the following characteristics: 1. Maintaining polarization transmission for a single-direction polarization vector laser; 2. Supporting the transmission and excitation of linearly polarized fiber modes or reducing the transmission and excitation of higher-order modes with radial and angular polarization vector directions. Furthermore, compared to non-polarization-maintaining fiber lasers, polarization-maintaining fiber lasers exhibit higher nonlinearity, theoretically with a nonlinearity threshold half that of non-polarization-maintaining fibers. Therefore, they are more prone to generating laser nonlinearities such as stimulated Raman, stimulated Brillouin, and four-wave mixing. Thus, to obtain high-power polarization-maintaining lasers, this is generally achieved by increasing the mode field or core diameter of the ytterbium-doped polarization-maintaining active fiber or shortening its length. Increasing the mode field or core diameter can increase the number of supported modes in an optical fiber. This requires reducing the numerical aperture (NA) of the fiber to limit the number of modes and thus reduce the excitation of higher-order modes during amplification. However, due to limitations in the fabrication process of double-clad polarization-maintaining active fibers (PSIFs), the NAF of the PIF core is limited to >0.04. This small NAF makes the fiber's fundamental mode loss highly sensitive to fiber coiling, leading to increased loss during coiling and hindering practical application. Therefore, the NAF of the core in double-clad ytterbium-doped polarization-maintaining large-mode-field active fibers is often limited to approximately 0.06. Shortening the polarization-maintaining length reduces effective absorption of pump light, thus limiting the application of this method. In conclusion, to obtain high-power polarization-maintaining lasers, a polarization-maintaining ytterbium-doped active fiber with a large mode field or core diameter, while simultaneously suppressing higher-order fiber modes with radial and angular vector components, is needed. Partial doping is a technique that selectively dops the fiber core region. Rare-earth ions are used to dope the overlapping region between the fundamental mode field region and the effective core region, while the overlapping region between the higher-order mode field region and the effective core region is lightly doped or undoped. Therefore, in polarization-maintaining laser amplification based on this polarization-maintaining active fiber, the fundamental mode with linear polarization vectors can achieve greater gain, thereby suppressing the excitation of higher-order modes with radial and angular polarization. Large-mode-field polarization-maintaining active fibers based on this partial doping technique can achieve high-power, high-polarization-degree, and high-beam-quality polarization-maintaining laser output.

[0003] To effectively suppress higher-order modes with different polarization vectors, current technologies often employ fiber coiling. The loss of the fundamental mode is much lower than that of higher-order modes for different coiling diameters, meaning the fundamental mode coiling loss is low. Under these conditions, fiber coiling significantly suppresses higher-order modes. However, for polarization-maintaining active fibers (especially double-clad active fibers) with large mode fields or large core diameters (e.g., greater than 30 μm), for fibers with a core diameter greater than or equal to 30 μm (numerical aperture ≈ 0.06), under the same coiling conditions, increasing the core diameter will lead to a significant increase in the fundamental mode loss coefficient. Utility Model Content

[0004] The purpose of this invention is to propose a polarization-maintaining active optical fiber based on partial doping.

[0005] To address the aforementioned problems, this invention provides a polarization-maintaining active optical fiber based on partial doping, comprising a core, a cladding, stress bars, and a coating layer. The core is located in the middle of the cladding, and stress bars are respectively disposed on both sides of the core, both of which are located inside the cladding and are spaced apart from the core. The two stress bars are used to maintain the polarization of the optical fiber. A coating layer is disposed outside the cladding. The fiber is characterized by supporting LP01, LP11, LP21, LP02, and LP31 modes; the core diameter is in the range of 20μm-35μm, and the effective diameter of the rare-earth ion doping region is less than 15μm.

[0006] Preferably, the optical fiber only supports LP01, LP11, LP21, LP02, and LP31 modes.

[0007] Preferably, the two stress bars can, under the action of the fiber winding mode, excite the vector polarization LP01 mode that is in the same direction as the stress direction, and suppress the polarization vector LP01 mode that is perpendicular to the stress direction.

[0008] Preferably, the two stress bars are arranged symmetrically with respect to the fiber core.

[0009] Preferably, a recessed cladding layer is added outside the fiber core.

[0010] Preferably, a coiled side core is added outside the fiber core.

[0011] Preferably, the undoped core region is a germanium-doped quartz layer, and the polarization-maintaining active fiber preform core is prepared by chemical vapor deposition; the rare earth ions doped in the partially doped polarization-maintaining active fiber include one or more of erbium, ytterbium, thulium, praseodymium, and neodymium, preferably ytterbium-doped.

[0012] Preferably, the stress bar has a diameter of 50μm-150μm, is made of borosilicate glass, and has a birefringence coefficient greater than 3.0*10^(-4).

[0013] Preferably, the polarization-maintaining active fiber has a cladding diameter of 400 μm and can output polarization-maintaining laser with a power greater than 3500 W and a polarization degree greater than 15 dB.

[0014] Preferably, the rare-earth ion polarization-maintaining active fiber has the following structural parameters: core diameter 30 μm, cladding diameter 400 μm, and numerical aperture 0.06.

[0015] The beneficial effects of this invention lie in the design of a polarization-maintaining rare-earth-doped active fiber with a large mode field or large core diameter that simultaneously suppresses higher-order fiber modes with radial and angular vector components, while achieving relatively low fundamental mode loss. Partial doping technology is employed to suppress higher-order modes with radial or angular polarization vectors in polarization-maintaining rare-earth-doped fibers with large core or mode field diameters. This patent utilizes partial doping technology, enabling the suppression of higher-order mode excitation with different polarization vectors without fiber coiling. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the optical fiber structure of this utility model.

[0017] Figure 2 This is a partial diagram of the modes supported by optical fiber. Detailed Implementation

[0018] The technical solution of this utility model will be further described below with reference to the accompanying drawings and specific embodiments.

[0019] like Figure 1 As shown, this utility model discloses a large core or mode field diameter panda-type polarization-maintaining active fiber based on partial doping technology and polarization-maintaining technology. Preferably, the fiber is a rare-earth-doped polarization-maintaining active fiber. Preferably, the structural parameters of the rare-earth-doped polarization-maintaining active fiber are 30 / 400 / 0.06 (the parameters adopt the common expression method of the prior art, and its format is "core diameter μm / cladding diameter μm / numerical aperture", and the fiber parameters in the following text are also expressed in the same format). The polarization-maintaining active fiber structure mainly consists of four parts (such as...). Figure 1As shown, the fiber includes a fiber core 31, a cladding 32, stress bars 33, and a coating layer 34. The fiber core 31 is located in the middle of the cladding 32, inside the cladding 32. Stress bars 33 are respectively arranged on both sides of the fiber core 31. The two stress bars 33 are used to achieve polarization maintenance of the optical fiber. The coating layer 34 is arranged outside the cladding. Both stress bars are located inside the cladding and distributed on both sides of the fiber core. The stress bars are spaced apart from the fiber core. Preferably, the two stress bars are arranged symmetrically with respect to the fiber core. The doped fiber core 31, preferably, supports the presence of LP01, LP11, LP21, LP02, and LP31 (preferably, the fiber only supports the presence of LP01, LP11, LP21, LP02, and LP31). For example, a core diameter of 30 μm and a numerical aperture of 0.06 can satisfy the above-mentioned mode distribution requirements. Considering that the linearly polarized LPmn mode has a vector degenerate mode and LP01 has two perpendicular vector polarization modes, suppressing a polarization mode in one direction can obtain linearly polarized mode laser (see Appendix). Figure 2 For higher-order modes, compared to LP02 and LP31 modes, the propagation constants of LP11 and LP21 modes are closer to the fundamental mode LP01. Therefore, they are easier to excite and amplify during laser amplification or transmission. Furthermore, the degenerate vector modes of LP11 and LP21 have radial and angular vector components (see appendix). Figure 2 Therefore, the existence of these two modes will seriously affect the polarization state of the transmitted laser. Compared with the fundamental mode, the higher-order mode has a larger mode field diameter region. In order to reduce the overlap between the higher-order mode field region and the effective region of rare earth ion doping in the fiber core, reduce the gain of the higher-order vector mode, and thus suppress the higher-order mode, experiments have shown that when the fiber core diameter is in the range of 20μm-35μm, the actual diameter of the effective region of rare earth ion doping is preferably less than 15μm (as shown in Figure 1, the area of ​​the doped region S2 is smaller than the area of ​​the fiber core region S1). The undoped fiber core region is a germanium-doped quartz layer, and the effective refractive index matches the rare earth ion doped region. Preferably, the absorption coefficient of the polarization-maintaining active fiber is ≥1.5dB / m@976nm. The polarization-maintaining active fiber preform core is prepared by chemical vapor deposition (MCVD).

[0020] For the manufacturing process, the stress rod diameter is approximately 50-150 μm, for example, 100 μm, and it is made of borosilicate glass. Before fiber drawing, this stress rod, as part of the preform, is also prepared by MCVD. The formation process of the stress rod is as follows: First, two holes are symmetrically drilled on the prepared preform with the fiber core as the center. Then, two borosilicate glass rods are inserted into the two holes respectively. During the fiber drawing process, the borosilicate glass rod and the preform generate stress and produce a birefringence effect. The birefringence coefficient brought by the stress rod is greater than 3.0*10^(-4). Due to the existence of this stress birefringence effect, under the action of fiber winding mode, the vector polarization LP01 mode in the same direction as the stress direction can be excited, while the vector polarization LP01 mode perpendicular to the stress direction can be suppressed.

[0021] The cladding material is pure quartz, and the cladding diameter of this large-mode polarization-maintaining active fiber is 400 μm.

[0022] The coating material is an acrylic resin with a low refractive index and a diameter of 530 μm, which can confine the cladding light.

[0023] Using this large mode field diameter polarization-maintaining fiber, high beam quality and high power polarization-maintaining laser with an output power greater than 3500W and a polarization degree greater than 15dB can be achieved.

[0024] Preferably, in order to further suppress higher-order modes without increasing fundamental mode loss, a recessed cladding is added outside the fiber core to increase higher-order mode loss.

[0025] Preferably, the rare earth ions doped in the partially doped polarization-maintaining rare earth-doped optical fiber include one or more of erbium, ytterbium, thulium, praseodymium, and neodymium, with ytterbium doping being the most preferred.

[0026] To further strip away and suppress unpolarized light, preferably, a coiled side core is added outside the fiber core to increase the coupling loss of higher-order modes.

[0027] The technical principles of this utility model have been described above with reference to specific embodiments. These descriptions are merely for explaining the principles of this utility model and should not be construed as limiting the scope of protection of this utility model in any way. Based on this explanation, those skilled in the art can readily conceive of other specific embodiments of this utility model without any inventive effort, and these embodiments will all fall within the scope of protection of this utility model.

Claims

1. A polarization-maintaining active optical fiber based on partial doping, comprising a core, a cladding, stress bars, and a coating layer, wherein the core is located in the middle of the cladding, stress bars are respectively disposed on both sides of the core, both stress bars are located inside the cladding, and the stress bars are spaced apart from the core, the two stress bars are used to achieve polarization maintenance of the optical fiber, and a coating layer is disposed outside the cladding; characterized in that, The optical fiber supports LP01, LP11, LP21, LP02, and LP31 modes; the core diameter is in the range of 20μm-35μm, and the effective diameter of the rare earth ion doping region is less than 15μm.

2. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, Fiber optic cables only support LP01, LP11, LP21, LP02, and LP31 modes.

3. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, Two stress bars can excite the vector polarization LP01 mode that is in the same direction as the stress direction under the action of the fiber winding mode, and suppress the polarization vector LP01 mode that is perpendicular to the stress direction.

4. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, The two stress bars are arranged symmetrically with respect to the fiber core; the undoped fiber core region is a germanium-doped quartz layer.

5. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, Add a recessed cladding layer outside the fiber core.

6. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, Add a coiled side core outside the fiber core.

7. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, The polarization-maintaining active optical fiber preform core is prepared by chemical vapor deposition; the rare earth ions doped in the partially doped polarization-maintaining active optical fiber include one or more of erbium, ytterbium, thulium, praseodymium, and neodymium.

8. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, The stress bar has a diameter of 50μm-150μm and is made of borosilicate glass. The birefringence of the stress bar is greater than 3.0*10^(-4).

9. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, The polarization-maintaining active fiber has a cladding diameter of 400 μm and can output polarization-maintaining lasers with a power greater than 3500 W and a polarization degree greater than 15 dB.

10. The polarization-maintaining active optical fiber based on partial doping according to claim 1, characterized in that, The rare-earth ion polarization-maintaining active fiber has the following structural parameters: core diameter 30 μm, cladding diameter 400 μm, and numerical aperture 0.06.

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