All-fiber mode converter and light system

[0047] The present invention will be further described with reference to the accompanying drawings.

[0048] like figure 1 As shown, the optical system of the all-fiber mode converter of the present invention includes an input fiber 10, two mode field matchers (20, 40), an all-fiber mode converter (30), an output fiber (50), and an input fiber (10). ), the first mode field matcher (20), the all-fiber mode converter (30), the second mode field matcher (40), and the output fiber (50) are fused together in sequence to achieve the purpose of mode conversion. The first propagation mode is transmitted from the input fiber 10 through the first mode field matcher (20) to the all-fiber mode converter (30) with the lowest loss. After the all-fiber mode converter (30), only the There are two propagation modes, and then the second mode field matcher (40) transmits the second propagation mode to the output fiber 50 with minimum loss for output.

[0049] like figure 2Shown is a schematic diagram of the structure of the mode converter of the present invention, which is composed of a mode-coupled fiber portion (31) and a mode-stripped fiber portion (32), which are joined. The first propagating mode is completely converted into the second mode and other unstable modes in the mode-coupled fiber portion (31), and the other unstable modes are: cladding mode, radiation mode and leaky mode. These unstable modes are stripped off in the mode stripped fiber portion (32), and finally output is a stable optical signal of the second propagation mode without any noise.

[0050] image 3 is the cross-sectional schematic diagram of the mode-coupled fiber portion of the all-fiber mode converter of the present invention, Figure 4 is its corresponding refractive index distribution. The specially designed fiber can realize the coupling of LP01 to LP02 modes. The mode-coupled fiber portion (31) includes: a cladding (31.1), a core (31.2), and an inner core (31.3). The core (31.2) and the cladding (31.1) form the first optical waveguide, and the core (31.2) and the inner core (31.3) form the second optical waveguide. When the first mode is transmitted in the mode-coupled fiber portion, the energy is periodically converted between the first optical waveguide and the second optical waveguide, and the coupling period is L. Therefore, at the propagation direction L, the energy in the second optical waveguide reaches a maximum. For predetermined parameters of cladding, core, and inner core, the transverse field distribution in the second optical waveguide is most similar to the transverse field distribution of the second propagating mode in the first waveguide. Here, the coupling efficiency of the second propagating mode reaches a maximum, while the first propagating mode is fully converted to the second propagating mode and other unstable modes. Then, through the mode stripping fiber section (32), the unstable modes are stripped out. Finally, only the second propagation mode remains to be transmitted into the output fiber.

[0051] The position of the inner core (31.3) of the above-mentioned mode-coupled fiber portion (31) is different in different mode conversion applications. Its location is at the maximum amplitude of the transverse field distribution of the converted higher-order modes. like image 3 Shown is a schematic cross-sectional view of the mode-coupling fiber portion of the LP01-to-LP02 mode converter. The amplitude of the transverse field distribution of the LP02 mode reaches the maximum at the fiber axis. Therefore, the position of the inner core (31.3) is in the fiber. at the axis; such as Image 6 Shown is a schematic cross-sectional view of the mode-coupling fiber portion of the LP01-to-LP11 mode converter. The amplitude of the transverse field distribution of the LP02 mode reaches a maximum at the fiber radius r = 4 μm. Therefore, the position of the inner core (31.3) At the fiber radius r=4μm; as Figure 7 Shown is a schematic cross-sectional view of the mode-coupling fiber portion of the all-fiber mode converter from LP01 to LP21. The amplitude of the lateral field distribution of the LP21 mode reaches the maximum at the fiber radius r=7.5μm and r=-7.5μm at the same time, so , the mode converter has two inner cores (31.3) located at the fiber radii r=7.5μm and r=-7.5μm, respectively.

[0052] The core and cladding parameters of the mode-coupled fiber portion (31) described above are different in different mode conversion applications. First, to ensure that the converted high-order mode (LPmn) can propagate in the fiber, the normalized frequency V corresponding to the cladding and the core is greater than the normalized cutoff frequency V1 of the converted high-order mode (LPmn). Secondly, in order not to excite higher-order modes (LPmn', where n'=n+1), the normalized frequency V corresponding to the cladding and the core is smaller than the normalized cut-off frequency V2 of the higher-order modes (LPmn'). According to the research, the larger the normalized frequency V corresponding to the cladding and the core, the higher the mode conversion efficiency. Therefore, the normalized frequency V calculated according to the parameters of the cladding and the core should be as close to V2 as possible.

[0053] The refractive index and core diameter of the inner core of the mode-coupled fiber portion described above are different in different mode conversion applications. The inner core and the core are combined to form the second optical waveguide, and the cladding and the core are combined to form the first optical waveguide. According to the above theory, at the coupling period L in the fiber transmission direction, when the transverse field of the second optical waveguide is The mode conversion efficiency is maximized when the distribution best matches the transverse field distribution of the second mode of the first optical waveguide. Therefore, the refractive index of the specific inner core and the size of the core diameter are designed to ensure a suitable transverse field distribution. like Figure 5 Shown are the lateral field distribution (dotted line) of the fundamental mode in the second optical waveguide of the LP01 to LP02 mode converter, and the lateral field distribution (solid line) of the second propagating mode (LP02) in the first optical waveguide .

[0054] Taking the mode converter from LP01 to LP02 as an example, the refractive index distribution of the fiber in the mode coupling conversion part is as follows Figure 4 As shown, the diameter of the cladding is 200 μm, the refractive index is 1.485; the diameter of the core is 31 μm, the refractive index is 1.487; the inner core is in the center of the fiber, the diameter is 9 μm, and the refractive index is 1.4894; the length of the mode coupling conversion part of the fiber is 271 μm. like Figure 8 As shown, in the optical signal propagation direction Z, the energy of the LP01 mode keeps decreasing, and the energy of the LP02 mode keeps increasing. At 271 μm, the energy of the LP02 mode reaches a maximum of 86%, and the energy of the LP01 mode reaches a minimum value close to 0%. At this time, the optical signal is transmitted into the mode stripped fiber part, the cladding diameter is 200μm, the refractive index is 1.485; the core diameter is 31μm, the refractive index is 1.487; the fiber length of the mode stripped part is at least 20mm. like Figure 9 As shown, the total energy of the transmitted signal in the fiber (solid line) gradually decreases until it is substantially equal to the energy of the LP02 mode (dashed line). After the optical signal of the LP01 mode passes through the mode converter of the present invention, 86% of the energy is converted into the energy of the LP02 mode, and the remaining 14% of the energy is converted into the energy of other modes and finally stripped out.