Highly nonlinear photonic crystal fiber and fiber amplifier

Abstract

This invention relates to a highly nonlinear photonic crystal fiber and an fiber amplifier. The fiber includes a core layer and a cladding layer, the cladding layer comprising an inner cladding layer and an outer cladding layer. The core layer is characterized by a refractive index profile exhibiting an α-power-law distribution, where the distribution index α is 2.0–8.0, the radius a is 0.8–2 μm, and the maximum relative refractive index difference Δ... 1max The refractive index is 2-5%, and the inner cladding is an air-hole lattice inner cladding, surrounded by an outer cladding, which is a pure silica glass outer cladding. The large refractive index difference between the high-refractive-index core and the low-refractive-index air-hole cladding of this invention is beneficial in two ways: firstly, it enhances the light-binding ability of the fiber core; secondly, it promotes a reduction in the effective mode area of ​​the optical fiber, thereby improving nonlinear performance. The graded-index high-nonlinear photonic crystal fiber provided by this invention is mainly used in the communication field and can be used as the optical gain medium for fiber amplifiers. The corresponding devices have advantages such as low noise and high gain.

Description

Technical Field

[0001] This invention relates to a highly nonlinear photonic crystal fiber and an optical fiber amplifier, belonging to the field of optical fiber technology. Background Technology

[0002] Highly nonlinear optical fibers can exhibit various nonlinear effects under specific conditions, such as self-phase modulation, soliton splitting, four-wave mixing, and stimulated Raman scattering. Their unique optical properties have led to their promising commercial prospects and significant research value in fields such as supercontinuum generation, optical frequency combs, wavelength division multiplexing (WDM) systems, broadband tunable light sources, optical soliton communication, and Raman amplifiers. However, traditional silica optical fibers exhibit a relatively low nonlinear coefficient at 1550 nm, which is insufficient to meet the key parameter requirements of fiber amplifiers in the communication band. Therefore, obtaining optical fiber media with a higher nonlinear coefficient at the 1550 nm band has become a crucial technological challenge that urgently needs to be overcome.

[0003] For silica optical fibers, nonlinearity can be improved to a certain extent through material manipulation or structural design. Chinese patent CN1721896A designs a highly nonlinear silica optical fiber that can achieve a W / W higher than 10 in the 1550nm band. - 1 km -1 The nonlinear coefficient is 6W. Compared with traditional silica fiber, the nonlinear performance has been significantly improved, but it still cannot meet the high nonlinearity requirements of related fields. In contrast, the rich tunable factors of microstructured fiber provide more options for the design of high nonlinear fiber. The literature [Gilles Mélin. et al. Photonic Crystal Fibers. (2008) SPIE. Vol. 6990, 699003] achieves a nonlinear coefficient of 6W based on the design of photonic crystal fiber with germanium-doped core. -1 km -1 Small effective area, high Raman gain fiber. The flexible structural arrangement of microstructured fibers can provide more design options for highly nonlinear fibers. However, from a manufacturing perspective, the precise proportions, tiny capillary apertures, and unique arrangement positions present technical challenges in realizing the high nonlinear design of microstructured fibers, including process tolerances and fixed positions. Therefore, most microstructured high nonlinear fibers exist only at the design level, and easily manufactured high nonlinear microstructured fibers are lacking.

[0004] From an application perspective, the lack of highly nonlinear optical fibers in communication bands limits the application of amplifiers in these bands. Taking Raman fiber amplifiers as an example, highly nonlinear optical fibers serve as the core gain medium, and key parameters such as the nonlinear coefficient significantly affect the gain performance of the Raman amplifier. Existing Raman amplifiers suffer from many problems, such as limited gain and relatively high noise. Summary of the Invention

[0005] The problem to be solved by the present invention is to provide a high nonlinear photonic crystal fiber and fiber amplifier that address the shortcomings of the prior art. The fiber has a high nonlinear coefficient and is used in the field of fiber amplifiers, where it has the effects of high gain and low noise.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows: It includes a core layer and a cladding layer, wherein the cladding layer includes an inner cladding layer and an outer cladding layer. The core layer's refractive index profile exhibits an α-power-law distribution, with the distribution index α ranging from 2.0 to 8.0, the radius a ranging from 0.8 to 2 μm, and the maximum relative refractive index difference Δ... 1max The content is 2-5%, the inner cladding is an air pore lattice inner cladding, and the outer cladding is a pure silica glass outer cladding.

[0007] According to the above scheme, the core layer distribution index α is 2.3 to 5.0.

[0008] According to the above scheme, the core layer is a silicon dioxide glass layer doped with germanium, bismuth, or a combination of both.

[0009] According to the above scheme, the inner cladding of the air hole lattice is a hexagonal or even-numbered regular polygonal air hole lattice inner cladding, and the core layer is located at the center of the air hole lattice.

[0010] According to the above scheme, each air hole in the air hole array has the same diameter, and the spacing between adjacent air holes is the same.

[0011] According to the above scheme, the air hole lattice is arranged in a dense packing pattern, and the air hole diameter and the spacing between adjacent air holes can be flexibly adjusted according to the dispersion property requirements.

[0012] According to the above scheme, the spacing Λ between adjacent air holes in the inner cladding of the air hole lattice is 2 to 12 μm, and the duty cycle D / Λ is 0.2 to 0.9.

[0013] According to the above scheme, the number of regular polygon layers in the inner cladding of the air hole lattice is 2 to 10.

[0014] According to the above scheme, the inner cladding of the air hole lattice is composed of a pure silica glass substrate and an air hole lattice.

[0015] According to the above scheme, the diameter of the outer cladding layer is 100-300 μm.

[0016] According to the above scheme, the formula for the refractive index distribution of the power function is:

[0017]

[0018] Where n1 is the refractive index of the fiber axis; r is the distance from the fiber axis; a is the fiber core radius; α is the distribution index; and Δ is the relative refractive index difference between the core center and the cladding.

[0019] According to the above scheme, the optical fiber has a strength of ≥18W in the 1550nm band. -1 km -1 The nonlinear coefficients.

[0020] The highly nonlinear photonic crystal fiber of the present invention is used in a backward-pumped fiber amplifier as a gain medium for the fiber amplifier.

[0021] The beneficial effects of this invention are as follows: 1. The large refractive index difference between the high-refractive-index core layer and the low-refractive-index air-hole cladding is beneficial to improving the core's ability to confine light and reducing the effective mode area of ​​the fiber, thereby improving nonlinear performance. 2. The core has a high refractive index and a high nonlinear coefficient, which improves nonlinear characteristics from the perspective of inherent material properties and promotes breakthroughs in the optical functions of nonlinear fibers. 3. The photonic crystal cladding layout surrounded by air-hole lattice can achieve waveguide dispersion modulation through the variable design of air holes, which is beneficial to dispersion compensation of the fiber in different wavelength ranges. 4. The graded-ratio core design can effectively avoid glass viscosity mismatch caused by abrupt changes in right-angle refractive index, reducing the risk of glass rod breakage during deposition. 5. The fabrication process of this invention relies on complete PCVD / MCVD deposition processes and photonic crystal fiber fabrication processes. The process route is mature, production is stable, and it is easy to mass-produce. 6. The graded-ratio high nonlinear photonic crystal fiber provided by this invention is mainly used in the communication field and can be used as the optical gain medium of fiber amplifiers. The corresponding devices have advantages such as low noise and high gain. Attached Figure Description

[0022] Figure 1 This is a radial cross-sectional structural diagram of an optical fiber embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of the refractive index profile of the fiber core layer according to an embodiment of the present invention.

[0024] Figure 3 This is an attenuation spectrum of an optical fiber in one embodiment of the present invention.

[0025] Figure 4 This is a gain diagram of an optical fiber in one embodiment of the present invention.

[0026] Figure 5 This is a radial cross-sectional structural diagram of another optical fiber embodiment of the present invention. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings.

[0028] Example 1: As Figure 1 , Figure 2 As shown, it includes a core layer 1 and a cladding layer, wherein the cladding layer includes an inner cladding layer 2 and an outer cladding layer 3. The refractive index profile of the core layer exhibits an α-power-law distribution, with a distribution index α of 2.6, a radius a of 1.87 μm, and a maximum relative refractive index difference Δ. 1max The fiber has a strength of 3.15% and a core layer made of germanium-doped silica glass. The inner cladding is a hexagonal lattice of air holes, with seven hexagonal layers. The core layer is located at the center of the air hole lattice. The spacing Λ between adjacent air holes in the inner cladding is 3.3 μm, and the duty cycle D / Λ is 0.8. The inner cladding consists of a pure silica glass substrate and the air hole lattice. An outer cladding, also made of pure silica glass, has a diameter of 110 μm. This fiber can achieve a strength greater than 20 W in the 1550 nm band. -1 km -1 The nonlinear coefficient has a decay value of less than 3 dB / km, and the decay spectrum is as follows. Figure 3 As shown. The gain effect of this highly nonlinear photonic crystal fiber is as follows. Figure 4 As shown, at an input power of -10dB, the maximum C-band gain can reach 18.1dB, and the noise is less than 5.3dB.

[0029] Example 2: Core radius a is 1.5 μm, core layer distribution index α is 2.5, and maximum relative refractive index difference Δ 1max The fiber has a strength of 3.2%, an inner cladding layer consisting of a 6-layer hexagonal air-hole lattice with an air-hole spacing of Λ of 2.6 μm, a duty cycle of 0.83, and an outer cladding diameter of 125 μm. This fiber can achieve a power output greater than 20W in the 1550nm band. -1 km -1 The nonlinear coefficient is small, and its attenuation is less than 3 dB / km. With an input power of 0 dB, the fiber achieves a maximum C-band gain of 18.8 dB and noise less than 5.9 dB. Everything else is the same as in Example 1.

[0030] Example 3: Core radius a is 1.25 μm, core layer distribution index α is 3.1, and maximum relative refractive index difference Δ 1max The fiber has a strength of 3.0%, an inner cladding layer consisting of a 7-layer hexagonal air-hole lattice with an air-hole spacing of Λ of 4 μm, a duty cycle of 0.75, and an outer cladding diameter of 118 μm. This fiber can achieve a strength greater than 18.7 W in the 1550 nm band. -1 km -1The nonlinear coefficient is small, and its attenuation is less than 3 dB / km. With an input power of -10 dB, the fiber can achieve a maximum C-band gain of 15 dB and noise less than 6 dB. Everything else is the same as in Example 1.

[0031] Example 4: Core radius a is 1.24 μm, core layer distribution index α is 3.0, and maximum relative refractive index difference Δ 1max The fiber has a strength of 2.95%, an inner cladding layer consisting of a 6-layer hexagonal air-hole lattice with an air-hole spacing of Λ of 4.6 μm, a duty cycle of 0.88, and an outer cladding diameter of 120 μm. This fiber can achieve a strength greater than 18.5 W in the 1550 nm band. -1 km -1 The nonlinear coefficient is small, and its attenuation is less than 3 dB / km. With an input power of 0 dB, the fiber achieves a maximum C-band gain of 17.2 dB and noise less than 5.8 dB. Everything else is the same as in Example 1.

[0032] Example 5: Core radius a is 1.4 μm, core layer distribution index α is 2.2, and maximum relative refractive index difference Δ 1max The fiber exhibits a strength of 3.02%, with the inner cladding employing a six-layer octagonal air-hole lattice, an air-hole spacing of Λ of 3.6 μm, a duty cycle of 0.85, and an outer cladding diameter of 115 μm. This fiber achieves a strength greater than 19.5 W in the 1550 nm band. -1 km -1 The nonlinear coefficient is low, and its attenuation is less than 3 dB / km. With an input power of 0 dB, the fiber can achieve a maximum C-band gain of 18 dB and noise less than 6 dB. See the schematic diagram. Figure 5 .

[0033] Example 6: An optical fiber amplifier is provided, wherein the optical fiber amplifier adopts a back-pumped method, and the optical gain medium of the optical fiber amplifier adopts the graded high nonlinear photonic crystal fiber described above (for example, the graded high nonlinear photonic crystal fiber as described in Examples 1 to 5 can be used).

Claims

1. A highly nonlinear photonic crystal fiber, comprising a core layer and a cladding layer, wherein the cladding layer comprises an inner cladding layer and an outer cladding layer, characterized in that... The core layer refractive index profile is an alpha power function distribution, the core layer distribution index alpha is 2.0-8.0, the radius a is 0.8-2 μm, the maximum relative refractive index difference Δ 1max 2-5%, the inner cladding layer is an air hole lattice inner cladding layer, the outer of the inner cladding layer is an outer cladding layer, and the outer cladding layer is a pure silica glass outer cladding layer.

2. The highly nonlinear photonic crystal fiber according to claim 1, characterized in that... The core layer distribution index α is 2.3 to 5.

0.

3. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The core layer is a silicon dioxide glass layer doped with germanium, bismuth, or a combination of both.

4. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The inner cladding of the air hole lattice is a hexagonal or even-numbered regular polygonal air hole lattice, and the core layer is located at the center of the air hole lattice.

5. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... In the aforementioned air hole lattice, each air hole has the same diameter, and the spacing between adjacent air holes is the same, exhibiting a periodic densely packed arrangement.

6. The highly nonlinear photonic crystal fiber according to claim 5, characterized in that... The spacing Λ between adjacent air holes in the inner cladding of the air hole lattice is 2 to 12 μm, and the duty cycle D / Λ is 0.2 to 0.

9.

7. The highly nonlinear photonic crystal fiber according to claim 5, characterized in that... The number of regular polygon layers in the inner cladding of the air hole lattice is 2 to 10.

8. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The inner cladding of the air hole lattice is composed of a pure silica glass substrate and an air hole lattice.

9. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The diameter of the outer cladding layer is 100–300 μm.

10. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The formula for the refractive index distribution of the α-power function is as follows: Where n1 is the refractive index of the fiber axis; r is the distance from the fiber axis; a is the fiber core radius; α is the distribution index; and Δ is the relative refractive index difference between the core center and the cladding.

11. The highly nonlinear photonic crystal fiber according to claim 1 or 2, characterized in that... The optical fiber has a nonlinear coefficient equal to 18 W -1 km -1 at the 1550 nm wavelength.

12. An optical fiber amplifier, characterized in that... Any of the highly nonlinear photonic crystal fibers of claims 1-11 can be used as the gain medium of a backward-pumped fiber amplifier.

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