Rare earth doped large mode field phosphate fiber

Abstract

A rare earth doped phosphate large mode field optical fiber comprises a core, a first inner cladding, a stress ring layer, a second inner cladding and an outer cladding from inside to outside. The first inner cladding wraps the core, the stress ring layer wraps the first inner cladding, the second inner cladding wraps the stress ring layer, and the outer cladding wraps the second inner cladding. The core has a higher refractive index than the first inner cladding, the stress ring layer has a higher refractive index than the first inner cladding, the second inner cladding has the same refractive index as the first inner cladding, and the second inner cladding has a higher refractive index than the outer cladding. The optical fiber has the characteristics of large mode field area and high batch stability, and can be used in large energy ultra-short pulse fiber lasers, which has great significance for the development of laser radar and atmospheric detection.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology and relates to a rare earth-doped large-mode-field phosphate optical fiber. Technical Background

[0002] In recent years, the demand for high-energy, high-peak-power pulsed fiber lasers has increased dramatically in important fields such as long-range laser sensing, Doppler laser wind radar, and atmospheric detection. The main operating mode of high-energy, high-peak-power pulsed fiber lasers is the master oscillating pulse amplification (MOPA) system, in which the gain fiber is made of rare-earth-doped fiber. Currently, commercial silica fiber is prone to nonlinear effects due to limited rare-earth doping concentration, large numerical aperture, small single-mode diameter, and long length, which limits its output pulse power and energy. Increasing the mode area of ​​the gain fiber can effectively solve these problems. However, increasing the mode area of ​​silica fiber is difficult due to manufacturing limitations.

[0003] In contrast, phosphate glass fiber has high optical uniformity, high rare earth doping concentration, ultra-low NA, ultra-large mode area, and short length, which can easily solve the above problems and effectively improve the pulse energy and power of the output laser.

[0004] Olson et al. used phosphate fiber as the gain medium for Er-doped large-mode-field fiber to achieve an ultra-large core diameter of 60 μm and obtain a pulse energy of 3 mJ (see IEEE Photonics Technology Letters, 32, 2020, 1481-1484).

[0005] Chinese patents CN105826799A and CN106356702B disclose the use of multi-component glasses (including phosphate glass, silicate glass, tellurate glass, etc.) as gain media for rare-earth-doped large-mode-field optical fibers, effectively increasing the threshold energy for nonlinear effects in pulsed lasers. However, in these schemes, multi-component glasses suffer from problems such as high content and easy diffusion of low-temperature components like alkali metals or alkaline earth metals, which severely affect the effective mode area and mode stability of the optical fiber, thereby significantly reducing the yield and batch stability of the fiber and greatly increasing the difficulty of practical application of this scheme. Summary of the Invention

[0006] The purpose of this invention is to provide a rare-earth-doped large-mode-area phosphate fiber. The fiber of this invention has the characteristics of large mode area and high batch stability, and can be used in high-energy ultrashort pulse fiber lasers, which has significant implications for the development of lidar, atmospheric detection and other fields.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A rare-earth-doped large-mode-field phosphate optical fiber is characterized by comprising a core, a first inner cladding, a stress ring layer, a second inner cladding, and an outer cladding. The first inner cladding surrounds the core, the stress ring layer surrounds the first inner cladding, the second inner cladding surrounds the stress ring layer, and the outer cladding surrounds the second inner cladding. The refractive index of the core is higher than that of the first inner cladding, the refractive index of the stress ring layer is higher than that of the first inner cladding, and the refractive index of the second inner cladding is the same as that of the first inner cladding, but higher than that of the outer cladding. The core is a phosphate glass doped with rare-earth activated ions, including Yb, Er, Tm, and Ho ions. The mode of the fiber core is guided by the step refractive index difference between the core and the first inner cladding. The numerical aperture (NA, equal to the square root of the difference between the squares of the refractive indices of the core and the first inner cladding) of the fiber core is between approximately 0.008 and approximately 0.026. The fiber core diameter is 30–100 μm, and the thickness of the first inner cladding is 5–20 μm.

[0009] The stress ring layer has a thickness of 1–10 μm and a refractive index higher than that of the first inner cladding layer, with a refractive index difference of less than 3 × 10⁻⁶. -5 Its coefficient of thermal expansion is greater than that of the first inner cladding glass, with a difference of 5 × 10⁻⁶. -7 K -1 ~30×10 -7 K -1 During the optical fiber drawing process, a stress-photoelastic effect occurs between the core glass and the cladding glass (see Applied Optics, 19, 1980, 2000-2006), causing a change in their refractive index. This is because the different coefficients of thermal expansion between the different glasses during fiber drawing and cooling result in different strains. This strain leads to a change in the refractive index, which can be calculated using the following formula:

[0010] Δn r =n r -n=-B2σ r -B1(σ θ +σ z (1)

[0011] Δn θ =n θ -n=-B2σ θ -B1(σ r +σ z (2)

[0012] Δn z =n z -n=-B2σ z -B1(σ r +σ θ (3)

[0013] Where B1 and B2 are constants; Δn represents the change in refractive index; σ represents stress; and r, θ, and z represent the radial, circumferential, and axial directions of the optical fiber, respectively. The stress ring layer of this invention increases the refractive index of the first inner cladding through the stress photoelastic effect generated during optical fiber drawing, offsetting the decrease in refractive index caused by the diffusion of the fiber core into the first inner cladding. This achieves the beneficial effects of maintaining the core's NA (Near-Nearest) mode area, improving the effective mode area, and enhancing mode stability.

[0014] The second inner cladding glass is the same as the first inner cladding glass.

[0015] The NA between the outer cladding glass and the second inner cladding glass is greater than 0.4.

[0016] Technical effects of the present invention:

[0017] 1. This invention utilizes a stress ring layer structure, which breaks through the limitation of numerical aperture of the fiber core in the existing scheme and solves the problem of low-temperature component diffusion, thereby achieving the beneficial effects of maintaining the core NA, improving the effective mode field area and mode stability.

[0018] 2. The stress ring layer reduces the difference between the refractive index of the first inner cladding and the refractive index of the fiber core, which increases the maximum single-mode diameter of the fiber core and also improves the absorption efficiency of the fiber core for pump light, further shortening the length of the optical fiber.

[0019] 3. The preparation method of the present invention does not employ complex microstructures, which also reduces the difficulty of optical fiber fabrication.

[0020] The optical fiber of this invention has the characteristics of large mode field area and high batch stability, and can be used in high-energy ultrashort pulse fiber lasers, which has significant implications for the development of lidar, atmospheric detection and other fields. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the cross-sectional structure of a rare-earth-doped large-mode-field phosphate optical fiber in Comparative Example 1 without a stress ring.

[0022] Figure 2 This is a schematic diagram of the cross-sectional structure of the rare earth-doped large-mode-field phosphate optical fiber of the present invention when a stress ring is present.

[0023] Figure 3 This is a cross-sectional micrograph of the rare-earth-doped large-mode-field phosphate optical fiber of the present invention in the presence of stress rings. Detailed Implementation

[0024] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0025] Please refer to Figure 2 , Figure 2 This is a schematic cross-sectional view of the rare-earth-doped large-mode-field phosphate fiber of the present invention. As shown in the figure, the rare-earth-doped large-mode-field phosphate fiber of the present invention comprises, from the inside out, a core 110, a first inner cladding 120, a stress ring layer 140, a second inner cladding 150, and an outer cladding 130. The first inner cladding 120 encloses the core 110, the stress ring layer 140 encloses the first inner cladding 120, the second inner cladding 120 encloses the stress ring layer 140, and the outer cladding 130 encloses the second inner cladding 150. The refractive index of the core is higher than that of the first inner cladding. The refractive index of the second inner cladding is higher than that of the first inner cladding. The refractive index of the second inner cladding is the same as that of the first inner cladding, and the refractive index of the second inner cladding is higher than that of the outer cladding. The fiber core is a rare-earth activated ion doped phosphate glass, and the rare-earth activated ions include Yb, Er, Tm, and Ho ions. The mode of the fiber core is guided by the step refractive index difference between the core and the first inner cladding. The numerical aperture (NA) of the fiber core is between approximately 0.008 and approximately 0.026, and the core diameter is 30–100 μm.

[0026] The thickness of the first inner cladding layer 120 is 5–20 μm.

[0027] The stress ring layer 140 has a thickness of 1–10 μm and a refractive index higher than that of the first inner cladding layer, with a refractive index difference of less than 3 × 10⁻⁶. -5 Its coefficient of thermal expansion is greater than that of the first inner cladding glass, with a difference of 5 × 10⁻⁶. -7 K -1 ~30×10 -7 K -1 .

[0028] The stress ring layer is used to increase the refractive index of the first inner cladding through the stress photoelastic effect generated during the fiber drawing process. This counteracts the decrease in refractive index caused by the diffusion of the fiber core into the first inner cladding, thereby maintaining the NA of the fiber core and improving the effective mode area and mode stability.

[0029] The second inner cladding glass is the same as the first inner cladding glass.

[0030] The NA between the outer cladding glass and the second inner cladding glass is greater than 0.4.

[0031] Table 1 summarizes the embodiments of the Yb-doped large-mode-field phosphate optical fiber of the present invention.

[0032]

[0033]

[0034]

[0035] The 60 embodiments shown in Table 1 can all achieve the expected effect of the present invention, that is, to realize the stable output and amplification of large mode field fiber continuous or pulsed laser using stress ring layer.

[0036] Example 1

[0037] Figure 2 An embodiment of the present invention is given (corresponding to the 8th embodiment in Table 1). Figure 2 This is a schematic diagram of the cross-sectional structure of the rare-earth-doped large-mode-field phosphate optical fiber of the present invention when a stress ring layer is present. The arrow indicates the positive direction of the refractive index n, and the solid line on the right represents the refractive index distribution. The core is 110, and the core diameter should be greater than or equal to 30 μm.

[0038] The cladding does not contain rare-earth activating ions and includes a first inner cladding 120, a second inner cladding 150, and an outer cladding 130. The numerical aperture between the core 110 and the first inner cladding 120 is 0.026. The refractive index of the outer cladding 130 is lower than that of the second inner cladding 150, and the numerical aperture between the second inner cladding 150 and the outer cladding 130 is 0.42. The first inner cladding 120 and the second inner cladding 150 should be made of the same glass. The thickness of the first inner cladding 120 should be less than 20 μm.

[0039] The refractive index of the stress ring layer 140 is greater than that of the first inner cladding layer 120, and the difference in refractive index between the two is 1×10⁻⁶. -5 The coefficient of thermal expansion of the stress ring layer 140 is greater than that of the first inner cladding layer 120, and the difference should be greater than 10 × 10. -7 K -1The thickness of the stress ring layer is 2 μm. As described in the previous principle, during the fiber drawing process, low-temperature elements in the fiber core, such as alkali metals or alkaline earth metals, diffuse into the inner cladding, causing a decrease in refractive index and a reduction in effective mode area. After introducing the stress ring layer, during the fiber cooling process, the difference in thermal expansion coefficients between the stress ring layer 140 and the first inner cladding layer 120 will generate strain. This strain will cause stress between the two. According to the stress photoelastic effect, the refractive index of the first inner cladding layer 120 increases, offsetting the change in refractive index caused by core diffusion, maintaining the original NA of the fiber core, and increasing the effective mode area. In this embodiment, the effective mode area of ​​the fiber core is 907 μm. 2 This represents an 85% improvement compared to Comparative Example 1# without the introduced stress ring layer. A micrograph of the optical fiber in this embodiment is shown below. Figure 3 As shown.

[0040] Furthermore, this embodiment only describes a Yb-doped large-mode-field phosphate fiber with an operating wavelength of 1 μm. For example, Er ions with an operating wavelength of 1.5 μm, or Ho ions with an operating wavelength of 2 μm, those skilled in the art can calculate the single-mode diameter corresponding to the operating wavelength using formulas. The single-mode diameter satisfies the following conditions, with normalized frequency: For example, in an optical fiber with a core NA of 0.026, the single-mode diameter is 30 μm for Yb ions operating at a wavelength of 1 μm, and 45 μm for Er ions operating at a wavelength of 1.5 μm. For other rare-earth activated ion-doped large-mode-field phosphate fibers, the requirements of this invention are still met, and the effectiveness of this invention is not affected.

[0041] Comparative Example 1

[0042] To highlight the beneficial effects of the stress ring layer in this invention Figure 1 A Yb-doped phosphate large-mode fiber without a stress ring layer is given as Comparative Example 1. Figure 1 This is a schematic diagram of the cross-sectional structure of a large-mode-field phosphate fiber without a stress ring layer. The arrow indicates the positive direction of the refractive index n, and the solid line on the right represents the refractive index distribution. The core is 110, and the core diameter is 30 μm.

[0043] The cladding does not contain rare earth activating ions and includes an inner cladding 120 and an outer cladding 130. The numerical aperture between the core 110 and the inner cladding 120 should be less than or equal to 0.026. The refractive index of the outer cladding 130 is less than that of the inner cladding 120, and the numerical aperture between the inner cladding 120 and the outer cladding 130 is 0.41.

[0044] Due to the diffusion of low-temperature components in the core 110, the refractive index of the region of the core 110 near the inner cladding 120 decreases, and the effective mode field area is reduced to 490 μm. 2 .

[0045] Experiments show that the optical fiber of this invention has the characteristics of large mode field area and high batch stability, and can be used in high-energy ultrashort pulse fiber lasers, which has significant implications for the development of lidar, atmospheric detection and other fields.

[0046] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the embodiments described above. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A rare-earth-doped large-mode-field phosphate optical fiber, characterized in that... The optical fiber comprises, from the inside out, a core (110), a first inner cladding (120), a stress ring layer (140), a second inner cladding (150), and an outer cladding (130). The first inner cladding (120) encloses the core (110), the stress ring layer (140) encloses the first inner cladding (120), the second inner cladding (120) encloses the stress ring layer (140), and the outer cladding (130) encloses the second inner cladding (150). The refractive index of the core is higher than that of the first inner cladding. The refractive index of the ring layer is higher than that of the first inner cladding layer. The refractive index of the second inner cladding layer is the same as that of the first inner cladding layer, and the refractive index of the second inner cladding layer is higher than that of the outer cladding layer. The fiber core is a phosphate glass doped with rare earth activated ions, including Yb, Er, Tm, and Ho ions. The mode of the fiber core is guided by the step refractive index difference between the core and the first inner cladding layer. The numerical aperture NA between the fiber core and the first inner cladding layer is between 0.008 and 0.026, and the core diameter is 30~100μm.

2. The rare-earth-doped large-mode-field phosphate optical fiber according to claim 1, characterized in that... The thickness of the first inner cladding layer (120) is 5~20μm.

3. The rare-earth-doped large-mode-field phosphate optical fiber according to claim 1, characterized in that... The stress ring layer (140) has a thickness of 1~10μm, and its refractive index is higher than that of the first inner cladding layer, with a refractive index difference of less than 10μm. ; Its coefficient of thermal expansion is greater than that of the first inner cladding glass, with a difference of 1 / 3. .

4. The rare-earth-doped large-mode-field phosphate optical fiber according to claim 1, characterized in that... The stress ring layer increases the refractive index of the first inner cladding through the stress photoelastic effect generated during the fiber drawing process, thereby offsetting the decrease in refractive index caused by the diffusion of the fiber core into the first inner cladding. This achieves the effect of maintaining the numerical aperture (NA) between the fiber core and the first inner cladding, improving the effective mode area and mode stability.

5. The rare-earth-doped large-mode-field phosphate optical fiber according to claim 1, characterized in that... The second inner cladding glass is the same as the first inner cladding glass.

6. The rare-earth-doped large-mode-field phosphate optical fiber according to claim 1, characterized in that... The numerical aperture NA between the outer cladding glass and the second inner cladding glass is greater than 0.4.

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