Double-clad single-mode strain optical fiber

Abstract

The invention relates to the technical field of optical fibers, in particular to a double-cladding single-mode strain optical fiber which comprises a fiber core, an inner cladding and an outer cladding, and a middle layer and a leakage layer are arranged between the inner cladding and the fiber core. The refractive index of the middle layer is lower than that of the fiber core, a refractive index barrier is formed, and a light field is limited from diffusing out of the fiber core; the refractive index of the leakage layer is equal to that of the fiber core, a leakage channel is provided for a high-order mode, the high-order mode cannot meet the total reflection condition due to the insufficient incident angle and can permeate into the leakage layer and be lost, only a fundamental mode (LP01) can be stably transmitted in the fiber core, and the phenomenon that when a laser and an amplifier carry out high-power laser output, the high-order mode is damaged is avoided. The invention solves the problem that the improvement of the laser power and the beam quality are limited due to the fact that the double-cladding single-mode strain optical fiber is easily influenced by nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), meanwhile, the problem of multimode interference is avoided, and the improvement of the laser power and the beam quality in the double-cladding single-mode strain optical fiber and the high stability and low mode field distortion of single-mode transmission are ensured.

Description

Technical Field

[0001] This invention relates to the field of optical fiber technology, specifically to a double-clad single-mode strained optical fiber. Background Technology

[0002] Double-clad single-mode strained fiber is a special type of fiber with a core design based on "double cladding" and "strain introduction." Double-clad fibers are widely used in cladding-pumped high-power fiber lasers and amplifiers; these devices offer relatively high conversion efficiency and beam quality. These fibers are primarily used in industrial cutting, welding, laser processing, medical surgery (such as laser minimally invasive surgery), and defense industries (such as lidar and directed energy weapons). Most existing double-clad fibers use solid glass as the inner cladding and a low-refractive-index polymer coating as the outer cladding. This type of fiber has significant limitations: firstly, the polymer coating has a low molding and softening temperature, limiting its operating temperature; secondly, in high-power applications, if the fiber generates significant heat due to laser absorption in the matrix but cannot dissipate it in time, or if the pump light or laser beam is not completely confined by the inner cladding and radiates to the outer cladding, it can easily lead to serious safety accidents such as coating burn-out and laser leakage.

[0003] To address the aforementioned problems, existing technologies have proposed several solutions. For example, Chinese Patent Application No. CN202022815344.1 discloses a cladding optical filtering device and a fiber laser. This solution includes a double-clad optical fiber and a filter assembly connected to the double-clad optical fiber. The double-clad optical fiber includes a fiber core, an inner cladding disposed around the outer wall of the fiber core, and an outer cladding and a coating disposed around the outer wall of the inner cladding. The inner cladding covers the outer wall of the fiber core and includes a covered section and an exposed section. The outer cladding and the coating are both disposed opposite to the covered section. The filter assembly includes two or more filters disposed against the outer wall of the exposed section, and the filters are spaced apart. The method involves stripping part of the outer cladding and coating of a double-clad fiber and attaching two or more filters at intervals to remove the cladding light in segments. This avoids the concentrated removal of a large amount of residual pump light and prevents the heat generated by removing residual pump light from concentrating. This increases the maximum power that the cladding light filtering device and the fiber laser can withstand. However, the above method still has some problems. When the laser and amplifier are outputting high-power lasers, they are easily affected by nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), which limits the improvement of laser power and beam quality. Summary of the Invention

[0004] The purpose of this invention is to provide a double-clad single-mode strained fiber to solve the problem that when double-clad single-mode strained fiber is used in lasers or amplifiers for high-power laser output, it is easily affected by nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), which limits the improvement of laser power and beam quality.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A double-clad single-mode strained optical fiber includes a core, an inner cladding, and an outer cladding. The inner and outer claddings sequentially wrap around the core. An intermediate layer and a leakage layer are disposed between the inner cladding and the core. The leakage layer is located outside the intermediate layer. The refractive index of the intermediate layer is lower than that of the core, and the refractive index of the leakage layer is equal to that of the core. The refractive index of the inner cladding is lower than that of the intermediate layer, and the refractive index of the outer cladding is lower than that of the inner cladding. The intermediate layer has a lower refractive index than the core, forming a "refractive index barrier" that restricts the diffusion of the light field outward from the core. The refractive index of the leakage layer is equal to that of the core. It provides a "leakage channel" for higher-order modes, which cannot meet the total internal reflection condition due to insufficient incident angle. They will seep into the leakage layer and be lost. Only the fundamental mode (LP01) can be stably transmitted in the fiber core. This avoids the problem that lasers and amplifiers are easily affected by nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) when outputting high-power lasers, which limits the improvement of laser power and beam quality. At the same time, it also avoids multimode interference problems, ensuring the improvement of laser power and beam quality in double-clad single-mode strained fiber, as well as the high stability and low mode field distortion of single-mode transmission.

[0007] Preferably, the radial cross-section of the fiber core is set as a transmission surface, the transmission surface is elliptical, the major axis of the transmission surface is 20-40 μm, the minor axis of the transmission surface is 10-20 μm, and the ellipticity of the transmission surface is controlled between 1.5 and 3 times. The thickness of the leakage layer is greater than 1.5 times the radius of the transmission surface and less than 2 times the radius of the fiber core. Traditional circular fiber cores are prone to random birefringence under external stress, causing fluctuations in the propagation speed difference between the two polarization states of the optical signal, resulting in pulse broadening during long-distance transmission (e.g., pulse broadening caused by PMD in 100km transmission > 5ps). Fixed geometric birefringence: ellipticity (1.5-3 times) makes the refractive index difference Δn in the major and minor axis directions ≈ 1×10 -4 This forms a stable polarization principal axis, controlling the PMD coefficient to <0.05ps / √km (1550nm wavelength), making it suitable for high-speed communication above 10Gbps, and ensuring high polarization retention and single-mode purity of double-clad single-mode strained fiber.

[0008] Preferably, a buffer layer is provided between the inner cladding and the outer cladding. The radial cross-section of the buffer layer is W-shaped, and the refractive index of the buffer layer is higher than that of the outer cladding. Under external mechanical loads (such as axial tension and radial compression), the stress of the outer cladding is directly transferred to the inner cladding, resulting in non-uniform strain (strain deviation > 15%) in the fiber core. At the same time, through the design of the W-shaped cross-section structure, double-clad fiber can simultaneously improve optical field confinement, heat dissipation capacity and cost control in high-power applications. It is especially suitable for high-temperature and high-power scenarios such as industrial laser processing and national defense, ensuring the improvement of laser power and beam quality in double-clad single-mode strained fiber.

[0009] Preferably, the outer layer is provided with a coating layer, which includes an inner coating and an outer coating. The outer coating is disposed outside the inner coating. The inner coating is made of low-modulus UV-curable acrylate or silicone rubber, and the outer coating is made of high-modulus UV-curable acrylate or polyimide.

[0010] Preferably, the coating layer is further provided with a chemically expandable flame retardant layer and a physically expandable heat insulation layer. The chemically expandable flame retardant layer is made of a material system including ammonium polyphosphate, pentaerythritol, and melamine. The physically expandable heat insulation layer is provided with multiple heat insulation holes, and the porosity of the multiple heat insulation holes on the physically expandable heat insulation layer is >90%. Traditional optical fiber coatings are mostly polymer materials, which are easily flammable when exposed to open flames, leading to damage to the fiber core structure. The flame-retardant layer material decomposes and dehydrates to form a carbonized layer at high temperatures, creating a dense, expanded carbon layer that isolates oxygen and heat conduction, preventing the flame from spreading along the optical fiber path. The melamine system releases non-flammable gases such as nitrogen during decomposition, diluting the concentration of flammable gases and reducing the generation of toxic gases such as carbon monoxide and hydrogen halides, thus mitigating safety hazards in a fire. High temperatures during a fire may cause a sudden change in the fiber core's refractive index; the carbonized layer's heat insulation effect buffers this temperature surge, preventing mode distortion or loss of transmitted optical signals due to thermal stress. Through this dual protection mechanism of "flame retardant and heat insulation," the structural damage and signal attenuation problems of traditional optical fibers in high-temperature and fire environments are solved, ensuring stable mechanical and optical transmission performance even in harsh environments such as oil exploration, power transmission, and aerospace.

[0011] Preferably, the inner cladding is made of fluoride glass, and an air layer is formed on the inner cladding. The air layer consists of two layers of air holes arranged in regular octagons. Traditional quartz inner claddings, when used for high-power laser transmission (e.g., >5W), suffer from nonlinear refractive index (n²≈2.6×10⁻⁶). ˉ2o m 2 / W) induces effects such as self-phase modulation (SPM) and stimulated Raman scattering (SRS), leading to spectral broadening and energy loss, while fluoride glasses (such as ZBLAN) have n2≈0.3×10 -20 m 2With a power density of only 1 / 9 that of quartz, the nonlinear threshold can be increased to >15W, making it suitable for high-power fiber lasers and amplifiers. Traditional circular inner cladding has limited ability to confine pump light, resulting in some energy remaining unabsorbed by the fiber core after multiple reflections (absorption efficiency <70%). The periodic arrangement of air holes forms a photonic bandgap, confining the pump light to the inner cladding region, extending the propagation path of light in the gain medium, and increasing the absorption efficiency to >90%. The regular octagonal structure destroys the rotational symmetry of the inner cladding, making the pump light mode more uniformly distributed, reducing the energy "dead zone" and ensuring the transmission capability of high-power single-mode lasers in double-clad single-mode strained fibers.

[0012] Preferably, the physically expanding thermal insulation layer is provided with multiple protective sleeves, each protective sleeve including a supporting part and a fixing part, with a connector between the supporting part and the fixing part. The supporting part and the fixing part are respectively provided with a convex ring and a concave ring at their ends, and the multiple protective sleeves are interlocked and connected together. When the optical fiber bending angle exceeds a critical value (e.g., approximately 11.5° when the bending radius of a single-mode fiber is <20mm), the optical field in the fiber core is disrupted due to total internal reflection, and some energy leaks to the cladding, resulting in a significant increase in loss (e.g., when the bending radius of a single-mode fiber is <20mm). (Loss can increase by more than 0.5dB when the diameter is 10mm). By cooperating with multiple protective sleeves, the bending degree of the double-clad single-mode strained fiber is controlled, so that the optical field mode is kept in the fiber core for transmission, avoiding energy attenuation that affects communication quality or sensing accuracy. At the same time, the interlocking of the convex and concave rings and the locking of the connectors of the support and fixing parts form a continuous and firm mechanical connection, preventing the protective sleeves from falling off or disintegrating under stress, significantly improving structural integrity, and ensuring the stability, mechanical reliability and service life of low-loss laser transmission in the double-clad single-mode strained fiber.

[0013] Preferably, multiple heat dissipation grooves are formed on the outer surface of the multiple protective sleeves, and heat dissipation fins are provided between the multiple heat dissipation grooves; when the optical fiber transmits at high power for a long time (such as laser energy transmission with power ≥1W) or the ambient temperature rises (such as ≥60℃ in industrial scenarios), heat accumulation causes the fiber core temperature to exceed the glass transition temperature (quartz glass is about 1100℃, but the physical expansion type heat insulation layer is only 80-120℃), resulting in thermal distortion of the fiber core refractive index, causing mode field mismatch, and increasing the loss by more than 0.2dB / km; the heat dissipation grooves and heat dissipation fins accelerate heat convection by increasing the surface area of ​​the protective sleeve, controlling the fiber core temperature at ≤60℃ (the safety threshold of the physical expansion type heat insulation layer), avoiding sudden loss changes, and ensuring the low-loss transmission stability of the double-clad single-mode strained optical fiber in high-temperature environments.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] 1. This invention provides an intermediate layer and a leakage layer between the inner cladding and the fiber core. The intermediate layer has a lower refractive index than the fiber core, forming a "refractive index barrier" that restricts the light field from spreading outward from the fiber core. The leakage layer has the same refractive index as the fiber core, providing a "leakage channel" for higher-order modes. Higher-order modes, due to insufficient incident angles, cannot meet the total internal reflection condition and will penetrate into the leakage layer and be lost. Only the fundamental mode can be stably transmitted in the fiber core. This avoids the influence of nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) on the laser and amplifier when performing high-power laser output, thus ensuring the improvement of laser power and beam quality in double-clad single-mode strained fiber.

[0016] 2. This invention designs the radial cross-section of the fiber core as the transmission surface. The transmission surface is elliptical, with a major axis of 20–40 μm and a minor axis of 10–20 μm. Traditional circular fiber cores are prone to random birefringence under external stress, causing fluctuations in the propagation speed difference between the two polarization states of the optical signal, resulting in pulse broadening during long-distance transmission. Fixed geometric birefringence: the ellipticity makes the refractive index difference Δn ≈ 1 × 10⁻⁶ along the major and minor axes. -4 This forms a stable polarization principal axis, controlling the PMD coefficient to <0.05ps / √km (1550nm wavelength), making it suitable for high-speed communication above 10Gbps, and ensuring high polarization retention and single-mode purity of double-clad single-mode strained fiber.

[0017] 3. This invention incorporates a buffer layer between the inner and outer cladding layers, with the buffer layer having a higher refractive index than the outer cladding layer. Under external mechanical loads (such as axial tension and radial compression), the stress in the outer cladding layer of a traditional planar buffer layer is directly transferred to the inner cladding layer, resulting in non-uniform strain (strain deviation > 15%) in the fiber core. Simultaneously, through the design of the W-shaped profile structure, the double-clad fiber can simultaneously improve optical field confinement, heat dissipation, and cost control in high-power applications. It is particularly suitable for high-temperature and high-power scenarios such as industrial laser processing and national defense, ensuring the improvement of laser power and beam quality in double-clad single-mode strained fiber. Attached Figure Description

[0018] Figure 1 This is a forward axonometric view of the double-clad single-mode strain fiber of the present invention.

[0019] Figure 2 This is a back-axis isometric view of the double-clad single-mode strain fiber of the present invention;

[0020] Figure 3 This is a front view of the double-clad single-mode strained optical fiber of the present invention.

[0021] Figure 4 This is a cross-sectional view at point AA in section 3 of the present invention;

[0022] Figure 5 This is an exploded view of the double-clad single-mode strain fiber of the present invention.

[0023] In the diagram: 101, fiber core; 102, transmission surface; 2, intermediate layer; 3, leakage layer; 4, inner cladding layer; 5, buffer layer; 6, outer cladding layer; 701, coating layer; 702, inner coating; 703, outer coating; 704, chemically expanding flame retardant layer; 705, physically expanding heat insulation layer; 706, heat insulation hole; 801, air layer; 802, air hole; 901, protective sleeve; 902, support part; 903, fixing part; 904, connector; 905, convex ring; 906, concave ring; 907, heat dissipation groove; 908, heat dissipation fins. Detailed Implementation

[0024] Please see Figures 1 to 5 This invention provides a double-clad single-mode strain optical fiber, the technical solution of which is as follows:

[0025] A double-clad single-mode strained fiber, please refer to [link / reference]. Figures 1 to 5The fiber includes a core 101, an inner cladding 4, and an outer cladding 6. The inner cladding 4 and the outer cladding 6 are sequentially wrapped around the core 101. An intermediate layer 2 and a leakage layer 3 are provided between the inner cladding 4 and the core 101. The leakage layer 3 is located outside the intermediate layer 2. The refractive index of the intermediate layer 2 is less than that of the core 101, and the refractive index of the leakage layer 3 is equal to that of the core 101. The refractive index of the inner cladding 4 is less than that of the intermediate layer 2, and the refractive index of the outer cladding 6 is less than that of the inner cladding 4. The radial cross-section of the core 101 is designated as a transmission surface 102. The transmission surface 102 is elliptical, and its major axis is... The fiber core 101 has a diameter of 35 μm, a minor axis of 16 μm, and an ellipticity controlled between 1.5 and 3 times. The thickness of the leakage layer 3 is greater than 1.5 times the radius of the transmission surface 102 and less than 2 times the radius of the fiber core 101. A buffer layer 5 is provided between the inner cladding layer 4 and the outer cladding layer 6. The radial section of the buffer layer 5 is W-shaped. The refractive index of the buffer layer 5 is higher than that of the outer cladding layer 6. A coating layer 701 is provided outside the outer cladding layer 6. The coating layer 701 includes an inner coating 702 and an outer coating 703. The outer coating 703 is disposed outside the inner coating 702. The inner coating 702 uses low-modulus ultraviolet light. The outer coating 703 is made of high-modulus UV-curable acrylate or silicone rubber. Outside the coating 701, there is a chemically expanded flame-retardant layer 704 and a physically expanded heat-insulating layer 705. The chemically expanded flame-retardant layer 704 is made of a material system including ammonium polyphosphate, pentaerythritol, and melamine. The physically expanded heat-insulating layer 705 has multiple heat-insulating holes 706 with a porosity >90%. The inner cladding layer 4 is made of fluoride glass and has an air layer 801. The air layer 801 consists of two layers of octagonal air holes 802. The physical expansion type heat insulation layer 705 is provided with multiple protective sleeves 901. The multiple protective sleeves 901 include a supporting part 902 and a fixing part 903. A connector 904 is provided between the supporting part 902 and the fixing part 903. The first and last ends of the supporting part 902 and the fixing part 903 are respectively provided with a convex ring 905 and a concave ring 906. The multiple protective sleeves 901 are interlocked and connected together. Multiple heat dissipation grooves 907 are opened on the outer surface of the multiple protective sleeves 901. Heat dissipation fins 908 are provided between the multiple heat dissipation grooves 907.

[0026] When working, please refer to Figures 1 to 5The laser first enters the fiber core 101, whose elliptical cross-section (major axis 35μm, minor axis 16μm, ellipticity 1.5 to 3 times) achieves single-mode transmission through geometric constraints, avoiding multimode interference. Because the refractive index of core 101 is higher than that of intermediate layer 2, the laser undergoes total internal reflection at the interface between core 101 and intermediate layer 2, propagating along the fiber axis. The refractive index of intermediate layer 2 is lower than that of core 101, further strengthening the total internal reflection condition and restricting the optical field distribution. The refractive index of leakage layer 3 is equal to that of core 101, allowing the higher-order modes leaking from core 101 to be lost here (because the refractive index of inner cladding layer 4 is lower than that of intermediate layer 2, higher-order modes cannot undergo total internal reflection in inner cladding layer 4 and are thus "absorbed" by leakage layer 3), ensuring that only the fundamental mode propagates in core 101. The refractive index of inner cladding layer 4 is lower than that of intermediate layer 2, forming a secondary constraint on the optical field. Its internal octagonal air hole 802 structure can adjust the mode field distribution and reduce nonlinear effects. At the same time, the low refractive index environment of air hole 802 further restricts the optical field expansion and improves the stability of single-mode transmission. The refractive index of outer cladding layer 6 is lower than that of inner cladding layer 4, constituting the outermost constraint boundary of the optical field. Buffer layer 5 is W-shaped and has a higher refractive index than outer cladding layer 6. Its geometry affects laser transmission. The outer cladding 6 acts as a buffer against thermal stress caused by temperature changes, preventing micro-cracks from forming due to thermal expansion and contraction. The inner low-modulus material (UV-cured acrylate or silicone rubber) has no direct impact on laser transmission, but its elasticity can adapt to the micro-deformation of the optical fiber, reducing the interference of mechanical stress on light transmission. The outer high-modulus material (UV-cured acrylate or polyimide) has high hardness, which can suppress excessive bending of the optical fiber and also provides preliminary thermal insulation against temperature rise during laser transmission. The chemically expanded flame-retardant layer 704 (ammonium polyphosphate and other systems) decomposes at high laser temperatures to produce inert gas and a carbonized layer, preventing heat diffusion. The physically expanded thermal insulation layer 705 (porosity > 90%) reduces heat conduction through the air insulation holes 706, preventing the outer material from overheating and failing during laser transmission. The interlocking structure of the protective sleeve 901 (convex ring 905 + concave ring 906) provides rigid support for the optical fiber, and the heat dissipation grooves 907 and fins can accelerate the dissipation of heat generated during laser transmission, preventing local overheating that could lead to a decrease in optical fiber performance.

[0027] The specific embodiment of the present invention has been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the embodiments described above. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principles and ideas of the present invention should still fall within the protection scope of the present invention.

Claims

1. A double-clad single-mode strain optical fiber, characterized in that, The fiber includes a core (101), an inner cladding (4), and an outer cladding (6). The inner cladding (4) and the outer cladding (6) are sequentially wrapped around the core (101). An intermediate layer (2) and a leakage layer (3) are provided between the inner cladding (4) and the core (101). The leakage layer (3) is located outside the intermediate layer (2). The refractive index of the intermediate layer (2) is less than that of the core (101). The refractive index of the leakage layer (3) is equal to that of the core (101). The refractive index of the inner cladding (4) is less than that of the intermediate layer (2). The refractive index of the outer cladding (6) is less than that of the inner cladding (4).

2. The double-clad single-mode strain optical fiber according to claim 1, characterized in that: The radial cross section of the fiber core (101) is set as the transmission surface (102), which is elliptical. The major axis of the transmission surface (102) is 20-40 μm, the minor axis is 10-20 μm, and the ellipticity of the transmission surface (102) is controlled between 1.5 and 3 times. The thickness of the leakage layer (3) is greater than 1.5 times the radius of the transmission surface (102) and less than 2 times the radius of the fiber core (101).

3. The double-clad single-mode strain optical fiber according to claim 2, characterized in that: A buffer layer (5) is provided between the inner cladding layer (4) and the outer cladding layer (6). The radial section of the buffer layer (5) is W-shaped, and the refractive index of the buffer layer (5) is higher than that of the outer cladding layer (6).

4. The double-clad single-mode strain optical fiber according to claim 3, characterized in that: The outer layer (6) is provided with a coating layer (701), the coating layer (701) includes an inner coating layer (702) and an outer coating layer (703), the outer coating layer (703) is disposed outside the inner coating layer (702), the inner coating layer (702) is made of low modulus UV-curable acrylate or silicone rubber, and the outer coating layer (703) is made of high modulus UV-curable acrylate or polyimide.

5. The double-clad single-mode strain optical fiber according to claim 4, characterized in that: The coating layer (701) is further provided with a chemically expandable flame retardant layer (704) and a physically expandable heat insulation layer (705). The chemically expandable flame retardant layer (704) is made of a material system including ammonium polyphosphate, pentaerythritol and melamine. The physically expandable heat insulation layer (705) is provided with a plurality of heat insulation holes (706), and the porosity of the plurality of heat insulation holes (706) on the physically expandable heat insulation layer (705) is >90%.

6. The double-clad single-mode strain optical fiber according to claim 1, characterized in that: The inner cladding (4) is made of fluoride glass, and an air layer (801) is provided on the inner cladding (4). The air layer (801) consists of two layers of air holes (802) arranged in regular octagons.

7. The double-clad single-mode strain optical fiber according to claim 5, characterized in that: The physical expansion type heat insulation layer (705) is provided with multiple protective sleeves (901). Each of the multiple protective sleeves (901) includes a supporting part (902) and a fixing part (903). A connector (904) is provided between the supporting part (902) and the fixing part (903). The first and last ends of the supporting part (902) and the fixing part (903) are respectively provided with a convex ring (905) and a concave ring (906). The multiple protective sleeves (901) are interlocked and connected together.

8. The double-clad single-mode strain optical fiber according to claim 7, characterized in that: Multiple heat dissipation grooves (907) are provided on the outer surface of the multiple protective sleeves (901), and heat dissipation fins (908) are provided between the multiple heat dissipation grooves (907).

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