Cylindrical vector optical fiber amplifier based on rotary conical optical fiber

Abstract

The invention discloses a cylindrical vector optical fiber amplifier based on a rotary conical optical fiber. The cylindrical vector optical fiber amplifier comprises a cylindrical vector optical seed source; an isolator, a beam combiner, a rotary conical optical fiber and a cladding light pumping stripper are sequentially arranged in the column vector light transmission direction of the column vector light seed source, and a first pumping source is further welded to the input end of the beam combiner. The rotary conical optical fiber is adopted as the gain optical fiber, the stability of light polarization can be effectively kept in the cylindrical vector light amplification process, the sensitivity to temperature is reduced, and meanwhile the mode purity is improved.

Description

Technical Field

[0001] The present invention relates to the technical field, and in particular to a cylindrical vector optical fiber amplifier based on a rotated tapered optical fiber. Background Art

[0002] Column vector lasers include radially polarized light (TM01 mode) and angularly polarized light (TE01 mode), which have the characteristics of axially symmetrical polarization and annular intensity distribution. Under strong focusing conditions, they will form a unique and strong local longitudinal electric field, and have unique application advantages in the fields of industrial processing, material processing, particle manipulation, microscopic imaging, etc. For example, the use of column vector light for laser welding or cutting can increase the processing speed by 1.5-2 times, and straight holes with high aspect ratio can be obtained when laser drilling, and better bionic surface structures can be obtained when processing micro-nano materials.

[0003] At present, the commonly used methods for generating cylindrical vector light include the following: one is to use a spatial conversion element (such as a birefringent crystal, a sub-wavelength grating mirror, a Q wave plate, an S wave plate, etc.) to convert the LP01 mode into a TM01 mode or a TE01 mode, and then inject it into a gain fiber (such as a large mode field ytterbium-doped fiber, a large mode field erbium-doped fiber, etc.) for amplification; another is to use a spatial conversion element to convert the LP01 mode into a TM01 mode or a TE01 mode, and then inject it into a single crystal fiber or a thermally conductive fiber rod for conversion; and another is to use a fiber conversion device (such as a mechanical long period grating) combined with a MOPA structure and inject it into the gain fiber for amplification.

[0004] The above scheme mainly adopts different mode selection schemes combined with various gain fibers for amplification. For example, single crystal fiber or thermal conductive fiber rod can be used as the amplification gain medium of column vector light, but due to the limitation of its own size and doping concentration, the amplification efficiency is low, the thermal effect is high, the heat generation is large, the manufacturing process is relatively complex, and the cost is high; and when isotropic large mode field fiber is used as the gain medium, the inhomogeneity inside the fiber (including bending, lateral compression, elliptical core, and inherent stress caused by changes in dopant distribution, etc.) causes the random change of internal birefringence, resulting in the distortion of spatial intensity and polarization distribution during transmission and amplification, which limits the beam quality performance, and the fundamental mode will be amplified synchronously with the high-order mode during the amplification process, resulting in a decrease in mode purity.

[0005] Several solutions have been proposed for the above problems: one is to add a half-wave plate between the column vector seed light and the amplifier stage for phase compensation; another is to effectively compensate for the depolarization of radially polarized light by combining a birefringent 90° quartz optical rotation crystal with a 4f imaging system; and another is to combine a half-wave plate with a Dove prism and rotate the birefringent gain medium for purity compensation. However, the above methods are only applicable to spatial light structures, not to all-fiber structures. In addition, an all-fiber polarization-maintaining fiber amplifier can be used, which can effectively solve the problem of polarization purity and mode purity degradation during power amplification. However, due to the need to use high-quality polarization-maintaining fiber and related optical devices (such as polarization-maintaining fiber couplers, isolators, etc.), the cost of all-fiber polarization-maintaining amplifiers is usually high, and high-power fiber amplifiers generate a lot of heat during operation, and the heat dissipation design is relatively complicated. Due to its compactness, the heat dissipation problem of the all-fiber polarization-maintaining structure may be more prominent. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a column vector fiber amplifier based on rotated tapered fiber, which adopts rotated tapered fiber (sT-DCF) as the gain fiber. It can effectively maintain the stability of light polarization during column vector light amplification, reduce its sensitivity to temperature, and improve mode purity.

[0007] The present invention discloses a column vector optical fiber amplifier based on a rotating tapered optical fiber, comprising: a column vector optical seed source;

[0008] An isolator, a beam combiner, a rotating tapered optical fiber and a cladding light pump stripper are sequentially arranged in the transmission direction of the column vector light of the column vector light seed source, and the input end of the beam combiner is also fused with a first pump source.

[0009] As a further improvement of the present invention, the first pump source is a semiconductor laser with a central wavelength of 976 nm, the beam combiner is a (2+1)×1 beam combiner, and the cladding pump stripper is connected at an angle of 8°.

[0010] As a further improvement of the present invention, the column vector light seed source includes a second pump source, a wavelength division multiplexer, a gain fiber, a coupler, a first polarization controller, a polarization-dependent isolator, a second polarization controller and a mode selective coupler;

[0011] The second pump source is fused to the input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is fused to the input end of the gain fiber (single-clad single-mode ytterbium-doped fiber SM-YSF-HI-HP) through a pigtail, the output end of the gain fiber is fused to the input end of a 1×2 coupler, the first output end of the coupler is used to output laser, the second output end of the coupler is fused to the input end of the first polarization controller, the output end of the first polarization controller is fused to the input end of the polarization-dependent isolator, the output end of the polarization-dependent isolator is fused to the input end of the second polarization controller, the output end of the second polarization controller is fused to the input end of the mode selective coupler, and the output end of the mode selective coupler is fused to the input end of the wavelength division multiplexer to form a ring cavity.

[0012] As a further improvement of the present invention, the second pump source is a semiconductor laser with a central wavelength of 976 nm, and the coupler is a 1×2 coupler.

[0013] As a further improvement of the present invention, the light emitted by the second pump source is coupled into the gain fiber through a wavelength division multiplexer, the light output from the gain fiber passes through a coupler, the coupler outputs a part of the light, and the other part is polarized by a first polarization controller. The output light passes through a polarization-dependent isolator to convert the light into linearly polarized light, and the light at the output end of the polarization-dependent isolator passes through a second polarization controller to convert it into elliptically polarized light. Due to the nonlinear effect in the cavity, when the light passes through the first polarization controller to achieve nonlinear deflection locking, the output light of the second polarization controller passes through a mode selective coupler and then outputs LP 11 The light of the mode resonates in the cavity.

[0014] As a further improvement of the present invention, the structure of the rotated tapered optical fiber is that a rotation pitch is formed on the outer surface of the tapered optical fiber.

[0015] As a further improvement of the present invention, the method for preparing the rotated tapered optical fiber is: in the process of drawing a preform rod along the X direction to form a tapered optical fiber, the preform rod is simultaneously rotated around the X axis to finally obtain the rotated tapered optical fiber.

[0016] As a further improvement of the present invention, the preform is a step-refractive index preform, and the refractive index of each layer is required to have a first refractive index n 1 The core part and a cladding with a refractive index n lower than the first refractive index 2 (n 1 >n 2), the pitch length of the optical fiber varies in the range of 30 to 31 mm with the length of the optical fiber of 2.5 m. In order to improve the absorption rate of the cladding to the pump, the cladding is cut into 4 times, and the shape is similar to a square with rounded corners. The core cladding diameter of the preform is 11.5, and the core varies from 12.5 to 37.5 μm. Its cone angle is 1×10 -5 .

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The column vector fiber amplifier of the present invention adopts a rotated tapered fiber (sT-DCF) as a gain fiber. The rotated tapered fiber can effectively solve the problem that the temperature sensitivity of birefringence causes a sharp change in birefringence during the amplification process of column vector light, which leads to the unstable polarization state of the output light, thereby improving the polarization purity of the fiber amplifier; the rotated tapered fiber combines the two structures of the rotated fiber and the tapered fiber, and can be used as a multifunctional composite technology, which has great potential for transmitting high-power short pulse signals with complex spatial and polarization distributions. At the same time, the reasonable design of the rotated tapered fiber can realize the mode coupling of the fundamental mode and the high-order mode during the amplification process, so that the mode purity of the column vector light after amplification can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of a cylindrical vector fiber amplifier based on a rotated tapered fiber disclosed in the present invention;

[0020] Figure 2 for Figure 1 Schematic diagram of the central column vector light seed source.

[0021] In the figure:

[0022] 1. Column vector light seed source; 2. Isolator; 3. First pump source; 4. Beam combiner; 5. Rotating tapered optical fiber; 6. Cladding light pump stripper; 7. Second pump source; 8. Wavelength division multiplexer; 9. Gain fiber; 10. Coupler; 11. First polarization controller; 12. Polarization-dependent isolator; 13. Second polarization controller; 14. Mode selective coupler. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0025] The present invention provides a column vector fiber amplifier based on a rotated tapered fiber. The column vector fiber amplifier adopts a rotated tapered fiber (sT-DCF) as a gain fiber. During the column vector light amplification process, the stability of light polarization can be effectively maintained, the sensitivity to temperature can be reduced, and mode purity can be improved. Since the fiber has both a rotation structure and a tapered structure, it has the advantages of both fibers.

[0026] like Figure 1 As shown, the column vector fiber amplifier of the present invention comprises: a column vector light seed source 1, an isolator 2, a first pump source 3, a combiner 4, a rotated tapered optical fiber 5 and a cladding light pump stripper 6; the isolator 2, the combiner 4, the rotated tapered optical fiber 5 and the cladding light pump stripper 6 are sequentially arranged in the column vector light transmission direction of the vector light seed source 1, and the first pump source 3 is also fused to the input end of the combiner 4. Specifically: a column vector light seed source 1 generates column vector light, and an isolator 2 is fused behind the column vector light seed source 1 to prevent reverse light from damaging the column vector light seed source 1; the output end of the isolator 2 is fused with the input end of the combiner 4, and the first pump source 3 uses a semiconductor laser with a central wavelength of 976nm, which is fused with the input end of the (2+1)×1 combiner 4, and the output end of the combiner 4 is fused with a rotated tapered optical fiber (special gain optical fiber) 5, and forward pumping is used to couple the pump light to the rotated tapered optical fiber for amplification; the other end of the rotated tapered optical fiber 5 is connected to a cladding light pump stripper 6 to strip off excess pump light; the cladding pump stripper 6 is connected to an 8° angle output to reduce return light.

[0027] like Figure 2As shown, the column vector light seed source 1 of the present invention includes a second pump source 7, a wavelength division multiplexer 8, a gain fiber 9, a coupler 10, a first polarization controller 11, a polarization-dependent isolator 12, a second polarization controller 13 and a mode selection coupler 14; wherein the second pump source 7 is fused with the input end of the wavelength division multiplexer 8, the output end of the wavelength division multiplexer 8 is fused to the input end of the gain fiber (single-clad single-mode ytterbium-doped fiber SM-YSF-HI-HP) 9 through a pigtail, and the output end of the gain fiber 9 is fused with the input end of the 1×2 coupler 10. The first output end of the coupler 10 is used to output laser, the second output end of the coupler 10 is fused to the input end of the first polarization controller 11, the output end of the first polarization controller 11 is fused to the input end of the polarization-dependent isolator 12, the output end of the polarization-dependent isolator 12 is fused to the input end of the second polarization controller 13, the output end of the second polarization controller 13 is fused to the input end of the mode selective coupler 14, and the output end of the mode selective coupler 14 is fused to the input end of the wavelength division multiplexer 8 to form a ring cavity. Based on this, the light emitted by the second pump source 7 is coupled into the gain fiber 9 through the wavelength division multiplexer 8, and the light output by the gain fiber 9 passes through the coupler 10. The coupler 10 outputs a part of the light, and the other part is polarized by the first polarization controller 11. The output light passes through the polarization-dependent isolator 12 to convert the light into linear polarized light. The light at the output end of the polarization-dependent isolator 12 passes through the second polarization controller 13 to convert it into elliptically polarized light. Due to the nonlinear effect in the cavity, when the light passes through the first polarization controller 14 to achieve nonlinear deflection locking, the output light of the second polarization controller 13 passes through the mode selection coupler 14 and then outputs LP 11 The light of the mode resonates in the cavity.

[0028] The structure of the rotating tapered optical fiber 5 of the present invention is that a rotating pitch is formed on the outer surface of the tapered optical fiber, and its preparation method is: using a step-index preform rod, and realizing the tapering of the optical fiber by continuously changing the drawing speed during the optical fiber drawing process, and rotating the preform rod at a certain speed during the drawing process, and controlling and changing the rotation speed to realize different pitches of the optical fiber, where the pitch can refer to the length of the optical fiber rotating 360°, recorded as ξ. At the same time, during the optical fiber drawing process, we will truncate the cladding of the optical fiber to improve the absorption of the pump light, and the number of cladding truncation is recorded as m; due to the periodic disturbance caused by the cladding rotation, mode coupling will occur in the optical fiber, and when the light propagates in the optical fiber and meets the following conditions:

[0029]

[0030] LP will occur 01 Module and LP 11 The mode coupling is and They represent the propagation constants of the sT-DCF transverse mode. Therefore, by optimizing the pitch length, core parameters and cladding shape to achieve mode conversion, the mode purity of the amplified light can be optimized in theory.

[0031] Furthermore, the present invention is a step-index preform, and the refractive index of each layer is required to have a first refractive index n 1 The core part and a cladding with a refractive index n lower than the first refractive index 2 (n 1 >n 2 ), the pitch length of the optical fiber varies in the range of 30 to 31 mm with the length of the optical fiber of 2.5 m. In order to improve the absorption rate of the cladding to the pump, the cladding is cut into 4 times, and the shape is similar to a square with rounded corners. The core cladding diameter of the preform is 11.5, and the core varies from 12.5 to 37.5 μm. Its cone angle is 1×10 -5 .

[0032] The result of forming a double-clad fiber with a tapered geometry is that light introduced into the thin end propagates in the wide core without changing the mode content. It is well known that increasing the diameter of multiple cylindrical fiber amplifiers in series usually increases the threshold of unwanted nonlinear effects. The T-DCF design incorporates this advantage in a single fiber. The result is that light amplification maintains excellent beam quality by increasing the stimulation threshold of nonlinear effects, including Brillouin and Raman scattering. The fiber's double-cladding structure means that its core can be pumped at higher powers than those propagated in the core alone. Compared with cylindrical fibers with similar levels of active ion doping, the absorption and conversion of pump light per unit length in the tapered fiber is higher due to improved cladding mode mixing and higher absorption at the thicker end of the tapered due to the thicker cladding. This also means that rare earth ion dopants can be effectively concentrated at the wide end of the T-DCF.

[0033] Birefringence (B) is an optical property of a material where the material has different refractive indices for different polarization directions of the optical signal. Birefringence can be defined based on the maximum difference in refractive indices for different polarizations: B = 2πΔn. Where Δn is the maximum difference between the refractive indices for different polarizations (e.g., "fast" and "slow" modes). Birefringence causes orthogonal polarization modes to have different propagation constants, causing them to propagate in the fiber at different phase velocities, which may result in unpredictable polarization states of the light.

[0034] The phase shift of the "fast" and "slow" waves with different polarization directions in the optical fiber due to birefringence can be expressed as:

[0035]

[0036] Where L is the length of the optical fiber, B λ represents the normalized birefringence of the optical fiber with respect to wavelength λ. At the same time, this formula can also characterize the sensitivity of the polarization state to temperature:

[0037]

[0038] From the above formula, we can know that the temperature sensitivity of polarization state depends on the fiber length L, the temperature sensitivity of birefringence dB / dT, the temperature sensitivity of fiber length dL / dT and the absolute value of birefringence B. Therefore, the temperature sensitivity of the polarization state of the passing light increases with the increase of the intrinsic birefringence and length of the fiber. During the amplification process, the temperature of the active fiber will rise sharply, thus causing the instability of the polarization state.

[0039] Therefore, it is necessary to use optical fiber with low intrinsic birefringence for amplification; if the optical fiber preform is rotated rapidly when drawing the optical fiber, the internal birefringence will become very low. The inherent residual birefringence in the optical fiber is mainly determined by the geometric defects of its core, such as ellipticity, frozen mechanical stress or local bending of the optical fiber. This local eigenstate of birefringence is generally linear polarization. When the optical fiber preform is rotated during the drawing process, the geometric anisotropy axis changes its angular position many times, and the time for each polarization component to propagate as a fast wave and a slow wave is roughly the same, thereby compensating the relative phase delay between the polarization eigenmodes segment by segment, thus obtaining the so-called rotated optical fiber. The intrinsic birefringence of this rotated optical fiber can be very small (B < 10 -5 ). The intrinsic polarization eigenstate may be close to circular polarization. Therefore, the polarization of light propagating through such fiber may be only very slightly perturbed. As a result, such fiber may not acquire any polarization, thereby reducing polarization distortion during amplification.

[0040] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A cylindrical vector fiber amplifier based on a rotated tapered fiber, characterized in that: include: Cylindrical vector light seed source; An isolator, a beam combiner, a rotating tapered optical fiber and a cladding light pump stripper are sequentially arranged in the transmission direction of the column vector light of the column vector light seed source, and the input end of the beam combiner is also fused with a first pump source.

2. The cylindrical vector fiber amplifier according to claim 1, characterized in that: The first pump source is a semiconductor laser with a central wavelength of 976 nm, the beam combiner is a (2+1)×1 beam combiner, and the cladding pump stripper is connected at an angle of 8°.

3. The cylindrical vector fiber amplifier according to claim 1, characterized in that: The column vector light seed source includes a second pump source, a wavelength division multiplexer, a gain fiber, a coupler, a first polarization controller, a polarization-dependent isolator, a second polarization controller, and a mode selective coupler; The second pump source is fused to the input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is fused to the input end of the gain fiber through a pigtail, the output end of the gain fiber is fused to the input end of the coupler, the first output end of the coupler is used to output laser, the second output end of the coupler is fused to the input end of the first polarization controller, the output end of the first polarization controller is fused to the input end of the polarization-dependent isolator, the output end of the polarization-dependent isolator is fused to the input end of the second polarization controller, the output end of the second polarization controller is fused to the input end of the mode selective coupler, and the output end of the mode selective coupler is fused to the input end of the wavelength division multiplexer to form a ring cavity.

4. The cylindrical vector fiber amplifier according to claim 3, characterized in that: The second pump source is a semiconductor laser with a central wavelength of 976 nm, and the coupler is a 1×2 coupler.

5. The cylindrical vector fiber amplifier according to claim 3, characterized in that: The light emitted by the second pump source is coupled into the gain fiber through the wavelength division multiplexer, and the light output from the gain fiber passes through the coupler. The coupler outputs a part of the light, and the other part is polarized by the first polarization controller. The output light passes through the polarization-dependent isolator to convert the light into linear polarized light. The light at the output end of the polarization-dependent isolator passes through the second polarization controller to convert it into elliptically polarized light. Due to the nonlinear effect in the cavity, when the light passes through the first polarization controller to achieve nonlinear deflection locking, the output light of the second polarization controller passes through the mode selection coupler and then outputs LP 11 The light of the mode resonates in the cavity.

6. The cylindrical vector fiber amplifier according to any one of claims 1 to 5, characterized in that: The structure of the rotated tapered optical fiber is that a rotation pitch is formed on the outer surface of the tapered optical fiber.

7. The cylindrical vector fiber amplifier according to claim 6, characterized in that: The method for preparing the rotated tapered optical fiber is as follows: in the process of drawing a preform rod along the X direction to form the tapered optical fiber, the preform rod is rotated around the X axis at the same time, so as to finally obtain the rotated tapered optical fiber.

8. The cylindrical vector fiber amplifier according to claim 7, characterized in that: The preform is a step-index preform. The refractive index of each layer is required to be a core part with a first refractive index n1 and a cladding refractive index n2 lower than the first refractive index. The pitch length of the optical fiber varies in the range of 30 to 31 mm with the optical fiber length of 2.5 m. In order to improve the absorption rate of the cladding to the pump, the cladding is truncated into 4 times. The core cladding diameter of the preform is 11.5, and the core varies from 12.5 to 37.5 μm. Its cone angle is 1×10 -5 .

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