All-fiber single-frequency narrow linewidth, single-polarization fiber laser device and method of manufacturing the same

Abstract

An all-fiber single-frequency narrow-linewidth single-polarization fiber laser device and a manufacturing method thereof, the method comprising: focusing a femtosecond laser pulse spot in a non-polarization-maintaining gain fiber core; linearly inscribing the spot in the core in a plane passing through the core center axis alternately in the radial direction and the axial direction to form a distributed feedback phase-shift grating with strong birefringence; the device comprising: a pump laser light source connected to an input end of a wavelength division multiplexing coupler A through a pigtail; one output end of the wavelength division multiplexing coupler A connected to an input end of a wavelength division multiplexing coupler B through a gain fiber; and the non-polarization-maintaining gain fiber inscribed with the distributed feedback fiber grating with strong birefringence. The method can simplify the structure of the fiber laser and achieve high-performance single-frequency single-polarization fiber laser output. The grating preparation method and the laser device can achieve single-polarization single-frequency narrow-linewidth laser output, and the manufacturing cost is low.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically relating to an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device and its manufacturing method. Background Technology

[0002] Optical fibers have the characteristics of bendability and waveguide modes with near-diffraction-limited beam quality, which makes fiber lasers have the advantages of compact structure and high output beam quality. Therefore, fiber lasers have become the main form of miniaturized, portable, and high-performance lasers.

[0003] Single-frequency narrow-linewidth fiber laser sources with low phase noise and high frequency stability have significant practical value and demand in fields requiring high monochromaticity and high coherence laser sources, such as remote sensing, spectral analysis, and optical communication. Polarization is one of the important characteristics of lasers; a laser with a certain polarization state can always be decomposed into components of two orthogonal polarization modes. Traditional communication optical fibers are non-polarization-maintaining fibers. Due to the influence of fiber manufacturing processes, stress, and temperature, non-polarization-maintaining fibers have a weak birefringence. When a laser with a certain polarization state propagates in a non-polarization-maintaining fiber, the random birefringence of the fiber causes random mode coupling between two orthogonal polarization modes with different propagation constants, resulting in random jitter in the polarization state of the output light wave. In many application fields such as high-precision fiber optic gyroscopes and fiber optic sensing, while requiring laser sources with high monochromaticity, high spatial coherence length, and high temporal coherence, the output laser must also have highly stable single-polarization output characteristics. Therefore, single-polarization, single-frequency narrow-linewidth fiber laser sources with high extinction ratios have significant research value and application prospects.

[0004] Typically, all-fiber single-frequency fiber lasers can be implemented using distributed feedback cavities or short-cavity distributed Bragg reflector cavities. Single-polarization, single-frequency fiber lasers can be achieved by further introducing polarization-maintaining fibers into the cavity; for example, by inscribing distributed feedback fiber gratings into the polarization-maintaining gain fiber, a polarization-maintaining fiber distributed feedback fiber laser cavity can be realized; another example is by adding a pair of polarization-maintaining fiber-based Bragg fiber gratings as resonant cavity mirrors at both ends of a short length of polarization-maintaining gain fiber, realizing a short-cavity distributed Bragg reflector fiber laser cavity. Such an all-fiber laser cavity composed of polarization fiber elements can achieve single-polarization, single-frequency, narrow-linewidth fiber laser output with a high polarization extinction ratio.

[0005] To realize the above-mentioned single-polarization, single-frequency narrow-linewidth all-fiber laser based on polarization-maintaining fiber, it is necessary to achieve precise alignment of the fast and slow axes of fiber polarization during fiber splicing, which undoubtedly increases the complexity of fiber laser fabrication; at the same time, the fabrication cost of polarization-maintaining fiber is also much higher than that of non-polarization-maintaining fiber. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device and its manufacturing method. This method is simple in process and low in manufacturing cost, effectively eliminating the dependence on polarization-maintaining fiber elements for achieving single-frequency narrow-linewidth, single-polarization fiber lasers. It also eliminates the complex polarization axis alignment steps between polarization-maintaining fiber elements during all-fiber fusion splicing, simplifying the fiber laser structure and enabling high-performance single-frequency, single-polarization fiber laser output. This device can generate polarization-dependent strong birefringence, achieve single-polarization, single-frequency narrow-linewidth laser output, and its low manufacturing cost facilitates widespread application.

[0007] To achieve the above objectives, the present invention provides a method for manufacturing an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device, specifically comprising the following steps:

[0008] Step 1: Using line-to-line femtosecond laser direct writing technology, a femtosecond laser pulse beam enters from the side of the fiber and is precisely focused and positioned on a plane passing through the central axis of the fiber core within the gain fiber core. The laser spot travels in a straight line along the radial and axial directions alternately within the core / cladding interface on this plane, writing a distributed feedback phase-shift grating with strong birefringence. The gain fiber is a non-polarization-maintaining fiber. The size of the femtosecond laser pulse focusing spot is preferably set between 0.5 and 5.0 times the operating wavelength of the femtosecond laser. The laser spot scanning speed is preferably set between 0.001 and 0.5 times the product of the femtosecond laser focusing spot size and the femtosecond laser repetition frequency.

[0009] S1: The femtosecond laser pulse is focused and precisely positioned on a plane passing through the central axis of the fiber core within the gain fiber, and this plane is perpendicular to the femtosecond laser beam;

[0010] S2: The femtosecond laser pulse focused spot travels radially along the central axis of the fiber core in a straight line on the above-mentioned plane, passing through the center of the fiber core at a uniform speed, marking a linear change in the optically induced refractive index. The travel length is preferably set to 0.2 to 1.1 times the diameter of the fiber core, and the straight line travel trajectory is symmetrically distributed on both sides of the center of the fiber core.

[0011] S3: The femtosecond laser pulse focused spot is then translated axially by a distance Λ on the above plane;

[0012] Where Λ=m*λ / (2*n) eff ), where Λ is the grating period, m is the grating order (a positive integer), λ is the grating wavelength, and n eff Effective refractive index;

[0013] S4: The femtosecond laser pulse focused spot then performs a linear scan in the radial direction of the fiber core center axis along the center direction of the fiber core on the above plane. Its travel length is the same as S2, and the travel linear trajectory is symmetrically distributed on both sides of the fiber core center.

[0014] S5: The femtosecond laser pulse focused spot is then translated axially by a distance Λ on the above-mentioned plane;

[0015] S6: Repeat S2-S5 in this way, alternating between radial and axial linear scans along the fiber core center axis;

[0016] S7: After the femtosecond laser pulse focused spot completes L1 = N1Λ cycles of scanning along the axial direction of the fiber core, it jumps one displacement interval L along the axial direction of the fiber core on the gain fiber core. p To achieve π phase shift;

[0017] Where N1 is any positive integer greater than 1; L p = (1 + 2 * M) * π / (2β), where M is any non-negative integer; β = 2π * n eff / λ,n eff λ is the fundamental mode propagation constant, and λ is the operating wavelength;

[0018] S8: The femtosecond laser pulse focuses the spot and repeats the S2-S5 process, completing L2 = N2Λ cycles of scanning along the axial direction of the fiber core center axis in the gain fiber core;

[0019] Where N2 is any positive integer greater than 1;

[0020] In this way, a distributed feedback fiber grating with π phase shift is finally inscribed in the gain fiber; due to the linear refractive index change formed in the core of the gain fiber by the line-to-line femtosecond laser direct writing method, the distributed feedback fiber grating with π phase shift has a strong birefringence.

[0021] Thus, a distributed feedback fiber grating embedded in the gain fiber constitutes a distributed feedback laser resonant cavity;

[0022] Step 2: Establish the connection between the output end of the pump laser source and the input end of the wavelength division multiplexing coupler A through a pigtail. Then, connect the first output fiber A of the wavelength division multiplexing coupler A to one end of the gain fiber through fusion splicing. Connect the other end of the gain fiber to the input fiber of the wavelength division multiplexing coupler B through fusion splicing. The pigtail and the fiber connected to the wavelength division multiplexing coupler A are both non-polarization-maintaining fibers.

[0023] Step 3: The separated residual pump signal is output through the first output fiber B of the wavelength division multiplexing coupler B, and a forward output laser signal is obtained through the second output fiber B in the wavelength division multiplexing coupler B; a reverse output laser signal is obtained through the second output fiber A of the wavelength division multiplexing coupler A; the polarization state of the forward output laser signal of the second output fiber B is adjusted by the fiber polarization controller B; the polarization state of the reverse output laser signal of the second output fiber A of the wavelength division multiplexing coupler A is adjusted by the fiber polarization controller A; wherein, the fibers connected to the wavelength division multiplexing coupler B are all non-polarization-maintaining fibers.

[0024] Furthermore, to reduce transmission loss, the gain fiber is connected to wavelength division multiplexing coupler A via low-loss fusion splicing; the gain fiber is also connected to wavelength division multiplexing coupler B via low-loss fusion splicing.

[0025] This method utilizes a femtosecond laser spot focused on the core of a gain fiber to etch an optically induced refractive index change within the fiber core. Because the optically induced refractive index change etched by the femtosecond laser pulse through the fiber side exhibits significant geometric differences in the directions parallel and perpendicular to the etch beam, a polarization-dependent strong birefringence is generated in the non-polarization-maintaining gain fiber. Thus, after laser oscillation, a single longitudinal mode located within the narrow linewidth window of the distributed feedback fiber grating has an absolute advantage in competition with other longitudinal modes, thereby achieving single-frequency narrow-linewidth laser output. Simultaneously, a single-frequency longitudinal mode with a polarization direction parallel to the fast axis of the high birefringence distributed feedback fiber grating constructed by line-to-line femtosecond laser direct writing has an absolute advantage in competition with another single-frequency longitudinal mode of orthogonal polarization state, thereby achieving single-polarization, single-frequency narrow-linewidth laser output. By pumping a distributed feedback fiber resonator, single-frequency, narrow-linewidth, single-polarization fiber laser output can be generated more effectively. This invention employs a technical approach based on line-to-line femtosecond fiber grating direct writing technology, implementing a polarization-dependent, strongly birefringent distributed feedback phase-shifting grating in a non-polarization-maintaining gain fiber, thereby achieving all-fiber distributed feedback laser output with single-frequency, narrow-linewidth, and single-polarization. This approach effectively eliminates the dependence on polarization-maintaining fiber elements (such as polarization-maintaining fiber gratings and polarization-maintaining gain fibers) for achieving single-frequency, narrow-linewidth, and single-polarization fiber lasers, and avoids the complex polarization axis alignment steps between polarization-maintaining fiber elements during all-fiber fusion splicing, simplifying the fiber laser structure and enabling high-performance single-frequency, single-polarization fiber laser output. This invention uses low-cost non-polarization-maintaining gain fiber, combined with the increasingly mature femtosecond laser fiber grating direct writing technology, to provide a method for realizing an all-fiber single-frequency, narrow-linewidth, single-polarization fiber laser based on non-polarization-maintaining fiber elements.

[0026] This invention also provides an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device, including a pump laser source, the output port of which is connected to a pigtail, and a wavelength division multiplexing coupler A, a gain fiber, and a wavelength division multiplexing coupler B. The end of the pigtail furthest from the pump laser source is connected to the input fiber in wavelength division multiplexing coupler A by fusion splicing. The gain fiber has a distributed feedback fiber grating with strong birefringence, fabricated using femtosecond line-to-line direct writing, internally inscribed. The gain fiber and the distributed feedback fiber grating constitute a distributed feedback laser resonant cavity. One end of the gain fiber is connected to the first output fiber A in wavelength division multiplexing coupler A by fusion splicing. The input fiber in wavelength division multiplexing coupler B is connected to the other end of the gain fiber by fusion splicing. The pigtail, gain fiber, and fibers connected to wavelength division multiplexing coupler A and wavelength division multiplexing coupler B are all non-polarization-maintaining fibers.

[0027] Furthermore, it also includes fiber polarization controller B and fiber polarization controller A; the fiber polarization controller B is connected to one output fiber of wavelength division multiplexing coupler B; the fiber polarization controller A is connected to the other output fiber of wavelength division multiplexing coupler A; the fibers used by the fiber polarization controller B and the fiber polarization controller A are both non-polarization-maintaining fibers.

[0028] In this invention, all fiber components constituting the fiber laser resonator are non-polarization-maintaining fibers, and the fiber laser resonator is a distributed feedback resonator. The distributed feedback fiber grating constituting the distributed feedback resonator is fabricated in a single-transverse-mode non-polarization-maintaining gain fiber using line-to-line femtosecond fiber grating direct writing technology. The distributed feedback resonator ensures that after oscillation, only one single-frequency longitudinal mode with an extremely narrow linewidth has a competitive advantage and is amplified during oscillation. The line-to-line femtosecond fiber grating direct writing technology generates a polarization-dependent strong birefringence in the non-polarization-maintaining gain fiber, ensuring that after oscillation, only one longitudinal mode in the linear polarization direction has a competitive advantage and is amplified during oscillation. Ultimately, single-frequency, single-polarization laser output can be achieved at the wavelength of the distributed feedback fiber grating. Furthermore, its manufacturing cost is low, which is beneficial for widespread application. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device based on non-polarization-maintaining fiber components according to the present invention.

[0030] Figure 2 A side view of a distributed feedback phase-shift grating being inscribed in a non-polarization-maintaining gain fiber using line-to-line femtosecond laser direct writing technology;

[0031] Figure 3 A cross-sectional view of a distributed feedback phase-shift grating being inscribed in a non-polarity-maintaining gain fiber using line-to-line femtosecond laser direct writing technology;

[0032] Figure 4 The images show the transmission spectrum of a distributed feedback fiber grating inscribed on a non-polarization-maintaining erbium-doped silica gain fiber and the laser output spectrum of an all-fiber single-frequency single-polarization fiber laser after oscillation, as examples of this invention.

[0033] Figure 5 The above is an example of the measured data and Lorentz fitting linewidth of the all-fiber distributed feedback single-frequency single-polarization fiber laser output based on non-polarization-maintaining erbium-doped silica gain fiber in the frequency domain using the time-delay self-heterodyne interferometry method in this invention.

[0034] Figure 6 This is an example of the measured spectrum of the all-fiber distributed feedback single-frequency single-polarization fiber laser output based on non-polarization-maintaining erbium-doped quartz gain fiber in the frequency domain, obtained by using the time-delay self-heterodyne interferometry method in this invention.

[0035] Figure 7 This is an example of the relative intensity noise spectrum in the frequency domain of the all-fiber distributed feedback single-frequency single-polarization fiber laser output based on non-polarization-maintaining erbium-doped silica gain fiber in this invention.

[0036] In the diagram: 1. Pump laser source, 2. Wavelength division multiplexing coupler A, 3. Gain beam, 4. Distributed feedback fiber grating, 5. Wavelength division multiplexing coupler B, 6. First output fiber B, 7. Second output fiber B, 8. Fiber polarization controller A, 9. Second output fiber A, 10. Fiber polarization controller B, 11. Pigtail. Detailed Implementation

[0037] The following description, in conjunction with the accompanying drawings and embodiments, further illustrates the all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device and manufacturing method of the present invention:

[0038] Specific examples illustrate the device and manufacturing method of an erbium-doped 1.55-micron all-fiber single-frequency narrow-linewidth, single-polarization fiber laser:

[0039] The specific example uses erbium-doped non-polarization-maintaining silica glass fiber with a core diameter of 4 micrometers.

[0040] In a specific example, the light source used to inscribe the phase-shifted distributed feedback fiber grating is an 800-nanometer femtosecond laser (pulse width 80 femtoseconds, repetition frequency 1 kilohertz);

[0041] The specific example illustrates the manufacturing method of a 1.55-micron erbium-doped all-fiber single-frequency narrow-linewidth, single-polarization fiber laser, which includes the following steps:

[0042] Step 1: As Figure 2As shown, using line-to-line femtosecond laser direct writing technology, a femtosecond laser pulse beam enters from the side of the fiber and is precisely focused and positioned on a plane passing through the central axis of the erbium-doped gain fiber 3. The laser spot travels along a linear trajectory alternately in the radial and axial directions on this plane within the core / cladding interface, thus writing a distributed feedback phase-shift grating with strong birefringence. The gain fiber 3 is an erbium-doped non-polarization-maintaining fiber. The size of the femtosecond laser pulse focusing spot is approximately 1 micrometer, and the laser spot scanning speed is 0.03 mm / s.

[0043] S1: The femtosecond laser pulse is focused and the spot is precisely located on a plane passing through the central axis of the fiber core within the erbium-doped gain fiber 3. This plane is perpendicular to the femtosecond laser beam.

[0044] S2: In a specific example, the initial position of the femtosecond laser spot is located at the interface between the gain fiber core and the first cladding; the laser spot and the fiber position are moved relative to each other by a high-precision program-controlled displacement platform. The femtosecond laser pulse focused spot travels radially along the central axis of the fiber core in a straight line along the aforementioned plane, passing through the center of the fiber core at a uniform speed, marking a linear change in photoinduced refractive index. The travel length is 4 micrometers (equal to the core diameter), and the straight-line trajectory is symmetrically distributed on both sides of the center of the fiber core; (see...) Figure 2 and Figure 3 (A schematic diagram showing the movement of the light spot along the side and cross-section of the optical fiber);

[0045] S3: The femtosecond laser pulse focused spot is then translated axially along the above plane by one periodic distance Λ = 1.072 micrometers (therefore, according to Λ = m*λ / (2*n)... eff (Λ is the grating period; m is the grating order, which is a positive integer, here equal to 2; λ is the grating wavelength, n) eff (This refers to the effective refractive index of the fundamental mode in the erbium-doped silica fiber core, corresponding to a fiber grating center wavelength of 1.55 micrometers);

[0046] S4: The femtosecond laser pulse focused spot then performs a linear scan along the direction of the fiber core center axis in the radial direction of the above plane. Its travel length is the same as S2 (i.e., equal to 4 micrometers), and the travel linear trajectory is symmetrically distributed on both sides of the fiber core center.

[0047] S5: The femtosecond laser pulse focused spot is then translated axially along the above plane by a distance Λ (equivalent to 1.072 micrometers);

[0048] S6: Repeat S2-S5 in this way, alternating between radial and axial linear scans along the fiber core center axis;

[0049] S7: After the femtosecond laser pulse focusing spot completes 12120 cycles of scanning along the axial direction of the fiber core (i.e., a total radial movement L1 of approximately 13 mm), it jumps one displacement interval L along the axial direction of the fiber core on the gain fiber core. p (=1.34 micrometers=1.25Λ), achieving π phase shift;

[0050] S8: The femtosecond laser pulse focuses the spot and repeats the S2-S5 process, completing the second 12120 cycles (i.e., a total radial movement L2 of about 13 mm) in the gain fiber core along the axial direction of the fiber core center axis.

[0051] In this way, a distributed feedback fiber grating 4 with π phase shift is finally inscribed in the gain fiber; due to the linear refractive index change formed in the core of the gain fiber by the line-to-line femtosecond laser direct writing method, the distributed feedback fiber grating 4 with π phase shift has a strong birefringence.

[0052] Thus, the distributed feedback fiber grating 4 embedded in the erbium-doped gain fiber 3 constitutes a distributed feedback laser resonant cavity.

[0053] Step 2: In this specific example, the pump laser source 1 is a 976 nm semiconductor laser. A connection is established between the output of the pump laser source 1 and the input of the wavelength division multiplexing coupler A2 via pigtail 11. Then, the first output fiber A of the wavelength division multiplexing coupler A2 is fused to one end of the erbium-doped silica gain fiber 3, and the other end of the gain fiber 3 is fused to the input fiber of the wavelength division multiplexing coupler B5. In this specific example, wavelength division multiplexing couplers A2 and B5 are 980 / 1550 nm wavelength division multiplexing couplers. Both pigtail 11 and the fibers connected to the wavelength division multiplexing coupler A2 are non-polarization-maintaining fibers.

[0054] Step 3: The separated residual pump signal is output through the first output fiber B6 of the wavelength division multiplexing coupler B5, and the forward output laser signal is obtained through the second output fiber B7 of the wavelength division multiplexing coupler B5; the reverse output laser signal is obtained through the second output fiber A9 of the wavelength division multiplexing coupler A2; the polarization state of the forward output laser signal of the second output fiber B7 is adjusted by the fiber polarization controller B8; the polarization state of the reverse output laser signal of the second output fiber A9 of the wavelength division multiplexing coupler A2 is adjusted by the fiber polarization controller A10; wherein, the optical fibers connected to the wavelength division multiplexing coupler B5 are all non-polarization-maintaining fibers.

[0055] Gain fiber 3 is connected to wavelength division multiplexing coupler A2 via low-loss fusion splicing; gain fiber 3 is connected to wavelength division multiplexing coupler B5 via low-loss fusion splicing.

[0056] The specific example of a 1.55-micron erbium-doped all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device includes a 976-nanometer semiconductor-pumped laser source 1, the output port of which is connected to a pigtail 11. It also includes a 980 / 1550-nanometer wavelength division multiplexing coupler A2, an erbium-doped silica gain fiber 3, and a 980 / 1550-nanometer wavelength division multiplexing coupler B5. The end of the pigtail 11 furthest from the pumped laser source 1 is connected to the input fiber in the wavelength division multiplexing coupler A2 by fusion splicing. The inner end of the erbium-doped silica gain fiber 3... The laser cavity is inscribed with a distributed feedback fiber grating 4, which has a strong birefringence and is fabricated by femtosecond line-to-line direct writing. The gain fiber 3 and the distributed feedback fiber grating 4 constitute a distributed feedback laser resonator. One end of the gain fiber 3 is connected to the first output fiber A in the wavelength division multiplexing coupler A2 by fusion splicing. The input fiber in the wavelength division multiplexing coupler B5 is connected to the other end of the gain fiber 3 by fusion splicing. The fibers connected to the pigtail 11, the gain fiber 3, the wavelength division multiplexing coupler A2, and the wavelength division multiplexing coupler B5 are all non-polarization-maintaining fibers.

[0057] It also includes fiber polarization controller B8 and fiber polarization controller A10; the fiber polarization controller B8 is connected to one output fiber of wavelength division multiplexing coupler B5; the fiber polarization controller A10 is connected to the other output fiber of wavelength division multiplexing coupler A; the fiber polarization controller B8 and fiber polarization controller A10 both use non-polarization-maintaining fiber.

[0058] Figure 4 The images show the transmission spectrum of the distributed feedback fiber grating inscribed on the non-polarization-maintaining erbium-doped silica gain fiber and the laser output spectrum of the all-fiber single-frequency single-polarization fiber laser after oscillation; the laser output wavelength is 1550 nm.

[0059] Figure 5 In this example, the all-fiber distributed feedback single-frequency single-polarization fiber laser output based on non-polarization-maintaining erbium-doped silica gain fiber is analyzed using a 50-kilometer-long delay fiber. The results are obtained by measuring the frequency domain data and Lorentz line shape fitting diagram using the self-heterodyne interferometry method. The fitting results show that the single-frequency narrow linewidth laser output has a 3 dB linewidth of 200 Hz.

[0060] Figure 6 The time-domain spectrum of the all-fiber distributed feedback single-frequency single-polarization fiber laser output based on non-polarization-maintaining erbium-doped quartz gain fiber in the example is displayed on the oscilloscope after passing through a Fabry-Perot interferometer, indicating that the fiber laser is a single-polarization single-frequency output.

[0061] Figure 7The example shows the relative intensity noise spectrum of the all-fiber distributed feedback single-frequency single-polarization fiber laser output in the frequency domain based on non-polarization-maintaining erbium-doped silica gain fiber. It can be seen that in the frequency domain above 1 MHz, the relative intensity noise of the above single-frequency narrow-linewidth, single-polarization fiber laser is below -110 dB / Hz.

Claims

1. A method for manufacturing an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device, characterized in that, Specifically, the following steps are included: Step 1: Using line-to-line femtosecond laser direct writing technology, a femtosecond laser pulse beam enters from the side of the fiber and is focused and positioned on a plane passing through the central axis of the fiber core of the gain fiber (3). The light spot travels in a straight line along the radial and axial directions alternately within the fiber core / cladding interface on this plane, thus writing a distributed feedback phase shift grating with strong birefringence. Among them, the gain fiber (3) is a non-polarization-maintaining fiber. Among them, the size of the femtosecond laser pulse focusing spot is set to be between 0.5 and 5.0 times the working wavelength of the femtosecond laser. Among them, the laser spot scanning speed is set to be between 0.001 and 0.5 times the product of the size of the femtosecond laser focusing spot and the repetition frequency of the femtosecond laser. S1: Position the femtosecond laser pulse focusing spot on a plane passing through the central axis of the fiber core inside the gain fiber (3), which is perpendicular to the femtosecond laser beam; S2: The femtosecond laser pulse focused spot travels along the radial direction of the fiber core's central axis in a straight line on the aforementioned plane, passing through the fiber core's center at a uniform speed, marking a linear change in the optically induced refractive index. The travel length is set to 0.2 to 1.1 times the fiber core diameter, and the straight-line travel trajectory is symmetrically distributed on both sides of the fiber core's center. S3: The femtosecond laser pulse focused spot is then translated axially by a distance Λ on the above plane; in, Λ represents the grating period, and m represents the grating order, which takes a positive integer value. The grating wavelength, Effective refractive index; S4: The femtosecond laser pulse focused spot then performs a linear scan in the radial direction of the fiber core center axis along the center direction of the fiber core on the above plane. Its travel length is the same as S2, and the travel linear trajectory is symmetrically distributed on both sides of the fiber core center. S5: The femtosecond laser pulse focused spot is then translated axially by a distance Λ on the above-mentioned plane; S6: Repeat S2-S5 in this way, alternating between radial and axial linear scans along the fiber core center axis; S7: After the femtosecond laser pulse focused spot completes L1=N1Λ cycles of scanning along the axial direction of the fiber core, it jumps one displacement interval L along the axial direction of the fiber core on the gain fiber core. p To achieve π phase shift; Where N1 is any positive integer greater than 1; ,in It can be any non-negative integer; , Let be the fundamental mode propagation constant. The operating wavelength; S8: The femtosecond laser pulse focuses the spot and repeats the S2-S5 process, completing L2=N2Λ cycles of scanning in the core of the gain fiber along the axial direction of the fiber core center axis; Where N2 is any positive integer greater than 1; In this way, a distributed feedback fiber grating (4) with π phase shift is finally inscribed in the gain fiber; due to the linear refractive index change formed in the core of the gain fiber by the line-to-line femtosecond laser direct writing method, the distributed feedback fiber grating (4) with π phase shift has a strong birefringence. Thus, the distributed feedback fiber grating (4) embedded in the gain fiber (3) constitutes a distributed feedback laser resonator. Step 2: Establish the connection between the output end of the pump laser source (1) and the input end of the wavelength division multiplexing coupler A (2) through the pigtail (11). Then, connect the first output fiber A of the wavelength division multiplexing coupler A (2) to one end of the gain fiber (3) by fusion splicing. Connect the other end of the gain fiber (3) to the input fiber of the wavelength division multiplexing coupler B (5) by fusion splicing. Among them, the fibers connected to the pigtail (11) and the wavelength division multiplexing coupler A (2) are all non-polarization-maintaining fibers. Step 3: The separated residual pump signal is output through the first output fiber B (6) of the wavelength division multiplexing coupler B (5), and the forward output laser signal is obtained through the second output fiber B (7) in the wavelength division multiplexing coupler B (5); the reverse output laser signal is obtained through the second output fiber A (9) of the wavelength division multiplexing coupler A (2); the polarization state of the forward output laser signal of the second output fiber B (7) is adjusted by the fiber polarization controller B (8); the polarization state of the reverse output laser signal of the second output fiber A (9) of the wavelength division multiplexing coupler A (2) is adjusted by the fiber polarization controller A (10); wherein, the optical fibers connected to the wavelength division multiplexing coupler B (5) are all non-polarization-maintaining fibers.

2. A single-frequency, narrow-linewidth, single-polarization fiber laser device, characterized in that, The device includes a pump laser source (1), the output port of which is connected to a pigtail (11), a wavelength division multiplexing coupler A (2), a gain fiber (3), and a wavelength division multiplexing coupler B (5); the end of the pigtail (11) away from the pump laser source (1) is connected to the input fiber in the wavelength division multiplexing coupler A (2) by fusion splicing; the inside of the gain fiber (3) is inscribed with a strong birefringence fiber prepared by the manufacturing method of an all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device according to claim 1. A distributed feedback fiber grating (4) is formed by a gain fiber (3) and a distributed feedback fiber grating (4); one end of the gain fiber (3) is connected to the first output fiber A in the wavelength division multiplexing coupler A (2) by fusion splicing; the input fiber in the wavelength division multiplexing coupler B (5) is connected to the other end of the gain fiber (3) by fusion splicing; the fibers connected to the pigtail (11), the gain fiber (3), the wavelength division multiplexing coupler A (2) and the wavelength division multiplexing coupler B (5) are all non-polarization-maintaining fibers.

3. The all-fiber single-frequency narrow-linewidth, single-polarization fiber laser device according to claim 2, characterized in that, It also includes fiber polarization controller B (8) and fiber polarization controller A (10); the fiber polarization controller B (8) is connected to one output fiber of wavelength division multiplexing coupler B (5); the fiber polarization controller A is connected to the other output fiber of wavelength division multiplexing coupler A; the fiber used by the fiber polarization controller B (8) and the fiber polarization controller A (10) are both non-polarization-maintaining fibers.

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