Method for eliminating birefringence filtering effect of full polarization-maintaining mode-locked fiber laser

Abstract

The invention discloses a method for eliminating a birefringence filtering effect of a full polarization-maintaining mode-locked fiber laser, which compensates birefringence of a polarization-maintaining fiber by inserting a birefringence adjusting component into a free space of a laser oscillation cavity, thereby eliminating a modulation phenomenon on a mode-locked spectrum. According to the method, the pulse output by the full-polarization-maintaining linear cavity fiber laser based on NPE mode locking can obtain a smooth spectrum, the practical application capability of the laser is improved, and the laser of the type can be applied to the fields of optical imaging, laser radar, optical frequency comb generation and the like.

Description

Technical Field

[0001] The invention belongs to the field of laser technology, and in particular relates to a method for eliminating the birefringence filtering effect of a full polarization-maintaining mode-locked optical fiber laser. Background Art

[0002] Mode-locked fiber lasers can directly output picosecond or femtosecond pulses. Compared with solid-state lasers, mode-locked fiber lasers have many advantages such as simple structure, good beam quality, and low manufacturing cost, and play a vital role in the field of ultrafast lasers. Ultrafast lasers have many applications in the fields of laser radar, spectroscopy, medical surgery, optical frequency comb seed sources, or supercontinuum generation. These applications have led to higher demands for fiber lasers with high robustness, high reliability, and high beam quality in industry and academia.

[0003] The output pulse performance and stability of fiber lasers are affected by the mode-locking mechanism and the laser cavity structure. Using polarization-maintaining fiber in fiber lasers can effectively maintain the polarization state and polarization direction of the laser beam during the oscillation of the resonant cavity, and is not easily affected by external environmental factors, such as temperature changes or mechanical vibrations of the fiber. Therefore, fiber lasers constructed using polarization-maintaining fiber have good stability.

[0004] All optical fibers in a fully polarization-maintaining fiber laser are polarization-maintaining fibers. This type of fiber laser has the advantages of high pulse quality and high stability. There are many ways to achieve mode locking in a fully polarization-maintaining fiber laser. Fully polarization-maintaining fiber lasers based on nonlinear optical loop mirrors, nonlinear amplifying loop mirrors, nonlinear polarization rotation, and self-stabilizing interferometric mode locking have been widely studied and have been implemented in optical fibers doped with ytterbium, erbium, thulium, and holmium.

[0005] There is a problem with the fully polarization-maintaining fiber lasers that have been reported so far. The introduction of polarization-maintaining fiber and Faraday rotator will bring about spectral modulation caused by birefringence filtering effect, which is manifested as the presence of modulation ripples similar to sine function in the spectrum of the mode-locked pulse. This is because the fully polarization-maintaining fiber laser uses a polarization-maintaining device. Due to problems such as the rotation angle deviation of the Faraday rotator and the welding angle error, the group velocity mismatch will not be fully compensated, thus forming soliton molecules in both the fast axis and the slow axis. The soliton molecules contain the main pulse that the experiment hopes to obtain and the irrational sub-pulses caused by the incomplete compensation of the group velocity mismatch, forming a coherent spectrum in the frequency domain, resulting in the phenomenon of periodic ripples in the output spectrum caused by birefringence filtering. The existence of spectral filtering and sub-pulses affects the beam quality and is not conducive to the commercial application of fiber lasers. Summary of the invention

[0006] In view of this, the purpose of the present invention is to provide a method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser, thereby eliminating the spectral modulation ripples caused by the birefringence filtering due to device errors, and obtaining a laser with a flatter output spectrum; and before passing through FR, the group velocity dispersion is compensated, so that the pulse lasers of the fast axis and the slow axis of the optical fiber coincide, and a single pulse laser with better beam quality is obtained.

[0007] A method for eliminating the birefringence filtering effect of a full polarization-maintaining mode-locked fiber laser, wherein the full polarization-maintaining mode-locked fiber laser comprises a pump source and a laser oscillator cavity connected to each other, wherein the laser oscillator cavity comprises a polarization-maintaining gain fiber, a polarization-maintaining single-mode fiber, and a birefringence adjustment component;

[0008] Wherein, the fast axis of the birefringence adjustment component is aligned with the slow axis of the polarization-maintaining single-mode optical fiber, and the slow axis of the birefringence adjustment component is aligned with the fast axis of the polarization-maintaining single-mode optical fiber;

[0009] Before the full polarization-maintaining mode-locked fiber laser generates laser light, the optical path difference rate of the full polarization-maintaining mode-locked fiber laser is adjusted by the birefringence adjustment component to reduce the optical path difference between two optical axes in the laser oscillation cavity and achieve spectrum optimization.

[0010] Preferably, the laser oscillation cavity is a linear cavity, and the pump source couples the pump light into the linear cavity through the second wavelength division multiplexer. After the pump light enters the linear cavity, it forms a light beam. The light beam passes through the Faraday rotator and the second wave plate in the linear cavity in sequence and reaches the first reflector. The first reflector folds the light back. The light beam passes through the second wave plate, the Faraday rotator, the second wave division multiplexer, the polarization-maintaining gain fiber, the polarization-maintaining single-mode fiber, the first wave division multiplexer, the birefringence adjustment component, the first wave plate and the polarization beam splitter in sequence. The light beam is divided into two beams of light at the polarization beam splitter, one of which is transmitted out of the linear cavity as the output light, and the other beam of light continues to be transmitted in the linear cavity and is reflected by the second reflector and then passes through the polarization beam splitter, the first wave plate, the birefringence adjustment component, the first wave division multiplexer, the polarization-maintaining single-mode fiber and the polarization-maintaining gain fiber in sequence to complete one cycle of transmission.

[0011] Preferably, the birefringence adjusting component is a birefringent crystal.

[0012] Preferably, the birefringent crystal is an electrically controlled birefringent crystal, which controls the refractive index of the crystal by adjusting an external electric field.

[0013] Preferably, the birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

[0014] Preferably, the optical path difference between the two axes of the birefringent crystal is adjusted so that the optical path difference between the two axes of the birefringent crystal is equal to the optical path difference between the two axes of the polarization-maintaining single-mode optical fiber, thereby achieving spectrum optimization;

[0015] The formula for calculating the optical path difference between the two axes of a birefringent crystal is: c L c =B f L f ;

[0016] Among them, L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, B f is the birefringence of the polarization-maintaining single-mode fiber, L f is the length of the polarization-maintaining single-mode fiber;

[0017] Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

[0018] Preferably, the laser oscillation cavity is a 9-shaped cavity, which includes an annular part and a linear part connected to each other. The pump source couples the pump light into the annular part of the 9-shaped cavity through the second wavelength division multiplexer. The pump light propagates clockwise in the annular part of the 9-shaped cavity to form a light beam. The light beam passes through the polarization-maintaining single-mode fiber, the polarization-maintaining gain fiber, the polarization-maintaining single-mode fiber and the output coupler in sequence. Then, part of the light energy of the light beam is output as output light to the outside of the 9-shaped cavity at the output coupler. The remaining light energy of the light beam reaches the polarization-maintaining fiber circulator and enters the linear part of the 9-shaped cavity. Then, the light beam passes through the polarization-maintaining gain fiber, the polarization-maintaining single-mode fiber, the first wavelength division multiplexer, the birefringence adjustment component, the Faraday rotator, the first wave plate and the second reflector in sequence. After being reflected by the second reflector, it propagates back to the polarization-maintaining fiber circulator. The light beam re-enters the annular part of the 9-shaped cavity and propagates clockwise.

[0019] Preferably, the birefringence adjusting component is a birefringent crystal.

[0020] Preferably, the birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

[0021] Preferably, the optical path difference between the two axes of the birefringent crystal is adjusted so that the optical path difference between the two axes of the birefringent crystal is equal to the optical path difference between the two axes of the polarization-maintaining single-mode optical fiber, thereby achieving spectrum optimization;

[0022] The formula for calculating the optical path difference between the two axes of a birefringent crystal is:

[0023] L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, λ is the optimized central wavelength of the fully polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained when the fully polarization-maintaining mode-locked fiber laser is optimized without adding the birefringent crystal;

[0024] Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

[0025] The present invention has the following beneficial effects:

[0026] The present invention compensates for the birefringence of the polarization-maintaining fiber by inserting a birefringence adjustment component in the free space of the laser oscillation cavity, thereby eliminating the filtering phenomenon on the locked mode spectrum. This method can enable the pulses output by the full polarization-maintaining fiber laser to obtain a smooth spectrum, improve the practical application capability of the laser, and enable this type of laser to be applied in the fields of laser processing, optical imaging, laser radar, optical frequency comb generation, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the structure of a linear cavity fully polarization-maintaining mode-locked fiber laser without optimization of the birefringence adjustment component;

[0028] Figure 2 This is the experimental spectrum of the linear cavity fully polarization-maintaining mode-locked fiber laser without optimization of the birefringence adjustment component;

[0029] Figure 3 Schematic diagram of the structure of a linear cavity fully polarization-maintaining mode-locked fiber laser in Embodiment 1 of the present invention;

[0030] Figure 4 The simulated spectrum diagram of the linear cavity full polarization-maintaining mode-locked fiber laser in Example 1 of the present application after and without the optimization of the birefringence adjustment component;

[0031] Figure 5 This is a schematic diagram of the structure of the 9-cavity fully polarization-maintaining mode-locked fiber laser in Example 2 of the present application.

[0032] Figure numerals: 1, polarization beam splitter; 2, wave plate one; 3, birefringence adjustment component; 4, wavelength division multiplexer one; 5, wavelength division multiplexer two; 6, Faraday rotator; 7, wave plate two; 8, reflector one; 9, reflector two; 10, pump source; 11, polarization-maintaining single-mode optical fiber; 12, polarization-maintaining gain optical fiber; 13, first optical fiber fusion point; 14, spatial optical path; 15, polarization-maintaining optical fiber circulator; 16, output coupler; 17, second optical fiber fusion point. DETAILED DESCRIPTION

[0033] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0034] Embodiment 1:

[0035] A method for eliminating the birefringence filtering effect of a full polarization-maintaining mode-locked fiber laser, wherein the full polarization-maintaining mode-locked fiber laser comprises a pump source 10 and a laser oscillator cavity connected to each other, wherein the laser oscillator cavity comprises a polarization-maintaining gain fiber 12, a polarization-maintaining single-mode fiber 11, and a birefringence adjustment component 3;

[0036] Wherein, the fast axis of the birefringence adjustment component 3 is aligned with the slow axis of the polarization-maintaining single-mode optical fiber 11, and the slow axis of the birefringence adjustment component 3 is aligned with the fast axis of the polarization-maintaining single-mode optical fiber 11;

[0037] Before the full polarization-maintaining mode-locked fiber laser generates laser light, the optical path difference rate of the full polarization-maintaining mode-locked fiber laser is adjusted by the birefringence adjustment component 3 to reduce the optical path difference between two optical axes in the laser oscillation cavity and achieve spectrum optimization.

[0038] The laser oscillation cavity is a linear cavity. The pump source 10 couples the pump light into the linear cavity through the wavelength division multiplexer 2 5. After the pump light enters the linear cavity, it forms a light beam. The light beam passes through the Faraday rotator 6 and the wave plate 2 7 in the linear cavity in sequence and reaches the reflector 1 8. The reflector 1 8 returns the light. The light beam passes through the wave plate 2 7, the Faraday rotator 6, the wavelength division multiplexer 2 5, the polarization-maintaining gain fiber 12, the polarization-maintaining single-mode fiber 11, the wavelength division multiplexer 1 4, the birefringence adjustment component 3, the wave plate 1 2 and the polarization beam splitter 1 in sequence. The light beam is divided into two beams of light at the polarization beam splitter 1, one of which is used as the output light to transmit out of the linear cavity, and the other beam of light continues to transmit in the linear cavity and passes through the polarization beam splitter 1, the wave plate 1 2, the birefringence adjustment component 3, the wavelength division multiplexer 1 4, the polarization-maintaining single-mode fiber 11 and the polarization-maintaining gain fiber 12 in sequence after being reflected by the reflector 2 9 to complete a cycle of transmission. The light beam propagates along the spatial optical path 14 between the wavelength division multiplexer 2 5, the Faraday rotator 6, the wave plate 2 7 and the reflector 1 8, and the light beam propagates along the spatial optical path 14 between the wavelength division multiplexer 1 4, the birefringence adjustment component 3, the wave plate 1 2, the polarization beam splitter 1 and the reflector 2 9.

[0039] In this embodiment, wave plate 1 2 is a half-wave plate, wave plate 2 7 is a λ / 8 wave plate, and polarization-maintaining gain fiber 12 and polarization-maintaining single-mode fiber 11 are fused at 0° at the first fiber fusion point 13. The central wavelength of the Faraday rotator 6 is 1550 nm, and the rotation angle is 45°±2°. In order to obtain a wide spectrum and narrow pulse width, the laser operates near the zero dispersion point, and the polarization-maintaining single-mode fiber 11 and the polarization-maintaining gain fiber 12 are respectively a positive dispersion polarization-maintaining erbium-doped gain fiber (PM-EDF, Liekki Er80-4 / 125-HD-PM) and a negative dispersion polarization-maintaining single-mode gain fiber.

[0040] The birefringence adjusting component 3 is a birefringence crystal.

[0041] The birefringent crystal is an electrically controlled birefringent crystal, which controls the refractive index of the crystal by adjusting an external electric field.

[0042] The birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

[0043] Adjust the optical path difference between the two axes of the birefringent crystal so that the optical path difference between the two axes of the birefringent crystal is equal to the optical path difference between the two axes of the polarization-maintaining single-mode optical fiber 11, thereby achieving spectrum optimization;

[0044] The formula for calculating the optical path difference between the two axes of a birefringent crystal is: c L c =B f L f ;

[0045] Among them, L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, B f is the birefringence of the polarization-maintaining single-mode fiber 11, L f is the length of the polarization-maintaining single-mode optical fiber 11;

[0046] Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

[0047] Figure 2 The experimental spectrum of the linear laser oscillator cavity in the fully polarization-maintaining mode-locked fiber laser without the birefringence adjustment component 3 is given. The experimental spectrum has obvious periodic filtering.

[0048] Figure 4 The simulated spectrum of the laser oscillator cavity without the birefringence adjustment component 3 in the conventional technology and the simulated spectrum of the laser oscillator cavity after the birefringence adjustment component 3 in this embodiment are given. The spectrum after the birefringence adjustment component 3 is obviously flatter than the spectrum without the birefringence adjustment component 3, and the periodic spectrum filtering effect is eliminated (the unevenness still existing in the spectrum is caused by factors other than the filtering effect, such as the cross-phase modulation effect in the optical fiber, which is not within the scope of discussion of the present invention).

[0049] Embodiment 2

[0050] like Figure 5Another embodiment of the invention is given, which provides a method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser. The laser oscillator cavity is a 9-shaped cavity, which includes an annular part and a linear part connected to each other. The pump source 10 couples the pump light into the annular part of the 9-shaped cavity through the wavelength division multiplexer 25, and propagates clockwise in the annular part of the 9-shaped cavity to form a light beam. The light beam passes through the polarization-maintaining single-mode fiber 11, the polarization-maintaining gain fiber 12, the polarization-maintaining single-mode fiber 11 and the output coupler 16 in sequence, reaches the polarization-maintaining fiber circulator 15 and enters the annular part of the 9-shaped cavity, and then the light beam passes through the polarization-maintaining gain fiber 12, the polarization-maintaining single-mode fiber 11, the wavelength division multiplexer 14, the birefringence adjustment component 3, the Faraday rotator 6, the wave plate 12 and the reflector 29 in sequence, and propagates back to the polarization-maintaining fiber circulator 15 after being reflected by the reflector 29, and the light beam re-enters the annular part of the 9-shaped cavity and propagates clockwise. At the output coupler 16, 10% of the energy is output to the outside of the cavity to form an output laser. The light beam propagates along the spatial light path 14 between the birefringence adjustment component 3, the Faraday rotator 6, the wave plate 1 2 and the reflector 2 9.

[0051] In this embodiment, wave plate 2 is a half-wave plate.

[0052] In the annular part of the 9-shaped cavity, the polarization-maintaining gain fiber 12 and the polarization-maintaining single-mode fiber 11 are fused at 0° at the first fiber fusion point 13 .

[0053] In the linear part of the figure-9 cavity, the polarization-maintaining gain fiber 12 and the polarization-maintaining single-mode fiber 11 are fused at a second fiber fusion point 17 at an angle of 20°.

[0054] The birefringence adjustment component 3 is a birefringence crystal, and the thickness of the crystal is calculated by the formula:

[0055]

[0056] L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, λ is the optimized central wavelength of the fully polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained when the fully polarization-maintaining mode-locked fiber laser is optimized without adding the birefringent crystal;

[0057] Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

[0058] The birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

[0059] In the above two embodiments, the principle of eliminating the filtering effect by adjusting the optical path difference between the two axes of the birefringent crystal is as follows:

[0060] Polarization-maintaining fiber will generate an optical path difference when passing through the two perpendicular polarization axes of the optical fiber. These two main polarization axes are called the fast axis and slow axis of the optical fiber. The fast axis is an optical axis with a small refractive index and a faster light transmission speed; the slow axis is an optical axis with a large refractive index and a slower transmission speed. The ideal value of the fast axis and the slow axis of the Faraday rotator 6 is 45°, to ensure that after being reflected by the mirror and passing through the Faraday rotator 6 twice, the optical axis rotates 90°, so that the fast axis and the slow axis are exchanged during the return process, and the optical path difference between the fast axis and the slow axis of the polarization-maintaining fiber is completely compensated in a round trip process to ensure that there is only one main pulse in both axes.

[0061] However, the actual rotation angle of the Faraday rotator 6 is not completely equal to 45° (usually 45°±2°). This deviation is caused by the manufacturing process, and this manufacturing error is inevitable. In addition, when the wavelength of light deviates from the central wavelength of the Faraday rotator 6, the rotation angle of the Faraday rotator 6 will also deviate from 45°. Therefore, after a round trip, the rotation angle is not equal to 90°, and the optical path difference between the fast axis and the slow axis of the polarization-maintaining fiber is not completely compensated. In the fast axis and the slow axis, there is a main pulse and an irrational sub-pulse, forming soliton molecules, and then forming interference fringes on the spectrum, resulting in an uneven spectrum.

[0062] After adding the birefringent crystal, the fast axis of the birefringent crystal is aligned with the slow axis of the optical fiber, so that the optical path difference between the fast axis and the slow axis is completely compensated before passing through the Faraday rotator 6, that is, when reaching the Faraday rotator 6, the light of the fast axis and the slow axis are at the same position. In this way, even if the rotation angle is not equal to 90° after passing through the Faraday rotator 6, the main pulse and the irrational sub-pulse will overlap in position, so that soliton molecules will not be formed, thereby eliminating the spectral unevenness caused by the interference fringes.

[0063] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser, characterized in that: The fully polarization-maintaining mode-locked fiber laser comprises a pump source (10) and a laser oscillator cavity which are connected to each other, and the laser oscillator cavity comprises a polarization-maintaining gain fiber (12), a polarization-maintaining single-mode fiber (11) and a birefringence adjustment component (3); Wherein, the fast axis of the birefringence adjustment component (3) is aligned with the slow axis of the polarization-maintaining single-mode optical fiber (11), and the slow axis of the birefringence adjustment component (3) is aligned with the fast axis of the polarization-maintaining single-mode optical fiber (11); Before the full polarization-maintaining mode-locked fiber laser generates laser light, the optical path difference rate of the full polarization-maintaining mode-locked fiber laser is adjusted by the birefringence adjustment component (3) to reduce the optical path difference between two optical axes in the laser oscillation cavity and achieve spectrum optimization.

2. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser according to claim 1, characterized in that: The laser oscillation cavity is a linear cavity. The pump source (10) couples the pump light into the linear cavity through the second wavelength division multiplexer (5). After the pump light enters the linear cavity, it forms a light beam. The light beam passes through the Faraday rotator (6) and the second wave plate (7) in the linear cavity in sequence and then reaches the first reflector (8). The first reflector (8) folds the light back. The light beam passes through the second wave plate (7), the Faraday rotator (6), the second wavelength division multiplexer (5), the polarization-maintaining gain fiber (12), the polarization-maintaining single-mode fiber (11), the wavelength division multiplexer (5), the polarization-maintaining single-mode fiber (12), the wavelength division multiplexer (5), the polarization-maintaining single-mode fiber (1 ... wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the wavelength division multiplexer (5), the A device (4), a birefringence adjustment component (3), a wave plate (2) and a polarization beam splitter (1) are used. The light beam is split into two light beams in the polarization beam splitter (1), one of which is used as output light to be transmitted out of the linear cavity, and the other light beam continues to be transmitted in the linear cavity and is reflected by a reflector (9) and then passes through the polarization beam splitter (1), the wave plate (2), the birefringence adjustment component (3), the wavelength division multiplexer (4), the polarization-maintaining single-mode optical fiber (11) and the polarization-maintaining gain optical fiber (12) in sequence, completing a transmission cycle.

3. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 2, characterized in that: The birefringence adjustment component (3) is a birefringence crystal.

4. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 3, characterized in that: The birefringent crystal is an electrically controlled birefringent crystal, which controls the refractive index of the crystal by adjusting an external electric field.

5. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 3, characterized in that: The birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

6. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 3, 4 or 5, characterized in that: Adjusting the optical path difference between the two axes of the birefringent crystal so that the optical path difference between the two axes of the birefringent crystal is equal to the optical path difference between the two axes of the polarization-maintaining single-mode optical fiber (11) to achieve spectrum optimization; The formula for calculating the optical path difference between the two axes of a birefringent crystal is: c L c =B f L f ; Among them, L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, B f is the birefringence of the polarization-maintaining single-mode fiber (11), L f is the length of the polarization-maintaining single-mode optical fiber (11); Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

7. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser according to claim 1, characterized in that: The laser oscillation cavity is a 9-shaped cavity, which includes an annular part and a linear part connected to each other. The pump source (10) couples the pump light into the annular part of the 9-shaped cavity through the second wavelength division multiplexer (5). The pump light propagates clockwise in the annular part of the 9-shaped cavity to form a light beam. The light beam passes through the polarization-maintaining single-mode fiber (11), the polarization-maintaining gain fiber (12), the polarization-maintaining single-mode fiber (11) and the output coupler (16) in sequence. Then, part of the light energy of the light beam is output as the output light at the output coupler (16). The light beam is output outside the 9-shaped cavity, and the remaining light energy of the light beam reaches the polarization-maintaining fiber circulator (15) and enters the linear part of the 9-shaped cavity. Then, the light beam passes through the polarization-maintaining gain fiber (12), the polarization-maintaining single-mode fiber (11), the wavelength division multiplexer 1 (4), the birefringence adjustment component (3), the Faraday rotator (6), the wave plate 1 (2) and the reflector 2 (9) in sequence. After being reflected by the reflector 2 (9), the light beam propagates in the reverse direction to the polarization-maintaining fiber circulator (15). The light beam re-enters the annular part of the 9-shaped cavity and propagates clockwise.

8. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 7, characterized in that: The birefringence adjustment component (3) is a birefringence crystal.

9. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser as claimed in claim 8, characterized in that: The birefringent crystal is one of yttrium vanadate, calcite or α-barium borate.

10. A method for eliminating the birefringence filtering effect of a fully polarization-maintaining mode-locked fiber laser according to claim 8 or 9, characterized in that: Adjusting the optical path difference between the two axes of the birefringent crystal so that the optical path difference between the two axes of the birefringent crystal is equal to the optical path difference between the two axes of the polarization-maintaining single-mode optical fiber (11) to achieve spectrum optimization; The formula for calculating the optical path difference between the two axes of a birefringent crystal is: L c is the thickness of the birefringent crystal, B c is the birefringence of the birefringent crystal, λ is the optimized central wavelength of the fully polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained when the fully polarization-maintaining mode-locked fiber laser is optimized without adding the birefringent crystal; Adjusting the optical path difference between two axes of the birefringent crystal includes adjusting the birefringent index of the birefringent crystal or the thickness of the birefringent crystal.

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