A method for eliminating birefringence filtering effect in fully polarization-maintaining mode-locked fiber lasers

Abstract

This invention discloses a method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser. By inserting a birefringence adjustment component into the free space of the laser oscillation cavity to compensate for the birefringence of the polarization-maintaining fiber, the modulation phenomenon in the mode-locked spectrum is eliminated. This method enables the pulse output of a fully polarization-maintaining linear cavity fiber laser based on NPE mode-locking to obtain a smooth spectrum, improving the practical application capability of the laser and enabling this type of laser to be used in fields such as optical imaging, laser radar, and optical frequency comb generation.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically relating to a method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser. Background Technology

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

[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 fibers in fiber lasers effectively maintains the polarization state and direction of the laser beam during resonant cavity oscillation, making it less susceptible to external environmental influences such as temperature changes or mechanical vibrations of the fiber. Therefore, fiber lasers constructed using polarization-maintaining fibers exhibit excellent stability.

[0004] In a fully polarization-maintaining fiber laser, all fibers are polarization-maintaining fibers. This type of fiber laser has advantages such as high pulse quality and high stability. There are various methods to achieve mode-locking in fully polarization-maintaining fiber lasers. Fully polarization-maintaining fiber lasers based on nonlinear optical loop mirrors, nonlinear amplifying loop mirrors, nonlinear polarization rotation, and self-stabilizing interference mode-locking have been extensively studied and have been implemented in ytterbium-doped, erbium-doped, thulium-doped, and holmium-doped fibers.

[0005] A problem has been reported with fully polarization-maintaining fiber lasers: the introduction of polarization-maintaining fiber and Faraday rotators introduces spectral modulation due to birefringence filtering, manifesting as sinusoidal modulation ripples in the mode-locked pulse spectrum. This is because fully polarization-maintaining fiber lasers use polarization-maintaining devices, and issues such as Faraday rotator rotation angle deviation and fusion angle errors can lead to incomplete compensation for group velocity mismatch. This results in solitons forming on both the fast and slow axes. These solitons contain the desired main pulse and irrational sub-pulses resulting from the incomplete compensation of group velocity mismatch, forming a coherent spectrum in the frequency domain, thus exhibiting periodic ripples in the output spectrum due to birefringence filtering. The presence of spectral filtering and sub-pulses affects beam quality and is detrimental to the commercial application of fiber lasers. Summary of the Invention

[0006] In view of this, the purpose of this 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 ripple caused by birefringence filtering due to device errors and obtaining a laser with a flatter output spectrum; and furthermore, by compensating for the velocity dispersion of the front group of the FR, the pulsed lasers of the fast axis and slow axis of the fiber are made to coincide, resulting in a single-pulse laser with better beam quality.

[0007] A method for eliminating the birefringence filtering effect in a polarization-maintaining mode-locked fiber laser, wherein the polarization-maintaining mode-locked fiber laser includes a pump source and a laser oscillation cavity connected to each other, and the laser oscillation cavity includes 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 fiber, and the slow axis of the birefringence adjustment component is aligned with the fast axis of the polarization-maintaining single-mode fiber.

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

[0010] Preferably, the laser oscillation cavity is a linear cavity. The pump source couples the pump light into the linear cavity through wavelength division multiplexer II. After entering the linear cavity, the pump light forms a beam. The beam passes sequentially through the Faraday rotator mirror and waveplate II within the linear cavity and reaches mirror I. Mirror I reflects the light back. The beam then passes sequentially through waveplate II, Faraday rotator, wavelength division multiplexer II, polarization-maintaining gain fiber, polarization-maintaining single-mode fiber, wavelength division multiplexer I, birefringence adjustment component, waveplate I, and polarization beam splitter. The beam is split into two beams by the polarization beam splitter. One beam is output and exits the linear cavity. The other beam continues to propagate within the linear cavity and is reflected by mirror II before passing sequentially through the polarization beam splitter, waveplate I, birefringence adjustment component, wavelength division multiplexer I, polarization-maintaining single-mode fiber, and polarization-maintaining gain fiber, completing one cycle of transmission.

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

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

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

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

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

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

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

[0018] Preferably, the laser oscillation cavity is a figure-9 cavity, which includes an interconnected annular portion and a linear portion. The pump source couples the pump light into the annular portion of the figure-9 cavity through a wavelength division multiplexer (WDM) II. The pump light propagates clockwise within the annular portion of the figure-9 cavity to form a beam. The beam passes sequentially through a polarization-maintaining single-mode fiber, a polarization-maintaining gain fiber, another polarization-maintaining single-mode fiber, and an output coupler. Then, part of the beam's energy is output as output light at the output coupler and exits the figure-9 cavity. The remaining energy of the beam reaches the polarization-maintaining fiber circulator and enters the linear portion of the figure-9 cavity. The beam then passes sequentially through a polarization-maintaining gain fiber, a polarization-maintaining single-mode fiber, a wavelength division multiplexer (WDM) I, a birefringence adjustment component, a Faraday rotator, a waveplate I, and a mirror II. After being reflected by mirror II, the beam propagates in the opposite direction to the polarization-maintaining fiber circulator. The beam then re-enters the annular portion of the figure-9 cavity and propagates clockwise.

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

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

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

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

[0023] L c B is the thickness of the birefringent crystal. c λ is the birefringence of the birefringent crystal, λ is the center wavelength of the optimized polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained by the optimized polarization-maintaining mode-locked fiber laser without the addition of a birefringent crystal.

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

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

[0026] This invention compensates for the birefringence of polarization-maintaining fiber by inserting a birefringence adjustment component into the free space of the laser oscillation cavity, thereby eliminating the filtering phenomenon in the mode-locked spectrum. This method enables the pulse output of the fully polarization-maintaining fiber laser to obtain a smooth spectrum, improving the practical application capability of the laser and enabling this type of laser to be used in laser processing, optical imaging, laser radar, optical frequency comb generation and other fields. Attached Figure Description

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

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

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

[0030] Figure 4 The simulated spectra of the linear cavity fully polarization-maintaining mode-locked fiber laser in Embodiment 1 of this application are obtained after optimization with the birefringence adjustment component and without optimization with the birefringence adjustment component.

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

[0032] Reference numerals in the attached diagram: 1. Polarization beam splitter; 2. Waveplate 1; 3. Birefringence adjustment assembly; 4. Wavelength division multiplexer 1; 5. Wavelength division multiplexer 2; 6. Faraday rotator; 7. Waveplate 2; 8. Mirror 1; 9. Mirror 2; 10. Pump source; 11. Polarization-maintaining single-mode fiber; 12. Polarization-maintaining gain fiber; 13. First fiber fusion splice; 14. Spatial optical path; 15. Polarization-maintaining fiber circulator; 16. Output coupler; 17. Second fiber fusion splice. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0034] Example 1:

[0035] A method for eliminating the birefringence filtering effect of a polarization-maintaining mode-locked fiber laser, wherein the polarization-maintaining mode-locked fiber laser includes a pump source 10 and a laser oscillation cavity connected to each other, and the laser oscillation cavity includes 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 fiber 11, and the slow axis of the birefringence adjustment component 3 is aligned with the fast axis of the polarization-maintaining single-mode fiber 11.

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

[0038] The laser oscillation cavity is a linear cavity. The pump source 10 couples the pump light into the linear cavity through wavelength division multiplexer 2 5. After entering the linear cavity, the pump light forms a beam. The beam passes sequentially through Faraday rotator 6 and waveplate 2 7 in the linear cavity and then reaches mirror 1 8. Mirror 1 8 reflects the light back. The beam then passes sequentially through waveplate 2 7, Faraday rotator 6, wavelength division multiplexer 2 5, polarization-maintaining gain fiber 12, polarization-maintaining single-mode fiber 11, wavelength division multiplexer 1 4, birefringence adjustment component 3, waveplate 1 2, and polarization beam splitter 1. The beam is split into two beams by polarization beam splitter 1. One beam is output and exits the linear cavity. The other beam continues to propagate in the linear cavity and is reflected by mirror 2 9 before passing sequentially through polarization beam splitter 1, waveplate 1 2, birefringence adjustment component 3, wavelength division multiplexer 1 4, polarization-maintaining single-mode fiber 11, and polarization-maintaining gain fiber 12, completing one cycle of transmission. The beam propagates along the spatial optical path 14 between wavelength division multiplexer 2 5, Faraday rotator 6, wave plate 2 7 and reflector 1 8, and between wavelength division multiplexer 1 4, birefringence adjustment component 3, wave plate 1 2, polarization beam splitter 1 and reflector 2 9.

[0039] In this embodiment, waveplate 2 is a half-waveplate, waveplate 7 is a λ / 8 waveplate, and polarization-maintaining gain fiber 12 and polarization-maintaining single-mode fiber 11 are fused at 0° at the first fiber fusion splice 13. The center wavelength of Faraday rotator 6 is 1550nm, and the rotation angle is 45°±2°. To obtain a wide spectrum and narrow pulse width, the laser operates near the near-zero dispersion point. Polarization-maintaining single-mode fiber 11 and 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 adjustment 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 the applied electric field.

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

[0043] The optical path difference between the two axes of the birefringent crystal is adjusted so that it is equal to the optical path difference between the two axes of the polarization-maintaining single-mode fiber 11, thereby achieving spectral optimization.

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

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

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

[0047] Figure 2 Experimental spectra of the linear laser oscillator cavity in a fully polarization-maintaining mode-locked fiber laser without birefringence adjustment component 3 are presented. The experimental spectra show obvious periodic filtering.

[0048] Figure 4 Simulated spectra of a laser oscillator cavity without birefringence adjustment component 3 in conventional technology are presented, as well as simulated spectra of a laser oscillator cavity with birefringence adjustment component 3 in this embodiment. The spectrum after birefringence adjustment component 3 is significantly flatter than the spectrum without birefringence adjustment component 3, and the periodic spectral filtering effect is eliminated (the unevenness still present 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 this invention).

[0049] Example 2

[0050] like Figure 5Another embodiment of the invention is provided, offering a method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser. The laser oscillation cavity is a figure-9 cavity, comprising interconnected annular and linear portions. The pump source 10 couples the pump light into the annular portion of the figure-9 cavity via a wavelength division multiplexer 5. The light beam propagates clockwise within the annular portion, forming a beam. The beam sequentially passes through a polarization-maintaining single-mode fiber 11, a polarization-maintaining gain fiber 12, another polarization-maintaining single-mode fiber 11, and an output coupler 16, reaching a polarization-maintaining fiber circulator 15 and entering the annular portion of the figure-9 cavity. Then, the beam sequentially passes through a polarization-maintaining gain fiber 12, a polarization-maintaining single-mode fiber 11, a wavelength division multiplexer 4, a birefringence adjustment component 3, a Faraday rotator 6, a waveplate 2, and a reflector 9. After reflection by the reflector 9, the beam propagates backward to the polarization-maintaining fiber circulator 15, re-entering the annular portion of the figure-9 cavity and propagating clockwise. At the output coupler 16, 10% of the energy is output outside the cavity to form the output laser. The light beam propagates along the spatial light path 14 between the birefringence adjustment component 3, the Faraday rotator 6, the waveplate 2, and the mirror 9.

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

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

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

[0054] Birefringence adjustment component 3 is a birefringence crystal, and the crystal thickness is manufactured using a formula:

[0055]

[0056] L c B is the thickness of the birefringent crystal. c λ is the birefringence of the birefringent crystal, λ is the center wavelength of the optimized polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained by the optimized polarization-maintaining mode-locked fiber laser without the addition of a birefringent crystal.

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

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

[0059] In the two embodiments described above, 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 generates an optical path difference when passing through its two perpendicular polarization axes. These two main polarization axes are called the fast axis and the slow axis of the fiber, respectively. The fast axis is the optical axis with a lower refractive index and a faster light transmission speed; the slow axis is the optical axis with a higher refractive index and a slower light transmission speed. The ideal value of the Faraday rotator 6 for both the fast and slow axes is 45°. This ensures that after reflection by the mirror and two passes through the Faraday rotator 6, the optical axis rotates 90°, allowing the fast and slow axes to exchange during the return journey. This completely compensates for the optical path difference between the fast and slow axes of the polarization-maintaining fiber within a round trip, guaranteeing only one dominant pulse on each axis.

[0061] However, the actual rotation angle of the Faraday rotator 6 is not exactly equal to 45° (it is usually 45° ± 2°). This deviation is caused by the manufacturing process, and such manufacturing errors are unavoidable. Furthermore, when the wavelength of light deviates from the center wavelength of the Faraday rotator 6, the rotation angle of the Faraday rotator 6 will also deviate from 45°. Therefore, after one round trip, the rotation angle is not equal to 90°, and the optical path difference between the fast and slow axes of the polarization-maintaining fiber is not fully compensated. In both the fast and slow axes, there is a main pulse and an irrational sub-pulse, forming solitons, which in turn create interference fringes in the spectrum, resulting in spectral unevenness.

[0062] By incorporating a birefringent crystal, the fast axis of the birefringent crystal is aligned with the slow axis of the optical fiber. This ensures that the optical path difference between the fast and slow axes is fully compensated before reaching Faraday rotator 6, meaning that the light from the fast and slow axes is in the same position when it reaches Faraday rotator 6. Thus, even if the rotation angle is not equal to 90° after passing through Faraday rotator 6, the main pulse and irrational sub-pulses will coincide in position, preventing the formation of solitons and thus eliminating spectral unevenness caused by interference fringes.

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

Claims

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

2. The method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in 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 wavelength division multiplexer two (5). After entering the linear cavity, the pump light forms a beam. The beam passes through the Faraday rotator (6) and waveplate two (7) in the linear cavity in sequence and then reaches the reflector one (8). The reflector one (8) refracts the light back. The beam passes through waveplate two (7), Faraday rotator (6), wavelength division multiplexer two (5), polarization-maintaining gain fiber (12), polarization-maintaining single-mode fiber (11), and wavelength division multiplexer two (5) in sequence. Using device 1 (4), birefringence adjustment component (3), waveplate 1 (2) and polarization beam splitter (1), the beam is split into two beams in polarization beam splitter (1). One beam is output as the output beam and passes out of the linear cavity. The other beam continues to propagate in the linear cavity and is reflected by mirror 2 (9) and then passes through polarization beam splitter (1), waveplate 1 (2), birefringence adjustment component (3), wavelength division multiplexer 1 (4), polarization-maintaining single-mode fiber (11) and polarization-maintaining gain fiber (12) in sequence to complete one cycle of transmission.

3. The method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in claim 2, characterized in that, The birefringent crystal is an electrically controlled birefringent crystal, which controls the refractive index of the crystal by adjusting the applied electric field.

4. The method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in claim 2, characterized in that, The birefringent crystal is one of yttrium vanadate, calcite, or barium α-borate.

5. A method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in claim 2, 3, or 4, characterized in that, Adjust the optical path difference between the two axes of the birefringent crystal so that it is equal to the optical path difference between the two axes of the polarization-maintaining single-mode fiber (11) to achieve spectral optimization; The formula for calculating the optical path difference between the two axes of a birefringent crystal is: B c L c =B f L f ; Among them, L c B is the thickness of the birefringent crystal. c B is the birefringence of a birefringent crystal. f L is the birefringence of polarization-maintaining single-mode fiber (11). f It is the length of the polarization-maintaining single-mode fiber (11); Adjusting the optical path difference between the two axes of a birefringent crystal includes adjusting the birefringence of the crystal or the thickness of the crystal.

6. The method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in claim 1, characterized in that, The laser oscillation cavity is a figure-9 cavity, which includes an interconnected annular section and a linear section. The pump source (10) couples the pump light into the annular section of the figure-9 cavity through wavelength division multiplexer (5). The pump light propagates clockwise within the annular section of the figure-9 cavity to form a beam. The beam passes sequentially through polarization-maintaining single-mode fiber (11), polarization-maintaining gain fiber (12), polarization-maintaining single-mode fiber (11), and output coupler (16). Then, part of the light energy of the beam is used as output light at the output coupler (16). The remaining light energy of the beam is output to the outside of the figure-9 cavity. It reaches the polarization-maintaining fiber circulator (15) and enters the linear part of the figure-9 cavity. Then the beam passes through the polarization-maintaining gain fiber (12), the polarization-maintaining single-mode fiber (11), the wavelength division multiplexer (4), the birefringence adjustment component (3), the Faraday rotator (6), the waveplate (2), and the mirror (9) in sequence. After being reflected by the mirror (9), the beam propagates in the opposite direction to the polarization-maintaining fiber circulator (15). The beam then re-enters the ring part of the figure-9 cavity and propagates clockwise.

7. The method for eliminating the birefringence filtering effect in a fully polarization-maintaining mode-locked fiber laser as described in claim 6, characterized in that, The birefringent crystal is one of yttrium vanadate, calcite, or barium α-borate.

8. A method for eliminating the birefringence filtering effect in a polarization-maintaining mode-locked fiber laser as described in claim 6 or 7, characterized in that, Adjust the optical path difference between the two axes of the birefringent crystal so that it is equal to the optical path difference between the two axes of the polarization-maintaining single-mode fiber (11) to achieve spectral optimization; The formula for calculating the optical path difference between the two axes of a birefringent crystal is: L c B is the thickness of the birefringent crystal. c λ is the birefringence of the birefringent crystal, λ is the center wavelength of the optimized polarization-maintaining mode-locked fiber laser, and Δλ is the filtering interval of the mode-locked spectrum obtained by the optimized polarization-maintaining mode-locked fiber laser without the addition of a birefringent crystal. Adjusting the optical path difference between the two axes of a birefringent crystal includes adjusting the birefringence of the crystal or the thickness of the crystal.

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