Linear-cavity nonlinear amplification ring mode-locked fiber laser

Abstract

The application discloses a linear cavity nonlinear amplification ring mirror mode-locked fiber laser, which comprises a first reflector, a first polarization beam splitter, a quarter-wave plate, a first Faraday rotator, a half-wave plate, a second polarization beam splitter and a third polarization beam splitter arranged in sequence along an optical path direction; one side of the third polarization beam splitter is connected with a pump light path; a pump input end of a wavelength division multiplexer is connected with a pump semiconductor laser through a single-mode optical fiber; emitted light of the wavelength division multiplexer passes through a second Faraday rotator and a second reflector in sequence; a third reflector and a fourth reflector are arranged on the sides of the third polarization beam splitter and the second polarization beam splitter respectively; and a second optical fiber collimator is arranged on the side of the first polarization beam splitter. The application forms an equivalent nonlinear amplification ring mirror structure through the polarization beam splitter, the reflector and the Faraday rotator, utilizes polarization orthogonal double beams to accumulate nonlinear phase shifts in the cavity and causes interference, forms a nonlinear transmission effect to realize self-oscillation mode-locking, and has compact structure and high stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of mode-locked laser, in particular to a linear cavity nonlinear amplifying loop mirror mode-locked fiber laser. BACKGROUND

[0002] In the field of mode-locked fiber lasers, how the optical path is transmitted, what level the repetition frequency is at and whether the operation is stable are all determined by the cavity structure, and the mainstream cavity structure of the current mode-locked fiber laser includes a ring cavity structure and a linear cavity structure. The ring cavity mode-locked fiber laser becomes an ideal platform for generating ultra-short pulses due to its simple structure and unidirectional traveling wave advantage, and its typical representative is a nine-character cavity mode-locked fiber laser. The earliest nine-character cavity mode-locked fiber laser was formally proposed as a passively mode-locked fiber laser structure by Menlo Systems Company, and a patent (EP2637265A1) has been applied for. The ring structure in the nine-character cavity mode-locked fiber laser is often limited in the high repetition frequency scene due to the disadvantage of fiber bending loss, and the linear structure can avoid this loss problem, so it is more suitable for the construction of high repetition frequency mode-locked fiber lasers.

[0003] In the field of the fusion application of mode-locked technology and linear cavity, there are several known schemes in the prior art. For example, a technical scheme of a linear cavity polarization maintaining fiber laser based on a Faraday rotator, which uses a nonlinear polarization rotation mechanism (NPR) to realize mode-locked operation. However, such a laser based on the NPR mechanism has inherent limitations in improving the repetition frequency. In order to improve the repetition frequency of the laser, shortening the resonant cavity length is a recognized effective technical approach in this field. After shortening the cavity length, the group velocity dispersion and nonlinear phase shift accumulated by the optical pulse in a single cycle are reduced, which helps to improve the pulse time domain stability. However, the short cavity structure puts forward more stringent requirements for the mode-locked self-starting condition, which is specifically manifested as follows: the modulation depth and response characteristics of the saturable absorption effect need to be further optimized, and at the same time, the insertion loss introduced by the elements in the cavity will more significantly affect the threshold and working stability of the laser, thereby posing a severe challenge to the overall design of the laser.

[0004] In a polarization maintaining fiber laser based on a nonlinear amplifying loop mirror (NALM), the optical field needs to form a pair of counter-propagating pulses in the polarization maintaining fiber loop, and then converge again at the beam splitter. The two pulses experience different degrees of nonlinear phase accumulation in the loop, and after interference, a transmission characteristic sensitive to light intensity is generated, thereby playing the role of equivalent saturable absorption. Since this mechanism essentially depends on the closed fiber loop and the reciprocal interference structure of counter-propagating, it is difficult to be transformed into a linear cavity form in terms of physical topology, so that further shortening the cavity length and improving the repetition frequency are limited. SUMMARY

[0005] The technical problem to be solved by the present application is to solve the problems of the prior art, and to provide a linear cavity nonlinear amplification ring mirror mode-locked fiber laser with compact structure, high stability and easy self-starting locking, which is equivalent to realizing the function of a nonlinear amplification ring mirror in a linear cavity physical topology through an innovative optical layout, thereby simplifying the structure, reducing the loss, and improving the repetition frequency and long-term running stability of the laser, and overcoming the problems of the prior art, such as the difficulty in effectively realizing a nonlinear amplification ring mirror mode-locked fiber laser in a linear cavity structure, and the difficulty in starting the mode-locked under a high repetition frequency.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0007] A linear cavity nonlinear amplification ring mirror mode-locked fiber laser comprises, in order along the optical path direction, a first reflector, a first polarization beam splitter, a quarter-wave plate, a first Faraday rotator, a half-wave plate, a second polarization beam splitter, and a third polarization beam splitter; the third polarization beam splitter is provided with a pump light path on one side;

[0008] The pump light path comprises, in order, a first fiber collimator, a gain fiber, and a half-polarization filter type wavelength division multiplexer; the light emitted by the half-polarization filter type wavelength division multiplexer passes through, in order along the optical path direction, a second Faraday rotator and a second reflector;

[0009] The optical path of the third polarization beam splitter is beside a third reflector; the optical path of the second polarization beam splitter is beside a fourth reflector;

[0010] The optical path of the first polarization beam splitter is beside a second fiber collimator;

[0011] The pump input end of the half-polarization filter type wavelength division multiplexer is connected to the output end of a pump semiconductor laser through a single-mode optical fiber; the pump light output by the pump semiconductor laser is coupled to the pump light path where the half-polarization filter type wavelength division multiplexer is located through the single-mode optical fiber, so as to supply pump energy to the gain fiber.

[0012] Further, the tail fiber slow axis of the first fiber collimator is aligned with the horizontal direction of the third polarization beam splitter, so as to ensure that the spatial light output by the third polarization beam splitter is coupled to the first fiber collimator; the first fiber collimator is connected to the half-polarization filter type wavelength division multiplexer through the gain fiber;

[0013] The tail fiber of the first fiber collimator is aligned and fused with the slow axis of one end of the gain fiber; the tail fiber of the polarization maintaining fiber is aligned and fused with the slow axis of the other end of the gain fiber; the slow axes of the tail fiber of the first fiber collimator and the tail fiber of the polarization maintaining fiber are aligned with each other;

[0014] The slow axis of the pigtailed polarization maintaining fiber is horizontally aligned with the second Faraday rotator, ensuring that the spatial light output by the second Faraday rotator is coupled to the half-polarization filter type wavelength division multiplexer.

[0015] Further, the external structure of the half-polarization filter type wavelength division multiplexer is a structure encapsulated by an outer steel tube; the internal structure of the half-polarization filter type wavelength division multiplexer comprises: a polarization maintaining fiber, a single-mode fiber, a double-fiber collimator, and a filter;

[0016] The polarization maintaining fiber and the single-mode fiber are coupled to the double-fiber collimator respectively; the double-fiber collimator converts the light beams respectively emitted from the polarization maintaining fiber and the single-mode fiber into two beams of high-quality collimated parallel light;

[0017] The filter is arranged at an inclination angle of 45° in the collimated light path emitted by the double-fiber collimator; the filter controls the light path based on wavelength: it has high transmittance for light in the 1550 nm band and high reflectivity for light in the 976 nm band; after the filter transmits 1550 nm light and reflects 976 nm light, the split parallel light returns to the double-fiber collimator, where it is refocused and efficiently coupled into the gain fiber.

[0018] Further, in the pump light path, the tail fiber of the first fiber collimator is a polarization maintaining fiber, and the slow axis direction is consistent with the horizontal direction;

[0019] The gain fiber is a polarization maintaining type erbium-doped fiber;

[0020] The pump semiconductor laser is selected to have a wavelength of 976 nm and a power of 1 W;

[0021] The polarization maintaining fiber is a PM1550 type polarization maintaining fiber, and the single-mode fiber is a HI1060 type single-mode fiber; the tail fiber lengths of the polarization maintaining fiber and the single-mode fiber are both 40 cm;

[0022] The tail fiber lengths of the first fiber collimator and the second fiber collimator are both 40 cm;

[0023] The length of the gain fiber is 42 cm.

[0024] Further, the third mirror and the third polarization beam splitter have a non-coaxial and parallel spatial distribution relationship, specifically a lateral light path layout in the vertical direction, for reflecting and regulating the S light beam split by the third polarization beam splitter, to realize the functions of polarization state control and mode locking in the laser cavity;

[0025] The fourth mirror and the second polarization beam splitter have a non-coaxial and parallel spatial distribution relationship, specifically a lateral light path layout in the vertical direction, for reflecting and regulating the S light beam split by the second polarization beam splitter, to realize the functions of polarization state control and mode locking in the laser cavity.

[0026] Furthermore, the slow axis of the pigtail of the second fiber collimator is aligned with the vertical direction of the first polarization beam splitter to ensure that the spatial light output from the first polarization beam splitter is coupled to the second fiber collimator.

[0027] Furthermore, the first Faraday rotator operates at a wavelength of 1550 nm or 1030 nm, and rotates at an angle of 45° clockwise or counterclockwise.

[0028] The second Faraday rotator operates at a wavelength of 1550 nm or 1030 nm, and rotates at an angle of 45° clockwise or counterclockwise.

[0029] Based on the foregoing embodiments, the present invention also provides another alternative:

[0030] An alternative technology for a linear cavity nonlinear amplifying ring mirror mode-locked fiber laser includes a first reflecting mirror, a first polarizing beam splitter, a quarter-wave plate, a first Faraday rotator, a half-wave plate, a first beam deflector, and a second beam deflector arranged sequentially along the optical path; a pump optical path is provided on one side of the second beam deflector.

[0031] The pump optical path includes a first fiber collimator, a gain fiber, and a semi-polarization-maintaining filter type wavelength division multiplexer connected in sequence; the light emitted from the semi-polarization-maintaining filter type wavelength division multiplexer passes through a second Faraday rotator and a second reflector in sequence along the optical path direction;

[0032] The second fiber collimator is located beside the optical path of the first polarization beam splitter.

[0033] The pump input of the semi-polarization-maintaining filter type wavelength division multiplexer is connected to the output of the pump semiconductor laser via a single-mode fiber. The pump light output from the pump semiconductor laser is coupled to the pump optical path of the semi-polarization-maintaining filter type wavelength division multiplexer via the single-mode fiber to supply pump energy to the gain fiber.

[0034] In this embodiment, the difference from the previous embodiment is that a first beam deflector and a second beam deflector are provided between the half-wave plate and the pump optical path, instead of the configuration in the previous embodiment where a second polarization beam splitter and a third polarization beam splitter are provided between the half-wave plate and the pump optical path.

[0035] In this embodiment, the first beam deflector is further configured as the second beam deflector by rotating 90° clockwise or counterclockwise around the straight line where the gain fiber is located as the rotation axis.

[0036] In this embodiment, the light output from the first fiber collimator is further incident on the second beam deflector and split into a first O-beam and a first E-beam. The first O-beam and the first E-beam continue to be transmitted to the first beam deflector, where a polarization state conversion occurs: the first O-beam is converted into a second E-beam, and the first E-beam is converted into a second O-beam. The second O-beam and the second E-beam are then re-combined after passing through the first beam deflector.

[0037] In this embodiment, the configuration of the first reflector, the first polarization beam splitter, the quarter-wave plate, the first Faraday rotator, and the half-wave plate arranged sequentially along the optical path, as well as the configuration of the pump optical path and the second fiber collimator, the configuration of the second Faraday rotator and the second reflector, and the configuration of the semi-polarization-maintaining filter type wavelength division multiplexer and the single-mode fiber are all the same as in the previous embodiment.

[0038] The working mechanism of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser in this embodiment is consistent with the working mechanism of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser in the previous embodiment.

[0039] The present invention has the following beneficial effects:

[0040] 1. The linear cavity nonlinear amplifying ring mirror mode-locked fiber laser provided by the present invention is characterized by the specific spatial arrangement of the first, second, and third polarization beam splitters and the third and fourth reflectors, which decomposes the incident light into two beams with orthogonal polarization. The two beams propagate in opposite directions along the slow and fast axes of the gain fiber in the pump optical path, undergo two amplifications and accumulate different nonlinear phase shifts, and then return to the polarization beam splitter to interfere. This process constructs an equivalent nonlinear amplifying ring mirror structure in the linear cavity, and utilizes the intensity-dependent transmission effect generated by the interference to achieve saturable absorption, thereby initiating and maintaining mode-locked operation.

[0041] 2. This invention adopts a compact linear cavity design, which breaks through the limitation of fiber ring on fiber length, thereby allowing the use of shorter intracavity fibers, creating conditions for increasing the repetition frequency of the laser; at the same time, environmental factors have less impact on this compact laser structure, enabling the laser to achieve reliable self-starting mode-locking and long-term stable output.

[0042] 3. This invention, based on polarization-maintaining fiber elements and precise polarization axis alignment, ensures the stability of the optical path polarization state. An equivalent nonlinear amplifying ring mirror structure is constructed using a polarization beam splitter, a reflector, and a Faraday rotator. The nonlinear phase shift accumulated within the cavity by orthogonally polarized double beams and their interference creates a nonlinear transmission effect, achieving self-oscillating mode-locking. This structure is compact and easy to adjust; by adjusting polarization control elements such as waveplates, the mode-locking state can be flexibly optimized to adapt to different application requirements.

[0043] 4. Through ingenious polarization control and optical path design, this invention successfully simulates the working mechanism of a nonlinear amplifying ring mirror in a linear cavity, realizing the equivalent saturable absorption effect without the need for a physical ring fiber loop, thus solving the problem that the principle of nonlinear amplifying ring mirrors is difficult to directly apply to linear cavities. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the overall structure of a linear cavity nonlinear amplification ring mirror mode-locked fiber laser provided in Embodiment 1 of the present invention.

[0045] Figure 2 This is a schematic diagram of a semi-polarization-maintaining filter type wavelength division multiplexer.

[0046] Figure 3 This is a schematic diagram of the overall structure of a linear cavity nonlinear amplification ring mirror mode-locked fiber laser provided in Embodiment 2 of the present invention.

[0047] Figure 4 The output characteristic diagram of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser under the hardware configuration of Embodiment 1 of the present invention is shown in the figure. (a) is the optical spectrum diagram, (b) is the autocorrelation curve of the pulse, (c) is the radio frequency spectrum diagram of the output pulse, and (d) is the time-domain pulse sequence diagram.

[0048] The components include: 1. First reflecting mirror; 2. First polarization beam splitter; 3. Quarter-wave plate; 4. First Faraday rotator; 5. Half-wave plate; 6. Second polarization beam splitter; 7. Third polarization beam splitter; 8. First fiber collimator; 9. Gain fiber; 10. Semi-polarization-maintaining filter type wavelength division multiplexer; 11. Second Faraday rotator; 12. Second reflecting mirror; 13. Pump semiconductor laser; 14. Third reflecting mirror; 15. Fourth reflecting mirror; 16. Second fiber collimator; 17. Polarization-maintaining fiber; 18. Single-mode fiber; 19. Dual-fiber collimator; 20. Filter; 21. Outer steel tube; 22. First beam deflector; 23. Second beam deflector; 24. First optical path; 25. Second optical path. Detailed Implementation

[0049] The present invention will now be described in further detail with reference to the accompanying drawings and specific preferred embodiments.

[0050] In the description of this invention, it should be understood that the terms "left side," "right side," "upper part," "lower part," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. "First," "second," etc., do not indicate the importance of the components, and therefore should not be construed as a limitation of this invention. The specific dimensions used in this embodiment are only for illustrating the technical solution and do not limit the scope of protection of this invention.

[0051] Example 1

[0052] like Figure 1 As shown, a linear cavity nonlinear amplifying ring mirror mode-locked fiber laser includes a first reflecting mirror 1, a first polarizing beam splitter 2, a quarter-wave plate 3, a first Faraday rotator 4, a half-wave plate 5, a second polarizing beam splitter 6, and a third polarizing beam splitter 7 arranged sequentially along the optical path direction; a pump optical path is provided on one side of the third polarizing beam splitter 7.

[0053] The pump optical path includes a first fiber collimator 8, a gain fiber 9, and a semi-polarization-maintaining filter type wavelength division multiplexer 10 connected in sequence; the light emitted from the semi-polarization-maintaining filter type wavelength division multiplexer 10 passes through a second Faraday rotator 11 and a second reflector 12 in sequence along the optical path direction.

[0054] The third polarizing beam splitter 7 has a third reflecting mirror 14 beside its optical path; the second polarizing beam splitter 6 has a fourth reflecting mirror 15 beside its optical path.

[0055] The second fiber collimator 16 is located beside the optical path of the first polarization beam splitter 2.

[0056] Furthermore, the pump input of the semi-polarization-maintaining filter type wavelength division multiplexer 10 is connected to the output of the pump semiconductor laser 13 via a single-mode fiber 18. The pump light output from the pump semiconductor laser 13 is coupled to the pump optical path of the semi-polarization-maintaining filter type wavelength division multiplexer 10 via the single-mode fiber 18, so as to supply pump energy to the gain fiber 9.

[0057] like Figure 1 As shown, the third polarization beam splitter is connected to a pump optical path on one side. This optical path is composed of a first fiber collimator, a gain fiber, and a semi-polarization-maintaining filter type wavelength division multiplexer, which are sequentially fused together. The slow axis of its pigtail is horizontally aligned with the spatial optical path. The pump input end of the semi-polarization-maintaining filter type wavelength division multiplexer is connected to a pump semiconductor laser through a single-mode fiber. The output light of the semi-polarization-maintaining filter type wavelength division multiplexer passes sequentially through a second Faraday rotator to a second reflector. A third reflector and a fourth reflector are respectively set next to the third polarization beam splitter and the second polarization beam splitter. A second fiber collimator is set next to the first polarization beam splitter.

[0058] The present invention provides a linear cavity nonlinear amplifying ring mirror mode-locked fiber laser, the core of which lies in the specific spatial arrangement of the first, second, and third polarization beam splitters and the third and fourth reflectors to decompose the incident light into two beams with orthogonal polarization. These two beams propagate in opposite directions along the slow and fast axes of the gain fiber in the pump optical path, undergo two amplifications and accumulate different nonlinear phase shifts, and then return to the polarization beam splitter to interfere. This process constructs an equivalent nonlinear amplifying ring mirror structure in the linear cavity, and utilizes the intensity-dependent transmission effect generated by the interference to achieve saturable absorption, thereby initiating and maintaining mode-locked operation.

[0059] Furthermore, the slow axis of the pigtail of the first fiber collimator 8 is aligned horizontally with the third polarization beam splitter 7 to ensure that the spatial light output from the third polarization beam splitter 7 is coupled to the first fiber collimator 8; the first fiber collimator 8 is connected to the semi-polarization-maintaining filter type wavelength division multiplexer 10 through the gain fiber 9.

[0060] Furthermore, the pigtail of the first fiber collimator 8 is fused to one end of the gain fiber 9, and the other end of the gain fiber 9 is fused to the pigtail of the polarization-maintaining fiber 17 on the semi-polarization-maintaining filter type wavelength division multiplexer 10.

[0061] Furthermore, the pigtail of the first fiber collimator 8 is fused with one end of the gain fiber 9 at a slow axis alignment; the pigtail of the polarization-maintaining fiber 17 is fused with the other end of the gain fiber 9 at a slow axis alignment; and the slow axes of the pigtails of the first fiber collimator 8 and the polarization-maintaining fiber 17 are aligned with each other.

[0062] Furthermore, the slow axis of the pigtail of the polarization-maintaining fiber 17 is aligned horizontally with the second Faraday rotator 11 to ensure that the spatial light output from the second Faraday rotator 11 is coupled to the semi-polarization-maintaining filter type wavelength division multiplexer 10.

[0063] Furthermore, such as Figure 2 As shown, the external structure of the semi-polarization-maintaining filter type wavelength division multiplexer 10 is a structure encapsulated by an outer steel tube 21; the internal structure of the semi-polarization-maintaining filter type wavelength division multiplexer 10 includes: polarization-maintaining fiber 17, single-mode fiber 18, dual-fiber collimator 19, and filter 20.

[0064] Furthermore, polarization-maintaining fiber 17 and single-mode fiber 18 are coupled to dual-fiber collimator 19 respectively; dual-fiber collimator 19 converts the beams emitted from polarization-maintaining fiber 17 and single-mode fiber 18 into two high-quality collimated parallel beams.

[0065] Furthermore, the filter 20 is positioned at a 45° angle in the collimated optical path emitted from the dual-fiber collimator 19; the filter 20 spatially controls the optical path based on wavelength: it has high transmittance for light in the 1550nm band and high reflectance for light in the 976nm band; after the filter 20 transmits 1550nm light and reflects 976nm light, the parallel light after beam splitting returns to the dual-fiber collimator 19, where it is refocused and efficiently coupled into the gain fiber 9.

[0066] Furthermore, the pigtail of the first fiber collimator 8 is a polarization-maintaining fiber, with the slow axis direction aligned with the horizontal direction.

[0067] Furthermore, gain fiber 9 is a polarization-maintaining erbium-doped fiber.

[0068] Furthermore, the pump semiconductor laser 13 is selected with a wavelength of 976nm and a power of 1W;

[0069] Furthermore, polarization-maintaining fiber 17 is a PM1550 type polarization-maintaining fiber; single-mode fiber 18 is a HI1060 type single-mode fiber, and the function of single-mode fiber 18 is to transmit 976nm pump light; the pigtail length of both polarization-maintaining fiber 17 and single-mode fiber 18 is 40cm.

[0070] Furthermore, the pigtails of both the first fiber collimator 8 and the second fiber collimator 16 are 40cm long; the gain fiber 9 is an Er80-4 / 125-HD-PM from nLight Liekki, with a length of 42cm.

[0071] Furthermore, the third reflector 14 and the third polarization beam splitter 7 are in a non-coaxial and parallel spatial distribution relationship, specifically a side optical path layout in the vertical direction, which is used to reflect and control the S-beam after it has been split by the third polarization beam splitter 7, so as to realize the function of polarization state control and mode locking in the laser cavity.

[0072] Furthermore, the fourth reflector 15 and the second polarization beam splitter 6 are in a non-coaxial and parallel spatial distribution relationship, specifically a side optical path layout in the vertical direction, which is used to reflect and control the S-beam after it has been split by the second polarization beam splitter 6, so as to realize the function of polarization state control and mode locking in the laser cavity.

[0073] Furthermore, the slow axis of the pigtail of the second fiber collimator 16 is aligned with the vertical direction of the first polarization beam splitter 2 to ensure that the spatial light output from the first polarization beam splitter 2 is coupled to the second fiber collimator 16.

[0074] Furthermore, the first Faraday rotator 4 operates at a wavelength of 1550 nm or 1030 nm, and rotates at an angle of 45° clockwise or counterclockwise.

[0075] Furthermore, the second Faraday rotator 11 operates at a wavelength of 1550 nm or 1030 nm, and rotates at an angle of 45° clockwise or counterclockwise.

[0076] The working principle of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser in this invention is as follows:

[0077] Along the direction of light propagation, the initial noise light exits from the first reflector 1 and is transmitted sequentially through the first polarization beam splitter 2, the quarter-wave plate 3, the first Faraday rotator 4, the half-wave plate 5, the second polarization beam splitter 6, and the third polarization beam splitter 7.

[0078] After the noise light is split by the first polarization beam splitter 2, it forms linearly polarized light in the first horizontal polarization direction and linearly polarized light in the first vertical polarization direction. The linearly polarized light in the first vertical polarization direction is coupled out of the cavity and completely lost. The linearly polarized light in the first horizontal polarization direction passes through the quarter-wave plate 3, the first Faraday rotator 4, and the half-wave plate 5 in sequence and is transformed into elliptically polarized light.

[0079] After the first elliptically polarized light is incident on the second polarization beam splitter 6, it is split into linearly polarized light with a second horizontal polarization direction and linearly polarized light with a second vertical polarization direction. The linearly polarized light with the second horizontal polarization direction is first transmitted through the third polarization beam splitter 7 and output. The linearly polarized light with the second vertical polarization direction is reflected by the fourth mirror 15 to the third mirror 14, and then reflected by the third mirror 14 and output by the third polarization beam splitter 7.

[0080] Linearly polarized light in the second horizontal polarization direction is coupled into the slow axis of the gain fiber 9 through the first fiber collimator 8, amplified by the gain fiber 9, passes through the semi-polarization-maintaining filter type wavelength division multiplexer 10, and then enters the second mirror 12 through the second Faraday rotator 11. It is reflected by the second mirror 12 and passes through the second Faraday rotator 11 again. After passing through the second Faraday rotator 11 twice, the polarization direction of the linearly polarized light in the second horizontal polarization direction is rotated 90° clockwise, transforming into linearly polarized light in the third vertical polarization direction. The linearly polarized light in the third vertical polarization direction is coupled into the fast axis of the gain fiber 9 through the semi-polarization-maintaining filter type wavelength division multiplexer 10, amplified again in the gain fiber 9, output through the first fiber collimator 8, reflected by the third polarization beam splitter 7 to the third mirror 14, reflected by the third mirror 14 to the fourth mirror 15, and finally reflected by the fourth mirror 15 and output from the second polarization beam splitter 6.

[0081] The linearly polarized light in the second vertical polarization direction is coupled into the fast axis of the gain fiber 9 through the first fiber collimator 8. After being amplified by the gain fiber 9, it passes through the semi-polarization-maintaining filter type wavelength division multiplexer 10, and then through the second Faraday rotator 11 before being incident on the second mirror 12. It is then reflected by the second mirror 12 and passes through the second Faraday rotator 11 again. After passing through the second Faraday rotator 11 twice, the polarization direction of the linearly polarized light in the second vertical polarization direction is rotated 90° clockwise, transforming into linearly polarized light in the third horizontal polarization direction. The linearly polarized light in the third horizontal polarization direction is coupled into the slow axis of the gain fiber 9 through the semi-polarization-maintaining filter type wavelength division multiplexer 10. After being amplified again in the gain fiber 9, it is output through the first fiber collimator 8 and then transmitted through the third polarization beam splitter 7 and the second polarization beam splitter 6 before being output.

[0082] Due to the difference in optical path length and nonlinearity experienced when propagating along the fast and slow axes in the gain fiber 9, different nonlinear phase shifts accumulate between the linearly polarized light in the third horizontal polarization direction and the linearly polarized light in the third vertical polarization direction; the two orthogonally polarized linearly polarized beams are combined into a second elliptically polarized beam at the second polarization beam splitter 6.

[0083] The second elliptically polarized light passes sequentially through the half-wave plate 5, the first Faraday rotator 4 and the quarter-wave plate 3, and then enters the first polarization beam splitter 2, where it undergoes Sagnac interference.

[0084] Two orthogonally polarized beams continuously transform and superimpose during their circulation within the cavity, forming an equivalent closed-loop optical path. Its working mechanism is similar to that of a figure-nine cavity mode-locked fiber laser: the optical signal generated by the gain medium is decomposed into two orthogonally polarized beams, with transmission paths equivalent to beams propagating in opposite directions. After circling the equivalent resonant cavity loop once, these two beams re-converge and interfere, utilizing the intensity-dependent transmission characteristics constructed by the fiber's nonlinear effect to achieve a saturable absorber function, thereby initiating and maintaining mode-locked operation. This process constitutes a nonlinear amplifying ring, and through cyclic feedback, the laser ultimately reaches and stabilizes in a mode-locked state.

[0085] This invention employs a compact linear cavity design, overcoming the limitation of fiber length imposed by fiber rings, thereby allowing the use of shorter intracavity fibers and creating conditions for increasing the laser repetition frequency. At the same time, environmental factors have less impact on this compact laser structure, enabling the laser to achieve reliable self-starting mode-locking and long-term stable output.

[0086] This invention, based on polarization-maintaining fiber elements and precise polarization axis alignment, ensures the stability of the optical path polarization state. An equivalent nonlinear amplifying ring mirror structure is constructed using a polarization beam splitter, a reflector, and a Faraday rotator. The nonlinear phase shift accumulated within the cavity by orthogonally polarized double beams and their interference creates a nonlinear transmission effect, achieving self-oscillating mode-locking. This structure boasts a compact design. Furthermore, the structure is easily adjustable; by adjusting polarization control elements such as waveplates, the mode-locking state can be flexibly optimized to adapt to different application requirements.

[0087] This invention, through ingenious polarization control and optical path design, successfully simulates the working mechanism of a nonlinear amplifying ring mirror in a linear cavity, realizing the equivalent saturable absorption effect without the need for a physical ring fiber loop, and solving the problem that the principle of nonlinear amplifying ring mirrors is difficult to directly apply to linear cavities.

[0088] The output characteristics of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser built based on the above hardware configuration are as follows: Figure 4 As shown, specifically:

[0089] like Figure 4 As shown, (a) is the optical spectrum diagram with a center wavelength of 1563 nm. A series of symmetrically distributed discrete sidebands with decaying intensity can be observed on both sides of the main spectral profile, namely Kelly sidebands, which are typical characteristics of soliton mode-locked operation.

[0090] like Figure 4 As shown in Figure (b), the autocorrelation curve of the pulse is obtained. The full width at half maximum (FWHM) of the curve is 1.35 ps. The actual pulse width is calculated to be 0.876 ps after fitting, indicating that the laser successfully outputs femtosecond-level ultrashort light pulses.

[0091] like Figure 4 As shown, (c) is the radio frequency spectrum of the output pulse. It shows sharp spectral peaks at the 26 MHz fundamental frequency and its harmonics. The signal-to-noise ratio of the fundamental frequency component exceeds 75 dB, which proves that the laser has extremely high repetition frequency stability, good cavity length control, and robust and reliable mode-locking state.

[0092] like Figure 4 As shown, (d) is a time-domain pulse sequence diagram, showing a pulse interval of 38 ns, which corresponds to the repetition frequency of 26 MHz in (c), further verifying the periodicity and stability of the mode-locked pulse sequence.

[0093] In summary, these superior performance results directly verify the feasibility and advantages of the described linear cavity nonlinear amplifying ring mirror mode-locked fiber laser. It successfully transplants the saturable absorption effect of the nonlinear amplifying ring mirror into the linear cavity structure, avoiding the bending loss of the ring cavity, and providing an effective technical path for realizing a high repetition rate, compact mode-locked laser.

[0094] Example 2

[0095] like Figure 3 As shown, the schematic diagram of the overall structure of the linear cavity nonlinear amplification ring mirror mode-locked fiber laser of the alternative embodiment of the present invention includes a first reflector 1, a first polarization beam splitter 2, a quarter-wave plate 3, a first Faraday rotator 4, a half-wave plate 5, a first beam deflector 22, and a second beam deflector 23 arranged sequentially along the optical path direction; a pump optical path is provided on one side of the second beam deflector 23.

[0096] The pump optical path includes a first fiber collimator 8, a gain fiber 9, and a semi-polarization-maintaining filter type wavelength division multiplexer 10 connected in sequence; the light emitted from the semi-polarization-maintaining filter type wavelength division multiplexer 10 passes through a second Faraday rotator 11 and a second reflector 12 in sequence along the optical path direction.

[0097] The second fiber collimator 16 is located beside the optical path of the first polarization beam splitter 2.

[0098] like Figure 3 As shown, the pump input of the semi-polarization-maintaining filter type wavelength division multiplexer 10 is connected to the output of the pump semiconductor laser 13 through a single-mode fiber 18. The pump light output from the pump semiconductor laser 13 is coupled to the pump optical path of the semi-polarization-maintaining filter type wavelength division multiplexer 10 through the single-mode fiber 18 to supply pump energy to the gain fiber 9.

[0099] In Example 2, the difference between Example 2 and Example 1 is that a first beam deflector 22 and a second beam deflector 23 are provided between the half-wave plate 5 and the pump optical path to replace the configuration in Example 1 where a second polarization beam splitter 6 and a third polarization beam splitter 7 are provided between the half-wave plate 5 and the pump optical path.

[0100] The following is in conjunction with Example 2 and the appendix. Figure 3 The configuration of the first beam deflector 22 and the second beam deflector 23 between the half-wave plate 5 and the pump optical path in this application will be described in further detail:

[0101] The first beam deflector 22 is configured as the second beam deflector 23 by rotating 90° clockwise or counterclockwise around the straight line where the gain fiber 9 is located.

[0102] In this alternative, the light output from the first fiber collimator 8 is incident on the second beam deflector 23 and split into a first O-beam and a first E-beam; the first O-beam and the first E-beam continue to propagate to the first beam deflector 22, where a polarization state conversion occurs: the first O-beam is converted into a second E-beam, and the first E-beam is converted into a second O-beam; the second O-beam and the second E-beam are then recombined after passing through the first beam deflector 22.Figure 3 The first optical path 24 indicates the optical path followed by the second E light after the first O light is converted, while the second optical path 25 indicates the optical path corresponding to the second O light after the first E light is converted.

[0103] In Embodiment 2, the configuration of the first reflector 1, the first polarization beam splitter 2, the quarter-wave plate 3, the first Faraday rotator 4, and the half-wave plate 5 arranged sequentially along the optical path, as well as the configuration of the pump optical path and the second fiber collimator 16, the configuration of the second Faraday rotator 11 and the second reflector 12, and the configuration of the semi-polarization-maintaining filter type wavelength division multiplexer 10 and the single-mode fiber 18 are all the same as in Embodiment 1.

[0104] The working mechanism of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser in Example 2 is consistent with that of the linear cavity nonlinear amplifying ring mirror mode-locked fiber laser in Example 1.

[0105] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0106] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all fall within the protection scope of the present invention.

Claims

1. A linear-cavity nonlinear amplifying loop mirror mode-locked fiber laser, characterized in that: The pump light path comprises a first optical fiber collimator, a gain optical fiber, and a half-polarization filter type wavelength division multiplexer connected in sequence. The pump light path comprises a first optical fiber collimator, a gain optical fiber, and a half-polarization filter type wavelength division multiplexer connected in sequence. The third polarization beam splitter is provided with a third mirror on the side of the light path. The first polarization beam splitter is provided with a second optical fiber collimator on the side of the light path. The pump input end of the half-polarization filter type wavelength division multiplexer is connected with the output end of the pump semiconductor laser through a single-mode optical fiber.

2. The linear cavity nonlinear amplification ring mirror mode-locked fiber laser according to claim 1, wherein: The tail fiber slow axis of the first optical fiber collimator is aligned with the horizontal direction of the third polarization beam splitter, so as to ensure that the spatial light output by the third polarization beam splitter is coupled to the first optical fiber collimator; the first optical fiber collimator is connected with the half-polarization filter type wavelength division multiplexer through the gain optical fiber. The tail fiber of the first optical fiber collimator is fused with the slow axis of one end of the gain optical fiber; the tail fiber of the polarization maintaining optical fiber is fused with the slow axis of the other end of the gain optical fiber; the slow axes of the tail fibers of the first optical fiber collimator and the polarization maintaining optical fiber are aligned with each other. The tail fiber slow axis of the polarization maintaining optical fiber is aligned with the horizontal direction of the second Faraday rotator, so as to ensure that the spatial light output by the second Faraday rotator is coupled to the half-polarization filter type wavelength division multiplexer.

3. The linear-cavity mode-locked fiber laser according to claim 2, wherein: The external structure of the half-polarization filter type wavelength division multiplexer is a structure encapsulated by an outer steel tube; the internal structure of the half-polarization filter type wavelength division multiplexer comprises a polarization maintaining optical fiber, a single-mode optical fiber, a double-fiber collimator, and a filter plate. The polarization maintaining optical fiber and the single-mode optical fiber are coupled to the double-fiber collimator respectively; the double-fiber collimator converts the light beams respectively output from the polarization maintaining optical fiber and the single-mode optical fiber into two beams of high-quality collimated parallel light. The filter plate is arranged in the collimated light path output by the double-fiber collimator at an inclination angle of 45°; the filter plate controls the light path based on wavelength: the light of the 1550nm wavelength band has high transmittance, and the light of the 976nm wavelength band has high reflectivity; after the filter plate transmits the 1550nm light and reflects the 976nm light, the split parallel light returns to the double-fiber collimator, is refocused by the double-fiber collimator, and is coupled into the gain optical fiber efficiently.

4. The linear cavity nonlinear amplification ring mirror mode-locked fiber laser according to claim 3, wherein: In the pump light path, the tail fiber of the first optical fiber collimator is a polarization maintaining optical fiber, and the slow axis direction is consistent with the horizontal direction; The gain optical fiber is a polarization maintaining type erbium-doped optical fiber; The pump semiconductor laser is selected to have a wavelength of 976nm and a power of 1W. The polarization maintaining fiber is a PM1550 polarization maintaining fiber, and the single-mode fiber is a HI1060 single-mode fiber, and the tail fiber lengths of the polarization maintaining fiber and the single-mode fiber are both 40 cm; The tail fiber lengths of the first fiber collimator and the second fiber collimator are both 40 cm; The gain fiber length is 42 cm.

5. The linear-cavity mode-locked fiber laser according to claim 1, wherein: The third mirror and the third polarization beam splitter are in a non-coaxial and parallel spatial distribution relationship, specifically a side light path layout in the vertical direction, for reflecting and regulating the S light beam after the light beam is split by the third polarization beam splitter, to realize the functions of polarization state control and mode locking in the laser cavity. The fourth mirror and the second polarization beam splitter are in a non-coaxial and parallel spatial distribution relationship, specifically a side light path layout in the vertical direction, for reflecting and regulating the S light beam after the light beam is split by the second polarization beam splitter, to realize the functions of polarization state control and mode locking in the laser cavity.

6. The linear-cavity mode-locked fiber laser according to claim 1, wherein: The tail fiber slow axis of the second fiber collimator is aligned with the vertical direction of the first polarization beam splitter, to ensure that the spatial light output by the first polarization beam splitter is coupled to the second fiber collimator.

7. The linear-cavity mode-locked fiber laser according to claim 1, wherein: The first Faraday rotator has a working wavelength of 1550 nm or 1030 nm, and a rotation angle of 45° clockwise or counterclockwise; The second Faraday rotator has a working wavelength of 1550 nm or 1030 nm, and a rotation angle of 45° clockwise or counterclockwise.

8. The linear cavity nonlinear amplifying ring mirror mode-locked fiber laser according to any one of claims 1-4 and 6-7, characterized in that: The first mirror, the first polarization beam splitter, the quarter-wave plate, the first Faraday rotator, the half-wave plate, the first beam shifter, and the second beam shifter are sequentially arranged along the light path direction; the second beam shifter is provided with a pump light path on one side; The pump light path comprises a first fiber collimator, a gain fiber, and a half-polarization filter type wavelength division multiplexer connected in sequence; the light emitted by the half-polarization filter type wavelength division multiplexer passes through the second Faraday rotator and the second mirror in sequence along the light path direction; The second fiber collimator is beside the light path of the first polarization beam splitter; The pump input end of the half-polarization filter type wavelength division multiplexer is connected to the output end of the pump semiconductor laser through a single-mode fiber; the pump light output by the pump semiconductor laser is coupled to the pump light path where the half-polarization filter type wavelength division multiplexer is located through the single-mode fiber, to supply pump energy to the gain fiber.

9. The linear-cavity mode-locked fiber laser of claim 8, wherein: The first beam shifter is configured as the second beam shifter, and rotates 90° clockwise or counterclockwise with the gain fiber as the rotation axis.

10. The linear-cavity mode-locked fiber laser of claim 9, wherein: The light output by the first fiber collimator is incident to the second beam shifter, and is split into a first O light and a first E light; the first O light and the first E light continue to transmit to the first beam shifter, and polarization state conversion occurs in the first beam shifter: the first O light is converted into a second E light, and the first E light is converted into a second O light; the second O light and the second E light are recombined after passing through the first beam shifter.

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