GHz high-repetition 9-shaped cavity mode-locked fiber laser and starting method thereof

Abstract

The application discloses a GHz high-repetition 9-shaped cavity mode-locked fiber laser and a starting method thereof, and particularly relates to the technical field of ultrafast laser, a nonlinear amplification ring mirror, and an auxiliary starting light path arranged outside the nonlinear amplification ring mirror and used in a mode-locked starting stage; the auxiliary starting light path comprises a second reflector, a focusing lens and a semiconductor saturable absorber mirror. The GHz high-repetition 9-shaped cavity mode-locked fiber laser and the starting method thereof have the advantages that by introducing an external and separable auxiliary starting light path, seed light with high peak power is formed in the starting stage, the problem that a nonlinear phase shift difference of an extremely short cavity length is insufficient to cause mode-locked difficulty is effectively solved, stable mode locking of GHz-level repetition frequency pulse output is realized, in a steady-state operation stage, the laser completely relies on the nonlinear amplification ring mirror with low noise and high stability to realize mode locking, and high-stability, low-noise and repetition frequency ≥1GHz pulse output is obtained.

Description

Technical Field

[0001] This invention relates to the field of ultrafast laser technology, and in particular to a GHz high repetition rate 9-cavity mode-locked fiber laser and its startup method. Background Technology

[0002] High-repetition-rate ultrafast fiber lasers have significant application value in fields such as optical frequency combs, high-speed optical sampling, and high-speed optical communication systems. Compared to low-repetition-rate lasers, ultrafast lasers with repetition rates reaching the GHz level can provide larger longitudinal mode spacing, which is beneficial for reducing inter-mode crosstalk and improving frequency resolution, showing significant advantages in precision measurement and high-speed signal processing scenarios. Therefore, realizing a high-stability and low-noise GHz-level high-repetition-rate mode-locked fiber laser has always been an important research goal in this field.

[0003] Currently, the main technical approaches to achieving GHz-level high repetition rate pulse output include ultrashort cavity schemes based on semiconductor saturable absorber mirrors, harmonic mode-locking technology, and schemes based on nonlinear polarization rotation. While ultrashort cavity schemes based on semiconductor saturable absorber mirrors can increase the fundamental frequency repetition rate by directly shortening the resonant cavity length, they face challenges in high-power or high-stability applications due to the low damage threshold and slow recovery time of the semiconductor saturable absorber mirrors themselves, as well as potential performance degradation over long-term operation. Harmonic mode-locking technology typically excites higher-order longitudinal modes in longer cavities, but this introduces significant phase noise and supermode noise, leading to increased pulse timing jitter. Furthermore, the system is highly sensitive to pump fluctuations and environmental disturbances, limiting its stability. Schemes based on nonlinear polarization rotation are extremely sensitive to environmental changes; the mode-locked state is easily affected by temperature, vibration, and other disturbances, and recovery after loss of lock is difficult, resulting in insufficient engineering practicality.

[0004] In recent years, figure-9 cavity mode-locked fiber lasers based on nonlinear amplifying ring mirrors have attracted widespread attention due to their structural stability, excellent noise characteristics, and ease of implementation in fully polarization-maintaining fiber systems. However, mode-locking in this laser relies on the accumulation of sufficient nonlinear phase shift difference as light propagates in both the clockwise and counterclockwise directions. This process typically requires fiber lengths on the order of meters, thus limiting the maximum repetition frequency of current figure-9 cavity mode-locked fiber lasers to 700 MHz. When the resonant cavity is compressed to the centimeter level to achieve a fundamental frequency repetition rate on the order of GHz, the gain and nonlinear effects of a single round trip within the cavity are significantly weakened. This prevents the initial pulse from being effectively amplified and accumulated to the phase shift difference required for mode-locking, resulting in the inability to self-initiate mode-locking.

[0005] Therefore, in a 9-cavity mode-locked fiber laser based on a nonlinear amplifying ring mirror, there is a physical contradiction between the extremely short cavity length required to achieve a GHz repetition rate and the sufficient nonlinear phase shift difference on which mode-locking self-starting depends. This contradiction restricts the further improvement of the repetition rate of this type of laser.

[0006] Based on the above problems, there is an urgent need in this field to propose a new GHz high repetition rate 9-cavity mode-locked fiber laser and its startup method, which can effectively overcome the problem of insufficient nonlinear phase shift accumulation under extremely short cavity conditions while maintaining the low noise and high stability characteristics of the 9-cavity laser, thereby realizing a GHz-level repetition rate 9-cavity mode-locked fiber laser. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this application proposes a GHz high repetition rate 9-cavity mode-locked fiber laser and its startup method, in order to solve the problem that current 9-cavity mode-locked fiber lasers cannot achieve GHz-level high repetition rate mode-locked pulse output due to insufficient accumulation of nonlinear phase shift difference at extremely short cavity lengths.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a GHz high repetition rate 9-cavity mode-locked fiber laser, comprising: a nonlinear amplifying ring mirror, and an auxiliary start-up optical path disposed outside the nonlinear amplifying ring mirror for the mode-locking start-up phase; The nonlinear amplifying ring mirror includes: a first pump source, a first wavelength division multiplexing collimator, a first half-wave plate, a second pump source, a second wavelength division multiplexing collimator, a second half-wave plate, a first polarization beam splitter cube, a Faraday rotator, an eighth-wave plate, a second polarization beam splitter cube, a grating pair, a first reflector, and a polarization-maintaining gain fiber. The first pump source is connected to the pump port of the first wavelength division multiplexing collimator, and the second pump source is connected to the pump port of the second wavelength division multiplexing collimator. The first wavelength division multiplexing collimator and the second wavelength division multiplexing collimator are fused together by their respective polarization-maintaining gain fiber pigtails to form the fiber portion of the laser resonant cavity. The Faraday rotator and the eighth-wave plate are configured such that, for clockwise propagating light pulses, their polarization direction is parallel to the slow axis of the eighth-wave plate, and for counterclockwise propagating light pulses, their polarization direction is parallel to the fast axis of the eighth-wave plate, thereby introducing a fixed phase bias between the clockwise and counterclockwise light paths. The auxiliary starting optical path includes: a second reflecting mirror, a focusing lens, and a semiconductor saturable absorber mirror; the second reflecting mirror, the focusing lens, and the semiconductor saturable absorber mirror are sequentially arranged in the transmission optical path of the first polarization beam splitter cube.

[0009] Preferably, the auxiliary start-up optical path is an external optical path, which only works during the mode-locking start-up phase and can be removed from the laser after mode-locking is established.

[0010] Preferably, the grating pair is a transmission diffraction grating pair, disposed between the second polarization beam splitter cube and the first reflecting mirror.

[0011] Preferably, the grating density of the transmission diffraction grating pair is from 600 lines / mm to 1800 lines / mm.

[0012] Preferably, the polarization-maintaining gain fiber is a single-mode polarization-maintaining gain fiber or a double-clad single-mode polarization-maintaining gain fiber.

[0013] Preferably, the Faraday rotator is a thin-film Faraday rotator, or a Faraday rotator consisting of a magneto-optical crystal inserted into a permanent magnet.

[0014] On the other hand, the present invention provides a starting method for the above-mentioned laser, comprising the following steps: S1: Start-up preparation and pulse shaping: The second reflector is placed in the transmission optical path of the first polarization beam splitter cube to construct an auxiliary start-up optical path, so that the pulse light transmitted from the first polarization beam splitter cube can be guided to the semiconductor saturable absorber mirror; at the same time, the first pump source and the second pump source are turned on, so that after the laser starts oscillating, the transmitted pulse propagating clockwise in the nonlinear amplification ring mirror is guided to the semiconductor saturable absorber mirror for pulse shaping and narrowing processing to form a narrowed pulse; S2: Seed injection nonlinear amplification ring mirror: After the narrowed pulse is reflected by the semiconductor saturable absorber mirror, it is collimated by the focusing lens and reflected by the second reflecting mirror in sequence, and then passes through the first polarization beam splitter cube again. It is injected back into the polarization-maintaining gain fiber to rapidly increase the peak power, thereby accelerating the accumulation of nonlinear phase shift difference between the two pulses in the nonlinear amplification ring mirror, so as to satisfy the mode-locking establishment condition. S3: Decoupling of auxiliary start-up optical path: After mode-locking is established, the auxiliary start-up optical path is decoupled from the nonlinear amplifying ring mirror; S4: Self-sustaining mode-locked operation: Under the condition of maintaining the operation of the pump source, the laser enters and maintains a steady-state mode-locked operation dominated by the nonlinear amplifying ring mirror, realizing pulse output with a repetition frequency on the order of GHz.

[0015] Preferably, the decoupling of the auxiliary starting optical path is achieved by removing the second reflecting mirror.

[0016] Preferably, the mode-locking conditions of the laser are determined by monitoring the output pulse sequence and the mode-locking spectrum.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a GHz high repetition rate (RFR) 9-cavity mode-locked fiber laser and its startup method. By introducing an external, separable auxiliary startup optical path, a high peak power seed light is formed during the startup phase, effectively solving the problem of insufficient accumulation of nonlinear phase shift difference due to extremely short cavity length, which leads to difficulty in mode-locking. Stable mode-locking of GHz-level repetition rate pulse output is achieved. During steady-state operation, the laser relies entirely on a low-noise, high-stability nonlinear amplifying ring mirror to achieve mode-locking, obtaining high-stability, low-noise pulse output with a repetition rate ≥1GHz. The auxiliary startup optical path has a simple structure and can be controlled by simply inserting and removing a single mirror, making it convenient and reliable to operate. It can be removed after mode-locking is established, avoiding the thermal effects and performance degradation that may occur in semiconductor saturable absorber mirrors under high RF operation. The laser uses grating pairs to achieve intracavity dispersion compensation and optimize pulse width, while maintaining the compact structure and high stability of the 9-cavity, thus improving the practicality of the system. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of the GHz high repetition rate 9-cavity mode-locked fiber laser provided in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the startup method for a GHz high repetition rate 9-cavity mode-locked fiber laser provided in Embodiment 2 of the present invention.

[0019] The following are the labels in the diagram: 1. First pump source; 2. First wavelength division multiplexing collimator; 3. First half-wave plate; 4. Second pump source; 5. Second wavelength division multiplexing collimator; 6. Second half-wave plate; 7. First polarization beam splitter cube; 8. Faraday rotator; 9. Eighth-wave plate; 10. Second polarization beam splitter cube; 11. Grating pair; 12. First mirror; 13. Polarization-maintaining gain fiber; 14. Second mirror; 15. Focusing lens; 16. Semiconductor saturable absorber mirror. Detailed Implementation

[0020] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0021] Example 1: As Figure 1 As shown, this embodiment provides a GHz high repetition rate 9-cavity mode-locked fiber laser. Its core is: in a 9-cavity mode-locked fiber laser based on a nonlinear amplifying ring mirror, an external and physically separable auxiliary start-up optical path is introduced. This optical path is based on a semiconductor saturable absorber mirror 16, which is used to solve the problem that the nonlinear amplifying ring mirror cannot achieve mode-locking due to insufficient accumulation of nonlinear phase shift difference under the extremely short cavity length condition of GHz.

[0022] Specifically, a GHz high repetition rate 9-cavity mode-locked fiber laser of this embodiment includes: a first pump source 1, a first wavelength division multiplexing collimator 2, a first half-wave plate 3, a second pump source 4, a second wavelength division multiplexing collimator 5, a second half-wave plate 6, a first polarization beam splitter 7, a Faraday rotator 8, an eighth-wave plate 9, a second polarization beam splitter 10, a grating pair 11, a first mirror 12, a polarization-maintaining gain fiber 13, a second mirror 14, a focusing lens 15, and a semiconductor saturable absorber mirror 16.

[0023] For ease of explanation, the component can be divided into two parts: a nonlinear magnifying ring mirror and an auxiliary starting optical path. The nonlinear amplifying ring mirror includes: a first pump source 1, a first wavelength division multiplexing collimator 2, a first half-wave plate 3, a second pump source 4, a second wavelength division multiplexing collimator 5, a second half-wave plate 6, a first polarization beam splitter 7, a Faraday rotator 8, an eighth-wave plate 9, a second polarization beam splitter 10, a grating pair 11, a first reflecting mirror 12, and a polarization-maintaining gain fiber 13; the nonlinear amplifying ring mirror acts as a fast saturable absorber during steady-state operation and is the core component for achieving steady-state, low-noise mode-locking in the laser. The Faraday rotator 8, the 1 / 8 waveplate 9, the second polarization beam-splitting cube 10, and the first reflecting mirror 12 introduce a fixed phase bias between the two propagating light paths, one clockwise and one counterclockwise. When the pulse is split into two paths in the ring mirror and undergoes different nonlinear phase shifts, its interference reflectivity depends on the difference in nonlinear phase shifts between the two paths. The center of the pulse has a higher light intensity and a larger nonlinear phase shift, resulting in a higher interference reflectivity; the edge of the pulse has a weaker light intensity, a smaller nonlinear phase shift, and a lower reflectivity. This intensity-dependent reflection characteristic allows the nonlinear amplifying ring mirror to selectively amplify high-peak pulses and suppress low-intensity noise, thereby achieving stable passive mode-locking.

[0024] The auxiliary start-up optical path includes: a second reflector 14, a focusing lens 15, and a semiconductor saturable absorber mirror 16; before the laser is started, the second reflector 14 is tilted at a 45° angle and placed in the transmission optical path of the first polarization beam splitter 7, which can realize the optical coupling between the auxiliary start-up optical path and the nonlinear amplifying ring mirror; after mode-locking is established, the auxiliary optical path can be decoupled from the main cavity by removing it from the optical path; In this process, when the auxiliary optical path is coupled with the nonlinear amplifying ring mirror, the clockwise propagating pulse transmitted from the first polarization beam-splitting cube 7 is guided to the semiconductor saturable absorber mirror 16 for pulse narrowing. This seed pulse is then reflected back along the original path and injected in the opposite direction into the polarization-maintaining gain fiber 13 for amplification, rapidly increasing the peak power. This process quickly establishes the nonlinear phase shift required for the nonlinear amplifying ring mirror to reach the mode-locking threshold, effectively overcoming the problem of insufficient naturally accumulated nonlinear phase shift under extremely short cavity lengths. Once mode-locking is stably established, the second reflecting mirror 14 is removed from the optical path. Afterward, the laser relies entirely on the fast saturable absorber characteristics of the nonlinear amplifying ring mirror itself to maintain a stable mode-locked pulse output at the GHz fundamental frequency repetition rate, while the auxiliary start-up optical path no longer participates in the operation.

[0025] The specific connection methods and working processes of each component are as follows: The first pump source 1 of the laser is connected to the pump port of the first wavelength division multiplexing collimator 2, and the second pump source 4 is connected to the pump port of the second wavelength division multiplexing collimator 5; the first wavelength division multiplexing collimator 2 and the second wavelength division multiplexing collimator 5 are connected by their respective polarization-maintaining gain fiber 13 pigtails through fusion splicing to form the fiber part of the laser resonant cavity. The oscillation of the laser forms initial small pulses with random fluctuations; An initial small pulse propagating clockwise exits from the first wavelength division multiplexing collimator 2, and after its polarization state is adjusted by the first half-wave plate 3, it is incident on the first polarization beam splitter 7. The p-polarization component of the initial small pulse passes through the first polarization beam splitter 7 and enters the auxiliary start-up optical path in a coupled state. The s-polarization component of the initial small pulse is reflected and enters the linear arm of the nonlinear magnifying ring mirror composed of the Faraday rotator 8, the eighth-wave plate 9, the second polarization beam splitter 10, the grating pair 11, and the first reflecting mirror 12. The initial small pulse of the s-polarized component entering the linear arm of the nonlinear amplifying ring mirror is rotated 45° by the Faraday rotator 8 to adjust its polarization direction to be parallel to the slow axis of the 1 / 8 waveplate 9. The beam after passing through the 1 / 8 waveplate 9 is incident on the second polarization beam splitter cube 10, where the s-polarized light is reflected and output, while the p-polarized light component interferes with the counterclockwise propagating p-polarized light at the second polarization beam splitter cube 10 and continues to propagate through the second polarization beam splitter cube 10. The initial small pulse of the p-polarized component that enters the auxiliary start-up optical path is reflected by the second reflecting mirror 14 and focused by the focusing lens 15, and then incident perpendicularly on the semiconductor saturable absorber mirror 16. The semiconductor saturable absorber mirror 16 uses its saturable absorption characteristics to shape and narrow the incident initial small pulse of the p-polarized component, and reflects the narrowed pulse. The reflected light returns along the original path, passes through the first polarization beam splitter 7 and enters the first wavelength division multiplexing collimator 2, and undergoes rapid gain amplification in the polarization-maintaining gain fiber 13 to form a strong counterclockwise pulse. A strong counterclockwise pulse exits from the second wavelength division multiplexing collimator 5, is adjusted to p-polarized light by the second half-wave plate 6, passes through the first polarization beam splitter 7 and enters the Faraday rotator 8. The Faraday rotator 8 rotates the polarization direction of the incident light by 45°, making the polarization direction parallel to the fast axis of the eighth-wave plate 9. The beam after passing through the eighth-wave plate 9 is incident on the second polarization beam splitter 10, where the s-polarized light is reflected and output, while the p-polarized light component interferes with the clockwise propagating p-polarized light at the second polarization beam splitter 10 and continues to propagate through the second polarization beam splitter 10. The interference pulse light passing through the second polarization beam splitter cube 10, after dispersion compensation by the grating pair 11, is incident perpendicularly on the first reflecting mirror 12. After being reflected by the first reflecting mirror 12, it returns along the same path and passes through the grating pair 11, the second polarization beam splitter cube 10, the 1 / 8 waveplate 9, and the Faraday rotator 8 in sequence. The pulse light undergoes a phase shift after passing through the 1 / 8 waveplate 9 and becomes elliptically polarized light. After passing through the Faraday rotator 8 again, its polarization direction is rotated by 45° and it is then incident on the first polarization beam splitter cube 7. The elliptically polarized pulse is split according to its polarization state by the first polarization beam splitter cube 7. The s-polarized component light is reflected by the first polarization beam splitter 7, and then incident on the first half-wave plate 3 and the second wavelength division multiplexing collimator 5, and enters the polarization-maintaining gain fiber 13, forming a counterclockwise propagation cycle. The p-polarized component light passes through the first polarization beam splitter 7, and then incident on the second half-wave plate 6 and the second wavelength division multiplexing collimator 5, and also enters the polarization-maintaining gain fiber 13. After being amplified in the fiber, it exits from the first wavelength division multiplexing collimator 2, forming a clockwise propagation cycle. Thus, both clockwise and counterclockwise pulses form closed loops, and the laser enters a steady-state oscillation. When the output monitoring device detects that the laser has entered the mode-locked working state, the second reflector 14 is moved out of the transmission optical path of the first polarization beam splitter 7, thereby decoupling the auxiliary start-up optical path from the nonlinear amplification ring mirror. The semiconductor saturable absorber mirror 16 no longer participates in the intracavity optical feedback process, and the laser enters a steady-state working state in which mode-locking is achieved only by the nonlinear amplification ring mirror. In the nonlinear amplifying ring mirror, due to the preset phase bias and the intensity-dependent nonlinear phase shift effect, the pulse center portion gains higher reflectivity and is enhanced, while the pulse edges and noise components are suppressed, thereby achieving stable passive mode-locked operation. Thus, the laser can achieve long-term stable operation at a GHz fundamental frequency, and avoids or reduces the damage threshold limitations, recovery time limitations, and thermal management problems that may arise from using the semiconductor saturable absorber mirror 16 as an intracavity element.

[0026] Example 2: Figure 2 As shown, this embodiment provides a startup method for the laser described in Embodiment 1, used to achieve mode-locked startup and steady-state operation of a nonlinear amplifying ring mirror under extremely short cavity length conditions, including the following steps: S1: During the laser startup phase, the second reflector 14 is placed in the transmission optical path of the first polarization beam splitter 7 at a 45° tilt angle, so that the auxiliary startup optical path is coupled with the nonlinear amplification ring mirror optical path of the laser; then the first pump source 1 and the second pump source 4 are turned on, and the pump power is adjusted to the preset startup power value. S2: Under the action of the pump source, the laser generates an initial oscillating optical signal. The p-polarized component pulse of the clockwise propagating small pulse generated by the nonlinear amplifying ring mirror is coupled to the auxiliary start-up optical path. After being reflected by the second reflecting mirror 14 and focused by the focusing lens 15, it is incident perpendicularly onto the semiconductor saturable absorber mirror 16. After the semiconductor saturable absorber mirror 16 performs pulse shaping and narrowing, it forms a narrowed pulse and returns the reflected light through the original path. After passing through the first polarization beam splitter 7, it enters the first wavelength division multiplexing collimator 2 and undergoes rapid gain amplification in the polarization-maintaining gain fiber 13, thereby increasing the peak power in the cavity and enhancing the accumulation of nonlinear phase shift to meet the nonlinear phase shift conditions required for the nonlinear amplifying ring mirror to establish mode-locking. S3: Monitor the laser output signal. When the output pulse sequence presents a stable periodic waveform in the time domain and the output spectrum presents the spectral shape corresponding to the mode-locked working state, move the second reflector 14 out of the transmission optical path of the first polarization beam splitter 7 to decouple the auxiliary start-up optical path from the nonlinear amplification ring mirror, thereby realizing the switching of the laser from the external trigger auxiliary mode to the fully self-sustaining nonlinear amplification ring mirror mode-locked mode. S4: After completing step 3, the pump source power is kept constant, and the laser enters a steady-state mode-locked operation dominated only by the nonlinear amplification ring mirror mechanism, continuously outputting stable ultrashort pulses at the GHz fundamental frequency repetition frequency.

[0027] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A GHz high repetition rate 9-cavity mode-locked fiber laser, characterized in that, include: A nonlinear amplifying ring mirror, and an auxiliary starting optical path disposed outside the nonlinear amplifying ring mirror for the mode-locking starting stage; The nonlinear amplifying ring mirror includes: a first pump source (1), a first wavelength division multiplexing collimator (2), a first half-wave plate (3), a second pump source (4), a second wavelength division multiplexing collimator (5), a second half-wave plate (6), a first polarization beam splitter (7), a Faraday rotator (8), an eighth-wave plate (9), a second polarization beam splitter (10), a grating pair (11), a first reflector (12), and a polarization-maintaining gain fiber (13). The first pump source (1) is connected to the first wavelength division multiplexing collimator (2). The pump port of the second pump source (4) is connected to the pump port of the second wavelength division multiplexing collimator (5). The first wavelength division multiplexing collimator (2) and the second wavelength division multiplexing collimator (5) are connected through the polarization-maintaining gain fiber (13). The Faraday rotator (8) and the eighth wave plate (9) are configured such that: for clockwise propagating light pulses, their polarization direction is parallel to the slow axis of the eighth wave plate (9) and for counterclockwise propagating light pulses, their polarization direction is parallel to the fast axis of the eighth wave plate (9). The auxiliary starting optical path includes a second reflector (14), a focusing lens (15), and a semiconductor saturable absorber (16), which are sequentially arranged in the transmission optical path of the first polarization beam splitter (7).

2. The GHz high repetition rate 9-cavity mode-locked fiber laser according to claim 1, characterized in that: The auxiliary start-up optical path is an external optical path that only works during the mode-locking start-up phase and can be removed from the laser after mode-locking is established.

3. The GHz high repetition rate 9-cavity mode-locked fiber laser according to claim 1, characterized in that: The grating pair (11) is a transmission diffraction grating pair (11), which is disposed between the second polarization beam splitter cube (10) and the first reflector (12).

4. A GHz high repetition rate 9-cavity mode-locked fiber laser according to claim 3, characterized in that: The grating density of the transmission diffraction grating pair (11) is 600 lines / mm to 1800 lines / mm.

5. A GHz high repetition rate 9-cavity mode-locked fiber laser according to claim 1, characterized in that: The polarization-maintaining gain fiber (13) is a single-mode polarization-maintaining gain fiber or a double-clad single-mode polarization-maintaining gain fiber.

6. A GHz high repetition rate 9-cavity mode-locked fiber laser according to claim 1, characterized in that: The Faraday rotator (8) is a thin-film Faraday rotator (8), or a Faraday rotator (8) composed of a magneto-optical crystal and a permanent magnet.

7. A startup method for a GHz high repetition rate 9-cavity mode-locked fiber laser, applied to the laser according to any one of claims 1-6, characterized in that, The method includes the following steps: S1. Start-up preparation and pulse shaping: The second mirror (14) is placed in the transmission optical path of the first polarization beam splitter (7) to construct an auxiliary start-up optical path so that the pulse light transmitted from the first polarization beam splitter (7) can be guided to the semiconductor saturable absorber mirror (16). At the same time, the first pump source (1) and the second pump source (4) are turned on so that after the laser starts oscillating, the transmission part of the pulse propagating clockwise in the nonlinear amplification ring mirror is guided to the semiconductor saturable absorber mirror (16) for pulse shaping and narrowing to form a narrowed pulse. S2. Seed injection into nonlinear amplification ring mirror: After the narrowed pulse is reflected by the semiconductor saturable absorber mirror (16), it is collimated by the focusing lens (15) and reflected by the second reflecting mirror (14) in sequence, and then passes through the first polarization beam splitter cube (7) again. It is injected into the polarization-maintaining gain fiber (13) to rapidly increase the peak power, thereby accelerating the accumulation of nonlinear phase shift difference between the two pulses in the nonlinear amplification ring mirror, so that it meets the mode-locking establishment condition; S3. Decoupling of auxiliary start-up optical path: After mode-locking is established, the auxiliary start-up optical path is decoupled from the main cavity of the laser; S4. Self-sustaining mode-locked operation: Under the condition of maintaining the operation of the pump source, the laser enters and maintains a steady-state mode-locked operation dominated by the nonlinear amplifying ring mirror, realizing the output of GHz-level repetitive frequency pulses.

8. The startup method according to claim 7, characterized in that: In step S3, the mode-locking stability is determined by monitoring the time-domain waveform of the pulse sequence output by the laser and the output spectrum morphology.

9. The startup method according to claim 7, characterized in that: The decoupling of the auxiliary start-up optical path is achieved by physically removing the second reflector (14).

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