Periodic bifurcation near-transform-limited nanosecond pulse mode-locked fiber laser and generating method

Abstract

The application discloses a near-transform limit nanosecond pulse mode-locked fiber laser with periodic bifurcation and a producing method. The laser is a fiber ring cavity which comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, a polarization detector, a dispersion control unit, an output coupler, a fiber isolator and a second polarization controller which are connected in sequence in a closed loop. The method comprises the following steps: coupling pump continuous light into the laser through the wavelength division multiplexer; absorbing the pump continuous light by the erbium-doped fiber and radiating gain pulses in a long-wave band; oscillating the generated pulses in the fiber laser cavity; through the cooperation of the first polarization controller, the polarization detector and the second polarization controller, generating equivalent saturated absorption effect by using the nonlinear polarization rotation generated when the gain pulses propagate in the fiber, and realizing pulse mode-locking; and making the fiber laser work in an abnormal dispersion region so that the generated pulses satisfy the soliton area theorem and near-transform limit pulses are generated. The application can realize nanosecond pulse periodic bifurcation output of a short-cavity fiber laser.

Description

Technical Field

[0001] This invention belongs to the fields of optical engineering, ultrafast nonlinear fiber optic dynamics, and fiber laser technology, and specifically relates to a near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation and its generation method. Background Technology

[0002] Fiber lasers come in a variety of types. Q-switched fiber lasers can produce pulse outputs with pulse widths on the order of nanoseconds. While Q-switching can produce large pulse widths, the corresponding pulse repetition frequencies are on the order of kilohertz, and the time and frequency domain parameters of the pulses do not satisfy the Fourier transform relationship. Mode-locked fiber lasers can produce pulse outputs on the order of femtoseconds or picoseconds, with repetition rates typically in the tens of megahertz range. Mode-locked fiber lasers can be used as seed sources for master oscillator power amplifier (MOPA) systems. Increasing the output pulse width of mode-locked fiber lasers can reduce the cost of MOPAs. By implementing dispersion management within the resonant cavity of a mode-locked fiber laser, stretched pulses or self-similar pulses can be generated in the near-zero dispersion region, overcoming the limitations of the soliton area theorem and increasing the pulse width. In a dispersion-managed cavity, pulses undergo significant pulse broadening as they propagate through an optical fiber with normal dispersion. The peak power of the pulse decreases, and the accumulation of nonlinear effects is less, making it possible to obtain higher pulse energies in this case. Under normal dispersion conditions, pulse width can be further increased by generating dissipative solitons or dissipative soliton resonance pulses. However, regardless of whether it is generated directly under dispersion management or under normal dispersion conditions, the pulse always has frequency chirp. The presence of chirp affects the subsequent amplification process in MOPA and introduces additional spectral broadening, thereby impairing the coherence and overall performance of the laser system. Mode-locked fiber lasers have significant advantages in directly generating chirp-free (transform-limited) pulses with pulse durations on the order of nanoseconds, which can meet the amplification requirements for pulses with large pulse energy, wide pulse duration, and high repetition rate. To achieve nanosecond pulse generation, dispersion management techniques can be used to effectively increase the cavity length, making pulse energy enhancement possible, but this introduces chirp into the output pulse. Adding a narrowband filter within the cavity can generate a large pulse width. However, when the pulse passes through the narrowband filter, a significant amount of energy is lost, resulting in relatively low single-pulse energy.

[0003] According to the soliton area theorem, under the condition that the intracavity nonlinearity remains unchanged, the product of the soliton pulse energy and pulse width is proportional to the absolute value of the intracavity dispersion. Increasing anomalous dispersion within the cavity to increase the pulse width is a possible strategy. However, to date, there have been no reports of generating nanosecond pulses in fiber lasers based on the soliton area theorem.

[0004] Periodic bifurcation in fiber lasers refers to the repetition of output pulse parameters with a period that is an integer multiple of the cavity length. Periodic bifurcation is an intrinsic characteristic of nonlinear systems and is widespread in all nonlinear systems. Because periodic bifurcation is a threshold effect, it only occurs under suitable conditions when the accumulated nonlinearity within the cavity exceeds a certain threshold. Therefore, reports of periodic bifurcation are common in picosecond / femtosecond mode-locked fiber lasers. One application of periodic bifurcation is to simultaneously achieve two or more different steady-state outputs in a single laser.

[0005] Traditional chirped pulse amplification techniques require broadening the seed source pulse to the nanosecond level before amplification. Directly using a near-transform-limit nanosecond pulse as the seed source pulse can greatly simplify the chirped pulse amplification system. Periodically bifurcated fiber lasers can serve as seed sources for optical frequency comb systems. Therefore, fiber mode-locked lasers capable of achieving near-transform-limit nanosecond pulse output with periodic bifurcation have significant application and research value. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation and a method for generating it. This laser has a simple structure and low manufacturing cost, and can generate near-transform-limited nanosecond pulse outputs, achieving periodic bifurcation output of nanosecond pulses in short-cavity fiber lasers. The method is simple to implement and has low implementation cost. Based on nonlinear polarization rotation mode-locking technology and pulse shaping technology determined by the soliton area theorem, it can output near-transform-limited nanosecond pulses with periodic bifurcation in short-cavity fiber lasers.

[0007] To achieve the above objectives, the present invention provides a near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation, the fiber laser comprising a pump source and a fiber ring cavity;

[0008] The fiber ring cavity includes a wavelength division multiplexer, erbium-doped fiber, a first polarization controller, a polarization analyzer, a second polarization controller, a fiber isolator, an output coupler, and a dispersion control unit.

[0009] The pump port of the wavelength division multiplexer is connected to the output port of the pump source via a single-mode optical fiber.

[0010] The erbium-doped fiber is a single-mode fiber, and one end of it is connected to the common port of the wavelength division multiplexer.

[0011] The input terminal of the first polarization controller is connected to the other end of the erbium-doped fiber;

[0012] The input end of the analyzer is connected to the output end of the first polarization controller via a single-mode optical fiber.

[0013] The output of the second polarization controller is connected to the signal port of the wavelength division multiplexer via a single-mode optical fiber;

[0014] The output of the fiber optic isolator is connected to the input of the second polarization controller via a single-mode fiber.

[0015] The first energy output port of the output coupler is connected to the input end of the fiber optic isolator via a single-mode fiber; the output ratio of the second energy output port of the output coupler is less than 20%, and it serves as the output port of the fiber laser.

[0016] The dispersion control unit consists of a grating pair, a prism pair, a chirped Bragg fiber grating, or a grating pair with a telescope system. The input of the dispersion control unit is connected to the output of the analyzer via a fiber collimator, and its output is connected to the energy input port of the output coupler via another fiber collimator. The dispersion control unit provides anomalous large dispersion, ensuring that the overall dispersion value of the fiber laser is less than -10. 5 ps 2 ;

[0017] The total length of the fiber portion in the fiber laser is less than 10 meters; the fiber laser is used to generate stable near-transform-limited nanosecond pulses and stable periodically bifurcated near-transform-limited nanosecond pulses, both of which are output through the second energy output port of the output coupler.

[0018] As a preferred embodiment, the pump source is a single-mode fiber-coupled semiconductor laser with a center wavelength of 976nm or 1480nm, and its output pigtail is a single-mode fiber that is single-mode in the 1550nm band, with an output power greater than 400mW.

[0019] As a preferred embodiment, the wavelength division multiplexer operates at a wavelength of 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

[0020] As a preferred embodiment, the erbium-doped fiber has an absorption coefficient greater than 10 dB / m at 1530 nm and a length greater than 3 meters; or the product of the absorption coefficient and the length is greater than 30 dB.

[0021] As a preferred embodiment, the first polarization controller is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

[0022] As a preferred embodiment, the analyzer is an optical fiber analyzer, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

[0023] As a preferred embodiment, the output pigtail of the output coupler is a single-mode optical fiber that is single-mode in the 1550nm band.

[0024] As a preferred embodiment, the optical fiber isolator is an isolator with a center wavelength of 1550nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

[0025] As a preferred embodiment, the second polarization controller is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

[0026] In this invention, the pump light emitted from the pump source can be coupled into the resonant cavity by setting up a wavelength division multiplexer (WDM). By setting an erbium-doped fiber on the output side of the common port of the WDM, photons can be absorbed and emitted, and the optical signal can be amplified in the range of 1550 nm, thereby effectively compensating for the loss of the optical signal during transmission, extending the transmission distance, and improving the signal quality. By setting a first polarization controller on the output side of the erbium-doped fiber, the polarization and loss of the optical pulse in the resonant cavity can be adjusted. By setting an analyzer on the output side of the first polarization controller, the polarization direction of the optical pulse transmitted through the analyzer can be limited. By setting a dispersion control unit on the output side of the analyzer, it is convenient to provide anomalous large dispersion, thereby making the overall dispersion of the fiber laser less than -10. 5 ps 2 This allows the laser to operate in the anomalous dispersion region. Because the laser operates in this region and the absolute value of the anomalous dispersion is large, the gain pulse propagation process undergoes pulse shaping, which satisfies the soliton area theorem. This results in nanosecond pulses that satisfy the near-transform limit. As the pump power increases, the nonlinearity accumulated during the propagation of the nanosecond pulse within the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, generating near-transform-limited nanosecond pulses with periodic bifurcation. By setting an output coupler on the output side of the dispersion control unit, both the near-transform-limited nanosecond pulses generated within the cavity and the periodically bifurcated near-transform-limited nanosecond pulses can be output. The use of fiber isolators restricts the unidirectional operation of the laser. By setting a second polarization controller on the signal port side of the wavelength division multiplexer, the polarization and loss of the optical pulses in the resonant cavity can be further adjusted. Simultaneously, the combined action of the first polarization controller, the analyzer, and the second polarization controller forms a mode-locking initiation device. This allows the nonlinear polarization rotation generated during the propagation of the gain pulse within the fiber to produce an equivalent saturable absorption effect, thereby achieving pulse mode-locking.

[0027] This laser has a simple structure and low manufacturing cost. It can generate near-transform-limited nanosecond pulse output and can realize the periodic bifurcation output of nanosecond pulses in short-cavity fiber lasers.

[0028] This invention also provides a method for generating a periodically bifurcated near-transform-limited nanosecond pulse, employing a periodically bifurcated near-transform-limited nanosecond pulse mode-locked fiber laser, comprising the following steps:

[0029] Step 1: Use a pump source to provide continuous pump light, and couple the continuous pump light into the fiber laser through a wavelength division multiplexer;

[0030] Step 2: The erbium-doped fiber absorbs the pump continuous light and is stimulated to emit a long-wavelength gain pulse, which oscillates within the fiber laser cavity.

[0031] Step 3: The first polarization controller, analyzer, and second polarization controller work together as a mode-locking initiation device. The nonlinear polarization rotation generated during the propagation of the gain pulse within the fiber produces an equivalent saturated absorption effect, thus achieving pulse mode-locking. Simultaneously, the counter-clockwise propagating gain pulse passes through the analyzer and enters the dispersion control unit. The anomalous dispersion value provided by the dispersion control unit ensures that the overall dispersion value of the fiber laser is less than -10. 5 ps 2 Because fiber lasers operate in the anomalous dispersion region and the absolute value of anomalous dispersion is large, the pulse shaping effect during gain pulse propagation makes the pulse parameters satisfy the soliton area theorem, thus obtaining nanosecond pulses and the pulses satisfying the near-transform limit; by introducing a large anomalous dispersion, the pulse width can reach the nanosecond level while maintaining the short cavity length of the fiber laser.

[0032] Step 4: Increase the pump power of the pump source. As the pump power increases, the nonlinearity accumulated by the nanosecond pulse propagating in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, that is, a near-transform-limited nanosecond pulse with periodic bifurcation is generated.

[0033] This invention proposes a method for generating near-transform-limited nanosecond pulses with periodic bifurcation. It utilizes a dispersion control unit to operate the fiber laser in the anomalous dispersion region, ensuring the generated gain pulse satisfies the soliton area theorem, thus producing near-transform-limited pulses. By introducing a large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining a relatively short cavity length of the fiber laser. This avoids the introduction of nonlinear enhancement due to extended cavity length. Near-transform-limited nanosecond pulses are directly generated in a mode-locked short-cavity single-mode fiber laser. By increasing the pump power, the nonlinearity accumulated during the propagation of the nanosecond pulse within the cavity gradually increases, thereby generating near-transform-limited nanosecond pulses with periodic bifurcation.

[0034] This method is simple to implement and has low implementation cost. It is based on nonlinear polarization rotation mode-locking technology and pulse shaping technology determined by the soliton area theorem. It uses a dispersion control unit to introduce large anomalous dispersion, aiming to produce near-transform-limited nanosecond pulses with periodic bifurcation in short-cavity fiber lasers. Attached Figure Description

[0035] Figure 1 This is a diagram of an experimental setup for realizing a near-transform-limited nanosecond pulse mode-locked short-cavity single-mode fiber laser capable of generating periodic bifurcation, according to an embodiment of the present invention.

[0036] Figure 2 The present invention provides a numerical simulation of the near-transform-limited nanosecond pulse output of a laser according to an embodiment of the present invention.

[0037] Figure 3 The present invention provides a numerical simulation of the near-transform-limited nanosecond pulse spectrum of a laser output according to an embodiment of the present invention.

[0038] Figure 4 This is a timing diagram of a near-transform-limited nanosecond pulse with periodic bifurcation in the laser output provided by an embodiment of the present invention.

[0039] Figure 5 This is a near-transform-limited nanosecond pulse spectrum of a periodically bifurcised laser output according to an embodiment of the present invention.

[0040] In the diagram: 1. Pump source, 2. Wavelength division multiplexer, 2a. Pump port, 2b. Signal port, 2c. Common port, 3. Erbium-doped fiber, 4. First polarization controller, 5. Analyzer, 6. Dispersion control unit, 7. Output coupler, 8. Fiber isolator, 9. Second polarization controller, 10. Fiber ring cavity. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, a near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation is disclosed, the fiber laser comprising a pump source 1 and a fiber ring cavity 10.

[0043] The fiber ring cavity 10 includes a wavelength division multiplexer 2, an erbium-doped fiber 3, a first polarization controller 4, a polarization analyzer 5, a second polarization controller 9, a fiber isolator 8, an output coupler 7, and a dispersion control unit 6.

[0044] The pump port 2a of the wavelength division multiplexer 2 is connected to the output port of the pump source 1 via a single-mode optical fiber.

[0045] The erbium-doped fiber 3 is a single-mode fiber, and one end of it is connected to the common port 2c of the wavelength division multiplexer 2.

[0046] The input end of the first polarization controller 4 is connected to the other end of the erbium-doped fiber 3;

[0047] The input end of the analyzer 5 is connected to the output end of the first polarization controller 4 via a single-mode optical fiber;

[0048] The output of the second polarization controller 9 is connected to the signal port 2b of the wavelength division multiplexer 2 via a single-mode optical fiber;

[0049] The output of the fiber optic isolator 8 is connected to the input of the second polarization controller 9 via a single-mode fiber.

[0050] The first energy output port of the output coupler 7 is connected to the input end of the fiber isolator 8 via a single-mode fiber; the output ratio of the second energy output port of the output coupler 7 is less than 20%, and it serves as the output port of the fiber laser.

[0051] The dispersion control unit 6 consists of a grating pair, a prism pair, a chirped Bragg fiber grating, or a grating pair with a telescope system. The input of the dispersion control unit 6 is connected to the output of the analyzer 5 via a fiber collimator, and its output is connected to the energy input port of the output coupler 7 via another fiber collimator. The dispersion control unit 6 provides anomalous large dispersion, ensuring that the overall dispersion value of the fiber laser is less than -10. 5 ps 2 ;

[0052] In a preferred embodiment, the dispersion control unit 6 is composed of a pair of mirrors and a pair of reflective gratings, which introduces large anomalous dispersion so that the pulse shaping of the laser satisfies the soliton area theorem.

[0053] The total length of the fiber portion in the fiber laser is less than 10 meters; the fiber laser is used to generate stable near-transform-limited nanosecond pulses and stable periodically bifurcated near-transform-limited nanosecond pulses, both of which are output through the second energy output port of the output coupler 7.

[0054] As a preferred embodiment, the single-mode optical fiber has a wavelength of 1550nm;

[0055] As a preferred embodiment, the pump source 1 is a single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm or 1480 nm, and its output pigtail is a single-mode fiber operating in the 1550 nm band, with an output power greater than 400 mW. Preferably, a pump source 1 with a center wavelength of 976 nm is used to correspond to the pump absorption peak of erbium-doped fiber, thereby improving pump efficiency. Furthermore, the output pigtail integrally connected to the pump source 1 is a single-mode fiber operating in the 1550 nm band. As a further preferred embodiment, the output pigtail integrally connected to the pump source 1 is Corning HI1060.

[0056] Preferably, the wavelength division multiplexer 2 couples the pump light into the resonant cavity. The operating wavelength of the wavelength division multiplexer 2 is 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode fiber operating in the 1550nm band. More preferably, the output pigtail type of the wavelength division multiplexer 2 is Corning HI1060.

[0057] As a preferred embodiment, the erbium-doped fiber 3 has an absorption coefficient greater than 10 dB / m at 1530 nm and a length greater than 3 meters; or the product of the absorption coefficient and the length is greater than 30 dB. As a further preferred embodiment, the erbium-doped fiber 3 is model ED1015-A, with a length of 300 cm, purchased from Yangtze Optical Fibre and Cable (YOFC), and has a high doping concentration, an absorption peak of 20 dB / m at 1529 nm, and strong gain. Of course, other single-mode erbium-doped fibers can also be used.

[0058] As a preferred embodiment, the first polarization controller 4 functions to adjust the polarization and loss of the optical pulses in the resonant cavity. The first polarization controller 4 is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band. The output pigtail of the first polarization controller 4 is Corning HI1060.

[0059] Preferably, the analyzer 5 is used to define the polarization direction of the light pulses transmitted through it. The analyzer 5 is an optical fiber analyzer, and its output pigtail is a single-mode optical fiber that operates in the 1550nm band. The pigtail type of the analyzer 5 is Corning HI1060.

[0060] Preferably, the output pigtail of the output coupler 7 is a single-mode optical fiber that is single-mode in the 1550nm band. More preferably, the output coupler 7 uses a 20:80 fiber coupler, which outputs near-transform-limited nanosecond pulses generated within the cavity and near-transform-limited nanosecond pulses with periodic bifurcation; the output pigtail of the output coupler 7 is Corning HI1060.

[0061] As a preferred embodiment, the fiber optic isolator 8 restricts the unidirectional operation of the laser. The fiber optic isolator 8 employs an isolator with a center wavelength of 1550nm, and its output pigtail is a single-mode fiber operating in the 1550nm band. The output pigtail type of the fiber optic isolator 8 is Corning HI1060.

[0062] As a preferred embodiment, the second polarization controller 9 functions to adjust the polarization and loss of the optical pulses in the resonant cavity. The second polarization controller 9 is a three-coil rotating polarization controller or a squeeze-type polarization controller, and its output pigtail is a single-mode optical fiber that operates in the 1550nm wavelength band. The output pigtail type of the second polarization controller 9 is Corning HI1060.

[0063] This invention innovatively introduces anomalous dispersion, making the overall dispersion of the fiber laser less than -10. 5 ps 2 The key to achieving near-transform-limited nanosecond pulses and periodically bifurcated near-transform-limited nanosecond pulses in this invention lies in introducing large anomalous dispersion using a dispersion control unit. When the laser operates in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, the pulse shaping during the gain pulse propagation process satisfies the soliton area theorem, thus obtaining nanosecond pulses that satisfy the near-transform limit. As the pump power increases, the nonlinearity accumulated during the propagation of the nanosecond pulse in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, resulting in near-transform-limited nanosecond pulses with periodic bifurcation.

[0064] The fiber laser of this invention was verified by numerical simulation. The time-domain plot and spectrum of the near-transform-limited nanosecond pulse output by the laser obtained from the numerical simulation are shown below. Figure 2 and 3 As shown, the timing diagram and spectrum of a near-transform-limited nanosecond pulse with periodic bifurcation are respectively as follows: Figure 4 and 5 As shown. From Figure 4 A clear periodic evolution can be observed, at which point the near-transformation-limit nanosecond pulse exhibits two output states: high pulse intensity and low pulse intensity. Since the pulse intensities are not significantly different and the pulse widths are both on the order of nanoseconds, the spectra of the two output states essentially overlap, as shown below. Figure 5 As shown.

[0065] This invention also provides a method for generating a periodically bifurcated near-transform-limited nanosecond pulse, employing a periodically bifurcated near-transform-limited nanosecond pulse mode-locked fiber laser, comprising the following steps:

[0066] Step 1: Use pump source 1 to provide continuous pump light, and couple the continuous pump light into the fiber laser through wavelength division multiplexer 2;

[0067] Step 2: The erbium-doped fiber 3 absorbs the pump continuous light and is stimulated to emit a long-wavelength gain pulse, which oscillates within the fiber laser cavity.

[0068] Step 3: The first polarization controller 4, the analyzer 5, and the second polarization controller 9 work together as a mode-locking initiation device. The nonlinear polarization rotation generated when the gain pulse propagates within the fiber produces an equivalent saturated absorption effect, thus achieving pulse mode-locking. Simultaneously, the counter-clockwise transmitted gain pulse passes through the analyzer 5 and enters the dispersion control unit 6. The anomalous dispersion value provided by the dispersion control unit 6 ensures that the overall dispersion value of the fiber laser is less than -10. 5 ps 2 Because fiber lasers operate in the anomalous dispersion region and the absolute value of anomalous dispersion is large, the pulse shaping effect during gain pulse propagation makes the pulse parameters satisfy the soliton area theorem, thus obtaining nanosecond pulses and the pulses satisfying the near-transform limit; by introducing a large anomalous dispersion, the pulse width can reach the nanosecond level while maintaining the short cavity length of the fiber laser.

[0069] Step 4: Increase the pump power of pump source 1. As the pump power increases, the nonlinearity accumulated by the nanosecond pulse propagating in the cavity gradually increases. After exceeding a certain threshold (which is a set value), periodic bifurcation occurs, that is, a near-transform limit nanosecond pulse with periodic bifurcation is generated.

[0070] The present invention is simple to implement and has low implementation cost. Based on nonlinear polarization rotation mode-locking technology and pulse shaping technology determined by the soliton area theorem, it generates near-transform-limited nanosecond pulse output by making the mode-locked short-cavity fiber laser work in the anomalous large dispersion region, and can realize the periodic bifurcation output of the nanosecond pulse of the short-cavity fiber laser.

[0071] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation, the fiber laser comprising a pump source (1) and a fiber ring cavity (10), characterized in that... ； The fiber ring cavity (10) includes a wavelength division multiplexer (2), an erbium-doped fiber (3), a first polarization controller (4), an analyzer (5), a second polarization controller (9), a fiber isolator (8), an output coupler (7), and a dispersion control unit (6). The pump port (2a) of the wavelength division multiplexer (2) is connected to the output port of the pump source (1) via a single-mode optical fiber; The erbium-doped fiber (3) is a single-mode fiber, one end of which is connected to the common port (2c) of the wavelength division multiplexer (2). The input end of the first polarization controller (4) is connected to the other end of the erbium-doped fiber (3); The input end of the analyzer (5) is connected to the output end of the first polarization controller (4) through a single-mode optical fiber; The output of the second polarization controller (9) is connected to the signal port (2b) of the wavelength division multiplexer (2) via a single-mode optical fiber; The output end of the fiber optic isolator (8) is connected to the input end of the second polarization controller (9) via a single-mode fiber; The first energy output port of the output coupler (7) is connected to the input end of the fiber isolator (8) through a single-mode fiber; the output ratio of the second energy output port of the output coupler (7) is less than 20%, and it serves as the output port of the fiber laser. The dispersion control unit (6) is composed of a grating pair, a prism pair, or a chirped Bragg fiber grating. The input end of the dispersion control unit (6) is connected to the output end of the analyzer (5) through a fiber collimator, and its output end is connected to the energy input port of the output coupler (7) through another fiber collimator. The dispersion control unit (6) is used to provide anomalous dispersion, and the anomalous dispersion value it provides makes the overall dispersion value of the fiber laser less than -10. 5 ps 2 ; The total length of the fiber portion in the fiber laser is less than 10 meters; the fiber laser is used to generate stable near-transform-limited nanosecond pulses and stable periodically bifurcated near-transform-limited nanosecond pulses, both of which are output through the second energy output port of the output coupler (7). The pump source (1) is a single-mode fiber-coupled semiconductor laser with a center wavelength of 976nm or 1480nm, an output pigtail of 1550nm single-mode fiber, and an output power greater than 400mW. The erbium-doped fiber (3) has an absorption coefficient greater than 10 dB / m at 1530 nm and a length greater than 3 meters, or the product of the absorption coefficient and the length is greater than 30 dB.

2. The near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The wavelength division multiplexer (2) operates at a wavelength of 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a 1550nm single-mode optical fiber.

3. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The first polarization controller (4) is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a 1550nm single-mode optical fiber.

4. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The analyzer (5) is an optical fiber analyzer, and its output pigtail is a 1550nm single-mode optical fiber.

5. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The output pigtail of the output coupler (7) is a 1550nm single-mode optical fiber.

6. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The fiber optic isolator (8) is an isolator with a center wavelength of 1550nm and its output pigtail is a 1550nm single-mode fiber.

7. A near-transform-limited nanosecond pulse mode-locked fiber laser with periodic bifurcation according to claim 1, characterized in that, The second polarization controller (9) is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a 1550nm single-mode optical fiber.

8. A method for generating a periodically bifurcated near-transform-limited nanosecond pulse, comprising a periodically bifurcated near-transform-limited nanosecond pulse mode-locked fiber laser as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Use a pump source (1) to provide continuous pump light, and couple the continuous pump light into the fiber laser through a wavelength division multiplexer (2); Step 2: Use erbium-doped fiber (3) to absorb pump continuous light and stimulate emission of long-wavelength gain pulses, which oscillate within the fiber laser cavity. Step 3: The first polarization controller (4), the analyzer (5), and the second polarization controller (9) work together as a mode-locking initiation device. The nonlinear polarization rotation generated when the gain pulse propagates in the fiber produces an equivalent saturated absorption effect, thereby achieving pulse mode-locking. At the same time, the counterclockwise transmitted gain pulse enters the dispersion control unit (6) after passing through the analyzer (5). The anomalous dispersion value provided by the dispersion control unit (6) makes the overall dispersion value of the fiber laser less than -10. 5 ps 2 Because fiber lasers operate in the anomalous dispersion region and the absolute value of anomalous dispersion is large, the pulse shaping effect during gain pulse propagation makes the pulse parameters satisfy the soliton area theorem, thus obtaining nanosecond pulses and the pulses satisfying the near-transform limit. By introducing anomalous dispersion, the pulse width can reach the nanosecond level while maintaining the short cavity length of the fiber laser. Step 4: Increase the pump power of the pump source (1). As the pump power increases, the nonlinearity accumulated by the nanosecond pulse in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, that is, a near-transform limit nanosecond pulse with periodic bifurcation is generated.

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