A single-mode fiber laser and method for generating bright and dark solitons

Abstract

A single-mode fiber laser and method for generating bright solitons and dark solitons, the laser comprising: a wavelength division multiplexer connected to a pump source; an erbium-doped fiber connected to one end of the wavelength division multiplexer; a first polarization controller connected to the other end of the erbium-doped fiber; a polarization analyzer connected to an input end of the first polarization controller; a second polarization controller connected to an output end of the polarization analyzer; an output coupler connected to an output end of the second polarization controller, a second output port of the output coupler serving as an output port of the fiber laser; a fiber isolator connected to a first output port of the output coupler; and a band-pass filter connected to an output end of the fiber isolator, the output end of the band-pass filter being connected to the wavelength division multiplexer. The method comprises: using a nonlinear polarization rotation mode-locked technique to make the laser operate in an equivalent saturated absorption region and an equivalent anti-saturated absorption region, respectively, to output bright solitons and dark solitons, respectively. The laser and method can generate bright solitons and dark solitons, respectively.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical engineering, ultrafast nonlinear fiber optics dynamics and fiber laser, and particularly relates to a single-mode fiber laser and a method for generating bright solitons and dark solitons. BACKGROUND

[0002] Soliton refers to a wave packet that can keep parameters unchanged when propagating in a medium. The nonlinear Schrodinger equation can be used to describe the pulse propagation in a fiber system. If the system is energy-conserved, such as the pulse propagation in a lossless fiber, solitons can be generated under appropriate conditions. According to the sign of dispersion, bright solitons and dark solitons can be realized in single-mode fibers with anomalous dispersion and normal dispersion, respectively. A fiber laser is a dissipative system because there is gain and loss in the laser cavity. Strictly speaking, the pulses generated in a fiber laser cannot be regarded as solitons. However, Kelly et al. proved that even in a high-gain fiber laser, as long as the soliton period is longer than the energy variation period, the stable evolution of the pulse can be realized. In the average sense, the stable pulse generated in a fiber laser can be regarded as a soliton. Even in the presence of a large period of energy variation, the pulse propagation in a fiber laser can be approximately described by the nonlinear Schrodinger equation.

[0003] It is found that a fiber laser operating in the anomalous dispersion region can generate bright solitons. Bright solitons refer to pulses with zero background, and dark solitons refer to concave depressions on a certain background. When a fiber laser operates in the normal dispersion region, bright solitons, known as gain-dominated solitons or dissipative solitons, can be obtained if appropriate spectral filtering is applied. A fiber laser operating in the normal dispersion region can also support the generation of dark solitons, so it is possible to support the generation of bright solitons and dark solitons simultaneously by specially designing a fiber laser operating in the normal dispersion region. The realization of bright solitons and dark solitons in a fiber laser will help to enhance the understanding of the pulse shaping mechanism of the fiber laser, and has great application and research value. SUMMARY

[0004] In view of the problems in the prior art, the application provides a single-mode fiber laser and a method for generating bright solitons and dark solitons. The laser has a simple structure and low manufacturing cost, and can generate bright solitons and dark solitons respectively. The method has a simple implementation process and low implementation cost, and can output bright solitons and dark solitons respectively in the same single-mode fiber laser.

[0005] To achieve the above purpose, the application provides a single-mode fiber laser for generating bright solitons and dark solitons, which comprises a pump source and a fiber ring cavity.

[0006] The fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, a polarization analyzer, a second polarization controller, an output coupler, a fiber isolator and a band-pass filter.

[0007] The pump port of the wavelength division multiplexer is connected with the output port of the pump source through a single-mode fiber.

[0008] The erbium-doped fiber is a single-mode fiber, one end of which is connected with the common port of the wavelength division multiplexer.

[0009] The input end of the first polarization controller is connected with the other end of the erbium-doped fiber.

[0010] The input end of the polarization analyzer is connected with the output end of the first polarization controller through a single-mode fiber.

[0011] The input end of the second polarization controller is connected with the output end of the polarization analyzer through a single-mode fiber.

[0012] The energy input port of the output coupler is connected with the output end of the second polarization controller through a single-mode fiber, and the output proportion of the second energy output port of the output coupler is not higher than 25%, which serves as the output port of the fiber laser.

[0013] The input end of the fiber isolator is connected with the first energy output port of the output coupler through a single-mode fiber.

[0014] The input end of the band-pass filter is connected with the output end of the fiber isolator through a single-mode fiber, and the output end thereof is connected with the signal port of the wavelength division multiplexer through a single-mode fiber.

[0015] The fiber laser is used for generating stable bright solitons and dark solitons, and the stable bright solitons and dark solitons are output through the second energy output port of the output coupler respectively.

[0016] As a preferred embodiment, the pump source is a semiconductor laser coupled with a normal dispersion fiber, the center wavelength of which is at 976 nm or 1480 nm, and the output pigtail thereof is a single-mode fiber with normal dispersion at the 1550 nm band, and the output power thereof is greater than 400 mW.

[0017] As a preferred embodiment, the working wavelength of the wavelength division multiplexer is 980 / 1550 nm or 1480 / 1550 nm, and the output pigtail thereof is a single-mode fiber with normal dispersion at the 1550 nm band.

[0018] As a preferred embodiment, the erbium-doped fiber is a fiber with normal dispersion at the 1550 nm band, and the absorption coefficient thereof at 1530 nm is 80 dB / m, and the length thereof is 3 meters.

[0019] As a kind of preference, the first polarization controller is three-piece coil rotation polarization controller or extrusion polarization controller, and the output pigtail of the polarization controller is single-mode fiber with normal dispersion in 1550nm band.

[0020] As a kind of preference, the polarizer is fiber polarizer, and the output pigtail of the polarizer is single-mode fiber with normal dispersion in 1550nm band.

[0021] As a kind of preference, the output coupler is fiber coupler with energy ratio of 25:75, and the output pigtail of the output coupler is single-mode fiber with normal dispersion in 1550nm band.

[0022] As a kind of preference, the fiber isolator adopts isolator with center wavelength of 1550nm, and the output pigtail of the fiber isolator is single-mode fiber with normal dispersion in 1550nm band.

[0023] As a kind of preference, the band-pass filter adopts isolator with center wavelength of 1550nm and 3dB bandwidth of 7.5nm~10.5nm, and the output pigtail of the band-pass filter is single-mode fiber with normal dispersion in 1550nm band.

[0024] In the present application, the pump light emitted by the pump source can be coupled into the resonant cavity through the setting of the wavelength division multiplexer. By setting the erbium-doped fiber on the output side of the common port of the wavelength division multiplexer, the photons can be absorbed and emitted, and the optical signal can be amplified in the range of 1550nm, so that the loss of the optical signal in the transmission process can be effectively compensated, the transmission distance can be prolonged, and the signal quality can be improved. By setting the 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 the polarizer on the output side of the first polarization controller, the polarization direction of the optical pulse passing through the polarizer can be limited. By setting the second polarization controller on the output side of the polarizer, the polarization and loss of the optical pulse in the resonant cavity can be further adjusted. At the same time, the first polarization controller, the polarizer and the second polarization controller arranged in sequence can jointly act, and the laser respectively works in the equivalent saturated absorption region and the equivalent anti-saturated absorption region through the nonlinear polarization rotation, so as to respectively generate bright soliton and dark soliton output. By setting the fiber isolator, the one-way operation of the laser can be limited. The laser has simple structure and low manufacturing cost, and can respectively generate bright soliton and dark soliton output.

[0025] The present application also provides a method for generating bright soliton and dark soliton, which adopts a single-mode fiber laser for generating bright soliton and dark soliton, and includes the following steps:

[0026] Step one: using a pump source to provide pump continuous light, and coupling the pump continuous light into the fiber laser through a wavelength division multiplexer;

[0027] Step two: using the erbium-doped fiber to absorb the pump continuous light, and emitting long-wave band gain pulses under excitation, and the generated gain pulses oscillate in the fiber laser cavity;

[0028] Step three: through the joint action of the first polarization controller, the polarizer and the second polarization controller, and using the nonlinear polarization rotation generated when the gain pulses propagate in the fiber, the fiber laser works in different regions, and at the same time, the gain pulses transmitted counterclockwise are filtered by using the band-pass filter, when the fiber laser works in the saturated absorption region, the dissipative soliton, i.e. the bright soliton, is obtained; when the fiber laser works in the anti-saturated absorption region, the dark soliton is obtained.

[0029] The present application provides a method for generating bright solitons and dark solitons, by making the laser work in the normal dispersion region, using the nonlinear polarization rotation technology, the fiber laser can work in different regions, when working in the equivalent saturated absorption region, i.e. the cavity transmission function is in the positive feedback state, the bright soliton can be generated; when working in the equivalent anti-saturated absorption region, i.e. the cavity transmission function is in the negative feedback state, the dark soliton can be generated. When the stable dark soliton is generated, the background of the dark soliton gradually increases when propagating through the gain fiber, and the darkness of the dark soliton (the ratio of the recessed intensity of the dark soliton to the background) remains unchanged. When the stable bright soliton is generated, because the spectral filtering plays a major role, the spectrum of the bright soliton presents obvious steep edges.

[0030] The method has simple implementation process and low implementation cost, which is based on the pulse shaping technology determined by the band-pass filtering effect, and the pulse shaping technology determined by the balance between the normal dispersion and the nonlinear effect satisfying the nonlinear Schrödinger equation, and uses the nonlinear polarization rotation mode-locking technology to make the laser work in the equivalent saturated absorption region and the equivalent anti-saturated absorption region respectively, so as to output the bright soliton and the dark soliton in the same single-mode fiber laser. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 An experimental device diagram of a single-mode fiber laser for generating bright solitons and dark solitons according to an embodiment of the present application is provided;

[0032] Figure 2 A time-domain diagram of the bright solitons output by the numerical simulation laser according to an embodiment of the present application is provided;

[0033] Figure 3 A spectrum diagram of the bright solitons output by the numerical simulation laser according to an embodiment of the present application is provided;

[0034] Figure 4 A time-domain diagram of a dark soliton output by a laser according to an embodiment of the present application is provided by numerical simulation;

[0035] Figure 5 A spectrum diagram of a dark soliton output by a laser according to an embodiment of the present application is provided by numerical simulation.

[0036] In the figure: 1, a pump source; 2a, a pump port; 2b, a signal port; 2c, a common port; 3, an erbium-doped fiber; 4, a first polarization controller; 5, a polarization analyzer; 6, a second polarization controller; 7, an output coupler; 8, a fiber isolator; 9, a band-pass filter; and 10, a fiber ring cavity. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be apparently and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0038] As shown in the figure, the present application provides a single-mode fiber laser capable of generating bright solitons and dark solitons, which comprises a pump source 1 and a fiber ring cavity 10. Figures 1 to 5

[0039] The fiber ring cavity 10 comprises a wavelength division multiplexer, an erbium-doped fiber 3, a first polarization controller 4, a polarization analyzer 5, a second polarization controller 6, an output coupler 7, a fiber isolator 8, and a band-pass filter 9.

[0040] The pump port 2a of the wavelength division multiplexer is connected to the output port of the pump source 1 through a single-mode fiber.

[0041] 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.

[0042] The input end of the first polarization controller 4 is connected to the other end of the erbium-doped fiber 3.

[0043] The input end of the polarization analyzer 5 is connected to the output end of the first polarization controller 4 through a single-mode fiber.

[0044] The input end of the second polarization controller 6 is connected to the output end of the polarization analyzer 5 through a single-mode fiber.

[0045] The energy input port of the output coupler 7 is connected to the output end of the second polarization controller 6 through a single-mode fiber, and the output proportion of the second energy output port of the output coupler 7 is not higher than 25%, which serves as the output port of the fiber laser. ​

[0046] The input end of the optical fiber isolator 8 is connected with the first energy output port of the output coupler 7 through a single-mode optical fiber;

[0047] The input end of the band-pass filter 9 is connected with the output end of the optical fiber isolator 8 through a single-mode optical fiber, and its output end is connected with the signal port 2b of the wavelength division multiplexer through a single-mode optical fiber;

[0048] The optical fiber laser is used to generate stable bright solitons and dark solitons, and the stable bright solitons and dark solitons are output through the second energy output port of the output coupler 7 respectively.

[0049] As a preferred, the total length of the optical fiber of the optical fiber laser is 5 meters.

[0050] As a preferred, the pump source 1 is a semiconductor laser coupled with a normal dispersion optical fiber, whose central wavelength is at 976 nm or 1480 nm, and the output pigtail is a single-mode optical fiber with normal dispersion at 1550 nm band, and the output power is greater than 400 mW. Preferably, the pump source 1 with a central wavelength of 976 nm is used to correspond to the pump absorption peak of the erbium-doped optical fiber, to improve the pumping efficiency, and the output pigtail integrally connected to the pump source 1 is a single-mode optical fiber with single mode at 1550 nm band; as a further preferred, the output pigtail integrally connected to the pump source 1 is a Corning MetroCor.

[0051] As a preferred, the wavelength division multiplexer is used to couple the pump light into the resonant cavity, and the working wavelength of the wavelength division multiplexer is 980 / 1550 nm or 1480 / 1550 nm, and the output pigtail is a single-mode optical fiber with normal dispersion at 1550 nm band. As a further preferred, the type of the output pigtail of the wavelength division multiplexer is Corning MetroCor.

[0052] As a preferred, the erbium-doped optical fiber 3 is a normal dispersion optical fiber at 1550 nm band, and the absorption coefficient at 1530 nm is 80 dB / m, and the length is 3 meters. As a further preferred, the type of the erbium-doped optical fiber 3 is EDF80, the length is 300 cm, and it is purchased from OFS company, and it has high doping concentration, the absorption peak is 80 dB / m@1530 nm, has strong gain, and the dispersion coefficient at 1550 nm is 40.8 ps 2 / km. Of course, as an alternative, other single-mode erbium-doped optical fibers with normal dispersion at 1550 nm band can also be selected.

[0053] As a preference, the first polarization controller 4 functions to adjust the polarization and loss of the light pulse in the resonant cavity, and the first polarization controller 4 is a three-coil rotating polarization controller or a squeeze polarization controller, and the output pigtail of the first polarization controller 4 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the first polarization controller 4 is a Corning MetroCor.

[0054] As a preference, the polarizer 5 functions to define the polarization direction of the light pulse passing through the polarizer, and the polarizer 5 is a fiber polarizer, and the output pigtail of the polarizer 5 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the polarizer 5 is a Corning MetroCor.

[0055] As a preference, the second polarization controller 6 functions to adjust the polarization and loss of the light pulse in the resonant cavity, and the second polarization controller 6 is a three-coil rotating polarization controller or a squeeze polarization controller, and the output pigtail of the second polarization controller 6 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the second polarization controller 6 is a Corning MetroCor.

[0056] As a preference, the fiber isolator 8 functions to limit the unidirectional operation of the laser, and the fiber isolator 8 uses an isolator having a center wavelength of 1550 nm, and the output pigtail of the fiber isolator 8 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the fiber isolator 8 is a Corning MetroCor.

[0057] As a preference, the output coupler 7 is a fiber coupler having an energy ratio of 25:75, and the output pigtail of the output coupler 7 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the output coupler 7 is a Corning MetroCor.

[0058] As a preference, the bandpass filter 9 uses an isolator having a center wavelength of 1550 nm and a 3 dB bandwidth of 7.5 nm to 10.5 nm, and the output pigtail of the bandpass filter 9 is a single-mode fiber having normal dispersion in the 1550 nm band. As a further preference, the output pigtail of the bandpass filter 9 is a Corning MetroCor.

[0059] The gain fiber and the passive fiber with normal dispersion are used in the fiber laser of the present application. The key to realize the bright soliton and dark soliton output of the laser of the present application is to make the fiber laser work in the normal dispersion region and introduce a 10 nm band-pass filter. When the laser works in the equivalent saturated absorption region, i.e. the cavity transfer function is in the positive feedback state, the pulse shaping of the gain pulse propagation is determined by the band-pass filtering effect of the band-pass filter, so that the bright soliton is generated. When the laser works in the equivalent anti-saturated absorption region, i.e. the cavity transfer function is in the negative feedback state, the influence of the wide band-pass filter on the gain pulse is weak, and the pulse shaping of the gain pulse propagation is determined by the balance between the normal dispersion and the nonlinearity satisfying the nonlinear Schrödinger equation, so that the dark pulse is generated.

[0060] The fiber laser of the present application is verified by numerical simulation. The time-domain graph and the spectrum graph of the bright soliton output by the laser are shown in Figure 2 and 3 respectively. The time-domain graph and the spectrum graph of the dark soliton are shown in Figure 4 and 5 respectively. It can be seen from Figure 2 that the typical steep spectral edge of the dissipative soliton (bright soliton) caused by the band-pass filtering effect in the normal dispersion region; and it can be seen from Figure 4 that the dark soliton is directly recessed to zero, i.e. the blackness of the dark soliton reaches 1.

[0061] The present application also provides a method for generating bright solitons and dark solitons, which uses a single-mode fiber laser for generating bright solitons and dark solitons, and includes the following steps:

[0062] Step one: a pump continuous light is provided by the pump source 1, and the pump continuous light is coupled into the fiber laser through the wavelength division multiplexer;

[0063] Step two: the pump continuous light is absorbed by the erbium-doped fiber 3, and the long-wavelength gain pulse is emitted by the stimulated emission, and the generated gain pulse oscillates in the fiber laser cavity; because all the devices work in the normal dispersion region, the gain pulse is not split, which ensures the stable output of the subsequent bright solitons and dark solitons;

[0064] Step three: through the joint action of the first polarization controller 4, the polarization analyzer 5 and the second polarization controller 6, and by using the nonlinear polarization rotation generated by the gain pulse when propagating in the fiber, the fiber laser works in different regions, and at the same time, the gain pulse propagating in the counterclockwise direction is filtered by the band-pass filter 9. When the fiber laser works in the saturated absorption region, the dissipative soliton, i.e. the bright soliton, is obtained. When the fiber laser works in the anti-saturated absorption region, the dark soliton is obtained.

[0065] In the process, the gain pulse transmitted counterclockwise is filtered by the band-pass filter since the fiber laser works in the normal dispersion region. When the laser works in the saturated absorption region, the pulse shaping of the gain pulse during propagation is mainly determined by the band-pass filtering, so that the dissipative soliton, i.e. the bright soliton, can be obtained. When the laser works in the anti-saturated absorption region, the pulse shaping of the gain pulse during propagation can be described by the nonlinear Schrödinger equation, and is mainly determined by the balance between the nonlinearity and the normal dispersion, so that the dark soliton can be obtained.

[0066] The method has simple implementation process and low implementation cost. The method is based on the pulse shaping technology determined by the band-pass filtering effect and the pulse shaping technology determined by the balance between the normal dispersion and the nonlinearity satisfying the nonlinear Schrödinger equation, and utilizes the nonlinear polarization rotation mode locking technology to make the laser work in the equivalent saturated absorption region and the equivalent anti-saturated absorption region respectively, so as to output the bright soliton and the dark soliton respectively in the same single-mode fiber laser.

[0067] The above only describes the preferred embodiments of the present application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the technical principles of the present application, and these improvements and modifications should also be considered as the protection scope of the present application.

Claims

1. A single-mode fiber laser capable of generating bright and dark solitons, 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, an erbium-doped fiber (3), a first polarization controller (4), an analyzer (5), a second polarization controller (6), an output coupler (7), a fiber isolator (8), and a bandpass filter (9). The pump port (2a) of the wavelength division multiplexer 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; 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 input end of the second polarization controller (6) is connected to the output end of the analyzer (5) via a single-mode optical fiber; The energy input port of the output coupler (7) is connected to the output end of the second polarization controller (6) through a single-mode optical fiber. The output ratio of the second energy output port of the output coupler (7) is not higher than 25%, and it serves as the output port of the fiber laser. The input end of the fiber optic isolator (8) is connected to the first energy output port of the output coupler (7) via a single-mode fiber; The input end of the bandpass filter (9) is connected to the output end of the fiber optic isolator (8) through a single-mode fiber, and its output end is connected to the signal port (2b) of the wavelength division multiplexer through a single-mode fiber. The fiber laser is used to generate stable bright solitons and dark solitons, and the stable bright solitons and dark solitons are output through the second energy output port of the output coupler (7).

2. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The pump source (1) is a semiconductor laser coupled with a normally dispersive fiber, with a center wavelength of 976nm or 1480nm, and its output pigtail is a single-mode fiber with normal dispersion in the 1550nm band, with an output power greater than 400mW.

3. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The wavelength division multiplexer operates at wavelengths of 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber with normal dispersion in the 1550nm band.

4. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The erbium-doped fiber (3) is a fiber with normal dispersion in the 1550nm band, an absorption coefficient of 80dB / m at 1530nm, and a length of 3 meters.

5. A single-mode fiber laser capable of generating bright and dark solitons 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 single-mode fiber with normal dispersion in the 1550nm band; the second polarization controller (6) is a three-coil rotating polarization controller or a squeeze polarization controller, and its output pigtail is a single-mode fiber with normal dispersion in the 1550nm band.

6. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The analyzer (5) is an optical fiber analyzer, and its output pigtail is a single-mode optical fiber with normal dispersion in the 1550nm band.

7. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The output coupler (7) is an optical fiber coupler with an energy ratio of 25:75, and its output pigtail is a single-mode optical fiber with normal dispersion in the 1550nm band.

8. A single-mode fiber laser capable of generating bright and dark solitons 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 single-mode fiber with normal dispersion in the 1550nm band.

9. A single-mode fiber laser capable of generating bright and dark solitons according to claim 1, characterized in that, The bandpass filter (9) uses an isolator with a center wavelength of 1550nm and a 3dB bandwidth of 7.5nm~10.5nm, and its output pigtail is a single-mode optical fiber with normal dispersion in the 1550nm band.

10. A method for generating bright and dark solitons, employing a single-mode fiber laser capable of generating bright and dark solitons as described in any one of claims 1 to 9, 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; 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: Through the combined action of the first polarization controller (4), the analyzer (5), and the second polarization controller (6), and by utilizing the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber, the fiber laser is made to work in different regions. At the same time, the gain pulse transmitted counterclockwise is filtered by the bandpass filter (9). When the fiber laser is working in the saturation absorption region, a dissipative soliton, i.e., a bright soliton, is obtained; when the fiber laser is working in the anti-saturation absorption region, a dark soliton is obtained.

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