Periodically-bifurcated near-transformation limit nanosecond pulse mode-locked fiber laser and generation method

Abstract

The invention discloses a periodic forked near-transformation limit nanosecond pulse mode-locked fiber laser and a generation method. The laser is characterized in that a fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, a polarization analyzer, a dispersion control unit, an output coupler, a fiber isolator and a second polarization controller which are sequentially connected in a closed loop mode. The method comprises the steps that pumping continuous light is coupled into the laser through the wavelength division multiplexer; pumping continuous light is absorbed by using an erbium-doped fiber and is stimulated to radiate a gain pulse with a long wave band, and the generated pulse oscillates in a fiber laser cavity; under the combined action of a first polarization controller, an analyzer and a second polarization controller, an equivalent saturated absorption effect is generated by utilizing nonlinear polarization rotation generated when gain pulses are propagated in an optical fiber, and pulse mode locking is realized; the optical fiber laser works in an abnormal dispersion region, so that generated pulses meet the soliton area theorem, and near-transformation limit pulses are generated. According to the invention, nanosecond pulse period bifurcation output of the short-cavity fiber laser can be realized.

Description

Technical Field

[0001] The invention belongs to the fields of optical engineering, ultrafast nonlinear fiber optical dynamics and fiber laser technology, and specifically relates to a periodically bifurcated near-transformation-limit nanosecond pulse mode-locked fiber laser and a generation method thereof. Background Art

[0002] There are many types of fiber lasers. Fiber lasers operating in Q-switched mode can produce pulse output with pulse width in the nanosecond range. Although Q-switched mode can produce large pulse widths, the corresponding pulse repetition frequency is in the kilohertz range, and the time and frequency domain parameters of the pulse do not satisfy the Fourier transform relationship. Mode-locked fiber lasers can produce pulse output in the femtosecond or picosecond range, with repetition rates typically in the tens of megahertz. Mode-locked fiber lasers can be used as seed sources for master oscillator power amplifier systems (MOPAs). Improving the output pulse width of mode-locked fiber lasers can reduce the cost of MOPAs. By implementing dispersion management in the resonant cavity of a mode-locked fiber laser, stretched pulses or self-similar pulses can be generated in the near-zero dispersion region, which can break the limitations of the "soliton area theorem" and increase the pulse width. In a cavity with dispersion management, when pulses propagate through an optical fiber with normal dispersion, they experience significant pulse broadening. The peak power of the pulse is reduced and the accumulation of nonlinear effects is less, which enables higher pulse energies to be obtained in this case. In the normal dispersion state, the pulse width can be further increased by generating dissipative solitons or dissipative soliton resonant pulses. However, whether it is generated directly by dispersion management or in the normal dispersion state, the pulse always has a frequency chirp. The presence of chirp will affect the amplification process in the subsequent MOPA and introduce additional spectral broadening, thereby damaging the coherence and overall performance of the laser system. The direct generation of chirp-free (transformation-limited) pulses with a pulse duration in the nanosecond range by a mode-locked fiber laser has significant advantages and can meet the needs of pulse amplification with large pulse energy, wide pulse duration and high repetition rate. In order to achieve nanosecond pulse generation, dispersion management technology can be used to effectively increase the cavity length, providing the possibility of pulse energy enhancement, but chirp will be introduced in the output pulse. Adding a narrowband filter in the cavity can produce a large pulse width. However, when the pulse passes through a narrowband filter, a large amount of energy is lost, resulting in a small single pulse energy.

[0003] According to the "soliton area theorem", under the condition that the nonlinear effect in the cavity remains unchanged, the product of the energy and pulse width of the soliton pulse is proportional to the absolute value of the dispersion in the cavity. Increasing the anomalous dispersion in the cavity to increase the pulse width is a possible strategy. However, so far, there has been no report on the generation of nanosecond pulses in fiber lasers based on the "soliton area theorem".

[0004] Periodic bifurcation in fiber lasers refers to the repetition of the parameters of the output pulses at an integer multiple of the cavity length. Periodic bifurcation is one of the intrinsic characteristics of nonlinear systems and is widely present in all nonlinear systems. Because periodic bifurcation is a threshold effect, it can only occur under appropriate conditions when the accumulated nonlinearity in the cavity exceeds a certain threshold. Therefore, reports on periodic bifurcation are common in picosecond / femtosecond mode-locked fiber lasers. One of the applications of the periodic bifurcation phenomenon is that two or more different steady-state outputs can be achieved simultaneously in one laser.

[0005] Traditional chirped pulse amplification technology requires that the seed source pulse be stretched to the nanosecond level before amplification. Directly using near-transformation-limited nanosecond pulses as seed source pulses can greatly simplify the chirped pulse amplification system. Periodically bifurcated fiber lasers can be used as seed sources for optical frequency comb systems. Therefore, fiber mode-locked lasers that can achieve periodic bifurcation near-transformation-limited nanosecond pulse output have great application and research value. Summary of the invention

[0006] In view of the problems existing in the above-mentioned prior art, the present invention provides a periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser and a generation method thereof. The laser has a simple structure and low manufacturing cost, can generate near-transformation-limited nanosecond pulse output, and can realize the periodic bifurcation output of nanosecond pulses of a short-cavity fiber laser; the method has a simple implementation process and low implementation cost, and is based on nonlinear polarization rotation mode-locking technology and pulse shaping technology determined by the soliton area theorem, and can output near-transformation-limited nanosecond pulses that can generate periodic bifurcation in a short-cavity fiber laser.

[0007] In order to achieve the above object, the present invention provides a periodically bifurcated near-transformation-limit nanosecond pulse mode-locked fiber laser, the fiber laser comprising a pump source and a fiber ring cavity;

[0008] The fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, an 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 through a single-mode optical fiber;

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

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

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

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

[0014] The output end of the optical fiber isolator is connected to the input end of the second polarization controller through a single-mode optical fiber;

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

[0016] The dispersion control unit is composed of a grating pair or a prism pair or a chirped fiber Bragg grating or a grating pair with a telescope system. The input end of the dispersion control unit is connected to the output end of the analyzer through a fiber collimator, and the output end is connected to the energy input port of the output coupler through another fiber collimator. The dispersion control unit is used to provide an abnormal large dispersion, and the abnormal dispersion value provided by the dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2 ;

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

[0018] Preferably, the pump source is a single-mode fiber-coupled semiconductor laser, whose central wavelength is 976nm or 1480nm, and whose output pigtail is a single-mode fiber that is single-mode in the 1550nm band, and whose output power is greater than 400mW.

[0019] Preferably, the operating wavelength of the wavelength division multiplexer is 980 / 1550 nm or 1480 / 1550 nm, and the output pigtail thereof is a single-mode optical fiber which is single-mode in the 1550 nm band.

[0020] Preferably, the absorption coefficient of the erbium-doped optical fiber at 1530 nm is greater than 10 dB / m, and the length is greater than 3 meters; or the product of the absorption coefficient and the length is greater than 30 dB.

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

[0022] Preferably, the polarizer is a fiber polarizer, and the output pigtail thereof is a single-mode fiber that is single-mode in the 1550 nm band.

[0023] Preferably, the output pigtail of the output coupler is a single-mode optical fiber which is single-mode in the 1550 nm band.

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

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

[0026] In the present invention, 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 an erbium-doped optical fiber on the output side of the common port of the wavelength division multiplexer, photons can be absorbed and emitted, and the optical signal can be amplified within the range of 1550nm, so that the loss of the optical signal during the transmission process can be effectively compensated, the transmission distance can be extended and the signal quality can be improved. By setting a first polarization controller on the output side of the erbium-doped optical 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 passing through the analyzer can be limited. By setting a dispersion control unit on the output side of the analyzer, it is convenient to use the dispersion control unit to provide abnormal large dispersion, so that the overall dispersion of the optical fiber laser can be made less than -10 5 ps 2 , and then the laser can work in the anomalous dispersion region. Because the laser works in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, the gain pulse will be shaped during the propagation process. This process satisfies the soliton area theorem, so that nanosecond pulses can be obtained and the pulses meet the near-conversion limit. With the increase of pump power, the nonlinearity accumulated by the nanosecond pulses in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, that is, near-conversion limit nanosecond pulses with periodic bifurcation are generated. By setting an output coupler on the output side of the dispersion control unit, the near-conversion limit nanosecond pulses generated in the cavity and the near-conversion limit nanosecond pulses with periodic bifurcation can be output. By setting the optical fiber isolator, the unidirectional operation of the laser can be limited. By setting the second polarization controller on the signal port side of the wavelength division multiplexer, 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 analyzer and the second polarization controller can form a mode-locking start device through the joint action of the first polarization controller, so that the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber can be used to produce an equivalent saturation absorption effect, thereby realizing pulse mode locking.

[0027] The laser has a simple structure and low manufacturing cost. It can generate nanosecond pulse output close to the transformation limit and can realize the periodic bifurcation output of nanosecond pulses of short-cavity fiber lasers.

[0028] The present invention also provides a method for generating periodically bifurcated near-transformation-limited nanosecond pulses, using a periodically bifurcated near-transformation-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: Utilize erbium-doped fiber to absorb continuous pump light and stimulate radiation of long-wave gain pulses, which oscillate in the fiber laser cavity;

[0031] Step 3: The first polarization controller, the analyzer, and the second polarization controller are used together as a mode-locking start device, and the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to produce an equivalent saturation absorption effect, thereby achieving pulse mode locking; at the same time, the counterclockwise transmitted gain pulse passes through the analyzer and enters the dispersion control unit. The anomalous dispersion value provided by the dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2 Because the fiber laser works in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, the pulse shaping effect on the gain pulse during propagation makes the pulse parameters satisfy the soliton area theorem, thereby obtaining nanosecond pulses and the pulses satisfy the near-transformation limit; by introducing a large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining the shorter 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 during the propagation of the nanosecond pulse in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, that is, a near-transformation limit nanosecond pulse with periodic bifurcation is generated.

[0033] The present invention proposes a method for generating near-transformation-limited nanosecond pulses with periodic bifurcation, which uses a dispersion control unit to make the fiber laser work in the anomalous dispersion region, so that the generated gain pulse satisfies the soliton area theorem and generates near-transformation-limited pulses; by introducing a larger anomalous dispersion, the pulse width reaches the nanosecond level while maintaining the shorter cavity length of the fiber laser. The introduction of nonlinear enhancement caused by extending the cavity length is avoided. Near-transformation-limited nanosecond pulses are directly generated in a mode-locked short-cavity single-mode fiber laser. By increasing the pump power, the nonlinearity accumulated by the nanosecond pulse propagating in the cavity is gradually enhanced, thereby generating near-transformation-limited nanosecond pulses with periodic bifurcation.

[0034] The method has a simple implementation process and 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 output near-transformation-limited nanosecond pulses that can produce periodic bifurcation in short-cavity fiber lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A diagram of an experimental device for realizing a near-transformation-limit 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 A time domain diagram of a near-transformation-limit nanosecond pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0037] Figure 3 A near-transformation-limit nanosecond pulse spectrum diagram of a numerically simulated laser output according to an embodiment of the present invention;

[0038] Figure 4 A timing diagram of near-transformation-limit nanosecond pulses of periodic bifurcation output by a numerically simulated laser according to an embodiment of the present invention;

[0039] Figure 5 This is a near-transformation-limit nanosecond pulse spectrum diagram of periodic bifurcation output by a numerically simulated laser according to an embodiment of the present invention.

[0040] In the figure: 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 DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, a periodically bifurcated near-transformation-limit nanosecond pulse mode-locked fiber laser, the fiber laser comprises 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, an 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 through a single-mode optical fiber;

[0045] The erbium-doped optical fiber 3 is a single-mode optical fiber, one end of which 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 optical fiber 3;

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

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

[0049] The output end of the optical fiber isolator 8 is connected to the input end of the second polarization controller 9 through a single-mode optical fiber;

[0050] The first energy output port of the output coupler 7 is connected to the input end of the optical fiber isolator 8 through a single-mode optical 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 optical fiber laser;

[0051] The dispersion control unit 6 is composed of a grating pair or a prism pair or a chirped fiber Bragg grating or a grating pair with a telescope system. The input end of the dispersion control unit 6 is connected to the output end of the analyzer 5 through a fiber collimator, and the 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 an abnormal large dispersion, and the abnormal dispersion value provided by the dispersion control unit 6 makes the overall dispersion value of the fiber laser less than -10 5 ps 2 ;

[0052] As a preferred embodiment, the dispersion control unit 6 is composed of a pair of reflectors and a pair of reflective gratings, and its function is to introduce large anomalous dispersion so that the pulse shaping of the laser satisfies the soliton area theorem.

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

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

[0055] As a preferred embodiment, the pump source 1 is a semiconductor laser coupled to a single-mode fiber, the central wavelength of which is 976nm or 1480nm, and the output pigtail thereof is a single-mode fiber that is single-mode in the 1550nm band, and the output power thereof is greater than 400mW. It is preferred to use a pump source 1 with a central wavelength of 976nm to correspond to the pump absorption peak of the erbium-doped fiber to improve the pumping efficiency, and the output pigtail integrally connected to the pump source 1 is a single-mode fiber that is single-mode in the 1550nm band; as a further preferred embodiment, the output pigtail integrally connected to the pump source 1 is Corning HI1060.

[0056] As a preferred embodiment, the function of the wavelength division multiplexer 2 is to couple the pump light into the resonant cavity, the working wavelength of the wavelength division multiplexer 2 is 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band. As a further preferred embodiment, the output pigtail type of the wavelength division multiplexer 2 is Corning HI1060.

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

[0058] As a preferred embodiment, the first polarization controller 4 is used to adjust the polarization and loss of the light pulse in the resonant cavity. The first polarization controller 4 is a three-coil rotating polarization controller or a squeezed 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] As a preferred embodiment, the polarization analyzer 5 is used to limit the polarization direction of the light pulse passing through the polarization analyzer. The polarization analyzer 5 is a fiber polarization analyzer, and its output pigtail is a single-mode fiber that is single-mode in the 1550nm band. The pigtail type of the polarization analyzer 5 is Corning HI1060.

[0060] As a preferred embodiment, the output pigtail of the output coupler 7 is a single-mode optical fiber that is single-mode in the 1550nm band. As a further preferred embodiment, the output coupler 7 uses an optical fiber coupler with an energy ratio of 20:80, which is used to output the near-transformation-limited nanosecond pulses generated in the cavity and the near-transformation-limited nanosecond pulses of periodic bifurcation; the output pigtail type of the output coupler 7 is Corning HI1060.

[0061] As a preferred embodiment, the optical fiber isolator 8 is used to limit the unidirectional operation of the laser. The optical fiber isolator 8 uses an isolator with a central wavelength of 1550nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band. The output pigtail type of the optical fiber isolator 8 is Corning HI1060.

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

[0063] The present invention innovatively introduces anomalous dispersion, making the overall dispersion of the fiber laser less than -10 5 ps 2 The key to realizing near-transformation-limited nanosecond pulses and periodically bifurcated near-transformation-limited nanosecond pulses by the laser of the present invention is to introduce large anomalous dispersion using a dispersion control unit; the laser operates in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, and the pulse shaping during the propagation of the gain pulse satisfies the soliton area theorem, thereby obtaining nanosecond pulses and the pulses satisfy the near-transformation limit; with the increase of the pump power, the accumulated nonlinearity of the nanosecond pulse propagation in the cavity gradually increases, and after exceeding a certain threshold, periodic bifurcation appears, that is, near-transformation-limited nanosecond pulses with periodic bifurcation are generated.

[0064] The fiber laser of the present invention is verified by numerical simulation. The time domain diagram and spectrum diagram of the near-transformation-limited nanosecond pulse output by the laser obtained by numerical simulation are shown in Figure 2 and 3 As shown in the figure, the timing diagram and spectrum diagram of the near-transformation-limited nanosecond pulse with periodic bifurcation are shown in Figure 4 and 5 As shown. Figure 4 It can be seen that the obvious period 2 evolution occurs. At this time, the near-transformation limit nanosecond pulse has two output states: high pulse intensity and low pulse intensity. Since the pulse intensity is not much different and the pulse width is in the nanosecond range, the spectra of the two output states basically overlap, such as Figure 5 shown.

[0065] The present invention also provides a method for generating periodically bifurcated near-transformation-limited nanosecond pulses, using a periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser, comprising the following steps:

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

[0067] Step 2: Utilize the erbium-doped fiber 3 to absorb the pump continuous light and stimulate the emission of long-wave gain pulses, and the generated gain pulses oscillate in the fiber laser cavity;

[0068] Step 3: The first polarization controller 4, the analyzer 5 and the second polarization controller 9 are used together as a mode-locking start device, and the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to produce an equivalent saturation absorption effect, thereby achieving pulse mode locking; at the same time, the counterclockwise 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 makes the overall dispersion value of the fiber laser less than -10 5 ps 2 Because the fiber laser works in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, the pulse shaping effect on the gain pulse during propagation makes the pulse parameters satisfy the soliton area theorem, thereby obtaining nanosecond pulses and the pulses satisfy the near-transformation limit; by introducing a large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining the shorter 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 during the propagation of the nanosecond pulse in the cavity gradually increases. After exceeding a certain threshold (the threshold is a set value), periodic bifurcation occurs, that is, a near-transformation limit nanosecond pulse with periodic bifurcation is generated.

[0070] The present invention has a simple implementation process and low implementation cost. It is based on nonlinear polarization rotation mode locking technology and pulse shaping technology determined by the soliton area theorem. By allowing the mode-locked short-cavity fiber laser to operate in an anomalous large dispersion region, a near-transformation-limited nanosecond pulse output is generated, and the nanosecond pulse periodic bifurcation output of the short-cavity fiber laser can be achieved.

[0071] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A periodically bifurcated near-transformation-limit nanosecond pulse mode-locked fiber laser, the fiber laser comprising a pump source (1) and a fiber ring cavity (10), characterized in that ； The optical fiber ring cavity (10) comprises a wavelength division multiplexer (2), an erbium-doped optical fiber (3), a first polarization controller (4), an analyzer (5), a second polarization controller (9), an optical 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 optical fiber (3) is a single-mode optical 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 optical fiber (3); The input end of the polarization analyzer (5) is connected to the output end of the first polarization controller (4) via a single-mode optical fiber; The output end 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 optical fiber isolator (8) is connected to the input end of the second polarization controller (9) via a single-mode optical fiber; The first energy output port of the output coupler (7) is connected to the input end of the optical fiber isolator (8) through a single-mode optical 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 optical fiber laser; The dispersion control unit (6) is composed of a grating pair or a prism pair or a chirped fiber Bragg grating or a grating pair with a telescope system. The input end of the dispersion control unit (6) is connected to the output end of the analyzer (5) through a fiber collimator, and the 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 an abnormal large dispersion, and the abnormal dispersion value provided by the dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2 ; The total length of the optical fiber part in the optical fiber laser is less than 10 meters; the optical fiber laser is used to generate stable near-conversion-limited nanosecond pulses and stable periodically bifurcated near-conversion-limited nanosecond pulses, and the near-conversion-limited nanosecond pulses and the stable periodically bifurcated near-conversion-limited nanosecond pulses are both output through the second energy output port of the output coupler (7).

2. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The pump source (1) is a single-mode fiber-coupled semiconductor laser, whose central wavelength is 976nm or 1480nm, whose output pigtail is a single-mode fiber that is single-mode in the 1550nm band, and whose output power is greater than 400mW.

3. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The working wavelength of the wavelength division multiplexer (2) is 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

4. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The erbium-doped optical fiber (3) has an absorption coefficient greater than 10 dB / m at 1530 nm and a length greater than 3 meters; or a product of the absorption coefficient and the length greater than 30 dB.

5. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The first polarization controller (4) is a three-coil rotating polarization controller or a squeezed polarization controller, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

6. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The polarizer (5) is a fiber polarizer, and its output pigtail is a single-mode fiber that is single-mode in the 1550nm band.

7. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The output pigtail of the output coupler (7) is a single-mode optical fiber that is single-mode in the 1550nm band.

8. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The optical fiber isolator (8) is an isolator with a central wavelength of 1550 nm, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550 nm band.

9. The periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser according to claim 1, characterized in that: The second polarization controller (9) is a three-coil rotating polarization controller or a squeezed polarization controller, and its output pigtail is a single-mode optical fiber that is single-mode in the 1550nm band.

10. A method for generating periodically bifurcated near-transformation-limited nanosecond pulses, using a periodically bifurcated near-transformation-limited nanosecond pulse mode-locked fiber laser as claimed in any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: using a pump source (1) to provide a pump continuous light, and coupling the pump continuous light into a fiber laser through a wavelength division multiplexer (2); Step 2: Utilize the erbium-doped fiber (3) to absorb the pump continuous light and stimulate the emission of a long-wave gain pulse, and the generated gain pulse oscillates in the fiber laser cavity; Step 3: The first polarization controller (4), the analyzer (5) and the second polarization controller (9) are used together as a mode-locking start device, and the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to generate an equivalent saturation absorption effect, thereby achieving pulse mode locking; at the same time, the gain pulse transmitted counterclockwise passes through the analyzer (5) and enters the dispersion control unit (6), and the anomalous dispersion value provided by the dispersion control unit (6) makes the overall dispersion value of the optical fiber laser less than -10 5 ps 2 , because the fiber laser works in the anomalous dispersion region and the absolute value of the anomalous dispersion is large, the pulse shaping effect on the gain pulse during propagation makes the pulse parameters satisfy the soliton area theorem, thereby obtaining nanosecond pulses and the pulses satisfy the near-conversion limit; By introducing larger anomalous dispersion, the pulse width can be reduced to nanoseconds while maintaining a shorter 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 during the propagation of the nanosecond pulse in the cavity gradually increases. After exceeding a certain threshold, periodic bifurcation occurs, i.e., near-transformation limit nanosecond pulses with periodic bifurcation are generated.

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